Study of enantioselective hydrogenation of bulky esters of phenylglyoxylic acid on Pt-CD and Pt- β-ICN chiral catalysts: Steric effect of ester groups and inversion of enantioselectivity

Abstract

Enantioselective hydrogenation of seven different esters of phenylglyoxylic acid was investigated by variation of the bulkiness at ester side of the substrates [Ph C(O) C(O)OR, R = Me ( l), cyclohexyl ( 2), adamantyl ( 3), cis-decahydro-1-naphthyl ( 4), phenyl ( 5), 1-naphthyl ( 6), 2-naphthyl ( 7)] on Pt-alumina-cinchonidine (CD) and Pt-alumina- β-isocinchonine ( β-ICN) chiral catalysts using mild experimental conditions (273 K and room temperature, hydrogen pressure of 1–25 bar) in solvents of AcOH and toluene. Hydrogenation on Pt-alumina-CD chiral catalyst produced high values of ee for all but the 5 and 7 compounds. The formation of ( R)-mandelic acid esters was 86–91% in AcOH and 79–94% in toluene. The experiments verified the general applicability of the Orito reaction in the preparation of enantiomers of α-hydroxyesters. Under the experimental conditions applied, the magnitude of ee is affected by the steric size of α-ketoesters and by solvents. In the enantioselective hydrogenation of substrates over a Pt-alumina- β-ICN catalyst in toluene, inversion of enantioselectivity occurred; ( R)-mandelic acid esters formed with a medium ee of 30–54%. The conformation of the adsorbed modifier plays a more determining role than the bulkiness of the substrate in the formation of the intermediate complex responsible for enantioselection.