Aqueous-Phase Oxidation of N-Substituted N-PhosphonomethylGlycines into Glyphosate with Hydrogen Peroxide in the Presence of Carbon-Supported Gold Catalysts

The catalytic performance of carbon-supported gold nanoparticles has been studied for the oxidative dealkylation of a series of N-substituted N-phosphonomethyl glycines into N-phosphonomethyl glycine (“glyphosate”) using dilute (0.11 m) aqueous H₂O₂ solution as oxidant. It was shown that the electron-donating inductive (+I) effect of the hydrocarbon substituent at the nitrogen atom greatly influences both the substrate reactivity and product distribution. Gold nanoparticles of 1.5-2.5 nm in diameter synthesized by “cationic adsorption” technique on the mesoporous Sibunit carbons, which are pre-oxidized with HNO₃ and containing a lot of surface acidic groups, were found to catalyze the formation of glyphosate from N-isopropyl-N-phosphonomethyl glycine at 70 °C with selectivity up to 94% at 98% substrate conversion. Oxidation of other substrates containing the substituents with a weaker +I effect than that of iPr group resulted a large amount of undesirable N-decarboxymethylation product in addition to glyphosate. Considering the literature data and our own experimental results, assumptions about the reaction mechanism and causes of the observed trends are made.