Impact of added ligand oxides in the sulfoxidation of phenylmercaptoacetic acids with oxovanadium(IV)‐salen complexes

Abstract

AbstractThe mechanistic aspects covering the effect of added ligand oxides, picoline N‐oxide (PicNO), pyridine N‐oxide (PyNO), and triphenyl phosphine oxide (TPPO) on the selective sulfoxidation of phenylmercaptoacetic acids to PSAA by H2O2 catalyzed by oxovanadium(IV)‐salen complexes have been studied spectrophotometrically in 100% acetonitrile medium. Hydroperoxovanadium(V)‐salen is the reactive species in this sulfoxidation. The rate of sulfoxidation is found to be enhanced significantly by increasing the concentrations of PMAA, salen, and H2O2. The ascertained order of reactivity among the oxovanadium(IV)‐salen complexes is 5,5′‐dimethyl > unsubstituted salen > 5,5′‐dichloro. The reaction is found to be impeded by the addition of ligand oxide and introduction of substituents in PMAA. The observed order of rate retardation among ligand oxides is TPPO < PyNO < PicNO. Downward Hammett plot obtained has been rationalized by change in rate‐determining step from coordination of PMAA to active species for electron‐withdrawing substituents to transfer of oxygen to PMAA for electron‐donating substituents within the same mechanism.