Chapter 6 - Pericyclic Reactions

Abstract

This chapter describes a variety of pericyclic reactions that include electrocyclic transformations, cycloadditions, sigmatropic rearrangements, and the ene reaction. Electrocyclic transformations involve intramolecular formation of a ring by bond-formation at the ends of a conjugated π system. The product has one more σ bond and one less π bond than the starting material. The reverse reaction, ring opening of a cyclic polyene, is also an electrocyclic process. Cycloadditions involve bonding between the termini of two π systems to produce a new ring. The product has two more σ bonds and two less π bonds than the reactants. Common examples are the Diels–Alder reaction and many 1,3-dipolar cycloadditions. In Sigmatropic reactions, an allylic σ bond at one end of a π system appears to migrate to the other end of the π system. The π bonds change position, but the total number of σ and π bonds is the same, as in the Cope and Claison rearrangements. Ene reactions combine aspects of cycloadditions and sigmatropic reactions. They may be inter- or intramolecular. Pericyclic reactions can be initiated either thermally or photochemically, but in either case, they show great stereospecificity. The conditions under which pericyclic reactions occur, and the stereochemistry of the products are formed, are dependent on the symmetry characteristics of the molecular orbitals involved. On this basis, pericyclic processes are classified as either symmetry-allowed or symmetry-forbidden.