Enantioselective hydrogenation of ethyl pyruvate in supercritical fluids

The enantioselective hydrogenation of ethyl pyruvate to (R)-ethyl lactate has been studied using gases under supercritical conditions as solvents. The catalyst was a 5 wt% Pt/alumina modified with cinchonidine. In supercritical ethane the reaction time could be reduced by a factor of 3.5 compared to toluene under similar conditions, without any loss in enantioselectivity. A further advantage of ethane is that the enantioselectivity remains high even at high catalyst/reactant ratio, which is interesting in view of a possible application of a continuous fixed-bed reactor for this reaction. A strong catalyst deactivation was observed in supercritical CO₂, which is due to the reduction of CO₂ on Pt as indicated by FTIR.