Electrochemical oxidation of sigma-complex-type intermediates in aromatic nucleophilic substitutions

A series of sigma -adducts (1H(=)...7H(=)) derived from the addition of 2-n itropropenide ion to various nitro-benzofurosans and nitrobenzofurazans hav e been oxidized electrochemically;. The results show that the rearomatizati on of the carbocyclic ring of these adducts as well as that of a few additi onal 4.6-dinitrobenzofuroxan adducts (8H(=) a - c) is associated with much higher oxidation potentials than found for the same process in the dinitro- and trinitrobenzene series. Especially high El values are measured for the : oxidation of the 2-nitropropenide 4.6-dinitro- and 4-nitro-6-trifluoromet hylsulfonylbenzofuroxan adducts 1H(-) and 4H(-) in acetonitrile: E-z(IH-)=1 .15 V versus SCE; E-z(4H(=)) = 1.33V versus SCE. These values fit well with the available evidence that the chemical oxidation of these adducts requir es the use of very strong oxidizing agents to proceed efficiently. The mech anism for the oxidation process has been established, It is shown to involv e transfer of two electrons and liberation of one proton per sigma -complex precursor with no evidence whatsoever for the intermediacy of radical anio nic species.