Azoxybenzenes, substituted

Additions of silyl-substituted synthons 965 to nitrones such as 962 a in the presence of LDA result in the products 966 which eliminate the lithium salt of O-tri-methylsilyl-N-phenylhydroxylamine 968 to give the olefins 967a or 967b in 72 and 39% yield, respectively [68, 69] (Scheme 7.20). The intermediate lithium salt 968 dimerizes with elimination of LiOSiMe3 98 to form azobenzene and azoxybenzene 961 [68, 69]. 

A kinetic study of reactions between 4-substituted nitrosobenzene and methoxide ions (in methanol), to yield 4-substituted azoxybenzenes in the presence of oxygen, indicated... 

In some way formally similar to the benzidine rearrangement is the Wallach rearrangement of azoxybenzene 23 to give 4-hydroxyazobenzene 24 in concentrated (typically 95%) H2SO4. The 2-hydroxy isomer is sometimes formed in low yield with some substituted azoxybenzenes, and it is the main product in the photochemically induced reaction. Much of what is known about the reaction has been covered in earlier review articles28-30. This contribution will report work published since 1981. 

Azoxybenzene was synthesized in 85% yield by reduction of nitrobenzene with sodium arsenite [221]. Nitrotoluenes and 2,5-dichloronitrobenzene were converted to the corresponding azoxy compounds by heating to 60-90° with hexoses (yields up to 74%) [316. Some ring-substituted nitrobenzenes were converted to azoxy compounds, some other to azo compounds by sodium bis 2-methoxyethoxy)aluminum hydride [575]. 

Reduction of substituted nitrobenzenes under alkaline conditions, usually with aqueous sodium acetate as electrolyte and a nickel cathode, is the classical method due to Elbs [45] for the formation of azo- and azoxy-compounds. Protons are used in the electrochemical reaction so that the catholyte becomes alkaline and under these conditions, phenylhydroxylamine reacts rapidly with nitrosobenzene to form azoxybenzene. Finely divided copper has long been known to catalyse the reduction of nitrobenzene to aniline in alkaline solution at the expense of azoxybenzene production [46]. Modem work confirms that whereas reduction of nitrobenzene at polycrystalline copper in alkaline solution gives mainly azoxybenzene, if the electrode is pre-oxidised in alkaline solution and then reduced just prior to the addition of nitrobenzene, high yields of aniline are obtained with good current efficiency... 

I. G. Laing in Rodd s Chemistry of Carbon Compounds. A Modern Comprehensive Treatise, Vol. 3, Part C (2.) Aromatic Compounds. Nuclear substituted benzenoid Hydrocarbons with more than one Nitrogen atom in a substituent group, S. 115-132, Elsevier, Amsterdam 1973 A.J. Floyd u. M. Sainsbury, Azo-xyaralkanes and Azoxybenzenes, Supplement to Vol. 3, Part C, S. 273 - 282, Elsevier Scientific Publishing Comp., Amsterdam Oxford New York 1981. 

Aniline may be made (I) hy Ihe reduction, with iron or tin in HOI, of nitrobenzene, and (2) by the amination of chlorobenzene by healing with ammonia to a high temperature corresponding to a pressure of over 200 atmospheres in the presence of a catalyst (a mixture of cuprous chloride and oxide). Aniline is the end-point of reduction of most mono-nitrogen substituted benzene nuclei, as nitrosobenzene, beta-phenylhydroxylamine. azoxybenzene, azobenzene, hydrazobenzene. Aniline is detected by the violet coloration produced by a small amount of sodium hypochlorite. 

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. 

Scheme 9 that both the molybdenum and iron complexes can catalyze the allylic amination of nonfunctionalized alkenes with an ene-like transposition of the double bond, but also that the yield of the allyl amine formed, 113, is moderate to high. It is generally found that higher substituted alkenes tend to give the best yields, and un-symmetrical alkenes (trisubstituted) react with virtually complete regioselectivity, as only one isomer is detected. The byproducts are primarily azoxybenzene and aniline, which arise from condensation of nitrosobenzene with phenyl hydroxylamine and reduction of phenyl hydroxylamine, respectively. 

The oxidation of aniline was carried out in the liquid phase over a series of transition metal - substituted molecular sieves. For low oxidant/aniline ratios, azoxybenzene (AZY) was the major product formed over Ti-containing catalysts, the reaction was limited by diffusion for medium pore zeolites like TS-l and mesoporous silicas were preferred as they permitted the use of both H2O2 and tert-butyl hydroperoxide as oxidants. Higher oxidant/aniline ratios (>2) led to the formation of nitrobenzene (NB), whose selectivity was proportional to the catalyst concentration. In contrast, vanadium containing molecular sieves were only active with TBHP and aniline was converted very selectively into nitrobenzene for all oxidant concentrations. 

Nitroarenes are reduced to azoxybenzenes by Bi KOH or Bi NaBH in alcoholic solvents (Scheme 14.113) [239]. Under microwave irradiation the reduction proceeds much more rapidly to give azobenzenes (Scheme 14.114) [239 b]. para-Substituted nitrobenzenes are reduced to para-substituted anilines by Bi-(NH4)2SO4 (Scheme 14.115) [240]. The combined use of BiClj and a reducing agent converts nitroarenes into azoxybenzenes, N-hydroxyanilines, or anilines, depending on the reagents and reaction conditions employed [241-244]. 

Substituted mesoporous silicas are very promising catalysts for the oxidation of arylamines in the liquid phase. Indeed Gontier and Tuel have reported that the performance of TS-1 was considerably poorer than large pore zeolite Ti- and V-substituted molecular sieves for the oxidation of aniline.36 At low oxidant/ aniline ratios it was found that azoxybenzene was the major product using Ti-substituted molecular sieves. In contrast, V-substituted molecular sieves were very selective towards the conversion of aniline to nitrobenzene. The difference between the Ti and V molecular sieves was attributed to the greater number of active oxidising sites in the V-HMS, leading to further oxidation of azoxybenzene into nitrobenzene. 

Occasionally, the reaction is complex and affords a multitude of products. For example, 2,4-dichloronitrobenzene and benzyl cyanide produce a mixture containing bis(4-chloro-2-benzoyl)azoxybenzene, 2-benzoyl-4-chlo-ronitrobenzene, 5,7-dichloroanthranil, and a-(5-chloro-2-nitrophenyl) benzyl cyanide.180 Using sodium hydride in dimethylsulfoxide as the base, the substituted benzyl cyanide, formed by direct nucleophilic displacement of chloride, becomes the major product (48%), whereas with sodium hydroxide and benzyltriethylammonium chloride the dichloroanthranil (33%) predominates. 

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. 

Sodium Hydroxide and Methyl Alcohol. On reacting nitrobenzene with methanol and caustic soda, azoxybenzene is the main reduction product formed. Sodium formate is obtained as the oxidation product of the methanol used. Naphthoquinone and its substitution products are promoters of this reaction. ... 

Manganese dioxide was also reactive toward other electron-rich aromatic compounds such as anilines (Laha and Luthy, 1990). The order of reactivity of substituted anilines toward Mn02 (alkoxy > alkyl > Cl > -COOH > NO2) was that expected for other oxidizing agents. The reaction products were mainly azobenzenes (PhN-= NPh) and azoxybenzenes... 

There are several methods reported in the literature for the oxidation of aniline, which include (i) the use of MnOs to produce azobenzene, (ii) the use of aqueous peracids to oxidize ortho-substituted aniline to nitrobenzene, (iii) the use of aqueous peracids to oxidize non-substituted anilines to azo- and azoxybenzene, (iv) the use of anhydric peracids to produce the corresponding nitrobenzenes, and (v) the use of catalytic systems, such as t-BuOjH-MoCVI), V(V), or t-BuOjH-TiCIV), to produce nitrobenzene or azoxybenzene. 

Stable mesophases are produced by substituted p-terphenyls and trans-stilbenes, whereas mesophases of lower thermal stabihty are formed by Schilf bases, esters and biphenyls. Stereochemical considerations are important and affect the position in the order, e.g. stibenes are planar, azoxybenzenes are shghtly twisted and Schiff bases are considerably twisted. The relatively low position of the tolanes (Z= =C ) in the order is surprising since crystalline diphenyl-acetylene contains planar molecules and possibly in the mesophase the rings become non-coplanar. The consequences, however, could be less serious than those in Schilf bases, since the cylinder of electron density associated with the molecular orbital of the =C linkage may still allow conjugation to occur even when the rings are non-coplanar. 

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