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This is to certify that the

dissertation entitled

SYNTHESIS, CHARACTERIZATION AND REACTIVITY
OF LATE TRANSITION METAL BORYL COMPLEXES

presented by

Carl N. Iverson

has been accepted towards fulfillment
of the requirements for

Ph. D. Chemistry

 

 

degree in

 

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Date 8/27/99

 

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1M GENO/0.0039659.“

SYNTHESIS, CHARACTERIZATION AND REACTIVITY OF LATE

TRANSITION METAL BORYL COMPLEXES

Carl N. Iverson

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY
Department of Chemistry

1999

ABSTRACT

SYNTHESIS, CHARACTERIZATION AND REACTIVITY OF LATE

TRANSITION METAL BORYL COMPLEXES

By

Carl N. Iverson

The hydroboration of unsaturated substrates is a well-known method used for
synthesizing organic compounds that contain a wide variety of functional groups.
Transition metal catalyzed hydroboration using simple borane analogues was introduced
in 1975 using Wilkinson’s catalyst and offer the potential control of chemo-, regio- and
stereochemical addition to organic substrates. More recently, alternative borane
compounds (E—BXZ, E = boryl, silyl, and stannyl groups) have been introduced into
catalytic addition reactions, allowing for multiple functionalizations of unsaturated
substrates.

For instance, catalytic diboration of alkynes using PinB—BPin (Pin = 02C6H122') with
(PPh3)4Pt has been reported. A catalytic cycle that involved alkyne association and
insertion into the Pt—B bond of a novel bisphosphinediborylplatinum(II) complex as the
key steps was invoked. To better understand the catalytic process, the reactivity of the
catalytically active species in simple stoichiometric reactions must be studied. Boryl
compounds, and in particular, (PPh3)2Pt(BCat)2, were synthesized via oxidative addition
of CatB—BCat (Cat = 02C6H42') to the platinum olefin complex, (PPh3)2Pt(n2—C2H4),

with loss of ethylene. The diboryl complex cleanly diborates alkynes to form

(PPh3)2Pt(n2—alkyne) and the 1,2—diborated—alkene. The clean reactivity allowed the
mechanistic details of this addition to be examined. Kinetic studies support phosphine
dissociation as the key step prior to association of the alkyne and subsequent insertion
between the Pt—B bond.

Kinetic studies of the catalytic reaction bore many similarities to the stoichiometric
addition. An analysis of the rate laws allowed us to directly compare rate constants for
the two mechanisms. The identical calculated values (within experimental error) are
consistent with a catalytic manifold that is indistinguishable to the stoichiometric
mechanism. These studies led to the use of base—free Pt complexes as catalysts that would
diborate olefins, since the bisphosphine compound accommodates only alkynes.

Activation and functionalization of hydrocarbons has been an area of fundamental
interest in organometallic chemistry. Systems where alkane and arene activation have
been studied in-depth and are well-understood are Cp*M(PMe3)(R)H (M = Ir, Rh; R = H,
alkyl, aryl). Cp*Ir(PMe3)H2 reacts with E—B(OR)2 (E = H, B(OR)2) reagents in
hydrocarbon solvents at elevated temperatures to produce thermally stable iridium boryls
and H—E. Alternatively, the iridium-borane adduct, Cp*Ir(PMe3)H[HBH(C6F5)2], is
formed when the electrophilic borane, [HB(C6F5)2]2, is used. Therrnolysis of this
compound results in formation of an iridium boryl with loss of H2. Cp*Ir(PMe3)(cy-
C6H”)H and Cp*Ir(PMe3)(C6H5)H react with HB(OR)2 at elevated temperatures to form
both B—Ir and B—C bond containing products. The ratio of products can be controlled
through manipulation of temperature and concentration of HB(OR)2. Exclusive B-C
bond formation, however, is observed when [HB(C6F5)2]2 is added to Cp*Ir(PMe3)(cy-
C6H“)H. Although inefficient, a combined thermal and photochemical approach would
constitute a catalytic cycle. Arene solutions of Cp*Ir(PMe3)(BPin)H were thermally
stable, but when heated in the presence of HBPin catalytic formation of aryl—BPin was

observed.

To my family

iv

ACKNOWLEDGEMENTS

This work would not have been possible without the assistance of my advisor, Mitch.
I would be remiss if I did not thank him for all he has done for me. He taught me the
qualities necessary for pursuing the truth in science and gave me the opportunity to
discover some exceptionally amazing chemistry. It was hard work but he never left me
hanging and I will always appreciate that.

My thanks also go out to the first rate staff and specialists at Michgan State
University; most especially, Scott and Manfred in the glass shop. They consistently
produced high quality craftsmanship and taught me how to blow glass the scalable NMR
tubes that were an integral part of my work.

I will value the friendships I gained in my time at MSU. First and foremost among
them are Dean Lantero, Doug Motry, and Baixin Qian. Working with them in the lab day
in and day out made my graduate experience a bit more passable. A good friend who
really gave me a second family is Lee Hoffman. I will always appreciate the times he
invited me to holiday dinners when I was unable to make it home. My relationships with
Gordon Galloway and Jim Hoeschele started while I was teaching for them but have
continued on as wonderful friendships. Others I must mention are those friends whom I
met early on, J .P., Calvin, Jason, and Joy; and friends of a more recent vintage, Joe, Brad,
Phillip, Heather, Sheila, Michelle, Chris, and the other Brad. I’ve had good times with all
of them.

Finally, I would like to thank my family for all the love, support, and inspiration they
have given me over the years. I could not have gotten through this without them. It really
is too difficult to express how much they all mean to me. So, to Mom and Dad, Jeanine
and her family, Jolene, Eric and Brooke, Jennifer and Kyle, Grandma, all sorts of Uncles

and Aunts, assorted cousins and other close friends, thank you so much.

TABLE OF CONTENTS

LIST OF TABLES ................................................................................... ix

LIST OF FIGURES .................................................................................. x

LIST OF SYMBOLS AND ABBREVIATIONS .............................................. xiv

CHAPTER 1

INTRODUCTION .................................................................................... 1
Reasons For Studying Transition Metal Boryl Complexes ................................. 1
Transition Metal Boryl Complexes ............................................................ 2
Transition Metal Catalyzed Boration .......................................................... 5
Other Boration Reagents ........................................................................ 8
Alkane Functionalization ...................................................................... 11

CHAPTER 2

SYNTHESIS, CHARACTERIZATION AND REACTIVITY OF GROUP 10 BORYL

COMPLEXES ....................................................................................... 16
Preparation and Reactivity of Diboryl Complexes of Platinum .......................... 17
Simple Reactivity of Diboryl Complexes ................................................... 22
Preparation and Characterization of a Stannylboryl Complex of Platinum ............. 24
Metathesis Reactivity of Group 10 Complexes ............................................. 28
Substituent Effects on Boryl Ligands ........................................................ 30

CHAPTER 3

PLATINUM MEDIATED DIBORATION OF OLEFINS AND ALKYNES .............. 33
Mechanism of Stoichiometric Diboration of Alkynes ..................................... 33
Mechanism of Catalytic Diboration of Alkynes ............................................ 45
Mechanistic Concerns Regarding cis-(PPh3)2Pt(BPin)2 ................................... 51
Catalytic Diboration of Olefins ............................................................... 54

vi

CHAPTER4
SYNTHESIS, CHARACTERIZATION AND REACTIVITY OF IRIDIUM BORYL

COMPLEXES ....................................................................................... 61
Reactions of Cp*Ir(PMe3)H2 With H—B(OR)2 and (RO)zB—B(OR)3 Bonds ............ 62
Kinetics Experiments ........................................................................... 65
Reaction of Cp*Ir(PMe3)H2 With the Electrophilic Borane, HB(C6F5)2 ................ 72
Stoichiometric C—B Bond Forming Reactions ............................................. 79
Catalytic Boration of Arenes .................................................................. 88

CHAPTER 5

EXPERIMENTAL ................................................................................. 100
General Considerations ....................................................................... 100
Syntheses ....................................................................................... 102
NMR Tube Reactions ........................................................................ 106
Kinetics Experiments ......................................................................... 1 1 1
Catalytic Diboration of Alkynes ............................................................ 113
Catalytic Stannaboration of Alkynes ....................................................... 114
Catalytic Diboration of Olefins .............................................................. 116
Catalytic Boration of Arenes ................................................................ 120
Crystal Structure Determinations and Refinement ........................................ 121

APPENDIX A

Summary of Crystal Data and Structure Refinement ........................................... 123

APPENDIX B

Derivation of Rate Expressions For Chapter 3 .................................................. 126

APPENDIX C

Kinetic Details ....................................................................................... 131
Platinum Kinetics .............................................................................. 131
Iridium Kinetics ................................................................................ 134

vii

REFERENCES ...................................................................................... 135

viii

Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Table 8.

LIST OF TABLES

Selected bond lengths and angles for 2 ................................................ 21
Selected bond lengths and angles for 2 from ref. 91 ................................. 21
Selected bond lengths and angles for 4 ................................................ 25
Products and crude yields for the diboration of selected olefins ................... 56
Selected bond lengths and angles for 29 ............................................... 63
Selected bond lengths and angles for 45 ............................................... 73
Selected bond lengths and angles for 46 ............................................... 78
Reaction conditions and yields for the addition of HBPin to 28 ................... 83

ix

LIST OF FIGURES

Figure 1. Boryl ligand 1t interactions involving A) boryl substituent orbitals and B) metal
frontier orbitals with the p orbital on boron ................................................... 2

Figure 2. Product characterization of the reaction of BF3 with szWHz from 1963 (top
reaction) and 1996 (bottom reaction) .......................................................... 4

Figure 3. Continuum of metal-boron bonding in simple metal-boron complexes ............ 5

Figure 4. Comparison of chemoselectivity for catalyzed and uncatalyzed

hydroborations .................................................................................... 6
Figure 5. Proposed catalytic cycle for olefin hydroboration by Wilkinson’s catalyst ....... 7
Figure 6. Comparison in reactivity of HBCat versus HB(C6F5)2 ............................... 9
Figure 7. Sequential substitution scheme utilizing heteroborane substrates ................. 10
Figure 8. Systems studied for C—H activation ................................................... 14
Figure 9. Scheme depicting the preparation of platinum boryl complexes .................. 17

Figure 10. 3'P NMR spectra of 2 at A) 20 OC and B) —50 °C. For both spectra, 512
transients were taken of a 32 mM solution in CDC13 ...................................... 19

Figure 11. 11B NMR spectrum (quartz NMR tube) of 2 at room temperature. The broad
features at -0.6 and -50 ppm are due to borosilicate glass in the probe ................ 19

Figure 12. ORTEP diagram of 2. Thermal ellipsoids are at 25 % probability. Only the
ipso carbons of the triphenylphosphine ligands are shown for clarity ................... 21

Figure 13. Scheme depicting the reactivity of 2 ................................................ 23

Figure 14. Two different views of compound 4. Thermal ellipsoids are at 30 %. Only the
ipso carbons of the triphenylphosphine ligands are shown for
clarity ............................................................................................. 25

Figure 15. Variable temperature NMR of the amino methyl groups of 4 in CD2C12.
Starred resonances at 5 2.3 and 2.55 indicate impurities .................................. 27

Figure 16. Reactivity of B2Cat2 with dialkyl and diaryl bisphosphine platinum
complexes ....................................................................................... 29

Figure 17. Proposed mechanism for the reaction of BzCatz with 9 ........................... 30

Figure 18. Simple orbital diagrams illustrating 1t bonding effects in boryl complexes. . ..31
Figure 19. Simple diagrams illustrating inductive effects in boryl complexes .............. 32

Figure 20. Pathway proposed by Miyaura and Suzuki for the platinum catalyzed
diborationofalkynes............................................................................34

Figure 21. Simple bimolecular pathways for the diboration reaction ......................... 36

Figure 22. Plots of 1n[2] versus time for the reaction of 2 with 4-octyne at 70 0C. ([2]., =

8.7 mM; 0 [4-octyne]., = 27 mM; [4-octyne]., = 52 mM) .............................. 37
Figure 23. Plot of l/k.,.,S versus [PPh3] for the reaction of 2 with 4-octyne at 90 °C in

C6D6. ([2]., = 8.7 mM, [4-octyne]., = 52 mM) .............................................. 38
Figure 24. Steady state pathway for the diboration reaction .................................. 39
Figure 25. Equilibrium pathway for the diboration reaction .................................. 40

Figure 26. Plot of ln{[2]./[2].,] versus time for the reaction of 2 with 4-octyne in the
presence of PPh3-d15 at 105 °C in C.,D.,. ([2]., = 8.7 mM, [PPh3-d15] = 27 mM; X [4-
octyne]., = 27 mM; O [4-octyne]., = 52 mM; [:1 [4-octyne]., = 105 mM; 0 [4-octyne].,
= 208 mM) ....................................................................................... 42

Figure 27. Eyring plot for the diboration of 4-octyne by 2 in the presence of PPh3-d15 in
C6D.,. ([2]., = 8.7 mM; [4-octyne]., = 52 mM; [PPh3-d15] = 8.7 mM; T = 343 to 383 K,

AH‘ = 25.90) kcal/mol, As1 = 4(2) e.u.) .................................................... 44
Figure 28. Steady state pathway for the catalytic diboration of alkynes ..................... 47
Figure 29. Equilibrium pathway for the catalytic diboration of alkynes ..................... 47

Figure 30. Plot of ln{[4-octyne]./[BZCat2].,} vs. time for the catalytic diborylation of 4-
octyne by BzCatz in the presence of 2 and PPh3 at 105 °C in C7D3 ([2]., = 7.0 mM,
[PPh3] = 55 mM; 0 [4-octyne]., = 42 mM, [BzCatz]., = 84 mM; 0 [4-octyne]., = 42
mM, [BzCatz]., = 42 mM; + [4-octyne]., = 84 mM, [B2Cat2]., = 42 mM) ................ 49

Figure 31. Plot of 1/k.,.,. vs. [PPh3] for the catalytic diborylation of 4-octyne by BzCatz in
the presence of 2 and PPh3 at 100 °C in C.,D., ([2]., = 7.0 mM, [4-octyne]., = 42 mM,
[B2Cat2]., = 42 mM) ............................................................................ 50

Figure 32. van’t Hoff plot for the equilibrium between 3 and 4-octyne. ([3]., = 8.7 mM;
[4-octyne]., = 52 mM; T = —80 °C to 0 °C; AH = 0.49 kcal/mol, AS = 6.6 cu). . . . . .53

Figure 33. Proposed mechanism for the diboration of olefins by base-free Pt(0)
complexes ...................................................................................... 58

Figure 34. Decomposition pathway for internal olefins via B-hydrogen elimination. .....59

xi

Figure 35. Selective di- and tetraboration of 1,5-hexadiene ................................... 60

Figure 36. Reaction scheme for the formation of iridium boryls from the reaction of 28
with HBCat and BzCatg ........................................................................ 63

Figure 37. Molecular structure of 29. Thermal ellipsoids are at 50% probability .......... 63

Figure 38. Reaction scheme for the formation of iridium boryls with HBPin and
szinz ............................................................................................. 65

Figure 39. Eyring plot for the reaction of 28 with HBCat in C6D6. from 20 °C to 70 °C
([28]., = 41 mM; [HBCat]., = 245 mM) ...................................................... 66

Figure 40. Plots of ln{ [Cp*Ir(PMe3)X2]./[Cp*1r(PMe3)X2]0} versus time for the reaction
of Cp*1r(PMe3)X2 with XBCat. ([Cp*Ir(PMe3)X2]., = 41 mM; [XBCat]., = 245 mM;

0 (X = D); (X = H)) ......................................................................... 67
Figure 41. Oxidative addition pathways for the formation of 30 ............................. 68
Figure 42. Ion pair and borane adduct pathways for the formation of 30 ................... 69

Figure 43. Degenerate hydride exchange in the reaction of Cp*Ir(PMe3)H2 with
DBCat ............................................................................................ 70

Figure 44. Plot of ln{[28]./[28]o} versus time for the reaction of Cp*Ir(PMe3)H2 with
HBPin. ([Cp*Ir(PMe3)I-12]., = 41 mM; [HBPin]o = 254 mM) ............................. 71

Figure 45. Two different views of 45. Thermal ellipsoids are at 30% probability. In (A)
all hydrogen atoms except H1 are omitted and only the ipso carbons of the
pentafluorophenyl groups are shown and in (B) all hydrogen atoms except H1 and all

fluorine atoms are omitted for clarity ........................................................ 73
Figure 46. 1H NMR of the hydride region of 45 in CDCle at —105 °C ..................... 74
Figure 47. Eyring plot for the exchange between Ha and Hb of 45 in CDCle from —105

°C to -55 °C ...................................................................................... 75
Figure 48. Exchange between H.I and H, in 45 .................................................. 76

Figure 49. Two different views of 46. Thermal ellipsoids are at 30% probability. In (A)
all hydrogen atoms except H1 and all fluorine atoms are omitted and in (B) all
hydrogen atoms except H1 are omitted and only the ipso carbons of the
pentafluorophenyl groups are shown for clarity ............................................ 78

Figure 50. Metathetic outcomes for the reaction of I-IBPin with Cp*Ir(PMe3)(cy-
C6H11)H ........................................................................................... 80

Scheme 51. Thermal decomposition pathway for the reaction of HBPin with 47 ......... 85

xii

Figure 52. Simple metathesis pathways for the reaction of HBPin with 47 ................. 85
Figure 53. Ring slip pathways for the reaction of HBPin with 47 ............................ 86
Figure 54. Reactivity of 48 with HBPin and HBCat ........................................... 87

Figure 55. Plot of ln{ [48]./[48]0} versus time for the reaction of 48 with 12 equivalents of
HBPin at 150 °C. The last data point indicates two half-lives
completion.([Cp*Ir(PMe3)H2]., = 35 mM; [HBPin]. = 407 mM) ........................ 88

Figure 56. Proposed catalytic cycle for the conversion of alkanes to organoboranes. .....89

Figure 57. 1H NMR of the aliphatic region from the catalytic borylation of C6D6 with 31
after 60 hours at 150 °C. The tall resonance at 8 1.1 is due to C6D5—BPin and the peak
at 5 0.98 (denoted with an asterisk) is due to residual HBPin ............................ 92

Figure 58. 1H NMR of selected regions of the crude material for the catalytic formation
of Ph-BPin ....................................................................................... 93

Figure 59. “B NMR of the crude material for the catalytic formation of Ph—BPin ........ 93

Figure 60. Product distributions for the catalytic conversion of monosubstituted
arenes ............................................................................................. 95

Figure 61. Solvent mixtures and relative ratios for competitive borylation of arenes. All
solvent mixtures are one-to-one C.,H.,:arene ................................................ 97

Figure 62. Proposed catalytic cycle involving a cationic iridium boryl species ............ 98

xiii

BzCatz
szing
Cat
cm'
COD
Cp
Cp*
cy
°C

kcal

LIST OF SYMBOLS AND ABBREVIATIONS

Angstrom

bis(catecholato)diboron, HaCoOzB—BOZCGHa
bis(pinacolato)diboron, MeanozB—BOZCzMea
catechol, 1,2—ozc6m2'

wave number

1,5-cyclooctadiene

cyclopentadienyl, nS-CsHs
pentamethylcyclopentadienyl. nS-C5(CH3)5
cyclohexyl

degrees Celcius

doublet

deuterium

deuterium substitution (x = number of substitutions)
entropy units

diethyl ether

hour

catecholborane, HBOZCfiI'L:
orthophenylenediamineborane, HB(NH)2C6H4
pinacolborane, HBOzCzMea

hertz

infrared

coupling constant

rate constant

temperature in Kelvin, equilibrium constant

kilocalorie

xiv

min
mL
mmol
mM
mol

nbe

OTf
Ph
PMe3
PPh3
Pin

thf

AH"
135*

AV 1/2

uL

ratio of isotope effect on observed rate constant
observed rate constant

liter, generic ligand

multiplet

methyl, -CH3

minute

milliliters

millimole

millimolar

mole

norbonadiene, bicyclo[2.21]heptadiene
nuclear magnetic resonance
trifluoromethanesulfonate, CF3SO3'
phenyl, —C.,H5

trimethylphosphine
triphenylphosphine

pinacol, 1,2—02C2Me42'

quartet

singlet, seconds

triplet

tetrah ydrofuran

delta, ppm for NMR spectroscopy
change in enthalpy

change in entropy

full width at half height

ligand hapticity of number “n”

microliter

XV

CHAPTER 1
INTRODUCTION

Reasons For Studying Transition Metal Boryl Complexes

The addition of BH3 to olefins was first described by H. C. Brown in 1950’s.1 In the
four decades following that initial report, the hydroboration reaction has been studied in
depth and has long been known as an effective synthetic tool for the preparation of
organic compounds containing a wide variety of functional groups. The complementary
transition metal catalyzed hydroboration was not discovered until 1985 when Mannig and
N6th reported the hydroboration of various functionalized alkenes using a simple borane
derivative in the presence of Wilkinson’s catalyst, (PPh;,)3RhC1.2 This result opened a
new arena in which to study B-C bond formation chemistry, and since that report the use
of well-defined systems in boration reactions has yielded an abundance of interesting
new results. Several reviews that have appeared in recent years attest to both the practical
importance and fundamental interest of metal-boron chemistry}9

In order to further understand metal boryl chemistry, the study of compounds
containing simple metal-boron linkages is needed. The preparation and study of the
chemistry of carborane and polyhedral borane transition metal complexes is vast due to
boron’s unusual electronic pr0perties and penchant for catenation. The chemistry of
borohydride complexes is also well established and so, the dearth of compounds
containing -BR2 and —BR3 ligands bound to metals is somewhat surprising. For example,
homoleptic boryl complexes and terminal borides are unknown, and only recently have
terminal borylene (M=B) compounds been described.”12 In contrast, many alkylidine

complexes are known and a terminal metal carbide was recently reported.13

Transition Metal Boryl Complexes

The most obvious and immediate comparisons of metal—boron chemistry should be
made to metal-carbon chemistry. The —BR2 fragment has the same formal charge as the —
CR3 fragment; however, due to the vacant boron p orbital boryl ligands compare more
favorably to carbene fragments. Hence, the strategy of using heteroatom substitution for
stabilizing carbene ligands may extend to the synthesis of boryl complexes as well. One
major difference between boryl and alkyl ligands is the possibility of it back-bonding to
the boron p orbital from appropriate filled frontier orbitals on the metal. Although such
back-bonding has not yet been proven conclusively, current arguments are based on
numerous structures of metal boryl complexes where the M—B bond distance is shorter
than expected based on the sum of covalent radii. Simple representations of both
interactions (boryl substituent/boron p orbital and dit—pn' back-bonding) are given in
Figure 1. Another major difference between the two ligands is in the value of
electronegativity; boron is 2.0 and carbon is 2.5 on the Pauling Scale. Therefore, M—B
bonds are predicted to be more covalent than M—C bonds and likely to have different

bonding properties.

“ X
'Y..9N\\ \\
" +O\x

  

A B

Figure l. Boryl ligand 1: interactions involving A) boryl substituent orbitals and B) metal

frontier orbitals with the p orbital on boron.

Boryl complexes have been produced through reactions of metal anion salts with
haloboranes, oxidative addition of E—B bonds, and metathesis reactions of boranes with
metal-ligand bonds (Equations l-3).14‘19 Although the first of these methods is somewhat
limited by the availability of suitable metal anions, the latter two lend a great deal of
latitude in terms of useful and interesting starting materials. Complexes containing
ligands that readily dissociate leaving an unsaturated metal center are ideal for the
oxidative addition of borane reagents. Notably, late metal phosphine systems and n2-
olefins in early and late metal complexes have proven effective in the synthesis of boryl
complexes.”22 Transition metal hydride, alkyl and olefin complexes also have been
used in metathesis reactions with boranes to produce compounds containing metal-boron

bondS.19,23,24

 

 

 

 

_ + X-BY
[LnM ]A’ _ AX 2 = LnM-BY2
A = alkaline earth metal
X = halide (1)
E-BY IE
LnML' = 1an 2 2 Law!
- L' \
3Y2 (2)
+ E-BY
LnM-R _ ER 2 ; LnM-BY2
R = H, alkyl (3)

The first compound containing a distinct metal-boron linkage was produced by

reacting triphenylborane with lithiumtriphenylgermanate to form a germaniumborate

adduct complex anion as reported by Grim in 1961.25 The first transition metal—boron
compound was reported a few years later by Parshall as rhenium pentacarbonyl anion
reacted with borane to form a rhenium—borane adduct.26 Following that, Noth and
Schmid reported the formation of several metal boryl compounds by reacting boron
halides with various transition metal compounds.27'29 The complexes contained metals in
the mid- to late transition block triads.

It should be noted that in some of the earliest reports of these types of compounds,
erroneous characterizations were made. For example, the reaction of
tungstenocenedihydride with BF; was reported to yield “Cp2WH2'BF3” based on IR data
and comparisons to related reactivity of protonations of hydride species.30 The
insolubility of the compound precluded NMR spectroscopy and X-Ray crystallography
was not a viable option at the time. Reinvestigation of the reactivity of CpZWHz with BF;
and other haloborane reagents revealed that zwitterionic and cationic trihydride species
are produced, as confirmed by X-Ray crystallography, and not tungstenoceneborane

adducts as earlier proposed (Figure 2).31»32

 

 

 

BF3
WHHH
K99
BF3
+ "HF' 7

Figure 2. Product characterization of the reaction of BF3 with CpZWHz from 1963 (top

reaction) and 1996 (bottom reaction).

In 1975, Kono and Ito reported the oxidative addition of 4,4,6-trimethyl-1,3,2-
dioxaborinane to Wilkinson’s catalyst.20 In spite of the obvious applications and well-
known catalytic ability of rhodium based systems, surprisingly little advancement was
made in this field until Mannig and Noth’s report. Since then, many accounts centered on
reacting common aryloxy and alkyl hydroboration reagents such as HBCat and 9-BBN
with late transition metal systems.

More recently, early transition metal systems have been studied as well. In particular,
bis-metallocene compounds have been useful for probing fundamental features of borane
coordination and activation. Indeed, one of the more interesting facets of these early
transition metal complexes is the fascinating array in binding of “simple” metal boryl
linkages. A continuum in M—B bonding can be illustrated in a series of bent metallocene
compounds. At one extreme, boryl complexes where B-H interactions are absent are
observed while borohydride complexes with strong B—H interactions lie at the other
extreme. These two extremes are linked by a o—complex where the borane is bound to the
metal through the H—B bond. Representative examples are given in Figure 3.143334

Compounds along this continuum can be described in terms of these limiting structures.

Cp.\T ..s\H Cp\T “,PMea Cp.\Nb 11"! /H
at Bcat i“ 0‘ '08
. \ \ BCat V I \
011/ H Cp/ H/ Cp'/ H

Figure 3. Continuum of metal-boron bonding in simple metal-boron complexes.

Transition metal catalyzed boration

Due in large part to the utility of the hydroboration reaction, the study of transition-

metal catalyzed boration chemistry has grown significantly in the past decade. Transition

metal complexes have shown considerable promise in promoting B—C bond formation
and although catalysis by late transition metal compounds is most prevalent, lanthanide
and early metal systems offer alternative pathways that can alter product distributions.
Besides increasing the efficiency (reduced reaction temperatures and faster rates) of a
process, catalysts can also dramatically influence the chemo,2 regio,35’38 and
stereoselective33'41 outcome of a reaction. For example, chemoselective addition is
observed when catecholborane hydroborates hex-5-en-2-one. In the uncatalyzed reaction
the ketone functionality is reduced while the olefin is preferentially attacked in the

presence of Wilkinson’s catalyst (Figure 4).

- WY
, 0‘

BCat
H—BCat ,. W

: CNBW
(PPh3)3RhC|

O

 

 

 

 

Figure 4. Comparison of chemoselectivity for catalyzed and uncatalyzed hydroborations.

Following Mannig and N'oth’s report of rhodium catalyzed hydroboration, most of the
research in the area of transition metal-boron catalysis was performed using mid- to late
transition metal systems, notably Wilkinson’s catalyst and other phosphine rhodium and
iridium compounds. Late transition metal systems are expected to follow a relatively
simple mechanistic pathway (shown in Figure 5 for Wilkinson’s catalyst): oxidative
addition of the borane, association and subsequent insertion of the unsaturated organic
substrate and reductive elimination of the organoborane regenerating the catalytically
active species.4244 However, strong evidence for a metathetic pathway has been

provided in the reaction of CpRu(PPh3)2R (R = Me, H) with catechol borane,24 and

computational investigations predict that oxidative addition and o-bond metathesis

pathways in Rh catalyzed hydroborations have similar energy barriers.43

L3RhCl

H
/_/
Y28 1 H-BY2

+ L - L
H /
CleRh’ C|L29h\

¥ 8Y2
8Y2 H
k I -L
\ ClLFilli—II
8Y2

L .—

 

 

Figure 5. Proposed catalytic cycle for olefin hydroboration by Wilkinson’s catalyst.

More recently, several groups have examined early metal systems, primarily utilizing
metallocene ligand sets. Also, the use of lanthanide compounds as hydroboration
catalysts have been reported, although these have been more sparse.45 Early transition
metal catalysts have been studied both to complement late transition metal systems and in
an effort to find less expensive catalyst sources.

Marks has reported the hydroboration of mono-, di—, and trisubstituted alkenes with
Cp'zLaH. This system features a reaction pathway that involves olefin insertion into the
metal hydrogen bond followed by metathesis of the alkyl complex with borane to
produce the organoborane and regenerating the lanthanum hydride.

Although a number of complexes have been shown to catalyze hydroboration, care

must be taken in labeling any system catalytic in nature. Burgess showed that a variety of

complexes thought to be catalytic actually promote the degradation of HBCat to BzCatg
and diborane.46 Thus, the organoborane products are produced by the reaction of BH3
and the olefin. Also, phosphine promoted degradation of HBCat in late metal systems has
been observed in late metal phosphine systems, leading to lower yields and eventual

catalyst poisoning.47

Other Boration Reagents

The utility of boration reagents other than diborane, HBCat and 9-BBN (and other
alkoxy and alkyl based boranes) has become apparent over the past several years. Amide
based boranes, HB(NR2)2, the very electrophilic borane, HB(C6F5)2,43 reported by Piers
and homo/heteroelement boranes, E—BXz (E = BXz, SiR3, SHR3; X = OR or NR2) have all
yielded significant results. Motry and Smith demonstrated aspects of tuning the. boryl
ligand substituents with their study on Cp'zTi(C2Ha) catalyzed hydroboration of
ethylene.49 Initial experiments in this system revealed that when stoichiometric amounts
of HBCat are added to the organometallic complex the organoborane isolated is the
vinylborane, CH2=C(H)BCat. Unfortunately, this dehydrogenative boration did not
extend into the catalytic manifold as the major organoborane product is ethylborane,
CH3CH2BCat. In contrast, when the aminoborane HBOp (Op = 1,2-(NPD2C51-142') is used,
dehydrogenative boration is observed under stoichiometric and catalytic conditions.

Like other non-“BH3” sterically hindered borating agents, Piers’ borane is useful for
its ability to hydroborate asymmetric and prochiral substrates more selectively. However,
it is unique from other reagents in that it reacts with olefins and alkynes facilely at room
temperature. This is primarily a result of the borane’s high electrophilicity and more rapid
dissociation to the monomeric species. As shown in Figure 6 these properties also lead to

a stark difference in reactivity with respect to dialkoxy— or dialkylboranes when reacted

in analogous fashion with transition metal compounds. The reaction of three equivalents
of HBCat with biscyclopentadieneyldimethyltitanium results in the formation of the
bis(borane) adduct of titanocene, Cp;Ti(HBCat)2.SO The analogous reaction with
HB(C6F5)2 is not known, however addition of two equivalents of the electrophilic reagent
to the zirconium analog results in methane loss and formation of a borane stabilized
carbene complex.51 A five-coordinate carbon center is featured in this unique compound.
The zirconium bound =CH2 bridges two boranes. that are, in turn, bound to the metal

center through the hydrogen atoms.

 

 

3 HBCat
H\
C '-H\\Me : C T / Boat '1' M98031
p21" \Me p2 ' \BCat + CH4
H
2 HB(CBF5)2
\\\\H_B\(CGF5)2
szerMe > szzri—ICHQ '1' CH4
Me H_B(C6F5)2

Figure 6. Comparison in reactivity of HBCat versus HB(C6F5)2.

Catalytic addition of B—E bonds (E = B, Si and Sn) to unsaturated organic compounds
has recently become an intense area of study. In the mid- to late eighties, a small number
of reports regarding B—Si and B—Sn reagents appeared.52r53 These findings utilized
trialkylborate silicon and tin compounds for the preparation of vinylsilanes and
vinylstannanes from alkynes in good yield. Separately, Ito and Tanaka, using neutral E—B
compounds, have further developed this field to include addition products with standard
olefins, a, malkenes and mixed alkene/alkyne substrates.54t55 Given the potential utility

of these reactions for sequentially and/or selectively functionalizing unsaturated organic

substrates (Figure 7), a basic understanding of the synthesis, characterization and
reactivity of transition metal compounds that simultaneously support M—B and M—E

bonds is of obvious interest.

 

E—B
[cat] E: _ :B
R ; R' R R'
E: :R" '"Fl ' :R"
R R' R R'

Figure 7. Sequential substitution scheme utilizing heteroborane substrates.

Diboron compounds of the formula XzB—BXZ (where X = halide) have been shown to
react with unsaturated organic substrates by adding across the multiple bond in a cis
fashion. This reaction, first discovered by Schlesinger in 1959, is an appealing method for
the synthesis of 1,2-diborated alkenes.56v57 However, only the halide species react readily
with unsaturated compounds. This is unfortunate as these compounds have an arduous
synthesis and are relatively unstable. The alkoxide and amide based diboron compounds,
on the other hand, have facile syntheses and are more stable than their halide
counterparts”,59 These compounds, though, are much less reactive.

Simple molecular orbital arguments suggest topological similarities between diboron
tetrahalides, and their derivatives, with carbon based fragments and dihydrogen. While
comparisons to olefinic fragments are supported by the empirical observation that the B—
B bond distance decreases when ArzB-BArz derivatives are reduced by two electrons,60
the comparison to H2 is not as strong. Nevertheless, early reports indicated that B—B
bonds oxidatively add to metal centers with B—B bond cleavage.17 Therefore, the

catalytic diboration of unsaturated organic substrates might be expected to parallel metal

10

mediated reactivity of H2, and other E—X transfer reactions. In fact, a catalyst is required
in order to promote B-B addition across the unsaturated carbon—carbon bond. Miyaura
and Suzuki were the first to report such a reaction in 1993 when they reacted
bis(pinacolato)diboron (BzPinz) with both internal and terminal alkynes in the presence of
catalytic amounts of tetrakis(triphenylphosphino)platinum, (PPh3)4Pt, to afford cis-
bis(bory1)alkenes (Equation 4).61

PinB—BPin

+

PinB BPin

(PPh3)4Pt ' .
R = R“ R R (4)

 

I

 

Alkane Functionalization

Alkanes make up a large fraction of the world’s supply of petroleum products and the
possibility of using this inexpensive source of C,,Hm compounds to make practical
organic molecules is a serious economic driving force. Because of this, alkane
functionalization has long been considered a “Holy Grail” by organometallic chemists.62
Unfortunately, the inert nature of these molecules toward organic substitution chemistry
makes this objective a challenging problem. Therefore, great effort has been made toward
the study of C-H and C—C bond activation with the ultimate goal of being able to
functionalize simple alkanes and alkyl groups. The greatest challenge appears to be in
designing systems that generate a catalytically viable intermediate following
functionalization of the M—C bond.

One issue concerning alkane functionalization that must be addressed is selectivity of
activation in the alkane. For instance, tertiary rather than primary C—H bonds are

preferentially activated in both radical and electrophilic reactions. A second issue is also

11

a matter of selectivity as the functionalized product molecule may be more activated
towards C—H cleavage than the parent alkane. For example , in the oxidation of methane,
methanol can undergo further reaction to produce C02 and H2.

An indirect route to functionalization, the “synthesis gas” process, uses steam
reformation of petroleum products (methane) to produce CO and H2. Once produced the
COM; mixture can be used in a variety of processes such as hydroformylation of olefins,
reduction of aromatic nitro compounds to amines and the production of methanol. The
last of these is important as it is a direct source for high-octane gasoline and as a primary
chemical feedstock. However, the preparation of methanol from methane in this case is a
cumbersome two—step process whose first step is highly endothermic (Eq 5 and 6). A
more efficient one—step process is certainly desirable.

[Ni]. A
CH4(g) + H20.» > 3 H...” + co...) AH°=+49.3 kcal/mol

 

(5)

[Cu/ZnO]
CO(g) + 2 H2(g) > CH30H(9) AH° = -21.7 kcal/mol

 

(6)

The first step of the functionalization process, C-H activation, has been studied in
depth by many groups.62'82 Mechanisms for C-H activation can be divided into three
fundamentally distinct classes: i) oxidative additions where new M—C and M-H bonds
are formed65t67r59t75t77 ii) o—bond metatheses between metal-ligand and C—H bonds73»79
and iii) activation by metal radicals.30'82 Historically, late—metal complexes were
believed to operate primarily by class I mechanisms, while C—H activations by (10 early
metal centers can only be explained by class II mechanisms. More recently, cationic Ir
and Pt complexes have been found to activate hydrocarbons under mild conditions. It has

been suggested that these reactions proceed via o—bond metathesis (class ii) pathways.

12

Figure 8 illustrates the activation process in four separate systems. Notably, Bergman
and Jones have separately investigated the reactivity of the Cp'M(PMe3) (M = Rh, 1r)
moiety.65v69 This reactive intermediate can be generated via photolysis of
CpM(PMe3)H2 in alkane or arene solutions, via dissociation of H2. C—H bonds of the
solvent molecules then oxidatively add to the metal. Whitesides has investigated C-H
activation in chelating bisphosphine complexes of platinum. The thermolysis of Pt(Il)
hydridoalkyls in this system results in the reductive elimination of the parent alkane and
subsequent activation of C-H bonds.76v77 Wolczanski has used bulky ligand systems to
stabilize early transition metal alkyl complexes. Titanium and zirconium complexes that
utilize the amide ligand ‘Bu;,SiNH° demonstrate 1,2-R—H elimination upon
thermolysis—”t72 A transient irnido complex is produced which forms new alkyl amido
complexes via concerted 1,2-R'—H addition. Bergman has also examined C—H activation
by the cationic species [Cp'Ir(PMe;t)Me][B(Ari=)4].63 This system is unique from the
other three in that it does not require an elimination reaction prior to activation and the

reaction occurs below room temperature.

13

 

 

 

 

 

 

 

 

., '__-’ Mn
’H - / ’
M9313, \H H2 Me3P M6313, Fl
- .J
' W
E \, . A E :Pt H'R :Pt'
P H - 'R P P ‘H
CYZ CY2 CY2
‘i . 1
B so. , _ . _
‘BuasiOuij __A__. tBE3STO;M=Nst‘Bu3 H R ‘Buastow-M\
. NHSi‘Bu3 -H-R 3 ' -’ NHSi‘Bu3
‘Buasio tBuaSiO
71 71
/ /
M6313 \Me CH4 M6313 \R

Figure 8. Systems studied for C—H activation.

Common themes to these (and other) systems are the requirement of a coordinatively
unsaturated metal center prior to C—H activation, selectivity of activation (generally aryl
> primary > secondary), and ligand sensitivity. Ligand substitution can have a dramatic
effect on the reactivity. For instance, if trimethylphosphine is replaced by
triphenylphosphine in Cp*1r(PR3)H2, then intramolecular C-H activation occurs as
orthometallation of a phenyl group of the phosphine is observed as well as C—H
activation of the solvent molecules following photolysis. Also, if non-chelating
phosphines are used in the Whitesides system, then C—H activation is blocked

completely. Lastly, Bergman notes that if the more coordinating anion,

14

trifluoromethanesulfonate (OTf), is used as the counterion to [Cp*Ir(PMe3)Me*] instead
of a non-coordinating [B(Arp)4’] anion, then higher temperatures are required to effect C—
H activation.

To this point, the number of reported examples where alkanes have been converted to
functionalized organic compounds has been limited. Shilov first reported the conversion
of methane to alcohols and methyl halides utilizing a platinum catalyst.83 Periana
reported the use of a mercury (11) salt to catalyze the oxidation of methane to methanol.84
More recently, there have been numerous reports appearing in the literature relating to
methane oxidation catalyzed by platinum (II) complexes that employ bidentate
bisnitrogen ligands.”86 In a non-methane functionalization reaction, Jones reported the

catalytic cyclization of 2,6—xylyl isocyanide to an indole using a ruthenium compound

”a 140 °c /
CN cat. (dmpe)2FluH2
H
(7)

This thesis describes our efforts toward the preparation of late transition metal boryl

(Equation 7).70

 

complexes, the use of non-traditional borating reagents to facilitate boration of
unsaturated organic substrates, and the boration of unsaturated organic molecules. In
particular, Chapter Two describes the preparation and reactivity of Group 10 boryl
complexes as well as the related diboration of alkynes. Chapter Three describes an
extension of the catalytic diboration to alkene moieties. Chapter Four describes the
metathesis reactivity and the catalytic boration of arenes in the Cp'M(PMe3) system.

Experimental procedures are detailed in Chapter Five.

15

CHAPTER 2
SYNTHESIS, CHARACTERIZATION AND REACTIVITY OF GROUP 10
BORYL COMPLEXES

We investigated the reactivity of borane reagents with late transition metal olefin
compounds for two main reasons. First, the rich reactivity of olefin complexes with
various carborane derivatives suggests that olefin complexes are particularly attractive
starting materials for preparing compounds with unsupported metal—boron bonds via E—B
oxidative addition. Second, reactions of olefin complexes with borane reagents offer the
possibility of pathways that are mechanistically distinct from other metal-mediated
hydroboration processes. Of particular interest is the synthesis of boryl complexes by
reaction of “non-traditional” borane reagents with olefin complexes in hopes of
complementing the application of HBCat as the borating reagent. The use of olefin
complexes should also avoid complications from side reactions involving dissociated

phosphine ligands with borane reagents since the olefin by—product is readily removed

(Eq 8).

 

 

Ph3P\ Ph3P\ IE
/Pt + E—BR2 > / \
PhsP ' H2c=CH2 Phap BR2 (8)

Also, the report by Suzuki that cis addition of the B—B bond in pinacol ester
derivatives to alkynes can be catalyzed by (PPh3)4Pt piqued interest in the reactivity of
platinum boryl complexes.61 It was postulated that cis-(PPh3)2Pt(BPin)2 is formed in situ

and serves as the active catalyst in this system. A direct synthesis of this compound,

16

however, was not reported, which hampers attempts for definitively probing the

mechanism for B—C bond formation.

Preparation and Reactivity of Diboryl Complexes of Platinum

(PPh3)2Pt(n2-C2P14) (1)87 reacts with various borane reagents to form platinum boryl
complexes (Figure 9). In each case ethylene is readily displaced by the borane, does not
react with the boryl complex, and is easily separated from the reaction mixture when the
volatile materials are removed in vacuo. In most cases (E—B = B2(OR)2, M63811-
B(NMe2)2), oxidative addition occurs in a cis fashion, although, trans addition has been

reported in the case of B-halocatecholboranes, (XBCat, X = C1 or Br).33

Ph3P\ [BCat

 

/ \
CatB-BCat Ph3P BCat
2
Ph3'D\PH_" PinB-BPin A Ph3P\ xBPi"
/ V \
phap Ph3P/ BPin
1 3

M938n-B(NM62)2

Ph3P SnMe3
\ /

Ph3P/ \B(NM62)2
4

Figure 9. Scheme depicting the preparation of platinum boryl complexes.

17

cis-(PPh3)2Pt(BCat)2 (2) is cleanly formed as the sole platinum containing product by
reacting (1) with BZCatz and is isolated in 75% yield. The reaction is accompanied by
rapid evolution of ethylene at temperatures as low as —80 °C, precluding quantitative
evaluation of the reaction rates. Room temperature 1H NMR in C6D6 exhibited
catecholate multiplets at 5 6.54 and 6.77 and the resonances for the triphenylphosphine
ligands were typical for a cis-bis(triphenylphosphine)platinum (II) complex. A relatively
broad singlet with 95Pt satellites (I‘erl = 1629 Hz, v,,2 = 60 Hz) was detected in the
31P{’H] NMR spectrum. Figure 10 demonstrates the effect of cooling a CDC13 solution
of 2 as the peaks sharpen at -50 °C (vm = 31 Hz). The 11B NMR spectrum (Figure 1])
exhibited a very broad resonances at 47 ppm (vm = 1845 Hz). This is an unusually large
vm value for a metalloboryl resonance. The failure to observe distinct coupling to either
31P or 195Pt nuclei can be explained by potential complications due to the quadrupolar llB
nucleus. Compound 3, cis-(PPh3)2Pt(BPin)2, is formed analogously by employing the
pinacolate derivative of the diboron compound. A single environment is observed for the
pinacol methyl groups at 8 1.02 and the 31P and 11B NMR spectroscopic data are similar
to that of 2. In contrast, the diboron compounds, C61-14(NH)2B—B(NH)2C6I-14 and
(MezN)2B—B(NMe2)2 do not react with (1) to produce the amide analogs of 2 and 3.
Evidence for a higher amount of it interaction between the boron atoms has been
reported; therefore, it may be more difficult to activate the B—B bond of the amide based

diboron compounds.89

18

 

 

 

 

 

 

 

 

 

 

’ ‘ ' l ' 11
49. I. 135' .. '39. '1 ‘23 111 29 IIIII :0. '1 13s. " '39. 1' I251 1' 129' .1 I

Figure 10. 31P NMR spectra of 2 at A) 20 CC and B) —50 °C. For both spectra, 512

transients were taken of a 32 mM solution in CDC13.

 

mnrrrzimnmriqrrrqmtgirrrgu'rrgimpirrmrqrmrrmrr;zirrmrrriqrrrtp‘rrrgmpram

80 40 0 -40 -80

Figure 11. 11B NMR spectrum (quartz NMR tube) of 2 at room temperature. The broad

features at -0.6 and —50 ppm are due to borosilicate glass in the probe.

The dicatecholatoboryl complex can also be cleanly formed from

bis(phosphine)platinum(112-alkyne) compounds. The alkyne complex (PPh3)2Pt(n2-4-

19

octyne) (5) reacts with BzCatz to generate 2 (62%) with loss of the alkyne. In the absence
of excess quantities of B2Cat2, thermolysis of the reaction mixture gives complicated
mixtures: however, when an excess of B2Cat2 is used, clean conversion to the 4,5-
bis(boryl)octene product is observed along with formation of 2. Presumably, excess
BZCatz prevents decomposition by trapping the transient “(PPh3)2Pt” fragment.
Diffraction quality crystals of cis-(PPh3)2Pt(BCat)2 were grown from a concentrated
toluene solution as colorless parallelepipeds. The structure of _cis-(PPh3)2Pt(BCat)2
(shown in Figure 12) reveals a cis geometry where AP(1)—Pt—P(2) (107.4(2)°) and
ZB(1)-Pt—B(2) (77.8(7)°) are among the most obtuse and acute angles reported for a cis-
(PPh3)2PtX2 complex. This is a common theme among other structurally characterized
cis-bis(phosphine)platinumdiboryl compounds.33v90'94 The platinum-boron distances of
207(2) and 208(2) A in cis-(PPh3)2Pt(BCat)2 are shorter than would be expected as has
been observed for other transition metal boryl complexes as well. An improved
refinement of 2 was reported by Marder et a1. and confirmed the general characteristics of
our structure.91 Although the Pt—B bonds in Marder’s paper were slightly shorter
(2.040(6) and 2.058(6) A) the angles ZB(l)—Pt—B(2) and AP(1)—Pt—P(2) were identical
within experimental error. A Pt—B bond distance can be estimated based on comparisons
to related aryl complexes. A typical Pt—le bond length is 2.08 A and spz-hybridized
CatB—BCat bonds are 0.17 A longer than spz-hybridized c.,,,.—caryl bonds.95-97 These
values yield an expected Pt-B bond distance should be approximately 2.16 A793 Also,
the BB distance of 2.60 A precludes significant interactions between the boron centers.

For comparison, the B—B distance in BzCatz is 1.678(3) A17

20

 

Figure 12. ORTEP diagram of 2. Thermal ellipsoids are at 25 % probability. Only the
ipso carbons of the triphenylphosphine ligands are shown for clarity.

Table 1. Selected bond lengths and angles for compound 2.

 

 

bond distance (A) bonds angle (deg)
Pt(l)—B(1) 208(2) B(1)—PI(1)—B(2) 77.9(7)
Pt(1)—B(2) 207(2) 1P(1)—PI(1)—P(2) 107.3(2)
Pt(1)—P(1) 2.350(5) P(1)—Pt(1)—B(2) 90.5(5)
Pt(1)—P(2) 2.375(5) B(1)—PI(1)—P(2) 84.6(5)
B(l)---B(2) 2-60 B(l)-Pt(1)-P(1) 168.1(5)
B(2)—Pt( 1 )—P(2) 159.9(5)

Table 2. Selected bond lengths and angles for compound 2 from ref. 91.

 

 

bond distance (A) bonds Angle (deg)
P1( 1 )¢B( 1) 2.058(6) B(1)—Pl(1)—B(2) 77.1(2)
PI(1)—B(2) 2.040(6) P(l)—Pt(l)—P(2) 107.14(4)
Pt(1)—P(1) 2.3465(12) P(l)—Pl(1)-B(2) 90.7(2)
Pt(1)-P(2) 2.3543(12) B(1)—Pt(l)—P(2) 85.3(2)
B(l)---B(2) 2.552 B(1)—Pt(l)—P(1) 167.6(2)
B(2)—Pt(1)—P(2) 160.1(2)

21

Simple Reactivity of Diboryl Complexes

Qualitative observations suggest that cis-(PPh3)Pt(BPin)2 is intrinsically less stable
than cis-(PPh3)Pt(BCat)2. Figure 13 illustrates the reactions of 3 with various small
molecules. For example, when 3 is dissolved in chloroform-d1 under ambient conditions,
cis-(PPh3)PtC12 forms readily while 2 exhibits moderate stability under these conditions.
For both complexes, reductive elimination can be observed when CO is added, however;
2 is stable with respect to appreciable concentrations of triphenylphosphine, whereas the
elimination of BzPinz from 3 is facile. Again, in contrast to 1, BzPinz is displaced when 3
is reacted with SnzMe6 and simple alkynes. In this reaction, some conversion of the
alkynes to di(pinacolatoboryl)a1kenes was observed. This led us to examine the catalytic
prospects of compounds 2 and 3. The details of this aspect of 2 and 3 are discussed in

depth in Chapter 3.

22

 

 

/Pt\ /\Pt ‘
Ph.,P2 BCat Ph3P 6\SnMe3
Measnz
CatB-BCat
R
Ph3p\ /BPIn — R Ph3P\
\Pt: . . /Pt “'1
Ph3P/3 BPin PinB-BPIn Ph3P R
5
Ph3P\ /CO
\Pt\ (Pph3)npt; n= 3, 4
PhaP/ Ba,b
7

Figure 13. Scheme depicting the reactivity of 3.

Finally, 3 can be converted to 2 when one equivalent of BzCatz is added to a solution
of 3. Qualitative disparities between rates for oxidative addition of BzCatz and szmz to
Wilkinson’s catalyst have been reported;17 however, the conversion of 3 to 2 obviously
has thermodynamic origins. Although we are not yet certain that Pt-B bond cleavage
occurs in the reaction of 3 with BzCatz, this observation clearly indicates that formation
of 2 + B2Pin2 is thermodynamically favored over 3 + BzCatz. It is tempting to argue that
the equilibrium is driven by the formation of stronger Pt—B bonds in 2; however, a
stronger B—B bond in BzPinz relative to BzCatz would also be consistent with the
experimental observations. The ease with which the elimination reactions proceed is in
marked contrast to reductive eliminations of alkanes from cis-dialkylplatinum

bisphosphine complexes.

23

Preparation and Characterization of a Stannylboryl Complex of Platinum

The heteroatom borane, Me:,Sn—B(NMe2)2 reacts with 1 at 45 °C for 3 hr to form the
oxidative addition product (PPh3)2Pt(SnMe3)(B(NMe2)2) (4)1599,100 1H NMR of the
reaction mixture revealed two new resonances for the tin and amino methyl groups at 8 —
0.46 and 2.53, respectively. An unsymmetrical cis configuration of the phosphine ligands
is apparent from the low temperature 31P NMR data. Two resonances are observed at 6
30.6 and 31.3. The phosphine position (trans-tin versus trans—boron) can be deduced
based on coupling constant values and appearance of the peaks. The resonance at 8 30.6
(I‘Jp.p.l = 2736 Hz ) has a central doublet with 121.-..1 = 14.6 Hz that is typical for
asymmetric cis—bis(phosphine)platinum complexes. Also, tin satellites are observed with
121p.s,,l = 1487 Hz, which is typical for a phosphine trans to a trimethyltin ligand. The
resonance at 6 31.3 (I‘Jp.p.l = 1404 Hz) is broad such that neither IZJp.pl nor IZJPSnI can be
distinguished. Expected phosphorous—tin cis-couplings are generally small and tin
satellites could easily be masked by the broad resonance. Also, phosphines that are trans
to boryl ligands have been observed to display broad resonances in other platinum boryl
complexes and the magnitude of I‘Jp.p.| is consistent with phohsphorous trans to boron.

Recrystallization of 4 from a CH2C12 solution layered with EtOH provided yellow
parallelipipeds. X-Ray crystallographic analysis confirmed the cis conformation (Figure
14). The compound is pseudo-square planar and distortion from planarity is reduced due
to the arrangement of the bis(dimethyl)amino ligand. The Pt—B bond distance is long
(2.130(8) A) relative to other late-metal boron linkages (see above). The amino groups
are oriented in a fashion favorable for n-bonding between the lone pair on nitrogen and
the p orbital on boron. An indication of this interaction may be found in the B—N

distances of 1.430(11) and 1.426(12) A, which are slightly shorter than the B—N bond

24

lengths obtained for the tris(amino)borane, B(NMe2)3 (1.441(1), 1.434(1), and 1.441(1)

A)_101
)
g "‘ . P1
Ptt
a .1, p2

N2

mg),
“'3' N1

 

‘
‘\‘
I.

N2

I’.'.
.’
.\
. .
c
p

Figure 14. Two different views of compound 4. Thermal ellipsoids are at 30 %. Only the

ipso carbons of the triphenylphosphine ligands are shown for clarity.

Table 3. Selected bond lengths and angles for 4.

 

 

bond distance (A) bonds angle (deg)
Pt(1)—B( 1) 2.130(8) B(1)-Pt(l)-P(1) 88.4(2)
Pi(1)-—P(l) 2.297(2) B(1)—Pt(1)—P(2) 169.7(2)
Pt(1)—P(2) 2.362(2) P(1)-Pt(1)-P(2) 101. 89(6)
Pt(l)—Sn(l) 2.6352(7) B(l)—Pt(1)—Sn(l) 75.1(2)
N(1)—B(1) 1.430(11) P(1)—Pt(l)—Sn(l) 163.16(5)
N(2)——B(1) 1.426(12) P(2)—Pt(1)—Sn(l) 94.67(5)
N(2)—B(1)—N( 1) 120.6(7)
N(2)—B(1)—Pi(1) 120.2(6)
N(1)—B(1)—Pt(1) 119.1(6)

Given the static arrangement of the methyl groups of the bis(amino)boryl ligand

obtained from the crystal structure, a single resonance in the 1H NMR indicates dynamic

behavior. Decreasing the temperature results in decoalescence of the fast exchange

singlet. The slow limit of exchange is reached at —70 °C where two sharp peaks appear

(Figure 15). The coalescence temperature is at -20 °C. If the static structure determined

by X-Ray analysis holds true at low temperature in solution, then the dynamic behavior is
readily explained. The view down the B-Pt—P(2) axis reveals two methyl environments.
One pair of methyl groups is internal and the other is external, i.e., pointing towards or
pointing away from the axis, respectively. At elevated temperatures, however, rotation

about the B—N bonds would lead to coalescence into a single resonance.

26

 

 

 

 

 

 

 

 

 

 

 

 

 

20 °C
f‘

10 °C J L -
-10 °C ' A
~20 °C A ‘—
.30 °C fl - W

.11 11
.50 °C J l¥ A A #4";

11 11
-70 °C L," w L

 

 

 

3.2 2.8 2.4 2.0 1.6

Figure 15. Variable temperature NMR of the amino methyl groups of compound 4 in

CD2C12. Starred resonances at 6 2.3 and 2.55 indicate impurities.

Given the ability of the diboryl compounds, 2 and 3, to catalytically add diboron
compounds to alkynes, and the desire to add heteroborane compounds to alkynes, the
catalytic reactivity of 4 was examined. Several alkynes were reacted with Me3Sn—

B(NMe2)2 in the presence of 4 (5 mole percent). Catalytic addition does occur but in a far

27

less efficient fashion than other known systems. For example, 4 requires 36 hours at 80
°C to convert 4—octyne to the 4-boryl-5-stannyl-oct-4-ene product. Related conversions
by Pd(0) catalysts occur within an hour at room temperature.100 Also, with the exception
of terminal aliphatic alkynes, selectivity of the addition was poor. Finally, unlike the
diboryl compounds 2 and 3, compound 4 did not survive the reaction conditions. Given

these results, this reaction was not pursued further.102

Metathesis Reactivity of Group 10 Complexes

Since various related dialkyl complexes are known and a wealth of mechanistic
information concerning reactivity of these compounds exist, these systems seemed
ideally suited for surveying potential metatheses between metal—carbon and boron-boron
bonds. With this in mind, we chose to investigate reactivity of the metallacyclopentane
complex, (PPh3)2Pt(CH2)4 (9) with BzCatz since 9 exhibits exceptional stability in
comparison to other Pt(II)-dialky1 species, and the decomposition pathways for 9 are well
documented. Compound 9 reacts cleanly with two molar equivalents of 82Cat2 at 95 °C
to form cis-(PPh3)2Pt(BCat)2 quantitatively as judged by 1H NMR. The organic portion of
the metallocyclopentane is converted quantitatively to CatB(CH2)4BCat. When the
reaction scaled was increased, 2 and the organodiborane were isolated in 68 and 47 %
yields, respectively. It is noteworthy that formation of 1- or 2-butenes, which are known
byproducts of the thermal decomposition of 9, are not observed. Metathesis in this system
appears to be general as CatB-BCat also exhibits similar reactivity with other dialkyl and
diaryl bisphosphine complexes of platinum (Figure 16). These results marked the first

clear demonstration of metathesis reactions of B—B bonds with M—C bonds.

28

 

Ph3P\ Ph3P\ ,BCat BCat
[Pt + 82Ca12 Pt + CatB/W

 

PhaP Phap’ ‘sca

9 2
Ph3P\ ,CHs Ph3P\ ,BCat

[Pr + 820312 /Pt\ + 2 Me—BCat
PhaP CH3 PhsP BCat

10 2

Et3P\ ,BCat CatB 0
Pt 4»

/
EtaP \BCat CatB 0
12

Figure 16. Reactivity of BZCatz with dialkyl and diaryl bisphosphine platinum

 

11

complexes.

Kinetic analysis of this reaction revealed a first order dependence in [9] and zero
order in [82Cat2]. Since bisphosphine complexes of platinum are well known for
dissociative pathways the phosphine dependence was examined. Unfortunately, when
triphenylphosphine was added significant amounts of l-butene were observed in the 1H
NMR. This result was not entirely unexpected as Whitesides has reported accelerated
decomposition of the platinacyclopentane with the addition of triphenylphosphine. Even
though a dissociative pathway cannot be independently confirmed, examination of the
activation parameters extracted from an Eyring plot could argue in favor of or against
phosphine dissociation. The sign and magnitude of the entropic term would seem to lend
support to a dissociative pathway; AH* = 29.7 kcal/mol and AS1 = 11 cu

A reasonable mechanism (Figure 17) that can be envisioned for this reaction is
similar to related hydro genations.103 Initial phosphine dissociation generates a reactive

three coordinate Pt (11) intermediate. Subsequent oxidative addition of BzCatz forms a Pt

29

(IV) intermediate which rapidly loses organodiborane. Trapping of the platinum fragment

by phosphine and BzCatz would give compound 2.

 

 

 

PPh3
- PPh CatB-BCat I
P“3P;Pt: J 3 Ph3P—P1<J CatB—PicI
PhaP [130 t
9 13 a
14
+ PPhB
CatB-BCat
v

 

Ph3P\ IBCat

CatB
NBCat +

Ph3P/ \BCat
2

Figure 17. Proposed mechanism for the reaction of B2Cat2 with 9.

Substituent Effects on Boryl Ligands

Some discussions regarding the trans-influence of boryl ligands and the related issue
of it back-bonding to the boryl from a transition metal center have recently appeared],104
The relatively long Pt—P bond distances trans to —BR2 moieties indicated that boryl
ligands have a strong trans influence. The magnitude of the trans influence has been
estimated by comparing I’Jp._pl values within the series of cis-(PPh3)2PtX2 compounds,
which has been shown to decrease with increasing trans influence of X for a series of
compounds. Thus, the progression of I‘Jp._pl for X = Cl >> Ph > BR; does support the
notion} that boryl ligands are stronger field ligands than their aryl counterparts.105 An

ordering of trans influence strength according to substitution on boron can also be made

30

using this method. 31P data for compounds that contain triphenylphosphine ligands trans
to boryl ligands support the progression of BCat < BPin < B(NMe2)2.

Although it is clear that boryl ligands have a strong trans influence, the explanations
given so far tend to rely simply on the stability of boron to act as a n—acceptor. The real
reasons are more likely due to a combination of factors. The arguments put forth so far
have not taken into account substituents on the boryl itself. For instance, substituents with
atoms that are able to interact electronically with the boron p orbital would be expected to
affect the ability of the metal to back-bond with boron (Figure 18). In the case of oxygen
based boryl ligands (example A), the lone pairs on oxygen are not aligned for meaningful
interaction with the empty p orbital of boron. It is convenient, therefore, to assume that
the short bond length is caused by strong it back-bonding with the metal. In contrast, the
bis(dimethylamino)boryl ligand is perfectly aligned for an interaction between the empty
p orbital of boron and the lone pairs on the nitrogens (example B). Electron donation into
boron’s p orbital should hinder 1t back-bonding from the platinum metal center and
prevent significant shortening of the Pt-B bond. The longer Pt—B bond in cis-
(PPh3)2Pt(SnMe3)[B(NMe2)2] appears to contrast the expected trend in trans influence

based solely on llJp._pl. Clearly, there are other factors at work

His?

952:. \

Q

  
 

.....
....

 

Figure 18. Simple orbital diagrams illustrating 1t bonding effects in boryl complexes.

The influence of the boryl ligand on I‘Jp..p| and the apparent absence of a correlation

to Pt—B bond distances could be explained by inductive effects. The difference in

31

electronegativity for boron and oxygen is significantly larger than the difference between
boron and nitrogen (Figure 19, examples C and D). Therefore, alkoxide substituents
should have a greater capacity to remove electron density from boron. In turn, this would
lessen the 0' donating ability of the bis(alkoxy)boryl ligand relative to the

bis(dimethylamino)boryl ligand.

 

 

NR2
2.2 2.1/ 2.2 2%
Pt B 3.5 Pt 8 .
\
.__.. Q0 ._. ,\~a.
C D

Figure 19. Simple diagrams illustrating inductive effects in boryl complexes.

Certainly, more work is needed to determine the extent of substituent effects and their

consequences on the trans influence and bond length in metal boryl systems.

32

CHAPTER 3
PLATINUM MEDIATED DIBORATION OF OLEFINS AND ALKYNES

Mechanism of Stoichiometric Diboration of Alkynes

Prior to this work, detailed mechanistic information regarding the metal-mediated C—
B bond formation was sparse. The earliest work involved H—B activation relating to
hydroboration promoted by Wilkinson’s catalystlgv‘iévlofi107 More recent work has been
performed in early to mid transition metal systems, again involving H—B
activation.24v108v109 Despite this, examples where insertions into metal boron bonds
proceed cleanly are rare, and factors which determine product selectivities are not well
understood.

The previous chapter described the reactivity of simple B—B bonds with olefin,
alkyne, and organometallacyclic bis(triphenylphosphine) group 10 complexes. In the case
of platinum complexes, (RO)zB—B(OR)2, ((OR); = Catz’ or Pinz’) reacted smoothly to
generate a common diboryl complex, (PPh3)2Pt(BCat)2 (2) or (PPh3)2Pt(BPin)2 (3). This
species has been postulated by Miyaura and Suzuki to be the active catalyst in the
catalytic diboration of alkynes mediated by (PPh3)4Pt (8b).90’110 Preliminary experiments
showed that the pure diboryl complex, cis-(PPh3)2Pt(BCat)2, catalyzed the addition of
CatB—BCat to alkynes, consistent with the supposition put forth by Miyaura and Suzuki.
Under catalytic conditions, 2 was the sole platinum containing species in solution as
judged by NMR spectroscopy. Since 2 can be prepared and isolated cleanly, this system
is ideally suited for addressing fundamental issues of B—C bond formation in this
system.111

The proposed mechanism for the diboration of alkynes in the presence of 8b is shown

in Figure 20. In this mechanism, a species related to 2 was suggested to be the active

33

catalyst, and the initial B-C bond forming reaction is insertion of the alkyne into the Pt—B
bond of 3. Reductive elimination from an alkenylboryl species generates “(PPh3)2Pt” and
the diborated alkene product. Subsequent oxidative addition of PinB-BPin to “(PPh3)2Pt”

completes the catalytic cycle.

 

R R
PinB BPin
P110)
PinB-BPin
R
Pt‘ BPin
Ln \BPin PinB—Pt—BPin
R Z R

Figure 20. Pathway proposed by Miyaura and Suzuki for the platinum catalyzed

diboration of alkynes.

While NMR data indicated that 3 was present under reaction conditions, no
experimental evidence was offered to support the postulated mechanism. A better
understanding of the steps preceding B—C bond formation could prove useful in

designing systems that diborate other unsaturated substrates, however, direct study of the

34

catalytic reaction would likely be complex. Therefore, in order to gain insights into the

mechanism, stoichiometric restraints were applied to diboration mediated by 2 (Eq 9).

 

(PP h312':’l(112-4-octyne)
(PPh3)2Pt(BCat)2 + XS 4-octyne , 5
2 +
CatB 8031
"03117 ' "C3H7 (9)

In the absence of added diboron reagent, a clean reaction was observed when excess
alkyne was present to trap “(PPh3)2Pt” as (PPh3)2Pt(n2-alkyne) (5). Two distinct
pathways that involve bimolecular reactions between 2 and alkyne are shown in Figure
21. In both cases, alkyne insertion is presumed to be rate limiting. Both rate laws (Eqs 10
and 11) require a first order dependence in [2] and [RCECR], and, although these two
pathways are mechanistically distinct, they can not be distinguished if intermediate 15 is

undetectable.

35

 

 

 

 

 

 

 

 

L\ ”Boat PR : R
\
L/ BCat “I k L ....... AFB...”
2 k_1 L/l
+ 80211
R : R 15
k3 k2
R R 1
L >—<__
\Pt\ BCat
L/ BCat
16
+R : R
11
R

L
\

Pt~11 + R — R
L/ CatMCat

R

Figure 21. Simple bimolecular pathways for the diboration reaction.

4112]
dt

 

-012l
dt

krkz
_ [2][RCECR]
(If-1+ k2)
= [(3 [2][RCECR]

36

 

When the reaction in Eq 9 was monitored under pseudo first-order conditions with
respect to alkyne concentration, plots of 1n[2] vs. time were linear over three half-lives.
This suggested that the reaction in Eq 9 is first order in [2]. Although the limited
solubility of [2] prevented determination over a broad concentration range, Rob, appears to
be invariant with respect to [2]. Significantly, varying the alkyne concentration had no
effect on kw, (Figure 22). Thus, the mechanisms corresponding to the rate laws in Eqs 10

and 11 can be excluded as the observed kinetics are consistent with the rate law in Eq 12.

 

Inttzuzp

 

 

 

 

0 2000 4000 6000 0000 110‘ 1210'

time(sec)

Figure 22. Plots of 1n[2] versus time for the reaction of 2 with 4-octyne at 70 °C. ([2]o =
8.7 mM; 0 [4-octyne]o = 27 mM; [4-octyne]0 = 52 mM).

«12]
= 2
dt W l (12)

 

37

Another possible mechanism takes into account a pathway which involves phosphine
dissociation. Dissociative pathways leading to three-coordinate intermediates have been
invoked for reactions involving reductive elimination in a1kyl(bisphosphine)M(II)
systems (M = Pd, P0103011}114 The exclusion of the mechanisms in Figure 21,
therefore, suggested this route. The reaction in Eq 9 was monitored at various phosphine
concentrations in order to assess this possibility. The addition of phosphine clearly

suppressed the reaction rate, and plots of l/kob, vs. [PPh3] were linear (Figure 23).

 

200

150 '

(«0)

Mt
ob.
a
O
O
I
\
~°\

50'

U/

 

 

 

0 5 10 15 20 25 30 35 40

[PPh11(mH)

Figure 23. Plot of llkob, versus [PPhg] for the reaction of 2 with 4-octyne at 90 °C in
C5D6. ([2]0 = 8.7 mM, [droctyne]o = 52 mM).

Two mechanisms that involve dissociative first steps are shown in Figures 24 and 25.

In Figure 24, phosphine dissociation is presumed to be rate limiting, while the subsequent

38

coordination and insertion steps are rapid. In Figure 25, alkyne insertion is presumed to
be the rate-limiting step, and 2, 17, and 18 are in rapid equilibrium. Unfortunately, the
reverse reactions of alkyne coordination or insertion have not been able to be studied in
this system, however, crossover experiments showed that elimination of the diborated
alkene is irreversible. When a solution of 5 and 4,5-diboryl—4-octene are heated in the
presence of 3-hexyne, the formation of 3,4-diboryl-3-hexene is not observed. It should
also be. noted that the inability to observe intermediates has no bearing on the choice of
mechanism, since 2 can be thermodynamically favored to the extent that 17 and 18 are

not detectable.

 

 

 

 

 

 

 

 

P R : R _
L BCat k BCat _
\ / 4 / k [n—=—R]
/Pt\ T- L + L_Pt\ V 5 k_ ‘ L—Pl—BCat
L BCat BCat 5 C31
2 17 18
k6
R _ R R _
L , Cat BCat >—<
\ —::—— '-
/Pt“’| i + 3=< ‘ R R ‘ L L—Pt BCat
L R R
R BCat
L. 4
5 19

Figure 24. Steady state pathway for the diboration reaction.

39

L BCat K /BCat
\pt: ==12= L + L—Pt + R—E—R
L/ BCat BCat
2 17

R
. CatB>__-<BCat AR : R g L
+ V

 

 

R R

 

3

figure 25. Equilibrium pathway for the diboration reaction.

 

R\-_—..—-R

 

L—Pt

L—Pl—BCat

1
ACat
6

1

BCat
BCat

 

Simplified rate expressions can be derived by applying steady-state or equilibrium

approximations to Figures 24 and 25, respectively.115 Application of the steady state

approximation to Figure 24 gives the rate law and kob, expression shown in Eqs 13 and

14, while the equilibrium treatment for Figure 25 gives the expressions in Eqs 15 and 16.

_ = kobslzl

k4k5k6 [RC ECR]

 

kAké + [@[PPhQ] + k5k6 [RC ECR]

-o[2 + 17+ 18]
d!

 

40

= k0,,[2 +17+1a]

(13)

(14)

(15)

-K1K2k6 [RC ECR]
K, + [PPha] + K1K21RCECR] (l6)

 

An inverse dependence on phosphine concentration is consistent with either
mechanism, as Eqs 14 and 16 reduce to Eqs 17 and 18 when the phosphine term
dominates the denominator. When [PPh3] z 0, Eq 6 reduces to kob, = k.., while eq 14
yields Eq 19. In principle, the two mechanisms are distinguishable in the absence of
phosphine if K1 >> K1K2[RCECR]. In this regime the mechanism in Figure 25 requires
an alkyne dependence. We do not see an alkyne dependence at any concentration of

alkyne. Thus, if

k4k5k6 [RCECR]

 

 

k =

°"°‘ loans + ks)[PPhal (17)
k -K,K2k6 [RCECR]

0‘” ‘ [PPh31 (18)

-K,K2k6 [RCECR]

 

the mechanism in Figure 25 operates, K. << K1K2[RCECR], and kob, z k6. Both Eqs 17
and 18 predict that an alkyne dependence should be observed for both mechanisms at
sufficiently high phosphine concentrations (k_4[PPh3] >> (k5k6/k5 + k6)[RC.=CR], or
[PPh3] >> K1 + K1K2[RCECR]). This is indeed the case, as kob, exhibits a first order
dependence on [RC-=CR] in the presence of added phosphine (Figure 26). Thus, the

kinetic data are consistent with either proposal As the concentration of alkyne is

41

increased at constant [PPhg], three regimes are possible; k4[PPh3] >> (k5k6/k5 +
ks)[RCECR]. MIPPhsl = (kskdks + ks)[RCECR], and k4[PPh3] << (kskdks +
k6)[RC.=CR], or [PPh3] >> K1 + K1K2[RCECR], [PPh3] = K, + K1K2[RC.=.CR], and
[PPh3] << K, + K1K2[RC:—CR]. Therefore, at sufficiently high concentrations of alkyne,
the value for kw, could be approached for rates observed in the absence of added
phosphine. Unfortunately, the concentrations of alkyne needed were exceedingly high to

make to make this comparison.

 

 

 

 

 

9’
5r
2‘.
s
3s.
3!.
0 so 100 150 200 250

Tine (min)

Figure 26. Plot of ln{[2]./[2]o} versus time for the reaction of 2 with 4-octyne in the
presence of PPh3-d15 at 105 °C in C6D6. ([2]() = 8.7 mM, [PPh3-d15] = 27 mM; X [4-
octyne]0 = 27 mM; 0 [4-octyne]o = 52 mM; El [4—octyne]o = 105 mM; 6 [4-octyne]o =

208 mM)

42

Attempts to determine activation parameters from the temperature dependence of km
were not successful due to the rapid rate of reaction and poor solubility of 2. This
problem was circumvented by suppression of the reaction rate via addition of phosphine,
and kobs was measured from 70 to 110 °C. The Eyring plot in Figure 27 yielded the
following activation parameters: AH" 25.9(7) kcal/mol and AS1 = 4(2) en. The magnitude
of ASIt would seems to disfavor phosphine dissociation as the rate-limiting step in the
.mechanism;116 nonetheless, rate-limiting dissociation is not categorically excluded as
contributions from other steps preceding the rate determining step could cause a decrease
in the value of 1351.117 In most cases, a negative value for AS1 might be expected for a
pathway that involves rate—limiting alkyne insertion into the Pt-B bond. Investigations of
CEC and C=C insertions into M—X bonds, however, indicate that the magnitude of AS‘t

can be small, and its sign can be either positive or negative.113v119

43

 

I"(k/'1')

-11 .-

42 L

 

 

43

 

25 255 2] 215 23 235 23 235

HMO”

Figure 27. Eyring plot for the diboration of 4-octyne by 2 in the presence of PPh3-d15 in
C6D5. ([2]o = 8.7 mM; [4-octyne]0 = 52 mM; [Pphrdm] = 8.7 mM; T = 343 to 383 K,
AH* = 25.90) kcal/mol, AS1 = 4(2) e.u.)

In the absence of phosphine, kw, reduces to the rate constant for the rate-determining
step in the proposed mechanisms. Thus, boration rates for different alkynes should be
identical if phosphine dissociation is rate limiting. On the other hand, if alkyne insertion
is rate limiting, then it might be expected that steric and/or electronic variations in the
alkyne could affect k0,”. In order to assess this question l-hexyne, phenylacetylene, and
diphenylacetylene were reacted with 2. For l-hexyne and phenylacetylene, 31P NMR
indicated that several metal-containing products form, which unfortunately prevents
comparisons between internal and external alkynes. The probable source of the
decomposition is the acetylenic proton; therefore, the reaction between 2 and
diphenylacetylene was examined. In this case, boration proceeded smoothly, though at a

much slower rate in comparison to the dialkyl substituted alkyne, 4-octyne. This result

44

clearly implies that kob, is affected by the nature of the alkyne and that phosphine
dissociation is not slower than the alkyne coordination and insertion steps.

The effects of electronic perturbation of the alkyne on the rate of boration was
explored by utilizing 4,4’-substituted diarylacetylenes. Boration of 1,2-di-p-
anisylacetylene proceeded at a slower rate than the analogous reaction with
diphenylacetylene. Unfortunately, boration of 1,2-di-p-(trifluoromethyl)phenylacetylene
was too rapid for rigorous kinetic analysis, and other diarylacetylenes did not yield
tractable data. The unexpected rapid rate for the trifluoromethyl moiety is somewhat
curious as the reaction rate was expected to increase by a factor of only two based on
linear free-energy relationships and tabulated 0'pm values.120 In this case, rates for
substituted diphenylacetylenes appear to offer little information regarding the boration
mechanism. Regardless, the results of the data argue that the rate of boration is indeed
sensitive to electronic perturbations in substituted diarylacetylenes.

From this stoichiometric investigation emerges an obvious feature concerning
reactivity in this system. Specifically, if 2 serves as the active catalyst under
stoichiometric conditions, catalysis by 2 liberates PPhg. This is indeed the case, as
alkynes that were diborated sluggishly by catalysis originating from 8b undergo smooth

diboration when pure 2 is used.90

Mechanism of Catalytic Diboration of Alkynes

Solutions of the diboryl complexes, 2 and 3, both exhibit catalytic activity for the
diboration of alkynes by BZCatz and B2Pin2, respectively. The rate of catalysis by 2 or 3 is
reasonably similar, however, quantitative comparisons of rates were not possible due to
deviations from pseudo first-order consumption of alkynes in the absence of added

phosphine. The cause of these deviations is not clear; however, boration by other boryl

45

species generated in the presence of excess diboron reagent could account for the
observations.

When phosphine is present during catalytic borations mediated by 2, well-behaved,
pseudo first-order consumption of 4-octyne was observed. Since 2 was the only Pt
containing complex detected (as judged by 1H and 3'P NMR) when catalytic diborations
were run in the presence of added phosphine, and comparisons to the stoichiometric
reactivity of 2 could be made, the kinetics of the diboration of 4—octyne, catalyzed by 2,
were investigated.

Two potential mechanisms for this catalysis are illustrated in Figures 28 and 29.
Similarities to the mechanisms proposed for the stoichiometric reaction of 2 with alkyne
are readily apparent, such as phosphine dissociation from 2, alkyne insertion into the Pt-B
bond of 18, and reductive elimination of the diborated alkene from 19. In contrast to the
stoichiometric reaction, however, the concentration of 2 remains constant throughout the
course of the catalysis. Rates for equilibration between species 2 and 17 and species 17
and 18 are considered to be rapid compared to the rate for alkyne insertion. As in the
treatment of the stoichiometric reaction, application of either steady-state or equilibrium

approximations gives rate laws for the consumption of alkyne.

46

L BCat k4 BCat

 

 

 

 

 

 

 

 

 

\ / /
P1\ k * L—Pl + L
L/ BCat .4 \Bcat
2 17
l +
+ CatB-BCat R : R
[1
R R k k
+ L - >—<— ‘5 5
CatB BCat
F R R R I R
>=<B k6 1
L—Pt Cat ~ L—Et—BCat
_ Cat d _ Cat J
19 1 8

Figure 28. Steady state pathway for the catalytic diboration of alkynes.

 

 

 

 

 

 

 

L\ /BCat K, lBCat
Pt 4 * L—Pt\ + L
L/ \BCat BCat
2 17
‘ +
+ CatB-BCat R — R
R R K2
+ L - >——<—
CatB BCat
R R R : R
>28 Re 1
L—Pt\ Cat % L—l-i’t—BCat
BCat BCat
19 18

Figure 29. Equilibriumqpathway for the catalytic diboration of alkynes.

47

-0[RC-="CR] k4k5kel2]
= [RCECR]
d! k_4k5 + k6)[PPh3] + ngdRC ECR] (20)

 

 

-qncaomnnal 'Kerkdz] RC—CR 17 1a
a: = [PPhal + K1K2121 [ _ + + l (21)

 

 

The rate laws in Eqs 20 and 21 simplify to Eqs 22 and 23 and Eqs 24 and 25,
respectively when phosphine dominates the denominator of kobs. Under these conditions
both mechanisms require that the reaction is first order in [2] and [RCECR], and exhibits

an inverse dependence on phosphine concentration. Even at modest

 

 

-0[RC"='CR]
= k, RCECR
dt bs [ l (22)
k4k5k6[2]
kobs -

 

-aIRC‘E-CR + 17 + 18] k [RC—CR 17 18]
= = + +
dt obs (24)

' K1K2k612]
[Pphal (25)

 

phosphine concentrations, plots of 1n[RC_=CR] vs. time in Figure 30 are linear through
three half-lives confirming a first order dependence on alkyne and [2]. Also, plots of
l/kob, vs. [PPh3] are linear as required by Eqs 23 and 25 (Figure 31).

48

Indecent/111,)

Figure 30. Plot of ln{[4-octyne]‘/[B2Cat2]o} vs. time for the catalytic diborylation of 4-
octyne by B2Cat2 in the presence of 2 and PPh; at 105 °C in C7D3 ([2]0 = 7.0 mM, [PPh3]
= 55 mM; 0 [4—octyne]o = 42 mM,. [B2Cat2]o = 84 mM; 0 [4-octyne]o = 42 mM,

.6
m

.
d

.‘s
in

via

r'o
in

 

 

‘+
+
+
'0».
++‘
Q ~
\0 ‘
x
\ s
C" s
23‘ .
c \ \
-..I 0‘ \ \
5... 0\ ‘
°\
0 ‘0‘
0 am, 5‘ O
6'... \ \
'93.
.5...
.
""9.
l J; J ‘

 

 

0

100 200 300 400 500 600 700

“no (min)

[B2Cat2]o = 42 mM; + [4-0Ctyne]o = 84 mM, [BzCatz]o = 42 mM).

49

[ESU
COH]
10110
PhD;

mech

d1b0f€
[h60r}

Roth]

 

 

 

 

60
,i’
/
/
/
50 " /
/
/
/
/
- 40 - ’
8 /
. /
a f’
S 30 _ /
/
/
/
/
/
20 ' ,o
/
/
I
I J
10
0 2 4 6 8
[PPh’](mM)

Figure 31. Plot of llkob, vs. [PPh3] for the catalytic ciborylation of 4-octyne by BZCatz in
the presence of 2 and PPhg at 100 °C in C6D6 ([2]0 = 7.0 mM, [4-octyne]o = 42' mM,
[BzCat2]o = 42 mM).

The concentration of CatB-BCat had no effect on the rate of alkyne consumption. This
result is in agreement with the ease with which CatB—BCat oxidative addition to Pt(0)
complexes occurs, is consistent with the hypothesis that regeneration of 2 is rapid, and
follows the rate determining step. The data from catalytic runs in the presence of
phosphine are reliably reproducible, and exhibit dependencies that are required by the
mechanisms in Figures 28 and 29.

The dependence of km on [alkyne], [PPh3], and [2] suggest that the mechanisms for
diboration within the stoichiometric and catalytic manifolds are essentially identical. This
theory could be verified by a direct comparison of rate constants between the

stoichiometric and catalytic systems. By rearranging Eqs 18 and 25 we see that a

50

comparison can be made, as K1K2k6 can be expressed in terms of kobs, [PPh3], and
[alkyne] for the stoichiometric system and kobs, [PPh3], and [2] for the catalytic system
(Eqs 26 and 27). The calculated values for K1K2k6 from the stoichiometric and catalytic
measurements are 0.019(3) and 0.024(3) sec", respectively. Thus, the data, which are
identical within experimental error, support the notion that the mechanisms for the

diboration in both systems are identical.

 

 

kobslpphiil
K K =
1 2k6 [RCECR] (26)
[(0 5 PPh
K1K2k6 = b 1 3]
[2] (27)

Mechanistic Concerns Regarding cis-(PPh3)2Pt(BPin)2

Although the mechanism proposed for the diboration of alkynes is different than the
original proposal by Miyaura and Suzuki, the results of this investigation support their
assertion that diboryl complex, cis-(PPh3)2Pt(BPin)2 (3), acts as the active catalyst.90
Since 3 is readily prepared, comparison of the stoichiometric boration reaction was
expected to be straightforward. In contrast to the catecholatoboryl chemistry, however,
spectra recorded under ambient conditions prior to initiation of the diboration reaction
indicated that an equilibrium existed between 3, 5, 4-octyne, and PinB—BPin (Eq 28). To

test for reversibility of this reaction,

51

(PPh3)2Pt(BPin)2 (PPh3)2Pt(n2-RCECR)

 

a , s
+ +
RCECR BgPinz (28)

the analogous experiment was performed using a volatile alkyne, 2-butyne. The addition
of 2-butyne to a C6D6 solution of 3 in an NMR tube showed that the alkyne complex was
readily formed with simultaneous elimination of PinB—BPin. Subsequent removal of 2-
butyne, in vacuo, regenerated 3 and confirmed the reversibility of the elimination.
Another complicating factor is the ease of BzPinz reductive elimination from cis-
(PPh3)2Pt(BPin)2 by the addition of PPh3. The observed equilibria between 3, PPh;,,
PinB-BPin, and (PPh3)nPt (n = 3, 4; 8a-b) would obscure comparisons of the relative
rates of boration for BzCatz and BzPinz under stoichiometric conditions. Since the
diboration reaction prevents an accurate evaluation of the equilibrium between 3 and
alkynes under usual reaction conditions (50 °C), the equilibrium constant at elevated
temperature was determined by extrapolating data from a Van’t Hoff plot for the
equilibrium in Eq 28. (Figure 32). From these data the equilibrium constant at 50 °C was
calculated to be 12.9. Thus, under typical conditions for catalytic diboration of 4ooctyne
(in the presence of added phosphine), 3 is the principal Pt containing species. As would
be expected, the equilibrium is sensitive to the alkyne substrate. For example, when
equimolar quantities of 3 and diphenylacetylene are dissolved in C6D6 the equilibrium is
observed to lie far to the side of the alkyne complex as (PPh3)2Pt(n2-PhCECPh) is the

only phosphine-containing species detected by 31P NMR.

52

 

-3 .5

I

 

 

 

-5.5
3.5 4 4.5 5 5.5

1000”

Figure 32. van’t Hoff plot for the equilibrium between 3 and 4-octyne. ([3]o = 8.7 mM;
[4-octyne]o = 52 mM; T = —80 °C to 0 °C; AH = 0.49 kcal/mol, AS = 6.6 en.)

The equilibria in the reactions of 3 with alkynes raise another possibility: that
diboration in the catecholate system could be due to a bimolecular reaction between trace
quantities of an alkyne complex and CatB—BCat. Despite the fact that in stoichiometric
reactions between 2 and 4-octyne neither CatB—BCat nor 5 are detected in solution,
alkyne complexes in this system are known to react by both associative and dissociative
pathways.121 This seems unlikely, however, as the boration rates in the stoichiometric
reaction between 2 and 4-octyne do not exhibit an alkyne dependence in the absence of
added phosphine. If boration proceeded through pathways that are first (or higher) order
in 5, an alkyne dependence would be expected since equilibrium concentrations of 5,

though small, are dependent on alkyne concentration.

53

Catalytic diboration of olefins

In the previous section details of the Pt-catalyzed addition of B-B bonds to alkynes
mediated by cis-(PPh3)2Pt(BCat)2 (2) were addressed. Although clean chemistry is
observed in this system for alkynes, the analogous addition to olefins does not occur. It
became apparent that formation of low-coordinate diboryl complexes, via phosphine
dissociation, was necessary to mediate coordination and insertion of the alkyne into the
Pt—B bond. Following that line of reasoning it seemed conceivable that catalytic activity
could be enhanced, and possibly olefin diboration would be observed, if the requirement
for phosphine dissociation could be eliminated. The most obvious candidates for testing
this hypothesis are the base-free Pt(0) compounds reported by Stone and co-workers.

In the hopes of forming an olefin platinum diboryl complex to examine this
possibility we attempted to react one and two equivalents of CatB-BCat, respectively,
with Pt(nbe)3 (20, tris(bicyclo[2.2.1]heptene)platinum(0)). In both cases a reaction takes
place readily at room temperature as the solution immediately turns red in color. The
inability to isolate or identify any platinum containing compounds hampered our efforts
toward this goal, however, the chief organic product was a
bis(boryl)bicyclo[2.2.l]heptane. From these results we reasoned that if we could not
isolate an olefin boryl complex that perhaps catalysis from the olefin complexes
themselves would be feasible. With this in mind equimolar amounts of norbomene and
CatB-BCat were reacted in the presence of 20 (3 molar percent) in an NMR tube.
Catalysis occurs at a rapid rate as diboration was complete within minutes at room
temperature.

The facility with which diboration in this reaction occurred might be due to the ring
strain associated with the bicyclic organic substrate, norbomene. Thus, in order to

ascertain whether this procedure would be a general route to diborated alkanes, a variety

54

of unactivated olefins (Table 4) were examined with Pt(nbe)3 and

bis(cyclooctadiene)platinum(0) (21, Pt(COD)2).1109122

55

Table 4. Products and crude yields for the diboration of selected olefins. Yields are based

on the crude material following solvent evaporation.

 

entry olefin product % yield
1 Z CatB BCat 84
CatB BCat

BCat
3 M 08mm 95
BCat
4 MCI 0313M 85
CI
O 803! O

5 Wok “RAM, 8“

CatB BCat
6 \ \‘—< 90
CBH4'p'C F3
C6H4-p-C F3
CatB BCat
7 c‘ 6H4-p-0Me ‘—< 95
CGH4'p'OMB
BCat

8 W swam 82

56

The reactions were typically complete after stirring for 10 minutes at room
temperature. As stated above, the solution turns deep red following addition of the
diboron reagent and appears to remain homogenous. Addition of mercury to the reaction
mixture had no effect on the catalytic activity; however, we have not been able to identify
the catalytically active species. Conversion is clean, as judged by 1H and ”B NMR and
product yields are high. Significantly, B2Cat3, the decomposition product that is often
. observed when catecholate-substituted borane reagents are employed, is formed in only
minor quantities.

A mechanism similar to the Pt-catalyzed diboration of alkynes can be envisioned for
the diboration of alkenes. Following B—B bond addition to the Pt metal center, boration
proceeds via insertion of the unsaturated substrate into the Pt—B bond of a Pt-diboryl
species and the diborated alkane is released by B—C reductive elimination from a boryl-
alkyl intermediate, as depicted in Figure 33. It is reasonable to assume that a-olefm
insertion into the Pt-B bond gives a primary alkyl intermediate as related insertions into

metal-carbon and metal-hydrogen bonds usually give primary metal-alkyl complexes.

57

R

CatB BCat
ant CatB-BCat
)R/
r—< BCat
LnPt\ BCat ant
BCat . BCat
24 22
_/R
T —/R
LnPt—BCat '—
CatB
23

Figure 33. Proposed mechanism for the diboration of olefins by base-free Pt(0)

complexes.

1,2-diborated products are observed exclusively for a-olefins and functionality on the
olefin substrates is tolerated so long as the functional group resides in a position beyond
the allylic carbon. For internal olefins, clean diboration is observed only for the bicyclic
compounds norbomene and norbornadiene. Only one stereoisomer is formed for these
substrates as the B—B bond adds preferentially to the exo olefin face. The stereochemistry
was confirmed by deuterolysis of the boryl groups and comparison of the NMR data to
the known dideuterionorbomane. Unfortunately, clean reactivity is not observed for other
internal olefins. For example, the reaction between CatB-BCat and trans-3-hexene gave a
complex mixture of products. 11B NMR spectra indicated significant quantities of HBCat

and BzCat3, in addition to a broad resonance in the region typical for alkylborane

58

products. Similar complications were observed when diborations of other internal olefins
were attempted.

Presumably, B—hydrogen elimination is the source of HBCat in the diboration
reactions of internal olefins. Figure 34 shows a likely pathway that can compete with the
reductive elimination of the diborated alkane from intermediate 25. The vinylborane that
is formed can then react with additional CatB-BCat to account for the complex mixture of
organoborane products._ Interestingly, the fi-hydrogen elimination pathway does not
appear to occur in the diboration of a-olefins. Although the reasons for this discrepancy
are not obvious, a possible explanation is that fi-hydrogen elimination could be

disfavored for hydro gens attached to a boron-substituted B—carbon center.

HRHZCHS /H
ant Cat a ant\ ""' ant + H3031
E BCat
Cat - J

25 \ 01426143 25
B Cat
—\—<8Cat 2 3

Figure 34. Decomposition pathway for internal olefins via B~hydro gen elimination.

 

 

 

Clean reactivity observed for the boration of norbomene and norbomadiene is not
surprising since B—hydrogen elimination from the bridgehead position of intermediate 27

would form a highly strained olefin (Eq 29).

 

V /I.-l
H /\ 7 + l-"Pt\BC t
an‘ \B 081 Cat a
Cat
27 (29)

59

mwdienes are also catalytically diborated by base-free Pt(0) compounds (Figure 35).
Significantly, for 1,5-hexadiene, equimolar quantities of diene and CatB—BCat give the

1,2-diborated product exclusively.

1 eq. BZCatz BCat
3mol°/o Pt(COD)2 _ (:8th

 

2 eq. 820312 8031
3 mol% P1(COD)2 CatB BCat

 

 

BCat

Figure 35. Selective di- and tetraboration of 1,5-hexadiene.

This result is somewhat surprising given that competition between the diborated
olefin and 1,5-hexadiene could occur as the reaction proceeds giving rise to tetraborated
products. If C=C insertion into the Pt-B bond is slow with respect to olefin exchange at
the platinum center, the observed selectivity could be attributed to preferential olefin
binding by 1,5-hexadiene, which can form chelate intermediates. Reactivity of
conjugated dienes is less clean as 1,3-butadiene gave a mixture of products that included

CatBCHZCH=CHCH2BCaL

6O

CHAPTER 4
SYNTHESIS, CHARACTERIZATION AND REACTIVITY OF IRIDIUM BORYL
COMPLEXES

Metal-mediated B—H activation is notably less difficult than analogous C-H
activations, despite the fact that B—H bonds are. stronger than the corresponding C—H
bonds.123v124 B—H activation is kinetically more favorable due to the vacant p orbital on
boron, thus coordinatively unsaturated boranes are far more reactive than hydrocarbons,
lessening the requirements on the metal complex. Conversely, a highly reactive,
coordinatively unsaturated complex or intermediate is required for activation of the
kinetically inert C-H bonds. The role of Cp*Ir(PMe3)H2 (28) in C—H activation
chemistry is well documented and the following mechanism is supported by kinetic data:
photoinduced H2 elimination from compound 28 generates the coordinatively unsaturated
reactive intermediate, [Cp"‘Ir(PMe3)].64v65 AIkyl and aryl hydride complexes are formed
via C-H activation by this intermediate and can subsequently undergo thermal reactions
where hydrocarbon elimination is observed.66

Catalytic functionalization of hydrocarbons has been a “Holy Grail” in homogeneous
and heterogeneous catalysis.62 Over the past 20 years a detailed picture of metal-
mediated C—H activation has emerged from intensive study by several research groups
and was briefly discussed in the introduction. Activation of C—H bonds is necessary for
generating species with M-C bonds, however, ensuing reactive chemistry directed toward
the M—C bond must be designed with catalytic capabilities in mind. Prior to our work,
reported examples of alkane functionalization have been limited primarily to the
oxidation of methane to methanol,33,34v36. To date, the only example not involving
methane conversion was the ruthenium catalyzed conversion of 2,6-xyly1 cyanide to an

indole.

61

Combining C-H activation to borane reactivity is potentially useful for more
specialized applications, as Suzuki and Miyaura have shown the utility of phenylboronic
acids in palladium catalyzed coupling reactions.4'125’126 It would seem reasonable to
utilize Lewis acidic reagents to convert metal alkyl or aryl groups to functionalized
organic products due to the fact that the reactive complexes and intermediates responsible
for C—H activation can be coordinatively unsaturated or cationic reagents. Lewis basic
substrates have the potential to hinder the reactivity of the unsaturated complexes. For
borane reagents, thermochemical and computational data establish that the reaction in Eq
30 is essentially thermoneutral.124 Hence, catalysis is thermodynamically viable. Given
the facility with which C—H activation occurs within the “Cp*Ir(PMe3)” system, our
expectations were that the activation of B—H and B-B bonds would also occur relatively
easily. For these reasons, we have been examining metathetic reactions of B—B/B-H

bonds with M—C/M—H bonds.127

H-CH3 + H-BCat —> H2 + OHS-BCat AH°=+1.1kcal/mol (30)

Reactions of Cp*lr(PMe3)H2 with H-B(OR)2 and (RO)2B-B(OR)2 bonds

Thermolysis of Cp*Ir(PMe3)H2 (28) in the presence of two molar equivalents of
HBCat or BzCatz gives the diboryl complex, Cp*Ir(PMe3)(BCat)2 (29), with generation
of H2 or HBCat respectively (Figure 36). Compound 29 could be isolated in 60% yield as
colorless crystals when prepared from compound 28 and two equivalents of BzCatz. 1H
and 3'P NMR data are typical for Cp*Ir(PMe3) compounds and 11B NMR revealed
resonance at 5 36, which is typical for Ir catecholatoboryl complexes. The formulation

was crystallographically confirmed (Figure 37). Ir—B distances of 2.03 and 2.04 A

62

compare to those in related Ir complexes.128 There are no other remarkable features in

this piano-stool structural.

HBCat E : I HBCat E : Z

—————> IL, ————” IL,
- H2 MeaP’ \”H - H2 Me3P’ \"BCat

BCat BCat
W 30 29

Ir. —*
Me3P’ \"H
H

28 Bzcatz : 5 I BZCatZ : 5 I

 

——> lr. ———> lr.,
- HBCat MeaP’ \"H - HBCat MeaP’ \"BCat
BCat BCat

30 29

Figure 36. Reaction scheme for the formation of iridium boryls from the reaction of 28

With I'lBCflI and B2Cat2.
,l»‘. 5' r 0 ~09
'& - ”.1/
It
I _ 81 f." ‘4
\ A7" ..
3.1.3 . .
g P 82 R x
l” I J . 1‘ a,
A. ‘v
(3.; j -: , y "
. if" 30 (at; ,
@ f: 110 . ‘7’
‘ ./ .. A \.

 

Figure 37. Molecular structure of 29. Thermal ellipsoids are at 50% probability.

Table 5. Selected bond lengths and angles for 29.

 

 

bond distance (A) bonds angle (deg)
Il'(1)—B( l) 2.028(11) B(l)-II'(1)—B(2) 84.7(4)
Ir(1)—B(2) 2.037(11) B(1)—II‘(1)—P(1) 90.6(3)
Ir( 1)-P(1) 2.248(3) B(2)—Il'(1)—P( l) 92. 1 (3)

63

When compound 28 is reacted with one molar equivalent of HBCat or BzCatz the
monoboryl complex, Cp*lr(PMe3)H(BCat) (30), forms with concomitant generation of
H2 or HBCat (Figure 36). Unfortunately, all attempts to isolate analytically pure 30 gave
mixtures of compounds 28, 29, and 30 and borane decomposition products. Despite this,
the identity of compound 30 can be made conclusively from spectroscopic data. 1H NMR
spectra indicate new resonances for the catecholate, Cp*, and PMe3 ligands. In addition, a
high-field resonance at 6 -17.12 is observed forthe hydride. This signal appears as a
doublet with a typical 121ml value (29 Hz) for this system. The hydride resonance
integrates as one proton, and the presence of a single iridium-bound hydride was
confirmed by observation of a doublet in the selectively decoupled 31P NMR spectrum.
1'B NMR spectra indicate a new resonance at 8 36 for the boryl compound and a doublet
for HBCat for the reaction of 28 with BzCatz when performed in an NMR tube.

The pinacolate analogue, Cp*Ir(PMe3)(H)(BPin) (31), can be generated from the
reaction of 28 and HBPin in toluene (Figure 38); however, in contrast to the catecholate
analog, higher temperature (120 °C) and increased reaction time (several days) are
required for product formation. Interestingly, the concentration of thf also had an effect
on product formation as higher concentrations inhibit formation of 31 and lower
concentrations lead to the formation of small amounts of Cp*Ir(PMe3)(BPin)2 (32).129
Compound 31 was obtained in 82% yield as an analytically pure orange oil.
Decomposition similar to that observed for 30 occurs when this compound is placed
under vacuum for an extended period of time. The rate, however, appears to be much
slower than that observed for the catecholate analog. The pinacolate analog of 29 can be
formed from compound 28 and a large excess of HBPin (24 molar equivalents, 150 °C),
but efforts to isolate this compound have been hampered by borane decomposition

products. BzPinz does not react with 28 even at elevated temperatures.

64

 

HBPin E : I ‘ HBPin E : Z
——>

lr., ——> In,
- H2 MegP’ \”H - H2 MeaP’ \”BPin
BPin BPin
‘6? 31 32
I"? ——
M93P \’H
H
23 B Pi
A NR

 

Figure‘38. Reaction scheme for the formation of iridium boryls with HBPin and BzPinz.

Kinetics Experiments

Metathesis reactions of M—H bonds with B-H bonds have two potential
regiochemical outcomes: i) formation of a metal boryl complex with elimination of H2 or
ii) degenerate H exchange between metal and borane hydrides. While the former is
readily identified by the formation of new species in the reaction mixture, the latter
process can be inferred from labeling studies. In reactions of transition metal complexes,
both processes have been observed in the few cases that have been studied in detail, and
general trends in reactivity have not emerged.15,18,1933’24’130132 These reactions are
important in stoichiometric and catalytic metal—directed borations of unsaturated organic
substrates because metal hydride intermediates can result from B—H/M—C metathesis in
B-C bond forming steps. Clearly, an area where these metathesis reactions are potentially
relevant is in metal—mediated functionalization of hydrocarbons.

The reaction between Cp*Ir(PMe3)H2 and HBCat to form Cp*Ir(PMe3)(BCat)H was
deemed sufficiently clean for kinetic analysis since the diboryl compound, 29, could not
be detected until the third half—life. The loss of compound 28 as a function of [28] and
[HBCat] was consistent with the rate law shown in Eq 31. An Eyring plot (Figure 39) for

65

 

k0,, between 20 and 70 °C yielded the following activation parameters: AH" = 14.9(5)
kcal/mol and AS1 = —29(2) e.u. In an effort to examine kinetic isotope effects the reaction
of 28—dz and HBCat was attempted. Unfortunately, rapid scrambling of the label with
respect to the rate of formation of 30 was observed, precluding the extraction of
individual isotope effects for the iridium and borane positions. However, the reaction
between 28—d2 and DBCat gave a kH/kD = 1.7(1) (Figure 40), which is a composite of
primary and secondary kinetic isotope effects. Rates for the reaction between 28 and

BzCatz were not reproducible, preventing a more detailed examination of this reaction.

-Om - kobsl1][HBCat]
dt ' (31)

 

 

41

Intk/T)

 

 

 

L L 1

 

46
29 3 SJ 32 33 3A 35

1000/r(1n<)

Figure 39. Eyring plot for the reaction of 28 with HBCat in C6D6. from 20 °C to 70 °C
([28]o = 41 mM; [HBCat]o = 245 mM).

66

 

normal/[2310)

 

 

 

 

Figure 40. Plots of ln{[Cp*Ir(PMe3)X2]./[Cp*lr(PMe3)X2]o} versus time for the reaction
of Cp*Ir(PMe3)X2 with XBCat. ([Cp*Ir(PMe3)X2]o = 41 mM; [XBCat]o = 245 mM; 0 (X

= D); (X = H))-

The metathesis reactions in Figures 36 and 38 pose interesting mechanistic questions.
For reactions between the dihydride, 28, and substrates with B—H bonds, potential

mechanisms are shown in Figures 41 and 42.

67

A - Oxidative Addition

 

 

 

- PMea + HBCat
Cp'|r(PMe3)H2 2 [Cp‘lrmHz] = [Cp‘erHgBCat]
28 + we" 33 ’ ”Boat 34
+ H2 - H2
- PM93
Cp'lr(PMea)(BCat)H ‘ —‘ [Cp'lr’"(H)BCat]
+ PMe3
30 35
B - Ring Slip
+ H8031 v
contention2 a: [(n3-Cp*)IH"(PMe3)H21= [ms-cm" (PMeaXBCallHal
23 36 ' “Boa“ 37
+ H2 ‘ H2
Cp’lr(PMe3)(BCat)H = [(na-Cp')ll’"(PMe3)(BCat)H]
30 as

Figure 41. Oxidative addition pathways for the formation of 30.

68

C - Ion-pair

+ HBCat t
Cp'|r(PM63)H2= [Cp’|r(PMe3)H+][HzBCat’]—‘—-—-.— [Cp'|r(PM83)H”'HBHCat]
- HBCat
28 39 4o

 

 

 

+ H2 - H2
[Cp'lr(PMeg)]
Cp'lr(PMe3)(BCat)H 4=—-——- 33
30 +
HBCat
D - Borane Adduct
' " t
Cp'II(PM93)H2 H\
28 ‘
‘_-—_':_ [Cp'|r(PM93)H2—>BHCat] = Cp*H(PMeg)lr,‘ :H
+ x 0"
HBCat 41 Eat
42
+ H2 - H2
Cp‘lr(PMeg)(BCat)H
30

Figure 42. Ion pair and borane adduct pathways for the formation of 30.

The first two pathways proceed through a series of oxidative addition/reductive
elimination steps following an initial ligand dissociation step. Phosphine dissociation
could possibly be examined by running the reaction in the presence of PMe; or PMe3-d9.
Unfortunately, phosphines react with catecholborane, leading to eventual boron

decomposition products. Ring slip pathways are sometimes invoked in cyclopentadienyl

69

systems; however, it is difficult to definitively assess this possibility experimentally. The
third mechanism requires HBCat to abstract a hydride from compound 28 to form the ion
pair, [Cp*Ir(PMe3)H+][H2BCat‘] (39). Although the cation, [Cp*Ir(PMe3)H+], has not
been reported, related cationic alkyl and aryl complexes have been fully
characterized.133r134 In the fourth pathway, formation of a borane adduct precedes Ir—
H/B—H metathesis. The iridium adduct, Cp*Ir(PMe3)(H)2-A1Ph3, as well as an iridium
_ borane adduct we prepared, which will be described. later, provide precedence for the
intermediacy of a borane adduct.135 If a borane adduct lies along the reaction coordinate,
the differences in reactivity can be correlated to the Lewis acidity of the borane as
competitive binding between these boranes and NEt3 gave the following ranking:
HB(C6F5)2 > HBCat > HBPin. Regardless of the mechanism for boryl formation, a
nonproductive pathway that could account for label scrambling proceeds through a four
centered transition state analogous to that proposed for exchange between hydride

positions of CpRu(PPh3)2H and HBCat (Figure 43).24

 

 

 

 

' ' t
IH\
Cp*|r(PMe3)H2 Cp‘|r(PMes)H‘ {BCat = Cp‘lr(PMea)H(DB(H)Cat)
28 ‘o’ 44
+
DBCat 43

Cp*Ir(PMes)(H)D
28-d1

+
HBCat

Figure 43. Degenerate hydride exchange in the reaction of Cp*Ir(PMe3)H2 with DBCat.

70

We also attempted to examine the kinetics of the reaction of Cp*Ir(PMe3)H2 with
HBPin to form Cp*Ir(PMe3)(BPin)H, but non-linear plots for the loss of [28] were seen
(Figure 44). Despite the lack of quantitative kinetic data, the temperature and reaction
time needed to form 31 from 28 and HBPin clearly suggested that the rate for boration by
HBCat is faster than by HBPin.136 Also, HBPin is far more stable to decomposition in
the presence of phosphines, therefore a kinetic run was prepared where approximately a
one-to-one ratio of HBPin to PMe3-d9 was used. The reaction proceeded very slowly, and
after 38 hrs of heating at 120 °C, the reaction had proceeded to 10 % completion
according to 1H NMR. Integration of the P(CH3)3 resonances in the 1H NMR of
Cp*Ir(PMe3)H2 and free PMe3 indicated that exchange between PMe3-d9 and Ir-bound

PMe3 had occurred to only 4%.

 

42v

3.8 O

3.6

impel/[23101:

3 .4

32F

 

 

 

2.0
0 2000 4000 6000 8000 110‘

time (s)

Figure 44. Plot of ln{[28],/[28]o} versus time for the reaction of Cp*Ir(PMe3)H2 with
HBPin. ([Cp"'Ir(PMe3)H2]o = 41 mM; [HBPin]o = 254 mM).

71

Reaction of Cp*lr(PMe3)H2 with the electrophilic borane, HB(C6F5)2

When Cp*Ir(PMe3)H2 was reacted with the electron deficient borane, [(C6F5)ZBH]2,43

the borane complex, Cp*Ir(PMe3)(H)[HBH(C6F5)2] (45) was isolated in 84% yield.

Signals due to the Cp* and PMe3 ligands were visible in the room temperature .‘H NMR

spectrum, but resonances in the hydride region could not be detected. At -70 °C, a single

high-field doublet (2 H, IZJpHI = 27 Hz), corresponding to the Ir—H nuclei, was observed

at -15.9 ppm. A broad resonance at 4.4 ppm was assigned to a boron hydride that is not
coordinated to Ir. The high frequency van (2508 cm") in the IR spectrum of compound

45 is typical for a terminal boron hydride and corroborates the NMR data.43v137v138

The molecular structure of 45 was determined and is shown in Figure 45. In the

solid—state, the two Ir-H positions are chemically inequivalent, and terminal B-H group

is clearly present. The hydrides were refined isotropically, and their positions did not
deviate substantially from electron density maxima found in difference Fourier maps
generated from low angle data. Although these peaks were located in chemically
reasonable positions, the Ir—Hl distance of 1.38(3) A is short for a terminal Ir hydride.

The value is considerably shorter than a typical terminal Ir—H bond distance determined
from X—ray (1.47—1.70 A)16,66.139,140 or neutron (l.587—1.617 A)141’145 data, however,
shorter lengths for iridium hydrides have been reported.145'148 This is not unusual as X—
ray data commonly underestimate M—H distances for hydrides attached to third period
transition elements. The Ir—H2 and Ir—B distances are 168(2) and 2.614(2) A,
respectively, and significant elongation of the B—H2 bond (134(2) A) is observed. The
Ir—H(2)—B angle of ll9.6(8)° is outside of the range reported for n‘-meta1 borohydrides
(121—173.9°). The short angle may be a consequence of Ir—B bonding interactions;

however, an nz-borohydride complex with a substantially shorter M—H-B angle of

72

112(5°) has been reported by Girolami. In valence bond terms,
Cp*lr(PMe3)(H)[HBH(C6F5)2] can be described as a borohydride complex, a borane
adduct related to the recently reported Lewis acid-base adduct, Cp*IrPMe3(H)2-A1Ph3, or

a combination of these two extremes.135

‘2
it?)

 

Figure 45. Two different views of 45. Thermal ellipsoids are at 30% probability. In (A)
all hydrogen atoms except H1 are omitted and only the ipso carbons of the
pentafluorophenyl groups are shown and in (B) all hydrogen atoms except H1 and all

fluorine atoms are omitted for clarity.

Table 6. Selected bond lengths and angles for 45.

 

 

bond distance (A) bonds angle (deg)
Ir(1)—B(1) 2.613(2) B(1)—Il‘(1)-H(1) 77.8(12)
Ir(1)-H( 1) 138(3) B(1)—Il’(1)—H(2) 26.5(8)
Il‘(1)—H(2) 1.68(2) H(1)-Ir(1)-H(2) 87.1(14)
B(1)—H(2) 134(2) P(l)—Ir(1)—H(l) 77.8(11)
Ir(1)-P(l) 2.258(1) P(1)—Ir(l)—H(2) 82.9(8)
B(1)—H(3) 1.05(2) B(l)—II’(1)—P( 1) 104.88(6)

In light of the solid state structure, in which H1, H2, and H3 are chemically

inequivalent, the inability to observe resonances for these hydrogen atoms suggested

73

dynamic behavior. When a toluene-d3 solution of 45 was cooled to —50 °C a doublet was
observed at 5 -15.9 and a broad resonance was observed at 8 4.3. Upon cooling to —80
°C, the doublet appeared to broaden somewhat but further decoalescence was not
observed in this solvent. Therefore, the low melting (—135 °C) solvent, CDle, was used
to access a lower temperature regime.149 As in toluene-d3, a doublet in the hydride region
(8 -15.6) and a broad resonance for the borane hydrogen were observed upon cooling;
however, at —105 °C, the doublet decoalesced to a broad singlet at 5 —l4.6 and a doublet
at 8 —l6.7 (IZJPHI = 38 Hz). The broad singlet is assigned to the hydride corresponding to
the bridging position (H3) and the doublet is assigned to the terminal Ir hydride (H) of
the static structure (Figure 46). The broad nature of H3 can be attributed to coupling to the
quadrupolar boron nucleus and has been observed in other metalloboryl

complexes.14v50v51

‘src‘i‘zz‘

Ir.
Meap” \"Hb H

\\

 

 

Haxa‘
Pop:5
6 5
45
Ha Hb
.n M
J L LJ’JK...
-15 -16 -17

Figure 46. ’H NMR of the hydride region of 45 in CDCle at —105 °C.

Line shape analysis was performed for spectra from —105 °C to —55 °C and an Eyring
plot (Figure 47) yielded activation parameters for the exchange, AH* = 12.4(6) kcal/mol

and AS1 = 25(3) e.u..150 The magnitude of the entropy term is consistent with a

74

dissociative process (Figure 48). Intermolecular hydride transfer at room temperature
between all hydride positions was independently confirmed by reacting 28-d2 with
Cp*Ir(PMe3)(H)[HBH(C6F5)2], which gave statistical scrambling of the deuterium label
as determined by 2H NMR.

 

10

'"(W
a.

 

 

 

 

45 5 5.5 6
1000m1M)

Figure 47. Eyring plot for the exchange between H, and Hi, of 45 in CDCle from -—105
°C to ~55 °C.

75

in
lei

for

 

 

/|:';"B‘\H 23 M93? \ Hb\ \Cst
I‘ch5 Ha 2:3
CoFs + H \Cst
45 H‘B(C6F5)2

45

 

 

Figure 48. Exchange between H, and Hb in 45.

In CDCle or CDzClz solutions of 45, a two major new sets of Cp*, PMe3, and
hydride resonances gradually form irreversibly at low temperature in approximately a
one-to-one ratio. One set was identified as the dihydride compound, 28. For the other set
of resonances in particular, the two hydride resonances at 5 -12.5 and 5 -13.9 integrate in
a 1:2 ratio, and the triplet of doublets at -12.5 is coupled to the doublet of doublets at -
13.9. All of the 1H and 31P NMR data for this iridium species correspond to the
previously reported cation, Cp*Ir(PMe3)H3“.139 In the 11B spectrum, a triplet at 5 —31.4 is
assigned to the borohydride anion, H2B(C6F5)2‘. Resonances for 28 and the cation are not
observed in C7Dg spectra taken at the same temperatures. The formation of the ionic
compound might be a result of differences in solvent polarity. Polar solvents such as
CD2C12 and CDCle would be better able than a non-polar solvent like C7133, to support
ionic species.

The appearance of a single high-field resonance upon heating a toluene solution of
Cp*Ir(PMe3)(I-I)[HBH(C5F5)2] to 80 °C indicated rapid exchange between the borane
hydrogen and both iridium hydride positions. This resonance was approximately the
weighted average of limiting Ir-H and B—H chemical shifts. The cause of this deviation
from the exact weighted average is unknown. Perhaps an equilibrium between 45 and

transient Cp*Ir(PMe3)H2 results if the borane fully dissociates to a small extent.

76

Compound 45 reacts with Lewis bases such as PMe3, pyridine, and thf to give
Cp*Ir(PMe3)H2 and the corresponding Lewis base adducts of HB(C6F5)2 (Eq 32), but it is
stable towards the weaker Lewis base diethyl ether. This is interesting in light of the fact
that diethyl ether is known to form an adduct with HB(C6F5)2.48 The fact that 28 is a
stronger base than EtzO was corroborated by examination of this reaction from the
reverse direction. In situ generation of (EtzO)-HB(C5F5)2 (confirmed by 1H NMR)
followed by the addition of an equimolar amount of compound28 revealed only 45 and
EtZO in solution. A qualitative ordering of the Lewis basicity of Cp*Ir(PMe3)H2 with
respect to other bases can be determined based on these observations: pyridine > PMe3 >

thf > 28 > EtzO.

 

g1: L = PMea, thf, @ H
> +

"Wu L:->Bl
MeaP/ HH pyridine Ir.. 5‘0er
H I ”’H I
‘34,, - MeSP/ \H ‘3st
0st ’0st
28
45 (32)

At elevated temperatures, Cp*lr(PMe3)(H)[HBH(C6F5)2] eliminates H2 and is
gradually converted to a new species, 46, which was isolated in 39 % yield. A new high
field doublet at 5 -16.6 in the 1H NMR was accompanied by a doublet at 8 —44.7 in the
31P NMR spectrum, and a low field resonance at 6 81 in the 11B NMR spectrum. Based
on these data, compound 46 was identified as the boryl complex,
CP"‘II(PM¢3)(H)[13(Can)2].151'”2

Single crystals. of 46 were grown from pentane at —78 °C and the structure was

confirmed by X-Ray crystallographic analysis. The molecular structure of compound 46

77

is shown in Figure 49. The Ir—H distance (1.51(4) A) is well within the typical range of

iridium hydride bonds and the compound adapts a piano-stool geometry.

 

Figure 49. Two different views of 46. Thermal ellipsoids are at 30% probability. In (A)
all hydrogen atoms except H1 and all fluorine atoms are omitted and ill (B) all hydrogen
atoms except H1 are omitted and only the ipso carbons of the pentafluorophenyl groups

are shown for clarity.

Table 7. Selected bond lengths and angles for 46.

 

 

bond distance (A) bonds angle (deg)
Ir(1)—B(l) 2.014(4) B(1)—lr(1)—H(1) 78.1(16)
1I(1)-P(1) 2.255(1) P(1)—11'(1)-H(1) 82.8(16)
II(1)—H(1) 151(4) B(l)—Ir( l)-P(1) 9233(8)

The thermolysis of 45, although not very clean, also resulted in the appearance of new
Cp*, PMC3, and hydride resonances. In particular, two hydride resonances, a doublet of
doublets at 6 —l9.2 and a triplet of triplets at 8 -23.0, integrate in a 2:1 ratio, and are
coupled to each other. 1H and 31P NMR data for this Ir species correspond to the

previously reported cation, (Cp’l‘Ir(PMe3)H)2(]r-H)“.139 In the 11B spectrum, a doublet at

78

8 —23.9 is assigned to the borohydride anion, HB(C6F5)3'. The NMR spectroscopic data
for this species is identical to that observed when one equivalent of B(C6F5)3 is reacted
with two equivalents of Cp*Ir(PMe3)H2. 1H and 19F NMR also supported the formation of

HC6F5 as a minor product.

Stoichiometric C-B Bond Forming Reactions

The reaction of borane reagents with Cp*Ir(PMe3)(R)H (R = cy-CgH“ (47), C6H5
(48)) has four possible outcomes (Figure 50); i) M—C/B-H bond metathesis leading to
metal boryl and R—H, ii) M—C/H—B bond metathesis leading to metal hydride and
organoborane, iii) degenerate M—H/B—H bond metathesis, and iv) M—H/H-B metathesis
leading to a complex containing both M—C and M—B bonds. Outcome iii is non-
productive and outcome iv, although possible, seems unlikely due to the scarcity of

compounds that simultaneously support both M-C and M-B linkages.104

79

‘fi‘ :42”

Ir‘,

l -,
Me3P’ r\”3pi M9313, \I’H
cy-Cs 12 . BPin
49 HBP'" HBPin 31
iv 1
*4? % CI‘CRJ412
,,lnq,
M93P \’H
' Ci‘CRJ112 _
DBPin m 47 K
W HBPin cy-C5H11-BPin W
MaeP’lr'K'b ”RP, 8"“
CW‘CRJ412 '1
474:11 2a

Figure 50. Metathetic outcomes for the reaction of HBPin with Cp*Ir(PMe3)(cy-
C6H11)H-

Cp*Ir(PMe3)(H)(Cy) and Cp*Ir(PMe3)(H)(Ph) reacted with HBX; reagents in C5D6
to yield the alkylborane derivatives, CyBXz and PhBXz (X2 = (C5F5)2, Pin, Cat). The
rates for B-C bond formation and Ir product distributions were borane dependent. The
reaction between [HB(C6F5)2]2 and compound 47 gave 28 and cy-Cnl-ln—B(C5F5)2 (Eq
33). Conversions were quantitative and complete within 5 min at ambient temperature as
judged by 1H NMR. Elimination of cyclohexane from compound 47 was not observed,
and Ir products from solvent activation were not detected. Characterization of the organic
product, cy-CaHn—B(C6F5)2, was established by comparing 1H and 11B NMR data to an

authentic sample, prepared from [HB(C6F5)2]2 and cyclohexene.153 The reaction between

80

, [HB(C6F5)2]2 and compound 48 resulted in a mixture of products, indicatinga more

complicated reaction.

542‘

In, + H8 F ————> Ir. .-
MeaP’ \"H (06 5)2 MeaP’ \"H + CY'06H11 B(CSFS)2
Cl‘CeHli H
47 28 (33)

For oxygen-substituted boranes, B—C bond formation required more forcing
conditions as illustrated by the reactivity of pinacolborane (HBPin) with compound 47.
When a cyclohexane solution of Cp*Ir(PMe3)(H)(cy—C5Hn) and HBPin was heated at
100 °C for 65 hr, Cp*Ir(PMe3)H2 was not formed. Instead, Cp*Ir(PMe3)(H)(BPin)
formed as the major Ir containing product. In order to determine the fate of the
cyclohexyl and hydride ligands, thermolysis of Cp*Ir(PMe3)(H)(cy—C(,Hn) in the
presence of HBPin was monitored by 1H NMR. In benzene—dis solutions, this reaction
yielded cy-C6Hn-BPin and cyclohexane and the major Ir containing products were 31
and 4846.154 The amounts of products formed, both organic and metal based, depend on
the reaction conditions. Typical product distributions obtained for the reaction of 47 with
HBPin at various reaction conditions are shown in Table 8. Some general trends are
noteworthy. Higher concentrations of HBPin relative to 47 (comparison of entries 1 and
2, 3 and 4, and 5 and 6) results in increased percentages of the monoboryl complex 31
and the organoborane product. Higher reaction temperatures lead to an increased
percentage of phenyl hydride complex 48 (entries 3 and 5, and 4 and 6) but not
necessarily an increased yield of cyclohexane. Increased amount of thf contaminant in the
borane (entries 1 and 5) appears to lead to a decreased amount of 31. Although rigorous
kinetic analysis was precluded by irreproducible rate data, the qualitative rates for

cyclohexane formation were, in general, higher than expected for thermal elimination of

81

cyclohexane from compound 47. The amounts of Cp*Ir(PMe3)(Ph)H and cy-C6H12
expected can be calculated given the kob, value for the thermal decomposition of
Cp*Ir(PMe3)(Cy)H in C6D6. Buchanon and Bergman measured the rate for the clean, first
order conversion of 47 to 48 in C6H6 over a range of temperatures and displayed the data
in an Arrhenius plot.66 The rates were identical in C6H6 and C6D6 and so the appropriate
kw, values for the reaction performed at 95 and 135 °C were calculated from their data

and allowed for the estimation of the amounts of 20 and cy-C6H1 2.

82

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5% ow 58 2 8 mm m: 2 _ a o
35 2 $8 w? 8 N2 3 2 _ c m
m. :8 2 0:3 3 3 SN 3 n: 3 v
3% .m 25 : cm 82 3 n: o m
8% S cm? 3 om an m: E. S N
ME? 2 £3 3 mm m; m: E. c _
825263 825263 5.8 2:5 Gav Emma .6 we 5%:
:6 s .5815 as an s S s as 2:: 9:2 Eu 3 has

 

 

2.8V 2 5mm: Mo 5223 05 .8 8.2» 98 22:28 553% .w 935,—.

83

The reaction manifold for this conversion is likely quite complex, with multiple
pathways leading to the eventual product distributions. Pathway A (Figure 51) starts with
the thermal elimination of cyclohexane, to yield the coordinatively unsaturated
intermediate [Cp*Ir(PMe3)] (33), which can then activate the OD bonds (Ai) or B—H
bonds (Aii) to yield complexes 31 and 48-d6, respectively. Pathways B and C (Figure 52)
are complementary in that both are assumed to involve simple metathetic exchange
involving Ir—C/B-H bonds. Pathway B generates 31 and cy-C6H12, whereas pathway C
yields compound 28 and c r-C6H11-BPin instead. Cp*Ir(PMe3)H2 can then be converted
to 31 as discussed earlier. Other mechanisms involve oxidative addition/reductive
elimination steps following ligand dissociation. Phosphine dissociation seems unlikely,
however, given the small amount of PMe3-d9 incorporation into Cp*Ir(PMe3)H2 and
Cp*Ir(PMe3)(BPin)H when 28 is reacted with HBPin in the presence of labeled PMe3
(see above). A ring slip mechanism (Pathway D, Figure 53) would also account for an
increased amount of cy-C6H12. Formation of an n3-pentamethylcyclopentadienyl complex
followed by oxidative addition of HBPin to the Ir metal center would yield intermediate
51. Reductive elimination of cy-CGHu—BPin (Di) or cy-C6H12 from 51 and rearrangement

of the Cp* ring to nS-coordination gives 28 and 31, respectively.

84

3%?

0.0605 MeSP ’l \IIID

 

All 0st

48-d6

A - 32"?2‘
Ir. -
Map \"’H 0’06”” Me3P’"
CV HBPin

‘7 33 fir
M93 P’ "IH
\BPin

31

Figure 51. Thermal decomposition pathway for the reaction of HBPin with 47.

fl 8 + HBPin fl

 

Ir. Ir.
MeaP’ ”’H _ C C H , M63P’ ‘0’ H
\CY'CGHH y 6 ‘2 BPin

47 C 31

    
 

+ HBPin
" CY‘C6H1 1-BPin

Figure 52. Simple metathesis pathways for the reaction of HBPin with 47.

85

W “3'0!"r TIS'CP'
D l + HBPin I so“
# fl

 

lr., Ir. M P—l V—BP'
MeaP’ \"H MeaP’ K’H 9:; / Ir m
Cy Cy Y H
47 50 51
' CszHn'BPin ' CY‘CGH12
113-09* 113-09'
la" la"
MeaP’ {'IH MeaP’ {’IH
- H BPin
36 52
or on

W + HBPin fl

Ir. lr.,
MeaP’ (”H T MeaP’ \”H
H BPin
28 31

Figure 53. Ring slip pathways for the reaction of HBPin with 47.

Two potential mechanisms besides pathway B could account for enhanced
cyclohexane formation. These include the reaction of cy-Can—BPin with compound 28
to generate Cp*Ir(PMe3)(BPin)H and cyclohexane (Eq 34) or pathways that involve
metathesis between compound Cp*Ir(PMe3)(Cy)H and C5D6.155 Neither of these occur as
control experiments indicated that (i) cy-C6H1 l—BPin and Cp*lr(PMe3)H2 do not react and
(ii) deuterium incorporation in compound 31 was insignificant as judged by'GC-mass
spectrometry of an NMR tube reaction sample. The metathesis of C6D6 with 47 would
provide C6H11D and the mass envelopes for both cyclohexane and cyclohexane-d1 can be
calculated. The envelope of the parent peak for cyclohexane produced in this reaction

matched that calculated for C6H12.

86

+ CY'CSH11"B(CGF5)2 Ir. + CVCGH‘Z

In,
MeaP’ \"H Me3P’ \"H
H BPin
28 31 (34)

The reaction between compound 47 and HBCat also gave a mixture of C(5le and cy-
Cél-In—BCat. In addition to Cp*lr(PMe3)(BCat)H and phenyl hydride (48), the diboryl
compound (29), was also formed in this reaction.

Reactions of 48 with HBPin and HBCat required elevated temperatures (Figure 54.
150 °C or higher). In both cases Ph—BXZ was the major organoboron product although a
sharp C6116 resonance appears downfield from residual C6D5H in the deuterated solvent.
This observation supports the notion that metathesis of B-H bonds with M—C bonds can
result in formation of M—B and C—H bonds in this system. For HBCat, the diboryl
compound, 29, was the predominant Ir containing product. The reaction with HBPin
yielded the monoboryl compound, 31, as the major Ir containing compound. This last
reaction was performed at 150 °C with 12 and 24-fold excesses of HBPin and was
monitored intermittently by 1H NMR. Neither reaction was taken to completion as plots
of 1n{[48]./[48]o} versus time were not linear (Figure 55) and indicated that excessive

reaction times were necessary to complete conversion to 31.

W + HBX2 W + HBX2 @
——>

In, In, + Ph-BX2 ———-> In,
Meap/ \”H 150 °c Meap/ \”H 150 °c Meap’ ”sz
Ph 8X2 _ H2 8X2
48 X2 = Pin, Cat X2 = Cat
30, 31 29

Figure 54. Reactivity of 48 with HBPin and HBCat.

87

 

4.4

4.2

mum/[4310)
5"
CD

5"
m

3.4

32 o

 

 

 

o 510‘ 1105 1.5105 210‘
tlme(s)

Figure 55. Plot of ln{[48],/[48]o} versus time for the reaction of 48 with 12 equivalents
of HBPin at 150 °C. The last data point indicates two half-lives
completion.([Cp"‘Ir(PMe3)Hz]o = 35 mM; [HBPin]0 = 407 mM).

Catalytic Boration of Arenes

The stoichiometric chemistry of compounds Cp*Ir(PMe3)(Cy)I-I and
Cp*Ir(PMe3)(Ph)H suggested a potential catalytic conversion of unactivated
hydrocarbons to organoboranes in this system as shown in Figure 56. Since B—C bond
formation was most facile for reactions of compounds 47 and 48 with [I-IB(C6F5)2]2, the
catalytic viability of this borane reagent was assessed first. The dihydride product from

Eq 31 reacts with [I-IB(C6F5)2]2, to generate the borohydride complex,
Cp*lr(PMes)(PD[HBH(Cst)2l-

88

H-B(Cst)2 cy-CeH1 1'B(C6F5)2

”27% W

In, I.,
MesP’ \"H Meap’ \"H
47 CY‘CBHH H
H2 28
H‘B(CSF5)2'
hv
CY'CBth

‘Sfi'a’ 3%?

"'0 I '"I
MesP’ \" H MeaP’ ::H
H '1
28 V Csrgs ’CSFS
+ 45
HB(CsF5)2

Figure 56. Proposed catalytic cycle for the conversion of alkanes to organoboranes.

Following the C—B activation process, the 'dihydride product reacted with excess
[HB(C6Fs)2]2 in solution to generate compound 45. At elevated temperatures, a complex
mixture formed when benzene or cyclohexane solutions of 45 were heated with excess
[I-IB(C5Fs)2]2, eliminating the possibility of a clean thermal catalytic process. Since
photolyses of compound Cp*Ir(PMe3)H2 in the appropriate solvent generates compounds
47 and 48, similar photochemical generation of compounds 47 and 48 from
Cp*lr(PMe3)H[HBH(C6Fs)2] would complete a photocatalytic cycle. Unfortunately,
compound 45 was photochemically inert.

In the course of investigating the general reactivity of iridium boryl compounds,

thermal B-H or B—B elimination from compounds Cp*Ir(PMe3)(BCat)2 and

89

Cp*Ir(PMe3)(BPin)H was assessed by heating C6D6 solutions of the pure compounds. No
evidence for elimination or C—B bond formation was observed even after several days at
200 °C. Thus, photochemical reactivity of mono or diboryl derivatives seemed necessary
for catalytic activity. Although the diboryl compound, 29, was photochemically inert, the
phenylhydride compound, 48, was regenerated when a benzene solution of the
monoboryl compound, 31, was photolyzed (Eq 35). Although less efficient, the
regeneration of 47 also occurred when a cyclohexane solution of 31 was photolyzed.
These two reactions, thermal B—C and photochemical Ir—C bond formation, constitute a

catalytic cycle for HBPin when performed consecutively.

: : hV, H'C6H5

 

Ill, > Ir, '
MeaP’ \”H MeaP’ \”H + HBP'"
BPin C6H5
31 43 (35)

To evaluate the potential of photocatalytic cyclohexane and benzene activation, two
thick-walled flasks were charged with Cp*Ir(PMe3)H2 (0.15 mol) and HBPin (0.89
mmol) and the contents were dissolved in C(5le and C6H6, respectively. After the
solutions were photolyzed to generate compounds 47 and 48 (confirmed by 31P NMR),
the flasks were heated in an oil bath to convert the phenyl and cyclohexyl compounds to
to Cp*Ir(PMe3)(BPin)H and R-BPin. After the thermal conversion was complete, the
solutions were photolyzed to regenerate compounds 47 and 48. From the initial Ir and
borane concentrations, full conversion should have required five thermal/photolytic
cycles; however, 11B NMR indicated full conversion of the borane after the third
thermolysis in the QR, solution. On the other hand, no significant conversion to

organoborane was apparent in the “B NMR. In fact, 31P NMR indicated that probable

90

formation of the diboryl was occurring. As mentioned earlier, the diboryl complex, 29,
was photolytically inactive and it stands to reason that the pinacolate analog also is
unreactive. Indeed this appears to be the case as continued thermolysis/photolysis cycling
yielded no further reaction.

A thermal catalytic process, however, was clearly implicated in the case of the
benzene solution. Although we had determined that solutions of Cp*Ir(PMe3)(BPin)H do
. not react with C6H6, reexamination of the NMR spectra for the “stoichiometric” reaction
between 48 and 6 equivalents of HBPin in C6D6 indicated that the intensity of the
pinacolate resonance for Ph—BPin was greater than anticipated. When a solution of 31
and five molar equivalents of HBPin was heated in C6D6, 11B NMR indicated exchange
between the boron hydride and solvent deuterons after 18 h at 150 °C. Most importantly,
conversion to C6D5BPin was quantitative after 60 h at 150 °C (Eq 36), as judged by 1H
(Figure 57) and “B NMR.

— “6 so it 150 °c — ”5
HBPin / ’ = ' / HD
+ \ / 20% 31 PmB—Q +

 

(36)

91

 

 

. 4L A ll _._ JJLAL—

2.0 1.5 1.0

   

Figure 57. 1H NMR of the aliphatic region from the catalytic borylation of C6D5 with 31
after 60 hours at 150 °C. The tall resonance at 5 1.1 is due to C5D5-BP1D and the peak at

5 0.98 (denoted with an asterisk) is due to residual HBPin.

The reaction was repeated on a larger scale (31, 0.15 mmol; HBPin, 0.75 mmol) in
can, and C5HSBPin was isolated in 53% yield although “B NMR again indicated
quantitative conversion. The catalytic process is very clean as judged by 1H, “B and 31P
NMR spectra taken of the crude reaction material at the end of the large scale reaction
following removal of the solvent ('H and 11B NMR are shown in Figures 58 and 59,

respectively).

92

 

8L -ll - Lo-

".11YWffffgffff“!mflffY3211WfffflT?“W"Ffif}21mfngfl2‘11mr

8.0 7.0 2.0 1.0

 

Figure 58. 1H NMR of selected regions of the crude material for the catalytic formation
of Ph-BPin.

 

H
l

j.
‘1
*‘fi‘ *5 :‘J \‘W

wnnnmmnwnwmwwmw-

100 50 0 ~50 ~100

 

Figure 59. “B NMR of the crude material for the catalytic formation of Ph—BPin.

In order to assess the selectivity of the reaction towards aromatic versus alkyl C—H
bonds an NMR tube reaction utilizing toluene as the reaction solvent was performed.
When the reaction was complete (as judged by 11B NMR), the contents of the tube were

placed in a Schlenk tube and the solvent was removed. 1H NMR analysis of the residue

93

did indicate that more than one isomer was formed. Evaluation of the aromatic region,
however, indicated that boration had occurred at only the para and meta positions of the
arene. The para and meta isomers were identified by comparison to literature values or to
an authentic sample prepared according to the method of Ishiyama et al.125 Other
functionalized arenes were used as reaction solvents in order to probe the generality of
this reaction. In each case boration was observed to occur at only the para and meta
positions. Steric interactions have been used to explain the lack of substitution at the
ortho position in similar systems. Consistent with this line of reasoning is the fact that
when boration of p-xylene was attempted the reaction was much slower and mesitylene
did not react. For comparison, after two and a half weeks at 150 °C 11B NMR indicated
that the reaction in p-xylene had not even reached two turnovers. Again, 1H NMR
analysis of the residue was consistent with attack at the arene C—H bond as only one

isomer could be detected in the aromatic region (Eq 37).

150°C
H-BPin + : PinB
2.5 weeks

20% 31 (37)

 

In the case of monofunctionalized arenes the ratio of para and meta substituted
products was essentially statistical; i.e. approximately two-thirds of the product was meta
substituted and one-third was para substituted (Figure 60). Another interesting feature of
these reactions was the relative rate of boration. The qualitative order, based on the
disappearance of the doublet due to HBPin in the 11B NMR, was R = CF; >> H > CH3 >
NMez.

94

 

selectivit

R % para % meta para: me¥a
CH3 34 66 1 : 0.97
CF3 30 7O 1 : 1.17
NMe2 35 65 1 :0.93

 

Figure 60. Product distributions for the catalytic conversion of monosubstituted arenes.

This observation is consistent with the progression for nucleophilic aromatic
substitution where electron withdrawing groups promote increased rates of reaction. The
usual substitution patterns for ortho and para directors (R = CF3) and meta directors
(CH3, NMez) in nucleophilic aromatic substitutions, however, are not observed. Metal
mediated nucleophilic aromatic substitutions have previously been reported for 116-arene
compounds.155v157 Coordination of the arene to a transition metal center produces a
highly electrophilic aromatic substrate that is more readily attacked by nucleophiles. In
general, this system has the same directing effects that are observed in the strictly organic
reactions; however, the similarities are seen primarily with the strongest donors, such as
amides and alkoxides (meta) and trifluoromethyl (para).

Similar qualitative observations regarding the rates and product distributions in the
thermolysis of Cp*Ir(PR3)Me2 in arenes have been reported by Diversi et al.158 Exclusive
meta and para activation of toluene has also been observed by the cationic Ir(III) species
Cp*Ir(PMe3)(Me)OTf at 25 °C and by the hydrido rhodium complex,
Cp*Rh(PMe3)(Pb)H, at 60 °C.133t159 On the other hand, an 113-benzylic complex is
formed when p-xylene is reacted with Cp*Ir(PMe3)(Me)OTf at 40 °C, benzylic and ortho
activation were observed to occur during photolysis of a p-xylene solution of 31 at room
temperature.55vl33Low temperature photolysis of a toluene solution of Cp*Rh(PMe3)H2

produces a small amount (7 %) of the ortho activated product and a trace amount (< 1 %)

95

of the benzylic product along with the major para and meta products (36 % and 56 %,
respectively).69

Competitive borylation experiments between benzene and substituted arenes were
performed in order to determine selectivities for various arenes. The relative selectivities
were calculated by the following method: equimolar amounts of C6H5 and the arenes
were used for all catalytic runs and the product ratios were determined by integration of
appropriate resonances. The product ratios were then divided by the number of aromatic
hydrogens available for substitution. For instance, the product ratio from the
C5H5CF3/C6H6 experiment was 86:14 (the value for C6H5CF3 includes both the meta and
para substituted products). This product ratio is then divided by the available hydrogens
(three for C6H5CF3 and six for C6116) to give a ratio of 28.7 :3. Normalizing the result
yields the final relative selectivity of 12.3/ 1. A summary of the results is given in Figure
61. The values for C6H5CF3 and p-xylene are consistent with the earlier observations
regarding the general rates of borylation; electron withdrawing groups promote
substitution and steric interactions hinder the rate of substitution. At first glance, the
values for C6H5CH3 and C6H5NMe2 appears contrary to the rate observations; however,
the molar concentration of the solvent mixture probably was not equal considering the
differences in boiling point (C6H6, bp 80 °C; C6H5CH3, bp 111 °C; C6H5CF3, bp 102 °C;
CJISNMez, bp 193 °C) and the temperature at which the catalytic reactions take place. A
significant vapor pressure due to C6116 may have been present in the NMR tube, thus
skewing all the values for the substituted arenes. A competition experiment was also
performed to examine the selectivity of the catalysis for C—H bonds versus C—D bonds. A
one to one molar mixture of C6115 and C6D6 was used under otherwise typical reaction
conditions and a ratio of kH/kp = 2.15 for the catalytic reaction was calculated based on
integration of the aromatic resonances with respect to the pinacolate methyl resonance.
This observation is consistent with the slow step of the reaction involving the activation

of an arene C—l-I bond.

96

solvent mixture relative
selectivity
CeHe 3 arene CgHg : arene

Q05, 1 : 12.3
QCHa 1 : 2.41
QM...

/X 2.15:1

H30©>CH3 19: 1

 

 

Figure 61. Solvent mixtures and relative ratios for competitive borylation of arenes. All

solvent mixtures are one-to-one Cgngarene.

At this time, the mechanism of this process is unknown; however, some speculations
can be made. Clearly, the thermal stability of 31 in benzene solutions is significant.
Arene borylation cannot occur by either simple C—H oxidative addition/C—B reductive
elimination (Eq 38) or Ir—B/C—H metathesis pathways (Eq 39). Also, the concentration of
28 does not significantly increase throughout the course of the reaction, as would be
expected if these pathways were in operation. The observation that catalysis by 31 occurs
faster than the conversion of 48 to 31 and the fact that 48 is not observed in solution
implies that [Cp*Ir(PMe3)], a postulated intermediate in related. reactions, is not being

generated.

97

L lr.n\\H __—_> L - lr’H
H
i“ rar
e|\\H '
- ——-> -
LnIr‘Bpin H-Ar L(n.1)|f\ .B'Pin

Ar

H

——.> Lnlr

————> Lnlr'\
H

“\‘H

.||\‘H
‘H

+ Ar—BPin

(38)

+ Ar-BPin

(39)

The formation of a cationic boryl species via hydride abstraction by HBPin (Figure
62) is unlikely. The opposite trend in the rate of reaction of arenes is observed with a
related cationic complex. CD2C12 solutions of Cp*Ir(PMe3)(Me)OTf react sluggishly with

C6H5CF3 in comparison to benzene and toluene.160

E : I HBPin

 

 

 

 

 

 

Ir. Ir
MeaP’ \"’H Me P \ - - -
BPin .. 3 BPin _ [HZBPln]
a1 55
H-Ar
H P'
T1428 '" Ar—BPin
_ +-
‘5?” -HBPin ”Q
<
Me P’""'H ’"\
a \H _MeaP H _ [HZBPin']
2a 56

Figure 62. Proposed catalytic cycle involving a cationic iridium boryl species.

98

Looking beyond these possibilities, the mechanism must include the following
empirical facts; 1) HBPin must be present in order for C—H functionalization to occur, 2)
steric interactions appear to affect the substitution patterns, and 3) the kH/kD value
indicates that C—H bond breaking process is likely a part of the rate limiting step of the
reaction.

To date, few examples of hydrocarbon functionalization have been reported;
therefore, the results presented in this chapter represent a significant step toward realizing
this ultimate goal. Although the catalytic boration proceeds at high temperature and high
catalyst loading, aromatic hydro gens are selectively substituted in the presence of alkyl
groups. Further work is needed ill this system to elucidate the mechanism by which
borylation occurs. Also, new systems that have recently been reported to activate C—H
bonds, such as (PCP)MH2 (M = Ir, Rh) and tris(pyrazolylborate)platinumalkyl complexes

may prove fruitful in these endeavors as well.161‘153

99

 

CHAPTER 5
EXPERIMENTAL

General Considerations

All manipulations were performed using glove box, Schlenk or vacuum-line
techniques. Benzene, diethyl ether, heptane, pentane, tetrahydrofuran and toluene were
pre-dried over sodium and distilled from sodium/benzophenone ketyl. Methylenechloride
was pre-dried over CaClz and distilled from CaHz. DME (dimethoxyethylene) was dried
with CaHz and distilled from LiAlH... Cyclohexane was purified and dried according to
the method reported by Perrin and Armarego. CDC13 was dried with and stored over 3A
sieves. CDzClz was dried with 4A sieves. C5D5 and toluene-d8 were dried and distilled
from 3A sieves and stored over a sodium mirror. CDCle was prepared according to
literature method. NEt; was refluxed over cm; and freshly distilled prior to use. Argon
and nitrogen were purified by passage through a column of MnO on silica or vermiculite.
(PPh3)2PI(le‘C2H4).87 (PPh3>2m(CH2)4.113 Pt(nbe)3.164 Freeman“ Cp*Ir(PMe3)H2,65
Cp*Ir(PMe3)(Ph)H,66 Cp"‘Ir(PMe3)(cy-CgH11)H,56 HC5Me5,165 Me3SnB(NMe2)2,166 1,2-
bis[p-(trifluoromethyl)phenyl]acetylene,1‘57 l-chloro—5-hexene,168 acetic acid 5-hexenyl
ester,168 [HB(C6F5)2]2,153 and PMe3169 were prepared according to literature methods.
B;(OR2)2 derivatives were prepared according to the literaature method except that trace
HCl impurities were removed by washing the solid with triethylamine prior to pentane
extraction and crystallization of the product.170 HBCat and HBPin were prepared by
reacting BH3~THF (Aldrich) with catechol (Aldrich) and pinacol (Aldrich) respectively.
The THF was then distilled from the solution at atmospheric pressure and the remaining
solution vacuum distilled to give the boranes as clear viscous liquids. DBCat and DBPin
were prepared by bubbling BzDs, generated from NaBDa and BF3'EI20, through a THF

solution of the appropriate diol. Ar—BPin125 and CyBPin171 were prepared according to

100

 

literature methods. Cp*Ir(PMe3)D2 was prepared by stirring an ether solution of
Cp*Ir(PMe3)H2 with degassed D20, followed by drying with MgS04 and crystallization
from pentane. l-Hexyne, 4-octyne, phenylacetylene, l-phenyl-l-propyne, methyl
propargyl ether, dirnethyl acetylene dicarboxylate, l-hexene, 1,5-hexadiene, norbomene,
norbomadiene, vinyl anisole, and p-(trifluoromethyl)styrene were all purchased from
commercial sources and distilled prior to use. Ethylene was purchased from Matheson
and used as received. . _

1H NMR spectra were recorded on Varian Gemini—300, VXR—300, inova—300 or
VXR-SOO spectrometers and referenced to residual proton solvent signals. 2H, 11B, 13C,
19F and 31P spectra were recorded on Varian VXR—300 or inova—300 spectrometers
operating at 46.04, 96.19, 75.43, 282.22, 121.44, and 111.86 MHz, respectively.
Deuterium chemical shifts are referenced to residual deuterium solvent signals of an
external standard of the same solvent that the sample is dissolved in. Boron chemical
shifts are referenced to a neat BFg-EtzO external standard. Carbon chemical shifts are
referenced to solvent signals. Fluorine chemical shifts are referenced to a neat CFCl3
external standard. Phosphorous chemical shifts are referenced to an 85% phosphoric acid
external standard. Tin chemical shifts are referenced to a neat SnMea external standard.
Coupling constants involving tin are taken as the average of the 117Sn and 119Sn satellites
except for those values given in the 119Sn NMR data. Photolytic experiments were
performed in a 450 W Rayonet photolysis unit equipped with 300 nm lamps. The area in
which NMR tubes were exposed to photolysis was cooled by an internal fan, keeping the
temperature at approximately 30 °C. Elemental analyses were performed on a Perkin
Elmer Series II CHNS/O Analyzer 2400 or by Desert Analytics (Tuscon, AZ). Mass
spectroscopic data were obtained on a portable Trio-l VG Masslab Ltd. Mass
spectrometer. GC analysis of catalytic mixtures were performed on a Hewlett Packard
58980A Gas Chromatograph. Mass spectral data were obtained at the Michigan State
University Mass Spectrometry Facility which is supported, in part, by a grant (DR-

101

 

00480) from the Biotechnology Research Technology Program, National Center for

Research Resources, National Institutes of Health.

Syntheses

cis-(PPh3)2Pt(BCat)z (2). (PPh3)2Pt(n2—CzHa)56 (100 mg, 0.134 mmol) and BzCatz
(35 mg, 0.15 mmol) were placed in a flask and dissolved in 5 mL of toluene. A white _
precipitate formed immediately. The solution was stirred for 2 h at room temperature, and
the supernatant was removed -via cannula. Additional product was obtained by removing
the solvent under reduced pressure and washing the residue with toluene (2 mL). The
overall yield was 96 mg (75%). 1H NMR (C6D6): 6.54 (m, 4 H, BOzC5H4), 6.77 (m, 4 H,
1302mm), 6.80 (m, 18 H, P(C6H5)3), 7.55 (m, 12 H, P(C6H5)3). “B NMR (CDC13 was
used for the sake of making a solution concentrated enough, quartz NMR tube): rt, 8 48.5
(v 1,2 = 1845 Hz). 31P NMR (CDC13 was used in order to take low temperature spectra): rt,
5 28.9 (11...... = 1629 Hz, vlrz = 60 Hz); —50 °C, 5 28.9 (11,...P = 1646 Hz, v,,2 = 31 Hz).
Anal. Calcd for CagsHssBanPZClPt: C, 57.29; H, 5.45. Found: C, 55.28; H, 5.27. Mp:
193—195 °C (dec).

cis-(PPh3)th(BPin)2 (3). (PPh3)2Pt(n2—C2Ha) (100 mg, 0.134 mmol) and BzPinz (35
mg, 0.15 mmol) were placed in a flask and dissolved in 5 mL of toluene. The golden
yellow solution was stirred for 2 h at room temperature, and subsequent solvent
evaporation yielded a light orange-yellow powder. The residue was recystallized from
CH2C12/ethanol, washed with hexanes, and dried in vacuo to give white crystals (91 mg,
70%). 1H NMR (CeDe): 1.02 (s, 24 H, 802C611”), 6.88 (m, 18 H, P(C6H5)3). 7.58 (m, 12
H, P(C,H5)3). “B NMR ((2.06, quartz NMR tube): 46.3 (vm = 1830 Hz). ”P NMR
(caDa): 29.77 (11...... = 1506 Hz). Mp: 183—187 °C (dec). Anal. Calcd for
CagsHssBzOlthClPt: C, 57.29; H, 5.45. Found: C, 55.28; H, 5.27 . Analyses for crystalline

samples of X-(CH2C12)m were consistently low in carbon. Although this compound has

102

been structurally characterized, the combustion analysis was not reported. Our melting
points for crystalline materials were consistently 40 °C higher than the reported value.

cis-(PPh3)th(SnMea)[B(NMe;)z] (4). (PPh3)2Pt(n2-C2Ha) (300 mg, 0.402 mmol)
was placed in a flask and dissolved in 5 mL toluene. Me3Sn—B(NMe2)2 (105 mg, 0.402
mmol) was then added via syringe. The solution was heated at 45°C for 3 hrs after which
the solvent was removed in vacuo. The residue was dissolved in methylene chloride,
layered with ethanol and placed in a -5°C freezer. Yellow crystals (260 mg, 66%) were
collected after 12 hrs. An additional crop of crystals (28 mg, 7%) was obtained after
reducing the mother liquor and replacing it in the —5°C freezer. 1H NMR (CDCl3); 5 -
0.46 (s, 9 H, Sn(CH3)3, 213.5,, = 38 Hz, 3J3.“ = 8 Hz); 5 2.53 (s, 12 H, B(N(CH3)2)2); 5
7.01—7.31 (m, 30 H, P(C6H5)3). “B NMR (CDC13, quartz NMR tube); 5 45.9 (broad). 3‘P
NMR (CDC13,—45°C); 5 30.6 (d, '1”, = 2736 Hz, 21“. = 14.6, Hum...” = 1487 Hz), 5
31.3 (broad, ‘1”, = 1404 Hz). “98n (CDClg); 5 —24.62 (dd, 11M, = 12,594 Hz, 215m...)
= 156 Hz, 215mm,, = 1519 H2,). mp; 201—202°C (dec.). anal Calc. C (52.57%), H
(5.23%), N (2.85%). Found C (52.29%), H (5.15%), N (2.77%).

cis-(PPh3)2Pt(SnMe3)2 (6). This preparation is analogous to that reported in the
literature. (PPh3)2Pt(n2—C2Ha) (100 mg, 0.134 mmol) was placed in a flask and dissolved
in 7 mL of toluene. A bright yellow solution formed on addition of MeGSnz (28 11L, 0.134
mmol), and within 15 min a bright yellow precipitate formed. After the solution was
stirred for 2 h, the solvent was removed under reduced pressure to afford a bright yellow
powder. 1H NMR (C60,): 0.40 (s, 18 H, Sn(CH3)3, ‘19,.“ = 8 Hz, 315,3, = 20 Hz), 6.86
(m, 18 H, P(C5H5)3), 7.46 (m, 12 H, P(C6H5)3). 31P NMR (C60,): 35.1 (11,32P = 2621 Hz,
213.... = 614, 642 Hz).

Cp‘lr(PMe3)(BCat)z (29). Cp*Ir(PMe3)H2 (150 mg, 0.37 mmol) and CatB—BCat
(167 mg, 0.70 mmol) were loaded into a Schlenk tube and dissolved in 3 mL toluene. The
solution was stirred at reflux for 24 hr. The solution was cooled to room temperature and

layered with 10 mL pentane. White crystals (145 mg, 60%) deposited after several days

103

at -80 °C. 'H NMR (C613,); 5 1.48 (d, 9 H, P(CH3)3, 2111.1» = 11 Hz), 1.97 (d, 15 H,
C5(CH3)5, “1H.P = 2 Hz), 6.75 (m, 2 H, 802mm), 7.05 (m, 2 H, 1302mm). “B NMR
(c613,); 5 36. 311>{‘H} NMR (C613,); 5 —39.5. Calc. C(46.82), H(5.03). Found C(48.53),
H(5.24). M.p. 170—172 °C.

Cp*lr(PMe3)(BPin)H (31). A solution of Cp*Ir(PMe3)H2 (305 mg, 0.75 mmol) and
HBPin (contaminated with 5% thf, 288 mg, 2.3 mmol), dissolved in 4 mL toluene, was
heated at 125 °C for 4 days. The solution was then transferred to a Schlenk flask via
cannula and the volatile materials removed under vacuum (three hours).
Cp*Ir(PMe3)(BPin)H (316 mg, 79%) was collected as an orange oil. 1H NMR (C6D6); 5 —
17.64 (d, 1 H, Ir—H, 21,” = 29 Hz), 1.17 (12 H, BOthsth), 1.50 (d, 9 H, P(CH3)3, 21,”.
= 10 Hz), 2.10 (d, 15 H, C5(CH3)5, ‘th4: = 2 Hz). “B NMR (C60,); 5 33. 31F{‘H) NMR
(Cst); 5 -42.8. Calc. C(42.79), H(7.02). Found C(42.79), H(7.31).

Cp‘Ir(PMe3)H[HBH(C6F5)2] (45). Cp*lr(PMe3)H2 (200 mg, 0.493 mmol),
dissolved in 2 mL toluene, was transferred via cannula to a suspension of [HB(C5F5)2]2
(170 mg, 0.246 mmol) in 3 mL toluene. The resulting solution was stirred at room
temperature for 30 min. The volume of the solution was reduced to 3 mL and layered
with 10 mL pentane. Yellow crystals (312 mg, 84%) deposit upon cooling at -78 °C for
24 hr. 1H NMR (toluene—r18, —70 °C); 5 —15.91 (d, 2 H, lr—H,.,,,. and Ir—H—B, 211,4. = 27
Hz), 0.88 (d, 9 H, P(CH3)3, 21m = 11 Hz), 1.35 (15 H, c5(CH:)). 4.35 (br, 1 H,
HB(C.F,)2). ”B NMR (Cst); 5 -43. 19F NMR (C6D5); 5 —131.2 (2 F), -160.1 (1 F), —
165.3 (2 F). 3‘P{’H} NMR (C60,); 5 —40.3. IR(Nujol mull, cm”) 2093, 1842, 1640. Calc.
C(39.96), H(3.62). Found C(39.65), H(3.45). M.p. 139—144 °C.

Cp*1r(PMe3)(B(C6F5)2)H (46)- Cp*1f(PMes)H(HBH(C6Fs)2) (510 mg. 0-68 mmol)
was dissolved in 7 mL toluene and stirred in a closed vessel at 110 °C for 4 hr. The
solvent was removed in vacuo and the oily residue is extracted with 3 x 7 mL pentane.
The pentane extracts were reduced in volume to 4 mL and cooled to -78 °C. Yellow
crystals (187 mg, 37 %) deposit after 12 hr. 'H NMR (c613,); 5 -16.64 (d, l H, Ir—H, 21...

104

p = 30 Hz), 5 0.96 (d, 9 H, P(CH3)3, 21,”. = 11 Hz), 1.55 (d, 15 H, C5(CH3)5, 41...? = 2
Hz). ”B NMR (C6D6); 5 80. ”F NMR (C613,); 5 —134.1 (2 F), -157.8 (1 F), —l64.2 (2 F).
31l>(‘H) NMR (c613,); 5 —44.7. Calc. C(40.07), H(3.36). Found C(40.26), H(3.37). M.p.
179—182 °C.

Formation of Cp*Ir(PMe3)(BCat)H (30). A solution of Cp*Ir(PMe3)H2 (10.0 mg,
0.0245 mmol), dissolved in 400 11L C6D6, was placed in a sealable NMR tube. HBCat
(2.6 11L, 0.025 mmol) was added to the tube via a microliter syringe and enough C5D5
was added to bring the volume of the solution to 600 11L. The solution was heated at 90
°C for 24 hr. Only one Ir containing product, consistent with formulation as
Cp*lr(PMe3)(BCat)H, was observed. 1H NMR (C6D6); 5 -17.12 (d, 2 H, Ir—H, 21“.}: = 29
Hz), 1.39 (d, 9 H, PMe3, 2111.1: = 10 Hz), 2.08 (d, 15 H, C5Me5, 413.1: = 2 Hz), 6.84 (m, 2
H, Bozcnn), 7.18 (m, 2 H, 802cm.) ”B NMR (c613,); 5 36. “Pl 1H) NMR (C60,); 5
-42.9.

Formation of [(Cp*lr(PMe3)H)2)(ll-H)*]. The non-pentane soluble portion of the
residue from the thermolysis of Cp*Ir(PMe3)H[HBH(C6F5)2] contains primarily a product
that was spectroscopically identical to the known cation [(Cp*Ir(PMe3)H)2(1l-H)*] with
the accompanying anion HB(C6F5)3’. Attempts at crystallizing this compound have thus
far been unsuccessful. 1H NMR (CDzClz, —80 °C); 5 —23.01 (tt, 1 H, lr—Hbfidg, 21,...l = 5
Hz, 21H... = 21 Hz), —19.18 (dd, 2 H, Ir-Hmm), 21...}, = 5 Hz, 211,... = 34 Hz), 1.48 (d, 18
H, P(CH3)3, 21,“. = 10 Hz), 1.93 (30 H, C5(CH3)5), 3.4 (br, 1 H, HB(Cst)s'). 1H NMR
(CD2C12, 20 °C); 5 -20.47 (br, 3 H, (l-I)Ir—H—lr(H)), 1.60 (d, 9 H, PMC3, 2111.!) = 10 Hz),
2.02 (d, 15 H, C5Me5, ‘er = 2 Hz), 3.6 (q, 1 H, H—B, ‘13.}, = 90 Hz). “8 NMR (CDzClz,
20 °C); 5 —23.9 (Um, = 90 Hz). l9F NMR (coach, 20 °C); 5 -—134.1 (2 F), —165.0 (1 F),
-l67.9 (2 F). 3‘P{‘H} NMR (CD2C12, 20 °C); 5 —47.0.

Formation of [Cp*Ir(PMe3)Ha”]. Compound 45, when dissolved in CDle or
CDzClz resulted in the slow formation of 1 and [Cp*Ir(PMe3)H3][HzB(C5Fs)2]. 1H NMR
(CD2C12, —80 °C); 5 —l4.13 (dd, 2 H, Ir-Hm 213.3 = 48 Hz, 211-1.}, = 13 Hz), 5 ~12.7l

105

(td, 1 H, Ir-H—Ir, 21,...I = 48 Hz, 211,... = 22 Hz), 1.71 (d, 9 H, P(CH3)3, 21.1.1. = 10 Hz),
2.17 (15 H, C5(CH3)5), 3.6 (br, 2 H, H2B(C6F5)2'). 1H NMR (c0202, 20 °C); 5 —20.47
(br, 3 H, (H)Ir—H—Ir(H)), 1.60 (d, 9 H, PMe3, 21.1... = 10 Hz), 2.02 (d, 15 H, C5Me5, ‘me
= 2 Hz), 3.6 (q, 1 H, H—B, ‘1” = 90 Hz). “B NMR (c0202, 20 °C); 5 —31.8 (11H.B = 85
Hz). l9F NMR (c0202, 20 °C); 5 -134.1 (2 F), -165.0 (1 F), —l67.9 (2 F). 31l>(‘H} NMR
(C0202, —-80 °C); 5 —37.9.

NMR Tube Reactions

cis-(PPh3)2Pt(n2-4-octyne) with BzCatz. cis-(Pph,)21>t(n2—4—octyne) (7.2 mg, 0.0087
mmol) and B2Cat2 (4.1 mg, 0.017 mmol) were mixed in C6D5 and transferred to a
scalable NMR tube. After sealing, the tube was thawed, and traces of a white precipitate
indicated cis-(PPh3)2Pt(BCat)2 deposition. 1H NMR indicated that cis-(PPh3)2Pt(BCat)2,
4-octyne, and BzCatz were the sole soluble species. The tube was heated for 4 h at 80 °C
to yield cis-(PPh3)2Pt(BCat)2 (by 1H and 31P NMR) as the free alkyne was converted to
the cis-4,5-diboryl-4-octene. 1H NMR (C613,): 0.90 (tr, 6 H, —CH2CH2CH3), 1.53 (sextet,
4H, —CH2CH2CH3), 2.49 (tr, 4 H, —CH2CH2CH3), 6.73 (m, 4 H, B02C6H4), 6.91 (m, 4 H,
302C6H4). “B NMR (C6D5): 32.7.

Reaction of (PPh3);Pt(BCat); with MegSng. cis-(PPh3)2Pt(BCat)2 (100 mg, 0.104
mmol) and Me58n2 (21.6 1.1L, 0.208 mmol) were placed in a Schlenk tube and dissolved
in 5 mL of toluene in a glovebox. The solution was stirred at 55 °C for 3 h. The solvent
was removed in vacuo and the residue washed with 10 and 5 mL of pentane to yield 51
mg (49% yield) of a light tan solid. 1H, ”F, and “B NMR spectroscopy indicated
formation of cis-(PPh3)2Pt(BCat)(SnMe3). 1H NMR (coca): —0.45 (m, 9 H, Sn(CH3)3
(51pm = 0.9 Hz, “Jr...H = 12 Hz, 31ng = 42 Hz), 6.72 (m, 2 H, B02C6H4), 6.81 (m, 2 H,
BOzCsHa), 6.92-7.37 (m, 30 H, P(C6H5)3). 31P NMR (CDC13, —50 °C): 26.63 ((1, trans

106

SnMeg, 21p.P = 23 Hz, 11....P = 2371 Hz, 215.2,. = 1268 Hz), 32.84 (br (1, trans BCat, 21p.P =
20 Hz, ‘Jm = 1761 Hz, 215...]. = 815 Hz). 5

Reaction of cis-(PPh3)2Pt(BPin)2 with Messnz. (PPh3)2Pt(BPin)2 (10 mg, 0.010
mmol) was dissolved in 600 11L of C6D6. Addition of Me6Sn2 (2.0 11L, 0.010 mmol) via
syringe immediately generated a bright yellow solution. 1H, 31P, and 11B NMR
spectroscopy all indicated complete conversion to trans-(PPh3)2Pt(SnMe3)2 and B2Pin2.

Reaction of cis-(PPh3)2Pt(BPin)2'with BzCatz. (PPh3)2Pt(BPin)2 (12.0 mg, 0.013
mmol) and BzCatz (3.0 mg, 0.013 mmol) were placed in a sealable NMR tube, and 600
11L of C6D6 was added via vacuum transfer. Upon warming of the sample to room
temperature, traces of a white precipitate indicated (PPh3)2Pt(BCat)2 deposition. 1H, 31P,
and 11B NMR spectroscopy all indicated conversion to (PPh3)2Pt(BCat)2 and BzPinz, as
cis-(PPh3)2Pt(BPin)2 was not detected.

Equilibrium of cis-(PPh3)2Pt(BCat)z with CO. (PPh3)2Pt(BCat)2 (10.0 mg, 0.010
mmol) was dissolved in 600 11L of CDC]; in a J. Young NMR tube. After the solution
was frozen, the argon atmosphere was evacuated and replaced with CO. Upon warming
of the sample to room temperature conversion to (PPh3)2Pt(CO)2 had occurred as
evidenced by 1H and 31P NMR spectra, and reductive elimination of B2Cat2 was
confirmed by 11B NMR. Chemical reversibility was confirmed by the regeneration of
(PPh3)2Pt(BCat)2 upon evacuation of the CO atmosphere.

Equilibrium of cis-(PPh3)2Pt(BPin)2 with C0. (PPh3)2Pt(BPin)2 (10.0 mg, 0.010
mmol) was dissolved in 600 1.1L of C7D3 in a J. Young NMR tube. After the solution was
frozen, the argon atmosphere was evacuated and replaced with CO. Upon warming of the
sample to room temperature, 1H and 31P NMR indicated complete conversion to
(PPh3)2Pt(CO)2. llB also confirmed reductive elimination of BzPinz. After evacuation of
the CO atmosphere, a trace of (PPh3)2Pt(BPin)2 was regenerated.

Equilibrium of cis-(PPh3)2Pt(BPin)z with 2-butyne. (PPh3)2Pt(BPin)2 (7.5 mg,
0.0077 mmol) was dissolved in 600 1.1L of C6D6 in a J. Young NMR tube. The

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atmosphere above the frozen solution was evacuated and replaced with approximately 10
molar equiv of 2-butync. Upon warming of the sample to room temperature, 1H and 31P
NMR indicated nearly complete conversion to cis-(PPh3)2Pt(n2-CH3CCCH3), and
reductive elimination of BzPinz was confirmed by 11B NMR. Reversibility was
demonstrated by regeneration of cis-(PPh3)2Pt(BPin)2 upon evacuation of the 2-butyne
atmosphere. Evacuation to dryness and dissolution of the residue ill C6D6 showed cis-
(PPh3)2Pt(BPin)2 to be the major product (“P NMR); however, 1H NMR indicated that
some diboration of 2-butyne had occurred.

Equilibrium of cis-(PPh3)2Pt(BPin)¢ with 4-octyne. (PPh3)2Pt(BPin)2 (5.1 mg,
0.0052 mmol) was placed in a scalable NMR tube, 4.6 uL of 4-octyne was added via
syringe, and C7D3 was added to 600 mL. After sealing, 31P NMR spectra were recorded at
193, 213, 233, 253, and 273 K The concentrations of cis-(PPh3)2Pt(BPin)2, (PPh3)2Pt(n2-
4-octyne), BzPinz, and 4-octync were determined by the relative intensities of the Pt-
containing species.

Reaction of cis-(PPhshPflBCat); with PPhn. cis-(PPh3)2Pt(BCat)2 (10 mg, 0.010
mmol) and PPh3 (30 mg, 0.11 mmol) were placed in an NMR tube and dissolved in 600
11L of CDzClz to form a clear, colorless solution. 1H, 31P, and 11B NMR (—80 and 20 °C)
showed 2 to be the exclusive Pt-containing species.

Reaction of cis-(PPh3)th(BPin)¢ with PPhs. cis-(PPh3)2Pt(BPin)2 (10 mg, 0.010
mmol) and PPh3 (30 mg, 0.11 mmol) were dissolved in 600 uL of CD2C12 in an NMR
tube. NMR spectra (‘H and 31P at -80 °C) of the yellow solution indicated reductive
elimination of B2Pin2 and formation of (PPh3)3Pt and (PPh3)4Pt.

Crossover Experiment. cis-(PPh3)2Pt(BCat)2 (4 mg, 0.0042 mmol), BzCatz (1 mg,
0.0042 mmol), and 4-octync (4.6 L, 0.032 mmol) were placed in a scalable NMR tube
and 595 1.1L of C6D6 was added while in a glovebox. The tube was flame-scaled, and the
mixture was heated at 80 °C in a constant-temperature oil bath until 1 was consumed.

The sealed tube was opened in the glovcbox. The solution was transferred to a new

108

scalable tube, and 3-hexyne (2 11L, 0.02 mmol) was added. After flame-scaling, the
solution was heated at 80 °C for 5 h. 1H NMR spectra showed no evidence for diboration
of hcxyne.

Cp*Ir(PMe3)D2 + [HB(C6Fs)2]2. Cp*lr(PMe3)D2 (10.0 mg, 0.0245 mmol) and
[HB(C6F5)2]2 (8.5 mg, 0.012 mmol) were dissolved in 600 11L toluene and transferred to a
suba-capped NMR tube at room temperature. 2H NMR at —70 °C indicated complete
deuterium scrambling between the hydride and borane sites. _

Cp*1r(PMe3)Dz + HBCat. Cp*lr(PMe3)D2 (10.0 mg, 0.025 mmol) and HBCat (2.6
11L, 0.025 mmol) were dissolved in 600 ttL toluene in a scalable NMR tube. The tube
was flame-sealed under an atmosphere of argon. 2H NMR indicated complete deuterium
scrambling between the hydride and borane sites after XX hr at 75 °C.

Cp*1r(PMe3)D¢ + HBPin. Cp*Ir(PMe3)D2 (10.0 mg, 0.025 mmol) and HBPin (3.6
11L, 0.025 mmol) were dissolved in 600 11L toluene in a scalable NMR tube. The tube
was flame-sealed under an atmosphere of argon. 2H NMR indicated complete deuterium
scrambling between the hydride and borane sites after XX hr at 75 °C.

Cp*1r(PMes)(cy-C6Hit)H + 6, 12, or 24 equiv. HBPin. Cp*Ir(PMe3)(cy-C6H“)H
(10 mg, 0.021mmol) was dissolved in 300 11L C6D6 and transferred to a scalable NMR
tube. HBPin was added to the scalable NMR tube via microsyringe and C6D6 was added
to make the total volume of the solution 600 1.1L. The tube was frozen in liquid nitrogen
and flame-sealed under an atmosphere of argon. After thawing, the reaction was heated at
95 °C or 135 °C and examined by 1H, "B, and 31P NMR. Compounds 31 and 48415 were
the primary Ir containing products and cy-Can and cy-Csl-lu-BPin were the organic
products. Product distribution was dependent on the reaction conditions (see Chapter 4
for details).

Cp*1r(PMe3)(Cy)H with [HB(C6F5)2]z. Cp*Ir(PMe3)(Cy)H (10.6 mg, 0.021 mmol)
and [HB(C6F5)2]2 (7.6 mg, 0.011 mmol) were placed in a suba-capped NMR tube and

dissolved in 600 11L C6D5. The reaction was complete at room temperature in the amount

109

of time needed to walk to the NMR spectrometer and take a spectrum (15 minutes). 1H,
“B, 19F and 31P NMR indicate quantitative formation of Cp*Ir(PMe3)H2 and CyB(C(,F5)2.

Cp*lr(PMe3)(Ph)H with HCat. Cp*Ir(PMe3)(Ph)H (8.9 mg, 0.019 mmol) was
placed in a scalable NMR tube. The sides of the tube were washed down with 200 11L
C5D6 and then HBCat (11.8 11L, 0.11 mmol) was added via microsyringe. Enough C6D5
was added to bring the total volume of the solution to 540 11L. The tube was then frozen
in N20) and quickly sealed. Upon thawing of the solution, the reaction was heated at 150
°C in an oil bath and monitored by 1H, 11B and 31P NMR.

Cp*Ir(PMes)(Ph)H with HBPin. Cp*Ir(PMe3)(Ph)H (9.0 mg, 0.019 mmol) was
placed in a scalable NMR tube. The sides of the tube were washed down with 200 11L
C6D5 and then HBPin (32 11L, 0.22 mmol) was added via microsyringe. Enough C6D;
was added to bring the total volume of the solution to 540 11L. The tube was then frozen
in Nza) and quickly sealed. Upon thawing of the solution, the reaction was heated at 150
°C in an oil bath and monitored by 1H, ”B and “P NMR.

Cp*lr(PMe3)H(HBH(C6F5)2) + PMea. Cp*lr(PMe3)H(HBH(C6F5)2) (10.0 mg, 0.013
mmol) was dissolved in 600 uL toluene-d3 in a J. Young NMR tube. The tube was frozen
and the head space above the sample evacuated. Two molar equivalents of PMe3 were
then added by vacuum transfer. The tube was thawed and immediately placed in a
spectrometer pre-set to -70 °C. 'H and 31P NMR indicated complete conversion to
Cp*Ir(PMe3)H2 and a Lewis acid-base adduct between HB(C6F5)2 and PMe; (see below).

[HB(C¢F5)2]2 + PMe3. [HB(C6Fs)2]2 (5.0 mg, 0.0072 mmol) was dissolved in 600 11L
toluene-d3 in a J. Young NMR tube. The tube was frozen and the headspace above the
sample evacuated. Two molar equivalents of PMe3 were then added by volumetric gas
transfer. 'H NMR (C613,); 5 0.76 (d, 9 H, CH3, 21,” = 1.5 Hz), 3.25 (br quart, 1 H, BH,
‘1“ = 76 Hz). “8 NMR (C613,); 5 —25.7 (tr, average 113...,” = 88 Hz). 19F NMR
(€st); 5 —131.0 (2 F), -157.8 (1 F), —l63.6 (2 F). “P NMR (C613,); 5 -9.1 (br quart, Up.
3 = 90 Hz).

110

HBCat + NEta. HBCat (4.4 11L, 0.042 mmol) and NEt3 (5.8 11L, 0.042 mmol) were
dissolved in 600 ttL C6D6 in a suba-capped NMR tube. 1H and “B NMR indicated a
Lewis acid-base interaction. 1H NMR (C60,); 5 0.69 (tr, 9 H, NCHZCCH3, 411,4, = 7 Hz),
2.27 (quart, 6 H, NCHZCH3, “Jr...I = 7 Hz), 6.82 (m, 2 H, BOzC6H4), 7.04 (m, 2 H,
B02C6H4). “B NMR (C613,) :5 11 (11H.B = 299Hz).

HBPin + NEt3. HBPin (6.1 11L, 0.042 mmol) and NEt; (5.8 11L, 0.042 mmol) were
dissolved in 600 11L C6D6 in a suba-capped NMR tube. 1H and 11B NMR indicated no .
Lewis acid-base interaction. 1H NMR (C6D6); 5 —17.64 (tr, 9 H, NCHZCHg, 21,“, = 29
Hz), 1.17 (quart, 6 H, NCH2CH3), 1.50 (m, 2 H, 302C6H4), 2.10 (m, 2 H, 302C6H4). ”B
NMR (C60,) ; 5 —17.64.

[HB(C6F5)2]2 + NEta. [HB(C(,F5)2]2 (10 mg, 0.014 mmol) and NEt; (4.0 11L, 0.029
mmol) were dissolved in 600 uL C60, in a suba-capped NMR tube. 1H NMR (C613,); 5
0.39 (tr, 9 H, NCHZCHg, 21,1.“ = 7 Hz), 2.54 (quart, 6 H, NCH2CH3, 21,“, = 7 Hz), 1.50
(m, 2 H, 802C611.) 2.10 (m, 2 H, BOZC6H4). “B NMR (C6D6) ; 5 —9 (11H.B = 98 Hz). l9F
NMR (C60,); 5 —128.2 (2 F), 4572 (1 F), —163.5 (2 F).

Kinetic Experiments

All kinetic experiments were run in sealed NMR tubes with a total solution volume of
600 11L. PPh3 was added to the tubes in the form of 0.2 M solutions of PPh3 in C6D6 or
Cst. The reactions were heated in constant temperature oil baths (Cole-Parmer Polystat
Constant Temperature Circulator or Tekmar RCT) or in the Varian 300 MHz
spectrometer when the reaction temperature and completion times were convenient.

A typical experimental run for the stoichiometric diboration of alkynes by cis-
(PPh3)2Pt(BCat)2 is described as follows: cis-(PPh3)th(BCat)2 (5.0 mg, 0.0052 mmol)
and 4-octyne (4.6 uL, 0.032 mmol) were placed in an NMR tube. In a drybox, 595 11L of

C6D5 was added via syringe. The solution was frozen in liquid nitrogen, and the tube was

111

flame-sealed. The tube was heated briefly to dissolve the Pt complex. The tube was then
placed in a constant-temperature oil bath. At specific intervals the tube was removed
from the oil bath and the reaction was quenched by rapid cooling in an ice bath. 1H NMR
spectra were then recorded at room temperature. The progress of the reaction was
monitored to 3 half-lives by measuring the disappearance of the catecholate resonance at
6.54.

A typical experimental run under catalytic conditions is described as follows: cis-
(PPh3)2Pt(BCat)2 (4 mg, 0.0042 mmol), CatB-BCat (6 mg, 0.025 mmol), 4-octyne (3.7
11L, 0.025 mmol), and C6D6 (595 11L) were added to an NMR tube. The tube was then
brought out of the glovebox and flame-sealed. The mixture was heated to 50 °C in the
Varian 300 MHz NMR spectrometer, and the reaction was monitored to 3 half-lives
completion. A program supplied within the spectrometer's software was used to take
scans of the mixture at specified intervals, and the time-dependent alkyne concentration
was calculated by integrating the resonance of the methylene group adjacent to the alkyne
triple bond.

A typical experimental run for the reaction of HBCat or HBPin with Cp*lr(PMe3)H2
is described as follows: In a drybox, Cp*Ir(PMe3)H2 (10.0 mg, 0.025 mmol) was
dissolved in 300 mL C6D; and transferred to a scalable NMR tube. HBCat (15.7 uL, 0.15
mmol) was added to the tube via microsyringe and C6D6 was added to the tube to make
the solution volume 600 11L. The solution was frozen in liquid nitrogen, and the tube was
flame-scaled with an atmosphere of argon gas. The tube was stored in the -15 C freezer
for an hour and then heated in a constant temperature oil bath. At specific intervals the
tube was removed from the oil bath and the reaction quenched by rapid cooling in an ice
bath. 1H NMR spectra were then recorded at room temperature. The progress of the
reaction was monitored through 3 half-lives by measuring the disappearance of the Cp*

and PMe3 resonances of 28.

112

Catalytic Diboration of Alkynes

Diboration 0f 1,2-Bis(p-anisolyl)acetylene. 1,2-bis(p-anisolyl)acetyleneS8 (50 mg,
0.21 mmol), BzCatz (50 mg, 0.21 mmol), and 2.5 mol % 1 were dissolved in 4 mL of
toluene. The solution was heated at reflux for 4 h, and then the solvent was removed in
vacuo. A 20 mL volume of EtzO was added, and the mixture was filtered. Evaporation of
the filtrate yielded 74 mg (74%) of the diborylated product. 1H NMR (CDC13): 3.76 (s, 3
H, OCH3, 6.76 (m, 2 H, 302C6H4), 7.05 (m, 4 H, C61140CH3), 7.11 (m, 2 H, 302C6H4).
“B NMR (CDCl;): 32.3. EI/MS: m/z = 476 (M”), I = 100 (100); m/z = 477 ([M + 11*), 1
= 30 (30); m/z = 475 ([M — 1]*), I = 48 (45). Mp; 146—151 °C.

Diboration of 1,2-Bis[p-(trifluoromethyl)phenyl]acetylene. l,2-Bis[p-
(trifluoromethyl)phenyl]acetylene (57 mg, 0.18 rrunol), BzCatz (43 mg, 0.18 mmol), and
2.5% 1 were dissolved in 4 mL of toluene. The solution was heated at reflux for 2 h, and
then the solvent was removed in vacuo. A 5 mL volume of EtzO was added, and the
mixture was filtered. Evaporation of the filtrate yielded 85 mg (85%) of the diborylated
product. 1H NMR (CDCl;): 5 7.09 (m, 4 H, C,H,CF3), 7.27 (m, 2 H, 802C414), 7.49 (d,
2 H, 302C6H4). 11B NMR (CDC13): 5 31.5. EI/MS: m/z = 552 (M*), I = 100 (100); m/z =
553 ([M + 1]”), I = 31 (30); m/z = 551 ([M — 1]*), I = 51 (45). Mp; 140-145 °C.

Protonolysis of Diborylated Phenylacetylenes.172 A 51 mg amount of the alkene
derived from diboration of l,2-bis(p-anisolyl)acetylene (0.11 mmol) was dissolved in 0.5
mL of diglyme. Several drops of glacial acetic acid were added, and the solution was
heated at 45 °C for 15 h. The solvent was removed in vacuo. Formation of the cis-
stilbene was confirmed by comparing the 1H NMR shift of the olcfmic protons (6.43
ppm) to the literature value (6.44 ppm).

A 50 mg amount of the alkene derived from diboration of 1,2-bis[p-
(trifluoromethyl)phenyl]acetylene (0.091 mmol) was dissolved in 0.5 mL of diglyme.

Several drops of glacial acetic acid were added, and the solution was heated at reflux for

113

12 h. The solvent was removed in vacuo. Formation of the cis-stilbene was confirmed by
comparing the 1H NMR shift of the olefmic protons (6.70 ppm) to the literature value
(6.72 ppm).

Catalytic Stannaboration of Alkynes

A series of scalable NMR tubes experiments were performed in order to examine the
catalytic activity of 1. In each tube was placed 1 (2 mg, 0.002 mmol), Me3Sn-B(NMe2)2
(10 mg, 0.042 mmol), and an alkyne (0.042 mmol). 600 mL of benzene-d6 was added and
the tubes were flame-sealed under vacuum. The reactions were followed by 1H and 11B
NMR. l-Hexyne and 4—octyne reacted smoothly at 75°C to yield single products in
approximately 8 hrs. Diphenylacetylenc, phenylacetylene, and l-phenyl-l-propyne
reacted slowly at 75°C. Upon raising the temperature to 90°C the reactions proceeded to
near completion but were accompanied by catalyst decomposition. Diphenylacetylene
yielded a single product as opposed to phenylacetylene and l-phenyl-l-propyne, which
both formed more than one isomer. None of 1,2-bis[p-(trifluoromethyl)phenyl]acetylene,
methyl propargyl ether, nor dimethyl acetylene dicarboxylate showed clean reactivity at
lower temperatures and catalyst decomposition occurred at higher temperatures.

Stannaboration of l-hexyne. l-hexync (118 mg, 1.45 mmol), Me;Sn-B(NMe2)2
(382 mg, 1.45 mmol) and 5 mole % catalyst l (71 mg, 0.073 mmol) were placed in a
flask and dissolved in 4 mL toluene. The solution was heated at 80°C for 36 hr at which
time the reaction was judged complete by GC analysis. The solvent was removed in
vacuo leaving an oily brown residue. The residue was taken up in hexane and washed
through neutral Alumina. The hexane was removed leaving a golden oil (353 mg, 71%).
In order to remove a small amount of PPh; that was present due to decomposition of
catalyst, the pure oil can be isolated by kugelrohr distillation(60°C, 0.1 mmHg). 1H
NMR (C60,); 5 0.23 (s, 9 H, Sn(CH3)3, 21“,, = 52 Hz), 5 0.92 (t, 3 H, CH3), 5 1.37 (m, 2

114

H, CH2) 5 1.46 (m, 2 H, CH3) 5 2.46 (t, 2 H, C=CCH2), 5 2.61 (s, 12 H, B(N(CH3)2)2), 5
6.51 (s, 1 H, HC=C, 31,”, = 193 Hz). 13C NMR (C,D,); 5 —8.7 (SnMc3, ‘19,, = 312
Hz), 5 14.2 (CH3), 5 22.8 (CH2), 5 40.9 (B(NMe2)2), 5 32.7 (CH2), 5 44.7 (C=C(Sn)CH2,
210,, = 67 Hz), 5 145.3 (broad, BC=CSn), 5 156.7 (BC=CSn, ‘JC_,, = 489 Hz). “8 NMR
(C,D,); 5 30.7. “98n (C,D,); 5 —50.3.

Stannaboration of 4-octyne. 4-octyne (148 mg, 1.34 mmol), Me;Sn—B(NMc2)2 (352
mg, 1.34 mmol and 5 mole % catalyst 1 (66 mg, 0.067 mmol) were placed in a flask and
dissolved in 4 mL toluene. The solution was heated at 80°C for 36 hr at which time the
reaction was judged complete by GC analysis. The solvent was removed in vacuo leaving
an oily brown residue. The residue was taken up in hexane and washed through neutral
Alumina The hexane was removed leaving a dark golden oil (340 mg, 68%). In order to
remove a small amount of PPh3 that was present due to decomposition of catalyst, the
pure oil can be isolated by kugelrohr distillation(70°C, 0.1 mmHg). 1H NMR (C6D6); 5
0.23 (s, 9 H, Sn(CH3)3, 2111.51, = 50 Hz), 5 0.98 (t, 6 H, CH3), 5 1.43 (m, 4 H, CH2) 5 2.31
(m, 2 H, CH2(B)C=C), 5 2.47 (t, 2 H, C=C(Sn)CH2), 5 2.57 (s, 12 H, B(CH3)2). 13C NMR
(C,D,); 5 -8.3 (SnMeg, ‘Jcs. = 306 Hz), 5 14.4 (CH3), 5 15.4 (CH3), 5 23.2 (CH2), 5 23.8
(CH2), 5 35.5 (CH2(B)C=C, 41cm, = 80 Hz), 5 36.5 (C=C(Sn)CH2, 2]c.sn = 67 Hz), 5 40.8
(B(NMe2)2), 5 147.1 (BC=CSn, 11cm, = 542 Hz), 5 156.2 (broad, BC=CSn,). “B NMR
(C,D,); 5 30.7. “9Sn (C,D,); 5 —52.8. EI/MS; m/z = 359.2 ([M-Me]”), l=100(100); m/z =
360.3 ([M-Me]+1”), I=20.9(19.2); m/z = 361.2 ([M-Me]+2*), I=16.3(14.8); m/z = 358.3
([M-Me]-1*), I=75.53(57.9); m/z = 357.3 ([M-Me]-2*), I=92.5(78.9); m/z = 356.3 ([M-
Me]-3”), I=66.7(44.2); m/z = 355.2 ([M-Me]-4*), I=57.0(46.1).

Stannaboration of diphenylacetylene. Diphenylacetylene (167 mg, 0.938 mmol),
Me;Sn—B(NMe2)2 (246 mg, 0.938 mmol) and 6 mole % catalyst 1 (54 mg, 0.056 mmol)
were placed in a flask and dissolved in 4 mL toluene. The solution was heated at 75°C
for 60 hr at which time the reaction was judged only 30% complete by GC analysis. The
temperature of the bath was raised to 85°C for another 60 hr. 1H NMR analysis of the

115

mixture indicated the absence of free stannyl-borane and approximately 20% unreacted
diphenylacetylene. An additional 50 mg Me3Sn—B(NMe2)2 was added to the flask and
heated for an additional 48 hrs. The solvent was removed in vacuo leaving an oily brown
residue. The residue was taken up in hexane and washed through neutral Alumina,
Activity 1. The hexane was removed leaving a dark golden oil (353 mg, 60%). The oil
was dissolved in a small amount of heptane and placed in a —80°C freezer to precipitate
PPh3 that was present due to decomposition of the catalyst. 1H NMR (C6D6); 5 0.19 (s, 9
H, Sn(CH3)3, 2J1”, = 53 Hz), 5 2.66 (s, 12 H, B(N(CH3)2)2), 5 6.82—7.18 (m, 10 H,
CsHs(B)C=C(Sn)C,H,. 13C NMR (C6D6); 5 -8.1 (SnMc3, 11,4, = 312 Hz), 5 40.8
(B(NMc2)2), 5 143.6 (C(B)C=CSn), 5 147.3 (BC=C(Sn)C), 5 151.5 (BC=CSn, lJc_sn =
489 Hz), 5 157.5 (broad, BC=CSn). “B NMR (C,D,); 5 31.2. “9Sn (C,D,); 5 —43.1.
EI/MS; m/z = 427.1 ([M-Mc]+), l=100(100); m/z = 428.1 ([M-Mc]+1*), I=23.9(24.9);
m/z = 429.1 ([M-Me]+2+), I=16.0( 15.7); m/z = 426.1 ([M-MeJ-l“), I=60.8(60.8); m/z =
425.1 ([M-Me]-2*), I=83.0(78.8); m/z = 424.1 ([M-Mc]-3"), I=48.0(45.5); m/z = 423.1
([M-Me]-4*), Ifl6.2(45.0).

Catalytic Diboration 0f Olefins

A typical catalytic reaction involving a gaseous olefin was prepared in the following
manner: CatB—BCat (224 mg, 0.94 mmol) and 3 mol% Pt(COD)2 were loaded into a
FisherPorter flask and dissolved in 3 mL toluene. The mixture was taken out,
immediately frozen and placed under vacuum, whereupon ethylene (1 atm) was
introduced. The mixture was warmed to room temperature and allowed to stir for several
hours. The solvent was removed in vacuo and the crude product collected.

A typical catalytic reaction involving a solid or liquid olefin was prepared in the
following manner: CatB—BCat (262 mg, 1.10 mmol) and 3 mol% Pt(COD)2 were loaded
into a Schlenk flask 4-Vinylanisole (148 mg, 1.10 mmol), dissolved in 3 mL toluene,

116

was then added to the solids in the flask. After stirring for 30 minutes at room
temperature. The solvent was removed in vacuo and the crude product collected.

Yields reported are based on the crude material collected from bottom of the flask
except where further purification is indicated. The solid products were collected by
scraping down the sides and bottom of the flask and transferring to a tared vial. The oil
products were collected by pipetting the contents from the bottom of the flask to a tared
vial. .

1,2-bis(catccholboryl)hexane (500 mg, 1.55 mmol) was converted to the
corresponding diol in 46% yield by a basic oxidative workup and was identical to 1H and
13C NMR of purchased 1,2-hexancdiol.

Diboration of Ethylene. The crude product was isolated as a light tan solid in 84%
yield and can be sublimed as a white solid in 75% yield. 1H NMR (CDC13); 5 1.61 (s, 4
H, HzC-CHZ), 7.05 (m, 4 H, BOzC6H4), 7.18 (m, 4 H, BOZC6H4). l3C{‘H} NMR
(CD00: 5 4.0 (br, 2 C, C—B), 112.3 (2 C, 302C6H4), 112.5 (2 C, BOZC,Ha), 122.5 (4 C,
B02C,Ha), 148.3 (4 C, BOzC,I-14). “B('H) NMR (CDClg); 5 35. EI/MS; m/z = 266.1
(M1), 1 = 100(100); m/z = 267.3 ([M+1]+), I = 11(16): m/z = 264.9 ([M-1J1“), I = 74 (47).
Analysis calculated for ClaleBan: C, 63.25; H, 4.55. Found: C, 63.36; H, 4.63.

Diboration of l-Hexene. The crude product was isolated as a brown oil in 83% yield.
The product decomposes upon Kugclrohr distillation. 1H NMR (CDC13); 5 0.90 (t, 3 H,
CH3, 7.1 Hz), 1.38 (m, 4 H, CHzCHz), 1.50—1.70 (m, 3 H, (BCat)CH(H)CH(BCat)CH2),
1.80 (m, 1 H, (BCat)CH(H)CH(BCat)), 1.93 (m, l H, (BCat)CHZCH(BCat)), 7.05 (In, 4
H, BO;C,H4), 7.18 (m, 4 H, BOzc6H4). l3C(‘Hi NMR(CDC13);5 12.1 (br, 1 C, C—B),
14.0 (CH3), 18.1 (br, 1 C, C—B), 22.8 (CH2), 31.0 (CH2), 33.2 (CH2), 112.3 (4 C,
802C,H,) , 122.4 (4 C, 802C,H,) , 148.2 (4 C, 1302C,H,). “8(1H} NMR (CDCla); 5 35.
EI/MS; m/z = 322.2 (M+), I = 100(100); m/z = 323.2 ([M+l]+), I = 21(20); m/z = 321.2
([M-1]"), 1 = 47(44).

117

Diboration 0f vinylanisole. The crude product was isolated as a light brown solid in
94% yield. The product does not sublime, melt or decompose at temperatures up to
180°C. 1H NMR (CDCls); 5 1.89 (dd, J = 16.8, 7.1 Hz, 1 H, CHZCH), 2.13 (dd, J = 16.8,
9.7 Hz, 1 H, CHZCH), 3.29 (t, 1 H, CHZCH, 31,”, = 9 Hz), 3.76 (s, 3 H, OCH3), 6.85(m, 2
H, C6H4), 7.03 (m, 4 H, B02C6H4), 7.16 (m, 4 H, BOZC,H4), 7.29 (m, 2 H, C,H4).
l3C(‘Hi NMR(CDC13);5 14.2 (br, 1 C, C—B), 24.9 (br, 1 C, C—B), 55.2 (OCH3), 112.3
(2 C, ‘BOZC,I-la), 112.5 (2 C, B02C,Ha), 114.2 (2 C, C,HaOMe), 122.5 (2 C, B02C61-14),
122.6 (2 C, BOZC,Ha), 128.9 (2 C, C,H.,0Me), 134.8 (1 C, C,H.,OMe), 148.2 (4 C,
B02C,Ha), 157.8 (1 C, C,H..0Me). ”B{’H} NMR (CD00; 5 35. EI/MS; mlz = 372.2
(M+), I = 100(100); m/z = 373.3 ([M+1]'*'), I = 21(23); m/z = 371.2 ([M-1P”), I = 46(48).

Diboration of 4-(triflu0romethyl)styrene. The crude product was isolated as a light
brown solid in 95% yield and can be sublimed as a white solid in 60% yield. 1H NMR
(CDCl3); 5 1.95 (dd, J = 16.7, 7.0 Hz, 1 H, CHZCH), 2.21 (dd, J = 16.7, 9.3 Hz, 1 H,
CHZCH), 3.45 (dd, J = 9.3, 7.1 Hz, 1 H, CHZCH), 7.07 (m, 4 H, B02CsH4), 7.21 (m, 4 H,
Bozc,H,), 7.55 (m, 4 H, C,H4CF3). l3C(‘Hl NMR(CDC13);5 13.6 (br, 1 C, C—B), 26.0
(br, 1 C, C—B), 112.4 (2 C, BOZC,H.,), 112.6 (2 C, BOZCJL), 122.7 (2 C, BOZC,H4),
122.8 (2 C, B02C6Ha), 125.7 (2 C, C,H,CF3), 128.3 (2 C, C6H4CF3), 147.1 (1 C,
C6114CF3), 148.1 (4 C, B02C61-14), (resonances due to CE; and ipso C attached to CF;
were not observable). “B{‘H) NMR (CDCl;); 5 34. EI/MS; m/z = 410.2 (W), I =
100(100); m/z = 411.2 ([M+1]+), I = 22(22); m/z = 409.1 ([M-1]+), I = 51(51). Mp: 95-
96°C.

Diboration of 6-Chloro-l-hexene. The crude product was isolated as a brown oil in
85% yield. The product decomposes upon Kugclrohr distillation. 1H NMR (CDC13); 5
1.48-1.70 (m, 5 H, CHZCH3), 1.82(m, 3 H, CHZCH), 1.93 (m, 1H, CHZCH), 3.54 (t, J =
6.9 Hz, 2 H, CHzCl), 7.05 (m, 4 H, Bozc,H,), 7.18 (m, 4 H, B02C,H4). l3C(‘Hl NMR
(CDC13); 5 12.1 (br, 1 C, C—B), 18.0 (br, 1 C, C-B), 26.1 (CH2), 32.56 (CH2), 32.65
(CH2), 44.9 (CH2), 112.3 (2 C, B02C61-14), 112.4 (2 C, BO;C,H,), 122.47 (2 C,

118

802C,H,), 122.53 (2 C, 802C,H,), 148.2 (4 C, 802C,H,). ”B{ 1H} NMR (CDCla); 5 35.
EI/MS; m/z = 356.1 (M+), I = 100(100); m/z = 357.2 ([M+1]+), I = 32(34); m/z = 358.2
([M+2]+), I = 33(34); m/z = 355.0 ([M-1]+), I = 55(46).

Diboration of acetic acid S-hexenyl ester. The crude product was isolated as a
brown oil in 88% yield. The product decomposes upon Kugclrohr distillation. 1H NMR
(CDCla); 5 1.39—1.78 (m, 6 H), 1.84 (in, 1 H), 1.95 (m, l H), 2.02 (s, 3 H, CH3), 2.03 (m,
1 H, CHZCH), 4.08 (t, J = 6.8 Hz, 2H, CHZOCOCH3) 7.06 (m, 4 H, BO;C,H4), 7.19 (m, 4
H, BOZC,H4). l3C(‘H} NMR (CDCls); 5 12.0 (br, 1 C, C—B), 18.0 (br, 1 C, C—B), 20.94
0, 25.1 (CH3), 28.6 (CH2), 32.9 (CH2), 64.3 (Cf’leCH3), 112.3 (2 C, BOZC,H,), 112.4 (2
C, Bozc,H.,), 122.45 (2 C, BOZC6Ha), 148.1 (4 C, 802C514), 171.2 (C=O). “B('H}
NMR (CDCls); 5 35. EI/MS; m/z = 380.2 (M+), I = 100(100); m/z = 381.3 ([M+1]+), I =
48(22): m/z = 379.3 ([M-1]+), I = 50(46).

Diboration of Norbomene. The crude product was isolated as a light tan solid in
93% yield and can be sublimed as a white solid in 85% yield. 1H NMR (CDC13); 5 1.44
(n1, 3 H), 1.73 (m, 3 H), 1.83 (s, 4 H), 2.65 (s, 2 H), 6.93 (s, 8 H, B02C61-14). l3C(‘Hl
NMR (CDCl;): 5 28.4 (br, 2 C, C-B), 31.9 (2 C), 38.5 (1 C, bridgehead CH2), 39.0 (2 C,
bridgehead CH), 112.0 (4 C, BOZC,H.,), 122.2 (4 C, BOzCsHa). 148.1 (4 C, BOzC,Ha).
“B(‘H) NMR (CDCl,); 5 35. EI/MS; m/z = 332.2 (W), I = 100(100); m/z = 333.2
([M+1]+), I = 17(21): m/z = 331.0 ([M-l]+), I = 62(46). Mp 89—90°C.

Tetraboration of Norbomadiene. The crude product was isolated as a tan solid in
89% yield. Attempts to sublime result in melting and decomposition at temperatures
nearing 180°C. 1H NMR (CDC13); 5 1.99 (s, 2 H), 2.15 (2, 4 H), 3.12 (s, 2 H), 6.96 (s, 8
H, 802C,H,). l3C{'H) NMR (CD00; 5 30.8 (br, 4 C, C—B), 39.0 (1 C, bridgehead
CH2), 41.7 (2 C, bridgehead CH), 112.1 (8 C, BO;C,H.,), 122.4 (8 C, 302C6H4)r 148.1 (8
C, 80205114). “B{ 1H} NMR (CDC13);5 35. EIIMS: m/z = 568.2 (W), I = 100(100); m/z
= 569.2 ([M+1]+), l = 29(32): m/z = 567.2 ([M-1]‘*), 1 = 92(82): m/z = 566.1 ([M-2]+), I
= 29(33).

119

Diboration of 1,5-Hexadiene. The crude product was isolated as a brown oil in 82%
yield. The product decomposes upon Kugclrohr distillation. 1H NMR (CDCl;): 5 1.49-
2.07 (m, 5 H, (BCat)CH2(BCat)CHCH2R), 2.23 (m, 2 H, RCHZCHCHZ), 4.94 (m, 1 H,
CHCH(H)), 5.02 (m, l H CHCH(H))), 5.82 (m, l H, CHCHZ), 7.04 (m, 4 H, BOzC,I-Ia),
7.15 (m, 4 H, 1302C,H,). l3C(‘Hi NMR (CDCl;); 12.0 (br, 1 C, C—B), 18.] (br, 1 C, C—
B), 32.5 (CH2), 32.8 (CH2), 112.3 (4 C, BOzC,H,), 115.0 (CHZCHR), 122.4 (4 C,
BOZC,1-14), 138.3 (CHZCHR),.148.2 (4 C, 802C,H,). “B(‘H} NMR (CDC13); 5 35.
EI/MS; m/z = 320.1 (Mi), I = 100(100); m/z = 321.2 ([M+1]+), I = 18(20): m/z = 319.0
([M-1]+), I = 55(46).

Tetraboration of 1,5-Hexadiene. The crude product was isolated as a brown solid in
87% yield. Attempts to sublime result in melting and decomposition at temperatures
nearing 180°C. 1H NMR (CDCls); 5 1.54—1.77 (m, 4 H, RCHZCHZR), 1.83 (m, 2 H),
1.98 (br m, 4 H), 7.03 (m, 4 H, (B02C6H4)), 7.12 (m, 4 H, (BOZC,H4)). l3C(‘H} NMR
(CDC13); 5 12.0 (br, 2 C, C—B), 18.1 (br, 2 C, C—B), 32.2 and 32.3 (2 C, equal intensity,
probably due to R,S-enantiomers, CHzCHz), 112.3 (4 C, BOzC,Ha), 112.4 (4 C,
BOZC,I-Ia), 122.4 (8 C, Bo,C,H,), 148.2 (8 C, 802C414). “B(‘H} NMR (CDC13); 5 35.
EI/MS; m/z = 558.3 (M‘l'), I = 100(100); m/z = 559.3 ([M+1]+), I = 28(31): m/z = 557.2
([M-1]"), 1 = 86(83).

Catalytic Boration of Arenes

A typical experimental run for the catalytic borylation of arenes with HBPin by
Cp*Ir(PMe3)(BPin)H is described as follows: In a drybox, Cp*Ir(PMe3)(BPin)H (9.8 mg,
0.018 mmol) was weighed into a test tube and dissolved in 300 mL C,D,. HBPin (13.4
11L, 0.092 mmol) was added to the solution via microsyringe and C6D, was added to
make the solution volume 540 (AL. The solution was thoroughly mixed and transferred to

a scalable NMR tube. The tube was frozen in liquid nitrogen and flame-sealed with an

120

atmosphere of argon gas. Upon thawing the tube was heated at 150 °C in an oil bath. The
reaction was monitored by 1H, 11B and 31P NMR. The resonance due to C6D5-BPin was
clearly increasing in intensity. The reaction was judged complete (60 hrs) by monitoring
the disappearance of the resonance due to pinacolborane in the 1H and llB NMR.
Isolation of Ph-BPin From a Catalytic Experiment. A solution of 31 (80 mg, 0.15
mmol) and HBPin (100 mg, 0.75 mmol), dissolved in 4.3 mL toluene, was heated at 150
°C 5 days in a thick—walled air—free flask. The solution was then transferred to a round
bottom flask and the benzene removed on the rotary evaporator. The residue was

chromatographed on silica with CHzClz to yield Ph-BPin (85 mg, 53% based on HBPin).

Crystal Structure Determinations and Refinement

Crystals of 2 for X-ray analysis were grown from a saturated toluene solution at -15
°C. Crystals of 4 for X-ray analysis were grown by diffusion of EtOH into CH2C12
solutions of the complex at 5 °C. The crystals were covered in a heavy hydrocarbon oil. A
suitable crystal was chosen and mounted on a glass fiber. 2 crystallized with a molecule
of toluene. 4 crystallized with a molecule of CHzClz. The data sets were collected at 183
K on a Siemens P3N diffractometer using Mo K01 X—radiationOc = 0.71069 A). Intensity
data were collected in the range 4.0 Q 20 9 550° using (ll/20 scans. The final unit cell
parameters were obtained by least-squares refinement of 25 reflections that had been
accurately centered on the diffractometer. Three standard reflections were recorded after
every 150 scans, and no significant change in intensity was observed. The structures were
solved using SHELXL—93 and was refined by full-matrix least-squares on all P2 data. All
non—hydro gen atoms were solved with anisotropic thermal parameters. Hydrogen atoms
were included in calculated positions and allowed to ride on the parent carbon atoms with
isotropic thermal parameters. Crystallograpic computations were performed on a Silicon

Graphics Indigo computer.

121

Crystals of 29 for X-ray analysis were grown from a saturated toluene solution at —80
°C. Crystals of 45 for X-ray analysis were grown by layering a toluene solution of 45
with heptane and cooling at —30 °C. Crystals of 46 for X-ray analysis were grown from a
saturated pentane solution at -80 °C. The crystals were covered in a heavy hydrocarbon
oil and a suitable crystal for each was mounted on a glass fiber. The data sets were
collected at 183 K on a Siemens CCD diffractometer using Mo K01 X-radiationOt =
. 0.71073 A). Data were collected as 90 second frames in a hemisphere of space. The final
unit cell parameters were obtained by least-squares refinement of accurately centered
reflections obtained from 60 frames of collected data. Smart was used to obtain a unit cell
and Saint was used to integrate the collected frames. SADABS was tehn used to apply
absorption corrections to the data. The structures were solved using SHELXL—86.
Atomic coordinates and thermal parameters were refined using the full-matrix least-
squares program, SHELXL—97, and calculations were based on F2 data. All non-
hydrogen atoms were refined anisotropically. All crystallograpic computations were
performed on Silicon Graphics Indigo computers. All hydrogen atoms not bound to metal
centers were placed in calculated positions and allowed to “ride” their parent carbon
atoms without further restraint. The hydride hydro gens in compounds 45 and 46 were
refined isotropically and found in chemically reasonable positions in Fourier difference
maps. In compounds 45 and 46, the pentamethylcyclopentadicnyl ligands were
disordered (rotated along the Ir-Cp*,,,,,,,;d axis) and an appropriate model was used in the
refinement.

Tables of pertinent data collection parameters for all compounds crystallographically

characterized are given in Appendix A

122

Appendix A. Summary of crystal data and structure refinement

 

 

2 4

Empirical formula C43H33B204P2P1 C44H53BC12N2P2PISD
Formula weight 1049.57 1067.31
Temperature 293(2) 1 83(2)
Wavelength 0.7 1069 0.71069
Crystal system Triclinic Triclinic
Space group P/1(no. 2) P—l
a (A) 11.304(3) 11.278(2)
b (A) 21.248(4) 12.117(2)
c (A) 11.206(3) 17.219(3)
ct, deg 103.33(2) 73.300(14)
[3, deg 116.l4(2) 79.050(12)
1, deg 7464(2) 88.749(14)
v A3 2309.7(10) 2211.5(6)
Z 2 2
pm, Mg m'3 1.509 1.603
Absorption coefficient 3.154 3.949
F(000) 1052 1056
Crystal size (mm) 0.10 x 0.10 x 0.40 0.98 x 0.49 x 0.22
20 range collected, deg 1.00 to 22.59 2.02 to 27.59
Index ranges 0511510 —4$h$ l4

—22$k$22 —15$k515

—12Sl$lO 4151522
Reflections collected 5231 10763
Unique reflections 5231 10223

Rint
Refinement method

Data / restraints / par
GOF on F2

Final R indices
[I>26(I)]

R indices (all data)

Largest diff. peak and
hole (e A3)

R(int) = 0.00851
Full-matrix least-
squares on 15‘2
5174 / 407 / 614
1.045

R1 = 0.0692, WRZ
= 0.1751

R1: 0.1148, WRZ
= 0.3791

2.119 and —3.991

123

R(int) = 0.0211
Full-matrix least—
squarcs on I“2
10206 / 18 / 508
1.088

R] = 0.0647, wR2 =
0.1632

R1 = 0.0745, wR2 =
0.1931

9.938 and —5.855

 

Appendix A (cont). Summary of crystal data and structure refinement

 

 

29 45
Empirical 10111111121 C25H32B211'O4P C25H27BF1011'P
Formula weight 641.30 751.45
Temperature 173(2) K 173(2) K
Wavelength 0.71069 A 0.71073 A
Crystal system monoclinic monoclinic
Space group P2(1)/n P2(l)/n
a (A) 11.359(3) 12.807(3)
b (A) 14.658(4) 14.017(3)
c (A) 15.306(3) 15.155(3)
0t, deg 90 90
[3, deg 91.77(2) 9548(3)
7, deg 90 90
v A3 2547.2(11) 2708.1(9)
Z 4 4
13..., Mg m‘3 1.672 1.843
Absorption coefficient 5.333 mm‘1 5.072 mm‘1
F(000) 1264 1456
Crystal size (mm) 0.25 x 0.36 x 0.48 0.36 x 0.45 x 0.54
20 range collected, deg 2.20 to 24.99 1.98 to 28.26
Index ranges 0<=h<=13 -16<=h<=16
0<=k<=l7 -17<=k<=18
-18<=l<=18 -19<=l<=19
Reflections collected 4725 31128
Unique reflections 4481 6469
Rim 0.0679 0.0230
Refinement method Full-matrix least- Full-matrix least-
squares on F2 squares on F2
Data / restraints / par 4451 / 0/ 298 6469 / 25 / 470
GOF on F2 1.000 1.051
Final R indices R1 = 0.0471 R1: 0.0166
[I>20’(I)] wR2 = 0.1194 wR2 = 0.0378
R indices (all data) R1 = 0.0934 R1 = 0.0218
wR2 = 0.1930 wR2 = 0.0390
Largest diff. peak and 2.706 and -3.753 0.387 I -0.879

hole (e A3)

124

Appendix A (cont). Summary of crystal data and structure refinement

 

 

46
Empirical formula CstszF 10er
Formula weight 749.43
Temperature 173(2) K
Wavelength 0.71073 A
Crystal system monoclinic
Space group P2(l)/n
a (A) 12.678(3)
b (A) 13.933(3)
c (A) 15.192(3)
01, deg 90
B, deg 9279(3)
'11, deg 90
v A3 2680.4(9)
Z 4
Dale, Mg m'3 1.857
Absorption coefficient 5.124 mm'1
F(000) 1448
Crystal size (mm) 0.13 x 0.31 x 0.34
20 range collected, deg 1.98 to 28.22
Index ranges -16<=h<=l 6
-l8<=k<=18
-l9<=l<=19
Reflections collected 20638
Unique reflections 6160
Rim 0.0213
Refinement method Full-matrix least-
squarcs on F2
Data/restraints/par 6160/25/438
GOF on F2 1.048
Final R indices R1 = 0.0201
[I>20(I)] wR2 = 0.0464
R indices (all data) R1 = 0.0273
wR2 = 0.0483
Largest diff. peak and 1.261 / -0.932

hole (e A3)

125

Appendix B. Derivation of Rate Expressions For Chapter 3

In Figure 24, the time dependent concentrations for 2, 17, 18 and 5 are governed by

the Eqs Bl—4:

'42] - k [2] - K 17][PPh]
dt - 4 41 3 (Bl)

 

01171 = R4121 + (,5 [13] - (K4[PPh3] + k5[RCECR])[17]
dt (32)

 

0118] _.
_ = k5[RC=CR][17] - (K5 + 4,9118]

dt (B3)

DIS] k 18
d1 - 61 1 (B4)

 

Application of the steady state approximation to [18] yields Eq BS, and combination of
the steady state expression for [17] and Eq B5 gives B6. The first order rate law in Eq B7
follows from substitution of the expression for [17] (Eq B6) in Eq B l:

 

 

one] mncacn]
dt 1* o z) [18] =( “H‘s )[17]

(35)

1 k.
fl- 0 z) [17] =( “ks )[2]
(B6)

 

d‘ K4(Ks+ke) +k5kg[RCE-CR]

126

 

 

-42] _ k4k5k6 2
dt - K4(k.5 + ks) [PPh3] + 181911905091 I 1 (B7)

In applying the equilibrium approximation to Figure 25, the assumption in Eq B8 is

made:

 

 

.012 + 17 + 18] a 45] = k6[18]
dt at (B8)

From the equilibrium constants, K, and K2, [2] and [17] can be expressed in terms of [18]

 

 

 

(Eqs B9 and B10):
K [13] 17 [13]
2 = [17][RCECR] : I I: K2[RCECR] (39)
(17(th,} [17][PPh3] [18][PPh3]
K = —— z) 2 = —— :3 2 =
’ [2] l l K, I l K, K2[RC-ECR] (310)

Eqs B9 and 810 provide an expression of [18] in terms of [2 + 17 + 18] (Eq B12), and
substitution for [18] in Eq B8 gives the final form of the rate law in Eq Bl3:

 

 

2 17 13 1 1 + [PPM [18]
I + + l = + K2 [ROE on] K, K2 [RCECR]
(311)

127

( K,K2[RCECR] )
[18] = [2 + 17 + 18]
K, + [PPhg] + K1K2[RCECR]
(BIZ)

 

 

-o{2 + 17 + 18] ( k6K1K2[RCECR] )
= [2 + 17 + 18]
dt K1 + [Ppha] + K1K2lRCECR]
(B13)

Under catalytic conditions the diboryl complex, 2, is regenerated. The rate laws that

govern the time dependent concentrations of the species in Figure 28 are given below:

 

 

 

 

.42] - k412] K [17][PPh3] ksl18]
dt ’ ‘ (314)
014] MR] K [1810‘ [PPh3] [RC—CR1) [17]
= + =
dt 5 -4 + k5 (BIS)
one] Rc—CR 17 K ) 18
d, = ksl — Ill-(5+ksll (m6)
dRCECR] k6[18] K [18] k5[RC—CR 17
= 1' + " =
dt 5 ll 1 (1317)

Applying the steady state approximation to [17] and [18] in Eqs BIS and B16 yield the
expressions in Eqs B18 and B19:

k412] + Kle
K4[PPh3] + macacn]

[17] =

 

(318)

128

 

K
[17] = ( k [350;kgn )[18]
5 "' l (319)

Application of the steady-state approximation to [2] in Eq B14, with substitution of the

value for [17] in Eq B18, yields Eq B20. Similarly, substitution of the expression for [17]

 

 

 

 

in Eq B19 in Eq B17 gives Eq‘BZl:
k, k5 [ROE CR]
[18] = _ [2]
K4 (K5 + k6) [PPh3] + k5 k6 [no =03]
(B20)
otnc_=_cm
= - 17
dt ksl l (321)
Combining Eqs B20 and 321 gives the rate law in Eq 322:
-o[RCE CR] k, k5 k6 [ROE CR] [2]
dt - K4 (K5 + k6) [PPh3] + 115 Re [RC sea]
(322)

Applying the equilibrium approximation to the cataytic reaction in Figure 29 gives Eq
823:

-qRCECFl +13] ~ «15] = kenal
dt d? (B23)

 

 

129

 

From the equilibrium constants in Eqs B24 and 825, [18] can be expressed in terms of

[18 + RCECR] (Eq 327):

 

 

[18] [18]
2:) ECR =
K2 [RCECR] [17] [RC 1 K2 [17]
[17] [PPh3] K1[2] [PPh3] [15]

 

 

 

K = 17:
1 [2] :>[]

[PPha]

PP
[RCECR + 13] = (1 + —[—'h-]—) [13]

 

18 K1K2l2] RECR 18
I] ‘ [PPh3]+K,K2[2] I + 1

Substituting this expression for [18] in Eq B23 gives the rate law in Eq B28:

 

 

«130203 + 13] _ k6K1K212]
d! ' [PPha] + K, K2 [2]

)[RCECR + 181

130

2 I 1 K3912]

(324)

(325)

(1326)

(B27)

(1328)

APPENDIX C. Kinetic Details

Platinum Kinetim

Data for stoichiometric [PPh3] dependence at 90 °C.

The rates were determined by measuring the disappearance of catecholate peaks of 2. The
experiments were performed in a constant temperature oil bath.

[2] = 8.7 mM; [4-octyne] = 52 mM

 

 

[33113] km

0 mM 0.050 sec"
2.2 0.041

8.7 0.018

17 0.01 1

22 0.0095

35 0.0058

Data for variable temperature stoichiometric with 1 eq. PPhs.
The rates were determined by measuring the disappearance of catecholate peaks of 2. The

experiments were performed in a constant temperature oil bath.

[2] = 8.7 mM; [4-octyne] = 52 mM; [PPh3] =8.7 mM

 

 

temp km

343 K 0.0019 sec"
353 0.0065

363 0.018

373 0.046

383 0.12

131

Data for stoichiometric [4-octyne] dependence with 8 eq. PPh3.at 105 °C.

The rates were determined by measuring the disappearance of catecholate peaks of 2. The

experiments were performed in a constant temperature oil bath.

[2] = 8.7 mM; [PPh3] = 70 mM

 

 

[4-octyne] Rob:

26 mM 0.0099 sec"
52 0.019

104 ‘ 0.037

209 0.082

261 0.098

Data for stoichiometric various alkynes at 70 °C.

The rate for ¢octyne was determined by measuring the disappearance of catecholate

peaks of 2. The 4-octyne experiment was performed in the spectrometer. The rates or the

substituted diphenyl acetylenes were determined by measuring the relative ”P intensities

of 2 and (PPh3)2Pt(n2-acetylene) at —40 °C. The acetylene experiments were performed in

a constant temperature oil bath.

[2] = 8.7 mM; [alkyne] = 52 mM

 

 

alkyne kobs
4-octyne 0.014 sec“1
Diphenylacetylene 0.0064
Bis(p-anisy1)acetylene 0.0047
Bis[p-(uifluoromethyl)phenyl] acetylene fast

132

Data for stoichiometric [4-octyne] dependence with 0 PPh3 at 70 °C
The rates were determined by measuring the disappearance of catecholate peaks of 2. The

experiment was performed in the spectrometer.

 

 

[2] = 8.7 mM
[4-octyne] km
26 mM 0.013 sec’fi

52 0.014

Data for catalytic dependence of [4-octyne] and [82Catz] with 8 eq. PPh3 at 105 °C.
The rates were determined by measuring the integrals of methylene protons alpha to
unsaturated bond in 4-octyne and diborylated 4-octyne. Experiments were heated in a

constant temperature oil bath.

[2] = 7.0 mM; [PPh3] = 56 mM

 

 

[82Catzl [4-0ctynel Rob.

42 mM 42 mM 0.0030 sec"
42 84 0.0031

84 42 0.0030

133

Data for catalytic [PPh3] dependence at 100 °C

The rates were determined by measuring the integrals of methylene protons alpha to
unsaturated bond in 4-octyne and diborylated 4-octyne. Experiments were heated in a
constant temperature oil bath.

[2] = 7 .0 mM; [BZCatz] = 42 mM; [4-octyne] = 42 mM

 

 

[PPh3] kobs

7.0 0.017 sec'1
3.5 0.029

1.7 0.052

0.9 0.074

Iridium Kinetim
Data for the addition of HBCat to 28.
The rates were determined by measuring the integrals of the Cp* resonance of

compounds 28, 29, and 30. Experiments were performed in C6D6 and heated in a constant

 

 

temperature oil bath.

[28] [HBCat] Temp km

42 mM 250 mM 40.0 °C 2.9 x 10“ sec'1 "
42 500 40.0 4.0 x 10“
84 500 40.0 8.0 x 10“
42 250 21.5 4.2 x 10'5
42 250 30.0 1.5 x 10“
42 250 50.0 5.8 x 10"
42 250 60.0 1.3 x 10'3
42 250 70.0 2.5 x 10'3
42 (28-d2) 250 (DBCat) 40.0 1.8 x 10“

134

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140

 

(166) Noth, H.; Schwerthoffer, R. Chem. Ber. 1981, 114, 3056-3062.

(167) Cummins, C. H. Tetrahedron Lett. 1994, 35, 857-860.

(168) Ashby, E. C.; Oswald, J. J. Org. Chem. 1988, 53, 6068-6076.

(169) Luetkens, M. L. Jr.; Sattellberger, A P.; Murray, H. H.; Basil, J. D.;
Fackler, J. P. Inorg. Syn. 1990, 28, 305-310.

(170) Noth, H. Z Naturforsch. 1984, 39b, 1463-1466.

(171) Tokuda, M.; Chung, V. V.; Inagaki, K; Itoh, M. J. Chem Soc., Chem.
Commun. 1977, 690-691.

(172) Brown, H. C.; Gupta, S. K J. Am Chem Soc. 1975, 97, 5249-5255.

(173) % conversion refers to the amount of Cp*Ir(PMe3)(Cy)H that has been used

up in the reaction (at least three 1/2 lives). “% 31” and “% actual 48” refer to the amounts
of Cp*Ir(PMe3)(BPin)H and Cp*Ir(PMe3)(C6D5)D produced at the point of % converson
listed. “% Theor. 48” and “% Theor. CyH” refer to the amount of Cp*Ir(PMe3)(C5D5)D
and cy-C6H12 expected based on just the thermolysis of pure Cp*Ir(PMe3)(cy-C6H11)H.
Ref. 66 was used to calculate the appropriate kw, values needed (as discussed in the text).
“% Cy-BPin” and “% actual CyH” refer to the percent yield of organic products at the

point of % conversion listed.

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