HOMOGENEOUS CATALYSTS FOR SELECTIVE MOLECULAR-OXYGEN DRIVEN OXIDATIVE DECARBOXYLATIONS

Colbalt(II) ion has been found to catalyze the molecular oxygen driven oxidation of N-(phosphonomethyl)iminodiacetic acid (PMIDA) to N-(phosphonomethyl)glycine (PMG) in aqueous solution. 1 This homogeneous catalytic conversion is novel and represents, in effect, an oxidative dealkylation of one carboxymethyl moiety yielding the N-substituted glycine. The reaction is selective to the desired product PMG when carried out at the natural pH of the free acid substrate (approximately 1-2) and when carried out at substrate loadings less than 5% by weight. In addition, the catalytic system is selective for the PMIDA substrate; i.e., other closely related ligands show no reactivity, e.g., NTA, EDTA, etc. The results of kinetic and mechanistic studies on dilute systems are presented and discussed with special emphasis on how an understanding of the mechanism can make it possible to generate a catalyst system that gives high yields even with high substrate loadings. The reactions are first-order in substrate and [Co]t. The oxygen pressure dependence exhibits saturation kinetics, while the selectivity increases as oxygen pressure increases. The rate is also inversely proportional to [H+]. The high selectivity of the oxidation and the unique selectivity of the cobalt catalytic system for the PMIDA substrate are discussed in terms of the magnitude of the metal ligand binding constant at the low pH of the reaction.