Theoretical investigation of solvent effects on glycosylation reactions: Stereoselectivity controlled by preferential conformations of the intermediate oxacarbenium-counterion complex

The mechanism of solvent effects on the stereoselectivity of glycosylation reactions is investigated using quantum-mechanical (QM) calculations and molecular dynamics (MD) simulations, considering a methyl-protected glucopyranoside triflate as a glycosyl donor equivalent and the solvents acetonitrile, ether, dioxane, or toluene, as well as gas-phase conditions (vacuum). The QM calculations on oxacarbenium-solvent complexes do not provide support to the usual solvent-coordination hypothesis, suggesting that an experimentally observed β-selectivity (α-selectivity) is caused by the preferential coordination of a solvent molecule to the reactive cation on the α-side (β-side) of the anomeric carbon. Instead, explicit-solvent MD simulations of the oxacarbenium-counterion (triflate ion) complex (along with corresponding QM calculations) are compatible with an alternative mechanism, termed here the conformer and counterion distribution hypothesis. This new hypothesis suggests that the stereoselectivity is dictated by two interrelated conformational properties of the reactive complex, namely, (1) the conformational preferences of the oxacarbenium pyranose ring, modulating the steric crowding and exposure of the anomeric carbon toward the α or β face, and (2) the preferential coordination of the counterion to the oxacarbenium cation on one side of the anomeric carbon, hindering a nucleophilic attack from this side. For example, in acetonitrile, the calculations suggest a dominant B_2,5 ring conformation of the cation with preferential coordination of the counterion on the α side, both factors leading to the experimentally observed β selectivity. Conversely, in dioxane, they suggest a dominant ⁴H₃ ring conformation with preferential counterion coordination on the β side, both factors leading to the experimentally observed α selectivity.