Thianthrenes, oxidation radical-cations

Unsaturated ketones react with phenyUiydrazines to form hydrazones, which under acidic conditions cyclize to pyrazolines (35). Oxidation, instead of acid treatment, of the hydrazone with thianthrene radical cation (TH " ) perchlorate yields pyrazoles this oxidative cyclization does not proceed via the pyrazoline (eq. 4). 

The most evident of these is the marked stability of radical cations formed in an aprotic medium by the oxidation of compounds where the first ionization potential (in the sense of photoelectron spectroscopy) is for the removal of an electron from a non-bonding orbital, e. g. thianthrene... 

The homogeneous oxidation of compounds 14 and 15 with thianthrene radical cation perchlorate (Th C104) was studied later by Kovelski and Shine... 

Thianthrene, 68 X = Y = S, radical-cation is obtained by oxidation in trifluoro-acetic acid containing perchloric acid and the evaporation of the solvent [232], It shows electrophilic behaviour on the sulphur atom. When the electrochemical oxidation of thianthrene is carried out in aqueous acetic acid, the monoxide is obtained... 

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. 

Azoadamantane exposed to 2 mol equivalents of T CIO4 at room temperature rapidly and quantitatively evolved nitrogen, and thianthrene and products derived from the adamantyl cation were obtained. Equations (38)-(40) (AA, azo-adamantane Ad, adamantane) make clear why 2 mol equivalents of the radical oxidant are required (85JA2561). The comparable interaction of T with phenylazotriphenylmethane and di-ter/-butyl diazene, using a 2 1 ratio of radical cation to substrate, also leads to the formation of thianthrene and nitrogen (85PS111). 

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]. 

A different approach, MSRS, involves modulation of the electrode potential by a periodic function, e.g., a sine wave or a pulse [340-345]. Absorbance variations other than those that are synchronized with the modulation frequency are discriminated by help of a lock-in amplifier, leaving the system almost insensitive to absorbance of products built up during the experiment. In this way it has been possible to obtain the visible spectrum of perylene radical anion in the presence of acetic acid (Fig. 50) and thianthrene radical cation in the presence of water [341]. Other examples include the oxidation of N,N-... 

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... 

Thianthrene radical cation is also an excellent one-electron oxidant of iron porphyrin complexes. Such oxidation of Fem(0Cl03)(TPP), where TPP is meso-tetraphenylporphyrin, provides the corresponding porphyrin 7r-cation radical analytically pure [32]. Similar oxidation of the AT-methyl porphyrin complex (N-MeTPP)FenCl, where AT-MeTPP is AT-methyl-meso-tetraphenylporphyrin, afforded [N-MeTPPFemCl]+ which was not further oxidized [33]. Thus thianthrene radical cation selectively oxidized the aromatic porphyrin ligand in one case and the metal center in the other. Ligand oxidation at a phenolic moiety has also been reported [34] on treatment of a 1,4,7-triazacyclononane appended with one or two phenol moieties ligated to Cu(II) complex with thianthrene radical cation. 

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. 

Oxidation of azo compounds by thianthrene radical cation has been reported. Thus treatment of l,l -azoadamantane 19 with two equivalents of thianthrene radical cation perchlorate in acetonitrile produced AT-adamantylacetamide 20 in 90% yield and thianthrene quantitatively [53]. 

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. 

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). 

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. 

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). ... 

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... 

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] ... 

Oxidation takes place at sulfur but charge and spin are highly delocalized. There will be no further elaboration on this transient here since this review is primarily devoted to radiation chemical contributions and aliphatic systems. But it should be mentioned that a wealth of information is available on thianthrene radical cations, particularly in the work of Shine [110]. Further examples for these species, including many thianthrene derivatives, have recently been reviewed by Glass [111], and informative reference may also be found in an article by Russell and Law [112]. 

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