Solvent effects on hydrogen bonding and rotation barriers in α-alkyl-substituted 2-alkoxybenzyl alcohols

Abstract

2-Alkoxyphenyl(α,α-dialkyl)methanols exist in two conformations, where the hydroxy hydrogen is either intramolecularly hydrogen-bonded to the alkoxy oxygen (syn rotamer) or is remote from the alkoxy group (anti rotamer) and therefore “free”. The anti and syn rotamers of 2-anisyldi(tert-butyl)methanol, and of derivatives where one or both tert-butyls are replaced by 1-adamantyl, can be separated by chromatography. The activation energy for anti→syn rotation of 2-anisyldi(tert-butyl)methanol (about 28 kcal mol⁻¹ at 373 K) varies insignificantly with the solvent, while that for the reverse reaction decreases in the order: chloroform ≈ toluene > pyridine > DMSO. Stabilization of the anti rotamer and the rotation transition state by hydrogen-bonding solvents would appear to be of equal importance, whereas the syn rotamer has no requirement for solvation of the polar OH group. Very similar solvent effects on equilibrium constants and rotation barriers are found for 2-anisyl(isopropyl)(tert-butyl)methanol, the rotamers of which are separable on the NMR time-scale. The free energy differences for the rotamers of this alcohol in a variety of solvents correlate with those for 3,4-(ethylenedioxy)-2-thienyldi(tert-butyl)methanol and with solute hydrogen bond basicity parameters.