874
Chemistry Letters Vol.36, No.7 (2007)
Immobilization of Palladium Acetate on Ionic Liquid Copolymerized Polystyrene:
A Way to Eliminate Inhibiting Effect of Imidazolium Chloride
and Enhance Catalytic Performance
Rie Sugimura, Kun Qiao,Ã Daisuke Tomida, Yohei Kume, and Chiaki YokoyamaÃ
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
2-1-1 Katahira, Aoba-ku, Sendai 980-8577
(Received March 19, 2007; CL-070278; E-mail: kunqiao@tagen.tohoku.ac.jp)
Immobilization of palladium acetate on a novel polymeric
-
N
N
Cl
Cl
support that is prepared by copolymerization of 3-butyl-1-vinyl-
imidazolium chloride with styrene is demonstrated to be an
effective way to eliminate inhibiting effect of imidazolium
chloride and enhance catalytic performance.
+
N
N
Styrene, AIBN
-
Cl
N
N
PS
Ethanol, reflux, 24 h
Pd(OAc)2, r.t.
Pd/PS-IL
Ethanol, 24 h
Over the past few years, ionic liquids have gained an in-
creasing reorganization as a kind of novel and versatile solvents
for synthetic chemistry. One of the most attractive applications
of ionic liquids is that they can be used as reaction medium
for a variety of transition-metal-catalyzed reactions.1 In most
cases, replacing common organic solvent with ionic liquid can
result in a number of improvements in terms of product separa-
tion, reaction rate enhancement, and catalyst recycling.1
A few examples of palladium-catalyzed alkoxycarbonyla-
tion and hydrogenation reactions in the presence of ionic liquids
have been reported, which revealed that the components of
ionic liquids played an important role in both reactions.2–8
PdCl2(PPh)3 or Pd(OAc)2 catalyzed methoxycarbonylation
of styrene could afford medium to high yields of products in
1-methylimidazolium-based ionic liquids consisted of anions
such as BF4À, PF6À, or NTf2À. Interestingly, however, ClÀ as
anion has been observed to bear a strong inhibiting effect
towards this reaction.2 When [BMIm]BF4 or [BMIm]NTf2 were
used as reaction solvent, for example, yields of products were 53
and 60%, respectively, but yet a addition of 0.5 wt % [BMIm]Cl
in [BMIm]NTf2 would lead to a nearly 40% decrease of products
yield, and only a 3% yield of products was obtained in pure
[HMIm]Cl.4 Similar inhibiting effect of imidazolium chloride
was also observed in the case of palladium-catalyzed methoxy-
carbonylation of iodobenzene.3,6 It had been proposed that the
formation of palladium carbene complex between imidazolium
chloride and Pd(OAc)2 is the major reason that lead to this inhib-
iting effect.6 Moreover, ClÀ is also known for its negative effect
for transition-metal-catalyzed hydrogenation reaction performed
in ionic liquid.9
Scheme 1. Preparations of PS-IL.
understanding of the behavior of PS-IL as solid support, another
two kinds of supported Pd catalysts were also prepared. One, de-
noted as Pd/PS, was prepared by immobilization of Pd(OAc)2
on polystyrene that was synthesized using the same way as that
of PS-IL. Another one, Pd/SiO2-IL, was prepared following a
similar route reported previously, where Pd(OAc)2 was fixed
on the surface of imidazolium chloride pretreated silica gel.10
The details of experiments regarding catalysts preparations,
characterizations as well as their catalytic uses for alkoxycar-
bonylation and hydrogenation reactions can be found in the
supporting information.
Then ethoxycarbonylation reactions of styrene were carried
out in the presence of different Pd catalysts, and the results were
summarized in Table 1. As can be seen, this reaction can be
smoothly carried out using Pd(OAc)2 as homogeneous catalyst
(Run 1). However, when Pd(OAc)2–[BVIm]Cl system was used,
Table 1. Alkoxycarbonylation reactions catalyzed by Pd
catalysta
Pd cat.
O
O
PPh (4.4%)
3
p-TsOH (5%)
R
OEt
R
OEt
R
CO (5 MPa)
EtOH (2.0 g)
70 °C, 6 h
l
b
Run Substrate
Cat.
Yield/%b l/b
1c
2c
3c
4c
5c
6c
7d
8d
9d
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
p-CSe
Pd(OAc)2
50
—
89
—
81
21
42
98
38
1.76
—
1.5
—
1.08
1.18
1.2
3
In this paper, we report the preparation of a kind of novel
copolymer by copolymerization of styrene with [BVIm]Cl (3-
butyl-1-vinylimidazolium chloride), which was denoted as
PS-IL, and its use as solid support to immobilize transition-met-
al. Immobilization of Pd(OAc)2 on PS-IL was demonstrated to
be an effective way to eliminate the inhibiting effect mentioned
above. Supported Pd catalyst so formed can give high catalytic
performances, even much better than its IL-free homogeneous
counterpart, for both alkoxycarbonylation and hydrogenation
reactions.
Pd(OAc)2–[BVIm]Cl
Pd(OAc)2–[BDMIm]Cl
Pd/SiO2-IL
Pd/PS-IL
Pd/PS
Pd/PS-IL
1-Hexene
IBf
Pd/PS-IL
Pd/PS-IL
—
aSubstrate: 1.0 g. bGC yield. cStyrene/Pd: 880. dSubstrate/Pd:
f
6000. ep-CS: p-chlorostyrene. IB: iodobenzene, 20% NEt3
was also used and the product was ethyl benzoate.
Preparations of PS-IL supported palladium catalyst, denoted
as Pd/PS-IL, were showed in Scheme 1. In order to get a better
Copyright Ó 2007 The Chemical Society of Japan