Phosphoramidate-assisted alkyne activation: Probing the mechanism of proton shuttling in a N,O-chelated Cp*Ir(III) complex

Ligand lability offers a unique opportunity for access to metal-ligand cooperativity (MLC), helping to direct new organometallic and catalytic reactions. In recent years, ligand-assisted C-H bond activation, and more generally, proton migration, have been of particular interest. This contribution describes a detailed computational study into the mechanism, regio-, and stereoselectivity observed in a recently reported transformation where MLC in a 16-electron iridium(III) phosphoramidate complex plays a critical role in directing the activation of terminal alkynes toward the generation of novel five-membered (E)-vinyloxyirida(III)cycles. Five possible pathways for the formation of such products were investigated. Based on our findings, it is proposed that the reaction proceeds via a ligand-assisted proton shuttle (LAPS) mechanism, where the phosphoramidate phosphoryl (P=O) group assists in both alkyne C-H bond activation and C-H bond formation to form a vinylidene intermediate. Next, C-O bond formation occurs via nucleophilic attack at the -carbon of the vinylidene giving the observed product. Although C-N (and not C-O) bond formation is thermodynamically favored in this model system, this trend is not observed experimentally and the computational study suggests that the observed regioisomer is simply the kinetic reaction product. In terms of stereoselectivity, formation of the (E)-irida(III)cycle is explained by its thermodynamic stability, when compared to the (Z)-isomer, and the relatively low barrier to interconversion between them.