Relief of steric strain by intramolecular C–H · · · O interactions: structural evidence for the 1,4-disubstituted cyclohexanes

Abstract

Two crystal structures are reported where trans-cyclohexane-1,4-diol is observed in the unexpected biaxial conformation, even with its associated steric conflict between the substituents and the axial H-atoms of the cyclohexane moiety. In a consecutive database analysis of trans- and cis-1,4-disubstituted cyclohexanes, it is found that the steric distortions caused by axial substituents, X, depend strongly on the chemical nature of X. These distortions are surprisingly small for X = O and N, larger for X = S, and more pronounced for X = C. Cyclohexanes with axial O-substituents are almost unstrained. This is explained by intramolecular C–H· · ·O interactions between the electron lone pair of the O-atom and the axial H-atoms, which allow shorter approach of O to H than of C to H. Reasons are given why the classification of these interactions as ‘hydrogen bonds’ is inadequate.