C4-bound imidazolylidenes: from curiosities to high-impact carbene ligands.

This feature article summarizes the progress achieved thus far in using C4-bound imidazolylidenes as a new class of ligands for transition metals. Since the discovery of this unusual carbene bonding mode in 2001, various rational routes towards complexes containing C4-bound carbenes have evolved. These advances allowed for studying the impact of this new type of ligand on the transition metal center, both from a fundamental point of view as well as from a more applied perspective, in particular for catalytic applications. The promising results accomplished in this relatively short period of time demonstrate the potential of C4-bound imidazolylidenes as unique carbene ligands for inducing catalytic activity and for mediating unprecedented transformations.