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

Abstract

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.