Related rhenium(V) catalysts adopt different mechanisms for oxygen atom transfer

The chemical mechanisms for three related rhenium(V) catalysts of oxygen atom transfer reactions are reviewed. Two of the catalysts are compounds with a single rhenium atom, [MeReE(mtp)(PPh3)], where E is either O (1) or S (2) and mtpH2 is 2-(mercaptomethyl)thiophenol; the third is the di-rhenium compound {MeReO(mtp)}2 (3). In each of them, the Re atom is square-pyramidal, sitting slightly above the approximate basal plane defined by S, S, C, and P; the oxo or thio group occupies the axial position. They all catalyze oxygen atom transfer from pyridine N-oxides to PAr3. The reaction rates are in the orders 3 ≫ 1 ≫ 2; even more striking is that the rate laws for the trio of catalysts is distinctly different. This signals the intervention of different chemical steps and intermediates.