Enantioselective heterogeneous catalysis .2. Examination of the formation of the individual (R) and (S) lactates in the cinchonidine modified platinum hydrogenation of pyruvate

The formation of the individual (R) and (S) lactates was measured during the dihydrocinchonidine modified platinum catalyzed hydrogenation of ethyl pyruvate at atmospheric pressure in a methyl acetate solvent. It was found that the product ee increased during the initial phase of the reaction. Plotting the amount of each enantiomer formed in one minute intervals against reaction time showed clearly that, as previously suggested, this reaction proceeds through an initial stage characterized by a significant increase in ee followed by a period of essentially stable ee. When (R) methyl lactate was added as a co-modifier with the dihydrocinchonidine, the production of (R) ethyl lactate decreased somewhat and the incremental changes observed during the first ten minutes of the reaction were less than those seen in the absence of the (R) methyl lactate co-modifier. With (S) methyl lactate as the co-modifier, the production of (R) ethyl lactate decreased significantly and the incremental changes were markedly different from those observed in the absence of a co-modifier. The production of (S) ethyl lactate was affected only slightly by the presence of either (R) or (S) methyl lactate as the co-modifier.