Hydrolysis of α-amino-acid esters in mixed-ligand complexes with 2,2′-bipyridylpalladium(II)

Abstract

α-Amino-acid esters, NH₂CH(R)CO₂R′(L), interact with [Pd(bipy)(OH₂)₂]²⁺(bipy = 2,2′-bipyridyl) in aqueous solution according to the equilibrium (i)(R and R′= alkyl). The kinetics of hydrolysis of the ester group in the complexes [Pd(bipy)L]²⁺ have been studied by pH-stat methods and rate constants [Pd(bipy)(OH₂)₂]²⁺+ NH₃CH(R)CO₂R′ [graphic omitted] [Pd(bipy){NH₂CH(R)CO₂R′}]²⁺+ H₃O⁺+ H₂O (i) have been obtained for the kinetic processes (ii) and (iii) where A = NH₂CH(R)CO₂^– and L = methyl glycinate, ethyl glycinate, ethyl L-α-alaninate, ethyl L-β-phenylalaninate, ethyl picolinate, ethyl L-cysteinate, and methyl L-histidinate. For the first five esters substantial rate accelerations are [Pd(bipy)L]²⁺+ H₂O [graphic omitted] [Pd(bipy)A]⁺+ R′OH + H⁺(ii), [Pd(bipy)L]²⁺+ OH^– [graphic omitted] [Pd(bipy)A]⁺+ R′OH (iii) observed for base hydrolysis (factors of 1.6 × 10⁵ for methyl glycinate to 3.3 × 10⁷ for ethyl picolinate). The effects with methyl L-cysteinate and methyl L-histidinate are much less marked, as the mixed-ligand complexes with these ligands do not involve alkoxycarbonyl donors. Possible mechanisms for these reactions are discussed, and alternative reaction schemes involving a monodentate (N-bonded) ester species are considered. Activation Parameters have been determined for reactions (ii) and (iii) where L = methyl glycinate.