Fluorinated dithietanes

Proton NMR spectra of the phosphathietanes have been discussed and illustrated along with the C-NMR spectrum for thiazetidine derivative 154 and the F-NMR spectrum for a fluorinated dithietane ether." °... 

The replacement of chlorine in the dithietane by fluorine occurs stepwise. As a consequence, small amounts of the partially fluorinated dithietanes II, III, and IV are ordinarily obtained. Since efforts are usually made to fluorinate... 

Whereas access to fluoromethylene ylides is easy, higher homologues are more difficult to prepare because ot the facile (1-elimination of fluorine However hexafluoroisoprapylidene ylides may be prepared in situ from tetrakisftnfluoro-methyl)-l,3-dithietane and tnphenylphosphme [54 (equation 51) or 2,2-dichloro-hexafluoropropane and tnphenylphosphme [55, 56, 57 (equation 52) (Table 21) Pcrfluoro-2-buteiie reacts with tnbutylphosphine to cleanly form a solution of a fluonnated ylide [5/ ] (equation 53)... 

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. 

Table 2 Selected bond distances and angles for fluorinated 1,3-dithietanes 8-10, calculated at the HF/6-31G level...

The oxidation of fluorinated derivatives, which requires chromium trioxide and hot, fuming nitric acid, usually stops at the mono-sulfone stage,although 2,2,4,4-tetrafluoro-l,3-dithietane is converted directly to the disulfone by treatment with chromium trioxide and boiling, fuming nitric acid. The conversion of 2,2,4,4-tetrachloro- or 2,2,4,4-tetrabromo-dithietane to the disulfone may be effected by oxidation to the monosulfone with potassium permanganate followed by treatment which chromium trioxide- nitric acid. Potassium permanganate oxidizes the dimer of thiocyclohexanone to the disulfone in 78% yield. 

Thiocarbonyl fluoride, 9, is prepared in good yield by pyrolysis of 6 at ca. 600°C in the vapor phase or by carrying out the reaction of sulfur and tetrafluoroethylene at higher temperatures. A cleaner synthesis involves fluorination of thiophosgene dimer to tetrafluoro-l,3-dithietane, 10, followed by cracking of 10 to two molecules of 9 (7). 

Rail, K. Sundermeyer, W. Synthesis of perhalogenated 1,3-dithietane 5-oxides and thiiranes. J. Fluorine Chem. 1990, 47, 121. 

The most easily accessible compounds of this class are 1,3-dithietanes. They are formed by a spontaneous dimerization of fluorinated thioketones or acyl halides. Thus, hexafluorothioacetone 34, generated in the reaction of bis(perfluoroisopropyl) mercury with molten sulfur at about 450°C (Scheme 2.18), undergoes at ambient temperature slow dimerization to form dithietane 35. This dimerization is significantly accelerated by catalytic amount of base, such as pyridine. At elevated temperature, the dimer 35 can be converted back into 34. 

Facile dimerization to 1,3-dithietanes is also characteristic of other fluorinated thioketones, such as 4-hydro- and 4-chloroperfluorobutan-2-thiones. Trifluorothioa-cetyl fluoride does not dimerize spontaneously however, when it is irradiated by UV light, it is converted to a mixture of cis and trans isomers of compound 36a. Dithietanes 36b, 37a, and 37b were also obtained by photochemical dimerization of the corresponding thiocarbonyls." ... 

Since the simplest F-l,3-dithietane (38) cannot be prepared by dimerization of thiocarbonyl fluoride, probably due to propensity of the latter to polymerize, a different synthetic approach is employed for its synthesis. Two-step process includes photocyclodimerization of Cl2C=S and Swarts-type fluorination of the dimer by antimony trifluoride to give 38 (Scheme 2.20). Compound 38 can be a source of very pure thiocarbonyl fluoride, when pyrolyzed at 500°C." ... 

Fluorinated 1,2-dithietenes 39a-C are the only known representatives of fluorinated four-membered ring systems containing two sulfur atoms in 1,2-position, while fluorinated 1,2-dithietanes have not been prepared so far. 

Synthetic applications of dithietane 35 are not limited to its use as a source of hexafluorothioacetone. This compound is also a valuable precursor for the synthesis of a variety of fluorinated sulfur-containing derivatives. For example, the pyrolysis of 35 at 325 C results in the high-yield formation of (CF3)2C=C(CF3)2. The reaction of 35 with PhaP was found to be a convenient route to (Ph)3P=C(CF3)2, which was used for the preparation of l,l-bis(trifluoromethyl) alkenes and CF2=C(CF3)P(0)(0R)2. Recently reported transformation of fluorinated thietanes involves reductive ring expansion to substituted dihydrothiophenes. For example, the treatment of compound 47 with aluminum powder in the presence of a catalytic amount of PbCl2 resulted in an interesting ring expansion process leading to the formation of compound 48 in excellent yield (Scheme 2.23). Readily available fluorinated cycloadducts 49... 

England, D.C. 2,4-Bis(hexafluoroisopropyUdene)-l,3-dithietane and fluorinated polysulfides useful as solvents for fluorinated polymers. US Patent 3,544,591, 1970. 

Abe, T Shreeve, J.n.M. Perfluorotetramethylenesulfur difluoride and its derivatives. Perfluoro-l,3-dithietane octafluoride and perfluoro-1,4-dithiane octafluoride. J. Fluorine Chem. 1973, 3(1), 17. 

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