Thianthrene electron transfer

To decrease the stationary concentration of complex (HetH- - - ArH) +, it will suffice to lower the concentration of the oxidizer, that is, substrate (HetH)+. This also decreases the equilibrium concentration of the cation-radical complex (HetH- - ArH)+. The rate of anisylation—the main process—drops sharply. The side process, one-electron transfer from anisole to the cation-radical of thianthrene, also decelerates, but not so markedly. So this side process (route b on Scheme 5.11) remains the only one. 

Other dyes which participate efficiently as electron transfer catalysts include the xanthenes (217,218), thianthrenes (219), phenothiazines (220), and anthraquinonesulfonates (221). 

Electron transfer may also dominate the excited state chemistry of open shell radical ions. The fluorescence of the radical anions of anthraquinone and 9,10-dicyanoanthracene and the radical cation of thianthrene are quenched by electron acceptors and donors, respectively, although detailed kinetic analysis of the electron exchange do not correspond exactly either with Weller or Marcus theory (258). The use of excited radical cations as effective electron acceptors represents a... 

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... 

Reversible oxidation potentials for two consecutive one-electron transfers were also observed for alkoxy-substituted thianthrene [192, 193]. 

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]. 

Disporportionation, as shown in Eq. (8) involves electron transfer from one thianthrene radical to another ... 

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. 

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). 

Very interesting behavior toward redox probes is demonstrated by the thin films obtained from l-(2-bisthienyl)-4-aminobenzene and l-(2-thienyl)-4-aminoben-zenediazonium (Figure 3.37, Cb and Cc, respectively). When decamethylferrocene (p° = -0.21 V/SCF) is used as a redox probe, its reduction is shifted to more negative potentials and becomes irreversible (Figure 3.37a) that is, the layer behaves as a diode. When the redox probe is thianthrene (E° = 1.13 V/SCF), both its reduction potential and its reversibility are maintained, and the films are now transparent to electron transfer (Figure 3.37b) [269]. The same phenomenon is observed with thin films obtained from the 2-terthiophenyldiazonium cation [270]. 

A greater probability of detecting the sulfurany radical intermediate in such reactions is obtained when the electron transfer to the sulfonium salt occurs at low temperatures. The formation of a 2 1 mixture of thianthrene (23) and 2-phenylthiothianthrene (24) from 5-(2-thianthreniumyl)-thianthrene perchlorate (25) and sodium naphthalenide in THF in the presence of thiophenol at -78°C strongly suggests [29] the intermediacy of radical (26) (Scheme 10). 

Thianthrene radical cation undergoes self exchange, i.e., transfer of an electron from thianthrene to its radical cation and the rate of this process measured [43]. In addition, this exchange has been studied by measurement of the equi-... 

Deubel feels that experiments coneeming fhianthrene 5-oxide (SSO) as a probe for the electronic character of oxygen-transfer reactions need to be reinterpreted. The SSO molecule has a sulfide group, which is attacked by electrophiUc oxidants, and a sulfoxide moiety, which is oxidized by nucleophilic oxidants. An AIM analysis of thianthrene 5-oxide reveals that there is an area of charge depletion at the sulfoxide group. The location of this area indicates that the attack of nucleophiUc oxidants on SSO is stericaUy hindered. Therefore, the SSO probe makes oxidants such as dioxiranes appear to be more electrophilic than they actually are. 

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