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 cat alyst system has been investigated in different solvents. Experimental variables used were as follows: (i) substrate and modifier concentrat ion, (ii) presence of acetic acid, (iii) presence of (R)-ethyl lactate , (iv) mode of introduction of the interacting components. A completel y different kinetic pattern was observed in ethanol and toluene. This difference is attributed to the lack of half-ketal formation in toluen e. The optical yield vs. conversion dependencies show a monotone incre ase at low conversion, i.e. the optical yield extrapolated to zero con version is close to zero. It is suggested that the rate acceleration i s due to the enhanced reactivity of the substrate induced by substrate -modifier interaction. It is proposed that the induction of enantio-di fferentiation is attributed to the following interactions: (i) conform ational 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.