An Unexpected Possible Role of Base in Asymmetric Catalytic Hydrogenations of Ketones.Synthesis and Characterization of Several Key Catalytic Intermediates

摘要

We report direct observations and reactivity studies of several catalytic intermediates in enantioselective ketone hydrogenations that reveal an unexpected role of added base in these systems.Among the most significant advances in enantioselective catalysis is Noyori et al.'s development of the catalyst system trans-Ru-(diphosphine)Cl_2(diamine)plus base in 2-PrOH.This remarkable system and its variants hydrogenate aryl-alkyl,vinyl-alkyl,and certain alkyl-alkyl ketones with high enantiomeric excess,turnover numbers,and frequencies.The mechanism of these ketone hydrogenations has been the subject of intense study.Most attempts to observe intermediates in these hydrogenations are impeded by long initiation times to convert stable precursors into less stable catalytic intermediates and by decomposition of active catalyst species via beta-hydride elimination from the amine ligand.