Enantioselective reduction of ketones
Enantioselective ketone reductions convert prochiral ketones into chiral, non-racemic alcohols and are used heavily for the synthesis of stereodefined alcohols. Introduction Carbonyl reduction, the net addition of H2 across a carbon-oxygen double bond, is a straightforward way to generate alcohols. Stoichiometric reducing agents to accomplish this task include lithium aluminium hydride, sodium borohydride, alkoxy borohydrides, alkoxy aluminium hydrides, and boranes. Initial efforts toward enantioselective ketone reductions focused on the development of chiral, non-racemic reducing agents. Although stoichiometric chiral reducing agents often afford products with high enantioselectivity, the necessity of a stoichiometric amount of chiral material is a disadvantage of these reagents. The catalytic, asymmetric reduction of ketones may be accomplished through the use of catalytic amounts of an oxazaborolidine catalyst in conjunction with borane or catecholborane as the stoichiometric reducing agent. Oxazaborolidines remain in common use for reductions of simple ketones. More recently, efforts in the field of enantioselective reduction have focused on the development of transition metal catalyzed reactions, which employ cheap reductants such as hydrogen gas (H2), formic acid (HCO2H), or isopropanol ((CH3)2CHOH). The latter two reagents are used for transfer hydrogenations, which represent the formal transfer of an H2 molecule from the reductant to the substrate. Asymmetric