Additional data to the origin of rate enhancement in the enantioselective hydrogenation of activated ketones over cinchonidine modified platinum catalyst

摘要

Kinetic behaviour of different substrates such as purified ethyl pyruvate, dimer containing (20%) ethyl pyruvate, methylbenzoyl formate and ketopantolactone was investigated in both racemic and enantioselective hydrogenation over Pt/Al_2O_3 catalysts. Upon introducing the chiral modifier by injection under condition of racemic hydrogenation an immediate increase in reaction rate is observed in the case of all substrates. Consequently, significant rate enhancement (RE) was obtained in the case of all substrates. The RE increased in the following order: ketopantolactone < ethyl pyruvate < methylbenzoyl formate. This order does not follow the ability of substrates to be involved in various undesired side reactions with the formation of poisonous surface residues. Accordingly, results obtained in this study confirm that the RE must be an intrinsic feature of the asymmetric hydrogenation of activated ketones in the presence of cinchona alkaloids. However, our results also indicate that the poisoning effect by organic residues originated from ethyl pyruvate cannot be neglected.