Diazonium ions, aromatic halides

R—N=N=. Aryl diazonium ions are formed by treatment of primary aromatic amines with nitrous acid. They are extremely useful in the preparation of aryl halides, phenols, and aryl cyanides. 

A second group of aromatic substitution reactions involves aryl diazonium ions. As for electrophilic aromatic substitution, many of the reactions of aromatic diazonium ions date to the nineteenth century. There have continued to be methodological developments for substitution reactions of diazonium intermediates. These reactions provide routes to aryl halides, cyanides, and azides, phenols, and in some cases to alkenyl derivatives. 

Synthetically important substitutions of aromatic compounds can also be done by nucleophilic reagents. There are several general mechanism for substitution by nucleophiles. Unlike nucleophilic substitution at saturated carbon, aromatic nucleophilic substitution does not occur by a single-step mechanism. The broad mechanistic classes that can be recognized include addition-elimination, elimination-addition, and metal-catalyzed processes. (See Section 9.5 of Part A to review these mechanisms.) We first discuss diazonium ions, which can react by several mechanisms. Depending on the substitution pattern, aryl halides can react by either addition-elimination or elimination-addition. Aryl halides and sulfonates also react with nucleophiles by metal-catalyzed mechanisms and these are discussed in Section 11.3. 

Although aromatic halides are inert to both SN1 and SN2 reactions (see Chapter 8), aromatic diazonium ions can act as the electrophilic partner in a nucleophilic substitution reaction. These ions are highly reactive because the leaving group, N2, is an extremely weak base ... 

Coupling of a few aliphatic diazonium compounds to aromatic rings has been reported. All the examples reported so far involve cyclopropanediazonium ions and bridgehead diazonium ions, in which loss of N2 would lead to very unstable carbocations. ° Azobenzenes have been prepared by Pd-catalyzed coupling of aryl hydrazides with aryl halides, followed by direct oxidation. ... 

To further decorate M-aromatic groups, several modifications were carried out by diazotization of 4-aminophenylthiohydantoins 31 with sodium nitrite. The diazonium ion 32 underwent a Sandmeyer reaction with cuprous halides to furnish several halobenzene derivatives 33. 

One of the best methods for the introduction of iodine into aromatic rings is the reaction of diazonium salts with iodide ions. Analogous reactions with chloride, bromide, and fluoride ions give poorer results, and 14-25 and 13-20 are preferred for the preparation of aryl chlorides, bromides, and fluorides. However, when other diazonium reactions are carried out in the presence of these ions, halides are usually side products. Aniline has also been converted to fluorobenzene by treatment with t-BuONO and Sip4 followed by heating. A related reaction between PhN=N—N C4Hg and iodine gave iodobenzene. ... 

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. 

Treatment of an aromatic amine with nitrous acid gives dia-zonium compounds. These couple with phenols to give dyes. Displacement of nitrogen from a diazonium compound by a halide or cyanide ion takes place with copper catalysis and is a useful synthetic method. 

Sulphonyl halides may be prepared by reaction of some organic compounds with sulphur dioxide in the presence of a halogen or halide ion. Compounds which undergo such reactions include aromatic diazonium salts and some aliphatic species. 

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