ELECTRONIC AND CONFORMATIONAL EFFECTS IN THE PHOTOCHEMISTRY OF ALPHA-ALKENYL-SUBSTITUTED VINYL HALIDES

The photochemical reactions in methanol of the vinylic halides 2-halo-1-phenyl-1,3-butadienes 1-3Z-X (beta-halo-beta-alkenylstyrenes) and 1-halo-1,2-diphenylethenes 4Z-X (alpha-halostilbenes), with X = Cl or Br, have been studied quantitatively. E/Z isomerization, dehydrohalogenation, nucleophilic substitution, a [1,3]-halogen shift, a [1,3]-hydrogen shift, and oxidation are observed as the primary reactions. No reductive dehalogenation products are formed. The efficiencies of product formation are dependent on the halogen used, the electron-donating capacity of the a-substituent, the ground state conformation of the starting material, and the wavelength of excitation. Apart from the photoinduced E/Z isomerization and the oxidation reaction typical for alkenes, product formation occurs exclusively via vinyl-cationic intermediates, which are formed upon photolytic cleavage of the carbon-halogen bond. These ionic species, or part of them, are present as vinyl cation/halide anion-pairs.