Cycloaddition reaction in organic synthesis- mechanism-Chemarticle

 Introduction

Hello everyone, today we will discuss cycloaddition reactions. Welcome to the chem article today we will briefly discuss the cycloaddition reactions mechanism in pericyclic reaction class. This is the pericyclic reaction's third part. We are already discussing what are the properties of pericyclic reactions, their molecular orbitals symmetry. Pericyclic reactions exist in cyclic transition states to the formation of products. 

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What is Electrocyclic Reaction?

We are already discussing Electrocyclic Reactions under thermal and photochemical conditions. What are the Conrotatory and Disrotatory under thermal and photochemical conditions? Both terminal orbitals lobes rotate in the same direction - that is called conrotatory. And both terminal orbitals lobes are rotated in opposite directions- this rotation is called disrotatory. For different types of pi electrons systems have different types of isomer product yield. They are stereoselective products under thermal and photochemical conditions/ maybe a 4n pi-electron system or (4n+2) pi electrons system. To form a cyclic isomeric product. 

Now, Today we will discuss pericyclic reactions type two- that is Cycloaddition Reactions. 

What is cycloaddition reaction? 

 Why call cycloaddition reactions?

In cycloaddition reaction, the reactant is two parts one is diene and another is dienophile. Diene and dienophile are added with cyclization reactions and the formation of cyclic products. That's why we call cycloaddition reactions. 

Cycloaddition reaction definition

In cycloaddition reactions take place two components are involved and the feasibility of a concerted process, which occurs overlaps. The overlap to two components is one component of HOMO and another component of LUMO.

 Cycloaddition reaction under thermal condition

Suprafacial addition

Two components are one diene and another monoene. 

The situation of bonding will be feasible when one concerted addition of components which have HOMO or the LUMO: this cycloaddition is called symmetry allowed. Two same phases of orbital lobes are interacted between ( one component HOMO phase orbital lobes and another component LUMO phase orbital lobes) resulting from that formation of bonding situation. And this type of interaction is called supra-supra interaction. This type of addition is called Suprafacial addition. 

• Supra-supra interaction is symmetry allowed. 
• Duo to supra-supra interaction between two-component HOMO and LUMO formation of sigma bonding situation. 

Cycloaddition reaction under thermal condition-Chem Article: the situation of bonding formation-Cycloaddition said symmetry allowed. That is Suprafacial addition

Antarafacial addition

  In the case of two components, they are monoene. Then we have to see that...

The situation of non-bonding, concerted addition is not feasible. This cycloaddition is called symmetry forbidden. Two different phase orbitals lobes do not interact with each other resulting from the formation of an anti-bonding situation. This is called supra-Antara interaction. Or we can say that Antarafacial addition.

•  Antarafacial addition or supra-Antara interaction is symmetrically not allowed. Supra-Antara interaction is symmetry forbidden. 
• Due to antarafacial addition, there is not any bond formation. 
• Due to antarafacial addition formation of anti-bonding situation.

Cycloaddition reaction under thermal condition-Chem Article: the situation of Anti-bonding formation-Cycloaddition said symmetry forbidden. That is Antarafacial addition.

  This whole situation is generally a thermal condition. In the presence of Heat, we see the whole cycloaddition reaction. 

Example of cycloaddition reactions under thermal condition

• A general example of a cycloaddition reaction of the (4πe + 2πe) system is the Diels-Alder reaction.
• A cycloaddition reaction of the (2πe + 2πe) system is the cyclo-dimerization of alkenes is not possible. Due to the Supra-Antara interaction of two components HOMO and LUMO. That is of course the impossibility of cyclo-dimerization of alkenes under thermal conditions.

 What happens is that two components are irritated by hv? Or what happens is that cycloaddition reaction under photochemical conditions?

Cycloaddition reaction under photochemical condition

Cycloaddition reaction of (2πe + 2πe) under photochemical condition

We expect that cyclo-dimerization of alkenes (2πe + 2πe) would be symmetry allowed due to irritation of an electron of HOMO (ψ1) promoting the next higher energy level (ψ2), i.e. π + hv→π*.  Now using the hv, the HOMO becomes ψ2. The consider the HOMO becomes π* (ψ2) orbital monoene. Therefore the feasibility of supra-supra interaction between one component orbital to another component orbital is symmetrically allowed. 

Cycloaddition reaction under photochemical condition-Chem Article: reaction of (2πe + 2πe) under photochemical condition. Cycloaddition reaction of (4πe + 2πe) under photochemical condition is not possible.

We know that the only symmetry allowed interactions to form a bond. That is why two monoene are participating in cycloaddition reactions under photochemical conditions. Hence the cyclo-dimerization of alkenes is possible under photochemical conditions.

Now

Cycloaddition reaction of (4πe + 2πe) under photochemical conditions is not possible.

We consider two components, one is diene and another is monoene ( 4πe + 2πe). The cycloaddition reaction of the (4πe + 2πe) system is not possible under photochemical conditions due to the cycloaddition being symmetrically forbidden. The HOMO of diene becomes ψ3 after irritation of an electron and the LUMO of monoene becomes ψ2. Their Two terminals orbitals occur in Supra-Antara interaction. That is symmetrically forbidden. Therefore formation or anti-bonding situation. 

Cycloaddition reaction of (4πe + 2πe) system. Cycloaddition reaction of (2πe + 2πe) system  Cycloaddition reaction of (4πe + 2πe) under photochemical conditions is not possible due to symmetry forbidden. Cycloaddition reaction of (2πe + 2πe) under photochemical conditions is possible due to symmetry allowed.

Example of cycloaddition reactions under the photochemical condition

• An example of cycloaddition reaction under photochemical conditions is cyclo-dimerization of alkene (2πe + 2πe).

Cycloaddition reaction examples:

Mainly Cycloaddition reactions are two types. The cycloaddition reactions examples are as follows below.

• Diels-Alder Reactions
• 1,3-Cycloaddition Reactions

Diels-Alder Reaction

The cycloaddition of the (4πe + 2πe) system is known as Diels-Alder Reaction. Cycloaddition of one diene and one dienophile occurs 1,4-addition of an alkene and diene under thermal conditions. First of all, we are considering the example of the Diels-Alder reaction between butadiene and maleic anhydride. 

Diels-Alder Reaction: occurs 1,4-addition of an alkene and diene under thermal conditions-Chem Article.

In case this involves the cycloaddition or 1,4-cycloaddition of alkene (Maleic Anhydride) and Diene (butadiene). The reaction of cycloaddition causes the formation of carbon-carbon single bonds. In case the diene must be cisoid, Otherwise transoid diene does not react in Diels- Alder reactions. 

Conclusion

We have to conclude that the cycloaddition reaction proceeds in the photochemical and thermal processes. What is the bonding formation for allowed symmetry? And What is symmetry forbidden in cycloaddition reactions? We briefly discuss cycloaddition reactions in synthesis organic. And their mechanisms Change for conditions changing like photochemical conditions and thermal conditions. And finally, we see the Diels-Alder reaction (4πe + 2πe) under thermal conditions is Symmetrically allowed. We get class notes of cycloaddition reactions with mechanisms. We follow cycloaddition reaction properties in thermal or photochemical conditions. 

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