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Pericyclic Reactions: Introduction

Pericyclic Reactions: Definitions

Pericyclic reactions are an important class of concerted reactions, i.e. they do not pass through intermediates. These processes are defined in two ways.

1. Formal definition
Breaking and forming of bonds takes place in a concerted manner (simultaneously) in a cyclic arrangement.
Cyclic arrangement
2. Theoretical definition
A cyclic conjugated system is formed in the transition state. The animation of the ACID function shows the density of the delocalized electrons. The electrons are cyclic delocalized only in the transition state.

Likely mechanisms of reactions with cyclic arrangements

Not every reaction in which bonds are formally broken or formed in a cyclic arrangement is necessarily a pericylic reaction. Principally, a reaction like the Diels-Alder reaction can proceed through four different mechanism all of which lead to the same product starting from the same educt.

1. Stepwise, diradical with diradical intermediate
2. Stepwise, dipolar with zwitterionic intermediate
Energy diagram of reactions proceeding stepwise
3. Concerted, pericyclic with unsymmetrical transition state
4. Synchronous, pericyclic with symmetrical transition state
Energy diagram of one-step reactions

Only the last two mechanisms are pericyclic. It has been known for some time that the Diels-Alder reaction between butadiene and ethylene illustrated above takes place in a pericyclic manner. However, theoreticians could not agree whether the transition state is exactly symmetrical or slightly distorted. Now there is agreement that the transition state is symmetrical, i.e. the reaction takes place in a synchronous manner. However, the addition of just one substituent leads to a distortion of the transition state and in extreme cases to a dipolar, stepwise mechanism.

Symmetrical transition state
Asymmetrical transition state
Zwitterionic intermediate

In the last example stabilization of the positive and negative charges by the phenyl substituent and two nitril substituents, respectively, is so effective that the reaction proceeds through an intermediate. The examples illustrate that principally each case has to be thoroughly examined whether a reaction that on first sight looks pericyclic is indeed concerted or synchronous.

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