Recent Developments of Expanded Ring N-Heterocyclic Carbenes

The tremendous advances in N-Heterocyclic carbenes (NHCs) have made them the ligand of choice for many transition-metal-catalyzed transformations. Due to the stability of the transition metal complexes of NHCs as well as their stability to air and moisture, NHCs are regarded as an attractive alternative to their phosphine counterparts. To date, research has largely focused on five-membered ring carbenes. However, several reports have demonstrated that changes to the size of the heterocyclic ring (six-, seven-, and eight-membered derivatives) can affect the properties including increased basicity, greater steric demand, and larger N-C-NHC-N angle. In this review, we survey recent developments in the design and synthesis of some expanded ring NHCs with a particularly strong influence on the metal's coordination.