Unprecedented Regioregular Poly(1,4-arylene)s Prepared by Nickel(II)-Catalyzed Cross-Coupling Polymerization of 2,5-Disubstituted Bromo(chloro)arylene

Article abstract of DOI:10.1002/anie.201903791

The unprecedented synthesis of regioregular head-to-tail-type poly(1,4-arylene)s bearing different substituents at the 2- and 5-positions is described. They were prepared by the polymerization of 2,5-disubstituted bromo(chloro)arylenes by selective halogen–metal exchange with a Grignard reagent and subsequent cross-coupling polymerization with a nickel catalyst [NiCl₂(dppp)]. Formation of the regioregular poly(1,4-arylene)s were confirmed by NMR spectroscopy, and showed remarkable differences to those polymers having uncontrolled regioregularity. Polymerization of bromo(chloro)arylenes with a chiral alkoxy substituent also led to the regioregular head-to-tail-type polyarylene, which demonstrated circular dichroism, thus suggesting formation of a structure with higher-order regularity.

Full text of DOI:10.1002/anie.201903791

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Accepted Article
Title: Unprecedented regioregular poly(1,4-arylene)s prepared
by nickel(II)-catalyzed cross-coupling polymerization of 2,5-
disubstituted bromo(chloro)arylene
Authors: Yushin Shibuya, Naoki Nakagawa, Naoki Miyagawa, Toyoko
Suzuki, Kentaro Okano, and Atsunori Mori
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Angew. Chem. 10.1002/ange.201903791
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http://dx.doi.org/10.1002/ange.201903791

Angewandte Chemie International Edition
10.1002/anie.201903791
COMMUNICATION
Unprecedented regioregular poly(1,4-arylene)s prepared by
nickel(II)-catalyzed cross-coupling polymerization of 2,5-
disubstituted bromo(chloro)arylene
[a]
[a]
[a]
[a]
[a]
Yushin Shibuya, Naoki Nakagawa, Naoki Miyagawa, Toyoko Suzuki, Kentaro Okano, and
Atsunori Mori*[a]
Dedication ((optional))
Abstract: Unprecedented synthesis of regioregular head-to-tail (HT)-
type poly(1,4-arylene) bearing different substituents at the 2- and 5-
positions is described. The regioregular poly(1,4-arylene) was
arylene) has not been attempted so far, to the best of our
knowledge,[ albeit involving potential constitution of higher order
orientation. It is therefore challenging to prepare and characterize
such unprecedented regioregular poly(1,4-arylene). Herein, we
describe synthesis of head-to-tail-type regioregular poly(1,4-
arylene) bearing different alkoxy substituents at the 2- and 5-
positions through the cross-coupling polymerization of
bromo(chloro)arylene with a nickel catalyst.
5]
prepared
bromo(chloro)arylene by sequential selective halogen-metal
exchange with Grignard reagent followed by cross-coupling
polymerization with a nickel catalyst NiCl (dppp). Formation of the
by
the
polymerization
of
2,5-disubstituted
a
2
regioregular poly(1,4-arylene) was confirmed by measurement of
NMR spectrum, which showed remarkable difference from that of the
related polymer with uncontrolled regioregularity. Polymerization of
bromo(chloro)arylene with a chiral alkoxy substituent under similar
conditions also led to the regioregular HT-type polyarylene, which
showed circular dichroism suggesting formation of a structure of a
higher order regularity.
Chart 1. Regioregular HT-type poly(hetero)arylenes
We first examined the polymerization of a 1,4-dibromobenzene
derivative 3a bearing different alkoxy substituents at the 2- and 5-
positions, which may distinguish steric bulkiness of each
substituent, to undergo regioselective bromine-metal exchange at
the bromine atom adjacent to the less sterically congested
Synthesis and design of π-conjugated polymers attract much
attention in materials science pursuing outstanding electronic
properties. Conjugated polymers have been shown to be
applicable
to
electron-conducting
materials,
organic
semiconductors, photovoltaic cells, etc.[1] Concerning five-
membered heteroaromatic compounds like polythiophenes 1, it is
particularly important to possess high head-to-tail (HT)
regioregularity to exhibit remarkable properties as materials.[2]
Much efforts have been devoted for the development of
preparative methodologies for the regioregular polythiophenes, in
which the cross-coupling strategy has been a powerful tool for the
designed synthesis of 1.[3] By a marked contrast to polythiophene
synthesis, much less attention has been paid to the study on the
design of poly(1,4-arylene)s 2 of controlled regioregularity. In
poly(1,4-arylene) synthesis, it is also effective to undergo
Kumada-Tamao-type catalyst transfer polymerization (KCTP),
which has been developed by Yokozawa and McNeil, by the
catalysis of a transition metal complex.[4] However, poly(2,5-
disubstituted-1,4-arylene) bearing the same substituent at the 2-
and 5-positions do not show regioregularity, while such an issue
appears in the case of 2,5-differently substituted polyarylene.
Nevertheless, synthesis of head-to-tail-type regioregular poly(1,4-
substituent followed by
coupling polymerization leading to the regioregular poly(1,4-
a transition metal-catalyzed cross-
[6]
arylene). Synthesis of 1,4-dibromobenzene 3a was carried out
employing 4-methoxyphenol (8a) as
a starting material.
Introduction of the hexyl substituent by the Williamson ether
synthesis followed by dibromination at the 2,5-positions afforded
3
a. Polymerization of the thus obtainted 3a was carried out by
t
Kumada-Tamao-type catalyst-transfer polymerization with BuLi
and nickel catalyst NiCl
poly(1,4-arylene) 2a in 20% yield with the average molecular
(dppe) (1 mol%)[4d] to furnish the resulting
2
weight of
M
n
=
5800 (M
w
/M
n
=
1.52). (Scheme 1) The
1
3
1
measurement of C{ H} NMR spectrum of 2a suggested the
formation of partly regioregular polymer but the regularity was
[7]
suggested to be ca. 4:1 despite attempted metalation at −78 °C.
The result shows that the halogen-metal exchange of 3a
preferencially takes place at the less hindered bromine atom of
the 1-position adjacent to the methoxy substituent whereas the
metallation reaction is also competitive at the alternative bromine
atom.
[
a]
Y. Shibuya, N. Nakagawa, N. Miyagawa, Dr. T. Suzuki, Dr. K.
Okano, Prof. Dr. A. Mori
Department of Chemical Science and Engineering
Kobe University
1-1 Rokkodai, Nada, Kobe 657-8501, Japan
E-mail: amori@kobe-u.ac.jp
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
Scheme 1. Polymerization of 1,4-dibromoarylene 3a
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COMMUNICATION
We next envisaged that the use of bromo(chloro)-2,5-
disubstituted arylene 5, with which the halogen-metal exchange
would occur at the bromine atom predominantly to the chlorine
atom, to afford the regioregular poly(1,4-arylene) although a
transition metal-catalyzed synthesis of poly(1,4-arylene) through
metalated chloroarene as a monomer from 5 as well as any
preparation pathways to 5 also have not yet been reported so far.
Preparation of 4-chloro-5-hexyloxy-2-methoxy-bromobenzene
regioregular 2a. Considering the high regioregularity
polymerization proceeds in a chain growth manner despite
polycondensation whereas
w n
a slightly broader M /M value
suggests that the polymerization proceeds in a less sufficient
catalyst transfer-type[2a] manner probably because of difficulties
of the intra-chain oxidative addition of the nickel complex to the
carbon-chlorine atom at the polymer end.
(
5a) was found to be performed with commercially available 2-
chloro-4-methoxyphenol (4a). Etherification of the phenol moiety
followed by bromination led to 5a in excellent yields. It should also
be pointed out that a broad range of differently substituted
bromo(chloro)benzene 5 can be, in general, synthesized as
outlined in Scheme 2 employing 4-hydroxy-benzaldehyde 6.
Etherification of phenol to give 4-alkoxybenzaldehyde 7[8] and the
Dakin reaction to give 4-alkoxyphenol 8[9,10] and following
chlorination at the ortho-position of phenol afforded 4. The second
phenol was then transformed into the alkoxy group and the
bromination led to the differently substituted 2,5-dialkoxy-
bromo(chloro)benzene 5.
Scheme 3. Polymerization of bromo(chloro)benzene with different substituents
Figure 1. 13C{ H} NMR spectra of (a) regioregular poly(1,4-arylene) 2a from 5a;
1
(b) polyarylene of statistical copolymer from 5a and 5b
It is remarkable that the polymerization protocol using
bromo(chloro)arylene is applicable to the preparation of a wide
range of differently substituted poly(1,4-arylene)s with alkoxy
groups. The results are summarized in Table 1. Although the
regioregularity can be hardly confirmed by a spectroscopic or
chromatographic analysis, the reaction of 5c bearing n-butyl and
n-hexyl substituents proceeded under similar conditions and
afforded the regioregular polymer 2c, which control would not be
Scheme 2. Preparation of bromo(chloro)benzene 5
As the synthetic pathway of the monomer precursor 5 in hand,
polymerization of 5a was performed. After examination of the
conditions of halogen-metal exchange of bromo(chloro)benzene,
which was found to take place under similar conditions to those
of dibromoarylene 3a[4] and hardly to observe the metalation at
the chlorine atom, polymerization of 5a was carried out by the
possible
by
the
polymerization
with
corresponding
dibromoarylene. Attempted polymerization of 5d bearing n-hexyl
and benzyl groups was found to proceed with much less efficiency
to afford the corresponding polymer 2d in an inferior yield. It was
found that polymerization took place at the elevated temperature
i
reaction of PrMgCl∙LiCl at 60 °C for 1 h in THF followed by the
addition of NiCl
2
(dppp) (1 mol%) as a catalyst. The polymerization
proceeded to afford the 1,4-polyphenylene 2a in 56% yield. SEC
analysis of 2a revealed to exhibit M
n
=22000; M
w
/M
n
= 2.01 as
n w n
(100 ºC) to afford 2d in 67% yield with M = 12700, M /M = 2.20.
shown in Schme 3. Figure 1 shows 13C{ H} NMR spectra of
1
Polymerization of 5e with an oligoethereal substituent was also
found to hardly take place under similar conditions (60 ºC),
however, the reaction of 5e was found to proceed when the
temperature was elevated to 100 ºC to afford polyphenylene 2e in
poly(1,4-arylene)
2a
obtained
by
the
reaction
of
bromo(chloro)benzene 5a (a) and the corresponding statistical
1
copolymer (b) employing 5a and the oppositely substituted 5b (R
2
=
n-hexyl; R = Me). Characteristic signals of methylene (red) and
n w n
59% yield (M = 17400, M /M = 2.85).
methyl (blue) clearly show the formation of regioregular 2a by the
reaction of 5a while the statistical polymer 2a indicated two
signals of different chemical shifts suggesting formation of non-
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10.1002/anie.201903791
Table 1. Polymerization of bromo(chloro)arylene 3 catalyzed by NiCl
2
(dppp)[a]
[d]
[d]
n
monomer precursor
R1
nC
nC
R2
nC
temp (°C) [b]
yield of 1 (%)[c]
M
n
M
w
/M
5
c
d
6
H
H
13
13
4
H
9
60
60
53
26
67
0
15500
6200
2.28
4.24
2.20
5
6
2
CH Ph
1
00
60
00
12700
5e
nC
6
H
13
2 2 2 2 2 2 3
(CH ) O(CH ) O(CH ) OCH
1
59
17400
2.85
[
a] The reaction was carried out with 3 (0.3 mmol) and a metallating agent (1.0 eq) in 0.6 mL of THF at 60 °C for 1 h to form the
corresponding metallated monomer followed by addition of NiCl (dppp) (1 mol %). [b] The reaction temperature in the
polymerization. [c] Isolated yield.[d] Determined by SEC analysis on the basis of 6 polystyrene standards.
2
Bromo(chloro)benzene 5f bearing a chiral substituent was also
prepared in a similar manner employing a chiral alkyl bromide
derived from natural terpene (S)-(-)-β-citronellol. The monomer
absorption maximum of the polyphenylene moiety at the polymer
main chain (ca. 340 nm). A weak CD peak emerged in a mixed
solution of 1,2-dichloroethane/methanol = 9:1. The remarkably
1
2
precursor 5f (R = Me, R =(CH
3
)
2
CH(CH
2
)
3
CH(CH
3
)(CH
2
)
2
) was
strong Cotton effect was observed in a solution of 1,2-
prepared from 2-chloro-4-methoxyphenol (4a) in 2-steps as
dichloroethane/methanol = 7:3 (ca. 300 mdeg). The CD spectrum
shown in Scheme 4. Although attempted polymerization at 60 ºC
of non-regioregular polyphenylene 2f prepared by the
1
2
catalyzed by NiCl
proceeded at 100 ºC to afford 2f in 64% yield (M
2.39).
2
(dppp) was unsuccessful at all, the reaction
polymerization of the related dibromoarylene 3f (R = Me, R
=(CH CH(CH CH(CH )(CH ) was also measured to observe
n
= 13200; M /M
w
n
3
)
2
2
)
3
3
2 2
)
=
a much smaller CD (less than 5 mdeg) value (See Supporting
Information). The results suggested that the chirality of the
regioregular side chain induced a higher order regularity such as
helix structure[11,12] based on a specific interaction of the
phenylene moiety and such characteristics was only observed in
the poly(1,4-arylene) bearing a head-to-tail-type regioregularity,
while CD of the related non-regioregular analog was much
weaker.[13]
Scheme 4. Preparation of regioregular poly(1,4-arylene) 2f bearing a chiral side
chain
Optical rotation of the obtained 2f bearing a chiral side chain
3
2
showed [α]
D
2 2
= +25.0 (c 0.5, ClCH CH Cl) and measurement of
the CD spectrum of 2f was also carried out. The results are shown
in Figure 2. Although the CD peak was hardly observed in a 1,2-
dichlorethane solution, it was found to exhibit the Cotton effect by
the addition of methanol at the wavelength corresponding to the
Figure 2. UV-vis (a) and CD (b) spectra of polymer 2f as 1.0 x 10-5 M 1,2-
dichloroethane solution (UV-vis) and 3.0 x 10-4 M mixed solution of 1,2-
dichloroethane/methanol (CD)
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In summary, we have shown that synthesis of unprecedented
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tail regioregularity was found to be successful by the use of
bromo(chloro)benzene derivatives, while such regularity has not
been realized by the related polymerization of 1,4-
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magnetic stirring bar were added 4-chloro-2-hexyloxy-5-methoxy-
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[4]
bromobenzene (5a, 96 mg, 0.3 mmol), PrMgCl·LiCl (1.1 M in THF, 273 μL,
0
.3 mmol) and THF (0.33 mL) at room temperature under a nitrogen
atmosphere. The mixture was warmed to 60 ºC and stirring was continued
for 1 h. NiCl (dppp) (1.6 mg, 3.0 µmol) was added and further stirring was
2
continued at 60 ºC for 3 h. The reaction mixture was poured into a mixture
of hydrochloric acid (1.0 M, 2 mL) and methanol (10 mL) to form a
precipitate, which was filtered off to leave a solid. After washing with
methanol repeatedly, the solid was dried under reduced pressure to afford
1
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3
= 2.01; 1H NMR
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See Supporting Information.
36.0 mg of 2a (56% isolated yield). M = 22000, M /M
n w n
(
1
400 MHz, CDCl
3
) δ 7.10 (s, 1H), 7.07 (s, 1H), 3.98 (s, 2H), 3.82 (s, 3H),
.81-1.59 (m, 2H), 1.42-1.22 (m, 6H), 0.89 (t, J = 6.4 Hz, 3H); 13C{ H} NMR
1
(100 MHz, CDCl
3
) δ 150.80, 150.26, 127.65, 127.39, 117.50, 115.12,
[7]
6
2
9.80, 56.41, 31.79, 29.64, 25.92, 22.78, 14.21; IR (ATR) 2951, 2932,
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-
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This work was supported by JSPS Kakenhi B Grant Number JP
25288049, Special Coordination Funds for Promoting Science
and Technology, Creation of Innovation Centers for Advanced
Interdisciplinary Research Areas (Innovative Bioproduction Kobe),
MEXT, Japan, and Cooperative Research Program of “Network
Joint Research Center for Materials and Devices”. The authors
thank Professor Satoshi Wakai of Kobe University for discussions
on the measurement of CD spectra.
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Keywords: regioregular poly(1,4-arylene) •
bromo(chloro)arylene • halogen-metal exchange • nickel catalyst
7
051. (h) P. Leysen, J. Teyssandier, S. De Feyter, G. Koeckelberghs,
•
Cotton effect
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1
[
13] See Supporting Information.
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
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Y. Shibuya, N. Nakagawa, N. Miyagawa,
T. Suzuki, K. Okano, A. Mori*
Page No. – Page No.
Unprecedented regioregular poly(1,4-
arylene)s prepared by cross-coupling
polymerization of
bromo(chloro)arylene catalyzed by
nickel(II) complex
Polymerization of differently substituted 1,4-bromo(chloro)benzene afforded
unprecedented head-to-tail (HT) regioregular poly(1,4-arylene), while the regio-
controlled synthesis from the 1,4-dibromoarylene is unsuccessful. Regioregular HT
poly(1,4-arylene) bearing a chiral substituent showed the Cotton effect in a CD
spectrum with a mixed solvent system of 1,2-dichloroethane/methanol suggesting a
helix structure.
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