Aromatic diazohydroxide

Fragmentation reactions which give rise to multiple bonding between nitrogen atoms are exemplified by decompositions of aromatic diazohydroxides in acidic media to give aryl cations, nitrogen and water. Such reactions generally occur in two steps and the intermediate diazonium ion is well characterised by its electrophilic properties. 

The reversibility of aromatic diazotization in methanol may indicate that the intermediate corresponding to the diazohydroxide (3.9 in Scheme 3-36), i. e., the (Z)-or (is)-diazomethyl ether (Ar — N2 — OCH3), may be the cause of the reversibility. In contrast to the diazohydroxide this compound cannot be stabilized by deprotonation. It can be protonated and then dissociates into a diazonium ion and a methanol molecule. This reaction is relatively slow (Masoud and Ishak, 1988) and therefore the reverse reaction of the diazomethyl ether to the amine may be competitive. Similarly the reversibility of heteroaromatic amine diazotizations with a ring nitrogen in the a-position may be due to the stabilization of the intermediate (Z)-diazohydroxide, hydrogen-bonded to that ring nitrogen (Butler, 1975). However, this explanation is not yet supported by experimental data. 

The complexity of the system consisting of the diazonium ion and the four reaction products shown in Scheme 5-14 is evident. In contrast to the two-step reaction sequence diazonium ion <= (Z)-diazohydroxide <= (Z)-diazoate (Scheme 5-1 in Sec. 5.1), equilibrium measurements alone cannot give unambiguous evidence for the elucidation of the mechanistic pathway from, for example, diazonium ion to ( )-diazoate. Indeed, kinetic considerations show that, depending on the reaction conditions (pH etc.) and the reactivity of a given diazonium ion (substituents, aromatic or heteroaromatic ring), different pathways become dominant. 

Conversion of the diazonium salt to the diazohydroxide by means of alkali in the presence of a liquid aromatic compound with which the... 

The second step is carried out by adding a 15-40% aqueous solution of sodium hydroxide dropwise to a well-stirred mixture of the cold dia-zonium salt solution and the aromatic liquid until a slight excess of alkali is present. This procedure avoids the isolation of the unstable, explosive diazohydroxides of Bamberger,1 which are formed from the diazonium salt and the alkali. The diazohydroxide is extracted by the aromatic liquid as it is formed. The biaryl reaction takes place in the organic liquid and is complete when evolution of nitrogen ceases. In the example given, 4-bromobiphenyl is obtained in 35-46% yield. 

The reactions involving the diazohydroxides, the diazoacetates, and the nitrosoacetylamines are closely related. Not only do these substances yield the same products when they react with aromatic compounds, but also they appear to react according to the same mechanism. This, perhaps, is not unexpected in view of the statements that nitroso-acetanilide is a tautomer of benzenediazoacetate. 110-11... 

It is not possible to prepare biaryls containing a free carboxyl group directly by the diazo reaction. No biaryl is formed when (a) diazotized aniline and sodium benzoate, (b) diazotized anthranilic acid and aqueous sodium benzoate, or (c) diazotized anthranilic acid and benzene are used as components in the reaction.13 On the other hand, the reaction proceeds normally if methyl benzoate is used in reaction (a) or when methyl anthranilate replaces the anthranilic acid in (b) and in (c). The success of the diazohydroxide reaction appears to lie in the ability of the non-aqueous liquid to extract the reactive diazo compound from the aqueous layer.4 However, esters and nitriles can be prepared from esters of aromatic amino acids and cyanoanilines and also by coupling with esters of aromatic acids, and from the products the acids can be obtained by hydrolysis. By coupling N-nitrosoacetanilide with ethyl phthalate, ethyl 4-phenylphthalate (VIII) is formed in 37% yield. 

Arylpyridines. The amino group of aromatic amines can be replaced by the pyridyl group by the normal diazohydroxide reaction,3 by treatment of the sodium diazotate in pyridine with acetyl chloride,17 by adding the dry diazonium salt to pyridine,18 and from the nitrosoacetyl... 

The synthesis of aryl azides in high yields and under mild conditions by the reaction of aromatic nitroso compounds with hydrazoic acid has been reported by Maffei and co-workers . Their results are summarized in Table 14. No intermediates have yet been isolated from this reaction, although Maffei suggested the intervention of a diazohydroxide. If the quantity of hydrazoic acid employed is less... 

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