The one-electron oxidation of biphenyl-2-ylethylenes. Subsequent chemical reactivity controlled by electron return or proton transfer

Biphenyl-2-ylethylenes react with (4-BrC6H4)3N˙+SbCl6– to give the radical cation of the electrocyclization product which initiates a protic catalysis to the related fluorenes or, in presence of 2,6-di-t-butylpyridine or water, lead stoicheiometrically to phenanthrenes; with photochemical oxidants only phenanthrenes or 9,10-dihydrophenanthrenes are obtained, with an efficiency related to the polarity of the solvent and the relative energies of the triplet donor and of the radical ion pair.