Arylhydroxylamines rearrangement

The change which arylhydroxylamines undergo by the action of mineral acids, especially when warm, is worthy of special note. If the position para to the NHOH-group is free, a rearrangement takes place to the isomeric p-aminophenol, e.g. in the case of phenylhydroxylamine according to the equation ... 

The A-acyl-O-arylhydroxylamine 166 rearranges in the presence of trifluoroacetic acid to 168 by a [3,3]-sigmatropic shift. An isourea intermediate 167 was proposed to explain the observed transformation (equation 49). 

In 1894 Bamberger began reporting on the rearrangement that bears his name. In its simplest form (Scheme 2) the Bamberger rearrangement is the conversion of an N-arylhydroxylamine (3) into the isomeric para-amino-... 

The decomposition of aryl azides in aqueous acid was known to produce para-aminophenols even before Bamberger began his work on the rearrangement of N-arylhydroxylamines. Bamberger recognized that N-arylhydroxyl-amines and aryl azides produced identical product mixtures when decomposed under the same conditions in aqueous acid, and he proposed a... 

N-Substituted arylhydroxylamines add to methyl propynoate and rearrangement occurs to give indolc-3-carboxylate esters[3]. With unsubstituted... 

High yields of 3-aminocoumarins (390) are formed rapidly when solutions of dimethyl butynedioate and O-arylhydroxylamines are mixed at low temperature (72TL3941). Their formation has been rationalized through a rearrangement of the substituted hydroxylamines (389). 

Friedlander - assumed that the phenols arose by way of a rearrangement of a hydroxylamine which was formed by the reaction of water with a nitrene. Bamberger compared the behaviour of arylhydroxylamines and aryl azides under similar conditions and concluded that hydroxylamines are not intermediates in the acid-catalysed decompositions of aryl azides. He proposed instead that with phenyl azide, for example, the initially formed nitrene was... 

Bamberger , were acid-catalysed, since many were carried out, at or near, room temperature or on a water-bath. For example, 2,4-xylyl azide was decomposed in sulphuric acid-ethanol medium at a temperature <35° , or in a 1 2 mixture of sulphuric acid and water at 65° , / -Tolyl azide was decomposed in concentrated sulphuric acid at —20° . On the other hand, it was also decomposed under conditions in which some thermolysis may well have been occurring, i.e. in a 1 3 (v/v) mixture of boiling sulphuric acid-water . More quantitative data are needed in this area, and kinetic data along with activation parameters would aid in clarifying the mode of decomposition. This type of data has been obtained for the acid-catalysed decomposition of a similar system, the arylhydroxylamines, and has helped in establishing that this is a nucleophilic intermolecular rearrangement . 

Alkyl nitroarenes may on reduction lose a proton from the side chain either during the acid-catalyzed rearrangement of the arylhydroxylamine or from the parent compound, from which an electrogenerated base (EGB) may abstract one of the relatively acidic protons a. to the nitroaryl. 

Aromatic hydroxylamines are generally prepared by chemical reduction or selective hydrogenation of aromatic nitrocompounds by using metal catalysts promoted with dimethylsulfoxide. However, such methods of synthesis are characterized by difficult products purification and low yields /1,2/. The low cost production of arylhydroxylamines can be of great practical interest because these compounds can undergo rearrangement to yield a variety of important chemicals 111. 

Fishbein, J. C., McClelland, R. A., Halide Ion Trapping of Nitrenium Ions Formed in the Bamberger Rearrangement of N Arylhydroxylamines. Lifetime of the Parent Phenylnitrenium Ion in Water, Can. J. Chem. 1996, 74, 1321 1328. 

Arylhydroxylamines react at low temperatures with DMAD and a rearrangement occurs to give high yields of 3-aminocoumarins. The yield is much lower when R is an electron-withdrawing group. 

BAMBERGER Phenylhydroxylamine Rearrangement Rearrangement of N-arylhydroxylamine to aminoptienol. 

Amount of Acid Used. This reaction requires the presence of small amounts of ferrous ion to act as a catalyst. Generally about 0.05-0.2 of an equivalent of acid is used. The acids usually employed in the reduction process are hydrochloric and sulfuric. It should be borne in mind that hydrochloric acid sometimes causes the formation of small amounts of chlorinated amines whereas sulfuric acid may rearrange the intermediate arylhydroxylamines to hydroxyarylamines and cause the formation of darker amines in lower yields, particularly in the case of solid amines. Where the danger of hydrolysis or contamination by such products exists, acetic or formic acid is employed instead. The disadvantages in the use of sulfuric acid appear to be minimized when the sulfuric acid is introduced as sodium acid sulfate (niter cake). When used alone or preferably in conjunction with a calculated quantity of sodium chloride, a very economical and satisfactory promoter is obtained. For example, 2.4 lb of sodium chloride with 6 lb of niter cake per 100 lb of nitro compound gives satisfactory results. The niter cake is first ground and is added, along with the sodium chloride, to the water and finely divided iron in the reducer. 

Scheme 14.10 Rearrangements of O-vinyl-jV-arylhydroxylamines for the preparation of interrupted Fischer-indole intermediates.—...

The Bartoli-indole synthesis utilizes the [3,3]-rearrangement of an 0-vinyl-iV-arylhydroxylamine intermediate to form indoles through a similar rearrangement and condensation process as the Fischer-indole synthesis f Scheme 14.7a). The rearrangement precursor 59 is generated from the addition of a vinyl Grignard reagent to a nitroarene. The... 

Rearrangements related to the Bartoli-indole synthesis have also been designed for the preparation of ortho-alkylated anilides and oxindoles from O-acetyl-JV-arylhydroxylamines. 

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