Origin of enhanced rate in the platinum-catalysed enantioselective hydrogenation of methyl pyruvate

Abstract

The remarkable 20-fold rate enhancement that accompanies the achievement of enantioselectivity in Pt-catalysed methyl pyruvate hydrogenation at 293 K and 10 bar pressure has been shown to be the consequence of three distinct roles played by the cinchona alkaloid modifier. The modifier promotes the rate (i) by its action as an alicyclic N-base, (ii) by its ability when adsorbed to facilitate the establishment of an enhanced surface coverage of adsorbed hydrogen, and (iii) by its effect as an aromatic adsorbate on the apparent specific activity of platinum. These effects have been demonstrated in experiments in which various N-bases have been used to promote the racemic rate of methyl pyruvate hydrogenation at 293 K, and in which H 2/D 2 exchange at 195 K has been used to estimate variations in the surface coverage of adsorbed hydrogen. The importance of the three effects is: (i) > (ii) ≈ (iii) and their combined effect accounts for the observed enhancement. It is proposed that the N-bases stabilise one of the half-hydrogenated states by H-bonding.