4-Arylazo-3-pyrazolin-5-ones

Arylazo-2-pyrazolin-5-ones are strongly colored and have good dyeing properties. These compounds have been prepared in vast numbers for use in dyeing all types of fabrics and are of great importance in the dye industry. They will be discussed fully from this viewpoint in the section dealing with pyrazolinone dyes. The 4-arylazo-2-pyrazolin-5-ones are listed in Table XXIII. 

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... 

A method frequently used and capable of a large number of variations is the cyclization of a-substituted-jS-ketoesters, apiides or hydra-zides with hydrazines to 4-arylazo-2-pyrazolin-5-ones. One variant of this is the treatment of an a,/J-diketoester with hydrazine (eq. 182).627,1573,1574,1576 Instead of the ester, hydrazides of a j8-diketoacids... 

Cyclization of a variety of bishydrazones of a,)3-diketoacids,52,285 a,j8-diketoesters52,264 397,732,782 and a,j3-diketoamides,352 using either acid, base or heat, to 4-arylazo-2-pyrazolin-5-ones has been reported (eq. 184). In this reaction the aryl groups have usually been the same... 

Huebner and Link870 have prepared 4-arylazo-2-pyrazolin-5-ones by the reaction of hydrazines with 4-hydroxycoumarin, 3,3 -methylene-bis(4-hydroxycoumarin) and with 2,3,4-triketochroman derivatives. In the case of the 4-hydroxycoumarins oxidation by phenylhydrazine occurs to give the bisphenylhydrazone of an a,/ -ketoester. The reaction then proceeds as previously discussed for such compounds. 

Biilow and Hecking263 have treated 4-arylazoisoxazolidones with hydrazines to form 4-arylazo-2-pyrazolin-5-ones (eq. 186) and other heterocycles undergo similar transformations.118... 

Although in this discussion the 4-arylazo-2-pyrazolin-5-ones have been written as 5-oxo compounds, it is likely that they exist largely in the tautomeric enol form.262... 

The methylation of 4-arylazo-2-pyrazolin-5-ones has already been mentioned.52 Nitration occurs with replacement of the 4-arylazo group.262 Reduction occurs readily but two different paths are followed. As mentioned earlier, 4-amino-2-pyrazolin-5-ones can be obtained by catalytic reduction670 and with zinc and hydrochloric acid.397 However, reduction with zinc and acetic acid leads to rubazonic acids (eq. 189). 

The most important reactions of 4-arylazo-2-pyrazolin-5-ones are those with various salts to form metal complexes. These products are of great importance in the dye industry and will be discussed more completely in the section devoted to dyes. Chromium complexes are prepared by reaction of a 4-arylazo-2-pyrazolin-5-one with inorganic compounds213,1457 including chromium sulfate475 and chromium trifluoride,1616 with organic chromium compounds, such as chromium... 

Various substituted 2-pyrazolin-5-ones react with metal salts with replacement of hydrogen and formation of semipolar bonds with electron donating atoms. The most important of these compounds are the metal complexes of 4-arylazo-2-pyrazolin-5-ones. These complexes are widely used in the dye industry to give desired properties to 4-arylazo-2-pyrazolin-5-one dyes. Although a large number of metal-dye complexes have been used in dyeing, only a few such compounds have been characterized and reported in the literature. Usually a solution of the azo compound and an appropriate salt such as cupric chloride or nickel sulfate are mixed in solution and heated.326 The reaction is... 

Only half-a-dozen of these compounds are known (Table XLI) and at least this number of methods of preparing them have been reported. Borsche and Manteuffel125 have found that 2-pyrazolin-4,5-diones are formed as the by-products when oc-ketoesters are treated with aryl-diazonium salts. The principal products are 4-arylazo-2-pyrazolin-5-ones. Nitric acid oxidation of 3-methyl-l-phenyl-2-pyrazolin-5-one forms the corresponding 4,5-dione.809 Wislicenus and Gtiz1842 heated 4 -bromo - 4 - nitro -1 - (4 - bromophenyl) - 3 - methyl - 2 -pyrazolin - 5 - one in water and obtained the analogous 4-oxo compound. Acid hydrolysis of rubazonic acids, which are 4-imino-2-pyrazolin-5-ones, leads to 2-pyrazolin-4,5-diones.424 809 Oxidation of 4,4 -bis(2-pyrazolin-5-ones) or of rubazonic acids with nitric acid809 forms the 4,5-diones. 

In many photographic processes it has been found advantageous to use a colored coupler. In these processes a 4-arylazo-2-pyrazolin-5-one is the coupler and it reacts with a developer as shown in eq. 272 (p. 138).1539 This results in the formation of an azomethine dye which counteracts the overlapping absorption of the negative image dyes present and leads to improved color reproduction. These compounds usually have a negative substituent at N-l 1058,1305 as this enhances the... 

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. 

A variety of functional groups can be introduced directly into the 4-position of 2-pyrazolin-5-ones. Perhaps the most important of these is the arylazo group which is introduced by reaction with diazonium salts (eq. 41).333,309 813 This reaction has been used to prepare a vast... 

Arylazo-3-pyrazolin-5-ones have been used as dyes. They form complexes with chromium which have good dyeing properties.1060 Miehaelis, Kotelmann and Drews984 report the reduction of 4-aryl-azo-3-pyrazolin-5-ones having no 1-substituent to pyrazoles by use of phosphorus pentasulfide (eq. 202). Reaction of such azo compounds... 

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... 

The azo dyes were identified by elemental analysis and MS,IR and NMR spectra, and it was proved by the studies of tautomerism of the azo dyes with IR and NMR spectra that hydrazone form was predominant in the azo dyes. First,the absorption peak of 1660, 1600, 1560 and 1260 cm" in IR spectrum of 4- were all compatible with the absorption peak CO, Endo C N, Exo C—N and NH of hydra-zone, respectively, that were exactly identified on arylazo-3-methyl-1-phenyl-2-pyrazolin-5-one... 

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