Showing papers on "Hydrogen atom abstraction published in 2017"
TL;DR: In this paper, the first step of SET reactions for 76 aromatic contaminants (ACs) with SO4 − was investigated, and it was shown that the Gibbs free energy ( Δ G SET ∘ ) of the reaction increases with a decrease of the electron donating character of the substituents on the ACs.
281 citations
TL;DR: The history of radical rebound mechanisms, their general features, and key intermediates involved are recounted, and it is envisioned that new chemistry will continue to arise by bridging enzymatic “radical rebound” with synthetic organic chemistry.
Abstract: 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.
206 citations
TL;DR: Despite the resonance stabilization of the π system of triplet O2, the weakness of the O-O σ bond makes reactions of O1, which eventually lead to cleavage of this bond, very favorable thermodynamically.
Abstract: Experimental heats of formation and enthalpies obtained from G4 calculations both find that the resonance stabilization of the two unpaired electrons in triplet O2, relative to the unpaired electrons in two hydroxyl radicals, amounts to 100 kcal/mol. The origin of this huge stabilization energy is described within the contexts of both molecular orbital (MO) and valence-bond (VB) theory. Although O2 is a triplet diradical, the thermodynamic unfavorability of both its hydrogen atom abstraction and oligomerization reactions can be attributed to its very large resonance stabilization energy. The unreactivity of O2 toward both these modes of self-destruction maintains its abundance in the ecosphere and thus its availability to support aerobic life. However, despite the resonance stabilization of the π system of triplet O2, the weakness of the O–O σ bond makes reactions of O2, which eventually lead to cleavage of this bond, very favorable thermodynamically.
116 citations
01 Jan 2017
TL;DR: In this article, temperature and pressure-dependent rate coefficients for acetylene addition reactions to the C6H5, C6C2H, C 6H4C 2H 3 and C 6C2C 2 H 3 radicals were evaluated under low pressure flame conditions.
Abstract: RRKM-Master Equation calculations have been performed to evaluate temperature- and pressure-dependent rate coefficients for acetylene addition reactions to the C6H5, C6H4C2H, C6H5C2H2, and C6H4C2H3 radicals. These calculations indicate a strong pressure dependence for the role of various Hydrogen-Abstraction-C2H2-Addition (HACA) sequences for the formation of naphthalene from benzene. At atmospheric and lower pressures the C8H7 radicals, C6H4C2H3 and C6H5C2H2, cannot be stabilized above 1650 K. As a result, both the Bittner–Howard HACA route, in which a second acetylene molecule adds to C6H5C2H2, and the modified Frenklach route, where a second C2H2 adds to the aromatic ring of C6H4C2H3 obtained by internal hydrogen abstraction, are unrealistic under low pressure flame conditions. At the higher pressures of some practical combustion devices (e.g., 100 atm) these routes may be operative. Naphthalene is predicted to be the main product of the C6H5C2H2 + C2H2 and C6H4C2H3 + C2H2 reactions in the entire 500–2500 K temperature range independent of pressure (ignoring the issues related to the instability of C8H7 species). Frenklach's original HACA route, where the second C2H2 molecule adds to the aromatic ring activated by intermolecular H abstraction from C8H6, involves the C6H4C2H + C2H2 reaction, which is shown to predominantly form dehydrogenated species with a naphthalene core (naphthyl radicals or naphthynes) at T
110 citations
TL;DR: The first example of aryl radical formation via the visible light-mediated decarboxylation of aRYl carboxylic acids using photoredox catalysis is presented.
Abstract: Herein we present the first example of aryl radical formation via the visible light-mediated decarboxylation of aryl carboxylic acids using photoredox catalysis. This method constitutes a mild protocol for the decarboxylation of cheap and abundant aryl carboxylic acids and tolerates both electron-rich substrates and those lacking ortho-substitution. The in situ formation of an acyl hypobromite is proposed to prevent unproductive hydrogen atom abstraction and trapping of the intermediate aroyloxy radical, enabling mild decarboxylation.
102 citations
TL;DR: This work reports the first photoredox-mediated hydrogen-atom transfer method for the efficient synthesis of ynones, ynamides, and ynoates with high regio- and chemoselectivity by direct functionalization of Csp2 (O)-H bonds.
Abstract: The development of new hydrogen-atom transfer (HAT) strategies within the framework of photoredox catalysis is highly appealing for its power to activate a desired C−H bond in the substrate leading to its selective functionalization. Reported here is the first photoredox-mediated hydrogen-atom transfer method for the efficient synthesis of ynones, ynamides, and ynoates with high regio- and chemoselectivity by direct functionalization of Csp2 (O)−H bonds. The broad synthetic application of this method has been demonstrated by the selective functionalization of C(O)−H bonds within complex molecular scaffolds.
76 citations
TL;DR: In this paper, a visible-light-mediated atom transfer radical cyclization of unactivated alkyl iodides is described, which operates under mild conditions and exhibits high chemoselectivity profile while avoiding parasitic hydrogen atom transfer pathways.
Abstract: A visible-light-mediated atom transfer radical cyclization of unactivated alkyl iodides is described. This protocol operates under mild conditions and exhibits high chemoselectivity profile while avoiding parasitic hydrogen atom transfer pathways. Preliminary mechanistic studies challenge the perception that a canonical photoredox catalytic cycle is being operative.
69 citations
TL;DR: A dual ammonia activation approach has been discovered whereby reversible M-L cooperativity and coordination induced bond weakening likely contribute to dihydrogen formation and enabled hydrogen atom abstraction and synthesis of a terminal nitride from coordinated ammonia, a key step in NH3 oxidation.
Abstract: Treatment of the bis(imino)pyridine molybdenum η6-benzene complex (iPrPDI)Mo(η6-C6H6) (iPrPDI, 2,6-(2,6-iPr2C6H3N═CMe)2C5H3N) with NH3 resulted in coordination induced haptotropic rearrangement of the arene to form (iPrPDI)Mo(NH3)2(η2-C6H6). Analogous η2-ethylene and η2-cyclohexene complexes were also synthesized, and the latter was crystallographically characterized. All three compounds undergo loss of the η2-coordinated ligand followed by N–H bond activation, bis(imino)pyridine modification, and H2 loss. A dual ammonia activation approach has been discovered whereby reversible M–L cooperativity and coordination induced bond weakening likely contribute to dihydrogen formation. Significantly, the weakened N–H bonds in (iPrPDI)Mo(NH3)2(η2-C2H4) enabled hydrogen atom abstraction and synthesis of a terminal nitride from coordinated ammonia, a key step in NH3 oxidation.
68 citations
TL;DR: A combined spectroscopy, kinetics and computational study on aldehyde deformylation by two side-on manganese(III)-peroxo complexes with bispidine ligands finds a novel mechanism for the reaction that is initiated by a hydrogen atom abstraction reaction, which enables a keto-enol tautomerization in the substrate.
Abstract: Oxygen atom transfer by high-valent enzymatic intermediates remains an enigma in chemical catalysis. In particular, manganese is an important first-row metal involved in key biochemical processes, including the biosynthesis of molecular oxygen (through the photosystem II complex) and biodegradation of toxic superoxide to hydrogen peroxide by superoxide dismutase. Biomimetic models of these biological systems have been developed to gain understanding on the structure and properties of short-lived intermediates but also with the aim to create environmentally benign oxidants. In this work, we report a combined spectroscopy, kinetics and computational study on aldehyde deformylation by two side-on manganese(III)-peroxo complexes with bispidine ligands. Both manganese(III)-peroxo complexes are characterized by UV–vis and mass spectrometry techniques, and their reactivity patterns with aldehydes was investigated. We find a novel mechanism for the reaction that is initiated by a hydrogen atom abstraction reactio...
61 citations
TL;DR: The energetics of hydrogen abstraction from benzene and naphthalene are investigated using both high level-of-theory quantum chemistry methods and a series of density functional theory methods, among which M06-2X/6-311g(d,p) has the best performance with a mean unsigned deviation from the CCSD(T)/CBS calculations.
Abstract: Hydrogen abstraction reactions of polycyclic aromatic hydrocarbons (PAH) by H atoms play a very important role in both PAH and soot formation processes. However, large discrepancies up to a few orders of magnitude exist among the literature rate constant values. To increase the reliability of the computed rate constants, it is critical to obtain highly accurate potential energy surfaces. For this purpose, we have investigated the energetics of hydrogen abstraction from benzene and naphthalene using both high level-of-theory quantum chemistry methods and a series of density functional theory (DFT) methods, among which M06-2X/6-311g(d,p) has the best performance with a mean unsigned deviation from the CCSD(T)/CBS calculations of 1.0 kcal mol-1 for barrier heights and reaction energies. Thus, M06-2X/6-311g(d,p) has then been applied to compute the potential energy surfaces of the hydrogen abstraction reactions of a series of larger PAH. Based on the quantum chemistry calculations, rate constants are computed using the canonical transition state theory. The effects of the PAH size, structure, and reaction site on the energetics and rate constants are examined systematically. Finally, the hydrogen abstraction rate constants for application in PAH and soot surface chemistry models are recommended.
50 citations
TL;DR: Electrochemical analysis revealed a pH-dependent and remarkably high FeIII-OH/FeII-OH2 reduction potential of 680 mV vs Ag/AgCl at pH 5.2 and nernstian behavior from pH 2 to pH 8 indicates a one-proton, one-electron interconversion throughout that range.
Abstract: A reactive hydroxoferric porphyrazine complex, [(PyPz)FeIII(OH) (OH2)]4+ (1, PyPz = tetramethyl-2,3-pyridino porphyrazine), has been prepared via one-electron oxidation of the corresponding ferrous species [(PyPz)FeII(OH2)2]4+ (2). Electrochemical analysis revealed a pH-dependent and remarkably high FeIII–OH/FeII–OH2 reduction potential of 680 mV vs Ag/AgCl at pH 5.2. Nernstian behavior from pH 2 to pH 8 indicates a one-proton, one-electron interconversion throughout that range. The O–H bond dissociation energy of the FeII–OH2 complex was estimated to be 84 kcal mol–1. Accordingly, 1 reacts rapidly with a panel of substrates via C–H hydrogen atom transfer (HAT), reducing 1 to [(PyPz)FeII(OH2)2]4+ (2). The second-order rate constant for the reaction of [(PyPz)FeIII(OH) (OH2)]4+ with xanthene was 2.22 × 103 M–1 s–1, 5–6 orders of magnitude faster than other reported FeIII–OH complexes and faster than many ferryl complexes.
TL;DR: This work has found through various energy decomposition analysis methods and natural bond orbital (NBO) calculations that, along with electrostatics and polarization, charge transfer interactions are important to understand Se/S hydrogen bonding and there is a delicate balance between the various interactions that plays the crucial role rather than a single component of the interaction energy.
Abstract: Subsequent to the recent re-definition of hydrogen bonding by the IUPAC committee, there has been a growing search for finding the presence of this ever interesting non-covalent interaction between a hydrogen atom in an X–H group and any other atom in the periodic table. In recent gas phase experiments, it has been observed that hydrogen bonding interactions involving S and Se are of similar strength to those with an O atom. However, there is no clear explanation for the unusual strength of this interaction in the case of hydrogen bond acceptors which are not conventional electronegative atoms. In this work, we have explored the nature of Se hydrogen bonding by studying indole⋯dimethyl selenide (indmse) and phenol⋯dimethyl selenide (phdmse) complexes using gas phase IR spectroscopy and quantum chemistry calculations. We have found through various energy decomposition analysis (EDA) methods and natural bond orbital (NBO) calculations that, along with electrostatics and polarization, charge transfer interactions are important to understand Se/S hydrogen bonding and there is a delicate balance between the various interactions that plays the crucial role rather than a single component of the interaction energy. An in-depth understanding of this type of non-covalent interaction has immense significance in biology as amino acids containing S and Se are widely present in proteins and hence hydrogen bonding interactions involving S and Se atoms contribute to the folding of proteins.
TL;DR: In this paper, a low-pressure, fuel-rich premixed laminar flame with 2-methyl-2-butene employing flame-sampling molecular-beam mass spectrometry with vacuum-ultraviolet single-photon ionization was investigated.
TL;DR: The reaction pathways during thermochemical formation of PCDDs from 2,3,6-trichlorophenol (TCP) over a Cu(II)O/silica matrix, which was used to simulate fly ash, were clarified and the proposed mechanism could be used for controlling PCDD/F formation during industrial thermal processes.
Abstract: Few studies have investigated the free radical intermediates involved in the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from chlorophenol. This study clarified the reaction pathways during thermochemical formation of PCDDs from 2,3,6-trichlorophenol (TCP) over a Cu(II)O/silica matrix, which was used to simulate fly ash, at 298–523 K. The reaction was studied using electron paramagnetic resonance (EPR) spectroscopy and theoretical calculations. In situ EPR indicated the TCP radical (TCPR) formed by hydrogen abstraction of TCP. Five elementary processes including dimerization of TCPR, ortho-chloride abstraction, Smiles rearrangement, ring closure, and intra-annular elimination of Cl were proposed to occur during formation of PCDDs. The proposed mechanism was further confirmed by the detection of PCDD products from thermochemical experiments in a tube furnace. Several dominant congeners, including 1,2,6,9-tetrachlorodibenzo-p-dioxin (TeCDD), 1,2,6,7-TeCDD, 1,2,8,9-TeCDD, and 1...
TL;DR: Density functional theory computations on the structure established here show that the protonation activates peroxide for electrophilic/single-electron-transfer reactivity.
Abstract: Binuclear non-heme iron enzymes activate O2 for diverse chemistries that include oxygenation of organic substrates and hydrogen atom abstraction. This process often involves the formation of peroxo-bridged biferric intermediates, only some of which can perform electrophilic reactions. To elucidate the geometric and electronic structural requirements to activate peroxo reactivity, the active peroxo intermediate in 4-aminobenzoate N-oxygenase (AurF) has been characterized spectroscopically and computationally. A magnetic circular dichroism study of reduced AurF shows that its electronic and geometric structures are poised to react rapidly with O2. Nuclear resonance vibrational spectroscopic definition of the peroxo intermediate formed in this reaction shows that the active intermediate has a protonated peroxo bridge. Density functional theory computations on the structure established here show that the protonation activates peroxide for electrophilic/single-electron-transfer reactivity. This activation of p...
TL;DR: Kinetic and thermodynamic analyses evidence a concerted proton-electron transfer pathway for these processes.
Abstract: Pyrazolate-based μ-1,2-peroxo dicopper(II) complex 1 undergoes clean 1e– oxidation at low potential (−0.59 V vs Fc/Fc+) to yield the rather stable μ-1,2-superoxo dicopper(II) complex 3, which was characterized by spectroscopic methods (ν(O–O) = 1070 cm–1, Δ(18O–16O) = −59 cm–1) and analyzed by DFT calculations. 3 is also formed via H-atom abstraction from the corresponding μ-1,1-hydroperoxo dicopper(II) complex 2, while 3 itself is able to abstract H-atoms from weaker X–H bonds such as TEMPO-H to re-form 2. Kinetic and thermodynamic analyses evidence a concerted proton–electron transfer pathway for these processes. The thermodynamic square scheme reveals a bond dissociation free energy of 71.7 ± 1.1 kcal mol–1 for the hydroperoxo OO–H bond of 2.
TL;DR: It is shown that the trans conformer of trifluoromethylhydroxycarbene preferentially rearranges through a facile quantum-mechanical hydrogen tunnelling pathway, while its cis conformer is entirely unreactive, presenting the first example of a conformer-specific hydrogen Tunnelling reaction.
Abstract: Conformational control of organic reactions is at the heart of the biomolecular sciences. To achieve a particular reactivity, one of many conformers may be selected, for instance, by a (bio)catalyst, as the geometrically most suited and appropriately reactive species. The equilibration of energetically close-lying conformers is typically assumed to be facile and less energetically taxing than the reaction under consideration itself: this is termed the ‘Curtin–Hammett principle’. Here, we show that the trans conformer of trifluoromethylhydroxycarbene preferentially rearranges through a facile quantum-mechanical hydrogen tunnelling pathway, while its cis conformer is entirely unreactive. Hence, this presents the first example of a conformer-specific hydrogen tunnelling reaction. The Curtin–Hammett principle is not applicable, due to the high barrier between the two conformers. Control over the selectivity of chemical reactions and biological molecules requires an intimate understanding of how conformation impacts reactivity. It is now shown that the trans conformer of trifluoromethylhydroxycarbene preferentially rearranges through a quantum-mechanical hydrogen-tunnelling pathway, whereas its cis conformer is unreactive.
TL;DR: A novel access to 2-substituted benzoimidazoles, through unprecedented denitrogenative imidoyl radical cyclization of 1-azido-2-isocyanoarenes, has been developed by adding a C- or P-centered radical to isocyanide, followed by cycloaddition of the imidoysl radical to the azido group.
TL;DR: The results clearly indicate the need for a more detailed investigation of the combustion kinetics of toluene oxidation and its key pyrolysis and oxidation intermediates and the computed barriers and rate constants retain an important internal consistency.
Abstract: Hydrogen atom abstraction from allylic C–H bonds by molecular oxygen plays a very important role in determining the reactivity of fuel molecules having allylic hydrogen atoms. Rate constants for hydrogen atom abstraction by molecular oxygen from molecules with allylic sites have been calculated. A series of molecules with primary, secondary, tertiary, and super secondary allylic hydrogen atoms of alkene, furan, and alkylbenzene families are taken into consideration. Those molecules include propene, 2-butene, isobutene, 2-methylfuran, and toluene containing the primary allylic hydrogen atom; 1-butene, 1-pentene, 2-ethylfuran, ethylbenzene, and n-propylbenzene containing the secondary allylic hydrogen atom; 3-methyl-1-butene, 2-isopropylfuran, and isopropylbenzene containing tertiary allylic hydrogen atom; and 1–4-pentadiene containing super allylic secondary hydrogen atoms. The M06-2X/6-311++G(d,p) level of theory was used to optimize the geometries of all of the reactants, transition states, products and ...
TL;DR: Nitrogen rich carbon nanotubes have been used as a metal free catalyst for the conversion of glycerol into dihydroxyacetone using tert-butyl hydroperoxide as an oxidant.
Abstract: Nitrogen rich carbon nanotubes have been used as a metal free catalyst for the conversion of glycerol into dihydroxyacetone using tert-butyl hydroperoxide as an oxidant. Pyridine nitrogen groups embedded in a carbon matrix are identified as active sites for the reaction. Computational studies have demonstrated that oxidation of pyridine groups to pyridine oxime followed by hydrogen abstraction from secondary alcohol is likely responsible for the oxidation process.
TL;DR: Important aspects that may affect the radical scavenging capacity of carotenoids, such as synergistic effects and solubility, are sometimes overlooked, and a greater number of such compounds should be explored.
TL;DR: In this paper, a density functional theory was used to study the mechanism and kinetics of benzoic acid with hydroxyl radicals in both gas and aqueous phases.
Abstract: Density functional theory was used to study the mechanism and kinetics of benzoic acid with hydroxyl radicals in both gas and aqueous phases as well as benzoate with hydroxyl radicals in the aqueous phase at the M06-2X/6-311+G(d,p) level of theory. The results show that all reaction pathways involved the formation of pre-reactive complexes which in turn alter reaction energy barriers. The reaction rate constants, calculated based on classical transitional theory, followed the order of meta addition > para addition > ortho addition for the reaction of benzoic acid and hydroxyl radicals in both gas and aqueous media. The energy barrier analysis reveals that the ortho adducts were also less vulnerable to subsequent reaction. In addition, the rate constants for the addition reactions were highest for benzoate in the aqueous phase, followed by benzoic acid in the aqueous phase, then by benzoic acid in the gas phase, consistent with electrostatic potential analysis. However, the rate constants of hydrogen abstraction in the aqueous phase were much lower than that in the gas phase and thus, gas phase reactions are preferred. The incorporation of one explicit water molecule, for addition reactions between benzoic acid and hydroxyl radicals, lowered reaction rates in the aqueous phase by increasing the bond length between the oxygen and reacting carbon in the benzene ring.
01 Jan 2017
TL;DR: In this paper, the authors presented a thermodynamically consistent mechanism to describe the thermal decomposition of titanium tetraisopropoxide (TTIP), which is based on an analogy between the decomposition of the isopropoxysynthetic branches and the decocomposition of isopsopropanol.
Abstract: This work presents the first systematically derived and thermodynamically consistent mechanism to describe the thermal decomposition of titanium tetraisopropoxide (TTIP). The mechanism is based on an analogy between the decomposition of the isopropoxide branches and the decomposition of isopropanol. Flux and sensitivity analyses were used to identify the main reaction pathways in the proposed mechanism as the step-wise release of C3H6 via four-member ring transition states, the successive abstraction of CH3 radicals via C–C bond cleavage followed by hydrogen abstraction to form C = C double bonds, and hydrogen abstraction from the isopropoxide methyl groups followed by the release of C3H6. The final decomposition product was titanium hydroxide, Ti(OH)4. Rate constants were calculated using conventional and variational transition state theories for reactions in the first two pathways. The calculated rates are similar to the rates calculated for the corresponding isopropanol reactions, providing support for the analogy with isopropanol. The mechanism was used to simulate the ignition delay of isopropanol and TTIP. Excellent agreement was observed with experimental data for isopropanol. However, the mechanism over predicted the ignition delay for TTIP. The discrepancy was shown to be unlikely to be caused by the modest difference between the true reaction rates for the TTIP system and those assumed based on the analogy with isopropanol. It was found that the sensitivity of the TTIP decomposition to the presence of water must be caused by additional chemical pathways than the ones given by isopropanol analogy.
TL;DR: The synthesis of the divinylamido PNP nickel(II) complex [NiBr{N(CHCHPtBu2 )2 }] is reported and the resulting pincer chemical non-innocence can be utilized for benzylic C-H hydrogen atom abstraction.
Abstract: The synthesis of the divinylamido PNP nickel(II) complex [NiBr{N(CHCHPtBu2)2}] is reported. This compound exhibits reversible, ligand-centered oxidation and protonation reactions. The resulting pincer chemical non-innocence can be utilized for benzylic C−H hydrogen atom abstraction. The thermochemistry and kinetics of hydrogen atom transfer were examined.
TL;DR: It can be well explained the experimental results that the oxidation of some oligosaccharides with hydroxyl free radicals can produce molecules of glucose, fructose and other monosACcharides.
TL;DR: An accurate and efficient electronic structure method is determined by considering eight hydrogen abstraction reactions and comparing their barrier heights and reaction energies computed using several exchange-correlation density functionals to those obtained from CCSD(T)-F12a/jun-cc-pVTZ coupled cluster calculations to find the M06-2X/ma-TZVP method to have the best performance.
Abstract: In order to explore the hydrogen abstraction reaction kinetics of unsaturated methyl esters by hydrogen atoms, we selected two molecules for study, in particular methyl 3-butenoate and methyl 2-butenoate, whose CC double bonds are at different locations We first determined an accurate and efficient electronic structure method for the investigation by considering eight hydrogen abstraction reactions and comparing their barrier heights and reaction energies computed using several exchange–correlation density functionals to those obtained from CCSD(T)-F12a/jun-cc-pVTZ coupled cluster calculations In this way, we found the M06-2X/ma-TZVP method to have the best performance with a mean unsigned deviation from the CCSD(T) calculations of 051 kcal mol−1 Based on quantum-chemical calculations by using the M06-2X/ma-TZVP method, we then computed rate constants for 298–2500 K by direct dynamics calculations using multi-structural canonical variational transition state theory including tunneling by the multi-dimensional small-curvature tunneling approximation (MS-CVT/SCT) The computed transmission coefficients were compared with those obtained using the zero-curvature tunneling (ZCT) and one-dimensional Eckart tunneling (ET) approximations We employed the multi-structural torsional method (MS-T) to include the multiple-structure and torsional potential anharmonic effects The results show that the variational recrossing transmission coefficients range from 06 to 10, and the multi-structural torsional anharmonicity introduces a factor of 05–25 into the rate constant, while the tunneling transmission coefficients obtained by SCT can be as large as 174 and differ considerably from those determined by the less accurate ZCT and ET approximations In addition, independent of the location of the CC double bond, the dominant hydrogen abstraction reactions occur at the allylic sites
TL;DR: A low-coordinate Lewis acidic nickel( II) complex has been synthesized that reacts with O2 to give a nickel(II) organoperoxide, as proposed for the enzymatic system and for the functional model.
Abstract: In metal-mediated O2 activation, nickel(II) compounds hardly play a role, but recently it has been shown that enzymes can use nickel(II) for O2 activation. Now a low-coordinate Lewis acidic nickel(II) complex has been synthesized that reacts with O2 to give a nickel(II) organoperoxide, as proposed for the enzymatic system. Its formation was studied further by UV/Vis absorption spectroscopy, leading to the observation of a short-lived intermediate that proved to be reactive in both oxygen atom transfer and hydrogen abstraction reactions, while the peroxide efficiently transfers O atoms. Both for the enzyme and for the functional model, the key to O2 activation is proposed to represent a concomitant electron shift from the substrate/co-ligand.
TL;DR: DFT calculations experimentally calibrated against 2 reveal that [Cu2O]2+ is likely thermodynamically viable in copper-dependent methane monoxygenase enzymes.
Abstract: The mono-μ-hydroxo complex {[Cu(tmpa)]2-(μ-OH)}3+ (1) can undergo reversible deprotonation at -30 °C to yield {[Cu(tmpa)]2-(μ-O)}2+ (2). This species is basic with a pKa of 24.3. 2 is competent for concerted proton-electron transfer from TEMPOH, but is an intrinsically poor hydrogen atom abstractor (BDFE(OH) of 77.2 kcal/mol) based on kinetic and thermodynamic analyses. Nonetheless, DFT calculations experimentally calibrated against 2 reveal that [Cu2O]2+ is likely thermodynamically viable in copper-dependent methane monoxygenase enzymes.
TL;DR: In this article, an energy-efficient sequence of elementary steps for propene oxidation to acrolein, propene ammoxidation to acrylonitrile, and acro-rolein is proposed.
Abstract: The mechanisms and energetics for the propene oxidation and ammoxidation occurring on the (010) surface of Bi2Mo3O12 were investigated using density functional theory (DFT). An energetically feasible sequence of elementary steps for propene oxidation to acrolein, propene ammoxidation to acrylonitrile, and acrolein ammoxidation to acrylonitrile is proposed. Consistent with experimental findings, the rate-limiting step for both propene oxidation and ammoxidation is the initial hydrogen abstraction from the methyl group of propene, which is calculated to have an apparent activation energy of 27.3 kcal/mol. The allyl species produced in this reaction is stabilized as an allyl alkoxide, which can then undergo hydrogen abstraction to form acrolein or react with ammonia adsorbed on under-coordinated surface Bi3+ cations to form allylamine. Dehydrogenation of allylamine is shown to produce acrylonitrile, whereas reaction with additional adsorbed ammonia leads to the formation of acetonitrile and hydrogen cyanide....