The photochem. isomerization of trans- and cis-difluorostilbene is reported. In the presence of a photosensitizer (biacetyl, 0.05 mol譫m-3) about 75% of the trans form can be converted into the crystalline cis form. The reactions of :CF2 from the Seyferth reagent (PhHgCF3) with cis- and trans-stilbene obey the Skell Rule, i.e., stereospecific cycloaddn. However, although the reaction of :CF2 with trans-1,2-difluorostilbene yields trans-1,2-diphenylperfluorocyclopropane as the only isolable product, the reaction of :CF2 with cis-1,2-difluorostilbene also gives the same trans-cycloadduct as the only isolable product. A possible mechanistic path involving the homolytic cleavage of the highly activated cyclopropane C-C bond facing the CF2 group is discussed.

Key words: PHOTOCHEMICAL REACTION, PHOTOSENSITIZATION, BENZENE P, CARBENE, CYCLOADDITION REACTION, CHEMICAL BONDS, ORGANO FLUORINE COMPOUNDS, ISOMERIZATION REACTION, SUBSTITUENT EFFECT, BUTANEDIONE