Chiral Molecular Tweezers: Synthesis and Reactivity in Asymmetric Hydrogenation

We report the synthesis
and reactivity of a chiral aminoborane
displaying both rapid and reversible H2 activation. The
catalyst shows exceptional reactivity in asymmetric hydrogenation
of enamines and unhindered imines with stereoselectivities of up to
99% ee. DFT analysis of the reaction mechanism pointed to the importance
of both repulsive steric and stabilizing intermolecular non-covalent
forces in the stereodetermining hydride transfer step of the catalytic
cycle.