Diazotization of 2- and 4-Aminophenols

The diazotization products of 2- and 4-aminophenols, -naphthols (etc.), possess a mesomeric (zwitterionic) phenolate-diazonium and quinone-diazide structure. We discussed these structures in the context of aromatic diazotization (Zollinger, 1994 Sect. 2.4) because the synthetic methods used are closely related to those used for aromatic diazonium salts. This is also the case for the diazotization of amino-di-, tri- and tetrazoles, which, in their neutral form, contain a heterocyclic NH group in the )8-position to the amino group. After diazotization, the NH group is very acidic. Following deprotonation the product corresponds to a heterocyclic diazoalkane. Similarly, the diazotization product of 4-(dicyano)methylaniline ((4-amino-phenyl)malonitrile) may lose the CH proton. This compound is, therefore, sometimes called a vinylene homolog of diazomalonitrile (Regitz and Maas, 1986, p. 205). 

Diazotized 2- and 4-aminophenols as well as corresponding diazotized aminonaphthols and hydroxy derivatives of higher condensed aminoaromatic systems exist in neutral aqueous solutions as zwitterions (23b) which are mesomeric with the corresponding quinone diazides (23a). They can therefore be classified either as diazonium ions or as diazoketones. Indeed, preparative methods for these compounds include those typical for diazonium ions and those used for diazoketones. 

A compilation of some 240 examples in the book by Ershov, Nikiforov and de Jonge443 demonstrates, however, that the most frequently used method is diazotization, applied to 2- or 4-aminophenols in the same or a similar way as described in Section II.A. It may be recalled that the very first diazotization (Griess, 1858) was carried out with an aminophenol derivative. 

Dichlorophenol has been prepared by the chlorination of phenol with chlorine gas in the presence of nitrobenzene and fuming sulfuric acid, by the decomposition of the diazotate of 2,6 dichloro-4-aminophenol, eind by the decarboxylation of 3,5-dichloro-4-hydroxybenzoic acid in quinoline or dimethyl-aniline. ... 

The major problem of these diazotizations is oxidation of the initial aminophenols by nitrous acid to the corresponding quinones. Easily oxidized amines, in particular aminonaphthols, are therefore commonly diazotized in a weakly acidic medium (pH 3, so-called neutral diazotization) or in the presence of zinc or copper salts. This process, which is due to Sandmeyer, is important in the manufacture of diazo components for metal complex dyes, in particular those derived from l-amino-2-naphthol-4-sulfonic acid. Kozlov and Volodarskii (1969) measured the rates of diazotization of l-amino-2-naphthol-4-sulfonic acid in the presence of one equivalent of 13 different sulfates, chlorides, and nitrates of di- and trivalent metal ions (Cu2+, Sn2+, Zn2+, Mg2+, Fe2 +, Fe3+, Al3+, etc.). The rates are first-order with respect to the added salts. The highest rate is that in the presence of Cu2+. The anions also have a catalytic effect (CuCl2 > Cu(N03)2 > CuS04). The mechanistic basis of this metal ion catalysis is not yet clear. 

For many decades intramolecular O-coupling was considered not to take place in the diazotization products of 2-aminophenol and its derivatives (for a contrary opinion see, however, Kazitsyna and Klyueva, 1972). The compounds were assumed to be present as one structure only, which can be represented as a mesomer of a phenoxide diazonium zwitterion 6.63 b and a diazocyclohexadienone 6.63 a (see reviews by Kazitsyna et al., 1966 Meier and Zeller, 1977 Ershov et al., 1981). In IUPAC nomenclature 6.63 is called 1,2-quinone diazide, in Chemical Abstracts 6-diazo-2,4-cyclohexadien-one (see Sec. 1.3). More recently, however, Schulz and Schweig (1979, 1984) were able to identify the intramolecular product of O-coupling, i.e., 1,2,3-benzooxadiazole (6.64) after condensation of 6.63 in vacuo at 15 K in the presence of argon (see Sec. 4.2). 

The properties of the diazotization product of 2-aminothiophenol are completely different from those of 2-aminophenol. In the reaction shown in Scheme 6-44 the diazo-thiophenolate (6.66) is not detected at all. The benzo-l-thia-2,3-diazole (6.67) is the stable product. A comparison of spectral data for diazophenoxide (6.63), benzo-l-oxa-2,3-diazole (6.64), and benzo-l-thia-2,3-diazole (6.67) is given in Section 4.2. The l-thia-2,3-diazole structure was first postulated by Le Fevre et al. (1954) on the basis of infrared spectra. 

C.I. Acid Yellow 76, 18850 [6359-88-2] (14), is obtained by coupling diazotized 4-aminophenol onto the pyrazolone component and then esterifying with -toluene-sulfonic acid chloride in an alkaline medium. The toluenesulfonic ester group substantially improves the fastness to milling and makes the shade obtained largely independent of pH the lightfastness is not quite as good as that of C.I. Acid Yellow 17. 

Acid YeUow 23 (31), commonly known as Tartraziae, stiU maintains sales of nearly 0.5 million /yr ia the United States. It was first discovered ia 1884 and is made by coupling equimolar quantities of diazotized sulfarulic acid to 3-carboxy-l- -sulfophenyl)-5-pyrazolone. Other monoazopyrazolone dyes of commercial importance iaclude Acid YeUow 17 (32) (sulfarulic acid — l-(2,5-dichloro-4-sulfophenyl)-3-methyl-5-pyrazolone and Acid YeUow 40 [6372-96-9] (33) (Cl 18950) (p-aminophenol l-(4-chloro-2-sulfophenyl)-3-methyl-5-pyrazolone) foUowed by esterification of the phenoUc hydroxy group with -toluenesulfonyl chloride. 

In the synthesis of the unsymmetrical 1 2 cobalt complex 18 [68928-31-4] [30], the 1 1 cobalt complex 17 and the azo dye made by coupling of 1-acetoacetyl-amino-2-ethylhexane with diazotized 2-aminophenol-4-sulfonamide are intro-... 

Trihydroxyazobenzene was prepared by coupling diazotized p-aminophenol with resorcinol (100% excess) in the presence of sodium hydroxide. The product was purified by dissolving it in sodium hydroxide and reprecipitating with acid and finally by two crystallizations from 70% alcohol. 

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