Deuterium isotope and anomeric effects in the conformational equilibria of molecules containing CHD-O groups

The authors have recently shown that the deuterium in a CHD group in cyclohexane-d/sub 1/ or cyclohexane-d/sub 11/ prefers the equatorial over the axial position by 6.3 +/- 1.5 cal/mol. They now report that the deuterium in 5,5-dimethyl-1,3-dioxane-2-d/sub 1/ (I) prefers the equilibrium isotope effect is understandable in terms of n-sigma* hyperconjugation (no-bond resonance) of a type which is also responsible, in large part, for the anomeric effect. The latter is a stabilization of an axial vs. an equatorial electronegative substituent, as in II. The bond to the axial substituent is weakened and lengthened, whereas the adjacent C-O bond is changed in an opposite fashion. The weakening of the axial bond, albeit in reduced form, should exist when X = H in II.