Reduction of azo compounds

Walker R, Ryan AJ (1971) Some molecular parameters influencing rate of reduction of azo compounds by intestinal microflora. Xenobiotica 4—5 483486... 

Ueda, T., Yamaoki, T., Miyamoto, M., Kimura, Y., Sasatani, H., and Kim, S.I., Baeterial reduction of azo compounds as a model reaction for the degradation of azo-eontaining polyurethane by the action of intestinal flora. Bull Chem. Soc. Jpn., 69 1139-1142 (1996). Krishnaiah, Y.S., Satyanarayana, S., and Prasad, Y.V., Studies of guar gum eompression-coated 5-aminosalicylic acid tablets for colon-specific drug delivery. Drug-Dev. Ind. Pharm., 25 651-657 (1999). 

The reduction of azo compounds to hydrazo compounds is also achieved by means of aryltellurols or sodium hydrogen telluride. The last reagent (generated from NaBH4 and... 

Further complications of the reduction of aromatic nitro compounds are the possibility of complete reduction to aromatic amines (which may condense with nitroso compounds to give the desired azo compounds), reduction of azo compounds to the corresponding hydrazo compounds, followed by a benzidine (or semidine) rearrangement. It is clear, therefore, that the level of reducing agent used and other reaction conditions are quite critical. 

Nitroaromatic compounds (NACs) are one of the widespread contaminants in the environments. Sources of NACs are numerous they originate from insecticides, herbicides, explosives, pharmaceuticals, feedstock, and chemicals for dyes (Agrawal and Tratnyek, 1996). Under anaerobic conditions, the dominant action is nitro reduction by zero-valent iron to the amine. Other pathways do exist, such as the formation of azo and azoxy compounds, which is followed by the reduction of azo compounds to form amines. Also, in addition to the possibility of azo and azoxy compounds, phenylhydrox-ylamine may be an additional intermediate (Agrawal and Tratnyek, 1996). Nitrobenzene reduction forms the amine aniline. Known for its corrosion inhibition properties, aniline cannot be further reduced by iron. Additionally, it interferes with the mass transport of the contaminant to the surface of the iron. The overall reaction is as follows ... 

Most interest in reductive transformations of environmental chemicals involves dehalo-genation of chlorinated aliphatic or aromatic contaminants and the reduction of nitroar-omatic compounds. Other reductive transformations that may occur abiotically in the environment include reduction of azo compounds, quinones, disulfides, and sulfoxides (Table 16.2). See other reviews (2, 9, 11) for additional discussion of the mechanisms of these reactions. 

The electrochemical reduction of azo compounds leads to amines, with cleavage of the N—N bond. The reaction can be used for introducing amino groups into aromatics ... 

The reduction of azo compounds using sodium hydrosulfite (Na2S204) and NaOH is an important reaction, as it provides an indirect method for the amination of phenols and naphthols (Fig. 13.49). The reduction of nitro groups in anthraquinone compounds works best when a mild reducing agent (e.g., sodium hydrosulfide, NaSH) is used. In this way one avoids reducing the quinoid system. 

These compounds are prepared in part by methods similar to those for amines in addition, specific methods are employed including the reduction of diazonium compounds, reduction of azo compounds, and reduction of nitrosamines leading to sym or unsym-substituted hydrazines. 

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. ... 

The stability of the radical anions of the azo compound makes them useful as electron transfer reagents, either in solution or bonded to an electrode surface. These aspects are treated in Chapter 29. The preparation of heterocyclic compounds by reduction of azo compounds in the presence of electrophiles is discussed in Chapter 18. 

Azobenzenes and azoxybenzenes are readily reduced to hydrazobenzenes [60]. The reduction of azo compounds was found to be chemically reversible, since the corresponding hydrazo compounds afford easily azo compounds under similar experimental conditions (note that the mercury electrode - easily anod-ically oxidized - cannot be conveniently used for such experiments). The use of redox cells with both porous electrodes permits the synthesis of azocompounds (a one-pot electrolysis) from azoxy compounds. 

The hydrazo-compounds are formed by the reduction of azo-compounds with ammonium sulphide or zinc dust and an alkali. Zinc dust with caustic soda acts as follows ... 

Reduction of azo compounds to amines, with fission of the N=N double bond, assumes practical importance because so many azo compounds are readily accessible, even on an industrial scale, by coupling of diazonium salts with reactive molecules. In this way a large number of primary amines can be obtained free from isomers, which is not always easy to achieve by other... 

Most of the early product studies used the convenient reduction of azo compounds with stannous chloride in acid (although many other reductants and even halogens can effect reductive rearrangement) but intrusion of tin complexes cannot be ruled out—although the one relevant kinetic analysis of reduction by titanous salts indicates hydrazo intermediates . Reaction of separately prepared substrate under kinetically controlled conditions is essential for mechanistic investigations. 

Reductive Reactions Catafysed by Microsomal Enzymes 9.2.2.I. Reduction of Azo Compounds... 

Azo-compounds are the most numerous class of pyrrole nitrogen derivatives. The a-compounds have been stated to be less stable to air and light than the jS-compounds, but this is certainly not generally true . Azocompounds seem to be more basic than the parent pyrroles, and in some cases to be markedly acidic neither of these facts is difficult to understand if the structures of the cations (p. 76) and anions are considered. The ability of many a-compounds to couple a second time, as with diazotized /)-nitroani-line, is the basis of a colour test for distinguishing them from j8-compounds. Azopyrroles give picrates, styphnates, picrolonates and trinitrobenzene complexes, but the usefulness of some of these is limited by their lacking true melting points /. The reduction of azo compounds was mentioned above. 

Irreversible changes can be caused by a chemical change of the substance, because of which the conditions for the coloring of the substances may cease to exist, or vice versa for example, the reduction of azo compounds with stannous chloride to colorless products (p. 364). and the reactions of anthraquinone with sodium dithionite solution leading to an intensely red coloration (p. 3(X)). 

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