Density Functional Theory Study of the Mechanism of Ni-Catalyzed Carboxylation of Aryl C(sp2)–S Bonds with CO2: Computational Evidence for the Multifaceted Role of Additive Zn

The mechanism of Ni-catalyzed carboxylation of aryl C(sp²)-S bonds with CO₂ was studied for the first time by density functional theory calculations. We first proposed another possible reaction pathway in which CO₂ insertion occurs prior to reduction. Then, we performed calculations on all proposed reaction pathways, and our calculation results show that the pathway in which reduction occurs prior to CO₂ insertion is the favored pathway for this reaction. Additionally, our calculations disclose that additive Zn⁰ acts in multifaceted roles. (1) Zn⁰ acts as a reductant to reduce the Ni^I and Ni^II intermediates. (2) The simultaneously formed Zn^IIBr₂ can undergo transmetalation with Ni^I or Ni^II intermediates to produce an aryl reservoir by forming arylzinc species. (3) Zn^IIBr₂ can also coordinate to the CO₂ to lower the energy barrier of the CO₂ insertion step. Moreover, the calculation results suggest that CO₂ insertion is the rate-determining step of the reaction, and CO₂ is easier to insert into the Ni^I-Ph bond rather than into the Ni^II-Ph bond. These calculation results can improve our understanding of the mechanism of the carboxylation process and the multifaceted roles of metal additive Zn⁰ and provide theoretical guidance for improving the carboxylation reaction.