Title:
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Synthesis of heterogeneous palladium catalyst assemblies by molecular imprinting
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Heterogeneous palladium catalysts for use in Suzuki-type reactions have been
prepared using the molecular imprinting technique. Using this technique allows the
preparation of a heterogeneous catalyst with uniform active sites which have a metal
complex configuration that favours processes in the catalytic cycle, thus enhancing
the rate of reaction. It was reasoned, from the general catalytic cycle for palladium
cross-couplings, that a bisphosphine complex with a cis ligand geometry would be
beneficial. Polymerisable phosphines were synthesised and the cis bisphosphine
palladium complexes prepared. The geometry of the complexes was fixed by addition
of a bidentate ligand (catechol). The resulting complexes were incorporated into a
macroporous polymer framework using the molecular imprinting technique. This
produced the heterogeneous palladium catalysts with uniform active sites and known
palladium content.
The catalysts were ~ested in model Suzuki reactions and were shown to give far
superior yields to the homogeneous catalyst and control heterogeneous catalyst
(derived from commercial ligand). The favoured square-planar cis-geometry was
likely to increase the rate of reductive elimination, which takes place from that
conformation. It is also reasonable to imply that the catalytically active palladium(O)
species would be strained, being tetrahedral not square-planar, and so the rate of
oxidative addition (the slow step in some cases) could also be accelerated. The
improved yields shown by the imprinted catalysts for the Suzuki reactions suggest
that this is the case.
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