Phenylhydroxylamines rearrangements involving

O-Substituted phenylhydroxylamines also undergo rearrangement to give the 2-isomers. For example O-(arenesulphonyl) phenylhydroxylamines 47 readily form the 2-sulphonyl derivatives 48. Experiments with 180-labelled compounds led to the suggestion54 of a mechanism involving an ion pair which has only a very short lifetime. 

Electrolytic reduction, as in the preparation of hydrazobenzene from nitrobenzene, or of p-aminophenol from nitrobenzene. The latter reduction involves a simultaneous rearrangement of the intermediate phenylhydroxylamine. 

Blechert has developed an interesting synthesis of 2-substituted indoles which involves the conjugate addition of V-phenylhydroxylamine salts (or V-phenylnitrones) to electron-deficient allenes, followed by carbanion-accelerated hetero-Cc rearrangement of the Michael adduct. For exanple, addition of the hydroxylamine salt (46) to the allenyl sulfone (47) produces the anion (48), which undergoes rapid 3.3-sigmatropic rearrangement to afford the -keto sulfone (49). Cyclization to the indole proceeds smoothly upon exposure to formic acid (Scheme 3). 

While most of the information regarding this reaction is well documented in patents, very little has been published in the open literature (6-8). The commonly accepted reaction mechanism involves the partial hydrogenation of nitrobenzene to give phenylhydroxylamine (PHA) which desorbs from the catalyst into the aqueous phase where it is converted to p-aminophenol (PAP) through an acid catalyzed rearrangement (8-12) (Scheme 1). Some of the PHA is not desorbed and is hydrogenated to aniline. 

Blechert et developed a multicomponent cascade reaction for the synthesis of indole derivatives as depicted in Scheme 1.8.5.17. The first step of the sequence involves formation of a nitrone derivative starting from phenylhydroxylamines and aldehydes. The resulting nitrones were not isolated but captured by a cyanoallene in a 1,3-dipolar cycloaddition reaction followed by hetero-Cope rearrangement, ring-opening and condensation to yield an indole derivative. 

Kohnstam, G., W. A. Fetch and D. L. H. Williams. 1984. Kinetic substituent and isotope effects in the acid-catalyzed rearrangement of N-phenylhydroxylamines. Are nitrenium ions involved J. Chem. Soc. Perkin Trans. II 1984 423-427. 

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