Thianthrene reduction

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. 

A complex reducing agent (CRA), dubbed NiCRA-bpy, was 99% effective in converting thianthrene into diphenyl over 18 hrs over 89 hrs, benzene (83%), diphenyl (8%), and dibenzothiophen (3%) were the products. The reductant was a4 2 1 2 mixture of NaH, /-AmONa, Ni(OAc)2, and bpy (88TL2963). 

Sulfur, at 345°C for thianthrene tetroxide, or at 250°C for thianthrene 5-oxide, produced thianthrene in good yields. S-Labeling experiments showed that the former took place with 80% replacement of ring sulfur and the latter took place with 91% replacement (73BCJ650), so these processes, whatever their detailed mechanism, do not involve simple reductive cleavage of the S—O bond. In accordance with this, thianthrene 5,5,10,10-tetroxide is converted into selenanthrene by reaction with elementary selenium (1896CB443). 

In the rings containing sulfur, reduction of sulfoxides of phenoxathiin and thianthrene can be performed in excellent yield with the aluminium chloride/sodium iodide or zinc dust/l,4-dibromobutane systems <1996CHEC-II(6)447>. 

For the purpose of this review the term heteroaromatic is applied to 7r-radical species when they may be regarded as arising from aromatic heterocyclic molecules or ions by addition or removal of an odd number (usually one) of electrons. Thus, entities such as the anion- and cation-radicals of pyridine (1 and 2) are clearly heteroaromatic. The dilemma whether or not to regard thianthrene (3) as aromatic (its central ring possessing eight electrons) does not arise for the thianthrene cation radical (4) it is heteroaromatic on the grounds that it may formally arise by one-electron reduction of the aromatic thianthrenium dication (5). 

Redox processes involving 178 have also been studied.Anodic oxidation of thianthrene has been eifected in a wide variety of solvents. Use of trifluoracetic acid gives stable solutions of 178 and, if perchloric acid is included, the solid perchlorate salt may be isolated on evaporation of the solvent after electrolysis. Dichloromethane at low temperatures has been used and, at the opposite extreme, fused aluminum chloride-sodium chloride mixtures. " Propylene carbonate permits the ready formation of 178, whereas the inclusion of water in solvent mixtures gives an electrochemical means of sulfoxidizing thianthrene. Reversible oxidation of 178 to thianthrenium dication may be brought about in customary solvents such as nitriles, nitro compounds, and dichloromethane if the solvent is treated with neutral alumina immediately before voltammetry addition of trifluoracetic anhydride to trifluoracetic acid equally ensures a water-free medium. The availability of anhydrous solvent systems which permit the reversible oxidation and reduction of 178 has enabled the determination of the equilibrium constants for the disproportionation of the radical and for its equilibria with other aromatic materials. ... 

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 reductive opening of thianthrene (68c) has been reported in Section IV.C for homologation purposes. However, the lithiation of (68c) and reaction with appropriate electrophiles led to other interesting organic compounds. Thus, after the reductive opening lithiation of (68c) and reaction with a carbonyl compound as the... 

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... 

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. 

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]. 

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