Showing papers on "Hydrogen atom abstraction published in 1999"

Polarity-reversal catalysis of hydrogen-atom abstraction reactions: concepts and applications in organic chemistry

Abstract: The rates and selectivities of the hydrogen-atom abstraction reactions of electrically-neutral free radicals are known to depend on polar effects which operate in the transition state. Thus, an electrophilic species such as an alkoxyl radical abstracts hydrogen much more readily from an electron-rich C–H bond than from an electron-deficient one of similar strength. The basis of polarity-reversal catalysis (PRC) is to replace a single-step abstraction, that is slow because of unfavourable polar effects, with a two-step process in which the radicals and substrates are polarity-matched. This review explores the concept of PRC and describes its application in a variety of situations relevant to mechanistic and synthetic organic chemistry.

An ab Initio Investigation of the Reactions of 1,1- and 1,2-Dichloroethane with Hydroxyl Radical

Abstract: The hydrogen abstraction reactions by OH radical from 1,1-dichloroethane and 1,2-dichloroethane have been investigated by ab initio molecular orbital theory. Optimized geometries and harmonic vibrational frequencies have been calculated for all reactants, transition structures, and products at the (U)HF/6-311G(d,p) and (U)MP2=full/6-311G(d,p) levels of theory. Single point QCISD(T)/6-311G(d,p)//(U)MP2=full/6-311G(d,p) calculations have also been carried out for the inclusion of higher order electron correlation. Three distinct transition structures have been located for the H3C−CHCl2 + OH reaction (one for α-abstraction and two for β-abstraction). Four transition structures have been located for the reaction ClH2C-CH2Cl + OH. The calculated barrier heights, reaction enthalpies, and change in entropy are found to be in good agreement with available experimental values. In addition, the rate constants calculated by using the transition state theory are found to be in good agreement with the experimental res...

Bond Dissociation Energies of the N−H Bond and Rate Constants for the Reaction with Alkyl, Alkoxyl, and Peroxyl Radicals of Phenothiazines and Related Compounds

Abstract: The results of a detailed thermodynamic and kinetic investigation on the homolytic reactivity of phenothiazine, phenoxazine, and phenoselenazine, of several substituted phenothiazines, and of related tricyclic aromatic amines are reported. All these compounds give, by hydrogen atom abstraction from the N−H group, persistent aminyl radicals. Equilibration of each of these radicals with the parent amine and a reference compound having an easily abstractable hydrogen allowed us to determine, by using EPR spectroscopy, the N−H Bond Dissociation Energies (BDE) of the amines. These are characterized by low BDE values (in some cases lower than the O−H bond strength of α-tocopherol, i.e 78.3 kcal/mol) and therefore are very good hydrogen atom transfer reagents. To check the efficiency of tricyclic amines as antioxidants and as polymerization inhibitors, absolute rate constants were determined for their reaction with alkyl, alkoxyl, and peroxyl radicals by using competitive techniques in the first two cases and by...

Theoretical Model Studies of the Iron Dimer Complex of MMO and RNR.

Abstract: Hybrid DFT calculations are used to study the structures and chemistry of the diiron complex of MMO and RNR. The chemical model used is larger than the ones used previously and contains the full first ligand sphere. New types of structures are suggested for the intermediates observed in the reactions of these enzymes. To obtain the very short Fe−Fe distances recently measured by EXAFS, it is suggested that both compound Q of MMO and compound X of RNR have two bridging carboxylates like the reduced diferrous complex but unlike the diferric complex. Antiferromagnetic coupling is shown to have significant effects on the electronic structure of the complexes and is also important for achieving the short Fe−Fe distance. The new model is also used to study the activation of methane, and a pure hydrogen abstraction transition state is located where a hydrogen of methane is abstracted by a bridging μ-oxo ligand. The combination of the methyl radical with the bridging μ-OH ligand formed occurs without a barrier an...

Theoretical study of the kinetics of the hydrogen abstraction from methanol. 3. Reaction of methanol with hydrogen atom, methyl, and hydroxyl radicals

Abstract: Ab initio calculations at different levels of theory have been performed for the title H-abstraction reactions. Total energies at stationary points of the potential energy surfaces for the reaction systems were obtained at MP2 and MP4 levels and improved by using Gaussian-2 (G2) methodology. The calculated G2 heats of reaction agree well with the experimental ones for both methoxy (product resulting from hydroxyl-side attack) and hydroxymethyl (product resulting from methyl-side attack) reaction channels. Calculations of the potential energy surfaces for the reaction systems show that H-abstraction from methanol by H, CH3, and OH (for methoxy reaction channel) proceeds by simple metathesis. The mechanism of the hydroxymethyl channel of reaction CH3OH + OH appears to be more complex, and it may consist of two consecutive processes. The reaction rate is determined by the energy barrier of the first process. Differences in the heights of the calculated energy barriers explain the differences in the reactivit...

Absolute Rate Constants for the β-Scission and Hydrogen Abstraction Reactions of the tert-Butoxyl Radical and for Several Radical Rearrangements: Evaluating Delayed Radical Formations by Time-Resolved Electron Spin Resonance

Abstract: An analysis is presented for the determination of absolute rate constants for radical transformation reactions in complex reaction systems by time-resolved ESR during intermittent photochemical radical production. The technique is applied to obtain absolute rate constants in large temperature ranges for the β-scission of the tert-butoxyl radical in various solvents and its hydrogen abstraction from cyclohexane, cyclopentane, tert-butylbenzene, and anisole. Kinetic studies of the neophyl and the 2-methyloxiran-2-yl rearrangements and the decarboxylation of the tert-butoxycarbonyl radical also demonstrate the versatility of the method. Further, rate data are given for the addition of the methyl radical to benzene and fluorobenzene, as well as for the hydrogen abstraction of methyl from di-tert-butyl peroxide.

Hydrogen Transfer Reactivity of a Ferric Bi-imidazoline Complex That Models the Activity of Lipoxygenase Enzymes.

Abstract: The mono-deprotonated ferric complex [Fe(Hbim)(H2bim)2](ClO4)2 (2) oxidizes organic substrates with weak C−H bonds. A hydrogen atom abstraction mechanism is indicated. The ΔH° for addition of H• to 2 (the strength of a N−H bond in 1) is determined to be 76 kcal/mol, consistent with the reactivity observed.

Rates and mechanisms of the thermal solvolytic decomposition of arenediazonium ions

Abstract: Arenediazonium tetrafluoroborates have been prepared and the kinetics of solvolysis have been investigated in water, trifluoroethanol, water–trifluoroethanol mixtures, hexafluoropropan-2-ol, trifluoroacetic acid, and ethanol by a UV method. A heterolytic mechanism involving short-lived aryl cations leads to products derived from nucleophilic capture of the aryl cations by solvent or a solute. Ionic solutes in aqueous trifluoroethanol and trifluoromethoxybenzene in trifluoroethanol have no kinetic effect and neither does replacement of the tetrafluoroborate counter-ion by chloride in trifluoroethanol. Rate constants for any one compound are not very solvent dependent, the reactions generally being characterised by high enthalpies of activation and appreciably positive entropies of activation. Compounds with 4-Cl, 4-F, 4-NO2, and 4-MeO substituents proved too unreactive for kinetic studies, but for different reasons. In ethanol, a radical reaction with characteristically different activation parameters competes with the heterolytic path and leads to hydrodediazoniation (reduction) by hydrogen atom abstraction from the CH2 group of ethanol.

Mammalian 15-Lipoxygenases

Abstract: Lipoxygenases are enzymes which dioxygenate polyunsaturated fatty acids to hydroperoxy derivatives. Although the mechanism of the enzyme catalysis is not entirely clear, it has been suggested that the lipoxygenase reaction involves the formation of an enzyme-bound fatty acid radical which is formed via a stereoselective removal of a hydrogen from a doubly allylic methylene group.1 It should be stressed that there are alternative explanations of the reaction mechanism implicating an electron removal from a double bond forming a fatty acid cation and a subsequent abstraction of a proton which would require a strong basic residue at the active site of the enzyme.2 Assuming the radical mechanism, the lipoxygenase reaction resembles that of the non-enzymatic lipid peroxidation. In principle, both reactions can be divided into three steps (Figure 1): i) hydrogen abstraction, ii) radical rearrangement and iii) oxygen insertion. During the lipoxygenase reaction each of the three steps is enzyme-controlled which leads to a specific pattern of oxygenation products. If, for instance, more than one doubly allylic methylene is present in the substrate fatty acid, lipoxygenases select one of them for initial hydrogen abstraction.3 In contrast, during non-enzymatic lipid peroxidation, hydrogen is removed from all doubly allylic methylenes. Similarly, the introduction of dioxygen proceeds stereoselectively in the case of the lipoxygenase reaction, whereas a stereorandom oxygenation is observed for the non-enzymatic lipid peroxidation.

Photochemistry and photoinduced chemical crosslinking activity of type I & II co-reactive photoinitiators in acrylated prepolymers

Abstract: The photoreactivity of Type I and II photointiators, namely, 4-(2-hydroxyethoxy)-phenyl-(2-hydroxy2-methylpropyl) ketone (Irgacure 2959, Ciba-Geigy) and 4-hydroxybenzophenone, respectively, have been examined in the free and bound form in melamine and urethane acrylate prepolymers. Co-reaction in the prepolymers was undertaken by partial replacement of the hydroxyacrylate components with the photoinitiators in the reaction with the isocyanate component. Both the photoinitiators were also converted into their corresponding acrylate derivatives for study. The free, bound and acrylated derivatives were then analysed by various spectroscopic techniques to evaluate the relationship between their photophysical properties and photoinitiation activity for photocrosslinking of commercial acrylated monomers and prepolymers. Photocuring studies using Hg sources (conveyor and RTIR) indicated that whilst binding the Type I photointiator reduced its efficiency in cure, the Type II photoinitiator was enhanced. The presence of an amine co-synergist, however, generally eliminated these differentials. Acrylation of the Irgacure 2959 also reduced photoactivity. Absorption spectroscopy indicated the presence of a strongly absorbing nπ * transition in the far UV region for both chromophore types which undergoes a blue shift on binding to the prepolymer. Phosphorescence spectra, lifetimes and quantum yields also indicate the presence of low lying triplet nπ * states for both types of chromophore. Binding appears to enhance the triplet lifetime and reduce the quantum yield of emission due to increased electron donation into the chromophore by the resin component and may in part account for their lower photoactivity. Microsecond flash photolysis identified the formation of benzoyl radicals in the case of the Type I system with a small enhancement in radical formation on binding. The Type II system gave ketyl radicals formed by hydrogen atom abstraction which were enhanced in the bound resin system. The latter would account for increased photoactivity for the bound Type II system. Nanosecond laser flash photolysis experiments identified the triplet–triplet absorption in the case of the Type II initiator. In the presence of a tertiary amine (triethylamine) the transient absorption and its lifetime were significantly enhanced due to exciplex formation. However, binding the benzophenone initiator to the resin significantly quenched the transient absorption and significantly reduced its lifetime. In the case of the Type I initiator the benzoyl radical was observed directly, confirming the microsecond flash photolysis data. Binding of the initiator to the melamine acrylate resin also reduced benzoyl radical formation and reduced the radical lifetime. This may account for the reduced photoactivity of the Type I bound initiator. Radical formation was also reduced when bound to the urethane acrylate but the lifetime was enhanced. For the initiator bound resin systems the presence of an amine co-synergist enhanced benzoyl radical formation. The implications of initiator co-reactions are discussed in terms of both the photochemical and commercial benefits.

Direct ab Initio Dynamics Studies of the Hydrogen Abstraction Reactions of Hydrogen Atom with Fluoromethanes

Abstract: A direct ab initio dynamics study on the gas-phase reactions of atomic hydrogen with different fluoromethanes has been carried out. The thermal rate constants were calculated using canonical variational transition state (CVT) theory augmented by multidimensional semiclassical zero and small curvature tunneling approximations. The potential energy surfaces for the reactions were calculated using hybrid density functional theory, namely, Becke’s half-and-half (BH) nonlocal exchange and the Lee -Yang-Parr (LYP) nonlocal correlation functionals using the cc-pVDZ basis set. The reaction energies and barrier heights were improved by single-point energy calculations along the minimum energy path (MEP) at the spin-projected fourth order Moller-Plesset perturbation theory (PMP4) using the cc-pVTZ basis set. The calculated forward and reverse thermal rate constants are in the good agreement with the experimental data. The electronic effects of fluorine substitution on the rate of this class of reactions are examined.

Hydrogen Abstraction from Methane and Hydrofluoromethanes by •OH Radical: Modified GAUSSIAN-2 Study

Abstract: The reactions of methane and hydrofluoromethanes (CH4-nFn, n = 0, 1, 2, or 3) with a hydroxyl radical have been investigated by a modified GAUSSIAN-2 (G2M) method. Reaction enthalpies have been compared to those obtained by original G2 and G2MP2 schemes. The average absolute error for the reaction enthalpies calculated with the G2M method was smaller than those calculated with the G2 and G2MP2 schemes. G2M reaction enthalpies were of chemical accuracy. Moreover, the G2M method and selected MP2 schemes (those with the smallest average errors in the calculations for the reaction enthalpies) were used to compute the classical barrier heights. According to the transition state theory, the reaction rates were computed in the temperature range of 280−420 K and then the activation energies were obtained by the least-squares fitting to the Arrhenius equation. The activation energies calculated by using G2M barrier heights showed the best agreement with the experimentally derived values. The influence of the fluor...

Dynamics of the O(1D)+CH4 reaction: Atomic hydrogen channel vs molecular hydrogen channel

Abstract: The O(1D)+CH4 reaction has been investigated using a new universal crossed molecular beam apparatus. Both the atomic hydrogen channel (CH3O/CH2OH+H) and the molecular hydrogen channel (H2CO/HCOH+H2) have been experimentally observed in this reaction. The experimental results suggest that the main atomic hydrogen channel in the O(1D)+CH4 reaction should be CH2OH (hydroxymethyl)+H, while the CH3O (methoxy)+H channel is at most a minor process. From the product angular distribution measurements, it is clear that the radical products (CH2OH and/or CH3O) in the hydrogen atom channel are only slightly backward scattered relative to the O(1D) beam direction, indicating that this product channel mainly goes through a long-lived intermediate pathway. The slightly backward scattered products are possibly due to other reaction mechanisms. For the molecular hydrogen channel, the product angular distribution obtained from simulation also seems isotropic, implying that this channel also likely goes through a long-lived...

Solvent effects on rates of photochemical reactions of rose bengal triplet state studied by nanosecond laser photolysis

Abstract: Bimolecular rate constants of energy transfer, electron-transfer, and H-atom abstraction reaction have been determined by following the transient absorption bands of the triplet states of rose bengal [ 3 ( RB 2− ) ∗ ] at about 1020 nm with various substrates using nanosecond laser flash photolysis. For electron acceptor (1,4-benzoquinone), electron-transfer takes place via an exciplex. Exciplex formation was found to be predominant in high viscosity solvents rather than low viscosity solvents. For electron donor (3,3′,5,5′-tetramethylbenzidine), the rate constants are independent of solvent viscosity, indicating that the contribution to an exciplex formation is small. The H-atom abstraction reactions of 3 ( RB 2− ) ∗ with H-donors show that the rate constants in water are higher than those in ethanol and in THF. The negative slopes of the Hammett plots indicate that the reaction center of [ 3 ( RB 2− ) ∗ ] has a highly electrophilic nature, which increases with solvent polarity, suggesting the polar transition states.

Ab Initio/Density Functional Theory and Multichannel RRKM Calculations for the CH3O + CO Reaction

Abstract: The potential energy surface for the reaction of methoxy radical with carbon monoxide has been studied using the G2(B3LYP/MP2/CC) method. Two reaction mechanisms were revealed. The hydrogen abstraction of CH3O by CO produces CH2O + HCO via a barrier of 24.19 kcal/mol. The addition of CH3O to CO proceeds to an intermediate CH3OCO via a barrier of 6.39 kcal/mol. The products, CH3 and CO2, can be formed in two ways. One is the C−O bond cleavage of the CH3OCO radical. The other involves the isomerization of CH3OCO to the CH3CO2 radical and the subsequent C−C bond fission. CH2O and HCO can be formed via the path CH3OCO → TS6 → IM4 → TS7 → CH2O + HCO. A radical product, CH2COOH, is formed through the hydrogen rearrangement of the CH3CO2 radical. Multichannel RRKM calculations have been carried out for the total and individual rate constants for various channels over a wide range of temperatures and pressures using the ab initio data. At lower temperatures, the title reaction is dominated by the stabilization of...

Magnetic Field Effects Due to the Δg Mechanism upon Chemical Reactions through Radical Pairs under Ultrahigh Fields of up to 30 T

Abstract: Magnetic field effects (MFEs) due to the isotropic Δg mechanism were studied for the hydrogen abstraction reactions of 4-methoxy-benzophenone with thiophenol in several solvents at 293 K by a laser flash photolysis technique under ultrahigh magnetic fields of up to 30 T. In 2-propanol, ethanol, and methanol, the yield of the escaped ketyl radical decreased with increasing magnetic field from 0 to 30 T. In 2-methyl-1-propanol, however, the decrease was almost saturated at 20 T and the yield above 20 T reduced to be 2/3 of that at 0 T. Such saturation of the MFEs due to the isotropic Δg mechanism in the reaction through radical pairs in solution was found for the first time.

Benzannelation Effect on Enediyne Cycloaromatization: An ab Initio Molecular Orbital Study

Abstract: The mechanism of altering the rate-limiting step in enediyne cycloaromatization (Bergman reaction) by benzannelation has been studied using ab initio molecular orbital (MO) methods. The calculated results indicate that the benzannelation effect cannot be interpreted by the energy separation between the lowest singlet and triplet states in p-benzyne-type intermediates 2 and 4, as revealed for p-benzyne 6 and 1,4-diylnaphthalene 7. The energy barriers for the retro-cyclizations of the intermediates to produce cyclodec-3-ene-1,5-diyne (1) and 3,4-benzo-cyclodec-3-ene-1,5-diyne (3) are estimated to be 15.3 and 5.9 kcal/mol, respectively. Hydrogen abstractions from methane by p-benzyne-type intermediates in the Bergman reaction of 1 and 3 are calculated to have energy barriers of 12.7 and 11.8 kcal/mol. From these theoretical results on the energy barriers of retro-cyclizations and hydrogen abstraction, we concluded that the rate-limiting step in Bergman reaction of 3 is hydrogen abstraction rather than cycliz...

Probing the mechanism of C-H activation: oxidation of methylcubane by soluble methane monooxygenase from Methylosinus trichosporium OB3b.

Abstract: The soluble form of methane monooxygenase (MMO) isolated from methanotrophic bacteria catalyzes the O2-dependent conversion of methane to methanol, as well as the adventitious oxidation of many other hydrocarbons. In past studies, it was reported that the oxidation reaction of methylcubane, a radical clock substrate, catalyzed by MMO from Methylococcus capsulatus (Bath) gave only cubylmethanol as the product rather than methylcubanol(s) or rearranged products characteristic of a radical formed on the methyl group [Choi, S.-Y., Eaton, P. E., Hollenberg, P. F., Liu, K. E., Lippard, S. J., Newcomb, M., Putt, D. A., Upadhyaya, S. P., and Xiong, Y. (1996) J. Am. Chem. Soc. 118, 6547−6555]. Such a substrate radical intermediate would be expected if the mechanism of MMO involves hydrogen atom abstraction as indicated by many previous mechanistic studies. Here it is shown that the reaction of methylcubane with the reconstituted MMO system from Methylosinus trichosporium OB3b yields both cubylmethanol and methylcu...

Hydrogen-atom number in platinum-hydrogen complexes in silicon

Abstract: Four different platinum-hydrogen complexes are identified in silicon by transient capacitance spectroscopy. From deep-level depth profiling, we are able to identify the number of hydrogen atoms in the defect complexes. Three of the platinum-hydrogen complexes are electrically active, and contain one, two, and three hydrogen atoms, respectively. A complete electrical passivation of substitutional Pt is achieved by at least four hydrogen atoms. Our results are in agreement with experimental data on a $\mathrm{Pt}\ensuremath{-}{\mathrm{H}}_{2}$ complex and a theoretical study on the structure and level positions of these defect complexes.

Iron Porphyrin and Cysteine Mediated Reduction of Ten Polyhalogenated Methanes in Homogeneous Aqueous Solution: Product Analyses and Mechanistic Considerations

Abstract: Reductive dehalogenation reactions of polyhalogenated C 1 - and C 2 -compounds are presently of particular interest because of the potential applicability of such processes in the treatment of wastes as well as in remediation approaches to removing such compounds from contaminated soils and aquifers. In this context, it is not only important to know the reaction kinetics of a specific compound with relevant reductants but also the type of product(s) formed under given conditions. In this study we have identified reaction intermediates as well as the final products of the reduction of 10 polyhalogenated methanes (PHMs) by an iron porphyrin in the presence of cysteine. Cysteine was chosen for two reasons: (i) as bulk electron donor and (ii) as an aqueous organic compound exhibiting functional groups (i.e., -NH 2 , -SH) that may undergo reactions other than just hydrogen abstraction with reactive intermediates such as radicals and carbenes. The data obtained support our hypothesis postulated in an earlier kinetic study that the initial and rate-determining step in the reduction of PHMs by the iron porphyrin is a dissociative one-electron transfer. Furthermore, it is shown that in fast consecutive reactions involving primarily the mercapto group of cysteine, all compounds were completely dehalogenated. Except for the fluorine containing compounds, the carbon of a given PHM was quantitatively recovered as N-formylcysteine. In the case of the fluorinated compounds, carbene intermediates could be trapped, which reacted further to some unidentified product(s), possibly including carbon monoxide. Finally, it is shown that the reduction of tetrahalomethanes by cysteine as sole reductant leads predominantly to the formation of the corresponding haloforms, suggesting that, in this case, the reaction occurred primarily by an X-philic dissociative two-electron transfer. The results of this study offer an interesting perspective for a fast complete dehalogenation of PHMs by using a very reactive one-electron donor (i.e., a reactive iron species) in the presence of organic matter exhibiting reduced sulfur groups.

Enantioselective Discrimination in the Intramolecular Quenching of an Excited Aromatic Ketone by a Ground-State Phenol

Abstract: Chiral recognition is a subject of increasing interest in photochemistry1 In principle, this can be achieved with optically active photosensitizers if an enantiodifferentiating interaction occurs in the relevant excited state Photosensitized electron transfer processes appear specially suitable for such purposes, as the donor and the acceptor must form a highly coupled system for some time2 The use of chiral compounds as both excited sensitizers and quenchers could enable examination of pure chiral discrimination through comparison of the quenching rates, since enantiomers have otherwise the same electronic nature and the same size Leigh et al3 have suggested that hydrogen abstraction from phenols by carbonyl π,π* triplets involves electron transfer within a hydrogen-bonded triplet exciplex, followed by proton transfer In this context, we have prepared bichromophoric compounds containing enantiomerically pure aryl ketones and phenols, to check the stereochemical aspects of intramolecular electron transfer by means of laser flash photolysis studies Benzoylthiophene is a heterocyclic diaryl ketone with a lowest lying π,π* triplet4 This excited state involves predominantly the thenoyl chromophore; it has an energy4 of ca 63 kcal mol-1 and is able to react with phenols, such as p-cresol and tyrosine methyl ester5 Both substructures are present in the four enantiomerically pure bichromophoric compounds prepared in this work: TPATyr [(R,S)-1, (S,S)-1] and SUP-Tyr [(R,S)-2, (S,S)-2] (Scheme 1) We have studied the intramolecular electron-transfer process by laser flash photolysis through phenol quenching of the ketone triplet state and found a high degree of stereoselectivity, which agrees with a geometry where interaction between the π-systems of the phenol and the aromatic ketone is maximized Compounds 1 and 2 differ in the relative aromatic ketone versus phenol orientation They were obtained by condensation of the corresponding racemic acid, tiaprofenic acid (TPA) or suprofen (SUP), with the methyl ester of the natural occurring amino acid (S)-tyrosine, in the presence of a carbodiimide {(1-ethyl-3-[3dimethylamino]propylcarbodiimide), EDC} In both cases, after purification of the resulting amides, one of the diastereoisomers crystallized The other stereoisomers were further purified by HPLC chromatography [potassium dihydrogen phosphate (006 M):acetonitrile:triethylamine (317:183:01 by volume)] All the bichromophoric compounds were fully characterized by 1H NMR, 13C NMR, and elemental analysis