Azoxy compounds: reduction

They are always coloured but give colourless products upon reduction. Hydrazo and azoxy compounds are reduced in acid solution to the parent amine. 

Azoxy compounds can be obtained from nitro compounds with certain reducing agents, notably sodium arsenite, sodium ethoxide, NaTeH, NaBH4—PhTeTePh, and glucose. The most probable mechanism with most reagents is that one molecule of nitro compound is reduced to a nitroso compound and another to a hydroxylamine 119-42), and these combine (12-51). The combination step is rapid compared to the reduction process. Nitroso compounds can be reduced to azoxy compounds with triethyl phosphite or triphenylphosphine or with an alkaline aqueous solution of an alcohol. ... 

DAS (11.7) is synthesised from 4-nitrotoluene-2-sulphonic acid (11.6) by the route outlined in Scheme 11.1. An important factor in the preparation of DAST brighteners in the purity necessary for good performance is the purity of the DAS used as starting material. At one time DAS made in this way contained significant amounts of yellow azoxy compounds similar to 11.8, which formed the main components of the obsolescent dye Sun Yellow (Cl Direct Yellow 11) made by the partial reduction and self-condensation of intermediate 11.6. Today the major manufacturers supply DAS essentially free from these undesirable impurities [37]. 

The reduction of nitroarenes to azoxy compounds ArN=N(0)Ar is promoted by bismuth trichloride/powdered zinc427. Aromatic amines are formed in excellent yields in the reduction of nitroarenes with BH3/NiCl2428 or nickel boride429. Acyl, ester, amide and cyano groups are not affected. Reaction of the nitro compounds RCH2N02 (R = Ph, Bz... 

A special use of sodium arsenite (applied in aqueous alkaline solutions) is partial reduction of trigeminal halides to geminal halides [220] and reduction of aromatic nitro compounds to azoxy compounds [221]. 

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... 

Nitro compounds in presence of carbonyl group are selectively reduced to amines in the presence of Raney nickel catalyst. Hydrazine reduces nitrdes yielding hydrazones. Under controlled reaction conditions other functional groups, including nitroso and oxime, may be reduced. Many partially hydrogenated derivatives, such as azo-, hydrazo-, and azoxy compounds may be obtained by partial reduction with hydrazine. Reaction with chlorobenzene yields benzene. 

The reductive methods of preparing azo compounds involve, as starting materials, aromatic nitro compounds, azoxy compounds, and azines. 

A large variety of reducing agents have been proposed for this reduction. However, zinc and sodium hydroxide offer the most common system, and lithium aluminum hydride merits consideration. The reduction of azoxy compounds with lithium aluminum hydride has value mainly in structural determinations. Its importance as a preparative procedure is limited normally such a reaction sequence would be a matter of putting the cart before the horse. The reduction of azines has potential value because of the accessibility of azines unfortunately, only under specialized circumstances has it been possible simply to add the required gram-molecule of hydrogen to the structure. Usua-ally, chlorine is added to an azine structure to produce dichloro azo compounds. An extension of the reaction permits the preparation of a,a -diacyl-oxyazoalkanes from azines. 

Historically this reaction developed from the assumption that the formation of azoxy compounds by the reduction of aromatic nitro compounds probably involved the intermediate formation of C-nitroso compounds and hydroxylamines. In the all-aliphatic series, this reaction appears to be quite general. Symmetrically and unsymmetrically substituted azoxy compounds have been prepared by it, the only major problems being the usual ones of developing procedures that afford good yields and of determining the exact position of the azoxy oxygen in unsymmetrically substituted products. 

A method for the preparation of unsymmetrical azoxy compounds involves the reaction of certain diimide dioxides with Grignard reagents [5]. This reaction has somewhat limited applicability because the diimide dioxides which were used were prepared by alkylation of organonitrosohydroxylamines, a class of compounds of which cupferron is perhaps the best-known example. The reaction is, in effect, a reduction of a diimide dioxide to an azoxy compound by use of a Grignard reagent. The overall process is represented by Procedure 2-3. Since the starting materials are, in effect, unsymmetrically substituted nitroso dimers, extension of the reaction to nitroso dimers would be interesting. 

When sodium methylate or ethylate was prepared by direct reaction of sodium with an excess of alcohols and the resulting mixture was used as a dispersion in benzene to reduce aromatic nitro compounds, yields of azoxy compounds were quite low. With the higher alcohols, substantial production of azoxy compounds was observed. However, the reduction product mixture usually contained a 40 % yield of amino compounds. In a few examples, where benzyl alcohol was used to prepare sodium benzylate, only azoxy products and no amino by-products were formed. The scope of this preparation requires further study. 

It was found that certain substituents render nitrobenzene inert to reduction by potassium borohydride, whereas other substituents activate the reduction to azoxy compounds. The substituents which favored this reduction all had positive Hammett sigma constants (e.g., p-C 1, p-Br, p-I, p-COOH, m-C 1, m-Br, m-1, m-CHO reduced to m-azoxybenzyl alcohol and m-OC2H5). Among the by-products of the reaction of a p-halogen compound, when carried out in ethanol solution, was p-nitrophenetole. The reduction ofp-fluoronitrobenzene afforded only p-nitrophenetole [48]. 

The nature of the azo bond is such that only a very limited number of possible functional groups can be considered to have the necessary features to serve as starting materials for reductive methods of preparation. In a sense, the Bogo-slovskii reaction [17, 18] may be considered a reduction of a diazonium salt by copper(I) ions. However, because the reaction resembles the other condensations of diazonium salts, its classification among the condensation reactions seems appropriate. The direct reduction of azoxy compounds as such is of minor preparative importance except as a method of identification of an azoxy compound. However, in the various bimolecular reduction procedures of aromatic nitro compounds, it has been postulated that an azoxy intermediate forms in the course of the reaction. This intermediate azoxy compound is ultimately reduced to an azo compound. 

The reduction of aromatic nitro compounds is believed to proceed to an intermediate mixture of nitroso compounds and substituted hydroxylamines which are not isolated but condense to form an azoxy compound which, in turn, is reduced to an azo compound. Contributing evidence to substantiate this mechanism is that the reduction of a mixture of two aromatic nitro compounds leads to a mixture of azo compounds consistent with that predicted if each of the nitro compounds were reduced to a nitroso compound and a hydroxylamine and these, in turn, reacted with each other in all possible combinations. This observation also implies that the bimolecular reduction of nitro compounds is practical only from the preparative standpoint for the production of symmetrically substituted azo compounds. Spectrophotometric studies of the reaction kinetics of the reduction of variously substituted nitro compounds may, however, uncover reasonable procedures for the synthesis of unsymmetrical azo compounds. 

Among the reductive methods of preparing azoxy compounds is the reduction of aliphatic nitroso compounds with stannous chloride. Triethyl phosphite has been used for the bimolecular reduction of fully fluorinated aromatic nitroso compounds. 

The bimolecular reduction of aromatic nitro compounds, depending on reaction conditions, may produce azoxy compounds, azo compounds, hydrazo compounds (1,2-diarylhydrazines), benzidines, or amines. Whereas the reduction with zinc and sodium hydroxide leads to azo compounds, zinc and acetic acid/acetic anhydride produces azoxy compounds. Other reducing agents suggested are stannous chloride, magnesium with anhydrous methanol, a sodium-lead alloy in ethanol, thallium in ethanol, and sodium arsenite. 

Traces of azo compounds were detected in the reaction mixture. They probably were formed by the reduction of the azoxy compound by the Grignard reagent. 

The bimolecular reduction of aliphatic nitroso compounds is complex and somewhat unreliable. With careful control of reaction conditions, a-nitroso ketones (in dimeric form) may be reduced with stannous chloride in an acidic medium at room temperature to the azoxy compounds, while dimeric a-nitroso acid derivatives may be reduced at about 50°C [10, 35, 36]. Nitrosoalkanes, on the other hand, are decomposed at room temperature to alcohols and nitrogen, and are reduced to amines at 50°-60°C. It has been postulated that only the dimeric nitroso compounds can be reduced to azoxy compounds and, in fact, that the dimer has a covalent nitrogen-nitrogen bond. Equations (31)—(34) summarize these data [10]. 

As indicated in the preceding chapter, the reduction of aromatic nitro compounds with zinc and sodium hydroxide solution leads to the azo product [39, 40]. On the other hand, in an acetic acid-acetic anhydride medium, reduction with zinc produces a symmetrical azoxy compound [41]. 

The reduction to produce azoxy compounds appears to be successful for a wide range of aromatic nitro compounds. The following compounds, however, could not be reduced 1-nitronaphthalene, m-dinitrobenzene, 3,5-dinitroben-zoic acid (although o-, m-, and p-nitrobenzoic acids were reduced smoothly), compounds containing an amino group o-, orp- to a nitro group (except sodium... 

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