Aromatic rearrangements azoxy compounds

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... 

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). 

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. 

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