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RSIRARITYLB

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This is to certify that the

dissertation entitled

Synthesis of Thiophene Oligomers
Containing 3,4—Dibuty1thiophenes and
Synthesis of Related Mixed Oligomers

presented by

Michael E. Benz

has been accepted towards fulfillment
of the requirements for

Ph.D. degreein Chemistry

 

 

Zia/W

Major professor,

Date Dec. 16, 1992

MSU is an Affirmative Action/Equal Opportunity Institution 0-12771

  
 

  
 

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TO AVOID FINES return on or before date due.

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SYNTHESIS OF THIOPHENE OLIGOMERS CONTAINING 3.4-
DIBUTYLTHIOPHENES AND SYNTHESIS OF RELATED MIXED
OLIGOMERS

BY

Michael Eric Benz

A DISSERTATION

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

DOCTOR OF PHILOSOPHY

Department of Chemistry

1992

ABSTRACT

SYNTHESIS OF THIOPHENE OLIGOMERS CONTAINING 3,4-
DIBUTYLTHIOPHENES AND SYNTHESIS OF RELATED MIXED
OLIGOMERS

BY

Michael Eric Benz

Alkyl-substituted thiophene oligomers are of interest both as
precursors to electrically conducting polythiophenes and as models of
these polymers. Efficient routes to oligomers containing dibutyl
substituted thiophenes separated by unsubstituted thiophenes were
developed. The key compound, 3',4'-dibutyl-2,2':S',2"-terthiophene
12, was synthesized in 49% net yield from tetrabromothiophene.
The principal reactions used were transition metal-catalyzed
couplings of Grignard reagents with arylbromides. Repetition of the
coupling procedure resulted in the synthesis of 3",4"-dibutyl-
2,2':5',2":5",2"':5"',2""-quinquethiophene 27. Dimerization of
terthiophene 12 by treatment with n-butyllithium followed with
cupric chloride was also achieved. The X-ray structures of all three

compounds have been determined.

Another route, using a 1,4-diketone intermediate, allowed synthesis

of terthiophene 12 in 22% net yield, and additionally, allowed

synthesis of 3,4-dibutyl-2,S-dithienylpyrrole 25 and 3,4-dibutyl—
2.5-dithienylfuran 24.

The UV spectra of all oligomers were taken and compared with that
of the corresponding unsubstituted and monosubstituted oligomer
(when available). This comparison indicated that the terthiophene,
quinquethiophene, and sexithiophene had a moderate decrease in
conjugation. The pyrrole 25 and the f uran 24 showed little effect

due to substitution.

A new reagent for converting 1,4-diketones to pyrroles (the Paal-
Knorr reaction) was found: methylchloroaluminum amide. This
reagent converts aryl 1,4-diketones to pyrroles efficiently. In most
cases, the reaction is run at room temperature for a few hours. Use
of this reagent allows the synthesis of pyrroles by a method which
is rapid, employs mild conditions, and allows synthesis of pyrroles

not available using tradition Paal-Knorr conditions.

This dissertation is dedicated with love to my family
and especially

to Nancy.

iv

Acknowledgments

It is a pleasure to thank Professor LeGoff for his support and
enthusiasm throughout my studies at Michigan State University, and
especially, for always taking my ideas and suggestions seriously. His
deep knowledge of his field combined with a f ar-ranging imagination
are the essence of creativity.

I greatly appreciate the interest shown in this work by Professors
Blanchard and Kanatzidis. It is always good to feel that your work
has application. Thanks also go to Professor Kanatzidis and J. H. Liao
for providing the X—ray data used in this dissertation.

I am indebted to Dave Gale and Evaldo de Armas for teaching me the
basics of electrochemistry and for investigating my compounds.
Thanks also go to Paul Weipert for teaching me about his research,
some of which was applied to this work.

I am grateful to Eric Lind for getting me started in the lab, and to
Bryon Merrill, for his friendship and for teaching me so much
chemistry. His thoroughness and thoughtful approach to chemistry
are the models I strive to emulate. My interactions with the other
graduate students greatly enlivened my experiences here ("can you
name a reagent that won't work for that reaction?"), and I would
especially like to thank Michael Waldo for his friendship, humor, and
for helping me out in various times of need.

Special thanks go to my family for their support throughout my
studies, and especially to my wife, Nancy Tomboulian, for her love,
understanding, and unending support.

TABLE OF CONTENTS

Page
LIST OF TABLES ................................................................................................. Vii
LIST OF SCHEMES .............................................................................................. viii
LIST OF FIGURES ............................................................................................... Xi
INTRODUCTION .................................................................................................. 1
I. SYNTHESIS OF THIOPHENE-BASED ANNULENES ................ 1
2. SYNTHESIS OF 3,4-DIALKYLSUBSTITUTED THIOPHENE
OLIGOMERS ....................................................................................... 16
RESULTS AND DISCUSSION ............................................................................ 27
1. PROGRESS TOWARD THE SYNTHESIS OF THIOPHENE-
BASED ANNULENES ........................................................................ 27
2. CONCLUSIONS ................................................................................... 56
3. CONVERSION OF 1,4-DIKETONES TO PYRROLES .................. 57
4. CONCLUSIONS ................................................................................... 63
EXPERIMENTAL ................................................................................................ 64
APPPENDIX .......................................................................................................... 82
LIST OF REFERENCES ........................................................................................ 123

vi

LIST OF TABLES

2222

Table 1 1H-NMR of [1.3.1.31Platyrin and

[1.5.1,SIPlatryin ......................................................................... 7
Table 2 1H-NMR of [341Porphyrin ...................................................... 9
Table 3 Conductivities of Some 3-Substituted

Polythiophenes .......................................................................... 2 2
Table 4 Conductivities of Some 3,4—Disubstituted

Polythiphenes ............................................................................. 23
Table 5 UV Spectra of Various Sexithiophenes ............................ 52

vii

Scheme

1

Scheme 2

Scheme 3

Scheme 4

Scheme 5

Scheme 6

Scheme 7

Scheme 8

Scheme 9

Scheme

Scheme

Scheme

Scheme

Scheme

Scheme

10
11
12
13

14

15

LIST OF SCHEMES

Synthetic Plan for Synthesis of Thiophene
Macrocycle ................................................................................

Synthesis of 3,4—Dibutylthiophene .................................

Bromination of 3,4-Dibutylthiophene ...........................

Initial Attempt to Synthesize Terthiophene 12 .......

Optimized Synthesis of 12 .................................................
The Rathke-Weipert Reaction ..........................................

Coupling of 2-Hexanoylthiophene by the Method
of Rathke and Weipert ..........................................................

Mixed Systems Available From Diketone 23 .............
Synthesis of Terthiophene 12 from Diketone 23....
Synthesis of 3,4-Dibutyl-2,S-bis-(2'-thienyl)-furan
Synthesis of Quinquethiophene 27 ................................
The Michael—Stetter Re action ...........................................
Vilsmeier Formylation of Quinquethiophene 27 ......

Proposed Mechanism for the Michael-Stetter
Reaction ......................................................................................

Vilsmeier For mylation of Terthiophene 12 ...............

viii

28
30
3O

31

32

34

42

42

43

43

Scheme 16

Scheme 17

Scheme 18
Scheme 19
Scheme 20
Scheme 21

Scheme 22

Scheme 23

Scheme 24
Scheme 25
Scheme 26

Scheme 27

Scheme 28

Scheme 29

Scheme 30

Scheme 31

Application of the Michael-Stetter Reaction to
Compound 31 ............................................................................

Plan for Synthesis of Bis—1,4-diketone 8 Through
the Mannich Base ....................................................................

Coupling of Methyl Ketones ................................................
Acetylation of Quinquethiophene 27 .............................
Acylation of Quinquethiophene 27 .................................
Acylation of Terthiophene 12 ...........................................

Luo's Synthesis of Sexithiophenes by the Method
of Colon .......................................................................................

Coupling of Terthiophene 12 by the Method of
Colon ..............................................................................................

Synthesis of Sexithiophene 40 ..........................................
Further Oligomerization of 12 to Give 42 ....................
Attempted Synthesis of Duodecithiophene 43 ..........

Attempted Synthesis of 43 by the Method of
Horowitz ......................................................................................

Transition Metal-catalyzed Polymerization of
Thiophene ..................................................................................

The Paal—Knorr Reaction ......................................................

Attempted Conversion of Diketone 23 to the
Pyrrole ........................................................................................

Formation of the Methylchloroaluminum Amide
Reagent .......................................................................................

ix

Base

44

45
45
46
47

48

49

49
50
53
54

55

55

57

58

58

13.2.39.

Scheme 32 Synthesis of 2,5-Diphenylpyrrole .................................... 59
Scheme 33 Synthesis of 2,5-Bis-(2'-thieny1)-3,4-
dibutylpyrrole .......................................................................... 59
Scheme 34 Synthesis of the Trimer 48 and the Attempted
Synthesis of 50 ........................................................................ 60
Scheme 35 One Step Synthesis of Poly(2-aminopyrrole) .............. 63

Figure 1

Figure 2

Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9

Figure 10

Figure 11
Figure 12
Figure 13
Figure 14
Figure 15

Figure 16

LIST OF FIGURES

Isomers implied by Kekule's Proposed Structure for
Benzene .........................................................................................

Relative Energies of p Electrons by the Huckel
Method ..........................................................................................

Calculated Resonance Energies of [NI-Annulenes .......
Ring Current Induced by External Field .........................
[1,3,1,3]Platyrin and [1,5,1 ,SIPlatryin ..............................
Expanded Porphyrin-type Molecules ...............................
[34IPorphyrin .............................................................................
Badger's ThIOphene-containing Heteroannulene ........
Kauffmann's Cyclopolythiophenes ....................................

Representation of a Generalized Bpicycle Made of a-
Linked Heteroaromatic Units ...............................................

Space Filling Representation of 10-Epicycle .................
Space Filling Representation of lZ—Epicycle .................
Planar Bpicycle Made of a-Linked Thiophene Units.
Metallophthalocyanine ...........................................................
Polyacetylene .............................................................................

Proposed Mechanism of Polymerization of
Thiophene ....................................................................................

10

10

ll
12
13
14
15

17

18

Figure 17

Figure 18

Figure 19

Figure 20

Figure 21

Figure 22

Figure 23

Figure 24
Figure 25
Figure 26

Figure 27

Figure 28
Figure 29
Figure 30
Figure 31
Figure 32

Figure 33

Pig;

Two Modes of Coupling in Thiophene Polymers ......... 18
Spin Density of Thiophene and Bithiophene by

Position .......................................................................................... l9
Polarons and Bipolarons in Polythiophene .................... 20

Energy Level Diagram Showing the Smaller Band Gap

of the Bipolaron Phase of Doped Polythiophene ......... 21
Model Segment of Poly(3,4-dimethylthiophene) ........ 24
Space Filling Representation of 3,4,3',4',3",4"-
Hexamethyl-2,2':5',2"-terthiophene ................................. 25
Symmetrical Bpicycle Compared to Isomeric Pair of
Bpicycles Generated by Use of Single Substituents.... 27
3',4'—Dibutyl-2,2':5',2"-terthiophene ................................. 29
X—ray Structure of Terthiophene 12 ................................ 33
UV Spectra of Various Terthiophenes ............................. 34
UV Spectra of Thiophene-Pyrrole-Thiophene

Trimers .......................................................................................... 38
X—ray Structure of Quinquethiophene 27 ...................... 40
UV Spectra of Quinquethiophenes ..................................... 41
X—ray Structure of Sexithiophene 40 ............................... 51
Methylchloroaluminum amide ............................................ 58
Substrates Not Successfully Converted to Pyrroles... 61
Synthesis of Pyrroles Using Dichloromethane .............. 61

Figure A 1

Figure A2
Figure A3

Figure A4

Figure A5

Figure A6

Figure A7

Figure A8

Figure A9

Figure A10
Figure A1 1
Figure A12
Figure A13
Figure A14
Figure A15

Figure A16

P289.

300 MHz lH-NMR spectrum of Cambridge
99%+ CD013 ................................................................................... 82

300 MHz 1H-NMR spectrum of terthiophene 12 ........ 83
7 5.4 MHz 13c-NMR spectrum of terthiophene 12 ..... 84

300 MHz 1H-NMR spectrum of

3,4-dibromothiophene 14 .................................................... 85

75.4 MHz 13c-NMR spectrum of

3,4-dibromothiophene 14 .................................................... 86

300 MHz 1H—NMR spectrum of

3,4-dibutylthiophene 15 ....................................................... 87

75.4 MHz 13c-NMR spectrum of

3,4—dibutylthiophene 15 ....................................................... 88

300 MHz 1H-NMR spectrum of compound 16 ............. 89

75.4 MHz l3C—NMR spectrum of compound 16 .......... 90
250 MHz 1H-NMR spectrum of compound 21 ........... 91
62.9 MHz 13C—NMR spectrum of compound 21 ........ 92
300 MHz 1H-NMR spectrum of compound 24 ........... 93
75.4 MHz 13C-NMR spectrum of compound 24 ........ 94
300 MHz 1H-NMR spectrum of compound 25 ........... 95
75.4 MHz 13C-NMR spectrum of compound 25 ........ 96
300 MHz 1H-NMR spectrum of compound 26 ........... 97

xiii

Figure A17
Figure A18
Figure A19
Figure A20
Figure A21
Figure A22

Figure A23

Figure A24

Figure A25
Figure A26
Figure A27
Figure A28
Figure A29
Figure A30
Figure A31
Figure A32
Figure A33

Figure A34

75.4 MHz l3C-NMR spectrum of compound 26 ........
300 MHz 1H—NMR spectrum of compound 27 ...........
75.4 MHz 13C-NMR spectrum of compound 27 .......
300 MHz 1H-NMR spectrum of compound 30 ..........
300 MHz lH-NMR spectrum of compound 31 ..........
75.4 MHz 13C—NMR spectrum of compound 31 .......

300 MHz 1H-NMR spectrum of N-methoxy—N-
methylhexanamide ...............................................................

75.4 MHz 13C-NMR spectrum of N-methoxy-N-
methylhexanamide ...............................................................

300 MHz 1H-NMR spectrum of compound 35 ..........
75.4 MHz 13C—NMR spectrum of compound 35 .......
300 MHz 1H—NMR spectrum of compound 37 ..........
75.4 MHz 13C-NMR spectrum of compound 37 .......
300 MHz 1H—NMR spectrum of compound 38 ..........
75.4 MHz l3C-NMR spectrum of compound 38 .......
300 MHz 1H—NMR spectrum of compound 39 ..........
75.4 MHz l3C—NMR spectrum of compound 39 .......
300 MHz 1H-NMR spectrum of compound 40 ..........

75.4 MHz 13C—NMR spectrum of compound 40 .......

xiv

Bags:

98
99
100
101
102

103

104

105
106
107
108
109
110
111
112
113
114

115

Figure A35
Figure A36
Figure A37
Figure A38
Figure A39
Figure A40

Figure A41

112.22

300 MHz 1H-NMR spectrum of compound 42 .......... 116
300 MHz 1H-NMR spectrum of compound 46 .......... 117
75.4 MHz 13C-NMR spectrum of compound 46 ....... 118
300 MHz 1H—NMR spectrum of compound 48 .......... 119
75.4 MHz 13C-NMR spectrum of compound 48 ....... 120
300 MHz 1H-NMR spectrum of compound 50 .......... 121
75.4 MHz 13C—NMR spectrum of compound 50 ....... 122

XV

INTRODUCTION

1. Synthesis of Thiophene-based Annulenes

Investigation of the concept of aromaticity has been a source of
inspiration to chemists since the early nineteenth century. The
prototypical aromatic molecule, benzene, has been known since its
isolation by Faraday in 1825. Benzene was an intriguing molecule
to nineteenth century chemists. It contained four units of
unsaturation, yet did not show the reactivity usually associated
with olefins. Kekulé1 proposed that the structure of benzene was a
hexagonal ring containing three double bonds. It was soon
observed that his proposed structure implied isomers should be
found for the ortho and meta disubstituted benzenes, because of the

arrangement of bonds and substituents (Figure 1).

Figure l: Isomers Implied by Kekulé's Proposed Structure for
Benzene

X X X X
Y0 "O D <1
Y Y
Subsequent investigations determined that no such isomerization
existed, and that in fact, the six bonds connecting the carbon atoms
were equivalent. Kekulé then suggested that the three double
bonds could be interconverted by 'mechanical motion', giving

equivalent structures. This early suggestion of resonance, which

was derided at the time as an expedient designed to save the

2

proposed structure, has developed into one of the most important

concepts in organic chemistry.

The concept of resonance was first suggested by Kekulé to explain
the structure of benzene. The explanation for its surprising stability
was developed in the theoretical work of Huckel.2 Huckel's
approach considered the n—electrons of benzene separately from the
other electrons of the molecule, which greatly simplified
calculations. This separate consideration of n-electrons is known as
the HUckel approximation. The resulting molecular orbital
calculations using this method gave a straight-forward explanation
of its stability: benzene has all of its electrons in filled bonding
orbitals, and these orbitals are of lower energy than the equivalent
acyclic compound, hexatriene. Application of the Hijckel method to
cyclobutadiene and cyclooctatetraene shows that these molecules
would not be expected to be as stable, because they have unpaired

electrons in nonbonding orbitals. These orbitals are shown in Fig. 2.

Figure 2: Relative Energies of it Electrons by the Huckel Method

cyclob utadiene benzene cyclooctatetraene

 

 

 

 

3
Hilckel further generalized this approach to state that molecules
containing 4n+2 conjugated, out-of-plane n—electrons would show
additional stability. He also stated that molecules with a 4n 1:-
electron system would not be as stable. Both theoretical and
experimental investigations have confirmed these statements.
Hijckel's method has several drawbacks, however, the most serious
being a large overestimation of resonance energy for all 4n and

4n+2 systems larger than benzene.

The next major step3 in quantifying resonance stabilization was
made by Dewar and Gleicher. They improved on HUCkel's method
by introducing two elements: (1) use of the semiempirical Pople-
Pariser-Parr (PPP) molecular orbital approach to calculate the
energy of the delocalized structure, and (ii) calculation of the net
resonance stabilization by comparison to an acyclic polyene with the
same number of single and double bonds. This change in the
reference structure was the more important innovation, and
resonance energies calculated using it are referred to as 'Dewar
resonance energies“, regardless of the molecular orbital approach
used. Dewar determined by his method that as the size of a
conjugated system increases, the resonance stabilization (or
destabilization) of an aromatic (or antiaromatic) compound

decreases, eventually tailing toward zero (Figure 3).

4

Figure 3. Calculated Resonance Energies of [NI-Annulenes

I
i ERIN)

0&5-

k Aromatic

I0

 
 
  

Anti - aromatlc

-0,5r—

-I.O '—

 

The concept of aromaticity had, as developed by Kekulé and Hackel,
come to embrace two different properties. These are bond
delocalization, as suggested by Kekulé, and thermodynamic stability,
as shown by Huckel. Other criteria for determining aromaticity
have been suggested, including lack of reactivity and reaction by
electrophilic substitution. Of these criteria, thermodynamic stability
and bond delocalizaton are most generally accepted as evidence of

aromaticity.

Direct evaluation of the additional thermodynamic stability
attributible to aromatic character has often proven to be an elusive
target for experimentalists. An investigator must determine the

heat of formation, and then compare it to that of the equivalent

5
bond-localized molecule. While good estimates of the energy of this
hypothetical bond-localized molecule may be made from
thermodynamic cycles, exact solutions are unavailable.
Furthermore, determining the heat of formation involves the
destruction of the compound. Qualitative experimental evidence of
aromatic character, in the form of a diamagnetic ring current effect
in the proton nuclear magnetic resonance (lH-NMR) spectra4, is
more accessible. This diamagnetic ring current arises from
precession of the it electrons in the presence of an external magnetic
field (Figure 4). The precession produces a small magnetic field in
the opposite direction of the external field. The effect of the
induced field is to deshield the protons outside the aromatic ring
and shield the protons on the interior of the ring. Conversely,
antiaromatic character is determined by the presence of a
paramagnetic ring current. Protons which are deshielded appear in
the spectrum at a lower field than expected, and the shielded

protons at a relatively higher field.

Figure 4: Ring Current Induced by External Field

 

H°+H‘

 

6

Since both properties are functions of the number of electrons in
the 1: system, the resonance energy of simple annulenes may be
calculated to close approximation using the ring current, and this
relationship has been developed by HaddonS. Thus, examination of
the ring current gives qualitative evidence of it electron
delocalization for all 4n and 4n+2 systems and allows an
approximate determination of the resonance energy for simple
annulenes by a method which is readily available, nondestructive,

and requires only a small amount of material.

Attempts to investigate and explore the limits of 'aromatic
character' have inspired many syntheses. Synthesis of annulenes
was led by Sondheimer6, who found that annulenes containing 14,
18, and 22 carbons showed diamagnetic ring current shielding in
their NMR spectra, indicating they are aromatic. Analogously,
annulenes containing 12, 16, 20, and 24 carbons showed
paramagnetic ring current shielding in their NMR spectra, indicating

they are antiaromatic.

All of these annulenes are fluxional molecules at room temperature,
and transpose inner and outer protons in a manner similar to the
flexing of cyclooctatetraene. This property is thought to contribute
to tendency of the larger annulenes to give spectra similar to that of
polyenes. The desire to eliminate this fluxional character and to
ensure a rigid, planar molecule led to the synthesis of

dehydroannulenes. Dehydroannulenes use triple bonds to help fix

7

the geometry of the molecule. Dehydro[26]- and [3OIannulenes

have been synthesized which show a diamagnetic ring current7.

Another method of ensuring planarity has been to incorporate
heterocyclic rings into the annulene structure. An ongoing project
in this laboratory has been the synthesis of macrocycles based on
expansion of the porphyrin system by extending the bridging units
between pyrrole rings8. Two examples of this type of expansion are
shown in Figure 5. These compounds have been named

[l,3,1,3lplatyrin and [l,5,l,5]platyrin.

Figure 5. [l,3,1,3]Platyrin and [l,5,l,51Platyrin

 

[1,3, l ,3IPlatyrin [1.5.1.51Platyrin

These compounds (22 and 26 nelectron systems, respectively) both

show strong evidence of a diamagnetic ring current, as indicated by

the proton shifts shown in Table 1.

Table 1: 1H-NMR of [l,3,1,3]Platyrin and [1.5.1.51Platyrin

[l,3,1,3]Platyrin [1.5.1.51Platyrin
internal C-H(8) -8.97 -l4.26
N-H(5) -5.6 -10.58

ext. methine(8) 1 1.64 1 1.75

8
Other investigations devoted to the synthesis of hetero-bridged
annulenes include the 22 n—electron systems pentaphyrin9a l,
decamethylsapphyrin"b 2, texaphyrin9° 3, and the 26 n-electron
system, hexaphyrin9d 4 (Figure 6). All show strong diamagnetic

ring current shielding in their 1H-NMR.

Figure 6: Expanded Porphyrin-type Molecules

      

MeP
Pentaphyrin (l) Decamethylsapphyrin (2)
(PMe=CH2CH2C02Me)

Et

 

Et
Texaphyrin (3) Hexaphyrin (4)

The largest heteroannulene synthesizedlo to date is shown in Figure
7. It has a conjugated ring containing 34 1t electrons. The 1H-NMR
spectrum (Table 2) shows evidence of a strong diamagnetic ring
current, clearly demonstrating that the limit for showing electron

delocalization has not yet been reached.

Figure 7: [3 4IPorphyrin

 

Table 2: 1H—NMR of [34IPorphyrin

(8)

Ha, H6 16.18
Hb, Hd 44.27
HC 17.19

Relatively few heteroannulenes containing thiophene have been
synthesized. One, synthesized by Badgerll and coworkers, is shown
in Figure 8. Because of the large sulfur atoms, this molecule is not
planar, and its aromaticity is only that of its three component

thiophenes acting independently.

10

Figure 8: Badger's Thiophene-containing Heteroannulene

The compounds shown in Figure 9 were synthesized by
Kauffmann12 and coworkers. These compounds are also nonplanar.
The X—ray structure determination of the center compound shows it
to have adopted a saddle conformation, with opposing pairs of
thiophenes tipped toward each other. The 1H-NMR spectra show

only signals normally expected for disubstituted thiophenes.

Figure 9: Kauffmann's Cyclopolythiophenes

 

Thus, it seems that conjugated ring systems that (1) contain 4n+2 1t-
electrons, and (ii) have a rigid planar structure, generally show
evidence of aromatic character in the form of a diamagnetic ring
current, up to the limits so far obtained for n. Synthesis and

investigation of larger ring systems must both meet the challenge of

11

guaranteeing the desired planarity of the ring system and cope with

the highly reactive nature of large n-electron systems.

Our conceptual approach to such a macrocyclic system is shown in
Figure 10. The system is created by linking heteroaromatic
molecules (such as f uran, pyrrole, or thiophene) in a consecutive

2:2“ fashion.

Figure 10: Representation of a Generalized Bpicycle Made of
a-Linked Heteroaromatic Units

This macrocycle (or 'epicycle', as it has been named in this
laboratory), with proper selection of X and n, will be planar. We
investigated various epicycles using the MM2 modeling system13
and found that for n=8 or 10, furan and pyrrole give a planar
structure. For thiophene, n=10 gives an interesting bowl-shaped
epicycle (Figure 11) and n=12 gives a planar epicycle (Figure 12).
The chemical robustness of thiophene relative to that of f uran or
pyrrole14 indicates that it would be the best starting material for

synthesis of such an epicycle.

12

 

Figure 11: Space Filling Representation of IO-Epicycle

13

 

Space Filling Representation of 12—Epicycle

Figure 12

14

The epicycle containing twelve thiophenes (Figures 12 and 13)
could be considered a sulfur-bridged [48]annulene. The bridging
sulfur atoms provide the necessary rigidity to ensure planarity.

The aromatic nature of thiophene would enhance the stability of the
neutral molecule, which is supported by the relative stability of
thiophene polymers, Vide infra. Two electron oxidation or reduction
would provide 4n+2 systems of 46 and 50 1t electrons, respectively,
which could be examined for evidence of aromaticity in the form of

a diamagnetic ring current.

Figure 13: Planar Bpicycle Made of a-Linked Thiophene Units

 

In addition to the study of aromaticity, this compound would also
be of interest as a potential electric conductor because of its
similarity to known molecular organic conductors. It has been
found that metallophthalocyanines15 (Figure 14) form crystals
which conduct electricity upon partial oxidation (doping). These
crystals have phthalocyanine molecules arranged in stacks, allowing
interaction between the n—orbitals of adjacent molecules. Since this
feature may also be found in crystals of the macrocycle 5, its

electrical conductivity could be investigated. The thiophene

 

15

macrocycle would also represent an 'ideal' polythiophene model,

because it is defect free, all (it-linked, and has no end effects. The
space between sulfur atoms across the molecule, which we calculate
by MM2 to be 12.2 A, Opens the possibility of using it as a

complexant.

Figure 14: Metallophthalocyanine

N
3:, N\ IN:<‘

M
N

N/\N
C9N

 

2. Synthesis of 3,4—Dialkylsubstituted Thiophene Oligomers

Thiophene oligomers have attracted interest due to their biological
activity and electronic properties. Terthiophene, a compound found
in some species of marigoldsl6, has a phototoxic effect on
nematodesl7 and the larvae of the mosquito and blackfly18. Longer
oligomers have been investigated for their potential to be
incorporated into electronic devices19 and as models for higher
polymerszo. High molecular weight thiophene polymers have been
shown to be relatively good organic conductors, with conductivities
in excess of 1000 50cm'1 after doping21. Thiophene-based
polymers have been investigated for use in many applications.
Among these are batterieszz, solar cellsZ3, and electronically

controlled drug delivery systems“.

Polymers which contain a conjugated n-electron system are found to
conduct electricity upon partial oxidation, or ”doping". This
property was discovered by Shirakawa and coworkers in 197725.
They found that polyacetylene (Figure 15) doped with iodine,
bromine, or chlorine vapor showed an improvement in conductivity
of some seven orders of magnitude. Polyacetylene is highly labile

in the presence in oxygen, limiting its possibilities as a raw material

16

17

for devices. Indeed, it has been suggested that polyactetylene may

be useful as an oxygen scavenger.

Figure 15: Polyacetylene

\\\\\\

11

Other organic compounds that could be converted to conjugated
polymers were investigated, and many found to conduct electricity
upon doping. Thiophene was found to be readily chemically or
electrochemically polymerized, and to form polymers of good
conductivity26. It also proved to be air and moisture stable in both
the doped and neutral form. These properties have made thiophene
and especially B-substituted thiophenes among the most interesting
and most widely investigated building blocks for conducting
polymer synthesis. The use of electrochemical synthesis gives a
polymer which is free of impurities and has relatively few

interruptions of conjugation”.

The electropolymerization27'28 of thiophene (Figure 16) starts with
electrochemical generation of radical cations. These couple to give
the dihydrobithiophene dication 6, which forms bithiophene upon
elimination of a pair of protons. Rearomatization has been
suggested to be the driving force for the reaction. The newly
formed oligomers (in Figure 16, bithiophene, 7), have lower

oxidation potentials than the starting monomer.

18

Figure 16: Proposed Mechanism of Polymerization of Thiophene

I [Q]
10] 53-10; Q—Q

0+

/\/\-e‘ [6/\/\] [(5)]
s s _*

/\ \— ,_-_2_H_:./\ /\ /\
S S

The coupling of the radical cations can also occur in an oc-B', rather
than the oc-oc' fashion, as shown in Figure 17. Initially, oc-oc' coupling
is favored, because the singlet electron density resides primarily on
the a-carbon. As polymerization procedes, ct-oc' coupling is not as
strongly favored over oc—B‘ coupling, because as oligomer length
increases, n-delocalization of the radical over the chain decreases

the relative probability of oc—oc' coupling”.

Figure 17: Two Modes of Coupling in Thiophene Polymers

/\
/\ /\ /\ 5
S

S S
oc—oc' coupling oc—B' coupling

19
Molecular orbital calculations29 indicate that the spin density of the
thiophene radical cation (Figure 18) strongly favors the a-position.
The bithiophene radical cation, while still having the greatest spin
density at the (it-position, also has high spin density at the internal
B-position. Steric interactions disfavor coupling at this site, but it
may still make an appreciable contribution. These a-B' couplings
are thought to decrease the conductivity of polymers because the
interruption of conjugation prevents the transport of charge carriers
such as bipolarons. While up to one third30 of the linkages in

polypyrrole are a-B', spectroscopic evidence indicates that this

defect occurs rarely in polythiophene“. 39drg

Figure 18: Spin Density of Thiophene and Bithiophene by Position

5 S

s
l l
5 . 1e .
. . I. I6 6 I .tl . l
1 -.2o -.14
2 .67 .67
3 -.01 -.50
4 .57

Neutral polythiophene is an insulator. To convert it to the
conducting phase, it must be oxidized. Removing electrons from the
1: system creates vacant p orbitals, which act as charge carriers.
Low levels of doping cause the formation of polarons (radical-

cations). At higher levels of doping (approximately seven percent

20

or greater), bipolarons (dications) are formed, which are thought to
be the major charge carriers in conjugated polymers32 (Figure 19).
Maximum doping levels33 for polythiophenes are typically on the

order of 30-50 %.

Figure 19: Polarons and Bipolarons in Polythiophene

/\+5__—_5_—'__s /\

S _ S _ S _ S
polaron
/ww_“_s___8+m
S _ S _ S _ S
bipolaron

As can be seen in figure 19, the creation of bipolarons forms a
"quinoidal” phase. The reorganization of the polymer to form this
quinoidal phase raises the energy of the highest occupied molecular
orbital (HOMO), with a corresponding decrease in the energy of the
lowest unoccupied molecular orbital (LUMO) (figure 20). Brédas34
has shown that the band gap decreases as a function of increasing
quinoidal character and that the conductivity of the polymer is

proportional to the degree of doping.

21

Figure 20: Energy Level Diagram Showing the Smaller Band Gap
of the Bipolaron Phase of Doped Polythiophene

 

 

i533331EIEIEIEIEIEIEIEIEIEIE 21.129.131.9311flM9 333131EIEIEIEIEIEIEIEIEIEIE '

 

 

OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO

 

Aromatic Quinoid
Phase - — Phase

 

nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn
-------------------------------------------------------
OOOOOOOOOOOOOOOOOOOOOOOOOOOOO

C323232313:32323232323232:.1.3.113.913.1311}.3.191119.52323Z323232323232323232323231A

 

 

 

 

 

 

 

 

IIIIII/I’ll],III/IIIIIIIIIIIIIIII[III/IIIIII,lllI/II/I'IIIIIIIIIIIII
r1111111 111111111111111111111111111111111111111111111111111111111.
r1111111 1111 1111111111111111111111111111111111111111 1 11 111111.
r111111111111111111111111111111111111111111111111111111111 111111111.
r11111111111111111111 --------------------------- 1111111111111 111111.
—'IIIIIIIIIIIIIIIIIIII ’IIII I I IIIIIIII-
r11111111111111111111 () IIIEB 1111111111111 111111
'IIIIIIIIIIIIIIIIIIII ’IIII I I I III/III
'l/l/l/l/l/l/ll/IIIIII11111111111111111111111111!!! ll/l/l/l/IIIIIII
r1111111111111111111111111111111111111111111111111111111111 11111111
1111111111111111111111111111111111111111111 11111111111111111111111
r1111111111111111111111111111111111111111111 11111111111111111111111
lllllllllllllllllllllllll lllllll ll lllll [11111111lllllllllllllllll

 

Much research has been directed toward investigating the changes
in physical and electrical properties of conducting polythiophenes
caused by modifying their structure. It is desirable to develop the
ability to 'tune' the properties of the polymer by appropriate
selection of the reaction conditions and monomer. These properties
include conductivity, environmental stability, solubility, melting
point (for melt processing), and optical response. Synthetic
chemists may contribute to this effort by synthesizing monomers to
be incorporated into conducting polymers. Initial efforts in this
area were directed toward synthesis of 3-substituted thiophenes.

It was shown that use of 3-substituted thiophenes gave improved

22

electronic conductivities in the resulting polymers (Table 3). 3-
Methyl thiophene gave a polymer which has higher conductivity
than thiophene. Many of these substituents have the additional
advantage of improving the physical properties of the resulting

polymer.

Table 3: Conductivities of Some 3-Substituted Polythiophenes

Monomer Conductivity (80cm'1) Reference
CH3

[/ \S 1975 35(a)
s CZHS

(/ \S 270 3500)
s CSHH

[/ \S 140 35(e)
3 C6Hl3

[/ \g 95 35(d)
S C7HIS

/ \ 110 35(c)
5 C9Hl9

[/ \g 100 35(e)
S CH20(CH2CHZO)2CH3

[/ \g 1050 35(e)
5 CHchoan23

Z/ \I 200 35(6)
5 (CHZCH20)2CH3

((5)1 250 35(1)

Polymers of 3,4-disubstituted thiophenes have also been studied,
but they were generally found to be poorer conductors than

monosubstituted polythiophenes (Table 4). This relative lack of

23
conductivity is thought to be caused by poor n-orbital overlap due

to the inability of the rings to adopt a coplanar conformation.

Table 4: Conductivities of Some 3,4-Disubstituted Polythiophenes

Monomer Conductivity (5- cm" 1) Reference

0
III
u
0
:1:
u

\
U)
/

0.5 36(a)

M 1110‘5 36(b)
Z/'—\S <1x10'S 36(b)

CH3 5 oc,,H9
Z/ \1 2.0 36(b)
S
C4H90 OC4H9
2 S '5 36(b)
/ \ 1x10
8
Ets Sm Does not
2/ \S electrochemically 29
S DOIymerize.
5/ g 10-20 36(c)
S
3 Does not
electrochemically 36(c)
/ \ DOIymerize.
S
OCH3 36(d)
220
2/ \S
S 0C2H4OCH3
2’ \S 30 36(d)

(I)

36(d)

\
U)
/
O
9
N
m
N
VI
K11

24

We have tested this pr0posal by using MM2 calculations to
investigate the effect of adding B-methyl groups to terthiophene
(Figure 21). The calculations performed on this oligomeric model of
the polymer (Figure 22) gave torsional angles of 47‘. The degree of
n-orbital overlap follows the cosine of the angle between rings, so a
small degree of nonplanarity will not have a great effect on the
conductivity37. It has been proposed that torsional angles less than

40° permit sufficient overlap for good conductivity37.

Figure 21: Model Segment of Poly(3.4-dimethylth10phene)

Me Me Me Me

/\ 5 /\
s \/ 5

Me Me

The relatively higher conductivities of the fused cyclopentyl- and
dialkoxythiophene polymers in Table four have inspired the
synthesis of several related systems33, but the 3,4-disubstituted
systems so far synthesized have conductivities typically an order of
magnitude less than that of polythiophene or the polymers of 3—

substituted thiophenes.

Other studies devoted to exploring the properties of polythiophenes
have focused on the effect of coupling longer oligomer units to form
the polymer. Electropolymerization of thiophene, bithiophene,

terthiophene, and quaterthiophene has given different results in the

hands of different investigators”. It has been suggested that the

25

 

\ \ \:\\ .
§§\X\\\ \\\\§ 3
\‘ \\\‘

V \,,‘

 

Figure 22: Space Filling Representation of 3,4,3',4'.3",4"—
Hexamethyl—Z,2':S'.2"-terthiophene

 

26

basis for the differences ob served in these materials is a reflection
of differences in the conjugation length. These differences could be
caused by the relative degree of polymerization, the number of a-B'
couplings of monomer units, or the morphology of the polymer.
While the degree of polymerization may be optimized by changes in
electropolymerization conditions“), it is possible that the relative
number of (at-[3' couplings could be minimized by synthesizing longer
oligomers. Recently developed techniques for the electrochemical
synthesis of polythiophenes should further reduce the occurence of

a—B' linkages”. The use of symmetrical oligomers is desirable in

order to know the connectivity of the subsequent polymer.

This study synthesizes oligomers appropriate for incorporation into
conducting polymers. It combines dialkyl substitution to provide
good solubility with oligomer synthesis in discrete lengths. These
symmetrical oligomers contain central 3,4-dibutylthiophenes
flanked by unsubstituted thiophenes. We have synthesized
compounds with the following goals in mind: 1. improve solubility
of oligomers by having alkyl groups attached to the beta position of
heterocyclic rings; and 2. synthesize oligomers which would give

copolymers of known stoichiometry and connectivity.

1. Progress Toward the Synthesis of Thiophene-based Annulenes

Any strategy for the synthesis of a macrocycle such as 5 must
address the insolubility of thiophene oligomers. Previous work in
this laboratory14(¢) has shown that the solubility of thiophene
oligomers drops sharply as the number of thiophenes in the
oligomer increases. As described in the introduction, B-alkyl groups
are commonly used to improve the solubility of thiophene oligomers
and polymers. We chose to use butyl groups to improve solubility.
The use of 3,4-dib utylthiophenes in the target macrocycle 8, as
shown in Figure 23, has the additional advantage of eliminating the
possibility of forming isomers such as 9 and 10 due to

unsymmetrical incorporation of oligomers.

Figure 23: Symmetrical Bpicycle Compared to Isomeric Pair of
Epicycles Generated by Use of Single Substituents

Bu Bu
Bu Bu

Bu Bu Bu
Bu

27

28

Synthesis of such an epicycle called for soluble, symmetrical
oligomers containing dibutyl-substituted thiophenes. We planned to
synthesize these oligomers, f unctionalize them, and then cyclize
through formation of 1,4-diketones to give compound 1 1.
Conversion of the 1,4-diketones to thiophenes would be carried out
by treatment with Lawesson's reagent (Scheme 1). This strategy
has an additional advantage, because it can also be used to generate
other macrocycles by converting the 1,4-diketones in 1 l to f uran or

pyrrole.

Scheme 1: Synthetic Plan for Synthesis of Thiophene Macrocycle

O O
/ \
/ \
/ \ Lawesson's

S s -------- -> ------------ -> 8

\ / reagent
/ \
/ \

0 O

1 l
8,8.
CH3. P\ ’13 OCH3

Lawesson‘s Reagent

29

Accordingly, compound 12 (Figure 24) was seen as a key
intermediate in the synthesis of macrocycle 8. Although many
terthiophenes have been synthesized,41 none of them contain a 3,4—
disubstituted central thiophene with alkyl substituents larger than
methyl. Of the possible methods available to prepare 12, the most

direct method was considered to be the use of the transition metal-

Figure 24: 3',4‘-Dibutyl-2,2':5',2"—terthiophene

/\ /\ l\
S S S

12
mediated coupling reactions developed by Kumada. Kumada‘“ID has
used this strategy to perform aryl—aryl and aryl—alkyl couplings in
high yield, and in fact had synthesized 3,4-dibutylthiophene and
2,2':5',2"-terthiophene.

3,4-Dibutylthiophene was synthesized as shown in Scheme 2,
starting with selective debromination of tetrabromothiophene with
n-buLi. Although the original publication“2 called for running the
reaction at 0°C, running the reaction at -78°C improved the yield
from 74% to 84%. The 3,4-dibromothiophene was converted to 3,4-
dibutylthiophene by coupling with n-butylmagnesium bromide in
the presence of a catalytic amount of [l,3-bis(diphenylphosphino)-
propane]Ni(II) chloride (Ni(dppp)C12). 3,4—Dibutylthiophene 12 was

produced in 77% yield upon distillation.

30

Scheme 2: Synthesis of 3,4-Dibutylthiophene

Br Br 2.2 equivalents Br Br n-butylmagnesium
/ \ n-buLi _ 2'1, \5 bromide _ I \
etITer ' Niidppplz 7

Br Br 8
S -78'C S ether

13 84% 14 77% 15

 

 

Compound 15 was treated with tetramethylammonium tribromide
(TMAT) in a 1:1 mixture of acetic acid and dichloromethane (Scheme
3). This resulted in the formation of 2,5-dibromo-3,4-

dibutylthiophene 16 in 92% yield.

Scheme 3: Bromination of 3,4-Dibutylthiophene

/ \ 2.2 eq.TMAT->HF
s AcOH/CHZCIZVBT Br

 

s
15 92% 16

Initial attempts to couple 16 with thiophenemagnesium bromide
using Ni(dppp)Clz as catalyst resulted in the formation of 12 in 1 1%
yield (Scheme 4). Although the desired product was obtained, the
reaction did not go to completion and resulted in the formation of
undesired side products, with starting material remaining. Since it
has been reported that lithioalkenes may be coupled with alkyl and
aryl bromides using Ni(dppp)Clz, the analogous reaction using 2-
lithiothiophene was attempted. This gave the desired terthiophene
12 in 20% yield. Use of bis(triphenylphosphino)nickel(II) chloride

as the coupling catalyst produced only trace amounts of 12.

31

Scheme 4: Initial Attempt to Synthesize Terthiophene 12

2.2 eq. thiophene-
, \ magnesium bromide / \ / \ / \
‘ %Ni d Cl ,
B, S B, S ( ppp) 2 S S S
16 12

A
r

11%

Examination of other coupling catalysts showed that [l,3-
bis(diphenylphosphino)ferrocene]Pd(II) chloride (Pd(dppf)ClZ) has
been used by Kumada‘i3 to couple aryl bromides with sterically
crowded alkyl magnesium halides. Using this catalyst gave an
immediate improvement in yield to 31%. More vigorous conditions
were tried using the higher boiling solvents tetrahydrofuran and t-
butyl-methyl ether. These attempts to push the reaction to
completion were not successful. Use of tetrahydrofuran resulted in
no formation of the desired product. It was found that
thiophenemagnesium bromide was not soluble in t-butyl—methyl

ether.

In previous reactions, I had observed that when the addition of
thiophenemagnesium bromide to the flask containing Ni(dppp)Clz
and 16 was made at 0°C, a clearer solution resulted (although the
yield did not increase). Based on this observation, I attempted the
reaction (Scheme 5) using Pd(dppf)Cl2 as catalyst and adding the
Grignard reagent to the solution of 16 at -78°C. The cold bath was
removed after one hour and the reaction allowed to continue an
additional twelve hours. The reaction went to completion and 12

was isolated in 83% yield after flash chromatography. The only side

32
product observed was bithiophene, which is necessarily formed in
the initial activation of the catalyst. It appears that using a catalyst
with a more highly hindered catalytic site eliminates the side
reactions observed using Ni(dppp)Clz. This, combined with the low
initial temperature of the reaction, prevented subsequent

deactivation of the catalyst.

Scheme 5: Optimized Synthesis of 12

2.2 eq. thiophenemagnesium bromide

57: Pd(d f)C1 ,
B / \ pp 2 e / \ / \ / \
r Br S

S -78'C —> room temperature 3 s

16 83% 12

 

Terthiophene 12 forms pale green crystals with a melting point of
36.0—36.5’C. It is extremely soluble in chloroform, dichloromethane,
and ether. It is moderately soluble in most other organic solvents.
The isolation of high quality crystals resulted in the determination
by M.G. Kanatzidis and 1H. Liao of the structure by X-ray diffraction
(Figure 25). This structure is very similar to that of the
unsubstituted terthi0phene,44 whose thiophene rings are in a
similar antiplanar conformation with a torsional angle of about 6-9‘.
The UV spectrum of terthiophene 12 showed a maximum at 331
nm. Comparison with nonsubstituted terthiophene 17 (364 nm)
and the mono-alkyl terthiophenes of Ferraris4le (18, 350 nm; 19,
342 nm) and Delabouglise“1d (20, 347 nm) shows a regular
hypsochromic shift of approximately 16 nm associated with each

alkyl group (Figure 26). According to the Woodward-Fieser rules

33

 

Figure 25: X-ray Structure of Terthiophene 12

 

34
for calculating the Xmax of conjugated polyenes, addition of alkyl
substituents should produce a bathochromic shift of 5 11111 per
substituent. Apparently, the alkyl groups prevent maximum
coplanarity from being achieved. A test for this proposal would be
to synthesize the 2,5"-dialkyl-2,2':5',2“-terthiophene, since alkyl
groups on the end positions should not adversely affect conjugation.
Uhlenbroek and Bijlool7b have synthesized the 2,5"-dimethyl-
2,2':5',2"-terthiophene, which shows a bathochromic shift of 9 nm.
This corresponds well with the prediction of the Woodward-Fieser

rules.

Figure 26: UV Spectra of Various Terthiophenes

C7HIS C18H37 CIOHZI
/s\/s7\/SW\/\/\szs\l\/s\
S S
19
UV (tmaxi364 350 342 347

Another route to terthiophene 12 was opened by the efforts of
Rathke and Weipert“S (Scheme 6). They have developed a new
method to couple ketones Via an intermediate bis-silyl enol ether.
The bis-silyl enol ether is oxidized by the action of cerric

ammonium nitrate to form the 1,4—diketone.

Scheme 6: The Rathke-Weipert Reaction

R12 MeZSiClz Et3N R' 0.5i.0 R‘ (NH412Ce(N03)s. _ 0
YR REF \E - R‘ R'
NaI CH3CN R2 M92 R2 NaHCO3

R2 R2

 

 

 

 

35

This route is unique in that it works only for alkyl ketones larger
than methyl (R2 cannot be H). Use of this method gives the coupled
product diketone 23 in 49% yield from the starting 2-
hexanoylthiophene 21 (Scheme 7).

Scheme 7: Coupling of 2-Hexanoylthiophene by the Method of
Rathke and Weipert

 

/ \ a _ / \ / \
S V S I 0‘5i.0 I S
0 M62
21 22
b

a) MeZSiClz, Et3N, NaI, CH3CN
b) (NH4)2C€(NO3)6, N3HCO3
net yield = 49% l \ I \

This route to 12 also opened the possibility of forming mixed
oligomers, by converting the 1,4—diketone 23 to furan 24, or
pyrrole 25 (Scheme 8). These mixed heterocyclic systems would
allow comparison of the properties within the trio of 12, 24, and

25.

 

36

Scheme 8: Mixed Systems Available From Diketone 23

/\ /\
s s

0230

/\/\/\ /\/\/\ /\/\/\
S S S S

S 0 S N S
H

12 24 25

Conversion of 23 to 12 was achieved in 83% by treatment with

phosphorous pentasulfide in refluxing toluene (Scheme 9).

Scheme 9: Synthesis of Terthiophene 12 from Diketone 23
P S
’5‘ ’5‘ 2 5 . / \ / \ / \
S S S
0 O
23 83% 12

Comparison of the two routes to 12 show them to be quite similar

 

in the amount of work required. Both take four steps and three
purification operations. The higher yield (49% versus 22%) obtained
using the transition metal-catalyzed coupling reaction make it the
preferred route to 12. The other route has the advantages of not

requiring expensive coupling catalysts and of providing the

37

diketone 23, which can be used to synthesize the corresponding

f uryl and pyrryl compounds.

Initial attempts to dehydrate the diketone 23 by treating with
hydrochloric acid in acetic anhydride gave the f uran 24 in yields of
less than 10%. Although the reaction was run for up to four days,
TLC showed a large amount of unreacted starting material.
However, a later attempt gave a yield of 66% after a reaction time of
24 h (Scheme 10). Comparison of these results led to the realization
that the intial attempts, giving the poorer yields, were made using
the easily purified higher melting isomer of 23. The latter attempt
was made using a mixture consisting mainly of the lower melting
isomer. This difference in rate of reaction of 2,3-dialkyl-l,4-
diketones has been previously noted46 in studies of the Paal-Knorr
reaction, with the d,l isomers reacting 4—40 times the rate of the
mesa. Since the dehydration of 1,4-diketones to form furans has
some similarities to the Paal-Knorr reaction, it is not surprising that
it shows a similar difference in rates. Compound 24 is a green oil
(Amax = 351 nm). This value for the UV absorption is almost
unchanged from that of the unsubstituted oligomer (lmax = 352 nm),
perhaps indicating less steric crowding by the butyl groups with the

smaller furan ring.

 

38

Scheme 10: Synthesis of 3,4-Dibutyl—2,5-bis-(2'-thienyl)-furan

\ HCI’ACZO e /\ /\ /\
S S O S
24

/\ /
s
023 0 66%

 

Conversion of diketone 23 to the pyrrole 25 was not achieved by
standard methods. The method developed for converting this and
other 1,4-diketones to pyrrole is described in part two of this
section. Pyrrole 25 (Figure 27) is a pale green solid (mp. = 45.5—
46.5°C) with Xmax = 342 nm, which again, is little changed from the
unsubstituted trimer. Ferraris4le has also synthesized a similar

mono-alkyl pyrrole 26, with Xmax = 315 nm.

Figure 27: UV Spectra of Thiophene-Pyrrole~Thiophene Trimers

C18H37
/\ /\ /\ /\/\/\ /\/\/\
S N S S N S S N S
H H H
25 26 27
UV (Amax) 342 315 349

In continuing with the plan for the synthesis of 8, 12 was
dibrominated using TMAT and the nickel-catalyzed coupling
reaction was repeated (Scheme 1 1). This produced the bright

golden-yellow quinquethiopohene 27 in 53% yield.

 

39

Scheme 11: Synthesis of Quinquethiophene 27

/\/\/\12

 

S S S
2.2 eq. TMAT
AcOH/CHZCIZ
Br/\ /\ /\Br26
S S S
2.2 eq. thiophenemagnesium
bromide
5% N1(dppp)C12
ether
11
/\ /\ /\ /\ /\ 27 53%netfrom12
S S S S S

Quinquethiophene 27 forms golden yellow needles with a melting
point of 102°C. It has good solubility in common organic solvents.
In fact, in one case where a modification of the synthetic procedure
resulted in contamination of the product with quaterthiophene, 27
was easily purified by dissolving the mixture in hot hexanes,
cooling, and filtering off the solid quaterthiophene from the solution
of 27. Slow recrystallization of 27 from hexanes gave crystals
suitable for X-ray structure determination by M.G. Kanatzidis and
j.H. Liao (Figure 28). Quinquethiophene 27 displays a UV
absorbtion at 389 nm. The Xmax of the nonsubstituted

quinquethiophene 29 and Wynberg's47 butylsubstituted thiOphene

 

40

P a
a.
a
. a
a
I.
’ ’ i o
" a
, ’ 1 an 4..
' t,
, m"
a
a
a ' ,,
a a a. ,,
Q ’ . 91’.

Figure 28: X—ray Structure of Quinquethiophene 27

41

28 are shown below in Figure 29. This series follows the trend of
the di- and monosubstituted terthiophenes in showing a regular
hypsochromic shift in the UV absorbance with increasing alkyl

substitution.

Figure 29: UV Spectra of Quinquethiophenes

kmax
/\/\/\/\/\ 389
s

S S S S
27

/\/\/\/\/\ 412

S S S S S
28

/\/\/\/\/\ 416
S S S S S

29

An alternative route to 27, using Pd(dppf)C12 as catalyst and the
conditions shown in Scheme 5, gave the quinquethiophene in 39%
yield. Although this method results in a slightly lower yield, it gives
more consistent results, and is the preferred method for

synthesizing quinquethiophene 27.

The initial strategy for cyclization was to diformylate 27 and join
two molecules of the resulting dialdehyde using the Michael-

Stetter‘i8 reaction, which would give 1 1. This reaction (Scheme 12)

 

42

has been used with great success in this laboratory in the synthesis
of numerous oligomers of pyrrole, thiophene, and furan14.

Scheme 12: The Michael-Stetter Reaction

thiazolium salt (cat), NaOAc A ,< > A
Ar-CHO > r r

 

divinyl sulfone, EtOH, reflux 0 0
Ar Yield (%) CHZPh
phenyl 46 _ . 11+
2-thienyl 48 thiazolium salt XI \>1-
2-furyl 75 H0 5
2-pyrryl 70

Treating quinquethiophene 27 with excess phosphorous oxychloride
in dimethylformamide (DMF) resulted in formation of the
dialdehyde 28 in 69% yield (Scheme 13). This brick red compound
had much lower solubility than 27.

Scheme 13: Vilsmeier Formylation of Quinquethiophene 27

/\ /\ /\ /\ /\ P00131DHVF /\ /\ /\ /\ /\

S S S S S I S S S S S

27 69% 0 30 0

 

 

Subjecting 30 to Michael-Stetter conditions resulted in recovery of
starting material. An examination of the proposed mechanism for
the Michael-Stetter reaction (Scheme 14) shows it to have several
equilibria which are concentration dependant. The dilute solution
necessitated by the low solubility of 30 appeared to be the source

of the lack of reactivity.

 

43

necessitated by the low solubility of 30 appeared to be the source

of the lack of reactivity.

Scheme 14: Proposed Mechanism for the Michael-Stetter Reaction

 

 

R R
1+ 1+0-
Ar—CHO + A): \>- T T «)1 HH
H0 5 HO 5 Ar

0 OZSKL 13+ 0H
ArMsog ‘ HO/J H -

 

 

5 Ar

ylbase

i

+ OH

N
. gt ~>+

JO 53:?» . ,.
Ar (Michael addition) 0 0

This possibility was investigated by synthesizing the dialdehyde 31
and subjecting it to Michael—Stetter conditions. Treating 12 with
POCl3 and N—methylformanilide produced 31 in 76% yield (Scheme

15). This golden yellow solid was much more soluble than 30.

Scheme 15: Vilsmeier Formylation of Terthiophene 12

P0C13’ N-methyl-
/\ /\ /\ ML,

8 S S 100 °C,2hr
12 76%

 

 

44

When compound 31 was subjected to Michael-Stetter conditions at
0.0012 M (1 gram 31 in 2 L anhydrous ethanol), no reaction
occured after 15 days. When the same reaction was attempted at
0.012 M, reaction was swift and complete in a few hours (Scheme
16). This great difference in reactivity seems to indicate that the
Michael—Stetter reaction is strongly concentration dependent. The
reaction produced 890 mg of an orange solid and a small soluble
fraction (100 mg), which contained a plethora of products. The solid
fraction was insoluble in all solvents. Separation of the soluble
compounds was attempted by both flash chromatography and
preparative thin layer chromatography, neither of which was
successful. Treating the solid, presumably the polymer shown in
Scheme 16, with Lawesson's reagent resulted in the formation of a

glossy black material, which was equally intractable.

Scheme 16: Application of the Michael—Stetter Reaction to
Compound 31

vinyl sulf one,

. 0 thiazolium saltr H
0 3 l NaOAc, EtOH, reflux

 

   

An alternative method‘19 for performing the Michael-Stetter
reaction is to use the addition of aldehydes to Mannich bases.
Applying this methodology to the synthesis of l 1 could be done as

shown in Scheme 17.

 

45

Scheme 17: Plan for Synthesis of Bis-1,4—diketone 8 Through the
Mannich Base

 

o o
:s
:s
:s
:5
:s

o o

27 33 34 ll

Synthesizing the bisacetquuinquethiophene 33 would also open
another route to 1 1 (Scheme 18). This is the oxidative coupling of
methyl ketones Via their enolates, which is a procedure50 known to

proceed in good yields.

Scheme 18: Coupling of Methyl Ketones

O 0‘

©)k LDA O/T CuCl2 O 0 o a 95%

Attempts to synthesize the bisacetquuinquethiophene 33 were

undertaken using several standard methods of Friedel—Crafts

 

46

acylation. Treating 27 with acetic anhydride and phosphoric acid as
catalyst resulted in no reaction. Treating 27 with acetyl chloride
using tin chloride as catalyst at room temperature resulted in no
reaction, but warming caused reaction to occur, quickly followed by
the formation of black material. Quenching the reaction at the
moment it started to throw down black material allowed the

isolation of 2-acetquuinquethiophene 35 in 16% (Scheme 19).

Scheme 19: Acetylation of Quinquethiophene 27

/\ /\ /\ /\ /\ 27
s s s s s

acetic anhydride,
SDCI4 16 %

 

The ketone coupling reaction developed by Rathke and Weipert was
also considered for its potential to generate an intermediate similar
to l 1. In addition to the methods used in the attempted synthesis
of 25, which were unsuccessful, Weinreb‘s ketone synthesi55l
through use of N,N-methoxymethylamides, which does not subject

the substrate to acid conditions, was attempted (scheme 20).

 

47

Scheme 20: Acylation of Quinquethiophene 27

/\l\ /\ /\ /\
s s 5 5—9
27

1.2 eq. n-buLi
O
2. 2 eq. Me-NW

OMe

yield=6 %

 

 

This method gave the dihexanoquuinquethiophene 36 in very poor
yield. Upon addition of N-methoxy-N-methylhexanamide, the
solution quickly turned dark red. Flash chromatography resulted in
significant decomposition of material on the column, with the result
that diketone 36 was isolated in 6% as an impure sample. The
inability of Weinreb's method to generate more than milligram
quantities of the desired compound was surprising. I therefore
subjected 12 to these conditions (Scheme 21 ), resulting in the

formation of 37 in good yield.

48

Scheme 21: Acylation of Terthiophene 12

/\ /\ /\ 12
s s s

1.2 eq. n-buLiO
2. 2 eq. Me—NW

OMe
/\ /\ /\ 37
S S S .
o 0 61% yield
.1.
/\ /\ /\ 38
5 5 5 0 15% yield

The lack of success obtained by the routes examined above led to
attempts to create an epicycle by sequential oxidative coupling of
12. Two methods were examined. The first was developed by
Colon and Kelsey,52 which reductively couples aryl chlorides using
N10 catalyst and Zn as the reducing agent. This method has been
used by Luol‘ic in this laboratory to synthesize two sexithiophenes

in good yield (Scheme 22).

49

Scheme 22: Luo‘s Synthesis of Sexithiophenes by the Method of
Colon

RR RR RR

/\ /\ /\BrNiClz.Zn.TPP>/\ /\ /\ /\ /\ /\

S S S DMAC, bipy S S S S S S
R=H, 54%;
R=Methyl, 41%

 

2—Bromoterthiophene 38 was synthesized by treating 12 with
pyridinium hydrobromide in 53% yield. Subjecting it to the
conditions of Colon (Scheme 23) resulted in the formation on

sexithiophene 39 in 1.8% yield.

Scheme 23: Coupling of Terthiophene 12 by the Method of Colon

(1&1fo :53: 431—13
Br 39

a) pyridine hydrobromide, chloroform b, 1.8%
b) NiClz, Zn, triphenyl phosphine,
bipyridine

 

/\ /\ /\/\ /\ /\
s s s s s s
40

Although the reaction was very clean, the poor yield and inability to

consistently generate the catalyst system prompted us to switch to
Kagon's method53 of making symmetrical thiophene oligomers.
Accordingly, compound 12 was treated sequentially with n—buLi

and CuClz (Scheme 24).

50
Scheme 24: Synthesis of Sexithiophene 40

-B'
/\ /\ /\ n "“= /\ /\ /\L,
S S S S S S
12

I CUCIZ

 

/\ /\ /\ /\ /\ /\ 40

9.4% from 12 s S S S s s

This method generated the sexithiophene 40 in 9.4% yield. The
sexithiophene is a bright orange solid, m.p.= 1 18—1 18.5°C.
Recrystallization from hexanes gave orange crystals suitable for X-
ray structure determination by MC. Kanatzidis and j.H. Liao , Figure
30. It shows the almost planar arrangement of the central four
thiophenes, in an antiparallel orientation similar that shown in
terthiophene 12. The outer thiophenes are variably tilted, lying
parallel in 11% of the molecules and antiparallel in 89% of the
molecules. Sexithiophene 40 is the longest oligomer yet
characterized by single crystal X—ray. The three X—ray structures
made available through the compounds synthesized in this work

doubles the number of oligothiophene structures known.

Sexithiophene 40 shows a long wavelength maximum in its UV
spectrum at 406 nm. The UV spectra of 40 and other
sexithiophenes are given in Table 5. The first four examples
demonstrate a correspondence of UV absorbence with alkyl

substitution that is similar to that found in terthiophene 12 and

 

51

Figure 30: X-ray Structure of Sexithiophene 40

 

52

quinquethiophene 27. The UV spectra of the last two examples
indicate that conjugation is interrupted by the opposing pairs of
methyl groups, and shows the value of using unsubstituted

thiophenes as spacers between substituted thiophenes.

Table 5: UV spectra of Various Sexithiophenes

Xmax reference

/\ /\ /\ /\ /\ /\ 406
s s s s s s

/\ /\ l\/\ /\ /\ 434 14(c)
s s s s s s

S S S S

SC101121 SCIOHZI

O—I’W’W 54

/\ /\ /\ /\ /\ /\ 368 55
S S S S S S

/\ /\ /\/\ /\ /\ 368 55
s s s s s s

Synthesizing sexithiophene 40 by forming the dilithioterthiophene
41 and coupling with cupric chloride (Scheme 25) not only
improved the yield to 27%, it also generated 27 mg (1.5%) of a light
and acid sensitive brick red powder. Integrating the B and or peaks
in its proton NMR spectrum gave a ratio of 6:1. Examination of its

UV spectrum showed peak absorbances at 426 and 690 nm. This

 

53

information indicated that the compound was the nonathiophene
42. FAB-MS provided confirming evidence, showing a peak at 1076
daltons. Features around 700 nm in the UV spectrum of
polythiophene made by polymerization of terthiophene have been

ob served.39111

Scheme 25: Further Oligomerization of 12 to Give 42

/\/\/\ =/\/\/\.
s s s s s 5]“141
12

J 5 eq. CuClz

 

/\ /\ /\ /\ /\ /\ 4o 27%
s s s s s s

+

/\ /\ /\ /\ /\ /\ /\ /\ /\ 421.5%
s s s s s s s s s

The successful synthesis of 40 in useful yields using these couplings
led to the attempt to use this method to synthesize the
duodecithiophene 43 (Scheme 26). Using 1 equivalent n-buLi and 4
equivalents CuClz gave 87% recovered 40 and 41 mg of a dark red
material. TLC analysis indicated that this material contained
several components. These fractions could not be separated by
flash chromatography or preparative thin layer chromatography.

The UV spectrum of this material showed a maximum at 435 nm,

 

54

which does not correspond well with Horowitz'54 duodecithiophene,

which has Xmax = 465 nm.

Scheme 26: Attempted Synthesis of Duodecithiophene 43

/\ /\ /\ /\ /\ /-\
s s s s s s

40

/\ /\ /\/\/\ /\/\ /\ /\/\ /\ /\
s s s s s s s s s s s s
43

Since CuClz is sparingly soluble in ether, such a small amount of
CuClz may have dissolved before reacting. (Additional evidence for
this possibility is the lack of reaction when attempting to make 40
using THF as solvent. CuClz is relatively soluble in THF.) To
investigate this possibility, the reaction was attempted using fifty
equivalents CuClz. This method gave identical results. Horowitz'
report54 of such a coupling being done in very concentrated solution
led to an attempt to replicate their results with sexithiophene 40.
This method gave none of the desired product, with extensive

decomposition of starting material (Scheme 27).

55

Scheme 27: Attempted Synthesis of 43 by the Method of Horowitz

/\ /\ /\ /\ /\ /\
s s s s s s

1. 2 eq. n-buLi
in 30 mL ether
2. 5 eq. CUCIZ

ii

51% recovery of 40

 

The coupling of 2,5-dibromothiophenes by forming the Grignard and
coupling with a transition metal catalystS6 (Scheme 28) showed
potential as a way to produce higher oligomers, and perhaps

provide the epicycle directly.

Scheme 28: Transition Metal-catalyzed Polymerization of
Thiophene

Br-Q—Br + Me —-—>- Br-QMgBr pd(dppf)Clz-= HflH

S n

 

This methodology was attempted using 2,5"—dibromoterthiophene
26 (Scheme 29). When 26 was added to 1.1 equivalents Mg and
stirred overnight, TLC showed decomposition products, but no trace
of terthiophene 12 or bromoterthiophene 39. The presence of
these products on the TLC plate would have provided evidence that
the Grignard was being formed. Bringing the mixture to reflux only

accelerated decomposition.

2. Conclusions

A variety of oligomers containing 3,4-disubstituted thiophenes have
been synthesized, resulting in the thiophene, pyrrole, f uran series
12, 24, 25. These syntheses complement the existing series of
unsubstituted and monosubstituted oligomers. Also, they have
paved the way for the doubling the number of known X-ray crystal

structures.

Attempts aimed toward the synthesis of an epicycle based on 5
probed the outer limits of solubility, stability, and separability. Use
of the Michael-Stetter reaction was prohibited by the low solubility
of 30. It was found that conditions strong enough to initiate a
reaction often were strong enough to initiate polymerization, as
shown in attempts to f unctionalize 27. Many separations were
hampered by the poor resolution of these compounds by flash

chromatography.

Any future work on the synthesis of epicycles should attempt to
make use of shorter oligomers which are f unctionalized and reacted
in a manner which would maximize differences in the

chromatographic properties of the reactants and expected products.

56

 

Conversion of 1,4-Diketones to Pyrroles

Treating 1,4-diketones with an amine source in the presence of an
acid catalyst, known as the Paal-Knorr reaction, is the most widely
reported method for producing pyrroles57 (Scheme 30). The
reaction is commonly run using acetic acid as catalyst and
ammonium carbonate or ammonium acetate as the amine source.

Primary amines may be used to form N-substituted pyrroles.

Scheme 29: The Paal-Knorr Reaction

0
RlMRZ + R3NH2 acid catalyst = Iii—Q—Rz

O

 

When these conditions were applied to diketone 23 (Scheme 31). no
reaction was observed using acetic acid or tin(IV) chloride as
catalyst. Even refluxing four days in propionic acid with propionic
anhydride to act as water scavenger resulted in complete recovery
of starting material. Since we desired to have the pyrrole 25 to
complement the thiophene 12 and furan 24, and since other
attemptsl4(b) in this group to convert 1,4-diketones to pyrroles had
proceeded only with difficulty, a more effective route to pyrroles

was desired, preferably one more mild than refluxing in acid for

several days.

57

58

Scheme 30: Attempted Conversion of Diketone 23 to the Pyrrole

proprionic anhydride

 

/ \ / \ ammonium carbonate > recovery
5 S acetic acid of 23
0 23 0 reflux, 4 days

Methylchloroaluminum amide, 44, has been used by Weinreb58 and
coworkers to convert esters to amides with good result. The
reagent appeared to contain all elements desired in the Paal-Knorr
synthesis: it provided an amine source, it could function as a Lewis

acid catalyst, and it could also function as a dehydrating agent.

Figure 31: Methylchloroaluminum Amide

('31
Me-Al-NHZ

This reagent is made by treating an amine hydrochloride with
trimethylaluminum (Scheme 32). Although trimethylaluminum is
extremely pyrophoric, it is available as less dangerous solutions in

hexanes and toluene.
Scheme 31: Formation of the Methylchloroaluminum Amide

Reagent

Cl
Me-Al-NH2 + CH4

 

Me3Al + NH.,c1

59
Treating 1,4-diphenyl-l,4-butanedione 45 with four equivalents of
methylchloroaluminum amide in toluene at room temperature
(Scheme 33) resulted in conversion to 2,5-diphenylpyrrole 46 in

85% yield .

Scheme 32: Synthesis of 2,5-Diphenylpyrrole

a
O r r O 1‘ Me-Al-NHZ M O /N\ O
o o 85% H

45 44 46

Heartened by this result, diketone 23 was submitted to the same
conditions. After 17 hours at room temperature, TLC indicated
partial conversion to the pyrrole. Heating to reflux for 2 hours
pushed the reaction to completion. The pyrrole 25 was isolated in
73% yield after flash chromatography. Bringing the reaction to
reflux immediately after addition of the amide 44 resulted in an

isolated yield of 89% (Scheme 34).

Scheme 33: Synthesis of 2,5—Bis-(2'—thienyl)-3,4-dibutylpyrrole

Cl
/\ /\ + 4eq.Me-Al-NH2 toluene ; /\ /\ /\

S S reflux, 2h 5 S
23 89% 25

 

1112

Similar results were obtained for l,4-bis-(2'-thienyl)-—1,4—
butanedione 47 when treated with 4 equivalents of amide 44 and

stirred two hours at room temperature. The 2,5-bis—(2'-thienyl)-

60

DYI‘I‘OIG 48 isolated in 75% (Scheme 35). Application of this method
to 1,4-bis-(2'-furyl)-1,4-butanedione 49 resulted in decomposition

of starting material upon addition of the amide.

Scheme 34: Synthesis of the Trimer 48 and the Attempted
Synthesis of 50

/ \ / \ Tl toluene / \ / \ / \
5 5 + 4 eq. Me- Al-NHZ —-—> 5 N s
O O 75% H
47 48

0 0

CI
/\ /\ + Mane-111111, x /0\ /N\ /O\
H

toluene
4 9 5 0

Attempts were also made to convert alkyl-substituted 1,4-
diketones by the same method. 3,4-Dimethy1-2,5-hexanedione 51
and 5,8-dodecanedione 52 were submitted to the conditions above
(Figure 32). One product was formed in each case, as indicated by
TLC, but the products could not be seperated from the toulene
solvent. The l,4-(2—pyrryl)-1,4-butanedione 53 did not react under
these conditions. Previous work by Merrilll‘fi’ directed toward
converting this compound to the terpyrrole, and observation of the
reaction mixture in this particular case, indicate that low solubility

was responsible for the lack of reactivity.

 

61
Figure 32: Substrates Not Successfully Converted to Pyrroles

M (I /\ /\
\/\/‘\/\n/\/\ N N

0 0 R 0 O R
51 52 53

Since the use of toluene made solvent removal unwieldy, the
reactions were attempted using trimethyl aluminum in hexanes
solution and dichloromethane as solvent. These reactions were run
at room temperature overnight, with the results summarized in

Figure 33.

Figure 33: Room Temperature Synthesis of Pyrroles with

 

 

Dichloromethane
1,4-diketone pyrrole yield
/ \
O 5 50 O N O m

H
45 46
/\ /\ /\/\/\ 34%
S S S N S
0 O H
4.7 48
/\ /\ /\/\/\ 52%
0 O 0 N O
o o H

49 50

/ \ / \ recovered

N N starting material
R 0 0 R

53

All 1,4-diketones treated with 4 equivalents
methylchloroaluminum amide in dichloromethane

 

62

Again, the 1,4-bis(2‘—pyrryl)-1,4—butanedione 52 failed to react,
probably due to its insolubility in dichloromethane. Applying the
reaction to alkyl-substituted 1,4-diketones failed to produce alkyl-
substituted pyrroles, for when 3,4-dimethyl-2,5-hexanedione 51
and 5,8-dodecanedione 52 were treated with 4 equivalents
methylchloroaluminum amide in dichloromethane, no reaction was
observed. It is interesting to note that 49, which decomposed when
toluene was the solvent, gave a yield which is more than twice that
obtained (52% versus 22%) when more traditional Paal-Knorr

conditions are used.”21

Conclusions

Methylchloroaluminum amide is an efficient reagent for converting
aryl 1,4-diketones to pyrroles. Its advantages over the traditional
Paal-Knorr reaction are its rapid rate of conversion, good yield, mild
conditions, and ability to generate pyrroles such as 25, which
cannot otherwise be synthesized. It is possible that further
investigation will allow its application to the conversion of alkyl-
substituted 1,4-diketones to pyrroles. While this reagent should be
examined for its applicability to all transformations Paal-Knorr
conditions are used for, its use in the reaction below may provide a
new conducting polymer in one step from easily obtainable

reagents.

Scheme 36: One Step Synthesis of Poly(Z-aminopyrrole)

HCl-H N NH oHCl _
2 fl 2 “W3 ._ 449 N}
o o ' n

 

63

Experimental

General: Melting points were determined using a Thomas-Hoover
capillary melting point apparatus and are uncorrected. Proton
nuclear magnetic resonance (lH-NMR) were obtained using either a

Bruker WM-250 (250 MHz) or Varian Gemini (300 MHz)

spectrometer. Chemical shifts are reported in parts per million (8)
using either the residual solvent proton resonance (chloroform, 8
7.24), or tetramethyl silane as internal standard (8 0.00). lH-NMR
data are reported as the chemical shift, chemical shift multiplicity (s
for singlet; d, doublet; t, triplet; q, quartet; dd, doublet of doublets;
m, multiplet) and number of hydrogens. 13C nuclear magnetic
resonance (13C-NMR) spectra were obtained using either a Bruker
WM-250 (62.9 MHZ) or Varian Gemini (75.4 MHz) spectrometer.
Chemical shifts are reported in parts per million (8) using the
residual solvent resonance as internal reference (chloroform, 8
77.0). Ultraviolet (UV) spectra were obtained using a Beckman DU—
64 spectrometer. Electron impact mass spectra (El-MS) were
obtained using either a Finnegan 400 mass spectrometer or a VG
Instruments Trio-l mass spectrometer. Fast atom bombardment
mass spectra (FAB-MS) were obtained by the MSU-NIH Mass
Spectrometry Facility, Biochemistry Building, Michigan State
University. Flash chromatography was performed according to the

method of sun,” Kahn, and Mitra. Trimethyl aluminum solutions,

64

65
n-butyllithium solutions, anhydrous tetrahydrofuran, diethyl ether,

dimethyl acetimide, and dimethyl formamide were purchased from
the Aldrich Chemical Company, Milwaukee, WI and used as
received. Dichloromethane and triethyl amine were distilled from
calcium hydride. Toluene was distilled from sodium-benzophenone
ketyl. Tetramethyl ammonium tribromide was prepared
according to the method of Chattaway and Hoyle6O and used without
recrystallization. Ethanol was dried by distillation from magnesium.
Cupric chloride was dried in an oven at 150°C overnight, ground to a
fine powder, and replaced in the oven until used. Sodium iodide
was dried in an Abder-Halder apparatus under vacuum with
refluxing toluene. Chloroform was washed with water, predried
with magnesium sulfate, and distilled from phosphorous pentoxide.
All reactions were performed under an argon or nitrogen

atmosphere unless otherwise mentioned.

3,4-Dibromothiophene 14: A dry 1000 ml flask containing 47 g
(0.1 17 mole) of tetrabromothiophene in 300 ml dry ether was
placed in a dry ice/acetone bath. One hundred mL 0.25 M n-butyl
lithium (in hexanes) in 150 mL dry ether was added dropwise over
a period of one hour. Twenty minutes after the addition was
complete, the cold bath was removed and 50 mL ice was added to
quench. The solution was allowed to come to room temperature and
placed in a 1000 mL separatory funnel. The organic layer was
washed with water, dried with MgSO4, and concentrated under

reduced pressure. The product was distilled (74 °C at 2.75 mm Hg)

66
through a twelve inch vacuum—jacketed Vigreux column, yielding

23.83 gm (84%) of 3,4-dibromothiophene as a clear liquid.
1H-NMR(CDCl3): 57.3 (s); 13C-NMR(chloroform-d): 8123.5. 113.6;
EI-MS m/z(relative intensity) 242 (m+. 52.9). 241 (43.7), 162
(52.4). 160 (44.2). 82 (51.0). 81 (87.9). 40 (base).

3.4—Dibutylthiophene 15: A dropping funnel containing 22 mL
(0.205 mole) of n-butyl bromide and 80 mL dry ether was attached
to a 500 mL flask containing 6 gm (0.25 g.atom) Mg. Dry ether was
added to cover the Mg. The flask was placed in an ice-water bath.
The solution was slowly dripped in with vigorous stirring. After the
addition was complete, the ice bath was removed and the mixture
refluxed for one hour. A 1000 mL flask was charged with 23.4
grams 3,4-dibromothiophene, 500 mg Ni(dppp)Clz and 300 ml dry
ether. This flask was placed in an ice-water bath. The Grignard
solution was then transferred to the well stirred mixture by double
ended needle. The ice bath was removed after addition was
complete and the solution was refluxed for 16 h. The reaction was
quenched with 50 mL ice, washed with water, dried with MgSO4.
and concentrated under reduced pressure. The product was
distilled (9 95-10 1 'C) through a twelve inch vacuum-jacketed
Vigreux column, yielding 14.6 g (77%) of 3,4-dibutylthiophene.
lH-NMR (chloroform-d) 7.0 (s, 2H). 2.6 (t, 4H). 1.7 (quintet, 4H), 1.5
(sextet, 4H), 1.0 (t,6H); 13C-NMR (chloroform-d) 6 141.7, 119.5.
31.2, 27.8, 22.0, 13.3; EI-MS m/z (relative intensity) 196 (m+, 7.62).
179, 167 (2.65). 154 (31.54). 139 (34.01). 125 (21.99). 111 (base);
UV(acetonitrile) 241 nm (2 11,500).

67

2.5-Dibromo-3.4-dibutylthiophene 16: To 18.7 g (0.0954 mol)
3,4-dibutylthiophene was added to 66 g (.21 mmol) tetramethyl
ammonium tribromide (TMAT) in 100 ml of 1:1 acetic
acid/dichloromethane. The reaction was monitored by TLC and
complete in 25 minutes. The mixture was diluted to 200 mL with
dichloromethane and filtered to remove the tetramethyl ammonium
bromide. The filtrate was washed with water (3x50 ml) and then
with saturated N aHCO3 solution until all acetic acid odor was gone.
It was dried with MgSOq, the solvent removed under reduced
pressure. Purification of the crude product by flash
chromatography gave 31.1 g (92%) as a clear liquid.

lH—NMR (CDCL3) 8 2.5 (t, 4H), 1.1-1.3 (m, 8H) 0.95 (t, 6H); l3C-NMR
(CDCL3)8141.4, 107.8, 31.7, 28.7, 22.6, 13.9; EI-MS m/z (relative
intensity) 356 (m+2, 9.6). 354 (m+, 18.9). 352 (9.1). 269 (49.6). 233
(31.8).191 (92.8). 189 (base).

3',4'-Dibutyl-2,2':5’,2"-terthiophene 12: Three grams 3,4-
dibutylthiophene (0.0325 mol) was brominated with 20.1 g (0.064
mol) TMAT as described above. A dry 100 mL flask containing 2.0
g Mg was placed in an ice bath and dry ether was added to cover.
To it was added dropwise 6.1 mL 2-bromothiophene (0.063 mol) in
20 mL ether. After addition was complete, 100 mL dry ether was
added and the solution refluxed for one hour. A dry 500 ml flask
containing the 2,5-dibromo—3,4-dibutylthiophene, 100 mg
Pd(dppf)Clz (0.4 mol %) and 300 mL dry ether was placed in a Dry

Ice/ acetone bath. The Grignard solution was transferred to this

68

flask by double ended needle. The Grignard solution must be added
with care to ensure that vigorous stirring continues. The cold bath
was removed and the mixture was stirred for 72 h at room
temperature. It was then quenched and washed with water. The
organic layer was dried with MgSO4 and the solvent removed under
reduced pressure. The product was purified by flash
chromatography using hexanes as eluent. Yield was 4.58 g (83%) of
the pale green terthienyl 12 (m.p.=36.0-36.5°C).
lH—NMR (CDCL3) 8 7.35 (d, 2H). 7.19 (d, 2H), 7.10 (dd, 2H), 2.77 (t,
4H), 1.61 (quintet, 4H), 1.50 (sextet, 4H), 1.00 (t,6H); 13C-NMR
(CDCL3) 8 139.7, 135.9, 127.0, 124.5, 124.9, 123.4, 32.3, 27.1, 22.3.
13.1; EI-MS m/z(relative intensity) 360 (m+. 3.63). 317 (6.41), 303
(2.56), 275 (32.40), 166 (18.79), 127 (22.74); UV (acetonitrile) 331
nm (8 15,000). anal. Calcd. for C20H2453: C, 66.62; H, 6.71
Found: C, 67.04; H, 6.83

Alternate procedure for 12: To 200 mg 1,4-diketone 23, in 100 mL
dry toluene, was added 123 mg phosphorous pentasulfide. The
reaction was heated to 80°C for one hour. The solution was cooled,
washed with water, and dried with MgSO4. The solvent was
removed under reduced pressure and the crude product purified by
flash chromatography (hexanes eluent). The product 12 was

obtained (165 mg) in 83% yield, identical to above.

2,5"-Dibromo-3',4'—dibutyl-2,2':5',2"—terthiophene 26:
Terthienyl 12 (2.00 g) was brominated with 2.1 equivalents (3.66
g) TMAT in 20 ml 1:1 acetic acid/dichloromethane. After 25

69

minutes, 50 mL dichloromethane was added. The solution was then
filtered, washed with water (3x200 mL), and then saturated sodium
bicarbonate solution. Flash chromatography (hexanes eluent) gave
the product, 2.724 g, (m.p.=40.0-41.0°C) as a pale green solid (95%).
lH-NMR (CDCL3) 8 6.99 (d, 2H). 6.85 (d, 2H), 2.63 (t, 4H), 125-1.6
(m, 8H), 0.93 (t, 6H); 13C-NMR (CDCL3)8140.4, 137.3. 130.1, 129.2,
126.1, 111.9, 32.8, 27.7, 22.9, 13.7; EI-MS m/z (relative intensity)
520 (m+2, 55.1). 518 (111*, base). 516 (47.6). 396 (37.9). 352 (26.5).
286 (5.8), 272 (12.2); UV (acetonitrile) 342 nm (8 20,000).

2-Bromo-3',4'-dibutyl-2,2':5',2"-terthiophene 39: One g
terthiophene 12 was placed in a 250 mL flask and 100 mL dry
chloroform added. 890 mg pyridinium bromide was added. The
reaction was quenched after stirring for 40 minutes. The
chloroform solution was washed with water, and the solvent
removed under reduced pressure. Flash chromatography (hexanes)
gave 651 mg (53.3%) of 39,233 mg (16.1%) of the dibrominated
product 26, and 274 mg (27.4%) recovered 12.

lH-NMR (CDCL3): 8 68-73 (m, 5H). 2.32 (m, 4H). 1.35-1.6 (m, 8H),
0.90(m,6H);13C-NMR(CDCL3):8140.1, 139.6,137.2,135.5, 129.9,
129.7, 128.3, 125.6, 125.5, 125.0, 111.2; EI-MS m/z (relative
intensity) 440 (m+1, 91.2), 439 (111*, 19.4),438 (base), 397 (20.7),
383 (7.0). 355 (20.0), 316 (59.0), 274 (73.7).

3",4"-Dibutyl-2,2':5',2":5",2"':5"',2""-quinquethiophene ,
27: Dibromoterthiophene 26 (597 mg, 1.15 mmol) was placed in a
flask with 100 mg Ni(dppp)Clz and 50 mL dry ether. Five

70

equivalents of 2—thienyl magnesium bromide were added by
cannula. The reaction was refluxed for four hours. The mixture
turned yellow-green after 45 minutes. The reaction was quenched
with ice, washed with water and dried with MgSOx. The product
was purified by flash chromatography (hexanes eluent) to give 324
mg (53.7%) of 3. m.p.=102'C.
lH-NMR (CDCL3): 8 7.0-7.3 (m. 10H). 2.75 (t, 4H), 1.4-1.6 (m, 8H). .9
(t, 6H); l3C—NMR (CDCL3): 8 140.2, 137.1, 137.0, 135.0, 129.7, 127.9,
126.3, 124.4, 124.0, 123.8, 32.8, 27.8, 23.0, 13.8; EI-MS m/z
(relative intensity) 524 (m+. 53.15). 481 (9.20), 439 (8.15). 424
(1.48). 262 (2.01). 207 (3.27). 179 (1.53). 149 (2.29), 127 (3.68).
101 (5.74), 85 (base); UV (heptane) 389 nm (8 33.000).
anal. Calcd. for ngHzgSS: C, 64.01; H, 5.38

Found: C, 64.58; H, 5.40

Alternate procedure for 27: One gram terthiophene 12 (0.00278
mol) is brominated with 1.83 g TMAT (0.00584 mol, 2.1
equivalents) as above. The product is placed in a flask with 100 mg
Pd(dppf)C12 and 300 mL dry ether. The flask is placed in a Dry
Ice/acetone bath and 2.1 equivalents thiophenemagnesium bromide
in 100 mL ether is added by double ended needle. The cold bath is
remove one hour after addition is complete and the reaction stirred
18 h. The reaction is quenched and washed with water, dried, and
the ether removed under reduced pressure. Flash chromatography
(hexanes eluent) gave 420 mg (29%) of 27, identical to above.
Although the yield is lower, this reaction is more consistant than

using Ni(dppp)C12.

71

3".4"-Dibutyl-2.5""-diformyl-2,2':5',2":5",2”':5”',2""-
quinquethiophene 30: Quinquethiophene 27 (324 mg, 0.00061
mol) was treated with 1.5 g P0Cl3 in 10 ml DMF and heated on a
steam bath for one hour. The material was decomposed by cautious
addition of sodium bicarbonate solution. The product was a brick
red solid (286 mg) that was obtained in 69% after flash
chromatography (hexanes eluent).

lH-NMR (CDCL3): 8 9.9 (s, 2H). 7.7 (d, 2H). 7.35 (d, 2H), 7.26 (d, 2H).
7.1 (d, 2H). 2.75 (t, 2H), 1.4-1.6 (m, 8H), 0.9 (t, 6H); EI—MS m/z 580.
495, 277, 238, 211, 185. 139.

Bis(2-thienoyl)-5,6-decane 23: A dry flask containing 7.5 g

(50 mmol) dry NaI was charged with Ar and purged. Three mL (25
mmol) dichlorodimethyl silane in 100 mL freshly distilled
acteonitrile were added to produce an opaque yellow mixture. After
stirring for ten minutes, the flask was placed in a water bath and
7.0 mL (50 mmol) freshly distilled triethyl amine followed by 9.0 g
(2 5 mmol) 2-hexanoylthiophene were added dropwise. The
reaction was allowed to run for 18.5 hr at room temperature and
was then quenched by addition of 200 mL 1:1 ice/saturated NaHCO3
solution. It was extracted with pentane (2x50mL) and dried over
sodium sulfate. lH-NMR showed a 3:1 ratio of the bis—silyl enol
ether to starting the ketone. This crude product was used without
further purification. A dry flask containing 27.41 g (50 mmol)

cerium ammonium nitrate and 8.4 g (100 mmol) sodium

72

bicarbonate was charged with Ar and purged. One hundred mL dry
acetonitrile were added to the flask. The bis—silyl enol ether in 150
mL dry acetonitrile were added over 2.5 h. The reaction was
worked up 24 h after the addition was complete by addition of 500
mL saturated sodium bicarbonate solution, followed by extraction
with ether (2x100mL). The organic phase was washed with 50 mL
brine and then dried over sodium sulfate. The product was purified
by flash chromatography (9:1 hexanes/ether eluent) to give 4.325 g
(48% from 2-hexanoylthiophene) of the coupled dione as a
diastereomeric mixture. One of the diastereomers was easily
isolated by recrystallization from hexanes (M.P.=l62-163°C) and
was used to obtain the analytical data.

lH-NMR (CDCL3): 8 7.82 (d, 2H). 7.68 (d, 2H), 7.16 (dd, 2H). 3.95 (m.
2H), 1.7 (m, 2H), 1.1 (m, 2H), 1.0-1.2 (m, 8H). 0.65 (t, 6H); l3c-NMR
(CDCL3):8196.5, 146.4, 134.7, 132.5. 128.4, 50.7, 32.2, 29.7, 22.6.
13.6. IR (CC14) 2970, 2940, 2840, 2820, 1654, 1410; EI-MS m/z
(relative intensity) 362 (m+, 8.5). 345 (2.6). 306 (31.0). 251 (19.2),
195 (95.3). 182(88.2),l68(56.5).139(77.7).111 (base).

3.4-Dibutyl—2.5-(2-thienyl)-furan 24: One hundred

milligrams diketone 23 (lower melting diastereomer) was placed in
a flask to which 100 mL acetic anhydride and 2 mL hydrochloric
acid were added. The reaction was stirred for 24 h in the absence
of light. The solution was poured over 50 g ice and stirred for one
hour, transferred to a separatory funnel and extracted with carbon
tetrachloride. The carbon tetrachloride solution was washed with

sodium bicarbonate solution and dried with magnesium sulfate.

73

The solvent was removed under reduced pressure and the crude
product purified by flash chromatography (hexanes eleunt), giving
63 mg 24 (66%) as a pale green oil.

lH-NMR (CDCL3): 8 7.09-7.31 (m, 6H). 2.62 (t, 4H), 1.47-1.75 (m, 8H).
1.00 (t, 6H); l3C-NMR (CDCL3):8143.8, 133.8, 127.4, 123.7, 123.4,
122.5, 32.0, 24.0. 23.1, 14.0; EI-MS m/z (relative intensity) 344 (m+,
base), 301 (16.9). 273 (9.2), 259 (23.1), 231 (11.5). 111 (79.8); UV
(acetonitrile) 351 (8 29,000).

3,4-Dibutyl-2.5-(2-thienyI)-pyrrole 25: Fifty mL toluene was
placed in a dry 100 mL flask with 1.773 mg ammonium chloride.
To this was added 16.5 mL of 2.0 M trimethyl aluminum in toluene
solution by syringe. The reaction was stirred for one hour, and
transferred to a dry 250 mL flask containing 3.00 g diketone 23 in
100 mL toluene Via double—ended needle. The reaction was heated
to reflux and complete in two hours. The reaction was quenched by
cautiously adding water dropwise. The toluene solution was
filtered, dried, and the toluene removed under reduced pressure.
Purification by flash chromatography gave the product (2.541 g,
89%) as a green solid (mp=45.5-46.5°C).

lH-NMR (CDCL3): 8 8.0 (br s, 1H). 7.0-7.24 (m, 6H), 2.66 (t, 4H), 1.4-
1.65 (m, 8H), 0.97 (t,6H); l3C-NMR (CDCL3):8127.0,122.9, 122.8 (2C).
122.4, 121.8, 33.1, 24.3, 22.6, 13.5; EI-MS m/z (relative intensity)
345 (m+2, 10.5). 344 (m+l, 23.1). 343 (111*, base). 300 (62.8) 258
(96.3), 129 (17.0); UV (acetonitrile) 342 nm (825,000).

74

2,5-Diphenylpyrrole 46: Ninety milligrams ammonium chloride
were placed in a dry 100 mL flask to with 50 mL toluene. To this
was added 0.84 mL of 2.0 M trimethylaluminum in toluene solution
by syringe. The reaction was stirred for one hour, and transferred
to a dry 250 mL flask containing 100 mg l,4—diphenyl-1,4-
butanedione 45 in 100 mL toluene. The reaction was stirred at
room temperature 2 h, cooled, quenched (carefully) with water and
filtered. The solution was washed with water, dried with
magnesium sulfate, and the solvent removed under reduced
pressure. Flash chromatography of the crude product gave 78 mg
(85%) of 46 as a white solid (m.p.=139.8-140.0°C).

lH—NMR (CDCL3): 8 8.6 (br s, 1H), 7.55 (d, 2H), 7.41 (dd, 4H), 7.25 (m.
4H), 6.60 (s, 2H); l3C-NMR (CDCL3):8133.1, 132.5, 129.0, 126.4,
123.8, 107.9; EI-MS m/z (relative intensity) 220 (m+1, 18.6). 219
(111*, base), 217 (9.5). 191 (4.1). 165 (3.1), 140 (2.1). 115 (43.3).

Alternatively, the reaction may be performed using
dichloromethane as solvent and trimethylaluminum in hexanes
solution. The reaction was run at room temperature overnight and

worked up as above, yielding 81%.

2,5-Di-(2-thienyl)-pyrrole 48: One gram 1,4-di-(2-thienyl)—
1,4-butanedione 48 was treated according to the toluene procedure
above, yielding 690 mg 48 (75%) as a white solid (mp=8l.0-81.5°C).
lH-NMR (CDCL3): 8 8.3 (br s, 1H), 7.00-7.20 (m, 6H), 6.4 (s, 2H); l3C—
NMR(CDCL3):8135.6,127.7,127.4, 122.9,121.1,108.5;E1—MS m/z

75

(relative intensity) 233 (m+2, 10.7), 232 (m+1, 16.5). 231 (mt, base),
199 (6.0), 186 (17.4). 171 (3.7),154(3.8). 122 (29.5), 115 (24.7).

2.5-Di-(2-furyl)-pyrrole 50: One gram 1,4-di-(2-fury1)-1,4-
butanedione 47 is treated using the dichloromethane procedure,
yielding 444 mg 50 (52%) as a white solid, m.p.e80.5-81.0°C.
lH-NMR (CDCL3): s 8.78 (br s, 1H). 7.39 (s, 2H). 6.4-6.7 (m, 6H); 13c-
NMR (CDCL3):8147.8, 140.5, 124.2, 111.6, 108.8, 102.7; EI-MS m/z
(relative intensity) 200 (m+1, 14.4), 199 (m+, base), 170 (24.6), 142
(36.3). 115 (10.6), 100 (12.6). ‘

3',4'-Dibutyl-2.5”—diformyl-2,2':5'.2"-terthiophene 31: To 2

g terthiophene 12 in a dry 50 mL flask was added 4.54 g N-
methylformanilide and 5.2 g phosphorous oxychloride. The mixture
was heated on a steam bath for two hours, forming a bright red
solution. This was cooled and decomposed by cautious addition of
sodium bicarbonate solution. The material was extracted with
ether, washed with water, dried with magnesium sulfate, and the
ether removed under reduced pressure. Recrystallization from
hexanes gave 1.78 g (76%) 31.

1H-NMR (CDCL3): 8 9.88 (s, 2H), 7.70 (d, 2H). 7.25 (d, 2H), 2.75 (t.
4H), 1.40-1.56 (m, 8H), 0.94 (t, 6H); l3C-NMR (CDCL3): 8181.2, 147.6,
142.9, 1136.8, 130.8, 127.1, 111.1, 32.8, 28.0, 14.0; MS-BI m/z
(relative intensity) 418 (m+2, 16.7), 417 (m+l, 25.1 ), 416 (mt, base),
359 (5.2), 345 (27.6), 331 (7.7). 303 (51.1), 275 (7.0), 240 (9.1).

76
3'.4'-Dibutyl-2,5"-dihexanoyl-2,2':5',2"-terthiophene 37:
Two grams 12 were placed in a flask with 200 mL dry ether. The
flask was placed in an ice/water bath and 2.1 equivalents n-buLi
(2.5 M in hexanes) was added. The cold bath was removed and the
solution stirred for 2 h. Then 2.0 g (2.2 equivalents) N-methyl-N-
methoxyhexanoic amide was added. The reaction was stirred for
one hour and then quenched and washed with dilute HCl. The
ethereal solution was dried with magnesium sulfate and the solvent
removed under reduced pressure. Flash chromatography (hexanes
eluent) gave 1.88 g 37 (61%) as deep green plates (m.p.=93.5-
94.5°C) and 388 mg 38 (15%) as a dark green oil.
For Compound 37: lH-NMR (CDCL3): 8 7.61 (d, 2H), 7.13 (d, 2H). 2.88
(t, 4H), 2.76 (t, 4.4), 1.77 (quintet, 4H). 1.1-1.6 (m, 16H), 0.86-1.00
(m, 12H); 13C-NMR (CDCL3):8193.7, 143.9, 143.3. 142.1, 132.0,
130.6, 126.5, 39.2, 32.8, 31.6, 24.7, 21.6, 21.4 (2C), 13.9 (2C); EI-MS
m/z (relative intensity) 557 (m+1, 30), 556 (111*, 99.1). 500 (58.1).
485 (18.8). 444 (11.2), 415 (19.3). 373 (9.5). 222 (53.6), 43 (base).

For Compound 38: lH-NMR (CDCL3): 8 7.62 (d, 1H). 7.32 (dd, 1H).
7.06—7.15 (m, 3H), 2.88 (t, 2H), 2.70 (m, 4H), 1.88 (quintet, 2H). 1.2-
1.7 (m, 12H), 0.90-1.00 (m, 9H); 13c-NMR (CDCL3):8193.3. 144.5.
142.8, 141.7, 140.5,135.7,132.0, 131.5, 129.1, 127.4, 126.2, 126.0,
125.7, 39.0, 32.8, 32.6, 31.5, 28.0, 27.7, 24.8, 23.0, 22.9, 22.5. 13.9,
13.8 (2C); EI-MS m/z (relative intensity) 460 (m+2, 16.4). 459
(m+l, 29.4). 458 (m+, base). 415 (9.9), 402 (54.0). 387 (12.8). 360
(13.0). 317 (14.1), 275 (11.6).

77
3'.3""'.4',4""-Tetrabutyl-2,2':5'.2":5".2"':5"'.2"":5"".2"'”-
sexithiophene 40:

Method one: Two grams terthiophene 12 were placed in a 500 mL
flask with 250 mL dry ether. 2.5 mL (1.0 equivalent) n-buLi
solution (2.5 M in hexanes) was added by syringe and the reaction
stirred for one hour. The flask was placed in a Dry Ice/ acetone bath
and 1.882 g (2.5 equivalents) anhydrous cupric chloride were
added. The cold bath was removed and the reaction stirred
overnight. The resulting greenish-brown solution was quenched by
addition of five mL water and filtered. The filtrate was washed,
dried with magnesium sulfate, and the solvent removed under
reduced pressure. Flash chromatography (hexanes eluent) gave 47
mg sexithienyl (9.4%) and 1.623 g (81%) recovered 12. The bright
orange product formed prismatic crystals, m.p.=118.0-118.5°C.
lH-NMR (CDCL3): 8 7.33 (dd, 2H), 7.05—7.18 (m, 6H), 2.68-2.82 (m,
8H), 1.4-1.7 (m, 16H), 0.95-1.06 (m, 12H); l3C-NMR (CDCL3): 8140.2.
136.7,136.1,135.2,130.0,129.6,127.3,1263,1259, 1253,1238.
32.9, 32.8, 27.9, 27.8, 23.0, 22.0, 13.8; EI-MS m/z (relative
intensity) 721 (m+3. 9.5), 719 (m+l, 39.0). 718 (m+, base), 675
(11.2). 633 (6.7). 360 (48.5). 295 (16.5). 274 (33.8); UV (heptane)
406 nm (8 25.000). anal. Calcd. for C40H46S6: C, 66.80; H, 6.45
Found: C, 66.86; H, 6.50

Method two: The above procedure was followed, except that 2.0
equivalents n-buLi (4.0 mL solution) and five equivalents cupric
chloride (3.36 g) were used. Accordingly, 1.75 g terthiophene 12
(0.00486 mol) gave 478 mg sexithiophene 40 (27%) and 27 mg

78

nonathiophene 42 (1.5%). Nonathiophene 42 was isolated as a light
and acid sensitive brick red powder, m.p.=128°C (dec.).

For compound 42: lH-NMR (CDCL3): 8 7.3 (d, 2H), 7.0—7.2 (m, 12H).
2.6-2.95 (m. 12H), 1.4-1.7 (m, 12H), 0.90-1.05 (m. 18H); FAB-MS:
1076.4 (m+). 993.3. 882.3. 800.3, 765.3. 749.3. 679.2, 636.3. 596.3.
485.2, 443.2, 403.2. 371.3. 341.3; UV (acetonitrile) 12 710 nm (a
3,400) kmax 426 nm (8 36,000).

N-Methoxy-N—methylhexanamide: Ten grams hexanoyl

chloride and 7.4 g N-methoxy-N-methylamine hydrochloride were
placed in a dry flask with 100 mL dichloromethane. The flask was
placed in an ice/water bath and 12.2 mL (.15 mol) pyridine was
added dropwise. The cold bath was removed and the reaction
stirred overnight. The solution was washed with water and dried
with magnesium sulfate. The compound was distilled (79°C at 2.25
mm Hg) to give 9.77 g product.

lH-NMR (CDCL3): 8 3.6 (s, 3H). 3.08 (s. 3H). 2.34 (t. 2H), 1.56
(quintet, 2H), 1.22 (m, 4H). 0.8 (t, 3H); l3C-NMR (CDCL3): 8 61.0, 33.8,
31.7. 31.5, 24.2, 22.1, 13.8; EI-MS m/z (relative intensity) 160 (m+1,
2.3). 159 (m+. 2.0). 130 (3.1), 116 (2.7). 103 (30.5). 99 (base), 71
(85.6).

2-Hexanoylthiophene 21: Ten gram thiophene (0.1 19 mol) and
13.5 hexanoyl chloride (0.1 mol) were placed in a dry flask with
100 mL dichloromethane. Tin(IV) chloride (26 g. 0.1 mol) in 100
mL dichloromethane was added dropwise. The reaction turned

bright red as the first few drops entered the solution. The reaction

79

was stirred 24 h. The mixture was quenched and washed with
water, dried with magnesium sulfate, and the solvent removed
under reduced pressure. The crude product was distilled (79°C at
0.5 mm Hg) to give 11.94 g (55%) 2-hexanoylthiophene.

lH-NMR (CDCL3): 8 7.7 (d, 1H). 7.68 (d, 1H). 7.13 (dd, 1H). 2.91 (T,
2H), 1.78 (quintet, 2H), 1.37 (m, 4H), 0.91 (t, 3H); 13C-NMR (CDCL3): 8
193.4, 144.5. 133.2. 131.5, 127.9, 39.3. 31.4, 24.4, 22.4, 13.8; EI-MS
m/z (relative intensity) 183 (m+l, 45.2), 182 (m+, 8.7). 165 (1.9),
139(4.1). 126 (56.6). 111 (base).

2-Acetyl-3",4"—dibutyl-2,2':5',2":5",2"':5"',2""-
quinquethiophene 35: One hundred milligrams

quinquethiophene 27 was placed in a dry flask with 50 mL dry
toluene. Acetic anhydride (0.06 mL) and 0.15 mL tin(IV) chloride
were added and the reaction stirred at room temperature until it
started to deposit black material at the meniscus, about one hour.
The reaction was immediately stopped and the crude product
purified by flash chromatography (hexanes eluent) to give 34 mg
35 (16%) (m.p.=116-117°C).

lH-NMR (CDCL3): 8 7.54 (d, 1H). 6.95-7.25 (m, 8H). 2.68 (m, 4H). 2.50
(s. 3H). 1.25-1.65 (m, 8H). 0.93 (m, 6H); 13C-NMR(CDCL3):8190.3.
145.5. 142.3, 140.8. 140.4, 137.5. 137.2, 135.6, 134.8, 133.4, 130.4.
129.2, 127.9, 126.5. 126.0, 124.5. 124.1, 124.0, 123.9, 123.8, 123.7.
32.8, 32.7, 27.93, 27.86, 26.5. 26.4, 23.0, 13.8 (2C); EI-MS m/z
(relative intensity) 569 (m+3. 6.4). 568 (m+2, 20.8). 567 (m+1, 28.3).
566 (m+, base). 523 (16.8). 481 (15.9). 433 (3.5). 405 (2.6), 283
(21.6).

80

Attempted synthesis of duodecithiophene 43:

Method 1: A flask containing 500 mg 40 (0.0007 mol) and 300 mL
dry ether was placed in an ice/water bath. One equivalent (0.3 mL)
n-buLi (2.5 M in hexanes) was added by syringe. It was then
placed in a Dry Ice/acetone bath and 376 mg (4 equivalents)
anhydrous cupric chloride . The cold bath was immediately
removed and the reaction stirred 14 h. The reaction was quenched
by adding 2 mL water, stirred 15 minutes, and decanted. The
ethereal solution was washed with water, dried with magnesium
sulfate, and the solvent removed under reduced pressure. Flash
chromatography (1% ethyl acetate in hexanes) gave 436 mg 40
(87% recovery) and 41 mg of a dark reddish material. TLC of this
material showed it to contain five bands. Prepartive TLC was
attempted, whith no resolution of the material. Resubmission of the
material to flash chromatography using a 10 mm column showed
that most of the material had polymerized, and no pure products

were obtained.

Method 2: The technique described in method 1, above, was
applied to 333 mg 40 , with the exception that 50 equivalents
cupric chloride were used. Flash chromatography gave no pure
products, excepting 300 mg 40 (90% recovery). lH-NMR of the
crude product showed peaks (8 4.2—4.6) indicative of chlorination of
the benzylic carbons of the side chains. Preparative TLC was

attempted without the isolation of pure product.

81
Method 3: To 300 mg 40 in 30 mL THF, 2 equivalents n-buLi were

added and the reaction stirred 30 minutes. Anhydrous cupric
chloride (5 equivalents, 280 mg) was added and the resulting slurry
stirred 2h. After one hour, the mixture started to turn dark. TLC
prior to workup showed no material other than 40. Flash

chromatography gave 153 mg (51%) recovered 40.

82

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(a) Fichou, D.; Horowitz, 6.; Garnier, F. Synth. Met. 1990, 39
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1983, 78.3212.

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Roncali, J.; Garreau, R.; Delabouglise, D.; Garnier, F.; Le maire, M. J.
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Chem. Soc. Chem. Comm. 1987, 1500. (d) Feldhues, M.; Kampf,
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Chem. 1981, 121, 355. (b) Chung, T.-C,; Kaufman, J.H.; Heeger,
A.J.; Wudl, F. Phys. Rev. B 1984, 30, 702. (c) Waltman, R.J.;
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