| Abstract
| - The 1H NMR solution spectra of the title compounds display anisochronous lines for the o-methylsubstituents below −170 °C, due to the existence of two propeller-like M and P conformationalenantiomers. The free energies of activation for the interconversion were determined to be 4.5 and5.0 kcal mol-1, respectively, for dimesityl sulfoxide and dimesityl sulfone. Molecular mechanicscalculations indicate that the enantiomerization process occurs via a correlated rotation (cog-wheeleffect) entailing a one-ring flip (gear-meshing) pathway. 13C NMR (CP-MAS) spectra and X-raydiffraction show that these helical enantiomers are stable in the crystalline state.
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