**5.4 NHC as ligands in asymmetric synthesis**

There are different approaches for inducing chirality by NHC ligands. One of the ways is N-substituents containing centers of chirality. But initially this method did not show great stereoselectivity [110] but later on the development of bidentate ligands enhanced the enantioselectivity. Grubbs et al. developed another method wherein, NHC ligands had the chiral elements within the N-heterocycles [111]. These ligands showed great selectivity. Another interesting method involves an element of chirality like axial chirality or planer chirality to make the NHC ligands as stereo directing ligands for asymmetric synthesis [112, 113] (**Figure 29**).

## **5.5 NHC as organocatalysts**

Carbene can act as an organocatalyst was demonstrated long back in 1940s [114]. Since then, several attempts have been undertaken to develop NHC mediated Benzoin, Acyloin condensation reactions [115–119] (**Figure 30**).

**Figure 28.** *NHC ligands in metathesis reactions (a) first complex (b) combination catalysts.*

**Figure 29.** *(a) Chiral NHC ligand structure, (b) synthesis of chiral ligand.*

**Figure 30.** *NHC ligands used as organocatalysts.*

*Recent Development of Carbenes: Synthesis, Structure, Photophysical Properties… DOI: http://dx.doi.org/10.5772/intechopen.101413*
