By treatment with dibromodifluoromethane and triphenylphosphine, four unfunctionalized olefins, four enethers and one bis-enether were converted to the gem-difluorocyclopropanes 1-9. The yields were considerably increased and isomerizations at the double bonds were avoided if the reactions were carried out in the presence of a macrocyclic polyether such as “18-crown-6” and if, in addition, the reagents were employed in moderate excess.
[Kim, Choongik; Wang, Zhiming; Choi, Hyuk-Jin; Ha, Young-Geun; Facchetti, Antonio; Marks, Tobin J. Journal of the American Chemical Society, 2008 , vol. 130, # 21 p. 6867 - 6878]