La photochimie des alcènes sous haute pression V: l'irradiation des cyclopentène et cyclohexène au voisinage de leur seuil d'absorption et en présence d'oxygène

Abstract

The photochemistry of cyclopentene and cyclohexene has been restudied in a region close to their absorption threshold and in the presence of oxygen. In the 203 – 214 nm region, the irradiation of cyclopentene results in the formation of methylenecyclobutane and bicyclo[2.1.0]pentane provided a high pressure of propane is added (P > 50 Torr). The products are similar to those reported at 184.9 nm; isomers and 1,4-pentadiene as well as vinylcyclopropane are the major ones. The lifetime of the excited methylenecyclobutane molecules is close to 0.16 ns. As far as the 1,3-pentadiene formation is concerned, that of the cis isomer is favoured in the high pressure region in agreement with the structure of the starting alkene.

The cyclohexene photoisomerization observed between 184.9 and 213.8 nm is similar to that of cyclopentene. However, owing to the high quantum values of the retro-Diels-Alder process, the observed isomers have low quantum values.