16.11:
Pericyclic Reactions: Introduction
Based on the mechanism, organic reactions can be broadly classified as ionic, radical, and pericyclic. While ionic and radical reactions have well-defined intermediates, pericyclic reactions proceed without any intermediates.
There are three classes of pericyclic reactions: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements.
In an electrocyclic reaction, the ends of a conjugated π system join to form a new σ bond, resulting in a cyclic product with one π bond less than the reactant.
In cycloaddition reactions, two different π systems interact to form a ring, with two π bonds of the reactants, transforming into two new σ bonds.
A sigmatropic rearrangement involves the net movement of a σ bond from one position to another.
Unlike ionic or radical reactions, pericyclic reactions are concerted, involving a flow of electrons in a circular path leading to a cyclic transition state.
Lastly, they are thermally or photochemically activated, and show high stereospecificity.
16.11:
Pericyclic Reactions: Introduction
Pericyclic reactions are organic reactions that occur via a concerted mechanism without generating any intermediates. The reactions proceed through the movement of electrons in a closed loop to form a cyclic transition state, where rearrangement of the σ and π bonds yields specific products.
Pericyclic reactions can be classified into three categories: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Electrocyclic reactions and sigmatropic rearrangements are intramolecular reactions, while cycloaddition reactions are intermolecular processes. The subclasses can be differentiated based on the number and the type of bonds broken and formed during the reaction.
- Electrocyclic reaction: one π bond is broken in the reactant to form a σ bond in the product.
- Cycloaddition reaction: two π bonds are broken in the reactants to form two σ bonds in the product.
- Sigmatropic rearrangement: None of the π bonds are broken. The reaction involves the migration of a σ bond with the simultaneous rearrangement of the adjacent π bonds.
The characteristic features of pericyclic reactions are listed below:
- Solvent polarity does not influence the rate of the reaction.
- The reactions do not require the presence of catalysts.
- They are thermally or photochemically activated.
- The product configuration depends on the number of electrons participating in the reaction (number of conjugated double bonds).
- The reactions are stereospecific.
The stereospecificity of pericyclic reactions can be explained based on the π molecular orbital symmetries of the reactant(s) and products.