Reduction, azobenzenes

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). 

Further nitration gives w-dinilrobenzene sulphonation gives w-nitrobenzene sulphonic acid. Reduction gives first azoxybenzene, then azobenzene and aniline depending upon the conditions. Used in the dyestufTs industry as such or as aniline. 

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. 

The most noteworthy reaction of azo-compounds is their behaviour on reduction. Prolonged reduction first saturates the azo group, giving the hydrazo derivative (C NH-NH C), and then breaks the NH NH linkage, with the formation of two primary amine molecules. If method (1) has been employed to prepare the azo-compound, these two primary amines will therefore be respectively (a) the original amine from which the diazonium salt was prepared, and (6) the amino derivative of the amine or phenol with which the diazonium salt was coupled. For example, amino-azobenzene on complete reduction gives one equivalent of aniline, and one of p-phenylene diamine, NHaCeH NH benzene-azo-2-naphthoI similarly gives one equivalent of aniline and one of... 

The azoxybenzene in turn, when heated with iron filings, readily undergoes C4HjNO NC,Hs + Fe = C.HjNtNC.Hs + FeO further reduction to azobenzene. 

Further reduction in alkaline solution (say, with zinc powder) leads to azobenzene and hydrazobenzene ... 

Reduction of nitrobenzene in methyl or ethyl alcoholic sodium hydroxide solution with zinc powder leads to azobenzene or hydrazobenzene according to the proportion of zinc powder employed ... 

The reduction of the nitro group to yield aniline is the most commercially important reaction of nitrobenzene. Usually the reaction is carried out by the catalytic hydrogenation of nitrobenzene, either in the gas phase or in solution, or by using iron borings and dilute hydrochloric acid (the Bechamp process). Depending on the conditions, the reduction of nitrobenzene can lead to a variety of products. The series of reduction products is shown in Figure 1 (see Amines byreduction). Nitrosobenzene, /V-pbenylbydroxylamine, and aniline are primary reduction products. Azoxybenzene is formed by the condensation of nitrosobenzene and /V-pbenylbydroxylamine in alkaline solutions, and azoxybenzene can be reduced to form azobenzene and hydrazobenzene. The reduction products of nitrobenzene under various conditions ate given in Table 2. 

In certain cases, alkanolamines function as reduciag agents. For example, monoethanolamine reduces anthraquiaone to anthranols, acetone to 2-propanol, and azobenzene to aniline (17). The reduction reaction depends on the decomposition of the alkan olamine iato ammonia and an aldehyde. Sinulady, diethan olamine converts o-chloronitrobenzene to 2,2 -dichloroazobenzene and y -dinitrobenzene to 3,3 -diamiQoazobenzene. 

We have noted above that benzocrowns may be nitrated quite readily. This approach was used in the formation of a photoresponsive bis-crown (see Sect. 3.8) wherein the nitrobenzo crowns reductively dimerize to the corresponding azobenzene. Kikukawa, Nagira and Matsuda have utilized 4-nitrobenzo-15-crown-5 in a somewhat different way during the synthesis of 4 -vinylbenzo-l 5-crown-5Nitration is effected using nitric acid in a mbcture of chloroform and acetic acid. 

Another interesting variation on the linkage holding together two benzocrowns was reported by Shinkai and coworkers In their case, the benzo groups on the crowns were also part of a photolabile azobenzene system. The azo-linked bis-crown was prepared as follows (see Eq. 3.34). 4 -Nitrobenzo-l 5-crown-5 was reductively dimerized... 

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). 

A similar reduction of nitrobenzene with (Me3Si)2Hg to give azobenzene and azoxybenzene has been described [86]. The dehydration of tetrabutylammonium fluoride di- or trihydrate by hexamethyldisilane 857 is discussed in Chapter 13. 

Sljukic B, Bank CE, Compton RG (2004) The search for stable and efficient sonoelectroca-talysts for oxygen reduction and hydrogen peroxide formation azobenzene and derivatives. Phys Chem Chem Phys 6 4034-4041... 

The hydroxyphenylbenzotriazole structure was constructed by a coupling of the diazonium salt of o-nitroaniline with 4-ethyl-phenol, followed by reduction of the nitro-azobenzene to the benzotriazole with zinc powder and NaOH. After blocking of the phenol by acetylation, bromination and dehydrobromination were performed as described earlier, and treatment with aqueous NaOH... 

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... 

The highly oxophilic nature of the cobalt powder was readily demonstrated by its reaction with nitrobenzene at room temperature. Reductive coupling was quickly effected by 2 to give azo- and azoxy derivatives. Nitrobenzene reacted with 2 to give azobenzene in yields up to 37%. In some cases small amounts of azoxybenzene were also formed. With 1,4-diiodonitrobenzene, 2 reacted to give low yields of 4,4-diiodoazoxy-benzene and 4,4-diiodoazobenzene. 

The oxidative properties of ditelluronium and diselenonium dications are similar to S-S dications. For instance, interaction of thiophenol or 1,2-diphenylhydrazine with either the diselenonium dication 113 or the ditelluronium dication 114 leads to almost quantitative formation of diphenyldisulfide or azobenzene and reduced bis-chalcogenides.113,114,124 Reduction of 113 and 114 with NaBH4 gives the same products as above but proceeds quantitatively via direct electron transfer (Scheme 47).128... 

Since azoxybenzene is attacked by more powerful reducing agents, e.g. zinc dust and sodium hydroxide solution or ammonia, the use of such agents converts nitrobenzene to azobenzene and hydrazobenzene, by passing at once beyond the azoxybenzene stage. The three reduction products with paired " nitrogen atoms, therefore, stand in very close genetic relation to each other. 

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. 

Optimal conditions were found for analysis of the azo dye 2,6-dichloro-4-nitro-2,-(acetylamino)-4 -(diethylamino)azobenzene (257) by various polarographic reduction methods and a mechanism was proposed for the process531. 

Azobenzene, 340/13/6.5 azoformamide, 260/194/925 diketene, 125/140/- 4-nitroisopropylbenzene, 250/> 182/830 4-nitrophenol, 280/> 199/1,030 4-nitrosophenol, 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 -... 

The catalytic effect of tetra-n-butylammonium fluoride in the homogeneous reduction of heterocyclic A-oxides and nitroarenes by hexamethyldisilane in tetra-hydrofuran can occur with EXPLOSIVE violence, but can be controlled by the slow addition of the disilane to the A-oxide (or nitroarene) and tetra-n-butylammonium fluoride to yield the parent heterocycle (>70%) (or azobenzene 84%). In a similar manner, azoxybenzene is converted into azobenzene (95%), and 4-nitropyridine-l-oxide, is reduced to azoxypyridine-l,l -dioxide (78%), with minor amounts of azopyridine-1, l -dioxide and azopyridine-1-oxide [5,6]. 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilhene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

Fig. 14 Reduction of PhjCSPh by electrochemically generated aromatic anion radicals (in DMF at 25°C). Variation of the rate-determining step rate constant, A , with the standard potential of the aromatic anion radical, p,g (from left to right azobenzene, benzo[c]cinnoline, 4-dimethylaminoazobenzene, terephthalonitrile, naphthacene, phlhalonitrile, perylene, fluoranthene, 9,10-diphenylanthracene). The dotted lines are the theoretical limiting behaviours corresponding to the concerted (right) and stepwise (left) pathways. (Adapted from Severin et al 1988.)...

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