A selective and direct synthesis of 2-bromo-4-alkylthiophenes: Convenient and straightforward approaches for the synthesis of head-to-tail (HT) and tail-to-tail (TT) dihexyl-2,2′-bithiophenes

Article abstract of DOI:10.1016/j.tetlet.2010.06.100

A straightforward method for the synthesis of 2-bromo-4-alkylthiophenes was developed, and the desired products were obtained in the highest chemical yields (>90%) reported to date. 2-Bromo-4-alkylthiophenes were synthesized by regioselective lithiating of 3-alkylthiophenes with n-BuLi and quenching with bromine at -78°C. Moreover, a simple and efficient protocol for the synthesis of dihexyl-2,2′-bithiophenes was developed by employing 2-bromo-4-hexylthiophene instead of the commonly used monomer, 2-bromo-3-hexylthiophene. Kumada and Suzuki cross-coupling reactions were conducted to synthesize the desired products as head-to-tail (HT) and tail-to-tail (TT) regioisomers in high yields and excellent selectivity.

Full text of DOI:10.1016/j.tetlet.2010.06.100

Tetrahedron Letters 51 (2010) 4526–4529
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Tetrahedron Letters
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A selective and direct synthesis of 2-bromo-4-alkylthiophenes: Convenient
and straightforward approaches for the synthesis of head-to-tail (HT) and
tail-to-tail (TT) dihexyl-2,20-bithiophenes
Ashraf A. El-Shehawy a,b, , Nabiha I. Abdo a,c, Ahmed A. El-Barbary c, Jae-Suk Lee a,
*
*
a Department of Nanobio Materials and Electronics, School of Materials Science and Engineering, and Research Institute for Solar & Sustainable Energies (RISE),
Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, Republic of Korea
b Department of Chemistry, Faculty of Science, Kafr El-Sheikh University, Kafr El-Sheikh 33516, Egypt
c Department of Chemistry, Faculty of Science, Tanta University, Tanta 31527, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
A straightforward method for the synthesis of 2-bromo-4-alkylthiophenes was developed, and the
desired products were obtained in the highest chemical yields (>90%) reported to date. 2-Bromo-4-alkyl-
thiophenes were synthesized by regioselective lithiating of 3-alkylthiophenes with n-BuLi and quenching
with bromine at ꢀ78 °C. Moreover, a simple and efficient protocol for the synthesis of dihexyl-2,20-bithi-
ophenes was developed by employing 2-bromo-4-hexylthiophene instead of the commonly used mono-
mer, 2-bromo-3-hexylthiophene. Kumada and Suzuki cross-coupling reactions were conducted to
synthesize the desired products as head-to-tail (HT) and tail-to-tail (TT) regioisomers in high yields
and excellent selectivity.
Received 21 April 2010
Revised 12 June 2010
Accepted 21 June 2010
Available online 25 June 2010
Keywords:
Alkylthiophenes
Bromination
Grignard reagents
2,20-Bithophenes
Ó 2010 Elsevier Ltd. All rights reserved.
Dihexylbithiophenes
HH- and TT-cross-couplings
Kumada and Suzuki cross-couplings
Oligo- and polythiophenes have received substantial attention
as potential materials in photovoltaic and electroluminescence de-
vices.1 Poly(3-alkylthiophenes) (P3ATs) are one of the most exten-
sively studied conductive polymers of the past decade due to their
high solubility and interesting electrochemical and physical prop-
erties.2 Recently, oligo- and poly-3ATs with dramatically improved
properties have been produced by a regioregular synthesis.3,4 The
substrates used in the preparation of oligo- and poly-3ATs are nei-
ther centrosymmetric nor do they possess a symmetry axis; thus,
three possible isomeric diads can be formed from the coupling of
3-alkylthiophenes. As a result, four types of spectroscopically dis-
tinct nonequivalent triads are detected in the NMR spectra.4–7
However, head-to-head couplings are unfavorable due to steric
repulsion between the alkyl chains and the lone pairs of electrons
on the adjacent sulfur atoms.4,6,7 Thus, an efficient synthesis of
alkylthiophenes would provide access to oligo- and polythio-
phenes, which may lead to the development of new devices and
applications of polymer-based materials.
Kumada4,6,8,9 and Suzuki10 cross-coupling methods have been
used extensively for the synthesis of head-to-tail and tail-to-tail
oligo- and poly-ATs. Alternatively, cryogenic lithiation of 2-halo-
3-alkylthiophenes and the subsequent treatment with organotin
or organoboron electrophiles is an efficient method for generating
monomers for the regiospecific synthesis of oligo- and poly-
ATs.4,11,12 Thus, halothiophenes are one of the most important
structures for the homocoupling of thiophenes. Upon treatment
with a stoichiometric amount of N-bromosuccinimide, 3-alkylthio-
phenes quantitatively transformed into 2-bromo-3-alkylthio-
phenes.13,14 The reaction is complete within a few minutes at
ambient or slightly elevated temperatures, and overbromination
typically leads to the corresponding 2,5-dibromo-3-alkylthio-
phenes.14 Although brominating the
a-position of 3-alkylthio-
phenes has been investigated extensively, few methods allow for
the selective bromination of the 5th position of 3-
alkylthiophenes.6,15
Reinecke et al. have investigated the bromination of 3-methyl-
thiophene by reacting the substrate with n-BuLi and quenching with
bromine.15b An inseparable mixture of products was obtained, and
the desired product (2-bromo-4-methylthiophene) was difficult to
isolate in reasonable yield and purity.15b Interestingly, de Meijere
et al. have also recently reported on the synthesis of 2-bromo-4-
methylthiophene in 55% yield by lithiating 3-methylthiophene with
* Corresponding authors. Tel.: +20 47 3215173; fax: +20 47 3215175 (A.A.E.-S.);
tel.: +82 62 9702306; fax: +82 62 9702304 (J.-S.L.).
E-mail addresses: elshehawy2@yahoo.com (A.A. El-Shehawy), jslee@gist.ac.kr (J.-
S. Lee).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.06.100

A. A. El-Shehawy et al. / Tetrahedron Letters 51 (2010) 4526–4529
4527
n-BuLi followed by treating the resulting 2-lithio-4-methyl deriva-
tive with N,N,N0,N0-tetramethyl-1,2-ethylenediamine (TMEDA) and
quenching with CBr4.15d
distillation failed to afford good yields; however, the desired prod-
ucts could be obtained in excellent purity with high yields by care-
ful silica gel flash chromatography.
As shown in Scheme 1, McCullough and co-workers synthesized
2-bromo-4-butylthiophene (5a) in four distinct steps starting from
3-butylthiophene (1a).6 To generate 2-bromo-4-butylthiophene,
the TMS-blocking group was removed from 2-trimethylsilyl-3-bu-
tyl-5-bromothiophene (4) and the desired product 5a was obtained
in 73% yield. Moreover, the synthesis of 2-bromo-4-dodecylthioph-
ene has been achieved by lithiating 3-dodecylthiophene and react-
ing the lithiated intermediate with carbon tetrabromide (CBr4).6
The crude product mixture consisted of 3-dodecylthiophene and
both isomers of the desired product in a 9:1 ratio. However, due
to side reactions and poor recovery, the overall yield of 2-bromo-
4-dodecylthiophene was very low (23%).
Recently, Luscombe et al. synthesized 2-bromo-4-hexylthioph-
ene (5b) by lithiating 3-hexylthiophene (1b) with n-BuLi followed
by treating the lithiated intermediate with TMEDA and CBr4
(Scheme 2).15e However, after fractional distillation, the desired
product was obtained in only ꢁ11% yield.
The structure of 2-bromo-4-alkylthiophenes (5a–c) was charac-
terized and confirmed by each of 1H and 13C NMR spectroscopy
(see Supplementary data). The 1H and 13C NMR spectra of the
resulting 2-bromo-4-alkylthiophenes were fully consistent with
the expected structures.
Kumada cross-coupling of thienylmagnesium bromides and
bromothiophenes is a useful method for the homologation of thio-
phenes and the synthesis of oligo- and polythiophenes.4,6,8,9 Thus,
we attempted the biaryl coupling of 3-hexylthiophene according to
the Kumada cross-coupling procedure. 2-Bromo-4-hexylthiophene
(5b) reacted readily with magnesium, resulting in the exclusive
formation of 4-hexyl-2-thienylmagnesium bromide (7) (Scheme 4).
To verify the formation of the Grignard reagent 7 in a good yield,
5b was reacted with magnesium and quenched with 2 M HCl.
Nearly quantitative conversion to 3-hexylthiophene (1b) was ob-
served, along with a negligible amount of 2-bromo-4-hexylthioph-
ene (5b), as shown by GC–MS, 1H, and 13C NMR analyses.
Kumada cross-coupling of the intermediate 7 with 2b and/or 5b
in the presence of catalytic amounts of Ni(dppp)Cl2 yielded exclu-
sively the corresponding coupled products 3,40-dihexyl-2,20-bithi-
ophene (8) and 4,40-dihexyl-2,20-bithiophene (9), in high yields
(Scheme 4). The coupled product 8 was also readily prepared in
high yield (92%) via Kumada cross-coupling of 3-hexyl-2-thienyl-
magnesium bromide (10)5h,13,15 and 5b in the presence of catalytic
amounts of Ni(dppp)Cl2 (Scheme 4). It is worth to mention that
bithiophenes 8 and 9 have not been synthesized by this synthetic
pathway; however, 3,40-dihexyl-2,20-bithiophene (8) was previ-
ously prepared in 75% yield by a metal-free oxidative biaryl-cou-
Due to the limitations in the aforementioned synthesis, we
developed an efficient and straightforward method for the synthesis
of 2-bromo-4-alkylthiophenes, and obtained the highest chemical
yields reported to date. Moreover, Kumada and Suzuki cross-cou-
pling of 2-bromo-4-hexylthiophene was conducted to synthesize
dihexyl-2,20-bithiophenes in high chemical yields and excellent
selectivity.
Lithiation of 3-alkylthiophenes occurs predominantly at the
carbon-c to the alkyl chain, however, sterically hindered alkyl sub-
stituents increase the selectivity of lithiation.6,11,12,15e Thus, 3-
alkylthiophenes (1a–c) were lithiated with n-BuLi at ꢀ78 °C and
subsequently quenched with bromine at the same temperature.
2-Bromo-4-alkylthiophenes (5a–c) were produced, along with
pling reaction of 3-hexylthiophene (1b) and
a recyclable
hypervalent iodine (III) reagent in the presence of TMSBr.16 More-
over, 4,40-dihexyl-2,20-bithiophene (9) was previously prepared in
52% yield by reacting 3-hexylthiophene with n-BuLi at ꢀ78 °C
and treating the lithiated intermediate with CuCl2.11a
trace
amounts
of
2,5-dibromo-3-alkylthiophenes
(6a–c)
(Scheme 3). After the addition of bromine, the color of the reaction
mixture changed within only few minutes, indicating the reaction
was completed. A slight excess of bromine ensured complete con-
version to the desired product.
When bromine was added to the reaction mixture in one por-
tion at ꢀ78 °C or at elevated temperatures, the desired products
5a–c were formed in low to moderate yields (43–67%) due to
The reactivity of 2-bromo-4-hexylthiophene (5b) in the Kumad-
a cross-coupling reaction was satisfactory. Compared to 2-bromo-
3-hexylthiophene (2b), the bromine in 2-bromo-4-hexylthiophene
(5b) was more reactive and the corresponding Grignard reagent
was formed rapidly. However, the oxidative addition of Ni(cod)2
(cod = 1,5-cyclooctadiene) to 2,5-dibromo-3-alkylthiophenes oc-
curs preferentially at the fifth position of the thiophene ring due
to the less amount of steric hindrance relative to the second posi-
tion.2f,4,17 Similar to the results obtained by Grignard Metathesis
Method (GRIM) for the synthesis of HH and TT-poly-ATs, the lack
of steric congestion facilitates the formation of an intermediate be-
tween the bromine atom in 2-bromo-4-hexylthiophene (5b) and
Ni(dppp)Cl2, which leads to TT and HT couplings.4,6,7
The lithiation of 2-bromo-4-hexylthiophene (5b) with n-BuLi at
ꢀ78 °C and the subsequent reaction with triisopropylborate and
pinacol afforded 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)-4-
hexylthiophene (11) in excellent yield (95%, Scheme 5). However,
boronic ester 11 has been previously synthesized in 89% yield by
reacting 3-hexylthiophene (1b) with n-BuLi at ꢀ78 °C and subse-
quently treating the lithiated intermediate with 2-isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane.11
scrambling at the
a-carbon adjacent to the alkyl groups of 1a–c
by hydrogen–bromide exchange.12–14 Significant amounts of the
starting material 1, as well as 2-bromo- and 2,5-dibromo-3-alkyl-
thiophenes (2 and 6, respectively), were detected and fully charac-
terized. Very slow addition of Br2 (as a solution in THF) was
necessary to achieve satisfactory results. Moreover, after the com-
plete addition of bromine, when water and/or aqueous NaOH was
added to the reaction mixture, at ambient or even lower tempera-
ture, the desired products 5a–c were obtained in low yields as a
mixture of isomeric products. To achieve selective formation of
the desired products, the reaction must be ended at ꢀ78 °C. It
was realized that adding a few drops of an aqueous methanolic
solution of sodium thiosulfate at ꢀ78 °C was found to be a suitable
method for ending the reaction. These modifications provided
2-bromo-4-alkylthiophenes (5a–c) in high yields (>90%). Several
attempts to isolate the desired products 5a–c by fractional
Bu
Bu
Bu
Br
Bu
TMS
Bu
TMS
(i) n-BuLi, -78 oC
(i) n-BuLi, -78 oC
(ii) TMS-Cl
NBS
HF (3 equiv)
Br
S
Br
MeOH.CH2Cl2
Reflux, 12 h
S
S
S
S
(ii) CBr4
HOAc:CHCl3
4
3
5a
1a
2a
(85%)
(90.4%)
(73%)
(70%)
Scheme 1. Synthesis of 2-bromo-4-butylthiophene in four distinct steps, starting from 3-butylthiophene, with an overall yield of 73%.6

4528
A. A. El-Shehawy et al. / Tetrahedron Letters 51 (2010) 4526–4529
C6H13
C6H13
(i) n-BuLi, TMEDA, Et2O, -78 oC to reflux, 30 min
Br
(ii) CBr4, Et2O, -78 oC, 8 h
S
S
5b
1b
(Isolated yield ~ 11%)
Scheme 2. Synthesis of 2-bromo-4-hexylthiophene (5b) by lithiating 3-hexylthiophene (1b) with n-BuLi followed by treating the lithiated intermediate with N,N,N0,N0-
tetramethyl-1,2-ethylenediamine (TMEDA) and CBr4.15e
R
R
R
R
(i) n-BuLi, THF, -78 oC
+
Br
Br
Br
Br
S
S
S
S
(ii) Br2, THF, -78 oC
5a-c
(Isolate yield > 90%)
2a-c
6a-c
(Traces)
1a-c
a. R = C4H9
b. R = C6H13
c. R = C8H17
Scheme 3. Synthesis of 2-bromo-4-alkylthiophenes (5a–c) by lithiating 3-alkylthiophenes (1a–c) with n-BuLi at ꢀ78 °C followed by quenching with bromine at the same
temperature.
C6H13
Br
C6H13
MgBr
5b
Mg / Ether
Reflux, 2h
C6H13
Ni(dppp)Cl2 / Ether
(92 %)
S
S
S
2b
10
S
C6H13
2M-HCl
2b
5b
(Traces)
1b
+
8
Ni(dppp)Cl2 / Ether
(95 %)
(> 97%)
C6H13
C6H13
C6H13
Mg / THF
5b
S
BrMg
S
S
0 oC to r.temp
Br
S
Ni(dppp)Cl2 / Ether
C6H13
5b
7
9
(93 %)
Scheme 4. Synthesis of 3,40-dihexyl-2,20-bithiophene (8) and 4,40-dihexyl-2,20-bithiophene (9) via Kumada cross-coupling method.
C6H13
C6H13
C6H13
S
Pd(PPh3)4 / Toluene
O
S
B
O
+
Br
2M-aq. K2CO3 / 90 oC
S
S
C6H13
5b
12
8
(90 %)
C6H13
C6H13
(i) n-BuLi / -78 oC
5b
O
S
B
S
S
(ii) B(O-iPr)3
(iii) Pinacol
Pd(PPh3)4 / Toluene
2M-aq. K2CO3 / 90 oC
O
C6H13
11
(95%)
9
(92%)
Scheme 5. Synthesis of 3,40-dihexyl-2,20-bithiophene (8) and 4,40-dihexyl-2,20-bithiophene (9) via Palladium-catalyzed Suzuki cross-coupling method.
Palladium-catalyzed Suzuki cross-coupling of boronic ester 11
with 2-bromo-4-hexylthiophene (5b) in the presence of Pd(PPh3)4
preceded efficiently and afforded 4,40-dihexyl-2,20-bithiophene (9)
in 92% yield as the tail-to-tail regioisomer. Interestingly, Suzuki
cross-coupling of 2-bromo-4-hexylthiophene (5b) with 2-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)-3-hexylthiophene (12)
in the presence of Pd(PPh3)4 afforded 3,40-dihexyl-2,20-bithiophene
(8) in excellent head-to-tail regioselectivity and high yield (90%).
It is worth to mention that 3,40-dihexyl-2,20-bithiophene (8) has
been previously prepared in 62% yield by the Suzuki cross-cou-
pling of 11 and 2b in the presence of Pd(PPh3)4.11a Moreover,
bithiophene 8 has also been synthesized in 70% yield by reacting

A. A. El-Shehawy et al. / Tetrahedron Letters 51 (2010) 4526–4529
4529
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the 5th position compared to lithiation of 3-hexylthiophene (1b).
In conclusion, we have developed a selective method for the
synthesis of 2-bromo-4-alkylthiophenes (5a–c) in high yields
(>90%). The present method is superior to the previously reported
methods for their synthesis.6,15e Dihexyl-2,20-bithophenes 8 and 9
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superior to those reported in the literatures. We believe that the
proposed synthetic method can be used for the step-wise construc-
tion of oligo- and polythiophenes with well-defined structures. To
investigate the optical and electrical properties of oligo- and poly-
ATs, further syntheses are currently being developed.
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This work was supported by the Program for Integrated Molec-
ular Systems (PIMS) and the World Class University (WCU) Pro-
gram, Korea (Project No. R31-20008-000-10026-0). (RISE), Korea.
The author (A.A.E.-S.) would like to thank the Korean Brain Pool
Program supported by the Korean Federation of Science and Tech-
nology Societies (KOFST).
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Supplementary data
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Supplementary data (experimental details and the spectral data
for the synthesized 2-bromo-4-alkylthiophenes) associated with
this article can be found, in the online version, at doi:10.1016/
j.tetlet.2010.06.100.
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