16.12: Thermal and Photochemical Electrocyclic Reactions: Overview
Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
Electrocyclic reactions are highly stereospecific. For a substituted polyene, the stereochemical outcome depends on the configuration of the reactant and the condition of the reaction, such as thermal or photochemical.
The stereochemistry of the product can be predicted from the symmetry of the frontier orbitals of the reactant; more specifically, the HOMO.
Under thermal conditions, the reactant’s ground state HOMO is symmetric, and the outermost p orbitals rotate in opposite directions, called disrotatory motion. In contrast, under photochemical conditions, the reactant’s excited state HOMO is asymmetric, and the outermost p orbitals rotate in the same direction, called conrotatory motion.
So, under thermal conditions, a 6 π electron system has a symmetric ground state HOMO, which undergoes a disrotatory motion to give a cis product. However, under photochemical conditions, a 6 π electron system has an asymmetric excited-state HOMO that undergoes a conrotatory motion to form a trans product.
Interestingly, the 4 π electron system has an asymmetric ground-state HOMO under thermal conditions, which undergoes a conrotatory motion to give a trans product. In contrast, the disrotatory motion of the symmetric excited-state HOMO of the 4 π system results in a cis product under photochemical conditions.