Rearrangement azoxy compounds

The conversion of azoxy compounds, (144) on acid treatment, to p-hydroxy azo compounds (145, or sometimes the o-hydroxy isomers ) is called the Wallach rearrangement. When both para positions are occupied, the t -hydroxy product... 

Oxidation of organonitrogen compounds is an important process from both industrial and synthetic viewpoints . N-oxides are obtained by oxidation of tertiary amines (equation 52), which in some cases may undergo further reactions like Cope elimination and Meisenheimer rearrangement . The oxygenation products of secondary amines are generally hydroxylamines, nitroxides and nitrones (equation 53), while oxidation of primary amines usually afforded oxime, nitro, nitroso derivatives and azo and azoxy compounds through coupling, as shown in Scheme 17. Product composition depends on the oxidant, catalyst and reaction conditions employed. 

Aromatic and aliphatic primary amines can be oxidized to the corresponding nitro compounds by peroxy acids and by a number of other reagents. The peroxy acid oxidations probably go by way of intermediate hydroxylamines and nitroso compounds (Scheme 2). Various side reactions can therefore take place, the nature of which depends upon the structure of the starting amine and the reaction conditions. For example, aromatic amines can give azoxy compounds by reaction of nitroso compounds with hy-droxylamine intermediates aliphatic amines can give nitroso dimers or oximes formed by acid-catalyz rearrangement of the intermediate nitrosoalkanes (Scheme 3). 

Nevertheless, azoxy compounds can be transformed into o-hydroxy azo derivatives by photolysis, the reaction being known as the photo-Wallach rearrangement . Irradiation of these compounds leads to migration of the oxygen to the aromatic ring far from the original N-O function. For instance, (phenyl)4-methoxyphenyldiazene-l-oxide (59) under photolysis affords 2-hydroxy-4-methoxyphenylazobenzene (60) in 79% yield (equation 69). ... 

The Af-aryl-Af-acylhydroxylamines 289 and 290 rearrange to aminophenol derivatives in the presence of sulfonyl chlorides (equation 138) as well as of iodonium salts in a reaction similar to the benzidine rearrangement (equation 139). The A-aryl-N,0-diacylhydroxylamine 291 undergoes isomerization on heating to produce dibenzoylated aminophenol 292 (equation 140) . The Wallach rearrangement consists of isomerization of aromatic azoxy compounds 293 to form the hydroxyazobenzenes 294 on heating in... 

Evidence suggests that the nitrous acid-catalyzed rearrangement of 3-(p-methoxyphenyl)anthranils also proceeds via a spiro intermediate, although in some cases azoxy compounds are isolated rather than acridones.212... 

Rearrangement of Azffxy Compounds (WaUach Rearrangement) The Wallach rearrangement is a reaction that converts an aromatic azoxy compound (170) into an azo compound (174) with sulfuric acid, wherein one ring is substituted by a hydroxyl group in the para position (Scheme 18.27). 

The hydrazides arise from the rearrangement of the azoxy compounds in the presence of acetic acid. B. T. Gillis and K. F. Schimmel, J. Org. Ghem. 27, 413 (1962). 

Like azo compounds, ,Z-isomerization should have taken place in the photoirradiation of the azoxy compounds. However, the reported formation of 2-hydroxyazobenzene mentioned above required either prolonged sunlight irradiation or UV light irradiation at a temperature of 150 to 200°C, although the inefficiency of the rearrangement suggested some other competitive energy dissipation process. In fact, a paper published by Milller in 1932 reported that isoazoxybenzene in alcohol was transformed into normal azoxybenzene upon UV irradiation at 0°C. ... 

The influence of the cathode metal is much more manifest when acid electrolytes are employed than in alkaline reduction. In alkaline solution at copper electrodes, if we except the last-mentioned process, the rapidly occurring condensation of the first reduction phases—of the nitroso- and hydroxylamine body—always leads immediately to the azoxy-body and makes this appear to be the typical product of the alkaline reduction, which can in turn be further reduced. In acid solution this condensation takes place so slowly that the molecular rearrangement of the hydroxylamine and its further reduction to amine has time to take place alongside the formation of the azoxy-body and the reduction of the latter to the hydrozo-compound or benzidine.4... 

Reductions of aromatic nitro compounds often proceed to generate mixtures of nitroso and hydroxyl-amine products which then condense to form azoxy and, eventually, azo compounds. This bimolecular reduction is practical only for the generation of symmetrically substituted azo compounds. The situation can be further complicated if the reduction continues such that aromatic amines are formed the amines may then condense with the intermediate nitroso compounds to generate hydrazo compounds which can then undergo a benzidine rearrangement. 

Unsymmetrical azo- or azoxy-compoimds on reduction with tin and hydrochloric add produce a mixture of two amines, while hydrazo-compounds may yield amine(s) formed by normal reduction of the hydrazo-link and a diamine produced by a benzidine rearrangement. Nitro-samines, on reduction with tin and acid, give the secondary amines from which they were derived.)... 

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