Rearrangements of aryl hydroxylamines

Rearrangement of Aryl Hydroxylamines 1 / C-Hydro-5 / N-hydroxy-interchange... 

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... 

The aryl azides are very reactive compounds. By the action of acids, for example, they lose the two terminal N-atoms as N2 the residual C8H5N< combines with water to form aryl hydroxylamine, but this at once undergoes rearrangement and becomes an aminophenol. 

Saczewski and Debowski reported a l-thia-3-oxa-4-aza-Cope rearrangement of the adduct 163 formed when the A-aryl-Af-(4,5-dihydro-177-imidazol-2-yl)-hydroxylamine 162 reacted with carbon disulfide in the presence of triethylamine at room temperature (equation 48). The intermediate 164 formed after the [3,3] rearrangement lost a fragment and after successive or simultaneous rearomatization afforded a thiophenol derivative that provided 165. 

Thermal hetero Cope [3,3] - rearrangement of O-arylated oximes (Sheradsky) or acid catalyzed anionic hetero [3,3] and [3,5] - rearrangement of hydroxylamines with N-O bond cleavage.(see 1st edition). 

With secondary amines, R2N-H, it appears that similar oxidation initially results in formation of the A -oxide, and this is often followed by a tautomeric rearrangement to generate the corresponding iV,A -disubstituted hydroxylamine (Equation 10.30). However, with primary amines the product mixture is generally not easily analyzed and extensive decomposition (except in the case of aryl amines such as aminobenzene [aniline], vide infra) may intrude (although, as shown in Scheme 10.4, it is common to find some hydroxamic add [genetically, RCONH(OH)] among the products). 

Saczewski and colleagues reported a similar pathway when the A-aryl-A-(4,5-dihydro-l//-imidazol-2-yl)hydroxylamine hydrochlorides 37 were refluxed in acetone in the presence of acetylenic derivatives 38 carrying electron-withdrawing substituents (equation 10). However, when 37 free bases were used, the adducts rearranged at room temperature, producing only the ketene aminals 39 in poor yields (equation 11). 

Acyl substituents at the 3- and/or 4-positions result in decreased hydrolytic stability compared with the alkyl and aryl derivatives described above. Despite this constraint most of the usual reactions of the carbonyl group are possible. Aldehydes <9ILA1211> and ketones are oxidized to the carboxylic acid, borohydride reduction affords the expected alcohols, and epoxides are formed on reaction with diazomethane. Oximes and arylhydrazones are formed with hydroxylamine and arylhydrazines, and the products may subsequently undergo monocyclic rearrangement involving the oxadiazole to give the corresponding isomeric furazans and 1,2,3-triazoles (Section 4.05.5.1.4). 

Amination of 5-amino-6-aryl-l,2,4-triazine 4-oxides 133 with hydroxylamine hydrochloride or O-methylhydroxyl-amine in DMF results in 5-hydroxylamino-6-aryl-l,2,4-triazines 137 in 70-80% yields, thus indicating that the Dimroth rearrangement is involved (Scheme 72) <2000TL7379>. 

---