Chiral Macrocyclic Borane-Catalyzed Asymmetric Hydrogenation of 2-Substituted Quinolines.

A metal-free asymmetric hydrogenation of 2-substituted quinolines was achieved utilizing a chiral macrocyclic borane catalyst. Macrocyclic boranes exhibited higher catalytic activity and enantioselectivity than acyclic analogues. A broad scope of 2-alkylquinoline substrates was transformed into optically active tetrahydroquinolines in 87-98% yields with 79-96% ee. In contrast, only moderate stereoselectivity was acquired for 2-phenylquinoline.