Conformation and Stereodynamics of Symmetrically Ortho-Disubstituted Aryl Carbinols and Aryl Ethers.

By making use of low temperature dynamic NMR spectroscopy the rotation barriers about the sp3-sp2 bond have been determined in a number of hindered benzyl alcohols symmetrically substituted in the ortho positions, the substituents being F, Cl, Br and Me. The free energies of activation covered the range 4.6 - 10.1 kcal mol-1. Ab-initio computations matched satisfactorily the trend of these values and predicted the conformation adopted by these compounds. In one case this result could be also confirmed by the X-ray diffraction structure. In the case of the corresponding methyl ethers two barriers could be measured, corresponding to the passage across two distinguishable transition states: the higher barriers covered the range 5.0-8.1 kcal mol-1 and the lower ones the range 4.7-6.2 kcal mol-1.