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. [ABSTRACT FROM AUTHOR]