Catalyst-Controlled Nitrene Transfer by Tuning Metal:Ligand Ratios: Insight into the Mechanisms of Chemoselectivity

Catalyst-controlled,
selective nitrene transfer is often challenging
when both C–H and CC bonds are present in a substrate.
Interestingly, a simple change in the Ag­(I):L ratio (L = bidentate
N,N-donor ligand) enables tunable, chemoselective nitrene transfer
that favors either CC bond aziridination using an ∼1:1
Ag:L ratio (AgLOTf) or insertion into a C–H bond when the Ag:L
ratio in the catalyst is 1:2 (AgL2OTf). In this paper,
mechanistic studies, coupled with kinetic profiling of the entire
reaction course, are employed to examine the reasons for this unusual
behavior. Steady-state kinetics were found to be similar for both
AgLOTf and AgL2OTf; both complexes yield electronically
similar reactive intermediates that engage in nitrene transfer involving
formation of a short-lived radical intermediate and barrierless radical
recombination. Taken together, experimental and computational studies
point to two effects that control tunable chemoselectivity: suppression
of aziridination as the steric congestion around the silver center
is increased in AgL2OTf and a decrease in the rate of C–H
insertion with AgLOTf in comparison to AgL2OTf. The observation
that the sterics of Ag catalysts can be varied, with minor effects
on the electronic features of the putative nitrene, has important
implications for the development of other silver catalysts that enable
tunable, site-selective C–H bond aminations.