Transition-State Effects in the Divalent Metal Ion Catalyzed Hydrolysis of Esters. Hydrolysis of 2-Pyridylmethyl Hydrogen Phthalate

Article abstract of DOI:10.1021/ja00544a023

Rate constants have been obtained for hydrolysis of 2-pyridylmethyl hydrogen phthalate, an ester with a poor leaving group, at 90 deg C in the pH range 6 - 11.The pH-rate constant profile shows hydroxide ion catalysis at high pH and a plateau due to nucleophilic carboxyl group participation.Divalent metal ions (Ni²⁺, Co²⁺, Zn²⁺) exert a large catalytic effect, although binding to the substrate is weak.Saturation effects were not observed even at 0.01 M metal ion (>100-fold excess over ester).Catalysis is dependent upon the presence of the pyridine nitrogen in the 2-position; metal ions are without effect in hydrolysis of the 4-pyridyl analogue.The metal ion catalyzed reactions are pH independent (pH 4.74 - 7.15 at 50 deg C).Therefore, metal ion catalysis in reactions of the phthalate ester is associated with carboxyl group participation.Since breakdown of a tetrahedral intermediate must be rate determining in the carboxyl nucleophilic reactions of esters with poor leaving groups, the metal ions must exert their effect in the transition state by stabilization of the leaving group.In contrast, the metal ion effects observed in hydrolysis of 2-pyridylmethyl benzoate represent metal ion promoted OH^- catalysis.