Design of Heterogeneous Catalysts for Selective Asymmetric Hydrogenation

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1. Asymmetric hydrogenation of (E)-alpha-phenylcinnamic acid on cinchonidine-modified Pd catalysts(1) The effects of the catalyst preparation method, supports, and the pre-reduction on the activity and enantioselectivity of supported Pd catalysts have been investigated. A titania-supported catalyst pre-reduced at around 473K was found most effective.(2) The amounts of the modifier and the substrate adsorbed on Pd are strongly influenced by the support employed, indicating the importance of the surface concentration of the modifier for obraining a high enantioselectivity. A support having appreciable amounts of both acidic and basic sites with a moderate specific surface area is preferable.(3) The Pd dispersion and the residual chlorine on Pd affect the enantioselectivity of the catalyst.(4) Polar solvents. lower hydrogen pressures, and lower substrate concentrations are preferable for this reaction, in contrast with the hydrogenation of alpha-ketoesters on cinchona alkaloid-modified Pt catalysts.(5) The highest optical yield of 72%ee in the enantioselective hydrogenation of alkenes with heterogeneous catalysts, was obtained with a 5%PdTiO_2 catalyst under optimal reaction conditions.2. Studies on the reaction mechanism(1) The conversion dependencies of optical yields showed the importance of adsorption equilibrium between the substrate and the modifier on Pd surface.(2) Kinetic studies based on the effects of reaction conditions and studies of the effects of the structures of the substrate and the omdifier are now currently underway to interpret the mechanism of this reaction.