Metal-catalyzed asymmetric reduction of unsaturated functions is a highly useful and fundamental transformation to give diverse chiral synthons. In particular, the enantioselective reduction of prochiral ketones is of great synthetic interest, since it can provide optically active chiral alcohols which have wide applications in organic synthesis, materials science, and pharmaceutical chemistry. Numerous and diverse metal catalytic systems for asymmetric hydrogenation and hydrosilylation of ketones extensively evolved in terms of activity, selectivity, and practicality, while only limited varieties of metal catalysts for the asymmetric hydroboration had been documented until 2010. Diverse and new metal complexes with a range of multi-dentate chiral ligands have recently emerged as catalysts for the enantioselective ketone hydroboration, which are highly differentiated from the precedence in several aspects. This Minireview summarizes recent examples of the metal catalyst systems for the asymmetric hydroboration of ketones published from 2015 to 2020. Diverse catalytic working modes involved in a process of enantiodifferentiating hydride transfer, are discussed with a strong emphasis on the steric and electronic effects of chiral ligands.
- chiral metal catalysts
- enantioselective hydroboration
- hydride transfer pathway
- prochiral ketones