β-Terthiophene aldehyde and phosphonate: Key building blocks for the synthesis of functionalised conducting polymers

Article abstract of DOI:10.1016/S0040-4039(01)01894-9

A simple and efficient methodology has been developed for the synthesis of β-functionalised terthiophene monomers, conducting polymer precursors. Using a building block approach, the key materials, β-terthiophene aldehyde and phosphonate, were constructed with Suzuki chemistry. Reactions of these building blocks with the appropriate coupling partner under Wittig/Horner-Emmons conditions provides conjugatively-linked functionalised monomers.

Full text of DOI:10.1016/S0040-4039(01)01894-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 8733–8735
b-Terthiophene aldehyde and phosphonate: key building blocks
for the synthesis of functionalised conducting polymers
Gavin E. Collis, Anthony K. Burrell and David L. Officer*
Nanomaterials Research Centre, Massey University, Private Bag 11222, Palmerston North, New Zealand
Received 10 August 2001; accepted 5 October 2001
Abstract—A simple and efficient methodology has been developed for the synthesis of b-functionalised terthiophene monomers,
conducting polymer precursors. Using a building block approach, the key materials, b-terthiophene aldehyde and phosphonate,
were constructed with Suzuki chemistry. Reactions of these building blocks with the appropriate coupling partner under
Wittig/Horner–Emmons conditions provides conjugatively-linked functionalised monomers. © 2001 Elsevier Science Ltd. All
rights reserved.
1. Introduction
or the terthiophene phosphonium salt (3) (Scheme 1).
This obviates the need to develop new synthetic
methodology for the formation of the terthiophene
moiety for each individual monomer (1), which con-
tains a different R group. A simple synthetic route to
(2) was envisioned using metal mediated coupling
strategies (Scheme 2). Functional Group Interconver-
sion (FGI) of (2) would then provide the phosphonium
salt (3).
There has been an enormous interest in functionalised
conducting polymers over the last thirty years. In par-
ticular, derivatised thiophene oligomers have received
considerable attention as the chemistry of thiophene
allows easy modification at both the a- and b-posi-
tions.1 The many interesting and novel functionalised
monomers that have been reported1–3 commonly
require separate synthetic strategies. A general method
in which various derivatised monomers can be formed
from a key precursor, thereby reducing the number of
specific syntheses, has yet to be reported.
Metal-mediated coupling reactions, such as the
Kumada, Suzuki, Stille and Negishi, have become
accepted methods for the synthesis of poly-aromatic
systems.4 Of these methods, the Kumada coupling
approach is advantageous due to the in-situ formation
of Grignard reagents and cheap nickel catalysts
employed.5 With these benefits in mind, we began gram
scale coupling of the acetal (5) with an excess of the
organomagnesium reagent (4) in the presence of
NiCl2(dppp) (Scheme 2). After an aqueous workup of
We have been interested in synthesising a series of
functionalised terthiophene monomers (1) for the gener-
ation of application-specific conducting polymers. The
key to this approach is the facile synthesis, using Wittig
chemistry, of the functionalised monomers from the
appropriate precursors, the terthiophene aldehyde (2)
R
O
CHO
PPh3X
S
R
PPh3X
Base
R
H
S
S
S
S
S
S
S
S
Base
(2)
(1)
(3)
Scheme 1.
Keywords: Suzuki reaction; building blocks; Wittig; Horner–Emmons; b-functionalised terthiophene monomers; conducting polymers.
* Corresponding author. Tel.: +64-0-356 9099; fax: +64-6-350 5682; e-mail: d.officer@massey.ac.nz
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01894-9

8734
G. E. Collis et al. / Tetrahedron Letters 42 (2001) 8733–8735
R
R
metal
FGI
+
S
S
Br
Br
M
(3)
catalysis
S
S
S
(4) M = MgBr
(5) R = acetal
(8) R = CHO
(6) R = acetal
(2) R = CHO
(7) M = B(OH)2
Scheme 2.
the reaction mixture, followed by chromatography and
recrystallisation, the terthiophene acetal (6) was
obtained in good yield (71%). The acetal group was
easily hydrolysed to give the terthiophene aldehyde (2)†
in excellent yield (95%). However, attempts to perform
the Kumada reaction on a larger scale were met with
incomplete reaction of the dibromide, resulting in poor
yields and complications in isolating pure product. To
eliminate these problems alternative coupling protocols
were considered.
extremely unstable. With sufficient quantities of (3) in
hand, Wittig condensation using 4-pyridyl carboxalde-
hyde (9) and DBU in THF gave the crude product (10)
(Scheme 3) as determined by proton NMR. However,
attempts to obtain the pure pyridyl derivative (10) were
thwarted as the by-product, triphenylphosphine oxide
(TPPO), could not be easily removed. Given the poor
yield of (3) and TPPO contamination, an alternative
approach using Horner–Emmons chemistry was
explored.
The key advantages of the Suzuki reaction6 are that the
mild conditions favour a large number of functional
groups and the by-product from the reaction, boric
acid, is non-toxic and easily removed. It was found that
reaction of the dibromide (8) with an excess of the
boronic acid (7) under Suzuki conditions7 gave the
target terthiophene aldehyde (2) in multigram quanti-
ties and in high yield (78%). This aldehyde (2) afforded
materials of the type (1) (Scheme 1) in Wittig reactions
with several p-substituted aryl ylides. The physical,
chemical and electrochemical properties of these mate-
rials have been studied and the results of these findings
are to be published shortly.8
The synthesis of thienylmethylphosphonates is achieved
using the Michaelis–Arbuzov rearrangement, which
involves reacting halomethylthiophene with triethyl
phosphite. This route provides simple access to both a-
and b-thiophene methylphosphonates in good yields.9
To the best of our knowledge, this method has not been
used to make the bithiophene and terthiophene homo-
logues. Recently, Dalton and Wang10 described an
efficient facile synthesis of oligothiophene phospho-
nates. The method involves deprotonation of a thio-
phene substrate with butyllithium, followed by in-situ
formation of an organocuprate species which is then
coupled with diethyl iodomethylphosphonate. How-
ever, due to the higher reactivity of the a-protons this
strategy only allows the formation of phosphonates at
this position.
With the ready availability of aldehyde (2), it was
envisioned that FGI would afford the phosphonium
salt (3) (Scheme 2). Sodium borohydride reduction of
(2) gave the corresponding alcohol in excellent yield.
Direct treatment of this alcohol with triphenylphos-
phine hydrogen bromide in refluxing toluene gave the
desired phosphonium bromide salt, albeit in low yields
(34%). Attempts to obtain greater quantities of the salt
(3) by converting the alcohol to the halide first and then
heating in the presence of triphenylphosphine also
proved difficult; the halide precursor was found to be
With the successful facile synthesis of b-terthiophene
aldehyde, we envisioned a similar approach could be
used to form the required b-substituted terthiophene
phosphonate (12). As depicted in Scheme 4, the target
phosphonate (12) should be accessible via the coupling
of the known dibromide (11) with 2-thienylboronic acid
(7). Given the difficulties of coupling electron rich
N
CHO
PPh3X
S
S
S
S
DBU
+
+
TPPO
S
S
N
(3)
(10)
(9)
Scheme 3.
† Selected data of (2): mp 68°; 1H NMR (400 MHz, CDCl3): l 10.08 (s, 1H, CHO); 7.57 (s, 1H, H 4%); 7.51 (dd, 1H, J=5.1, 1.2 Hz, H 5); 7.32
(dd, 1H, J=3.6, 1.2 Hz, H 3); 7.30 (dd, 1H, J=5.1, 1.2 Hz, H 5%%); 7.23 (dd, 1H, J=3.6, 1.2 Hz, H 3%%); 7.17 (dd, 1H, J=5.1, 3.6 Hz, H 4); 7.06
(dd, 1H, J=5.1, 3.6 Hz, H 4%%); 13C NMR (67.8 MHz): l 184.9, 145.8, 137.5, 136.7, 135.4, 132.0, 129.1, 128.6, 128.2, 127.9, 125.7, 124.9, 122.3;
Anal. calcd for C13H8OS3: C, 56.49; H, 2.92. Found: C, 56.73; H, 2.75.

G. E. Collis et al. / Tetrahedron Letters 42 (2001) 8733–8735
8735
O
O
P(OEt)2
see text
P(OEt)2
+
S
S
(7)
Br
Br
S
S
(11)
(12)
Scheme 4.
Acknowledgements
halides with boronic acids, a number of catalysts and
conditions were studied.‡ Three of these catalyst sys-
tems afforded significant quantities of the terthio-
phene phosphonate (methods 1, 5 and 6). The best
conditions, using Pd2(dba)3 catalyst system, gave short
reaction times and clean reaction mixtures. A typical
reaction was performed on a gram scale with the
gradual addition of an excess of boronic acid and
monitoring by HPLC to ensure that all the phospho-
nate (11) had been consumed. After an aqueous
workup, the crude oil was purified by high vacuum
distillation to give the terthiophene (12)§ as a viscous
bright yellow oil in 70% yield. Horner–Emmons reac-
tion of the pyridine aldehyde (9) with (12) then gave,
after a simple aqueous workup and recrystallisation,
the desired pure product (10) in good yield as a yel-
low microcrystalline solid.14
The authors would like to thank MURF and NERF
(Contract no. MAUX9913) for financial support.
References
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Chem. 1999, 9, 1957–1974.
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Rev. 2000, 100, 2537–2574.
4. Kalinin, V. N. Synthesis 1992, 5, 413–432.
5. Kumada, M. Pure Appl. Chem. 1980, 52, 669–679.
6. Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457–2483.
7. Gronowitz, S.; Peters, D. Heterocycles 1990, 30, 645–658.
8. Collis, G. E.; Scott, S. M.; Burrell, A. K.; Officer, D. L.,
manuscript to be submitted.
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Chem. 1980, 17, 1259–1264.
10. Wang, C.; Dalton, L. R. Tetrahedron Lett. 2000, 41,
617–620.
In conclusion, by using Suzuki coupling methodology,
we have established a synthetic route to the key
building blocks, b-terthiophene aldehyde (2) and
phosphonate (12). Wittig or Horner–Emmons conden-
sation of these substrates with the appropriate cou-
pling partner allows ready access to conjugatively
linked novel functionalised terthiophene monomers.
Recent research has been directed towards using this
method to synthesise functionalised terthiophene
monomers to produce conducting polymers for appli-
cation in the areas of photovoltaics, bio-sensors,
metal-ion detectors and electro-catalysis. The results
from some of this work will be published elsewhere in
the near future.
11. Dyer, U. C.; Shapland, P. D.; Tiffin, P. D. Tetrahedron
Lett. 2001, 42, 1765–1767.
12. Cockerill, G. S.; Easterfield, H. J.; Percy, J. M.; Pintat, S.
J. Chem. Soc., Perkin Trans. 1 2000, 16, 2591–2599.
13. Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 1999, 37,
3387–3388.
14. All new compounds obtained have been fully character-
ised by elemental analysis, mass spectrometry, NMR,
UV–vis and IR spectroscopy.
‡ (1) Pd(PPh3)4/1 M Na2CO3/DME;7 (2) Pd/C/aq. Na2CO3/isopro-
panol;11 (3) Pd(PPh3)4/K3PO4/toluene;12 (4) PdCl2(PPh3)2/K3PO4/
toluene;4 (5) Pd(PPh3)4/NEt3/DMF12 and (6) Pd2(dba)3/P(But)3/
K3PO4/THF.13
§ Selected data of (12) 1H NMR (400 MHz, CDCl3): l 7.35 (dd, 1H,
J=5.2, 1.2 Hz, H 5); 7.32–7.30 (m, 1H, H 3); 7.23 (s, 1H, H 4%);
7.23 (dd, 1H, J=5.1, 1.1 Hz, H 5%%); 7.18 (dd, 1H, J=3.6, 1.1 Hz,
H 3%%); 7.10 (dd, 1H, J=5.2, 3.6 Hz, H 4); 7.02 (dd, 1H, J=5.1, 3.6
Hz, H 4%%); 4.07 (q, 4H, OCH2CH3); 3.31, (d, 2H, JH,P=21.3 Hz,
ThCH2P); 1.28, (bt, 6H, OCH2CH3). 13C NMR (100.6 MHz): l
136.7, 135.9 (d, JC,P=2.3 Hz), 131.9 (d, JC,P=11.5 Hz), 128.2 (d,
JC,P=9.4 Hz), 127.8, 126.8 (d, JC,P=2.8 Hz), 126.2, 124.7, 123.9,
62.3 (d, JC,P=6.6 Hz), 27.4 (d, JC,P=141.5 Hz), 16.4 (d, JC,P=6.1
Hz). HRMS EI (M+) calcd for C17H19O3PS3 398.0234. Found
398.0230.