Aminoanthraquinones, diazotization

The nitrosylsulfuric acid method is particularly suitable for the diazotization of di- and trinitroanilines and aminoanthraquinones. Such amines may be added directly to the nitrosylsulfuric acid, but it is preferable to run the appropriate amount of nitrosylsulfuric acid into a solution of the amine in 96% sulfuric acid. In general, these diazotizations can be carried out at room temperature. The end-point is determined in the usual manner with iodide paper, but only after first diluting a few drops with ice. On completion, the whole is diluted with ice. The test with iodide paper fails in the case of polynitrodiazonium salts. 

Aminoanthraquinone is diazotized advantageously with nitrosylsulfuric acid after being dissolved in sulfuric acid. Coupling the diazo compound onto barbituric acid, for instance, affords a yellow pigment [3] with the structure 79 ... 

The compounds are obtained by coupling diazotized 1-aminoanthraquinone onto 2-hydroxy-3-naphthoic acid, followed by separation, drying, and conversion into the azo dye acid chloride. Condensation with amines (structure 81) is achieved in an aprotic organic solvent. 

Bersier and coworkers29 published a list of 20 aromatic and heteroaromatic amines whose stabilities in diazotization systems have been investigated. Aqueous systems are harmless, even with amines containing one or two nitro groups (provided that they can be diazotized at all in water). In 96% sulfuric acid, diazotizations of aminoanthraquinones are not dangerous this is also the case for heteroaromatic amines in mixtures of sulfuric acid... 

The procedure given can be used for diazotizing all primary amines whose basicity is so low that they do dissolve in aqueous acids, such as the tri- and tetrahalogenanilines, dihalogennitroanilines, dinitroaniline, aminoanthraquinone, etc. If necessary, the amount of sulfuric acid can be increased in order to obtain complete solution. The sulfates of diazo-anthraquinones are very difficultly soluble in dilute sulfuric acid. Hence, they can be filtered off after pouring the concentrated sulfuric acid solution onto ice, and thus freed from most of the excess acid. 

Aminoanthraquinones undergo acylation with acid chlorides, generally in organic solvent, and with carboxylic acids or anhydrides in oleum. Benzoylation is an important reaction, since many vat dyes contain benzamido groups in the a- (1-) position. The free amino groups may be diazotized to afford biaryl compounds. The great lability of substituents in anthraquinones, as compared with similar benzene and naphthalene compounds, offers a number of routes to aminoanthraquinones. 

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