Photochemistry of 2-vinylstilbenes: Configurational and conformational conditions for the [2 + 2]-photocycloaddition intramolecular trapping of the twisted singlet state of stilbene

The photoproduct formation and relative reaction rates of 2-vinylstilbenes bearing alkyl substituents at the vinyl group are compared. The normal photoproduct, a 5-exo-phenylbenzobicyclo2.1.1]hex-2-ene, appears to originate from the conformer 1-s-trans-(styryl)-2-s-cis-(vinyl)benzene.The vinyl moiety has to be in the E configuration (when applicable); the configuration of the stilbene moiety is irrelevant for formation of the product, which can be regarded as the intramolecular trapping of the twisted singlet biradical of stilbene by the vinyl group.According to this concept 2-(1-cycloheptenyl)stilbene forms 1,6-pentamethylene-5-exo-phenylbenzobicyclo2.1.1]hex-2-ene. However, 2-(1-cyclopentenyl)stilbene and 2-(1-cyclohexenyl)stilbene do not form any photocyclization product. This is ascribed to a poor orbital overlap and/or the occurrence of other relaxation processes of the intermediate biradical.