Aryl Hydroxylamines

We now turn to the two aryl hydroxylamines, Af-phenylhydroxylamine and its o-nitro derivative. As liquids, the former compound has an enthalpy of formation that is ca 20 kJmoH more positive than that of the latter. For comparison, we find the enthalpies of the corresponding hquid species without the NHOH group, Le. benzene and nitrobenzene, differ by 36.5 kJmol . Alternatively said, equation 4 is endothermic by 16.1 kJmol . 

Similarly, removing only the NH and thus comparing the resulting phenols with hydroxy-lamines, as in equation 5, the formal reaction enthalpy is 13.3 kJ mol using the enthalpies of fusion in Reference 5. 

We might have thought that the o-nitro group would provide stabilization for the hydrox-ylamine and the phenol by two mechanisms (a) enhanced (i.e. zwitterionic, dipolar, quinonoid-like) resonance as is often discussed for o- and p-nitroaniline, cf. Reference 12, and (b) intermolecular H—O hydrogen bonding between the nitro and hydroxyl groups. 

However, as seen from the data in Reference 1, reaction 6 

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Aryl hydroxylamines treated with acids rearrange to aminophenols. Although this reaction (known as the Bamberger rearrangement) is similar in appearance to... 

Rondevstedt, C. S., Chem. Eng. News, 1977, 55(27), 38 Synthesis, 1977, 851-852 In an attempt to reduce 2-chloro-5-methylnitrobenzene to the aniline by treatment with excess hydrazine and Pd/C catalyst, the hydroxylamine was unexpectedly produced as major product. During isolation of the product, after removal of solvent it was heated to 120°C under vacuum and exploded fairly violently. Many aryl-hydroxylamines decompose violently when heated above 90-100°C, especially in presence of acids. GLC is not suitable as an analytical diagnostic for arylhydroxyl-amines because of this thermal decomposition. 

Alkyl and aryl C-nitroso compounds contain a nitroso group (-N=0) directly attached to an aliphatic or aromatic carbon. As compounds with a nitroso group attached to a primary or secondary carbon exist primarily as the oxime tautomer, the stable examples of C-nitroso compounds contain nitroso groups attached to tertiary carbons, such as 2-methyl-2-nitroso propane (1, Fig. 7.1) or nitroso groups attached to carbons bearing an electron-withdrawing group (-CN, -N02, -COR, -Cl, -OAc, Fig. 7.1). Oxidation of alkyl and aryl hydroxylamines provides the most direct route to alkyl and... 

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. 

Many of the 3-hydroxytriazene derivatives produced by diazo-coupling onto N-alkyl or A-aryl hydroxylamines decompose explosively above their m.p.s. However, the heavy metal derivatives are stable and used in analytical chemistry. 

Although 3-aminoquinazoline derivatives are normally prepared de novo from hydrazine starting materials, they can also be prepared by N-amination reactions. Thus, treatment of l-substituted-2,4(177,377)-quinazolinediones 32 with 0-aryl hydroxylamines under basic conditions gives 3-amino-l-substituted-2,4(177,377)-quinazolinediones 33 which are intermediates in the synthesis of 3-amino-2,4(l/7,3/7)-quinazolinedione antibacterial agents <20020PD230, 2005JHC669, 2006JME6435>. 

M Fe over the rate in the absence of Fe" ". The reaction was hindered by complexation of Fe " by CN , and produced pivalic acid as a by-product. Reduction was promoted more effectively by Cu than by Fe, and N-aryl-hydroxylamines added to the reaction mixture were oxidized to nitroso... 

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

Aryl hydroxylamines treated with acids rearrange to aminophenols.255 Although this reaction (known as the Bamberger rearrangement) is similar in appearance to 1-32 to 1-36, the attack on the ring is not electrophilic but nucleophilic. The rearrangement is intermolecular, with the following mechanism ... 

Table 3.2 Oxidation of aryl hydroxylamines with FeCI3. NHOH NO...

Pyrroloindoles/ A simple route to this system, which is present as the corresponding quinone in mitomycins, can be obtained by reaction of N-aryl-hydroxylamines (1) with ethyl 6-oxo-2-hexynoate (2) to form a nitrone that is reduced without isolation with NaBHsCN (1.2 equiv.). 

Because of their reactivity, aryl hydroxylamines can sometimes be trapped by further reaction with another material present in the hydrogenation system. Aryl nitrones such as 17 are produced by the in situ condensation of the aryl hydroxylamine with an aldehyde which is present in the reaction medium (Eqn. 19.16).35 These reactions take place in good yield over a Urushibara nickel catalyst (Chapter 12) in aqueous alcohol at room temperature and four... 

Bis(silyl)ketene acetals undergo silatropic ene reaction with nitrosobenzene to give N-hydroxyamino acid derivatives. When allylmagnesium chloride is reacted with nitroarenes, unstable adducts result. Reduction of these adducts with LAH in the presence of palladium on charcoal leads to A -allyl-W -aryl-hydroxylamines (73 Scheme 15). With alkyl Grignard reagents this reaction is negligible. ... 

Transformations that are mechanistically analogous to the Fischer, and also produce indoles, use aryl-hydroxylamines instead of arylhydrazines, as shown below. ... 

N-Aryl-hydroxylamine, die im sauren Medium zu Amino-phenolen umlagern, las-sen sich durch Aluminium-Amalgam zu Aminen reduzieren (Bd. XI/1, S. 515 s.a. Bd. X/l, S. 1242). 

The preparation of nitrosobenzene by oxidation of phenylhydroxylamine with dichromate-sulfuric acid has been described by Gattermann and Wieland.la 2,6-Dihaloanilines can be oxidized to nitroso compounds in good yield by a mixture of 30% hydrogen peroxide and glacial acetic acid.205 Nitroso compounds are also obtained in good yield by oxidation of aryl-hydroxylamines with diethyl azodicarboxylate.206... 

The reduction of aromatic nitro compounds to N-aryl-hydroxylamines by o-xylene-a,a -dithiol occurs when an iron... 

Benzofurans and Other Annelated Furans.-A new synthesis of benzofurans (73) is by the reaction of the phosphonium salt (72) with acid chlorides RCOCl in the presence of triethylamine. The benzofuran (74) results from the condensation of hexafluorobenzene with acetylacetone. Three instances of the formation of benzofurans from O-aryl-hydroxylamines, i.e. the oxygen analogue of the Fischer indole synthesis, have been reported O-phenylhydroxylamine hydrochloride and benzenesulphonylacetone give a 2 1 mixture of compounds (73 R = CH2S02Ph) and (75), the oxime ether (76) is converted into the aldehyde (77 ... 

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