Diazonium salts, from arylamines, reactions

Arylamines are converted by diazotization with nitrous acid into arenediazonium salts, ArN2+ X-. The diazonio group can then be replaced by many other substituents in the Sandmeyer reaction to give a wide variety of substituted aromatic compounds. Aryl chlorides, bromides, iodides, and nitriles can be prepared from arenediazonium salts, as can arenes and phenols. In addition to their reactivity toward substitution reactions, diazonium salts undergo coupling with phenols and arylamines to give brightly colored azo dyes. 

Aryldiazonium salts, usually obtained from arylamines, undergo replacement of the diazonium group with a variety of nucleophiles to provide advantageous methods for producing aryl halides, cyanides, phenols and arenes by reductive removal of the diazo group. Coupling reaction of aryldiazonium salts with phenols or arylamines give rise to the formation of azo dyes. 

Salts of primary aromatic amines react with nitrous acid to produce diazonium salts. The reaction is usually performed by adding a cold solution of sodium nitrite to a cold solution of the arylamine in aqueous mineral acid. The end point of the reaction is conveniently determined by the detection of excess nitrous acid with potassium iodide-starch paper. Sulfamic acid has long been used both in industry and in the laboratory to remove excess nitrous acid. It has been found to react with the more active diazo compounds. In most cases, high temperatures are avoided to prevent the formation of phenols and the decomposition of the unstable nitrous acid. An excess of mineral acid is necessary to prevent coupling between the diazonium salt and unreacted amine (cf. method 494). If the amine salt is somewhat insoluble, a fine crystalline form, which is produced by rapid crystallization from a warm aqueous solution, may be employed. ... 

Heck reactions are compatible with water (see Chapter 3.2.4) [35], which increases the speed of reaction in the presence of quaternary ammonium salts [36 a]. It is not surprising, then, that aqueous solvents (e. g. CH3CN/H2O) and water-soluble catalysts such as Pd(TPPTS)3 where TPPTS = P(C6H4-m-S03Na)3 [35, 67] can be employed successfully (eq. (10)). However, only aryl and vinyl iodides and aromatic diazonium salts (generated in situ from arylamines in aqueous media) are, up to the present, accessible to this method [36 b-h]. 

We have seen that primary arylamines react with nitrous acid to form stable arene-diazonium salts (Section 16.12). Arenediazonium salts are useful to synthetic chemists because the diazonium group can be replaced by a wide variety of nucleophiles. This reaction allows a wider variety of substituted benzenes to be prepared than can be prepared solely from electrophilic aromatic substitution reactions. 

Arylamines from triazenes. Arylamines can be protected as triazenes (formation by reaction of the diazonium salts with a secondary amine). Regeneration of ArNH2 is accomplished by reductive cleavage using Al-Ni alloy in a basic solution. 

To exploit the synthetic versatility of aryl diazonium salts, be prepared to reason backward. When you see a fluorine attached to a benzene ring, for example, realize that it probably will have to be introduced by a Schiemann reaction of an arylamine realize that the required arylamine is derived from a nitroarene, and that the nitro group is introduced by nitration. Be aware that an unsubstituted position of a benzene ring need not have always been that way. It might once have borne an amino group that was used to control the orientation of electrophilic aromatic substitution reactions before being removed by reductive deamination. The strategy... 

The amino-1,3-azoles behave as normal arylamines, for example undergoing carbonyl condensation reactions, easy electrophilic substitutions, and diazoti-sation, though 2-aminooxazoles cannot be diazotised, presumably due to the greater electron-withdrawal by the oxygen. In appropriate cases, good yields of diazoazoles, as opposed to azole-diazoniums salts, can be obtained from aminoimidazoles. ... 

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