Phenylhydroxylamine, rearrangement

Phenylhydroxylamine rearranges in sulfuric acid to give mainly p-aminophenol. Industrial routes to this compound have been developed in which phenylhydroxylamine, formed by hydrogenation of nitrobenzene in sulfuric acid over platinum-on-carbon, is rearranged as it is formed. Conditions are adjusted so that the rate of rearran ment is high relative to the rate of hydrogenation of hydroxylamine to aniline (15,17,86). An easy way to obtain a favorable rate ratio is to carry out the reduction with about 1% DMSO present in the sulfuric acid (79,81). 

Phenylhydroxylamine rearranges rapidly in sulphuric acid at medium temperatures to give 4-aminophenol 5. Formation and rearrangement of the phenylhy-... 

BAMBERGER Phenylhydroxylamine Rearrangement Rearrangement of N-arylhydroxylamine to aminoptienol. 

Partial rate factor 251 Pauli principle 4 p-Phenylenediamine, basicity 183 Phenylhydroxylamine rearrangement 638... 

N-substituted phenylhydroxylamine derivatives, e.g. N-acetyl and N-sulphonic acid, also form the para-aminophenol", but more bulky groups prevent reaction, it is thought by steric hindrance to the approach of the hydronium ion. -substituted phenylhydroxylamines, on the other hand form only the ortho product, it is thought via an intramolecular rearrangement, e.g. 

A rearrangement reaction which has been much studied, and in many ways resembles the reaction of phenylhydroxylamines, is the Wallach rearrangement111 of azoxybenzene (LXVI) to give para hydroxyazobenzene (LXVII), viz. 

Heesing and coworkers have reported the rearrangement of 0-alkylsulfinyl-iV-benzoyl-iV-phenylhydroxylamine (143) at — 70°C to the corresponding sulfonamide together with the o- and p-alkylsulfonyl derivatives 144 and 145 (equation 89). The reaction has been suggested to proceed by an intramolecular radical pair mechanism, as evidenced by experiments with oxygen-18 labeling and C-CIDNP effects. 

On the other hand, phenylhydroxylamines undergo some interesting reactions that are commercially useful. For example they rearrange (Fig. 2.34) and undergo condensations into products such as p-aminodiphenylamine278,279 and others that are useful as antioxidants, antiozonants, dyes, medicinals, and resins. 

The corresponding reaction of diethyl 1,2-propadienyl phosphonates (sulfones or sulfoxides) with N-phenylhydroxylamine afforded a-anionic N-phenylvinyloxyamine 159, which upon [3,3]-sigmatropic rearrangement led to anionic 2-(2 -oxoalkyl)phe-nylamine 160. Further cyclization provides an efficient synthesis of indole derivatives 161 [84]. 

With phenylhydroxylamine and an aldehyde, 1,2-propadienyl methyl ketone afforded indole derivatives via the intermediacy of 437 the latter was formed by a sequential addition, in situ aldol condensation and sigmatropic rearrangement process [195]. 

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 ... 

A process for the direct reduction of nitrobenzene to -p-ammophenol, an important intermediate for the production of dyes, depends on the above interesting transformation. Nitrobenzene in alcoholic solution is mixed with concentrated sulphuric acid and electrolysed with a lead cathode. This process proves that phenylhydroxylamine is also an intermediate in the reduction of nitrobenzene in acid solution, as was mentioned above. Here, as a result of the rapidity of the rearrangement which takes place, it is not converted into aniline. 

This is the best known rearrangement reaction of phenylhydroxylamines and is an acid catalysed reaction leading principally to the formation of 4-amino phenols 37, although a little of the 2-isomers 38 are also sometimes formed. Reaction proceeds quite smoothly in relatively dilute acid at room temperature. Reaction is quite general for a range of R and X substituents. Much of the early work was carried out by Bamberger38 and the position up to 1967 has been very well reviewed39. 

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. 

A synthetic application of this reaction has been reported57 when the rearrangement of 2-aryl-O-phenylhydroxylamines is followed by a ring enlargement to give an aryldihy-droazepinone (Scheme 10). The 2-aryl-2-phenyl intermediate was also trapped out as the N-trifluoroacetamide. 

Montmorillonite KIO clay and its various cation-exchanged forms have been found to promote the formation of an unexpected product, p-nitrosodiphenylamine, from iV-phenylhydroxylamine, rather than the typical Bamberger products. A Bamberger rearrangement has been shown to occur during the metabolism of 2,4,6-trinitrotoluene... 

The intermediate generated in pyrolysis of IV-acylacetyl-iV-phenylhydroxylamines 48 can form a 3-aza-4-oxa-l,5-diene system (50) (equation 16). A homolytic cleavage of the O—H bond with subsequent rearrangement to the aniline radical, followed by recombination with the hydrogen radical to give the corresponding 0-acyl hydroxylamine 49,... 

Recently, a Bamberger rearrangement in gas phase was reported. The rearrangement of A-phenylhydroxylamine (210, R, R, R = H) into 4-aminophenol (214, R , R, R = H) occurred on the acid sites of H-ZSM-5 zeolite. However, A-phenylhydroxylamine with K-10 montmoriUonite clay and its various cation-exchanged forms failed to rearrange into aminophenols ... 

As a last example we may mention the rearrangement of an arylhy-droxylamine, such as N-phenylhydroxylamine, which when heated with sulfuric acid is converted into p-aminophenol ... 

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