STEREOELECTRONIC EFFECTS IN NEGATIVELY AND POSITIVELY (PROTONATED) CHARGED SPECIES - AB-INITIO STUDIES OF THE ANOMERIC EFFECT IN 1,3-DIOXA SYSTEMS

A systematic computational ab initio study of the conformational depen dent proton affinities of methoxymethoxide, tetrahydropyran 2-oxide, m ethoxymethanol, dimethoxymethane, 1, 3-dioxane, and tetrahydropyran ha s been carried out at the MP2/6-31+G level of theory. In addition, me thoxide, propoxide and methanol, propanol, and dimethyl ether have bee n computed at the same level as reference systems. Methoxymethoxide an d tetrahydropyran 2-oxide exhibit a strong anomeric effect, e.g., the equatorial oxide is a stronger base than the axial one and all are wea ker bases than the simple alkoxides. Axial (n(pi)) protonation is pref erred over equatorial (n(sigma)) by 2 - 3 kcal/mol. The COCOC acetals are stronger bases (at the acceptor O) then the simple ethers. The str uctural changes between bond lengths and bond angles for different con formers correlate well with the On(pi)-sigma(C-O) lone pair delocaliz ation interactions. Thus, the anomeric effect plays an important role in the charged species and in the process of their formation.