Azoxybenzenes, rearrangements

W A L L A C H Azoxybenzene Rearrangement Acid catalyzed rearrangement of azoxybenzenes to p-hydroxyazobenzenes. 

WALLACH Azoxybenzene rearrangement Acid catalyzed rearrangement of azoxybenzene to p-trydroxyazobenzenes C 5-N=N-C U-OH-p 0-1 A 2... 

A rearrangement reaction which has been much studied, and in many ways resembles the reaction of phenylhydroxylamines, is the Wallach rearrangement111 of azoxybenzene (LXVI) to give para hydroxyazobenzene (LXVII), viz. 

The interesting rearrangement which azoxybenzene undergoes as a result of the action of concentrated sulphuric acid may also be mentioned here, p- Hydroxyazobenzene, the parent substance of the acid azo-dyes, is formed by this rearrangement (Wallach). 

In some way formally similar to the benzidine rearrangement is the Wallach rearrangement of azoxybenzene 23 to give 4-hydroxyazobenzene 24 in concentrated (typically 95%) H2SO4. The 2-hydroxy isomer is sometimes formed in low yield with some substituted azoxybenzenes, and it is the main product in the photochemically induced reaction. Much of what is known about the reaction has been covered in earlier review articles28-30. This contribution will report work published since 1981. 

The removal of potassium cations makes the results of the liquid-phase and electrode reactions similar. In the presence of crown ether, the eight-membered complex depicted in Scheme 2.16 is destroyed. The unprotected anion-radicals of azoxybenzene are further reduced by cyclooctatet-raene dianion, losing oxygen and transforming into azodianion. The same particle is formed in the electrode reaction shown in Scheme 2.13. In the chemical reduction, stabilization of azodianion is reached by protonation. Namely, addition of sulfuric acid to the reaction results in the formation of hydrazobenzene, which instantly rearranges into benzidine (4,4 -diamino-l,l"-diphenyl). The latter was isolated from the reaction, which proceeded in the presence of crown ether. 

The observation that azoxybenzene (19) rearranges to 2-hydroxyazobenzene (20) on direct irradiation but is reduced to azobenzene (21) in the presence of benzophenone was originally explained by a difference in reactivity between the singlet and triplet states.117 However, three new lines of evidence argue in... 

Aryldiazonium dications are thought to be intermediates in the Wallach rearrangement.9713 Azoxybenzene is shown to form the monoprotonated... 

Benzidine.—That nitrobenzene, by electrolytical reduction in acid solution, can directly yield benzidine, was first proved by Hiiussermann,1 who used sulphuric acid. Lob 2 later proved the same to be true for hydrochloric-, acetic- and formic-acid electrolytes. However, several reactions predominate in this direct acid reduction, which prevent the carrying out of the reaction up to hydrazobenzene, or the formation of benzidine. Phenylhydroxylamine may particularly be mentioned in this connection. In alcoholic-acid solution it is partly rearranged to amidophenol or its ethers, and partly reduced to aniline. Azoxybenzene, in acid solution, is the starting-point in the benzidine formation however, in this case, the combining velocity of nitrosobenzene and phenylhydroxylamine is not very great, so that the latter is to a very considerable extent subject to the more rapidly acting influence of the acid. 

Azinemonoxides are cleaved by two distinct photochemical pathways. One involves oxygen migration and the other involves a pericyclic ring closure. 4) Both processes are represented in the mass spectra of the compounds. Similar rearrangements occur in both the photochemistry and mass spectrometry of aryl nitrones, aryl N-oxides and aromatic azoxybenzenes. )... 

Azoxybenzenes are reduced in a four-electron reduction to hydrazobenzenes and are generally in protic solvents more difficult to reduce than the azobenzenes in this respect the system azoxybenzene-azobenzene is similar to that of nitrobenzene-nitrosobenzene. The analogy can be extended Further reduction of azobenzene yields hydrazobenzene, which can be reoxidized to azobenzene and which easily rearranges in acid solution analogously, phenylhydroxylamine can be reoxidized to nitrosobenzene and can be rearranged to p-aminophenol or similar products in acid solution. 

---