Azoxybenzenes, formation

Oxidation. Aromatic amines can undergo a variety of oxidation reactions, depending on the oxidizing agent and the reaction conditions. For example, oxidation of aniline can lead to formation of phenyUiydroxylamine, nitrosobenzene, nitrobenzene, azobenzene, azoxybenzene or -benzoquinone. Oxidation was of great importance in the early stages of the development of aniline and the manufacture of synthetic dyes, such as aniline black and Perkin s mauve. 

The least powerful method of reduction, boiling nitrobenzene with sodium methoxide in solution in methyl alcohol, provides azoxybenzene in excellent yield (Zinin) the methoxide is converted into formate. (Write the equation.)... 

Azoxybenzene (1 g.) is dissolved in 5 c.c. of alcohol, the solution is heated to boiling, and 3 c.c. of 50 per cent sodium hydroxide solution and 2-3 g. of zinc dust are added with shaking. At first the mixture becomes red, because of the formation of azobenzene, but on more prolonged boiling a colourless solution is obtained just as in the reduction of nitrobenzene. When this stage has been reached, the mixture is filtered with suction through a small Buchner funnel and the hydrazobenzene is finally isolated in the manner described on p. 183 et seq. 

Anilines are converted into nitrosoarenes ArNO by the action of hydrogen peroxide in the presence of [Mo(0)(02)2(H20) (HMPA)]224, whereas catalysis of the reaction by titanium silicate and zeolites results in the formation of azoxybenzenes ArN (0)=NAr225. Azo compounds ArN=NAr are formed in 42-99% yields by the phase-transfer assisted potassium permanganate oxidation of primary aromatic amines in aqueous benzene containing a little tetrabutylammonium bromide226. The reaction of arylamines with chromyl chloride gives solid adducts which, on hydrolysis, yield mixtures of azo compounds, p-benzoquinone and p-benzoquinone anils 234227. 

The photoreduction of nitrobenzene using p5o ex filtered light from a medium pressure mercury arc was studied in petroleum, toluene, ether, 2-propanol, tert-butyl alcohol, diethylamine, triethylamine, aqueous solutions of 2-propanol and diethylamine and also in aqueous t-butylalcohol containing sodium boro-hydiide 3 >. Varying amounts of aniline, azo- and azoxybenzene were obtained. In the presence of a fourty-fold excess of benzophenone, a six-fold increase in the rate of aniline formation in ethereal solution was observed, and aniline formation was completely suppressed by addition of biacetyl or octafluomaphthalene Since unreacted nitrobenzene could be recovered in these experiments, it is demonstrated that the triplet state of nitrobenzene was quenched. 

The triplet state of 4-nitrobiphenyl has been observed in laser flashed benzene solution (. max 540 nm, t 10 ns at room temperature) 32). 4-Nitrobiphenyl and 4,4 -dinitrobiphenyl have been photoreduced by sodium formate in buffered aqueous methanolic solution 43i>) 15% 4-aminobiphenyl and 11% 4,4 -azobi-phenyl as well as 49% 4-amino-4 -nitrobiphenyl and 20% 4,4 -(p-nitrophenyl)-azoxybenzene, respectively, could be isolated and identified by comparison with authentic samples. 

The logical conclusion reached while considering these data is as follows. In liquid phase (THF), under the conditions of a regular volume continuum without gradients of concentration and potential, all anion-radicals of azoxybenzene can be stabilized just after formation due to their bonding with potassium cations. This yields the coordinative complex. The complex is diamagnetic and, therefore, azoxybenzene anion-radicals cannot be revealed by ESR spectroscopy (Scheme 2.15). 

The removal of potassium cations makes the results of the liquid-phase and electrode reactions similar. In the presence of crown ether, the eight-membered complex depicted in Scheme 2.16 is destroyed. The unprotected anion-radicals of azoxybenzene are further reduced by cyclooctatet-raene dianion, losing oxygen and transforming into azodianion. The same particle is formed in the electrode reaction shown in Scheme 2.13. In the chemical reduction, stabilization of azodianion is reached by protonation. Namely, addition of sulfuric acid to the reaction results in the formation of hydrazobenzene, which instantly rearranges into benzidine (4,4 -diamino-l,l"-diphenyl). The latter was isolated from the reaction, which proceeded in the presence of crown ether. 

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... 

From Figure 5 it can be clearly seen that nitrosobenzene totally inhibited nitrobenzene hydrogenation. The rapid adsorption and formation of azoxybenzene indicated that nitrosobenzene was more strongly adsorbed than nitrobenzene and that, given its high surface concentration, the principal surface reaction was coupling to form azoxybenzene with loss of water as shown in the reaction sequence ... 

Note that the rate of hydrogen up-take and the rate of formation of azoxybenzene were identical over the first 20 min of reaction (0.02 mol.min g )... 

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). 

It is likely that on highly reduced materials, like metals, a nitrene intermediate is formed upon reduction of nitrobenzene151. Although direct evidence for nitrene formation has not been obtained in this study, an indirect indication for such an intermediate can be found in the production of 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. 

The oxidation of aniline with H202 gives rise to a number of products, some of which are accounted for by the reactivity in solution of the primary products. A possible reaction scheme is the sequential oxidation of aniline (VIII) to phenyl hydroxylamine (IX) to nitrosobenzene (X). The condensation of unreacted VIII with X results in the formation of azobenzene (XI), while reaction of IX with X produces azoxybenzene (XIII). Only when excess H202 is used does nitrobenzene (XII) form. 

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. 

Depending on the type of iron catalyst, the reaction seems to take different mechanistic pathways. According to Johannsen and Jorgensen s results, the catalytic cycle starts with the formation of nitrosobenzene 32 either by disproportionation of hydroxylamine 29a to 32 and aniline in the presence of oxo iron(IV) phthalocyanine (PcFe4+=0) or by oxidation of 29a [131]. The second step, a hetero-ene reaction between the alkene 1 and nitrosobenzene 32, yields the allylic hydroxylamine 33, which is subsequently reduced by iron(II) phthalocyanine to afford the desired allylic amine 30 with regeneration of oxo iron(IV) phthalocyanine (Scheme 3.36). That means the nitrogen transfer proceeds as an off-metal reaction. The other byproduct, azoxybenzene, is probably formed by reaction of 29a with nitrosobenzene 32. 

In non-aqueous medium (e.g., acetonitrile) an aromatic nitro compound is reduced to the anion radical in a reversible one-electron step 1S3. Addition of proton donors (e.g., p-toluenesulphonic,o-phthalic, benzoic acid) gives a new many-electron, irreversible wave at more positive potentials. Cpe at potentials past the many-electron wave demonstrated the formation of the hydroxylamine and the azoxybenzene. 

Asotobacter chroococcum, polysaccharide formation by, IV, 220 Azoxybenzene, IV, 98 —, m-dinitro-, IV, 99... 

Nitration of azoxybenzene may lead to the formation of various nitro derivatives, differing in the number of nitro groups. 

Benzidine.—That nitrobenzene, by electrolytical reduction in acid solution, can directly yield benzidine, was first proved by Hiiussermann,1 who used sulphuric acid. Lob 2 later proved the same to be true for hydrochloric-, acetic- and formic-acid electrolytes. However, several reactions predominate in this direct acid reduction, which prevent the carrying out of the reaction up to hydrazobenzene, or the formation of benzidine. Phenylhydroxylamine may particularly be mentioned in this connection. In alcoholic-acid solution it is partly rearranged to amidophenol or its ethers, and partly reduced to aniline. Azoxybenzene, in acid solution, is the starting-point in the benzidine formation however, in this case, the combining velocity of nitrosobenzene and phenylhydroxylamine is not very great, so that the latter is to a very considerable extent subject to the more rapidly acting influence of the acid. 

The nature of the electrolyte sometimes has an important influence on the products of electrolytic reduction. The alkalinity or acidity, for example, plays an essential part in determining the nature of the substance obtained in the reduction of nitrobenzene in this case the effect is mainly due to the influence of the hydrogen ion concentration on various possible side reactions. The formation of azoxybenzene, for example, in an alkaline electrolyte is due to the reaction between phenyl-hydroxylamine and nitrosobenzene, viz.,... 

Photorearrangement of azoxybenzenes to hydrazobenzenes is a well-studied process that has now been adapted to provide a solid-state actinometer system/ The azoxybenzene is incorporated into a block of poly(methyl methacrylate) which may then be cut into appropriately sized pieces. Formation of the o-hydroxyazobenzene is monitored by its absorption at 420 nm, and the quantum yield for this conversion is given as 4.2x10 the total photon dose on any sample can be derived from a single absorbance measurement using an empirical relationship. 

Back in 1899, Werner and Stiasny (49) had studied the action of nitric acid on azobenzene and produced a series of nitro-azobenzenes and nitro-azoxybenzenes, but Werner carried the work no further. Werner then became interested in the analogy between the lakes of mordant dyes and the metallic derivatives of /3-diketones and proposed the view that mordant dyes were internal metallic complexes. This resulted in three papers published in the 1908-09 period. The first (43) reporting the complex metal salts formed from oximes, diketones, and several metals, came to the conclusion that the formation of mordant dyes depends on the formation of complex metal salts. He found that dyes capable of combining with mordants possessed both a salt-forming complex and a group capable of forming a coordinate link with a metal ion. 

Nitrosobenzene may act as an electrophilic center, and in alkaline solution phenylhydroxylamine reacts with it with formation of azoxybenzene [130-132]. The reaaction may go through an initial, homogeneous one-electron transfer followed by a radical coupling [132]. This reaction occurs generally during the reduction of simple nitrobenzenes in alkaline solution and makes azo and hydrazobenzenes as well as benzidines easily available. Azoxybenzenes may, however, also be formed in acid solution [94,104,121] ... 

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]. 

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