Insights into the Regioselective Hydrocarboxylation of Styrenes with CO2 Controlled by the Ligand of Nickel Catalysts

The carboxylation of unsaturated hydrocarbons using CO2 offers a highly appealing access to valuable carboxylic acids, while selectively obtaining the anti-Markovnikov product still remains a challenge. In this work, a density functional theory (DFT) study on the ligand-controlled regioselective hydrocarboxylation of styrenes with CO2 mediated by Ni­(II) catalysts is presented. Our calculations support that the regioselectivity origins from the difference in reaction mechanisms and the Ni center of active catalytic species has different valence states when Ni­(II) catalysts with the ligand of different properties are used. For the neocuproine ligand, styrene ligating to the NiI species initiates its hydrocarboxylation, and subsequent protonation preferentially occurs at the Cβ atom, which promotes the following CO2 insertion at the Cα-position and leads to a unique α-selective product. However, a key Ni-hydride intermediate is first formed by the protonation of Ni0 species in the presence of 1,4-bis­(diphenylphosphino)­butane (dppb) as the ligand. The excellent β-selectivity is attributed to the superiority of Cβ ligating to the Ni center rather than its protonation.