Unified mechanistic analysis of polar reactions of diaryliodonium salts

Reactions of aryl(m-carboran-9-yl)iodonium tetrafluoroborates [aryl = phenyl, (1); 4-methoxyphenyl, (2a); 4-fluorophenyl, (2b); 3-nitrophenyl, (2c); mesityl, (2d)] with Br–, NO2–(polar reactions) and PPh3(radical reactions) have been studied. Polar reactions of 1, 2a–c with Br– and NO2– lead regiospecifically to the corresponding iodoarene and substituted m-carborane. In contrast to this, reactions of 2d, containing a bulky aryl ligand, with Br– and NO2– are not regiospecific, since they produce all four possible products: 9-iodo-m-carborane, iodomesitylene, substituted m-carborane, and substituted mesitylene. These results are rationalized in terms of a new mechanism which includes formation of a 10-I-3 hypervalent intermediate and its subsequent synchronous (cheletropic) decomposition during intramolecular rotation. The mechanism can account for some novel, known chemistry of diaryliodonium ions, including the so-called ‘ortho-effect’ and the unusual reactivity of cyclic halonium cations.