Solvent effects on hydrogen bonding and rotation barriers in α-alkyl-substituted 2-alkoxybenzyl alcohols †
作者:John S. Lomas、Alain Adenier
DOI:10.1039/b102416g
日期:——
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 âfreeâ. 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 antiâsyn rotation of 2-anisyldi(tert-butyl)methanol (about 28 kcal molâ1 at 373 K) varies insignificantly with the solvent, while that for the reverse reaction decreases in the order: chloroform â 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.
2-烷氧基苯基(α,α-二烷基)甲醇存在两种构象,其中羟基氢要么与烷氧基氧形成分子内氢键(顺式旋转异构体),要么远离烷氧基(反式旋转异构体)从而“自由”存在。2-茴香基二(叔丁基)甲醇及其衍生物(其中一个或两个叔丁基被1-金刚烷基取代)的反式和顺式旋转异构体可以通过色谱法分离。2-茴香基二(叔丁基)甲醇的反式至顺式旋转活化能(在373 K时约为28 kcal mol-1)随溶剂变化不大,而相反反应的活化能则按以下顺序递减:氯仿≈甲苯>吡啶>DMSO。氢键溶剂对反式旋转异构体和旋转过渡态的稳定作用似乎同等重要,而顺式旋转异构体则不需要极性OH基团的溶剂化。2-茴香基(异丙基)(叔丁基)甲醇的平衡常数和旋转能垒受到与前述类似的溶剂效应影响,其旋转异构体在NMR时间尺度上可分离。这种醇在各种溶剂中旋转异构体的自由能差异与3,4-(亚乙基二氧基)-2-噻吩基二(叔丁基)甲醇的相应数值以及溶质氢键碱度参数相关联。