ABSTRACT
THE CRYSTAL STRUCTURES OF
BIS(TRIPHENYLPHOSPHINE)TETRAMETHYLENEPLATINUM(II)é
2, ,6,7,7,8-HEXAMETHYL-1,S-DIPHENYLTETRACYCLO[3.3.0.02, .03,6]

3
OCT -4-0NE; AND 6,6-DIMETHYL-2,3-BENZO-2,4-CYCLOHEPTADIENONE
PHOTODIMER A

By

Carol Greenup Biefeld

The assessment of the effects of internal and external stresses
on ring conformations are described gia_a comparison of the common
conformations of cycloalkanes with those found from X-ray structure
determinations of the title compounds.

Precession camera techniques have been used to gain space group
information and lattice parameters. Three-dimensional, single-crystal
intensity data have been collected with a four—circle, computer-
controlled diffractometer at 23(2)°.

The crystal structure of bis(triphenylphosphine)tetramethylene-
platinum(II), Pt[P(C6HS)3]2C4H8, is triclinic, space group 22; with
a_= 9.78(l), b_= l7.7S(2) and g_= 9.66(l) X; 2_= 94.06(4), §_=
102.74(7) and 1_= 98.S4(6)°; with §.= 2. The structure has been
solved by conventional Patterson and Fourier techniques with 3504
independent reflections and refined to a discrepancy index, based on
‘5, of 0.074. The phosphine ligands and terminal carbon atoms of the

tetrahydroplatinole ring comprise a distorted square-planar configuration

around the platinum atom in which the coordinated carbon atoms are

G8)8#é—_

Carol Greenup Biefeld
2.12(2) and 2.05(2) X from the metal. The angle between the tetrahydro—
platinole ring and the plane defined by the platinum and phosphorus
atoms is l75(1)°. The tetrahydrOplatinole ring is in an envelope
.configuration.
The compound 2,3,6,7,7,8-hexamethy1-l,S-diphenyltetracyclo-

2,8003,6

[3.3.0.0 ]octan-4-one, C26H280’ crystallizes in the monoclinic

0
space group P21/c with g_= 9.155(3), 9_= l4.635(9), g_= 15.425(4) A

 

and §_= 100.7(2)°, with §_= 4. The structure has been solved by

direct methods and refined (1577 independent reflections) by

full-matrix, least-squares techniques to a final R-value of 0.082.

The cyclobutane ring is nonplanar with a dihedral angle of 133(1)°

and contains two exceptionally long C-C bonds of 1.608(8) and

1.602(10) X which reflect the internal strain of the cage system.

The molecule, disregarding substituents, exhibits near mirror symmetry.

The three five-membered rings are in distorted enve10pe configurations.
The crystal structure of 6,6-dimethy1-2,3-benzo—2,4-cycloheptadienone,

photodimer A, C26H2802’ is monoclinic, space group 9312! with a_=

30.141(l6), b_= 8.536(4), g_= 15.979(6) Z and 8': 103.1(2)° with §_=

8. The structure has been solved by direct methods and refined (1761

independent reflections) by block diagonal, least-squares techniques

to a final Brvalue of 0.050. The five- to six-membered ring juncture

has been found to be gig, while the six- to seven-membered ring

juncture is trans; a long C-C bond length, 1.580(4) X, at the latter

juncture is indicative of strain at this site in the molecule. The

five-membered ring assumes a half-chair conformation, and the seven-

membered ring, a pseudo half-boat configuration. The cyclohexene is

Carol Greenup Biefeld
substantially flattened. Comparison of this structure is made with

that of its stereoisomer, dimer B.

THE CRYSTAL STRUCTURES OF
BIS(TRIPHENYLPHOSPHINE)TETRAMETHYLENEPLATINUM(II);
2,3,6,7,7,8-HEXAMETHYL-l,5-DIPHENYLTETRACYCLO[3.3.0.02:8.03,6]
OCTAN-4-ONE; AND 6,6-DIMETHYL—2,3-BENZO-2,4-CYCLOHEPTADIENONE
PHOTODIMER A

By

Carol Greenup Biefeld

A DISSERTATION
Submitted to
Michigan State University

in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry

1973

ACKNOWLEDGMENTS

There have been many people whose assistance has contributed
to the completion of this work and I would like to thank each of them
with special appreciation to the following: I wish to extend my
sincere gratitude to Dr. Harry A. Eick for his guidance, encouragement,
understanding and patience without which this work would not have
been finished. My special thanks to my parents and especially my
'“husband Bob for their understanding, encouragement and self—denial
which made attainment of this goal possible.

Thanks are also due to my colleagues, past and present members
of the high temperature research group, for their concern and vital
dialogue; to Dr. Robert Grubbs and Dr. Harold Hart for their
contribution of the compounds studied and their discussions concerning
the chemistry of the systems; and finally to Dr. Bobby Barnett.
Dr. Melvin Neuman and Dr. Richard Vandlen for thtir willingness to
always answer one more question about crystallography.

The financial support of the National Science Foundation is

acknowledged gratefu11y.

ii

TABLE OF CONTENTS

CHAPTER
1. Introduction
2. The Crystal Structure of Bis(triphenylphOSphine)tetra-

5.

methyleneplatinum(II)

Introduction

Experimental

Solution and Refinement of the Structure
Discussion

The Crystal Structure of 2,366,7,7,8-Hexamethy1-l,5-
diphenyltetracyclo[3.3.0.02» .0326]octan-4-one

Introduction

Experimental

Phase Determination
Refinement of the Structure
Discussion

The Crystal Structure of 6,6-Dimethy1-2,3—benzo-
2,4-cycloheptadienone Photodimer A

Introduction

Experimental

Phase Determination
Refinement of the Structure
Discussion

Summary and Conclusions

REFERENCES

In Text
General

iii

Page

11

11
12
16
26

39
42
44
SO
57

79
81
84
86
100

130
135

135
139

TABLE

10.

11.

12.

13.

14.

15.

16.

17.

18.

LIST OF TABLES

Crystal Data for PLAT
Fractional Coordinates and Thermal Parameters for PLAT

Observed and Calculated Structure Factor Magnitudes
for PLAT

Bond Distances Within PLAT
Bond Angles Within PLAT
Crystal Data for HDTO

Statistical Distribution of E's Calculated from the
Wilson Plot for HDTO

Starting Set of Signs for HDTO

Positions and Relative Heights of Assigned Peaks in the
E-Map for HDTO

Positions and Relative Heights of the Hydrogen Atom
Peaks for HDTO

Fractional Coordinates (X 104) of Nonhydrogen Atoms in HDTO
Anisotropic Thermal Parameters of Nonhydrogen Atoms in HDTO

Fractional Coordinates (X 103) and Isotropic Temperature
Factors for Hydrogen Atoms in HDTO

Observed and Calculated Structure Factor Magnitudes
for HDTO

Bond Distances Within HDTO
Bond Angles Within HDTO
Crystal Data for Dimer A

Statistical Distribution of E's Calculated from the
Wilson Plot for Dimer A

iv

Page

14

43

47

51

52

55
58

6O

63

65
68
69

83

85

LIST OF TABLES (Continued)

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

Hand-Determined Phases for Dimer A

Positions and Relative Heights of Assigned Peaks in the
E-Map for Dimer A

Positions and Electron Density (X 103) of the Hydrogen
Atom Peaks for Dimer A

Fractional Coordinates (X 104) of Nonhydrogen Atoms in
Dimer A

Anisotropic Thermal Parameters (X 104) of Nonhydrogen Atoms
in Dimer A

Fractional Coordinates (X 104) and Isotropic Temperature
Factors of Hydrogen Atoms in Dimer A

Observed and Calculated Structure Factor Magnitudes for
Dimer A

Bond Distances Involving Nonhydrogen Atoms Within
Dimer A

Bond Distances Involving Hydrogen Atoms Within
Dimer A

Bond Angles Involving Nonhydrogen Atoms Within Dimer A
Bond Angles Involving Hydrogen Atoms Within Dimer A
Dihedral Angles in Dimer A

Comparison of the Dihedral Angles in Dimers A and B

87

89

92

96

98

101

107

108

109

111

116

128

LIST OF FIGURES

FIGURE Page

in
1;:

1. Stereoscopic View of PLAT

2. Stereosc0pic View of the Packing of Four Molecules of
PLAT in the Front Half of Four Unit Cells 33

3. Dihedral Angles Calculated for the Ideal Half—Chair
and Envelope Conformations and Observed for the
Tetrahydroplatinole Ring in PLAT 37
4. Stereoscopic View of HDTO 67

O
5. (a) Bond Lengths (A) in HDTO. E.s.d.'s Lie in the Range
0.008 to 0.013 A. (b) Bond Angles (°) in HDTO. E.s.d.'s

Lie in the Range 0.4 to 0.8° 67
6. Comparison of the Dihedral Angles in Norbornane47 and
HDTO 7S
7. Projection of HDTO Along C(6)...C(8) 77
8. Conformation of Cyclopentane III in HDTO 77
9. Stereoscopic View of Dimer A 102
O
10. Bond Lengths (A) in Dimer A. E.s.d.'s Lie in the
Range 0.004 to 0.005 A 103
11. Bond Angles (°) in Dimer A. E.s.d.'s Lie in the
Range 0.2 to 0.3° 105
O
12. Bond Distance (A) in Dimer B 121
13. Bond Angles (°) in Dimer B 123

vi

CHAPTER 1

INTRODUCTION

Since 1885 when Baeyer's strain theory for cyclic compounds
appeared, chemists have been increasingly concerned with the conformation
of molecules. Conformational analysis has aided in the prediction of
molecular stability and reactivity, and in the discovery of synthetic
routes for the preparation of natural products and the elucidation of
their structures. The conformation of cyclic systems is still a major
area of both experimental and theoretical interest.

In 1885 Baeyer predicted that the strain (angle or Baeyer strain)
present in cyclic systems could be defined as l/2[109°28' - (actual
bond angle)°]. The factor of 1/2 results from the strain being spread
over two bonds and the actual bond angle in this case is taken to be
that of the planar polygon. Thus, the strain present in a planar
cyclic system would decrease to a minimum for cyclopentane and then
increase indefinitely. Even though heats of combustion--which later
proved conclusively that Baeyer's strain theory broke down for rings of
more than five members-~were not yet available, there were indications
that six-membered rings were more stable than the five-membered ones.
Sachse in 1890 resolved this contradiction with the postulation of
a puckered rather than planar conformation for cyclohexane, and
postulated furthermore, that this puckered form could exist in two
configurations--the chair and boat conformations. It wasn't until
1918, however, that Mohr realized that substituted cyclohexane in the

chair form could interconvert with the concomitant absence of

2

diastereomers. In 1928 X—ray diffraction studies verified the existence
of the chair form of cyclohexane, and in 1936 Kohlrausch demonstrated
the existence of two types of bonds in cyclohexane--axial and equatorial;
however, only in 1950 did Barton demonstrate the chemical consequences
of these two types of bonds. After the chemical implications of axial
and equatorial bonds were established, the scope of the field of
conformational analysis expanded tremendously, and in 1969 Barton and
Hassel were awarded a Nobel Prize for their work in this field.

Conformational analysis began with a concern for an explanation
of the relative energies of small— to medium-sized rings. Through
theoretical, physical, and chemical means the configurations of these
cyclic systems have become well-established. The conformation
assumed by a ring has been found to be that which affords the best
compromise in the minimization of Baeyer or angle strain, torsional
strain due to bond eclipsing of substituents, and van der Waal inter-
action of substituent atoms. A cyclopropane ring must be planar;

however, cyclobutane has been found to assume a puckered conformation, I,

by electron diffraction1 and spectroscopic and thermodynamic measurements.2

The puckered form is apparently favored slightly since puckcring

3
relieves torsional strain at the expense of a slight increase in angle
strain. Even though planar cyc10pentane would exhibit bond angles
of 108°, bond eclipsing causes the ring to assume a puckered configuration.
There are two puckered forms of higher symmetry than the other possible

ones--the envelope (CS), II, and the half-chair (C2), III. Substituted

II III

cyc10pentanes seem to prefer one of these two conformations. The
principal conformations of cyclohexane are the rigid form or chair, IV,

the flexible form or boat, V, and the twist form, VI. The chair form

IV

 

is the one of lowest energy since it is free of both angle and torsional
strain. The flexible form, V, is that of highest energy due to
torsional strain and van der Waal interaction of the hydrogen atoms
shown. The twist form is of slightly lower energy than the boat

due to a small decrease in torsional strain and the absence of the

van der Waal interaction of the aforementioned hydrogen atoms. The

last cycloalkane which is relevant to the compounds to be considered is
cycloheptane, which has two main conformations, the chair, VII, and

the boat, VIII, configurations which are similar to those assumed by

cyclohexane.

VII VIII

The conformations discussed thus far are the common ones associated

with the particular alkane. Substitution on the ring can greatly

alter the relative energy of a particular conformer because strain

is induced by the alteration of the valence angle which influences
Baeyer strain, by the eclipsing of bulky substituents, or by their
interaction. The environment in which a cycloalkane is found should
have an effect on its configuration; the magnitude of this effect
depends on the complexity of the ring's surroundings. The simple
replacement of one carbon atom by a heteroatom may or may not have a
noticeable effect dependent upon how much the valence angles are

altered. If two or more cycloalkanes are joined to form either fused

S

or bridged ring systems, conformational analysis becomes more complicated
due to the fact that ring juncture, particularly of small cyclic
compounds, forces the rings to assume certain shapes in order that
they be joined. If the effect of ring juncture is coupled with that
of substitution, in particular by bulky groups or by carbonyl groups
which cause part of the ring to be planar, the configurational
changes could become somewhat complicated. To assess the actual
conformational changes in such situations, a method which will
determine the configuration of the rings unambiguously must be used.
X-Ray diffraction is such a method and was chosen for this study.

Very little conformational information on heterocycles of less
than six atoms is available; but of that available the most seems
to exist for five-membered heterocycles. Recent electron diffraction
studies on tetrahydrofuran,3 (THF), tetrahydrothiOphene,4 (THS), and
tetrahydroselenophane,S (THSe), suggest that the latter two compounds
exist primarily in the C2 conformation while THF exists as a mixture
of conformers. The angle C-heteroatom-C decreases from SE: 107.5°
for THF to 93.4(5)° for THS to 89.1(5)° for THSe, while the bond
lengths C-heteroatom are 1.428(3), 1.839(2) and 1.975(3) A, respectively.
From this information it can be postulated that as the bond angle
decreases and bond length increases in this series, the half-chair
conformation is favored. In order to test this hypothesis a compound
which could have a smaller bond angle and larger bond length is
needed. Bis(triphenylphosphine)tetramethyleneplatinum(lI), IX, could
fit these requirements; however, in interpreting the results of an

X-ray structural investigation, the packing forces between molecules

t’\
./ /<j

IX

must be considered to insure that the conformation of the ring is not
a result of the packing of the bulky phOSphine ligands.

Conformational effects in extended ring systems become greater
as the number of atoms the cyclic structures have in common increases.
If there is only one common atom, little or no effect should be noted
on ring conformation except perhaps that due to substitution. However,
as mentioned previously, in fused (two atoms in common) and bridged
(three atoms in common) systems there should be a marked effect on
ring configuration due to the geometrical constraints of ring juncture.
A good illustration of this point is the bridged compound

bicyclo[2.l.l]hexane, X. In this compound the cyclobutane moiety

must be puckered in order to be joined through three atoms to the
cyc10pentane ring. Substitution also influences ring conformation in
bridged systems. For example, bicyclo[3.3.l]nonane cannot have both
cyclohexane rings in the chair conformation because of interaction

between the hydrogens at C and C7 (XI), thus one ring assumes the

3

7
boat configuration (XII). However, if one of the methylene groups

containing a troublesome hydrogen atom is replaced by a carbonyl

 

 

XI XII

group, both cyclohexane rings can now exist in the low—energy chair

conformation. Norbornane systems, XIII, which consist of two

XIII

cyclopentane rings sharing three atoms, are particularly strained
bridged systems, both rings being in the envelope, Cs’ configuration.
These rigid systems are also affected by substitution, particularly if
the substitution occurs at the non-common atoms. The norbornane
skeleton has been observed to twist6 in order to partially alleviate
the strain induced by substitution. The question arises as to how

the norbornane frame would shift to relieve strain induced by

substituents at each carbon atom when further rigidity is added to

8
the skeleton by the linking of the ends of each cyclopentane to form
two additional rings as in 2,3,6,7,7,8-hexamethy1tetracyclo-

2,8 03 6

[3.3.0 ’ ]octan-4-one, XIV.

/

 

IO
XIV

The requirements of ring juncture and substitution are not the only
factors which influence the conformation of rings in fused systems;
there is the added complication that ring juncture may be either £12.
or trans: For example, hydrindane exhibits both the SEE: XV, and

trans, XVI, isomers. Due to smaller van der Waal interactions,

XV XVI

equatorial substitution is favored over axial in cyclohexane, thus
the trans-isomer should be the preferred one; however, the trans—
form is more strained than the cis—form because only a slight flattening

of the cyclohexane ring accommodates juncture with the C2 or half-chair

9
form of cyclopentane. The van der Waal strain of the Eisfform and
torsional strain of the Egangfform tend to cancel one another with
the result that both forms have similar conformational energies;
thus,a small change in substitution may result in a conformational

change. If a substitution is made in the system to l-hydrindanone, XVII,

0*

XVII

for example, the cis-isomer becomes the lower energy form due to the
fact that the preferred conformation of cyclopentanone is C2 which
makes cis fusion easier. Whereas, the introduction of a double bond

into l-hydrindanone, XVIII, makes the trans-isomer more stable than

 

XVIII

the gi§_due to the flattening effect of the double bond on the chair
form of cyclohexane. Thus, small changes in substitution on and in
hybridization in the rings which comprise a fused system can alter

the conformation preferred by a cyclic moiety. As the basic hydrindane
system is extended to include more rings, in particular rings of fixed
geometry such as aryl groups, the conformations of all the flexible

cyclic components could be altered. A portion of the

10
6,6—dimethy1-2,3-benzo-2,4-cycloheptadienone photodimer molecule,

XIX, is related to hydrindane. A structural study of this compound

 

XIX

not only provides a means of further investigation of substitution
effects on the hydrindane system but also provides an opportunity to
study the configuration of a seven-membered ring in a complex fused
ring system.

In summary, a structural analysis of the three compounds, IX,
XIV and XIX, was undertaken to provide additional experimental data
on the conformational changes small- and medium-sized rings undergo
when one of these rings is either internally altered by replacement
of one carbon atom by a platinum atom or is placed either in a bridged

or fused ring system.

CHAPTER 2

THE CRYSTAL STRUCTURE OF

BIS(TRIPHENYLPHOSPHINE)TETRAMETHYLENEPLATINUM(II)

Introduction

 

Both concerted and nonconcerted mechanisms have been formulated
to account for the fact that certain transition metal compounds

catalyze olefin metathesis reactions (2-1) which are thermally

 

___ metal \ 1==:
2CH-2--—CHR\~catalyst CH2 CH2 + CHR==CHR (2-1)

forbidden by Woodward-Hoffmann rules of conservation of orbital

symmetry. In the concerted mechanism it is proposed that the new

carbon-carbon bonds are formed simultaneously7.11 (2-2) and that the
."““’“'I
2CH§:(:HR + M éFRJLMqLflFBer—fi
R R
(3-2)
‘T‘
M :2: M + CH=CH + CHR=CHR
| 2 2

/"‘““x
R R

function of the metal is to provide a template8 of atomic orbitals
through which electron pairs of the metal and ligand can interchange.
The nonconcerted mechanism involves a metallocyclic intermediate”.14
and can be represented by a scheme (2-3) in which intermediate

A rearranges to intermediate B: Evidence to support a non-concerted

mechanism with a metallocyclic intermediate is provided both by

11

12

ZCH'f—‘CHR + M(L) “22/.(Mlg :rjzsz ..__.

A

dRP/ILMAC :nmn + CH2_CH2 + CHRZ‘ZCHR

(L)n

(2-3)

)n
EL

preliminary structural data on an iridium compound involved in the

"symmetry forbidden" dimerization of norbornadiene15 and by the

reaction of labelled 1,4-dilithiobutane with tungsten hexachloride to
14

produce ethylenes with scrambled labels , (2-4). In the latter case

WClfi
Li—CH2—-CHD—CHD—CH-2——Li .‘—__T CH

2—-—CH2 +
(2-4)

CHDZZCHD + CHEZZCHD

the instability of the tungsten metallocycle prevented its isolation;
however, reaction of 1,4-dilithiobutane with group VIII bis(triphenyl-
phosphine)diha1ide compounds did produce isolatable compounds.16
Consequently, the structure of bis(triphenylphosphine)tetramethylene—
platinum(II) has been determined both to demonstrate further the
formation of metallocyclic species in these reactions and to determine

if conformational features which could indicate a probable mode of

rearrangement of the cyclic intermediate exist in the molecule.

Experimental

 

Treatment of cis-bis(triphenylphosphine)dichloroplatinum(II) with

one equivalent of 1,4-dilithiobutane produced the title compound16

13

(hereafter PLAT). Recrystallization of the purified product from a
mixture of hexane and benzene produced clear, gold crystals of
irregular shapes. Preliminary measurements of lattice parameters to
aid in crystal alignment and space group indications were effected
gig precession camera techniques with Zr-filtered Mo radiation.
Lattice parameters and intensity data measurements were made yia_a
computer-controlled, four-circle, Picker goniostat with Mo Ka
radiation (graphite monochromator, mounted adjacent to the X-ray
tube) at a temperature of 23(2)°. A crystal roughly triangular
(0.44 X 0.36 X 0.44 mm and SE: 0.15 mm at its thickest point) was
mounted parallel to a triangular face such that the [0,14,5] was
coincident with the phi-axis of the diffractometer. Cell constants
were obtained from least-squares refinement of 12 reflections which
had been hand-centered on the goniostat. The density was obtained
by flotation in mixtures of chloroform and carbon tetrachloride.
Pertinent crystal data are given in Table 1.

Two common scanning techniques exist for data collection:
the moving—crystal-stationary-detector or w-scan and the moving-crystal-
moving-detector or 20 scan. [The background is measured by a
stationary-crystal-stationary-detector technique in both cases.]
In the 26 scan the path of the scan lies along the reciprocal lattice
vector, whereas, in the w-scan the path of the scan is tangential
to the reciprocal lattice vector. When monochromatized radiation is
used, both techniques yield equivalent results. If, however, the
mosaic spread of the crystal is large so that the peaks scanned are

broad or the reflections are very close together due to large lattice

14

Table 1. Crystal Data for PLAT

-1
Pt[P(C6HS)3]2C4H8, M — 775.8 g mole

a_= 9.78(l), g. 17.75(2), g_= 9.66(1) A

g_= 94.06(4), §_ 102 74(7), 1_= 98.54(6)°

Systematic Absences: None

Space Group: PT, No. 2

§_= 2, F(OOO) = 772 e, 3.: 1608.2 A3

u = 47.1 cm’1 (Mo Ka)

D = 1 57(1), 0C = 1.60 g cm'3

exp alc
—_ O
1(Mo K0, graphite monochromator) = 0.7093 A

15

parameters, the w-scan is the better data collection method since
resolution of adjacent peaks is improved. The scan range is determined
experimentally by examination of reflections in several different
regions of reciprocal space. The range chosen is the shortest one
needed to include the major area of the most diffuse diffraction
peak. Ideally, the scan rate is chosen to provide good counting
statistics and minimization of crystal decay in the X-ray beam.

In this work the w-scan technique was used for data collection

with a scan range of 1°, scan rate of 0.5°/min, and a Ka ~Ka7

l
dispersion factor of 0.692. The counting system employed a scintillation
detector with pulse height discrimination to eliminate high and low
energy noise. Three-dimensional single-crystal intensity data were
collected in one hemisphere (hkz, hkl, bk?) and hkifi in two sets,

that of an inneresphere for which 3.5° < 20 < 20° and that of an
outer-sphere for which 20° < 20 < 50°, to a minimum d-spacing of

0.84 A. Individual background measurements were made at the endpoints

of the scan range for 10 sec each. Neither filters nor attenuators

were used, and three standard reflections, the 050, 023 and 144,

were checked every 200 reflections. About one-third of the way through
the data—collection process, the intensities of the standards

decreased markedly because the position of the monochromator had

changed. Readjustment of the monochromator brought the intensities

of the standards to 7% above the initial values. A correction factor

was applied in the program INCOR17 to bring all data to a similar

scale. After correction the variation in the intensities of the

standards was approximately 3%, indicative that the crystal had not

16

suffered radiation damage during data collection. A total of 476
and 6000 reflections were collected in the inner and outer sets,
respectively, excluding standard reflections.

The data were corrected for background and considered to be of
measurable intensity by the criterion I > 20(1) where I = P - B
and 02(1) = P + CB + [D(I)]2. In these equations P = 10(Ic) + 5,
(Ic = integrated peak count), B = C[10(IBl + IBZ + 1)], (I81 and
132 are integrated background counts) [factors of 10 arise because
of truncation in the data collection program], C the ratio of total
peak count time to total background count time and D a 2% instrumental
drift factor. An absorption correction was not attempted because
the irregular shape of the crystal with possible reentrant angles
made definition of the bounding planes virtually impossible. Further-
more, examination of 33 Friedel pairs in the inner-sphere data revealed
only five whose intensities were not within one standard deviation of
each other. The program INCORl7 altered to include a perpendicular
monochromator correction, was used to correct the data for Lorentz

and polarization effects.

Solution and Refinement of the Structure

 

Solution of the structure was effected by the heavy-atom method.
A Patterson synthesis was computed17 with the structure factors
derived from the inner sphere data. The function used in this

+++
calculation can be represented by (2—5), in which A(uvw) is the

. + + +
A(K‘A’m = lTZZZIFOIZethU + kv + 11w) (2-5)
V h k 2

17
average electron density product at points at the ends of a vector
whose components are (3,3,3), V is the volume of the unit cell, and
F0 is the observed structure factor. Peaks in the Patterson map
correspond to interatomic vectors with the heights of the peaks
proportional to the product of the number of electrons associated
with the atoms located at the endpoints of the vector. Since the
equivalent positions in space group PT are (x,y,z) and(§,y32), a
peak which corresponds to an intermetallic vector should appear at
(2x,2y,22) and (2§,2§,2E). The two strongest peaks, excluding the
origin peak, were found at (0.7,0.s,o.7) and (0.3,0.s,o.3). Because
there are two symmetry-related molecules of PLAT in the unit cell,
the atomic positions derived from only one Patterson peak are needed
to define the positional parameters of the metal atoms. The
platinum atom was placed at (0.35,0.25,0.35). After two cycles of
full—matrix, least-squares refinement,17 the reliability factor,
which was the function minimized, decreased to 0.377. This factor is
defined as [2w(AF)2/£w(FO)2], where AF is the difference between the
observed structure factors (F0) and those calculated from the known
structural fragment (Fc)’ and w is the weight associated with F0.
With unit weights, 133,, w = l, the reliability factor is represented
by R, and if w f l, R". The weights used in this structure are based

on counting statistics and are defined in equation 2-6.

w = l/ozp, OF = o(I)F/ZI (2-6)

18
Once the positions of the heavy atoms are known, there are two
methods commonly used to determine the remaining light atom structure.
The first of these is the FO Fourier synthesis, which is calculated

from (2-7) where p(x,y,z) is the electron density at (x,y,z). Since

1 -2ni(hx + ky + £2)

o(x.y.z) =V12; 12. 2 F e
'l

0 (2-7)

both the magnitude of F0 and its associated phase angle are required

in the Fourier calculation and since only the magnitude is observable,
the heavy-atom structural fragment is used to calculate a second set

of structure factors, Fc' The phase angles of the Fc's and magnitudes
of the Fo's are then combined to calculate the F0 Fourier map from
(2-7). If there is a large discrepancy between the number of electrons
possessed by the heavy atoms and that by the light atoms, the heavy-
atom peaks overshadow those of the lighter atoms. However, if the large
peaks are subtracted out, the peaks which correspond to the missing
atoms become more distinct. This subtraction process corresponds to

the computation of a difference Fourier synthesis, (2-8), which is

e-2ni(hx + ky + £2)

Ao(X.y.Z) = % (2-8)

the other method used to determine light-atom structures. But,

since spurious peaks appear in difference syntheses due both to the
incompleteness of the model and to series termination effects in the
Fourier summation (effects due to the restriction of the data to a
finite rather than an infinite set), the two techniques are best used
in tandem, with the Fo Fourier maps used to verify the peak positions

in the difference maps. Since the heavy-atom model of the PLAT

19
structure consisted only of two platinum atoms, both PC and difference
Fourier maps were computed to determine the light-atom structure.
Several cycles of Fourier calculations followed by least-squares
refinement of the accumulated atomic coordinates (and isotropic
thermal parameters) were necessary to effect the solution of the
light-atom structure.

Before much of the light-atom structure had been determined,
the graphite monochromator was found to have been set to diffract
from the (004) rather than the (002) plane. The net result of this
error is that instead of monochromatic radiation of wavelength A,
there will be radiation of two wavelengths, A and X/Z. This second
wavelength arises from the combination of the substantial white
streak in molybdenum radiation with the strongly diffracting (002)
plane of graphite, since when the (004) is in position to diffract

Mo Ka radiation, the (002) will diffract a white radiation component.

1
The presence of the two wavelengths causes a special type of multiple
reflection, such that whenever a (h'k'i') plane is in the diffracting

position, any (hkz) plane related to it by d 2d

hkl = h'k'l' will also

be in a diffracting position, and thus the intensity of the (h'k'R')
plane may be in error. As a result, the intensity of any reflection
with hkz all even was suspect, and 776 such reflections were deleted
from the data set. Sufficient data remained (3505 unique reflections)
to determine the structure (SE: 18 reflections per parameter), so

the solution process was resumed after Lorentz and polarization factors

had been recalculated with the proper monochromator interplanar

d-value (that corresponding to the (004) reflection plane).

20

A set of the 1684 strongest reflections was selected from the
data for use in the procedure outlined above, and all nonhydrogen
atoms were located successfully. The structure was refined by a
full-matrix, least-squares procedure17 with the molecule divided
into three sections each of which included platinum and phosphorus
atoms: (1) four carbon atoms in the tetrahydroplatinole ring,
'(2) all the carbon atoms in one phosphine group,(3) all the carbon
atoms in the other phosphine group. With only the platinum and
phosphorus atoms treated anisotropically, R reduced to 0.064 and
Rw to 0.065 for the 1684 strongest reflections. When all 3505
reflections were used with the same parameters, R = 0.077. Examination
of the Fo's at this point revealed one reflection, T014 for which
the initial intensity had been mispunched; it was deleted. The
planarity of the phenyl rings was checked with the program LSQPLN18
and the greatest deviation from planarity was found to be 5%.

The final step was location of the hydrogen atoms. Examination
of a difference Fourier map indicated large peaks and valleys, of
the order of 4-5 e A73, surrounding the platinum atom position and
apparently due to series termination effects in the Fourier summation.
Therefore, the hydrogen atom positions were determined by geometrical

19

considerations. The phenyl hydrogen atom positions were calculated

from (2-9), where r”, rCB, and rCA

coordinates of a hydrogen atom, the carbon atom to which it is bonded,

represent orthogonal Angstrom

 

C 2:8 (rC ‘ rC ) (3'9)

21
and the carbon atom para_to CB, respectively. The hydrogen atom
positions on the methylene carbon atoms were calculated from tetrahedral
geometry and a carbon-hydrogen bond length of 1.10 A. Refinement of
the hydrogen atom positions was effected by recalculation of their
parameters after each of four cycles of least-squares analysis on
the corresponding carbon atoms. The final unweighted and weighted
R values were 0.074 and 0.068, respectively, for the 3504 independent
reflections with I > 30(1). Final shifts in all parameters were
less than 1% of their estimated standard deviations; negative and
positive residual peaks of the order of 1.7(1) e R-3 were arranged
symmetrically around the platinum atom in the final difference Fourier
map and were attributed to series termination effects in the Fourier
summations.

Cromer and Waber's20 scattering factors for neutral atoms were
used for Pt, P, and C while that for hydrogen was taken from the
International Tables.21 The real and imaginary parts of the anomalous
scattering of platinum and phosphorus22 were included in the calculated
structure factors. A correction for secondary extinction did not
seem necessary and so was not included.

The final positional and thermal parameters are given in Table 2.
Since the absolute weights associated with the observed structure
factors are unknown, only estimated standard deviations can be

obtained for these parameters.23

These estimated standard deviations
were calculated from the inverse matrix of the final least-squares

cycle. The hydrogen atom numbers match those of the carbon atom to

22

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26

which the hydrogen is bonded. Observed and calculated structure
factor magnitudes are listed in Table 3.

Selected bond distances and angles presented in Tables 4 and S,
were calculated with the program ORFFE.24 A stereoscopic view of a
single molecule (produced by the program ORTEPZS) is presented in
Figure l and that of the packing of four molecules in the front half
of four unit cells in Figure 2. A 20% probability ellipsoid as shown
in the ORTEP drawings represents an equiprobability surface of thermal

displacement and contains 20% of the probability distribution, 1:3,,

20% of the thermal motion resides within the boundaries of the ellipsoid.

Discussion

 

The molecular configuration is essentially a square plane, consisting
of the platinum atom bonded to two phosphorus atoms and two carbon
atoms, the latter connected by a two carbon bridge. The Pt-P bond
distances of 2.279(5) and 2.285(6) A in PLAT may be compared with
values of 2.29(1) and 2.30(1) A in Pt[P(C6HS)3]2(C2(CN)4),26 2.25(l)
to 2.28(l) A in Pt[P(C6H5)3]3,27 and 2.24(l) and 2.35(l) A in
Pt[P(C6H5)3]2CSZ.28 The Pt-P bond length in PLAT is shorter than
that which would be predicted from the usual sum rules for a normal
covalent bond and probably reflects slight n-interactions. The
Pt-C bond distance of 2.05(2) and 2.12(2) A may be compared to values

0
of 2.10(3) and 2.11(3) A in Pt[P(C6H5)3]2C2(CN)4,26 2.105(17) in

9 and 2.04(3) A in Pt(Cl)(0Me)(1,5-C8H12)(py).30

[Pt(acac)2Cl]-,2
It is interesting to compare the Pt—C bond distance in PLAT to those

0
in o-allyl complexes: l.95(5) and 2.00(5) A in [Pt2(acac)2(ally1)2]31

O
and 1.9S(8) A in PtBrSOEt(oA)232 where 0A is o-allylphenyldimethylarsine.

27

Table 3. Observed and Calculated Structure Factor Magnitudes for PLAT

   

      

 

   

           

 

 

 

   

   

 

 

   

     

    

 

 

         

 

 

 

 

 

 

     

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28

Table 3 (Continued)

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29

Table 4. Bond Distances Within PLAT.

E.s.d.'s appear in parentheses

O
Atoms DiSt. A Atoms

(A) Distances from Central Platinum Atom

Pt-P(1) 2.279(5) Pt-C(37)

Pt-P(2) 2.285(6) Pt-C(40)

(B) Phosphorus Phenyl Carbon Distances

P(1)-C(l) l.83(2) P(2)-C(19)
P(1)-C(7) l.80(2) P(2)-C(25)
P(1)—C(13) 1.82(2) P(2)-C(31)

Mean P-C(Pheny1)a

(C) Carbon-Carbon Distances Within C4H8 Ligand

C(37)-C(38) 1.S7(3) C(39)-C(40)

C(38)-C(39) 1.45(3) Mean c—ca

(D) Carbon-Carbon Distances Within the Phenyl Rings

C(1)-C(2) 1.41(3) C(19)-C(20)
C(2)-C(3) 1.41(4) C(20)-C(21)
C(3)-C(4) ‘ 1.38(S) C(21)-C(22)
C(4)-C(5) 1.36(3) C(22)-C(23)
C(S)-C(6) 1.39(3) C(23)-C(24)

C(6)-C(l) 1.39(3) C(24)-C(19)

O
Dist, A

.12(2)

.05(2)

.83(2)
.84(2)
.82(2)

.82(1)

.48(3)

.SO(6)

.35(3)
.39(3)
.43(3)
.39(3)
.34(3)

.35(3)

Table 4 (Continued)

Atoms

C(7)-C(8)
C(8)-C(9)
C(9)-C(10)
C(10)-C(ll)
C(11)-C(12)

C(12)-C(7)

C(13)-C(14)
C(14)-C(15)
C(15)-C(16)
C(16)-C(17)
C(17)-C(18)

C(18)-C(13)

l.

O
Dist, A

37(2)

.40(3)
.37(3)
.35(3)
.41(3)

.39(3)

.39(3)
.38(3)
.28(3)
.40(3)
.43(3)

.37(2)

30

Atoms

C(25)-C(26)
C(26)-C(27)
C(27)-C(28)
C(28)-C(29)
C(29)-C(30)

C(30)—C(25)

C(31)-C(32)
C(32)-C(33)
C(33)-C(34)
C(34)-C(35)
C(35)-C(36)
C(36)-C(31)

Mean C-C

O
Dist, A

1.37(3)
1.40(3)
.36(4)
.31(4)
.43(3)

.35(3)

.39(4)
.44(4)
.34(4)
.32(4)
.40(4)
.39(3)

.38(4)

aE.s.d.'s for mean bond distances are calculated from the equation

0 = [[z::T (xi -‘i)2]/(N - 1)]1/2, where xi is the ith bond length and

‘Y is the mean of the N equivalent bond lengths.

31

Table 5. Bond Angles Within PLAT.

E.s.d.'s appear in parentheses
Atoms Angles Atoms

(A) Angles Around Platinum Atom

P(])-Pt-P(2) 98.8(2)° P(2)-Pt-C(37)
P(l)-Pt-C(37) 173.4(6) P(2)-Pt-C(40)
P(l)-Pt-C(40) 93.9(5) C(37)-Pt-C(40)

(B) Angles Around PhOSphorus Atoms

Pt-P(l)-C(l) 118.3(6) Pt-P(2)-C(19)
Pt-P(1)-C(7) 111.8(6) Pt-P(2)-C(25)
Pt-P(l)-C(13) 113.8(6) Pt-P(2)-C(31)
C(1)-P(1)-C(7) 100.8(9) C(19)-P(2)-C(25)
C(7)-P(l)-C(13) 111.1(8) C(25)-P(2)-C(31)
C(l)-P(l)-C(13) 99.9(9) C(19)-P(2)-C(31)
(C) Angles Within the C4H8 Ligand

Pt-C(37)-C(38) 104.5(14) C(38)-C(39)-C(40)
Pt-C(40)-C(39) 116.1(15) C(37)-C(38)-C(39)

(D) Angles Within the Phenyl Rings

C(IJ-C(2) -C(3) 116.6(26) C(19) -C(20)-C(21)
C(2)-C(3) -C(4) 123.5(29) C(20)-C(21)-C(22)
C(3)-C(4)-C(5) 118.3(24) C(21)-C(22)-C(23)

C(4)-C (5)-c(6) 121.3(22) C(22)-C(23)-C(24)

Angles

86.

167.

80.

124.

115.

110.

99

103.

101

107

110.

123.

118.

114.

124.

5(6)°
2(5)

9(8)

0(6)
6(7)

1(7)

.4(8)

8(9)

.6(9)

.3(18)

6(18)

7(23)
6(21)
1(19)

5(24)

Table 5 (Continued)

Atoms

C(5)-C(6) -C(1)
C(6)-C(l)-C(2)
C(7)-C(8)-C(9)
C(8)-C(9)-C(10)
C(9)-C(10)-C(ll)
(:(10)-C(11)-C(12)
C(11)-C(12)-C(7)

(I(12)-C(7)-C(8)

C(13) -C(14)-C(15)
C(14)-C(15)-C(16)

C(lS)-C(16)-C(17)

C(16)-C(17)-C(18)'

C(17)-C(18)-C(13)

C(18)-C(13)-C(l4)

Angles

120.

119.

119.

120

119

121

118.

120.

123.

118.

123

118.

118.

117

4(19)
9(20)

8(19)

.4(18)
.8(23)

.3(23)

5(17)

3(19)

3(16)

5(21)

.4(24)

1(19)

9(19)

.8(18)

3
See footnote a to Table 4.

32

Atoms

C(23)-C(24)-C(19)
C(24)-C(19)-C(20)
C(25)-C(26)-C(27)
C(26)-C(27)-C(28)
C(27)-C(28)-C(29)
C(28)-C(29)-C(30)
C(29)-C(30)-C(25)

C(30)-C(25)-C(26)

C(31)-C(32)-C(33)
C(32)-C(33)-C(34)
C(33)-C(34)-C(3S)
C(34)-C(35)-C(36)
C(35)-C(36)-C(31)
C(36)-C(31)-C(32)

Mean

Angles

121.

117

118.

118.

124

117

118.

122.

116.
120.

123.

118

120.

120.

120.

1(19)

.4(17)

4(20)

1(23)

.8(24)

.8(22)

5(19)

1(19)

9(23)
4(27)

2(28)

.9(26)

3(23)
1(22)

0(24)3

33

 

Figure 1. Stereoscopic view of PLAT.

 

Figure 2. Stereoscopic view of the packing of 4 molecules of PLAT

in the front half of four unit cells.

34
‘ o
The P-C6H5 bond lengths average l.82(1) A, a value comparable to the

0
sum of the covalent radii, 1.87 A,33 and to values reported elsewhere,

°. 34
for example, 1.835(13) to 1.850(14) A in (C3H5)2Ru[P(C6HS)3]2

O
and a mean P-C6H5 distance of 1.83(1) A in RhH(CO)[P(C6H 5

S)3]3'3
The average C-C bond length of 1.38(4) A in the phenyl rings compares
well with the normal value of 1.394(5) 1.36 The c-c bond distances
in the tetrahydrOplatinole ring range from 1.4S(3) to 1.S7(3) A
compared to a single C-C bond length of 1.537(5) A.36 In view of the
magnitude of the estimated standard errors, the differences in the
three C-C bond lengths is probably insignificant.

The square planar geometry around the platinum is distorted.
The degree of distortion can be expressed quantitatively by the angle
of 175(1)° between the planes through the atoms (Pt,P1,P2) and
(Pt,C40,C37), and,compared to an angle of 0(l)° (between the plane
defined by the Pt and two C atoms of the cyclopropene ring and that
by the Pt and two P atoms) in the related compound Pt[P(C6H5)3]2CSH10,37
the degree of distortion is significant. A second way to visualize
the distortion from square planar geometry is to examine the placement
of the remaining atoms around the platinum with respect to these two
planes. The relationship of the four carbon atoms in the tetrahydro-
platinole ring to the platinum-phosphorus plane is that C37 and C39 are
above the plane (0.15(2) and 0.02(2) A, respectively) and C38 and C40
are below it (0.49(2) and 0.05(2) A, respectively). In turn the
relationship of the phosphorus atoms and the carbon atoms,
C38 and C39, of the tetrahydroplatinole ring to the platinum-
C37-C40 plane is that P1 is 0.165(5) A above the plane while P2,
C38, and C39 are below it by 0.098(5), 0.62(2), and 0.002(16) A,

reSpectively. A third measure of the distortion from square planar

35

geometry is the angles around the platinum atom. The Pl-Pt-PZ and
C37-Pt-C40 angles are 98.8(2) and 80.9(8)°, respectively. A sizable
difference would be expected since the angles involve different types
of atoms. But one would expect the Pl-Pt-C40 and P2-Pt-C37 angles to
be comparable since they do involve the same types of atoms; they
were found to be 93.9(5) and 86.5(6)°, respectively. The 7.4°
difference is well outside the estimated standard deviation (120).
Finally the angles Pl-Pt-C37 and P2-Pt-C40 are 173.4(6) and 167.2(5)°,
respectively, markedly different from the 180° angle expected for
square planar geometry.

The geometry around the phosphorus atoms is, as expected,
tetrahedrally distorted, the angles around P1 range from 99.9(9) to
118.3(6)°, those around P2 from 99.4(8) to 124.0(6)°. The phenyl groups
are planar; the deviations from the best plane through the carbon
atoms of each ring range from 0.001(15) to 0.040(14) A, while the
greatest deviation of a hydrogen atom from the associated phenyl
plane is 0.04(3) A.

The conformation of the tetrahydroplatinole ring can be
determined in two ways. In the ideal enve10pe (Cs) configuration
four atoms are in a plane with the fifth atom 0.75 A from that plane,
while in the ideal half-chair conformation (C2) only three atoms are in
a plane with the remaining two atoms 0.39 A above and below the plane.38
In the tetrahydroplatinole ring, the maximum deviation from the best
plane through atoms C37, C39, C40 and Pt is 0.007(17) A, and the
fifth atom C38 lies 0.61(2) A below this plane. On the basis of these

data one can argue that the ring exhibits the envelope conformation.

36

A second method to determine conformation is to examine the dihedral
angles in the ring. The dihedral angle, w, around a bond, 3,5,,
C38-C39, can be determined by looking down this bond such that C38
superimposes on C39; the magnitude of w is then the angle the front
bond C37-C38 makes with the back bond C39-C40, and the sign is negative
if the angle is taken clockwise from the front bond to the rear bond

and vice versa, 1.39 The dihedral angles for the half-chair,

 

39,40
envelope,

and tetrahydroplatinole rings are shown in Figure 3.

As can be seen from the diagram, the dihedral angles of the metallocycle
resemble those of the envelope rather than the half-chair form.

[The discrepancy in signs arises from C2 being above the plane of the
other fOur atoms, while C38 is below the plane in the metallocycle.]

The two sets of dihedral angles are remarkably similar if the
dissimilarity of the bond angles and bond distances is considered.

The tetrahydroplatinole ring is, thus, in a deformed envelope

configuration. A close examination of the packing of molecular units

37

.3 .. 7 \i.»

4

 

 

half-chair envelope

C2 CS

638
/ «a
c <39
P, 0

C40

tetrahydroplatinole ring

 

Figure 3. Dihedral angles calculated for the ideal half-chair and
envelope conformations and observed for the tetrahydro-

platinole ring in PLAT.

38

(Figure 2) indicates that this ring conformation does not result
from packing forces.

The geometry of the tetrahydroplatinole ring provides an
indication of the mechanism whereby rearrangement of the metallocycle
intermediates in the olefin metathesis reaction occurs. Two reasonable
analogous mechanisms which fit the requirements of the observed

geometry are represented in (2-10) and (2—11). The former illustrates

R R R R R
T: /H _' R (2-10)
M =C M
(1.1,, (3).. \H (L),
R'
(2-11)
R

 

II
(1.1.. 0.1,.

a nonconcerted reaction path with a carbene intermediate; whereas,
the latter represents a concerted process in which two bonds are
forming while two others are breaking. In metals with readily

10,41

accessible higher coordination numbers, the above mechanisms

could provide a mode for facile rearrangement.

CHAPTER 3

THE CRYSTAL STRUCTURE OF

28 3,6

2,3,6,7,7,8-HEXAMETHYL-1,5-DIPHENYLTETRACYCLO[3.3.0.0 ’ .0 ]—

OCTAN-4-ONE

Introduction

 

The existence of a novel intermediate, (CH)5+, with pyramidal
geometry was predicted in the carbonium ion rearrangements of the
tricyclo[2.1.0.02’5]pentane system.42 Subsequently many reports“)-45
appeared concerning (CH)5+-type carbonium ions as intermediates in
other carbonium ion rearrangements. In one case Hart and Kuzuya46
reported on the basis of spectroscopic data and various labelling
experiments that the tetracyclic alcohol (I,R=H) ionizes in fluoro—

sulfonic acid at temperatures below ~50° to the pyramidal cation (11),

7
a (CH)5+-type carbonium ion. This ion then rearranges to III4' above

 

I, Ran II III

39

40
~50°. However, they found that if R=CH3, the intermediate in the

above reaction was the bicyclo[3.2.l]octadieny1 cation (IV)48

A .
I (R=CH3) fl . III (R=c113)

rather than the (CH)5+-type ion. In the first ionization, formation

of the cation involves participation of a bond in the cyclopropane ring,
while in the second,I(R=CH3)+IV, a 1,2-shift of a bond in the cyclo-
butane ring is involved in the initial step of the rearrangement (3-1).
Thus, different structural features of the molecule are involved

in these two ionization processes.

 

41

The geometry of the highly strained tetracyclic species, I, is
unknown. Both X-ray structural studies49 and theoretical calculations50
on the related bicyclic norbornyl systems have shown that the endo-
cyclic angles in the five-membered rings are smaller than the normal
tetrahedral value. Altona and Sundaralingam6 have found both by
observation and by theoretical calculations that these bicyclic
systems adapt themselves to strain induced by substitutions through
subtle changes in ring conformation. However, these workers did not
investigate the effect on ring conformation due to the linking of the
two five-membered rings as exemplified in I.

The crystal structure of a species closely related to l,

2,8.03,6

2,3,6,7,7,8-hexamethyl-l,5-dipheny1tetracyclo[3.3.0.0 ]octan-

4-one, V, has been undertaken to determine the molecular parameters

 

in order to: (l) establish additional evidence for the existence of
(CH)S+-type carbonium ions, (2) provide an explanation for the chemistry

of I, and (3) demonstrate the effect of strain in the tetracyclic system.

42

Experimental

 

A sample of 2,3,6,7,7,8-hexamethy1-l,S-diphenyltetracyclo-

[3.3.0.02’8.03’6

]octan-4-one (HDTO) was supplied by Professor H. Hart.
Recrystallization51 from petroleum ether produced clear, colorless
crystals in the form of flat plates. Preliminary measurements of the
lattice parameters and space group determination were effected as
described previously. Subsequent measurements of the crystal para-
meters were made via_the computer-controlled, four-circle goniostat
(described in Chapter 2) at a temperature of 23(2)°. The crystal was
roughly a rectangular prism (0.14 X 0.18 X 0.33 mm) mounted with the
long dimension [100] parallel to the phi-axis of the goniostat.
Cell constants were obtained from least-squares refinement of 12
reflections which had been hand-centered on the goniostat. The
density was determined by flotation in aqueous potassium iodide.
Pertinent crystal data are given in Table 6.

Three-dimensional, single-crystal intensity data in one
quadrant (hkz, hki) to the limit 20 = 45°, which corresponds to a
minimum d-spacing of 0.93 A, were collected as described previously
by the w-scan technique with a scan range of 0.8°, scan rate of

O.5°/min, and a Ka -Ka2 dispersion factor of 0.692. The standard

1
deviation from the average intensities of two periodically monitored
reflections, 060 and 401, was 3.0%, indicative that the crystal had

not suffered radiation damage during the eight days of data collection.
A total of 3010 reflections was collected, exclusive of standards.

The data were corrected for background and considered to be of

measurable intensity by the criterion I > 30(1) (see Chapter 2 for

43

Table 6. Crystal Data for HDTO

C26H280, M = 356.51

a_= 9.155(3), b_= l4.635(9), 2.: 15.425(4) A
8.: 100.7(2)°
Systematic absences: hpfi, &_= Zn_+ l; 050, k_= Zn_+ 1

Space Group: P21/c, No. 14

 

z_= 4; F(OOO) = 768 e; v_= 2030.7 A3

n = 0.75 cm’1 (Mo Km)

0 = 1.155(2), 0C = 1.166 g cm‘3

exp alc
A(Mo Kc, graphite monochromator) = 0.7093 A

 

44

definitions). An absorption correction was considered unnecessary in
view of the magnitude of u. The data were corrected for Lorentz and
polarization effects (see Chapter 2 for details). After Friedel
pairs and equivalent reflections had been averaged 2675 independent
reflections remained with 1577 of these with intensities greater than

30(1).

Phase Determination

 

Since HDTO does not contain a heavy atom, the solution of the
structure, or more precisely the determination of the phase angles
associated with the observed structure factors can be obtained by
direct method procedures. Since the phase angle in a centrosymmetric
space group can be only 0 or n (which corresponds to structure factor
signs of + or -, respectively), one might think that the phase angles
could be assigned randomly. But the random assignment of signs to only
ten structure factors would mean calculation and examination of
210 Fourier maps! Thus, a more systematic method is needed and is
provided by the symbolic addition procedure developed by Karle and
Karle.52 This method is based on the £2 equation represented in

general terms in (3-2), where 5 means the "sign of", E is the

E B (3-3)

0;
5E N s 5 2:3-

2
h .15.
normalized structure factor-~a structure factor for a system of point
atoms, and h, k, and hfk_represent Miller index triplets.

The normalized structure factors, E's, needed to utilize the £2

relationship are computed from the observed structure factors through

45

(3-3), in which 6 is a number which corrects for space group extinctions,

2
[~12 = J-F-l— (3-3)

0
If; is the sum of the atomic scattering factors over all the atoms,
and F is a structure factor on an absolute scale at 0 K (all thermal

notion removed) derived from (3-4). In (3-4) K is an overall scale

2 e-ZBsinZB/Az

2 2
IFI = (UK) Fobs (3-4)

factor, B an overall temperature factor and A the wavelength. The
overall scale and temperature factors are determined from a Wilson

0 2
plot which is a plot of 1n(<lFobsI>2/2(fi) ) versus the average of

sinze (see 3-5) in the sine/A interval over which the Fobs's were

2

(IFobs|> ZBsinZO

ln -———————§—' = In K - -_——7T—-
2(fi) A

(3-5)

averaged. The physical basis for the plot is seen more easily from

a rearrangement of (3-5) to (3-6). Equation 3-6 is deduced from

2 2 e-(2Bsin20)/12

= K£(f;) (3-6)

the knowledge that the structure factors are proportional to the sum

of the scattering factors of all the atoms in the unit cell, (3-7).

2 2

<|F = Kzfi (3—7)

obsl>

However, the situation is more complicated because the scattering power
of an atom is not only a function of the number of electrons associated

with the atom, but also a function of their spatial distribution. Large

46

thermal vibrations are indicative of a large electron cloud, and a
scattering power which falls off rapidly. Thus the actual scattering

power of an atom, fi’ is represented by (3-8), in which f; is only

f. = f? 6-851n0/A

1 1 (3-8)

a function of number of electrons. Combination of (3-7) and (3-8)
yields (3-6) on which the Wilson plot is based.

A statistical distribution of the intensities of the E's is
examined for an indication of whether or not the crystal is centro-
symmetric since theoretically this distribution depends only on the
presence of a center of symmetry. Theoretical values of a statistical
intensity distribution of E's based on a random distribution of atoms
in centric and acentric cells are shown in Table 7, together with values
observed for HDTO.

The E's calculated from (3-3) can now be subjected to a symbolic
addition procedure.52 One of the computer programs available to perform
this task is MULTAN,53 which consists of three main sections. In the
first section all the £2 relationships--all the combinations of k_and
hit for a given h_(see eq. 3-1) for as many h_reflections as needed to
effect a phase solution, usually 8-10 reflections per nonhydrogen
atom-~are set up, each with an associated weight, (3-9). [Zi in (3-10)

is the atomic number of the ith atom in the unit cell.] The next

3/2
N n
o = 22. (3-10)

Table 7.

Quantity

<|E|>
2

<5)

(IE2

.. 1|)
IEI > 1
[El >2

[El > 3

Plot for HDTO

Calculated

 

HDTO

_

0.819

1.027

0.960

32.80

4.93

0.36

47

Statistical Distribution

of E's Calculated from the Wilson

Centric

0.798

1.000

0.968

32.00

5.00

0.30

Theoretical

 

Acentric

.886

.000

.736

.00

.80

.01

48
section of MULTAN, CONVERSE,54 has two functions: (1) to apply the

£1 formula (3-11), and (2) to find the best reflections to use for

s{E 2l-l} (3-11)

'11
2h2k2£} “ S{|Ehk£

defining the origin. The first function of CONVERSE is accomplished

by use of the Z formula and the associated probability formula,

1
(3-12) (see (3-10) for the definition of Oi)’ both of which must be

C53
2
P+{E } = 1/2 + l/2tanh— 3/2 E2h2k2£(IEhk£I - 1) (3—12)

02
modified to take into account space group symmetry. The former yields
indications for the signs of the centric reflections (which are
structure invariant) and the latter the probability of the signs being
correct. The second function--that of finding the best reflections to
use for defining the origin--is performed by the definition of an

for each reflection either by (3-13) (th is defined by 3-9) if

“i

=[fith°°S(¢k* ¢h_ k) )12 + [:thsinc¢k+ 4h_ 112 (3-13)

some phase information is known or by (3-14) (where I1 and ID are

modified Bessel functions) in the absence of any. Since ah gives a

11(K1315)11(Kh(h-£))

 

' 3,131.: 0. [‘15 0 .01-1)
measure of the reliability with which the phase eh may be determined
from the tangent formula (to be defined below), the ah's for the

reflection list, which is identical to that chosen for the 22 listing,

are scanned, and the reflection with the lowest ah, that is, the most

49

unreliable reflection, is eliminated along with all the phase
relationships in which it is involved. All ah's are then recalculated,
and the least reliable reflection and all phage relationships involving
it are again deleted. The process is repeated through the reflection
list with the result that the last reflections eliminated are those
whose phase relationships are known most reliably. The origin-
defining reflections and any others needed to provide sufficient £2
relationships to give the phase determining formula a good starting

set are selected from these more reliable reflections.

The final section of the program, FASTAN, determines a set of
phases for each set of reflections (the origin reflections plus any
others needed, the signs of the latter being specified arbitrarily)
chosen by CONVERGE via the weighted tangent formula (3-15), where the

"i"i-8'EiEi-1'Si"‘¢l ' “ll-1) ‘1

73M

F

taneh = (3-15)

wEwETEJEEEh_£)cos(¢E - ¢h-k) Bh

WM

weight associated with the phase ¢h is given in (3-16) and (3-17).

w = tanh(1/2ah) (3—16)

2 2
ah = [Eb—[(Th - Bh) (3-17)

Each set of phases has a number of figures of merit associated with it
to help minimize the number of E-maps which must be examined. That
chosen for use in this study is called the absolute figure of merit,

ABS FOM, which should theoretically equal 1.0 for the most probable

50
set, but may in practice be higher due to the method of calculation.

It is defined by (3-18) where Ede is the sum of the estimated a's

Z -Xa
Ila-11 r

ABS FOM (3-18)

 

201 201
e r

from equation (3-14) and Ear = 3(EKZBB)—- the value of Zeb with the
assumption of random phases. '—'_

The phase determination for HDTO was commenced with the program
FAME,SS which was used to calculate a Wilson plot of seven intervals
of sinze, the 5'5, and the intensity distribution (Table 7) from the
observed structure factors of HDTO. The intensity statistics of the
E's reveals the centric distribution required by space group P21/c .
The 272 largest E's from FAME were processed through MULTAN. Origin-
defining and other reflections used to initiate the phase determination
are listed in Table 8. Four sets of phases were calculated by MULTAN
of which only one had an ABS FOM near 1.000 (0.992). It proved to be
the correct one since the largest 27 peaks in the corresponding E-map
revealed the positions of all nonhydrogen atoms. A list of these

peaks with their associated peak heights relative to a maximum height

of 999 is given in Table 9.

Refinement of the Structure

 

Three cycles of full-matrix least-squares refinement17 of
positional and isotropic thermal parameters of the nonhydrogen atoms
yielded an R-value (Chapter 2) of 0.132. A difference Fourier map
(2-8) calculated17 at this point contained peaks which corresponded to

all 28 hydrogen atoms, the largest peak height was equivalent to

51

Table 8. Starting Set of Signs for HDTO

Origin-specifying
reflections

21 results

Reflections whose
signs were permuted

A |::r'
l7:
|z<>

M

on c> n1 n1 ea
owlowlo

N
“4

In:

3.44

3.41

3.11

2.78

3.42

3.39

52

Table 9. Positions and Relative Heights of Assigned Peaks in the

E-Map for HDTO

Atom x y 2 Peak Height
C(l) 0.55 0.17 0.30 899
C(2) 0.58 0.19 0.40 802
C(3) 0.69 0.12 0.45 703
C(4) 0.65 0.04 0.38 801
C(S) 0.66 0.09 0.30 880
C(6) 0.80 0.14 0.38 667
C(7) 0.80 0.25 0.36 866
C(8) 0.63 0.25 0.34 758
C(9) 0.57 0.35 0.32 635
C(10) 0.87 0.26 0.28 734
C(11) 0.87 0.31 0.42 674
C(12) 0.44 0.21 0.45 542
C(13) 0.75 0.12 0.54 425
C(14) 0.95 0.09 0.40 635
C(15) 0.69 0.04 0.23 999
C(16) 0.65 0.08 0.14 839
C(17) 0.69 0.05 0.07 676
C(18) 0.76 -0.03 0.08 782
C(19) 0.81 -0.08 0.16 623
C(20) 0.78 -0.03 0.23 787
C(21) 0.41 0.16 0.25 888
C(22) 0.35 0.22 0.18 542

C(23) 0.19 0.21 0.14 464

Table 9 (Continued)

Atom

C(24)
C(25)
C(26)

0(1)

0.11

0.19

0.32

0.61

53

0.14

0.07

0.08

-0.04

0.14

0.21

0.26

0.40

Peak Height

599
696
763

804

S4

0.6(1) e A's. The positions and peak heights (relative to 999) which
were assigned as hydrogen atoms are listed in Table 10. Cycles of
refinement were then carried out alternatively, first on the carbon
and oxygen atom positions and their anisotropic temperature factors,
then on the hydrogen atom positions with their isotropic thermal
parameters fixed at 7.00 A2. The least-squares refinement of the
hydrogen atom positions proved unsuccessful; thus, further refinement
of these parameters was effected by recalculating the hydrogen atom
positions after each 2 cycles of refinement on the carbon and oxygen
atom parameters. The phenyl hydrogen atom positions were calculated
from (2-9); the methyl hydrogen atom positions from tetrahedral
geometry (with the hydrogen atom position associated with the highest
peak height used as one reference point) and a carbon-hydrogen bond
length of 1.10 A. An additional six cycles of refinement with carbon
and oxygen atom positional and anisotrOpic thermal parameters varied
and hydrogen atom positional parameters recalculated after each two
cycles served to complete the refinement of the structure. The largest
parameter shift in the final refinement cycle was less than 0.3 of
one estimated standard deviation.23 A final difference Fourier map
contained no features other than a randomly fluctuating background
below 0.3(1) e 3-3. Weights, as defined in (2-6), were used in the
least-squares, full-matrix calculations, which included only the
reflections of intensity greater than 30(I). Cromer and Waber's20

scattering factors for neutral atoms were used for carbon and oxygen,

while those for hydrogen were taken from the International Tables

 

for X-Ray Crystallography.21

 

 

1,.

55

Table 10. Positions and Relative Heights of the Hydrogen Atom Peaks

for HDTO
Atom x y 2 Peak Height
H(9A) 0.63 0.40 0.36 672
H(QB) 0.48 0.38 0.32 617
H(9C) 0.57 0.37 0.26 766
H(lOA) 0.96 0.24 0.28 580
H(10B) 0.81 0.22 0.22 707
H(lOC) 0.83 0.32 0.26 744
H(llA) 1.00 0.28 0.46 533
H(llB) 0.85 0.29 0.50 675
H(11C) 0.86 0.36 0.41 895
H(IZA) 0.43 0.15 0.48 640
H(lZB) 0.38 0.24 0.42 772
H(lZC) 0.48 0.24 0.50 779
H(13A) 0.75 0.08 0.54 599
H(l3B) 0.66 0.12 0.58 560
H(13C) 0.77 0.17 0.56 759
H(14A) 0.94 0.04 0.42 765
“(148) 1.00 0.09 0.34 597
H(14C) 1.02 0.12 0.46 764
H(l6) 0.59 0.14 0.14 933
H(17) 0.65 0.07 0.02 887
“(13) 0.76 -0.06 0.02 731
”(19) 0.87 -0.15 0.16 525

“(20) 0.80 -0.06 0.28 999

Table 10 (Continued)

Atom

H(22)
H(23)
H(24)
H(25)

H(26)

0.41

0.18

0.02

0.13

0.36

56

0.29

0.27

0.14

0.00

0.04

0.18

0.08

0.10

0.22

0.32

Peak Height

638
776
380
661

857

57

Final atomic positional parameters appear in Tables 11, 12, and
13. The estimated standard deviations were calculated from the inverse
matrix of the final least-squares cycle. The final R-values, weighted
and unweighted, respectively, are 0.062 and 0.082 for the 1577
reflections of intensity greater than 30(1). Observed and calculated

structure factor magnitudes are listed in Table 14.

Discussion

 

The structure of HDTO is illustrated in the stereoscOpic drawing
of Figure 4 which shows the 20 per cent equiprobability ellipsoids
(see Chapter 2), derived from the anisotropic thermal parameters.

Bond lengths and angles, shown in Figure 5 and also in Tables 15 and
16, were calculated with the program ORFFE.24

The structure of HDTO is related closely to that of the norbornyl
system; however, due to the strain induced by the cyclopropane and
cyclobutane rings, the bond distances and angles appear to differ
from those usually found for norbornyl systems. [For a summary of
bond distances and angles for these systems, see reference 49.]

For example, the distance C(1)-C(8) of 1.524(9) A and C(2)-C(8) of
1.507(9) A are shorter than those found for comparable bonds in either

56

O
unsubstituted gaseous norbornane, l.54(1) A, or substituted

5 58 while the distances

norbornane, 1.535(4) X, 7 and l.53(3) X,
C(3)-C(6), 1.602(10) A, and C(5)-C(6), 1.608(8) A, are much longer.
Such results are expected, however, because in this molecule these
bonds comprise cyclopropane and cyclobutane rings, respectively.

The bonds C(2)-C(8) and C(l)-C(8) which are in the cyclopropane ring

0
are expected to be shorter than the usual 1.537(5) A36 for a

58

Table 11. Fractional Coordinates (X 104) of Nonhydrogen Atoms in HDTO

E.s.d.'s X 104 are given in parentheses

x y 2
C(1) 5559(7) 1693(4) 3060(4)
C(2) 5763(7) 1936(4) 4056(4)
C(3) 6952(7) 1254(5) 4475(4)
C(4) 6504(7) 399(6) 3915(4)
C(S) 6733(6) 922(4) 3097(4)
C(6) 8034(7) 1412(4) 3783(4)
C(7) 8016(7) 2415(4) 3555(4)
C(8) 6333(7) 2571(4) 3425(4)
C(9) 5708(7) 3516(4) 3209(4)
C(10) 8594(7) 2599(5) 2690(4)
C(11) 8850(7) 3004(5) 4322(4)
C(12) 4422(8) 2140(5) 4496(4)
C(13) 7443(8) 1200(5) 5454(4)
C(14) 9532(7) 913(5) 3994(4)
C(15) 6985(7) 459(5) 2778(5)
C(16) 6489(7) 822(5) 1448(5)
C(17) 6761(8) 401(6) 692(5)
C(18) 7589(9) - 396(7) 769(6)
C(19) 8090(8) - 762(5) 1574(7)
C(20) 7811(7) - 356(5) 2337(5)
C(21) 4071(7) 1602(5) 2498(4)
C(22) 3467(8) 2242(5) 1875(4)

Table 11 (Continued)

C(23)
C(24)
C(25)
C(26)

0(1)

X

2061(9)
1242(8)
1817(8)
3231(8)

6313(6)

59

Y

2122(6)
1350(7)
707(6)
829(5)

- 385(3)

Z

1354(5)
1464(6)
2080(6)
2606(4)

4091(3)

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63

Table 13. Fractional Coordinates (X 103) and Isotropic Temperature

Factors for Hydrogen Atoms in HDTO

x y z B
H(9A) 638 401 365 7.0 £2
H(QB) 456 353 333 7.0
H(9C) 570 372 252 7.0
H(IOA) 973 266 310 7.0
H(10B) 868 234 204 7.0
H(lOC) 824 332 258 7.0
H(llA) 1006 289 443 7.0
H(llB) 844 288 494 7.0
H(11C) 856 370 406 7.0
H(lZA) 385 150 461 7.0
H(IZB) 364 260 408 7.0
H(12C) 490 247 513 7.0
H(13A) 837 72 563 7.0
H(l3B) 651 98 577 7.0
H(13C) 780 190 566 7.0
H(14A) 934 18 409 7.0
H(14B) 1013 100 344 7.0
H(14C) 1020 120 460 7.0
H(16) 591 139 140 7.0
H(l7) 639 67 10 7.0
H(18) 780 - 71 23 7.0
H(19) 867 -133 163 7.0

H(ZO) 819 - 63 293 7.0

Table 13 (Continued)

H(22)
H(23)
H(24)
H(ZS)

H(26)

406

164

23

128

365

64

279

259

126

16

37

180

91

110

215

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«uniqunu

 

 

 

g poo! oozooooognouoooo 00.2.:::0:00000~::=oc:-::00:00' o‘fi==C-:=:OOOD:=~===o=~o=o~oooo=voo=o 2:::':‘0-0::“00h0=O-30000‘fl-0000c300c-e
géoogo ooznooooooocoooogg -~:=:::otsooooo:==...o::oo:uoo70‘0220o:::u.oo::c:==o:uoguooopcgooozogozgz o=ov00::'=o0..:0.:.o=o::oo:0:o:ooo=:
'E’T..!'.T.“.TT.T‘??°‘i‘."=1"7'.”2'1""‘""2'?"-i‘."TT""“2TTn1'o H.‘.'T' Q'T1f‘22f"'i““"“‘:;;1YTf?"""‘.:'71"""""2‘7
. .-_. ==-- -1- a — -... ---- 7.000.000. .

 

.. — --.—< - .......— —-.....--

Table 14

67

 

Figure 4. Stereoscopic View of HDTO.

 

 

 

(a) (b)

0
Figure 5. (a) Bond lengths (A) in HDTO. E.s.d.'s lie in the range 0.008
O
to 0.013 A. (b) Bond angles (°) in HDTO. E.s.d.'s lie in
the range 0.4 to 0.8°.

68

Table 15. Bond Distances Within HDTO

E.s.d.'s are given in parentheses

Endo-Cage Carbon-Carbon Bonds

C(1)-C(2) 1.554(9) Z C(3)-C(6) .602(10) Z
C(1)-C(S) 1.551(9) C(4)-C(S) .523(10)
C(1)-C(8) 1.524(9) C(5)-C(6) 1608(8)
C(2)-C(3) 1.529(9) C(6)-C(7) .509(9)
C(2)-C(8) 1.507(9) C(7)-C(8) .534(9)
C(3)-C(4) 1.533(10)
Exo-Cage Carbon-Carbon Bonds
C(1)-C(21) 1.479(8) C(6)-C(14) .535(9)
C(2)-C(12) 1.539(10) C(7)-C(10) .548(10)
C(3)-C(13) 1.495(9) C(7)-C(11) .547(9)
C(S)-C(IS) 1.489(10) C(8)-C(9) .510(9)
Carbon-Carbon Distances Within the Phenyl Rings
C(15)-C(16) 1.383(10) C(21)-C(22) .380(9)
C(16)-C(17) 1.382(11) C(22)-C(23) .396(10)
C(17)-C(18) 1.385(13) C(23)-C(24) .384(13)
C(18)-C(19) 1.352(13) C(24)-C(25) .371(12)
C(19)-C(20) 1.385(13) C(25)-C(26) .406(10)
C(20)-C(15) 1.406(10) C(26)-C(21) .395(10)
Mean c-ca .385(15)
c=o bond
C(4)-0(1) 1.199(10)

3See footnote a Table 4.

Table 16.

E.s.d.'s
C(2)-C(l)-C(S) 100.
C(2)-C(l)-C(8) 58.
C(2)-C(1)-C(21) 121
C(S)-C(1)-C(8) 109.
C(S)-C(l)-C(21) 120.
C(8)-C(l)-C(21) 127
C(1)-C(2)~C(3) 102.
C(1)-C(2)-C(8) 59.
C(1)-C(2)-C(12) 121
C(3)—C(2)-C(8) 111
C(3)—C(2)-C(12) 120.
C(8)-C(2)-C(12) 124
C(2)-C(3)-C(4) 101
C(2)-C(3)-C(6) 96.
C(2)-C(3)-C(13) 121
C(4)-C(3)-C(6) 82.
C(4)-C(3)-C(13) 121
C(6)-C(3)-C(13) 125.
C(3)-C(4)-C(S) 89.
C(3)-C(4)-0(1) 133.
C(S)-C(4)-O(1) 135.
C(1)-C(5)-C(4) 101
C(l)-C(S)-C(6) 97

Bond Angles Within HDTO

are given in parentheses

0(5)°
6(4)
.9(6)
2(5)
9(5)
.3(5)
4(5)
7(4)
.5(5)
.8(6)
2(6)
.4(6)
.5(5)
0(5)
.3(6)
9(5)
.7(6)
0(6)
6(5)
5(6)

7(7)

.3(5)

.1(5)

69

C(4)-C(S)-C(15)
C(6)-C(S)-C(15)
C(3)-C(6)-C(5)
C(3)-C(6)-C(7)
C(3)-C(6)-C(14)
C(5)-C(6)-C(7)
C(S)—C(6)-C(l4)
C(7)-C(6)-C(l4)
C(6)-C(7)-C(8)
C(6)-C(7)-C(ll)
C(6)-C(7)-C(10)
C(8)-C(7)-C(10)

C(8)-C(7)-C(ll)

C(10)—C(7)-C(ll)

C(1)-C(8)-C(7)
C(1)-C(8)-C(9)
C(2)-C(8)-C(1)
C(2)-C(8)-C(7)
C(2)-C(8)-C(9)

C(7)-C(8)-C(9)

C(S)-C(15)-C(16)
C(5)-C(15)-C(20)

C(1)-C(21)-C(22)

122

123.

84

108.

115

107

116.

118

98.

112.

112

110

111

111

108.

124.

61

106.

123.

119.

122

119

123.

.8(6)°

3(5)

.3(5)

3(5)

.4(5)

.8(5)

8(5)

.9(6)

2(5)

5(5)

.3(6)
.9(5)
.0(5)

.3(5)

0(5)

0(5)

.7(4)

5(5)
0(6)

7(6)

.4(6)

.8(6)

6(6)

Table 16 (Continued)

C(1)-C(S)-C(15)

C(4)-C(S)—C(6)

121

83.

Angles Within the Phenyl

C(15)-C(16)-C(l7)
C(16)-C(17)-C(18)
C(17)-C(18)-C(19)
C(18)-C(19)-C(20)
C(19)—C(20)-C(15)

C(l6)-C(15)-C(20)

122

119.

120.

121

119

117.

.0(5)

0(5)

Rings

.O(7)

0(7)

0(8)

.7(7)

.4(7)

8(7)

70

C(1)-C(21)-C(26)

C(21)-C(22)-C(23)
C(22)-C(23)-C(24)
C(23)-C(24)-C(25)
C(24)-C(25)-C(26)
C(25)-C(26)-C(21)
C(22)-C(21)-C(26)

a
Mean

118.

121

119.

119.

120.

120.

118

120.

a .
E.s.d.'s were calculated from the formula in Footnote a,

except x1 is now the ith bond angle and i'is the mean of

equivalent bond angles.

1(6)

.5(7)

7(7)
8(7)
5(8)

2(6)

.3(6)

0(13)

Table 4,

theifl

71
. 59
Csp3-Csp3 bond because of the phenomenon of bent bonds. For example,
the average carbon-carbon bond distance in the three-membered ring in

O
gaseous cyclopropane is 1.510(2) A,60 in cyclopropanecarboxamide,

1.50 A,61 in cis-1,2,3-tricyanocyc10propane, 1.518(3) A,62 and in

2’4]heptan-6-—y1 pfnitrobenzoate, henceforth

O
exo[3.l.1.02’4], 1.SO(2) A.63 The structure of this latter compound

exo-anti-tricyclo[3.l.l.O

is also related closely to that of HDTO, in that it is missing only

the bridging carbon atom. In contrast to the C(1)-C(8) and C(2)-
C(8) bond lengths, the C(1)-C(2) bond is longer than would be expected,
particularly for a cyclopropane bond, 1.554(9) A. The elongation of the
C(1)-C(2) bond is probably due to the strain produced in the molecule
by the bonding of atom C(1) to C(2), the atoms being opposite sides

of the basic norbornyl skeleton. The exocyclic bond lengths and angles
are expected to be shorter and larger, respectively, than the normal
values, characteristic of bent bonding. The average exocyclic
Csp3-Csp3 bond length for the bonds C(2)-C(12) and C(8)-C(9) of
1.524(9) A compares to 1.487 A in bicyclopropane64 and 1.478(5) A

in a pentacyclic compound.65 The average exocyclic angle is 123.4(6)°,

much greater than the predicted 116°59

and a similar angle, 120.0(2)°,
in bicyclopropane.64 This opening probably reflects one of the ways
strain in the molecule is relieved.

The carbon—carbon bonds in cyclobutane rings have been found to
be somewhat longer than expected and vary from 1.547 to 1.57 A.
[For a summary of dimensions of cyclobutane rings, see reference 66.]
This observation explains in part the long C(3)-C(6) and C(5)-C(6)
bonds observed in HDT --the remaining two bonds C(3)-C(4), 1.533(10) A,

O
and C(4)-C(S), 1.523(10) A, are slightly longer than the normal

72

value for a carbon-carbon bond in a C-C=0 group of 1.506(5) A.36
The average bond angle in the cyclobutane ring, 84.8(5)°, compares to
87.8° in cyclobutane,“ 84.8° in 3&[3.1.1.02’4], and 88.0° in
transfbicyclo[4.2.0]octy1 1-3,5-dinitrobenzoate.68 The bond lengths

and angles of those exocyclic groups which involve atoms C(5) and C(6)
approximate the values eXpected for sp2- and spS-hybridized carbon

atoms, while those which involve atom C(3) (sps-hybridized) do not and
probably reflect an uneven distribution of strain in this part of

the molecule.

The bond distances C(2)-C(3), 1.529(9) A, and C(1)-C(5), 1.551(9) A,
do not differ significantly from those found in either norbornyl
systems (1.539(25) to 1.578(18) A) or in SEEI3'1°1'02’4]» 1.56(2)
and 1.59(2) A. The distances which involve the bridgehead carbon
atom, C(7), are also comparable to those found in norbornyl systems.

The bridging angle of 98.2(5)° is slightly larger than those found
in norbornyl derivatives which range from 92(1) to 96(l)°. The
values of the remaining endo angles in the cage of HDTO are less than
the normal tetrahedral ones, an observation which is consistent both
with observed49 and predicted50 values in the norbornyl system.

The phenyl groups are planar; the deviations from the best plane
through the carbon atoms of each ring range from 0.000(8) to 0.004(6) A,
while the greatest deviation of a hydrogen atom from the associated
phenyl plane is 0.04(1) A. The average carbon-carbon distance and
angle in the phenyl rings are 1.385 A and 120.0° with r.m.s. deviations
of 0.015 A and l.3°, respectively, compared to the normal values of

0
1.394(5) A36 and 120°. The distances C(1)-C(21) and C(5)—C(15) of

73

1.479(8) and 1.489(10) A are close to the expected C-C6HS distance of
1.505(5) A.36 Finally the angles involving the cage-phenyl carbon
atoms are substantially larger than the expected tetrahedral angle;
C(8)-C(1)-C(21) being particularly large at 127.3(S)°, again probably
due to a combination of steric strain and the observed opening of
the exocyclic bonds of cyclopropane rings.62

The 0(1)-C(4) distance is 1.199(10) A, similar to the normal value

0
of 1.215(5) A.36

The angles around C(4) might be expected to be 120°
since it is an spZ-hybridized carbon atom, but as it is also a member
of the cyclobutane ring, one angle closes to 89.6(5)°, and the other
two open to an average of 134.6(7)°.

The closest intermolecular (nonhydrogen atom) contact is 3.51(l) A,
indicative that the molecular structure is composed of discrete
molecules. Thus, packing would not seem to be the cause of the
significantly long C(3)-C(6) and C(5)-C(6) bonds. The section of the
structure containing these bonds is shielded from the neighboring
molecules by methyl carbon atoms C(13) and C(14), by the bridgehead
carbon atom C(7) and its methyl groups C(10) and C(11), and by a phenyl
group. Therefore, the lengthening of the bonds must be due to internal
strain. Further proof that packing is not an influence on these bonds
is the almost perfect mirror plane of symmetry exhibited by the
molecule when phenyl and methyl groups are neglected. The methyl
carbon atoms on one side of the molecule and the phenyl rings on the
other would surely create different packing environments.

In order to investigate further the strain present in the molecule,

the conformations and dihedral angles of the rings which make up the

74
structure have been investigated. The cyclobutane ring is puckered,
as expected, with a dihedral angle of 133°, considerably less than
those found (145 to 160°) for other puckered cyclobutane rings,66
but not significantly different from that of 132° found for
2,4].

There are three five-membered rings in HDTO. The first two to

exo[3.l.1.0

be discussed are those which relate this structure to that of the
norbornyl system. These cyc10pentane rings are expected to assume the
envelope conformation as Altona and Sundaralingam6 have calculated.

In norbornane C(7)--see Figure 6(a)--is the atom out of the plane formed
by the other four atoms in each ring. The five-membered rings in

HDTO do assume the envelope conformation, but_the envelope is severely
deformed from that in norbornane. First, the out-of—plane atom in the
analogous five-membered rings in HDTO is not the tnddging carbon
atom, C(7), as in norbornane (see Figure 6(a) for numbering scheme)
but C(6). Furthermore, the envelope is slightly flattened. The
greatest deviation from the best plane through atoms C(1), C(5),

C(7) and C(8) is 0.052(6) A with C(6) 0.640(6) A below it--in the

38--while

‘ 0
ideal envelope conformation this distance would be 0.75 A
that from the plane formed by C(2), C(3), C(7) and C(8) is 0.055(6) A

O
with C(6) 0.634(6) A above it. Comparison of the dihedral angles of

the five—membered rings of norbornane and HDTO (see Figure 6) emphasizes
the distortion even further. As discussed previously, the HDTO cage
exhibits near mirror symmetry, thus one would expect the dihedral angles
of one cyclopentane to parallel those of the other, and to within a degree

they do with the exception of those around bond CB'

This disparity

seems to indicate an uneven distribution of strain, i.e., torsional or

75

 

 

 

 

 

a Bond Dihedral Angle
4K.b a SS
0
' \ b -55
5 ' c 35
d d 0
d c
c e -35
6
Norbornane
Bond Dihedral Angle
O . .
Ring 1 Ring II
b a -33 32
O\
' b 45 -45
c -38 39
CB -39 32
d 16 -17
e 10 - 9
HDTO

Figure 6. Comparison of the dihedral angles in norbornane47 and HDTO.

76

angular distrotion, in this particular region in the molecule. The
presence of such distortion would correlate with the long bonds
involving atom C(6). The size of the phenyl groups coupled with the
extreme degree of substitution must be contributing factors. It

can be speculated that since the end of the molecule closed off by the
three-membered ring is relatively rigid, any steric strain involving the
substituents would have to be relieved by a distortion of the more
flexible cyclobutane portion of the molecule. A projection along

the C(6)...C(8) vector illustrates the angles between the cyclobutane
and cyclopropane rings. In unsubstituted norbornane these would, of
course, be zero. In HDTO the rear bond C(2)-C(8) and C(1)-C(8) have,

as expected, moved down (see Figure 7) 9° and 7°, respectively,

from the front bonds C(3)-C(6) and C(5)-C(6). The near-mirror

symmetry is reflected once more since these angles are not significantly
different from each other.

The remaining cyclopentane ring (outlined in Figure 8) is also a
distorted envelope, but the distortion is opposite that of the
preceding two. The greatest deviation from the best plane involving
atoms C(1), C(2), C(3) and C(5) is 0.011(6) A with C(4) 0.952(7) A
below the plane and the dihedral angles, shown in Figure 8, are larger
than those for an ideal envelope (compare with Figure 3). In summary
HDTO is very much distorted from the conformation assumed by norbornane,
and this deformation can be rationalized on the basis not only of the
strain induced by closure of the ends of the norbornane skeleton to
small rings, but also of the minimization of the steric interaction

of the substituents.

77

 

Figure 7. Projection of HDTO along C(6)...C(8).

 

//1 Bond Dihedral Angle
Q\ a 2

 

Figure 8. Conformation of cyclopentane III in HDTO.

78

The molecular parameters of HDTO are found to correlate with the
chemistry of the molecule. One of the most significant features of
the molecule is the distance of 2.145(16) A from C(4) to the mid-point
of the C(1)-C(2) bond. The same distance is significantly larger--
2.23 A--in 35213.1.1.02’4]. This short contact facilitates the
participation of the C(1)-C(2) bond in the ionization of I to II.

The exceptional length of the C(3)-C(6) and C(5)-C(6) bonds allows
rationalization of why the 1,2-shift occurs at this point in the
molecule in the ionization of I to IV. Thus, both ionizing paths have
a strong structural basis. The path chosen by I is observed to vary
with the R group, and thus it can be rationalized that the reaction
path chosen depends on the electron demand at C(4). If it is large

as is the case when R = H, the demand is met by the electrons of the
C(1)-C(2) bond; however, if it is small as when R = CH3 rearrangement

occurs via the rupture of one of the long cyclobutane bonds.

CHAPTER 4

THE CRYSTAL STRUCTURE OF
6,6-DIMBTHYL—2,3-BENZO-2,4-CYCLOHEPTADIENONE

PHOTODIMER A

Introduction

 

An unusual photolysis reaction has been reported69 for 6,6-dimethy1-

2,3-benzo-2,4-cycloheptadienone, I. The photolysis of this compound

yields dimeric products exclusively--no monomeric compounds have been
observed. An infrared spectrum established that only one of the carbonyl
groups remained conjugated in the two major products, dimer A and

dimer B. Thus, photodimerization could not have been the simple

addition of two molecules of I to yield products such as II and III,

 

79

80

but must have been a more complicated process in which at some stage
an aryl-carbonyl bond had been broken--an unusual type of cleavage for
aryl-ketones. Investigation of the dimers via pmr and chemical means

has established them to be stereoisomers with the general structure IV

 

IV

which contains four asymmetric centers. Mechanistic considerations
(only two of eight possible dimers were formed, so the mechanism must
be geometrically constrained) coupled with the knowledge gained from
the chemical and spectroscopic investigations leads to two possible
sets of structures. If a nonconcerted mechanism with diradical

intermediates is postulated,69 dimers with structures V and VI would

 

81

result; however, if a concerted cycloaddition mechanism (g,g,, the
Woodward-Hoffmann allowed n45 + «28) is considered,70 dimers of

structures VII and VIII would be produced. Spectroscopic and chemical

 

VII

VIII

data can not conclusively distinguish between the two sets of

possibilities. To obtain a valid mechanism for this unusual photolysis
reaction, the structures of the dimers have to be established unambiguously.
For this reason an X-ray structural determination of dimer A has been

undertaken.

Experimental

 

A crystalline sample of dimer A was supplied by Professor H. Hart.
The crystals were in the form of clear, colorless, flat plates.
Preliminary measurements of the lattice parameters and space group
determination were effected as described previously. All subsequent
measurements were made via_a computer-controlled, four-circle,
Picker goniostat described in Chapter 2 at a temperature of 23(2)°.
A roughly rectangular prismatic crystal (0.29 X 0.07 X 0.44 mm) was

mounted with the long dimension [001] parallel to the phi-axis of

82

the goniostat. Cell parameters were obtained from least-squares
refinement of 12 reflections which had been machine-centered71 on
the goniostat. The density was determined by flotation in aqueous
potassium bromide. Pertinent crystal data are given in Table 17.

Three-dimensional, single-crystal intensity data in one quadrant
(hkl and hkT) to the limit 26 = 45°, which corresponds to a minimum
d-spacing of 0.93 A, were collected by the w-scan technique with a
scan range of 0.7°, scan rate of 0.5°/min, and a Kol-Kaz dispersion
factor of 0.692. Individual background measurements were made at the
endpoints of the scan range for 10 sec each. The degree of attenuation
of each of the three attenuators used was determined by counting for
periods of 20 sec five strong reflections both with and without the
attenuator a minimum of 20 times. The maximum of the standard
deviations from the average intensities of the three periodically
monitored reflections (004, 204, 131) was 1.2%, indicative that the
crystal did not suffer appreciable radiation damage during the seven
days of data collection. A total of 2916 reflections was collected,
exclusive of standards.

The data were corrected for background and considered to be of
measurable intensity by the criterion I > 20(1) (see Chapter 2 for
definitions). An absorption correction was not considered necessary
in View of the small u. The data were corrected for Lorentz and
polarization effects (see Chapter 2 for details). After Friedel
pairs and equivalent reflections had been averaged 2635 independent

reflections remained with 1761 of these with intensities greater than

20(1).

83

Table 17. Crystal Data for Dimer A

26H2802’ M = 372.51

§_= 30.141(16), b_= 8.536(4), 2.: 15.979(6) A

C

§_= 103.1(2)°
Systematic Absences: hkg, h + k = 2n + l;
nog, g = 2n + 1; OkO, k = 2n + 1
Space Group: 9312, No. 15
z_= 8; F(OOO) = 1600 g; v = 4003.7 13
p = 0.823 cm_1 (Mo Ka)

0 = 1.228(2); 0c = 1.236 g cm“3

exp alc
—"—""" o
A(Mo K0, graphite monochromator) = 0.7093 A

84

Phase Determination

 

Normalized structure factor magnitudes were calculated from the
corrected intensity data gig the program FAME.SS The statistical
distribution calculated from a Wilson plot of 20 intervals with the
1761 observed reflections (shown in Table 18) was consistent with
that for a centrosymmetric crystal; thus, the space group was
assumed to be gglg_(No. 15).

The 300 largest E's were then processed through the program
MULTAN;53 however, the program chose a seminvariant as an origin-
specifying reflection. Seminvariants are the linear combinations of
phases whose values are determined only by the observed intensities
once a space group is chosen. Thus, reflections of this type cannot
be used to specify the origin. For centered space groups, the
seminvariants are identified by transforming the centered cell to the
corresponding primitive cell. In the space group C215! there are
two classes of seminvariants: those of the even, even, even (BBB)
and the odd, odd, even (OOE) reflections. Since only four of the
eight classes of reflections are observed for space group £312_

(EEE, EEO, OOE, OOO), phases must be specified for one reflection

of each of the non-seminvariant classes, 1,3,, EEO and 000, to define
the origin. Thus in this case two reflections are needed to fix the
origin uniquely in contrast to the three nonlinearly dependent
reflections needed for a primitive cell. Several different sets of
reflections (EEO and 000) were processed with the E's through MULTAN;
however, all E-maps examined placed the molecule too near a symmetry
element (intermolecular distances were unreasonable). Consequently a

hand-determination of the phases was initiated.

85

Table 18. Statistical Distribution of E's Calculated from the Wilson

Plot for Dimer A

 

 

Quantity Calculated Theoretical

Dimer A Centric Acentric
<IE|> 0.728 0.798 0.886
<E2> 1.036 1.000 1.000
<|E2 — 1|> 1.074 0.968 0.736
|El > 1 31.88 32 00 37.00
[E] > 2 5.58 5.00 1.80

IEI > 3 0.57 0.30 0.01

86
The 200 strongest E's were used to generate a list of all the

possible 2 relationships via MULTAN. With this list and the

2
probability formula (4-1) (see Chapter 3 for definition of terms) the

”3 ' 59' (5351-1:
h) = 1/2 + 1/2 tanh 3/2

phases of 39 reflections were either assigned or calculated through

 

P+(E (4-1)

utilization of (3-2) with a probability minimum of 0.99. The minimum
probability requirement was relaxed at some points in the determination
so that the process could continue with a minimum number of symbols
(Table 19). The two origin reflections, the 21 reflection, and the
three reflections assigned symbols in the hand-determination
(arbitrary signs were assigned to these reflections to produce the
eight possible solutions) along with the 200 largest E's were
processed through MULTAN. The phases from the solution with the
highest ABS FOM, 1.180, (3-18) produced an E—map which contained
peaks corresponding to the positions of all 28 nonhydrogen atoms. A
list of these peaks with their associated peak heights relative to
1175 is given in Table 20. The phases generated by MULTAN for the
correct solution (in which a_was - and b_and g were +) were compared
to those determined by hand, and only one phase, that of the 021

(#171), was found to differ between the two determinations.

Refinement of the Structure

 

Four cycles of fu1l-matrix least-squares refinement17 of positional
and isotropic thermal parameters followed by two cycles of refinement
on positional and anisotropic thermal parameters yielded an R-value

(see Chapter 2) of 0.124. The large number of parameters made the

87

Table 19. Hand—Determined Phases for Dimer A

No.* h k 2 Sign E Probability
27 20 6 3 + 2.73 origin
47 13 l 11 + 2.54 origin

5 7 5 8’ + 3.42 0.95

66 20 4 3 - 2.34 0.985
157 0 2 0 - 1.94 21

4 6 6 7' a 3.52 assigned
24 19 5 4 -a 2.79 0.95

52 25 1 3' - 2.48 0.95

92 26 2 4' a 2.20 0.999

140 6 4 7' -a 1.99 0 999

138 13 1 15' a 2.00 0 999
75 19 3 4 a 2.29 0.999
40 12 2 12 a 2.58 0.985

158 25 3 3' + 1.94 0 999

8 16 2 6 b 3.21 assigned
63 22 4 1' ab 2.38 0.97

59 4 2 3 -b 2.42 0.989

80 3 3 '5 -b 2.27 0.99

130 10 4'10 ab 2.03 0.99
97 3 1 2' ab 2.17 0 999

141 10 2 13 ab 1.99 0.99

139 9 3 14 b 2.00 0.99

‘0
N
O
H
\H
O
(N

.18 assigned

22 10 6 11' -ac 2.87 0.97

Table 19 (Continued)

No.*

129

55

16

13

56

186

90

171

86

132

83

19

177

109

h

25

25

19

16

17

14

24

2

“I
A] (3 Al

“I

“I

“H

“N

a”

88

Sign

-abc
-c
-c

bc

bc

abc

3C

bc

3C

abc

2.

03

.45
.93
.07
.45
.86
.21

.92

.24

.01

.28
.25
.88
.89

.11

Probability

.999
.995
.999
.995
.999
.999

.99

of be
of -bc

.995

of abc
of -abc

.999

.99

.99

.995

.99

*This number correSponds to the magnitude of the E of the reflection

in relation to the rest, 1.6.

the greatest E, etc.

#1 corresponds to the reflection with

89

'Table 20. Positions and Relative Heights of Assigned Peaks in the

E-Map for Dimer A

Atom x y 2 height
C(1) 0.479 -O.115 0.379 1056
C(2) 0.446 -0.215 0.400 938
C(3) 0.400 -0.200 0.350 1175
C(4) 0.388 -0.015 0.325 926
C(S) 0.338 0.000 0.283 881
C(6) 0.325 0.115 0.208 849
C(7) 0.275 0.150 0.191 715
C(8) 0.267 0.133 0.283 633
C(9) 0.304 0.067 0.341 832
C(10) 0.329 0.185 0.408 983
C(11) 0.300 0.285 0.440 1095
C(12) 0.321 0.385 0.509 815
C(13) 0.367 0.385 0.542 844
C(14) 0.396 0.300 0.500 940
C(15) 0.375 0.185 0.441 1166
C(16) 0.404 0.100 0.408 1030
C(17) 0.450 0.150 0.391 963
C(18) 0.458 0.300 0.391 909
C(19) 0.496 0.367 0.375 864
C(20) 0.530 0.285 0.359 1002
C(21) 0.525 0.115 0.359 822
C(22) 0.487 0.050 0.377 934

C(23) 0.392 -O.28S 0.267 801

Table 20 (Continued)

Atom

C(24)
C(25)
C(26)
0(1)

0(2)

0.371
0.350
0.337
0.512

0.225

90

-0.285

0.280

0.067

-0.210

0.167

0.392

0.217

0.133

0.350

0.291

height

685
635
535
911

641

91

cost of performing a full-matrix least-squares calculation prohibitive
and, because of computer memory capacity, prevented simultaneous
refinement of nonhydrogen and hydrogen atom parameters. Thus, block
diagonal (bd) least-squares refinement procedures73 were used hereafter
in the calculations. Four cycles of bd least-squares treatment on the
nonhydrogen atom positional and anisotropic thermal parameters yielded
an R = 0.119. A difference Fourier map74 calculated at this point
revealed the positions of all 28 hydrogen atoms, the largest peak
height was equivalent to 0.4(1) e 2-3. The positions and peak
heights relative to the actual electron density (x 1000) assigned as
hydrogen atoms are given in Table 21. Three cycles of bd refinement
were carried out with the carbon and oxygen atomic parameters variable
and the hydrogen atom positional and isotropic thermal (6.00 A2)
parameters fixed; the R-value decreased to 0.080. Nine cycles of bd
refinement with all atomic parameters varying served to complete the
refinement of the structure. The largest parameter shift in the final
cycle was less than 0.4 of one estimated standard deviation.23 A
final difference Fourier map contained no features other than a
randomly fluctuating background below 0.2(1) e A's. Weights equal
to l/o(|FOI)2 were used in the least-squares calculations, which
included only the observed reflections. Cromer and Waber's20
scattering factors for neutral atoms were used for carbon and oxygen,
while those of Stewart, Davidson and Simpson75 were used for hydrogen.

The final atomic positional and thermal parameters appear in
Tables 22, 23, and 24. The estimated standard deviations23 were

calculated from the inverse matrices of the final least-squares bd

92
Table 21. Positions and Electron Density (X 103) of the Hydrogen

Atom Peaks for Dimer A

Atom x y 2 height
H(ZA) 0.454 -0.350 0.396 386
H(ZB) 0.450 -0.285 0.475 346
H(4) 0.408 0.015 0.280 445
H(S) 0.321 -0.115 0.250 374
H(7A) 0.250 0.050 0.150 351
H(7B) 0.250 0.200 0.141 309
H(9) 0.288 -0.050 0.375 415
H(ll) 0.262 0.267 0.408 361
H(lZ) 0.296 0.467 0.542 309
H(13) 0.379 0.467 0.609 412
H(14) 0.433 0.300 0.542 450
H(16) 0.421 0.015 0.475 470
H(l8) 0.429 0.385 0.391 344
H(19) 0.500 0.515 0.367 319
H(20) 0.567 0.285 0.342 435
H(21) 0.554 0.050 0.342 423
H(23A) 0.358 -0.300 0.217 321
H(23B) 0.417 -0.267 0.233 347
H(23C) 0.418 -0.415 0.283 319
H(24A) 0.329 -0.267 0.359 463
H(24B) 0.379 -0.385 0.425 392
H(24C) 0.358 -0.200 0.442 418

H(25A) 0.387 0 300 0.241 330

Table 21 (Continued)

Atom

H(ZSB)
H(ZSC)
H(26A)
H(26B)

H(26C)

0.342
0.350
0.371
0.333

0.308

93

0.333

0.350

0.050

0.150

0.000

0.291

0.158

0.125

0.083

0.067

height

303
262
331
303

281

94

Table 22. Fractional Coordinates (X 104) of Nonhydrogen Atoms in

Dimer A

E.s.d.'s appear in parentheses

Atom x y 2
C(1) 4839(1) -1272(4) 3755(2)
C(2) 4465(1) -216l(4) 4032(2)
C(3) 3979(1) —1975(4) 3481(2)
C(4) 3866(1) - 233(4) 3258(2)
0(5) 3362(1) 60(3) 2839(2)
C(6) 3258(1) 1185(4) 2050(2)
C(7) 2744(1) 1424(4) 1919(2)
C(8) 2643(1) 1269(4) 2791(2)
C(9) 3058(1) 648(4) 3429(2)
C(10) 3279(1) 1860(4) 4086(2)
C(11) 3002(1) 2810(4) 4465(2)
C(12) 3187(1) 3833(4) 5124(2)
C(13) 3652(1) 3892(4) 5421(2)
C(14) 3931(1) 2959(4) 5059(2)
C(15) 3751(1) 1948(4) 4370(2)
C(16) 4071(1) 824(4) 4064(2)
C(17) 4517(1) 1497(4) 3918(2)
C(18) 4588(1) 3095(4) 3858(2)
C(19) 4985(1) 3712(4) 3694(2)
C(20) 5324(1) 2711(4) 3584(2)
C(21) 5259(1) 1132(4) 3612(2)

Table 22 (Continued)

Atom

C(22)
C(23)
C(24)
C(25)
C(26)
0(1)

0(2)

X

4864(1)
3950(1)
3655(1)
3500(1)
3375(1)
5135(1)

2280(1)

95

Y

497(4)
-2911(4)
-2715(4)

2759(4)

399(5)

-2022(3)

1561(4)

Z

3776(2)
2651(2)
3976(2)
2230(2)
1262(2)
3528(2)

2958(2)

96
Table 23. Anisotropic Thermal Parameters (X 104) of Nonhydrogen Atoms

in Dimer A

The temperature factor expression used was

2

2 2
exp[-(h 311 + k 822 + 2 833 + th12 + hILB13 + k1823)]

E.s.d.'s appear in parentheses

Atom

8

B

8

B

11 22 33 12 13 23
C(1) 9(0) 184(7) 38(2) 10(2) -1(1) -14(3)
C(2) 13(1) 121(6) 43(2) 9(2) 2(1) - 8(3)
C(3) 10(0) 112(6) 33(2) 2(1) 2(1) - 9(3)
C(4) 8(0) 115(6) 33(2) 2(1) 4(1) 0(2)
0(5) 8(0) 107(6) 35(2) - 1(1) 2(1) - 1(2)
C(6) 10(0) 164(7) 34(2) 2(1) 2(1) 7(3)
C(7) 11(0) 237(8) 44(2) 8(2) 1(1) 15(3)
C(8) 8(0) 240(8) 56(2) 3(2) 2(1) - 6(3)
C(9) 9(0) 163(7) 40(2) 1(1) 6(1) 4(3)
C(10) 10(0) 133(6) 34(2) 2(1) 7(1) 6(3)
C(11) 11(0) 177(7) 45(2) 7(2) 8(1) 5(3)
C(12) 16(1) 184(7) 52(2) 10(2) 16(1) - 2(3)
C(13) 17(1) 159(7) 35(2) 6(2) 10(1) — 7(3)
C(14) 12(0) 141(6) 32(2) 3(1) 5(1) - 0(3)
C(15) 10(0) 116(6) 33(2) 1(1) 5(1) 5(3)
C(16) 9(0) 113(6) 33(2) 2(1) 2(1) - 6(3)
C(17) 9(0) 142(6) 30(1) - 3(1) 2(1) -13(3)
C(18) 11(0) 157(6) 46(2) - 2(1) 6(1) - 7(3)

Table 23 (Continued)

Atom

C(19)
C(20)
C(21)
C(22)
C(23)
C(24)
C(25)
C(26)
0(1)

0(2)

811

14(1)
12(1)

9(0)

8(0)
13(1)
15(1)
16(1)
15(1)
13(0)

10(0)

822

180(7)
225(8)
221(8)
154(6)
155(7)
156(7)
178(7)
283(9)
221(5)

514(9)

97

33

56(2)
58(2)
48(2)
36(2)
50(2)
55(2)
49(2)
39(2)
82(2)

74(2)

812

- 9(2)
—14(2)
1(2)
0(1)
1(2)
2(2)

- 9(2)
4(2)
18(1)

24(2)

813

10(1)
11(1)

. 5(1)
2(1)
3(1)
9(1)
1(1)
3(1)
9(1)

6(1)

23

-19(3)
-25(3)
-28(3)
-15(3)
-14(3)
21(3)
28(3)
10(4)
-27(3)

- 7(3)

98

Table 24. Fractional Coordinates (X 104) and Isotropic Temperature

Factors of Hydrogen Atoms in Dimer A

E.s.d.'s appear in parentheses

Atom x y z 3(42)
H(2A) 4549(8) -3339(30) 4041(15) 4.0(6)
H(2B) 4483(9) -1859(32) 4620(17) 5.4(7)
H(4) 4065(7) 22(26) 2804(14) 2.6(6)
H(S) 3232(7) - 977(27) 2611(14) 3 0(6)
H(7A) 2575(10) 460(37) 1612(19) 7.0(8)
H(7B) 2605(10) 2460(34) 1558(18) 6.3(8)
H(9) 2945(9) - 370(34) 3757(17) 5.4(7)
H(ll) 2642(8) 2759(30) 4182(15) 3 9(6)
H(12) 2983(9) 4538(33) 5359(17) 5 8(8)
H(13) 3789(8) 4512(33) 5916(16) 5.0(7)
H(14) 4281(9) 2947(33) 5348(17) 5.6(8)
H(16) 4169(7) 90(28) 4615(15) 3.6(6)
H(18) 4332(8) 3798(30) 3888(15) 4.2(7)
H(19) 5027(10) 4893(37) 3660(18) 7.8(10)
H(20) 5615(8) 3028(32) 3479(16) 4.7(7)
H(Zl) 5496(8) 456(30) 3525(15) 3 7(6)
H(23A) 3631(10) -2833(35) 2221(18) 6.5(8)
H(ZSB) 4178(9) -2464(32) 2345(17) 5.5(8)
H(23C) 4010(10) -4094(38) 2766(20) 7 9(9)
H(24A) 3301(9) -2694(35) 3631(17) 6.3(8)
H(24B) 3772(9) -3741(32) 4233(17) 5.6(8)

Table 24 (Continued)

Atom

H(24C)
H(25A)
H(ZSB)
H(ZSC)
H(26A)
H(268)

H(26C)

X

3628(8)
3847(11)
3408(12)
3401(10)
3709(9)
3288(9)

3176(13)

99

Y

-2119(30)

2634(37)
3484(42)
3353(37)

344(33)

1196(35)

- 680(49)

Z

4468(15)
2443(20)
2728(22)
1714(19)
1356(17)

819(18)

1037(25)

B(A2)
4.4(7)
8.0(9)
10.3(11)
7.4(9)
5.5(8)
6.3(8)

12.2(13)

100

cycle. Final R-values, weighted and unweighted, respectively, are
0.043 and 0.050 for the 1761 reflections with intensities greater than
20(1). Observed and calculated structure factor magnitudes are

listed in Table 25.

Discussion

 

The X-ray structural analysis has established the configuration
of dimer A as that of VII. The structure of dimer A is illustrated in
the stereoscopic drawing25 of Figure 9 which shows 20 percent
equiprobability ellipsoids (see Chapter 2) derived from the anisotropic
thermal parameters. The five- to six-membered ring juncture has been
found to be cis, while the six- to seven-membered juncture has been
found to be traps, Bond distances and angles, shown in Figures 10 and
11 and also in Tables 26, 27, 28 and 29 were calculated with the program

DAESD.76 The average lengths77 of chemically equivalent bonds compare

36

well, on the whole, with the expected values. The average of eleven

C SP3- -C 5P3 bond lengths is 1.541(16) A compared to the usual value of

1. 537(5)A 36

The longest bond of this type, C(4)-C(16) is 1. 580(4) A,
and is at the six- to seven-membered ring juncture. The lengthening
of this bond is probably a result of strain at this ring juncture.

There are two types of Cspz-C 3 bonds, one in which the spZ-hybridized

5P
carbon atom is part of an aryl group and the other in which it is part
of a carbonyl group. The average bond lengths of these types are

0
1.521(5) and 1.508(9) A, respectively, compared to the usual values

1. 505(5) and 1. 506(5) A. 36

The one Cspz'cspz bond length (excluding
O
aryl carbon atoms) C(1)-C(22) is 1.513(5) A compared to a normal value

0
of 1.47(2) A. The average of the aryl carbon atom bond lengths is

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107

Tkable 26. Bond Distances Involving Nonhydrogen Atoms Within Dimer A

E.s.d.'s are given in parentheses

O O
Atcun(l)-Atom(2) Distance (A) Atom(1)-Atom(2) Distance (A)

(3(1)-C(2) 1.506(5) C(8)-0(2) .209(4)
(3(1)-C(22) 1 513(5) C(9)-C(10) .517(4)
(3(1)-0(1) 1.219(4) C(10)-C(11) .398(4)
C(2)-C(3) 1.537(4) C(10)-C(15) .392(4)
C(3)-C(4) 1.550(4) C(11)-C(12) .384(5)
C(3)-C(23) 1.533(4) C(12)-C(13) .376(5)
C(3)-C(24) 1.529(4) C(13)-C(14) .378(5)
C(4)-C(S) 1.534(4) C(14)-C(15) .407(4)
C(4)-C(16) 1.580(4) C(15)-C(16) .520(4)
C(S)-C(6) 1.559(4) C(16)-C(17) .527(4)
C(S)-C(9) 1.541(4) C(17)-C(18) .387(5)
C(6)—C(7) 1.529(4) C(17)-C(22) .409(4)
C(6)-C(25) 1.525(5) C(18)-C(19) .387(5)
C(6)-C(26) 1.537(5) C(19)-C(20) .373(5)
C(7)-C(8) 1.500(5) C(20)-C(21) .365(5)
C(8)-C(9) 1.518(4) C(21)-C(22) .387(4)

Table 27.

C(2)-H(2A)
C(2)-H(ZB)
C(4)-11(4)
C(S)-H(S)
C(7)-H(7A)
C(7)-H(7B)
C(9)-H(9)
C(11)-H(11)
C(12)-H(12)
C(13)-H(13)
C(14)-H(14)
C(16)-H(16)
C(18)-H(18)

C(19)-H(19)

Bond Distances Involving Hydrogen Atoms Within Dimer A

108

E.s.d.'s are in parentheses

0
Atom (1 ) -Atom(2) Distance (A)

1.04(3)
0.97(3)
1.07(2)
1.00(2)
1.03(3)
1.09(3)
1.11(3)
1.08(3)
0.99(3)
0.96(3)
1.05(3)
1.07(2)
0.99(3)

1.02(3)

Atom(l)-Atom(2)

C(20)-H(20)

C(21)-H(21)

C(23)-H(23A)
C(23)-H(238)
C(23)-H(23C)
C(24)-H(24A)
C(24)-H(24B)
C(24)-H(24C)
C(25)-H(25A)
C(25)-H(ZSB)
C(25)-H(25C)
C(26)-H(26A)
C(26)-H(26B)

C(26)-H(26C)

0
Distance (A)

0.97(3)
0.95(2)
1.05(3)
1.01(3)
1.04(3)
1.08(3)
1.00(3)
O.96(3)
1.03(3)
1.09(4)
0.96(3)
0.99(3)
0.97(3)

1.11(4)

109
ikable 28. Bond Angles Involving Nonhydrogen Atoms Within Dimer A

E.s.d.'s appear in parentheses

Aton1(1)-Atom(2)—Atom(3) Angle (°) Atom(l)-Atom(2)-Atom(3) Angle (°)

C)(l)-C(l)-C(2) 118.0(3) C(S)-C(9)-C(8) 102.6(2)
()(1)-C(1)-C(22) 119 6(3) C(S)-C(9)-C(10) 114.6(2)
(3(2)-C(1)-C(22) 122.3(3) C(8)-C(9)-C(10) 113.1(3)
(2(1)-C(2)-C(3) 117.4(3) C(9)-C(10)-C(11) 118.9(3)
(:(2)-C(3)-C(4) 111.3(2) C(9)-C(10)-C(15) 121.3(3)
(:(2)-C(3)-C(23) 107.6(3) C(11)-C(10)-C(15) 119.6(3)
(:(2)-C(3)-C(24) 107.4(2) C(10)-C(11)-C(12) 121.2(3)
C(4)-C(3)-C(23) 109.6(2) C(11)-C(12)-C(13) 119.2(3)
C(4)-C(3)-C(24) 112.4(2) C(12)-C(13)-C(14) 120.4(3)
C(23)-C(3)-C(24) 108.3(3) C(13)-C(14)-C(15) 121 3(3)
C(3)-C(4)-C(S) 113.7(2) C(10)-C(15)-c(14) 118.2(3)
C(3)-C(4)-C(16) 109 6(2) C(10)-C(15)-C(16) 122.6(3)
C(S)-C(4)-C(16) 115.4(2) C(14)-C(15)-C(16) 118.6(3)
C(4)-C(S)-C(6) 116.8(2) C(4)-C(16)-C(15) 117.5(2)
C(4)-C(5)-C(9) 117.0(2) C(4)-C(16)-C(17) 106.6(2)
C(6)—C(5)-C(9) 104.8(2) C(15)-C(16)-C(l7) 117.3(3)
C(5)-C(6)-C(7) 101.8(2) C(16)-C(17)-C(18) 122.4(3)
C(5)-C(6)-C(25) 113.0(3) C(16)-C(17)-C(22) 120.5(3)
C(S)-C(6)-C(26) 110.7(3) C(18)-C(17)-C(22) 116.9(3)
C(7)-C(6)-C(25) 109.5(3) C(17)-C(18)-C(19) 122.7(3)
C(7)-C(6)-C(26) 111.0(3) C(18)-C(19)-C(20) 119.1(3)
C(25)-C(6)-C(26) 110.6(3) C(19)-C(20)-C(21) 119.7(3)

(2(6)-C(7)-C(8) 105.9(3) C(20)-C(21)-C(22) 121.8(3)

110

Table 28 (Continued)
Atom(l)-Atom(2)-Atom(3) Angle (°) Atom(1)-Atom(2)-Atom(3) Angle (°)

O(2)—C(8)-C(7) 124.9(3) C(1)-C(22)-C(17) 125.0(3)
O(2)-C(8)-C(9) 125.0(3) C(1)-C(22)-C(21) 115.3(3)

C(7)-C(8)-C(9) 110.2(3) C(17)-C(22)-C(21) 119.7(3)

Table 29.

111

E.s.d.'s are given in parentheses

Bond Angles Involving Hydrogen Atoms Within Dimer A

Aton1(1)-Atom(2)-Atom(3) Angle(°) Atom(l)-Atom(2)-Atom(3) Angle (°)
(3(1)-C(2)-H(2A) 107(1) C(19)-C(18)-H(18) 120(2)
(2(1)-C(2)-H(28) 106(2) C(18)-C(19)-H(19) 121(2)
(:(3)-C(2)-H(2A) 108(1) C(20)—C(19)-H(19) 120(2)
(:(3)-C(2)-H(28) 111(2) C(19)-C(20)-H(20) 125(2)
ld(2A)-C(2)-H(ZB) 107(2) C(21)-C(20)-H(20) 115(2)
C(3)-C(4)-H(4) 103(1) C(20)-C(21)-H(21) 119(2)
C(5)-C(4)-H(4) 109(1) C(22)-C(21)-H(21) 120(2)
C(16)-C(4)-H(4) 105(1) C(3)-C(23)-H(23A) 113(2)
C(4)-C(5)-H(5) 106(1) C(3)-C(23)-H(238) 108(2)
C(6)-C(S)-H(S) 105(1) C(3)-C(23)-H(23C) 113(2)
C(9)-C(S)-H(S) 106(1) H(23A)-C(23)-H(238) 107(2)
C(6)-C(7)-H(7A) 110(2) H(23A)-C(23)-H(23C) 106(2)
C(6)-C(7)-H(7B) 116(2) H(23B)-C(23)-H(23C) 110(2)
C(8)-C(7)-H(7A) 101(2) C(3)-C(24)-H(24A) 114(2)
C(8)-C(7)-H(7B) 115(2) C(3)-C(24)-H(24B) 112(2)
H(7A)-C(7)-H(7B) 108(2) C(3)—C(24)-H(24C) 113(2)
C(S)-C(9)- H(9) 109(1) H(24A)-C(24)-H(24B) 116(2)
C(8)-C(9)-H(9) 107(1) H(24A)-C(24)-H(24C) 99(2)
C(10)-C(9)-H(9) 110(1) H(24B)-C(24)-H(24C) 103(2)
C(10)-C(ll)-H(ll) 116(1) C(6)-C(25)-H(25A) 112(2)
C(12)—C(11)-H(11) 123(1) C(6)-C(25)-H(ZSB) 117(2)
C(11)-C(12)-H(12) 120(2) C(6)-C(25)-H(25C) 105(2)

112

Table 29 (Continued)

Atom(l)-Atom(2)eAtom(3) Angle(°) Atom(l)-Atom(2)-Atom(3) Angle (°)

C(13)—C(12)-H(12) 121 (2) H(25A)-C(25)-H(ZSB) 103(3)
C(12)-C(13)-H(13) 121(2) H(25A)-C(25)-H(25C) 116(3)
C(14)-C(13)-H(13) 118(2) H(ZSB)-C(25)-H(25C) 104(3)
C(13)-C(14)-H(14) 118(2) C(6)-C(26)-H(26A) 108(2)
C(15)-C(14)-H(14) 121(2) C(6)-C(26)-H(268) 103(2)
C(4)-C(16)-H(l6) 109(1) C(6)-C(26)-H(26C) 115(2)
C(15)-C(16)-H(16) 101(1) H(26A)-C(26)-H(263) 104(2)
C(17)-C(16)-H(16) 105(1) H(26A)-C(26)—H(26C) 118(3)
(2 (17)-C(18)-H(18) 117(2) H(26B)-C(26)-H(26C) 108(3)

113
0 O
.l.387(13) A compared to the usual value of 1.394(5) A.36 The

differences between the eXpected and observed distances in all these
cases are not significant, _i_.g_., the observed values lie approximately
within 30 of those expected. The normal C-O bond length in carbonyl

6 In dimer A the C(1)-0(1) bond length is

groups is 1.215(5) R3
1.1219(4) A and that of C(8)-0(2) is 1.209(4) A. The direction of the
deviation in the two C-O bond lengths from the expected value, although
the: deviations themselves are insignificant, was anticipated. The
(3(1[)-0(l) bond is part of an extended conjugated system (see below)
aJui therefore should be longer than the expected value, while the
C(}3)-0(2) bond appears artificially shortened due to the large 822
artisotropic thermal parameter. The average C-H bond distance is
1 .02(5) A, and the C-H bond distances range from 0.95(2) to 1.11(4) A.
In contrast to the bond lengths, the bond angles in dimer A do
(liffer significantly from those expected for sp2 and sp3 hybridized
(:arbon atoms in the nonaromatic rings. In the five-membered ring the
.Smaller than normal endocyclic angles and the large exocyclic angles

\vhich involve the carbonyl carbon atom are comparable to those found

in S-benzyl-S-p-bromobenzylidenecyclopentanone photodimer78 (see IX

 

114

in which only the cyclopentanone ring is shown for simplicity). The
somewhat enlarged exocyclic angles at C(6), particularly C(S)-C(6)-C(25),
probably result from crowding in the ggmfdimethyl group. The opening
of the angle C(4)-C(5)-C(6) to 116.8(2)°, may also be attributed to
the steric crowding of the ggmfdimethyl group with atoms C(4), C(8),
C(9) and C(10) (intramolecular distances: C(4)~C(25), 3.103(5) A;
C(8)-C(25), 3.185(5) A; C(9)-C(25), 3.138(5) A; C(10)-C(25), 3.274(4) A;
C(4)-C(25), 3.243(4) A) as well as to strain involved in the five-
to six-membered ring juncture.
Theoretical calculations on 4,5-disubstituted cyclohexene rings79’8O
have led to predictions that angles at the trigonal carbon atoms
should be 124° and those at other carbon atoms 110.5°. A comparison of
these predictions with the bond angles observed in the nonaromatic
six-membered ring of dimer A reveal substantial differences. As will
be discussed in detail later, the cyclohexene ring in dimer A is
substantially flattened with the result that all the endocyclic and
exocyclic angles have to approach 120°. This flattening effect
probably results from strain induced by juncture of the cyclohexene
ring with three other cyclic moieties.

In two reported structures which contained cycloheptane rings,
the observed endocyclic angles were greater than those found in
paraffin chain molecules and range from an average of 115.3° in one

case81 to 118.1° in the other.82

Thus the angles at spS-hybridized
atoms in a seven-membered ring are larger than those usually expected
and are probably normal; however, the angles C(4)-C(16)-C(l7) and
C(3)-C(4)-C(16) of 106.6(2) and 109.6(2)° are substantially smaller

than those observed in seven-membered rings. These small bond angles

115
.and the long C(4)-C(16) bond provide additional evidence for the strain

.at.the six- to seven-membered ring juncture. The bond angles around
(3(3), the atom to which a ggmfdimethyl group is bonded, are both larger
:and smaller than those expected, no doubt because of steric interaction
‘both within the group and between it and atoms C(1), C(5) and C(16)
(intramolecular distances: C(1)-C(23), 3.179(5) A; C(5)-C(23),
3.148(4) A; C(S)-C(24), 2.994(4) A; and C(16)-C(24), 3.263(5) A).
The angles C(1)-C(22)-C(21) (115.3(3)°) and C(18)-C(17)-C(22)
(116.9(3)°) are significantly smaller than the 120° expected at aryl
carbon atoms, while the angle C(1)-C(22)-C(17) (125.0(3)°) is
significantly larger. These angles involve the atoms through which
the aryl and seven-membered ring are joined and provide evidence for
strain produced by the crowding of the aryl groups (C(14) to C(18),
3.058(4) A). The average angle in the aromatic rings is 120.0(16)°77
and the angles involving the hydrogen atoms are normal within the
accuracy expected.

The conformations of cyclic systems are described most precisely
by the dihedral angles. These angles for the five-, six- and seven-
membered nonaromatic rings are listed in Table 30. [For the definition
of a dihedral angle, see Chapter 2; C(X) + C(Y) indicates that the
bond C(X)-C(Y) is viewed from atom C(X) to atom C(Y).] A more
descriptive method of specifying ring conformation is to indicate the
deformation of the ring by calculating planes and determining out-of—plane
distances fer atoms in the ring. In the cyclopentanone ring, atoms
C(5) and C(6) lie 0.358(3) and 0.267(3) A below and above the plane
formed by atoms C(7), C(8) and C(9), an indication that the ring is

in the half-chair conformation. The deviation from the ideal half-chair

116

Table 30. Dihedral Angles in Dimer A

Bond Angle (°)

Five ~Membered Ring

(3(5) + C(6) 39
C(6) + C(7) —30
C(7) + C(8) 10
C(8) + C(9) 14
C(9) r C(S) -32

S i x-Membered Ring

C(4) + C(S) 29
C(S) + C(9) -41
C(9) + C(10) 25
C(10) + C(15) 2
C(15) + C(16) -13
C(16) + C(4) - 3

Seven-Membered Ring

C(1) + C(2) -67
C(2) + C(3) 46
C(3) + C(4) 40
C(4) + C(16) -81
C(16) + C(17) 57
C(17) + C(22) 0

C(22) + C(1) ll

117
configuration is reflected both in the fact that atoms C(5) and
C(6) are unequal distances from the plane and in the differences of
the observed dihedral angles from those shown in Figure 3 (Chapter 2).
Calculations have suggested that cyclopentanone rings tend to exist
in the half-chair conformation.83 The presence of this configuration
in dimer A, albeit slightly deformed, is consistent with these
calculations, but not with the conformation found for the cyclopentanone

rings of the cyclopentanone photodimer, X.84 In X the out-of—plane

 

O

atoms were 0.21 and 0.45 A abgvg_the plane formed by the other atoms;
however, the cyclopentanone rings in this structure may not be as
flexible as that in dimer A due to the strain induced by juncture
with a planar cyclobutane ring.

The cyclohexene ring in dimer A is substantially flattened for
a six-membered ring, as evidenced by a comparison of the observed

dihedral angles with those predicted for cyclohexene, XI.79 The

 

XI

118

dihedral angles shown in XI are for the conformation of unsubstituted
cyclohexene which most closely resembles that found for the non-
aromatic six-membered ring in dimer A. The flattening of the
cyclohexene ring is illustrated not only by the substantial decrease
in the dihedral angles observed from those predicted, but also by
the relatively small deviations of the ring from planarity. Two
different planes can be found for the cyclohexene ring in dimer A,
and each of these allows the conformation to be described as a pseudo
half-boat. The maximum deviation from the best plane through atoms
C(9), C(10), C(15) and C(16) is 0.007 A with atoms C(4) and C(5),
respectively, 0.286(3) and 0.622(3) A above the plane; whereas, the
maximum deviation from the best plane through atoms C(4), C(5),
C(15) and C(16) is 0.015 A with atoms C(9) and C(10) 0.620(3) and
0.320(3) A, respectively, below the plane.

The pseudo half-boat configuration for the cyclohexene ring
in dimer A contrasts to that found for a cyclohexene moiety, ring
C, of a steroid.85 The conformation of this latter ring is that of
a distorted half-chair with the out-of—plane atoms (Opposite the
double bond) 0.205 and 0.582 A above and below the plane of the

86 have shown that the half-chair

86

other four atoms. Calculations79’
conformation with the out-of-plane atoms 0.48 A above and below
the plane of the other atoms is probably the low-energy form.
However, the fact that the cyclohexene ring in dimer A is the central
ring of the structure with three other cyclic moieties fused to it,

probably causes it to assume the observed configuration.

119

No structures of a cycloheptenone ring or a compound containing
such a ring have been reported, and although predictions have been
made as to the geometrical shapes assumed by cycloheptene and
cycloheptanone, none for a cycloheptenone ring could be found.

It is only possible to compare the experimentally determined

81’82 all of

conformation with those found for cycloheptane rings,
which are the skew chair.87 As for the cyclohexene ring, two four
atom planes exist in the cycloheptenone ring, each of which leads
to a distorted chair conformation for the ring when the out-of-
plane atoms are considered. The maximum deviation from the best
plane through atoms C(2), C(3), C(4) and C(17) is 0.008 A with

atom C(16) 0.922(3) A above the plane and atoms C(1) and C(22)
0.981(3) and 0.865(3) A, respectively, below it; whereas, that from
the best plane through atoms C(1), C(16), C(17) and C(22) is 0.0003 A
with atom C(2) 0.233(3) A below and atoms C(3) and C(4) 0.835(3)
and 1.265(3) A, respectively, above the plane. When atom C(2) is
considered one of the atoms of this latter plane, the maximum
deviation from planarity is 0.079(3) A, and the conformation is a
distorted half-boat. The tolerably good plane formed by atoms
C(1), C(2), C(16), C(17) and C(22) is one indication that the
carbonyl group C(1)-0(1) is conjugated with the double bond of the
aryl group. As a further check on the planarity of this section of
the molecule, the angle between the plane formed by atoms C(1),
C(2), C(22) and 0(1) and that formed by the aryl group was found to
be 9°. This angle is the same as that found between the
carboxylate and the phenyl planes in transfbicyclo[4.2.0]octyl

l-3,5-dinitrobenzoate.68 Thus, the carbonyl group at atom C(1) is

120
considered to be conjugated while that at atom C(8) is definitely

not, in agreement with infrared data.69’70

The aryl groups are
planar--the maximum deviations from the best plane through atoms
C(10) to C(15) are 0.015(3) X and 0.13(3) R for carbon and hydrogen
atoms, respectively, while those from the plane through atoms

C(17) to C(22) are 0.016(3) and 0.04(2) R.

The dihedral angles involving the hydrogen atoms attached to
the four asymmetric carbon atoms remain to be discussed. These
angles, designated by the hydrogen atoms involved where the hydrogen
atoms are labelled as in IV, are 35, 85 and 124°, for hydrogen
atoms ab, bg_and SQ) respectively. The dihedral angle involving
hydrogen atoms b_and g_is particularly important since in the pmr

studies on dimer A69’70

no coupling was found between these atoms.
This observation led to the prediction that the dihedral angle
involving these atoms must be approximately 90°; this structural
investigation has confirmed this prediction.

The comparison of bond lengths and angles and conformational
angles is most valid between compounds of very similar chemical and
structural components. In the case of dimer A, such a comparison
would be with its stereoisomer, dimer B. The structure of dimer B
was elucidated88 concomitantly on the same diffractometer, but with
different data collection and reduction procedures. The configuration
of dimer B was found to be that of VIII; thus in both dimers the
five- to six-membered ring juncture is £33, while the six- to seven-

membered juncture is trans. The bond lengths and angles found for

dimer B are shown in Figures 12 and 13. Overall the two structures

121

.m uoeflo :H A<v moocmumfla ccom
0

.NH champ“

122

 

Figure 12

123

.m uoEHo :fl may moawcm ncom

.mH otsmfla

11241

nu

0N
:IIIIIIIIIIHHWMW_
Nfi. _

  

.hgwnv_ .llllllllllmmw
0.8

VN

EZJY: as: \

mH engage

So
3.
Eu.
. . 3w.
3.0. 3: ow:
to: . ad: to:
A; mom. «m1

him
now. m.o~_/
mu. _
mm. _

 

 

wd: ON

 

 

9.2 39 . w m. _
i mm: 6.8. mu «3. mm:
a ado.
N8. .
~.m9\.w t: 3w. ._ to:
mom. m3. 3: ode/
_._N_ Na: n8. 4.
go
__ 2m. mm:

 

 

125

have similar bond lengths and angles, as a comparison of the average
lengths of chemically equivalent bonds helps demonstrate. The average

0
CS 3-Csp3 bond length77 in dimer B is 1.550(19) A compared to

P

O
1.541(16) A in dimer A. The two types of CS 2-C5p3 bonds, the

P
spz-hybridized carbon atom being part of an aryl or carbonyl group,
have average lengths of 1.527(7) and 1.508(7) A, respectively,
compared to analogous bond lengths in dimer A of 1.521(5) and
1.508(9) A. The one nonaryI Cspz'cspz bond length is 1.491(8) A

in dimer 8 and 1.513(5) A in dimer A. The two C-O conjugated and
nonconjugated bond distances, 1.225(7) and 1.208(7) A in dimer B,
compare to 1.219(4) and 1.209(4) A in dimer A. The average aryl

bond length and angle are 1.385(11) A and 120.0(15) compared to

l 387(13) A and 120.0(16)° in dimers B and A, respectively. None

of these average values is significantly different in the two dimers;
however, there are several individual differences which are important.
The dimer 8 C(4)-C(16) bond length of 1.562(7) A, although still
longer than the normal Csp3-Csp3 bond length of 1.537(5) A,36 is not
the longest bond of this type in the molecule as was the comparable
bond in dimer A. Instead, the longest bond in dimer B occurs at
bond C(5)-C(6) (1.582(7) A). All the substantially different bond
lengths occur in this same region of the molecule. The bonds
C(4)-C(5) and C(6)-C(7) are also much longer in dimer 8 than in
dimer A, probably because the ggmfdimethyl groups at C(3) and C(6)
are much more severely crowded. This crowding is further emphasized
by a critical comparison of the bond angles in the two dimers. In
the area of the molecule where the ggmfdimethyl groups are located,

the angles in dimer B differ from those expected to a much greater

126

extent than do the same ones in dimer A. For example, angles
C(4)-C(5)-C(6) and C(3)-C(4)-C(5) of 128.2(4) and 117.2(4)°,
respectively, in dimer B are much larger than the same angles of
116.8(2) and 113.7(2) in dimer A, while the angle C(2)-C(3)-C(23)
of 104.4(4)° in dimer B is smaller than the same angle of 107.6(3)°
in dimer A. However, the angles in the area of the six- to seven-
membered ring juncture and aryl- to seven-membered ring juncture
are more normal in dimer B than in dimer A. For instance, the angle
C(15)-C(16)-C(17) of 109.6(4)° in dimer B is much more normal than
that of 117.3(3)° in dimer A. The endocyclic angles of the
Csps-hybridized carbon atoms in the seven—membered ring in dimer B
more closely resemble those observed previously in cycloheptanes--
in particular C(3)-C(4)-C(16) and C(4)-C(16)-C(17) which are greater
than the normal tetrahedral values as expectedf-than are the same
angles in dimer A. The angle C(1)-C(22)-C(21) of 118.4(5)° in dimer
B approximates the expected 120° more closely than that of 115.3(3)°
in dimer A. Finally, the closing of the angle C(2)-C(1)-C(22) to
115.6(5)° in dimer B contrasted to a slight opening of the same
angle in dimer A is probably due to a difference in ring conformation
at these atoms.

In summary most of the major differences in bond angles and
bond lengths between the two structures are probably due to a
different distribution of strain in the two molecules. In dimer A
the greatest areas of strain are at the ring junctures, whereas,

in dimer B the greatest stress is at the gem-dimethyl groups.

127

A tabular comparison of dihedral angles in dimers A and B is
given in Table 31. From this table it can be seen that the rings
of dimer B are much more puckered than those of dimer A, since with
only two exceptions the dihedral angles for the cyclic moieties in
dimer B are larger than those in dimer A. The exceptions, both of
which occur in the five-membered ring, arise because this ring is
in the envelope configuration in dimer B rather than the half-chair
conformation as in dimer A, thus the angle down the bond C(7) + C(8)
should approach 0° as the conformation of the ring approaches that
of an ideal envelope. Finally, the dihedral angles involving the
hydrogen atoms at the four asymmetric carbon atoms are 49, 45 and
146° in dimer B (at hydrogen atoms ab, bg_and Ed, see IV), qualitatively
in agreement with the complex coupling pattern observed in its pmr
spectrum.69’70

With the structures of both dimers established unambiguously
as those of VII and VIII, the mechanism of the photodimerization of
the parent compound I can be discussed. As has been indicated this
mechanism must be geometrically constrained. A diradical mechanism
proposed and discussed69 before X-ray structural data were available
meets this requirement, but is now refuted since it would yield
structures V and V1 for the dimers. The required mechanism must
account for the common structural features of atoms H8 and Hb being
“cis and atoms Hc and Hd being 53323! and the unique orientations of
atoms Hb and Hc’ which are tran§_in dimer A and $35.1“ dimer B.

In addition to the structural similarities and differences at the

asymmetric carbon atoms, the mechanism must allow for the experimental

128

Table 31. Comparison of the Dihedral Angles in Dimers A and B

Bond Dihedral Angle
Dimer A Dimer B

Five-Membered Ring

C(S) + C(6) 39 -39
C(6) + C(7) —30 27
C(7) +C(8) 10 - 5
C(8) + C(9) 14 -20
C(9) + C(S) —32 37

Six-Membered Ring

C(4) + C(S) 29 -49
C(S) + C(9) -41 59
C(9) + C(10) 25 -31
C(10) + C(15) z _ 9
C(15) + C(16) -13 19
C(16) + C(4) - 3 12

Seven-Membered Ring

C(1) + C(2) -67 30
C(2) + C(3) 46 -61
C(3) + C(4) 4o 62
C(4) + C(16) -81 -85
C(16) + C(17) 57 74
C(17) + C(22) 0 -10

C(22) + C(1) 11 -58

129

69,70

observation that atoms “b and Hc are derived from the doubly

bonded carbon atom that is 8 to the aryl in 1, whereas

atoms Ha and H are from the corresponding a carbon atom. One

d

mechanism which accounts for all of these features includes a
(n4g + n23) cycloaddition followed by a 1,3-suprafacial acyl shift,7O
illustrated below, for the formation of dimer A. [For a complete

mechanistic discussion see reference 70.]

 

 

CHAPTER 5

SUMMARY AND CONCLUSIONS

The basic conformations of rings of four to seven atoms have been
presented in the introduction. The structures of the compounds
PLAT, HDTO, and dimer A have been determined, and the conformations
of the cyclic moieties in these compounds established unambiguously.
A comparison of conformations is now made to assess the magnitude of
the effects of internal alteration of a ring by replacement of a
carbon atom with a heteroatom and of external alteration of a
cyclic moiety by changing the environment in which it is found.

Formulated earlier in this work was a hypothesis which stated
that as the bond length (heteroatom-C) and bond angle (at the
heteroatom) in five-membered heterocycles increases and decreases,
respectively, the half-chair conformation is favored. As the
structural data on the tetrahydroplatinole ring in PLAT show, the
Pt—C bond distances average 2.08(2) A and the C37-Pt-C4O bond angle
is 80.9(8)°, larger and smaller than the bond length of 1.975(3) A
and angle of 89.1(5)° observed for tetrahydroselenophane (THSe).

For the hypothesis to be correct, the tetrahydroplatinole ring should
have been observed in a half-chair conformation; it has been found

in an almost ideal envelope configuration (see Figure 3). As

Figure 2 shows the argument that packing requirements force the

ring to assume this shape cannot be used as substantiation of the
hypothesis. However, direct comparison of the heterocyclic moiety in

PLAT with heterocycles such as THF (tetrahydrofuran),

130

131
'FHS (tetrahydrothiOphene) and THSe is not fully valid since in the
latter compounds there is no substitution at the heteroatom. The
fact that the tetrahydroplatinole ring has bulky triphenylphosphine
substituents at the platinum atom in addition to the ring atoms
may have a profound effect on the ring conformation.

In the norbornane system both five-membered rings are in the
envelope configuration with the out-of—plane atom being the bridgehead
atom. In the more rigid HDTO system, the corresponding five-
membered rings are again in the envelope configuration. Thus, on
the surface it appears that rigidity added to the basic norbornane
system by conversion of the bicyclic system into a tetracyclic system
and by substitution at each carbon atom has not altered the basic
conformation of the five-membered rings. However, a critical
examination of these conformations reveals that the bridgehead
carbon atom has been replaced by C(6) as the out-of-plane atom
(see Figure 4). This observation means that the norbornane skeleton
has undergone a drastic change. In the substituted norbornane
systems examined by Altona and Sundaralingam,6 changes in the
norbornane frame occurred such that the dihedral angles were
altered significantly, but no basic change in ring conformation was
noted. They did observe that the entire frame would twist in what

they termed synchro and contra motions, illustrated in I and II,

   

1 II

132

respectively, and that this twisting phenomenon, which was attributed
to effects of substitution, seemed to be an additive property.
Therefore, with a highly substituted molecule such as HDTO, a large
twist of the norbornane frame would be expected. But the actual
twist the molecular skeleton could undergo is severaly limited by

the rigidity introduced into the norbornane frame by the closing

of the two ends to form additional rings. The cumulative effect is
the change in the position of the out-of—plane atom in the envelope
configuration of the five-membered rings.

The importance of ring juncture on the conformation of cyclic
moieties in a complicated fused system such as dimer A can be assessed
by a comparison of the ring conformations found in this compound
with those found in its stereoisomer, dimer B. The difference in
these compounds is not one of gross ring junctures which are SEE.
in the case of the five- to six-membered ring juncture and 33325
in the six- to seven-membered ring juncture for both dimer A and
dimer B, but in the relationship of the hydrogen atoms attached to

the carbon atoms at these junctures, III. In both dimers H3 and

 

III

133
Hb are Ei§_and HC and Hd are trans; however, the relationship of
Hb and HC is Erafls_in dimer A but Ei§_in dimer B. The conformational
effects due to this small junctional difference are apparent from a
comparison of the dihedral angles of the two stereoisomers (see
Chapter 4). The majority of the dihedral angles of dimer B are
larger than those of dimer A, an indication that the conformations
of the rings of dimer A are more flattened than those of dimer B.
Since the carbonyl group in the seven-membered ring is conjugated
with the aryl ring, the seven-membered rings are substantially
flattened in both dimers. This flattening effect yields ring
conformations unlike those found for other seven-membered rings.

The section of the molecule related to the hydrindane system most

closely resembles IV in which the five- to six-membered ring juncture

IV
15 trans in contrast to that observed in the two dimers; however, this
difference may be due to geometrical constraints on the reaction
mechanism rather than to energy considerations. But, what is significant
is that the five-membered ring ad0pts a half-chair conformation in
dimer A contrasted to an evenlope configuration in dimer B. From
examination of models it appears that the different relationships
of atoms Hb and Hc cause the van der Waal interactions of the
ggmfdimethyl groups to be greater in dimer B than in dimer A. This
greater interaction could be one reason for the conformational

difference. An additional (or alternate) explanation for this

134

difference is that since the cyclohexene ring in dimer A is more
flattened than that in dimer B, fusion with a five-membered ring
in a half-chair conformation is less strained than with one in an
envelope configuration.

The conformations of cyclic systems have been shown to be
highly dependent on both internal and external constraints. Although
explanations can be found to rationalize these differences, the
effects of eclipsing and van der Waal interactions of substituents
and other structural features due to such constraints are so
intertwined as to be virtually impossible to separate. In order
to be able to state explicitly which effects cause which changes,
a more systematic study would have to be made with a series of closely
related compounds in which structural and substitutional changes
would be made stepwise, and the conformational changes noted

at each step.

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In Text

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’Vb

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”UL

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