Polyarylene synthesis by cross-coupling with HOMSi reagents

Article abstract of DOI:10.1246/cl.2013.45

Cross-coupling reaction of dibromoarenes with HOMSi reagents (organo[2-(hydroxymethyl)phenyl]dimethylsilanes), or alternatively bromoarenes with arylene-bisHOMSi reagents, proceeded smoothly in the presence of a Pd catalyst and a weak base, and ter- or quaterarenes are produced in excellent yields. The present reaction was successfully applied to polyarylene synthesis using 4,7-dibromobenzothiadiazole or a 2,7-dibromofluorene derivative along with a 2,7-fluorenylenebisHOMSi reagent.

Full text of DOI:10.1246/cl.2013.45

doi:10.1246/cl.2013.45
Published on the web December 22, 2012
45
Polyarylene Synthesis by Cross-Coupling with HOMSi Reagents
Kenta Shimizu,1 Yasunori Minami,2 Yoshiaki Nakao,3 Ken-ichiro Ohya,4 Hideyuki Ikehira,5 and Tamejiro Hiyama*2
1Department of Applied Chemistry, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551
2Research and Development Initiative, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551
3Department of Material Chemistry, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510
4Tsukuba Research Laboratory, Sumitomo Chemical Co., Ltd., Kitahara, Tsukuba, Ibaraki 300-3294
5Organic Synthesis Research Laboratory, Sumitomo Chemical Co., Ltd., Kasugade-naka, Konohana-ku, Osaka 554-8558
(Received October 12, 2012; CL-121045; E-mail: thiyama@kc.chuo-u.ac.jp)
Cross-coupling reaction of dibromoarenes with HOMSi
reagents (organo[2-(hydroxymethyl)phenyl]dimethylsilanes), or
alternatively bromoarenes with arylene-bisHOMSi reagents,
proceeded smoothly in the presence of a Pd catalyst and a
weak base, and ter- or quaterarenes are produced in excellent
yields. The present reaction was successfully applied to
polyarylene synthesis using 4,7-dibromobenzothiadiazole or a
2,7-dibromofluorene derivative along with a 2,7-fluorenylene-
bisHOMSi reagent.
of high-molecular-weight polymers. In this regard, HOMSi
reagents have advantages of easy handing and purification as
well as cyclic silyl ether formation. On the other hand, for a
single carbon-carbon bond formation, HOMSi reagents are
generally used slightly in excess.6 Thus, we started our research
by scrutinizing the cross-coupling conditions for the stoichio-
metric coupling of 4,7-dibromobenzothiadiazole (1a) with
phenyl-HOMSi reagent 2a.
After screening various parameters of reaction conditions,
we found that the reaction of 1a and 2a in a molar ratio of
1.0:2.1 proceeded smoothly in the presence of [Pd{P(o-tolyl)3}2]
(2.0 mol %), DPPF (= 1,1¤-bis(diphenylphosphino)ferrocene)
(2.1 mol %), CuBr¢SMe2 (3.0 mol %), Cs2CO3 (4.2 equiv), MS
3A (200 mg mmol¹1) in THF/NMP at 50 °C for 3 h to give 4,7-
diphenylbenzothiadiazole (3aa) in 95% yield (eq 1). The scope
of dibromoarenes 1 was proven to be broad as demonstrated in
Table 1. Substituted aryl electrophiles such as 1,4-dibromo-2,5-
dihexylbenzene (1b) and 2,7-dibromo-9,9-dioctylfluorene (1c)
gave the corresponding terphenyl and quaterphenyl derivatives
3ba and 3ca in 94% and 92% yields respectively (Entries 1 and
Polymers containing conjugated ³-electron systems play
key roles in many electronic organic materials such as sensors,
semiconductors, photovoltaic cells (PVC), field-effect transistors
(FET), and optical devices (e.g., organic light-emitting diodes).1
Thus invention of efficient synthetic methods for ³-conjugated
polymers has grown to be an important issue in synthetic organic
chemistry. Metal-catalyzed cross-coupling reaction of dihaloar-
enes with organobimetallic reagents is a powerful tool to
synthesize polyarylenes straightforwardly.2,3 To obtain high-
molecular-weight polymers, use of highly reactive cross-cou-
pling reaction is essential. So far, the Suzuki-Miyaura coupling
has been employed to this end. However, the polymer synthesis
is often accompanied by nontrivial contaminant formation
attributed mainly to the boron reagents.4,5 In this sense,
organosilicon-based cross-coupling is considered to be advanta-
geous. In particular, recently invented aryl[2-(hydroxymethyl)-
phenyl]dimethylsilanes, simply called HOMSi reagents,
smoothly cross-couple with a range of haloarenes to give the
corresponding biaryls in the presence of a Pd catalyst and weak
base in preference to the coupling active groups like boryl and
stannyl groups.6,7 Thus, it is reasonable to apply the HOMSi-
based cross-coupling to simultaneous multiple bond forming
reactions and polymer synthesis. Herein we report that the Pd-
catalyzed cross-coupling reaction of dihaloarenes with HOMSi
reagents, or haloarenes with arylene-bisHOMSi reagents, works
well to give ter- to quinquaryl derivatives. The cross-coupling
reaction is finally shown to be applicable to polyarylene
synthesis.
Table 1. Cross-coupling reaction of dihaloarenes 1 with 2aa
[Pd{P(o-tolyl)3}2] (2.0 mol%)
DPPF (2.1 mol%)
CuBr·SMe2 (3.0 mol%)
HO
O
Br Ar Br
+
Ph Ar Ph
+
Si
Ph Si
Me2
Cs2CO3 (2.1 mmol)
MS 3A
Me2
THF/NMP, 50 °C, 5 h
1
2a
3
4
(0.50 mmol)
(1.05 mmol)
quant
Entry
1
1
Product
Yield/%b
94
Hex
1b
3ba
Br
Br
Hex
Oct Oct
2
92
1c
3ca
Br
Br
3
4
98
90
1d
1e
3da
3ea
i-Bu
N
Br
2
Et
N
Br
Br
S
S
[Pd{P(o-tolyl)3}2] (2.0 mol%)
DPPF (2.1 mol%)
HO
N
N
N
N
CuBr·SMe2 (3.0 mol%)
Ph Si
+
Br
Br
Ph
Ph
5
90
1f
3fa
ð1Þ
Br
Br
Me2
S
Cs2CO3 (2.1 mmol)
MS 3A
THF/NMP, 50 °C, 5 h
1a
2a
3aa
95%
aUnless otherwise noted, a mixture of 1 (0.50 mmol), 2a (1.05
mmol), [Pd{P(o-tolyl)3}2] (0.010 mmol), DPPF (0.011 mmol),
CuBr¢SMe2 (0.015 mmol), Cs2CO3 (2.10 mmol), MS 3A (100
mg), THF (0.75 mL), and NMP (0.25 mL) was heated at 50 °C
(0.50 mmol)
(1.05 mmol)
It is well recognized that the polymer synthesis by the cross-
coupling reaction requires careful control of the catalyst system
including ligand and base in addition to stoichiometic ratio of
dihaloarenes and organobimetallic reagents for the preparation
for 5 h.11 NMP: N-methylpyrrolidone. Isolated yield.
b
Chem. Lett. 2013, 42, 45-47
© 2013 The Chemical Society of Japan
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46
Table 2. Cross-coupling reaction of 1a with 2a
Table 3. Cross-coupling reaction of bromoarenes 5 with 6a
[Pd{P(o-tolyl)3}2] (2.0 mol%)
DPPF (2.1 mol%)
CuBr·SMe2 (3.0 mol%)
[Pd{P(o-tolyl)3}2] (5.0 mol%)
DPPF (5.3 mol%)
CuBr·SMe2 (7.5 mol%)
S
Oct Oct
HO
N
N
Ar Br
+
+
Si
Si
Ar Si
Cs2CO3 (2.1 mmol)
MS 3A
THF/NMP, 50 °C, 5-9 h
Cs2CO3 (2.0 mmol)
MS 3A
PhMe/DME, 50 °C
24-37 h
Br
Br
Me2
1a
(0.50 mmol)
2
5
6
(1.05 mmol)
(1.05 mmol)
(0.50 mmol)
S
N
N
O
Oct Oct
HO
+
Si
Si =
Ar
Ar
O
Me2
+
Ar
Si
Ar
Si
Me2
Me2
3
4
quant
7
4
quant
Entry
1
2
Product
Yield/%b
Entry
1
5
Product
Yield/%b
91
Hex
Si
7a
(= 3ca)
99
2b
3ab
5a
Br
Hex
Hex
Br
Oct Oct
2
3
72
80
5b
5c
7b
7c
2
3
4
87
76
91
2c
2d
2e
3ac
3ad
3ae
Si
Hex
Ph2N
Si
Br
S
S
Et
N
N
N
4c
94
Si
5d
7d
Br
5
99
2f
3af
aUnless otherwise noted, a mixture of 5 (1.05 mmol), 6 (0.50
mmol), [Pd{P(o-tolyl)3}2] (0.025 mmol), DPPF (0.026 mmol),
CuBr¢SMe2 (0.038 mmol), Cs2CO3 (2.0 mmol), MS 3A (100
mg), PhMe (0.75 mL), and DME (0.25 mL) was heated at
50 °C.11 DME: 1,2-dimethoxyethane. bIsolated yield. cReac-
tion run on a 0.20 mmol scale.
Si
S
aUnless otherwise noted, a mixture of 1a (0.50 mmol),
2 (1.05 mmol), [Pd{P(o-tolyl)3}2] (0.010 mmol), DPPF (0.011
mmol), CuBr¢SMe2 (0.015 mmol), Cs2CO3 (2.1 mmol), MS
3A (100 mg), THF (0.75 mL), and NMP (0.25 mL) was heated
at 50 °C for 5-9 h.11 bIsolated yield.
bromobenzene 5b did not seriously hamper the reaction to
give the corresponding bisarylated fluorene 7b in 72% yield
(Entry 2). Heteroaryl electrophiles also reacted with 6 to give
the corresponding quateraryls 7c and 7d in good and excellent
yields (Entries 3 and 4).
2). Dibromo derivative of triarylamine 1d reacted with 2a to
afford 3da in a quantitative yield (Entry 3). Dibromoheteroar-
enes 1e and 1f also underwent the phenyl coupling in excellent
yields (Entries 4 and 5).
Having succeeded in the cross-coupling of dibromoarenes
with phenyl-HOMSi reagent 2a, we turned our attention to the
reaction of other aryl- and heteroaryl-HOMSi reagents 2b-2f
with 4,7-dibromobenzothiadiazole (1a) as a coupling partner.8
As summarized in Table 2, the teraryl and quateraryl synthesis
proceeded without any problems. Perturbation by an ortho
substituent is minimum (Entry 1). When fluorenyl-HOMSi
reagent 2c was used, the corresponding quinquaryl derivative
3ac was obtained in 87% yield (Entry 2). Diphenylamino group
did not hamper the reaction, but the isolated yield of 3ad was
slightly lower (Entry 3). Heteroaryl groups such as 2-carbazoyl
and 2-thienyl groups tolerated the coupling reaction conditions
to give 3ae and 3af in 91% and 99% yields, respectively (Entries
4 and 5). To the best of our knowledge, the cross-coupling
reaction of 4,7-dibromobenzothiadiazole with a 2-thienylsilane
reagent has never been performed before.
S
Oct Oct
[Pd{P(o-tolyl)3}2] (5.0 mol%)
DPPF (5.3 mol%)
CuBr·SMe2 (7.5 mol%)
N
N
Si
Si
Br
Br
+
Cs2CO3 (2.0 mmol)
MS 3A
PhMe/DME, 85 °C, 24 h
1a
(0.50 mmol)
6
(0.50 mmol)
S
Oct Oct
N
N
O
+
X
Y
n
Si
ð2Þ
Me2
4
8
Mw = 2.3 x 104
quant
X = Br, H
Y = Si, H PDI = 2.97
[Pd{P(o-tolyl)3}2] (5.0 mol%)
Ruphos (10 mol%)
CuBr·SMe2 (7.5 mol%)
Oct Oct
Oct Oct
Br
Br
+
Si
Si
Cs2CO3 (2.0 mmol)
MS 3A
PhMe/DME, 50 °C, 24 h
1c
6
(0.50 mmol)
(0.50 mmol)
Oct Oct
Oct Oct
O
The scope of the reaction of bromoarenes with a fluoreny-
lene-bisHOMSi reagent 6, as an organobimetallic coupling
partner is summarized in Table 3. The reaction of 6 (0.50 mmol)
with bromobenzene (5a) (1.05 mmol) proceeded smoothly in the
presence of [Pd{P(o-tolyl)3}2] (5 mol %), DPPF (5.3 mol %),
CuBr¢SMe2 (7.5 mol %), Cs2CO3 (4.0 equiv) and MS 3A
(200 mg mmol¹1) in toluene/DME at 50 °C for 24 h to give 7a
(identical to 3ca) in 91% yield (Entry 1). Dihexyl substituents in
+
X
Y
Si
Me2
4
ð3Þ
n
9
X = Br, H Mw = 1.7 x 104
Y = Si, H
quant
PDI = 3.73
Finally, the cross-coupling reaction was applied to dibro-
moarenes 1 and fluorenylene-bisHOMSi reagent 6 to give the
corresponding conjugated polyarylenes (eqs 2 and 3). Copoly-
merization of 1a and 6 in strictly equimolar amounts under the
Chem. Lett. 2013, 42, 45-47
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

47
optimized conditions proceeded smoothly to give poly(9,9-
dioctylfluorene-co-benzothiadiazole) (F8BT) with Mw of 2.3 ©
104 (PDI 2.97) (eq 2).9 F8BT is attracting specific interest as a
photovoltaic and light-emitting material.10 4,7-Dibromo-9,9-
dioctylfluorene (1c) also reacted with 6 to give a polyfluorene
with Mw of 1.7 © 104 (PDI 3.73) in almost quantitative yield
(eq 3).
In summary, we have demonstrated that HOMSi-based
cross-coupling is an efficient and straightforward strategy for
simultaneous multiple cross-coupling and polymer synthesis.
Further application to construction of polyarylene frameworks is
in progress in our laboratories.
reaction is accompanied by formation of boronic acids,
which tend to precipitate in organic phase and hamper the
formation of high-molecular-weight polymers. Thus poly-
mer synthesis by this cross-coupling reaction is carried out
often in an organic-aqueous biphase system to let boronic
acid residue move to aqueous phase. However, the presence
of water often is apt to trigger protodeborylation. In sharp
contrast, HOMSi-based cross-coupling reaction coproduces
cyclic silyl ether 4 which acts like an organic solvent, and
water cosolvent is not necessary. Therefore, HOMSi-based
cross-coupling reaction is free of salt precipitation and
protodesilylation.
6
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8
9
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Polymer synthesis by the Suzuki-Miyaura cross-coupling
Chem. Lett. 2013, 42, 45-47
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/