additives such as phosphines and amines enhance the
catalytic activity or selectivity in heterogeneous systems,5,6
leaching of Pd during or after reaction is often observed due
to those additives.
Recently, we have reported the polymer incarcerated (PI)
method for immobilization of palladium catalysts into
polystyrene-based polymers.1,8 The heterogeneous PI pal-
ladium catalysts are highly active for hydrogenation and
carbon-carbon bond-forming reactions. These catalysts are
recovered quantitatively by simple filtration and can be
reused several times without loss of activity. For example,
PI Pd-catalyzed Suzuki-Miyaura couplings proceed in high
yields in the presence of an externally added phosphine
ligand such as tris(o-methoxyphenyl)phosphine without
leaching of palladium.7d In this case, however, the phosphine
ligand is lost during filtration of the catalyst, and addition
of the phosphine ligand is necessary in every recycling use.
In this paper, we describe new PI Pd catalysts using polymer
supports which contain the diphenylphosphino group instead
of adding external phosphine ligands, and their applications
to Suzuki-Miyaura couplings.
PI Pd 2a-g were prepared from Pd(PPh3)4 and copolymer
1a-g by the PI method (microencapsulation and cross-
linking of polymer chains) as reported previously (Figure
1). Since some phosphines were oxidized to the correspond-
ing phosphine oxides during the preparation of the catalyst,
HSiCl3 reduction gave phosphinated PI Pd. It was revealed
from 31P SR-MAS NMR analysis9 that the phosphine oxides
on the copolymer were completely reduced to the corre-
Figure 1. Preparation of phosphinated PI Pd 2a-f and nonphos-
phinated PI Pd 2g incarcerated Pd with copolymers.
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A. S. C. Chem. ReV. 2002, 102, 3385.
sponding phosphines. Furthermore, 4 equiv of triphenylphos-
phine from Pd(PPh3)4 were eliminated during the immobi-
lization and recovered from the filtrate after washing (THF
and hexane). PI Pd 2a-g thus prepared were then analyzed
by transmission electron microscopic (TEM). Judging from
the magnified TEM image of PI Pd 2b (Figure 2), small Pd
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Figure 2. TEM image of PI Pd 2b.
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Chem. Soc. 1998, 120, 2985. (b) Nagayama, S.; Endo, M.; Kobayashi, S.
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clusters were dispersed on the polymer support without
formation of large Pd clusters (TEM detection limit ) ca. 1
nm). Formation of extremely small Pd clusters might be due
to a stabilization effect of the polymer.
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