Electron spin resonance analysis of the oxidation reactions of nitrone type spin traps with gold(III) ion

When cyclic nitrones, such as 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), 4-phenyl-5,5-dimethyl-l-pyrroline-N-oxide (PDMPO), and 3,3,5,5-tetramethyl-1-pyrroline-N-oxide (M4PO) were mixed with hydrogen tetrachloroaurate(III), DMPOX (5,5-dimethyl-1-pyrrolid-2-one-N-oxyl) type free radicals appeared with the precipitation of AU(0). The reaction did not proceed with noncyclic nitrones, such as N-tert-butyl-alpha-phenyl-nitrone (PBN) and alpha-(4-pyridyl-1-oxide)-N-tert-butyl-nitrone (POBN). The order of the HAuCl4 decrease was DMPO > PDMPO > M4PO. The reaction was depressed by the addition of chloride or hydroxide ions. 1-Hydroxy-5,5-dimethyl-1-pyrrolid-2-one (HDMPN), the precursor of DMPOX, was also oxidized to DMPOX by HAuCl4. Every step of the gold reduction from Au(III) to Au(0) can be used for the oxidation of HDMPN to DMPOX. Based on these and previous results, the reaction was assumed to proceed by the following scheme consisting of a ligand exchange interaction of AuCl4 with > N+-O- in DMPO, then nucleophilic addition of a water molecule to DMPO, then the stepwise intramolecular transfer of three electrons from DMPO to Au(III), and finally the precipitation of Au(0).