Evidence on the Nature of the Active Oxidants Involved in the Oxidation of Alcohols with Oxone Catalyzed by an Electron‐Deficient Manganese Porphyrin: A Combined Kinetic and Mechanistic Study

Abstract

A study of the kinetics and mechanism of the oxidation of a series of electron‐rich and electron‐deficient benzyl alcohols and cyclohexanol with high‐valent manganese oxo species formed by the reaction of meso‐tetrakis(2,6‐dichlorophenyl)porphyrinatomanganese(III) acetate [Mn(TDClPP)(OAc)] and tetra‐n‐butylammonium oxone (TBAO) in the presence of imidazole (ImH) are reported. The use of Mn(TDClPP)(OAc), ImH, and TBAO in a 1:15:1000 molar ratio led to the exclusive formation of [(ImH)MnV(O)(TDClPP)]+ (λmax = 408 nm), whereas a mixture of [(ImH)MnV(O)(TDClPP)]+, (ImH)MnIV(O)(TDClPP) (λmax = 421 nm), and (ImH)(peroxymonopersulfate)MnIII(TDClPP) (λmax = 473 nm) was observed for molar ratios lower than this. The formation of [(ImH)MnV(O)(TDClPP)]+ and (ImH)MnIV(O)(TDClPP) from the catalyst precursor was evidenced in the UV/Vis absorption spectrum of the intermediates at 273 K in dichloromethane. Owing to the different reactivities of the high‐valent intermediates towards the oxidation of alcohols, a biphasic mechanism was observed. The observed (kobs) and second‐order (k2) rate constants of oxygen‐atom transfer from high‐valent MnV(O) species to the substrates were evaluated by monitoring the absorption changes at λ = 473 nm. The Hammett treatment of the oxidation data gave a slope (ϱ) of –0.87, which is consistent with a radical mechanism. Furthermore, a strong correlation between the rate constants and the C–H bond dissociation energies (BDEs) of the alcohols suggests the involvement of H‐atom abstraction in the mechanism of the oxidation reaction. Interestingly, a negative temperature dependence of the reaction rate was observed for the oxidation of benzyl alcohol from 273 to 298 K. Furthermore, the high‐valent MnV(O) and MnIV(O) species may be distinguished through their significantly different activities towards the oxidation of alcohols.