Ring contraction of cyclic olefins: chemical processes specific to electronically excited states?

The photochemical ring cleavage and ring contraction reactions of cyclic olefins are reviewed. The retro Diels-Alder processes cannot be unambiguously linked to a particular electronic state. Although there seem to be some differences between the bicycloalkane products observed in the photosensitization (triplet) and the direct photolysis (singlet) of cyclo-olefins, these differences do not warrant assignment of the formation of a given bicycloalkane to a particular electronic state. Ring contraction to the corresponding vinylcycloalkane can occur from either the triplet or the “hot” ground state. The formation of methylenecycloalkanes appears to be a reaction specific to a Rydberg singlet state of the cycloalkenes.