Conformational studies by dynamic NMR. 80.(1) cog-wheel effect in the stereolabile helical enantiomers of dimesityl sulfoxide and sulfone.
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.
Casarini
D
D
Chemistry Department, Università della Basilicata, Potenza, Italy.
Grilli
S
S
Lunazzi
L
L
Mazzanti
A
A