A novel double insertion of the difluoromethylene unit from trifluoromethylcopper into the carbon–copper bond of perfluoroaryl- and perfluorovinylcopper reagents: preparation, mechanism and applications of new fluorinated copper reagents

Reaction of CF3Cu with pentafluorophenylcopper, prepared from CuBr and pentafluorophenylcadmium C6F5CdX, gave the double CF2 unit insertion product, C6F5CF2CF2Cu, in 70–80% yields. The insertion reaction also worked well with CF3MX (M = Cd, Zn; X = Br, CF3) and C6F5CdX in the presence of CuBr at room temperature to afford C6F5CF2CF2Cu. With a vinylcopper analog, (Z)-CF3CFCFCu, double CF2 insertion also occurred resulting in the formation of (E)-CF3CFCFCF2CF2Cu, which could be trapped with allyl bromide to give (E)-CF3CFCFCF2CF2CH2CHCH2. When CF2CFCu was used as a substrate, (E)-CF3CFCFCF2CF2Cu was observed, but in low yield. A mechanism for this novel double CF2 insertion is proposed to involve a copper difluorocarbenoid complex intermediate, which inserted into the carbon–copper bond of the fluorinated phenyl and vinyl coppers to form fluorinated benzyl or allyl copper reagents. The intermediate benzyl or allyl coppers are more reactive than the pentafluorophenyl or vinylcoppers and rapidly undergo insertion of another CF2 unit. Perfluorobenzylcopper was prepared independently and gave C6F5CF2CF2Cu on reaction with CF3Cu. The resultant new copper reagent, such as C6F5CF2CF2Cu, showed good thermal stability and reactivity. Halogenation of the copper reagent gave the corresponding halides C6F5CF2CF2X, and insertion of sulfur dioxide into the carbon–copper bond followed by trapping with allyl halide gave the corresponding allylic sulfones. C6F5CF2CF2Cu also participated in coupling reactions with allyl halides, vinyl halides and aryl halides to give the coupled products in good yields. Selective functionalization of both aryl rings of the fluorinated diarylethanes with a variety of reagents has also been demonstrated.