Mechanism of Synergistic Cu(II)/Cu(I)-Mediated Alkyne Coupling: Dinuclear 1,2-Reductive Elimination after Minimum Energy Crossing Point

An in-depth theoretical study of synergistic Cu­(II)/Cu­(I)-mediated alkyne coupling was performed to reveal the detailed mechanism for C–C bond formation, which proceeded via an unusual dinuclear 1,2-reductive elimination. Because the reactant for dinuclear 1,2-reductive elimination was calculated to be triplet while the products were singlet, the minimum energy crossing point (MECP) was introduced to the Cu/TMEDA/alkyne system to clarify the spin crossing between triplet state and singlet state potential energy surfaces. Computational results suggest that C–H bond cleavage solely catalyzed by the Cu­(I) cation is the rate-determining step of this reaction and Cu­(II)-mediated dinuclear 1,2-reductive elimination after the MECP is a facile process. These conclusions are in good agreement with our previous experimental results.