Hydrolysis of (2-Deoxy-α- d -Glucopyranosyl)pyridinium Salts: The 2-Deoxyglucosyl Oxocarbenium Is Not Solvent-Equilibrated in Water

The hydrolysis reactions of four 2-deoxy-α- d -glucopyranosyl pyridinium salts exhibit first-order rate constants that are independent of pH in the range of 4.4−10.5 pH units. Derived second-order rate constants for the hydrolysis reactions of 2-deoxy-α- d -glucopyranosyl 4‘-bromoisoquinolinium tetrafluoroborate ( 4d ) conducted in the presence of nucleophilic monoanions (μ = 2.0) including AcO -, Cl -, Br -, and N 3 - exhibit a Swain−Scott parameter ( s ) of 0.03 ± 0.10, indicating that these reactions show no sensitivity to the nature of the anion. In the presence of azide ion, a substantial quantity of the 2-deoxy-α-glucopyranosyl 4‘-bromoisoquinolinium salt hydrolysis product results from a post rate-limiting reaction of a cationic intermediate with azide. Analysis of the hydrolysis product ratios indicates that the 2-deoxyglucosyl oxocarbenium ion is not solvent-equilibrated in water. Furthermore, the reaction of solvent occurs about 2-fold faster with the cationic intermediate that is formed during solvolysis of the β-anomeric salt than with the corresponding intermediate produced from the reactions of the α-anomer 4d .