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For the first time, the potential of acidic cation-exchange resin (sulfonated polystyrene) to catalyze the Wallach rearrangement of azoxybenzene into 4-hydroxyazobenzene has been proved. This finding reveals an alternative reaction path possible in a heterogeneous process using solid acids and may help to clear some doubts concerning the rearrangement mechanism postulated so far. The resin-induced reaction was found to proceed exclusively in a non-polar medium. Reasonable yield was obtained particularly in isooctane due to favorable distribution of azoxybenzene throughout the resin's matrix. On the contrary, the HY type zeolite did not activate the rearrangement, most probably because of steric hindrance. Experimental results favor the hypothesis of a quinoid intermediate controlling the chemistry of azoxybenzene conversion, which also follows from thermodynamic considerations involving DFT calculations.

Ab initio calculations have been performed on several different structures for the dicationic intermediate proposed for the Wallach rearrangement of aromatic azoxy compounds to hydroxy-substituted azo systems in strongly acidic media. For the unsubstituted parent compound azoxybenzene, these calculations reveal that the preferred structure for the intermediate is planar, as previously assumed, but bent rather than linear as we have formulated it. The presence of two methyl groups at the para positions of both aromatic rings does not change this situation, but six methyl groups at all para and ortho ring positions lead to a different preferred structure — still bent but with the two aromatic rings now at 90° to one another rather than being coplanar — undoubtedly due to steric interference between the ortho methyl groups. In all the cases examined the two positive charges reside primarily in the aromatic rings rather than on the nitrogens, which are sp2 hybridized and still have their lone pairs. The overall structures can best be regarded as two six-electron π systems joined together, with little communication between the two rings. For the most part the calculations are in good agreement with experimental observations. Recent calculations on other possible reaction intermediates by other groups are also discussed.Key words: Ab initio calculations, Wallach rearrangement, azoxyarenes, reaction intermediate, acid catalysis, intermediate structure, reaction mechanism.

When the starting materials 4-acetyl-ONN- and -NNO-azoxybenzene(1α and 1β, respectively) were heated in 70/80% sulfuric acid, α/β isomerization and the Wallach rearrangement to 4-acetyl-4′-hydroxyazobenzene (2) were observed. However, neither the o-hydroxyazobenzene nor 4-acetyl azobenzene was detected in the reaction product. The isomerization of 1α was faster than that of 1β, demonstrating the higher themodynamic stability of the latter.

Acidic zeolites HY and CaY catalyse the Wallach rearrangement of azoxybenzene ( I ) leading mainly to the formation of para -hydroxyazobenzene ( II ) and ortho -hydroxyazobenzene ( III ). In this transformation, increasing the loading level of I results in the formation of a larger amount of para -isomer. As observed in isotropic media, photochemical Wallach rearrangement in the presence of various cation-exchanged faujasites results in the predominant formation of the ortho -isomer from the S 1 state and this rules out any appreciable heavy atom effect.

Product studies on the oxidation of two 1-arylazo-2-hydroxynaphthalene-6-sulfonate dyes by peroxy acids and by tert-butyl hydroperoxide catalysed by iron(III) 5,10,15,20-tetra(2,6-dichloro-2-sulfonatophenyl)porphyrin in solution have been carried out. In single turnover experiments at pH 11.88, 9.30 and 6.93, dye bleaching by peroxy acids is a two-step process; an initial fast reaction, during which the oxoiron(IV) porphyrin is also formed, is followed by a slow phase involving the regeneration of the iron(III) porphyrin from the oxoiron(IV) porphyrin. The peroxy acid∶dye stoichiometry is 1∶1 for the initial fast dye bleaching and 2∶1 for the slow phase; the latter is equivalent to a 4∶1 stoichiometry of oxoiron(IV) porphyrin to dye. In multiple turnover reactions the dye bleaching occurs in two sequential oxidations by oxoiron(IV) porphyrin π-cation radical, each with a peroxy acid∶substrate stoichiometry of 1∶1.Evidence is presented to show that, in multi-turnover reactions at pH 11.88 where the substrate is the dye anion, the initial product arises from the hydroxylation of the 1-position of the naphthol ring which in the second step is oxidised further to give an azoxy compound. At pH 9.30, where the dyes exist predominantly as the hydrazone tautomers, the initial oxidation generates an azoxy compound which following a Wallach rearrangement is further oxidised on the naphthol ring. More extensive oxidation results in the cleavage of the N–N bond.The iron(III) porphyrin-catalysed dye oxidation with excess of tert-butyl hydroperoxide at pH 6.93, which occurs by hydrogen atom-abstraction, has a hydroperoxide∶dye stoichiometry approaching 2∶1 and gives the same azoxy product as obtained at pH 11.88 with the peroxyacids.Mechanisms to account for the observed products and stoichiometries are discussed.

Treatment of azoxybenzene and its derivatives with acids is known to result in the Wallach rearrangement, which leads to 2- or4-hydroxyazobenzenes. Starting in the 1960s, experimental findings have lead to the proposal of several mechanisms for this rearrangement. In this work, molecular orbital theory employing the semiempirical AM1 method is used to locate and discuss the energetics of the intermediates and the transition states for this rearrangement. Based on the results of AM1 calculations in vacuum and in solution, the most plausible mechanistic pathways are proposed and discussed.

The Jencks principle, that postulated mechanistic intermediates have to have a finite lifetime in the reaction medium, has in general not been applied to the mechanisms of organic reactions. In particular, oxygen-protonated species in which the positive charge cannot be delocalized, such as H3O+, R2HO+ and tetrahedral intermediates, and even some of those where it can, such as acylium ions and protonated esters, do not exist in aqueous media that are more dilute than concentrated acid. Nor do primary and secondary carbocations. This has considerable consequences; many accepted organic reaction mechanisms have to be modified. Examples of this are provided, particularly for reactions that take place in acidic solutions. For instance, ether hydrolysis is a general-acid-catalyzed process in which an oxygen-protonated species is not formed, and nor is a carbocation unless it is stable in the medium. Amide hydrolysis involves a second proton transfer in stronger acid media. Ester hydrolysis involves tetrahedral intermediates which are neutral, not charged, whether the medium is acidic, neutral or basic. Many other reactions are discussed.

A series of para-substituted azoxybenzenes was obtained as model compounds for the investigation on the mechanism of Wallach rearrangement. Oxidation ofazobenzenes with hydrogen peroxide in acetic acid solution, provided mixtures of a and β isomers. Some couples of the products were separated, using chromatography and crystallization techniques, and identified on the basis of their carbon NMR and mass spectra. The SCSD algorithm can be applied to the interpretation of the 1 3 C-NMR spectra. Recognition of the ipso and para carbons suffice to the identification of an isomer, hence there are no strong interactions between the azoxy group and another substituent across the aromatic ring. Fragmentation of azoxybenzenes under electron impact occurs preferentially on the oxidized side of the azoxy bridge. Relative intensities of daughter ions differentiate the 0NN = a and NNO = βisomers in most cases.

1 Introduction 2 Rearrangement of Hydrazobenzenes (The Benzidine Rearrangement) 3 Rearrangement of Azoxybenzenes (The Wallach Rearrangement) 4 Rearrangement Involving Phenylhydroxylamines 5 Rearrangement of N-Halo Compounds 6 Rearrangement Involving Nitro Groups 7 Rearrangement Involving Nitroso Groups Keywords: hydrazobenzene rearrangement; rearrangement reactions and amino, nitro and nitroso groups; quinamine rearrangement; azoxybenzene (Wallach rearrangement) rearrangement; rearrangement involving phenylhydroxylamines; Bamberger rearrangement; Orton rearrangement of N-chloroanilides; rearrangement and nitroso groups - Fischer Hepp rearrangement

The crystal structure of the title compound (common name: trans-azoxybenzene, t-AXB), C12H10N2O, was determined at 299 ± 0.5 K, i.e. 6 K below its melting point. The az­oxy group has the expected geometry, and the O atom is affected by libration normal to the N=N—O plane. The O atom is disordered over two positions; the site-occupancy ratio is ∼3:2. Phenyl rings are out of the NNO plane, with dihedral angles ranging from 20 (1) to 34 (1)°. The dihedral angle between the two phenyl rings is 46.19 (11)°. This geometry contrasts strongly with the gas-phase conformation of t-AXB, previously determined by electron diffraction combined with ab initio calculations, which showed that the stable conformation is planar. This discrepancy is, however, consistent with low rotational barriers for phenyl rings. These features could be of inter­est in relation to the actual reaction mechanism of the Wallach rearrangement (acid-catalyzed isomerization of t-AXB into 4-hydroxy­azobenzene), which remains a controversial matter.

The major product in the reaction of 4-alkylazoxybenzenes with sulfuric acid is not 4-(p-alkylphenylazo)phenol, which is regarded as the usual product of Wallach rearrangement. The following azophenols were obtained as major products; 2-alkyl-5-(phenylazo)phenol (R : Me), 2-alkyl-4-(phenylazo)phenol (R : Et), and 4-(phenylazo)phenol (R : i-Pr and t-Bu).

Treatment of 4,4′-dihaloazoxybenzenes with sulfuric acid afforded 4,4′-dihaloazobenzenes (6) as the major product and 5-halo-2-(p-halophenylazo)phenols, the Wallach rearrangement products as minor products, besides 2-halo-5-(p-halophenylazo)phenols, 2-halo-4-(p-halophenylazo)phenols, and 4-(p-halophenylazo)phenols as abnormal rearrangement products. The reaction of fluoro compound gave the azophenols (3a or 5a) in the best yields. However, the ratio of 3a to 5a was found to vary with the concentration of sulfuric acid. Treatment of 4-haloazoxybenzenes with sulfuric acid gave 4-(p-halophenylazo)phenols and 4-haloazobenzenes (13) as major products together with 2-(p-halophenylazo)phenols and 5-halo-2-(phenylazo)phenols as minor products. Yields of reduction products 6 and 13 increased as the halosubstituent became heavier in the following sequence: F→Cl→Br→I.

The reactions of 4,4′-dialkylazoxybenzenes (1) with sulfuric acid afforded 2-alkyl-5-(p-alkylphenylazo)phenols (2) and 2-alkyl-4-(p-alkylphenylazo)phenols (3) as novel rearrangement products; in certain cases 4-(p-alkylphenylazo)phenols (4) were also obtained. The ratios of these azophenols were greatly affected by the alkyl group type. In addition, the reactions gave a very small amount of 5-alkyl-2-(p-alkylphenylazo)phenols (5), which were considered as normal Wallach rearrangement products.

In an extension of Wallach rearrangement studies into the phenylazoxypyridine series, an investigation of 4-, 3-, and 2-phenylazoxypyridines, the N-oxides, and methiodides is reported. Oxidation of 4- and 3-phenylazopyridine with peracetic acid gives rise to the α and β phenylazoxypyridine-N-oxides, contrary to previous literature reports on the obtention solely of the α isomers. 2-Phenylazopyridine, however, yields only the 2-(phenyl-α-azoxy)pyridine-N-oxide. These results are rationalized on the basis of field, resonance, and steric effects. An unprecedented reactivity difference has been observed in the reactions of the α,β isomers of phenylazoxypyridines under conditions of the Wallach rearrangement. This reactivity difference permits the isolation of the α-azoxy isomers from the α,β mixtures. Unequivocal confirmation of the structures has been obtained from X-ray crystal structure determinations of two representative compounds in this series, viz. 4-(phenyl-β-azoxy)pyridine-N-oxide (11) and 4-(phenyl-α-azoxy)pyridinium methiodide (12), which itself was formed by deoxygenation of 4-(phenyl-α-azoxy)pyridine-N-oxide, followed by methylation with methyl iodide. The crystal structure of 11 was solved by direct methods and refined by block-diagonal least squares to R = 0.041 for 2479 observed reflections. The asymmetric unit contains two independent molecules, both of which are planar. The structure of 12 was determined by the heavy-atom method and refined by full-matrix least squares to R = 0.043 for 1718 observed reflections. The molecules are not planar, the pyridine ring being rotated by 36.5° from the phenylazoxy plane. Evidence is presented for the existence of intramolecular [Formula: see text] and [Formula: see text] bonds in crystal structures of trans-azoxyarenes. The carbon atoms involved in these hydrogen bonds are ortho to the azoxy group and can act as proton donors as a result of the inductive effect of the adjacent nitrogen.

The photo-rearrangement of mixtures of azoxybenzene 4 and, successively, [15N, 15N′]4, [18O]4, and [2-14C]4 were carried out. Kinetic isotope effects (KIE) were calculated from measurements of isotopic ratios in both recovered 4 and the product, 2-hydroxyazobenzene (6). Analogous rearrangement of mixtures of 2,2′-azoxynaphthalene (8) with [15N, 15N′]8 and [1,1′-13C2]8 were carried out and KIE were calculated from isotope ratios in the product. The results (particularly the lack of nitrogen KIE) collectively indicate that if an oxadiazole-like intermediate is involved in these rearrangements, an activation barrier exists in its formation rather than its decomposition.

The present study discusses the behaviour of fluorinated azoxybenzenes in strong acids. 2,2′,3,3′,5,5′,6,6′-Octafluoro- azoxybenzene I reacted with chlorosulphonic acid giving the Wallach rearrangement product, the chlorosulphonate ester of 4-hydroxy-2,2′,3,3′,5,5′,6,6′-octafluoroazobenzene. In HF, H 2 SO 4 and HSO 3 F compound I was stable, whereas in SbF 5 -HSO 3 F (1:1) at 20 °C, it gave quantitatively the reduction product, 2,2′,3,3′,5,5′,6,6′-octafluoroazobenzene. Using 1 H, 13 C, 15 N and 19 F NMR, we studied cationoid species generated from fluorinated azoxybenzenes by treatment with strong acids. The role of these species in the Wallach rearrangement and formation of azobenzenes has been examined. In HSO 3 Cl, cleavage of the CN bond takes place, leading, e.g. in the case of decafluoroazoxybenzene, to pentafluorochlorobenzene and the pentafluorophenyldiazonium cation. A route for this reaction is suggested.

Data concerning the use of various acids to effect the Wallach rearrangement in the series of azoxybenzenes are analysed and the influence of the chemical nature and position of the substituents on the reactions of azoxybenzenes in the presence of acids, including the Wallach rearrangement and side processes, are examined. Data obtained in the study of the mechanism of the Wallach rearrangement by kinetic and tracer atom methods are discussed and the effect of Lewis acids and UV radiation on azoxybenzenes is considered. The bibliography includes 63 references.

When an equimolar solution of various azoxybenzenes and SbCl 5 in carbon tetrachloride were mixed, a 1:1 complex immediately deposited as orange crystals in high yield. The thermal reaction of these complexes in inert solvents gives o -hydroxy-azobenzene selectively. On the contrary, other Lewis acids such as TiCl 4 , AlCl 3 , FeCl 3 and ZnBr 2 failed to give an isolatable complex with azoxybenzene, and their direct thermal reaction with azoxybenzene resulted in deoxygenation to yield azobenzene as a main product.

The acid-catalyzed Wallach rearrangement of azoxybenzenes has been extended to the naphthyl azoxy series in order to evaluate the effect of annelation of benzene rings on the course of rearrangement. The six known azoxy derivatives containing the α-naphthyl, β-naphthyl, and phenyl moieties, in various combinations, have been examined with respect to product orientation in moderately strong sulfuric acid solutions. The course of rearrangement (eqs. 2–6) is discussed on the basis of current mechanisms.

The Wallach rearrangement of 3-chloro-3′-methyl- and 3-methyl-3′-nitro-azoxybenzene gave rise to the corresponding 4′-hydroxyazo derivatives with the hydroxyl group attached to the benzene ring adjacent to the trivalent nitrogen bridge, while 2,2′,5,5′-tetrachloro-4′-nitro- and 2′-nitro-4,4′,5,5′-tetrachloro-azoxybenzene were hydroxylated in the ortho and para positions respectively on the ring adjacent to the pentavalent nitrogen bridge.Oxidation of 3-methyl-3′-nitro-and 3-chloro-3′-methyl-azobenzene gave rise to the corresponding α-isomers, which was proved by the bromination and reduction products.The ultraviolet and visible absorption spectra of the unsymmetrically and symmetrically substituted azo- and azoxy-benzenes prepared have been recorded and briefly discussed.

The rearrangement reaction of azoxybenzenes into hydroxyazobenzenes in strongly acidic solution has been studied by an U. V. spectrophotometric method and by isolation of the rearranged compound. In all cases under investigation, it appeared that the oxygen atom in the azoxy group migrated to the unsubstituted ring, depending neither on the substituent already present in the other ring, nor on the distance between the oxygen atom and the eligible position; whereas, the position in the open ring, ortho or para, to which the oxygen migrates depends on the substituent already present in the other ring. In all compounds besides -and -4-methyl azoxybenzene, the oxygen atom migrates to the para position. In the case of and -4-methylazoxybenzene, the oxygen atom migrates to the ortho position of the unsubstituted ring.

Study of the acid-catalyzed Wallach rearrangement of azoxybenzene is extended into the 100% H2SO4 region. The rate of formation of 4-hydroxyazobenzene can be followed spectrally in a straight-forward manner until close to 99% H2SO4, but in higher acidities sulfonation of this product becomes kinetically important. The advent of second equilibrium protonations of 4-hydroxyazobenzene and of 4-hydroxyazobenzene-4′-slfonic acid further complicate the azoxybenzene rearrangement as followed spectrally. Above 100% H2SO4 a second sulfonation is also observed. A method is given for dissecting the rate data for the primary rearrangement process from the first of the sulfonation reactions.The rate of the azoxybenzene rearrangement is observed to increase continuously up to 99.99% H2SO4 (the upper limit of the present kinetic method). This suggests that the second proton transfer step to azoxybenzene is rate-determining and not an equilibrium process. These results permit a clarification of a previously proposed mechanism (1).

The rate of rearrangement of azoxybenzene to p-hydroxyazobenzene has been measured in 75.3–96.4% sulfuric acid at 25° and in 65.0–90.4% sulfuric acid at 75.5° by spectrophotometric methods. The pKa of azoxybenzene in aqueous sulfuric acid has also been determined. It is found that although azoxybesssnzene is almost completely protonated over the entire range of acid concentration studied, the rate increases by more than 1 000-fold. A two-proton process is therefore indicated and mechanisms are proposed involving a dication (II) as the key intermediate. The rate data do not allow differentiation between two proposed mechanisms, one involving two equilibrium protonations, and the other a single equilibrium protonation followed by rate-determining proton transfer. Past mechanisms of the Wallach rearrangement are discussed.

An azoxy compound can be treated with concentrated sulphuric acid to produce a nydroxyazo product consisting chiefly of the para isomer. This reaction, the Wallach rearrangement, is the basis for a suggested generic test for an azoxy compound. For the identification of a given azoxy compound, two derivatives are suggested: the hydrazo compound produced by the reduction of the azoxy compound by zinc dust, and the azo compound produced by the oxidation of the hydrazo compound. These compounds are described for the first time: 2,2′-dichloro-4-hydroxyazobenzene, 2,5′,2,5′-tetrachloro-4-hydroxyazobenzene, 2,2′-dimethoxy-5,5′-dichloroazobenzene, 3,3′,4,4′-tetrachlorohydrazobenzene, 2,2′-dimethyl-3,3′-dichlorohydrazobenzene, and 2,2′-dimethoxy-5,5′-dichlorohydrazo-benzene.

In relation to the intermediacy of azobenzene-4-hydrogen sulfate in the Wallach rearrangement of azoxybenzene, the potassium salt has been characterized and subjected to examination under acidic conditions. A pKa value of −2.14 has been obtained, for N-protonation. The sulfate salt hydrolyzes to p-hydroxyazobenzene in acid media and the rate of the reaction has been measured over the region 22–42% H2SO4 at 25° spectrophotometrically. A correlation between rate and acidity function indicates that a two-proton process is involved; a reactive species is proposed having a nitrogen and the phenolic oxygen protonated. The relevance of these findings to Wallach rearrangement studies is discussed.

Under the reaction conditions for the Wallach rearrangement, 4-methyl-ONN-(1α), 4-methyl-NNO-(1β), 4-nitro-ONN-(2α), and 4-nitro-NNO-azoxybenzene (2β) were found to undergo αβ-interconversion, while 4-bromo-ONN-(3α) and 4-bromo-NNO-azoxybenzene (3β) do not. The αβ-interconversion occurs with both mono- and di-protonated forms of the azoxybenzenes.

Naphthalene-1-ONN-azoxybenzenes and some unsymmetrically ortho-substituted ONN- and NNO-azoxybenzenes were prepared, and their photochemical behavior in ethanol was compared. The crowded azoxyarenes tend to undergo a facile Wallach rearrangement via the known azoxy-ortho oxygen mirgation and a concurrent pathway involving ONN–NNO isomerization. The relative contribution of the two pathways was affected by substituents.

In the Wallach rearrangement of α-p-methylazoxybenzene with concentrated sulfuric acid as a catalyst, 2-hydroxy-4-methylazobenzene and 4-hydroxy-4'-methylazobenzene were formed as rearrangement products. The ο-isomer, 2-hydroxy-4'- methylazobenzene, has been recognized in the previous research.It is interesting that the yield of the p-isomer decreased and that of the ο-isomer increased as the concentration of sulfuric acid approached 100%.When α-p-methylazoxybenzene is irradiated by ultraviolet ray in various solvents, we obtained 2-hydroxy-4-methylazobenzene. In addition, it bas deen reported that 2-hydroxy-4'-methylazobenzene was formed by photo-chemical rearrangement.

Die Umsetzung von Azoxybenzolen mit Schwefelsaure oder anderen Protonensauren wie Chlorsulfonsaure oder Polyphosphorsaure liefert neben den Azobenzolen 0- und p-Hydroxyazobenzole.

Treatment of 4,4′-dihaloazoxybenzenes with sulfuric acid afforded 4,4′-dihaloazobenzenes (6) as the major product and 5-halo-2-(p-halophenylazo)phenols, the Wallach rearrangement products as minor products, besides 2-halo-5-(p-halophenylazo)phenols, 2-halo-4-(p-halophenylazo)phenols, and 4-(p-halophenylazo)phenols as abnormal rearrangement products. The reaction of fluoro compound gave the azophenols (3a or 5a) in the best yields. However, the ratio of 3a to 5a was found to vary with the concentration of sulfuric acid. Treatment of 4-haloazoxybenzenes with sulfuric acid gave 4-(p-halophenylazo)phenols and 4-haloazobenzenes (13) as major products together with 2-(p-halophenylazo)phenols and 5-halo-2-(phenylazo)phenols as minor products. Yields of reduction products 6 and 13 increased as the halosubstituent became heavier in the following sequence: F→Cl→Br→I.

The major product in the reaction of 4-alkylazoxybenzenes with sulfuric acid is not 4-(p-alkylphenylazo)phenol, which is regarded as the usual product of Wallach rearrangement. The following azophenols were obtained as major products; 2-alkyl-5-(phenylazo)phenol (R : Me), 2-alkyl-4-(phenylazo)phenol (R : Et), and 4-(phenylazo)phenol (R : i-Pr and t-Bu).

The treatment of azoxybenzenes bearing electron-withdrawing substituents at 4,4′-positions with sulfuric acid gave the corresponding 2-hydroxyazobenzenes, together with 4′-substituted 4-hydroxyazobenzenes. The treatment of 4,4′-dimethoxyazoxybenzene with p-toluenesulfonic acid and acetic anhydride afforded 4,4′-dimethoxy-3-acetoxyazobenzene and 4,4′-dimethoxy-2-tosyloxyazobenzene as the main rearrangement products.