Enantioselective Hydrogenation Of Alpha-Beta-Unsaturated Ketones

In the asymmetric synthesis of chiral compounds the reduction of prochiral unsaturated reactants has a great importance. The use of proline as a chiral agent in hydrogenations, where the proline is applied as homogeneous chiral catalyst in ring-closure reactions of triketones, was reported previously. In this paper Proline is used in the hydrogenations of α-β unsaturated ketones or acetophenone with palladium on carbon catalysts, producing saturated ketones or α-phenyl-ethyl-alcohol of 5–60% optical purity. The influence of parameters on optical yield is also investigated. UV, NMR and circular dichroism measurements allow to propose the following mechanism: the zwitterionic form of proline gives a Schiff-base or enamin with the ketones; in the hydrogenation of this compound asymmetric induction takes place, which results in optically active ketones after hydrolizing the saturated product.