Mechanistic Studies of O 2 -Based Sulfoxidations Catalyzed by NO x /Br Systems

Many studies have noted that mixtures of nitrogen oxides (or oxyanions; NOx = NO3-, NO2-, NO-) and halide (particularly bromine) with or without a transition metal salt (most frequently, copper and iron complexes) are fast and highly selective catalysts for aerobic sulfoxidation and decontamination of many oxidizable toxic compounds. However, the lack of extensive and rigorous kinetic information has kept unequivocal mechanistic inferences on these important catalyst systems to a minimum. Through a series of spectroscopic and kinetics studies on NOx/bromine systems in acetonitrile, the following clarifying technical points have been established: (1) bromine is the reactive species in these oxidations; (2) nitrosonium and hypobromite are not kinetically important, (3) tribromide (Br-3(-)) functions as a stable reservoir for Br-2. (4) salts of Br-3(-) and nitrate are stable, storable decontamination catalyst precursors; and (5) most importantly, the multiple complex roles of water (required for catalyst inhibitory at most concentrations) have been clarified.