Chapter 14 - Cycloaddition Reactions Involving 4n Electrons: (4+4) Cycloaddition Reactions between Unsaturated Conjugated Systems

Abstract

The (4 πs + 4 πs) photocycloadditions are among the oldest known and, with the (2 πs + 2 πs) cycloadditions, constitute an important group of photochemical reactions. Molecules known to date as being involved in such processes are mainly of the anthracene type. Fritzsche, who discovered the photodimerization of anthracene, also observed that the monomer could be quantitatively regenerated from photoproduct by heating; Chandross recognized that photodissociation could also be effective. A great deal of work has been devoted to anthracene and anthracene derivatives since the early days and the research in this field is still active. It includes other aromatic compounds such as naphthalene derivatives, benzanthracenes and some heterocyclic compounds. The reaction can be either intermolecular or intramolecular; in the latter case, although kinetically monomolecular, the fundamental process is a bimolecular photocycloaddition since the two reacting parts linked by a chain (bichromophores) have to diffuse together.