1 INTRODUCTION

The enantioselective hydrogenation of pyruvate esters catalysed by cinchona-modified supported Pt has been intensively investigated from the standpoint of molecular mechanism. ^{1 5} Most models conceive of the active form of the modifier (e.g. cinchonidine) as adsorbed at the Pt surface with the quinoline ring system oriented parallel to a flat metal surface; D-tracer, NEXAFS and ATRIR studies have supported this view. ^{6 8} Cinchonidine molecules exhibit four low energy conformations and their equilibrium populations have been measured.4 ^,9 The populations of the conformations in the adsorbed state are not known, but are assumed to be similar to those in solution. For cinchonidine adsorbed in the open-3 conformation, adjacent surface sites exist at which pyruvate ester may undergo selective enantioface adsorption and, by subsequent hydrogenation, may give preferential formation of one enantiomer in the product. Such 1:1 reactant-modifier interactions require a substantial area of surface involving possibly as many as 25 metal atoms.1 ^,4 Enantioselective reaction might thus be expected to be structure-sensitive, and indeed enantiomeric excess (ee) tends to increase with increasing Pt particle size10. Whether catalyst particles contain sufficiently large terraces to accommodate the proposed 1:1 complexes is uncertain.

In terms of chiral performance, cinchonidine-modified Pt/alumina catalyses the hydrogenation of pyruvate esters giving an enantiomeric excess in the lactate product in the range 65 80% without optimisation1, 70 90%...