Thianthrene radical cations dimerization

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]. 

Dimerization of heterocyclic cation radicals was discovered in recent times. The well known thianthrene cation radical perchlorate forms a dimer in propionitrile solution. This association is, in fact, tetrameric (de Sorgo et al., 1972) that is, the anions are included in the aggregrate (Th +C104)2. Phenoxaselenine cation radical has also been found to form an (M +)2 dimer in sulfuric acid and nitro-methane solutions (Cauquis and Maurey-Mey, 1973). It is not surprising that the very stable metalloporphyrin cation radicals should dimerize, but this was demonstrated only recently with zinc (Fuhrhop et al., 1972), and magnesium (Fajer et al., 1970) octaethyl-porphyrin (MOEP). 

Mann and Cottrell have established that electrochemical oxidation of aliphatic sulfides proceeds through a relatively stable radical-cation 15 localized on the sulfur atom. However, the more stable configuration corresponds to a molecular complex 16 derived from two sulfide molecules according to ESR data. " ° The weak bond that is formed between the interacting sulfur atoms is a two-center three-electron (2c, 3e) bond." Further oxidation results in formation of an ordinary single bond between the two positively charged onium ions. Similar observations were made recently for electrochemical behavior of thianthrene where the ratio of radical-cation to dimer could be easily controlled by concentration and temperature and studied quantitatively using in situ UV/Vis-NIR- and ESR-spectroelectrochemical measurements (Scheme 6). ... 

Dimerizations are highly solvent dependent. Thus, thianthrene cation radical does not dimerize as readily in acetonitrile as in... 

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