Diazonium salts, coupling stability

The alternative approach is to pad the fabric with the alkaline naphthol and dry, foUowed by printing directly onto this prepared fabric diazonium salts or stabilized diazonium salts. Coupling is instant and the only further treatment needed is to remove aU the uncoupled naphthol and surface azo pigment in a subsequent washing treatment. Because the choice of colors is limited from one naphthol component, other shades are obtained by using other classes of dye alongside the azoic colors, eg, reactives. This approach is widely used in the production of African prints. 

Aminopyridine /V-oxides can be diazotized, and the diazonium salts undergo coupling, etc. These diazonium salts are stabilized by mesomerism (728), cf (726). Amino groups in pyridazine /V-oxides can also be diazotized and the diazonium group further replaced by other functionality. Nitrosation of 3-amino-1,2,4-triazine 2-oxides and subsequent thermolysis of the diazonium tetrafluoroborate salts afforded 3-fluoro-1,2,4-triazine 2-oxides (85H(23)1969). 

Coupling of aromatic diazonium salts with stabilized methylenephosphoranes leads to arylazometh-ylenephosphoranes (equation 30). ... 

Because of the small concentration of the 2 1 complex the last term can be ignored. From the extreme rate values in the absence of zinc and with an excess of zinc, 2i and 22 are determined as 2.4 X 104 min.-1 and 1.57 min.-1 respectively. These values can be combined with the trend in the rate constants to give the stability constant of the reactive complex, presumably Zn(OR)(OAc), as 3 X 107. For the simple zinc complex in water the literature values of the stability constant for the 1 1 complex vary from 2.5 X 108 to 6.3 X 108. The diazo coupling reaction of the complex indicates the smaller effect of coordination vis a vis protonation since this reaction is very sensitive to such effects and does not proceed with phenols. Unfortunately the choice of cations for such a reaction is restricted since the cation should not interfere with the analytical methods used to obtain the kinetic data nor should it introduce additional reactions such as occur with transition metal cations which can catalyze the decomposition of the diazonium salt via a redox process. 

As might be expected, the stability and reactivity of diazonium salts are affected by substituents as well as by the anion. The rate of coupling and the location of the azo bond are influenced by substituents on the coupling reagent. Some primary and secondary aromatic amines derived from benzene initially form A-azo compounds, which rearrange to p-aminoazo compounds in acidic media [8b, c]. Tertiary amines, on the other hand, behave normally. Preparation 2-3, while of a rather complex molecule, illustrates the simple techniques commonly used in coupling in an aqueous system. Note the presumed preferential tendency of coupling to take place predominantly in the para position. 

Pyridine-2- and -4-diazonium ions are far less stable than benzenediazonium cations. Azole-diazonium salts generally show intermediate stability provided diazotization is carried out in concentrated acid, many of the usual diazonium reactions succeed. Indeed, azolediazonium salts are often very reactive in coupling reactions. 

Azoic components (stabilized diazonium salts) are applied to fibers containing a coupling component, and the color form on the substrate. Azoic components can be used on cotton, polyester, rayon, and cellulose acetate. 

Azoic components and compositions cotton, rayon, cellulose acetate and polyester fiber impregnated with coupling component and treated with a solution of stabilized diazonium salt azo... 

A diazonium salt is a weak electrophile and hence reacts only with highly electron rich species such as amino and hydroxy compounds. Even hydroxy compounds must be ionized for reaction to occur. Consequently, hydroxy compounds such as phenols and naphthols are coupled in an alkaline medium (pH >pKa of phenol or naphthol typically pH 7-11), whereas aromatic amines such as N,N-dialkylamines are coupled in a slightly acid medium, typically pH 1-5. This provides optimum stability for the diazonium salt (stable in acid) without deactivating the nucleophile (protonation of the amine). 

The use of stabilized diazonium salts is a recent modification, and the exact limits of this reaction have not yet been established. It has been found to be particularly good for nitroanilines (4-nitrobiphenyl is obtained in 70% yield from p-nitroaniline) and for coupling with nitrobenzene. Coupling of the stabilized salt of aniline failed with toluene, biphenyl, naphthalene, and 1-nitronaphthalene, and the stabilized salt of /3-naphthylamine gave no /3-phenylnaphthalene when coupling with ben-... 

Compared to diazonium salts, diazo compounds are generally much less reactive towards nucleophiles than towards electrophiles. As a result of this azo coupling reactions of diazo compounds are the exception rather than the rule. Electron withdrawing substituents on the diazo carbon increase the reactivity towards nucleophiles. Consequently the ability to undergo azo coupling reactions increases from diazomethane to diazocarbonyl- and 2-diazo-l, 3-dicarbonyl compounds. Among the earliest reactions known were those with cyanide and sulfite ions Tertiary phosphines, as opposed to amines, can form stable addition complexes with diazoalkanes probably due to the ability of phosphorus to stabilize the betaine with its empty d orbitals (6). 

Biaryls are produced in low yields by several related coupling reactions. When benzenediazonium chloride is treated with aqueous sodium hydroxide in the presence of benzene, nitrogen is eliminated and biphenyl is formed in 22% yield. Better results are sometimes obtained when the sodium hydroxide is replaced by sodium acetate or when stabilized diazonium salts are employed. In the former modification the aryl-diazoacetate, ArN = NOCOCHj, is an intermediate product. The tautomeric forms of the aryldiazoacetates are N-nitrosoamides, ArN(N0X30CH, which are obtained by the action of nitrous fumes or nitrosyl chloride on... 

Indoles have been prepared from reactions of o-aminophenylketones with reactive , or stable " arsonium ylides. Oxo-stabilized ylides reacted with 2-chloro-oximes to give trans-5-acyl-A -isoxazolines, and isoxazoles have been obtained from reactive arsonium ylides and a-isonitrosoketones, and from triphenylarsonium methylide and nitrile oxides The latter ylide reacts similarly with nitrile imines to give pyrazoles. With triphenylarsonium benzylides and benzoylylides,benzene diazonium salts give 1,3,4,6-substituted 1,4-dihydro-1,2,4,5-tetrazines in a reaction in which initial coupling of the reagents is followed by a dimerisation. ... 

Instead, these heterocycles and their derivatives most commonly undergo electrophilic substitution nitration, sulfonation, halogenation. Friedel-Crafts acylation, even the Reimer-Tiemann reaction and coupling with diazonium salts. Heats of combustion indicate resonance stabilization to the extent of 22-28 kcal/ mole somewhat less than the resonance energy of benzene (36 kcal/mde), but much greater than that of most conjugateci dienes (about Tlccal/mole). On the basis of these properties, pyrrole, furan, and thiophene must be considered aromatic. Clearly, formulas I, II, and III do not adequately represent the structures of these compounds. 

Brase and coworkers were the first to introduce triazene linkers for use in SPOC in a seminal report in 2000 [18]. They found immobilized diazonium ions have high stability, which enabled their application as linkers and scavengers. Their synthesis involves the reduction of 3-amino-6-chlorobenzoic acid, which was subsequently coupled onto a Merrifield resin via standard etherification. Subsequent conversion of the amino group into a diazonium salt generated the supported diazonium tet-rafluoroborate, which was found to sequester amines (Scheme 8.12). 

Amino-l,2,4-thiadiazoles are diazotized at — 10°C in phosphoric acid, and couple with naphthol to give moderate yields of diazo dyes.182,388 In their limited stability, the 3-diazonium salts resemble those of 3-aminoiso-thiazole, but differ from the more stable 1,2,4-thiadiazole- and isothiazole-5-diazonium salts a consideration of the 7t-electron densities leads to the opposite conclusion.182... 

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