Photochemical transformations. 42. Photoreactions of certain bridged bicyclic and tricyclic chlorides and bromides containing aromatic chromophores

Abstract

Irradiations of a number of bicyclic and tricyclic bromides and chlorides containing aromatic rings have been carried out. With the allylic chlorides 2-Cl and 3-Cl, triplet-sensitized photoreactions in acetone (or in acetonitrile with acetophenone sensitizer) lead to allylic scrambling and allyl-to-cyclopropyl isomerization to 5-Cl, while the corresponding bromides, under similar conditions, give allylic scrambling, photo-Wagner–Meerwein rearrangement, and photosolvolysis, but no allyl-to-cyclopropyl isomerization. The differences in reaction types are ascribed to the requirement for intramolecular electron transfer for the "ionic" reactions, which may be exothermic in the triplet states of the bromides, but not in those of the chlorides. In direct irradiations in acetic acid with 254-nm light, the singlet state of the chloride 2-Cl is photoactive in the "ionic" sense, giving a mixture of acetates, benzofluorenes, and 1-methylfluoranthene. The latter hydrocarbon is the major product in the direct irradiation of 5-Cl in acetic acid, along with a mixture of acetates. The photochemical results are contrasted with ground-state solvolyses of these compounds.