Abstract:Oxidation of alcohols, ethers, and sulfoxides by ozone in acetonitrile is catalyzed by sub-millimolar concentrations of Fe(CH 3 CN) 6 2+ . The catalyst provides both rate acceleration and greater selectivity toward the less oxidized products. For example, Fe(CH 3 CN) 6 2+ -catalyzed oxidation of benzyl alcohol yields benzaldehyde almost exclusively (>95%) whereas uncatalyzed reaction generates a 1:1 mixture of benzaldehyde and benzoic acid. Similarly, aliphatic alcohols are oxidized to aldehydes/ketones, cyclo… Show more
“…Thus, only part of the reaction takes place by one-electron transfer, the remaining ∼40% apparently involving attack by Fe IV aq O 2+ at one of the reactive sites and/or rings in the ABTS molecule causing oxidative degradation. This path is reminiscent of the reaction of the acetonitrile complex Fe IV AN O 2+ with benzyl alcohol and of reactions of other Fe(IV) complexes with aromatic compounds. , At the low concentrations used, no products could be identified by NMR. The protonated form of the reductant, HABTS – , which is dominant at pH 1 (p K a 2.07) is expected to be unreactive in electron transfer, but it is possible that it engages in a slower side reaction with Fe IV aq O 2+ at the ring.…”
The kinetics of oxidation of organic and inorganic reductants by aqueous iron(IV) ions, Fe(IV)(H2O)5O(2+) (hereafter Fe(IV)aqO(2+)), are reported. The substrates examined include several water-soluble ferrocenes, hexachloroiridate(III), polypyridyl complexes M(NN)3(2+) (M = Os, Fe and Ru; NN = phenanthroline, bipyridine and derivatives), HABTS(-)/ABTS(2-), phenothiazines, Co(II)(dmgBF2)2, macrocyclic nickel(II) complexes, and aqueous cerium(III). Most of the reductants were oxidized cleanly to the corresponding one-electron oxidation products, with the exception of phenothiazines which produced the corresponding oxides in a single-step reaction, and polypyridyl complexes of Fe(II) and Ru(II) that generated ligand-modified products. Fe(IV)aqO(2+) oxidizes even Ce(III) (E(0) in 1 M HClO4 = 1.7 V) with a rate constant greater than 10(4) M(-1) s(-1). In 0.10 M aqueous HClO4 at 25 °C, the reactions of Os(phen)3(2+) (k = 2.5 × 10(5) M(-1) s(-1)), IrCl6(3-) (1.6 × 10(6)), ABTS(2-) (4.7 × 10(7)), and Fe(cp)(C5H4CH2OH) (6.4 × 10(7)) appear to take place by outer sphere electron transfer (OSET). The rate constants for the oxidation of Os(phen)3(2+) and of ferrocenes remained unchanged in the acidity range 0.05 < [H(+)] < 0.10 M, ruling out prior protonation of Fe(IV)aqO(2+) and further supporting the OSET assignment. A fit to Marcus cross-relation yielded a composite parameter (log k22 + E(0)Fe/0.059) = 17.2 ± 0.8, where k22 and E(0)Fe are the self-exchange rate constant and reduction potential, respectively, for the Fe(IV)aqO(2+)/Fe(III)aqO(+) couple. Comparison with literature work suggests k22 < 10(-5) M(-1) s(-1) and thus E(0)(Fe(IV)aqO(2+)/Fe(III)aqO(+)) > 1.3 V. For proton-coupled electron transfer, the reduction potential is estimated at E(0) (Fe(IV)aqO(2+), H(+)/Fe(III)aqOH(2+)) ≥ 1.95 V.
“…Thus, only part of the reaction takes place by one-electron transfer, the remaining ∼40% apparently involving attack by Fe IV aq O 2+ at one of the reactive sites and/or rings in the ABTS molecule causing oxidative degradation. This path is reminiscent of the reaction of the acetonitrile complex Fe IV AN O 2+ with benzyl alcohol and of reactions of other Fe(IV) complexes with aromatic compounds. , At the low concentrations used, no products could be identified by NMR. The protonated form of the reductant, HABTS – , which is dominant at pH 1 (p K a 2.07) is expected to be unreactive in electron transfer, but it is possible that it engages in a slower side reaction with Fe IV aq O 2+ at the ring.…”
The kinetics of oxidation of organic and inorganic reductants by aqueous iron(IV) ions, Fe(IV)(H2O)5O(2+) (hereafter Fe(IV)aqO(2+)), are reported. The substrates examined include several water-soluble ferrocenes, hexachloroiridate(III), polypyridyl complexes M(NN)3(2+) (M = Os, Fe and Ru; NN = phenanthroline, bipyridine and derivatives), HABTS(-)/ABTS(2-), phenothiazines, Co(II)(dmgBF2)2, macrocyclic nickel(II) complexes, and aqueous cerium(III). Most of the reductants were oxidized cleanly to the corresponding one-electron oxidation products, with the exception of phenothiazines which produced the corresponding oxides in a single-step reaction, and polypyridyl complexes of Fe(II) and Ru(II) that generated ligand-modified products. Fe(IV)aqO(2+) oxidizes even Ce(III) (E(0) in 1 M HClO4 = 1.7 V) with a rate constant greater than 10(4) M(-1) s(-1). In 0.10 M aqueous HClO4 at 25 °C, the reactions of Os(phen)3(2+) (k = 2.5 × 10(5) M(-1) s(-1)), IrCl6(3-) (1.6 × 10(6)), ABTS(2-) (4.7 × 10(7)), and Fe(cp)(C5H4CH2OH) (6.4 × 10(7)) appear to take place by outer sphere electron transfer (OSET). The rate constants for the oxidation of Os(phen)3(2+) and of ferrocenes remained unchanged in the acidity range 0.05 < [H(+)] < 0.10 M, ruling out prior protonation of Fe(IV)aqO(2+) and further supporting the OSET assignment. A fit to Marcus cross-relation yielded a composite parameter (log k22 + E(0)Fe/0.059) = 17.2 ± 0.8, where k22 and E(0)Fe are the self-exchange rate constant and reduction potential, respectively, for the Fe(IV)aqO(2+)/Fe(III)aqO(+) couple. Comparison with literature work suggests k22 < 10(-5) M(-1) s(-1) and thus E(0)(Fe(IV)aqO(2+)/Fe(III)aqO(+)) > 1.3 V. For proton-coupled electron transfer, the reduction potential is estimated at E(0) (Fe(IV)aqO(2+), H(+)/Fe(III)aqOH(2+)) ≥ 1.95 V.
Since our initial report in 1976, the oxygen rebound mechanism has become the consensus mechanistic feature for an expanding variety of enzymatic C–H functionalization reactions and small molecule biomimetic catalysts. For both the biotransformations and models, an initial hydrogen atom abstraction from the substrate (R–H) by high-valent iron-oxo species (Fen=O) generates a substrate radical and a reduced iron hydroxide, [Fen−1–OH ·R]. This caged radical pair then evolves on a complicated energy landscape through a number of reaction pathways, such as oxygen rebound to form R–OH, rebound to a non-oxygen atom affording R–X, electron transfer of the incipient radical to yield a carbocation, R+, desaturation to form olefins, and radical cage escape. These various flavors of the rebound process, often in competition with each other, give rise to the wide range of C–H functionalization reactions performed by iron-containing oxygenases. In this review, we first recount the history of radical rebound mechanisms, their general features, and key intermediates involved. We will discuss in detail the factors that affect the behavior of the initial caged radical pair and the lifetimes of the incipient substrate radicals. Several representative examples of enzymatic C–H transformations are selected to illustrate how the behaviors of the radical pair [Fen−1–OH ·R] determine the eventual reaction outcome. Finally, we discuss the powerful potential of “radical rebound” processes as a general paradigm for developing novel C–H functionalization reactions with synthetic, biomimetic catalysts. We envision that new chemistry will continue to arise by bridging enzymatic “radical rebound” with synthetic organic chemistry.
“…The decrease in TCE removal indicated that HO• was the dominant species responsible for TCE degradation, but the contribution of other species cannot be neglected as well. Because IPA reacts with Fe(IV) at a very slow rate (3.2×10 3 M −1 s −1 ) compared to HO• (1.9×10 9 M −1 s −1 ) [44], the results of the IPA scavenging tests strongly support the predominant role of HO• rather than Fe(IV) in TCE degradation. Meanwhile, minor inhibition was found in TCE degradation in the presence of CF, though it was much smaller than that of IPA.…”
The enhancement effect of an environmentally friendly reducing agent, ascorbic acid (AA), on trichloroethene (TCE) degradation by Fe(III)-activated calcium peroxide (CP) was evaluated. The addition of AA accelerated the transformation of Fe(III) to Fe(II), and the complexation of Fe(III)/Fe(II) with AA and its products alleviated the precipitation of dissolved iron. These impacts enhanced the generation of reactive oxygen species (ROSs). Investigation of ROSs using chemical probe tests, electron paramagnetic resonance (EPR) tests, and radical scavenger tests strongly confirm large production of hydroxyl radicals (HO•) that is responsible for TCE degradation. The generation of Cl− from the degraded TCE was complete in the enhanced CP/Fe(III)/AA system. The investigation of solution matrix effects showed that the TCE degradation rate decreases with the increase in solution pH, while Cl−, SO42− and NO3− anions have minor impact. Conversely, HCO3− significantly inhibited TCE degradation due to pH elevation and HO• scavenging. The results of experiments performed using actual groundwater indicated that an increase in reagent doses are required for effective TCE removal. In summary, the potential effectiveness of the CP/Fe(III)/AA oxidation system for remediation of TCE contaminated groundwater has been demonstrated. Additional research is needed to develop the system for practical implementation.
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