Thianthrene dication

Thianthrene dication 17 is formally a fully aromatic species but evidence for electronic stabilization of these species has yet to be adduced. 

Cation radicals may be oxidized further, and some of them become stable dications. This process is often characterized as the second wave in two successive anodic one-electron oxidations. The thianthrene dication is now well known (Shine and Piette, 1962 Hammerich and Parker, 1973), but has not been isolated. On the other hand, the dipcrchloratc of 2,3,7,8-tetramethoxythianthrene dication is a blue solid (Glass el al., 1973). Dications of some aromatics which are well-known for forming stable cation radicals (e.g. perylene) have been made in FS03H-SbF5 solution (Brouwer and van Doom, 1972). Irradiation of hexachlorobenzcne cation radical in SbFs-Cl2 causes the formation of the dication (in this case, a triplet state) (Wasserman et al., 1974). Oxidation of metallo-porphyrin cation radicals to stable (but reactive) dications is quite common (Dolphin et al., 1973). 

The thianthrene cation-radical (56) shows enhanced stability in trifluoro-acetic acid, which is recommended generally as a solvent in which to prepare cation-radicals. Thianthrenium perchlorate (56 C10 as counter-ion) reacts with substituted benzenes PhR at the para position, rapidly where R = MeO, more slowly where R = Me. The product is a sulphonium salt (58). Kinetic studies show that the reaction is second-order in the thianthrene cation-radical (56). The suggested mechanism features the thianthrene dication (57) as the reactive species, formed in low concentration by disproportionation of the thianthrene cation-radical. 

The first thianthrene dication (32) has been isolated. Work on the thianthrene cation radical (33) continues. Its reaction with aliphatic amines yielded the sulphimides (34 X = NR) phenols and aromatic amines were attacked in the pflra-position to afford (34 X = / -HOCeH4 or p-R2NC H4), and enolizable ketones yielded the j8-keto-sulphonium salts (34 X = CHR COR ). The latter compounds reacted with nucleophiles Y to give the a-substituted ketones YCHR COR, and might prove to be useful intermediates for their synthesis. Reinvestigation of the kinetics of the reaction of (33) with phenol and with anisole has provided evidence which appears to disprove the disproportionation mechanism postulated earlier for these reactions. 

Thianthrene, 2,3,7,8-tetramethoxy-dication, 3, 968 Thianthrenedicarboxylic acids applications, 3, 993 Thianthrene-2,6-dicarboxylic acids... 

Thianthrene 5-oxide is converted, in concentrated sulfuric acid, into a solution of T (62JA4798) sulfuric or perchloric acid in nitromethane can also be used (63TL993). One view is that this transformation involves homolysis of the O-protonated sulfoxide, with hydroxyl radical as byproduct, though the involvement of a dication has also been suggested (63JOC2828). 

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. 

This would correspond to an ECEC process at the anode 37 In contrast, the reaction between thianthrene cation radical and water 119- has been shown to occur with the dication as the kinetically active species due to equilibrium (56), corresponding to an electrochemical ECCC process ... 

For the purpose of this review the term heteroaromatic is applied to 7r-radical species when they may be regarded as arising from aromatic heterocyclic molecules or ions by addition or removal of an odd number (usually one) of electrons. Thus, entities such as the anion- and cation-radicals of pyridine (1 and 2) are clearly heteroaromatic. The dilemma whether or not to regard thianthrene (3) as aromatic (its central ring possessing eight electrons) does not arise for the thianthrene cation radical (4) it is heteroaromatic on the grounds that it may formally arise by one-electron reduction of the aromatic thianthrenium dication (5). 

Redox processes involving 178 have also been studied.Anodic oxidation of thianthrene has been eifected in a wide variety of solvents. Use of trifluoracetic acid gives stable solutions of 178 and, if perchloric acid is included, the solid perchlorate salt may be isolated on evaporation of the solvent after electrolysis. Dichloromethane at low temperatures has been used and, at the opposite extreme, fused aluminum chloride-sodium chloride mixtures. " Propylene carbonate permits the ready formation of 178, whereas the inclusion of water in solvent mixtures gives an electrochemical means of sulfoxidizing thianthrene. Reversible oxidation of 178 to thianthrenium dication may be brought about in customary solvents such as nitriles, nitro compounds, and dichloromethane if the solvent is treated with neutral alumina immediately before voltammetry addition of trifluoracetic anhydride to trifluoracetic acid equally ensures a water-free medium. The availability of anhydrous solvent systems which permit the reversible oxidation and reduction of 178 has enabled the determination of the equilibrium constants for the disproportionation of the radical and for its equilibria with other aromatic materials. ... 

This equilibrium has attracted much attention because of the mechanism first suggested for the reaction of thianthrene radical cation with water. In this reaction, the second order dependence of the rate on the concentration of TH was explained by disproportionation followed by nucleophilic attack on the dication. The equilibrium constant KD for the reaction shown in Eq. (8) may be determined electrochemically. The difference in E° for the two one-electron processes shown in Eqs. (9) and (10) determine KD. Interestingly, this value depends strongly on solvent and counterion and pKD ranges between 5-13 [46-48] ... 

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. 

Benzopentathiepin (35) reacts with many aromatic compounds, such as benzene, 1,4-dimethyl-benzene, and naphthalene in the presence of Lewis acid to give various thianthrenes in moderate yields. This benzopentathiepin was found to serve as a 1,4-dication synthon (Scheme 9) <88H(27)2563>. 

A more detailed study" has shown that this oxidation occurs as a stepwise process with the second oxidation potential 20 mV lower than the first. Such anomalous electrochemical behavior of 10 presents more evidence for the formation of a S-S dication." The first evidence that 10 can be oxidized chemically to a stable disulfonium dication was obtained by Shine and Kette." While investigating the oxidative properties of the thianthrene radical cation... 

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