Oxygen Atom Transfer Reactions Research Articles

Oxygen atom transfer promoted nitrate to nitric oxide transformation: a step-wise reduction of nitrate → nitrite → nitric oxide.

Nitrate reductases (NRs) are molybdoenzymes that reduce nitrate (NO3−) to nitrite (NO2−) in both mammals and plants. In mammals, the salival microbes take part in the generation of the NO2− from NO3−, which further produces nitric oxide (NO) either in acid-induced NO2− reduction or in the presence of nitrite reductases (NiRs). Here, we report a new approach of VCl3 (V3+ ion source) induced step-wise reduction of NO3− in a CoII-nitrato complex, [(12-TMC)CoII(NO3−)]+ (2,{CoII–NO3−}), to a CoIII–nitrosyl complex, [(12-TMC)CoIII(NO)]2+ (4,{CoNO}8), bearing an N-tetramethylated cyclam (TMC) ligand. The VCl3 inspired reduction of NO3− to NO is believed to occur in two consecutive oxygen atom transfer (OAT) reactions, i.e., OAT-1 = NO3− → NO2− (r1) and OAT-2 = NO2− → NO (r2). In these OAT reactions, VCl3 functions as an O-atom abstracting species, and the reaction of 2 with VCl3 produces a CoIII-nitrosyl ({CoNO}8) with VV-Oxo ({VV Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O}3+) species, via a proposed CoII-nitrito (3, {CoII–NO2−}) intermediate species. Further, in a separate experiment, we explored the reaction of isolated complex 3 with VCl3, which showed the generation of 4 with VV-Oxo, validating our proposed reaction sequences of OAT reactions. We ensured and characterized 3 using VCl3 as a limiting reagent, as the second-order rate constant of OAT-2 (k2/) is found to be ∼1420 times faster than that of the OAT-1 (k2) reaction. Binding constant (Kb) calculations also support our proposition of NO3− to NO transformation in two successive OAT reactions, as Kb(CoII–NO2−) is higher than Kb(CoII–NO3−), hence the reaction moves in the forward direction (OAT-1). However, Kb(CoII–NO2−) is comparable to Kb{CoNO}8, and therefore sequenced the second OAT reaction (OAT-2). Mechanistic investigations of these reactions using 15N-labeled-15NO3− and 15NO2− revealed that the N-atom in the {CoNO}8 is derived from NO3− ligand. This work highlights the first-ever report of VCl3 induced step-wise NO3− reduction (NRs activity) followed by the OAT induced NO2− reduction and then the generation of Co-nitrosyl species {CoNO}8.

Nonclassical oxygen atom transfer reactions of an eight-coordinate dioxomolybdenum(vi) complex

Eight-coordinate MoO2(DOPOQ)2 can donate two oxygen atoms to substrates such as phosphines in a four-electron nonclassical oxygen atom transfer reaction.

Unprecedented Reactivities of Highly Reactive Manganese(III)-Iodosylarene Porphyrins in Oxidation Reactions.

We report that Mn(III)-iodosylarene porphyrins, [MnIII(Porp)(sArIO)]+, are capable of activating the C-H bonds of hydrocarbons, including unactivated alkanes such as cyclohexane, with unprecedented reactivities, such as a low kinetic isotope effect, a saturation behavior of reaction rates, and no electronic effect of porphyrin ligands on the reactivities of [MnIII(Porp)(sArIO)]+. In oxygen atom transfer (OAT) reactions, the sulfoxidation of para-X-substituted thioanisoles by [MnIII(Porp)(sArIO)]+ affords a very unusual behavior in the Hammett plot with the saturation behavior of reaction rates and no electronic effect of porphyrin ligands on reactivities. The reactivities and mechanisms of [MnIII(Porp)(sArIO)]+ are then compared with those of the corresponding MnIV(Porp)(O) complex. The present study reports the first example of highly reactive Mn(III)-iodosylarene porphyrins with unprecedented reactivities in C-H bond activation and OAT reactions.

Acid Catalysis via Acid‐Promoted Electron Transfer

This feature article focuses on acid catalysis in various redox reactions of not only organic compounds but also metal complexes, in particular metal‐oxygen complexes via acid‐promoted electron transfer (APET). On the other hand, proton‐coupled electron transfer (PCET) has been extensively studied in relation with important biological PCET reactions such as O2 reduction in respiration and water oxidation in photosynthesis. Because PCET is normally defined as simultaneous electron and proton transfer from the same substrate, acid‐promoted electron transfer using external acids is called APET in this article. Various types of chemical transformations exhibiting acid catalysis via APET discussed in this article include CC bond formation, CH bond oxidation, aromatic hydroxylation, and oxygen atom transfer reactions such as sulfoxidation and epoxidation reactions.

Corrole photochemistry

Abstract The rapid expansion of photoredox catalysis and artificial photosynthesis has garnered renewed interest in the field of photochemistry. While porphyrins have been widely utilized for a variety of photochemical applications, corrole photochemistry remains underexplored, despite an exponential growth in corrole chemistry. Indeed, less than 4% of all corrole-related publications have studied the photochemistry of these molecules. Since corroles exhibit chemical properties that are distinct from porphyrins and related macrocycles, it is likely that this divergence would also be observed in their photochemical properties. This review provides a comprehensive summary of the extant corrole photochemistry literature. Corroles primarily serve as photosensitizers that transfer energy or an electron to molecular oxygen to form singlet oxygen or superoxide, respectively. While both of these reactive oxygen species can be used to drive chemical reactions, they can also be exploited for photodynamic therapy to treat cancer and other diseases. Although direct photochemical activation of metal–ligand bonds has been less explored, corroles mediate a variety of transformations, particularly oxygen atom transfer reactions. Together, these examples illustrate the diversity of corrole photochemistry and suggest that there are many additional applications yet to be discovered.

Oxygen Atom Transfer Reactivity of Molybdenum(VI) Complexes Employing Pyrimidine- and Pyridine-2-thiolate Ligands.

Four dioxidomolybdenum(VI) complexes of the general structure [MoO2L2] employing the S,N-bidentate ligands pyrimidine-2-thiolate (PymS, 1), pyridine-2-thiolate (PyS, 2), 4-methylpyridine-2-thiolate (4-MePyS, 3) and 6-methylpyridine-2-thiolate (6-MePyS, 4) were synthesized and characterized by spectroscopic means and single-crystal X-ray diffraction analysis (2-4). Complexes 1-4 were reacted with PPh3 and PMe3, respectively, to investigate their oxygen atom transfer (OAT) reactivity and catalytic applicability. Reduction with PPh3 leads to symmetric molybdenum(V) dimers of the general structure [Mo2O3L4] (6-9). Kinetic studies showed that the OAT from [MoO2L2] to PPh3 is 5 times faster for the PymS system than for the PyS and 4-MePyS systems. The reaction of complexes 1-3 with PMe3 gives stable molybdenum(IV) complexes of the structure [MoOL2(PMe3)2] (10-12), while reduction of [MoO2(6-MePyS)2] (4) yields [MoO(6-MePyS)2(PMe3)] (13) with only one PMe3 coordinated to the metal center. The activity of complexes 1-4 in catalytic OAT reactions involving Me2SO and Ph2SO as oxygen donors and PPh3 as an oxygen acceptor has been investigated to assess the influence of the varied ligand frameworks on the OAT reaction rates. It was found that [MoO2(PymS)2] (1) and [MoO2(6-MePyS)2] (4) are similarly efficient catalysts, while complexes 2 and 3 are only moderately active. In the catalytic oxidation of PMe3 with Me2SO, complex 4 is the only efficient catalyst. Complexes 1-4 were also found to catalytically reduce NO3- with PPh3, although their reactivity is inhibited by further reduced species such as NO, as exemplified by the formation of the nitrosyl complex [Mo(NO)(PymS)3] (14), which was identified by single-crystal X-ray diffraction analysis. Computed ΔG⧧ values for the very first step of the OAT were found to be lower for complexes 1 and 4 than for 2 and 3, explaining the difference in catalytic reactivity between the two pairs and revealing the requirement for an electron-deficient ligand system.

Enhanced Redox Reactivity of a Nonheme Iron(V)-Oxo Complex Binding Proton.

Acid effects on the chemical properties of metal-oxygen intermediates have attracted much attention recently, such as the enhanced reactivity of high-valent metal(IV)-oxo species by binding proton(s) or Lewis acidic metal ion(s) in redox reactions. Herein, we report for the first time the proton effects of an iron(V)-oxo complex bearing a negatively charged tetraamido macrocyclic ligand (TAML) in oxygen atom transfer (OAT) and electron-transfer (ET) reactions. First, we synthesized and characterized a mononuclear nonheme Fe(V)-oxo TAML complex (1) and its protonated iron(V)-oxo complexes binding two and three protons, which are denoted as 2 and 3, respectively. The protons were found to bind to the TAML ligand of the Fe(V)-oxo species based on spectroscopic characterization, such as resonance Raman, extended X-ray absorption fine structure (EXAFS), and electron paramagnetic resonance (EPR) measurements, along with density functional theory (DFT) calculations. The two-protons binding constant of 1 to produce 2 and the third protonation constant of 2 to produce 3 were determined to be 8.0(7) × 108 M-2 and 10(1) M-1, respectively. The reactivities of the proton-bound iron(V)-oxo complexes were investigated in OAT and ET reactions, showing a dramatic increase in the rate of sulfoxidation of thioanisole derivatives, such as 107 times increase in reactivity when the oxidation of p-CN-thioanisole by 1 was performed in the presence of HOTf (i.e., 200 mM). The one-electron reduction potential of 2 (Ered vs SCE = 0.97 V) was significantly shifted to the positive direction, compared to that of 1 (Ered vs SCE = 0.33 V). Upon further addition of a proton to a solution of 2, a more positive shift of the Ered value was observed with a slope of 47 mV/log([HOTf]). The sulfoxidation of thioanisole derivatives by 2 was shown to proceed via ET from thioanisoles to 2 or direct OAT from 2 to thioanisoles, depending on the ET driving force.

Diverse Reactivity of a Rhenium(V) Oxo Imido Complex: [2 + 2] Cycloadditions, Chalcogen Metathesis, Oxygen Atom Transfer, and Protic and Hydridic 1,2-Additions.

We present a wide range of reactivity studies focused on the rhenium(V) oxo imido complex (DippN)(O)Re(BDI) (1, Dipp = 2,6-diisopropylphenyl and BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate). This complex, which was previously shown to possess a highly polarized Re oxo moiety, has proven to be a potent nucleophile and a valuable precursor to a variety of rare structural motifs in rhenium coordination complexes. For example, the Re oxo moiety of 1 undergoes [2 + 2] cycloadditions with carbodiimides, isocyanates, carbon dioxide, and isothiocyanates at room temperature. In the case of CO2, the cycloadduct with 1 (a carbonate complex) undergoes the facile ejection of CO2, demonstrating that this binding process is reversible. In the case of isothiocyanate, chalcogen metathesis with 1 takes place readily as the inclusion of a second equivalent of substrate in the reaction mixture rapidly yields a dithiocarbamate complex. This metathesis process was extended to the reactivity of 1 with phosphine chalcogenides, leading to the isolation of terminal sulfido imido and selenido imido complexes. Attempts to complete this series and generate the analogous terminal telluride led to the formation of a bidentate tritelluride (Te32-) complex. Triethylphosphine could only undergo oxygen atom transfer (OAT) with 1 under pressing thermal conditions that also led to C-N cleavage of the BDI ligand. In contrast, OAT between 1 and CO or 2,6-xylylisocyanide (XylNC) was found to be much more facile, proceeding within seconds at room temperature. While the addition of excess CO led to a rhenium(III) imido dicarbonyl complex, we found that the addition of 2 equiv of XylNC was necessary to promote OAT, resulting in the isolation of a rare example of a stable metal isocyanate complex. Our experimental observations of CO and XylNC and their OAT reactions with 1 inspired a mechanistic computational study to probe the intermediates and kinetic barriers along these reaction pathways. Finally, we describe 1,2-additions of both protic and hydridic substrates with the Re oxo moiety of 1, which most notably led to the syntheses of an uncommon example of a terminal rhenium hydroxide complex and an oxo-bridged Re-O-Zr hetero-bi-metallic complex that was generated using Schwartz's reagent (Cp2ZrHCl). A brief discussion of a potential alternative route to 1 is also presented.

Structure and Unprecedented Reactivity of a Mononuclear Nonheme Cobalt(III) Iodosylbenzene Complex

AbstractA mononuclear nonheme cobalt(III) iodosylbenzene complex, [CoIII(TQA)(OIPh)(OH)]2+ (1), is synthesized and characterized structurally and spectroscopically. While 1 is a sluggish oxidant in oxidation reactions, it becomes a competent oxidant in oxygen atom transfer reactions, such as olefin epoxidation, in the presence of a small amount of proton. More interestingly, 1 shows a nucleophilic reactivity in aldehyde deformylation reaction, demonstrating that 1 has an amphoteric reactivity. Another interesting observation is that 1 can be used as an oxygen atom donor in the generation of high‐valent metal‐oxo complexes. To our knowledge, we present the first crystal structure of a CoIII iodosylbenzene complex and the unprecedented reactivity of metal‐iodosylarene adduct.

A kinetic study of the photoinduced oxo-transfer using a Mo complex anchored to TiO2

Kinetic study of the photo-assisted oxygen atom transfer reaction (OAT) using a dioxo-Mo complex anchored on TiO2, stimulated by light, was performed at ambient conditions using triphenylphosphine (PPh3) as a model molecule. The kinetic of the OAT reaction was studied with three catalytic systems: 4,4’-dicarboxylate-2,2’-bipyridine-dioxochloromolybdenum (MoO2L/TiO2), H2MoO4 (H2MoO4/TiO2) and molybdenum oxide (MoO3/ TiO2) anchored to TiO2. The MoO2L/TiO2 gives conversion higher than 90% and selectivity (to phosphine oxide) close to 100%. MoO3/TiO2 did not allow the oxo-transference, suggesting the importance of the bipyridine ligand as an electronic connector between MoO2L unit and TiO2. With the MoO2L/TiO2 system was observed that when the photonic flux increases, the quantum yield, and the OAT reaction rate increases.

Oxophosphonium-Alkyne Cycloaddition Reactions: Reversible Formation of 1,2-Oxaphosphetes and Six-membered Phosphorus Heterocycles.

While the metathesis reaction between alkynes and carbonyl compounds is an important tool in organic synthesis, the reactivity of alkynes with isoelectronic main-group R2E═O compounds is unexplored. Herein, we show that oxophosphonium ions, which are the isoelectronic phosphorus congeners to carbonyl compounds, undergo [2 + 2] cycloaddition reactions with different alkynes to generate 1,2-oxaphosphete ions, which were isolated and structurally characterized. The strained phosphorus-oxygen heterocycles open to the corresponding heterodiene structure at elevated temperature, which was used to generate six-membered phosphorus heterocycles via hetero Diels-Alder reactions. Insights into the influence of the substituents at the phosphorus center on the energy profile of the oxygen atom transfer reaction were obtained by quantum-chemical calculations.

Effect of Ligand Fields on the Reactivity of O2‐Activating Iron(II)‐Benzilate Complexes of Neutral N5 Donor Ligands

Three new iron(II)-benzilate complexes [(N4Py)Fe II (benzilate)]ClO 4 ( 1 ), [(N4Py Me2 )Fe II (benzilate)]ClO 4 ( 2 ) and [(N4Py Me4 )Fe II (benzilate)]ClO 4 ( 3 ) of neutral pentadentate nitrogen donor ligands have been isolated and characterized to study their dioxygen reactivity. Single crystal X-ray structures reveal a mononuclear six-coordinate iron(II) centre in each case, where benzilate binds to the iron center in monodentate mode via one carboxylate oxygen. Introduction of methyl groups in the 6-positions of the pyridine rings makes the N4Py Me2 and N4Py Me4 ligands weaker field compared to the parent N4Py ligand. All the complexes ( 1-3 ) react with dioxygen to decarboxylate the coordinated benzilate to benzophenone quantitatively. The decarboxylation takes lesser time for the complex of more sterically hindered ligand and follows the order 3 > 2 > 1 . The complexes display oxygen atom transfer reactivity to thioanisole and also exhibit hydrogen atom transfer reactions with substrates containing weak C-H bonds. Based on interception studies with external substrates, labelling experiment and Hammett analysis, a nucleophilic iron(II)-hydroperoxo species is proposed to form upon two-electron reductive activation of dioxygen by each iron(II)-benzilate complex. The nucleophilic oxidants are converted to the corresponding electrophilic iron(IV)-oxo oxidant upon treatment with a protic acid. The high-spin iron(II)-benzilate complex with the weakest ligand field results in the formation of a more reactive iron-oxygen oxidant.

Molybdenum and Tungsten Cofactors and the Reactions They Catalyze.

The last 20 years have seen a dramatic increase in our mechanistic understanding of the reactions catalyzed by pyranopterin Mo and W enzymes. These enzymes possess a unique cofactor (Moco) that contains a novel ligand in bioinorganic chemistry, the pyranopterin ene-1,2-dithiolate. A synopsis of Moco biosynthesis and structure is presented, along with our current understanding of the role Moco plays in enzymatic catalysis. Oxygen atom transfer (OAT) reactivity is discussed in terms of breaking strong metal-oxo bonds and the mechanism of OAT catalyzed by enzymes of the sulfite oxidase (SO) family that possess dioxo Mo(VI) active sites. OAT reactivity is also discussed in members of the dimethyl sulfoxide (DMSO) reductase family, which possess des-oxo Mo(IV) sites. Finally, we reveal what is known about hydride transfer reactivity in xanthine oxidase (XO) family enzymes and the formate dehydrogenases. The formal hydride transfer reactivity catalyzed by xanthine oxidase family enzymes is complex and cleaves substrate C-H bonds using a mechanism that is distinct from monooxygenases. The chapter primarily highlights developments in the field that have occurred since ~2000, which have contributed to our collective structural and mechanistic understanding of the three canonical pyranopterin Mo enzymes families: XO, SO, and DMSO reductase.

Addressing Ligand-Based Redox in Molybdenum-Dependent Methionine Sulfoxide Reductase.

A combination of pulsed EPR, CW EPR, and X-ray absorption spectroscopies has been employed to probe the geometric and electronic structure of the E. coli periplasmic molybdenum-dependent methionine sulfoxide reductase (MsrP). 17O and 1H pulsed EPR spectra show that the as-isolated Mo(V) enzyme form does not possess an exchangeable H2O/OH- ligand bound to Mo as found in the sulfite oxidizing enzymes of the same family. The nature of the unusual CW EPR spectrum has been re-evaluated in light of new data on the MsrP-N45R variant and related small-molecule analogues of the active site. These data point to a novel "thiol-blocked" [(PDT)MoVO(SCys)(thiolate)]- structure, which is supported by new EXAFS data. We discuss these new results in the context of ligand-based and metal-based redox chemistry in the enzymatic oxygen atom transfer reaction.

Concerted proton-electron transfer reactions of manganese-hydroxo and manganese-oxo complexes.

The enzymes manganese superoxide dismutase and manganese lipoxygenase use MnIII-hydroxo centres to mediate proton-coupled electron transfer (PCET) reactions with substrate. As manganese is earth-abundant and inexpensive, manganese catalysts are of interest for synthetic applications. Recent years have seen exciting reports of enantioselective C-H bond oxidation by Mn catalysts supported by aminopyridyl ligands. Such catalysts offer economic and environmentally-friendly alternatives to conventional reagents and catalysts. Mechanistic studies of synthetic catalysts highlight the role of Mn-oxo motifs in attacking substrate C-H bonds, presumably by a concerted proton-electron transfer (CPET) step. (CPET is a sub-class of PCET, where the proton and electron are transferred in the same step.) Knowledge of geometric and electronic influences for CPET reactions of Mn-hydroxo and Mn-oxo adducts enhances our understanding of biological and synthetic manganese centers and informs the design of new catalysts. In this Feature article, we describe kinetic, spectroscopic, and computational studies of MnIII-hydroxo and MnIV-oxo complexes that provide insight into the basis for the CPET reactivity of these species. Systematic perturbations of the ligand environment around MnIII-hydroxo and MnIV-oxo motifs permit elucidation of structure-activity relationships. For MnIII-hydroxo centers, electron-deficient ligands enhance oxidative reactivity. However, ligand perturbations have competing consequences, as changes in the MnIII/II potential, which represents the electron-transfer component for CPET, is offset by compensating changes in the pKa of the MnII-aqua product, which represents the proton-transfer component for CPET. For MnIV-oxo systems, a multi-state reactivity model inspired the development of significantly more reactive complexes. Weakened equatorial donation to the MnIV-oxo unit results in large rate enhancements for C-H bond oxidation and oxygen-atom transfer reactions. These results demonstrate that the local coordination environment can be rationally changed to enhance reactivity of MnIII-hydroxo and MnIV-oxo adducts.

Formation and kinetic studies of manganese(IV)-oxo porphyrins: Oxygen atom transfer mechanism of sulfide oxidations

Visible light irradiation of photo-labile porphyrin-manganese(III) chlorates or bromates (2) produced manganese(IV)-oxo porphyrins [MnIV(Por)(O)] (Por = porphyrin) (3) in three porphyrin ligands. The same oxo species 3 were also formed by chemical oxidation of the corresponding manganese(III) precursors (1) with iodobenzene diacetate, i.e. PhI(OAc)2. The systems under study include 5,10,15,20-tetra(pentafluorophenyl)porphyrin‑manganese(IV)-oxo (3a), 5,10,15,20-tetra(2,6-difluorophenyl)porphyrin‑manganese(IV)-oxo (3b), and 5,10,15,20-tetramesitylporphyrin‑manganese(IV)-oxo (3c). As expected, complexes 3 reacted with thioanisoles to produce the corresponding sulfoxides and over-oxidized sulfones. The kinetics of oxygen atom transfer (OAT) reactions of these generated 3 with aryl sulfides were studied in CH3CN solutions. Second-order rate constants for sulfide oxidation reactions are comparable to those of alkene epoxidations and activated CH bond oxidations by the same oxo species 3. For a given substrate, the reactivity order for the manganese(IV)-oxo species was 3a > 3b > 3c, consistent with expectations on the basis of the electron-withdrawing capacity of the porphyrin macrocycles. Free-energy Hammett analyses gave near-linear correlations with σ values, indicating no significant positive charge developed at the sulfur during the oxidation process. The mechanistic results strongly suggest [MnIV(Por)(O)] reacts as a direct OAT agent towards sulfide substrates through a manganese(II) intermediate that was detected in this work. However, an alternative pathway that involves a disproportionation of 3 to form a higher oxidized manganese(V)-oxo species may be significant when less reactive substrates are present. The competition product studies with the Hammett correlation plot confirmed that the observed manganese(IV)-oxo species is not the true oxidant for the sulfide oxidations catalyzed by manganese(III) porphyrins with PhI(OAc)2.

A Reversible Electron Relay to Exclude Sacrificial Electron Donors in the Photocatalytic Oxygen Atom Transfer Reaction with O2 in Water

AbstractUsing light energy and O2 for the direct chemical oxidation of organic substrates is a major challenge. A limitation is the use of sacrificial electron donors to activate O2 by reductive quenching of the photosensitizer, generating undesirable side products. A reversible electron acceptor, methyl viologen, can act as electron shuttle to oxidatively quench the photosensitizer, [Ru(bpy)3]2+, generating the highly oxidized chromophore and the powerful reductant methyl‐viologen radical MV+.. MV+. can then reduce an iron(III) catalyst to the iron(II) form and concomitantly O2 to O2.− in an aqueous medium to generate an active iron(III)‐(hydro)peroxo species. The oxidized photosensitizer is reset to its ground state by oxidizing an alkene substrate to an alkenyl radical cation. Closing the loop, the reaction of the iron reactive intermediate with the substrate or its radical cation leads to the formation of two oxygenated compounds, the diol and the aldehyde following two different pathways.

A Reversible Electron Relay to Exclude Sacrificial Electron Donors in the Photocatalytic Oxygen Atom Transfer Reaction with O2 in Water.

Using light energy and O2 for the direct chemical oxidation of organic substrates is a major challenge. A limitation is the use of sacrificial electron donors to activate O2 by reductive quenching of the photosensitizer, generating undesirable side products. A reversible electron acceptor, methyl viologen, can act as electron shuttle to oxidatively quench the photosensitizer, [Ru(bpy)3 ]2+ , generating the highly oxidized chromophore and the powerful reductant methyl-viologen radical MV+. . MV+. can then reduce an iron(III) catalyst to the iron(II) form and concomitantly O2 to O2.- in an aqueous medium to generate an active iron(III)-(hydro)peroxo species. The oxidized photosensitizer is reset to its ground state by oxidizing an alkene substrate to an alkenyl radical cation. Closing the loop, the reaction of the iron reactive intermediate with the substrate or its radical cation leads to the formation of two oxygenated compounds, the diol and the aldehyde following two different pathways.

Cross-linked poly(4-vinylpyridine-N-oxide) as a polymer-supported oxygen atom transfer reagent

Oxygen atom transfer (OAT) reagents are common in biological and industrial oxidation reactions. While many heterogeneous catalysts have been utilized in OAT reactions, heterogeneous OAT reagents have not been explored. Here, cross-linked poly(4-vinylpyridine-N-oxide), called x-PVP-N-oxide, was tested as a heterogeneous OAT reagent and its oxidation chemistry compared to its molecular counterpart, pyridine-N-oxide. The insoluble oxidant x-PVP-N-oxide demonstrated comparable reactivity to pyridine-N-oxide in direct oxidation reactions of phosphines and phosphites in acetonitrile, but x-PVP-N-oxide did not react in other solvents. The polymer backbone of x-PVP-N-oxide, however, allowed for easy filtering and recycling in sequential oxidation reactions. In addition, x-PVP-N-oxide was tested as the stoichiometric oxidant in a copper-catalyzed OAT reaction to α-diazo-benzeneacetic acid methyl ester. The heterogeneous oxidant was much less reactive than pyridine-N-oxide, indicating that interaction with the metal catalyst was challenging. These results demonstrated a proof-of-concept that recyclable, polymer-supported OAT reagents could be a viable OAT reagents in direct oxidation reactions without metal catalysts.

Dimethylphenylphosphine oxide coordinated trivalent rhenium featuring pyridylbenzazole chelation: Oxygen atom transfer kinetics, isomer preference, metal oxidation and computational analysis

The present work embodies outward oxygen atom transfer (OAT) reaction from pyridylbenzoxazole (L1) chelated oxorhenium(V) complex of type [ReOCl3(L1)], 1 towards oxophilic PMe2Ph resulting in the formation of reduced Re(III) derivative [Re(OPMe2Ph)Cl3(L1)], 4 containing oxidised phosphine trans to azole nitrogen. Another possible isomer (4a) having oxidised phosphine trans to pyridyl nitrogen has not been isolated. Structural elucidation of 4 reveals meridional disposition of three Cl atoms around the metal featuring distorted octahedral geometry. DFT analysis of isomers of type [Re(OPMe2Ph)Cl3(L1)] (4 and 4a) confirms isoenergetic nature in gas phase, however, stereospecific binding of L1 to Re(III) centre affording 4 as exclusive product is attributed to crystal packing force. 1 → 4 redox transformation has also been attended with the change in singlet → triplet spin state of the metal. Triplet state of 4 is more stable by ca. 15 kcal mol−1 compared to the singlet state in gas phase. Two electron paramagnetic nature (2.13 μB) in solid state affirms triplet ground state characterised with strong orbital coupling. Kinetic data for 1 → 4 transformation suggests associative pathway (ΔS≠ = ∼−36 J/K/mol) with no major structural reorganisation in the primary coordination sphere of the substrate and the reagent (ΔH≠ = ∼+10 kcal mol−1) to reach the transition state. Pyridylbenzthiazole (L2) chelated oxorhenium(V) complex of type [ReOCl3(L2)], 1b also reacts with PMe2Ph in a same manner to from [Re(OPMe2Ph)Cl3(L2)], 4b where the pyridyl nitrogen lies trans to phosphineoxide moiety unlike 4. The second order rate constants of OAT to PMe2Ph for [ReOCl3(L1)] are nearly two times higher than that of [ReOCl3(L2)] at all recorded temperatures due to difference in azole heteroatom electronegativity, ReVO bond length and ReVI/ReV reduction potential.