1.3- Dithietanes compounds

Thioketenes can be prepared in several ways, from carboxyHc acid chlorides by thionation with phosphoms pentasulfide [1314-80-3] 2 5 ketene dithioacetals by -elimination, from l,2,3-thiadia2oles with flash pyrolysis, and from alkynyl sulfides (thioacetylenes). The dimeri2ation of thioketenes to 2,4-bis(alkyHdene)-l,3-dithietane compounds occurs quickly. They can be cleaved back pyrolyticaHy (63). For a review see Reference 18. 

The conversion of cyclic sulfides to sulfones is accompbshed by more energetic oxidations. Perhalogenated thiolanes [106] and 1,3-dithietanes [107] are oxidized to sulfones and disulfones, respectively, by a mixture of chromium trioxide and nitric acid (equation 98) The same reagent converts 2,4-dichloro-2,4-bis(tnfluoromethyl)-l,3-dif/u cto cs to disulfone derivatives [107], whereas trifluoromethaneperoxysulfonic acid converts the starting compound to a sul-fone-sulfoxide derivative [2(equation 99). 

Tlie present chapter reviews the chemistry of three- and four-membered ring compounds containing an S—S bond in their ring. Dithiiranes 1,2-dithietanes and 1,2-dithietes are the compounds of this type. Although 1,3-dithietanes are four-membered heterocycles which are prepared much more easily and are seemingly more familiar, they have no S-S bond in the ring and hence are not included in this chapter. 

Tlie chemistry of 1,2-dithietanes is still emerging. Isolable and well-characterized 1,2-dithietanes are limited to only two compounds, 3,4-diethyl-l,2-dithietane 1,1-dioxide (77) and dithiatopazine (73).The synthesis of 1,2-dithietanes has been overshadowed by their thermal instability, which arises most probably from repulsive interactions between the lone-pair electrons on the sulfur atoms, as we have already seen in the chemistry of dithiiranes. 

Thermolysis of the dicarbamic acid silyl ester (CH2 N[SiMe3]C02SiMe3)2) gives the 1,3-diazetane derivative 26 <96JOM93>. Alkylation of the 1,3-dithietane tetraoxide 27 with a,co-dihaloalkanes yields the dispiro compounds 28 (n = 1-4) <95ZOR589>. The first 1,2-dithiete S-oxide 29 is reported <95TL8583>. 

As indicated above, the most important of the thiocarbonyl compounds is thiocarbonyl fluoride, CF2=S. It has been synthesized by a number of routes (43), the most convenient of which for laboratory investigation is dimerization of thiophosgene followed by conversion of the dimer to 2,2,4,4-tetrafluoro- i, 3-dithietane (I) and thermal cracking of the dithietane (44). 

A number of routes to thiocarbonyl fluoride that do not involve tetrafluoro-dithietane have been developed. In one (50), phosgene is chlorinated to give tri-chlorosulfenyl chloride, which is converted to chlorodifluorosulfenyi chloride by reaction with antimony trifluoride, and the fluorinated compound is then dehalogenated by reaction with tin. 

Pressed films of the polymer are white and elastic. When kept at room temperature for several days, the polymer slowly degrades. Eventually it is all converted to the dithietane. It must be emphasized that these products should be handled with great care because the dithietane is a very toxic compound. 

The parent compound 1,3-dithietane was successfully prepared according to Equation (3) (76JA5715, S2JA3119). 

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. 

We have reported the first electroactivity of a thioketene dimer compound [116]. The CV measurement of 2,4-dibenzylidene-l,3-dithietane (31), which was prepared by a basic dimerization of phenylthioketene derived from ben-zyltriphenylphosphonium chloride, showed irreversible two-step oxidation peaks at 0.25 and 0.61 V vs Ag/Ag+, indicating that 31 acts as a stronger electron donor than 2,6-bisphenyl-l,4-dithiafulvene (30) and TTF (2). The dimer (31) can form a 1 1 CT complex with TCNQ in DMSO. Cycloaddition polymerization of bisthioketene derived from p-xylenebis(triphenylphosphoni-um chloride) gave a -conjugated polymer (32) with thioketene dimer unit in the main chain (Scheme 12). This polymer was the first polymer contain-... 

The chemistry of some ring systems having two heteroatoms, i.e. dioxetanes, dithietanes, oxathietanes and thiazetidines are described. Next, the review considers compounds having either silicon or boron in a four-membered ring. Some thermolysis processes are interesting in the silicon series and the first thermally stable 1,2-dihydro-1,2-diborete is described. 

The geometric structural parameters of a number of A-oxides generated from 1,3-dithietane and keto and thioketo derivatives generated from 2,2,4,4-tetrafluoro-l,3-dithietane have been recently calculated at the Hartree-Fock (HF)/6-31-G level, within the molecular orbital (MO) theory framework <1999JMT(466)111>. Structures shown here show the optimized geometries of the parent 1,3-dithietane 1, its mono- and bis-oxides 2-4, l,l -dioxide 5, l,3,3 -trioxide 6, and l,l, 3,3 -tetraoxide 7, whereas Table 1 lists the selected bond distances and angles for these compounds. 

The reaction of dispiro-l,3-dithietane 78 with PPh3 and 2,3-dihydro-l,3-diisopropyl-4,5-dimethylimidazol-2-yli-dene 79 giving the zwitterionic compounds 80 and 81 was described. The use of aqueous ammonia resulted in the formation of the dianionic disulfide 82 (Scheme 8) <2004ZFA1659>. 

The reactions of carbon disulfide with compounds 191 under phase-transfer catalysis (PTC) conditions followed by the reaction of the formed intermediate with 5,5-dibromo-3-phenyl-2-thioxo-thiazolidin 4-one 192 gave spiro-1,3-dithietane derivatives 193 (Scheme 23) <1997PS173>. 

The synthesis of poly-dibenzylidene-l,3-dithietane 201 is based on the Wittig reaction of/ -xylcnc-bis(triphcnyl-phosphonium) chloride 199 with carbon disulfide <2001MM346, 2002MM3806>. The phosphonium salt 199 was converted to the ylide 200, which reacted with carbon disulfide, yielding, after methanolysis, a thioketene. The latter was stirred at room temperature for 12 h to provide the polymeric compound 201, bearing 1,3-dithietane moieties in 54% yield (Scheme 25) <2001MM346, 2002MM3806>. 

Thiatriazolines are reported as possible transient intermediates in reactions of azide ion with thiobenzophenone (equation 49a) and thiobenzophenone S-oxides (equation 49b) <76ACS(B)997>. l,3-Dithietane-2,4-diylidene bis(cyanoacetic ester) (76) reacts with sodium azide to give a compound assumed to be the sodium salt of alkylidene-1,2,3,4-thiatriazolidine (77). Nitrogen is evolved on acidification of the sodium salt and a hitherto unknown perhydro-l,4,2,5-dithiadiazine (78) is formed (equation 49c) (81ZC102). 

Head-to-tail dimerization of thiocarbonyl compounds to 1,3-dithietanes (Scheme 105) occurs thermally <1977JOC2345> and photochemically <1981JOC3911>, or catalyzed either by a base <1973T2759> or a sulfonic acid <1997BCJ509>. For example, synthesis of dithietane 213 in excellent yield was performed by the room temperature dimerization of the thione 212 (Scheme 106) <2004TL7655, CHEC-III(2.18.4.2)838>. Diaryl thioke-tones are resistant to dimerization. Some thiones dimerize to 1,3-dithietanes <1975BSF(2)1670>. 

The reaction between o-phenylenediamine and an equimolar amount of an aromatic or heterocyclic aldehyde has been shown to proceed by initial formation of a monoanil (74). In the presence of oxidizing agents (e.g. nitrobenzene, which also acts as the solvent) this can form the 2-substituted benzimidazole. With two moles of aldehyde the bis-anil (75) forms, giving rise to a 1,2-disubstituted benzimidazole (Scheme 42). This aldehyde route to benzimidazoles is particularly suited to the synthesis of compounds with a heterocyclic group (e.g. 2-thienyl-, 2-pyridyl-) at C-2. Reaction of 2,2,4,4-tetrakis(trifluoromethyl)-l,3-dithietane with o-phenylenediamine gives 2,2-bis(trifluoromethyl)benzimidazoline (7430785). 

---