Thermal Routes to Substituted Cyclopentadienes

An investigation has been made with the aim of finding a route to cyclopentadienes, particularly those with a single substituent at the 5-position, in which the final step is a thermal process of low enough activation energy that the cyclopentadiene produced does not undergo rearrangement by hydrogen migration under the reaction conditions. In the routes studied the final intermediates have been suitably substituted norbornenes which can lead to the required cyclopentadiene and an aromatic fragment by a retro-Diels–Alder reaction. In one series the intermediate was a phenyl azide adduct of norbornadiene which fragmented to cyclopentadiene and phenyltriazole, but the temperature required for this was clearly too high to allow substituted cyclopentadienes to be isolated before they had rearranged. In the course of this work, in addition to the exo-adduct of phenyl azide, the endo-adduct and a number of bisadducts were obtained and studied. In another series, adducts of tetraphenylcyclopentadienone and dimethyldiphenylcyclopentadienone with norbornadiene and 7-phenylnorbornadiene were prepared and studied. Decarbonylation of the adducts was effected both thermally and photochemically; in the latter case the reaction could be carried out at low temperature and the dihydrobenzene (a dihydrobenzonorbornadiene) produced was isolable. With or without isolation, the dihydrobenzene underwent a subsequent retro-Diels–Alder reaction to provide the cyclopentadiene and a substituted benzene. Kinetic studies showed that the activation energy for this process was almost but not quite low enough to satisfy the conditions stipulated, and in no case was a 5-substituted cyclopentadiene isolated before it had rearranged.