Diazonium salts with aromatic compounds

A. Coupling of Diazonium Salts with Aromatic Compounds. 291... 

Diazonium salts react with various nucleophiles in water (Eq. 11.62).106 In acidic aqueous solution, p-pheny I e ncbis di azo ni um ion reacts with alcohols more rapidly than it does with water.107 In the presence of nucelophiles such as halides, the substitution products are obtained. Furthermore, diazonium salts of aromatic compounds are excellent substrates for palladium-catalyzed coupling reactions such as the Heck-type reactions in water. 

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]. 

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

Some effective cross-linking agents are less suitable for the conjugation of enzyme to antibody. Diazonium salts of aromatic compounds react preferentially with histidine and tyrosine which are very often located in or near the active site of the enzyme. Protecting the active sites with inhibitors (Jansen et al., 1971) is often cumbersome and, consequently, not very popular. 

Reactions with Diazonium Salts The reaction with diazonium salts of aromatic compounds yields directly arylated derivatives of carbon nanotubes (Figure 3.79). It has first been reported for HiPCo tubes as these are very reactive due to their small diameters and the related high curvature. Other nanotubes are lager in diameter and, consequently, less reactive. Hence, the reaction occurs only under more drastic conditions, but still good yields may be achieved. 

Synthesis. Almost without exception, azo dyes ate made by diazotization of a primary aromatic amine followed by coupling of the resultant diazonium salt with an electron-rich nucleophile. The diazotization reaction is carried out by treating the primary aromatic amine with nitrous acid, normally generated in situ with hydrochloric acid and sodium nitrite. The nitrous acid nitrosates the amine to generate the N-nitroso compound, which tautomerizes to the diazo hydroxide. 

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. 

The coupling of aromatic diazonium salts with a variety of aromatic compounds is the basis of the azo dye industry [2-5]. A variety of dying techniques are available but, fundamentally, two procedures are involved ... 

The formation of diazonium salts from aromatic primary amines by reaction with nitrous acid undoubtedly involves the intermediate formation of V-nitroso compounds. The Demjanov and Tiffeneau-Demjanov ring expansions also involve V-nitroso compounds [2]. Some V-nitroso compounds have been used as sources of free radicals and as blowing agents. 

Diazonium salts of aromatic amines are very useful as intermediates to other compounds. Because aromatic diazonium salts are only stable at very low temperatures (zero degrees and below), warming these salts initiates decomposition into highly reactive cations. These cations can react with any anion present in solution to form a variety of compounds. Figure 8-2 illustrates the diversity of the reactions. 

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],... 

Three methods for the preparation of aromatic arsonic adds are de- scribed in this review. By far the most widely applicable of these is the Bart reaction, which involves the interaction of a diazonium salt with an inorganic arsenic compound. In the original Bart process 1 and most of its modifications an alkali arsenite is used, as shown in the following equation. 

To conduct the substitution reactions of Figure 5.56, one neutralizes an acidic solution of the aromatic diazonium salt with diethylamine. This forms the diazoamino compound B (called a triazene ). It is isolated and subjected to the substitution reactions in an organic solvent. In each case, at first a leaving group is generated from the NIT, moiety of the dia-... 

Aryl thiocyanates, ArSCN, are formed by direct thiocyanation of the aromatic nucleus or by treating diazonium salts with metallic thiocyanates. Methods for the thiocyanation of organic compounds have been reviewed. ... 

Much of Landsteiner s pioneer work was carried out with haptens that were aromatic amines. The compounds were converted to diazonium salts with nitrous acid and aUowed to react with proteins at alkaline pH (approximately 9). Reaction occurred primarily with histidine, tyrosine, and tryptophan residues of the protein carrier. For a representative procedure, see Kabat (p. 799 seq.). An interesting application of this procedure was the preparation of a chloramphenicol-protein conjugate which was used to elicit antibodies specific for chloramphenicol. In this case, a prior reduction of the nitro group of chloramphenicol to an amino group was required. As early as 1937, carcinogenic compounds were conjugated to protein carriers by means of their isocyanate derivatives which were prepared from amines. Immune sera were raised, and their properties were studied. - ... 

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. 

Opgenorth, H. J., Ruechardt, C. Aromatic diazo compounds. V. Reaction of aromatic diazonium salts with nitrite ions. Liebigs Ann. Chem. 1974, 1333-1347. 

The preparation of unsymmetrical biaryls,6 the Sandmeyer reaction,6 the replacement of an aromatic diazonium group by hydrogen,7 and the reaction of diazonium salts with ,0-unsaturated carbonyl compounds 8 appear to proceed by free radical mechanisms and will not be discussed. Diazonium coupling reactions, however, have the characteristics of ionic processes. 

Azo coupling of aromatic or heteroaromatic diazonium salts with 0x0 TPs at the 6-position gives azo compounds (92JSC165) or tautomers (e.g., 95) (97JCR(M)2378). 

One of the earliest means of introducing fluorine selectively into specific positions of aromatic compounds is the Balz-Schiemann reaction [77] which dates back to the 1920s. An isolated arene diazonium tetrafluoroborate is thermolyzed at up to 120 °C to yield the corresponding fluoroaromatic compound. Because of the infamously hazardous nature of isolated diazonium salts the scope of the classical variant of the Balz-Schiemann reaction was limited to the small scale. The high exothermicity of the reaction is most conveniently controlled by diluting the diazonium salt with a solid inert medium such as sea sand. In addition to the danger to the experimenter, the reproducibility of the reaction yield is quite poor. 

The colors of azo compounds vary with the nature of the aryl group, with its substituents, and with pH. Substituents also affect the water-solubility of azo dyes and how well they bind to a particular fabric. Countless combinations of diazonium salts and aromatic substrates have been examined with a view toward obtaining azo dyes suitable for a particular application. 

Azo coupling (Section 22.19) Formation of a compound of the type ArN=NAr by reaction of an aryl diazonium salt with an arene. The arene must be strongly activated toward electrophilic aromatic substitution that is, it must bear a powerful electron-releasing substituent such as —OH or —NR2. 

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