Mechanistic studies and synthetic applications of oxygen atom transfer reactions catalyzed by rhenium(V) dithiolato complexes and methyltrioxorhenium (MTO)

A series of Re(V) dithiolato complexes catalyze the rapid and efficient transfer of an oxygen atom from a wide range of ring-substituted pyridine N-oxides to triphenylphosphine, yielding the pyridines in high yield. Some of these complexes can also catalyze the oxidation of a variety of sulfides to sulfoxides and sulfones with tert-butyl hydroperoxide in excellent yield with high selectivity and generality. Heavily substituted thiophenes can also be converted to the corresponding S-oxides with moderate to excellent yields using this catalytic system.; The kinetics and mechanism of these two oxygen transfer reactions were also studied. The mechanisms feature a common aspect in that phosphines and sulfides, being the substrates, inhibit the reaction. Rate constants were obtained for the oxygen atom transfer reactions between pyridine N-oxides and phosphines catalyzed by Re(V) dithiolato complexes. In the case of oxygen atom transfer reaction between tert-butyl hydroperoxide and sulfide catalyzed by Re(V) dithiolato compounds, an induction period is observed due to the slow ligand exchange step. Reaction schemes are proposed to interpret the kinetic data. In both cases, the active intermediates are Re(VII) dioxo species, which were detected at the low temperature.; Organic disulfides with both alkyl and aryl substituents are oxidized by hydrogen peroxide when CH₃ReO₃ (MTO) is used as a catalyst. Thiosulfinate is formed in the first step in about an hour with nearly quantitative yield. Kinetics studies of the first oxidation reaction established that two peroxorhenium compounds are the active forms of the catalyst. Rate constants were obtained and a mechanism was proposed in which the electron-rich sulfur attacks the peroxo oxygen of intermediates.