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

Transition\nmetal-catalyzed alkyne dimerization represents a powerful\nmethod for the construction of enynes. However, the ambiguous hydrogen\ntransfer mechanism during the dimerization has resulted in controlling\nthe regio-, stereo-, and, where applicable, chemoselectivity remaining\na long-standing challenge. Herein, a combination of DFT calculations\nand quasi-classical MD simulations was used to interrogate the dynamic\nmotion of hydrogen in cobalt-catalyzed alkyne dimerization. The collective\nresults inspired us to propose, for the first time, a substrate-dependent\ndifferential hydride transfer model involving either concerted oxidative\nhydrogen transfer or stepwise oxidative addition, followed by alkyne\ninsertion. The practicability and universality of this oxidative hydride\ntransfer mechanism were further validated by the theoretical studies\nof experimentally observed selective cross- and homo-dimerization.\nCharge distribution analyses depicted that the differentiation between\nthose two hydride transfer mechanisms originates from the α-silicon\neffect, which can stabilize the neighboring negative charge of the\nalkyne. Furthermore, a comprehensive DFT study of the substituent\neffects of alkynes reveals that the electron-withdrawing group will\naccelerate the oxidative hydride transfer process, which can open\nup avenues for mechanistic-oriented selective dimerization.