Comparative study of the catalytic activity of a series of β-brominated Mn–porphyrins in the oxidation of olefins and organic sulfides: Better catalytic performance of the partially brominated ones

The oxidation of olefins and organic sulfides with tetra-n-butylammonium periodate (TBAP) in the presence of a Mn(III) complex from a series of partially and fully brominated meso-tetraphenylporphyrins, MnTPPBrx(OAc) (x = 0, 2, 4, 6 and 8) has been studied in a comparative manner. With the exception of MnTPPBr2(OAc), the half-wave potential for the Mn(III)/Mn(II) metal-centered process was gradually shifted to higher values on going from MnTPP(OAc) to MnTPPBr8(OAc). However, a complex order has been observed for the catalytic activity of the complexes; while the stability of the Mn–porphyrins towards oxidative degradation under the reaction conditions decreases in the order MnTPPBr8(OAc) > MnTPP(OAc) > MnTPPBr6(OAc) > MnTPPBr2(OAc) > MnTPPBr4(OAc), their catalytic activity has been found to follow the order MnTPPBr8(OAc) ≪ MnTPP(OAc) ⩽ MnTPPBr6(OAc) < MnTPPBr2(OAc) ⩽ MnTPPBr4(OAc). The maximum conversion (42%) was obtained in the case of MnTPPBr4(OAc) and the minimum conversion (14%) is related to MnTPPBr8(OAc). Oxidation of methyl phenyl sulfide with TBAP in a 1:1 molar ratio leads to the following pattern of catalytic activity for the used Mn–porphyrins: MnTPPBr4(OAc) > MnTPP(OAc) > MnTPPBr2(OAc) ⩾ MnTPPBr6(OAc) ≫ MnTPPBr8(OAc). The highest methyl phenyl sulfide conversion is 69% at room temperature.

Oxidation of alkenes and organic sulfides with tetra(n-butyl)ammonium periodate in the presence of the Mn(III) complexes of meso-tetraphenylporphyrin and the corresponding β-brominated analogues shows a complex order of catalytic activity for the used Mn–porphyrins which cannot be simply explained by the relative electron-deficient character of the Mn–porphyrins.Figure optionsDownload as PowerPoint slideHighlights
► Catalytic activity of a series of partially and fully brominated porphyrins.
► Partially brominated porphyrins are more efficient catalysts compared to MnTPPBr8(OAc).
► Anodic shifts for the half-wave potential on going from MnTPP(OAc) to MnTPPBr8(OAc).