**5. Cascade reactions through anion-**π **catalysis**

Cascade reactions are also known as domino or tandem reactions and comprises of at least two simultaneous consecutive reactions. Herein, the preceding reaction develops a chemical functionality on which a subsequent reaction occurs. Such reactions are of vital importance in the synthesis of complex natural products possessing various chiral centers [35]. During these cascade cyclization reactions,

charge displacements are stretched over longer distances. Matile's group has revealed that anion-π catalysis in terms of anion-π interactions is highly capable in the stabilization of these charge displacements. With the help of anion-π catalysis, the cascade reactions on the π-acidic catalytic surface leads to generation of bicyclic asymmetric products possessing four chiral centers (**Figure 16**). Moreover, cascade cyclization reactions through anion-π catalysis are also in action in the generation of asymmetric cyclohexane moieties containing five chiral centers generated in a single step on π-acidic catalytic surface (**Figure 17**). The concept of anion-π catalysis also play a central role in other cascade reactions on the π-acidic aromatic surface. For instance, the reaction of **93** with **94** takes place in a cascade way on the π-acidic aromatic surface (**Figure 18**). The greatest outcome of anion-π catalysis in cascade cyclization reactions is anion-π cinchona fusion catalyst [36].

Epoxide ring opening followed by ether and polyether cascade cyclic reactions are considered as conventional reactions in chemical and biological sciences. Nowadays, these reactions are also considered as attractive tools for anion-π catalysis. To this context, Matile's group has reported some functional systems

#### **Figure 16.**

*Schematic representation of the generation of anion-*π *catalyzed bicyclic cascade products and the structures of involved anion-*π *transition states.*

#### **Figure 17.**

*Schematic illustration of asymmetric anion-*π *catalyzed cyclohexane rings through cascade cyclisation reaction. Moreover, the structures of anion-*π *transition states are also given.*

**103**

**Figure 20.**

**Figure 19.**

*virtue of anion-*π *catalysts (***6, 19,** *and* **23***).*

**Figure 18.**

*acetoacetate (***94***).*

*Anion-*π *Catalysis: A Novel Supramolecular Approach for Chemical and Biological…*

*Schematic depiction of anion-*π *catalyzed cascade reaction of benzaldehyde derivative (***93***) with ethyl* 

which operate through anion-π interactions and show autocatalysis. Studies have revealed that aromatic π-acidic surfaces involve epoxide ring opening followed by ether cyclization without any activating group (**Figure 19**) [37]. Quite recently, they have also observed exceptional high autocatalysis on the π-acidic surfaces of hexafluorobenzene and substituted NDI's as far as epoxide ring opening followed by cyclisation reactions are concerned. This unique characteristic of autocatalysis not only adds complexity to reaction mechanisms but also offers intriguing perspec-

*Schematic illustration of epoxide ring opening followed by ether and polyether cascade cyclization reactions by* 

*Photoamidation reaction between* **104** *and* **105** *assisted by anion-*π *interactions.*

Besides the above-mentioned catalytic relevances of anion-π interactions in the domain of catalysis, amidation reactions driven by light have also been carried out by means of these interactions. It has been observed that anion-π interactions helps in the stabilization of transient complex formed between electron deficient moiety **104** and carbonate or phosphate anion. This complex in turn undergoes cleavage of N-O bond in the presence of light to offer amidyl radical, which is later trapped by heteroaromatic system (**105**) to offer the desired product **106** (**Figure 20**) [40].

Anion-π catalysis in general operates on the fundamental principle of anionic

transition state stabilization on π-acidic aromatic surfaces and offers a novel

tives towards future developments [38, 39].

**6. Conclusions and outlook**

*DOI: http://dx.doi.org/10.5772/intechopen.95824*

*Anion-*π *Catalysis: A Novel Supramolecular Approach for Chemical and Biological… DOI: http://dx.doi.org/10.5772/intechopen.95824*

#### **Figure 18.**

*Current Topics in Chirality - From Chemistry to Biology*

charge displacements are stretched over longer distances. Matile's group has revealed that anion-π catalysis in terms of anion-π interactions is highly capable in the stabilization of these charge displacements. With the help of anion-π catalysis, the cascade reactions on the π-acidic catalytic surface leads to generation of bicyclic asymmetric products possessing four chiral centers (**Figure 16**). Moreover, cascade cyclization reactions through anion-π catalysis are also in action in the generation of asymmetric cyclohexane moieties containing five chiral centers generated in a single step on π-acidic catalytic surface (**Figure 17**). The concept of anion-π catalysis also play a central role in other cascade reactions on the π-acidic aromatic surface. For instance, the reaction of **93** with **94** takes place in a cascade way on the π-acidic aromatic surface (**Figure 18**). The greatest outcome of anion-π catalysis in

cascade cyclization reactions is anion-π cinchona fusion catalyst [36].

Epoxide ring opening followed by ether and polyether cascade cyclic reactions are considered as conventional reactions in chemical and biological sciences. Nowadays, these reactions are also considered as attractive tools for anion-π catalysis. To this context, Matile's group has reported some functional systems

*Schematic representation of the generation of anion-*π *catalyzed bicyclic cascade products and the structures of* 

*Schematic illustration of asymmetric anion-*π *catalyzed cyclohexane rings through cascade cyclisation reaction.* 

*Moreover, the structures of anion-*π *transition states are also given.*

**102**

**Figure 17.**

**Figure 16.**

*involved anion-*π *transition states.*

*Schematic depiction of anion-*π *catalyzed cascade reaction of benzaldehyde derivative (***93***) with ethyl acetoacetate (***94***).*

**Figure 19.**

*Schematic illustration of epoxide ring opening followed by ether and polyether cascade cyclization reactions by virtue of anion-*π *catalysts (***6, 19,** *and* **23***).*

**Figure 20.**

*Photoamidation reaction between* **104** *and* **105** *assisted by anion-*π *interactions.*

which operate through anion-π interactions and show autocatalysis. Studies have revealed that aromatic π-acidic surfaces involve epoxide ring opening followed by ether cyclization without any activating group (**Figure 19**) [37]. Quite recently, they have also observed exceptional high autocatalysis on the π-acidic surfaces of hexafluorobenzene and substituted NDI's as far as epoxide ring opening followed by cyclisation reactions are concerned. This unique characteristic of autocatalysis not only adds complexity to reaction mechanisms but also offers intriguing perspectives towards future developments [38, 39].

Besides the above-mentioned catalytic relevances of anion-π interactions in the domain of catalysis, amidation reactions driven by light have also been carried out by means of these interactions. It has been observed that anion-π interactions helps in the stabilization of transient complex formed between electron deficient moiety **104** and carbonate or phosphate anion. This complex in turn undergoes cleavage of N-O bond in the presence of light to offer amidyl radical, which is later trapped by heteroaromatic system (**105**) to offer the desired product **106** (**Figure 20**) [40].

#### **6. Conclusions and outlook**

Anion-π catalysis in general operates on the fundamental principle of anionic transition state stabilization on π-acidic aromatic surfaces and offers a novel

approach towards diverse molecular transformations. Over the past seven years, steady advancement has been made in the domain of anion-π catalysis with regard to the design of catalyst and the scope of the reaction. Considering the significance of polarizability, it is believed that there will be the emergence of more hidden occurrences of immature anion-π catalysis in the near future. The unconventional anion-π catalysis gains an optimistic outlook from the immense impact of current developments made with conventional cation-π and ion-pairing interactions. It is thus expected that anion-π catalysis will eventually offer new mechanisms and access to new reactivities. However so far, anion-π catalysis fails in the general expectation to produce novel products. Nevertheless, efforts are being carried out all across the globe to meet the general expectations of anion-π catalysis to offer access to novel products with exceptional features, which are far outside the scope of conventional catalysis.
