Enantioselective Reduction of Ketones Catalyzed by Rare-Earth Metals Complexed with Phenoxy Modified Chiral Prolinols

Enantioselective reduction of ketones and alpha,beta-unsaturated ketones by pinacolborane (HBpin) has been well established by using chiral rare-earth metal catalysts with phenoxy modified prolinols. A number of highly optically active alcohols were obtained from reduction of simple ketones catalyzed by ytterbium complex 1 [(LYb)-Yb-4((LH)-H-4)] (H2L4 = (S)-2- tertbutyl-6-(2-(hydroxydiphenylmethyl)pyrrolidin-1-yl)methyl)phenol). Moreover, alpha,beta-unsaturated ketones were selectively reduced to a wide range of chiral allylic alcohols with excellent yields, high enantioselectivity, and complete chemoselectivity, catalyzed by a single component chiral ytterbium complex 2 [(LYb)-Yb-1((LH)-H-1)] (H2L1 = (S)-2,4-di-tert-butyl-6-(2-(hydroxydiphenylmethyl)pyrrolidin-1-yl)methyl)phenol).