Aryl radicals, heterolytic radical additions

Simple mechanistic considerations easily explain why heterolytic dissociation of the C — N bond in a diazonium ion is likely to occur, as a nitrogen molecule is already preformed in a diazonium ion. On the other hand, homolytic dissociation of the C —N bond is very unlikely from an energetic point of view. In heterolysis N2, a very stable product, is formed in addition to the aryl cation (8.1), which is a metastable intermediate, whereas in homolysis two metastable primary products, the aryl radical (8.2) and the dinitrogen radical cation (8.3) would be formed. This event is unlikely indeed, and as discussed in Section 8.6, homolytic dediazoniation does not proceed by simple homolysis of a diazonium ion. 

Triplet sensitization of sulfonium salts proceeds exclusively by the homolytic pathway, and that the only arene escape product is benzene, not biphenyl or acetanilide. However, it is difficult to differentiate between the homolytic or heterolytic pathways for the cage reaction, formation of the isomeric halobiaryls. Our recent studies on photoinduced electron transfer reactions between naphthalene and sulfonium salts, have shown that no meta- rearrangement product product is obtained from the reaction of phenyl radical with diphenylsulfinyl radical cation. Similarly, it is expected that the 2- and 4-halobiaryl should be the preferred products from the homolytic fragments, the arene radical-haloarene radical cation pair. The heterolytic pathway generates the arene cation-haloarene pair, which should react less selectively and form the 3-halobiaryl, in addition to the other two isomers. The increased selectivity of 2-halobiaryl over 3-halobiaryl formation from photolysis of the diaryliodonium salts versus the bromonium or chloronium salts, suggests that homolytic cleavage is more favored for iodonium salts than bromonium or chloronium salts. This is also consistent with the observation that more of the escape aryl fragment is radical derived for diaryliodonium salts than for the other diarylhalonium salts. 

We generally think of electrophilic and nucleophilic additions as being heterolytic processes, but there can be electrophilic and nucleophilic character to radical additions also. Alkyl or aryl substituents on a carbonmethyl radical, while electron-withdrawing substituents make it more electrophilic. For a discussion, see Zipse, H. He, J. Houk, K. N. Giese, B. /. Am. Chem. Soc. 1991,113,4324. Perfluoroalkyl radicals are strongly electrophilic see, for example, Avila, D. V. Ingold, K. U. Lusztyk, J. Dolbier, W. R. Pan, H.-Q. /. Am. Chem. Soc. 1993,115,1577. 

The remaining two reactions, which will be described in this section, are related through the well-known formation of azo compounds 39 by the addition of nucleophiles to diazonium salts (40). The former compounds lose nitrogen by homolytic and the latter by heterolytic processes to give, respectively, aryl radicals and sometimes, as has been recently demonstrated, aryl cations. Because the equilibrium between 39 and 40 is so dependent on solvent, gegenion, and additives, both reactions [equations (8) and (9)] often occur simultaneously. ... 

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