Stereomutation of cyclic sulfonium and selenonium compounds

New cyclic sulfonium and selenonium compounds have been synthesized in procedures of a few steps: the diastereoisomeric mixtures cis/trans-3-methyl-1-phenylthiolanium perchlorate 1, cis/trans-3-methyl-1-ethylthiolanium hexafluorophosphate 2, cis/trans-3-methyl-1-phenylselenolanium hexafluorophosphate 3, and the diastereoisomerically pure trans-4-tert-butyl-1-phenylthianium hexafluorophosphate 4. After enrichment of the main isomer, the rates of stereomutation at the chalcogenonium centre were determined by NMR spectroscopy, and the activation enthalpies and entropies were calculated. The selenonium compound 3 showed an increased configurational stability in water. Inversion of configuration was about 50 times slower than that of the sulfonium compound 1. Unexpectedly, the selenonium compound 3 showed a 70-fold acceleration of the stereomutation rate in the solvent DMF and a large activation entropy.