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.