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 alpha-substituted acrylic acids under mild conditions and avoided the use of high-pressure hydrogen gas was developed. The products included chiral beta-amino acids and alpha-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 alpha-phenylacrylic acid which was hydrogen bonded with formic acid. An alternative conventional pathway of nickel hydride insertion cannot explain selective deuteration at beta-position of alpha-phenylacrylic acid when HCO2D was used.