DOI: 10.1002/chem.201203331
Concise Synthesis of Well-Defined Linear and Branched Oligothiophenes
with Nickel-Catalyzed Regiocontrolled Cross-Coupling of 3-Substituted
Thiophenes by Catalytically Generated Magnesium Amide
Shota Tanaka, Daiki Tanaka, Go Tatsuta, Kohei Murakami, Shunsuke Tamba,
Atsushi Sugie, and Atsunori Mori*[a]
Abstract: The synthesis of linear and
branched oligothiophenes of well-de-
fined structures is performed with re-
gioselective deprotonation of 3-substi-
tuted thiophenes and nickel-catalyzed
cross-coupling of the thus formed
metalated species with a bromothio-
phene. The reaction of 3-hexylthio-
phene with EtMgCl and 2,2,6,6-tetra-
methylpiperidine (TMP-H, 10 mol%)
induces the metalation selectively at
the 5-position by use of the catalytical-
ly generated hindered magnesium
amide (TMPMgCl) and the subsequent
reaction of a 2-halo-3-hexylthiophene
(bromide or chloride) in the presence
of a nickel catalyst affords a head-to-
tail (HT)-type dimer. By repeating the
same sequence, the linear oligothio-
phene up to a 4-mer is synthesized in
good yield. The reaction of 3-hexylthio-
phene with 2,3-dibromothiophene also
takes place to afford a branched oligo-
thiophene 3-mer in quantitative yield.
The obtained 3-mer is also metalated
at the sterically less-hindered position
in a regioselective manner furnishing a
7-mer in >99% yield after a further
coupling reaction with 2,3-dibromo-
thiophene. These dendrimers react
with several multifunctionalized organ-
ic electrophiles, leading to a variety of
branched oligothiophenes.
Keywords: cross-coupling
·
den-
drimers · NHC ligands · nickel cata-
lysts · oligothiophenes
Introduction
cross-coupling reactions of organoboron[11] and organotin[12]
reagents and the synthesis requires a two-step sequence in
each extension of the number of generations involving tran-
sition-metal-catalyzed coupling and further transformation
of the thus coupled dendron into the corresponding organo-
metallic reagent.[13] Accordingly, a rather multistep synthetic
pathway has been repeated when oligothiophene dendrimers
of higher order generations are designed. Development of a
more concise preparative method for thiophene dendrimers
is, therefore, intriguing.
Oligothiophenes and polythiophenes have recently attracted
remarkable attention as materials showing conductive, semi-
conductive, nonlinear optical, and liquid–crystalline charac-
teristics, among others. They are employed as thin-film or-
ganic transistors,[1] liquid–crystalline compounds,[2] and dye-
sensitized organic photovoltaic cells.[3] Oligothiophenes com-
posed of a thiophene unit bearing a substituent at the 3-po-
sition form three types of possible regioisomers, head-to-
head (HH), tail-to-tail (TT), and head-to-tail (HT). Among
these, the HT isomer generally shows a superior perform-
ance due to decreased steric congestion and thus extended
p-conjugation.[1d,3b–d,4] Development of a concise preparative
method of HT-type oligothiophenes is, therefore, an impor-
tant issue in organic synthesis.[5,6]
On the other hand, we have been engaged in the coupling
À
reaction at the C H bond of heteroaromatic compounds
with various organic electrophiles.[14,15] The method allows
À
remarkable step efficiency by the direct activation of the C
H bond or in situ formation of metallic species by deproto-
native metalation prior to the coupling compared with exist-
ing cross-coupling with the isolated organometallic com-
pounds, such as boron, silicon, and tin reagents.
Branched oligothiophene dendrimers have also been stud-
ied as represented by the pioneering work of Advincula and
Bꢀuerle as potentially applicable light-harvesting,[7] self-as-
sembly,[8] and optoelectronic materials.[9,10] These oligothio-
phene dendrimers have been synthesized by employing
We have recently reported HT-type oligothiophene[16,17]
and polythiophene[18] syntheses with nickel-catalyzed cross-
À
coupling at the C H bond of thiophene, in which a stoichio-
metric amount of magnesium amide TMPMgCl·LiCl (TMP-
H=2,2,6,6-tetramethylpiperidine), recognized as a Knochel–
Hauser base,[19] served as an efficient deprotonating agent
for in situ formation of the metalated species. A series of
linear oligothiphenes can be synthesized by repeating the
coupling reaction, in which a single one-pot reaction has ex-
tended a thiophene unit in a facile manner in contrast to
multistep conventional cross-coupling pathways. Although
[a] S. Tanaka, D. Tanaka, G. Tatsuta, K. Murakami, S. Tamba,
Dr. A. Sugie, Prof. Dr. A. Mori
Department of Chemical Science and Engineering, Kobe University
1-1 Rokkodai, Nada, Kobe 657-8501 (Japan)
Fax : (+81)78-803-6181
Supporting information for this article is available on the WWW
1658
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Chem. Eur. J. 2013, 19, 1658 – 1665