Electrocyclizations

Electrocyclizations are pericyclic reactions in which a linear conjugated polyene closes to form a ring by forming a sigma bond between the terminal carbons of the pi system. The process is concerted and proceeds through a cyclic transition state whose symmetry is governed by orbital considerations, as described by Woodward–Hoffmann rules. The number of pi electrons and the mode of activation (thermal or photochemical) determine the stereochemical outcome of the ring closure.

Under thermal conditions, a 4n+2 pi-electron system closes disrotatorily, while a 4n pi-electron system closes conrotatorily. Photochemical excitation reverses these outcomes: 4n+2 pi electrons close conrotatorily and 4n pi electrons close disrotatorily. The reaction is typically suprafacial at the termini, and the rotation sense dictates the relative stereochemistry of substituents on the resulting ring.

Common examples include the thermal closure of butadiene (four pi electrons) to cyclobutene, which proceeds conrotatorily, and the thermal closure of hexatriene (six pi electrons) to cyclohexadiene, which proceeds disrotatorily. Substituents at the ends influence the product’s stereochemistry in a way that is predictable from the chosen mode of rotation.

Electrocyclizations are valuable tools in organic synthesis for constructing cycloalkene frameworks and are frequently employed in natural product synthesis and photochemical studies. They form part of the broader class of pericyclic reactions and illustrate how orbital symmetry controls reaction pathways and stereochemical outcomes.