Introduction to N-Heterocyclic Carbenes: Synthesis and Stereoelectronic Parameters
N-Heterocyclic carbenes (NHCs) are cyclic compounds containing a divalent carbon atom bound to at least one nitrogen atom within the heterocycle. Variation of the size of the carbene ring, the substituents on the nitrogen atoms or the additional atoms within the heterocycle lead to an array of different NHCs exhibiting a broad range of electronic properties. Their ability to act as donors and the resulting stable bonds to most transition metals make them excellent ligands in coordination chemistry. In addition, free NHCs have found applications as organocatalysts in metal-free chemical transformations. In this chapter, synthetic procedures leading to different NHCs and important structural and electronic features of this class of compounds are discussed.