**2. Kemp elimination: a classical tool for anion-**π **catalysis**

The Kemp elimination is a well-known reaction, which involves the abstraction of a proton from the carbon of the benzisoxazole substrate with the help of a catalytic amount of base. This reaction plays an essential biological role and has been documented as an ideal conventional tool for anion-π catalysis by Matile's group. They have carried out this reaction by virtue of the NDI-based anion-π catalysts possessing covalently linked carboxylate base and a solubilizer (alkyl tail) on the π-acidic surface [11]. There occurs the formation of phenolate in the anionic transition-state **(30**) after the proton is abstracted by a covalently attached carboxylate base of NDI based anion-π catalyst (**9**) from the carbon of the substrate (**27**). The anionic transition-state (**30**) acquires stability on the π-acidic surface of catalyst (**9**) by means of anion-π interactions with a magnitude of ΔΔGTS = 31.8 ± 0.4 kJ mol−1 along with a catalytic ability of 3.8 × 105 M−1 and the transition state recognition (KTS) = 2.7 ± 0.5 *μ*M (**Figure 3**). In order to circumvent the inhibition of the desired

*Current Topics in Chirality - From Chemistry to Biology*

through anion-π interactions (**Figure 1**) [4].

*Structures of diverse* π*-acidic aromatic systems.*

inverted the intrinsic negative Qzz of aromatic systems into positive one (Qzz > 0) by attaching strong electron withdrawing substituents on aromatic systems. By virtue of this, they have generated various π-acidic aromatic scaffolds possessing strong positive quadrupole moment, which in turn interact with diverse anions

The recognition of anions is of paramount importance as anions are abundant in nature and play very essential biological role through participation in enzymatic reactions. The transport of anions across biomembranes during different biochemical events makes their recognition even more important [5]. Scientists around the globe are constructing diverse artificial anion receptors, which mimic the function of biosystems and involve anion-π interactions besides other non-covalent interactions in the recognition phenomenon of anions [6]. From the recent theoretical, computational, and experimental investigation, it has been observed that anion-π interactions have shown a promising role in supramolecular catalysis and has given rise to a new concept of anion-π catalysis. The emerging field of anion-π catalysis has not yet much explored in chemistry and until now only few reports are available in literature [7–9]. This is because of the fact that anion-π interactions have recently got experimental evidence, and also there is a dearth of π-acidic aromatic systems being the supreme prerequisite of these interactions [10]. Theoretical and experimental studies have revealed that anion-π catalysis works on the fundamental principle of the stabilization of anionic transition state on π-acidic aromatic surfaces. This stabilization in turn lowers the activation barrier of a particular reaction and hence leads to the formation of a selective desired product quickly under normal reaction conditions [11, 12]. The first evidence of anion-π catalysis came from the Matile's group after carrying out the transmembrane transport of anion by virtue of anion-π interactions [13]. Researchers have developed various anion-π catalysts by adapting different synthetic methodologies [7–9]. It is not feasible herein to discuss such methodologies, but for the convenience of the readers, we have assembled a group of anion-π catalysts used in this chapter (**Figure 2**) [11, 12]. In this chapter, we will discuss the role of these anion-π catalysts in various chemical reactions like

**92**

**Figure 1.**

#### **Figure 3.**

*Schematic illustration of Kemp elimination reaction along with a mechanistic cyclic pathway. Blue color depicts electron deficient and red color implies electron rich.*

product, reactive intermediate (**31**) involves protonation of phenolate to yield the desired product **28** with the regeneration of the catalyst (**9**) (**Figure 3**) [11].
