Studies of the Platinum Cinchona Alkaloid Catalyst for Enantioselective α-Ketoester Hydrogenation

Abstract

The enantioseleetive hydrogenation of α-ketoesters catalysed by a cinchona alkaloid modified supported platinum catalyst was first reported by Orito in 1978 [1]. Since then extensive research by Wells and co-workers and Blaser, Baiker and co-workers has established the optimal reaction conditions and suggested a reaction mechanism [2,3]. Modification of the platinum surface by naturally occurring cinchona alkaloids not only effects the product enantioselectivity, cinchonidine gives the (R)-product whilst cinchonine, its pseudo enantiomer, furnishes the (S)-product, (schemes 1 and 2), but it also enhances the reaction rate by up to one hundred times over the racemic reaction. The reaction is well behaved giving quite respectable optical yields, as high as 94% under optimised conditions. However the many variables associated with the physical and chemical properties of supported catalysts make mechanistic elucidation difficult. In order to alleviate these problems we have been examining the application of novel colloidal catalysts to this reaction. Colloids have been shown to be more active than analogous conventional heterogeneous catalysts because of an increased active metal surface area [4]. This feature may allow the enantioselective hydrogenation of less reactive substrates than α-ketoesters, thus expanding the presently limited scope of the reaction. Colloidal metals can be prepared cleanly by condensing metal vapour into an organic solvent at 77 K (metal vapour synthesis), leaving none of the surface halide contaminants that often reside on supported catalysts, contaminants which can interfere with the adsorption characteristics of the surface and modifier.