Alkenes in the presence of light undergo cycloaddition reactions to form four-membered rings. These reactions are called [2 + 2] cycloadditions.

photochemical reactions 01

Photochemical cyclization of Norbornadiene

An interesting application of this reaction is the conversion of solar energy into chemical energy through the intramolecular photochemical transformation of norbornadiene to quadricyclane. The reaction is thermodynamically unfavorable, but the presence of light allows the product to be obtained. The quadricyclane in the presence of metallic catalysts is broken, obtaining norbornadiene again and the release of 26 Kcal/mol.
photochemical reactions 02