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.

Carbonyl reduction, the net addition of H₂ across a carbon-oxygen double bond, is an important way to prepare alcohols. Stoichiometric reducing agents to accomplish this task include lithium aluminium hydride, sodium borohydride, alkoxy borohydrides, alkoxy aluminium hydrides, and boranes. Although stoichiometric chiral reducing agents often afford products with high enantioselectivity, the necessity of a stoichiometric amount of chiral material is a disadvantage.

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.

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 induction in transition metal catalyzed reactions is achieved through the use of a chiral Lewis basic ligand in catalytic amounts. For ketone substrates that can chelate the metal catalyst, enantioselectivities of transition metal catalyzed reactions may be higher (and side reactions less prevalent) than the corresponding oxazaborolidine reductions.

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