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It is well-known that the equilibrium isotope fractionation of mercury (Hg) includes classical mass-dependent fractionations (MDFs) and nuclear volume effect (NVE) induced mass-independent fractionations (MIFs). However, the effect of the NVE on these kinetic processes is not known. The total fractionations (MDFs + NVE-induced MIFs) of several representative Hg-incorporated substances were selected and calculated with ab initio calculations in this work for both equilibrium and kinetic processes. NVE-induced MIFs were calculated with scaled contact electron densities at the nucleus through systematic evaluations of their accuracy and errors using the Gaussian09 and DIRAC19 packages (named the electron density scaling method). Additionally, the NVE-induced kinetic isotope effect (KIE) of Hg isotopes are also calculated with this method for several representative Hg oxidation reactions by chlorine species. Total KIEs for 202Hg/198Hg ranging from − 2.27‰ to 0.96‰ are obtained. Three anomalous 202Hg-enriched KIEs (δ202Hg/198Hg = 0.83‰, 0.94‰, and 0.96‰,) caused by the NVE are observed, which are quite different from the classical view (i.e., light isotopes react faster than the heavy ones). The electron density scaling method we developed in this study can provide an easier way to calculate the NVE-induced KIEs for heavy isotopes and serve to better understand the fractionation mechanisms of mercury isotope systems. [ABSTRACT FROM AUTHOR]

We report a comprehensive kinetic and product study of the oxidation of mandelic acid (MA) by permanganate in the pH range of 1–13, including a full account of total oxidizing equivalents (five and three-electron change in acidic and basic media, respectively). In the entire pH range, the reaction shows a primary kinetic deuterium isotope effect (kH/kD ≥8–9), indicating rate-limiting hydride transfer. The deuterium label in α-deutero-mandelic acid is retained in benzaldehyde. Benzaldehyde (BZ) is formed in post-rate limiting steps due to reactions involving manganese intermediates. In alkaline pH (≥13), in the presence of barium acetate, Mn(VI) is removed as insoluble blue barium manganate; the stoichiometry of the first step of reduction was found to be: MA + 2Mn(VII) → PGA + 2Mn(VI). Manganate, MnO42−, is directly reduced to MnO2 giving an additional mole of phenylglyoxylic acid (PGA). The experimentally observed ratio of benzaldehyde to phenylglyoxylic (BZ/PGA) provides a basis for discrimination between mechanistic choices that include direct reduction of Mn(V) to Mn(III) (in an acidic medium), disproportionation to Mn(IV) and Mn(VI) or oxidation to Mn(VI) by a second mole of permanganate. Interestingly, at pH 4, a stoichiometric, soluble Mn(IV) is observed for the first time for hydroxy-acid oxidation, reminiscent of the Guyard reaction. Because of the widespread use of permanganate as an environmentally green oxidant, results from mandelic acid oxidation have implications for the remediation of dissolved organic matter (DOM) including hydrocarbons and nitroaromatics in waste and groundwater. [ABSTRACT FROM AUTHOR]

An efficient oxidant, isoquinolinium dichromate (IQDC), was used to assess the oxidation kinetics of several secondary alcohols in a dimethylformamide medium. The reaction order for IQDC was found to be first order, whereas, for alcohols, it was less than two. The reaction was conducted at various temperatures, and the parameters for the development and disintegration of the alcohol-IQDC complexes were computed. The kinetic isotopic effect on the reaction rates was studied at various temperatures and kH/kD = 5.43 at 25°C suggests the primary kinetic isotope effect. [H+] helps to catalyze the oxidation process. Multiparametric Swain's equation is used in 19 distinct solvents to assess the effect of the solvent. It has been proposed a mechanism for the oxidation process that is compatible with rate law. [ABSTRACT FROM AUTHOR]

Elucidating the reaction mechanism in heterogeneous catalysis is critically important for catalyst development, yet remains challenging because of the often unclear nature of the active sites. Using a molecularly defined copper single‐atom catalyst supported by a UiO‐66 metal–organic framework (Cu/UiO‐66) allows a detailed mechanistic elucidation of the CO oxidation reaction. Based on a combination of in situ/operando spectroscopies, kinetic measurements including kinetic isotope effects, and density‐functional‐theory‐based calculations, we identified the active site, reaction intermediates, and transition states of the dominant reaction cycle as well as the changes in oxidation/spin state during reaction. The reaction involves the continuous reactive dissociation of adsorbed O2, by reaction of O2,ad with COad, leading to the formation of an O atom connecting the Cu center with a neighboring Zr4+ ion as the rate limiting step. This is removed in a second activated step. [ABSTRACT FROM AUTHOR]

Highly oxygenated organic molecules (HOMs) from α -pinene ozonolysis have been shown to be significant contributors to secondary organic aerosol (SOA), yet our mechanistic understanding of how the peroxy-radical-driven autoxidation leads to their formation in this system is still limited. The involved isomerisation reactions such as H-atom abstractions followed by O2 additions can take place on sub-second timescales in short-lived intermediates, making the process challenging to study. Similarly, while the end-products and sometimes radical intermediates can be observed using mass spectrometry, their structures remain elusive. Therefore, we propose a method utilising selective deuterations for unveiling the mechanisms of autoxidation, where the HOM products can be used to infer which C atoms have taken part in the isomerisation reactions. This relies on the fact that if a C-D bond is broken due to an abstraction by a peroxy group forming a -OOD hydroperoxide, the D atom will become labile and able to be exchanged with a hydrogen atom in water vapour (H2O), effectively leading to loss of the D atom from the molecule. In this study, we test the applicability of this method using three differently deuterated versions of α -pinene with the newly developed chemical ionisation Orbitrap (CI-Orbitrap) mass spectrometer to inspect the oxidation products. The high mass-resolving power of the Orbitrap is critical, as it allows the unambiguous separation of molecules with a D atom (mD=2.0141) from those with two H atoms (mH2=2.0157). We found that the method worked well, and we could deduce that two of the three tested compounds had lost D atoms during oxidation, suggesting that those deuterated positions were actively involved in the autoxidation process. Surprisingly, the deuterations were not observed to decrease HOM molar yields, as would have been expected due to kinetic isotope effects. This may be an indication that the relevant H (or D) abstractions were fast enough that no competing pathways were of relevance despite slower abstraction rates of the D atom. We show that selective deuteration can be a very useful method for studying autoxidation on a molecular level and likely is not limited to the system of α -pinene ozonolysis tested here. [ABSTRACT FROM AUTHOR]

The mechanism and the reactive species involved in the oxidation of alkenes, and alcohols with H2O2, catalysed by an in situ prepared mixture of a MnII salt, pyridine‐2‐carboxylic acid and a ketone is elucidated using substrate competition experiments, kinetic isotope effect (KIE) measurements, and atom tracking with 18O labelling. The data indicate that a single reactive species engages in the oxidation of both alkenes and alcohols. The primary KIE in the oxidation of benzyl alcohols is ca. 3.5 and shows the reactive species to be selective despite a zero order dependence on substrate concentration, and the high turnover frequencies (up to 30 s−1) observed. Selective 18O labelling identifies the origin of the oxygen atoms transferred to the substrate during oxidation, and is consistent with a highly reactive, e. g., [MnV(O)(OH)] or [MnV(O)2], species rather than an alkylperoxy or hydroperoxy species. [ABSTRACT FROM AUTHOR]

A few substituted mandelic acids are oxidized by one of the halochromates 2-picolinium chlorochromate (PiCC) under the influence of dimethyl-sulphoxide (DMSO) as a solvent medium, leading to the form of some oxo-acids. It was observed that this reaction is found to be of unit order with oxidant and less than unity with the reactant. It was also observed that the reaction is not proceeding through the free radical mechanism. A considerable kinetic isotope effect was observed in the oxidation of the deuteriated form of mandalic acid (DMA). The solvent isotope effect was also absent in this reaction. The hydrogen ion effect was observed as the rate is increasing with an increase in the acidity of the medium. To ascertain the importance of solvent media, the reaction of the acids is studied in nineteen different solvent media, which was analyzed by the multipara metric LSER model of Taft and solvating power model of Swain. We are proposing the transfer of hydride ions from acid to the oxidant as its mechanism. [ABSTRACT FROM AUTHOR]

Cytochrome P450 monooxygenase enzymes are versatile catalysts, which have been adapted for multiple applications in chemical synthesis. Mutation of a highly conserved active site threonine to a glutamate can convert these enzymes into peroxygenases that utilise hydrogen peroxide (H2O2). Here, we use the T252E‐CYP199A4 variant to study peroxide‐driven oxidation activity by using H2O2 and urea‐hydrogen peroxide (UHP). We demonstrate that the T252E variant has a higher stability to H2O2 in the presence of substrate that can undergo carbon‐hydrogen abstraction. This peroxygenase variant could efficiently catalyse O‐demethylation and an enantioselective epoxidation reaction (94 % ee). Neither the monooxygenase nor peroxygenase pathways of the P450 demonstrated a significant kinetic isotope effect (KIE) for the oxidation of deuterated substrates. These new peroxygenase variants offer the possibility of simpler cytochrome P450 systems for selective oxidations. To demonstrate this, a light driven H2O2 generating system was used to support efficient product formation with this peroxygenase enzyme. [ABSTRACT FROM AUTHOR]

We report the catalytic oxidation of organic substrates by a cobalt complex, [CoIII(TAML)]− (TAML = tetra‐amido macrocyclic ligand), and meta‐chloroperbenzoic acid (m‐CPBA) under the mild reaction conditions and the identification of a cobalt(IV)–oxo intermediate, [CoIV(TAML)(O)]2−, as an active oxidant in the catalytic oxidation reactions. In the hydroxylation of cyclohexane, cyclohexanol is the sole product and the hydroxylation reaction occurs via an oxygen non‐rebound mechanism. Kinetic isotope effect value of 3.1, obtained in the oxidation of cyclohexane‐h12/d12, indicates that a hydrogen atom abstraction from cyclohexane by the cobalt(IV)–oxo complex is the rate‐determining step. The catalytic oxidation of cyclohexene prefers the C═C bond epoxidation to the C–H bond activation, yielding cyclohexene oxide as a sole product. In the epoxidation of cis‐ and trans‐stilbenes, cis‐ and trans‐stilbene oxides, respectively, are yielded stereoselectively. The present study reports a highly selective catalytic oxidation of organic substrates by a cobalt complex and m‐CPBA and the involvement of a cobalt(IV)–oxo intermediate as an active oxidant in the catalytic oxidation reactions. [ABSTRACT FROM AUTHOR]

Four cobalt(II)–carboxylate complexes [(6-Me3-TPA)CoII(benzoate)](BPh4) (1), [(6-Me3-TPA)CoII(benzilate)](ClO4) (2), [(6-Me3-TPA)CoII(mandelate)](BPh4) (3), and [(6-Me3-TPA)CoII(MPA)](BPh4) (4) (HMPA = 2-methoxy-2-phenylacetic acid) of the 6-Me3-TPA (tris((6-methylpyridin-2-yl)methyl)amine) ligand were isolated to investigate their ability in H2O2-dependent selective oxygenation of C–H and C＝C bonds. All six-coordinate complexes contain a high-spin cobalt(II) center. While the cobalt(II) complexes are inert toward dioxygen, each of these complexes reacts readily with hydrogen peroxide to form a diamagnetic cobalt(III) species, which decays with time leading to the oxidation of the methyl groups on the pyridine rings of the supporting ligand. Intramolecular ligand oxidation by the cobalt-based oxidant is partially inhibited in the presence of external substrates, and the substrates are converted to their corresponding oxidized products. Kinetic studies and labelling experiments indicate the involvement of a metal-based oxidant in affecting the chemo- and stereo-selective catalytic oxygenation of aliphatic C–H bonds and epoxidation of alkenes. An electrophilic cobalt–oxygen species that exhibits a kinetic isotope effect (KIE) value of 5.3 in toluene oxidation by 1 is proposed as the active oxidant. Among the complexes, the cobalt(II)-benzoate (1) and cobalt(II)-MPA (4) complexes display better catalytic activity compared to their α-hydroxy analogues (2 and 3). Catalytic studies with the cobalt(II)–acetonitrile complex [(6-Me3-TPA)CoII(CH3CN)2](ClO4)2 (5) in the presence and absence of externally added benzoate support the role of the carboxylate co-ligand in oxidation reactions. The proposed catalytic reaction involves a carboxylate-bridged dicobalt complex in the activation of H2O2 followed by the oxidation of substrates by a metal-based oxidant. [ABSTRACT FROM AUTHOR]

The oxidation of some vicinal, non-vicinal organic diols and one of their monoethers by isoquinolinium dichromate (IQDC), in aqueous acetic acid medium, in the presence of toluene p-sulphonic acid (TsOH), resulted in the formation of corresponding hydroxy carbonyl as a main product of the oxidation. The reaction is first-order with respect to IQDC and [H+]. The reaction is catalysed by [H+]. The Michaelis-Menten type kinetics is observed with respect to diols. The oxidation of [1,1,2,2-²H4] ethanediol exhibited primary kinetic isotope effect (kH/kD = 6.14 at 293 K). The reaction has been studied in different percentage of acetic acid-water mixture. [ABSTRACT FROM AUTHOR]

Oxidation of secondary alcohols is an important part of synthetic organic chemistry. Various studies are carried out at different reaction conditions to determine the best mechanistic pathways. In our study, oxidation of different secondary alcohols was done by using Benzimidazolium Fluorochromate in dimethyl sulphoxide, which is a non-aqueous solvent. Oxidation resulted in the formation of ketonic compounds. The reaction showed first order kinetics both in BIFC and in the alcohols. Hydrogen ions were used to catalyze the reaction. We selected four different temperatures to carry out our study. The correlation within the activation parameters like enthalpies and entropies was in accordance with the Exner's criterion. The deuterated benzhydrol (PhCDOHPh) oxidation exhibited an important primary kinetic isotopic effect (kH/kD = 5.76) at 298 K. The solvent effect was studied using the multiparametric equations of Taft and Swain. There was no effect of addition of acrylonitrile on the oxidation rate. The mechanism involved sigmatropic rearrangement with the transfer of hydrogen ion taking place from alcohol to the oxidant via a cyclic chromate ester formation. [ABSTRACT FROM AUTHOR]

S=2 oxoiron(IV) species act as reactive intermediates in the catalytic cycle of nonheme iron oxygenases. The few available synthetic S=2 FeIV=O complexes known to date are often limited to trigonal bipyramidal and very rarely to octahedral geometries. Herein we describe the generation and characterization of an S=2 pseudotetrahedral FeIV=O complex 2 supported by the sterically demanding 1,4,7‐tri‐tert‐butyl‐1,4,7‐triazacyclononane ligand. Complex 2 is a very potent oxidant in hydrogen atom abstraction (HAA) reactions with large non‐classical deuterium kinetic isotope effects, suggesting hydrogen tunneling contributions. For sterically encumbered substrates, direct HAA is impeded and an alternative oxidative asynchronous proton‐coupled electron transfer mechanism prevails, which is unique within the nonheme oxoiron community. The high reactivity and the similar spectroscopic parameters make 2 one of the best electronic and functional models for a biological oxoiron(IV) intermediate of taurine dioxygenase (TauD‐J). [ABSTRACT FROM AUTHOR]

Ferrate(VI), [FeO4]2−, is a very powerful oxidant that can oxidize a wide variety of inorganic and organic compounds. However, the mechanisms of many of these oxidation reactions have not been studied in detail. In this work, we have investigated the kinetics and mechanism of the oxidation of 4-alkylbenzenesulfonates by ferrate in aqueous solutions at pH 7.45–9.63 by UV/Vis spectrophotometry. The reactions are first order with respect to both [ferrate] and [4-alkylbenzenesulfonate]. The second-order rate constants for the oxidation of 4-isopropylbenzenesulfonate by ferrate at 25 °C and I = 0.3 M are found to be (5.86 ± 0.08) × 10−1 M−1 s−1 and (4.11 ± 1.50) × 10−3 M−1 s−1 for [Fe(O)3(OH)]− and [FeO4]2−, respectively, indicating that [Fe(O)3(OH)]− is two orders of magnitude more reactive than [FeO4]2− and is the predominant oxidant in neutral and slightly alkaline solutions. This is further supported by the effect of the ionic strength on the rate constant. No solvent kinetic isotope effect (KIE) was found but a moderate primary KIE = 1.6 ± 0.1 was observed in the oxidation of 4-ethylbenzenesulfonate and 4-ethylbenzenesulfonate-d9. Alkyl radicals were trapped by CBrCl3 in the oxidation of alkylarenes by ferrate. Combined with DFT calculations, a hydrogen atom transfer (HAT) mechanism was proposed for the reactions between [Fe(O)3(OH)]− and 4-alkylbenzenesulfonates. [ABSTRACT FROM AUTHOR]

We report detailed mechanistic investigations of an iron‐based catalyst system, which allows the α‐C−H oxidation of a wide variety of amines. In contrast to other catalysts that effect α‐C−H oxidations of tertiary amines, the system under investigation exclusively employs peroxy esters as oxidants. More common oxidants (e. g. tBuOOH) previously reported to affect amine oxidations via free radical pathways do not provide amine α‐C−H oxidation products in combination with the described catalyst system. The investigations described herein employ initial rate kinetics, kinetic profiling, DFT calculations as well as Eyring, kinetic isotope effect, Hammett, ligand coordination, and EPR studies to shed light on the Fe catalyst system. The obtained data suggest that the catalytic mechanism proceeds through C−H abstraction at a coordinated substrate molecule. This rate‐determining step occurs either through an Fe(IV) oxo pathway or a 2‐electron pathway at an Fe(II) intermediate with bound oxidant. DFT calculations indicate that the Fe(IV) oxo mechanism will be the preferred route of these two possibilities. We further show via kinetic profiling and EPR studies that catalyst activation follows a radical pathway, which is initiated by hydrolysis of PhCO3tBu to tBuOOH. Overall, the obtained mechanistic data support a non‐classical, Fe catalyzed pathway that requires substrate binding, inducing selectivity for α‐C−H functionalization. [ABSTRACT FROM AUTHOR]

In the search for highly reactive oxidants we have identified high‐valent metal–fluorides as a potential potent oxidant. The high‐valent Ni–F complex [NiIII(F)(L)] (2, L=N,N′‐(2,6‐dimethylphenyl)‐2,6‐pyridinedicarboxamidate) was prepared from [NiII(F)(L)]− (1) by oxidation with selectfluor. Complexes 1 and 2 were characterized by using 1H/19F NMR, UV‐vis, and EPR spectroscopies, mass spectrometry, and X‐ray crystallography. Complex 2 was found to be a highly reactive oxidant in the oxidation of hydrocarbons. Kinetic data and products analysis demonstrate a hydrogen atom transfer mechanism of oxidation. The rate constant determined for the oxidation of 9,10‐dihydroanthracene (k2=29 m−1 s−1) compared favorably with the most reactive high‐valent metallo‐oxidants. Complex 2 displayed reaction rates 2000–4500‐fold enhanced with respect to [NiIII(Cl)(L)] and also displayed high kinetic isotope effect values. Oxidative hydrocarbon and phosphine fluorination was achieved. Our results provide an interesting direction in designing catalysts for hydrocarbon oxidation and fluorination [ABSTRACT FROM AUTHOR]

The kinetics of oxidation of 16 carboxylic acid esters of the adamantane series in the system H2SO4–HNO3 have been studied, and the effective rate constants have been determined. The reaction is described by the pseudo-first-order kinetic equation. The primary kinetic isotope effect has been estimated at 2.9±0.3. The rate-determining step of the oxidation process is cleavage of the adamantane C–H bond. The presence of an ethyl group at the bridgehead position increases the reactivity of adamantane substrates toward oxidation, whereas methyl, ethoxycarbonyl, and ethoxycarbonylmethyl groups reduce the reactivity. [ABSTRACT FROM AUTHOR]

Thermal rate constants for the title reaction are calculated by using the quantum instanton approximation within the full dimensional Cartesian coordinates. The results reveal that the quantum effect is remarkable for the reaction at both low and high temperatures, and the obtained rates are in good agreement with experimental measurements at high temperatures. Compared to the harmonic approximation, the torsional anharmonic effect of the internal rotation has a little influence on the rates at low temperatures, however, it enhances the rate by about 20% at 1000 K. In addition, the free energy barriers for the isotopic reactions and the temperature dependence of kinetic isotope effects are also investigated. Generally speaking, for the title reaction, the replacement of OH with OD will reduce the free energy barrier, while substituting D for H (connected to C) will increase the free energy barrier. [ABSTRACT FROM AUTHOR]

A highly efficient palladium‐catalyzed functionalization of allenylsilanes to give regio‐ and stereodefined fully‐substituted alkenylsilanes has been developed. This oxidative coupling reaction showed good functional group compatibility with exclusive regio‐ and stereoselectivity. The pending olefin on the silyl group was shown to be an indispensable element for the initial allenic C(sp3)−H bond cleavage, and performs as the directing group to control the overall selectivity. The addition of substoichiometric amounts of Et3N was found to increase the reaction rate leading to a higher reaction yield. The reaction can be easily scaled up and applied for the late‐stage functionalization of natural products and pharmaceutical compounds, including amino acids and steroid derivatives. The newly introduced functional groups include aryl, alkynyl, and boryl groups. The highly strained four‐membered ring, silacyclobutene was obtained when B2pin2 was employed as the coupling partner. Mechanistic studies, including kinetic isotope effects, showed that the allenic C(sp3)−H bond cleavage is the rate‐limiting step. [ABSTRACT FROM AUTHOR]

Highlights • Water-assisted O 2 activation to form active OH∗ intermediates is in equilibrium. • The OH∗ species-assisted C H bond cleavage of glycerol is the possible RDS. • The oxidation of the primary and secondary hydroxyl groups have similar mechanism. Abstract Fundamental understanding of heterogeneously catalyzed aerobic, base-free oxidation of glycerol is highly desirable for sustainable and highly efficient chemical synthesis. In this work, Pt/CNTs catalyzed base-free oxidation of glycerol is investigated by combining experiments, DFT calculations and kinetics analysis. The solvent effects and kinetic isotope effects measurements as well as DFT calculations reveal that water-assisted O 2 activation to form active OH∗ intermediates is not in rate-determining step (RDS) but in equilibrium. Then, Langmuir-Hinshelwood kinetic models are developed and fitted with the experimental data. The OH∗-assisted C H bond cleavage of glycerol is discriminated as the RDS for the two parallel oxidation pathways of glycerol via primary or secondary hydroxyl groups, where the former case exhibits lower activation energy. The insights revealed here could guide the design and optimization of this catalytic process to generate the targeted products. [ABSTRACT FROM AUTHOR]

Mononuclear nonheme MnIII‐peroxo complexes are important intermediates in biology, and take part in oxygen activation by photosystem II. Herein, we present work on two isomeric biomimetic side‐on MnIII‐peroxo intermediates with bispidine ligand system and reactivity patterns with aldehydes. The complexes are characterized with UV/Vis and mass spectrometric techniques and reaction rates with cyclohexane carboxaldehyde (CCA) are measured. The reaction gives an unusual regioselectivity switch from aliphatic to aldehyde hydrogen atom abstraction upon deuteration of the substrate, leading to the corresponding carboxylic acid product for the latter, while the former gives a deformylation reaction. Mechanistic details are established from kinetic isotope effect studies and density functional theory calculations. Thus, replacement of C−H by C−D raises the hydrogen atom abstraction barriers and enables a regioselectivity switch to a competitive pathway that is slightly higher in energy. [ABSTRACT FROM AUTHOR]

Kinetic and mechanistic studies of the oxidation of some α-amino acids by benzimidazolium dichromate (BIDC), in glacial acetic acid, in the presence of toluene psulphonic acid (TsOH), were discussed. The reactions are of first order with respect to BIDC. However, a fractional-order dependence on the concentration of amino acid and a fractional second order dependence on the concentration of hydrogen ions were observed. The Michaelis-Menten type kinetics was suggested with respect to substrate. The oxidation of perdeuterioglycine showed the absence of a kinetic isotope effect. The reactions showed an excellent correlation with the Taft σ* substituent constants, the reaction constant being low negative. The experimental results suggest the formation of an intermediate complex between the protonated forms of amino acid and BIDC both, which decomposes in the rate-determining step forming iminium cation, followed by a fast step to yield the products. [ABSTRACT FROM AUTHOR]

Understanding the reaction mechanism underlying the functionalization of C−H bonds by an enzymatic process is one of the most challenging issues in catalysis. Here, combined approaches using density functional theory (DFT) analysis and transient kinetics were employed to investigate the reaction mechanism of C−H bond oxidation in d‐glucose, catalyzed by the enzyme pyranose 2‐oxidase (P2O). Unlike the mechanisms that have been conventionally proposed, our findings show that the first step of the C−H bond oxidation reaction is a hydride transfer from the C2 position of d‐glucose to N5 of the flavin to generate a protonated ketone sugar intermediate. The proton is then transferred from the protonated ketone intermediate to a conserved residue, His548. The results show for the first time how specific interactions around the sugar binding site promote the hydride transfer and formation of the protonated ketone intermediate. The DFT results are also consistent with experimental results including the enthalpy of activation obtained from Eyring plots, as well as the results of kinetic isotope effect and site‐directed mutagenesis studies. The mechanistic model obtained from this work may also be relevant to other reactions of various flavoenzyme oxidases that are generally used as biocatalysts in biotechnology applications. Which is the first step on the path? Combined approaches using density functional theory (DFT) analysis and transient kinetics were employed to investigate the reaction mechanism of C−H bond oxidation in d‐glucose, catalyzed by the enzyme pyranose 2‐oxidase (P2O) (see figure). [ABSTRACT FROM AUTHOR]

The alpha-hydroxy acids (AHA) are naturally occurring organic acid. The oxidation of mandelic acid by pyridiniumdichromate has been studied spectrophotometrically in presence of p-toluenesulphonic acid (PTSA) in dimethylsulphoxide (DMSO) as a solvent. The rate of reaction decrease & with an increase in polarity of solvent indicating an ion-dipole interaction in the slow step. The reaction exhibit no primary kinetic isotope effect. The activation parameters have been evaluated. [ABSTRACT FROM AUTHOR]

The oxidation of propane has been studied over silica-supported vanadium oxide and polycrystalline, bulk MoVTeNb oxide with M1 structure. Temperature-programmed reaction experiments were performed, and the reactivity of propane molecules labeled with deuterium and 13C, respectively, was analyzed under steady-state conditions. The measurement of kinetic isotope effects reveals fundamental differences in the activation of propane over the two catalysts. The reaction network of consecutive and parallel reactions of the formed propylene is comparable. However, oxygen insertion into the CHO group of acrolein under formation of acrylic acid is faster over M1 than oxidation at the CH2 group and decarbonylation to acetaldehyde. In contrast, the latter process is preferred over silica-supported vanadium oxide resulting in lower selectivity to unsaturated oxygenates. [ABSTRACT FROM AUTHOR]

In aqueous TiO2 photocatalytic hydroxylation of weakly polar aromatics, a series of inverse H/D KIEs of 0.7-0.8 were observed, which is different than the normal H/D kinetic isotope effects (KIEs) usually observed for polar aromatics. This result indicated that the oxidation started by photo-induced hvb+ can be the rate-determining step. [ABSTRACT FROM AUTHOR]

Engineered bacterial cytochrome P450s are noted for their ability in the oxidation of inert small alkanes. Cytochrome P450 BM3 L188P A328F ( BM3 PF) and A74E L188P A328F ( BM3 EPF) variants are able to efficiently oxidize n-butane to 2-butanol. Esterification of the 2-butanol derived from this reaction mediated by the aforementioned two mutants gives diastereomeric excesses ( de) of −56±1 and −52±1 %, respectively, with the preference for the oxidation occurring at the C−H S bond. When tailored (2 R,3 R)- and (2 S,3 S)-[2-2H1,3-2H1]butane probes are employed as substrates for both variants, the obtained de values from (2 R,3 R)-[2-2H1,3-2H1]butane are −93 and −92 % for BM3 PF and EPF, respectively; whereas the obtained de values from (2 S,3 S)-[2-2H1,3-2H1]butane are 52 and 56 % in the BM3 PF and EPF systems, respectively. The kinetic isotope effects (KIEs) for the oxidation of (2 R,3 R)-[2-2H1,3-2H1]butane are 7.3 and 7.8 in BM3 PF and EPF, respectively; whereas KIEs for (2 S,3 S)-[2-2H1,3-2H1]butanes are 18 and 25 in BM3 PF and EPF, respectively. The discrepancy in KIEs obtained from the two substrates supports the two-state reactivity (TSR) that is proposed for alkane oxidation in cytochrome P450 systems. Moreover, for the first time, experimental evidence for tunneling in the oxidation mediated by P450 is given through the oxidation of the C−H R bond in (2 S,3 S)-[2-2H1,3-2H1]butane. [ABSTRACT FROM AUTHOR]

A series of experiments were carried out to determine the clumped ( 13 CH 3 D) methane kinetic isotope effects during oxidation of methane by OH and Cl radicals, the major sink reactions for atmospheric methane. Experiments were performed in a 100 L quartz photochemical reactor, in which OH was produced from the reaction of O( 1 D) (from O 3 photolysis) with H 2 O, and Cl was from photolysis of Cl 2 . Samples were taken from the reaction cell and analyzed for methane ( 12 CH 4 , 12 CH 3 D, 13 CH 4 , 13 CH 3 D) isotopologue ratios using tunable infrared laser direct absorption spectroscopy. Measured kinetic isotope effects for singly substituted species were consistent with previous experimental studies. For doubly substituted methane, 13 CH 3 D, the observed kinetic isotope effects closely follow the product of the kinetic isotope effects for the 13 C and deuterium substituted species (i.e., 13,2 KIE = 13 KIE × 2 KIE). The deviation from this relationship is 0.3‰ ± 1.2‰ and 3.5‰ ± 0.7‰ for OH and Cl oxidation, respectively. This is consistent with model calculations performed using quantum chemistry and transition state theory. The OH and Cl reactions enrich the residual methane in the clumped isotopologue in open system reactions. In a closed system, however, this effect is overtaken by the large D/H isotope effect, which causes the residual methane to become anti-clumped relative to the initial methane. Based on these results, we demonstrate that oxidation of methane by OH, the predominant oxidant for tropospheric methane, will only have a minor (∼0.3‰) impact on the clumped isotope signature (Δ 13 CH 3 D, measured as a deviation from a stochastic distribution of isotopes) of tropospheric methane. This paper shows that Δ 13 CH 3 D will provide constraints on methane source strengths, and predicts that Δ 12 CH 2 D 2 can provide information on methane sink strengths. [ABSTRACT FROM AUTHOR]

A pair of carbohydrate model compounds, methyl β-d-glucopyranoside (MGP) and a deuterated MGP labeled at each C-H bond, was reacted with active oxygen species (AOS) generated from reactions between O2and a co-existing phenolic compound, 2,4,6-trimethylphenol (TMPh) or 4-hydroxy-3-methoxybenzyl alcohol (vanillyl alcohol, Valc), under O2delignification conditions. The pair were also reacted with O• generated in the alkaline H2O2treatment with the addition of FeCl3. Clear kinetic isotope effects were observed only in the reactions of two pairs using the deuterated MGPs labeled at the anomeric and C-2 positions in this system. These results suggest that at least a specific AOS is generated only in this system and that a major AOS in the O2-Valc system is highly reactive O•. It is also suggested that the AOS specifically generated only in this system is a peroxyl radical derived by the combination reaction between the TMPh phenoxyl radical and O2. [ABSTRACT FROM AUTHOR]

A new μ-oxo diiron(III) complex of the lithium salt of the pyridine-based unsymmetrical ligand 3-[(3-{[bis(pyridin-2-ylmethyl)amino]methyl}-2-hydroxy-5-methylbenzyl)(pyridin-2-ylmethyl)amino]propanoate (LiDPCPMPP), [Fe2(μ-O)(LiDPCPMPP)2](ClO4)2, has been synthesized and characterized. The ability of the complex to catalyze oxidation of several alkanes and alkenes has been investigated by using CH3COOH/H2O2 (1:1) as an oxidative system. Moderate activity in cyclohexane oxidation (TOF = 33 h-1) and good activity in cyclohexene oxidation (TOF = 72 h-1) were detected. Partial retention of configuration (RC = 53 %) in cis- and trans-1,2-dimethylcyclohexane oxidation, moderate 3°/2° selectivity (4.1) in adamantane oxidation, and the observation of a relatively high kinetic isotope effect for cyclohexane oxidation (KIE = 3.27) suggest partial metal-based oxidation, probably in tandem with free-radical oxidation. Low-temperature UV/Vis spectroscopy and mass spectrometric studies in the rapid positive detection mode indicate the formation of a transient peroxido species, [Fe2(O)(O2)(LiDPCPMPP)2]2+, which might be an intermediate in the metal-based component of the oxidation process. [ABSTRACT FROM AUTHOR]

Soluble methane monooxygenase (sMMO) is an enzyme that converts alkanes to alcohols using a di( μ-oxo)diiron(IV) intermediate Q at the active site. Very large kinetic isotope effects (KIEs) indicative of significant tunneling are observed for the hydrogen transfer (H-transfer) of CH4 and CH3CN; however, a relatively small KIE is observed for CH3NO2. The detailed mechanism of the enzymatic H-transfer responsible for the diverse range of KIEs is not yet fully understood. In this study, variational transition-state theory including the multidimensional tunneling approximation is used to calculate rate constants to predict KIEs based on the quantum-mechanically generated intrinsic reaction coordinates of the H-transfer by the di( μ-oxo)diiron(IV) complex. The results of our study reveal that the role of the di( μ-oxo)diiron(IV) core and the H-transfer mechanism are dependent on the substrate. For CH4, substrate binding induces an electron transfer from the oxygen to one FeIV center, which in turn makes the μ-O ligand more electrophilic and assists the H-transfer by abstracting an electron from the CH σ orbital. For CH3CN, the reduction of FeIV to FeIII occurs gradually with substrate binding and H-transfer. The charge density and electrophilicity of the μ-O ligand hardly change upon substrate binding; however, for CH3NO2, there seems to be no electron movement from μ-O to FeIV during the H-transfer. Thus, the μ-O ligand appears to abstract a proton without an electron from the CH σ orbital. The calculated KIEs for CH4, CH3CN, and CH3NO2 are 24.4, 49.0, and 8.27, respectively, at 293 K, in remarkably good agreement with the experimental values. This study reveals that diverse KIE values originate mainly from tunneling to the same di( μ-oxo)diiron(IV) core for all substrates, and demonstrate that the reaction dynamics are essential for reproducing experimental results and understanding the role of the diiron core for methane oxidation in sMMO. [ABSTRACT FROM AUTHOR]

We report the studies on the mechanism of oxidation of 3′,4′-dihydroxy- l-phenylalanine ( l-DOPA) to neurotoxic dopachrome catalyzed by enzyme horseradish peroxidase (EC 1.11.1.7) using the kinetic (KIE), and solvent (SIE), isotope effect methods. For kinetic studies two specifically deuterated isotopomers: [2′,5′,6′-H]- l -DOPA was synthesized by the acid catalyzed isotopic exchange between native l-DOPA and heavy water, and [5′-H]- l-DOPA was synthesized in two step reaction. The first step involved acid catalyzed isotopic exchange between l-tyrosine and deuterated water and resulting product [3′,5′-H]- l-tyrosine was hydroxylated by enzyme tyrosinase (EC 1.14.18.1). The values of deuterium KIEs and SIE's in the enzymatic oxidation of l-DOPA and its isotopomers are determined using non-competitive spectrophotometric method. The measured values were: KIE on V (1.1 and 2.2) and KIE on V/ K (1.7 and 3.2) for [2′,5′,6′-H]- l-DOPA and [5′-H]- l-DOPA, respectively, while the corresponding values of SIE were: SIE on V (2.1, 2.4, and 2.1) and SIE on V/ K (1.3. 1.6, and 1.1) for l-DOPA, [2′,5′,6′-H]- l-DOPA, and [5′-H]- l-DOPA, respectively. The size of KIE and SIE, typical for secondary isotope effects indicate that both the solvent and presence of deuterium at the 2′-, 5′, and 6′-positions of l-DOPA has the little impact on the enzymatic oxidation of this compound. [ABSTRACT FROM AUTHOR]

Highlights: [•] Carbon chain size influenced the rate and selectivity of α,ω-diol oxidation. [•] The proximity of a second alcohol decreased the rate of terminal alcohol oxidation. [•] Reaction orders for O2 and alcohol were evaluated at different start-up conditions. [•] Kinetic isotope effect verified kinetic relevance of C–H activation. [•] A two-site kinetic model for Pt-catalyzed alcohol oxidation was proposed. [ABSTRACT FROM AUTHOR]

The radical S-adenosyl-L-methionine enzyme DesII from Streptomyces venezuelae is able to oxidize the C3 hydroxyl group of TDP-D-quinovose to the corresponding ketone via an α-hydroxyalkyl radical intermediate. It is unknown whether electron transfer from the radical intermediate precedes or follows its deprotonation, and answering this questionwould offer considerable insight into themechanism by which the small but important class of radical-mediated alcohol dehydrogenases operate. This question can be addressed by measuring steady-state kinetic isotope effects (KIEs); however, their interpretation is obfuscated by the degree to which the steps of interest limit catalysis. To circumvent this problem, we measured the solvent deuterium KIE on the saturating steady-state concentration of the radical intermediate using electron paramagnetic resonance spectroscopy. The resulting value, 0.22±0.03, when combined with the solvent deuterium KIE on the maximum rate of turnover (V) of 1.8±0.2, yielded a KIE of 8±2 on the net rate constant specifically associated with the α-hydroxyalkyl radical intermediate. This result implies that electron transfer from the radical intermediate does not precede deprotonation. Further analysis of these isotope effects, along with the pH dependence of the steady-state kinetic parameters, likewise suggests that DesII must be in the correct protonation state for initial generation of the α-hydroxyalkyl radical. In addition to providing unique mechanistic insights, this work introduces a unique approach to investigating enzymatic reactions using KIEs. [ABSTRACT FROM AUTHOR]

Earlier studies of electron transfer (ET) from the nitrogenase Fe protein to the MoFe protein concluded that the mechanism for ET changed during cooling from 25 to S °C, based on the observation that the rate constant for Fe protein to MoFe protein ET decreases strongly, with a nonlinear Arrhenius plot. They further indicated that the k was reversible, with complete ET at ambient temperature bc with an equilibrium constant near unity at S °C. These studie were conducted with buffers having a strong temperature c-' ,. We have examined the temperature variation in the kinetics of oxidation of the Fe protein by the MoFe protein at a constant pH of 7.4 fixed by the buffer 3-(N-morpholino)propanesulfonic acid (MOPS), which has a very small temperature coefficient. Using MOPS, we also observe temperature-dependent ET rate constants, with nonlinear Arrhenius plots, but we find that ET is gated across the temperature range by a conformational change that involves the binding of numerous water molecules, consistent with an unchanging ET mechanism. Furthermore, there is no solvent kinetic isotope effect throughout the temperature range studied, again consistent with an unchanging mechanism. In addition, the nonlinear Arrhenius plots are explained by the change in heat capacity caused by the binding of waters in an invariant gating ET mechanism. Together, these observations contradict the idea of a change in ET mechanism with cooling. Finally, the extent of ET at constant pH does not change significantly with temperature, in contrast to the previously proposed change in ET equilibrium. [ABSTRACT FROM AUTHOR]

Abstract: Kinetic and isotopic data are used to interpret the mechanistic role of gaseous H2O molecules and of non-reducible (Al2O3) and reducible (TiO2, Fe2O3) supports on CO oxidation turnovers catalyzed by small Au clusters (<5nm). H2O acts as a co-catalyst essential for O2 activation and for catalyst stability in CO oxidation at near-ambient temperatures, but also inhibits rates via competitive adsorption at higher H2O pressures. The effects of CO, O2, and H2O pressures on CO oxidation turnover rates, the absence of 16O2/18O2 and 16O2/H2 18O exchange, and the small H2O/D2O kinetic isotope effects are consistent with quasi-equilibrated molecular adsorption of CO, O2, and H2O on Au clusters with the kinetic relevance of H2O-mediated O2 activation via the formation of hydroperoxy intermediates (*OOH), which account for the remarkable reactivity and H2O effects on Au clusters. These elementary steps proceed on Au clusters without detectable requirements for support interface sites, which are no longer required when H2O is present and mediates O2 activation steps. Rate enhancements by H2O were also observed for CO oxidation on Pt clusters (1.3nm), which is also limited by O2 activation steps, suggesting H2O-aided O2 activation and *OOH species in oxidations involving kinetically-relevant O2 activation. These intermediates have also been proposed to account for the ability of O2/H2O mixtures to act as reactants in alkene epoxidation on Au-based catalysts. [Copyright &y& Elsevier]

Second-order rate constants were determined for the oxidation of 27 alcohols (R1R2CHOH) by a carbocationic oxidizing agent, 9-phenylxanthylium ion, in acetontrile at 60 °C. Alcohols include open-chain alkyl, cycloalkyl, and unsaturated alcohols. Kinetic isotope effects for the reaction of 1-phenylethanol were determined at three H/D positions of the alcohol (KIEα-D = 3.9, KIEβ-D3 = 1.03, KIEOD = 1.10). These KIE results are consistent with those we previously reported for the 2-propanol reaction, suggesting that these reactions follow a hydride-proton sequential transfer mechanism that involves a rate-limiting formation of the α-hydroxy carbocation intermediate. Structure-reactivity relationship for alcohol oxidations was deeply discussed on the basis of the observed structural effects on the formation of the carbocationic transition state (Cδ+OH). Efficiencies of alcohol oxidations are largely dependent upon the alcohol structures. Steric hindrance effect and ring strain relief effect win over the electronic effect in determining the rates of the oxidations of open-chain alkyl and cycloalkyl alcohols. Unhindered secondary alkyl alcohols would be selectively oxidized in the presence of primary and hindered secondary alkyl alcohols. Strained C7C11 cycloalkyl alcohols react faster than cyclohexyl alcohol, whereas the strained C5 and C12 alcohols react slower. Aromatic alcohols would be efficiently and selectively oxidized in the presence of aliphatic alcohols of comparable steric requirements. This structure-reactivity relationship for alcohol oxidations via hydride-transfer mechanism is hoped to provide a useful guidance for the selective oxidation of certain alcohol functional groups in organic synthesis. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Deprotonation of the alkoxysulfonium intermediate has been shown to be rate-determining in the Swern oxidation of benzyl alcohol. Directly following this rate-determining step is the intramolecular syn-β-elimination of the ylide. In the present study, intramolecular 2H kinetic isotope effects (KIEs) are used to gain insight into this syn-β-elimination step. As a result of the stereogenic sulfur center in the ylide intermediate, two diastereomeric transition states (endo-TS1 and exo-TS1) must be assumed to contribute to the intramolecular KIE. The intramolecular 2H KIE determined at -78 °C is 2.82 ± 0.06. Attempts to reproduce this measurement computationally using transition state theory and a Bell tunneling correction yielded a value (1.58) far below that determined experimentally. Computational analysis is complicated by the existence of two distinct transition structures owing to the stereogenic center. Two extremes of Curtin-Hammett kinetics are explored using energies, vibrational frequencies, and moments of inertia from computed transition structures. Neither Curtin-Hammett scenario can reproduce the observed KIE to any acceptable degree of fidelity. Evidence based upon previous kinetics measurements and calculations upon a model system suggests that the stereogenic sulfur center is not likely to undergo inversion to a significant degree at the temperatures at which the Swern oxidation is performed here. Proceeding under the assumption of no stereoinversion at the sulfur center, calculations predict a nearly linear Arrhenius plot for the KIE-even with the inclusion of a one-dimensional tunneling correction. By contrast, the experimentally determined temperature dependence shows a significant concave upward curvature indicative of the influence of tunneling. Notably, KIEs measured in CCl4, CHCl3, CH2Cl2, dichloroethane, and chlorobenzene at -23 °C showed little variance. This finding discounted the possible influence from dynamical effects due to incomplete vibrational relaxation. An ad hoc amplification of the imaginary frequencies corresponding to the first-order saddle points corresponding to endo-TS1 and exo-TS1 allowed us to reproduce the experimental temperature dependence of the KIE using only two adjustable parameters applied to a kinetic scenario that involves four isotopomeric transition states. The cumulative data and computational modeling strongly suggest that, even though the intramolecular 2H KIE observed in these experiments is small, this reaction requires a multidimensional description of the tunneling phenomenon to accurately reproduce experimental trends. [ABSTRACT FROM AUTHOR]

[Display omitted] • Degradation of DEP in the PS/ZVI was studied at different pHs. • Isotope fractionation was used to understand the DEP oxidation pathway. • •OH was found to be the main functioning radical species at pH = 3 and 7. • •OH was the predominant species for PS/ZVI oxidation of DEP at pH = 11. • •OH addition to the DEP ring is assumed to be the main reaction mechanism. This study reports the 13C and 2H isotope fractionation associated with the oxidation of diethyl phthalate (DEP) by persulfate (PS) activated with zero-valent iron (ZVI) using three concentration levels (0.2, 0.5 and 1.0 g L−1) at different pH values, 3, 7 and 11, respectively. The results showed that the degradation of DEP followed a pseudo first-order kinetics. The fastest degradation was found at neutral conditions (pH 7). Similar carbon and hydrogen isotope fractionation (ε C and ε H) was observed during the oxidation of DEP by ZVI activated PS at pH 3, 7 and 11. At ZVI concentration of 0.5 g L−1, the correlation of 13C and 2H fractionation (Λ) were obtained to be 12.7 ± 3.5, 11.1 ± 4.2 and 12.0 ± 2.9 at pH 3, 7 and 11, respectively. The concentration of ZVI has no effect on the correlation of 13C and 2H fractionation (Λ). In addition, radical quenching approach and electron paramagnetic resonance (EPR) were combined to explore the dominant radical species in the ZVI activated PS reaction, and hydroxyl radical (•OH) was found to be the predominant radical at all pH studied. The results of CSIA show the addition of •OH to the aromatic ring of DEP is the main reaction mechanism, which is consistent with the results of radical quenching experiment and EPR study. Carbon and hydrogen apparent kinetic isotope effects (AKIEs) obtained from •OH reactions with DEP supported the hypothesis of C H bond cleavage. Thus, carbon and hydrogen isotope enrichment factors clearly distinguish the different reaction mechanisms and hence, are a promising approach to improve understanding of radical species reaction pathways for chemical oxidation of DEP. [ABSTRACT FROM AUTHOR]

Abstract: The first step in the oxidation of ascorbate with substituted nitrosobenzenes is a proton-coupled electron transfer (PCET) reaction and the observed kinetic isotope effects in the reaction, , change dramatically with a change in solvent polarity, in line with known theoretical predictions. [Copyright &y& Elsevier]

Abstract: A combination of time-resolved techniques (part I) with kinetic isotope effects studies (part II) has been used to investigate the mechanism of the decatungstate (W10O32 4−) photocatalyzed oxidation of aromatic alcohols, in acetonitrile. It is demonstrated that aromatic alcohols exclusively react by hydrogen atom transfer (HAT) mechanism, like to that proposed for the analogous alkane oxidations, and not by an electron transfer (ET) which is predominant in alkenes. The primary and β-secondary isotope effects provide strong evidence for a stepwise mechanism, in which the hydrogen atom abstraction occurs in the rate-determining step of the reaction. The positive slope in Hammett kinetics suggests a radical intermediate for this reaction and supports an early transition state. [Copyright &y& Elsevier]

A number of enzymes from the cytochrome P450 family show atypical (non-Michaelis–Menten) kinetic behavior resulting from substrate activation, inhibition, partial inhibition, biphasic saturation, or autoactivation. Herein, we provide a technique that can identify multiple substrate occupancy in the same active site of a P450 as a result of an altered kinetic profile. Using an isotope effect on product ratios confirms that the enzyme–substrate (ES) complex responsible for ω hydroxylation of palmitic acid (palmitate) is in rapid equilibrium with the ES complex that leads to ω-1 hydroxylation of palmitate. Co-incubation of a second substrate, lauric acid (laurate), results in a change in the ratio of ω to ω-1 hydroxylated palmitate. Furthermore, an isotope effect on palmitate is observed when deuterated laurate is co-incubated with non-deuterated palmitate. These results are only consistent with both substrates being in the same active site simultaneously. This mode of binding explains how the F87A mutant of P450BM3 is able to produce the ω alcohol, a product that arises from the high-energy primary radical. [Copyright &y& Elsevier]

A rate enhancement of one to two orders of magnitude can be obtained in the aldehyde deformylation reactions by replacing the –N(CH 3) groups of [NiIII(O 2)(Me 4 [12]aneN 4)]+ (Me 4 [12]aneN 4 = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) and [NiIII(O 2)(Me 4 [13]aneN 4)]+ (Me 4 [13]aneN 4 = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclotridecane) complexes by –NH in [NiIII(O 2)([12]aneN 4)]+ (2 ; [12]aneN 4 = 1,4,7,10-tetraazacyclododecane) and [NiIII(O 2)([13]aneN 4)]+ (4 ; [13]aneN 4 = 1,4,7,10-tetraazacyclotridecane). Based on detailed spectroscopic, reaction-kinetics and theoretical investigations, the higher reactivities of 2 and 4 are attributed to the changes in the secondary-sphere interactions between the [NiIII(O 2)]+ and [12]aneN 4 or [13]aneN 4 moieties, which open up an alternative electrophilic pathway for the aldehyde oxidation reaction. Identification of primary kinetic isotope effects on the reactivity and stability of 2 when the –NH groups of the [12]aneN 4 ligand are deuterated may also suggest the presence of secondary interaction between the –NH groups of [12]aneN 4 and [NiIII(O 2)]+ moieties, although, such interactions are not obvious in the DFT calculated optimized structure at the employed level of theory. [Display omitted] • Secondary Coordination Sphere affects the reactivity and stability of [NiO2] + cores. • A rate enhancement of 1–2 orders of magnitude is achieved in aldehyde oxidation reaction. • Aldehyde oxidation involves both electrophilic and nucleophilic pathways. [ABSTRACT FROM AUTHOR]

[Display omitted] • Selective oxidation of benzene to phenol can be achieved by using O 2 as an oxidant under ambient conditions. • The high phenol yield (23–41%) and selectivity (80–90%) can be obtained with H 3 PW 12 O 40 under light irradiation. • The addition of acetonitrile to the reaction solution significantly inhibited the oxidation of phenol. • The reaction mechanism was proposed by kinetic and spectroscopic studies and several controlled experiments. The synthesis of phenol by one-pot oxidation of benzene with dioxygen (O 2) is a significant conceptual and practical challenge because benzene has low reactivity to O 2 at room temperature and ambient pressure. Furthermore, phenol is more reactive than benzene and is easily overoxidized, so complete oxidation is more likely to proceed in the benzene oxidation reaction, which generally results in lower phenol yield and selectivity. Herein, we report that a heteropolyacid H 3 PW 12 O 40 functions as a photocatalyst for benzene oxidation reaction in an aqueous acetonitrile solution using O 2 in high phenol yield (23–41%) and selectivity (80–90%). The addition of acetonitrile to the reaction solution significantly inhibited the complexation between phenol and H 3 PW 12 O 40 , preventing phenol overoxidation. Furthermore, no decomposition of H 3 PW 12 O 40 during the reaction was evidenced by UV–VIS spectra. This photocatalyst can be used repeatedly with only a slight decrease in the rate of phenol formation. For the mechanistic study, the initial rates, which depended on the reaction conditions, were investigated in detail. The benzene oxidation under anaerobic conditions was also conducted as a control experiment. A color change of reaction solution from colorless to blue has confirmed the reduction of H 3 PW 12 O 40 during the catalytic process and the formation of reduced H 3 PW 12 O 40 detected in the UV–VIS spectra. A kinetic isotope effect (KIE), k H / k D = 1.15, is found for the oxidation of benzene/benzene‑ d 6 , indicating electron transfer is the principal C−H bond activation process in the mechanism. We identified hydrogen peroxide as an intermediate and found that phenol was generated in both the photocatalytic reduction and reoxidation process in our system. [ABSTRACT FROM AUTHOR]

Conclusions on the functional and structural organisation of photosynthetic water oxidation are gathered from a critical survey of the wealth of data reported in the literature and author's own experimental research: (1) the water oxidising complex (WOC) contains a tetranuclear manganese cluster of ‘dimer of dimers’ structure and functional heterogeneity of the metal centers, (2) the four step univalent oxidative pathway leading to water oxidation into molecular oxygen and four protons comprises only manganese, tyrosine YZ of polypeptide Dl and the substrate as redox active species, (3) the redox transitions S0 → S1 and S1 → S2 are manganese centered whereas S2 → S3 is most likely a ligand-centered reaction, (4) there exist several lines of evidence for a marked structural change that accompanies the redox transition S2 → S3, (5) one Ca2+ is an indispensible constituent of a functionally competent WOC while the role of Cl- is much less clear and a direct participation disputable, (6) substrate water is most likely bound in all redox states S0,..., S3 and exchangeable with the bulk phase. The protonation state is determined by the redox state Si and the protein microenvironment. A mechanism is proposed for water oxidation in the WOC that is based on three key postulates: (1) water oxidation takes place in the first coordination sphere of one manganese dimer [MnaMnb]; (2) the essential O-O bond is preformed in S3 as part of a rapid redox isomerism S3(I)⇔S3(II) where in S3(II) a nuclear geometry and electronic configuration is attained that corresponds to a peroxidic-type species; and (3) S3(II) is an ‘entatic state’ for the formation of complexed dioxygen triggered by Yzox induced electron abstraction from the WOC and electronic redistribution to S0(O2). [ABSTRACT FROM AUTHOR]

Deuterium kinetic isotope effects support a mechanism for the Boyland-Sims oxidation involving a nucleophilic displacement by the amine on the peroxide oxygen. [ABSTRACT FROM AUTHOR]

Cover Feature: Mechanistic Insights into Fe Catalyzed -C-H Oxidations of Tertiary Amines (ChemCatChem 1/2021) Keywords: Iron catalysis; C-H oxidation; kinetics; DFT calculation; mechanistic analysis EN Iron catalysis C-H oxidation kinetics DFT calculation mechanistic analysis 4 4 1 01/18/21 20210112 NES 210112 B The Cover Feature b shows cast iron catalysis: Trialkyl amines cooked with the right recipe will take the oxygen from water to form the corresponding trialkyl amide. Iron catalysis, C-H oxidation, kinetics, DFT calculation, mechanistic analysis. [Extracted from the article]

• The mechanism of the monoxygenation of methane is probed using computation. • Structures of intermediates at a Cu(Asp)(His) 2 active site are defined. • Transition states for H atom abstraction from alkanes by a copper oxo are located. • The copper oxo is not selective for methane over methanol oxidation. The coordinates of the copper site in the C subunit of particulate methane monooxygenases pMMO are used to optimize simplified structures of possible intermediates in the conversion of methane and dioxygen into methanol and water. The transition states for the hydrogen atom abstraction from methane or ethane by a copper oxo intermediate are located and found to have an energies consistent with a process occurring under ambient conditions and with the correct kinetic isotope effect. The fact that hydrogen atom abstraction from methanol has even a lower barrier indicates that the enzyme system utilizes other means of achieving selectivity for methane over methanol oxidation. [ABSTRACT FROM AUTHOR]