Polymer Bound Water-Soluble Metal Phosphine Complexes-Preparation, Characterisation and Catalytic Application
Abstract: Four new water-soluble phosphines have been prepared by utilising water-soluble polymers as charge carrying units. Coupling of methyl-4-(diphenylphosphino)-benzyl-amine or 4-diphenylphosphino benzaldehyde to poly-acrylic acid and poly-ethylene imine, respectively, afford the ligands PAA-PNH and PEI-PNH. Coupling of (2S,4S)-4-diphenylphosphino-2-di-phenylphosphino-methylpyrrolidine (PPM) or (R,R)-3,4-bis(diphenylphosphin)pyrrolidine (pyrphos) to poly-acrylic acid give the ligands PAA-PPM and PAA-pyrphos, respectively. Reaction of PAA-PNH with [Rh(CO)2(acac)] give a complex which is active in the hydroformylation reaction of different olefins, both as a supported aqueous phase catalyst (SAP) and as a catalyst in biphasic (H2O/toluene) hydroformylation reactions. Reaction of PAA-PNH and PEI-PNH with [Rh(NBD)2]OTf give the complexes [Rh(P)2(NBD)]+. Complexes derived from both these ligands are active as catalysts in hydrogenation reactions of both water-soluble and water-insoluble olefins. Reaction of PAA-PPM and PAA-pyrphos with [Rh(NBD)2]OTf give the complexes [Rh(P-P)(NBD)]+ (P-P=PAA-PPM and PAA-pyrphos), which have been applied as catalysts in biphasic enantioselective hydrogenations of different dehydro amino acids. The hydrogenation of a-acetamido cinnamic acid with [Rh(P-P)(NBD)]+ (P-P=PAA-PPM and PAA-pyrphos) give as best N-acetyl phenyl alanine in 89 and 82 % ee, respectively. The enantioselectivity for the PAA-PPM based catalysts are dependent on the phosphorus content of the ligand and the ionic strength of the aqueous solution. [Rh(DPPBTS)(NBD)]OTf (DPPBTS=tetrasulfonated bis(diphenyl phosphino) butane) has been prepared and used to study pH effects on the rate of hydrogenation in water. The hydrogenation of a-acetamido acrylic acid is extremely fast below the pKa of the substrate, while the reaction is dramatically slower at higher pH. This change in rate is due to a changed coordination mode of the substrate at higher pH.
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