NHCs
N-heterocyclic carbenes (NHCs) are a class of stable, predominantly neutral singlet carbenes in which the reactive carbene center is embedded in a nitrogen-containing heterocycle, most commonly an imidazole ring. The adjacent nitrogen atoms stabilize the carbene by resonance donation, making it persistent under ordinary conditions. In typical aryl-imidazol-2-ylidene derivatives, the carbene carbon is flanked by two nitrogens, and the substituents on the nitrogens tune steric and electronic properties.
NHCs are among the strongest organic sigma-donor ligands known, with relatively weak pi-acceptor ability. This leads
Preparation commonly involves deprotonation of imidazolium or related salt precursors with strong bases to generate the
Applications span major areas of homogeneous catalysis and organocatalysis. As ligands, NHCs enable effective catalysts for
Historical note: stable NHCs were first isolated in the early 1990s, a development that profoundly impacted