Mechanism of the Catalytic Carboxylation of Alkylboronates with CO2 Using Ni-NHC Complexes: A DFT Study

A new mechanism is proposed for the Ni-catalyzed carboxylation of organoboronates with CO2. DFT investigations at the PBE0-D3 level have shown that direct CO2 addition to the catalysts [Ni(NHC)(Allyl)Cl] (1(NHC), NHC=IMe, IPr, SIPr and IPr*) is kinetically disfavored and formation of the Aresta-type intermediate is unlikely to occur. According to the mechanism proposed here, the carboxylation process starts with addition of the borate species to 1(NHC), followed by transmetalation, CO2 cycloaddition and carboxylation. The rate-determining step was identified as being the transmetalation process, with computed relative free energy barriers of 34.8, 36.8, and 33.5 kcal mol(-1) for 1(IPr), 1(SIPr) and 1(IPr*), respectively.