Structural and Computational Analysis of 2-Halogeno-Glycosyl Cations in the Presence of a Superacid: An Expansive Platform

An expansive NMR-based structural analysis of elusive glycosyl cations derived from natural and non-natural monosaccharides in superacids is disclosed. For the first time, it has been possible to explore the consequence of deoxygenation and halogen substitution at the C2 position in a series of 2-halogenoglucosyl, galactosyl, and mannosyl donors in the condensed phase. These cationic intermediates were characterized using low-temperature in situ NMR experiments supported by DFT calculations. The 2-bromo derivatives display intramolecular stabilization of the glycosyl cations. Introducing a strongly electron-withdrawing fluorine atom at C2 exerts considerable influence on the oxocarbenium ion reactivity. In a superacid, these oxocarbenium ions are quenched by weakly coordinating SbF6- anions, thereby demonstrating their highly electrophilic character and their propensity to interact with poor nucleophiles.