Active methylene compounds reaction with diazonium

C-coupling is of outstanding importance in the azo coupling reaction for the synthesis of azo dyes and pigments. An aromatic or heteroaromatic diazonium ion reacts with the so-called coupling component, which can be an aromatic primary, secondary, or tertiary amine, a phenol, an enol of an open-chain, aromatic, or heteroaromatic carbonyl compound, or an activated methylene compound. These reactions at an sp2-hybridized carbon atom will be discussed in Chapter 12. In the... 

Diazonium salts are capable of reacting with active methylene compounds. In the case of linear compounds, isomerization usually takes place to afford the corresponding hydrazone [22], We believe that, since the azo hydrazone tauto-merism appears to be pH dependent (see below), it is possible that the normal reaction conditions favor formation of hydrazones. 

Diazonium salts (157) (Equation (1)) have been reacted with 2-chloro- 1,3-dicarbonyl compounds yielding heterocycles (158) (50-75%) <82JCS(Pl)989>. In a similar reaction sequence (Scheme 3), salt (157) yielded products (160) and/or (161) with activated methylene compounds <89ZN(B)95i>. The formation of compound (160) involves elimination of a molecule XH, which is usually hydrogen cyanide. 

Occasionally it is unnecessary to isolate the hydrazone since this is itself accessible by a coupling reaction. Numerous formazans can be obtained by treating compounds containing active methylene groups directly with 2 molar proportions of a diazonium salt. For example, pyruvic acid is converted into 3-oxalo-l,5-diphenylformazan (fi4formazylglyoxalic acid ) in 94% yield by benzenediazonium chloride in potassium hydroxide solution.351 Acyl groups (CH3CO or COOH) are often eliminated in such reactions 343,344,352 thus acetoacetic ester yields ethyl l,5-diphenylformazan-3-carboxylate almost... 

Intramolecular coupling of diazonium salts with ortho substituents bearing an active methylene grouping or its equivalent gives rise to cinnolines, which may be considered a class of cyclic azo compounds. Three name reactions are cited here for reference only. 

Azo dyes, characterized by the presence of one or more azo groups (-N=N-), are the most commercially important class of dyes. These compounds are synthesized using a diazoti-zation reaction in which a primary aromatic amine reacts with nitrous acid to form a diaz-onium salt. The diazonium compound then typically is coupled with phenols, napthols, aromatic amines, heterocycles, or a variety of other compounds containing active methylene groups. Azo dyes are used in acid, direct, disperse, fiber reactive, and mordant applications. 

Various aromatic amines, phenols, and compounds containing active methylene groups can be titrated with arenediazonium salts, from which 4-bromo-l-naphthale-nediazonium chloride seems to be the most widely applicable titrant. Compounds that react slowly with arenediazonium salts can be determined by back-titration when the excess of arenediazonium salt is back-titrated with either sodium tetraphenylborate or 2,4-diaminotoluene. Indirect determination is useful for secondary amines, which react with arenediazonium ions to form triazenes. The determination of diazonium salts of ampholytic character is based on the reaction of these salts with l-phenyl-3-methyl-5-pyrazolone, the excess of which is titrated with 4-bromo-l-naphthalenediazonium chloride solution. 

This reaction has been modified to treat the acidic diazonium salt with sodium acetate before the addition of compounds with active methylene groups. In addition, this reaction has been carried out in the presence of an ammonium salt. j... 

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