1.4- Dithiins radical cations

The cation radicals of type 39 condense to 1,4-dithiins and 1,4-dithiin radical cations, respectively. The cis-geometry of the radicals 39 was established by conversion into the radical cation 41, whose identity was confirmed by independent synthesis. The apparent instability of cation radicals of dithienes in solution does not mean that such species are incapable of existence. The ion Ni [8202(0113)2] 2 has been detected in the mass spectrum of the corresponding neutral dithiene (II). 

The one-electron oxidation of 1,2-dithiin 20 with 1.5 equivalent of SbCl5 under vacuum at room temperature gave a bright yellow solution that exhibited a nine-line ESR signal. The optimized structure obtained by theoretical calculations (B3LYP/6-31G(d)) for the radical cation 20 + was the one with a... 

The 1,4-dithiin annelated with two BCO units 21 also undergoes ready one-electron oxidation with 1.5 equivalents of SbF5 in CH2C12 to give radical cation salt 21 SbF6" in 67% yield as brown-colored single crystal, which was stable... 

Figure 17. 1,4-Dithiin 21 and the molecular structure of its radical-cation salt 2f+SbF6 determined by X-ray crystallography. The observed lengths (A) of Cl-S and C1-C2 in 2f+SbF6 are 1.72(1) and 1.31(2) to be compared with 1.324(3) and 1.760(2) determined for neutral 21, respectively. Also shown is the thianthrene radical cation 23 +. 

Mass spectra of the dibenzo-l,2-dithiins 52 and S3 display intense parent ions, indicating that loss of an electron gives rise to particularly stable radical cations <2000JA5052>. 

Recently, radical cations of other conjugated sulfur compounds70 have been prepared the radicals derived from 1,4-dithiin (56), 1,4-benzodithiin (57), and three derivatives of radicals 55 and 57. Analysis of the ESR spectra of these derivatives permitted the assignment of hyperfine-splitting constants to the six chemically non-equivalent... 

The electrochemical oxidation of 2,5-diaryl-1,4-dithiins (50) has been studied using various voltametric techniques and all compounds were found to undergo quasi-reversible one-electron transfers to the radical cations and dications.126 The first formal redox potential and the lifetime of the radical cation were found to decrease with increasing electron donation from the aryl ring. The major products were the 2,2 -dimers, which result via reaction of two radical cations for which rate constants are given. Dibenzothiophene radical cations reacted with tetranitromethane under... 

There is a long standing interest in the chemistry and the properties of cyclic compounds containing sulfur atom in modern material chemistry due to their redox chemistry. In particular, the focus has been on dithiole derivatives, e.g., dithiafulvenes and tetrathiafulvalenes, since the finding of metallic conductivity and low temperature superconductivity in radical cation salts. The quite low oxidation potentials of 1,4-dithiin compounds have been reported, recently [109]. On the other hand, thioketene dimers (2,4-bis(alkyli-dene)-l,3-dithietane) have been known for more than 100 years and synthesized by various methods [110-115]. The structure of these dimer compounds is similar to that of the redox-active sulfur compounds therefore, the potential electronic property of the thioketene dimer moiety is considerably attractive with the aim of application to a new and better -donor. 

The first examples of 3,6-(perfluoroalkyl)-l,2-dithiins have been obtained through the oxidative deprotection and cyclisation of (Z,Z)-l,4-6 (t-butylthio)-l,4-6w(perfluoroalkyl)-1,3-butadienes. They adopt a similar twist geometry to the parent dithiin and during electrochemical oxidation appear to form a planar radical cation (Scheme 29) <03JOC8110>. 

A typical example in heteroaromatic chemistry is the preparative electrolysis of some 2,5-diaryl-1,4-dithiins 13, which give two well resolved quasi-reversible one-electron transfer waves to the radical cation and the di-cation, respectively, in cyclic voltammetry. The electrolysis gives low yields of the 2,2 dimers (see Scheme 12) via the radical cation coupling mechanism [46]. 

Under conditions where the primary electrode product undergoes a slow chemical reaction, that is, ti/2 is of the order of seconds, the value of n determined by a relatively fast technique like LSV may differ from that obtained by a slow experiment like coulo-metry. This type of behavior was observed in the anodic oxidation of 2,3,5,6-tetraphenyl-1,4-dithiin in MeCN [278]. During CV the reversible oxidation to the radical cation is observed. However, when constant-current coulometry was carried out as described earlier, this time at i = 50 mA, 6.44 min was required to oxidize completely 0.1 mmol of the substrate to a product electroinactive in the potential region of interest, indicating an overall two-electron process (Fig. 43). Thus, apparently contradictory results may be obtained due to the difference in time scale between the two types of experiment. 

Similarly to tetrathioethylenes and tetrathiafulvalenes, 1,4-dithiin (LV) and its benzoder-ivatives (LVI and LVII) are electron-rich compounds that can be easily oxidized to their corresponding cation radicals and dications. There are several reviews [2, 3, 186, 187] on the chemistry of the radical cations of dibenzo-1,4-dithiin (LVH) and related compounds. 

The anodic oxidation of a series of 2,5-diaryl-1,4-dithiins (LVIII), as in Eq. (96), has been studied in detail [189]. Cyclic voltammetry (CV) experiments showed that all compounds undergo two quasi-reversible one-electron transfers to form the radical cations and the dications. Linear correlations between E° and a were observed. Preparative electrolyses gave the corresponding 2,2 -dimers (LIX) as the major products in yields up to 20%. The low yields are due to workup difficulties and formation of polymeric materials of unknown composition. 

Voltammetric oxidation of 1,4-dioxins and 1,4-dithiins in acetonitrile containing perchlorate yields a radical cation in the first one-electron oxidation and a dication in the second one-electron oxidation. Both compounds are typical r-electron-rich heterocycles, whereas the dication contains six 7r-electrons, and the relatively high stability of the dication is connected with this aromaticity [296]. 

The central 1,4-dithiin ring in 33 adopts a boat conformation, as is the case for TH, with a 48° fold along the S-S vector in the solid state as shown by X-ray crystallographic analysis [92]. Both 33 and 34 undergo reversible one-electron oxidation to the corresponding radical cation with E1/2=0.93 and 0.77 V, respectively, vs Ag/AgCl as shown by cyclic voltammetry in acetonitrile [92]. There is also an irreversible second oxidation for both compounds at E1/2 ca. 1.5 V. 

The radical cations of benzo-1,4-dithiin 36,1,4-dithiin 37a and some of their derivatives have been prepared and characterized by their EPR spectra [7, 93-96]. 

---