A Frustrated Lewis Pair Catalyzed Asymmetric Transfer Hydrogenation of Imines Using Ammonia Borane.

Inspired by the zwitterion species generated from the splitting of H2 by frustrated Lewis pairs, we put forward a novel frustrated Lewis pair by the combination of Hδ- and Hδ+ incorporated Lewis acid and base together. Piers' borane and chiral tert-butylsulfinamide were chosen as the FLP, and a metal-free asymmetric transfer hydrogenation of imines was realized with high enantioselectivities. Significantly, with ammonia borane as hydrogen source, a catalytic asymmetric reaction using 10 mol % of Piers' borane, chiral tert-butylsulfinamide, and pyridine additive, has been successfully achieved to furnish optically active amines in 78-99% yields with 84-95% ee's. Experimental and theoretical mechanistic studies reveal an interesting 8-membered ring hydrogen transfer transition state and an expected regeneration of reactive species with ammonia borane. Accordingly, a plausible catalytic pathway for this reaction is depicted.