Formation and kinetic studies of manganese(IV)-oxo porphyrins: Oxygen atom transfer mechanism of sulfide oxidations

Abstract

Visible light irradiation of photo-labile porphyrin-manganese(III) chlorates or bromates (2) produced manganese(IV)-oxo porphyrins [MnIV(Por)(O)] (Por = porphyrin) (3) in three porphyrin ligands. The same oxo species 3 were also formed by chemical oxidation of the corresponding manganese(III) precursors (1) with iodobenzene diacetate, i.e. PhI(OAc)2. The systems under study include 5,10,15,20-tetra(pentafluorophenyl)porphyrin‑manganese(IV)-oxo (3a), 5,10,15,20-tetra(2,6-difluorophenyl)porphyrin‑manganese(IV)-oxo (3b), and 5,10,15,20-tetramesitylporphyrin‑manganese(IV)-oxo (3c). As expected, complexes 3 reacted with thioanisoles to produce the corresponding sulfoxides and over-oxidized sulfones. The kinetics of oxygen atom transfer (OAT) reactions of these generated 3 with aryl sulfides were studied in CH3CN solutions. Second-order rate constants for sulfide oxidation reactions are comparable to those of alkene epoxidations and activated CH bond oxidations by the same oxo species 3. For a given substrate, the reactivity order for the manganese(IV)-oxo species was 3a > 3b > 3c, consistent with expectations on the basis of the electron-withdrawing capacity of the porphyrin macrocycles. Free-energy Hammett analyses gave near-linear correlations with σ values, indicating no significant positive charge developed at the sulfur during the oxidation process. The mechanistic results strongly suggest [MnIV(Por)(O)] reacts as a direct OAT agent towards sulfide substrates through a manganese(II) intermediate that was detected in this work. However, an alternative pathway that involves a disproportionation of 3 to form a higher oxidized manganese(V)-oxo species may be significant when less reactive substrates are present. The competition product studies with the Hammett correlation plot confirmed that the observed manganese(IV)-oxo species is not the true oxidant for the sulfide oxidations catalyzed by manganese(III) porphyrins with PhI(OAc)2.