Coupling of diazonium compound

Diazo A family of reprographic processes (including Diazotype), based on the coupling of diazonium compounds with dye couplers to form colored compounds. Exposure of the dia-zonium compounds to near-ultraviolet radiation destroys them, so illuminated areas do not develop color. 

Coupling of Diazonium Compounds with Hydrazones. The most common method for the production of formazans is the coupling of diazonium salts with aryl hydrazones in an alkaline medium, possibly in presence of an organic solvent. 

Double Coupling of Diazonium Compounds. Some compounds with an activated methyl group can form symmetric formazans with strong diazo compounds by double coupling ... 

Azo coupling of diazonium salts with aromatic (or pseudoaromatic) hydrocarbons is possible if the coupling agent is highly substituted. For example, azo compounds have been produced from pentamethylbenzene [12], benzpyrene [13], and azulene [1-4]. 

B. Coupling of Diazonium Salts with Active Methylene Compounds... 

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. 

Perhaps the best-known method of preparing aromatic azo compounds involves the coupling of diazonium salts with sufficiently reactive aromatic compounds such as phenols, aromatic amines, phenyl ethers, the related naphthalene compounds, and even sufficiently reactive aromatic hydrocarbons. Generally, the coupling must be carried out in media which are neutral or slightly basic or which are buffered in the appropriate pH range. The reaction may also be carried out in nonaqueous media. While some primary and secondary aromatic amines initially form an A-azoamine, which may rearrange to the more usual amino-C-azo compound, tertiary amines couple in a normal manner. 

Two molecules of a diazonium salt may couple with loss of some nitrogen. In the Bogoslovskii reaction, this reaction has been developed as a means of preparing o,o -dihydroxyazo compounds, which are difficult to obtain by other means. This reaction involves the use of a cuprous complex as the reaction catalyst. Self-coupling of diazonium salts also takes place in the presence of sodium sulfite. 

The phenolic group of tyrosine undergoes iodina-tion (Eq. 3-44), acylation, coupling with diazonium compounds, and other reactions. 

Since 1 mol. of R salt couples with 1 mol. of diazonium compound, the strength of the R salt solution can be easily calculated, and hence... 

Phenols, Naphthols (e.g., 39-47). Phenols mainly couple at the 4-position, or at the 2-position if the 4-position is occupied, p-Hydroxybenzoic acid couples with elimination of C02 resorcinol couples twice initially at the 4-position, and with a second equivalent of diazonium compound at the 2-position under acid conditions or at the 6-position under alkaline conditions. a-Naphthols mainly couple at the 4-position, in addition to which varying quantities of 2- and 2,4-coupling products are obtained, depending on the diazo component. p-Naphthol couples at the 1-position. Substituents in the 1-position, such as S03H, COOH, Cl, CH2OH, or... 

Coupling Components With Condensed Cyclic Ammo nium Residues. Heterocyclic compounds in which the condensed benzene ring is substituted by a hydroxyl or an amino group can be coupled with diazonium compounds and may also be quatemized, either prior or subsequent to the coupling reaction, to yield cationic azo dyes. l,2-Dialkyl-6-nitroindazolium salts are reduced to the 6-amino compounds and then coupled with diazonium salts of aromatic amines. These dyes (e g., 21) color polyacrylonitrile in bright yellow to orange shades [64],... 

Imidazole is reported159 to be coupled at position 2, in contrast with the other electrophilic substitutions which occur preferentially at position 4. This peculiar orientation, together with the observation that 1-methylimidazole does not couple with diazonium compounds,160 supports the view that ionization of the N-H bond is a prerequisite for coupling. 

It would perhaps be more correct to state that Robinson and Teuber arrived at the correct structure for calycanine on the basis of their calycanthine structure, which was derived from a presumed biogenesis from two molecules of A-methyltryptamine. It had the correct number of functional groups (two each of =NMe and =NH) and explained the readiness with which it could be coupled with diazonium compounds. Furthermore, its conversion to calycanine involved only a minor readjustment of bonds in addition to the usual dehydrogenation and loss of A-methyl. 

The fraction obtained by predpitation with potassium chloride is G salt which contains 5 per cent, at the most, of R salt as an impurity. In the laboratory, this crude G salt can be purified by recrystalhzation from water. The method used in the plant is based on the fact that G salt couples with diazonium compounds much less readily that either R salt or Schaeffer salt. A test is made to determine the content of easily coupling sulfonic acids by titrating a sample with diazobenzene or diazoxylene in dilute solution. The crude G salt is then treated with the correct amount, as indicated by the titration, of a slowly reacting diazo compound, usually diazoxylene. Under these conditions, only the impurities co le, leaving a solution of pure R salt after the dye which is formed is removed. The G salt solution can be used directly in the preparation of azo dyes. The dye from the impurities is sold under the name Ponceau. 

The coupling reaction is important in the industrial preparation of azo dyes as well as in the analytical determination of diazonium compounds. The reaction has been reviewed, and experimental procedures have been given,... 

Karmas and Spoerri (817) found that whereas 2-hydroxypyrazines couple with diazonium compounds at the 5(para)-position in neutral or mildly alkaline solution (1122), addition of phenyldiazonium chloride to hydroxypyrazines in molar aqueous sodium hydroxide resulted in a Gomberg type of phenyiation of the nucleus with evolution of nitrogen. In this reaction substitution occurred preferentially at a free 3-position but may also occur at positions 5 and 6 if these also originally bore hydrogen (817). For example, 2-hydroxypyrazine in 5% aqueous sodium hydroxide with benzenediazonium chloride produced 2-hydroxy-3-phenylpyrazine (47%) and 3-hydroxy-2,5-diphenylpyrazine (4%) (817). 

Oxycellulose and phenylhydrazine gave a yellow compound, which formed a diphenylformazan, showing that the oxycellulose had reacted in one of the two possible hemiacetal forms. Aminophenols and oxycellulose gave derivatives which coupled with diazonium compounds, enabling chemically colored fibers to be prepared. Reduction of oxycellulose oxime with lithium aluminum hydride, sodium borohydride, or sodium amalgam gave an amino-oxycellulose (109) in which up to 25 % of the oxime groups had been reduced. ... 

The coupling of diazonium salts with aromatic phenols and amines yields azo compounds, which are of tremendous importance to the dye industry. 

Problem 23.20 (a) Coupling of diazonium salts with primary or secondary aromatic amines (but not with tertiary aromatic amines) is complicated by a side reaction that yields an isomer of the azo compound. Judging from the reaction of secondary aromatic amines with nitrous acid (S. 23.10), suggest a possible structure for this by-product. 

Reduction of azo compounds to amines, with fission of the N=N double bond, assumes practical importance because so many azo compounds are readily accessible, even on an industrial scale, by coupling of diazonium salts with reactive molecules. In this way a large number of primary amines can be obtained free from isomers, which is not always easy to achieve by other... 

---