Insight into Reaction Mechanism of [2 + 2 + 1] Cross-Cyclotrimerization of Carboryne with Alkene and Trimethylsilylarylalkyne Mediated by Nickel Complex

A density functional theory (DFT) study was carried out to elucidate the mechanism of [2 + 2 + 1] cyclotrimerization of o-carboryne with CH2CHnBu and TMS––Ar mediated by a nickel complex. The calculated results reveal that the commonly proposed mechanism via a nickel vinylidene intermediate is kinetically and thermodynamically disfavored. An unprecedented reaction mechanism involving alkyne insertion, η1-alkenyl to η2-alkenyl rearrangement, and TMS migrations is found to be responsible for the favorable [2 + 2 + 1] cycloaddition. The new mechanism rationalizes well the substituent effect on the stereoselectivity of E/Z [2 + 2 + 1] products 3E/3Z and the chemoselectivity of [2 + 2 + 1] over [2 + 2 + 2] cyclotrimerization.