Divalent Metal Ion Catalysis in the Hydrolysis of Esters of Picolinic Acid. Metal Ion Promoted Hydroxide Ion and Water Catalyzed Reactions

Article abstract of DOI:10.1021/ja00290a048

Rate constants have been determined for hydrolysis of a series of phenolic and aliphatic esters of picolinic acid in H2O.Hydroxide ion, hydronium ion, and water catalyzed reactions were observed in hydrolysis of the phenolic esters.Catalysis by low concentrations of Ni²⁺ and Cu²⁺ occurs even though binding of the metal ions is weak (saturation effects were not observed).Both metal ion promoted water and OH^- catalyzed reactions were observed with the esters having leaving groups with pK_a values of 12.4 or less.Rate enhancements produced by 0.01 M Ni²⁺ and 0.001 M Cu²⁺ range from 10- to near 200-fold in the pH-independent water reactions and from 10²- to over 10⁵-fold in the OH^- catalyzed reactions.Significant metal ion catalysis was not observed in the hydrolysis of 4-nitrophenyl isonicotinate or 8-(5-nitroquinolyl) isonicotinate; therefore, metal ion catalysis in the hydrolysis of the esters with the pyridine nitrogen ortho to the ester function must be associated with a chelation effect.The rate constants k₀ and k_OH for hydrolysis of the picolinate esters in the metal ion promoted water and OH^- catalyzed reactions are little affected by the leaving group (β_1g ca. 0) for leaving groups with pK_a values ranging from 4.1 with 2,4-dinitrophenol to 12.4 with trifluoroethanol, and ratios of k_OH/k₀ are nearly constant.This indicates that there is little or no C-O bond breaking in the critical transition state, i.e., in both reactions the nucleophilic attack step is rate determining.When the leaving group is ethanol, then k_OH is markedly less than in the case of the trifluoroethyl ester, and a metal ion promoted water reaction is not detected even at pH values as low as 4.Thus, a change in rate-determining step has occurred with the change in the leaving group.Likewise only metal ion promoted OH^- catalysis is observed with ethyl 6-carboxypicolinate.Rate enhancements produced by saturating concentrations of Ni²⁺ and Cu²⁺ are in that the case 2.7*10⁴- and 1.3*10⁵-fold, respectively.Intramolecular general base catalysis does not occur in the metal ion promoted water reaction of 8-quinolyl picolinate or 8-(5-nitroquinolyl) picolinate.With the nitro substituted esters of picolinic acid a metal ion promoted formate and acetate ions are attacking the metal ion complexes as nucleophiles.