The influence of remote substituents upon ionization potential. Part I. The effect of two allylic oxygens upon the ionization of the π-bond

Abstract

The photoelectron (pe) spectrum of ascaridole (1) shows an unusually high binding potential for the π-bond. Several possible explanations both conjugative and inductive in nature can be envisioned to account for these observations. To probe which of these mechanisms was prominent, several symmetrical bisallylic oxygen compounds as well as their saturated analogues were synthesised and their pe spectra recorded. Substitution at the allylic centres was chosen to be nearly constant in each case, but the orientation of the symmetrically placed oxygens varied about the π-bond from 0° in ascaridole (1) to 90° in 1,4-dimethoxybicyclo[2.2.2]oct-2-ene (6). In each unsaturated case the position of the ionization attributable to the π-bond is the same. Conjugative interactions of the oxygens with the π-bond cannot account for the pe results since these are expected to show marked dependence upon geometrical constraints. It is postulated that in these systems, the greater electronegativity of oxygen relative to carbon is responsible for the observed result.