The prime reactive intermediate in the iron(III) porphyrin complex catalyzed oxidation reactions by tert-butyl hydroperoxide

Oxidative reactivity difference among the metal oxo and metal hydroxo moieties: pH dependent hydrogen abstraction by a manganese(IV) complex having two hydroxide ligands

Clarifying the difference in redox reactivity between the metal oxo and metal hydroxo moieties for the same redox active metal ion in identical structures and oxidation states, that is, M(n+)O and M(n+)-OH, contributes to the understanding of nature's choice between them (M(n+)O or M(n+)-OH) as key active intermediates in redox enzymes and electron transfer enzymes, and provides a basis for the design of synthetic oxidation catalysts. The newly synthesized manganese(IV) complex having two hydroxide ligands, [Mn(Me(2)EBC)(2)(OH)(2)](PF(6))(2), serves as the prototypic example to address this issue, by investigating the difference in the hydrogen abstracting abilities of the Mn(IV)O and Mn(IV)-OH functional groups. Independent thermodynamic evaluations of the O-H bond dissociation energies (BDE(OH)) for the corresponding reduction products, Mn(III)-OH and Mn(III)-OH(2), reveal very similar oxidizing power for Mn(IV)O and Mn(IV)-OH (83 vs 84.3 kcal/mol). Experimental tests showed that hydrogen abstraction proceeds at reasonable rates for substrates having BDE(CH) values less than 82 kcal/mol. That is, no detectable reaction occurred with diphenyl methane (BDE(CH) = 82 kcal/mol) for both manganese(IV) species. However, kinetic measurements for hydrogen abstraction showed that at pH 13.4, the dominant species Mn(Me(2)EBC)(2)(O)(2), having only Mn(IV)O groups, reacts more than 40 times faster than the Mn(IV)-OH unit in Mn(Me(2)EBC)(2)(OH)(2)(2+), the dominant reactant at pH 4.0. The activation parameters for hydrogen abstraction from 9,10-dihydroanthracene were determined for both manganese(IV) moieties: over the temperature range 288-318 K for Mn(IV)(OH)(2)(2+), DeltaH(double dagger) = 13.1 +/- 0.7 kcal/mol, and DeltaS(double dagger) = -35.0 +/- 2.2 cal K(-1) mol(-1); and the temperature range 288-308 K for for Mn(IV)(O)(2), DeltaH(double dagger) = 12.1 +/- 1.8 kcal/mol, and DeltaS(double dagger) = -30.3 +/- 5.9 cal K(-1) mol(-1).

Laser flash photolysis generation of high-valent transition metal-oxo species: insights from kinetic studies in real time

High-valenttransition metal-oxo species are active oxidizing species in many metal-catalyzed oxidation reactions in both Nature and the laboratory. In homogeneous catalytic oxidations, a transition metal catalyst is oxidized to a metal-oxo species by a sacrificial oxidant, and the activated transition metal-oxo intermediate oxidizes substrates. Mechanistic studies of these oxidizing species can provide insights for understanding commercially important catalytic oxidations and the oxidants in cytochrome P450 enzymes. In many cases, however, the transition metal oxidants are so reactive that they do not accumulate to detectable levels in mixing experiments, which have millisecond mixing times, and successful generation and direct spectroscopic characterization of these highly reactive transients remain a considerable challenge. Our strategy for understanding homogeneous catalysis intermediates employs photochemical generation of the transients with spectroscopic detection on time scales as short as nanoseconds and direct kinetic studies of their reactions with substrates by laser flash photolysis (LFP) methods. This Account describes studies of high-valent manganese- and iron-oxo intermediates. Irradiation of porphyrin-manganese(III) nitrates and chlorates or corrole-manganese(IV) chlorates resulted in homolytic cleavage of the O-X bonds in the ligands, whereas irradiation of porphyrin-manganese(III) perchlorates resulted in heterolytic cleavage of O-Cl bonds to give porphyrin-manganese(V)-oxo cations. Similar reactions of corrole- and porphyrin-iron(IV) complexes gave highly reactive transients that were tentatively identified as macrocyclic ligand-iron(V)-oxo species. Kinetic studies demonstrated high reactivity of the manganese(V)-oxo species, and even higher reactivities of the putative iron(V)-oxo transients. For example, second-order rate constants for oxidations of cis-cyclooctene at room temperature were 6 x 10(3) M(-1) s(-1) for a corrole-iron(V)-oxo species and 1.6 x 10(6) M(-1) s(-1) for the putative tetramesitylporphyrin-iron(V)-oxo perchlorate species. The latter rate constant is 25,000 times larger than that for oxidation of cis-cyclooctene by iron(IV)-oxo perchlorate tetramesitylporphyrin radical cation, which is the thermodynamically favored electronic isomer of the putative iron(V)-oxo species. The LFP-determined rate constants can be used to implicate the transient oxidants in catalytic reactions under turnover conditions where high-valent species are not observable. Similarly, the observed reactivities of the putative porphyrin-iron(V)-oxo species might explain the unusually high reactivity of oxidants produced in the cytochrome P450 enzymes, heme-thiolate enzymes that are capable of oxidizing unactivated carbon-hydrogen bonds in substrates so rapidly that iron-oxo intermediates have not been detected under physiological conditions.

Olefin Epoxidation by Alkyl Hydroperoxide with a Novel Cross-Bridged Cyclam Manganese Complex: Demonstration of Oxygenation by Two Distinct Reactive Intermediates

Olefin epoxidation provides an operative protocol to investigate the oxygen transfer process in nature. A novel manganese complex with a cross-bridged cyclam ligand, MnIV(Me2EBC)(OH)2(2+) (Me2EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane), was used to study the epoxidation mechanism with biologically important oxidants, alkyl hydroperoxides. Results from direct reaction of the freshly synthesized manganese(IV) complex, [Mn(Me2EBC)(OH)2](PF6)2, with various olefins in neutral or basic solution, and from catalytic epoxidation with oxygen-labeled solvent, H2 18O, eliminate the manganese oxo moiety, Mn(IV)=O, as the reactive intermediate and obviate an oxygen rebound mechanism. Epoxidations of norbornylene under different conditions indicate multiple mechanisms for epoxidation, and cis-stilbene epoxidation under atmospheric 18O2 reveals a product distribution indicating at least two distinctive intermediates serving as the reactive species for epoxidation. In addition to alkyl peroxide radicals as dominant intermediates, an alkyl hydroperoxide adduct of high oxidation state manganese(IV) is suggested as the third kind of active intermediate responsible for epoxidation. This third intermediate functions by the Lewis acid pathway, a process best known for hydrogen peroxide adducts. Furthermore, the tert-butyl peroxide adduct of this manganese(IV) complex was detected by mass spectroscopy under catalytic oxidation conditions.

Olefin Epoxidation by the Hydrogen Peroxide Adduct of a Novel Non-heme Mangangese(IV) Complex: Demonstration of Oxygen Transfer by Multiple Mechanisms

Olefin epoxidations are a class of reactions appropriate for the investigation of oxygenation processes in general. Here, we report the catalytic epoxidation of various olefins with a novel, cross-bridged cyclam manganese complex, Mn(Me2EBC)Cl2 (Me2EBC is 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane), using hydrogen peroxide as the terminal oxidant, in acetone/water (ratio 4:1) as the solvent medium. Catalytic epoxidation studies with this system have disclosed reactions that proceed by a nonradical pathway other than the expected oxygen-rebound mechanism that is characteristic of high-valent, late-transition-metal catalysts. Direct treatment of olefins with freshly synthesized [Mn(IV)(Me2EBC)(OH)2](PF6)2 (pKa = 6.86) in either neutral or basic solution confirms earlier observations that neither the oxo-Mn(IV) nor oxo-Mn(V) species is responsible for olefin epoxidization in this case. Catalytic epoxidation experiments using the 18O labels in an acetone/water (H2(18)O) solvent demonstrate that no 18O from water (H2(18)O) is incorporated into epoxide products even though oxygen exchange was observed between the Mn(IV) species and H2(18)O, which leads to the conclusion that oxygen transfer does not proceed by the well-known oxygen-rebound mechanism. Experiments using labeled dioxygen, (18)O2, and hydrogen peroxide, H2(18)O2, confirm that an oxygen atom is transferred directly from the H2(18)O2 oxidant to the olefin substrate in the predominant pathway. The hydrogen peroxide adduct of this high-oxidation-state manganese complex, Mn(IV)(Me2EBC)(O)(OOH)+, was detected by mass spectra in aqueous solutions prepared from Mn(II)(Me2EBC)Cl2 and excess hydrogen peroxide. A Lewis acid pathway, in which oxygen is transferred to the olefin from that adduct, Mn(IV)(Me2EBC)(O)(OOH)+, is proposed for epoxidation reactions mediated by this novel, non-heme manganese complex. A minor radical pathway is also apparent in these systems.

Influence of Solvent Composition on the Kinetics of Cyclooctene Epoxidation by Hydrogen Peroxide Catalyzed by Iron(III) [tetrakis(pentafluorophenyl)] Porphyrin Chloride [(F20TPP)FeCl]

The epoxidation of cyclooctene catalyzed by iron(III) [tetrakis(pentafluorophenyl)] porphyrin chloride [(F20TPP)FeCl] was investigated in alcohol/acetonitrile solutions in order to determine the effects of the alcohol composition on the reaction kinetics. It was observed that alcohol composition affects both the observed rate of hydrogen peroxide consumption (the limiting reagent) and the selectivity of hydrogen peroxide utilization to form cyclooctene epoxide. The catalytically active species are formed only in alcohol-containing solvents as a consequence of (F(20)TPP)FeCl dissociation into [(F20TPP)Fe(ROH)]+ cations and Cl- anions. The observed reaction kinetics are analyzed in terms of a proposed mechanism for the epoxidation of the olefin and the decomposition of H2O2. The first step in this scheme is the reversible coordination of H2O2 to [(F20TPP)Fe(ROH)]+. The O-O bond of the coordinated H2O2 then undergoes either homolytic or heterolytic cleavage. The rate of homolytic cleavage is found to be independent of alcohol composition, whereas the rate of heterolytic cleavage increases with alcohol acidity. Heterolytic cleavage is envisioned to form iron(IV) pi-radical cations, whereas homolytic cleavage forms iron(IV) hydroxo cations. The iron(IV) radical cations are active for olefin epoxidation, whereas the iron(IV) cations catalyze the decomposition of H2O2. Reaction of iron(IV) pi-radical cations with H2O2 to form iron(IV) hydroxo cations is also included in the mechanism, a process that is favored by alcohols with a high charge density on the O atoms. The proposed mechanism describes successfully the effects of H2O2, cyclooctene, and porphyrin concentrations, as well as the effects of alcohol concentration.

Cytochrome P-450 model compound catalyzed selective hydroxylation of C–H bonds: Dramatic solvent effect

Selective hydroxylation of cyclohexane and cyclohexene by t-BuOOH in presence of F2oTPPFe(III)Cl as the catalyst has been achieved at room temparature in high yields.

Olefin Oxygenation by the Hydroperoxide Adduct of a Nonheme Manganese(IV) Complex: Epoxidations by a Metallo−Peracid Produces Gentle Selective Oxidations

The reactive intermediates and mechanisms of oxygenation of olefins by manganese complexes were investigated by treating olefins with newly synthesized [MnIV(Me2EBC)(OH)2](PF6)2 in the presence and absence of peroxide and by studying its catalytic epoxidation reaction in normal aqueous solution and, individually, with isotopically labeled H218O, 18O2, and H218O2. The manganese oxo species is not the reactive intermediate for the oxygen transfer process mediated by this manganese complex. A novel manganese(IV) peroxide intermediate, MnIV(Me2EBC)(O)(OOH)+, was captured by mass spectrometry and is proposed as the intermediate that oxygenates olefins in this catalytic system.

Probing the Cytochrome P450-like Reactivity of High-Valent Oxo Iron Intermediates in the Gas Phase

Electrospray ionization in combination with Fourier transform ion cyclotron resonance spectrometry is used to prepare and characterize at a molecular level high-valent oxoiron intermediates formed in the reaction of [(TPFPP)Fe(III)]Cl (TPFPP= meso-tetrakis(pentafluorophenyl)porphinato dianion) (1-Cl) with H(2)O(2) in methanol. The intrinsic reactivity in the gas phase of the iron(IV) oxo porphyrin cation radical complex, [(TPFPP)(.+)Fe(IV)=O](+), has been probed toward selected substrates (S), chosen among naturally occurring and model compounds. Whereas CO and cyclohexane proved to be unreactive, olefins, sulfides, amines, and phosphites all undergo oxygen atom transfer in the gas phase yielding the reduction product 1 and/or an adduct ion ([1-S](+)). The reaction efficiencies show a qualitative correlation with the oxophilic character of the active site of S. A notable exception is nitric oxide, which displays a remarkably high reactivity, in line with the important role of NO reactions with iron porphyrin complexes. Furthermore, subsidiary information on the neat association reaction of 1 with selected ligands (L) has been obtained by a kinetic study showing that both the efficiency and the extent of ligation toward the naked ion 1 depend on the electron-donating ability of L.

Oxoiron(IV) porphyrin π-cation radical complexes with a chameleon behavior in cytochrome P450 model reactions

There is an intriguing, current controversy on the involvement of multiple oxidizing species in oxygen transfer reactions by cytochromes P450 and iron porphyrin complexes. The primary evidence for the "multiple oxidants" theory was that products and/or product distributions obtained in the catalytic oxygenations were different depending on reaction conditions such as catalysts, oxidants, and solvents. In the present work, we carried out detailed mechanistic studies on competitive olefin epoxidation, alkane hydroxylation, and C=C epoxidation versus allylic C-H hydroxylation in olefin oxygenation with in situ generated oxoiron(IV) porphyrin pi-cation radicals (1) under various reaction conditions. We found that the products and product distributions were markedly different depending on the reaction conditions. For example, 1 bearing different axial ligands showed different product selectivities in competitive epoxidations of cis-olefins and trans-olefins and of styrene and para-substituted styrenes. The hydroxylation of ethylbenzene by 1 afforded different products, such as 1-phenylethanol and ethylbenzoquinone, depending on the axial ligands of 1 and substrates. Moreover, the regioselectivity of C=C epoxidation versus C-H hydroxylation in the oxygenation of cyclohexene by 1 changed dramatically depending on the reaction temperatures, the electronic nature of the iron porphyrins, and substrates. These results demonstrate that 1 can exhibit diverse reactivity patterns under different reaction conditions, leading us to propose that the different products and/or product distributions observed in the catalytic oxygenation reactions by iron porphyrin models might not arise from the involvement of multiple oxidizing species but from 1 under different circumstances. This study provides strong evidence that 1 can behave like a "chameleon oxidant" that changes its reactivity and selectivity under the influence of environmental changes.

Oxidizing intermediates in cytochrome P450 model reactions

Oxoiron(IV) porphyrin pi-cation radicals have been considered as the sole reactive species in the catalytic oxidation of organic substrates by cytochromes P450 and their iron porphyrin models over the past two decades. Recent studies from several laboratories, however, have provided experimental evidence that multiple oxidizing species are involved in the oxygen transfer reactions and that the mechanism of oxygen transfer is much more complex than initially believed. In this Commentary, reactive intermediates that have been shown or proposed to be involved in iron porphyrin complex-catalyzed oxidation reactions are reviewed. Particularly, the current controversy on the oxoiron(IV) porphyrin pi-cation radical as a sole reactive species versus the involvement of multiple oxidizing species in oxygen transfer reactions is discussed.

Radical Autoxidation and Autogenous O2 Evolution in Manganese−Porphyrin Catalyzed Alkane Oxidations with Chlorite

A manganese porphyrin catalyst employing chlorite (ClO(2)(-)) as a "shunt" oxidant displays remarkable activity in alkane oxidation, oxidizing cyclohexane to cyclohexanol and cyclohexanone with >800 turnover numbers. The ketone is apparently formed without the intermediacy of alcohol and accounts for an unusually large fraction of the product ( approximately 40%). Radical scavenging experiments indicate that the alkane oxidation mechanism involves both carbon-centered and oxygen-centered radicals. The carbon-radical trap CBrCl(3) completely suppresses cyclohexanone formation and reduces cyclohexanol turnovers, while the oxygen-radical trap Ph(2)NH inhibits all oxidation until it is consumed. These observations are indicative of an autoxidation mechanism, a scenario further supported by TEMPO inhibition and (18)O(2) incorporation into products. However, similar cyclohexane oxidation activity occurs when air is excluded. This is explained by mass spectrometric and volumetric measurements showing catalyst-dependent O(2) evolution from the reaction mixture. The catalytic disproportionation of ClO(2)(-) into Cl(-) and O(2) provides sufficient O(2) to support an autoxidation mechanism. A two-path oxidation scheme is proposed to explain all of the experimental observations. The first pathway involves manganese-porphyrin catalyzed decomposition of ClO(2)(-) into both O(2) and an unidentified radical initiator, leading to classical autoxidation chemistry providing equal amounts of cyclohexanol and cyclohexanone. The second pathway is a "rebound" oxygenation involving a high-valent manganese-oxo intermediate, accounting for the excess of alcohol over ketone. This system highlights the importance of mechanistic studies in catalytic oxidations with highly reactive oxidants, and it is unusual in its ability to sustain autoxidation even under apparent exclusion of O(2).

Hydroxylation by the hydroperoxy-iron species in cytochrome P450 enzymes

Intramolecular and intermolecular kinetic isotope effects (KIEs) were determined for hydroxylation of the enantiomers of trans-2-(p-trifluoromethylphenyl)cyclopropylmethane (1) by hepatic cytochrome P450 enzymes, P450s 2B1, Delta2B4, Delta2B4 T302A, Delta2E1, and Delta2E1 T303A. Two products from oxidation of the methyl group were obtained, unrearranged trans-2-(p-trifluoromethylphenyl)cyclopropylmethanol (2) and rearranged 1-(p-trifluoromethylphenyl)but-3-en-1-ol (3). In intramolecular KIE studies with dideuteriomethyl substrates (1-d(2)) and in intermolecular KIE studies with mixtures of undeuterated (1-d(0)) and trideuteriomethyl (1-d(3)) substrates, the apparent KIE for product 2 was consistently larger than the apparent KIE for product 3 by a factor of ca. 1.2. Large intramolecular KIEs found with 1-d(2) (k(H)/k(D) = 9-11 at 10 degrees C) were shown not to be complicated by tunneling effects by variable temperature studies with two P450 enzymes. The results require two independent isotope-sensitive processes in the overall hydroxylation reactions that are either competitive or sequential. Intermolecular KIEs were partially masked in all cases and largely masked for some P450s. The intra- and intermolecular KIE results were combined to determine the relative rate constants for the unmasking and hydroxylation reactions, and a qualitative correlation was found for the unmasking reaction and release of hydrogen peroxide from four of the P450 enzymes in the absence of substrate. The results are consistent with the two-oxidants model for P450 (Vaz, A. D. N.; McGinnity, D. F.; Coon, M. J. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 3555), which postulates that a hydroperoxy-iron species (or a protonated analogue of this species) is a viable electrophilic oxidant in addition to the consensus oxidant, iron-oxo.

Oxidation and Oxygenation of Iron Complexes of 2-Aza-21-carbaporphyrin

Oxidation and oxygenation of (HCTPPH)Fe(II)Br an iron(II) complex of 2-aza-5,10,15,20-tetraphenyl-21-carbaporphyrin (CTPPH)H2 have been followed by 1H and 2H NMR spectroscopy. Addition of I2 or Br2 to the solution of (HCTPPH)Fe(II)Br in the absence of dioxygen results in one-electron oxidation yielding [(HCTPPH)Fe(III)Br]+. One electron oxidation with dioxygen, accompanied by deprotonation of a C(21)H fragment and formation of an Fe-C(21) bond, produces an intermediate-spin, five-coordinate iron(III) complex (HCTPP)Fe(III)Br. In the subsequent step an insertion of the oxygen atom into the preformed Fe(III)-C(21) bond has been detected to produce [(CTPPO)Fe(III)Br]-. Protonation at the N2 atom affords (HCTPPO)Fe(III)Br. The considered mechanism of (HCTPPH)Fe(II)Br oxygenation involves the insertion of dioxygen into the Fe-C bond. The 1H NMR and 2H NMR spectra of paramagnetic iron(III) complexes were examined. Functional group assignments have been made with use of selective deuteration. The characteristic patterns of pyrrole and 2-NH resonances have been found diagnostic of the ground electronic state of iron and the donor nature localized at C(21) center as exemplified by the 1H NMR spectrum of intermediate-spin (HCTPP)Fe(III)Br: beta-H 7.2, -10.6, -19.2, -20.6, -23.2, -24.9, -43.2; 2-NH -76.6 (ppm, 298 K). The structures of two compounds (HCTPP)Fe(III)Br and (HCTPPO)Fe(III)Br, were determined by X-ray diffraction studies. In the first case, the iron(III) is five-coordinate with bonds to three pyrrole nitrogen atoms (Fe-N distances: 1.985(8), 2.045(7), 2.023(8) A), and the pyrrolic trigonal carbon (Fe-C: 1.981(8) A). The iron(III) of (HCTPPO)Fe(III)Br forms bonds to three pyrrole nitrogen atoms (Fe-N distances 2.104(5), 2.046(5), 2.102(5) A). The Fe-O 2.041(5) A and Fe-C(21) 2.192(5) A distances suggests a direct interaction between the iron center and the pi electron density on the carbonyl group in a eta2 fashion.

Donor Ligand Effect on the Nature of the Oxygenating Species in MnIII(salen)-Catalyzed Epoxidation of Olefins: Experimental Evidence for Multiple Active Oxidants

The stereoselectivity of olefin epoxidation catalyzed by Mn(III)(salen)X (1a, X = Cl(-); 1b, X = BF(4)(-)) complexes is examined in the presence of neutral donor ligands, employing various iodosylarenes (ArIO: PhIO, C(6)F(5)IO, and MesIO) as the oxygen atom source. The cis/trans ratios of stilbene oxides and the enantiomeric excesses of styrene oxide and 1,2-dihydronaphthalene oxide are found to be strongly dependent on the nature of the iodosylarenes under certain conditions. In other cases, olefin epoxidation is shown to proceed with essentially identical diastereoselectivities or enantioselectivities, regardless of the oxygen atom source used. We propose that a Mn(V)(salen)-oxo intermediate and a complex between the catalyst and the terminal oxidant competitively effect the epoxidation when the stereoselectivities are markedly dependent on the oxygen atom source. A single Mn(V)(salen)-oxo species is considered to be the sole oxygenating intermediate when the terminal oxidants do not exert a notable influence on the product selectivities. Our results clearly demonstrate the existence of multiple oxidizing species and the conditions in which only a single oxygenating intermediate is involved. The axial donor ligands (both anionic ligands and neutral ligands) are shown to strongly influence both the identity and the reactivity of the oxygenating species.

Multiple active oxidants in competitive epoxidations catalyzed by porphyrins and corroles

We demonstrate the existence of multiple active oxygenating species in porphyrin and corrole-catalyzed competitive epoxidations of styrene and cis-cyclooctene.

Anionic ligand effect on the nature of epoxidizing intermediates in iron porphyrin complex-catalyzed epoxidation reactions

We have studied an anionic ligand effect in iron porphyrin complex-catalyzed competitive epoxidations of cis- and trans-stilbenes by various terminal oxidants and found that the ratios of cis- to trans-stilbene oxide products formed in competitive epoxidations were markedly dependent on the ligating nature of the anionic ligands. The ratios of cis- to trans-stilbene oxides obtained in the reactions of Fe(TPP)X (TPP = meso-tetraphenylporphinato dianion and X(-) = anionic ligand) and iodosylbenzene (PhIO) were 14 and 0.9 when the X(-) of Fe(TPP)X was Cl(-) and CF(3)SO(3)(-), respectively. An anionic ligand effect was also observed in the reactions of an electron-deficient iron(III) porphyrin complex containing a number of different anionic ligands, Fe(TPFPP)X [TPFPP = meso-tetrakis(pentafluorophenyl)porphinato dianion and X(-) = anionic ligand], and various terminal oxidants such as PhIO, m-chloroperoxybenzoic acid (m-CPBA), tetrabutylammonium oxone (TBAO), and H(2)O(2). While high ratios of cis- to trans-stilbene oxides were obtained in the reactions of iron porphyrin catalysts containing ligating anionic ligands such as Cl(-) and OAc(-), the ratios of cis- to trans-stilbene oxide were low in the reactions of iron porphyrin complexes containing nonligating or weakly ligating anionic ligands such as SbF(6)(-), CF(3)SO(3)(-), and ClO(4)(-). When the anionic ligand was NO(3)(-), the product ratios were found to depend on terminal oxidants and olefin concentrations. We suggest that the dependence of the product ratios on the anionic ligands of iron(III) porphyrin catalysts is due to the involvement of different reactive species in olefin epoxidation reactions. That is, high-valent iron(IV) oxo porphyrin cation radicals are generated as a reactive species in the reactions of iron porphyrin catalysts containing nonligating or weakly ligating anionic ligands such as SbF(6)(-), CF(3)SO(3)(-), and ClO(4)(-), whereas oxidant-iron(III) porphyrin complexes are the reactive intermediates in the reactions of iron porphyrin catalysts containing ligating anionic ligands such as Cl(-) and OAc(-).