Aromatic azo-compounds. Part VI. The action of light on azoxy-compounds

Photolysis of aromatic azoxy compounds in non-basic solvents in the presence of 2-naphthol leads quite generally to azonaphthols in addition to the normal photorearrangement product. Free diazonium ions are implicated as the precursors of the azonaphthols, both the latter and the usual photorearrangement product being derived from the same excited state of the azoxy compound. Variation of the solvent, light intensity, s~~bstrate concentration, and percent reaction all cause changes in the product distribution. Such changes can be accon~modated by postulating an intermediate which can either afford the normal rearrangement product by loss of a proton to a weak base, or expel a diazonium ion when no base is present. NIGEL J. BUNCE. Can. J. Chem. 55, 383 (19771 La photolyse de composis azoxyaromatiques dans des solvants non-basiques en presence de naphthol-2 conduit generalenlent a la formation d'azonaphtols aux cBtes du produit normal de photorearrangernent. Des ions diazonium libres sont impliques comme precurseurs des azonaphtols; ces derniers OLI de mCme que les produits habituels de rearrangement se formeraient a partir du mCme etat excite du compose azoxy. Des variations dans le solvant, dans l'intensite de la lumiere, dans la concentration du substrat et dans le pourcentage de la reaction causent toujours des changements dans la distribution des produits. On peut accommoder de tels changements en faisant l'hypothese qu'un intermediaire peut soit fournir le produit normal par perte d'un proton vers la base faible soit expulse un ion diazonium quand il n'y a pas debase presente.[Traduit par le journal]

mentioning

confidence: 77%

On the involvement of diazonium ions in the photorearrangement of azoxybenzene

Bunce¹

1977

“…In the case of the dinaphthyl azoxy compounds containing the 1-and/or 2-naphthyl ring systems, rearrangement occurs preferentially to the 4-position of the 1-naphthyl ring if such is available. If such a position is not available, as in the case of 14, then ortho rearrangement to the 1-position of the 2-naphthyl ring system results, yielding the same product as obtained in the photochemically induced rearrangement (15).…”

Section: Discussionmentioning

confidence: 90%

Orientation in the Wallach Rearrangement for the Naphthyl Azoxy Series

Dolenko¹,

Buncel²

1974

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 a-naphthyl, p-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.Le rbarrangement de Wallach, sous catalyse acide, d'azoxybenzenes a Ct C applique a la skrie d'azoxynaphtalene afin d'tvaluer I'effet d'annellation d'anneaux benziniques sur le micanisme du rearrangement. Les six derives azoxy connus, contenant les groupes a-naphtyle, p-naphtyle et phenyle dans des combinaisons differentes, ont e t t CtudiCs en fonction de I'orientation du produit dans des solutions d'acide sulfurique moderement fortes. On discute du processus de rearrangement (eqs. 2-6) en se basant sur les divers mecanismes connus.[Traduit par le journal]Can. J. Chem., 52, 623 (1974) The transformation of azoxybenzene (1) t o H p-hydroxyazobenzene (2) in strongly acidic media 0

“…The choice was made for convenience in preparation and also to avoid the raising of any question of ambiguity in our measurements from the possibility of isomerization of the azoxy group via an oxadiaziridine (16), prior to the photo-Wallach rearrangement. Such a possibility would appear to be ruled out by studies of orientation in the photo-Wallach rearrangement (10,17), but isomerization of 4'-into 4-methoxyazoxybenzene has been reported, implicating the participation of an oxadiaziridine intermediate (17,18), while the possible formation of diphenyloxadiaziridine as an initial side reaction in the photorearrangement of 4 has been proposed (19). Furthermore, Oae and co-workers found that a small amount of scrambling did, in fact, occur in the photo-rearrangement of [l-14C]4.…”

Section: Introductionmentioning

confidence: 99%

“…In order to specify particular extents of conversion, the disappearance of azoxy compound and the formation of product were first monitored spectroscopically with irradiations of unlabeled substrate. The formation of 2-hydroxyazobenzene (6) was monitored at 410nn-1, while the formation of 1-hydroxy-2,2'-azonaphthalene (10) was monitored at 505 nm. Solvents for photo-rearrangement were 95% ethanol for 4 and 1,2-dimethoxyethane (DME) for 8.…”

Section: Photo-rearrangementsmentioning

confidence: 99%

The photo-Wallach rearrangement. Heavy-atom kinetic isotope effects and mechanism

Shine¹,

Subotkowski²,

Gruszecka³

1986

The photo-rearrangement of mixtures of azoxybenzene 4 and, successively, [15~,15N']4, [180]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) Introduction In recent years sporadic attempts have been made to detect intermediates in photochemical reactions in solution by measuring heavy-atom (other than hydrogen isotopes) kinetic isotope effects (KIE). The basis for this approach was expressed by Schutte and Havinga in 1967, who used the photo-Fries rearrangement to try to answer "the question that has been raised whether photochemical reactions after electron excitation pass through a transition state of appreciable energy of activation" (1). Schutte and Havinga used 4-metho~~phenyl-['~~]acetate for measuring the carbon KIE in the photorearrangement of the ester into 2-acetyl-4-methoxyphenol, and found that within experimental error the rearrangement did not exhibit an isotope effect. They concluded, therefore, that rearrangement began in the electronically excited state (singlet) and proceeded very rapidly through vibrationally excited states in a process that required no thermal energy of activation.The quest was later pursued by Kwart and co-workers (2), who sought evidence for a vibrationally excited ground state intermediate in the photochemical rearrangement of the ylide 1 into the diazepine 3. In this case an inverse carbon KIE was found with the use of specifically labeled [13c]1, and the result was attributed to the formation of the intermediate 2 in the pathway from excited 1 to product (3) (reaction [I]). To our knowledge this is the first time in which a heavy-atom KIE has been found in a photochemical rearrangement.