Asymmetric hydrogenation of activated ketones on platinum: relevant and spectator species

The heterogeneous enantioselective hydrogenation of activated ketones over chirally modified platinum is reviewed with emphasis on identifying the role of the various species observed in this catalytic system. The past years have witnessed a continuous broadening of the scope of this catalytic system including new reactants and modifiers affording over 97 % ee. New reaction pathways have been uncovered and the kinetic and mechanistic studies have been faced with a number of complicating factors caused by spectator species and interactions in solution and on the Pt surface. The previously proposed mechanistic models are critically assessed in the light of these new findings. KEY WORDS: heterogeneous catalysis; enantioselective hydrogenation; Pt/alumina; cinchona alkaloids; chiral modifier; mechanism