Conformational Studies by Dynamic NMR. 80. Cog-Wheel Effect in the Stereolabile Helical Enantiomers of Dimesityl Sulfoxide and Sulfone.

The 1H NMR solution spectra of the title compounds display anisochronous lines for the o-methyl substituents below -170 °C, due to the existence of two propeller-like M and P conformational enantiomers. The free energies of activation for the interconversion were determined to be 4.5 and 5.0 kcal mol -1, respectively, for dimesityl sulfoxide and dimesityl sulfone. Molecular mechanics calculations indicate that the enantiomerization process occurs via a correlated rotation (cog-wheel effect) entailing a one-ring flip (gear-meshing) pathway. 13C NMR (CP-MAS) spectra and X-ray diffraction show that these helical enantiomers are stable in the crystalline state.