The Exceptional Conformational Bias of 1,5-Diene-3,4-diols Is Controlled by Electrostatic Interactions: A Theoretical Elucidation of the Origin in Rotational Isomer Stability.

Abstract

A conformational study of 1,5-hexadiene-3,4-diol (1) and 3-buten-1-ol (2) with the ab initio molecular orbital methods reveals that electrostatic interactions, rather than steric or tosional effects, control the conformational preferences of these compounds. It is found that the 1,2-dioxygen function prefers the anti arrangement due to the lone pair electron repulsion. A pseudo 1,3-diaxial oxygen/pi bond repulsion and a 1,3-diaxial attraction between an oxygen lone pair and a vinyl H are found to be responsible for the profound preference for the CO-eclipsed form in diol 1. Quantitatively, the repulsion between two gauche oxygen atoms is calculated to be approximately 1.5 kcal/mol at the MP2/6-31G//HF/6-31G level of theory. The repulsion between an oxygen atom and a pi bond at the pseudo 1,3-diaxial position is approximately 1 kcal/mol, and the attractive interaction between an oxygen atom and a vinyl H at the pseudo 1,3-diaxial position is approximately 0.5 kcal/mol, respectively, at the MP2/6-31G//MP2/6-31G level.