Conformational studies by dynamic NMR. 83. Correlated enantiomerization pathways for the stereolabile propeller antipodes of dimesityl substituted ethanol and ethers.
Below -100 degrees C, the NMR spectra of dimesityl derivatives of ethanol and of various ethers reveal how these molecules exist as M and P propeller-like 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 interconversion circuit produced good agreement between the computed and experimental barrier for a correlated dynamic process where a disrotatory one-ring flip pathway reverses the helicity of the conformational enantiomers. Introduction of a configurationally stable chiral center allowed two distinct NMR spectra to be detected at appropriate low temperature for two stereolabile diastereoisomers.
Grilli
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Department of Organic Chemistry A. Mangini, University of Bologna, Risorgimento, 4, Bologna 40136, Italy.
Lunazzi
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Mazzanti
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