Tetrathioethylenes

Other conditions being equal, the first oxidation potentials of tetrathiofulvalenes are usually less positive than those of tetrathioethylenes [141-151]. The first reversible oxidation of tetrathiofulvalenes normally occurs at 0.2-0.8 V versus SCE, whereas the second occurs at 0.6-1.2V... 

X-Ray diffraction analysis of a single crystal of 84 (2,5-bis(dithian-2-ylidene)-l,3,4,6-tetrathiapentalene (BDA-TTP)) showed that the three tetrathioethylene units are situated almost in a common plane, while the two trimethylene end groups are far out of that plane in opposite directions and the dihedral angle arormd the intramolecular sulfur-to-sulfur axis in each dithiole ring is 53.6° <2001JA4174>. 

The electrochemical behavior of trithio-orthoesters under oxidative conditions is far more complex from the mechanistic point of view than that of thioacetals. Examples, which may be considered as representatives of this class of compounds, are two types of thio oxalates, XXXVI and XXXVII. The oxidation of XXXVI in MeCN was shown to produce well-known tetrathioethylene derivatives, XXXIX and XL [Eq. (93)] [149]. 

Two successive C-S bond cleavages at the cation radical stage are suggested, the result of the first one being supposedly structure XXXVIII, which is followed by a homo-lytic dissociation to produce cation radical XXXIX. This can be oxidized to form XL. Product XXXIX was identified by comparison of its cyclic voltammetric behavior with that of tetrathioethylene, and by electron spin resonance (ESR) spectroscopy. 

In the first step, an electron transfer and a homolytic cleavage at the cation radical stage of one of the C-S bonds to the methylthio group are proposed, leading to carboca-tion XLI, which in two successive 1,2-shifts followed by elimination of MeS" " is transformed into the tetrathioethylene derivative (XLII). It was shown [150, 151] that compound XLII, after being oxidized in two reversible one-electron steps to the dication XLIL, rearranges to the more stable dication XLIII. Reactions similar to those described have also been observed as result of nonelectrochemical oxidations [152]. 

The oxidation of these compounds involves two successive reversible steps with formation of the radical cation (XLVIII) and the dication (XLIX), both quite stable, as shown for tetrathioethylenes (L) ... 

Table 4 Oxidation Potentials for Some Tetrathioethylenes (L) in MeCN...

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. 

This rearrangement occurs via the dication of 40 but not via its radical cation. A similar rearrangement for cyclic tetrathioethylene dications is known and is discussed below. 

Disproportionation constants, Aidisp, have been measured from oxidation potentials in acetonitrile and other solvents for the thianthrene, 4,4,-dimethoxybiphenyl, and 9,10-di-p-anisylanthracene cation radicals (Hammerich and Parker, 1973). In acetonitrile they are, respectively, 2 3 x 10-9, 2-7 x 10-5 and 1-9 x 10-4, indicating that these cation radicals, like the violenes have a very small tendency toward disproportionation. 7sfdiSp f°r thianthrene cation radical (l.xl0-9) and the tetrathioethylene cation radical (5xl0-8) have been measured in molten aluminum chloride-sodium chloride at 140° (Fung et al., 1973). 

The crystal structure of this salt (Fig. 3) is very simiar to that of K-(BEDT-TTF)2l3 [14]. Namely, two BEDT-TTF molecules form a dimerized pair, and the dimers are arranged over one another almost prependicularly. The interplanar spacing between the central tetrathioethylene moieties of a dimerized pair increases from 3.30 A at 104 K to 3.38 A at 298 K. 

Methods for the preparation of tetrathioethylenes, including cyclic derivatives of general structure (145), have been summarized, and the... 

All performed in protic media, the extensive exchange experiments discussed above carry intriguing implications for aprotic solvents. Here ylid concentrations are not suppressed ndi. are their dimers cleaved as a consequence of solvent acidity, so it mould be possible to prepare a host of yet-unknown heteroatom-substituted eAylenes by the ylid route. In addition to the peraminoethylenes synthesized via ylids, in fact, several 1,2-diamino-1,2-dithioethylenes and tetrathioethylenes have already been prepared by deprotonation of thiazoUum and 1,3-dithiolium salts, respectively. 

A survey of the methods for synthesizing tetrathioethylenes includes the new method of pyrolysis of orthothio-oxalates, e.g, (278). ... 

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