Radical Cations of Fulvenes: Planar or Twisted?

The reactions of several fulvene derivatives with photoexcited electron acceptors (chloranil, anthraquinone) give rise to strong nuclear spin polarization effects. These results offer insight into the structures of the radical cation intermediates. For all systems studied, the spin density is restricted to the ring carbons. Electron-donating substituents in the exomethylene position affect the delocalization of the positive charge and the strength of the exocyclic bond in the radical cations. For example, the (Z)- and (E)-2-terr-butyl-6-(dimethylamino)fulvene radical cations rearrange readily, whereas the di-tert-butylfulvene radical cations show no interconversion. The results are compatible with either planar or slightly twisted radical cations and preclude the intermediacy of perpendicular radical cations.