An Overview ofNHCs

This chapter provides a general overview of N-heterocyclic carbenes (NHCs) and highlights their common structural features and properties. It briefly summarizes some of the uses of NHCs outside of organocatalysis. The major synthetic routes to commonly employed NHC organocatalysts are discussed while a particular focus is given to understanding and comparing the various electronic and steric properties of different classes of NHC. In comparison to phosphines, NHCs are generally more electron rich and, upon binding to metals, afford complexes that are more thermodynamically stable. Indeed, metal-ligand bond distances measured in crystal structures are typically shorter in NHC-metal carbene complexes than in the corresponding phosphine-ligated species, whereas bond dissociation energies are often higher. Accessing NHCs for use as ligands or organocatalysts requires the synthesis of a suitable precursor. Most commonly, the corresponding cationic azolium salt is prepared with the free carbene being obtained upon simple deprotonation usually under mild conditions with various organic or inorganic bases.