Types of Pericyclic Reactions

In this article we will learn about Types of Pericyclic reactions. In organic chemistry, a pericyclic reaction is a chemical reaction where the reaction proceeds through concerted mechanism and it has cyclic transition state. 

Table of Contents

1. Introduction
2. Characteristics of Pericyclic Reactions
3. Types of pericyclic reactions
4. Applications of Pericyclic Reactions
5. Conclusion
6. FAQs

1. Introduction

In the world of organic chemistry, there exists a fascinating class of reactions known as pericyclic reactions. These reactions involve concerted rearrangements of electrons within cyclic systems, leading to the formation of new molecular structures. Pericyclic reactions have captivated the minds of chemists for decades, and their study has revolutionized the way we understand and manipulate chemical reactions. In this article, we will delve deep into the realm of pericyclic reactions, exploring their mechanisms, applications, and the incredible insights they provide into the world of molecular rearrangements.

What Are Pericyclic Reactions?

Pericyclic reactions are a class of organic reactions characterized by the concerted rearrangement of electrons within a cyclic system. Unlike stepwise reactions, which involve the formation of intermediate species, pericyclic reactions occur in a single, continuous step. This concerted nature gives rise to unique reaction profiles and exquisite stereochemical control.

2. Characteristics of Pericyclic Reactions

• Concerted reaction mechanism

• Cyclic transition state

• No intermediate forms in the reaction

• These are non-ionic reactions

• Bond making and bond formation occurs in single step with high stereospecificity.

• Thermal or Photochemical Reaction Conditions

3. Types of pericyclic reactions

The pericyclic reactions are further classified as following types;

• Electrocyclic reaction

• Cycloaddition reaction

• Sigmatropic reaction

• Group transfer reaction

Figure 1 : Types of Pericyclic Reactions

3.1 Electrocyclic reaction

These reactions are characterized by formation of a ring due to terminal π- bonds in conjugated systems. The Electrocyclic reactions further divided in to two types namely, 

• Ring closure electrocyclic reactions

• Ring opening electrocyclic reactions

In this types of reactions Ring closure reactions are most common as compare to ring opening reactions. 

For example,

1,3,5-hexatriene undergoes ring closure reaction to form 1,3-cyclohexadiene. And cyclobutene undergoes ring opening reaction to form 1-3-butadiene.

Figure 2 : Electrocyclic Reactions

3.2 Cycloaddition reaction

Cycloaddition reaction involves combination of two π- systems to form a ring. The Cycloaddition reaction is described as [i + j] addition reaction, where i and j are number of atoms of two π- systems. [4 + 2] cycloaddition reactions are most common reactions which form six membered ring compounds. 
For example, Reaction of 1,3-butadiene and ethylene gives cyclohexene compound. This is an example of Diels -Alder reaction. In Diels-Alder terminology, the conjugated system is termed as "diene" and alkene is recognized as "dienophile". 

Figure 3 : Cycloaddition Reaction

Another example can be given as; Ethylene molecule undergoes [2+2] cycloaddition reaction form butene.  

Figure 4 :  [2+2] Cycloaddition Reaction

3.3 Sigmatropic reaction

Sigmatropic reaction also known as Sigmatropic rearrangment is a pericyclic reaction in which migration of  σ-bond takes place. This is an intramolecular reaction. The reaction is described as [i,j] sigmatropic reaction where "i and j " stands for relative distance (in terms of atom) in each end of σ-bond which has transferred.  
For example; 2-butene undergoes [1,3] Sigmatropic rearrangement in presence of light to form 1-butene.

Figure 5 : [1,3] Sigmatropic Rearrangement 

Cope rearrangement is a [3,3] Sigmatropic rearrangement of 1,5-dienes. For example, 3-methylhexa-1,5-diene when heated to 300 ° C to form hepta-1,5-diene.

Figure 6 : Cope Rearrangement

Claisen rearrangement is a [3,3] Sigmatropic rearrangement of allyl vinyl ethers. For example, 

Figure 7: Claisen Rearrangement

3.4 Group Transfer Reaction

It is a pericyclic reaction where one or more groups of atoms transfer from one molecule to another. Group transfer reactions are less common as compare to other pericyclic reactions. They do not have a specific conversion of π- bond into σ-bond or vice versa. Ene Reaction is the most studied example of group transfer reactions. For example, reaction between alkene with allylic hydrogen (the Ene) and compound with multiple bond (the Enophile) provides substituted alkene product and migration of double bond at allylic position. 

Figure 8: Group Transfer Reaction (Ene Reaction)

4. Applications of Pericyclic Reactions

Pericyclic reactions find widespread applications in various fields, including organic synthesis, drug discovery, and materials science. The unique properties and selectivity of pericyclic reactions make them indispensable tools for chemists worldwide. Let's explore some of the exciting applications of pericyclic reactions.

Synthesis of Complex Organic Molecules: Pericyclic reactions offer efficient and stereo controlled routes to the synthesis of complex organic molecules. The Diels-Alder reaction, for instance, has been extensively employed in the synthesis of natural products and pharmaceuticals.

Materials Science: Pericyclic reactions play a vital role in the development of advanced materials. The photochemical and thermal EORs have been harnessed to create self-healing polymers, smart materials, and molecular switches.

Total Synthesis of Natural Products: Pericyclic reactions have enabled the total synthesis of various natural products, allowing chemists to unlock their biological activities and explore their therapeutic potential.

Drug Discovery: The ability of pericyclic reactions to access complex molecular scaffolds makes them invaluable in drug discovery. Chemists utilize pericyclic reactions to construct key intermediates and develop novel drug candidates.

5. Conclusion

• Pericyclic reactions takes place via a cyclic transition state and concerted mechanism.

• Pericyclic reactions are induced either thermally or photochemically.

• These reactions are highly stereospecific.

• There are four types of Pericyclic reactions : Electrocyclic reactions, Cycloaddition reactions, Sigmatropic rearrangements and Group transfer reactions.

• Electrocyclic reactions involves creation of cyclic product from open chain conjugated systems or opening of cyclic molecules with strained rings.

• Ring opening Electrocyclic reactions are more common.

• Cycloaddition reaction involves combination of two π electron system to form a cyclic molecule.

• Sigmatropic reactions are characterized by migration of a σ bond adjacent to one or more π systems with reorganization of π system in the process.

• Group transfer reactions involves transfer of one or more groups or atoms from one molecule to another.

• Ene Reaction is the most studied example of group transfer reactions.

6. Frequently Asked Questions (FAQs)

Q1: What is the significance of pericyclic reactions in organic synthesis?

Pericyclic reactions offer efficient and stereo controlled routes to the synthesis of complex organic molecules. Their unique properties and selectivity make them invaluable tools for chemists in their quest for new compounds and materials.

Q2: Are pericyclic reactions reversible?

Pericyclic reactions can be reversible under certain conditions. Factors such as temperature, concentration, and reaction conditions can influence the reversibility of pericyclic reactions. However, many pericyclic reactions are inherently irreversible due to the concerted nature of the rearrangement process.

Q3: Are there any limitations or challenges associated with pericyclic reactions?

While pericyclic reactions offer numerous advantages, they also present certain limitations and challenges. One challenge is the requirement for precise control of reaction conditions and stereochemistry to achieve the desired outcome. Additionally, the synthesis of substrates with the necessary π-systems can be challenging, limiting the scope of pericyclic reactions in certain cases.

Q4: Can pericyclic reactions be catalyzed?

Yes, pericyclic reactions can be catalyzed using a variety of catalysts, including transition metals, organic catalysts, and Lewis acids. Catalysts can facilitate the reaction by lowering the activation energy and promoting the desired rearrangements.

That's all for this topic, see you in next blog with another concept.