A MODEL FOR THE ENANTIOSELECTIVE HYDROGENATION OF PYRUVATE CATALYZED 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 napht halene forms an ordered ad-layer, and that quinoline and the alkaloid adsorb in a disordered state and without decomposition. These experime nts do not support the hypothesis of ordered adsorption of alkaloid th at forms the basis of the template model for the interpretation of ena ntioselectivity in Pt-catalysed pyruvate hydrogenation. The model is a ccordingly reviewed. Molecular modelling studies show that a highly sp ecific 1:1 interaction between cinchonidine (or cinchonine) and pyruva te interprets the observed sense of the enantioselectivity, provided r elative energy relationships derived for purely intermolecular interac tions 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 ra te that always accompanies enantioselective reaction over cinchona-mod ified Pt. The previously published dependencies of optical yield on (a ) surface concentration of adsorbed cinchonidine modifier, and (b) mod ifier composition for mixtures of quinine and quinidine, are shown to be in quantitative agreement with the proposed 1:1 interaction model a nd at variance with the ordered adsorption model. Catalysts modified a nd used under strictly anaerobic conditions show negligible activity a nd enantioselectivity demonstrating that oxygen plays a crucial role i n successful catalyst preparation. XPS experiments confirm that adsorp tion 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 ephedri ne, D-and L-histidine and the methyl esters of D- and L-tryptophan hav e been examined as modifiers for supported Pt. Although there is evide nce that these compounds can provide chiral direction to pyruvate hydr ogenation, rate enhancement is slight and enantioselectivity is corres pondingly low.