Nitro-aromatic Anions

A new dimension was added to at least the understanding of organic electrosyntheses by combining electrolysis with spectroscopy to yield information on the nature of the first-formed intermediate and the kinetics of the follow-up reaction(s). One early development was the characterization of nitro-aromatic anion radicals by electron-spin-resonance (e. s. r.) spectroscopy due to D. H. Geske and his associates. 

The reaction course has not been elucidated (cf. also sodium hydroxide reagent). Hydrolyzation reactions and aromatizations are probably primarily responsible for the formation of colored and fluorescent derivatives. Substituted nitrophenols - e.g. the thiophosphate insecticides — can probably be hydrolyzed to yellow-colored nitro-phenolate anions by sodium hydroxide or possibly react to yield yellow Meisenheimer complexes. Naphthol derivatives with a tendency to form radicals, e.g. 2-naphthyl benzoate, react with hydrolysis to yield violet-colored mesomerically stabilized 1,2-naph-thalenediol radicals. 

Some useful elaboration of the initially introduced nucleophiles has also been reported.113 Thus, for-mylation of nitro aromatic rings was achieved via VNS reaction of the nitroarene with the anions of tris-phenylthiomethane114 and chloroform,115 followed by hydrolysis, as in the example of Scheme 12. 

The different steps of the biotransformations that produce a primary amine from an aromatic nitro compound involve a nitro radical-anion, a nitroso derivative, a nitroxyl radical, a hydroxylamine and then the primary amine (Figure 33.15). 

This colored complex was used to develop colorimetric tests for many nitro aromatic compounds. Whether the Meisenheimer complex or 2,4,6-trinitrobenzyl anion (TNT ) resulting from proton abstraction reacted further with TNT (or with excess hydroxide) to form larger species is still not clear, but alkaline hydrolysis of TNT in aqueous media led to large molecular weight compounds (60% > 30 kDa [28] 40% > 1 kDa [30]). 

The halogenated biphenyls represent a class of compounds where reductive conditions [51] (solvated electron, aromatic radical anions, ketyl radicals) answer the purpose much better than the OH radical. However, this is to be seen in the context that the toxicity of the halogenated biphenyls exceeds that of biphenyl itself by such a wide margin that the latter compound is considered as relatively harmless. Complete removal of the pollutant would still have to rely on the oxidative pathway. A similar situation exists with respect to nitro-aromatics [52] which are also subject to reductive attack, indirectly by the OH radical via a-hydroxyethyl radical generated from the additive ethanol [53]. 

Data also exist for the base-catalyzed protiodesUylation of XCglltSiMes in aqueous DMSO . This reaction involves the base-catalyzed cleavage of the silicon compound to give an aromatic anion Ar , and the usual substituent effects in electrophilic substitution are reversed electron-attracting groups increase the reaction rate and electron-withdrawing groups decrease it. Thus the m- and p-nitro compounds are about 10 times more reactive than the parent (X = H) compound. 

Highly activated nitro-aromatics have been substituted, in a few examples, with KCN, as the cyanide anion source, in the presence of... 

Anions in the High Pressure Kinetics of Nitroalkanes and Nitro-Aromatics", Int.J.Chem.Kinet., 18 (1986), p 1205. 

A nitro group is a strongly activating substituent in nucleophilic aromatic substitution where it stabilizes the key cyclohexadienyl anion intermediate... 

These reversible reactions are cataly2ed by bases or acids, such as 2iac chloride and aluminum isopropoxide, or by anion-exchange resias. Ultrasonic vibrations improve the reaction rate and yield. Reaction of aromatic aldehydes or ketones with nitroparaffins yields either the nitro alcohol or the nitro olefin, depending on the catalyst. Conjugated unsaturated aldehydes or ketones and nitroparaffins (Michael addition) yield nitro-substituted carbonyl compounds rather than nitro alcohols. Condensation with keto esters gives the substituted nitro alcohols (37) keto aldehydes react preferentially at the aldehyde function. 

Solvent for Displacement Reactions. As the most polar of the common aprotic solvents, DMSO is a favored solvent for displacement reactions because of its high dielectric constant and because anions are less solvated in it (87). Rates for these reactions are sometimes a thousand times faster in DMSO than in alcohols. Suitable nucleophiles include acetyUde ion, alkoxide ion, hydroxide ion, azide ion, carbanions, carboxylate ions, cyanide ion, hahde ions, mercaptide ions, phenoxide ions, nitrite ions, and thiocyanate ions (31). Rates of displacement by amides or amines are also greater in DMSO than in alcohol or aqueous solutions. Dimethyl sulfoxide is used as the reaction solvent in the manufacture of high performance, polyaryl ether polymers by reaction of bis(4,4 -chlorophenyl) sulfone with the disodium salts of dihydroxyphenols, eg, bisphenol A or 4,4 -sulfonylbisphenol (88). These and related reactions are made more economical by efficient recycling of DMSO (89). Nucleophilic displacement of activated aromatic nitro groups with aryloxy anion in DMSO is a versatile and useful reaction for the synthesis of aromatic ethers and polyethers (90). 

A mechanism of this type permits substitution of certain aromatic and ahphatic nitro compounds by a variety of nucleophiles. These reactions were discovered as the result of efforts to explain the mechanistic basis for high-yield carbon alkylation of the 2-nitropropane anion by p-nitrobenzyl chloride. p-Nitrobenzyl bromide and iodide and benzyl halides that do not contain a nitro substituent give mainly the unstable oxygen alkylation product with this ambident anion ... 

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