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Showing papers on "Hydrogen atom abstraction published in 2014"


Journal ArticleDOI
TL;DR: An Fe(V)(O) complex has been synthesized from equimolar solutions of (Et4N)2[Fe(III)(Cl)(biuret-amide)] and mCPBA in CH3CN at room temperature as discussed by the authors.
Abstract: An Fe(V)(O) complex has been synthesized from equimolar solutions of (Et4N)2[Fe(III)(Cl)(biuret-amide)] and mCPBA in CH3CN at room temperature. The Fe(V)(O) complex has been characterized by UV-vis, EPR, Mossbauer, and HRMS and shown to be capable of oxidizing a series of alkanes having C-H bond dissociation energies ranging from 99.3 kcal mol(-1) (cyclohexane) to 84.5 kcal mol(-1) (cumene). Linearity in the Bell-Evans-Polayni graph and the finding of a large kinetic isotope effect suggest that hydrogen abstraction is engaged the rate-determining step.

136 citations


Journal ArticleDOI
TL;DR: By probing the phenylacetylene intermediate together with naphthalene under combustion-like conditions by photo-ionization mass spectrometry, the very first direct experimental evidence for the validity of the HACA mechanism which so far had only been speculated theoretically is reported.
Abstract: For almost half a century, polycyclic aromatic hydrocarbons (PAHs) have been proposed to play a key role in the astrochemical evolution of the interstellar medium (ISM) and in the chemistry of combustion systems. However, even the most fundamental reaction mechanism assumed to lead to the simplest PAH naphthalene--the hydrogen abstraction-acetylene addition (HACA) mechanism--has eluded experimental observation. Here, by probing the phenylacetylene (C8 H6 ) intermediate together with naphthalene (C10 H8 ) under combustion-like conditions by photo-ionization mass spectrometry, the very first direct experimental evidence for the validity of the HACA mechanism which so far had only been speculated theoretically is reported.

124 citations


Journal ArticleDOI
TL;DR: Monomeric and dimeric vanadium oxide species are both considered and modeled by vanadyl-substituted silsesquioxanes and Reactivity descriptors indicate that this trend will continue also for larger oligomers, and the size distribution of oligomeric species is predicted to be rather statistical.
Abstract: Possible reaction pathways for the oxidative dehydrogenation of propane by vanadium oxide catalysts supported on silica are examined by density functional theory. Monomeric and dimeric vanadium oxide species are both considered and modeled by vanadyl-substituted silsesquioxanes. The reaction proceeds in two subsequent steps. In a first step, hydrogen abstraction from propane by a vanadyl (O═V) group yields a propyl radical bound to a HOVIV surface site. Propene is formed by a second hydrogen abstraction, either at the same vanadia site or at a different one. VV/VIV redox cycles are preferred over VV/VIII cycles. Under the assumption of fast reoxidation, microkinetic simulations show that the first step is rate-determining and yields Arrhenius barriers that are lower for dimers (114 kJ/mol at 750 K) than for monomers (124 kJ/mol). The rate constants predicted for a mixture of monomers and dimers are 14% larger (750 K) than for monomers only, although the increase remains within experimental uncertainty lim...

119 citations


Journal ArticleDOI
TL;DR: Significant deuterium kinetic isotope effects and a positive correlation of second-order rate constants (k2) compared to rate constants for p-X-DTBPs plus cumylperoxyl radical reactions indicate a mechanism that involves rate-limiting hydrogen atom transfer (HAT).
Abstract: To obtain mechanistic insights into the inherent reactivity patterns for copper(I)-O2 adducts, a new cupric-superoxo complex [(DMM-tmpa)Cu(II)(O2(•-))](+) (2) [DMM-tmpa = tris((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)amine] has been synthesized and studied in phenol oxidation-oxygenation reactions. Compound 2 is characterized by UV-vis, resonance Raman, and EPR spectroscopies. Its reactions with a series of para-substituted 2,6-di-tert-butylphenols (p-X-DTBPs) afford 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) in up to 50% yields. Significant deuterium kinetic isotope effects and a positive correlation of second-order rate constants (k2) compared to rate constants for p-X-DTBPs plus cumylperoxyl radical reactions indicate a mechanism that involves rate-limiting hydrogen atom transfer (HAT). A weak correlation of (k(B)T/e) ln k2 versus E(ox) of p-X-DTBP indicates that the HAT reactions proceed via a partial transfer of charge rather than a complete transfer of charge in the electron transfer/proton transfer pathway. Product analyses, (18)O-labeling experiments, and separate reactivity employing the 2,4,6-tri-tert-butylphenoxyl radical provide further mechanistic insights. After initial HAT, a second molar equiv of 2 couples to the phenoxyl radical initially formed, giving a Cu(II)-OO-(ArO') intermediate, which proceeds in the case of p-OR-DTBP substrates via a two-electron oxidation reaction involving hydrolysis steps which liberate H2O2 and the corresponding alcohol. By contrast, four-electron oxygenation (O-O cleavage) mainly occurs for p-R-DTBP which gives (18)O-labeled DTBQ and elimination of the R group.

114 citations


Journal ArticleDOI
TL;DR: The potential energy surfaces for the pyrolysis relevant reactions of the biofuel candidate 2-methylfuran have been characterized using quantum chemical methods using Canonical transition state theory and Rice-Ramsperger-Kassel-Marcus theory to determine the high-pressure limiting kinetics of elementary reactions.
Abstract: Due to the rapidly growing interest in the use of biomass derived furanic compounds as potential platform chemicals and fossil fuel replacements, there is a simultaneous need to understand the pyrolysis and combustion properties of such molecules. To this end, the potential energy surfaces for the pyrolysis relevant reactions of the biofuel candidate 2-methylfuran have been characterized using quantum chemical methods (CBS-QB3, CBS-APNO and G3). Canonical transition state theory is employed to determine the high-pressure limiting kinetics, k(T), of elementary reactions. Rice–Ramsperger–Kassel–Marcus theory with an energy grained master equation is used to compute pressure-dependent rate constants, k(T,p), and product branching fractions for the multiple-well, multiple-channel reaction pathways which typify the pyrolysis reactions of the title species. The unimolecular decomposition of 2-methylfuran is shown to proceed via hydrogen atom transfer reactions through singlet carbene intermediates which readily undergo ring opening to form collisionally stabilised acyclic C5H6O isomers before further decomposition to C1–C4 species. Rate constants for abstraction by the hydrogen atom and methyl radical are reported, with abstraction from the alkyl side chain calculated to dominate. The fate of the primary abstraction product, 2-furanylmethyl radical, is shown to be thermal decomposition to the n-butadienyl radical and carbon monoxide through a series of ring opening and hydrogen atom transfer reactions. The dominant bimolecular products of hydrogen atom addition reactions are found to be furan and methyl radical, 1-butene-1-yl radical and carbon monoxide and vinyl ketene and methyl radical. A kinetic mechanism is assembled with computer simulations in good agreement with shock tube speciation profiles taken from the literature. The kinetic mechanism developed herein can be used in future chemical kinetic modelling studies on the pyrolysis and oxidation of 2-methylfuran, or the larger molecular structures for which it is a known pyrolysis/combustion intermediate (e.g. cellulose, coals, 2,5-dimethylfuran).

105 citations


Journal ArticleDOI
TL;DR: It is shown here that iron( IV)-tosylimido complex is much more reactive with sulfides than the corresponding iron(IV)-oxo complex; however, the reverse trend is obtained for hydrogen atom abstraction reactions.
Abstract: Nature often utilizes molecular oxygen for oxidation reactions through monoxygenases and dioxygenases. In many of these systems, a high-valent iron(IV)-oxo active species is found. In recent years, evidence has accumulated of possible iron(IV)-imido and iron(V)-nitrido intermediates in enzymatic catalysis, although little is known about their activity. In this work, we report a detailed combined kinetics and computational study on the difference in reactivity and chemical properties of nonheme iron(IV)-oxo compared with iron(IV)-tosylimido. We show here that iron(IV)-tosylimido complex is much more reactive with sulfides than the corresponding iron(IV)-oxo complex; however, the reverse trend is obtained for hydrogen atom abstraction reactions. The latter proceed with a relatively small kinetic isotope effect of kH/kD = 7 for the iron(IV)-tosylimido complex. Moreover, a Hammett analysis of hydrogen atom abstraction from para-X-benzyl alcohol reveals a slope of close to zero for the iron(IV)-oxo, whereas a ...

101 citations


Journal ArticleDOI
TL;DR: In this paper, visible light irradiation of gold-palladium alloy nanoparticles supported on photocatalytically inert ZrO2 significantly enhances their catalytic activity for oxidant-free dehydrogenation of aromatic alcohols to the corresponding aldehydes at ambient temperatures.

86 citations


Journal ArticleDOI
TL;DR: In this paper, for the hydrogen abstraction reaction of HCHO by OH radicals assisted by water, formic acid, or sulfur acid, the possible reaction mechanisms and kinetics have been investigated theoretically using quantum chemistry methods and transition-state theory.
Abstract: In this paper, for the hydrogen abstraction reaction of HCHO by OH radicals assisted by water, formic acid, or sulfur acid, the possible reaction mechanisms and kinetics have been investigated theoretically using quantum chemistry methods and transition-state theory. The potential energy surfaces calculated at the CCSD(T)/6-311++G(df,pd)//MP2(full)/6-311++G(df,pd) levels of theory reveal that, due to the formation of strong hydrogen bond(s), the relative energies of the transition states involving catalyst are significantly reduced compared to that reaction without catalyst. However, the kinetics calculations show that the rate constants are smaller by about 3, 9, or 10 orders of magnitude for water, formic acid, or sulfur acid assisted reactions than that uncatalyzed reaction, respectively. Consequently, none of the water, formic acid, or sulfur acid can accelerate the title reaction in the atmosphere.

78 citations


Journal Article
TL;DR: In this article, visible light irradiation of gold-palladium alloy nanoparticles supported on photocatalytically inert ZrO2 significantly enhances their catalytic activity for oxidant-free dehydrogenation of aromatic alcohols to the corresponding aldehydes at ambient temperatures.
Abstract: We find that visible light irradiation of gold–palladium alloy nanoparticles supported on photocatalytically inert ZrO2 significantly enhances their catalytic activity for oxidant-free dehydrogenation of aromatic alcohols to the corresponding aldehydes at ambient temperatures. Dehydrogenation is also the dominant process in the selective oxidation of the alcohols to the corresponding aldehydes with molecular oxygen. The alloy nanoparticles strongly absorb light and exhibit superior catalytic and photocatalytic activity when compared to either pure palladium or gold nanoparticles. Analysis with a free electron gas model for the bulk alloy structure reveals that the alloying increases the surface charge heterogeneity on the alloy particle surface, which enhances the interaction between the alcohol molecules and the metal NPs. The increased surface charge heterogeneity of the alloy particles is confirmed with density function theory applied to small alloy clusters. Optimal catalytic activity was observed with a Au : Pd molar ratio of 1 : 186, which is in good agreement with the theoretical analysis. The rate-determining step of the dehydrogenation is hydrogen abstraction. The conduction electrons of the nanoparticles are photo-excited by the incident light giving them the necessary energy to be injected into the adsorbed alcohol molecules, promoting the hydrogen abstraction. The strong chemical adsorption of alcohol molecules facilitates this electron transfer. The results show that the alloy nanoparticles efficiently couple thermal and photonic energy sources to drive the dehydrogenation. These findings provide useful insight into the design of catalysts that utilize light for various organic syntheses at ambient temperatures.

72 citations


Journal ArticleDOI
TL;DR: In this paper, a micro-reactor coupled with online gas chromatography/mass spectrometry with a flame ionization detector was used for lignin solvolytic conversion.
Abstract: The benefit of using hydrogen donor solvents in lignin solvolytic conversion was studied using a micro reactor coupled to online gas chromatography/mass spectrometry with a flame ionization detector. This system is able to achieve very high heating rates of reactants and capable of analyzing the reaction products immediately after the reaction is completed, thus allowing for probing “semi” time-resolved reactions occurring during lignin conversion. These features have been impossible for the batch reactors typically employed in previous solvent liquefaction studies. The study showed that the hydrogen donor solvents tetralin and isopropanol were effective in converting lignin into short-chain alkyl phenols (SCAP) at high yield. The yield of SCAP increased with an increasing reaction temperature over the range of 300–400 °C and reaction time over the range of 5–15 min, which is attributed to stabilization of lignin-derived products by hydrogen abstraction from the hydrogen donor solvents. The molecular weig...

66 citations


Journal ArticleDOI
TL;DR: In this article, N-Heterocyclic carbenes are shown to cleanly abstract dihydrogen from organotin di- and trihydrides to intermediately form the reactive stannylene species [R2Sn] and [R′SnH], respectively.
Abstract: N-Heterocyclic carbenes are shown to cleanly abstract dihydrogen from organotin di- and trihydrides to intermediately form the reactive stannylene species [R2Sn] and [R′SnH], respectively, as well as the corresponding dihydro imidazole [R = Ph, trip: 2,4,6-triisopropylphenyl; R′ = trip, Ar*: 2,6-bis(2′,4′,6′-triisopropylphenyl)phenyl]. Depending on the molar ratio of the carbene, these intermediates undergo further reactions to form tetraorgano distannanes and carbene adducts in excellent yields. Since the side products can be removed as volatiles, the method offers a facile, clean and high yielding route to stannylene derivatives with strongly decreased steric protection compared to previously known compounds. On stoichiometric reaction hydrogen was completely removed from an organotin trihydride and a novel tin cluster with overall composition R6Sn6 (R = trip) was structurally characterized.

Journal ArticleDOI
TL;DR: Rate constants were calculated using conventional transition state theory with an asymmetric Eckart tunneling correction, as implemented in Variflex and are generally in good agreement.
Abstract: We have performed a systematic, theoretical chemical kinetic investigation of H atom abstraction by Ḣ atoms and ȮH, HȮ2, and ĊH3 radicals from aldehydes (methanal, ethanal, propanal, and isobutanal) and acids (methanoic acid, ethanoic acid, propanoic acid, and isobutanoic acid). The geometry optimizations and frequencies of all of the species in the reaction mechanisms of the title reactions were calculated using the MP2 method and the 6-311G(d,p) basis set. The one-dimensional hindered rotor treatment for reactants and transition states and the intrinsic reaction coordinate calculations were also determined at the MP2/6-311G(d,p) level of theory. For the reactions of methanal and methanoic acid with Ḣ atoms and ȮH, HȮ2, and ĊH3 radicals, the calculated relative electronic energies were obtained with the CCSD(T)/cc-pVXZ (where X = D, T, and Q) method and were extrapolated to the complete basis set limit. The electronic energies obtained with the CCSD(T)/cc-pVTZ method were benchmarked against the CCSD(T)/CBS energies and were found to be within 1 kcal mol(-1) of one another. Thus, the energies calculated using the less expensive CCSD(T)/cc-pVTZ method were used in all of the reaction mechanisms and in calculating our high-pressure limit rate constants for the title reactions. Rate constants were calculated using conventional transition state theory with an asymmetric Eckart tunneling correction, as implemented in Variflex. Herein, we report the individual and average rate constants, on a per H atom basis, and total rate constants in the temperature range 500-2000 K. We have compared some of our rate constant results to available experimental and theoretical data, and our results are generally in good agreement.

Journal ArticleDOI
TL;DR: Using a previously developed full-dimensional analytical potential energy surface, it is found that the F + NH3 → HF + NH2 system is a barrierless reaction with intermediate complexes in the entry and exit channels.
Abstract: The hydrogen abstraction reaction of fluorine with ammonia represents a true chemical challenge because it is very fast, is followed by secondary abstraction reactions, which are also extremely fast, and presents an experimental/theoretical controversy about rate coefficients. Using a previously developed full-dimensional analytical potential energy surface, we found that the F + NH3 → HF + NH2 system is a barrierless reaction with intermediate complexes in the entry and exit channels. In order to understand the reactivity of the title reaction, thermal rate coefficidents were calculated using two approaches: ring polymer molecular dynamics and quasi-classical trajectory calculations, and these were compared with available experimental data for the common temperature range 276–327 K. The theoretical results obtained show behavior practically independent of temperature, reproducing Walther–Wagner’s experiment, but in contrast with Persky’s more recent experiment. However, quantitatively, our results are 1 ...

Journal ArticleDOI
TL;DR: Combined evidence from product distribution and isotope effects indicate that HAT, rather than SET, initiates N-dealkylation of atrazine to desethyl- and desisopropylatrazine by the microorganism Rhodococcus sp.
Abstract: Cytochrome P450 enzymes are responsible for a multitude of natural transformation reactions. For oxidative N-dealkylation, single electron (SET) and hydrogen atom abstraction (HAT) have been debated as underlying mechanisms. Combined evidence from (i) product distribution and (ii) isotope effects indicate that HAT, rather than SET, initiates N-dealkylation of atrazine to desethyl- and desisopropylatrazine by the microorganism Rhodococcus sp. strain NI86/21. (i) Product analysis revealed a non-selective oxidation at both the αC and βC-atom of the alkyl chain, which is expected for a radical reaction, but not SET. (ii) Normal 13C and 15N as well as pronounced 2H isotope effects (ecarbon: −4.0‰ ± 0.2‰; enitrogen: −1.4‰ ± 0.3‰, KIEH: 3.6 ± 0.8) agree qualitatively with calculated values for HAT, whereas inverse 13C and 15N isotope effects are predicted for SET. Analogous results are observed with the Fe(IV)O model system [5,10,15,20-tetrakis(pentafluorophenyl)porphyrin-iron(III)-chloride + NaIO4], but not with permanganate. These results emphasize the relevance of the HAT mechanism for N-dealkylation by P450.

Journal ArticleDOI
TL;DR: This work details hydrogen atom abstraction reactions from six ethers by the hydroperoxyl radical in order to test the effect of the functional group on the rate constant calculations, and reports the calculated individual, average, and total rate constants.
Abstract: In this work, we detail hydrogen atom abstraction reactions from six ethers by the hydroperoxyl radical, including dimethyl ether, ethyl methyl ether, propyl methyl ether, isopropyl methyl ether, butyl methyl ether, and isobutyl methyl ether, in order to test the effect of the functional group on the rate constant calculations. The Moller–Plesset (MP2) method with the 6-311G(d,p) basis set has been employed in the geometry optimizations and frequency calculations of all of the species involved in the above reaction systems. The connections between each transition state and the corresponding local minima have been determined by intrinsic reaction coordinate calculations. Energies are reported at the CCSD(T)/cc-pVTZ level of theory and include the zero-point energy corrections. As a benchmark in the electronic energy calculations, the CCSD(T)/CBS extrapolation was used for the reactions of dimethyl ether + HȮ2 radicals. A systematic calculation of the high-pressure limit rate constants has been performed us...

Journal ArticleDOI
TL;DR: In this article, a theoretical study on the mechanism and kinetics of the gas phase reactions of a volatile anaesthetic compound (CF3)2CHOCH2F (Sevoflurane) with the OH radicals has been carried out using the hybrid HF-density functional M06-2X/6-31+G(d,p) method.
Abstract: A theoretical study on the mechanism and kinetics of the gas phase reactions of a volatile anaesthetic compound (CF3)2CHOCH2F (Sevoflurane) with the OH radicals has been carried out using the hybrid HF–density functional M06-2X/6-31+G(d,p) method. Three conformations are predicted for the Sevoflurane molecule. Among the three conformers, the most stable one is considered for a detailed study. Reaction profiles are modeled including the formation of pre-reactive and post-reactive complexes at entrance and exit channels. Single point energy calculations have been performed by using the 6-311++G(d,p) basis set. The hydrogen abstraction from the –CH2F group is found to be the dominant reaction channel for hydrogen abstraction by OH radicals. Theoretically the calculated rate constant is found to be in good agreement with the experimentally measured ones. Using group-balanced isodesmic reactions, the standard enthalpies of formation for (CF3)2CHOCH2F, (CF3)2COCH2F and (CF3)2CHOCHF radicals are also reported for the first time. The atmospheric fate of the alkoxy radical, (CF3)2CHOCHFO, is also investigated for the first time using the same level of theory. Out of four prominent plausible decomposition channels including oxidation, our results clearly point out that reaction with O2 is the dominant path for the decomposition of (CF3)2CHOCHFO in the atmosphere involving the lowest energy barrier which is in accord with recent experimental findings.

Journal ArticleDOI
TL;DR: The high-pressure limit rate constants for every reaction channel were calculated by conventional transition-state theory, with an asymmetric Eckart tunneling correction, using the energies obtained with the G3 method.
Abstract: A systematic investigation of the rate constants for hydrogen atom abstraction reactions by hydroxyl radicals on esters has been performed. The geometry optimizations and frequency calculations were obtained using the second-order Moller–Plesset method with the 6-311G(d,p) basis set. The same method was also used in order to determine the dihedral angle potential for each individual hindered rotor in each reactant and transition state. Intrinsic reaction coordinate calculations were used in order to connect each transition state to the corresponding local minimum. For the reactions of methyl ethanoate with an •OH radical, the relative electronic energies were calculated using the G3 and the CCSD(T)/cc-pVXZ (where X = D, T, and Q) methods, which were extrapolated to the complete basis set (CBS) limit. The electronic energies obtained using the G3 method were then benchmarked against the CBS results and were found to be within 1 kcal mol–1 of one another. The high-pressure limit rate constants for every rea...

Journal ArticleDOI
TL;DR: The selective C(sp3)H activation at position 2 in oxetane has been accomplished by decatungstate photocatalysis under mild conditions and the resulting α-oxy radicals were trapped by electron-poor olefins resulting in the smooth preparation of 2-substituted oxetanes.
Abstract: The selective C(sp3)H activation at position 2 in oxetanes has been accomplished by decatungstate photocatalysis under mild conditions. The resulting α-oxy radicals were trapped by electron-poor olefins resulting in the smooth preparation of 2-substituted oxetanes. The chemoselectivity in hydrogen abstraction in substituted oxetanes containing other H donating groups, such as CH2OH, CH2OAc and CHO, has been demonstrated in intramolecular models.

Journal ArticleDOI
TL;DR: O-Carboryne is a very useful synthon for generating a variety of functionalized carboranes that have potential applications in medicine, materials science and organometallic/coordination chemistry.
Abstract: o-Carboryne (1,2-dehydro-o-carborane) is a very reactive intermediate that can be generated in situ from 1-Br-2-Li-1,2-C2B10H10, 1-I-2-Li-1,2-C2B10H10 or 1-Me3Si-2-[IPh(OAc)]-1,2-C2B10H10. It exists in two resonance forms, a bonding form vs. a biradical form. Each form demonstrates unique reactivity patterns. o-Carboryne can undergo various types of reactions such as [4 + 2]/[2 + 2] cycloaddition, ene reaction, hydrogen abstraction, and sp2/sp3 C–H bond insertion reaction with a broad spectrum of substrates including alkenes, alkynes, (hetero)aromatics, ethers, and ferrocenes. It is a very useful synthon for generating a variety of functionalized carboranes that have potential applications in medicine, materials science and organometallic/coordination chemistry. This perspective offers an overview of recent advances in this interesting research field.

Journal ArticleDOI
TL;DR: The dynamics of the hydrogen abstraction reaction between methane and hydroxyl radical is investigated using an initial state selected time-dependent wave packet method within a six-dimensional model and the newly proposed sudden vector projection model is rationalized.
Abstract: The dynamics of the hydrogen abstraction reaction between methane and hydroxyl radical is investigated using an initial state selected time-dependent wave packet method within a six-dimensional model. The ab initio calibrated global potential energy surface of Espinosa-Garcia and Corchado was used. Integral cross sections from several low-lying rotational states of both reactants have been obtained using the centrifugal sudden and J-shifting approximations. On the empirical potential energy surface, the rotational excitation of methane has little effect on the reaction cross section, but excited rotational states of OH inhibit the reactivity slightly. These results are rationalized with the newly proposed sudden vector projection model.

Journal ArticleDOI
TL;DR: The proposed reaction mechanism, supported by electronic structure calculations, involves hydrogen-atom abstraction from 1,4-cyclohexadiene to provide the cyclohexadienyl radical, leading to formation of the pyrrolide product.
Abstract: The iron(IV) nitrido complex PhB(MesIm)3Fe≡N reacts with 1,3-cyclohexadiene to yield the iron(II) pyrrolide complex PhB(MesIm)3Fe(η5-C4H4N) in high yield. The mechanism of product formation is proposed to involve sequential [4 + 1] cycloaddition and retro Diels–Alder reactions. Surprisingly, reaction with 1,4-cyclohexadiene yields the same iron-containing product, albeit in substantially lower yield. The proposed reaction mechanism, supported by electronic structure calculations, involves hydrogen-atom abstraction from 1,4-cyclohexadiene to provide the cyclohexadienyl radical. This radical is an intermediate in substrate isomerization to 1,3-cyclohexadiene, leading to formation of the pyrrolide product.

Journal ArticleDOI
TL;DR: In this paper, the authors used catalytic system Cu(NO 3 ) 2 /NHPI for the mild oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran with molecular oxygen.

Journal ArticleDOI
TL;DR: In this article, a photochemical RNR containing a [ReI] photooxidant site-specifically incorporated at position 355 ([Re]-β2), adjacent to PCET pathway residue Y356 in β.
Abstract: Ribonucleotide reductases (RNRs) catalyze the conversion of nucleotides to deoxynucleotides in all organisms. Active E. coli class Ia RNR is an α2β2 complex that undergoes reversible, long-range proton-coupled electron transfer (PCET) over a pathway of redox active amino acids (β-Y122 → [β-W48] → β-Y356 → α-Y731 → α-Y730 → α-C439) that spans ∼35 A. To unmask PCET kinetics from rate-limiting conformational changes, we prepared a photochemical RNR containing a [ReI] photooxidant site-specifically incorporated at position 355 ([Re]-β2), adjacent to PCET pathway residue Y356 in β. [Re]-β2 was further modified by replacing Y356 with 2,3,5-trifluorotyrosine to enable photochemical generation and spectroscopic observation of chemically competent tyrosyl radical(s). Using transient absorption spectroscopy, we compare the kinetics of Y· decay in the presence of substrate and wt-α2, Y731F-α2 ,or C439S-α2, as well as with 3′-[2H]-substrate and wt-α2. We find that only in the presence of wt-α2 and the unlabeled subst...

Journal ArticleDOI
TL;DR: In this paper, a comprehensive study at the B3LYP(QM)/CHARMM(MM) level of the hydrogen abstraction reaction from arachidonic acid (AA) catalyzed by rabbit 15-lipoxygenase (15-rLO) is presented.
Abstract: The regioselective activation of C–H bonds in complex molecules containing several of them is still an exciting challenge in chemistry. However, many enzymes catalyze these processes, and much can be learned from the way they do it. For example, each mammalian lipoxygenase isoenzyme abstracts a hydrogen atom from essentially a unique carbon position. This paper presents a comprehensive study at the B3LYP(QM)/CHARMM(MM) level of the hydrogen abstraction reaction from arachidonic acid (AA) catalyzed by rabbit 15-lipoxygenase (15-rLO). Most of the products of this reaction arise from the initial hydrogen abstraction from the carbon C13 of AA. Nevertheless, we have shown that 15-rLO seems able to catalyze not only the abstraction of H13 but also that of H10. After having studied 20 of these hydrogen transfers initiated from different snapshots of an extensive MD sampling of the Michaelis complex, we have even concluded that the reaction mechanisms for both abstractions are identical (proton-coupled electron t...

Journal ArticleDOI
TL;DR: This study provides new insights into the oxidative repair mechanism of DNA repair by nonheme FeII and α-ketoglutarate (α-KG) dependent dioxygenases and a possible explanation for the substrate preference of AlkB.
Abstract: AlkB is the title enzyme of a family of DNA dealkylases that catalyze the direct oxidative dealkylation of nucleobases. The conventional mechanism for the dealkylation of N1-methyl adenine (1-meA) catalyzed by AlkB after the formation of FeIV–oxo is comprised by a reorientation of the oxo moiety, hydrogen abstraction, OH rebound from the Fe atom to the methyl adduct, and the dissociation of the resulting methoxide to obtain the repaired adenine base and formaldehyde. An alternative pathway with hydroxide as a ligand bound to the iron atom is proposed and investigated by QM/MM simulations. The results show OH– has a small impact on the barriers for the hydrogen abstraction and OH rebound steps. The effects of the enzyme and the OH– ligand on the hydrogen abstraction by the FeIV–oxo moiety are discussed in detail. The new OH rebound step is coupled with a proton transfer to the OH– ligand and results in a novel zwitterion intermediate. This zwitterion structure can also be characterized as Fe–O–C complex an...

Journal ArticleDOI
TL;DR: Comparison with available rate data shows this inverse relationship between bond strengths and abstraction rates does not hold at all temperatures, and the conjectured association between bond strength and rates of hydrogen abstraction by hydroxyl radicals is discussed.
Abstract: As part of our ongoing investigation of the combustion chemistry of oxygenated molecules using multireference correlated wave function methods, we report bond dissociation energies (BDEs) in C1–C4 alcohols (from methanol to the four isomers of butanol) and C1–C4 aldehydes (from methanal to butanal). The BDEs are calculated with a multireference averaged coupled-pair functional-based scheme. We compare these multireference BDEs with those derived from experiment and single-reference methods. Trends in BDEs for the alcohols and aldehydes are rationalized by considering geometry relaxations of dissociated radical fragments, resonance stabilization, and hyperconjugation. Lastly, we discuss the conjectured association between bond strengths and rates of hydrogen abstraction by hydroxyl radicals. In general, abstraction reaction rates are higher at sites where the C–H bond energies are lower (and vice versa). However, comparison with available rate data shows this inverse relationship between bond strengths and...

Journal ArticleDOI
TL;DR: A computational study proposes a new mechanism for the reaction mechanism of Peptidylglycine α-hydroxylating monooxygenase, suggesting that the most likely abstracting species is [CuOOH]2+.

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TL;DR: The combined experimental and theoretical study on the elucidation of the intrinsic reactivity patterns of theHRP-I and HRP-II mimics provides a valuable tool for evaluating the particular role of the HRP active species in biological systems.
Abstract: For the exploration of the intrinsic reactivity of two key active species in the catalytic cycle of horseradish peroxidase (HRP), Compound I (HRP-I) and Compound II (HRP-II), we generated in situ [Fe(IV) O(TMP(+.) )(2-MeIm)](+) and [Fe(IV) O(TMP)(2-MeIm)](0) (TMP=5,10,15,20-tetramesitylporphyrin; 2-MeIm=2-methylimidazole) as biomimetics for HRP-I and HRP-II, respectively. Their catalytic activities in epoxidation, hydrogen abstraction, and heteroatom oxidation reactions were studied in acetonitrile at -15 °C by utilizing rapid-scan UV/Vis spectroscopy. Comparison of the second-order rate constants measured for the direct reactions of the HRP-I and HRP-II mimics with the selected substrates clearly confirmed the outstanding oxidizing capability of the HRP-I mimic, which is significantly higher than that of HRP-II. The experimental study was supported by computational modeling (DFT calculations) of the oxidation mechanism of the selected substrates with the involvement of quartet and doublet HRP-I mimics ((2,4) Cpd I) and the closed-shell triplet spin HRP-II model ((3) Cpd II) as oxidizing species. The significantly lower activation barriers calculated for the oxidation systems involving (2,4) Cpd I than those found for (3) Cpd II are in line with the much higher oxidizing efficiency of the HRP-I mimic proven in the experimental part of the study. In addition, the DFT calculations show that all three reaction types catalyzed by HRP-I occur on the doublet spin surface in an effectively concerted manner, whereas these reactions may proceed in a stepwise mechanism with the HRP-II mimic as oxidant. However, the high desaturation or oxygen rebound barriers during CH bond activation processes by the HRP-II mimic predict a sufficient lifetime for the substrate radical formed through hydrogen abstraction. Thus, the theoretical calculations suggest that the dissociation of the substrate radical may be a more favorable pathway than desaturation or oxygen rebound processes. Importantly, depending on the electronic nature of the oxidizing species, that is, (2,4) Cpd I or (3) Cpd II, an interesting region-selective conversion phenomenon between sulfoxidation and H-atom abstraction was revealed in the course of the oxidation reaction of dimethylsulfide. The combined experimental and theoretical study on the elucidation of the intrinsic reactivity patterns of the HRP-I and HRP-II mimics provides a valuable tool for evaluating the particular role of the HRP active species in biological systems.

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TL;DR: Paramagnetic CpCr[(RN)2C6H4] compounds were treated with R'N3 azides to generate chromium imido complexes that differed as the R and R' substituents were varied.

Journal ArticleDOI
TL;DR: The reaction system was successfully applied to three-component reactions, including radical carbonylation, and the rate constant for the hydrogen abstraction of a primary alkyl radical from tetrabutylammonium cyanoborohydride was estimated to be 3 orders of magnitude smaller than that of tributyltin hydride.
Abstract: Cyanoborohydrides are efficient reagents in the reductive addition reactions of alkyl iodides and electron-deficient olefins. In contrast to using tin reagents, the reaction took place chemoselectively at the carbon–iodine bond but not at the carbon–bromine or carbon–chlorine bond. The reaction system was successfully applied to three-component reactions, including radical carbonylation. The rate constant for the hydrogen abstraction of a primary alkyl radical from tetrabutylammonium cyanoborohydride was estimated to be <1 × 104 M–1 s–1 at 25 °C by a kinetic competition method. This value is 3 orders of magnitude smaller than that of tributyltin hydride.