Fluorinated cyclopropanecarboxylic acids and their derivatives

The addition of halofluorocarbenes such as :CF 2 , :CClF or :CBrF, generated from the appropriate halofluoromethanes with bases, to butadiene or isoprene gives the corresponding l-vinyl substituted halofluorocyclopropanes in moderate to good yield. The vinyl group facilitates the carbene addition and permits a subsequent wide derivatisation. Several transformations including hydroboration, oxidation and Curtius degradation are presented. Furthermore, replacement of the chlorine atom in 2-chloro-2-fluorocyclopropanecarboxylic acid derivatives by hydrogen is achieved catalytically with Raney nickel. This sequence provides a convenient route to mono-fluorine substituted cyclopropane carboxylic acids.