Asymmetric hydrogenation of ethyl pyruvate: Diffusion effects on enantioselectivity

Marked shifts in enantioselectivity from ca, 20 to 60 ee% as a function of reactor agitation speed were observed in the hydrogenation of ethyl pyruvate over cinchona-alkaloid-modified Pt. Concomitant with this shift in enantioselectivity was a change in the observed kinetics from the zero-order substrate dependence typical of gas-liquid diffusion control to a positive dependence on substrate concentration, even for a series of reactions carried out at constant pressure, A strong relationship between the solution hydrogen concentration and this rate and enantioselectivity behavior was confirmed, For systems where enantioselectivity exhibits a positive dependence on hydrogen concentration, the intrinsic ability of a catalyst to effect asymmetric hydrogenation may be masked in a reaction carried out under conditions where gas-liquid diffusion is the rate-limiting step. (C) 1996 Academic Press, Inc.