Reduction of a nitro-compound to an amine

REDUCTION OF A NITRO COMPOUND TO AN AMINE (a) Aniline from Nitrobenzene1... 

The reduction of a nitro compound to an amine requires six equivalents of reducing agent ... 

Reduction—Continued of a hydroxylamino acid to an amino acid by hydroxylamine, 33, 26 of a nitro compound to an amine, S3,9 of a nitro compound to an azo compound, 22, 28... 

The element tin has played an increasingly important role in organic and organome-tallic chemistry, serving as a source of Lewis acids for selective transformations [1]. The main activity in these fields has been focused on Sn(IV) compounds, and Sn(II) compounds have been used primarily as reductants of aromatic nitro compounds to aromatic amines [2]. During the last decade, however, asymmetric synthesis has developed increasingly, and in this field both Sn(II) and Sn(IV) reagents have played major roles. 

Laboratory studies have indicated an increasing number of further processes for which iron oxides may be used as catalysts. A sodium promoted iron oxide on a support of Si02 catalyses the gas phase oxidation (377-427 °C) by nitrous oxide, of pro-pene to propene oxide (Duma and Honicke, 2000). Ferrihydrite or akaganeite can be used to catalyse the reduction (at 55-75 °C) by hydrazine, of aromatic nitro compounds to aromatic amines (which are the starting materials for a huge range of chemicals) these Fe oxides have the potential to provide a safe and economical pathway to the production of these important organics (Lauwiner et al., 1998). 

The reduction of aromatic nitro compounds to the corresponding amines was catalyzed by [Ru3(CO)i2] in combination with aliphatic amine cocatalysts (95). A mixture of diglyme and water was used as a solvent, turnover frequencies were about 5000 h-1, and a CO partial pressure of 20-50 atm was necessary. The reaction is highly selective for aromatic amines. It was speculated that the reaction proceeds via an intramolecular hydrogen transfer in a hydrido-metal-nitrene intermediate without prior formation of H2 in the water gas shift reaction. 

Amination. The process of making an amine (RNH2) is generally referred to as amination. The methods commonly used are (a) reduction of a nitro compound and (b) action of ammonia on a chloro-, hydroxy-, or sulfonic acid compound. 

An amino group may be introduced into an aromatic compound by nitration, followed by reduction25. Most aromatic compounds, whether of high or low reactivity, undergo nitration with one of a wide variety of nitrating agents26-28. The reduction of the nitro compounds to amines can also be effected by a variety of methods (equation 3). This... 

Associated with some metals (Al, Fe, Zn), BiCl3 gives Bi(0) which is a catalyst for the allylation of aldehydes and amines (ref. 35), and for the reduction of aromatic nitro compounds to azoxy compounds (ref. 36). When associated with sodium borohydride, BiCl3 gives an efficient system for the selective reduction of nitroarenes and azomethines (ref. 37). 

Under suitable conditions it is possible to isolate the A-substituted hydroxylamines that are formed as intermediates in the reduction of nitro compounds. For this purpose it is essential in the reduction of aromatic nitro compounds to work with neutral or nearly neutral solutions suitable reducing agents are hydrogen and platinum oxide catalysts in glacial acetic acid,82,83 zinc dust in ammonium chloride solution,84 aluminum amalgam,85 and ammonium sulfide.86 Aliphatic nitro compounds may be reduced as their alkali salts (nitronates) by diborane in tetrahydrofuran, then giving A-alkylhydroxyl-amines 87 for instance, A-cyclohexylhydroxylamine is thus obtained from nitrocyclohexane in 53% yield. However, aliphatic nitro compounds are converted into A-alkylhydroxylamines more simply by catalytic hydrogenation in the presence of palladium-barium sulfate unlike aromatic nitro compounds, aliphatic nitro compounds require an acid medium for reduction to hydroxylamines an oxalic acid medium has proved the most suitable. 

Reductive conditions favor the hydroxylamine side of what can be considered an equilibrium reaction between the hydroxylamine and nitroso functional groups trapped between the arylamine and nitroaromatic oxidative extremes. This is why all the standard methods for the partial reduction of nitroaromatic compounds inevitably yield the hydroxylamine product rather than the nitroso product. For example, a common technique to accomplish such partial reduction is the use of zinc powder and ammonium chloride in hydroxylic solvents (19). Stronger reducing conditions, such as zinc powder in acidified hydroxylic solvents, result in the complete reduction of the nitro group to the amine. This is because the acidic conditions provide a catalyst for the N-0 bond breaking reaction. 

Dissolve 0-5 g. of the substance in 10 ml. of 50 per cent, alcohol, add 0-5 g. of solid ammonium chloride and about 0 -5 g. of zinc powder. Heat the mixture to boiling, and allow the ensuing chemical reaction to proceed for 5 minutes. Filter from the excess of zinc powder, and teat the filtrate with Tollen s reagent Section 111,70, (i). An immediate black or grey precipitate or a silver mirror indicates the presence of a hydroxyl-amine formed by reduction of the nitro compound. Alternatively, the filtrate may be warmed with Fehling s solution, when cuprous oxide will be precipitated if a hydroxylamine is present. Make certain that the original compound does not aflfect the reagent used. 

If we assume that it is possible to reduce an aliphatic nitro compound to the corresponding nitroso compound, the possibility of a simultaneous isomerisation to an oxime has to be kept in mind. It is known from the reduction of primary and secondary nitro paraffins by stannous chloride1121, that the reduction does not proceed to form amines, which could be explained by a rapid isomerisation of the adsorbed intermediate to an oxime. 

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