Oxidation of styrene and of some derivatives with H2O2 catalyzed by novel imidazolium-containing manganese porphyrins: A mechanistic and thermodynamic interpretation

The oxidation of styrene and derivatives with H2O2 catalyzed by manganese porphyrins in acetonitrile is described. The effect of the imidazolium substituent on the catalytic efficiency has been also considered. The thermodynamic analysis has indicated that enthalpy rules styrene oxidation when catalysts Mn(Porph)-1 and Mn(Porph)-2 are used whereas the entropy is the driven force for Mn(Porph)-3 and Mn(Porph)-4 catalyzed reactions. Interestingly, an enthalpy–entropy compensation phenomenon is observed when comparing the thermodynamic results obtained for catalysts Mn(Porph)-3 and Mn(Porph)-4. Hammett plots for the studied manganese porphyrins provided small ρ-values, and this is typical for multi-step reactions, indicating that there is no significant charge separation in the transition state. For Mn(Porph)-1 and Mn(Porph)-2 the formation of multiple active species can be put forward whereas for Mn(Porph)-3 and Mn(Porph)-4 a concerted-type mechanism, via metalloxetane intermediate, fits well with the values obtained for those catalysts. The imidazolium-based catalysts have shown to be efficient catalysts in styrene and derivatives oxidation with hydrogen peroxide.

The oxidation of styrene and derivatives with H2O2 catalyzed by manganese porphyrins in acetonitrile is described and the effect of the imidazolium substituent on the catalytic efficiency has been considered. The imidazolium-based catalysts showed to be efficient catalysts in styrene and derivatives oxidation with hydrogen peroxide.Figure optionsDownload high-quality image (130 K)Download as PowerPoint slideHighlights
► Thermodynamic activation parameters are affected by catalysts structural differences.
► Structural differences also change products’ selectivity.
► Hammett analysis provided small ρ-values.
► Results fit with a concerted reaction pathway, starting from an oxene-type approach.
► The determining step is a [π2a + π2s] metallooxetane intermediate.