The 3 + 2 cycloaddition mechanism of the rhodium-catalyzed redox-neutral cyclization of sulfoxonium ylides with 1,4-diphenyl-1,3-butadiyne has been investigated using density functional theory. The calculations indicate that the active catalyst is Cp*Rh(OAc)2. The reaction proceeds sequentially through C(sp2)-H bond cleavage, alkyne insertion into the Rh-C(sp2) bond, intramolecular nucleophilic addition, protodemetalation, and stepwise proton migration, ultimately affording the product 3-phenyl-2-(phenylethynyl)-1-indenone. Additionally, based on the computational results, we propose that the rate-determining step of the catalytic cycle is governed by both C-H activation and intramolecular nucleophilic addition, rather than being controlled by a single step alone.
Qiu et al. (Fri,) studied this question.