Double 1,2-Migration of Bromine and Silicon in Directed C–H Alkynylation Reactions with Silyl-Substituted Alkynyl Bromides through an Iridium Vinylidene Intermediate

The iridium­(III)-catalyzed C–H
alkynylation of 2-acylimidazoles
with alkynyl bromides, which was recently developed by our group,
provides an efficient strategy for the construction of both C­(sp)–C­(sp2) and C­(sp)–C­(sp3) bonds. The mechanism
for this reaction was extensively studied using density functional
theory (DFT) calculations. The computed catalytic cycle is initiated
by C–H activation, and the formed iridacycle undergoes a strain-controlled
regioselective migratory insertion of an alkynyl bromide. The resulting
α-bromovinyl iridium species is rapidly converted into a more
stable iridium vinylidene intermediate by a 1,2-bromine migration,
and the adjacent silyl group subsequently migrates to furnish a C–H-alkynylated
product. The origin of the unique difference in reactivity with respect
to the substituent on the alkynyl bromide is the 1,2-migration step,
in which a silyl group can highly stabilize the transition state by
the β-silicon effect.