Computational Insight into the Mechanism of Nickel-Catalyzed Reductive Carboxylation of Styrenes using CO2

DFT calculations have been carried out to study the detailed mechanisms for the nickel-catalyzed reductive carboxylation of ester-substituted styrenes H2CCHAr using CO2 to form α-carboxylated products. Two possible mechanisms, the oxidative coupling mechanism and the nickel hydride mechanism, were calculated and compared. Our calculations show that, for the oxidative coupling mechanism, a metallacycle thermodynamic sink is generated from oxidative coupling between CO2 and a styrene substrate molecule on the nickel(0) metal center, which should be avoided in order for smooth reductive carboxylation of styrenes. For the nickel hydride mechanism, a nickel hydride species is the active species, from which styrene insertion into the Ni–H bond followed by reductive elimination produces the α-carboxylated product. Calculations show that either of these two steps (insertion and reductive elimination) can be the rate-determining step, and both transition states are only slightly more stable than the oxidative coupling transition state leading to the thermodynamic sink. Because of the competitive nature between the two mechanisms, the reaction conditions and other factors (substituent, pressure, and ligand) significantly affect the reaction outcome, all of which have been discussed in detail.