The relative reactivity of 1,3-and 1,2-dimethoxybenzene 1o and 1d in thianthrenation and the lack of reactivity of 1,4-dimethoxybenzene 1y in both reactions prompted us to take a closer look at these substrates (Scheme 7). Electron-rich arenes, such as these, are known to be reductants via single electron transfer (SET), and arene radical cations have been suggested as intermediates in the formation of both aryl thianthrenium and diaryliodonium salts. 1,3-Dimethoxybenzene 1o has a reduction potential of 1.50 vs SCE, and readily forms 1o-IMes and 1o-TT in high yields (Scheme 7a). 1,2-Dimethoxybenzene 1d is a stronger reductant with a reduction potential of 1.43 vs SCE, and is only compatible in the thianthrenation reaction (Scheme 7b). In the case of iodination of 1d, both 1d and Me-sI(OAc)2 are completely consumed in the reaction, but no product is formed (Scheme 7b). Moreover, a competition experiment between 1d and 1o in the formation of aryl(Mes)iodonium salts results in neither 1d-IMes or 1o-IMes. This data suggests that 1d inhibits the formation of 1o-IMes by decomposing MesI(OAc)2 via SET. A similar scenario is observed for the attempted formation of an aryl(Mes)iodonium salt from 1,4-dimethoxybenzene 1y; both the arene and MesI(OAc)2 are consumed, but no product is observed (Scheme 7c). 1,4-Dimethoxybenzene 1y also inhibits the formation of 1o-IMes in a competition experiment. Thianthrenation of 1y is somewhat different from iodination. Although thianthrenation of 1y is unsuccessful and 1y is completely consumed, it does not inhibit Scheme 6. Arene relative reactivity scale for the formation of aryl thianthrenium and diaryliodonium salts. a,b a Conditions: 1 (1 mmol, 1 equiv.), MesI(OAc)2 (MIDA) (1 mmol, 1 equiv.), TfOH (1 mmol, 1 equiv.), MeCN (1 mL), r.t., 3 hours. b Conditions: 1 (0.5 mmol, 1 equiv.), TTO (0.5 mmol, 1 equiv.), Tf2O (0.6 mmol, 1.2 equiv.), MeCN (2 mL), 0-25 °C, 30 minutes. the formation of 1o-TT in a competition experiment. Collectively, the data presented in Scheme 7 suggest that there is a key difference in the mechanisms for forming aryl thianthrenium and aryl(Mes)iodonium salts. Although both reactions are somewhat sensitive to the reduction potentials of arene substrates, thianthrenation reactions are not inhibited by strongly reducing arenes. This may suggest that arene radical cations are not productive intermediates in the formation of diaryliodonium salts.