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

  • Szori K
  • Balázsik K
  • Felföldi K
 et al. 
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Abstract

Enantioselective hydrogenation of seven different esters of phenylglyoxylic acid was investigated by variation of the bulkiness at ester side of the substrates [Ph{single bond}C(O){single bond}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. © 2006 Elsevier Inc. All rights reserved.

Author-supplied keywords

  • Chiral hydrogenation
  • Cinchonidine and β-isocinchonine
  • Conformation
  • Intermediate
  • Inversion of enantioselectivity
  • Phenylglyoxylic acid esters
  • Pt-alumina

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Authors

  • Kornél Szori

  • Katalin Balázsik

  • Károly Felföldi

  • Mihály Bartók

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