Mechanistic Revision of Ir(III)-Catalyzed Allylic C–H Alkynylation through DFT Calculations: An Ir(III)-Only Pathway via Alkyne Insertion and Ag-Assisted Debromination

The mechanism of Ir(III)-catalyzed allylic C-H alkynylation of unactivated olefins with bromoalkynes has been investigated using density functional theory (DFT). The previously proposed Ir(III)-Ir(V)-Ir(III) redox cycle was found to be energetically inaccessible under the reported mild conditions. In contrast, our study reveals a distinct Ir(III)-only catalytic pathway, in which all elementary steps are mediated within the Ir(III) oxidation state. In this mechanism, C-C coupling of the two substrates is achieved through insertion of the bromoalkyne into the Ir-C bond of an η³-allyl-Ir(III) intermediate, a step identified as turnover-limiting and responsible for the observed regioselectivity favoring the linear product. Silver salts, beyond acting as halide scavengers, play multiple essential roles by promoting C-H activation and enabling debromination, thereby ensuring efficient product release and catalyst regeneration. These findings revise the mechanistic picture of this transformation and establish Ir(III) catalysis as the key driving force, while highlighting a distinct functional role of silver salts in redox-neutral C-C coupling reactions.