Pericyclic Reactions

by Gaurav Kayal, Assistant Professor, Pharmaceutical Chemistry Page 1 of 24

Reactions of Organic Compounds

A pericyclic reaction, a reaction that occurs as a result of

reorganizing the electrons in the reactant(s)

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Pericyclic Reaction I

An intramolecular reaction in which a new s bond is

formed between the ends of a conjugated p system by Gaurav Kayal, Assistant Professor, Pharmaceutical Chemistry Page 3 of 24

Electrocyclic Reactions Are Reversible

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Pericyclic Reaction II

Two different p bond-containing molecules react to form

a cyclic compound by Gaurav Kayal, Assistant Professor, Pharmaceutical Chemistry Page 5 of 24

Pericyclic Reaction III

A s bond is broken in the reactant, a new s bond is

formed in the product, and the p bonds rearrange by Gaurav Kayal, Assistant Professor, Pharmaceutical Chemistry Page 6 of 24

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• The electrocyclic reactions and sigmatropic

rearrangements are intramolecular reactions

Note

• The cycloaddition reactions are usually intermolecular

reactions

Common features among the three pericyclic reactions

• are concerted reactions

• are highly stereoselective

• are not affected by catalysts by Gaurav Kayal, Assistant Professor, Pharmaceutical Chemistry Page 8 of 24

The configuration of the product formed depends on:

• the configuration of the reactant

• the number of conjugated double bonds or pairs of

electrons in the reacting system

• whether the reaction is a thermal or a photochemical

reaction

A photochemical reaction takes place when a reactant absorbs light

A thermal reaction takes place without the absorption of light by Gaurav Kayal, Assistant Professor, Pharmaceutical Chemistry Page 9 of 24

An electrocyclic reaction is completely stereoselective

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A symmetry-allowed pathway is one in which in-phase

orbitals overlap

If a reaction is symmetry-forbidden, it cannot take place

by a concerted pathway

The symmetry of the HOMO of the compound undergoing

ring closure controls the stereochemical outcome of an

electrocyclic reaction

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Cycloadditions are classified according to the number of

p electrons that interact in the reaction

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A [2 + 2] Cycloaddition Reaction

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Table 29.3

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Sigmatropic Rearrangements

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Table 29.4

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