Hydrogen Bond Enhanced Enantioselectivity in the Nickel-Catalyzed Transfer Hydrogenation of α‑Substituted Acrylic Acid with Formic Acid

A chiral nickel complex that catalyzed asymmetric transfer hydrogenation of α-substituted acrylic acids under mild conditions and avoided the use of high-pressure hydrogen gas was developed. The products included chiral β-amino acids and α-methyl carboxylic acids such as three nonsteroidal anti-inflammatory profens. Deuterium-labeling experiments and DFT studies pointed to an unconventional protonation of a metalacyclopropane complex formed by α-phenylacrylic acid which was hydrogen bonded with formic acid. An alternative conventional pathway of nickel hydride insertion cannot explain selective deuteration at β-position of α-phenylacrylic acid when HCO2D was used.