Stereochemical studies of the hydrogenation with asymmetrically modified nickel catalysts; the hydrogenation of methyl 2-alkyl-3-oxobutyrate.

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The hydrogenation of methyl 2-methyl-3-oxobutyrate (I) to methyl 3-hydroxy-2-methylbutyrate (III) was carried out with various types of asymmetrically modified nickel catalysts (MNi). The use of MNi always resulted in the deviation of the ratio, 2S/2R=((2S,3R)-III+(2S,3S)-III)/((2R,3R)-III+(2R,3S)-III, from 1/1, in spite of the use of racemic I. A catalyst giving a larger 2S/2R value always gave a larger 3R/3S value. These results led to the conclusion that the configurations at the C-2 and C-3 positions of the product were determined in the process of the formation of the complex between the substrate and the modifying reagent (the absorption step), not at the step of the hydrogen addition to the adsorbed substrate (the rate-determining step). The best optical yield and the highest ratio of erythro-III/threo-III were obtained when (R,R)-tartaric acid-MNi was used as a catalyst. The amounts of the isomers produced were in the following order: 2S,3R>>2R,3R≈2R,3S>2S,3S. Stereochemical models are proposed to account for the formation of the 2S,3R-isomer in a large excess.
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