Quantum chemical study of the preferred conformations, molecular geometries, and relative thermodynamic stabilities of 2-substituted 4-methylene-1,3-dioxolanes and 4-methyl-1,3-dioxoles

The preferred conformations, molecular geometries, and relative stabilities of carbon-carbon double-bond exo-endo isomeric 2-substituted 4-methylene-1 ,3-dioxolanes (a) and 4-methyl-1,3-dioxoles (b) have been studied by DFT ca lculations at the B3LYP/6-31G* level of theory. The main interest of this w ork was devoted to the contribution of alkoxy substituents on the relative thermodynamic stabilities of these isomeric unsaturated acetals. Comparison of the computational data with previous experimental findings shows both t he enthalpies and entropies of the a b isomerization to be accurately predi ctable by the DFT calculations. Most importantly, the good agreement betwee n experiment and theory proved also to be applicable to the previously obse rved unexpectedly large effect of 2-alkoxy substitution on the isomer equil ibria.