Solvolysis of D-Glucopyranosyl Derivatives in Mixtures of Ethanol and 2,2,2-Trifluoroethanol

Article abstract of DOI:10.1021/ja00526a043

The products of solvolysis of α- and β-D-glucopyranosyl fluorides, 2,4-dinitrophenyl β-D-glucopyranoside, and the trifluoromethanesulfonates of the β-D-glucopyranosyl 3-bromopyridinium and α-D-glucopyranosyl 4-methylpyridinium ions in an equimolar mixture of ethanol and trifluoroethanol buffered with ca. 2 equiv of 2,6-lutidine have been examined by GLC of their trimethylsilyl ethers.The initial products of the solvolyses of phenyl α- and β-D-glucopyranosides catalyzed by trifluoromethanesulfonic acid in an equimolar mixture of ethanol and trifluoroethanol, and the products of uncatalyzed solvolysis of β-D-glucopyranosyl-p-nitrophenyltriazene, have been likewise examined.The composition of the medium for solvolysis of the glucosyl fluorides has also been systematically varied from pure ethanol to pure trifluoroethanol.The percentage of products with the same anomeric configuration as the starting material is in the range 8.1-88.5percent; change of leaving group, at constant anomeric configuration, or of anomeric configuration, at constant leaving group, yields different product distributions.Therefore the transition state for the product-determining step contains the leaving group.The preference for attack by ethanol as compared with trifluoroethanol varies from 0.9 to 20 in a way which shows no general systematic distinction between pathways for retention or inversion.The nucleophilic selectivity for retention is lowered by anionic leaving groups, especially fluoride, which preferentially stabilize the transition state containing trifluoroethanol by hydrogen bonding.Nucleophilic attack at the α face is preferred over nucleophilic attack at the β face, and exibits a lower selectivity: this is ascribed to hydrogen bonding between the oxygen atom of the 2-hydroxyl group and the hydroxyl group of the approaching alcohol.A model for solvolysis involving a reversibly formed ion pair or encounter complex is incompatible with the selectivities still observed with leaving groups less nucleophilic than the solvent components: a model involving selection between the components of a pool of solvent molecules by an irreversibly formed ion pair or encounter complex requires an implausibly large pool to explain observed specificities.It is therefore concluded that the observed selectivities are a consequence of the facilitation of the departure of the leaving group by the solvent, from either side of the reaction center.