Classification of Pericyclic Reactions

 

Classification of Pericyclic Reactions

Introduction

            Most of the organic reactions proceeds in step wise manner.  However there are certain reactions, which proceed in a single concerted step via formation of a cyclic transition state involving π or σ electrons.

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Electron moves round a circle and there are no positive or negative charges on any intermediates-indeed, there are no intermediates at all, such type of reaction is called Pericyclic reactions.

eg. Diels-Alder reaction.

The energy for Pericyclic reaction is supplied by heat in thermally induced reaction or by ultraviolet in a photo-induced reaction. Pericyclic reactions are highly stereospecific. Thermal and photo-chemical process gives different products but specific stereochemistry.

            Pericyclic reactions do not involve ionic or free radical intermediates and so solvents and reagents have no effect on the source of reaction.

Types of Pericyclic reactions:

The three most common types of Pericyclic reactions are

1)      Cycloaddition reaction

2)      Electro cyclic reaction

3)      Sigma tropic reaction

  1)    Cycloaddition Reactions

In these reactions two or more π electron system combines to form a ring. The reaction involve conversion of two π bonds into two sigma bonds (or reveres).

An example of cycloaddition reaction is well known as Diels-Alder reaction.

 

2)2)Electrocyclic  Reactions

These are reversible reaction in which a compound with conjugation double bond undergoes cyclization. In this process two π electrons are used to form a σ-bond.

eg. 1,3,5 hexatriene on heating gives 1,3 cycloheaxadiene

 

 In the above illustration, an electrocyclic ring closer takes place. The reverse of this process is an electrocyclic ring opening.

 3)    Sigmatropic Rearrangements

            These are concerted intermolecular rearrangements. In these rearrangements an atoms (or group of atoms) shift from one position to another. The process involves breaking of bond and forming of a new bond.

(But number of π bonds remains same)

 eg.  3-methyl 1,5 hexadiene on heating gives 1,5 heptadiene.

Common example of sigmatropic reaction is Claisen rearrangement.

 

            Types of Pericyclic reaction

 Group Transfer Reaction: - a reaction in which one or more groups or atom transfer from one molecule. In this reaction both molecule are joined together by σ-bond.