New data on enantiomeric excess versus conversion during enantioselective hydrogenation of activated ketones on a platinum catalyst

The initial transient period (ITP) was studied in the enantioselective hydrogenation of activated ketones (ethyl pyruvate, pyruvaldehyde, dimethyl acetal, methyl benzoyl formate) on Pt-alumina (E4759) catalyst modified by cinchonidine (CD) or dihydrocinchonidine (DHCD) under mild experimental conditions in toluene (hydrogen pressure: I bar, temperature: 253-298 K, modifier concentration: 0.001-1 mmol/L). The effects of temperature, modifier concentration, different pretreatments of the catalyst, and structure of the reactants on the relationship of the enantiomeric excess (ee) and conversion were studied. Our results suggest that the ITP is affected not only by impurities/contaminants present in the catalytic system but also by the competitive adsorption of reactants, modifier, and solvent. Consequently it may not be excluded that the ITP can be considered an intrinsic feature of this type of enantioselective hydrogenation reaction. (C) 2004 Published by Elsevier Inc.