C-H functionalization polycondensation of chlorothiophenes in the presence of nickel catalyst with stoichiometric or catalytically generated magnesium amide

Article abstract of DOI:10.1021/ja2033525

Polymerization of 2-chloro-3-substituted thiophenes proceeded with a stoichiometric amount of magnesium amide, TMPMgCl?LiCl, or a combination of a Grignard reagent and a catalytic amount of secondary amine in the presence of a nickel catalyst. Although the nickel-catalyzed polymerization with NiCl ₂dppe, which exhibited high catalytic activity in the reaction of bromothiophenes, was less effective, use of a nickel catalyst bearing N-heterocyclic carbene as a ligand was found to induce polymerization with controlled molecular weight and molecular weight distribution.

Full text of DOI:10.1021/ja2033525

COMMUNICATION
pubs.acs.org/JACS
CÀH Functionalization Polycondensation of Chlorothiophenes in the
Presence of Nickel Catalyst with Stoichiometric or Catalytically
Generated Magnesium Amide
Shunsuke Tamba, Keisuke Shono, Atsushi Sugie, and Atsunori Mori*
Department of Chemical Science and Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan
S Supporting Information
b
Chart 1
ABSTRACT: Polymerization of 2-chloro-3-substituted
thiophenes proceeded with a stoichiometric amount of
magnesium amide, TMPMgCl LiCl, or a combination of
3
a Grignard reagent and a catalytic amount of secondary
amine in the presence of a nickel catalyst. Although the
nickel-catalyzed polymerization with NiCl dppe, which ex-
2
hibited high catalytic activity in the reaction of bromothio-
phenes, was less effective, use of a nickel catalyst bearing N-
heterocyclic carbene as a ligand was found to induce
polymerization with controlled molecular weight and mo-
lecular weight distribution.
Scheme 1
ligothiophenes and polythiophenes have recently attracted
O
considerable attention in materials science. Regioregular
thiophenes, which involve head-to-tail (HT) repeating units such
as poly(3-hexylthiophene) (HT-P3HT, 1), are of particular
interest since the compounds show remarkable physical proper-
When the polymerization of 2-chloro-3-hexylthiophene (3b)
was carried out with an equimolar amount of 4 and 0.5 mol % of
9
a
NiCl dppe at 25 °C for 24 h, only 18% of P3HT 1 was
2
1
ties for uses such as organic thin-film transistors, thin-film organic
obtained, with a much lower M = 4200 (M /M = 1.34). The
n
w
n
2
3
solar cells, and conductive polymer materials. Development of
a practical synthetic method for P3HTs is therefore important in
result is in sharp contrast to the polymerization with 2-bromo-
-hexylthiophene (3a), which brought about complete conver-
sion at room temperature within 24 h, affording 1 with M =
3
4
organic synthesis. Dehalogenative polycondensation developed
n
5
6
7
by Rieke, McCullough, and Yokozawa with a Grignard reagent
and a nickel catalyst is widely employed for the synthesis of the
regioregular polythiophenes. Considering the point of atom
efficiency in the polythiophene synthesis, the reaction with the
loss of two halogen atoms in the thiophene unit is problematic
issue; for example, the use of 2,5-dibromo-3-hexylthiophene (2a)
brings about 49% mass loss in the polymerization. We have
recently shown that dehydrobrominative polycondensation with
4
5 000 (M /M = 1.32) (Scheme 1).
w n
We then surveyed nickel catalysts with various ligands. Table 1
summarizes the polymerization of 2-chloro-3-hexylthiophene
(
3b) with KnochelÀHauser base 4. The reaction was slightly
9a
improved by the use of NiCl dppp (0.5 mol %) as a catalyst to
2
afford the polymer 1 in 57% yield; however, its molecular weight
was still low compared with the theoretical M (ca. 33 000) based
n
on the ratio of catalyst loading to the monomer. Drastic improve-
ment of the catalytic activity was observed when the catalyst was
2
-bromo-3-hexylthiophene (3a) proceeds with KnochelÀHau-
8
13a
ser base (chloromagnesium 2,2,6,6-tetramethylpiperidide lithium
switched to that bearing PPh and NHC ligands on the nickel
3
chloride salt, TMPMgCl LiCl, 4), and a nickel catalyst gives
3
to afford polythiophene with M
n
= 27 300 and a relatively narrow
9
highly regioregular HT-P3HT, in which the atom efficiency is
polydispersity (M /M = 1.27). As summarized in Figure 1, (a)
w
n
improved to 32% mass loss. A more improved efficiency is
possible if the polymerization is performed with the correspond-
ing chlorothiophene 3b, giving only 17% mass loss (Chart 1);
however, the reaction of 2-chlorothiophene has been difficult
the average molecular weight was found to decrease by increasing
the catalyst loading, while the molecular weight distribution was
1
.2 to 1.3, and (b) a linear increase of M was observed as
n
14
consumption of the monomer proceeded. The use of a mixture
10
under similar conditions. It is highly intriguing to investigate
a novel metal-catalyzed polymerization that can be used for
chlorothiophenes as a monomer. Herein, we report that a nickel
of Ni(cod) and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene
2
(
IPr; Ni/L = 1:2) also afforded a polythiophene of high molec-
ular weight and HT selectivity in 41% yield. However, the poly-
dispersity was considerably inferior. Similar reaction with Ni(cod)2
11,12
catalyst bearing N-heterocyclic carbene (NHC)
as a ligand is
highly effective for the polymerization of 2-chloro-3-hexylthiophene,
affording HT-P3HT. We also describe CÀH functionalization
with the combination of a Grignard reagent and a catalytic
amount of secondary amine in place of KnochelÀHauser base.
Received: April 12, 2011
Published: June 02, 2011
r 2011 American Chemical Society
9700
dx.doi.org/10.1021/ja2033525
|J. Am. Chem. Soc. 2011, 133, 9700–9703

Journal of the American Chemical Society
COMMUNICATION
Table 1. Nickel-Catalyzed Polymerization of Chlorothio-
Scheme 2
a
phene 3b with KnochelÀHauser Base
b
c
d
catalyst (mol %)
yield, %
M
n
(M
w
/M
n
)
% HT
NiCl
2
dppe (0.5)
dppp (0.5)
18
57
4200 (1.34)
9700 (1.39)
91
98
99
98
98
97
95
98
98
NiCl
2
NiCl
NiCl
NiCl
NiCl
NiCl
2
2
2
2
2
(PPh
(PPh
(PPh
(PPh
(PPh
3
)IPr (0.5)
3
)IPr (1.0)
3
)IPr (1.5)
3
)IPr (2.0)
3
)IPr (3.0)
67 (90)
(95)
(99)
(87)
(89)
41
29200 (1.29)
17400 (1.31)
10700 (1.26)
7870 (1.22)
5170 (1.15)
28400 (1.92)
31900 (6.30)
Ni(cod)
2
/2IPr (0.5)
Ni(cod)
2
/2SIPr (0.5)
70
a
The reaction was carried out with 3b and 1.0 equiv of TMPMgCl LiCl
3
Table 2. Generation of Thienyl Magnesium Species with a
4
in THF for the metalation, and nickel catalyst was employed for the
a
b
Grignard Reagent and a Catalytic Amount of Amine
polycondensation. Isolated yield. Conversion (%) of the monomer is
shown in parentheses. M
analysis using CHCl as an eluent. HT selectivity was estimated by H
c
n
and Md/M values were estimated by SEC
w n
1
3
NMR analysis.
b
Grignard reagent
EtMgCl
amine
none
time, h
conversion, %
10
2
26
73
85
85
88
46
17
Et
2
NH
NH
NH
i
Pr
2
10
1
Cy
2
TMPH
10
10
10
morpholine
Figure 1. (a) Relationship of [consumed monomer]/[catalyst] feed
3 2
(Me Si) NH
ratio toward M
n w n
and M /M in the polymerization of 3b/4 with
NiCl (PPh )IPr after stirring for 24 h. The solid line indicates theore-
2
3
i
PrMgBr
none
10
10
5
21
85
73
83
35
tical molecular weight at each ratio. (b) Relationship of monomer
conversion vs M , obtained with 0.5 mol % of NiCl (PPh )IPr. The
solid line indicates theoretical molecular weight at each conversion;
O and 0 show M and M /M , respectively.
Et
2
NH
NH
NH
n
2
3
i
Pr
2
n
w
n
Cy
2
2
TMPH
10
and 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene (SIPr;
Ni/L = 1:2) afforded 1in 70% yield with M =31900(M /M =6.3).
t
n
w
n
BuMgCl
none
10
10
10
10
10
6
71
90
95
5
The reaction was considered to proceed by the CÀH func-
Et
2
NH
NH
NH
tionalization of chlorothiophene 3b with TMPMgCl LiCl 4 to
i
3
Pr
2
form the corresponding thienyl Grignard reagent A along with
Cy
2
2
,2,6,6-tetramethylpiperidine (TMPH), and the addition of a
TMPH
nickel catalyst induced the polymerization of A, as illustrated in
Scheme 2. Concerning the atom efficiency, the reagent for the
generation of the metallic species should also be taken into con-
sideration. Thus, it is intriguing to perform the total polymeri-
zation process with catalytic use of the magnesium amide. Since
KnochelÀHauser base 4 is prepared by the reaction of 2-pro-
pylmagnesium chloride (LiCl salt) with 2,2,6,6-tetramethyl-
a
The reaction was carried out with 2-chloro-3-hexylthiophene (0.5 mmol),
Grignard reagent (0.5 mmol), and amine (0.05 mmol) in 0.5 mL of THF
at 60 °C. The conversion was estimated by H NMR analysis after
quenching the reaction mixture with iodine.
b
1
amount of secondary amines to form the corresponding thienyl
magnesium reagent. Table 2 summarizes the results. The reaction
was examined at 60 °C in THF, and the progress of the reaction was
monitored by quenching with iodine to form 2-chloro-3-hexyl-
5-iodothiophene (5). It was found that combination of several
Grignard reagents and 10 mol % of amine was effective. The
reaction with a Grignard reagent in the absence of amine afforded
5 in extremely low yield, which suggested that Grignard reagent
also abstracted the hydrogen atom, albeit with insufficient efficiency.
The reaction with sterically less-hindered ethylmagnesium chlo-
ride showed superior reactivity with a hindered secondary amine
8
,15
piperidine
by proton abstraction to form propane, the pro-
duced TMPH would be employed for the regeneration of 4.
This suggests that in situ formation of magnesium amide species
occurs with a catalytic amount of secondary amine and Grignard
reagent. Although metalation of 3-methylthiophene has been
shown recently with catalytic TMPH and isopropyl Grignard
16
reagent, it is still unrevealed if the reaction of the Grignard reagent
toward the halogen-containing thiophene is applicable.
Accordingly, we investigated the metalation of 2-chloro-
3-hexylthiophene (3b) with several Grignard reagents and a catalytic
9
701
dx.doi.org/10.1021/ja2033525 |J. Am. Chem. Soc. 2011, 133, 9700–9703

Journal of the American Chemical Society
COMMUNICATION
17
Table 3. Nickel-Catalyzed Polycondensation of 3-Substituted
leading to 2-thienyl Grignard reagent. Thus, quenching of the
metalated species with iodine afforded 2-iodo-3-hexylthiophene
exclusively (Scheme 3).
2
-Chlorothiophene Using Grignard Reagent and Catalytic
a
Amine
With the polymerization conditions determined for 3b, several
other chlorothiophenes were converted to poly-3-substituted
thiophenes, as shown in Table 3. Monomers 6 and 7 were
synthesized in a similar manner to that of 3b. Polymerization of 6
and 7 also proceeded to afford the corresponding polymers
highly efficiently.
In conclusion, 2-chloro-3-subsituted thiophenes were revealed to
polymerize with magnesium amide and a nickel catalyst. The
polymerization shows remarkable synthetic advantage in atom
efficiency, giving only 17% mass loss comparing with those
(
49À55%) of existing dehalogenative methods with 2,5-diha-
lothiophenes. The reaction using catalytically generated magne-
sium amide with Grignard reagent and catalytic secondary amine
is also worth mentioning from a mechanistic point of view as well
as for the practical synthetic efficiency of the total process. The
established new concept opens further improvement toward
various transition metal-catalyzed coupling reactions as well as
the related cross-coupling polycondensation reactions.
a
The reaction was performed with monomer (1.0 mmol), Cy NH
2
(0.1 mmol), EtMgCl (1.0 mmol), and NiCl
2
(PPh
3
)IPr (1.0 mol %) in
1
2
0 mL of THF at 60 °C for 1 h (metalation) and at room temperature for
’
ASSOCIATED CONTENT
b
4 h (polymerization). Estimated by SEC analysis using CHCl as an
3
c
1
eluent. Estimated by H NMR.
S
b
Supporting Information. Experimental details and char-
acterization of new compounds and polymers. This material is
available free of charge via the Internet at http://pubs.acs.org.
Scheme 3
’
AUTHOR INFORMATION
Corresponding Author
amori@kobe-u.ac.jp
’
ACKNOWLEDGMENT
This work was partially supported by a Grant-in-Aid for
Scientific Research (B) by Japan Society for the Promotion of
Science (JSPS). Thiophene monomers were kindly donated by
Shuji Okamoto and Hikaru Meguro of Soken Chemical &
Engineering Co., Ltd.
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