Mechanism of homogeneously and heterogeneously catalysed meerwein-ponndorf-verley-oppenauer reactions for the racemisation of secondary alcohols

作者：Klomp D; Maschmeyer T; Hanefeld U; Peters JA^*

来源：Chemistry - A European Journal, 2004, 10(8): 2088-2093.

DOI：10.1002/chem.200305460

• hydrogen transfer
• lanthanides
• Meerwein-Ponndorf-Verley reduction
• Oppenauer oxidation
• zeolites

The mechanism of hydrogen transfer from alcohols to ketones, catalysed by lanthanide(III) isopropoxides or zeolite Beta has been studied. For the lanthanide catalysed reactions, (S)1-phenyl-(1-H-2(1),)ethanol and acetophenone were used as case studies to determine the reaction pathway for the hydrogen transfer. Upon complete racemisation all deuterium was present at the 1-position, indicating that the reaction exclusively takes place via a carbon-to-carbon hydrogen transfer. Zeolite Beta with different Si/Al ratios was applied in the racemisation of (S)1-phenylethanol. In this case the racemisation does not proceed via an oxidation/reduction pathway but via elimination of the hydroxy group and its readdition. This mechanism, however, is not characteristic for all racemisation reactions with zeolite Beta. When 4-tert-butyl cyclohexanone is reduced with this catalyst, a classical MPV reaction takes place exclusively. This demonstrates that zeolite Beta has a substrate dependent reaction pathway.