Zn(II)-Catalyzed Asymmetric Transfer Hydrogenation of γ-Indolyl β,γ-Unsaturated α-Keto Amides and α-Keto Esters with Hantzsch Ester

Highly enantioselective transfer hydrogenation of gamma-indolyl beta,gamma-unsaturated alpha-keto amides and alpha-keto esters with Hantzsch ester catalyzed by chiral zinc complex is reported. Using 5 mol% of Zn(OTf)2-imidazolidine-pyrroloimidazolone pyridine as the catalyst, the C=C bond in gamma-indolyl beta,gamma-unsaturated alpha-keto amides is selectively reduced, affording hydrogenation products in high yields (up to 99% yield) and excellent enantioselectivities (up to 99% ee). Using the same catalyst system, the C=O bond in alpha-keto esters is reduced, generating alpha-hydroxy esters in good results (up to 99% yield and 98% ee). A retro-Michael pathway and subsequent transfer hydrogenation reaction are proposed for the explanation of the side products in the transfer hydrogenation of gamma-indolyl beta,gamma-unsaturated alpha-keto amides.