Enantioselective hydrogenation of alpha-keto esters over cinchona-Pt/Al2O3 catalysts - New interpretation of the rate acceleration and the induction of enantio-differentiation

The hydrogenation of ethyl pyruvate over the cinchonidine-Pt/Al2O3 catalyst system has been investigated in different solvents. Experimental variables used were as follows: (i) substrate and modifier concentration, (ii) presence of acetic acid, (iii) presence of (R)-ethyl lactate, (iv) mode of introduction of the interacting components. A completely different kinetic pattern was observed in ethanol and toluene. This difference is attributed to the lack of half-ketal formation in toluene. The optical yield vs. conversion dependencies show a monotone increase at low conversion, i.e. the optical yield extrapolated to zero conversion is close to zero. It is suggested that the rate acceleration is due to the enhanced reactivity of the substrate induced by substrate-modifier interaction. It is proposed that the induction of enantio-differentiation is attributed to the following interactions: (i) conformational changes of the modifier induced by substrate or acetic acid, (ii) formation of a substrate-modifier complex, (iii) shielding effect induced by the quinoline ring of the modifier.