Theoretical Investigation on Nickel-Catalyzed Hydrocarboxylation of Alkynes Employing Formic Acid

DFT calculations have been conducted to elucidate the mechanistic details of a novel Ni-catalyzed hydrocarboxylation reaction of alkynes, in which formic acid is atom-economically used through a catalytic CO recycling manner. On the basis of our theoretical investigations, the bisphosphine (dppbz, 1,2-bis­(diphenylphosphino)­benzene) ligated nickel monocarbonyl complex (dppbz)­NiCO was located as the active catalytic species for this process, and such a carbonyl ligand is found to be critical for the final reductive elimination. Our studies also revealed the addition of H to alkynes proceeds via a proton transfer process directly from formic acid (i.e., outer-sphere pathway) rather than through a proposed hydrometalation process (i.e., direct hydride shift from Ni–H). The thermal decomposition of formic anhydrides was found to be vital to a successful reaction, and its barrier must be slightly higher than the energetic span of the Ni catalytic cycle. Fast release of CO can poison the Ni catalyst, so that the reaction would be shut down. Other intriguing experimental observations, such as ligand effect, regioselectivity, and extraordinary compatibility of C–X (X = halogen) bonds, are also discussed in this article.