Unraveling the Competition of Two C-H and Two M-C Bonds in Guiding the Mechanism of Rhodium(III)-Catalyzed C-H Activation-Annulation

Imidazolium motif containing molecules are known to be of significant interest in diverse research areas, but there is a lack of functionalization protocols of these molecules. In a program to overcome this challenge, recently we developed a unique dual role of a latent imidazolium C-H bond which not only generated a metal-C-NHC bond upon activation but also directed further aryl/heteroaryl C-H activation to furnish a library of potentially valuable products. Mechanistic investigation of any newly discovered catalytic reaction is at the heart of its future development for potential application in diverse fields. Motivated by this philosophy, we delineate in the present work the key mechanistic insights of this annulation reaction which unravel the crucial competition of two C-H bonds (imidazolium and aryl C-H) and two M-C bonds (M-C-NHC and M-C-aryl) in establishing the rate-limiting step and the alkyne-insertion regioselectivity in the reaction. Through careful isolation and X-ray structural characterization of the key seven-membered inserted intermediate along with a DFT rationale, the exclusive regioselectivity of the alkyne insertion to the M-C-NHC bond was established. Kinetics studies were used to evaluate the rate-determining step of the reaction, which was found to be the initial nondirected imidazolium C-H activation step. These mechanistic insights should be useful in understanding similar C-H activation processes in general which are topical in the area of catalysis.