Thianthrene anodic oxidation

Radical 80 has been prepared as its perchlorate salt by anodic oxidation in ethyl acetate in the presence of hthium perchlorate. The reactivity toward nucleophiles of material so prepared was investigated nitrite and nitrate ions give 2-nitrodibenzo[l,4]dioxin although the mechanisms of the reactions are not clear. Pyridine gives 7V-(2-dibenzo[l,4]dioxinyl)pyridinium ion (84). Other nucleophiles acted as electron donors and largely reduced 80 back to the parent heterocycle they included amines, cyanide ion and water. In an earlier study, the reaction of 80 with water had been examined and the ultimate formation of catechol via dibenzo[l,4]dioxin-2,3-dione was inferred. The cation-radical (80) has been found to accelerate the anisylation of thianthrene cation-radical (Section lII,C,4,b) it has been found to participate in an electrochemiluminescence system with benzo-phenone involving phosphorescence of the latter in a fluid system, and it has been used in a study of relative diffusion coefficients of aromatic cations which shows that it is justified to equate voltammetric potentials for these species with formal thermodynamic redox potentials. The dibenzo[l,4]dioxin semiquinone 85 has been found to result from the alkaline autoxidation of catechol the same species may well be in-... 

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. ... 

The mechanism of reaction of organosulfur cation radicals with ketones is not yet known, but may well involve the enolic form of the ketone. Reactions occur either in the neat ketone or in acetonitrile solution. Anodic oxidation of thianthrene in acetone also gave the anticipated product [54 X = S R = Me]. 

Reactions with nitrate ion are similar to those of nitrite ion. That is, 6 " " gave 2-nitrodibenzodioxin (12) and I " gave thianthrene 5-oxide (O. The last reaction was interpreted analogously to eq. 44, but the mechanism of the ring nitration is entirely unknown. Anodic nitrations which appear to be cation radical-nitrate ion reactions can be found in the literature, and their mechanisms are also unsolved. 

The preparation of dibenzo[6,e ][ 1,4]dioxin cation radical (66) has been achieved by oxidation of the heterocycle in ethyl acetate-lithium perchlorate at a platinum anode, using a controlled potential of 1.2 volts vs. Ag-AgC104. The blue solid collected at the anode contained between 85-90% of (66) as the perchlorate (74JHC139). The purple cation radical perchlorate of phenoxathiin, (67), is obtained in high purity by oxidation of phenoxathiin in benzene with 70% perchloric acid-acetic acid (75JOC2756). Similar perchloric acid oxidation of thianthrene affords the dark reddish brown perchlorate of (68) (69JOC3368) and the heterocycle can also be oxidized on a preparative scale with antimony pentachloride (62JCS4963). 

Trifluoroacetic acid (TFA) has been used with considerable success in stabilizing cation radicals in anodic and, on some occasions, chemical oxidations. Solutions of thianthrene and... 

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). 

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