Catalytic transformations of thiiranes by (Thiirane)W(CO)(5) complexes

The thiirane complexes W(CO)(5)(SCH2CH2), 1, W(CO)(5)(cis-SCHMeCHMe), 2, and W(CO)(5)(trans-SCHMeCHMe), 3, have been prepared and characterized. The molecular structure of compound 2 was also determined crystallographically. It was found to contain an S-coordinated cis-dimethylthiirane ligand with a pyramidal sulfur atom coordinated to a W(CO)(5) group. Compounds 1-3 decompose slowly to yield sulfur and the corresponding olefin. Compound 1 and W(CO)(5)(NCMe) were both found to transform free thiirane into a mixture of cyclic polydisulfides (SCH2CH2S)(n), 4-7, n = 2, 3, 4, and 5, and ethylene, catalytically. The turnover frequency for the formation of 4 by 1 at 25 degrees C is 6.8 h(-1). When the catalytic reaction was performed in the presence of dimethyl acetylene dicarboxylate, DMAD, compounds 4 and 5 and the six-membered heterocycle SCH2CH2SC(CO(2)Me)C(CO(2)Me), 8 were formed, the last by the trapping of a suspected SCH2CH2S intermediate. Small amounts of polythioether macrocycles (CH2CH2S)(n), 12S4, n = 4, and 15S5, n = 5, were also formed. (2R,3S)-dimethylthiirane (cis-DMT) and (2R,3R(2S,3S))-dimethylthiirane (trans-DMT) undergo a combination of isomerization and desulfurization to yield a mixture of cis- and trans-butene by 2 and 3, respectively. In the presence of DMAD, the isomerization of the substituted thiiranes is suppressed and small amounts of the cyclic disulfides [C(H)CH3C(H)CH3SS](3), 9, and [C(H)CH3C(H)SS](2), 10, and the cyclic trisulfide (4S,5S(4R,5R))-[S3C(H)CH3C(H)CH3], 12, were formed. The molecular structure of 9 was determined crystallographically and established a pattern of identical R,R/S,S stereochemistries at adjacent stereogenic carbon atoms about the ring. A similar stereochemical result was indicated for the stereogenic carbon centers in 12 by preparing the osmium complex Os-2(Co)(6)[mu-(R,R(S,S))-SC(H)CH3C(H)CH3S], 13, from it and determining the configurations at the stereogenic carbon atoms in the SC(H)CH3C(H)CH3S ligand by an X-ray crystallographic analysis. All of the catalytic transformations can be explained by a back-side addition of a sulfur atom from an uncoordinated molecule of thiirane to a carbon atom of a thiirane ligand. This leads to an opening of the thiirane ligand and formation of a zwitterionic thiiranium/thiolato intermediate having the thiolato sulfur atom coordinated to the tungsten atom. Release of the added thiirane from a conformer for the zwitterion would result in isomerization of the thiirane ligand. Alternatively, loss of olefin from the thietanium ion group would yield a (SCH2CH2S)W(CO)(5) intermediate that could decompose by loss of SCH2CH2S groups, which could then combine to form the cyclic disulfides, 4-7. In the presence of DMAD, the SCH2CH2S group is trapped to yield 8. A series of similar ring-opening additions followed by a cyclization could lead to the formation of the polythioether macrocycles 12S4 and 15S5, but these compounds are minor products even under the best conditions. It was also found that compound 4 has moderate antimicrobial activity toward Escherichia coli.