title: Mechanism of homogeneously and heterogeneously catalysed Meerwein-Ponndorf-Verley-Oppenauer reactions for the racemisation of secondary alcohols.
abstract: 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-(2)H(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 re-addition. 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.
authors.affiliation: Laboratory of Applied Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands.
authors.forename: Dirk
authors.initials: D
authors.lastname: Klomp
authors.forename: Thomas
authors.initials: T
authors.lastname: Maschmeyer
authors.forename: Ulf
authors.initials: U
authors.lastname: Hanefeld
authors.forename: Joop A
authors.initials: JA
authors.lastname: Peters