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.