Variable Mechanisms for Cobalt-Catalyzed Alkyne Dimerization Pinpointed by Quasi-Classical Trajectory Simulations

Transition metal-catalyzed alkyne dimerization represents a powerful method for the construction of enynes. However, the ambiguous hydrogen transfer mechanism during the dimerization has resulted in controlling the regio-, stereo-, and, where applicable, chemoselectivity remaining a long-standing challenge. Herein, a combination of DFT calculations and quasi-classical MD simulations was used to interrogate the dynamic motion of hydrogen in cobalt-catalyzed alkyne dimerization. The collective results inspired us to propose, for the first time, a substrate-dependent differential hydride transfer model involving either concerted oxidative hydrogen transfer or stepwise oxidative addition, followed by alkyne insertion. The practicability and universality of this oxidative hydride transfer mechanism were further validated by the theoretical studies of experimentally observed selective cross- and homo-dimerization. Charge distribution analyses depicted that the differentiation between those two hydride transfer mechanisms originates from the α-silicon effect, which can stabilize the neighboring negative charge of the alkyne. Furthermore, a comprehensive DFT study of the substituent effects of alkynes reveals that the electron-withdrawing group will accelerate the oxidative hydride transfer process, which can open up avenues for mechanistic-oriented selective dimerization.