Mechanistic Elucidation of the Arylation of Non-Spectator N-Heterocyclic Carbenes at Copper using a Combined Experimental and Computational Approach

CuI(NHC)Br complexes (NHC = N-heterocyclic carbene) undergo a direct reaction with iodobenzene to give 2-arylated benzimidazolium products. The nature of the Nsubstituent on the NHC ligand influences the reactivity of the CuI(NHC)Br complex towards arylation. N-Benzyl or Nphenyl substituents facilitate arylation, whereas N-mesityl substituents hinder arylation. Density functional theory calculations show that an oxidative addition/reductive elimination pathway, involving CuIII species, is energetically feasible. A less hindered CuI(NHC)Br complex, with N-benzyl groups, is susceptible to oxidation reactions to give 1,3- dibenzylbenzimidazolium cations containing a CuIBrx anion (various polymorphs). The results described herein are of relevance to C-H functionalization of benz(azoles).