Azoxybenzene, preparation

Azoxybenzene is readily prepared by reduction of nitrobenzene in an alkaline medium with dextrose or sodium arsenite ... 

The relationships of azoxybenzene to azo- and hydrazobenzene are discussed in the explanations given for the next preparation. 

The aromatic nitramine (234) can be prepared by nitratingA, A -diphenylethylenediamine ° or 2,2, 4,4 -tetranitro-A,A -diphenylethylenediamine with mixed acid, the latter synthesized from 2,4-dinitrochlorobenzene. The azoxy-nitramine (235) is prepared by nitrating 4,4 -bis(dimethylamino)azoxybenzene with mixed acid. ... 

Under similar conditions, cw-azobenzene could be oxidized to cw-azoxy-benzene. Evidently the only major precaution to be taken in this preparation is the exclusion of ultraviolet light (by carrying the reaction out in a dark room) [24]. Whether this precaution is truly required is open to some doubt since cis-azoxybenzenes were prepared more recently by oxidation while warming with a heating lamp [28]. The isomerization by ultraviolet light is probably an equilibrium process in which equilibrium constants have a pronounced dependence on the chemical constitution of the materials involved. Therefore variations in the observations of the stability of the products are not entirely surprising. 

Preparation of 4 -Methylazoxybenzene (p-Tolyl-NNO-azoxybenzene or N-Phenyl-N -p-tolyldiimide N-Oxide) [18]... 

Azoxybenzene has been prepared by reduction of nitrobenzene with alcoholic potassium hydroxide,1 with sodium amalgam,2 with hydrogen in the presence of lead oxide,3 with methyl alcohol and sodium hydroxide,4 with sodium methylate and methyl alcohol,5 and by electrolytic reduction 6 by oxidation of azobenzene with chromic anhydride 7 by treatment of /9-phenylhydroxylamine with alkaline potassium permanganate,8 with nitrobenzene,9 with mineral adds,10 and with mercury acetamide,11 and by oxidation of aniline with hydrogen peroxide,12 and with acid permanganate solution in the presence of formaldehyde.13 The procedure described above is a slight modification of one described in the literature.14... 

Azoxybenzene is readily prepared by reduction of nitrobenzene in an alkaline medium with a variety of mild reducing agents. Reducing sugars have been used successfully for the reduction of substituted nitro compounds to the corresponding azoxyarenes33 and the use of D-glucose for the reduction of nitrobenzene is illustrated in Expt 6.89. 

When reduced by electrolysis, nitrobenzene and its homologues yield the same products as may be obtained by the various chemical methods of reduction. Aniline, azobenzene, azoxybenzene, hydrazobenzene, and -amino-phenol, as well as phenylhydroxylamine, can thus be obtained from nitrobenzene, and most, if not all, of these products could be prepared satisfactorily on an industrial scale by electrolysis, by adjusting the manner of working so that economy of energy is combined with maximum yields. Many of these products demand a comparatively high price, so that low power cost is not so important in this class of manufacture as high percentage yields. 

To obtain higher nitrated azoxybenzenes, indirect nitration methods must be applied. 3,5,3 ,5 -Tetranitroazoxybenzene (VII), for example, may be prepared from sym-trinitrobenzene either by boiling with a dilute sodium carbonate solution (Lobry de Bruyn and Leent [12]) or by partial reduction (Blanksma [13]). 

Ru(II)Cl2(cod)(PhNO)2, prepared by reacting [RuCl2(cod)]n with nitrosobenzene, has been shown to be an active catalyst for the synthesis of azoxybenzene from nitro-soarene and carbon monoxide (Eq. 11.57) [105]. 

There are several recent methods for the reduction of azobenzene to hydrazobenzene in near-quantitative yield. Samarium(II) iodide reduces azobenzene to hydrazobenzene rapidly at room temperature. Hydrogen telluride, generated in situ from aluminum telluride and water, reduces both azobenzene and azoxybenzene to hydrazobenzene a mixture of phenyllithium and tellurium powder has been used to reduce azobenzene. A complex of the coenzyme dihydrolipoamide and iron(II) is also effective for the reduction of azo- and azoxy-benzene to hydrazobenzene the reduction probably involves coordination of the azobenzene to iron(II) as shown in structure (1). Electrochemical reduction has been used to prepare a number of hydrazobenzenes from the corresponding azobenzenes. In the presence of an acylating agent a diacylhydrazine (e.g. the pyridazinedione derivative 2) can be isolated from the electrochemical reduction of azobenzene. 

The benefits of the use of micromembranes for the selective removal of one or more products during reaction have been demonstrated for equdibrium-limited reactions [289]. For example, the performance of hydrophilic ZSM-5 and NaA membranes over multichannel microreactors prepared from electro-discharge micromachining of commercial porous stainless steel plates was studied by Yeung et al. in the Knoevenagel condensation [290,291] and andine oxidation to azoxybenzene [292]. For such kind of reactions, the zeolite micromembrane role consists of the selective removal of water, which indeed yields higher conversions, better product purity, and a reduction in catalyst deactivation in comparison to the traditional packed bed reactor. 

The preparation of azoxy compounds is possible under a variety of conditions, and a number of patents [133] have been issued covering these processes. The yields are generally fair to good (40-90%) substituents ortho to the nitro group retard the formation of azoxybenzenes. A rotating cathode has been employed for this reaction [134,135]. 

The reduction of azoxy benzene to hydrazobenzene is of preparative interest [132,134], due to the easy conversion of hydrazobenzenes to benzidines and similar compounds. Several patents [177] describe the conversion of nitrobenzene to azoxybenzene and further to hydrazobenzene environmental considerations are in favor of the electrochemical reduction compared with the metal powder reductions, and the method stands a good chance to be competitive in the future. 

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. ... 

Zinc and Weak Alkali The mechanism of reduction with zinc and strongly alkaline solutions leads to the formation of azoxybenzene as the first stable product in the reduction of nitrobenzene. Bamberger has shown that, when faintly alkaline systems are employed, the principal initial stable product of reduction is N-phenylhydroxylamine. The systems —zinc + calcium chloride and zinc + ammonium chloride—are not neutral but quite alkaline to phenolphthalein. In the preparation of N-phenylhy-droxylamine from nitrobenzene by means of zinc dust and aqueous calcium chloride, the reduction liquid has a pH Between 10.5 and 11.7, which corresponds to the alkalinity of 0.01 N alkali. When NH4CI is substituted for CaCb in the reducing system, the pH is between 8 and 9. 

The preparation of a series of p-alkoxy-p -acyloxy-azoxybenzenes (3) was also reported recently. xhe azoxy compounds of this series were generally lower melting than the corresponding azo compounds from which they were derived because the former are mixtures of isomers. The higher nematic isotropic temperature of the azoxy materials compared to the azo compounds and to analogous Schiff base compounds (vide infra) was attributed to broadening of the molecule as a result of the presence of a lateral oxygen atom. 

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