Catalytic Transformations of Thiiranes by (Thiirane)W(CO)5Complexes

The thiirane complexes W(CO) 5 (SCH 2 CH 2 ), 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 (SCH 2 CH 2 S) n, 4 − 7, n = 2, 3, 4, and 5, and ethylene, catalytically. The turnover frequency for the formation of 4 by 1 at 25°C is 6.8 h - 1 . When the catalytic reaction was performed in the presence of dimethylacetylenedicarboxylate, DMAD, compounds 4 and 5 and the six-membered heterocycle SCH 2 CH 2 SC(CO 2 Me)C(CO 2 Me), 8 were formed, the last by the trapping of a suspected SCH 2 CH 2 S intermediate. Small amounts of polythioether macrocycles (CH 2 CH 2 S) n, 12S4, n = 4, and 15S5, n = 5, were also formed. (2 R,3 S )-dimethylthiirane ( cis -DMT) and (2 R,3 R (2 S,3 S ))-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)CH 3 C(H)CH 3 SS] 3, 9, and [C(H)CH 3 C(H)SS] 2, 10, and the cyclic trisulfide (4 S,5 S (4 R,5 R ))-[S 3 C(H)CH 3 C(H)CH 3 ], 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 [μ-( R,R ( S,S ))-SC(H)CH 3 C(H)CH 3 S], 13, from it and determining the configurations at the stereogenic carbon atoms in the SC(H)CH 3 C(H)CH 3 S 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 (SCH 2 CH 2 S)W(CO) 5 intermediate that could decompose by loss of SCH 2 CH 2 S groups, which could then combine to form the cyclic disulfides, 4 − 7 . In the presence of DMAD, the SCH 2 CH 2 S 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 .