Asymmetric Hydrogenation of α‑Boryl Enamides Enabled by Nonbonding Interactions

The
asymmetric hydrogenation of α-boryl enamides has been
developed using a bisphosphine-rhodium catalyst. The chelate coordination
of the amido group to rhodium and the nonbonding interactions between
the substrate and the ligand play important roles to afford chiral
α-amidoboronic esters with quantitative conversions, high chemoselectivity,
and excellent enantioselectivity (92–99% ee). Computation of
the catalytic cycle revealed selectivity both in the hydrogen activation
and migratory insertion steps, equally contributing to the high enantioselectivity.
In both cases, the nonbonding interactions provided by the Bpin group
contributed significantly to the stabilization of the transition states
in the lower energy pathway.