Rearrangements, of azoxybenzenes

Acid catalyzed rearrangement of azoxybenzenes to p-hydroxyazt enzsnes... 

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. 

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

Rearrangement of azoxybenzenes. SbCU and azoxybmzene in CCU form an orange 1 1 complex (9S.57o yield), which when heated for 5 hours at 86-88° rearranges mainly to o-hydroxyazobenzene. Rearrangement of azoxybenzenes-... 

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]. 

TABLE 3.5 Kinetic Data for the Rearrangement of Azoxybenzene To 4-hydroxyazobenzene At 25°C... 

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). 

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). 

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. 

A kinetic stndy of this reaction in media of different acid concentrations (Buncel, 2000) gave the resnlts shown in Table 3.5, which records as well the corresponding X and // valnes. Also given in Table 3.5 are the extents of protonation of azoxybenzene, according to Eqnation B, which are calculated (Buncel, 1975a, b) from the spectrophotometrically determined value, p/f =-5.15, corresponding to 50% protonation of azoxybenzene in 65% H SO (Hg=-5.15). From these resnlts, what can yon deduce about the role of protonation of 1 in its ease of rearrangement and hence on the mechanism of Reaction A ... 

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. 

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... 

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. )... 

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