16.14: Photochemical Electrocyclic Reactions: Stereochemistry
The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
Conjugated systems containing an even number of π-electron pairs undergo a disrotatory ring closure. For example, photochemical activation of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives cis-3,4-dimethylcyclobutene.
Conjugated systems with an odd number of π-electron pairs undergo a conrotatory ring closure. For example, (2E,4Z,6E)-2,4,6-octatriene, a conjugated diene containing three π-electron pairs, forms trans-5,6-dimethyl-1,3-cyclohexadiene.