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.