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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:249774 |
| Date | January 2002 |
| Creators | Baines, Nicholas John |
| Publisher | University of East Anglia |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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