Asymmetric Reduction of Quinolines: A Competition between Enantioselective Transfer Hydrogenation and Racemic Borane Catalysis.

A chiral phosphoric acid catalyzed asymmetric transfer hydrogenation of quinolines with regenerable dihydrophenanthridine derived by a borane-catalyzed hydrogenation of phenanthridine under H 2 has been successfully realized. Despite the competition of a racemic hydrogenation pathway, a variety of tetrahydroquinolines were furnished in high yields with up to 91% ee.