Thianthrene radical cations with nucleophiles

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... 

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] ... 

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,... 

The main emphasis of this chapter will be on our attempts to find evidence for radical coupling in reactions of the thianthrene cation radical (Th +) with nucleophiles, that is, for the occurrence of steps such as equations 7 and 8. However, because discussions of reactions of cation radicals with radicals are relatively sparse, we will refer first to other works that principally deal with aromatic nitration. 

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 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]. 

Shine and co-workers have carried out product studies of the reactions of fV-substituted phenothiazine cation-radicals with nucleophilic re-agents. As with the cation-radicals of thianthrene (178) and phenoxathiin (198), addition of certain nucleophiles at S may occur to give, ultimately, such adducts as 270-272 (see Sections III,C,4,b V,B,1). Attack by other nucleophiles may result in reduction by electron transfer or substitution at position 3. Shine has reviewed much of this work. The mechanisms of such reactions have been controversial (see Section... 

The cation radical of thianthrene (LVII" ") has been subject of extensive investigation [3, 187]. Voltammetric experiments with thianthrene (LVII) when carried out in the presence of TEA and its anhydride or activated neutral alumina gave reversible oxidation potentials in a variety of solvents, and disproportionation equilibrium constants, Xd,sp, were calculated [190]. Accurate values of X isp are important in the mechanistic evaluation for the reaction of LVII" with nucleophiles and have been used to rule out the disproportionation pathway, such as the reaction with water [191]. 

The structure of the thianthrene cation radical is shown in structure 1 as Th +. Its electron spin and charge are fully delocalized (2), yet reactions with nucleophiles occur principally at one of the sulfur atoms (3). Therefore,... 

Occasionally, electron transfer is a nuisance and competes with or overshadows a desired nucleophilic reaction. For example, reaction of thianthrene cation radical perchlorate with aniline leads to benzidine (among other products) instead of nucleophilic substitution in thianthrene (Shine and Kim, 1974). 

Because the cyanide ion is so easily oxidized its apparent ability to react with aromatic cation radicals instead of being oxidized by them reflects the competition so often encountered in cation radical chemistry between nucleophilicity and oxidizability of a nucleophile. The subject has not been treated analytically yet. In the present context, the tri-p-anisylaminium ion is reduced by cyanide ion (Papouchado et al., 1969) in a very fast overall second-order reaction (Blount et al., 1970). The cation radicals of thianthrene, pheno-thiazine, and phenoxathiin are also reduced by cyanide ion (Shine et al., 1974). In none of these cases, incidentally, is the fate known of the cyano radical presumed to be formed. Perylene cation radical perchlorate, on the other hand, reacts with cyanide ion in acetonitrile solution to give low (13%) yields of both 1- and 3-cyano-perylene (Shine and Ristagno, 1972). 

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. 

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. ... 

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. 

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-... 

Numerous electrochemical studies of systems which form stable cation radicals exist. For example, the CV oxidation of aromatic hydrocarbons which have the sites of high electron density blocked, such as 9,10-diphenylanthracene (DPA) or rubrenc, show volt-ammograms typical of nernstian one-electron systems (Phelps et al., 1967 Marcoux et al., 1967 Peover and White, 1967). Rotating disk and RRDE studies also provide evidence for their stability. Controlled potential coulometric oxidation of these show napp-values of one and CV studies of the oxidized solution showed a cathodic peak for reduction of the cation radical at the same potentials and of the same height as that of the original solution. Similar electrochemical behaviour is observed with phenothiazines, thianthrenes, and other heterocyclic compounds in solvents suitably freed from nucleophilic impurities. Not only do the electrochemical results demonstrate the production of a stable cation radical, but the measured Ep- or... 

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