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.