electrocyclization

Electrocyclization is a pericyclic reaction in which a conjugated polyene undergoes ring closure to form a cyclic product, generating a new carbon–carbon sigma bond while the pi system rearranges. The process is concerted and often involves rotation of the terminal p orbitals, described as conrotatory (ends rotate in the same direction) or disrotatory (ends rotate in opposite directions). The allowed pathway is dictated by orbital symmetry and depends on the number of pi electrons and whether the reaction occurs thermally or photochemically.

For thermal electrocyclizations, 4n pi electrons favor disrotatory closure, whereas 4n+2 pi electrons favor conrotatory closure. Under photochemical conditions, the stereochemical outcome is reversed: 4n pi electrons proceed conrotatory, and 4n+2 pi electrons proceed disrotatory. These rules derive from the symmetry properties of the frontier molecular orbitals involved.

A classic example is the thermal closure of hexa-1,3,5-triene (6 pi electrons) to cyclohexadiene, which occurs via a conrotatory pathway. The thermal closure of butadiene (4 pi electrons) to cyclobutene proceeds disrotatorily. Substituents can influence the preferred pathway and the stereochemical outcome.

Electrocyclization is often reversible, linking to electrocyclic ring-opening. It is widely used in synthetic organic chemistry to construct five- and six-membered rings and plays a role in the biosynthesis of certain natural products through polyene cyclizations.