Conformational analysis of the thiolane ring system. II. Proton magnetic resonance spectra and base-catalyzed H--D exchange of sulfonium cations and sulfoxides derived from 3,3-dimethylthiolane and trans-2-thiahydrindan

Proton NMR spectral analysis of 1,3,3-trimethylthiolanium iodide (III-CH3) and 3,3-dimethylthiolane 1-oxide (III-O) allows the unambiguous configurational assignment of these five-membered S heterocycles. Both systems appear to be conformationally homogeneous; however, while the sulfonium cation adopts a half-chair conformation with maximum twist at C3--C4, the sulfoxide has an envelope conformation with sulfur at the tip and the oxygen functionality quasi-axial. In base-catalyzed H--D exchange of the α protons, the two compounds display quite different diastereotopic reactivity patterns, perhaps related to their different ground state conformation. Thus, the exchange is highly stereospecific in the sulfonium cation, but essentially nonstereospecific in the sulfoxide. By contrast, in the trans-2-thiahydrindan series where, by virtue of the trans ring fusion, the five-membered heterocycle is rigidly held in the half-chair conformation, the exchange is stereospecific in the sulfoxide as well, though less so than in the sulfonium cation. These results are examined together with those in the literature for comparable systems. However, there seems to be no obvious logical connection between kinetic acidity and geometrical factors. (auth)