WALLACH Azoxybenzene rearrangement

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. 

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

Wallach rearrangement. Acid-catalyzed rearrangement of azoxybenzenes to p-hydroxyazo-benzenes. 

Wallach found that azoxybenzene in presence of acids rearranges into 4-hydroxyazobenzene, but his main work was on terpenes, on which he published 126 papers. He studied limonene (and its tetrabromide), pinene, and terpineol. Limonene tetrabromide was independently discovered by Guillaume Adolphe Renard (Rouen 10 May 1846-April 1919), who also obtained methylcyclohexane from rosin spirit, and worked on the electrolytic oxidation of alcohol, turpentine, benzene, toluene, etc. (1880 f.). 

Azoxybenzene is known to rearrange in concentrated sulfuric acid to />-hydroxyazobenzene. This acid-catalyzed Wallach rearrangement has been extended by using arenesulfonic anhydrides to furnish the corresponding /rarenesulfonyl-oxyazobenzenes in reasonable yields (eq 2). ... 

There are only a relatively small number of examples of this rearrangement in the chemical literature nevertheless, both a thermal and photochemical Wallach rearrangement of azoxybenzene has been achieved in zeolite cages [89] and an ortho-selective rearrangement has been developed on azoxybenzene-SbClj complexes [90]. 

Azoxybenzene (3) is stable for indefinite periods in aqueous, alkaline, or dilute acid media. In moderately concentrated snlfnric acid solution, however, it undergoes rearrangement to p-hydroxyazobenzene (4), a reaction known as the Wallach rearrangement, Eqnation A ... 

Antimony pentachloride, unlike other Lewis acids and proton adds, reacts with various azoxybenzenes to give 1 1 complexes (26) in high yield. Thermolysis of (26) in inert solvents gives o-hydroxyazobenzenes via a selective Wallach rearrangement (Scheme 40). 

The formation of the two types of ortho rearrangement products was not observed in the Wallach rearrangement of azoxybenzene derivatives with sulfuric acid. The Wallach rearrangement of perfluoroazoxybenzenes in chlorosulfonic acid has been studied. 2,2 -, 3,3 -, 5,5 - and 6,6 -Octafiuoroazoxybenzene 428 with the reagent at 20 °C afforded the chlorosulfonate ester of 4-hydroxyoctafluor-oazobenzene 429 (Equation 131). 

There appear to be no reports of the treatment of azoxybenzenes with warm excess chlorosulfonic acid which might yield the chlorosulfonyl derivatives of the corresponding hydroxyazobenzenes via the Wallach rearrangement and subsequent chlorosulfonation. 

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