16.11 : 周環反応: はじめに

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.