Anomeric effect in pyranose-ring derivatives containing carbon, silicon, and germanium as anomeric centers: an ab initio systematic study

Abstract

We have performed a systematic conformational analysis focused on the evaluation of the anomeric effect (AE) in a series of pyranose derivatives containing carbon, silicon, and germanium as anomeric centers (c*) using the MP2/aug-cc-pVDZ level of theory together with natural bond orbital (NBO) electronic structure calculations. Although, both endo- and exo-anomeric effects operate within all the systems studied; the conformational preference towards the axial (α) form can be explained in terms of the incidence of the endo-anomeric effect. The magnitude calculated for the AE is considerably higher for compounds containing carbon as c*. On the other hand, the lower magnitude of the hyperconjugative delocalizations towards antibonding exocyclic anomeric orbitals in Si- and Ge-containing compounds can be justified by the availability of energetically accessible vacant d-type orbitals in these atoms. While the conformational preference in the carbon group is purely related to a higher anomeric hyperconjugation in the α conformers, steric and electrostatic factors dictate the conformational α arrangement in the Si- and Ge-containing compounds. Implicit consideration of the solvent (water) produces a notable increase in the population of the β anomers in some of the systems into study. However, the results of NBO energy partition study performed reveal that the merely observation of a change in the α/β ratio for a given system upon solvation should not be taken as an indication of a predominant role of electrostatic effects as the origin for their anomeric preference.