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

Abstract

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 cou...