Journal of the American Chemical Society, Vol.138, No.39, 12956-12962, 2016
A Frustrated Lewis Pair Catalyzed Asymmetric Transfer Hydrogenation of Imines Using Ammonia Borane
Inspired by the zwitterion species generated from the splitting of H-2 by frustrated Lewis pairs, we put forward a novel frustrated Lewis pair by the combination of H delta- and H delta+ 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% ees. 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.