VANADIUM(IV,V) SALTS AS HOMOGENEOUS CATALYSTS FOR THE OXYGEN OXIDATION OF N-(PHOSPHONOMETHYL)IMINODIACETIC ACID TO N-(PHOSPHONOMETHYL)GLYCINE

From screening studies we have found that several metal ions are capable of catalyzing the molecular oxygen oxidation of N-(phosphonomethyl)iminodiacetic acid (PMIDA) to N-(phosphonomethyl)glycine (PMG), the active agent in the herbicide Roundup. Of these metal ions, vanadium, manganese, and cobalt salts were the most active, with the relative order of activity under a given set of conditions being V > Mn > Co. The vanadium-catalyzed PMIDA oxidations proceed at much faster rates and under milder conditions than with the other metals, but the chemistry suffers from lower selectivities. Kinetic and mechanstic studies reveal that the reactions are first-order in the [PMIDA], [V]tot., and [H+]. In addition, the rate exhibits O2 saturation kinetics, while increasing O2 pressure increases the selectivity to PMG. From spectrophotometric, deuterium isotope, and other mechanistic studies, key mechanistic features have been elucidated: (1) An OV(IV) (PMIDA(3-)) complex is first oxidized to V(V). (2) V(V) oxidation of a carboxyl moiety yields an intermediate carbon-centered N-methylenePMG radical. (3) The trapping of this radical with O2 results in the formation of N-formylPMG, which hydrolyzes to PMG. (4) H atom abstraction by the N-methylenePMG radical leads to the undesired byproduct N-methylPMG.