Iridium-Catalyzed Asymmetric Transfer Hydrogenation of 1-Aryl-1-alkylethenes with Ethanol.

Asymmetric transfer hydrogenation of 1-aryl-1-alkylethenes with ethanol was developed with a chiral (PCN)Ir complex as the precatalyst, featuring high enantioselectivities, good functional group tolerance, and operational simplicity. The method is further applied to formal intramolecular asymmetric transfer hydrogenation of alkenols without an external H-donor, producing a tertiary stereocenter and remote ketone group simultaneously. The utility of the catalytic system was highlighted by gram scale synthesis and the synthesis of the key precursor of ( R )-xanthorrhizol.