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

We have studied the competitive epoxidations of olefins with cis- and trans -stilbenes and with cyclooctene and trans-stilbene in iron porphyrin comple x-catalyzed epoxidation reactions by H2O2, tert-butyl hydroperoxide (t-BuOO H), and m-chloroperoxybenzoic acid (m-CPBA) in protic solvent (i.e., a solv ent mixture of CH3OH and CH2Cl2) and aprotic solvent (i.e., a solvent mixtu re of CH3CN and CH2Cl2) at room temperature under catalytic reaction condit ions. The competitive epoxidations were also carried out with in situ gener ated high-valent iron(IV) oxo porphyrin cation radical complexes in aprotic solvent under stoichiometric reaction conditions. By determining the ratio s of epoxide products formed in the competitive epoxidations, we were able to conclude unambiguously that the reactive species generated in protic sol vent are high-valent iron(IV) oxo porphyrin cation radical complexes 3 and the intermediates formed in aprotic solvent are oxidant-iron porphyrin inte rmediates 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 s olvent, the rate of O-O bond cleavage of 2 is relatively slow and 2 transfe rs its oxygen to olefins prior to the formation of 3. To further examine th e effect of the general-acid catalysis on the nature of epoxidizing interme diates, we carried out competitive epoxidations in the solvent mixtures of alcohol/CH2Cl2 using alcohols of varying pK(a) values and in the presence o f an acid (i.e., HClO4) in aprotic solvent. The product ratios were found t o vary depending on the strength of the solvent acidity, demonstrating that the reaction of 2 with olefin competes with the 0-0 bond cleavage of 2 tha t leads to the formation of 3. We also reported for the first time that a h igh-valent iron(IV) oxo porphyrin cation radical intermediate containing el ectron-deficient porphyrin ligand shows an unexpected preference for trans- stilbene over cis-stilbene in the competitive epoxidations of cis- and tran s-stilbenes.