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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
Author(s) -
Nasrollahi Rahele,
Zakavi Saeed
Publication year - 2017
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201601488
Subject(s) - chemistry , catalysis , manganese , reaction rate constant , cyclohexanol , porphyrin , medicinal chemistry , photochemistry , reaction mechanism , dichloromethane , inorganic chemistry , imidazole , dissociation (chemistry) , kinetics , stereochemistry , organic chemistry , solvent , physics , quantum mechanics
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)Mn V (O)(TDClPP)] + ( λ max = 408 nm), whereas a mixture of [(ImH)Mn V (O)(TDClPP)] + , (ImH)Mn IV (O)(TDClPP) ( λ max = 421 nm), and (ImH)(peroxymonopersulfate)Mn III (TDClPP) ( λ max = 473 nm) was observed for molar ratios lower than this. The formation of [(ImH)Mn V (O)(TDClPP)] + and (ImH)Mn IV (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 ( k obs ) and second‐order ( k 2 ) rate constants of oxygen‐atom transfer from high‐valent Mn V (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 Mn V (O) and Mn IV (O) species may be distinguished through their significantly different activities towards the oxidation of alcohols.