Abstract
The (1)H NMR solution spectra of the title compounds display anisochronous lines for the o-methyl substituents below -170 degrees 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. (13)C NMR (CP-MAS) spectra and X-ray diffraction show that these helical enantiomers are stable in the crystalline state.
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