Allylic Oxidation Products Research Articles

Efficient allylic oxidation of cyclohexene catalyzed by Ti-MCM-41 using H2O2 as oxidant

2-cyclohexen-1-ol and 2-cyclohexen-1-one are very important chemical raw materials, and it is a challenge to develop an environmental and efficient synthesis route with titanium-containing catalyst to obtain high yield of allylic oxidation product. Herein, we present an intensive study on the allylic oxidation of cyclohexene over titanosilicates/H2O2 system. Ti-MCM-41 is found as an efficient catalyst. The reaction parameters, including cyclohexene/H2O2 molar ratios, solvent, catalyst amount, titanium content, reaction temperature and reaction time, were studied in detail. And a high cyclohexene conversion (∼20.2 %) and total selectivity of 2-cyclohexen-1-one and 2-cyclohexen-1-ol (∼90.0 %) was achieved under optimized reaction conditions. Further the allylic oxidation of cyclohexene proceeds a radical mechanism as demonstrated by the addition of radical scavenger BHT. This will make it possible for the highly selective green generation of enols and ketenes.

Substituent effect on iron phthalocyanines as cyclohexene oxidation catalysts

Two iron phthalocyanines peripherally octasubstituted either with electron-withdrawing isobutylsulfonyl moities or electron-donating isobutoxy moieties were designed to investigate the effect of the substitution pattern on their oxidation catalytic activity, and were then tested in oxidation of cyclohexene as a reaction model. For both catalysts, the main product of oxidation was 2-cyclohexen-1-ol which is an allylic oxidation product. The electron-withdrawing isobutylsulfonyl substituted iron phthalocyanine 1 exhibited better catalytic activities than the electron-donating isobutoxy substituted iron phthalocyanine 2.

A further step to sustainable palladium catalyzed oxidation: Allylic oxidation of alkenes in green solvents

The palladium catalyzed oxidation of alkenes with molecular oxygen is a synthetically important reaction which employs palladium catalysts in solution; therefore, a solvent plays a critical role for the process. In this study, we have tested several green solvents as a reaction medium for the allylic oxidation of a series of alkenes. Dimethylcarbonate, methyl isobutyl ketone, and propylene carbonate, solvents with impressive sustainability ranks and very scarcely exploited in palladium catalyzed oxidations, were proved to be excellent alternatives for the solvents conventionally employed in these processes, such as acetic acid. Palladium acetate alone or in the combination with p-benzoquinone efficiently operates as the catalyst for the oxidation of alkenes by dioxygen under 5–10 atm. For most substrates, the systems in green solvents showed better selectivity for allylic oxidation products as compared to pure acetic acid; moreover, the reactions in propylene carbonate solutions occurred even faster than in acetic acid.

Activated Carbon-Supported Ruthenium as a Catalyst for the Solvent- and Initiator-Free Aerobic Epoxidation of Limonene

The activity and selectivity of ruthenium catalysts supported on various activated carbons have been investigated in the epoxidation of limonene using molecular oxygen, the greenest oxidizing agent, under solvent/reductant- and initiator-free reaction conditions. The catalysts were prepared via different methods including impregnation, sol immobilization, and cation exchange. Supported Ru catalysts with a metal particle size ranging from 1.8 to 21 nm were obtained. The catalyst prepared by cation exchange using Darco G60, with the highest mesopore surface area and pore volume and the lowest Ru particle size (1.8 nm), was found to give the best combination of limonene conversion of 35% and higher selectivity for epoxidation products (Sepox) of 57% at 80 °C and 3 bar oxygen pressure, compared to the total selectivity of 25.5% for allylic oxidation products (Sallyl). Other activated carbon supports, e.g., Darco MRX, Norit RB4C, and Norit RX3, were found to yield less effective catalysts. In addition, the sol immobilization method led to the highest conversion of limonene (40.4%) but with a lower total selectivity toward epoxidation products (Sepox) of 36.8% and that toward allylic oxidation products (Sallyl) of 33.6%. The textural parameters of carbon-supported Ru catalysts were determined by BET, TEM, SEM, TPR, and CO chemisorption. Highly dispersed Ru particles were found to promote the formation of limonene epoxides via the reaction of limonene hydroperoxide with limonene instead of hydroperoxide decomposition to allylic radicals. Finally, the appropriate reaction conditions to maximize the selectivity of the epoxides and the reusability of the catalyst were established.

Selective Oxidation of Citronellol over Titanosilicate Catalysts

Citronellol is one of the most widely used fragrances for bouquetting purposes and it is a starting material for synthesis of several other terpenoids. Nevertheless, few data have been reported on citronellol selective oxidation. Accordingly, we report our findings on the selective oxidation of citronellol with hydrogen peroxide using a set of titanosilicate catalysts with different morphologies and textural properties—conventional titanium silicalite 1 (TS-1), mesoporous TS-1, layered TS-1 and silica-titania pillared TS-1 and also studying the effect of the solvent used. Epoxidation of C6=C7 double bond was the main primary reaction in this system and trace signals of C5 allylic oxidation products were observed without formation of citronellal. Due to the presence of post-synthesis introduced additional Ti sites, the silica-titania pillared TS-1 (TS-1-PITi) provided the highest conversion among the tested catalysts; nevertheless, citronellol was oxidized over all the studied catalysts including conventional TS-1; therefore, showing that it penetrates even into MFI micropores (0.55 nm in diameter). When using acetonitrile as a solvent, the conversion was proportional to the titanium content of the catalyst. When studying the effect of the solvent, acetonitrile provided the highest epoxide selectivity (55%) while in methanol, 2-propanol and 1,4-dioxane, ring opening reactions caused epoxide decomposition.

New scents from bio-renewable cis-jasmone by aerobic palladium catalyzed oxidations

The palladium catalyzed oxidation is a versatile method for upgrading of cis-jasmone, a main aroma constituent of jasmine essential oil. Although both olefinic bonds in cis-jasmone are sterically hindered, the substrate has demonstrated a remarkable reactivity in Wacker type oxidations. Depending on the catalyst and the solvent, reactions can be directed to either allylic oxidation or ketone products. The oxidation in acetic acid solutions with a chloride-free Pd(OAc)2/p-benzoquinone catalyst gave with high selectivity an allylic acetate. An efficient oxygen-couple catalytic turnover can be achieved either under atmospheric pressure with catalytic amounts of Cu(OAc)2 to promote the regeneration of p-benzoquinone or at 5–10 at m of oxygen in the absence of any auxiliary co-catalyst. Alternatively, cis-jasmone can be oxidized to ketone products in aqueous dimethylacetamide solutions using molecular oxygen as a final oxidant and PdCl2 as a sole catalyst. Poly-functionalized jasmonoids obtained are potentially useful as components of perfumes and cosmetics.

Low temperature solvent-free allylic oxidation of cyclohexene using graphitic oxide catalysts

A range of graphitic oxides have been utilised as metal free carbocatalysts for the low temperature oxidation of cyclohexene. The activity of the catalysts was correlated with the amount of surface oxygen on the graphitic oxide. In the case of cyclohexene oxidation, major selectivity is observed to allylic oxidation products. This is in contrast to the epoxide being the major product in linear alkene oxidation. This selectivity was maintained over long reaction times and at a conversion of above 50 %. Only small amounts of epoxide were observed, which eventually decreases at higher conversion due to hydrolysis to cyclohexane diol. The similarity between the non-catalysed and the catalysed product distribution suggests that these catalysts act as a solid initiator, and the role of the graphitic oxide is to decrease the lengthy induction period observed in the blank non-catalysed reaction.

Mn(III)-Iodosylarene Porphyrins as an Active Oxidant in Oxidation Reactions: Synthesis, Characterization, and Reactivity Studies.

Mn(III)-iodosylarene porphyrin adducts, [Mn(III)(ArIO)(Porp)]+, were synthesized by reacting electron-deficient Mn(III) porphyrin complexes with iodosylarene (ArIO) at -60 °C and characterized using various spectroscopic methods. The [Mn(III)(ArIO)(Porp)]+ species were then investigated in the epoxidation of olefins under stoichiometric conditions. In the epoxidation of olefins by the Mn(III)-iodosylarene porphyrin species, epoxide was formed as the sole product with high chemoselectivities and stereoselectivities. For example, cyclohexene oxide was formed exclusively with trace amounts of allylic oxidation products; cis- and trans-stilbenes were oxidized to the corresponding cis- and trans-stilbene oxides, respectively. In the catalytic epoxidation of cyclohexene by an electron-deficient Mn(III) porphyrin complex and sPhIO at low temperature (e.g., -60 °C), the Mn(III)-iodosylarene porphyrin species was evidenced as the active oxidant that effects the olefin epoxidation to give epoxide as the product. However, at high temperature (e.g., 0 °C) or in the case of using an electron-rich manganese(III) porphyrin catalyst, allylic oxidation products, along with cyclohexene oxide, were yielded, indicating that the active oxidant(s) was not the Mn(III)-iodosylarene adduct but probably high-valent Mn-oxo species in the catalytic reactions. We also report the conversion of the Mn(III)-iodosylarene porphyrins to high-valent Mn-oxo porphyrins under various conditions, such as at high temperature, with electron-rich porphyrin ligand, and in the presence of base (OH-). The present study reports the first example of spectroscopically well-characterized Mn(III)-iodosylarene porphyrin species being an active oxidant in the stoichiometric and catalytic oxidation reactions. Other aspects, such as one oxidant versus multiple oxidants debate, also were discussed.

Redox-Active Cobalt(II/III) Metal-Organic Framework for Selective Oxidation of Cyclohexene.

We report herein a new cobalt(II/III) mixed-valence metal-organic framework formulated as [CoIICo2III(μ3-O)(bdc)3(tpt)]·guest 1, where bdc = benzene-1,4-dicarboxylate and tpt = 2,4,6-tri(4-pyridinyl)-1,3,5-triazine, which can be used as a redox-active heterogeneous catalyst for selective oxidation of cyclohexene on the allylic position without destroying the adjacent double bond. Two oxidants were chosen to demonstrate this result. For using tert-butyl hydroperoxide, the conversion rate is 63% and only allylic oxidation products ( tert-butyl-2-cyclohexenyl-1-peroxide, 86%; 2-cyclohexen-1-one, 14%) are found, whereas if using O2 as oxidant, a total conversion of 38% is achieved and also only the allylic oxidation products (cyclohexenyl hydroperoxide, 72%; 2-cyclohexen-1-one, 20%; and cyclohex-2-en-1-ol, 8%) are found. The absence of any adduct on the double bond may be due to the unique radical chain mechanism triggered by the mixed-valent [CoIICo2III(μ3-O)] centers.

Limonene oxidation over Ti-MCM-41 and Ti-MWW catalysts with t-butyl hydroperoxide as the oxidant

This paper presents the method of limonene epoxidation over Ti-MCM-41 and Ti-MWW catalysts in the presence of t-butyl hydroperoxide as an oxidant. Natural limonene used in the epoxidation process was obtained via steam distillation (97% purity). The purpose of the research was to obtain the highest yield of 1,2-epoxylimonene, but the performed studies showed that the process of limonene epoxidation is more complex because in addition to epoxidation products (1,2-epoxylimonene and its diol which were formed usually in small amount) also the following oxygenated derivatives of limonene are formed: carvone, carveol, and perillyl alcohol (products of allylic oxidation (hydroxylation) at positions 6 and 7 in limonene molecule). Therefore, ultimately, the most favorable conditions for the limonene oxidation process were selected at the highest conversion of limonene and at high values of perillyl alcohol and carveol or one of these two products. Perillyl alcohol and carveol have found numerous applications in medicine, among others, therefore this direction seems to be the most beneficial. This also work presents the short characteristization of the titanium silicate Ti-MCM-41 and Ti-MWW catalysts used in the research, taking into account their structure, properties and applications are presented.

Oxidation of Cyclohexene in the Presence of Transition‐Metal‐Substituted Phosphotungstates and Hydrogen Peroxide: Catalysis and Reaction Pathways

AbstractHomogeneous catalytic oxidations of cyclohexene by transition‐metal‐substituted phosphotungstates [PW11M(L)O39]m− (PW11M, M=CoII, CuII, FeIII, NiII, MnII, L=H2O or absence) with hydrogen peroxide in acetonitrile were experimentally studied. The catalytic activities of allylic oxidation were found to strongly depend on the transition metals, and PW11Co showed the highest activity. The product distribution and the catalyst stability were dominated by mole ratio of hydrogen peroxide to PW11M, whereby low or high mole ratios led to stable structure of PW11M and predominant formation of allylic oxidation products or decomposition of PW11M, respectively. Different from the activation of the allylic C−H bond by radicals, the oxidation of C=C double bond was based on tungsten‐peroxo species. A reaction mechanism composed of radical and nonradical processes was proposed from NMR, EPR, and kinetic data, to describe the reaction pathways of cyclohexene oxidation.

Hydrogen-Atom Transfer Oxidation with H2O2 Catalyzed by [FeII(1,2-bis(2,2'-bipyridyl-6-yl)ethane(H2O)2]2+: Likely Involvement of a (μ-Hydroxo)(μ-1,2-peroxo)diiron(III) Intermediate.

The iron(II) triflate complex (1) of 1,2-bis(2,2'-bipyridyl-6-yl)ethane, with two bipyridine moieties connected by an ethane bridge, was prepared. Addition of aqueous 30% H2O2 to an acetonitrile solution of 1 yielded 2, a green compound with λmax=710 nm. Moessbauer measurements on 2 showed a doublet with an isomer shift (δ) of 0.35 mm/s and a quadrupole splitting (ΔEQ) of 0.86 mm/s, indicative of an antiferromagnetically coupled diferric complex. Resonance Raman spectra showed peaks at 883, 556 and 451 cm-1 that downshifted to 832, 540 and 441 cm-1 when 1 was treated with H218O2. All the spectroscopic data support the initial formation of a (μ-hydroxo)(μ-1,2-peroxo)diiron(III) complex that oxidizes carbon-hydrogen bonds. At 0°C 2 reacted with cyclohexene to yield allylic oxidation products but not epoxide. Weak benzylic C-H bonds of alkylarenes were also oxidized. A plot of the logarithms of the second order rate constants versus the bond dissociation energies of the cleaved C-H bond showed an excellent linear correlation. Along with the observation that oxidation of the probe substrate 2,2-dimethyl-1-phenylpropan-1-ol yielded the corresponding ketone but no benzaldehyde, and the kinetic isotope effect, kH/kD , of 2.8 found for the oxidation of xanthene, the results support the hypothesis for a metal-based H-atom abstraction mechanism. Complex 2 is a rare example of a (μ-hydroxo)(μ-1,2-peroxo)diiron(III) complex that can elicit the oxidation of carbon-hydrogen bonds.

Catalytic allylic oxidation of internal alkenes to a multifunctional chiral building block.

The stereoselective oxidation of hydrocarbons represents one of the most significant advances in synthetic chemistry over the last fifty years1–3. Inspired by nature, chemists have developed enantioselective dihydroxylations, epoxidations, and other oxidations of unsaturated hydrocarbons. More recently, the catalytic enantioselective allylic C–H oxidation of alkenes has emerged as a powerful chemical strategy, streamlining the production of pharmaceuticals, natural products, fine chemicals and other functional materials4–7. Allylic functionalization provides a direct path to chiral synthons with a newly formed stereocenter from petrochemical feedstocks while preserving the olefin functionality as a handle for further elaboration. Various metal-based catalysts have been discovered for the enantioselective allylic C–H oxidation of simple alkenes with cyclic or terminal double bonds8–16. However, a general and selective allylic oxidation remains elusive with the more common internal alkenes. Here, we report the enantioselective, regioselective, and E/Z selective allylic oxidation of unactivated internal alkenes via a catalytic asymmetric hetero-ene reaction with a chalcogen-based oxidant. This method represents the first example of selectively converting unsymmetrical internal alkenes into allylic functionalized products with high stereoselectivity and regioselectivity. Stereospecific transformations of the multifunctional allylic oxidation products highlight the potential for rapidly converting internal alkenes into a broad range of enantioenriched structures that can be utilized in the synthesis of complex target molecules.

Highly selective allylic oxidation of cyclohexene over molybdenum-doped manganese oxide catalysts

A facile synthesis of molybdenum-doped manganese oxides was achieved by a single step reflux method. The catalytic activity of molybdenum-doped manganese oxides for the oxidation of cyclohexene was investigated under solvent-free conditions. Various techniques such as X-ray diffraction, transmission electron microscopy, hydrogen temperature-programmed reduction, thermogravimetric analysis and diffuse reflectance infrared Fourier transform spectroscopy of adsorbed pyridine were employed to indentify the features of catalysts as prepared. The characterization results indicate that Mo dopants were successfully incorporated into the crystal structure of manganese oxide and the concentration of Mo dopant had an influence on redox properties as well as the Lewis acid sites of Mo dopant catalysts. Furthermore, 1.0 mol%Mo-MnO2 catalyst exhibited highest total yield (32.4%) and almost 100% selectivity towards allylic oxidation products, which could be attributed to its relative lower redox properties and the highest Lewis acidity. These results thereby provide an insight of catalyst design in selective catalytic oxidation of cyclohexene.

Factors Controlling the Chemoselectivity in the Oxidation of Olefins by Nonheme Manganese(IV)-Oxo Complexes.

We report the oxidation of cyclic olefins, such as cyclohexene, cyclohexene-d10, and cyclooctene, by mononuclear nonheme manganese(IV)-oxo (Mn(IV)O) and triflic acid (HOTf)-bound Mn(IV)O complexes. In the oxidation of cyclohexene, the Mn(IV)O complexes prefer the C-H bond activation to the C═C double bond epoxidation, whereas the C═C double bond epoxidation becomes a preferred reaction pathway in the cyclohexene oxidation by HOTf-bound Mn(IV)O complexes. In contrast, the oxidation of cyclohexene-d10 and cyclooctene by the Mn(IV)O complexes occurs predominantly via the C═C double bond epoxidation. This conclusion is drawn from the product analysis and kinetic studies of the olefin oxidation reactions, such as the epoxide versus allylic oxidation products, the formation of Mn(II) versus Mn(III) products, and the kinetic analyses. Overall, the experimental results suggest that the energy barrier of the C═C double bond epoxidation is very close to that of the allylic C-H bond activation in the oxidation of cyclic olefins by high-valent metal-oxo complexes. Thus, the preference of the reaction pathways is subject to changes upon small manipulation of the reaction environments, such as the supporting ligands and metal ions in metal-oxo species, the presence of HOTf (i.e., HOTf-bound Mn(IV)O species), and the allylic C-H(D) bond dissociation energies of olefins. This is confirmed by DFT calculations in the oxidation of cyclohexene and cyclooctene, which show multiple pathways with similar rate-limiting energy barriers and depending on the allylic C-H bond dissociation energies. In addition, the possibility of excited state reactivity in the current system is confirmed for epoxidation reactions.

The epoxidation of limonene over the TS-1 and Ti-SBA-15 catalysts.

Limonene belongs to a group of very important intermediates used in the production of fine chemicals. This monoterpene compound can be obtained from peels of oranges or lemon which are a (biomass) waste from the orange juice industry. Thus, limonene is a renewable, easy available and a relatively cheap compound. This work presents preliminary studies on the process of limonene epoxidation over zeolite type catalysts such as: TS-1 and Ti-SBA-15. In these studies methanol was used as a solvent and as an oxidizing agent a 60 wt % hydrogen peroxide solution was applied. The activity of each catalyst was investigated for four chosen temperatures (0 °C, 40 °C, 80 °C and 120 °C). The reaction time was changed from 0.5 to 24 h. For each catalyst the most beneficial conditions (the appropriate temperature and the reaction time) have been established. The obtained results were compared and the most active catalyst was chosen. These studies have also shown different possible ways of limonene transformation, not only in the direction of 1,2-epoxylimonene and its corresponding diol, but also in direction of carveol, carvone and perillyl alcohol—compounds with a lot of applications. The possible mechanisms of formation of the allylic oxidation products were proposed.

Surfactant-assisted sol–gel synthesis of zirconia supported phosphotungstates or Ti-substituted phosphotungstates for catalytic oxidation of cyclohexene

The Keggin-type polyoxometalates (POMs) including phosphotungstate and Ti-substituted phosphotungstates were supported on zirconia (ZrO2) matrix by a surfactant-assisted sol–gel copolymerization technique. The resultant ZrO2 supported POMs composite materials (ZrO2–POMs) were characterized by X-ray power diffraction, transmission electron microscopy, Fourier transform infrared spectra, diffuse reflectance ultraviolet–visible spectra, thermogravimetry-differential scanning calorimetry analysis, and 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR) analysis. The results showed that the ZrO2–POMs materials have worm-like disordered mesoporous, and the primary structure of POMs is preserved after the formation of the composite materials. All the ZrO2–POMs materials are catalytically active for the H2O2-mediated oxidation of cyclohexene with relatively high selectivity to allylic oxidation products. Moreover, these materials exhibit high stability against leaching of active species, and can be easily recycled without obvious loss of activity and selectivity. The existence of relatively strong interaction between the POM units and the ZrO2 framework should play important role in fabricating the stable heterogeneous ZrO2–POMs catalysts.

Catalytic oxidation of cyclohexene by aqueous iron(III)/H2O2 in mildly acidic solution: Epoxidation versus allylic oxidation

Abstract Catalytic oxidation of cyclohexene (organic layer) at 25 °C in a stirred two phase system with fresh [Fe(OH2)6](ClO4)3/H2O2 in the pH range 2.0–4.0 (I = 0.1 M, NaClO4/HClO4) is complete within a 2-minute sampling time before deactivating in contrast to previous reports in the literature. Cyclohexene epoxide is a significant product alongside the more dominant allylic oxidation products; 2-cyclohexen-1-ol and 2-cyclohexen-1-one. Both epoxide and the accompanying allylic products appear over the same time period suggesting that they derive from the same catalytic species. The pH dependence of oxidation product yield (optimizing at ~ pH 3) suggests that the active catalyst derives from reactions involving [Fe(OH2)5OH]2 + (pKa 2.54) and is most likely the hydroperoxo ion [Fe(OH2)5OOH]2 +. Yields of cyclohexene epoxide are independent of the presence of dioxygen suggesting that an activated form of H2O2 ([Fe(OH2)5OOH]2 +) is responsible. Reduction in the relative yield of the two allylic oxidation products in the absence of dioxygen suggests that they derive from 2-cyclohexen-1-peroxy radicals resulting from iron(II)-promoted Fenton chemistry following homolytic Fe O or O O cleavage on [Fe(OH2)5OOH]2 +. A mechanism for the epoxide formation is proposed involving H3O+-assisted heterolytic O O cleavage on [Fe(OH2)5OOH]2 + accompanying O atom transfer to the cyclohexene double bond. Accompanying catalysis of H2O2 decomposition persists over several hours. For the first time in a Fenton-like system evidence has been obtained for different timescales for the catalysis of oxidation reactions versus catalysis of H2O2 decomposition.

Electrochemical and Catalytic Studies of a Manganese(III)Complex with a Tetradentate Schiff‐Base Ligand Encapsulated in NaY Zeolite

The manganese(III) complex with a Schiff-base salen-type ligand (1,5-bis{[(1E)-(2-hydroxyphenyl)methylene]amino}-1H-imidazole-4-carbonitrile) has been encapsulated in the nanopores of a Y zeolite by using two different methodologies, the flexible ligand and in situ complex preparation methods. Cyclic voltammetry studies revealed that the neat complex undergoes reversible oxidation in dmf, which has been attributed to the MnII/III redox couple, whereas the two heterogeneous catalysts show different electrochemical behaviour in aqueous medium. The encapsulated and non-encapsulated homogeneous MnIIIsalen complexes were screened as catalysts for olefin oxidation by using tBuOOH as the oxygen source in different solvents. Under the optimized conditions, the catalysts exhibited moderate activity. Both heterogeneous catalysts catalysed the oxidation of cyclohexene with tBuOOH, primarily to give the allylic oxidation products. These catalysts were found to be reusable after the catalytic cycle, but with some loss of activity. A DFT study confirmed the distortion of the complex in the zeolite cage, the main difference being observed for the bonded chloride.

Cyclohexene as a substrate probe for the nature of the high-valent iron-oxo oxidant in Fe(TPA)-catalyzed oxidations

Cyclohexene is shown to be a versatile substrate probe in shedding light on the nature of the high-valent oxidant generated by bio-inspired nonheme iron catalysts. Cyclohexene provides the oxidant a choice to attack an allylic C–H bond or a CC bond, and the selectivity observed should depend on the electronic properties of the oxidant. Allylic oxidation products are predominantly observed in the reaction with [FeIV(O)(TPA)(NCCH3)]2+ (TPA = tris(2-pyridylmethyl)amine), while epoxide and cis-diol are the major products found in the [FeII(TPA)(NCCH3)2]2+-catalyzed reaction with H2O2. This difference suggests that the oxidant generated in the latter case must be distinct from [FeIV(O)(TPA)(NCCH3)]2+ and supports an oxoiron(V)-hydroxo species that has been proposed as the oxidant.