The hydrolysis of alpha-amino-acid esters in mixed ligand complexes with copper(II)-iminodiacetate

alpha-Amino-acid esters (EH+) interact with [Cu(IMDA)]0 to give mixed ligand complexes according to the equilibrium, [Cu(IMDA)]0 + EH+ in equilibrium with [Cu(IMDA)(E)]0 + H+, where EH+ represents the protonated ester +NH3CH(R)CO2R' and IMDA2- is HN(CH2CO2-)2. The mixed ligand complexes are only formed over a rather narrow pH range (ca. pH 5.8-6.5). At higher pH there is kinetic evidence for the competing equilibrium, [Cu(IMDA)(E)]0 + OH- in equilibrium with [Cu(IMDA)OH]- +E Rate constants kOH have been obtained by pH-stat for the hydrolyses [where A- = NH2CH(R)CO2-] [Cu(IMDA)(E)]0 + OH- kOH leads to [Cu(IMDA)(A)]- + R'OH. The complexed alpha-amino-acid esters undergo base hydrolysis ca. 10(4) times faster than the free esters E. Values of kOH show little dependence on the nature of the alkyl substituent R but the normal leaving group effect of methyl esters hydrolyzing at ca. twice the rate of ethyl esters is observed. Activation parameters have been determined for base hydrolysis of [Cu(IMDA)(glyOMe)]0, and possible mechanisms for the reaction are considered.