**Abstract**

Arduengo et al., isolated the first 'bottleable' carbene, the first N-heterocyclic carbene (NHC) 1,3-di(adamantyl)imidazol-2-ylidene resulted to an explosion of experimental and theoretical studies of novel NHCs being synthesized and analyzed have huge practical significance. These compounds emerged as successful ligands for coordinating transition metals, the complexes with NHC show diverse applications in the field of catalysis and organic transformation, NHC as ligand to main group elements and their properties and applications. Here this chapter provides the concise overview of N-heterocycle carbene as an organocatalyst that provides different organic transformation on to a carbonyl group. The majority of the NHC catalyzed reactions are employed in the phenomenon of reversing the electrophilic character of carbonyl carbon to nucleophilic carbon (umpolung activity) on coordination suggests benzoin, Stetter and hydroacylation reactions. Also, nonumpolung activity of bis-electrophile α,β-unsaturated acylazoliums reaction with suitable bis-nucleophiles in the organic synthesis have been studied.

**Keywords:** NHC carbene, Breslow intermediate, benzoin reaction, Stetter reaction, hyroacylation reaction, annulation reaction, acylazoliums, α,β-unsaturated acylazolium

### **1. Introduction**

The first isolable carbene stabilized with adjacent phosphorous and silicon are reported by Bertrand et al. [1]. Soon after in 1991 Arduengo et al., isolated a bottleable carbene in N-heterocyclic scaffold, said to be N-heterocyclic carbene (NHC) [2]. Substituted bulky groups in NHCs adjacent to the carbene carbon provide kinetically stabilized and sterically avoids the dimerization to corresponding olefins. Remarkable stability and simple synthetic protocols revealed enormous applications of NHCs on transition metal, main group elements and as organo catalysts highlights new area of research. From the last two decades NHC organo catalysis has shepherded to extensive applications in the carbon–carbon and carbon–heteroatom bond formation. NHCs as organocatalyst involving umpolung activity of the functional group with carbonyl carbon (majority reaction employ aldehydes as substrate) acts as a transient nucleophile rather than an electrophile. The obtained nucleophile acyl anion is commonly called as "Breslow intermediate" [3]. The purpose of this

chapter, to explores the NHC catalyzed transformations in organic chemistry involving in the benzoin reaction, Stetter reaction, α-β-unsaturated aldehydes in construction of heterocycles, β-functionalization of enals, hydroacylation of double bonds and triple bonds. NHCs are also known as non-umpolung mode transformations catalysis. Specially, the mode of reactivity in the generation of α-β-unsaturated acylazolium intermediates and are intercepted with various bis-nucleophiles for the enantioselective construction of various heterocyclic compounds [4–8].
