Reduction, azobenzenes nitro-compounds

Aromatic sym-disubstituted hydrazines are obtained by reduction of azo compounds, which in turn are intermediates in properly controlled reductions of nitro compounds. The over-all reduction can be accomplished with zinc dust and alkali or electrolytically. For example, hydrazobenzene, the simplest member, is made by both procedures. Chemical reduction is carried out on o-nitrobromobenzene to form 2,2 -dibromohydrazobenzene (57%), the halo groups remaining intact. Many examples of the electrolytic procedure have been cited the yields vary from 50% to 95%. To a limited extent, a magnesium-magnesium iodide system has been employed as a reducting agent for the azobenzenes. ... 

N-phenylhydroxylamine, PhNHOH and further reduction can give azoxybenzene, azobenzene, hydrazobenzene and aniline. The most important outlet commercially for the nitro-compounds is the complete reduction to the amines for conversion to dyestufTs. This is usually done in one stage with iron and a small amount of hydrochloric acid. 

Because aromatic nitro compounds such as nitrobenzene had been reduced by hexamethyldisilane 857 at 240 °C to give azobenzene and aniline [84], we slowly added hexamethyldisilane 857 in THF to a solution of nitrobenzene and 0.05 equivalents of Bu4NF-2-3H20 and obtained, via the probable intermediates 1000-1002, azobenzene in 84% yield [85]. Because azoxybenzene 961 affords azobenzene in 95% yield, azoxybenzene 961 is a probable intermediate in the reduction of nitrobenzene [85] (Scheme 7.26). 

Amination of aromatic nitro compounds is a very important process in both industry and laboratory. A simple synthesis of 4-aminodiphenyl amine (4-ADPA) has been achieved by utilizing a nucleophilic aromatic substitution. 4-ADPA is a key intermediate in the rubber chemical family of antioxidants. By means of a nucleophibc attack of the anilide anion on a nitrobenzene, a o-complex is formed first, which is then converted into 4-nitrosodiphenylamine and 4-nitrodiphenylamine by intra- and intermolecular oxidation. Catalytic hydrogenation finally affords 4-ADPA. Azobenzene, which is formed as a by-product, can be hydrogenated to aniline and thus recycled into the process. Switching this new atom-economy route allows for a dramatic reduction of chemical waste (Scheme 9.9).73 The United States Environmental Protection Agency gave the Green Chemistry Award for this process in 1998.74... 

In this paper the differences between the behaviour of aliphatic and aromatic nitro compounds adsorbed on a-Mn304 are discussed. The presence of a hydrogen atom on the a-carbon of aliphatic nitro compounds prevents their selective reduction to the nitroso analogues. Suggestions are made concerning the mechanisms of the reduction of nitrobenzene to nitrosobenzene and of the formation of some side products of the reduction (azobenzene and azoxybenzene). 

Coordination chemistry reveals how two ArN species can be coupled into one azobenzene molecule. In the case of the reaction of Fe3(CO)12 with aromatic nitro compounds in benzene1161, formation of derivatives such as in structure 111 has been proven by X-ray diffraction. Azoxybenzene can be formed by reaction of nitrene with nitrosobenzene, formed by reduction of nitrobenzene. 

Aromatic and heterocyclic nitro compounds are readily reduced in good yield to the corresponding amines (e.g. o-aminophenol, Expt 6.50) by sodium borohydride in aqueous methanol solution in the presence of a palladium-on-carbon catalyst. In this reduction there is no evidence for the formation of intermediates of the azoxybenzene or azobenzene type, although if the reaction is carried out in a polar aprotic solvent, such as dimethyl sulphoxide, azoxy compounds may sometimes be isolated as the initial products. 

With some reducing agents, especially with aromatic nitro compounds, the reduction can be stopped at an intermediate stage, and hydroxylamines (19-46), hydrazobenzenes, azobenzenes (19-80), and azoxybenzenes (19-79) can be obtained in this manner. However, nitroso compounds, which are often postulated as intermediates, are too reactive to be isolated, if indeed they are intermediates. Reduction by metals in mineral acids cannot be stopped, but always produces the amine. 

Azobenzenes may be prepared by oxidation of hydrazobenzenes this may be made in an undivided cell by reduction of the nitro compound with constant current [178]. Additionally, azobenzenes may be obtained in low to fair yield by anodic oxidation of aromatic amines at a rotating platinum screen cylinder anode in aqueous DMF [179]. 

The method gives excellent results as applied to the preparation of toluidines, aminodiphenyls, phenylenediamines, aminophenols, p-aminobenzoic acid, as well as to the reduction of polycyclic aromatic nitro compounds. When applied to azobenzene or azoxybenzene it gives hydrazobenzene in 80-90% yield. ... 

An aromatic nitro function hydrogenates to the corresponding amine in high yield over palladium-on-carbon in a process that is slightly superior to that using platinum. Homogeneous reduction of aromatic compounds (1) by a triphenylphosphine complex of palladium yields anilines 2 along with minor quantities of azobenzene and azoxyben-zene derivatives . 

Azobenzene, 340/13/ 6.5 azoformamide, 260/194/925 diketene, 125/140/— 4-nitroisopropylbenzene, 250/ > 182/ 830 4-nitrophenol, 280/ > 199/ 1,030 4-nitro-sophenol, 120/ 23/ 5. It is concluded that the first and last compounds are of relatively low hazard [1]. Improved equipment has provided more accurate and detailed results for a further 7 compounds and has shown the effect of variations in the initial stage of decomposition on the final pressure attained, and of the increase in pressure causing a reduction in the rate of pressure rise. At 0.2 g/cm3 loadings, comparable results are — A diazonium salt, 112/ 200/ 66 azoisobutyronitrile, 80/ 130/ 8,800 1,3-diphenyl-triazene, 140/ 95/ 420 2-nitrobenzaldehyde, 200/ 945/ 8,700 3-nitrobenzaldehyde, 190/ 830/ 4,100, 4-nitrobenzaldehyde, 200/ 960/ 4,700. Solids which deflagrate give substantially higher rates of pressure rise because the rate of pressure rise is not depressed by increase in pressure, e.g. ammonium dichromate, 227/ 510/ 68,000 [2]. 

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