A model for the enantioselective hydrogenation of pyruvate catalysed by alkaloid-modified platinum†

A LEED and XPS study of the adsorption of naphthalene, quinoline, and 10,11-dihydrocinchonidine on Pt(111) at 300K has shown that only naphthalene forms an ordered ad-layer, and that quinoline and the alkaloid adsorb in a disordered state and without decomposition. These experiments do not support the hypothesis of ordered adsorption of alkaloid that forms the basis of the template model for the interpretation of enantioselectivity in Pt-catalysed pyruvate hydrogenation. The model is accordingly reviewed. Molecular modelling studies show that a highly specific 1:1 interaction between cinchonidine (or cinchonine) and pyruvate interprets the observed sense of the enantioselectivity, provided relative energy relationships derived for purely inter-molecular interactions are valid for the same molecules in the adsorbed state. Moreover, the ‘product’ of this 1:1 interaction is a satisfactory precursor to the H-bonded state considered responsible for the greatly enhanced rate that always accompanies enantioselective reaction over cinchona-modified Pt. The previously published dependencies of optical yield on (a) surface concentration of adsorbed cinchonidine modifier, and (b) modifier composition for mixtures of quinine and quinidine, are shown to be in quantitative agreement with the proposed 1:1 interaction model and at variance with the ordered adsorption model. Catalysts modified and used under strictly anaerobic conditions show negligible activity and enantioselectivity demonstrating that oxygen plays a crucial role in successful catalyst preparation. XPS experiments confirm that adsorption of cinchonidine from air-saturated ethanolic solution on Pt(111) provides an adlayer containing both alkaloid and adsorbed oxygen. (S)-(-)-1-benzyl-pyrrolidine-2-methanol, various configurations of ephedrine, D-and L-histidine and the methyl esters of D- and L-tryptophan have been examined as modifiers for supported Pt. Although there is evidence that these compounds can provide chiral direction to pyruvate hydrogenation, rate enhancement is slight and enantioselectivity is correspondingly low.