Kinetic analysis of the phenyl-shift reaction in β-O-4 lignin model compounds: a computational study

The phenyl-shift reaction for the β-radical of phenethyl phenyl ether (PhCH(2)C?HOPh, β-PPE) is an integral step in the pyrolysis of PPE, which is a model compound for the β-O-4 linkage in lignin. We investigated the influence of natural occurring substituents (hydroxy, methoxy) on the reaction rate by calculating relative rate constants using density functional theory in combination with transition state theory, including anharmonic correction for low-frequency modes. The phenyl-shift reaction proceeds through an oxaspiro[2.5]octadienyl radical intermediate and the overall rate constants were computed invoking the steady-state approximation (its validity was confirmed). Substituents on the phenethyl ring have only little influence on the rate constants. If a methoxy substituent is located in the para position of the phenyl ring adjacent to the ether oxygen, the energies of the intermediate and second transition state are lowered, but the overall rate constant is not significantly altered. This is a consequence of the dominating first transition from reactant to intermediate in the overall rate constant. In contrast, o- and di-o-methoxy substituents significantly accelerate the phenyl-migration rate compared to β-PPE.     

Selective cleavage of methoxy protecting groups in carbohydrates

The selective cleavage of methoxy protecting groups next to hydroxy groups is achieved using a radical hydrogen abstraction reaction as the key step. Under the reaction conditions, the hydroxy group generates an alkoxyl radical that reacts with the sterically accessible adjacent methoxy group, which is transformed into an acetal. In the second step, the acetals are hydrolyzed to give alcohols or diols. A one-pot hydrogen abstraction-hydrolysis procedure was also developed. Good yields were usually achieved, and the mild conditions of this methodology were compatible with different functional groups and sensitive substrates such as carbohydrates.     

Enhancement of chain amplification in photoreactions of N-methoxypyridinium salts with alcohols

Recently we reported a chain-amplified photochemical reaction, initiated by electron transfer from an excited sensitizer to N-methoxypyridinium salts, which leads to N-O bond cleavage (26). Hydrogen atom abstraction by the methoxy radical from an alcohol yields an alpha-hydroxy radical, which reduces another N-methoxypyridinium molecule and propagates the chain. We now report that the chain amplification can be significantly enhanced in the presence of water. Detailed kinetic studies of the reaction of 4-cyano-N-methoxypyridinium salt (CMP) with benzhydrol (BH) showed that the rate constant for reduction of CMP by the diphenyl ketyl radical (1.1 x 10(6) M(-1) s(-1)) increases by more than an order of magnitude in the presence of water. This increase in the rate constant is the result of coupling of the electron transfer to a proton transfer from the ketyl radical to water, which decreases the endothermicity of the reaction. Unfortunately, this increase in the rate constant for one of the two propagation steps is accompanied by a larger increase in the rate constant(s) of the competing termination reaction(s) of the ketyl radical. The observed enhancement in chain amplification is the result of a significant increase in the ratio of propagation to termination rate constants of the reactions of the methoxy radical. The main chain-terminating reactions of the methoxy radical are deuterium abstraction from the solvent, CD(3)CN, and reaction with the sensitizer, thioxanthone. The effect of increase in the ratios of the propagation rate constant of the methoxy radical (hydrogen abstraction from BH) to those of both termination reactions is larger than the unfavorable effect of water on the reactions of the ketyl radical. The increase in chain amplification depends on the concentration of the reactants; at 0.037 M of both reactants, the quantum yield increases form approximately 16 to approximately 45 in the presence of <1% water. The reaction of 4-phenyl-N-methoxypyridinium (PMP) with 4-methoxybenzyl alcohol does not proceed via chain amplification because of large endothermicity for electron transfer from the alpha-hydroxy radical to the pyridinium salt. However, chain amplification could be induced, simply by addition of water, where at approximately 10% water content, a quantum yield of approximately 5 was obtained. Water-induced, proton-coupled electron transfer increases the rate constant for reduction of PMP from a negligible level to becoming the dominant path.     

Multichannel RRKM-TST and CVT rate constant calculations for reactions of CH2OH or CH3O with HO2

The kinetics and mechanism of the gas-phase reactions between hydroxy methyl radical (CH(2)OH) or methoxy radical (CH(3)O) with hydroproxy radical (HO(2)) have been theoretically investigated on their lowest singlet and triplet surfaces. Our investigations indicate the presence of one deep potential well on the singlet surface of each of these systems that play crucial roles on their kinetics. We have shown that the major products of CH(2)OH + HO(2) system are HCOOH, H(2)O, H(2)O(2), and CH(2)O and for CH(3)O + HO(2) system are CH(3)OH and O(2). Multichannel RRKM-TST calculations have been carried out to calculate the individual rate constants for those channels proceed through the formation of activated adducts on the singlet surfaces. The rate constants for direct hydrogen abstraction reactions on the singlet and triplet surfaces were calculated by means of direct-dynamics canonical variational transition-state theory with small curvature approximation for the tunneling.     

Computational study of bond dissociation enthalpies for substituted β-O-4 lignin model compounds

The biopolymer lignin is a potential source of valuable chemicals. Phenethyl phenyl ether (PPE) is representative of the dominant β-O-4 ether linkage. DFT is used to calculate the Boltzmann-weighted carbon-oxygen and carbon-carbon bond dissociation enthalpies (BDEs) of substituted PPE. These values are important for understanding lignin decomposition. Exclusion of all conformers that have distributions of less than 5% at 298 K impacts the BDE by less than 1 kcal mol(-1). We find that aliphatic hydroxyl/methylhydroxyl substituents introduce only small changes to the BDEs (0-3 kcal mol(-1)). Substitution on the phenyl ring at the ortho position substantially lowers the C-O BDE, except in combination with the hydroxyl/methylhydroxyl substituents, for which the effect of methoxy substitution is reduced by hydrogen bonding. Hydrogen bonding between the aliphatic substituents and the ether oxygen in the PPE derivatives has a significant influence on the BDE. CCSD(T)-calculated BDEs and hydrogen-bond strengths of ortho-substituted anisoles, when compared with M06-2X values, confirm that the latter method is sufficient to describe the molecules studied and provide an important benchmark for lignin model compounds.     

Photoinduced C-Br homolysis of 2-bromobenzophenones and Pschorr ring closure of 2-aroylaryl radicals to fluorenones

A variety of diversely substituted 2-aroylaryl radicals, generated by photoinduced homolysis of 2-bromoarylketones, is shown to undergo Pschorr cyclization to yield fluorenones in moderate to excellent yields. The photochemical results illustrate that the substituents in the two phenyl rings of the 2-bromobenzophenone skeleton exert a dramatic influence on the reactivity of the derived 2-aroylaryl radicals. The disubstitution by methoxy groups in the radical ring renders the aryl sigma-radical highly electrophilic and unreactive for hydrogen abstraction and cyclization. On the other hand, the substituents in the non-radical ring that strongly stabilize the hydrofluorenyl pi-radical, formed subsequent to the attack of the 2-aroylaryl radical on the non-radical ring, promote cyclization to furnish fluorenones in excellent isolated yields.     

Ab initio study on the structures of fluorinated formates and hydrogen abstraction reaction with OH radical

The conformational potential energy surfaces for mono- and difluoromethyl formate have been determined by using a modified G2(MP2) level of calculations. The structures and vibrational frequencies for the conformers of mono- and difluoromethyl formate have been reported. The hydrogen abstraction reaction channels between these two formates and OH radicals have been studied at the same level of theory. Using the standard transition state theory and taking into account the effect of tunneling across the reaction barrier, we have estimated the rate constant for hydrogen abstraction by OH radical. The effect of successive fluorine substitution for methyl hydrogen on the conformational stability and on the hydrogen abstraction rate has been analyzed.     

Thiyl radical-mediated cleavage of allylic C-N bonds: scope, limitations, and theoretical support to the mechanism

Thiols mediate the radical isomerization of allylic amines into enamines. The reaction results in the cleavage of the allylic C-N bond, after treatment with aqueous HCl. The mechanism involves the abstraction of an allylic hydrogen alpha to nitrogen by thiyl radical, followed by a return hydrogen transfer from the thiol to the carbon gamma to nitrogen in the intermediate allylic radical. The scope and limitations of the reaction with respect to the nature of the thiol, to the structure of the allylic chain, and to the nature of the substituents at nitrogen were investigated. The experimental results were interpreted on the ground of DFT calculations of the C-Halpha BDE in the starting allylic amines, and of the C-Hgamma BDE in the resulting enamines. The efficiency of the initial hydrogen transfer is the first requirement for the reaction to proceed. A balance must be found between the S-H BDE and the two above-mentioned C-H BDEs. The incidence of stereoelectronic factors was analyzed through NBO calculations performed on the optimized geometries of the starting allylic amines. Additional calculations of the transition structures and subsequent tracing of the reaction profiles were performed for the abstraction of Halpha from both the allyl and the prenyl derivatives by p-TolS(*). The latter allowed us to estimate the rate constant for the abstraction of hydrogen by thiyl radical from an N-prenylamine and an N-allylamine.     

Criegee中间体RCHOO(R=H,CH_3)与NCO的反应机理

采用CCSD(T)/aug-cc-p VTZ//B3LYP/6-311+G(2df,2p)方法对Criegee中间体RCHOO(R=H,CH_3)与NCO反应的机理进行了研究,利用经典过渡态理论(TST)并结合Eckart校正模型计算了标题反应在298~500 K范围内优势通道的速率常数.结果表明,上述反应包含亲核加成、氧化和抽氢3类机理,其中每类又包括NCO中N和O分别进攻的两种形式.亲核加成反应中O端进攻为优势通道,氧化和抽氢反应则是N端进攻为优势通道;甲基取代使CH_3CHOO反应活性高于CH2OO;anti-CH_3CHOO的加成及氧化反应活性高于syn-CH_3CHOO,而抽氢反应则是syn-CH_3CHOO的活性高于anti-CH_3CHOO.anti-构象对总速率常数的贡献大于syn-构象,且总速率常数具有显著的负温度效应.     

Remarkable enhancement of catalytic activity of a 2 : 1 complex between a non-planar Mo(v)-porphyrin and a ruthenium-substituted Keggin-type heteropolyoxometalate in catalytic oxidation of benzyl alcohols

A 2?:?1 complex composed between a non-planar Mo(v)-porphyrin complex ([Mo(DPP)(O)](+), DPP(2-) = dodecaphenylporphyrin) and a ruthenium-substituted Keggin-type heteropolyoxometalate (Ru-POM), [SiW(11)O(39)Ru(III)(DMSO)](5-), acts as an efficient catalyst for oxidation of benzyl alcohols with iodosobenzene as an oxidant in CDCl(3) at room temperature. The catalytic oxidation afforded the corresponding benzaldehydes, whereas neither the ammonium salt of Ru-POM nor [Mo(DPP)(O)](+) alone exhibited catalytic reactivity under the same experimental conditions. This enhancement can be attributed to a large anodic shift of the redox potential of the ruthenium centre due to the complexation of the Ru-POM with two cationic {Mo(DPP)(O)}(+) units. The kinetic analysis demonstrated that the catalytic oxidation proceeded via formation of a catalyst-substrate complex, and electron-withdrawing substituents at the para position of benzyl alcohol accelerated the reaction. The rate constants of the oxidation reactions correlate to the bond dissociation energies of the C-H bonds of the substrate. A linear correlation was observed for logarithm of the rate constants of oxidation reactions of benzyl alcohols with that of hydrogen abstraction by cumyl peroxyl radical, indicating the reaction proceeds via hydrogen abstraction. The observed kinetic isotope effect (KIE) indicates that the hydrogen abstraction occurs from the benzyl group rather than the hydroxy group.     

Model compound studies of the beta-O-4 linkage in lignin: absolute rate expressions for beta-scission of phenoxyl radical from 1-phenyl-2-phenoxyethanol-1-yl radical

Arrhenius rate expressions were determined for beta-scission of phenoxyl radical from 1-phenyl-2-phenoxyethanol-1-yl, PhC*(OH)CH2OPh (V). Ketyl radical V was competitively trapped by thiophenol to yield PhCH(OH)CH2OPh in competition with beta-scission to yield phenoxyl radical and acetophenone. A basis rate expression for hydrogen atom abstraction by sec-phenethyl alcohol, PhC*(OH)CH3, from thiophenol, log(k(abs)/M(-1) s(-1)) = (8.88 +/- 0.24) - (6.07 +/- 0.34)/theta, theta = 2.303RT, was determined by competing hydrogen atom abstraction with radical self-termination. Self-termination rates for PhC*(OH)CH3 were calculated using the Smoluchowski equation employing experimental diffusion coefficients of the parent alcohol, PhCH(OH)CH3, as a model for the radical. The hydrogen abstraction basis reaction was employed to determine the activation barrier for the beta-scission of phenoxyl from 1-phenyl-2-phenoxyethanol-1-yl (V): log(k beta)/s(-1)) = (12.85 +/- 0.22) - (15.06 +/- 0.38)/theta, k beta (298 K) ca. (64.0 s(-1) in benzene), and log(k beta /s(-1)) = (12.50 +/- 0.18) - (14.46 +/- 0.30)/theta, k beta (298 K) = 78.7 s(-1) in benzene containing 0.8 M 2-propanol. B3LYP/cc-PVTZ electronic structure calculations predict that intramolecular hydrogen bonding between the alpha-OH and the -OPh leaving group of ketyl radical (V) stabilizes both ground- and transition-state structures. The computed activation barrier, 14.9 kcal/mol, is in good agreement with the experimental activation barrier.     

Chain amplification in photoreactions of N-alkoxypyridinium salts with alcohols: mechanism and kinetics

Photosensitized electron transfer from a variety of singlet- and triplet-excited donors to N-methoxypyridinium salts leads to N-O bond cleavage. Hydrogen atom abstraction by the resulting methoxy radical from an added alcohol generates an alpha-hydroxy radical that reduces another pyridinium molecule, thus leading to chain propagation. For example, thioxanthone-sensitized reactions of 4-cyano-N-methoxypyridinium, P1, with several aliphatic and benzyl alcohols gave quantum yields for products formation (an aldehyde or a ketone and protonated 4-cyanopyridinium) of approximately 15-20, at reactant concentrations of approximately 0.02-0.04 M. The reaction can also be sensitized with triplet benzopheone, which in this case acts as an electron donor. Energetic limitations on chain propagation are imposed by the relationship between the oxidation potential of the alpha-hydroxy radical and the reduction potential of the pyridinium salt. The chain reactions proceed despite approximately 0.25 eV endothermicity for the electron-transfer step. Chain reactions with the harder-to-reduce 4-phenyl-N-methoxypyridinium, however, are limited in scope because of increased endothermicity for electron transfer. The thioxanthone-sensitized reaction of P1 with benzhydrol was studied in detail by a combination of steady state and transient kinetics. The bimolecular rate constants for the chain propagation reactions:hydrogen atom abstraction by the methoxy radical and electron transfer from the diphenylketyl radical to P1 are approximately 6 x 10(6) and 1.1 x 10(6) M(-1) s(-1), respectively. The kinetic data indicate that deuterium atom abstraction by the methoxy radical from the solvent, acetonitrile-d(3), is a dominant chain-terminating process. Because of a large deuterium isotope effect, approximately 7, the quantum amplification is strongly suppressed when the reaction is carried out in acetonitrile.     

The selectivity of charged phenyl radicals in hydrogen atom abstraction reactions with isopropanol

The vertical electron affinity is demonstrated to be a key factor in controlling the selectivity of charged phenyl radicals in hydrogen atom abstraction from isopropanol in the gas phase. The measurement of the total reaction efficiencies (hydrogen and/or deuterium atom abstraction) for unlabeled and partially deuterium-labeled isopropanol, and the branching ratios of hydrogen and deuterium atom abstraction, by using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer, allowed the determination of the selectivity for each site in the unlabeled isopropanol. Examination of hydrogen atom abstraction from isopropanol by eight structurally different radicals revealed that the preferred site is the CH group. The selectivity of the charged phenyl radicals correlates with the radical's vertical electron affinity and the reaction efficiency. The smaller the vertical electron affinity of a radical, the lower its reactivity, and the greater the preference for the thermodynamically favored CH group over the CH3 group or the OH group. As the vertical electron affinity increases from 4.87 to 6.28 eV, the primary kinetic isotope effects decrease from 2.9 to 1.3 for the CD group, and the mixture of primary and alpha-secondary kinetic isotopes decreases from 6.0 to 2.4 for the CD3 group.     

A systematic computational study of the reactions of HO2 with RO2: the HO2 + C2H5O2 reaction

The reaction of HO2 with C2H5O2 has been studied using the density functional theory (B3LYP) and the coupled-cluster theory [CCSD(T)]. The reaction proceeds on the triplet potential energy surface via hydrogen abstraction to form ethyl hydroperoxide and oxygen. On the singlet potential energy surface, the addition-elimination mechanism is revealed. Variational transition state theory is used to calculate the temperature-dependent rate constants in the range 200-1000 K. At low temperatures (e.g., below 300 K), the reaction takes place predominantly on the triplet surface. The calculated low-temperature rate constants are in good agreement with the experimental data. As the temperature increases, the singlet reaction mechanism plays more and more important role, with the formation of OH radical predominantly. The isotope effect of the reaction (DO2 + C2D5O2 vs HO2 + C2H5O2) is negligible. In addition, the triplet abstraction energetic routes for the reactions of HO2 with 11 alkylperoxy radicals (CnHmO2) are studied. It is shown that the room-temperature rate constants have good linear correlation with the activation energies for the hydrogen abstraction.     

Quasi-classical trajectory and direct-dynamics CVT study on the initiation steps of methanol combustion

Direct-dynamics canonical variational transition-state theory (CVT) and quasi-classical trajectory (QCT) calculations have been performed to study the dynamics of the initiation steps in the methanol combustion at high oxygen concentration. The initiation steps in combustion of methanol is hydrogen abstraction from carbon or oxygen in methanol to produce hydroxymethyl radical (CH₂OH) or methoxy radical (CH₃O), respectively, and hydroperoxyl radical (HO₂). A new analytical potential energy function driven from our DFT calculations is constructed to study the dynamics of the title reactions. Reactive cross sections and reaction probabilities at various relative translational energies and initial vibrational and rotational reactant excitation were obtained to calculate the rate constants. The calculated rate constants from CVT and QCT calculations are compared.     

Characterization of the high molecular mass chlorinated matter in spent bleach liquors (SBL): 3—Mass spectrometric interpretation of aromatic degradation products in SBL

The aromatic content of the high molecular matter in pulp mill bleachery effluents has been characterized after chemical degradation in the laboratory followed by derivatization and gas chromatographic/mass spectrometric analysis. Aromatic molecules with carboxy, hydroxy, methoxy and chloro substituents were separated and identified by recording their computer-reconstructed ion current chromatograms. About 70 aromatic carboxylic acids were tentatively identified as degradation products. Aromatic compounds with hydroxy groups were derivatized to trideuteromethoxy groups in order to distinguish them by mass spectrometry from compounds with naturally occurring methoxy groups. Single and double hydrogen atom transfer between methyl ester and methoxy groups were observed in some molecules and given special attention when they occurred ortho to each other. A methoxy group substituted ortho to a methyl carboxylate resulted in an unusual fragmentation.     

Ab initio and NMR studies on the effect of hydration on the chemical shift of hydroxy protons in carbohydrates using disaccharides and water/methanol/ethers as model systems

Density functional theory (DFT) and Hartree-Fock (HF) quantum mechanical calculations have been performed on the disaccharides, [small beta]-l-Fucp-(1[rightward arrow]4)-[small alpha]-d-Galp-OMe, [small beta]-l-Fucp-(1[rightward arrow]4)-[small alpha]-d-Glcp-OMe, and [small beta]-l-Fucp-(1[rightward arrow]3)-[small alpha]-d-Glcp-OMe. The [capital Delta][small delta]-values (difference between the chemical shift in the disaccharide and the corresponding monosaccharide methyl glycoside) for the exchangeable hydroxy protons have been calculated and compared to experimental values previously measured by NMR spectroscopy for samples in aqueous solutions. The calculations performed on molecules in vacuum showed that hydroxy protons hydrogen bonded to the neighboring ring oxygens have large positive [capital Delta][small delta]-values, indicating that they are deshielded relative to those in the corresponding methyl glycoside. The NMR experiments showed instead that these hydroxy protons close to the neighboring ring oxygens were shielded. This discrepancy between calculated and experimental data was attributed to solvent effects, and this hypothesis has been confirmed in this work by monitoring the chemical shift of the hydroxy proton of methanol in water, ethers and water/ether solutions. Shielding of the hydroxy proton of methanol is observed for increased ether concentrations, whereas deshielding is observed for increased concentration of water. The shielding observed for hydroxy protons in disaccharides is a consequence of reduced hydration due to intermolecular hydrogen bonding or steric effects. In strongly hydrated systems such as carbohydrates, the hydration state of a hydroxy proton is the key factor determining the value of the chemical shift of its NMR signal, and the [capital Delta][small delta] will be a direct measure of the change in hydration state.     

Small ring compounds—XXX : Homolytical aromatic substitution and hydrogen abstraction reaction with cyclopropyl radical

Homolytic aromatic substitution and hydrogen abstraction reactions with cyclopropyl radical were carried out to determine the reactivity and ionic character of cyclopropyl radical by examination of the orientation effect, partial rate factor and influence of substituents. By thermal decomposition of biscyclopropaneformyl peroxide in a series of substituted benzenes, the corresponding cyclopropylated benzene derivatives (the mixture of ortho, meta and para isomers) were obtained in moderate yield. In view of the orientation effect and the partial rate factor, the cyclopropyl radical seems to be fairly free from polar effect, and to resemble the phenyl radical more than the common alkyl radical although the cyclopropyl radical has a slightly higher reactivity than the phenyl radical. The relative reactivity of the 2-phenylcyclopropyl radical in the hydrogen abstraction reaction toward the benzylic position of ring-substituted toluenes gave good Hammett's correlation with the slope of + 0·20 suggesting little ionic character in the transition state. This result was in good agreement with the conclusion obtained from homolytic aromatic substitution reaction and with the chemical reactivity to be expected from the non-planar nearly sp²-hybridized conformation of the cyclopropyl radical.