Enantioface-Differentiating (Asymmetric) Hydrogenation of β-Ketoester with Modified Raney Nickel Catalyst (MRNi). XXXI. A Comparative Study of Reaction Rates and Optical Yields

Abstract

Enantioface-differentiating hydrogenations of methyl acetoacetate over (S)-alanine-, (S)-2-aminobutyric acid, (S)-valine-, (S)-leucine-, (S) -malic acid-, and (R,R)-tartaric acid-MRNi were carried out under atmospheric pressure of hydrogen and the effects of the modifying reagent on the rates of the formation of enantiomer, v_S and v_R, were investigated. The rates of hydrogenation, v=v_S+v_R, were the same for all the amino acid-MRNi and also for all the hydroxy dicarboxylic acid-MRNi. However, the v_R⁄v_s ratios were different. These results are explained in terms of the independence of the rate-determining and enantioface-differentiating steps in the reaction pathway of hydrogenation. The rate-determining step of hydrogenation was shown to be the hydrogen addition to the adsorbed substrate, while the enantioface-differentiating step was expected to occur prior to the rate-determining step.