**3. The Stetter reaction**

In 1976, Stetter developed the thiazolium catalyzed highly selective conjugate addition reaction of aromatic/aliphatic aldehydes with an array of Michael acceptors and in most cases these reactions proceed in an intermolecular fashion [22, 23]. The Stetter reaction can be catalyzed by broad range of thiazolium, triazolium and imidazolium carbene, mostly α,β-unsaturated ketones are used as Michael acceptor. The formation of 1,4-diketone, γ-ketonitriles and γ-ketoesters resulting from NHC catalyzed Stetter reaction which is not easy by conventional method [24–27]. A mechanism of benzoin reaction of similar fashion is observed in the Stetter reaction, the in-situ generated free carbene (**B**) of azolium salt (**A**) when treated with base reacts with aldehyde, generating the nucleophilic Breslow intermediate (**D**). This intermediate undergoes irreversible addition to the Michael acceptor generating intermediate (**F**), which on proton transfer and subsequent release of free carbene affords the desired Stetter product (**G**) (**Figure 8**) [28].

**Figure 8.** *Proposed mechanism of Stetter reaction.*
