Thianthrenes, radical cations, reversible

To our knowledge no reaction of iodide ion as a nucleophile with a cation radical is known. Iodide ion reduces cation radicals very well and is frequently used for the iodimetric assay of cation-radical salts. Since the reduction is reversible and some compounds can be oxidized to the cation radical stage by iodine, an excess of iodide is used. Some cation radicals are also reduced by other halide ions for example, that of 9,10-diphcnylanthracene is reduced by bromide ion (Sioda, 1968), that of perylene by bromide and chloride ions (Ristagno and Shine, 1971b), and thianthrene radical cation to some extent by chloride ion (Murata and Shine, 1969). These reductions, particularly those by iodide ion, reflect again the competition between nucleophilicity and oxidizability of a nucleophile in reactions with cation radicals. 

Reversible dimerizations are observed less frequently, since often the reactions are not fast enough to be treated as thermodynamic equilibria. Examples are the dimerization of the radical cations of thianthrenes [110] and thiophene derivatives [111]. 

For mesitylene and durene, the kinetics have been followed by specular reflectance spectroscopy [17]. The results indicated that mesitylene produces a fairly stable radical cation that dimerizes. That of durene, however, is less stable and loses a proton to form a benzyl radical, which subsequently leads to a diphenylmethane. The stability of the radical cation increases with increasing charge delocalization, blocking of reactive sites, and stabilization by specific functional groups (phenyl, alkoxy, and amino) [18]. The complex reaction mechanisms of radical cations and methods of their investigation have been reviewed in detail [19a]. Fast-scan cyclovoltammetry gave kinetic evidence for the reversible dimerization of the radical cations of thianthrene and the tetramethoxy derivative of it. Rate constants and enthalpy values are reported for this dimerization [19b]. 

As we have pointed out previously, oxygen and water concentrations can be kept at extremely low levels with a properly maintained purification train. In fact, contamination by water is much more easily controlled in a dry box than on a vacuum line. This may result in part from the number of operations and manipulations necessary to use a vacuum line. The oxidation of the cation radical of thianthrene to the dication illustrates this point. The dication is very electrophilic and is rapidly attacked by any nucleophiles (e.g., water). The electrochemistry of the dication in solutions prepared in the dry box (with acetonitrile as the solvent purified as described earlier) is reversible if a little care is taken in preparing the solvent and in drying the glassware. It is more difficult to obtain such reversible behavior when solutions are prepared on a vacuum line. 

The cation radical of thianthrene (LVII" ") has been subject of extensive investigation [3, 187]. Voltammetric experiments with thianthrene (LVII) when carried out in the presence of TEA and its anhydride or activated neutral alumina gave reversible oxidation potentials in a variety of solvents, and disproportionation equilibrium constants, Xd,sp, were calculated [190]. Accurate values of X isp are important in the mechanistic evaluation for the reaction of LVII" with nucleophiles and have been used to rule out the disproportionation pathway, such as the reaction with water [191]. 

Apparently, the p-toluyl cation (not cation radical) is formed by oxidative scission of the ketone and is trapped by fluoride donation from the electrolyte Me4NBF4. In our context it is interesting that, in demonstrating the reversible one- and two-electron oxidation stages of thianthrene, Hammerich and Parker (1973) used the same electrolyte. Thus the BF4 ion is apparently not nucleophilic towards either the thianthrene cation radical or dication within the times of the cyclic voltammetry. 

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