Showing papers on "Hydrogen atom abstraction published in 2000"

The C2H5 + O2 Reaction Mechanism: High-Level ab Initio Characterizations

Abstract: The C2H5• + O2 reaction, central to ethane oxidation and thus of fundamental importance to hydrocarbon combustion chemistry, has been examined in detail via highly sophisticated electronic structure methods. The geometries, energies, and harmonic vibrational frequencies of the reactants, transition states, intermediates, and products for the reaction of the ethyl radical (X 2A‘) with O2 (X 3 , a 1Δg) have been investigated using the CCSD and CCSD(T) ab initio methods with basis sets ranging in quality from double-zeta plus polarization (DZP) to triple-zeta plus double polarization with f functions (TZ2Pf). Five mechanisms (M1−M5) involving the ground-state reactants are introduced within the context of previous experimental and theoretical studies. In this work, each mechanism is systematically explored, giving the following overall 0 K activation energies with respect to ground-state reactants, Ea(0 K), at our best level of theory: (M1) direct hydrogen abstraction from the ethyl radical by O2 to give e...

Flash Vacuum Pyrolysis of Methoxy-Substituted Lignin Model Compounds

Abstract: The flash vacuum pyrolysis (FVP) of methoxy-substituted β-O-4 lignin model compounds has been studied at 500 °C to provide mechanistic insight into the primary reaction pathways that occur under conditions of fast pyrolysis. FVP of PhCH2CH2OPh (PPE), a model of the dominant β-O-4 linkage in lignin, proceeds by C−O and C−C cleavage, in a 37:1 ratio, to produce styrene plus phenol as the dominant products and minor amounts of toluene, bibenzyl, and benzaldehyde. From the deuterium isotope effect in the FVP of PhCD2CH2OPh, it was shown that C−O cleavage occurs by homolysis and by 1,2-elimination in a ratio of 1.4:1, respectively. Methoxy substituents enhance the homolysis of the β-O-4 linkage, relative to PPE, in o-CH3O-C6H4OCH2CH2Ph (o-CH3O-PPE) and (o-CH3O)2-C6H3OCH2CH2Ph ((o-CH3O)2-PPE) by a factor of 7.4 and 21, respectively. The methoxy-substituted phenoxy radicals undergo a complex series of reactions, which are dominated by 1,5-, 1,6-, and 1,4-intramolecular hydrogen abstraction, rearrangement, and β−...

Hydrogen Atom Abstraction Kinetics from Intramolecularly Hydrogen Bonded Ubiquinol-0 and Other (Poly)methoxy Phenols

Abstract: The effect of methoxy substitution on the abstraction of the phenolic hydrogen atom involved in intramolecular hydrogen bonding by tert-butoxyl and cumyloxyl radicals has been investigated by laser flash photolysis. Also transition state calculations for methoxyl radical and 2-methoxyphenol have been carried out by a density functional theory (DFT) method. Hydrogen atom abstraction is surprisingly easy from intramolecularly hydrogen bonded methoxyphenols, in contrast to intermolecularly hydrogen bonded molecules. The kinetic solvent effect, investigated in six solvents with different hydrogen bond accepting properties, on the hydrogen atom abstraction reaction from o-methoxy phenols was shown to be smaller than for non-hydrogen bonded phenols, and is independent of further methoxy substitution. The high rate constant for hydrogen atom abstraction from ubiquinol-0 (2.8 × 109 M-1 s-1 in CCl4) and the small kinetic solvent effect make it a good antioxidant, even in a polar environment.

Photo- and radiation chemical induced degradation of lignin model compounds.

Abstract: The basic mechanistic aspects of the photo- and radiation chemistry of lignin model compounds (LMCs) are discussed with respect to important processes related to lignin degradation. Several reactions occur after direct irradiation, photosensitized or radiation chemically induced oxidation of LMCs. Direct irradiation studies on LMCs have provided supportive evidence for the involvement of hydrogen abstraction reactions from phenols, β-cleavage of substituted α-aryloxyacetophenones and cleavage of ketyl radicals (formed by photoreduction of aromatic ketones or hydrogen abstraction from arylglycerol β-aryl ethers) in the photoyellowing of lignin rich pulps. Photosensitized and radiation chemically induced generation of reactive oxygen species and their reaction with LMCs are reviewed. The side-chain reactivity of LMC radical cations, generated by radiation chemical means, is also discussed in relation with the enzymatic degradation of lignin.

The effects of weak magnetic fields on radical recombination reactions in micelles.

Abstract: Purpose : To demonstrate the effects of weak magnetic fields (> ˜ 1 mT) on chemical reactions involving free radicals, in the context of possible effects of environmental electromagnetic radiation on biological systems. Materials and methods : Transient absorption, flash photolysis experiments have been performed to study the kinetics and yields of radical reactions. The triplet state of benzophenone has been used as a convenient source of radical pairs, whose identity is largely immaterial to the investigation of the so-called Low Field Effect. Hydrogen abstraction from surfactant molecules in micelles yields a pair of neutral radicals, one large and one small, in a region of restricted translational and rotational motion. Results : In alkyl sulphate and sulphonate micelles a weak field increases the concentration of free radicals that escape from the micelle to an extent that depends on the structure, dynamics and volume of the space in which the radical pairs are confined. The effect (up to 10%) is typ...

Biomimetical catalytic activity of iron(III) porphyrins encapsulated in the zeolite X

Abstract: The approach adopted for the obtention of zeolite-encapsulated FeP led to clean syntheses of biomimetical catalyst. The catalysts were obtained through the zeolite synthesis method, where NaX zeolite was synthesised around one of the cationic FePs: iron(III) 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin (FeP1) or iron(III) 5-mono(2,6-dichloro-phenyl)10,15,20-tris(4-N-methylpyridyl)porphyrin (FeP2). The syntheses yielded pure FeP1NaX and FeP2NaX catalysts without any by-products blocking the zeolite nanopores. FeP1NaX and FeP2NaX efficiently catalysed the epoxidation of (Z)-cyclooctene by iodosylbenzene (PhIO) in DCE, giving rise to cis-epoxycyclooctane yields of 85% and 95%, respectively. Hydroxylation of adamantane shows a preferable alkane oxidation at the tertiary CH bond, indicating a hydrogen abstraction through the FeIV(O)P·+species in the initial step. The total adamantanol yields were 52% and 45% for FeP1NaX and FeP2NaX, respectively. Concerning selectivity, FeP1NaX and FeP2NaX gave an 1-adamantanol (Ad-1-ol)/2-adamantanol (Ad-2-ol) ratio of 20:1 and 11:1, respectively (after statistical correction). Therefore, these results indicate a free radical activation of the CH bonds of adamantane as expected for P-450 models. In the cyclohexane oxidation catalysed by FeP1NaX in DCE, a cyclohexanol (C6-ol) yield of 50% and an alcohol/ketone ratio of 10 was obtained. The hydroxylation occurs according to the so-called oxygen rebound mechanism, as expected for a P-450 model system. FeP2NaX is less selective (C6-ol yield=25% and alcohol/ketone=1.2). One possible explanation is that a Russell-type mechanism involving O2 imprisoned within the zeolite cages may be operating parallelly, generating both C6-ol and cyclohexanone.

A comparative study of galactose oxidase and active site analogs based on QM/MM Car-Parrinello simulations

Abstract: A parallel study of the radical copper enzyme galactose oxidase (GOase) and a low molecular weight analog of the active site was performed with dynamical density functional and mixed quantum-classical calculations. This combined approach enables a direct comparison of the properties of the biomimetic and the natural systems throughout the course of the catalytic reaction. In both cases, five essential forms of the catalytic cycle have been investigated: the resting state in its semi-reduced (catalytically inactive) and its oxidized (catalytically active) form, A semi and A ox, respectively; a protonated intermediate B; the transition state for the rate-determining hydrogen abstraction step C, and its product D. For A and B the electronic properties of the biomimetic compound are qualitatively very similar to the ones of the natural target. However, in agreement with the experimentally observed difference in catalytic activity, the calculated activation energy for the hydrogen abstraction step is distinctly lower for GOase (16 kcal/mol) than for the mimetic compound (21 kcal/mol). The enzymatic transition state is stabilized by a delocalization of the unpaired spin density over the sulfur-modified equatorial tyrosine Tyr272, an effect that for geometric reasons is essentially absent in the biomimetic compound. Further differences between the mimic and its natural target concern the structure of the product of the abstraction step, which is characterized by a weakly coordinated aldehyde complex for the latter and a tightly bound linear complex for the former.

A DFT Study on the C−H Bond Dissociation Enthalpies of Haloalkanes: Correlation between the Bond Dissociation Enthalpies and Activation Energies for Hydrogen Abstraction

Abstract: The C−H bond dissociation enthalpies of halomethanes were computed from the results of density functional theory (DFT) calculations at the B3LYP level with various basis sets, such as 6-311G(d,p) and 6-311++G(3df,2p). Reasonably accurate C−H bond dissociation enthalpies were obtained even at the B3LYP/6-311G(d,p) level when ROB3LYP method was used for radicals. Applying the same procedure, the C−H bond dissociation enthalpies for a series of haloethanes were also calculated. Good correlation has been observed between the activation energies for the hydrogen abstraction from haloalkanes by OH radical and the corresponding C−H bond dissociation enthalpy values.

Oxidation of organic substrates catalyzed by a novel mixed-ligand ruthenium(III) complex

Abstract: Cationic [Ru III (app)(bipy)(H 2 O)] 2+ ( 1 ) complex (H 2 app= N -(hydroxyphenyl)pyridine-2-carboxaldimine; bipy=2,2'-bipyridyl) has been synthesized and characterized by physico-chemical methods. Complex 1 is found to be an effective catalyst in the oxidation of both saturated and unsaturated hydrocarbons by using tert -butylhydroperoxide ( t -BuOOH). A mechanism involving formation of and transfer from a reactive high valent Ru(V)-oxo species as catalytic intermediate is proposed for the catalytic processes. The results of the product distribution in the present investigation clearly indicate the preference for side-on approach of olefins and the high electrophilic nature of RuO bond in [Ru V (app)(bipy)O] 2+ intermediate complex, which leads to the higher affinity of hydrogen atom/hydride abstraction than oxo-transfer to CC double bond.

Identification of the sites of hydroxyl radical reaction with peptides by hydrogen/deuterium exchange: prevalence of reactions with the side chains.

Abstract: Hydroxyl radical-effected protium/deuterium (1H/2H) exchange into the C−H bonds present in peptides has been used to identify the site of hydrogen atom abstraction by hydroxyl radical. Radiolysis of anaerobic, N2O-saturated D2O solutions containing peptide and dithiothreitol generates a hydroxyl radical that mediates 1H/2H exchange into the side chains of peptides of up to 66 atom % excess 2H. The 1H/2H exchange is determined by measuring the isotope ratio, [M + H + 1]+/[M + H]+, of the peptide using electrospray ionization−mass spectrometry. The 1H/2H exchange within each residue of the peptide was determined by measuring the isotope ratio of each isolated dansyl amino acid following hydrolysis and derivatization. Generation of 0.40 mM hydroxyl radical effected 1H/2H exchange into each of the five different residues of (Ala2)-leucine enkephalin (YAGFL). The propensity of the residues to undergo exchange was L > Y > A ≅ F > G, independent of whether they were radiolyzed separately or as the peptide. The m...

Reactivity of a parent amidoruthenium complex: a transition metal amide of exceptionally high basicity.

Abstract: Complexes with late metal–oxygen and –nitrogen single bonds (metal alkoxides and amides) play important roles in biological systems and have been implicated as intermediates in a variety of industrially important catalytic reactions.1–3 When the metal–heteroatom bond reacts with carbon–hydrogen bonds of organic molecules, an important question concerns whether the transformation takes place via odd- or even-electron intermediates. For example, many late metal alkoxides undergo rapid overall even-electron RO− transfer reactions with electrophiles.4–8 In other cases, metal–oxygen bonds have been proposed to react by initial hydrogen atom abstraction, an odd-electron process.9–11 Stimulated by this difference in behavior, we set out to determine the preferred mode of reactivity in the well-characterized parent hydroxo and amide complexes: trans-(DMPE)2(H)-Ru(NH2) (1) and trans-(DMPE)2(H)Ru(OH) (2) (DMPE = 1,2-bis(dimethylphosphino)ethane).8,12 This study has uncovered only 2-electron behavior for both the OH and NH2 metal-bound fragments, and has revealed that complex 1 bears a surprisingly basic amido ligand, a property that is expressed without dissociation of the NH2 group from the ruthenium center. In analogy to reactions reported by Stack and Mayer and their co-workers,9,11 our ruthenium amido and hydroxido complexes 1 and 2 cause the dehydrogenation of cyclohexadiene (1,4-CHD) and 9,10-dihydroanthracene (DHA) (eq 1). However, compounds 1 and 2 also catalyze the interconversion of 1,4- and 1,3-CHD more rapidly (t1/2 = <5 min) than dehydrogenation occurs. The dehydrogenation and isomerization reactions could proceed by initial abstraction of a hydrogen atom (although the required oxidation state change is less favorable here than in the earlier-reported systems) or a proton, in addition to pathways that might involve interaction of the diene with the metal center. To explore this question, we extended the above reactions to a larger series of organic compounds having C–H bonds characterized by a wide range of bond dissociation energies and acidities. Two interrelated modes of reactivity were observed with organic compounds containing relatively acidic C–H bonds (Scheme 1). The first includes the generation of a stable ion pair formed by overall proton transfer from carbon to ruthenium-bound nitrogen (e.g., 1 + fluorene → 4a in THF or benzene).13 The second is overall displacement of the nitrogen fragment from the Ru center (e.g., 1 + PhC≡CH → 5 in benzene;14 1 + arylacetonitriles → 6a, 6b in benzene).13 We have evidence that proton transfer to form an ion pair is the initial step in these reactions as well. Monitoring the reaction of 1 with cyclobutanone by NMR at 25 °C in THF-d8 reveals that ion pair 4b is initially formed, and this is transformed more slowly into the stable substitution product 7.13 It seems likely that the other displacement reactions proceed by analogous initial proton-transfer steps. Scheme 1 Surprisingly, amido complex 1 also reacts with exceedingly weakly acidic compounds, although these reactions are sufficiently endothermic that only H/D exchange is observed. One of the most striking reactions of 1 occurs upon its dissolution into toluene-d8 (pKa of toluene = 41,15 BDE = 89.8 kcal/mol16). Monitoring this reaction by 1H and 2H {1H}NMR spectroscopy showed full deuterium incorporation into both the N-bound positions and the DMPE ligand, concurrent with hydrogen incorporation into the benzylic position of toluene (Scheme 1). A similar, but much slower, exchange is observed with hydroxido complex 2. Consistent with this observation, complex 1 can be used to catalyze deuterium exchange between toluene-d8 and carbon acids of similar or weaker acidities but widely varying bond dissociation energies (BDE’s; listed in kcal/mol), including triphenylmethane (pKa = 31.5 (THF),17 BDE = 8118), cycloheptatriene (pKa = 38.8,15 BDE = 7319), ammonia (pKa = 41,20 BDE = 107.419), dihydrogen (pKa = 35,15 BDE = 10415), and even propene (pKa = 43,15 BDE = 88.816).21 No exchange is observed with benzene (pKa = 43,15 BDE = 110.9)19 or THF (BDE = 95 (est),22 pKa unknown), even at elevated temperatures.23 No detectable amounts of radical coupling products are observed in any of these reactions.24 Exchange is observed between 1 and the methyl hydrogens of toluene, but not between 1 and the benzylic hydrogens of p-toluidine, which is less acidic than toluene, but has a comparable bond dissociation energy.25,26 The fact that this selectivity exists, and that exchange occurs even with compounds having very high X–H bond energies (e.g., ammonia), provides strong evidence that amido complex 1 and hydroxido complex 2 abstract protons rather than hydrogen atoms from weak carbon (and other) acids. In contrast to its behavior with stronger acids, no substitution of the ammonia ligand by the transiently generated anion is observed in any of the exchange reactions between 1 and very weak acids, such as toluene and propene.27 Similarly, despite rapid H/D exchange between 1 and ND3, no labeled nitrogen is incorporated into amido complex 1 upon addition of 15NH3 to a solution of 1 in THF-d8, even after 1 month at room temperature.28,29 A strongly hydrogen bonded complex [(DMPE)2(H)Ru–NH3- - -15NH2], apparently incapable of interchanging the nitrogen groups, is presumably responsible for this intriguing behavior. A quantitative assessment of the basicity of amido complex 1 is complicated by the influence of ion pairing in the calculation of proton-transfer equilibrium constants in nonpolar solvents.30 However, we were able to obtain an approximate idea of the basicity of 1 and 2 by finding organic acids whose conjugate bases are of comparable proton-abstracting ability to 1 or 2. In the case of hydroxido complex 2, addition of fluorene (pKa = 22.9 (THF),17 BDE = 80 kcal/mol18) in THF-d8 led to an equilibrium mixture of fluorene, hydroxido complex 2, and the cationic aquo complex/fluorenide ion pair 10a as observed by 1H NMR spectrometry (eq 2, Keq = 6.4). The much stronger base 1 completely deprotonates fluorene. For this system, triphenylmethane is required to establish a solution containing detectable equilibrium concentrations of Ph3H, 1, and the ammonia complex/triphenylmethide ion pair 4c. This proton-transfer equilibrium is rapid on the NMR time scale at room temperature, but at lower temperatures each species can be observed (eq 3). The equilibrium constant (Keq) was measured at five different temperatures; a linear van’t Hoff plot allowed us to calculate an extrapolated equilibrium constant of 3.6 × 10−1 at 25 °C. If ion pairing effects are ignored, the basicity of hydroxido complex 2 is comparable to that of fluorenide (pKa = 22.9 in THF), and the basicity of amido complex 1 is comparable to that of triphenylmethide (pKa = 31.5 in THF). However, this conclusion must be modified to the extent that the ions formed by proton transfer in THF tend to experience mutual stabilization due to their presence in a contact ion pair, a conclusion that is supported by preliminary UV/visible investigation of these systems.30 Even though a realistic determination of the actual pKa of the ruthenium ammonia complex awaits further study, it is clear that complex 1 exhibits an unusually high thermodynamic proton-abstraction propensity. In summary, amido complex 1 can abstract hydrogen from a wide range of organic compounds, but in all cases that we have investigated, the hydrogen is removed as a proton rather than a hydrogen atom. By inference we believe the same to be true of hydroxido complex 2, although we have collected less evidence to support this extension of our conclusions. We have preliminary indications that the corresponding iron complexes can be prepared, and it will be interesting to see if first-row M–X bonds exhibit a higher propensity to undergo one-electron reactions. Most significantly, the NH2 group in 1 exhibits a degree of basicity reminiscent of alkali metal amides, despite its undoubtedly strong bond with the ruthenium center, which remains intact through many reversible proton-transfer events. We assume the high basicity of the amido complex 1 is either due to a remarkably polarized Ru–N bond, resulting in a highly reactive π-orbital,31 or a strong filled/filled (dπ-pπ) repulsion,6 a theory that has been invoked many times before but has never, to our knowledge, been exemplified so strongly as in amido complex 1.

Film growth precursors in a remote SiH/sub 4/ plasma used for high-rate deposition of hydrogenated amorphous silicon

Abstract: The SiH4 dissociation products and their contribution to hydrogenated amorphous silicon (a-Si:H) film growth have been investigated in a remote Ar–H2–SiH4 plasma which is capable of depositing device-quality a-Si:H at 10 nm/s. SiH3 radicals have been detected by means of threshold ionization mass spectrometry for different fractions of H2 in the Ar–H2-operated plasma source. It is shown that at high-H2 flows, SiH4 dissociation is dominated by hydrogen abstraction and that SiH3 contributes dominantly to film growth. At low-H2 flows, a significant amount of very reactive silane radicals, SiHx(x⩽2), is produced, as concluded from threshold ionization mass spectrometry on SiH2 and optical emission spectroscopy on excited SiH and Si. These radicals are created by dissociative recombination reactions of silane ions with electrons and they, or their products after reacting with SiH4, make a large contribution to film growth at low-H2 flows. This is corroborated by the overall surface reaction probability which d...

The Influence of Boryl Substituents on the Formation and Reactivity of Adjacent and Vicinal Free Radical Centers

Abstract: Radicals containing α-boronate substituents were generated by bromine abstraction from 1-bromoalkyldioxaborolanes (boronic esters), by addition to vinyl boronate, and by hydrogen abstraction from alkyldioxaborolanes and observed by EPR spectroscopy. Unsymmetrically substituted α-boronate radicals displayed selective line broadening in their low-temperature spectra from which barriers to internal rotation about •CH2−B(OR‘)OR bonds were found to be 3 ± 1 kcal mol-1. Use of an empirical relationship between barrier height and bond dissociation energy led to BDE[(RO)2BCH2−H] = 98.6 kcal mol-1. Rate constants for hydrogen abstraction from 2,4,4,5,5-pentamethyl-1,3,2-dioxaborolane by tert-butoxyl radicals were determined from competitive EPR and product studies and found to be relatively small, comparable to those of unactivated methyl groups. Hydrogen abstraction from bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)methane was found to be extremely difficult. The structures and energetics of α-boronate radical...

Hydrogen-atom Abstraction/Cyclization in synthesis. Direct syntheses of coumestan and coumestrol

Abstract: The synthesis of coumestrol has been achieved in five steps from 1,3-dimethoxybenzene. The key step is a photochemical cyclization of a glyoxylate ester.

Radical polymerization of phenyl acrylate as studied by ESR spectroscopy: Concurrence of propagating and mid-chain radicals

Abstract: The electron spin resonance (ESR) spectrum of the propagating radical of phenyl acrylate (PhA) was successfully recorded in benzene as a non-polar solvent. The hyperfine coupling constants for the a and b-protons were evaluated on the basis of the spectra of the propagating radicals of PhA and phenyl acrylate-a-d. The simulated spectrum satisfactorily fits those observed during polymerization, and the spectrum of the poly(PhA) radical obtained at low conversion (l15%) was assigned to the propagating radical. The spectra observed at higher conversions (g15%) indicated the presence of two types of radical species, a propagating radical and a mid-chain radical produced by abstraction of the a-hydrogen of the monomeric unit. The content of branching in the polymers as a result of the formation of a mid-chain radical was found to be 1n3% by 13C NMR spectroscopy. The absolute rate constants for propagation (kp) and termination (kt) of PhA at low conversions were determined based on the quantification of the propagating radical by ESR spectroscopy at 60dC: kp = 3 580 dm3mmoln1msn1 and kt = 6,8t106 dm3mmoln1msn1. However, these seemed to be apparent values because the propagating radical is expected to be converted to the mid-chain radical by intra- and intermolecular hydrogen abstraction before the loss of its activity by bimolecular termination. Conversion from the propagating radical to the mid-chain radical followed by reinitiation was estimated to occur more than twenty times during the lifetime of each polymer chain. c WILEY-VCH Verlag GmbH

What Are the Reasons for the Kinetic Stability of a Mixture of H2 and O2

Abstract: Calculations at the (14,10)CASSCF/6-31G** and the MR-(S)DCI/cc-pVTZ levels are employed to answer the title question by studying three possible modes of reaction between dioxygen and dihydrogen molecules at the ground triplet state and excited singlet state of O2. These reaction modes, which are analogous to well-established mechanisms for oxidants such as transition metal oxene cations and mono-oxygenase enzymes, are the following: (i) the concerted addition, (ii) the oxene-insertion, and (iii) the hydrogen abstraction followed by hydrogen rebound. The “rebound” mechanism is found to be the most preferable of the three mechanisms. However, the barrier of the H-abstraction step is substantial both for the triplet and the singlet states of O2, and the process is highly endothermic (>30 kcal/mol) and is unlikely to proceed at ambient conditions. The calculations revealed also that the lowest singlet state of O2 has very high barriers for reaction and therefore cannot mediate a facile oxidation of H2 in con...

Addition complex formation vs. Direct abstraction in the OH + C2H4 reaction

Abstract: The potential energy surface of the C2H5O system was studied by high level ab initio methods. Unimolecular rate constants have been computed using a simple transition state theory approach. The good agreement between predicted and experimental high pressure limiting rate constants supported the reliability of the proposed procedure. The direct bimolecular H-atom abstraction from ethylene by OH is unimportant and the reaction proceeds ia the intermediate adduct. We predict high pressure bimolecular rate constants above 600 K for the following reactions under the condition that the pre-equilibrium is established: We also predict that the addition of H-atoms to acetaldehyde proceeds without an appreciable barrier and that redissociation is efficient above 400 K and a thermal equilibrium will be established. We found the barrier for addition of CH3 to formaldehyde to be 12 kJ mol−1 lower than the currently accepted barrier for the competing hydrogen abstraction reaction leading to CH4+CHO.

Ab Initio Mechanism and Multichannel RRKM−TST Rate Constant for the Reaction of Cl(2P) with CH2CO (Ketene)

Abstract: The potential energy surface for the most important pathways of the reaction between Cl(2P) and ketene has been studied using the ab initio G2(MP2) method. A variety of possible complexes and saddle points along the minimum energy reaction paths have been characterized at the UMP2(full)/6-31G(d,p) level. The calculations reveal that the addition−elimination mechanism dominates the Cl + CH2CO reaction and the direct hydrogen abstraction pathway is negligible. It is interesting to note that the addition reaction starts by the formation of a p−π complex (PπC), and subsequently the chlorinated acetyl radical CH2ClCO(2A‘) and the chloroformyl methyl radical CH2CClO(2A‘ ‘) are formed through the isomerization of PπC. The C−C bond scission of CH2ClCO(2A‘) leads to the products CO and CH2Cl. The three-center HCl elimination from PπC, occurring via a high energy barrier (TS3) and a weakly bound hydrogen bonding (HBC1), was proposed to account for the minor yield of the HCl + HCCO observed experimentally. Multichan...

Hydrogen Atom Abstraction by Metal−Oxo and Metal−Superoxo Complexes: Kinetics and Thermodynamics

Abstract: The superoxochromium complex CraqOO2+ abstracts a hydrogen atom from CMe3CHO in acidic aqueous solution with k = 0.16 M-1 s-1. This rate constant is only ∼102 times smaller than that for the reaction of CraqO2+ with the same aldehyde, k = 23 M-1 s-1, in contrast to the much greater reactivity difference between alkoxyl and alkylperoxyl radicals, kt-BuO/kt-BuOO ≈ 106. The absolute rate constants for hydrogen atom abstraction from a common reagent by metal-oxo and -superoxo species and the corresponding organic oxygen-centered radicals, RO• and ROO•, can now be compared for the first time: kBuO (9 × 107 M-1 s-1) > kCrO(23) ≥ kBuOO(8) > kCrOO (0.16). The reactivity of individual species is explained by the energetics of the O−H bonds in ROH, ROOH, CraqOH2+, and CraqOOH2+.

Radical Addition to Isonitriles: A Route to Polyfunctionalized Alkenes through a Novel Three-Component Radical Cascade Reaction

Abstract: The reaction of aromatic disulfides, alkynes, and isonitriles under photolytic conditions affords polyfunctionalized alkenesβ-arylthio-substituted acrylamides or acrylonitrilesin fair yields through a novel three-component radical cascade reaction. The procedure entails addition of a sulfanyl radical to the alkyne followed by attack of the resulting vinyl radical to the isonitrile. A fast reaction, e.g., scavenging by a nitro derivative or β-fragmentation, is necessary in order to trap the final imidoyl radical, since addition of vinyl radicals to isonitriles seems to be a reversible process. The stereochemistry of the reaction is discussed, particularly with respect to the stereochemical outcome of related hydrogen abstraction reactions by the same vinyl radicals. The lower or even inverted preference for either geometrical isomer observed in our cases with respect to that encountered in hydrogen abstraction reactions is explained in terms of transition-state interactions and/or isomerization of the fina...