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 dependent proton affinities of methoxymethoxide, tetrahydropyran 2-oxide, methoxymethanol, dimethoxymethane, 1,3-dioxane, and tetrahydropyran has been carried out at the MP2/6-31+G* level of theory. In addition, methoxide, propoxide and methanol, propanol, and dimethyl ether have been computed at the same level as reference systems. Methoxymethoxide and tetrahydropyran 2-oxide exhibit a strong anomeric effect, e.g., the equatorial oxide is a stronger base than the axial one and all are weaker bases than the simple alkoxides. Axial (n_π) protonation is preferred over equatorial (n_σ) by 2-3 kcal/mol. The COCOC acetals are stronger bases (at the acceptor O) then the simple ethers. The structural changes between bond lengths and bond angles for different conformers correlate well with the On_π-σ*c-o lone pair delocalization interactions. Thus, the anomeric effect plays an important role in the charged species and in the process of their formation.