Fluorinated biphenyls from aromatic arylations with pentafluorobenzenediazonium and related cations. Competition between arylation and azo coupling

Abstract

High yields of the mixed perfluorinated biaryls (C₆F₅–Ar) are obtained by the catalytic dediazoniation of the pentafluorobenzenediazonium salt (C₆F₅N₂⁺BF₄^–) in acetonitrile solutions containing various aromatic substrates (ArH) together with small amounts of iodide salts. Activated (electron-rich) as well as deactivated (electron-poor) arenes are successfully pentafluorophenylated by this method. The arylation is distinct from the azo coupling of the same substrates, which takes place in the absence of the iodide catalyst and yields the corresponding diazene (C₆F₅NN–Ar) as product. The catalytic role of iodide, and the isomeric product distributions obtained with this procedure indicate that the arylation proceeds via the pentafluorophenyl radical in a efficient homolytic chain process. Since azo coupling involves electrophilic aromatic substitution of electron-rich ArH by C₆F₅N₂⁺, the two competing pathways are distinct and do not have reactive intermediates in common.