the axial C6 proton, consistent with extensive charge delocalization, -CHSS+CH-c-, -CHS+=SCH--. Unlike 1,2-dithianes, methylation of 1,2-dithiolanes 15-18 was nonselective. Methyl lipoate 18 gave the four possible regio-and stereoisomers 8a,b and 94b in the ratio 26:11:33:30. A remarkable dynamic N M R effect was observed whereby 'H and I3C resonances for pairs of diastereomers 6a,b, 7a,b, 84b, and 94b became indistinguishable on addition of traces of sulfide (e.g., Me2S). The ' H spectrum of the chiral ion 5 (six nonequivalent ring protons) collapsed to one with planar symmetry (three pairs of nonequivalent ring protons) on adding Me2& indicating rapid interconversion of enantiomers. No net change occurred as the salts could be recovered unchanged. We attribute these results to a rapid and reversible ring opening by the sulfide nucleophile whereby chirality at pyramidal sulfur is destroyed." Reclosure to either of two configurations results in the interconversion of enantiomers for 5 and diastereomers for 6-9 (eq 2).
n = i , zThe 'H spectrum of the chiral dithianium ion la also collapsed to that of a symmetrical ring on addition of Me,S-consistent with rapid equilibration of enantiomers through an achiral acyclic intermediate (19). Reclosure to an equatorial MeS' orientation does not occur. Similar sulfide-induced ring opening may be expected of ions 2-4 but there was no change in the 'H or 13C spectra on addition of Me2& and no interconversion of diastereomers of 2-4 could be detected. We regard this as evidence that reclosure of 19 to a configuration in which MeS+ is equatorial is energetically disfavored over reclosure to the axial form.In comparison, 1,2-dithiolanium diastereomers are comparable in energy, which suggests that the five-membered ring is torsionally flexible and can adopt twist conformations that avoid destabilizing interactions12 while retaining the stereoelectronic advantage of a pseudoaxial MeS+. Most importantly, ring-opening equilibria (eq 2) strongly favor ring closure, which means that ring strain is not the source of reactivity in 1,2-dithiolanium ions, as has been suggested for 1 ,2-dithiolanes.I3 Rather, reactivity is best attributed to the rapid cleavage of S-S+ bonds by nucleophiles. The ready polymerization of 1,2-dithiolane~'~ can likewise be attributed to initial thiolanium ion formation followed by monomer-induced ring cleavage. The process can be reversed, as we have shown by the successful preparation of 5 (eq 1) by methylation of 1,2-dithiolane polymer.
Acknowledgment.We are indebted to the NSF-supported Southern California N M R facility at Caltech for the high-field spectra and to partial support from General Medical Sciences, GM 27319.
