Factors affecting the reactivity and selectivity in the oxidation of sulfides with tetra-n-butylammonium peroxomonosulfate catalyzed by Mn(III) porphyrins: Significant nitrogen donor effects

The oxidation of aryl sulfides by tetra-n-butylammonium peroxomonosulfate (n-Bu₄NHSO₅) was carried out in the presence of six different manganese (III) tetraarylporphyrins Mn(Por)s] as biomimetic catalysts and a number of nitrogen donors as co-catalysts. There is no noticeable difference between the reactivity of sulfides, in the presence of electron-rich Mn(por)s, whereas, for electron-deficient catalysts, conversion rates are different. Nevertheless, the over-oxidation of sulfoxide is more sensitive to both the nature of substituents attached to the sulfur atom in substrates as well as porphyrin complex structure. The degree of catalytic activity of Mn(Por)s for the formation of sulfone product increases as the following order: Mn(TPFPP)OAc < MnT(4-NO₂P)P]OAc < Mn(TDCPP)OAc < Mn(TPP)OAc < Mn(TMP)OAc < MnT(4-OMeP)P]OAc. Our results show that in the presence of electron-rich Mn(Por)s, the strong π-donor N-H imidazoles possess co-catalytic activity greater than that of strong σ-donor amines and weak π-donor pyridines. When electron-deficient Mn(Por)s were employed as catalyst, pyridines demonstrated a higher co-catalytic activity than that of N-H imidazoles. The pronounced effect of protic solvents on the rate and selectivity of oxidation reactions, particularly in the presence of electron-deficient Mn(Por)s has been observed. The outcome of our investigations accompanied by UV-Vis and Raman spectral data confirms the involvement of different active oxidant such as a high valent Mn-oxo species as well as a six-coordinate (L)(Por)Mn-OHSO₄] complex.