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 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.