Conformational studies by dynamic NMR. 83.(1) Correlated enantiomerizationpathways for the stereolabile propeller antipodes of dimesityl substitutedethanol and ethers

Below -100 degreesC, the NMR spectra of dimesityl derivatives of ethanol an d of various ethers reveal how these molecules exist as M and P propeller-l ike stereolabile enantiomers, owing to the restricted rotation about the Ar -C bond. Single-crystal X-ray diffraction of one such derivative confirmed the existence of a two-blade propeller structure. Computer analysis of the NMR line shape allowed the barriers for the enantiomerization process to be determined. Theoretical modeling (Molecular Mechanics) of the interconvers ion circuit produced good agreement between the computed and experimental b arrier for a correlated dynamic process where a disrotatory one-ring flip p athway reverses the helicity of the conformational enantiomers. Introductio n of a configurationally stable chiral center allowed two distinct NMR spec tra to be detected at appropriate low temperature for two stereolabile dias tereoisomers.