Electrochemical Oxidation ofσ-Complex-Type Intermediates in Aromatic Nucleophilic Substitutions

Abstract

A series of sigma-adducts (1H-...7H-) 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 Eo values are measured for the oxidation of the 2-nitropropenide 4,6-dinitro- and 4-nitro-6-trifluoromethylsulfonylbenzofuroxan adducts 1H- and 4H- in acetonitrile: E (1H-)= 1.15 V versus SCE; Eo(4H-)=1.33V 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 sigma-complex precursor with no evidence whatsoever for the intermediacy of radical anionic species.

4-(D-Glucosamino)-7-nitrobenzoxadiazole: synthesis, anomers, spectra, TLC behavior, and applications

“…Like the NBD moiety, polynitrophenyl groups are strongly electron-attracting and they were the prototype for the formation of Meisenheimer complexes. [35][36][37]44 By reacting the nucleophile 1 (as hydrochloride) with picryl chloride 6 in the presence of NaHCO 3 , the new compound 8 was formed (Scheme 2). In order to obtain in an analogous way the known compound 9, 38-43 the electrophile was 2,4-dinitrofluorobenzene, 7.…”

Nucleophilic Aromatic Substitution of Hydrogen: A Novel Electrochemical Approach to the Cyanation of Nitroarenes

“…This radical undergoes first‐order C−H bond cleavage [Eqs. (3)] to give either the final rearomatized compound 1 , and hydrogen atom (as proposed by Terrier in related systems)15 or the radical anion of the rearomatized compound 1 . − , and a proton as earlier proposed by Sosonkin 25.…”

“…One variety of chemical oxidation is the electrochemical oxidation of σ H adducts. In a very recent paper Terrier et al have established,15 by electrochemical methods, the mechanism leading to the rearomatized products for the 2‐nitropropenide adducts of nitrosubstituted 1,2,3‐benzoxadiazoles and related 1‐oxides.…”

Synthesis of 4-aryloxy-7-nitrobenzofurazan derivatives from 4-chloro-7-nitrobenzofurazan and various phenoxide anions (including pharmaceuticals) in the presence of crown ethers

“…Among the three possible types of aromatic nucleophilic substitution (normal S N Ar displacement of activated halogen via a Meisenheimer complex [24,25,41], substitution of hydride, possibly in the presence of an oxidant [24,25,41], and vicarious nucleophilic substitution of an ®-situated hydride anion in the presence of an oxidant [41][42][43][44][45][46][47]), thē rst one was involved in our case [48]. The high yields indicated in Table 1 for the aroxide salts were possible by the appropriate choice of the cation: the sodium salts of 7e,g and 8 and the potassium salts of the remaining phenolic compounds.…”