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.