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Published on the web December 5, 2009
Synthesis and Catalysis of B12-Core-Shell Hyperbranched Polymer
Hisashi Shimakoshi,1 Masashi Nishi,1 Akihiro Tanaka,2 Katsumi Chikama,2 and Yoshio Hisaeda*1
1Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Fukuoka 819-0395
2Synthesis Research Department, Chemical Research Laboratory, Nissan Chemical Industries, Ltd., Funabashi 274-8507
(Received September 28, 2009; CL-090875; E-mail: yhisatcm@mail.cstm.kyushu-u.ac.jp)
A vitamin B12 derivative was immobilized on a core-shell
RO2C
RO2C
CO2R
OH
OH
42
hyperbranched polymer, and the hybrid polymer was charac-
terized by GPC, UV-vis, IR as well as TEM analyses. The
hybrid polymer exhibits catalysis for the dehalogenation of
phenethyl bromide to form ethylbenzene and 2,3-diphenylbutane
by irradiation with UV light at room temperature.
42
OH
CO2R
OH
42
42
X
N
N
N
42
HO
Co
Y
+
N
OH
42
RO2C
HO
HO
HO
42
42
42
CO2R
R = 1H, 6CH3, X = Y = CN
CO2R
core-shell HBP
The immobilization of catalysts onto various supports,
such as metals, clays, zeolites, and polymers has been performed
by many researchers in order to develop a good catalytic
system.1 Cobalt complexes, i.e., vitamin B12 derivatives, are
good catalysts for several molecular transformation reactions,2
and efforts for the immobilization of these vitamin B12
derivatives on various supports have been reported by several
groups.3
Most of these studies used a solid-phase support, and the
prepared hybrid materials exhibited both advantages and
disadvantages as a support due to the heterogeneous nature of
the reaction conditions. The use of a soluble polymer may be
applicable as an alternative to conventional solid-phase supports
to alleviate such disadvantages. Dendric polymers, particularly
perfect dendrimers and hyperbranched polymers (HBPs), offer a
wide range of new possibilities. Since dendrimers have to be
prepared in tedious multistep syntheses which are difficult for
large-scale syntheses, the HBPs offer a promising alternative
among these soluble polymers.4
The HBP is rapidly and conveniently prepared by a one-pot
procedure, and due to its globular (spherical) shape, the HBP
is soluble in various solvents. Furthermore, many functional
groups are present on the polymer surface due to its dendric
topology. Therefore, the HBP is expected to be a practical
support for the catalyst.5 In this study, the synthesis and
characterization of new hybrid catalysts composed of a vitamin
B12 derivative and a core-shell HBP are reported. As a large
number of functional groups are present in the HBP, propagation
of the shell polymer on the HBP was effectively generated to
form the unique core-shell topological HBP.
The core unit of the core-shell HBP was prepared by
the living radical polymerization of 2-(N,N-diethyldithiocarba-
moyl)ethyl methacrylate under UV irradiation,6 and subsequent
grafting from this HBP with 2-hydroxyethyl methacrylate
produced the core-shell HBP.7,8 Immobilization of the vitamin
B12 derivative on the core-shell HBP was achieved by
esterification of the hydroxy groups of the core-shell HBP with
the vitamin B12 derivative bearing a carboxylic group5 as shown
in Scheme 1. The B12-core-shell HBP was characterized by
GPC, UV-vis, IR, and TEM (see the Supporting Information12).
The hybrid polymer showed the typical UV-vis spectrum of B12
with absorption maxima at 372, 505, 546, and 587 nm in CH2Cl2
as shown in Figure S4.2,5 The hybrid polymer well dissolved in
=
CO2H
Co
Co
Co
EDC
O
OH
Co
O
O
O
in dry DMF
O
O
HO
O
OH
O
OH
O
Co
Co
O
Modification ratio of -OH group of
core−shell HBP with B12 : 1~20 %
Scheme 1.
other solvents, such as CHCl3, CH3CN, acetone, THF, DMF,
1,4-dioxane, and MeOH. The amount of the immobilized B12
(modified yields of hydroxy groups of core-shell HBP with B12)
was determined by UV-vis12 and was controlled by the added
equivalents of B12 under the synthetic conditions as shown in
Figure S7. The modified yields of the hydroxy groups of the
polymer were in good relationship to the added B12. But the
value reached a maximum at nearly 20%. It is assumed that B12
was immobilized on the surface of the polymer due to steric
hindrance of the B12. Therefore, the modified yield reached a
maximum at a certain value. From the TEM image, the spherical
shape of the polymer was clearly observed, and the topology of
the polymer is similar to that of the dendrimer as shown in
Figure S6. The size of the polymer is estimated to be 3-5 nm.
The catalytic activity of a hybrid catalyst, the B12-core-shell
HBP, was investigated for comparison to that of the monomeric
B12 (heptamethyl cobyrinate perchlorate). TiO2 was used as a
reducing reagent for the B12 moiety of the hybrid catalyst to
form the reactive CoI species.9 When phenethyl bromide was
used as a substrate, ethylbenzene (EB) and 2,3-diphenylbutane
(DB) were obtained as products by irradiation with UV light
(365 nm). Especially, the coupling product, DB, was obtained in
a high yield when compared to that of the monomeric B12 under
the same reaction conditions. In addition, the yields of DB were
independent of the modified yield of B12 and ranged from 16 to
2%. The yield then decreased to 8% with the modified 0.5%
yield. In the case of the monomeric B12, the yield of DB was
Chem. Lett. 2010, 39, 22-23
© 2010 The Chemical Society of Japan