Accessible syntheses of late transition metal (pre-)catalysts bearing N-heterocyclic carbene ligands

This microreview details and discusses some selected examples of the accessible syntheses of useful complexes bearing N-heterocyclic carbenes (NHCs). The term "accessible", as used here, is defined as a synthetic route that does not require the isolation of the free NHC. These routes typically rely on either the generation of low equilibrium concentrations of the free NHC, which is then trapped by a metal centre, the deprotonation of the azolium salt by a suitable ligand or the activation of the C–H bond. These approaches can help to overcome a key weakness of NHCs in that they must be handled under stringently inert conditions. Studies of complexes based on metal centres from groups 8–11 are presented and discussed.