Coupling Compounds with Reactive Methylene

Aromatic nitro compounds are often strongly colored. They frequently produce characteristic, colored, quinoid derivatives on reaction with alkali or compounds with reactive methylene groups. Reduction to primary aryl amines followed by diazotization and coupling with phenols yields azo dyestuffs. Aryl amines can also react with aldehydes with formation of Schiff s bases to yield azomethines. 

Considering the electrophilic nature of the reaction mechanism, suitable coupling components for the synthesis of azo pigments should carry a nucleophilic center on the aromatic ring system (naphthols) or should be enolizable compounds with reactive methylene functions. Naphthols enter the reaction as naphtholates compounds with reactive methylene groups participate as enolates. 

Compounds with Reactive Methylene Groups. In this group the coupling components with the greatest industrial importance are the A-acetoacetyl derivatives of aromatic amines (acetoacetarylides) CH3COCH2CONHAr. Anilines substituted by halogen, alkyl, alkoxy, nitro, and acylamino groups are most suitable (e.g., Napthol AS-IRG). 

Hydrazino groups attached to cationic rings (as 763) undergo oxidative coupling reactions with amines, phenols (to give e.g. 764) and reactive methylene compounds, e.g. (763) + CH2(CN)2 — (765). 

In color photography diethyl-p-phenylenediamine is an important developer because its oxidation product readily couples with a large number of phenol and reactive methylene compounds to form indoplieiiol and indoaniline dyes, which are the basis of most of the current color processes. 

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. 

Coupling with phenols is usually effected more easily than coupling with amines, and naphthols couple considerably more readily than hydroxybenzene derivatives. Reactive methylene compounds behave similarly to phenols of the benzene series. Negative substituents (halogen, nitro, sulfo, carboxyl, carbonyl, etc.) accelerate coupling when... 

The following types of compounds undergo coupling reactions with diazo compounds (a) Phenols, (b) Enolizable keto compounds of an aliphatic character, having a reactive methylene group which may be part of an open chain or a ring system, either homocyclic or heterocyclic. This group of compounds may be represented by the formula ... 

As an example of the procedure the coupling of 3-methyl-2(l//)benzothiazolone hydrazone to phenols or compounds containing a reactive methylene group will be described 381 Potassium ferricyanide (22 mmoles) is dissolved in a mixture of Water (50 ml), methanol (50 ml), and 25% aqueous ammonia solution (10 ml). A second solution is prepared from 3-methyl-2(l//)benzothiazolone hydrazone (5 mmoles) and the coupling component (5 mmoles) in a mixture of methanol (70 ml) and water (30 ml), and the ferricyanide solution is run into this at 25-30°, with stirring and cooling, during the course of 2-5 min. The dye is formed instantaneously. After 15 min the solution is diluted with water (250 ml), and the precipitated dye is filtered off, washed with water, and dried, in a vacuum over silica gel. 

Azo coupling reactions are frequently observed in diazo transfer processes in which 4-toluenesulfonyl and other azides react with highly reactive methylene compounds, e. g., with dicarbonyl compounds (see Sect. 2.6, Schemes 2-56, 2-59, and 2-63). Such reactions were not, however, investigated mechanistically. Thus, although it is likely that alkanediazonium ions are intermediates, there is no direct evidence for their intermediacy. 

Gu and coworkers reported an 2-iodoxybenzoic acid (IBX) induced oxidative coupling of in situ formed Baylis-Hillman adducts 114 with a nucleophile in water (Scheme 12.44) [68]. As nucleophiles, styrenes, P-dicarbonyl compounds, benzamides, and less reactive indoles can be used. This strategy offers an alternative path to the methylenylation of fS-dicarbonyl compounds with formaldehyde for the formation of a methylene intermediate. In the reaction of 112 and the styrenes 113 (R = Ph), a 5,6-dihydropyran 114 is formed. 

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