Electron spin resonance studies of radicals and radical-ions in solution. Part IV. Further intermediates in the reduction of aromatic nitro- and nitroso-compounds

Abstract

The oxidation and reduction of nitrosobenzene, phenylhydroxylamine, and some of their derivatives have been studied by electron spin resonance methods in a variety of solvents. From the observed spectra the species PhNOand PhfiHO have been identified. PhNOis observed only in strongly alkaline solution and in the immediate vicinity of the cathode during the electrolytic reduction of nitrosobenzene. In neutral or acid solution PhNOis replaced by P h i H O which is identical with the first oxidation product of phenylhydroxylamine. The role of these paramagnetic species in the reduction of aromatic nitro-compounds is discussed briefly.

Electrochemical reduction of C-4 nitrosophenyl 1,4-dihydropyridines and their parent C-4 nitrophenyl derivatives in protic media

“…This may be partly due to its chemical instability and the difficulty to be synthesized. Mostly, literature has been devoted to nitrosobenzene [25,26]. A good review about addition, reduction and oxidation reactions of nitrosobenzene was published some years ago by Zuman and Shah [27].…”

Proximity effects in diaryl derivatives. Part 7. Mechanism of base-catalysed rearrangement of 2-(hydroxyamino)aryl phenyl sulphones to 2-hydroxy-2′-(phenylsulphonyl)azoxybenzenes

“…On addition of zinc dust to the yellow solutions of 4-chloro-2-nitrodiphenyl sulphide in aqueous ethanol containing ammonium chloride stirred at 20 "C, the solution became The Role of Oxygen in the Reuction Of2-(Hydrosyamino)aryl Phenyl Sulphones with Bust>.-In the formation of azoxybenzenes ( 3) and ( 4) from 2-(hydroxyamino)aryl phenyl sulphones (l), the reactions were carried out ' under nitrogen ' for 2-24 h.' The corresponding rearrangements of the methyl sulphone (7), however, occurred in air, but not in a nitrogen a t m ~s p h e r e . ~ Our first objective was to resolve this apparent dichotomy by studying the role of oxygen in the base-catalysed reactions of the sulphones (1).…”

“…Reaction of phenylhydroxylamine with base in the presence of air is believed to give the radical anions PhNO' and PhN02'.6,7 An analogous reaction is suggested as the first step in the reaction of compounds (1) with base, and is supported by the need for oxygen, by the kinetic studies indicating that ionisation of the hydroxyamino-group is involved, and by the isolation of 2-nitroaryl phenyl sulphones as minor products of the reaction.2 Dimerization of the nitrosoaryl radical anion (9) or reaction of the anion of the hydroxylamine with the corresponding nitroso compound to give the dianion (10) of a symmetrical N,N-diol is regarded as the second stage of the reaction. A similar intermediate was proposed for the reaction of phenylhydroxylamine with nitrosobenzene in aqueous sodium hydroxide, the product, azoxybenzene, being formed from the partially protonated dianion in an elimination The corresponding symmetrically substituted azoxybenzenes (1 1 ; R = S0,Ph) were not detected in the reaction of 2-(hydroxyamin0)aryl phenyl sulphones (1) with base, although 2,2'-bis(methylsulphonyl)azoxybenzene ( 6 ; R = Me) was the major product from the methyl sulphone (7). 4 Since the hydroxy-group in product ( 4) is not derived from solvent and since the N-oxide group in compound ( 4) is adjacent to the ring bearing the hydroxy-function, the most likely mechanism for loss of a phenylsulphonyl group involves transfer of oxygen from the nitrogen atom furthest away from the ring under attack in intermediate (10); a mechanism satisfying these conditions and in accord with the stereochemistry of the final product, cj: (5), is proposed in Scheme 2.…”

“…In an independent investigation, Shaw et aL4 showed that reaction of 2-(hydroxyamino)phenyl methyl sulphone (7) with aqueous sodium hydroxide in air at ambient temperature gave 2-hydroxy-2'-(methylsulphonyl)azoxybenzene (8) and 2,2'-bis(methylsulphonyl)azoxybenzene (6; R = Me) as major products; it is likely that the reactions of methyl and phenyl sulphones with base occur by analogous routes, although they were studied under different conditions. Several mechanisms for the formation of the azoxybenzenes (8) were discussed, and the one preferred (Scheme 1) involved oxidation of the hydroxylamine to a nitroso-derivative from which a methylsulphonyl group is displaced by hydroxide ion; the nitrosophenol so formed then reacts with the starting hydroxylamine.…”

Electrochemical and ESR Characterization of Free Radicals from C-4 Nitroso Phenyl 1,4-Dihydropyridines in Aprotic Media

“…The assignments of the coupling constants to the specific protons can also be done by comparison with other nitro-and nitrosoaromatic anion radicals. 23,44,45 The comparison with nitroaromatic Scheme 1. Father-son type intermolecular reaction between the parent nitroso compound and the electrogenerated nitroso radical anion to form the corresponding nitranion.…”