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 paper describes a detailed computational study into the mechanism,
as well as the region 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 α-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 that of the Z-isomer, and the relatively low barrier to interconversion
between them.