N-Heterocyclic Carbenes

This review tries to shed light on the early history of different methods for the generation of N-heterocyclic carbenes (NHCs), i.e., the extrusion of heterocumulenes (decarboxylations) from suitable mesomeric betaines, deprotonations of hetarenium salts, α-eliminations, tautomerizations of mesomeric betaines, and reductive desulfurizations of cyclic thioureas. Selected examples of acyclic and three- to eight-membered NHCs are presented, as well the generation of selected five- and six-membered anionic NHCs.