Conformational effects in compounds with six-membered rings—VI : Conformational equilibria in 5-alkyl-1,3-dioxane

Abstract

Conformation equilibria in 5-alkyl-1,3-dioxanes have been studied by determining the equilibria between stereoisomeric pairs of 2,5-disubstituted-1,3-dioxanes in which the 2-substituent (t-butyl or p-nitrophenyl) was large enough to act as a conformational locking group; the results were independent of the 2-substituent. The more fully studied 2-t-butyl series gave enthalpy and entropy differences for the conformations of 5-methyl, 5-ethyl and 5-isopropyl-1,3-dioxanes which were smaller than those for the analogous alkylcyclohexanes, in agreement with the difference in van der Waals radii of oxygen atoms and methylene groups. cis-2,5-Di-t-butyl-1,3-dioxane has an axial 5-t-butyl group. The enthalpy and entropy differences vary with the size of the alkyl group in an irregular manner which is not consistent with the usual assumptions made in the conformational analysis of chair-chair equilibria in 6-membered rings and which shows the inadequacy of considering only free energies.