Lesson Introduction

Organic reactions can be classified into three major classes: polar reactions (reaction between nucleophile and electrophile), radical reactions (reaction using one electron from each of the reactants) and pericyclic reactions. A pericyclic reaction, that is, cyclic reorganization of electrons, is one in which bonds are made or broken in a concerted cyclic transition state. A concerted reaction is one that involves no intermediates during the course of the reaction. Pericyclic reactions have certain characteristic properties, although as usual it is not difficult to find exceptions to all these rules.

• There is little solvent effect on the rate of pericyclic reactions that can occur in the gas phase with no solvent.
• There is no nucleophilic or electrophilic component. This means that in the arrow-pushing sense, there is no beginning and no ending for the arrows, and the arrow-pushing can occur in either a clockwise or anti-clockwise direction.
• Normally, no catalyst is needed to promote the reactions. However, many transition metal complexes can catalyze pericyclic reactions by virtue of d-orbital participation. Lewis acids also catalyze many forms of pericyclic reactions, either directly or by changing the mechanism of the reaction so that it becomes a stepwise process and, hence, no longer a true pericyclic reaction.
• Pericyclic reactions normally show very high stereospecificity. The stereochemistry of pericyclic reactions depends on the symmetry of the interacting molecular orbitals and not on the overall symmetry of the molecules.
• Pericyclic reactions can be frequently promoted by light as well as heat. Normally, the stereochemistry under the two sets of conditions is different, and it was thought opposite.
• Pericyclic reactions are unusual in that very few enzymes that catalyze such reactions are known.

A set of molecular orbitals of the reactants is transformed into a corresponding set of molecular orbitals (MOs) of the products through a concerted process. If during the transformations, the symmetry of the concerted orbitals is conserved, that is, orbitals remain in phase, the reaction involves a relatively low energy transition state and is called symmetry-allowed. On the other hand, if bringing one or more orbitals out of phase destroys orbital symmetry, the transition state energy becomes very high due to an antibonding interaction, and the reaction is symmetry-forbidden (if it occurs, it will be nonconcerted).

Prior to the 1960s, organic reactivity was thought to be dominated by factors such as

• the relative stability of reactant, transition state and product (that is, thermodynamics);
• geometrical effects such as strain and steric interactions, hydrogen bonding,
• electrostatic effects such as the polarity of functional groups and
• the aromaticity of either the reactant or the product.

A new explanation was proposed based on ‘stereoelectronic’ factors, that is, on recognizing that the three-dimensional properties of the electrons and their phase relationship could dominate the other factors listed above. This theory became known as the conservation of orbital symmetry. The pericyclic reactions are highly diastereoselective under kinetically controlled conditions and are therefore of special interest in synthetic chemistry. For the same reacting systems, thermal and photochemical reactions give opposite stereochemistry.

For the sake of convenience, the pericyclic reactions are divided into five major categories: electrocyclic, cycloaddition, sigmatropic, cheletropic and group transfer reactions; of which we will discuss here three in rather greater detail than others.