Thermal and Photochemical Electrocyclic Reactions: Overview

An electrocyclic reaction is the intramolecular cyclization of a conjugated acyclic polyene, such as hexatriene, to form a new σ bond.

Interestingly, polyenes with substituents at the termini yield different products under different conditions.

For example, thermal activation of octatriene forms the cis product, whereas photochemical activation gives the trans product.

The stereochemical outcome depends on the symmetry of the frontier orbitals of octatriene.

Notice that the HOMO of the triene in the ground state and the excited state have different symmetries.

Here, the outermost p orbitals are either in-phase or out-of-phase.

When the p orbitals are in phase, they must rotate in opposite directions to cyclize, called a disrotatory motion.

In contrast, when the p orbitals are out of phase, they rotate in the same direction, resulting in a conrotatory ring closure.

Under thermal conditions, the cyclization proceeds via the ground state HOMO, following a disrotatory motion, giving the cis product.

In contrast, under photochemical conditions, the cyclization proceeds via the excited state HOMO, following a conrotatory motion, forming the trans product.