Thianthrene radical cations salts

Figure 17. 1,4-Dithiin 21 and the molecular structure of its radical-cation salt 2f+SbF6 determined by X-ray crystallography. The observed lengths (A) of Cl-S and C1-C2 in 2f+SbF6 are 1.72(1) and 1.31(2) to be compared with 1.324(3) and 1.760(2) determined for neutral 21, respectively. Also shown is the thianthrene radical cation 23 +. 

Species 4, produced from thianthrene by loss of an electron from a sulfur, is correctly known as thianthrene radical ion(l+) most authors have referred to it as the thianthrene radical cation or the thianthrene cation radical. The species produced by loss of two electrons from the central ring, and for which 5 is probably a resonance contributor, is termed thianthrenediium. Sulfonium salts (6) produced formally by utilizing a sulfur lone-pair in bonding to, are 5-R-thianthrenium salts. Thianthrene... 

These results support the suggestion that the energy difference between folded and planar thianthrene conformations is small. However, 2,3,7,8-tetramethoxy-thianthrene radical cation [22], as both the hexachloroantimonate and triiodide salts, and thianthrene radical cation [20], as its tetrachloroaluminate salt, have been shown to be planar in the solid state. The dihedral angles 0 are 180° and 174°, respectively. These results provide strong evidence for delocalization in... 

This cation radical rapidly deprotonates, undergoes further oxidation (or disproportionates) to the corresponding a-carbonyl cation. Cyclization of this cation and rearrangement [37] yield ultimately 11. It should be noted that an alternative reaction path has been identified in the reaction apparently of enols with thianthrene radical cation. Ketones [38, 39] and aldehydes [40] on treatment with thianthrene radical cation form sulfonium salts 12 and thianthrene. 

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. 

A great deal of work has been carried out on the thianthrene radical ion(l+), which can be produced from thianthrene by a variety of one-electron oxidations. The radical cation reacts at sulfur with nucleophilic species, giving rise to 5-substituted products, oxides, ylids, and 5-R-thianthrenium salts. 

A quite different mode of reaction was observed for the reactions between thianthrene radical ion(l+) and the heterocyclic bases pyridine (72JOC2691) and 2,3-diazabicyclo[2,2,2]oct-2-ene (88JA7880) thianthren-2-yl-N salts were obtained in each case. It was shown that 2 mol equivalents of the radical cation are required, the byproduct being thianthrene. 

Further interesting examples of C-S bond formation involve the reaction of previously prepared (or in situ anodically generated) thianthrene or phenothiazine radical cations with alkenes or alkynes, to give l,2-bis(hetaryl) alkanes (or the respective alkenes) [82]. With cyclooctene a 1 1 adduct is obtained instead. Another valuable application is the smooth reaction with ketones (Scheme 26). The thian-threnium salts (40) now obtained are readily deprotonated to the corresponding ylides (41) [83]. The latter compounds are directly obtained when yff-dicarbonyls are used. 

Thianthrene cation radical tetrafluoroborate 139 has been found to add to 2,3-dimethyl-2-butene 138 at —15 °C to give adduct 2,3-dimethyl-2,3-(5,10-thianthreniumdiyl)butane ditetrafluoroborate 140, which was isolated and characterized by 111 NMR spectroscopy at —15 °C (see Equation 40) <2006JOC3737>. The adduct 140 was stable in CD3CN solution at —15 °C but decomposed quickly at 23 °C, forming the salt of 2,4,4,5,5-pentamethyl-2-oxazoline with loss of thianthrene. 

The reaction in Scheme 5.11 gives the snlfoninm salt (anion CIO4 ) in a 90% yield (ronte a). One-electron reduction of the thianthrene cation-radical by anisole is the side reaction (ronte b). Route b leads to products with a 10% total yield. Addition of the dibenzodioxine cation-radical accelerates the reaction 200 times. The cation-radicals of thianthrene and dibenzodioxine are stable. Having been prepared separately, they are introdnced into the reaction as perchlorate salts. 

When the concentration of the thianthrene cation-radical drops from 10 -10 M to 10 M, the reaction results change The sulfonium salt is not produced at all (route a becomes closed) and... 

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-... 

The interhalogens iodine bromide and iodine chloride form stronger charge-transfer complexes than iodine. Therefore, oxidation of suitable substrates should be feasible. In the case of thianthrene, oxidation is very rapid and a cation radical salt with a complex anion is formed (22) (Murata and Shine, 1969). 

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. 

Synthesis and Properties.—The two most general methods for preparing sulphonium ylides continue to be a-deprotonation of a sulphonium salt and the reaction of a sulphide (or disulphide with a carbene. A new development involves the reaction of the thianthrene or phenoxathiin cation radical with a dicarbonyl compound, e.g. ethyl benzoylacetate, to give (4). An infrequently used but useful route to sulphonium ylides involves reaction of sulphides e.g. dithia[3,3]cyclophanes with benzyne. A detailed description of a preparation of the sulphonium salt precursor to Trost s diphenylsulphonium cyclopropylide has appeared. The selectivity of ylide formation in the reaction of cyclic and acyclic sulphides with carbenes has been examined and compared with the much... 

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. 

---