Triarylaminium Cations

Table 10. Formal potentials (V, relative to Fc) of triarylaminium cations [N(atyl)3] ...

Substituted 3-anilino-l,5-diphenylpyrazoles were oxidized with lead(iv) tetraacetate in benzene or methylene chloride solution <1997M261>. ESR measurements confirmed the formation of aminyl radicals from /irzra-CFIs-substituted pyrazoles. The radical intermediates from unsubstituted pyrazoles were recognized by their transformations to triarylaminium cation radicals. These were generated by consecutive oxidation of the dimeric products the structures of which were proved by NMR spectroscopy and labeling. 

A stable triarylaminium salt (2) was isolated by Wieland in 1907 [3]. Neither 1 nor 2 was recognized as a cation radical salt, since this concept had not yet even begun to emerge. It was not until 1926 that Weitz formulated the corresponding perchlorate salt of 2 as a delocalized cation which is also a free ammonium radical [4]. Michaelis then made Weitz s interpretation more certain via electrochemical oxidation studies, and he coined the term cationic free radical , which is the appropriate antecedent of the current term cation radical [5]. Since that time, many other cation radical salts have been prepared, but the salts S " and 4 have an especially important role in the electron transfer chemistry of cation radicals [6, 7],... 

In 1981, Bauld and Bellville demonstrated that substrate cation radicals can be generated at a rate sufficient to produce efficient ET cycloaddition chemistry by using triarylaminium salts such as S " in catalytic amounts [15]. Since 3+ is commercially available and shelf-stable, this chemical method of oxidation is a particularly convenient method for generating cation radicals of substrates which have oxidation potentials up to about 1.6 V vs SCE and for studying their unique ET chemistry in solution. The oxidation potential of 3 is 1.05 V vs. SCE [7]. Other triarylaminium salts such as 4+ which have even higher oxidation potentials (the oxidation potential of 4 is 1.59 V [7]) are also available and have been employed in many successful ET reactions of less readily oxidizable substrates. A particular advantage of 4+ is its much greater solubility than 3+, in dichloromethane, so that reactions can be run at temperatures as low as -78 °C or even lower. 

Recently, Ledwith (68) continuing his interest in the chemistry of cation radicals (69. 70) demonstrated that the photoinitiation by triarylaminium, sulfonium, and iodonium salts occurs by a mechanism that is different from that proposed by Crivello. 

Wieland, 1907 Wieland and Wecker, 1910). Oxidation was carried out in an inert solvent such as benzene, from which the salt precipitated. The easier and more reliable preparation of perchlorate salts by the iodine-silver perchlorate method (Weitz and Schwechten, 1926, 1927), to which we have referred earlier (p. 168), allowed a much clearer understanding of the nature of triarylaminium ions to be obtained. Isolation of perchlorates permitted chemical studies, and easy reduction to the triarylamine by iodide ion, ferrous ion, etc., was consistent with the cation-radical view that was developed. The name aminium ion was coined by Weitz. Other salts were prepared such as tritolylaminium picrate (by oxidation of the amine with lead dioxide in the presence of picric acid), and it was also recognized that conjugate anions in salts obtained by oxidation with antimony pentachloride, phosphorus pentachloride and ferric chloride had to be complex anions rather than simple anionic radicals. This is a particularly pertinent point in antimony pentachloride oxidations (p. 165). 

Electron-transfer-mediated cycloadditions of 2-(l-cyanovinyl)indoles can be carried out with cyclohexadienes and electron-rich styrenes the reactions are catalysed by photoexcited pyrylium salts or triarylaminium radical cations (Equation (91)) <93AG(E)90i>. 

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