Formation of Thiirenes

Evidence for the formation of thiiren by photoelimination of nitrogen from 1,2,3-thiadiazoles has been described,351 and several thiirens, prepared in this way, have been identified in an argon matrix at 8K.352 Phenylthiazirine (422) appears to be an intermediate in the related transformation of 5-phenyl-l,2,3,4-thiatriazole (423) into benzonitrile sulfide (424),353 and further... 

Replacement of disulfur dichloride by sulfur dichloride in reaction with acetylenes in dichloromethane led to formation of 2,3-dichlorothiiranes in quantitative yield <2001TL4017>. Acid hydrolysis of 2,3-dichlorothiirane (R = R = f-Bu) gave a mixture of 2,2,5,5-tetramethyl-4-oxo-3-hexanethione 289 and 2,3-di-f-butylthiirene-l-oxide 290 (Scheme 83). Exclusive formation of thiirene-A-oxides was observed during alkaline hydrolysis of 2,3-dichloro thiiranes. 

A formation of thiirene by secondary photolysis of thioketene, which is generated by photolysis of 1,2,3-thiadiazole, has been reported <92ACS482>. Irradiation of [4-13C]-l,2,3-thiadiazole together with diethylamine in an EPA glass (diethyl ether-isopentane-ethanol (5 5 2)) at 77 K gave N,N-diethylthioacetamide with 37% carbon randomization in the thioketene (Scheme 9). Experiments with [4-13C]-l,2,3-thiadiazole at room temperature demonstrate lack of carbon randomization in the thioacetyl group of the trapping product. Therefore, the reaction mechanism of photolysis of... 

Two further papers in a series on the generation of nitrilimines (RC=NNR ) by irradiation of sydnones suggest that the reactive excited state is a (n, n ) triplet state,and demonstrate again the use of the reactive nitrilimines in the synthesis of heterocyclic compounds. In continuing studies on the formation of thiirens from 1,2,3-thiadiazoles (23), the parent thiiren has been characterized in a low-temperature matrix after photolysis of (23 R = R = H). Product analysis in experiments with C-substituted substrates suggests that thiiren is formed to a considerable extent thermally but to a lesser extent photochemically from (23 R = H, Ri = Ph). 

All unexpected reaction is the formation of dithiete 175, thiirene 1-oxide 185, and thioketone 186 by the treatment of di-l-adamantylacetylene with S2CI2 (99UPl).Tlie yield of each compound is 30% at most. 

Verification of the molecular weight of thiirene dioxides by mass spectrometry, employing the conventional electron-impact (El) ionization method, has been unsuccessful due to the absence or insignificant intensity of molecular ion peaks in their mass spectra. The base peak is rather characteristic, however, and corresponds to the formation of the disubstituted acetylene ion by loss of sulfur dioxide91 (equation 3). 

To summarize under favorable conditions the acidity of a-hydrogens facilitates the generation of a-sulfoxy and a-sulfonyl carbanions in thiirane and thiirene oxides and dioxides as in acyclic sulfoxides and sulfones. However, the particular structural constraints of three-membered ring systems may lead not only to different chemical consequences following the formation of the carbanions, but may also provide alternative pathways not available in the case of the acyclic counterparts for hydrogen abstraction in the reaction of bases. 

Thus, like a, /1-unsaturated ketones and sulfones, both thiirene dioxides and thiirene oxides are preferentially attacked by the less basic nucleophiles on the vinylic carbon atom2. This would lead to formally 1,4 Michael-type adducts and/or other products resulting from further transformations following the initial formation of the a-sulfonyl and a-sulfoxy carbanions. 

Unexpectedly, neither direct complexation nor the deoxygenated complexes 95 or 96136,137 were observed in the reaction of diphenylthiirene oxide (18a) with iron nonacarbonyl. Instead, the red organosulfur-iron complex 97138 was isolated12, which required the cleavage of three carbon-sulfur bonds in the thiirene oxide system (see equation 33). The mechanism of the formation of 97 from 18a is as yet a matter of speculation. 

Dialkyl sulphones may be converted to sulphonic acids by reaction with carbon tetrachloride and base at 80 °C209. This reaction proceeds by initial formation of a-chloro sulphones which are then converted to a thiiren intermediate which decomposes to give a sulphonic acid (equation 92). 

The symmetric diarylthiirene oxides (18) are much more thermally stable than the corresponding saturated thiiranes and unsaturated thiirene dioxides. Thus, the thiirene oxide 18a shows only slight decomposition after 24 hours of reflux in benzene, whereas the analogous sulfone 19b fragments completely to SO2 and diphenylacetylene after less than six hours under the same conditions Irradiation of the oxide 18a, however, does result in the elimination of sulfur monoxide and formation of diphenylacetylene. Its thermolysis at 130 °C afforded benzil as the only isolable product, implying that SO is not being eliminated in this thermolytic process. 

The reaction of 1,2-diadamantylacetylene with disulfur dichloride led to the formation of the dithiete 232 (mentioned above), l,2-diadamantyl-2-thiooxoethanone 233, and the thiirene 1-oxide derivative 234 in 21%, 33%, and 27% yield, respectively (Equation 50) <2000TL8349, 2003JA12114, 1998BCJ1181 >. 

Dihydro-4,4,6,6-tetramethylfuro[3,4-rf]-l,2,3-thiadiazole (37) has been used to prepare some highly reactive intermediates. Photolysis of compound (37) in the presence of furan gave the adduct (38) in 11% yield a similar reaction in the presence of 2,5-dimethylfuran gave the adduct (39) in 12% yield. The authors claim that formation of adducts (38) and (39) unequivocally proves the intermediacy of the antiaromatic thiirene (40) <87TL2867>. 

Evidence for the formation of diradical (14 equation 6) from the photolysis of 1,2,3-thiadiazoles has been gained by ESR work (79JA3976). Photolysis of thiadiazoles at low temperature led to the characterization of thioketenes by IR and a thiirene was postulated as a reaction intermediate (74JA6768). 

Thiadiazoles extrude nitrogen and thiirenes can be formed, but their lifetime is fleeting as they rearrange to thioketenes Scheme 3 shows the eventual formation of propadienethione 33 from 32 <1988JA789, 1990GPL (168)1>. 

Dorofeeva and Gurvich evaluated the available structural parameters, fundamental frequencies, and enthalpies of formation of thiirane, thiirene, and other sulfur compounds as well as some other quantities <1995JPCRD1351>. For thiirane, ideal gas values at lb and 300K recommended in 1995 are Gp = 53.603, A = 255.569J K mol , AfAA = 81.917 kjmol , and AfG = 96.897. Analogous values for thiirene are 54.929, 255.678, 299.973, and 275.665. The latter were all calculated because experimental numbers were not available. The situation does not appear to have changed since that time. 

The thiirene 6 was proposed as an intermediate in the formation of a cycloadduct 23 obtained from the reaction of a thiophene endoperoxide 36 <1996CC177> and cyclooctyne under thermal conditions (Scheme 6). The structure of 23 has been verified by X-ray crystallography <2002JA8316>. 

Transition metal complexes catalyze desulfurization of thiiranes. Complexes of Rh <1986TL3573>, Re <1997TL7701, 1999CC1003>, and Mo <2003JA3871> have been successfully employed in catalytic quantities. The presence of a stoichiometric sulfur acceptor such as carbon monoxide or an alkene, phosphine, or arsine is required. In Re-catalyzed desulfurization, the formation of an intermediate Re =S species via initial coordination of the thiirene ligand has been suggested (Schemes 12 and 13) <1997TL7701>. Kinetics and mechanism of sulfur... 

Elimination of nitrogen by irradiation of argon matrix-isolated 4,5-bis(methoxycarbonyl)-l,2,3-thiadiazole (38) resulted in the almost quantitative formation of bis(methoxycarbonyl)thiirene (39), confirming earlier predictions that electron-withdrawing substituents should stabilize the thiirene ring. Benzoselenirene (40) has similarly been obtained and identified spectroscopically on irradiation of matrix-isolated 1,2,3-benzoselenadiazole (41) rearrangement to the fulveneselone (42) is observed on further irradiation. 

In the presence of hexafluoro-2-butyne, 2,3-di(trifluoromethyl) thiophene was isolated in addition to the above products, formed by the addition of C2H2S to the triple bond of 2-butyne. Methyl substitution either in the 4- or 5-position leads to the formation of only one thiophene, viz. 2,3-trifluoromethyl 5-methylthiophene in the presence of hexafluoro-2-butyne, suggesting the intermediacy of methyl thiirene. Aryl-substituted 1,2,3-thiadiazoles have been investigated by Kirmse and Horner and by Huisgen . 

---