Theoretical rationalisation for the mechanism of N-heterocyclic carbene-halide reductive elimination at Cu^III, Ag^III and Au^III

Reductive elimination of imidazolium salts from Cu^III is extremely sensitive to the anionic ligand (X or Y) type on Cu (e.g. ΔG^‡ ranges from 4.7 kcal mol⁻¹ to 31.8 kcal mol⁻¹, from chloride to benzyl). Weakly σ-donating ligands dramatically accelerate reductive elimination. Comparison with Ag/Au shows that the HOMO energy, strength of M–NHC and M–Y bonds and inherent stability of M^III with respect to M^I are critical to governing reaction feasibility.