pH-Rate Profile for Hydrolysis of 4-Nitrophenyl β-D-Glucopyranoside: Unimolecular, Bimolecular and Intramolecular Cleavage Mechanisms

Abstract

1,2-<i>trans</i>-Glycosides hydrolyze through different\nmechanisms at different pH values, but systematic studies are lacking. Here we\nreport the pH-rate constant profile for the hydrolysis of<i> </i>4-nitrophenyl\nβ-D-glucoside. An inverse kinetic isotope effect of <i>k</i>(H₃O⁺)/<i>k</i>(D₃O⁺ = 0.65 in the acidic region indicates that the mechanism requires the\nformation of the conjugate acid of the substrate for the reaction to proceed,\nwith heterolytic cleavage of the glycosidic C-O bond. Reactions in the\npH-independent region exhibit general catalysis with a single proton in flight,\na normal solvent isotope effect of <i>k</i>_H/<i>k</i>_D =\n1.5, and when extrapolated to zero buffer concentration show a small solvent\nisotope effect <i>k</i>(H₂O)/<i>k</i>(D₂O) = 1.1,\nconsistent with water attack through a dissociative mechanism. In the basic\nregion, solvolysis in ¹⁸O-labelled water and H₂O/MeOH\nmixtures allowed detection of bimolecular hydrolysis and neighboring group participation,\nwith a minor contribution of nucleophilic aromatic substitution. Under mildly\nbasic conditions, a bimolecular concerted mechanism is implicated through an\ninverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–)\n= 0.5 and a strongly negative entropy of activation (D<i>S</i>^‡ = –13.6 cal\nmol^–1 K^–1). Finally, at high pH, an inverse solvent\nisotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.6 indicates\nthat the formation of 1,2-anhydrosugar is the rate determining step.<br>