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^–(k_OH^Cl 15.2 dm³ mol^–1 s^–1) a slower base hydrolysis of the ester function occurs (k_OH 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 (k_OH^Cl 15.6 dm³ mol^–1 s^–1). For the analogous cis-[CoBr(en)₂{NH₂(CH₂)₃CO₂Me}]²⁺, k_OH^Br= 125 ± 3 dm³ mol^–1 s^–1 and k_OH^Br: k_OH^Cl= 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 (k_OH^Cl 10.4 dm³ mol^–1 s^–1 determined by pH-stat; k_OH^Cl 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 k_OH 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.