Mechanistic study of oxygen-transfer reaction catalyzed by an oxorhenium(V) compound

The new binuclear oxothiolatorhenium(V) compound. Re2O2(mtp)(3) (D-1, mtpH(2) = 2-mercaptomethylthiophenol) was found to be an efficient catalyst for oxygen-transfer reactions. Strong Lewis bases such as phosphines coordinate to one of the rhenium centers in D-1; we suggest that this opens one of the Re-S bridges. Dialkylsulfides coordinate weakly to D-1. Alkylarylsulfides, diarylsulfides, triphenylarsine and triphenylstibine, and dienes and alkenes do not coordinate to D-1. D-1 catalyzes the oxidation of phosphines, arsines, stibenes, sulfides, and dienes by pyridine N-oxides and catalyzes the oxidation of phosphines by dimethyl sulfoxide. The kinetics and mechanism for the oxidation of triarylphosphines by pyridine N-oxides were investigated. The relative reactivities of all substrates were studied by competitive reactions. The order was found to be phosphine > arsine > stibene > sulfide > diene. The reaction is proposed to go through a Re(VII) intermediate with pyridine N-oxide as one of the ligands. The N-O bond was activated through coordination to rhenium, and the oxygen atom was abstracted by a phosphine forming a phosphine oxide.