A novel in situ deprotection/coupling and iterative divergent/convergent strategy for the synthesis of oligo(1,4-phenyleneethynylene)s

Article abstract of DOI:10.1016/j.tetlet.2005.10.113

A novel in situ deprotection/coupling and iterative divergent/convergent strategy for the synthesis of oligo(1,4-phenyleneethynylene)s is described.

Full text of DOI:10.1016/j.tetlet.2005.10.113

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 8971–8973
A novel in situ deprotection/coupling and iterative
divergent/convergent strategy for the synthesis of
oligo(1,4-phenyleneethynylene)s
Guorong Li, Xianhong Wang* and Fosong Wang
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Graduate School
of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, PR China
Received 29 September 2005; revised 19 October 2005; accepted 24 October 2005
Available online 8 November 2005
Abstract—A novel in situ deprotection/coupling and iterative divergent/convergent strategy for the synthesis of oligo(1,4-phenyl-
eneethynylene)s is described.
Ó 2005 Elsevier Ltd. All rights reserved.
Conjugated oligomers of precise length and constitution
have received considerable attention both as models for
analogous bulk polymers and as candidates for molecu-
lar wires and molecular scale electronic devices.1 Shape
persistent oligo(1,4-phenyleneethynylene)s appear espe-
cially attractive for their excellent main-chain rigidity
and interesting electronic characteristics.2 Several
synthetic methods including solution and solid phase
synthesis have been reported for preparing oligo(1,4-
phenyleneethynylene)s.3–6 However, in most of the
previously reported methods, isolation of air unstable
terminal alkyne or bis(terminal alkyne)s was necessary.
Tour et al. therefore developed an in situ desilylation/
coupling bi-directional synthesis strategy to avoid the
problem of isolating air-sensitive intermediates, and
they have successfully prepared oligo(1,4-phenylene-
ethynylene)s up to heptamer.6 Some insoluble by-prod-
ucts, however, were obtained because of the undesired
oligomerization via alkyne/bromide coupling, and the
amount of the insoluble materials increased with the
increase of chain length of the oligomers. Moreover,
although the oligomers grow bi-directionally, this strat-
egy is a stepwise approach in essence, which is therefore
only suitable for the preparation of short-length oligo-
(1,4-phenyleneethynylene)s.
On the other hand, it is well known that the iterative
divergent/convergent strategy has become the most effi-
cient method for preparing long-chain oligomers, since
the product can be readily separated from by-products
and starting materials as a result of large differences in
molecular length. Thus, it would be particularly impor-
tant and attractive to develop a new strategy combining
the advantages of the iterative divergent/convergent
strategy and of the in situ desilylation/coupling strategy
for the synthesis of oligo(1,4-phenyleneethynylene)s. In
this letter, the feasibility and efficiency of such a novel
strategy is demonstrated.
The synthetic route is outlined in Scheme 1. 1,4-Di-
bromo-2,5-didodecyloxybenzene (compound 1) was
easily synthesized analogous to previously reported
procedures.7 4-Iodoaniline was converted to the dieth-
yltriazene 2,4 followed by coupling to trimethylsilyl-
acetylene and then desilylation to afford air stable
compound 3, which was then coupled with 1 to give
4, the desired Ôstarting monomerÕ for the in situ desilyl-
ation/coupling iterative divergent/convergent doubling
strategy. Compound 4 was divided into two parts. One
part was coupled with trimethylsilylacetylene in the
presence of Pd/Cu catalyst to afford compound 5 and
the other was converted to compound 6 with methyl
iodide. Then, 5 was coupled with 6 at room tempera-
ture in the presence of both Pd/Cu catalyst and
MeOH/K2CO3 yielding the tetramer 7. Iteration of
the above reaction sequence doubled the molecular
length of the tetramer 7 to afford the octamer 10,
Keywords: Synthesis; Oligo(1,4-phenyleneethynylene)s; Molecular
wires.
*
Corresponding author. Tel.: +86 431 5262250; fax: +86 431
5689095; e-mail: liguorong1976@yahoo.com.cn
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.10.113

8972
G. Li et al. / Tetrahedron Letters 46 (2005) 8971–8973
Scheme 1. Reagents: (a) trimethylsilylacetylene, Pd(dba)2, CuI, PPh3, THF, Et3N; (b) Pd(dba)2, CuI, PPh3, THF, Et3N; (c) MeI; (d) Pd(dba)2, CuI,
PPh3, THF, Et3N, MeOH, K2CO3.
which was quite soluble in common organic solvents,
such as THF, CHCl3 and so on. We found that
compounds bearing the diethyl triazine group easily
decomposed on silica gel, fortunately they were stable
to column chromatographic purification on neutral
alumina. Oligomers 4, 7, and 10 were fully character-
ized by 1H NMR, 13C NMR, and CHN analyses
and matrix-assisted laser desorption/ionization mass
spectrometry (MALDI-MS).
ric hindrance of the long-chain dodecyloxy substituent
which hinders the coupling reaction of terminal alkyne
with terminal aryl bromine.
In summary, the feasibility and efficiency of a novel
in situ deprotection/coupling and iterative divergent/
convergent strategy is demonstrated in this report. Since
isolation of air unstable terminal alkyne or bis(terminal
alkyne)s was avoided, and higher yields as well as easier
chromatographic purification of the final products were
also available, this strategy provides a facile and rapid
method to prepare long-chain monodisperse oligo(1,4-
phenyleneethynylene)s.
Compared with TourÕs in situ deprotection/coupling bi-
directional synthesis strategy,6 this novel in situ depro-
tection/coupling and iterative divergent/convergent
strategy has two main advantages: (1) easier chromato-
graphic purification of the final product because of
larger differences between the final product and by-
products and starting materials in molecular length,
(2) higher yield of in situ deprotection/coupling reaction
as a result of enhanced iodo-selectivity due to strong ste-
Acknowledgements
This work was supported by the National Natural Sci-
ence Foundation of China (Grant No. 29992535) and

G. Li et al. / Tetrahedron Letters 46 (2005) 8971–8973
8973
2. (a) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M.
Science 1999, 286, 1550; (b) Bumm, L. A.; Arnold, J. J.;
Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L.;
Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271,
1705; (c) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.;
Bumm, L. A.; Monell, J. D.; Stapleton, J. J.; Price, D. W.;
Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S.
Science 2001, 292, 2303; (d) Reed, M. A.; Zhou, C.; Muller,
C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252; (e)
Fan, F. R. F.; Yang, J.; Cai, L.; Price, D. W., Jr.; Dirk, S.
M.; Kosynkin, D. V.; Yao, Y.; Rawlett, A. M.; Tour, J. M.;
Bard, A. J. J. Am. Chem. Soc. 2002, 124, 5550; (f) Stewart,
M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton,
J. J.; McGuiness, C. L.; Allara, D. L.; Tour, J. M. J. Am
Chem. Soc. 2004, 126, 370; (g) Li, G. R.; Wang, X. H.; Li,
J.; Zhao, X. J.; Wang, F. S. Chinese Chem. Lett. 2005, 16,
719.
National Science Fund for Distinguished Young Schol-
ars of China (No. 20225414).
Supplementary data
General experimental procedure for the in situ deprotec-
tion/coupling reaction and characterization data includ-
1
ing H NMR, 13C NMR, CHN and MALDI-MS data
for oligomers 4, 7, and 10 are available. Supplementary
data associated with this article can be found, in the
online version, at doi:10.1016/j.tetlet.2005.10.113.
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