Evidence for the participation of two distinct reactive intermediates in iron(III) porphyrin complex-catalyzed epoxidation reactions

Abstract

We have studied the competitive epoxidations of olefins with cis- and trans-stilbenes and with cyclooctene and trans-stilbene in iron porphyrin complex-catalyzed epoxidation reactions by H2O2, tert-butyl hydroperoxide (t-BuOOH), and m-chloroperoxybenzoic acid (m-CPBA) in protic solvent (i.e., a solvent mixture of CH3OH and CH2Cl2) and aprotic solvent (i.e., a solvent mixture of CH3CN and CH2Cl2) at room temperature under catalytic reaction conditions. The competitive epoxidations were also carried out with in situ generated high-valent iron(IV) oxo porphyrin cation radical complexes in aprotic solvent under stoichiometric reaction conditions. By determining the ratios of epoxide products formed in the competitive epoxidations, we were able to conclude unambiguously that the reactive species generated in protic solvent are high-valent iron(IV) oxo porphyrin cation radical complexes 3 and the intermediates formed in aprotic solvent are oxidant-iron porphyrin intermediates 2. A protic solvent such as methanol is proposed to function as a general-acid catalyst, thereby increasing the rate of O-O bond cleavage of 2 to form 3. In the absence of general-acid catalysis such as in aprotic solvent, the rate of O-O bond cleavage of 2 is relatively slow and 2 transfers its oxygen to olefins prior to the formation of 3. To further examine the effect of the general-acid catalysis on the nature of epoxidizing intermediates, we carried out competitive epoxidations in the solvent mixtures of alcohol/CH2Cl2 using alcohols of varying pK(a) values and in the presence of an acid (i.e., HClO4) in aprotic solvent. The product ratios were found to vary depending on the strength of the solvent acidity, demonstrating that the reaction of 2 with olefin competes with the O-O bond cleavage of 2 that leads to the formation of 3. We also reported for the first time that a high-valent iron(IV) oxo porphyrin cation radical intermediate containing electron-deficient porphyrin ligand shows an unexpected preference for trans-stilbene over cis-stilbene in the competitive epoxidations of cis- and trans-stilbenes.