Nitro groups, reduction oximes

Interestingly, treatment of a-nitrostilbene in water-ethyl acetate mixture by the cation-radical of A,A -dioctyl-4,4 -bipyridinium (the octylviologen cation radical) leads to the formation of products derived from the nitro group reduction. No dimerization is observed (Tomioka et al. 1986). Water is responsible for the fixation of transferred electron within the nitro group. Further reactions result in the formation of the corresponding oxime and ketone as shown in Scheme 2.10. 

Formic acid is a good reducing agent in the presence of Pd on carbon as a catalyst. Aromatic nitro compounds are reduced to aniline with formic acid[100]. Selective reduction of one nitro group in 2,4-dinitrotoluene (112) with triethylammonium formate is possible[101]. o-Nitroacetophenone (113) is first reduced to o-aminoacetophenone, then to o-ethylaniline when an excess of formate is used[102]. Ammonium and potassium formate are also used for the reduction of aliphatic and aromatic nitro compounds. Pd on carbon is a good catalyst[103,104]. NaBH4 is also used for the Pd-catalyzed reduction of nitro compounds 105]. However, the ,/)-unsaturated nitroalkene 114 is partially reduced to the oxime 115 with ammonium formate[106]... 

The bromination-oxidation-reduction route has been used in the syntheses of many energetic polynitropolycycloalkanes. Some of these reactions are illustrated in Table 1.6 (see also Chapter 2). A common strategy in these reactions is to use the oxime functionality to incorporate the nitro group, followed by oxidative nitration to gem-dinitro functionality via the Kaplan-Shechter reaction. This has been used in the case of 2,5-dinitronorbornane to synthesize 2,2,5,5-tetranitronorbornane. ... 

The reduction of aliphatic nitrocompounds in acid solution proceeds in two steps. First the nitrosocompound is formed. A low steady state concen ation of 2-methyl-2-nitrosopropane has been detected during the reduction of 2-methyl-2-nitropropane [13]. At the cathode potential necessary to attach the first electron to a nitro group, the nitroso intermediate undergoes further reduction to the hydroxyla-mine. When the nitrocompound has one a-hydrogen substituent, tautomerism of the nitroso intermediate to an oxime is in competition with further reduction. Both temperature and proton availability affect the rate of this isomerisation. Reduction of aliphatic nitrocompounds to the hydroxylamine is usually carried out in acid solution at 0-5° C to minimise oxime formation [14, 15], The hydroxylamine is stable towards further reduction in acid solution. Oximes in acid solution are reduced... 

Aliphatic nitro compounds are reduced to various products, in all the published examples, only C-F bonds at a-positions are reduced, while the nitro group can be reduced in two steps. Catalytic hydrogenation of primary nitro compounds over palladium transforms them to the corresponding isonitroso compounds, i. e. oximes, while secondary nitro groups are converted into amines (Table 4). The reduction with Raney nickel alloy converts all types of nitro compounds into the corresponding amines, e. g. formation of 14.136... 

The nitro group is converted to versatile functionalities. A carbon-carbon double bond can be produced from nitroalkane by elimination of a nitro group with vicinal hydrogen as nitrous acid. The Nef reaction is also often used for transformation from nitroalkanes to ketones. The most useful chemical modification of a nitro group is the reduction furnishing oximes and amines, and further chemical conversion to various functionalities can be performed via diazonium salts. 

It was known for long time that the 1,5- and 1,8-dinitronaphthalenes react under the action of concentrated sulphuric acid to yield naphthazarine - a valuable compound for dyeing [30], The mechanism of the formation of this compound (based on experiments of Dimroth and Ruck [18a]) probably consists in the transformation of the nitro compounds to quinone-oximes and the reduction of one of the nitro groups by hydroxylamine split off the oxime ... 

On the other hand, conjugated nitroalkenes are very useful electron-poor alkenes, prone to act as nucleophilic acceptor, mainly in the Michael reaction (Berestovitaskaya et al., 1994) or in the Diels-Alder cycloaddition (Denmark and Thorarensen, 1996). Moreover, the nitro group can be easily turned into a respectable array of functional groups such as its reduction to a primary amine, replacement with hydrogen (Ballini et al., 1983 Ono, 2001), conversion into a carbonyl (Nef reaction) (Ballini and Petrini, 2004), and transformation into other important functionalities such as nitrile, nitrile oxide, oximes, hydroxylamines, and thiols (Colvin et al., 1979). 

Nitro compounds are versatile synthetic intermediates which have found widespread utility in industrial applications. Aromatic nitro compounds are the usual starting materials for commercial applications, but aliphatic compounds exhibit a greater diversity of chemical behavior under reducing conditions. " Nitroso compounds, hydroxylamines, oximes, amines, nitrones, ketones and silyl nitronates are frequently encountered during the reduction of nitro compounds. Several specialized reviews have appeared which highlight the versatility of the nitro group in organic chemistry. ... 

Various studies have been devoted to the synthesis of amino-substituted steroids. These compounds can be prepared either from an alcohol, by displacement of a toluene-p-sulphonyl group by an amine, ° from a ketone, either by reduction of the corresponding oxime ° ° or by Beckmann rearrangement of 20-keto-oximes, ° or by catalytic reduction of a nitro-group. ° Another... 

In pseudonitroles, such as 2-nitro-2-nitrosopropane [42], the first reduction in acid solution is a two-electron reaction that results in the loss of the nitro group as a nitrite ion. Protonation of the primarily formed anion on oxygen forms an oxime ... 

Since oximes are reduced to imines with buffered aqueous TiCb, one can assume that reduction of nitronate anions might progress through a similar mechanism. With regard to the reduction of nitro compounds themselves, there is some evidence for the intermediacy of a nitroso compound (178 Scheme 43). Indeed, at low pH tautomerization of (178) to oxime (179) would be rapid, whereas with TiCb-NH4OAC at pH 6 the nitro group is trapped to give only pyrroline (183), due to slow tautomerism. 

During catalytic hydrogenation of nitro derivatives, partial reduction of the nitro group yields azo, hydrazo, hydroxylamine, nitroso, or oxime functions, in side products, as a result of catalyst deactivation. Partial reduction may become synthetically useful under carefully chosen conditions, and in substrates where the new formed function cannot interact within the molecule. Nitrobenzene hydrogenation over platinum-on-carbon in a mixture alcohol-dimethyl sulfoxide (as promoter) produces high yields of phenylhydroxylamines ... 

Reductions. Lane has reviewed reductions with BMS. He notes that this reagent is somewhat less reactive than BH3 THF and usually requires a temperature of 20-25°. He recommends that the reagent be added at this temperature. Reduction of aliphatic carboxylic acids proceeds readily, but reduction of benzoic acids is slow unless trimethyl borate is added. The reagent reduces acids, esters, oximes, nitriles, and amides, but does not reduce halides or nitro groups. 

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