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.