Copper(I) activation of C-X bonds: bimolecular vs. unimolecular reaction mechanism.

Experimental kinetic studies and DFT calculations show that the oxidative addition of aryl halides (Ar-X) to complexes [Cu(NHC)R] follow different paths depending on the nature of X. For X = Br a concerted addition leads to cis -[Cu(NHC)XRAr] from which the usual C-C coupled product Ar-R eliminates. However, for X = I trans -[Cu(NHC)IRAr] is formed instead, leading to the elimination of R-I in a metathesis reaction. This behaviour is accounted for by a change in the reaction mechanism for Ar-I, which involves two molecules of copper(I) complex, the second one stabilising the incipient iodide formed in the C-I breaking (oxidative addition) and C-I forming (reductive elimination) processes.