Base hydrolysis of amino-acid esters and amides in the co-ordination sphere of cobalt(III). Part 3. Hydrolysis of methyl and ethyl 4-amino-butanoate

Abstract

The base hydrolysis of cis-[CoCl(en)2{NH2(CH2)3CO2Me}]2+(en = ethylenediamine) has been studied by pH-stat and stopped-flow spectrophotometry at 25 °C and l= 0.1 mol dm–3. Two consecutive reactions are observed in the pH-state measurements. Following loss of Cl–(kOHCl 15.2 dm3 mol–1 s–1) a slower base hydrolysis of the ester function occurs (kOH 0.72 dm3 mol–1 s–1). In the pH-stat measurements 2 mol of base are consumed per mol of complex and the final product of the reaction is the penta-amine hydroxo-complex which has been identified by visible spectra and potentiometric titration. In the stopped-flow measurements a single kinetic process is observed spectrophotometrically corresponding to chloride hydrolysis (kOHCl 15.6 dm3 mol–1 s–1). For the analogous cis-[CoBr(en)2{NH2(CH2)3CO2Me}]2+, kOHBr= 125 ± 3 dm3 mol–1 s–1 and kOHBr: kOHCl= 8 : 1. Complete base hydrolysis of the complex cis-[CoCl(en)2{NH2(CH2)3CO2}]+ containing the N-co-ordinated amino-acid requires 1 mol of base (kOHCl 10.4 dm3 mol–1 s–1 determined by pH-stat; kOHCl 9.83 dm3 mol–1 s–1 determined spectro-photometrically at l= 0.1 mol dm–1 and 25 °C). For the unprotonated ester NH2(CH2)3CO2Me, lactamisation to give 2-pyrrolidinone competes with base hydrolysis, but kOH for ester hydrolysis is ca. 0.1 dm3 mol–1 s–1 at 25 °C. Thus a small enhancement (ca. 7.2 times) occurs for hydrolysis of the N-co-ordinated ester.