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 (CF 2 ) from the Seyferth reagent (PhHgCF 3 ) with cis ‐ and trans ‐stilbene obey the Skell rule, i. e. , stereospecific cycloaddition. However, although the reaction of CF 2 with trans ‐1,2‐difluorostilbene yields trans ‐1,2‐diphenyl‐perfluorocyclopropane (9) as the only isolable product, the reaction of CF 2 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 CF 2 group is discussed.