Thianthrene radical cations reaction

Another general problem, perhaps with somewhat more impact on homogeneous solution chemistry, is whether the reactions of radical ions with nucleophiles or electrophiles, depending upon the charge, go directly via the radical ions or if reactive doubly charged ions must first be formed by disproportionation. The reaction of thianthrene radical cation with water (Murata and Shine, 1969 Shine and Murata, 1969) is a typical example which was proposed to involve the disproportionation mechanism (71 and 72). The... 

The reactions of thianthrene radical cation have recently been reviewed [10]. Consequently, this discussion will emphasize the major reaction motifs and reactions reported subsequent to the previous review. However, some of the previously reviewed material will be presented here so that a complete picture may be painted. 

In competition with this radical coupling reaction is electron transfer between the alkyl radical and thianthrene radical cation. If the radical R- is of sufficiently low oxidation potential (and the reorganization energy permits), e.g., terf-butyl radical, then electron transfer occurs to yield the corresponding cation R+. Evidence for electron-transfer in these reactions was also convincingly demonstrated by the use of unsymmetrical organometallic species. Kochi [29] has shown that the alkyl group that is preferentially cleaved... 

Formation of 7r-cation radicals has also been suggested [35] in the reaction of stable enols with thianthrene radical cation and other one-electron oxidants. Thus enol 10 on treatment with thianthrene radical cation affords benzofuran derivative 11 in 87 % yield. The initial step in this reaction is suggested to be one-electron transfer forming the cation radical of 10, which has been unequivocally identified in a related system [36]. 

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. 

Cyclized products 13 and 14 are formed in 0.7 and 12% yields, respectively, in the reaction of lithium dialkylamide 15 with thianthrene radical cation [41]. 

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] ... 

This led to the interesting suggestion that the azo compound underwent one-electron oxidation to the then little known azoalkane radical cation. This species subsequently lost dinitrogen to form 1-adamantyl cation and 1-adamantyl radical. This latter species was then oxidized by thianthrene radical cation to 1-adamantyl cation. The 1-adamantyl cation thus formed underwent Ritter reaction to produce AT-adamantyl acetamide 20. 

Oxidation of aryl hydrazones by thianthrene radical cation have also been suggested to occur via electron-transfer and such reactions have been reviewed previously [110]. Reaction of oximes with thianthrene radical cation produces cycloaddition products [56,57], nitriles, and carbonyl compounds. The cycloaddition products are believed to be formed via initial one-electron oxidation of the oxime. 

Finally it should be noted that the radical products obtained by nucleophilic attack on thianthrene radical cation are characteristically oxidized by this radical cation in a one-electron transfer reaction. This process is presented in detail in the subsequent section on nucleophilic attack. 

What is the mechanism of attack of nucleophiles on radical cation species such as anthracene or thianthrene radical cations Extensive studies by a number of groups have been conducted on the reaction of radical cations with nucleophiles (6, 23, 24). Two main mechanisms have been proposed the disproportionation pathway, equations 6 and 7, in which the nucleophile,... 

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. 

Intermediates in the thermal oxidations of thioethers with NO are generally difficult to detect owing to the limiting reaction rates. The exceptions are oxidations of strong sulphur-containing donors derived from thianthrene. The stoichiometric oxidation of thianthrene in dichloromethane at -78 °C produces a pink-coloured solution ( max =-544 nm) that finally leads to a pale-yellow solution from which thianthrene 5-oxide was isolated in 94% yield. This process involves the intermediate stage of the thianthrene radical cation formation [23] ... 

An energy-sufficient mixed chemiluminescent radical-ion reaction is that of thianthrene (TH) radical cation 101 and 2.5-diphenyl-1.3.4-oxadiazole (DPO) radical anion 102 156> ... 

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. 

A ring carbon can also be involved, however, as in the reaction of the thianthrene and phenothiazine radical cations in neat pyridine or with pyridine in an anhydrous solvent. In this reaction the 1-pyridinium group is inserted on to the benzo ring (43), apparently via nucleophilic attack on di-cations 42, in turn resulting from oxidation of the initially formed radical cation adducts (Scheme 27). In the presence of moisture the sulfoxides are again formed [84]. 

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]. 

Benzothiazolylhydrazones 281 react with the thianthrene cation radical to give [l,2,4]triazolo[3,4- ]benzothiazoles 282 in an oxidative intramolecular cylization reaction (Equation 36) <2000BKC425>. 

The restriction for a nucleophile to penetrate and react with the confined cation-radical sometimes leads to unexpected results. Comparing the reactions of thianthrene cation-radicals, Ran-gappa and Shine (2006) refer to the zeolite situation. When thianthrene is absorbed by zeolites, either by thermal evaporation or from solution, thianthrene cation-radical is formed. The adsorbed cation-radical is stable in zeolite for a very long time. If isooctane (2,2,4-trimethylpentane) was used as a solvent, tert-butylthianthrene was formed in high yield. The authors noted it is apparent that the solvent underwent rupture, but the mechanism of the reaction remains unsolved. ... 

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