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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.

Extension of our studies of the generic Wallach rearrangement (of azoxybenzene to 4-hydroxyazobenzene) to the heteroaromatic series (azoxypyridines and axoxypyridine N-oxides) has revealed some dramatic reactivity differences, particularly for the α and β compounds. We have studied the 3-isomers and the 4-isomers in each series, each with α and β forms, eight compounds in all, in the 100 wt% sulfuric acid region of acidity. In those cases in which a product could be observed, the α and β isomers both give the same one, the corresponding 4′-hydroxyazo compounds. All the compounds react much more slowly than does azoxybenzene itself, presumably because of the extra positive charge present in the substrates, but the β isomers have half-lives of seconds and the α isomers half-lives of hundreds of hours in the 100 wt% H2SO4 acidity region. The α compounds have measurable pKBH+ values, but the β compounds do not, exhibiting only a medium effect in the acidity region in which the α compounds protonate. This means that for the β compounds, the protonated intermediates must be much less stable and the postulated reaction intermediates must be much more stable than for the α compounds. To clarify this, we have obtained Mulliken charge distributions for the various species concerned, calculating the charge carried by each half of the molecule, larger charge separations being taken to indicate lesser stability. As far as we can establish, this is the first time that this technique has been used to indicate the stabilities of carbocationic species.Key words: azoxypyridines, azoxypyridine N-oxides, Wallach rearrangement, excess acidity, basicities, theoretical calculations, charge distributions, reactivities.

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.

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.

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 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.

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.

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.