**4. Anion-**π **catalysis in action for Diels-Alder reactions**

The Diels-Alder reaction discovered in 1928 (Noble prize 1950), a pericyclic [4 + 2] cycloaddition reaction unites diene and dienophile in an atom economic way to yield corresponding Diels-Alder adducts in a regio- and stereoselective manner. Interestingly, this reaction has been used for the synthesis of a plethora of medicinal as well as other compounds. With these thoughts in mind, Matile's group in recent years has successfully carried out Diels-Alder reactions by means of anion-π catalysts

**101**

**Figure 14.**

**Figure 15.**

*catalyst (***14***).*

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

based on fullerenes [34]. During the experimental studies, they have got thermodynamically more stable *exo*-products as compared to generally formed *endo*-products

*Michael product (***77***) catalyzed by triethylaluminium and exo-Diels-Alder product (***79)** *catalyzed by anion-*π

*Diels-Alder reaction between hydroxypyrone (***70***) and maleimides (***71***) by means of anion-*π *catalyst (***21***).* 

*Moreover, the transition states of exo-***74** *and endo-***75** *compounds are also given.*

There is no doubt that the main objective of anion-π catalysis is to discover the reactions of indefinite reactivities and the Diels-Alder reactions of anionic nature offers a first indication in this direction. Matile's group has revealed that that the reaction between **76** and **71** catalyzed by triethylaluminium yields Michael adduct **77** as a major compound along with a Diels-Alder side product **78**. Nevertheless, a reaction of concerted cycloaddition nature by means of anion-π catalyst (14) solely

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,

by virtue of the *exo*-transition state stabilization as shown in **Figure 14.**

offers *exo*-Diels-Alder compound (**79**) (**Figure 15**) [34].

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

*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 14.**

*Current Topics in Chirality - From Chemistry to Biology*

product (**40**). The existence of anion-π contacts in proteins has been established through the nitrate inhibition of mutant S112Y. The optimum performance has been found at acidic pH = 3, which clearly indicates that enolate gets formed by virtue of the stabilization on π-acidic surfaces. Moreover, K121 mutant has been found in concurrence with the docking results as far as the function of catalyst composed of tertiary amine is concerned at an ideal pH 3. By means of diverse mutants, it is established that enhancing enantioselectivity continuously agrees with the stabiliza-

*Schematic illustration of selective addition product (***39***) on the polarizability induced* π*-acidic carbon* 

More interestingly, the same group has reported innovative anion-π catalysis on the surfaces of carbon nanotubes and synthesized selective addition products on their π-acidic surfaces (**Figure 13**). Studies have revealed that tertiary amine based multi-walled carbon-nanotubes (MWCNT) display much higher efficiency as compared to single-walled carbon nanotubes (SWCNT). This is by virtue of the fact that between and along the nanotubes of MWCNT, there exists a polarizibility

The Diels-Alder reaction discovered in 1928 (Noble prize 1950), a pericyclic [4 + 2] cycloaddition reaction unites diene and dienophile in an atom economic way to yield corresponding Diels-Alder adducts in a regio- and stereoselective manner. Interestingly, this reaction has been used for the synthesis of a plethora of medicinal as well as other compounds. With these thoughts in mind, Matile's group in recent years has successfully carried out Diels-Alder reactions by means of anion-π catalysts

**4. Anion-**π **catalysis in action for Diels-Alder reactions**

tion of particular transition state [32].

*nanotubes (SWCNT and MWCNT).*

induced π-acidic surfaces [33].

**100**

**Figure 13.**

*Diels-Alder reaction between hydroxypyrone (***70***) and maleimides (***71***) by means of anion-*π *catalyst (***21***). Moreover, the transition states of exo-***74** *and endo-***75** *compounds are also given.*

#### **Figure 15.**

*Michael product (***77***) catalyzed by triethylaluminium and exo-Diels-Alder product (***79)** *catalyzed by anion-*π *catalyst (***14***).*

based on fullerenes [34]. During the experimental studies, they have got thermodynamically more stable *exo*-products as compared to generally formed *endo*-products by virtue of the *exo*-transition state stabilization as shown in **Figure 14.**

There is no doubt that the main objective of anion-π catalysis is to discover the reactions of indefinite reactivities and the Diels-Alder reactions of anionic nature offers a first indication in this direction. Matile's group has revealed that that the reaction between **76** and **71** catalyzed by triethylaluminium yields Michael adduct **77** as a major compound along with a Diels-Alder side product **78**. Nevertheless, a reaction of concerted cycloaddition nature by means of anion-π catalyst (14) solely offers *exo*-Diels-Alder compound (**79**) (**Figure 15**) [34].
