Hydrolysis of α-amino-acid esters in mixed-ligand complexes with ethylenediaminepalladium(II)

Abstract

α-Amino-acid esters react with [Pd(en)(OH₂)₂]²⁺(en = ethylenediamine) according to the equilibrium (i). The [Pd(en)(OH₂)₂]²⁺+ [graphic omitted]H₃CH(R) CO₂R′ [graphic omitted] [Pd(en){NH₂CH (R) CO₂R′}]²⁺+ H₃O⁺+ H₂O(i) kinetics of hydrolysis of [Pd(en) L]²⁺[L = NH₂CH(R) CO₂R′] have been studied by pH-stat and rate constants obtained for the kinetic processes (ii) and (iii) where A^–= NH₂CH(R) CO₂^– and L = ethyl glycinate (gly-OEt), [Pd(en)L]²⁺+ H₂O [graphic omitted] [Pd (en)A]⁺+ R′OH + H⁺(ii), [Pd(en)L]²⁺+ OH^– [graphic omitted] [Pd(en)A]⁺+ R′OH (iii) methyl glycinate (gly-OMe), ethyl α-alaninate (α-ala-OEt), ethyl β-phenylalaninate (phe-OEt), ethyl picolinate (pic-OEt), ethyl cysteinate (cys-OEt), and methyl histidinate (his-OMe). For the first five esters substantial rate accelerations are observed for base hydrolysis (factors of 4 × 10⁴ for gly-OEt to 1.4 × 10⁷ for pic-OEt). The effects with cys-OEt and his-OMe are much less marked, as the mixed-ligand complexes with these ligands do not involve alkoxycarbonyl donors. Possible mechanisms for these reactions are considered, and it is suggested that the reaction may involve kinetically important ion pairing between the complex and the incoming nucleophile prior to nucleophilic attack on the ester ligand.