Diazonium cations coupling

Whereas reaction of hydrazones disubstituted in the 4,5-position of the selenazole rings with diazotized arylamines only gives formazans, compounds unsubstituted in the 5-position can be attacked there by the diazonium cation. In fact, the azo coupling in this position is decidedly quicker reaction of 5-benzylidenehydrazino-4-phenyl-... 

Another classical electrophilic aromatic substitution reaction is diazo coupling, in which the effective electrophile has been shown to be the diazonium cation (cf. p. 120) ... 

The rearrangement has been shown under these conditions to be an infermolecular process, i.e. the diazonium cation becomes free, for the latter may be transferred to phenols, aromatic amines or other suitable species added to the solution. It is indeed found that the rearrangement proceeds most readily with an acid catalyst plus an excess of the amine that initially underwent coupling to yield the diazoamino compound (33). It may then be that this amine attacks the protonated diazoamino compound (39) directly with expulsion of PhNH2 and loss of a proton ... 

Thus the 2,4-dinitrophenyldiazonium cation will couple with PhOMe and the 2,4,6-compound with even the hydrocarbon 2,4,6-trimethylbenzene (mesitylene). Diazonium cations exist in acid and slightly alkaline solution (in more strongly alkaline solution they are converted first into diazotic acids, PhN=N—OH, and then into diazotate anions, PhN=N—O ) and coupling reactions are therefore carried out under these conditions, the optimum pH depending on the species being attacked. With phenols this is at a slightly alkaline pH as it is PhO , and not PhOH, that undergoes attack by... 

It has long been recognized that carbazole couples with diazonium salts just once, at C-3, to give monoarylazo products.9-Alkylcarbazoles react comparably. The use of sodium 4-dodecylbenzene sulfonate as a phase transfer catalyst was unsuccessful in an attempt to induce diazocoupling in both rings of 9-ethylcarbazole. 3,6-Dihydroxy-9-methylcarbazole likewise coupled only once, at C-2, with variously substituted benzene diazonium cations, whereas 3,6-dihydroxycarbazole itself coupled twice at 2- and 7-positions with a-naphthyldiazonium ion. ... 

Generally the coupling of diazonium salt is carried out under neutral to slightly alkaline conditions. In coupling with amines or phenols, it has been demonstrated that the active species are the diazonium cation, the free amine,... 

Like pyridine, the azines are not normally attacked by diazonium cations, but since coupling reactions seldom involve extensive protonation of the substrate, the reactions become viable provided that sufficient numbers of activating groups are present, and if reactive diazonium salts are used. Pyridazin-3,4(l//,2//)-diones are among the few pyridazines... 

Diazonium Coupling Reactions. The union of a diazonium ion with a phenol or an amine is known as a diazonium coupling reaction. The process appears to be an example of aromatic substitution in which the reactive species are the diazonium cation and phenoxide ion or the substituted aniline molecule 9... 

The term, azo coupling denotes all reactions of diazonium cations with anionic substances that lead to neutral molecules with removal of the charges on the reactants 315... 

Non-aromatic diazonium cations decompose in the presence of nucleophiles with nitrogen elimination, and do not effect coupling reactions. However, some heterocyclic diazonium compounds were found to have reactivity similar to that of arenediazonium compounds, including production of coupling derivatives. Such is the case of 1,3,4-thiadiazolinediazonium cations (reaction 32) and diazonium betaines (reaction 33). ... 

Azo-coupling, in which the diazonium cation functions as an electrophile, is also of great industrial significance, particularly in the production of dyes. Because of the weak electrophilic character of the diazonium cation, only reactive aromatic systems such as phenols and amines are normally attacked. Azo-coupling is performed in weakly acidic solutions, to ensure high concentrations of the diazonium cation. 

Azo compounds have the general formula R -N=N-R, where R and R are alkyl or aryl groups. Aromatic azo compounds are generally strongly colored and many are easily prepared via a so-called diazo coupling reaction, in which a diazonium cation is coupled with a relatively active (i.e., highly nucleophilic) arene such as a phenol or aniline. The dye called aniline yellow, for example, is prepared as follows ... 

Azo coupling is an example of aromatic electrophilic substitution in which the electrophile is the diazonium cation, Elec-... 

Catalysis by functional micelles continues to attract attention. A micellar diazonium ion couples to 2-naphthols up to 244-fold faster than a model diazonium ion at pH 7. This reaction is of interest as it represents the formation of a cationic intermediate catalysed by a cationic micelle. ... 

The existence of S /ace-N=N-aryl bonds are more difficult to establish because azo bonds are also formed during the growth of the film. A C(OH)-C-N=N-C6H5 has been observed by TOF-SIMS [126] and assigned to an electrophilic attack of the diazonium cation on an -OH-substituted aromatic ring at the surface of carbon. This is the well-known azo coupling reaction between diazonium salts and, for example, naphthol that has been the basis of the dye chemistry. It is different from the reaction of Figure 3.1 that involves a radical. 

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