**2. Benzoin reaction**

In 1958, Breslow proposed a mechanism in which the reaction precedes *via* an enaminol intermediate popularly known as Breslow intermediate, using thiazonium zwitterion nothing but the resonance structure of an NHC. Deprotonation of thiazonium salt (**A**) using base generates a nucleophilic thiazolidine on addition to aromatic aldehyde furnish the tetrahedral intermediate (**C**) followed by proton transfer in order to obtain enaminol (**D**). In case of benzoin condensation, nucleophilic attack of intermediate on to another equivalent of aldehyde leads to formation of 2-hydroxy ketone (**E**) with subsequent elimination of the thiazolidine (**B**) (NHC). Transformation of carbonyl compound as transient nucleophile with NHC example of umpolung reaction and Breslow intermediate can be thought of as acyl anion equivalent (**Figure 1**) [9].

#### **2.1 Homo benzoin reaction**

Homo benzoin reactions are less challenging due to chemoselectivity issue and oxidation of aldehyde to corresponding carboxylic acid that could be avoided by coupling exclusion of oxygen. An efficient benzoin reaction catalyzed by

**Figure 1.** *Proposed mechanism of benzoin reaction.*

*N-Heterocyclic Carbene Mediated Organocatalysis Reactions DOI: http://dx.doi.org/10.5772/intechopen.100642*

imidazonium carbene was shown by Xu and Xia in 2005 [10] (**Figure 2**). Iwamoto et al. shows NHC catalyzed benzoin reaction in aqueous media [11, 12]. Enantioselective benzoin reaction employing NHC catalyst have many illustrations by various groups [13] (**Figure 3**). The most efficient enantioselective benzoin reaction was reported by Connon et al. with >99% ee (**Figure 4**) [14].

#### **2.2 Cross benzoin reaction**

**Figure 2.** *Efficient enantioselective homo benzoin reaction.*

**Figure 3.** *An efficient benzoin reaction catalyzed by imidazonium carbene.*

**Figure 4.** *The most efficient enantioselective benzoin reaction by Connon et al. with >99%.*

In an NHC driven cross benzoin reaction, total four products are possible, a pair of homo benzoin and cross benzoin adducts each. Electronic or steric reasons one of the aldehydes may be significantly less reactive. Connon et al. found that cross benzoin reaction could be biased by synthetically useful amount for the synthesis of desirable cross coupling products (**Figure 5**) [15].

Glorious et al., demonstrated a highly selective cross benzoin reaction with broad substrate scope [16]. Young et al. showed reactivity controlled by the carbine catalyst. An intermolecular cross coupling of aromatic aldehyde for the formation of Breslow intermediate is followed by coupling with acetaldehyde using thiazonium carbene catalyst. In constant acyl anion generation from acetaldehyde preferred by triazolium carbine is followed by coupling with aromatic aldehyde (**Figure 6**) [17].

**Figure 5.** *Cross benzoin reaction by Connon et al.*

**Figure 6.** *Reactivity controlled reaction by using deferent carbene catalyst.*

**Figure 7.** *The first aza benzoin reaction by Enders et al.*
