Cobalt-Catalyzed Reductive Alkylation of Heteroaryl Bromides: One-Pot Access to Alkylthiophenes, -furans, -selenophenes, and -pyrroles

Article abstract of DOI:10.1002/ejoc.201500784

A practical and convenient Co-catalyzed alkylation method for the facile introduction of various alkyl chains into organic electronically significant heteroaryl compounds, including thiophenes, furans, selenophenes, and pyrroles, is reported. Under well-optimized reaction conditions, a wide range of alkylated heteroaryl compounds have beeen efficiently prepared in moderate to good isolated yields. Notably, 2- or 3-alkylthiophenes, which play a decisive role in polymer chemistry and organic materials, have been synthesized step-economically for the first time by this reductive-coupling methodology using inexpensive cobalt salts as catalysts. This straightforward synthetic procedure avoids the preparation of moisture-unstable organometallic reagents (RMgX or RZnX) required in conventional alkylation protocols. Various alkyl chains have been introduced into organic, electronically important heteroaryl compounds step-economically through Co-catalyzed reductive alkylation reactions. The resulting alkylheteroarenes are indispensable building blocks for polymer chemistry and π-functional organic materials.

Full text of DOI:10.1002/ejoc.201500784

FULL PAPER
DOI: 10.1002/ejoc.201500784
Cobalt-Catalyzed Reductive Alkylation of Heteroaryl Bromides: One-Pot
Access to Alkylthiophenes, -furans, -selenophenes, and -pyrroles
Deng-Jhou Cai,[a] Po-Han Lin,[a] and Ching-Yuan Liu*[a]
Keywords: Cobalt / Alkylation / Reduction / Heterocycles
A practical and convenient Co-catalyzed alkylation method
for the facile introduction of various alkyl chains into organic
electronically significant heteroaryl compounds, including
thiophenes, furans, selenophenes, and pyrroles, is reported.
Under well-optimized reaction conditions, a wide range of
alkylthiophenes, which play a decisive role in polymer chem-
istry and organic materials, have been synthesized step-eco-
nomically for the first time by this reductive-coupling meth-
odology using inexpensive cobalt salts as catalysts. This
straightforward synthetic procedure avoids the preparation
alkylated heteroaryl compounds have beeen efficiently pre- of moisture-unstable organometallic reagents (RMgX or
pared in moderate to good isolated yields. Notably, 2- or 3-
RZnX) required in conventional alkylation protocols.
tion-metal catalysts, such as palladium or nickel complexes
(Scheme 1). Therefore, the development of a convenient and
user-friendly alternative route to alkylthiophenes would be
highly desirable. Although direct Csp2–Csp3 bond formation
through reductive alkylation under nickel,[11] copper,[12]
iron,[13] and cobalt catalysis has been reported,[14] the sub-
strate scope of all these methods is limited to aryl halides,
and to date there are no examples or optimized reaction
conditions for the alkylation of heteroaryl compounds suit-
able for organic electronics. Hence, in this work we have
focused on the use of readily available and inexpensive co-
balt salts to carry out an in-depth optimization of reaction
parameters for the development of a general and convenient
one-pot reductive alkylation of various important het-
eroaryl bromides. To the best of our knowledge, this is the
first time that 3-alkylthiophenes, synthetically versatile pre-
cursors in polymer chemistry and modern organic elec-
tronics, have been step-economically synthesized by Co-cat-
alyzed direct reductive alkylation reactions (Scheme 1).
Introduction
Heterocycle-containing π-conjugated small molecules or
macromolecules have established their significance and in-
dispensability as a variety of organic optoelectronic materi-
als.[1] In particular, thiophene-based materials have been
well studied and extensively applied in modern organic elec-
tronics owing to their remarkable thermal and electrochem-
ical stability in various electronic devices.[2] For example,
thiophenes are employed as monomers in the synthesis of
donor-type homopolymers, such as the noted poly(3-hexyl-
thiophenes) (P3HT),[3] and are often incorporated into do-
nor–acceptor (D–A) type copolymers for photovoltaic ap-
plications.[4] In addition, some interesting π-conjugated
backbones, such as benzodithiophene-4,8-dione,[5] dithieno-
thiophene,[6] thiazolothiazole,[7] and Pechmann dyes,[8] have
also been prepared from thiophenes and applied in numer-
ous organic semiconducting materials. It is worth noting
that these organic electronic materials all require the pres-
ence of appropriate alkyl groups attached to target com-
pounds. These alkyl chains enable an increase in the solubil-
ity of materials in organic solvents, which is beneficial for
subsequent device fabrications and the study of different
crystalline behaviors or microscopic structures at inter-
faces.[9] However, the existing synthetic approaches to the
introduction of alkyl chains into thiophenes all rely on con-
ventional methods[10] that have shown shortcomings, in-
cluding the employment of organometallic reagents pre-
generated by metal insertions and the use of precious transi-
[a] Department of Chemical and Materials Engineering, National
Central University,
Jhongli District, Taoyuan, Taiwan 320, R.O.C.
E-mail: cyliu0312@ncu.edu.tw
Scheme 1. Accessing multifunctional alkylheteroaryl compounds
by traditional synthetic routes amd one-pot cobalt-catalyzed re-
ductive alkylation reactions.
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Eur. J. Org. Chem. 2015, 5448–5452

Reductive Alkylation of Heteroaryl Bromides
Table 1. Optimization of the Co-catalyzed reductive alkylation of
3-bromothiophene with 1-iodododecane.[a]
Results and Discussion
To obtain the optimum reaction conditions, the reductive
alkylation of 3-bromothiophene (1a) with 1-iodododecane
(2a) was carried out by using CoBr2 as catalyst with a vari-
ety of ligands, reducing agents, and (co)solvent systems
(Table 1). Initially, a number of triarylphosphine ligands
were tested with manganese powder as reducing agent in
DMAc/pyridine as solvent, and the desired product 3-dode-
cylthiophene (3a) was isolated in moderate to good yields
(42–84%; Entries 1–5). In particular, the sterically less hin-
dered tri(p-tolyl)phosphine gave a promising yield (84%;
Entry 4), presumably because the formation of a byproduct
from the homocoupling of I–C12H25 (2a) was considerably
reduced in this case. On the other hand, trialkylphosphines
were found to afford 3a in lower yields (41–54%; Entries 6–
9). The dicyclohexyl(biphenyl)phosphine ligands, including
XPhos and SPhos, were also employed, but we only ob-
served slightly improved yields (55–57%; Entries 10 and 11)
when compared with the reactions performed with trialkyl-
phosphine ligands. Bidentate ligands, such as dppe and
dppp, also furnished 3a in moderate yields (48–50%; En-
tries 12 and 13). In contrast, XantPhos, a structurally rigid
bidentate ligand with a wide bite angle, was shown to be
inefficient in the reductive alkylation, and product 3a was
generated in only trace amounts (Entry 14). Similarly,
DPEPhos also gave an unsatisfactory yield (31%; En-
try 15). Furthermore, the alkylation reactions with N,N-bi-
dentate ligands produced 3a in yields of 36–54% (En-
tries 16–18). Interestingly, we still observed and isolated the
desired product even under ligand-free conditions (45%;
Entry 19). Based on the above results, we selected P(p-
tolyl)3 as the optimal ligand for subsequent optimizations.
First, manganese powder was replaced by three different
metals, namely In, Zn, and Cu. However, none of these was
effective as reducing agent in the alkylation reactions (En-
tries 20–22). In the absence of Mn, the alkylation did not
occur, and both starting materials (1a and 2a) were reco-
vered (Entry 23). Solvent effects were then investigated. In
the absence of pyridine, the yield of 3a drastically dropped
(15%; Entry 24). We also tested other polar aprotic sol-
vents, such as N-methylpyrrolidinone (NMP), N,NЈ-dimeth-
ylpropyleneurea (DMPU), and DMSO, in combination
with DMAc (14–48%; Entries 25–27). It is worth noting
that low-polarity solvents (toluene and o-xylene) made the
alkylations less efficient, and the formation of 3a was not
observed (Entries 28 and 29). Finally, under the optimum
conditions [CoBr2, P(p-tolyl)3, Mn, and DMAc/pyridine],
we replaced 2a with its bromo congener and found that the
alkylation proceeded to yield 3a in only a moderate yield
(48%; Entry 30), Hence, the subsequent exploration of sub-
strate scope focused on the use of iodo-substituted alkanes.
As shown in Table 2, the substrate scope with respect to
Entry
Ligand
Reducing
agent
Solvent
Yield
[%][b]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19[c]
20
21
22
23[d]
24[e]
25
26
27
28
29
30[f]
PPh3
P(o-tolyl)3
P(m-tolyl)3
P(p-tolyl)3
L1
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
In
Zn
Cu
–
Mn
Mn
Mn
Mn
Mn
Mn
Mn
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc/pyridine
DMAc
55
58
60
84
42
42
44
41
54
55
57
50
48
trace
31
54
53
36
45
0
P(tBu)2(Me)·HBF4
P(tBu)3·HBF4
PCy3
L2
XPhos
SPhos
dppe
dppp
XantPhos
DPEPhos
bipyridyl
phenanthroline
neocuproine
–
P(p-tolyl)3
P(p-tolyl)3
P(p-tolyl)3
P(p-tolyl)3
P(p-tolyl)3
P(p-tolyl)3
P(p-tolyl)3
P(p-tolyl)3
P(p-tolyl)3
P(p-tolyl)3
P(p-tolyl)3
trace
0
0
15
48
14
43
0
NMP/pyridine
DMPU/pyridine
DMSO/pyridine
toluene/pyridine
o-xylene/pyridine
DMAc/pyridine
0
48
[a] Unless otherwise specified, the alkylation reaction was con-
ducted with 3-bromothiophene (1a; 1.00 mmol) and 1-iododode-
cane (2a; 1.10 mmol) in the presence of CoBr2 (0.20 mmol), ligand
(0.20 mmol), and reducing agent (4.00 mmol) in co-solvent
(1.50 mL/0.38 mL, 4:1, v/v) at 70 °C for 24 h. [b] Isolated yields. [c]
Ligand was not added. [d] Reducing agent was not added. [e] N,N-
Dimethylacetamide (DMAc) was used as sole solvent. [f] 1-Iododo-
decane was replaced with 1-bromododecane (1.1 mmol).
both reactants was investigated by treating bromothio- ford the synthetically useful products 3b–3g in generally
phenes 1a–1d with iodoalkanes 2b–2l using the optimum good yields (60–78%). The alkylation of 1a with branched
conditions (Table 1, Entry 4). First, linear alkyl chains of alkyl iodides possessing (2-ethylhexyl), (2-hexyldecyl), or (2-
different lengths (C6 to C18) were installed at the 3-position octyldodecyl) groups gave the corresponding 3-alkylthio-
of thiophene (1a) by using the present methodology to af- phenes 3h–3j in yields of 72–82%. Moreover, thiophene de-
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D.-J. Cai, P.-H. Lin, C.-Y. Liu
FULL PAPER
rivatives bearing phenyl- or ester-substituted alkyl chains or 2,3-dialkylthiophenes are often incorporated as end-
could be readily synthesized and isolated in good yields (65, groups in small-molecule semiconducting materials.[15]
63%; 3k, 3l). The above 3-alkylthiophenes are valuable to
Furthermore, the substrate scope with respect to other
polymer chemists, because they have been extensively em- important heteroaryl compounds was explored (Table 3).
ployed as monomers for the preparation of a variety of Initially, the alkylation of 5-bromo-2,2Ј-bithiophene (1e)
functional conjugated polymers, including the noted was performed with two different alkyl iodides, and we suc-
P3HT.[3] The direct alkylation of 2-bromothiophene (1b) cessfully isolated products 4a and 4b (62 and 74%, respec-
with linear or branched iodoalkanes also proceeded tively). Likewise, under the optimum conditions, 2-bromo-
smoothly to yield the desired 2-alkylthiophenes (73, 76%; 3,4-ethylenedioxythiophene (EDOT, 1f) was alkylated to
3m, 3n). Importantly, functional groups such as ester and give the desired products in moderate yields (48, 59%; 4c,
ketone (using 1c: 2-acetyl-5-bromothiophene as substrate) 4d). In addition to thiophene derivatives, bromofuran (1g),
were tolerated under the optimum reaction conditions, pro- -selenophene (1h), and -pyrrole (1i) were also examined,
viding the alkylation products in moderate yields (65, 42%; and the corresponding alkylation compounds 4e–4j were
3o, 3p). We also employed 2-bromo-3-hexylthiophene (1d) obtained in yields of 34–55%. In the cases of 4g–4j, 3,4-
as substrate in reductive alkylations with two different dibromoselenophene (1h) and 3,4-dibromopyrrole (1i) were
iodoalkanes; it was found that interesting dialkyl-substi- used as starting materials, because we were interested to see
tuted thiophenes could be obtained in good yields (70, whether dialkylation would take place. However, 1h gave
72%; 3q, 3r). In contrast to 3-alkylthiophenes, which oc- only trace amounts of 3,4-dialkylselenophenes, the reaction
cupy a central role in macromolecular chemistry, 2-alkyl- mixture also containing 3-alkylselenophene (4g or 4h; the
second bromine atom was reduced) and 3-alkyl-4-bromo-
selenophene (the second bromine atom retained), both of
Table 2. Exploration of the substrate scope in the reductive alkyl-
ation of bromothiophenes with various alkyl iodides.[a]
which were formed and isolated in almost equal amounts.
Interestingly, in the case of 3,4-dibromopyrrole (1i) under
identical reaction conditions we only obtained the 3-alkyl-
pyrroles 4i and 4j as the sole products, and the 3-alkyl-4-
bromopyrroles or dialkylpyrroles were not observed at all.
In the two alkylation reactions of 1i, a significant amount
of undesired reduction products (both bromine atoms re-
Table 3. Exploration of the substrate scope in the reductive alkyl-
ation of other bromoheteroarenes with various alkyl iodides.[a]
[a] Unless otherwise specified, the alkylation reaction was con-
ducted with bromoheteroarenes 1e–j (2.00 mmol) and various
iodoalkanes (2.20 mmol) in the presence of CoBr2 (0.40 mmol),
P(p-tolyl)3 (0.40 mmol), and Mn powder (8.00 mmol) in N,N-di-
methylacetamide (DMAc)/pyridine (3.00 mL/0.75 mL, 4:1, v/v) at
70 °C for 24 h. [b] Isolated yields. [c] 3,4-Dibromoselenophene was
used as the starting material. [d] 3,4-Dibromo-1-(2-ethylhexyl)pyr-
role was used as the starting material.
[a] Unless otherwise specified, the alkylation reaction was con-
ducted with bromothiophenes 1a–d (2.00 mmol) and iodoalkanes
2b–l (2.20 mmol) in the presence of CoBr2 (0.40 mmol), P(p-
tolyl)3 (0.40 mmol), and Mn powder (8.00 mmol) in N,N-dimethyl-
acetamide (DMAc)/pyridine (3.00 mL/0.75 mL, 4:1, v/v) at 70 °C
for 24 h. [b] Isolated yields.
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Reductive Alkylation of Heteroaryl Bromides
duced) were generated, thereby lowering the yields of the oxidative addition to the heteroaryl bromides to form het-
products 4i and 4j (40 and 43%, respectively). Finally, the eroarylcobalt intermediates. These intermediates are re-
alkylation of 5-bromopyrimidine (1j) was conducted with duced, again by excess manganese, prior to the second oxi-
1-iodooctane to afford the product 4k in good yield (70%). dative addition with alkyl iodides, which proceeds to gener-
As discussed above, 3-alkyl-4-bromoselenophenes were ate alkyl(heteroaryl)cobalt species. Reductive elimination
formed as a co-product when 3,4-dibromoselenophene (1h) then occurs to give the desired alkylation product and
underwent alkylations with iodoalkanes. Inspired by this re- simultaneously regenerate the cobalt catalyst for the next
sult, we anticipated that the reductive alkylation of 3,4-di- cycle.
bromothiophene (1k) would give a similar product distribu-
Conclusions
tion as 1h (Scheme 2). However, in contrast to the outcome
of the alkylation of 1h, the alkylation of 1k with 1-iodo-
decane yielded only 3-decylthiophene (3e; 65%), with the
formation of 3-bromo-4-decylthiophene or 3,4-didecyl-
thiophene not detected at all. The second alkylation of 3-
bromo-4-decylselenophene (5) with a branched alkyl iodide
generated a mixture of the debrominated product 4g (30%)
and the interesting unsymmetrical dialkylselenophene 6
(42%), which has not yet been accessed by other reported
alkylation methods.
The incorporation of alkyl chains into heteroaryl com-
pounds suitable for organic materials is an indispensable
process, because the presence of appropriate alkyl groups is
of vital importance for the solubility of materials and their
crystalline properties. In contrast to the conventional syn-
thetic methods that require the additional preparation of
air-sensitive organometallic reagents for Csp2–Csp3 forma-
tion, we have demonstrated herein a convenient one-pot ac-
cess to multifunctional alkylheteroaryl compounds by using
inexpensive cobalt salts as catalyst. Under the optimized
conditions, a variety of important heterocycles, such as
thiophenes, furans, selenophenes, and pyrroles, have been
efficiently alkylated in good isolated yields, thereby making
this reductive alkylation synthetically more useful and at-
tractive to polymer chemists and modern organic elec-
tronics. The development of cobalt-catalyzed reactions and
further applications in the step-economical synthesis of
functional π-conjugated organic materials are currently un-
derway in our laboratory.
Experimental Section
General Procedure for the Cobalt-Catalyzed Reductive Alkylation of
Heteroaryl Bromides. Synthesis and Characterization of Product 3a:
3-Bromothiophene (1a; 163 mg, 1.00 mmol) and 1-iodododecane
(2a; 326 mg, 1.10 mmol) were added to a solution of CoBr2 (44 mg,
0.20 mmol), P(p-tolyl)3 (61 mg, 0.20 mmol), and Mn powder
(220 mg, 4.00 mmol) in a co-solvent system (DMAc/pyridine =
1.5:0.38 mL, 4:1, v/v) in a flame-dried Schlenk tube under N2. Five
drops of trifluoroacetic acid were added slowly. The reaction mix-
ture was then gradually heated to 70 °C and kept at the same tem-
perature under N2 for 24 h. After the reaction mixture had cooled
to room temperature, the precipitates were separated by filtration
(Celite) and washed with ethyl acetate (60 mL). The filtrate was
mixed with saturated NH4Cl(aq) (20 mL) and the mixture stirred
for 10 min. The mixture was then extracted with ethyl acetate (2ϫ
50 mL), and the combined organic layers were washed with brine
(50 mL), dried (Na2SO4), and concentrated in vacuo. Purification
by flash chromatography (hexanes) yielded the desired product 3a
Scheme 2. Comparison of the reductive alkylations of 3,4-di-
bromoselenophene (1h) and 3,4-dibromothiophene (1k).
A tentative mechanism for the reductive alkylation of
heteroaryl bromides is proposed (based on the mechanism
postulated by Amatore and Gosmini[14b]). As shown in
Scheme 3, CoBr2 is first reduced by manganese to undergo
1
(212 mg, 84%) as a pale-yellow oil. H NMR (CDCl3, 300 MHz):
δ = 7.22–7.29 (m, 1 H), 6.90–7.00 (m, 2 H), 2.64 (t, J = 7.5 Hz, 2
H), 1.57–1.72 (m, 2 H), 1.20–1.45 (m, 18 H), 0.91 (t, J = 6.5 Hz, 3
H) ppm. 13C NMR (CDCl3, 75 MHz): δ = 143.2, 128.3, 125.0,
119.7, 31.9, 30.5, 30.3, 29.68, 29.67, 29.65, 29.6, 29.5, 29.36, 29.35,
22.7, 14.1 ppm.
Acknowledgments
Financial support provided by the Ministry of Science and Tech-
nology (MOST), Taiwan (MOST 103-2113-M-008-009-MY2) and
Scheme 3. Plausible reaction mechanism for the reductive alkyl-
ation of heteroaryl bromides.
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D.-J. Cai, P.-H. Lin, C.-Y. Liu
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Received: June 12, 2015
Published Online: July 20, 2015
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Eur. J. Org. Chem. 2015, 5448–5452