Enantioselective hydrogenation: chemistry of the modification of platinum by cinchonidine

Abstract

Effective modification of 6.3% Pt/silica (EUROPT-1) by the alkaloid cinchonidine to achieve fast enantioselective hydrogenation of methyl pyruvate to methyl lactate or of butane-2,3-dione to 3-hydroxybutan-2-one is shown to require adsorption of the alkaloid in competition with a strong co-adsorbate. Comparison of experiments in which catalyst is modified under aerobic conditions and under anaerobic conditions indicates that, normally, oxygen present as air dissolved in solvent and organic reactant functions as the strong co-adsorbate. Alternative co-adsorbates that facilitate effective modification include nitrous oxide, ethyne, propyne and buta-1,3-diene; ineffective co-adsorbates include propene and carbon monoxide. Product analysis shows that co-adsorbate propyne is removed as propene and propane in the early stages of reaction, and this creates the surface conditions necessary for fast enantioselective hydrogenation of pyruvate ester or alkadione. Modification under propyne provides reaction rates and values of the enantiomeric excess under standard conditions (10 bar, 293 K) that are comparable to, or higher than those obtained with normal aerobic modification. The ability to modify catalysts under reducing conditions has provided, for the first time, a reliable route to the preparation of enantioselective Ru catalysts.