Formation of a trans-cycloadduct from the reaction of difluorocarbene with cis-difluorostilbene

The photochemical isomerization of trans and cis-difluorostilbenes has been studied. In the presence of a photosensitizer (biacetyl, 0.05 mol·^?1) about 75% of the trans form can be converted into the crystalline cis form. The reactions of difluorocarbene (CF₂) from the Seyferth reagent (PhHgCF₃) with cis- and trans-stilbene obey the Skell rule, i. e., stereospecific cycloaddition. However, although the reaction of CF₂ with trans-1,2-difluorostilbene yields trans-1,2-diphenyl-perfluorocyclopropane (9) as the only isolable product, the reaction of CF₂ with cis-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.