**2. Chemistry of N-heterocyclic carbenes**

## **2.1 Classes of NHCs and related stable carbenes**

Many types of carbene compounds were labeled as NHCs in the past, and definitions of NHC based on their constitution are often subject to several interpretations [22]. N-heterocyclic carbenes can be defined as any chemical with a carbene center in a nitrogen-containing heterocyclic ring. A broad array of carbenes depending on substituents, size of the ring, and degree of stabilization of heteroatom are available, out of which a few important selected ones are shown in **Figure 2**. Another important classes of NHCs are imidazolinylidene (1), Tetrahydropyrimidinylidene (2), *N,N*-Diamidocarbene (DAC) (3), Benzimidazolylidene (4). These NHCs trigger small molecules like NH3 & can go for insertion into alkenes [23]. Out of these Imidazolylidene-derived NHCs (1) like IPr or IDipp (1a), IMes (1b), and IAd (1c) is the commonest NHCs that are mainly used as ligands with block elements. They function as catalysts in cross-coupling reactions or other significant organic–inorganic transformations [24, 25]. One more category of NHCs was reported by Bertrand et al. [26] where he reported only one nitrogen-containing NHCs like pyrrolidinylidene usually termed as cyclic on the basis of nature of the substituent present neighboring to the carbene center [27, 28].

These compounds are more attracted to the π-electron cloud than other categories of NHCs and help stabilize delicate representative elements and organic free radicals [29]. Another category of NHCs with more than two nitrogen atoms in their heterocyclic framework is also present. Triazolylidenes (8) are examples of this class, widely used as organocatalysts for many transformations [30]. NHC classes having a different heteroatom like oxygen [oxazolylidene (6)], sulfur thiazolylidene (7) are also accessible. Different NHCs moieties can be obtained by developing the carbine center at different positions. In usual NHCs, the carbene center is generally present between the two nitrogen atoms as in imidazolylidenes **1**, generating the carbene center at the 4-position. However, that species is labeled mesoionic or abnormal carbene (MIC or an NHC), which is not neutral, and it is not possible to draw its non-zwitterionic resonance structure (9) [31, 32]. It is termed as "remote" NHC (rNHC) (10) when the carbene center is away from the nitrogen atom [33].

All aforementioned act as NHCs, but some related non-NHCs have also been reported, exhibiting the same characteristics as shown in **Figure 3** [34]. One of these is acyclic diamino carbenes (ADCs) (11) [35–37]. A carbene species 12 stabilized by adjoining silicon & phosphorus substituent was reported by Bertrand in 1988 and after 3 years free NHC was also isolated [38]. Cyclopropenylidene compounds (13) with exocyclic nitrogens were also synthesized by Bertrand [39] while "bent allene" species (14) have themselves been used as ligands [40]. These compounds are considered as NHCs stabilized acyclic carbene.

*N-Heterocyclic Carbenes (NHCs): An Introduction DOI: http://dx.doi.org/10.5772/intechopen.102760*

**Figure 2.** *Some examples of important classes of NHC.*

**Figure 3.**

*Related classes of stable carbene.*

#### **2.2 Common structural features of NHCs**

Nitrogen heterocycles are fundamental in defining the characteristics of NHC. Ranging from four-membered heterocycles, NHCs can be poly NHCs also, but the five-membered NHCs are most common (**Figure 4**), and all are based on imidazole and imidazolydiene [41]. Several structural features of imidazolylidene (1) are similar in all variants of NHCs, which helps stabilize the carbenes. Alteration in the structure and substituents of imidazolylidene develops diverse behavior in NHCs.

In the basic structure of imidazolylidene, the carbene carbon is attached to the two nitrogen atoms of the heterocycle. The aliphatic/aromatic substituent on the nitrogen atom (s) is denoted as the N-substituent(s). The remaining positions, i.e., the 4- and 5-positions in imidazolylidenes, are referred to as the NHC backbone. The substituents present on the ring backbone do not contribute any steric effect at the carbene center and affect the electronic environment only. However, substituents present on the nitrogen of the ring greatly influence the steric properties of the carbene center (**Figure 5**).

#### *2.2.1 Stabilization of the carbene center*

The steadiness of the carbene center in NHCs can be attributed to kinetic and thermodynamic factors. A carbene is carbon with an incomplete octet formed as an intermediate but does not undergo dimerization to form an alkene. Similarly, NHC bearing bulky aryl or alkyl substituent on nitrogen atom does not dimerize due to steric clashing and is termed the Wanzlick equilibrium. Electronic properties of NHC further contribute toward stability as they bear a singlet ground state despite classical carbene, which carries a triplet ground state. The lone pair of electrons of NHCs (singlet) is confined in sp2 -hybridized orbital, which exists in the plane of the ring in the highest occupied molecular orbital and an empty p-orbital is lying

**Figure 4.** *Common five-membered heterocyclic rings.*

*N-Heterocyclic Carbenes (NHCs): An Introduction DOI: http://dx.doi.org/10.5772/intechopen.102760*
