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.