Enol radical cations in solution. 13. First example of a radical dication rearrangement. One-electron oxidation of dihydrobenzofuranyl cations leads to drastic rate enhancement in the oxidative benzofuran formation from enols

Article abstract of DOI:10.1021/jo980859n

The synthesis and electrochemical investigation of six stable, simple enols El-EG are reported that are characterized by electron-releasing substituents in the a-position. Oxidative benzofuran formation from these enols is unusually slow because a key intermediate in the reaction, the dihydrobenzofuranyl cation X⁺, is substantially stabilized vs rearrangement by the attached electronreleasing substituents. The persistent cations X⁺ were characterized by 1H NMR and cyclic voltammetry, and the kinetics of their rearrangement was followed by UV/vis. Notably, upon oneelectron oxidation of X⁺ to the radical dication, the formation of the benzofurans B was markedly accelerated by a factor of > 10⁶.