Diazonium salts aryl, reaction with aromatic compounds

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. 

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. 

As it is well known, nucleophilic substitution of a C-X bond, one of the key synthetic reactions with aliphatic compounds is severely limited with aromatic derivatives, where it occurs thermally only with electron-withdrawing substituted compounds and/or under severe conditions. Alternatives include time honored reactions involving the phenyl radical generated by decomposition of diazonium salts after a reductive step, such as the Meerwein and the Gomberg-Bachmann reactions, as well as the (often photoinitiated) SrnI reaction, where a (usually weak, e.g. carbon-iodine) bond is cleaved after monoelectronic reduction to give an aryl radical as the active inter-mediate that adds to an enolate, cyanide or other nucleophiles (and thus again with an aryl radical as the key intermediate. Scheme S). ... 

A related process that involves diazonium salts is the Pschorr reaction, hich also couples aryl diazo-nium compounds to other aromatic rings. This diazonium salt coupling can he done under acidic conditions, but addition of copper powder usually promotes the radical process. Aryl amines generate aryl diazonium salts upon treatment with nitrous acid.l An example is the reaction of 233 to give an aryl diazonium salt, which cyclized in the presence of copper to give thaliporphine (234) in 43% yield. Kupchan called this transformation an improved Pschorr reaction. 68... 

The Balz-Schiemann and Wallach reactions The Balz-Schiemann reaction (the thermal decomposition of an aryl diazonium salt. Scheme 46) was for many years the only practical method for the introduction of a fluorine atom into an aromatic ring not bearing electron-withdrawing substituents. This reaction, first reported in the late 1800s, was studied in fluorine-18 chemistry as early as 1967 [214]. It involves the generation of an aryl cation by thermal decomposition, which then reacts with solvent, nucleophiles or other species present to produce a substituted aromatic compound. Use of fluorine-18-labelled... 

Primary aromatic amines (e.g. aniline) and secondary aliphatic-aromatic amines (e.g. N-methylaniline) usually form diazoamino compounds in coupling reactions with ben nediazonium salts. If the nucleophilic reactivity of the aryl residue is raised by sutetituents or fused rinp, as in m-toluidine and I - and 2-naphthylamine, aminoazo formation takes place (C-coupling). Hov ver, the possibility has also been noted that in aminoazo formation, initial attack of the diazonium ion may still be at the amine N-atom, but the a -complex might rearrange too rapidly to allow its identification... 

Oxygen reacts readily with aromatic sulfides [341], AT-methionyl peptides [342], 1,3-dithianes [343], aziridines [344], and phosphorus [345] compounds after oxidation. On the other hand, aryl radicals are involved in the ET reaction of trivalent phosphorus compounds with aromatic diazonium salts [346]. 

In each case, the mechanism involves generation of an aryl radical from a covalent azo compound. In acid solution, diazonium salts are ionic and their reactions are polar. When they cleave, the product is an aryl cation (see p. 856). However, in neutral or basic solution, diazonium ions are converted to covalent compounds, and these cleave to give free radicals (Ar and Z"). Note that radical reactions are presented in Chapter 14, but the coupling of an aromatic ring with an aromatic compound containing a leaving group prompted its placement here. Note the similarity to the Suzuki reaction in 13-12. 

Methyl-, benzyl- and toluene-sulphonamide react with aromatic diazonium compounds to give isolable triazenes which decompose in the presence of alkali with the formation of the corresponding aromatic azide and the alkyl- or aryl-sulphinic acid salt . This sequence (equation 123), known as the Dutt-Wormall reaction, is mechanisti-... 

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. 

Aryl diazonium salts can also undergo coupling reactions with phenol or aromatic amines, which possess nucleophilic OH or NH2 groups, respectively. This electrophilic substitution reaction (with the diazonium salt as the electrophile) produces highly coloured azo compounds. 

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