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.

Abstract

The synthesis and electrochemical investigation of six stable, simple enols E1-E6 are reported that are characterized by electron-releasing substituents in the alpha-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 electron-releasing substituents. The persistent cations X(+) were characterized by (1)H NMR and cyclic voltammetry, and the kinetics of their rearrangement was followed by UV/vis. Notably, upon one-electron oxidation of X(+) to the radical dication, the formation of the benzofurans B was markedly accelerated by a factor of >10(6).