Hydrazobenzene from azobenzene

An electrolytically generated base, hydrazobenzene from azobenzene, has been used for a Wittig synthesis. This method permits the maintenance of any desired base concentration by controlling the current Certain ketones can be conveniently obtained through a Horner synthesis of thioenolethers... 

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. 

Diphenylpicrylhydrazyl, which is prepared from MA -diphenyl-A -(2,4,6-trinitrophenyl)hydrazine by oxidation with lead dioxide [982], is used for the dehydrogenation of 1,4-dihydronaphthalene to naphthalene and of hydrazobenzene to azobenzene [983]. Simpler and cheaper compounds are, however, available for such purposes (equation 21). 

We consider here the detailed consequences of the reduction in aprotic solvents, two sequential steps, of azobenzene and analogous aromatic azo compounds [12-15]. The dianion is considerably more basic than the radical anion, and the dianion is only long-lived in very dry nonacidic solvents [14-16]. Both the dianion and the radical anion derived from azobenzene and substituted azobenzenes have been used as EGBs. The anion resulting from protonation of the dianion is less basic (by several pATa units) than the dianion itself but more basic than the radical anion [15]. Using the dianion as EGB may therefore result in mono- or diprotonation, depending on the strength of the acid. The radical anion leads directly to hydrazobenzene due to the further reduction of the protonated radical anion (Scheme 3) and fast protonation of the more basic anion [pATa(Ph-NH-NH-Ph) =26.1, pATa(Ph-NH-N -Ph) 31, pAa(Ph-NH-N -Ph) < 20]. 

Oxidation. Bismuthines accept oxygen from iodosylbenzene under ultrasonic irradiation. These AraBiO are mild oxidants, which convert benzylic, allylic, and secondary alcohols to carbonyl compounds, hydrazobenzenes to azobenzenes. By contrast, oxides of lower pnictogen elements are devoid of oxidizing power for organic substances. 

It is difficult to remove the hydrazobenzene from the iron sludge at this point, and a number of modifications of this process have consequently been advocated. One method involves the removal of azobenzene and reducing this compound by zinc in alcoholic alkaline solution at 60 C. The reduction mass is filtered, and the zinc residues are boiled up with fresh alcohol. The filtrate separates into two layers, of which the lower contains aqueous sodium zincate, while the upper is an alcoholic solution of hydrazobenzene. The alcoholic layer is separated and saturated with carbon dioxide to precipitate the alkali. After filtering, the alcoholic solution is evaporated to obtain the hydrazobenzene, for which practically quantitative yields have been claimed. 

Method 2 (from hydrazobenzene). Prepare a solution of sodium hypobromite by adding 10 g. (3-2 ml.) of bromine dropwise to a cold solution of 6-0 g. of sodium hydroxide in 75 ml. of water immersed in an ice bath. Dissolve 9-5 g. of hydrazobenzene (Section IV,87) in 60 ml. of ether contained in a separatory funnel, and add the cold sodimn hypobromite solution in small portions. Shake for 10 minutes, preferably mechanically. Separate the ether layer, pour it into a 100 ml. distilling flask, and distil off the ether by warming gently on a water bath. Dissolve the warm liquid residue in about 30 ml. of alcohol, transfer to a small beaker, heat to boiling on a water bath, add water dropwise to the hot solution until the azobenzene just commences to separate, render the solution clear again with a few drops of alcohol, and cool in ice water. Filter the orange crystals at the pump, and wash with a little 50 per cent, alcohol. Dry in the air. The yield is 8 g. 

If the methyl alcohol is distilled off before thorough cooling in a freezing mixture, the yield of hydrazobenzene is appreciably increased, but the product is considerably more coloured due to admixture with a trace of azobenzene. About 12 g. of impure hydrazobenzene may be recovered by distilling off the methyl alcohol from the hltrato after the colourless hydrazobenzene has been collected. 

By Dismutation.—Hydrazobenzene (1-2 g.) is melted in a test tube over a small flame. The orange-red liquid thus produced is carefully heated until the aniline which has been formed begins to boil. On cooling, a semi-solid mixture of red azobenzene and aniline is obtained. The aniline can be shaken out with water and identified by means of the bleaching powder reaction. The azobenzene may be recrystallised from alcohol as described above. If it is desired to isolate the aniline also, when larger amounts of hydrazobenzene are used, the base is separated from the azobenzene by means of dilute acetic acid. From the solution of its acetate the aniline is then liberated with concentrated alkali hydroxide solution, extracted with ether, and purified in the manner already described. 

The hydrogen-substrate ratio of 4.3 indicates formation of azobenzene as the main product, as well as some hydrazobenzene. Both products were isolated from such a run in a 2 to 1 ratio, respectively. Addition of less than stoichiometric quantities of azobenzene to CoH (no added alkali) did not result in hydrogen absorption. However, absorption of 0.53 atom of hydrogen per mole of azobenzene was observed upon further injection of small amounts of the substrate after alkali was added a 69% yield of hydrazobenzene was isolated. 

Dinitroozobenzene, (N02)2 CeH,-N N -C Hj red ndls (from ale) mp 116-9° prepd by reacting yel mercuric oxide with a hot alcoholic soln of 2,4-dinitro-hydrazobenzene. Treatment with fuming nitric acid gives, according to the exptl conditions, either 2,4,4 -trinitro-azoxybenzene or 2,4,4 -trinitro-azobenzene Ref Beil 16, 58... 

It has been known for some time that irradiation of azobenzene (324) in either 22 N sulfuric acid350 351 or acetic acid with added ferric chloride 352 yields benzo[c]cinnoline (325). This is accompanied by the formation of an almost equal quantity of benzidine (326), undoubtedly arising by rearrangement of hydrazobenzene (327). The mechanism of this reaction differs, therefore, from that of the stilbene cyclodehydrogenation, and azobenzene itself functions as the hydrogen acceptor. Yields of not more than 50% of benzo[c]cinnoline are generally observed. 

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. 

According to more recent research hydrazobenzene and hydrazotoluene can also be used as hydrogen donors to oxygen. The azobenzene or azotoluene resulting from oxidation are reduced to the initial hydrazo- compounds by sodium amalgam. 

Aromatic hydrazines like hydrazobenzene are readily oxidized to azobenzenes with air in the presence of alkali or by the action of sodium hypobromite. Aliphatic azo compounds are also prepared from the corresponding hydrazo compounds. Thus azomethane, CH5N=NCHj, is jjte-pared by the oxidation of sym-dimethylhydrazine with cupric chloride (70%). The oxidation of -hydrazotoluene, CjHsCHjNHNHCHjCjHs, to the azo compound is accomplished with mercuric oxide in boiling ether (76%). ... 

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. ... 

Azobenzene has been manufactured by reduction of a suspension of nitrobenzene by Na(Hg). Further reduction to hydrazobenzene is possible if higher temperatures and concentrations of alkali are employed [79]. The corresponding arsenic compounds may be prepared from a phenylarsenoxide by amalgam reduction thus -aminophenylarsen-oxide yields 80% H9NC6H4As=AsC6H4NHt on treatment with 4% Na(Hg) in methanol [80]. 

Azobenzene and its derivative hydrazobenzene may be converted into a derivative of diphenyl, benzidine and the ease with which this reaction takes place seems to point to the existence of an unstable linkage between the benzene rings. This would be most readily explained by the assumption that a change in the linkage (from double to single) between the carbon atoms in the benzene chain takes place in a similar manner to the linkage in the quinone formula. 

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. 

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