Photosolvolysis of (E)-styryl(phenyl)iodonium tetrafluoroborate. Generation and reactivity of a primary vinyl Cation.

Abstract

The photochemistry of (E)-styryl(phenyl)iodonium tetrafluoroborate in methanol and 2,2,2-trifluoroethanol as well as in dichloromethane and toluene has been investigated. In all solvents the vinylic C [bond] I bond is more photoreactive than the aromatic C [bond] I bond. Homolysis as well as heterolysis of both bonds occurs, but the latter type of cleavage predominates. In alcoholic solvents, the incipient phenyl cation produces a nucleophilic substitution product. The primary styryl cation gives nucleophilic substitution, elimination, and rearrangement products. The dependence of the photoreaction on the nucleophilicity of the solvent indicates that in the presence of good nucleophiles a 10-I-3 compound is the reactive iodonium species. In this case the reaction proceeds via an S(N)i mechanism. In the absence of good nucleophiles an 8-I-2 species gives photoreaction via an S(N)1 mechanism. This is corroborated by the solvent dependence of the UV spectra, and the product composition upon photoreaction with bromide in varying concentration. Photoreaction of the iodonium salt in a chlorinated alkane yields (E)- and (Z)-beta-fluorostyrene in a Schiemann-type reaction. Reaction in toluene yields Friedel-Crafts products. The results of the photochemical reactions are compared to those of the thermal ones, and the implications of the differences are discussed.