Electrochemical Oxidation of σ-Complex-Type Intermediates in Aromatic Nucleophilic Substitutions

• Published in: Chemistry : a European journal 2001-04, Vol.7 (8), p.1712-1719
• Main Authors: Moutiers, Gilles, Pinson, Jean, Terrier, François, Goumont, Régis
• Format: Article
• Language: English
• Subjects: aromatic substitution; cyclic voltammetry; electrochemistry; Meisenheimer complexes; σ-adducts
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/1521-3765(20010417)7:8<1712::AID-CHEM17120>3.0.CO;2-G

A series of σ‐adducts (1 H−…︁7 H−) derived from the addition of 2‐nitropropenide ion to various nitrobenzofuroxans and nitrobenzofurazans have been oxidized electrochemically. The results show that the rearomatization of the carbocyclic ring of these adducts as well as that of a few additional 4,6‐dinitrobenzofuroxan adducts (8 H− a–c) is associated with much higher oxidation potentials than found for the same process in the dinitro‐ and trinitrobenzene series. Especially high E° values are measured for the oxidation of the 2‐nitropropenide 4,6‐dinitro‐ and 4‐nitro‐6‐trifluoromethylsulfonylbenzofuroxan adducts 1 H− and 4 H− in acetonitrile: E°(1 H−)=1.15 V versus SCE; E°(4 H−)=1.33 V versus SCE. These values fit well with the available evidence that the chemical oxidation of these adducts requires the use of very strong oxidizing agents to proceed efficiently. The mechanism for the oxidation process has been established. It is shown to involve transfer of two electrons and liberation of one proton per σ‐complex precursor with no evidence whatsoever for the intermediacy of radical anionic species. Very strong oxidizing agents are required for the chemical oxidation of σ adducts derived from the addition of 2‐nitropropenide ion (Nu−) to various nitrobenzofuroxans and nitrobenzofurazans (see scheme; X=NO2, CF3; Y=NO2, H, SO2CF3, CF3, CN, NO2). This oxidation has been achieved electrochemically and the mechanism has been shown to involve transfer of two electrons and liberation of one proton per σ‐complex precursor with no evidence for the intermediacy of radical anionic species.