Determination of Reactive Intermediates in Iron Porphyrin Complex-Catalyzed Oxygenations of Hydrocarbons Using Isotopically Labeled Water:  Mechanistic Insights

Abstract

We have studied iron porphyrin complex-catalyzed oxygenations of hydrocarbons by several oxidants (i.e., hydrogen peroxide, tert -butyl hydroperoxide, and m -chloroperoxybenzoic acid (MCPBA)) in the presence of H 2 18 O. In the olefin epoxidation and alkane hydroxylation reactions catalyzed by ( meso -tetrakis(pentafluorophenyl)porphinato)iron(III) chloride [Fe(F 20 TPP)Cl], the percentages of 18 O incorporated into the oxygenated products were found to be the same in all of the reactions of hydrogen peroxide, tert -butyl hydroperoxide, and MCPBA, leading us to conclude that a common high-valent iron oxo complex was the reactive intermediate responsible for oxygen atom transfer. When the epoxidation of cyclooctene by MCPBA and H 2 O 2 was performed at low temperature in the presence of H 2 18 O, it was found that there was no 18 O-incorporation from labeled water into cyclooctene oxide. We interpreted the lack of 18 O-incorporation in these reactions with that an electronegatively-substituted iron porphyrin complex forms a relatively stable (Porp)Fe III −OOR species and this intermediate transfers its oxygen to olefin prior to the O−O bond cleavage at low temperature. As the reaction temperature raised from −78 °C to room temperature, the amount of 18 O incorporated into the oxide product gradually increased in the reactions of cyclooctene epoxidation. This was attributed to the fast conversion of Fe III −OOR to the high-valent iron oxo complex via the O−O bond cleavage at higher temperature. We found, by studying the effects of the olefin and H 2 18 O concentrations on the amount of 18 O incorporated into the oxide product, that the rate of the oxygen exchange between high-valent iron oxo complex and labeled water was slower than that of the oxygen atom transfer from the intermediate to organic compounds in catalytic oxygenation reactions. Blocking an axial position of iron porphyrin complex with imidazole prevented the 18 O-incorporation from labeled water into the oxygenated products, explaining the phenomenon of no oxygen exchange in cytochrome P-450 systems.