Theoretical investigation on the regioselectivity of Ni(COD)2-catalyzed [2 + 2 + 2] cycloaddition of unsymmetric diynes and CO2

Reaction mechanisms of the Ni(COD)2-catalyzed [2 + 2 + 2] cycloaddition of unsymmetric diynes and CO2 have been theoretically studied by using the density functional theory calculations. Three major steps are included, oxidative coupling of CO2 with Et-substituted Ctriple bond; length of mdashC bond, the second Ctriple bond; length of mdashC bond insertion and reductive elimination of the product from the Ni center, in which the Ctriple bond; length of mdashC bond insertion was found to be rate-determinant. The steric arrangement of the N,P-bidentate ligand was demonstrated to be influential on reaction barriers. Based on the mechanistic study, the regioselectivity of the catalytic reaction was elucidated. In addition, we also explained why the mechanisms involving oxidative coupling of both the Ctriple bond; length of mdashC bonds are unavailable.