3- Pyrazolin-5-ones diazonium coupling

The reaction is very common in pyrazolone chemistry. Since alkoxypyrazoles and tautomerizable pyrazolones undergo this reaction and 3-pyrazolin-5-ones, like antipyrine, do not, it is assumed that the reaction takes place at C-4 of the OH tautomer. Pyrazolone diazo coupling is an important industrial reaction since the resulting azo derivatives are used as dyestuffs. For instance, tartrazine (Section 4.04.4.1.3) has been prepared this way. 3,5-Pyrazolidinediones react with aryldiazonium salts resulting in the introduction of a 4-arylazo group. As has been described in Section 4.04.2.1.4(v), diazonium salts couple in the 3-position with indazole to give azo compounds. 

Both 2-pyrazolin-5-ones and 3-pyrazolin-5-ones undergo substitution at C-4 in an aromatic fashion. Halogenation, nitration and coupling with diazonium salts occur readily. In 2-pyrazolin-5-ones such reactions... 

Of the many methods which have been used for preparing 4-arylazo-2-pyrazolin-5-ones by far the most important is the direct coupling of a diazonium salt with a 2-pyrazolin-5-one (eq. 41) 77.809,ei3,818.880.1251 xhis reaction goes extremely readily with... 

Amino-3-pyrazolin-5-ones (4-imino-3-pyrazolidinones) react with nitrous acid as do ordinary aromatic amines to form diazonium salts.426,984,1353 These then couple readily with aromatic amines,1017,1018,1060 phenols992,1323 and aliphatic compounds having active hydrogen atoms,428,1017 such as acetoacetic esters. The products are 4-azo-3-pyrazolin-5-ones (eq. 200). The diazonium salts also couple... 

A very few bis(3-pyrazolin-5-ones) in which the two rings are connected by the azo group have been prepared. The preparation of these via the 4-azido-3-pyrazolin-5-ones456 has already been mentioned. These bispyrazolinones have also been prepared by coupling diazonium salts with 3-pyrazolin-5-ones (eq. 203).984,1001 Scott and co-workers1284... 

A variety of 2-pyrazolin-5-ones having carboxyl or carboxyl derivatives as substituents and a second functional group are known (Table XXXIII). Such compounds are usually prepared by methods already described, and only those compounds having an arylazo substituent as the second functional group will be discussed. These compounds have been extensively investigated because of their commercial importance as dyes. The important dye tartrazine is of this class. It is the trisodium salt of 4-(4-sulfophenylazo)-l-(4-sulfophenyl)-5-oxo-2-pyrazolin-3-carboxylic acid. These dyes exist largely as enol isomers. The principal means of preparation is coupling of a diazonium salt with a 5-oxo-2-pyrazolin-3-carboxylic acid or its derivatives. Another important preparation of these compounds (eq. 231) is by the reaction of... 

In the presence of sulfuric acid 1-nitroso-3-pyrazolidinones lose nitrous acid by elimination, giving 2-pyrazolin-5-ones which are then nitrosated at C-4.335,1028 The amino group in 4-amino-3-pyrazol-idinones is diazotizable and the resulting diazonium salt undergoes coupling.12... 

The amino group of 4-amino-3-pyrazolin-5-ones can be diazotized easily and the resulting diazonium salt couples with the usual reagents. Other diazonium salts couple with 4-amino-3-pyrazolin-5-ones at the amino group. Mercuric halides replace the amino hydrogen in 2,3-dimethyl-l-phenyl-4-sulfamino-3-pyrazolin-5-one.543 The 4-amino groups react normally with isothiocyanates,49 epoxides,1261 ureas1367 and nitrosamines.527... 

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