Kinetics and mechanism of the hydrolysis of α-amino-acid esters in mixed-ligand complexes with copper(II) ethylenediaminemonoacetate

Abstract

Amino-acid esters (EH⁺) interact with [Cu(edma)]⁺ to give mixed-ligand complexes in accordance with the equilibrium, [Cu(edma)]⁺+ EH⁺⇌[Cu(edma)E]⁺+ H⁺, where EH⁺ is the protonated ester [graphic omitted]H₃CH(R)CO₂R′ and edma^–= ethylenediaminemonoacetate. The ternary complexes are only formed over a narrow pH range (7.3–8.2) and 8.4–8.8 with methyl L-histidinate. At higher pH values there is kinetic evidence for the competing equilibrium, [Cu(edma)E]⁺+ OH^–⇌[Cu(edma)(OH)]⁰+ E. Rate constants have been obtained at I= 0.1 mol dm^–3 and 25 °C for both base hydrolysis and solvolysis of the ester group of a series of α-amino-acid esters, while co-ordinated (probably chelated) in the ternary complexes. Base hydrolysis is some 10³ times faster than for the unprotonated esters (E), and possible mechanisms for these reactions are considered.