Showing papers on "Homolysis published in 2009"
TL;DR: In this article, a thermodynamic scheme representing the atom transfer radical polymerization (ATRP) equilibrium as the formal sum of equilibria involving carbon−halogen bond homolysis and three additional distinct thermodynamic contributions related to the catalyst is rigorously evaluated.
Abstract: A thermodynamic scheme representing the atom transfer radical polymerization (ATRP) equilibrium as the formal sum of equilibria involving carbon−halogen bond homolysis and three additional distinct thermodynamic contributions related to the catalyst is rigorously evaluated. The reduction/oxidation of both the metal complex and the halogen atom, and the affinity of the higher oxidation state of the catalyst for halide anions (or “halidophilicity”), are measured. The validity and self-consistency of the model are verified by independently measuring, computing, or calculating the overall ATRP equilibrium constant and all four contributing equilibrium constants for one catalyst/alkyl halide combination in acetonitrile. As a thorough demonstration of the value and effectiveness of the scheme, the different equilibrium constants were measured or calculated in 11 different organic solvents, and a comparison of their values was used to both understand and predict catalyst activity in ATRP with high accuracy. The ...
196 citations
TL;DR: It is found that the oxygen-carbon bond dissociation enthalpy is substantially lowered by oxygen substituents situated at the phenyl ring adjacent to the ether oxygen, which could be a competitive initial reaction under high-temperature pyrolysis conditions.
Abstract: Lignin is an abundant natural resource that is a potential source of valuable chemicals. Improved understanding of the pyrolysis of lignin occurs through the study of model compounds for which phenethyl phenyl ether (PhCH2CH2OPh, PPE) is the simplest example representing the dominant β-O-4 ether linkage. The initial step in the thermal decomposition of PPE is the homolytic cleavage of the oxygen−carbon bond. The rate of this key step will depend on the bond dissociation enthalpy, which in turn will depend on the nature and location of relevant substituents. We used modern density functional methods to calculate the oxygen−carbon bond dissociation enthalpies for PPE and several oxygen-substituted derivatives. Since carbon−carbon bond cleavage in PPE could be a competitive initial reaction under high-temperature pyrolysis conditions, we also calculated substituent effects on these bond dissociation enthalpies. We found that the oxygen−carbon bond dissociation enthalpy is substantially lowered by oxygen subs...
158 citations
TL;DR: The detection and kinetic characterization of a cytochrome P450 model compound I, [OFe(IV)-4-TMPyP](+) (1), in aqueous solution shows extraordinary reaction rates for C-H hydroxylations, suggested to result from a high porphyrin redox potential and spin-state-crossing phenomena.
Abstract: The detection and kinetic characterization of a cytochrome P450 model compound I, [OFe(IV)-4-TMPyP](+) (1), in aqueous solution shows extraordinary reaction rates for C-H hydroxylations. Stopped-flow spectrophotometric monitoring of the oxidation of Fe(III)-4-TMPyP with mCPBA revealed the intermediate 1, which displays a weak, blue-shifted Soret band at 402 nm and an absorbance at 673 nm, typical of a porphyrin pi-radical cation. This intermediate was subsequently transformed into the well-characterized OFe(IV)-4-TMPyP. Global analysis afforded a second-order rate constant k(1) = (1.59 +/- 0.06) x 10(7) M(-1) s(-1) for the formation of 1 followed by a first-order decay with k(2) = 8.8 +/- 0.1 s(-1). (1)H and (13)C NMR determined 9-xanthydrol to be the major product (approximately 90% yield) of xanthene oxidation by 1. Electrospray ionization mass spectrometry carried out in 47.5% (18)OH(2) indicated 21% (18)O incorporation, consistent with an oxygen-rebound reaction scenario. Xanthene/xanthene-d(2) revealed a modest kinetic isotope effect, k(H)/k(D) = 2.1. Xanthene hydroxylation by 1 occurred with a very large second-order rate constant k(3) = (3.6 +/- 0.3) x 10(6) M(-1) s(-1). Similar reactions of fluorene-4-carboxylic acid and 4-isopropyl- and 4-ethylbenzoic acid also gave high rates for C-H hydroxylation that correlated well with the scissile C-H bond energy, indicating a homolytic hydrogen abstraction transition state. Mapping the observed rate constants for C-H bond cleavage onto the Bronsted-Evans-Polanyi relationship for similar substrates determined the H-OFe(IV)-4-TMPyP bond dissociation energy to be approximately 100 kcal/mol. The high kinetic reactivity observed for 1 is suggested to result from a high porphyrin redox potential and spin-state-crossing phenomena. More generally, subtle charge modulation at the active site may result in high reactivity of a cytochrome P450 compound I.
154 citations
TL;DR: By regarding both steps as single electron versions of oxidative addition and reductive elimination, a mechanism-based platform for the design of catalysts and reagents for electron transfer reactions evolved that opens broad perspectives for further investigations.
Abstract: In a combined theoretical and experimental study, an efficient catalytic reaction featuring epoxide opening and tetrahydrofuran formation through homolytic substitution reactions at C-O and Ti-O bonds was devised. The performance of these two key steps of the catalytic cycle was studied and could be adjusted by modifying the electronic properties of the catalysts through introduction of electron-donating or -withdrawing substituents to the titanocene catalysts. By regarding both steps as single electron versions of oxidative addition and reductive elimination, a mechanism-based platform for the design of catalysts and reagents for electron transfer reactions evolved that opens broad perspectives for further investigations.
95 citations
TL;DR: Water plays a crucial role by stabilizing transition states for the proton migrations and providing easy intramolecular H-transfers in the absence of any N,O-ligands and saturating the Re coordination sphere what leads to a decrease of the activation barrier for the formation of HOO(*).
Abstract: Plausible mechanisms of radical formation in the catalytic system [MeReO3]/H2O2/H2O−CH3CN for the oxidation of alkanes to alcohols and ketones, via radical pathways, are investigated extensively at the density functional theory level. The most favorable route is based on the monoperoxo complex [MeReO2(O2)(H2O)] and includes the formation of an H2O2 adduct, water-assisted H-transfer from H2O2 to the peroxo ligand, and generation of HOO•. The thus formed reduced ReVI complex [MeReO2(OOH)(H2O)] reacts with H2O2, resulting, upon water-assisted H-transfer and O−OH bond homolysis, in the regeneration of the oxo−ReVII catalyst and formation of the HO• radical that reacts further with the alkane. Water plays a crucial role by (i) stabilizing transition states for the proton migrations and providing easy intramolecular H-transfers in the absence of any N,O-ligands and (ii) saturating the Re coordination sphere what leads to a decrease of the activation barrier for the formation of HOO•. The activation energy of th...
92 citations
TL;DR: Additional studies in natural water samples are required to assess the relative importance of these direct irradiation mechanisms relative to indirect photolysis and other abiotic and biotic degradation and environmental partitioning pathways across the continuum of marine, freshwater, and wastewater biogeochemical signatures.
88 citations
TL;DR: Gas-phase ion-molecule reactions, complemented by DFT and wave-function based electronic structure calculations, are presented, which permit rather detailed descriptions of two fundamental processes related to the activation of methane at room temperature.
Abstract: Gas-phase ion–molecule reactions, complemented by DFT and wave-function based electronic structure calculations, are presented, which permit rather detailed descriptions of two fundamental processes related to the activation of methane at room temperature. (i) Recent examples are discussed, which shed light on the role of oxygen-centered radicals in the first step of the oxidative dimerization of methane, i.e. the homolytic C–H bond cleavage of methane. (ii) Thermal ligand exchange processes of the type ML+ + CH4→ M(CH3)+ + LH are analyzed in detail with an emphasis on L = H and F attached to various transition-metal cations M+. It will be demonstrated inter alia that similar systems, e.g. MH+ (M = Ni, Pd, Pt) in their room-temperature reactions with methane, actually exhibit fundamentally different mechanistic scenarios.
85 citations
TL;DR: The rates of coloration, which increase in the order 1 < 3, 2 < 4, are related to the decrease in the concentration of reducible Mo(6+) centers with irradiation time and follow a second-order reaction law because the event of light absorption at a reducibleMo( 6+) site does not necessarily coincide with that of the N(+)-H bond breaking in the N-H...O hydrogen bond associated with the Mo(5+) site.
Abstract: The excellent photochromic properties of (H2DABCO)2(HDMA)0.5Na0.75(H3O)0.75[Mo8O27]·3H2O (4), a new member of the (H2DABCO)2(A)x[Mo8O27]·nH2O series, are compared with those of (H2DABCO)2(NH4)2[Mo8O27]·4H2O (1), (H2DABCO)2(H2pipz)[Mo8O27] (2), and (H2pipz)3[Mo8O27] (3). All these powdered materials turn from white to purple under illumination at 365 nm, which is associated with photoreduction of Mo6+ cations into Mo5+ cations. We show that the rates of coloration, which increase in the order 1 < 3, 2 < 4, are related to the decrease in the concentration of reducible Mo6+ centers with irradiation time and follow a second-order reaction law because the event of light absorption at a reducible Mo6+ site does not necessarily coincide with that of the N+−H bond breaking in the N+−H···O hydrogen bond associated with the Mo6+ site. First-principles density functional electronic structure calculations were carried out to find that this trend correlates with the homolytic dissociation energies of the N+−H bonds in...
77 citations
TL;DR: Two different methods for generating protonated S-nitrosocysteine in the gas phase are described and NO loss is found to be the lowest energy channel for the former ion.
72 citations
TL;DR: Thermal conversion of methyl beta-d-glucoside to levoglucosan was studied with the MP4//DFT(B3LYP) method, and the direct homolysis mechanism is clearly ruled out.
Abstract: Thermal conversion of methyl beta-d-glucoside to levoglucosan was studied with the MP4//DFT(B3LYP) method. The first step is conformational change of the reactant to (1)C(4) from (4)C(1). The second step is intramolecular nucleophilic substitution at the anomeric C1, which occurs via one step without oxacarbenium ion intermediate. The DeltaG(0)() value (52.5 kcal/mol) is smaller than the C1-O1 bond energy, indicating the direct homolysis mechanism is clearly ruled out. Bimolecular reaction also occurs with smaller activation energy via the similar transition state.
67 citations
TL;DR: The mechanisms for hydrogen-atom transfer from the cyanoisopropyl radical, the formation and homolysis of organo-cobalt complexes that mediate living radical polymerization of vinyl acetate, and cobalt-mediated hydrogenation of olefins are investigated and the results are compared with experimental data.
Abstract: The mechanisms for hydrogen-atom transfer from the cyanoisopropyl radical C-center dot(CH3)(2)CN to [Co-II(por)](center dot) (yielding [Co-III(H)(por)] and CH2=C(CH3)(CN); por = porphyrinato) and the insertion of vinyl acetate (CH2=CHOAc) into the Co-H bond of [Co(H)(por)] (giving [Co-III{CH-(OAc)CH3}(por)]) were investigated by DFT calculations. The results are compared with experimental data. These reactions are relevant to catalytic chain transfer (CCT) in radical polymerization of olefins mediated by [Co-II(por)](center dot), the formation and homolysis of organo-cobalt complexes that mediate living radical polymerization of vinyl acetate, and cobalt-mediated hydrogenation of olefins. Hydrogen transfer from C-center dot(CH3)(2)CN to [Co-II(por)](center dot) proceeds via a single transition state that has structural features resembling the products [Co(H)(por)] and CH2=C(CH3)CN. The separated radicals approach to form a close-contact radical pair and then pass through the transition state for hydrogen-atom transfer to form [Co-III(H)(por)] and CH2=C(CH3)CN. This process provides a very low overall barrier for the hydrogen-atom transfer reaction (Delta G((sic)) = + 3.8 kcal mol(-1)). The reverse reaction corresponding to the addition of [Co(H)(por)] to CH2=C(CH3)CN has a low barrier (Delta G((sic)) = + 8.9 kcal mol(-1)) as well. Insertion of vinyl acetate into the Co-H bond of [Co-III(H)(por)] also proceeds over a low barrier (Delta G((sic)) = + 11.4 kcal mol(-1)) hydrogen-transfer step from [Co-III(H)(por)] to a carbon atom of the alkene to produce a close-contact radical pair. Dissociation of the radical pair, reorientation, and radical-radical coupling to form an organo-cobalt complex are the culminating steps in the net insertion of an olefin into the Co-H bond. The computed energies obtained for the hydrogen-atom transfer reactions from C-center dot(CH3)(2)CN to [Co-II(por)](center dot) and from [Co(H)(por)] to olefins, as well as the organo-cobalt bond homolysis energies correspond well with the experimental observations. The mechanism of alkene insertion into the Co-H bond of [Co-III(H)(por)] is of general interest, because the species does not contain any cis-vacant sites to the hydride and the usual migratory insertion pathway is not available. The low barrier predicted here for the multistep insertion process suggests that (depending on the bond strengths) even for systems that do have a cis-vacant site, the radical-type insertion might compete with classical migratory insertion.
TL;DR: It is found that most of the title compounds are insensitive towards impact stimuli with their h50 larger than 60.0cm, and a correlation is developed between impact sensitivity h50 and the ratio (BDE/E) of the weakest bond BDE to the total energy E.
TL;DR: Data from continuous wave photolysis magnetic field effect (MFE) data reveal the ethanolamine ammonia lyase (EAL) active site to be an ideal microreactor in which to observe enhanced magnetic field sensitivity in the adenosylcobalamin radical pair, and support the notion that rapid radical quenching, through the coupling of homolysis and hydrogen abstraction steps, and subsequent radical pair stabilization make a contribution to the observed rate acceleration.
Abstract: The activation of the Co−C bond in adenosylcobalamin-dependent enzymes generates a singlet-born CoII−adenosyl radical pair. Two of the salient questions regarding this process are: (1) What is the origin of the considerable homolysis rate enhancement achieved by this class of enzyme? (2) Are the reaction dynamics of the resultant radical pair sensitive to the application of external magnetic fields? Here, we present continuous wave photolysis magnetic field effect (MFE) data that reveal the ethanolamine ammonia lyase (EAL) active site to be an ideal microreactor in which to observe enhanced magnetic field sensitivity in the adenosylcobalamin radical pair. The observed field dependence is in excellent agreement with that calculated from published hyperfine couplings for the constituent radicals, and the magnitude of the MFE (<18%) is almost identical to that observed in a solvent containing 67% glycerol. Similar augmentation is not observed, however, in the equivalent experiments with EAL-bound methylcobal...
TL;DR: The study underlines the complexity of the decomposition process in nitroaromatic compounds and casts some doubts on the characterization of the energetic properties of such molecules only on the basis of C-NO(2) homolysis.
Abstract: The pathways corresponding to the most energetically favorable decomposition reactions that can be envisaged for o-nitrotoluene (and 20 of its derivatives) have been studied, using density functional theory, in order to evaluate the influence of substituents’ nature (nitro, methyl, amino, carboxylic acid, and hydroxyl) and position. The first mechanism consists of the direct dissociation (homolysis) of the carbon nitrogen bond (CH3C6H4NO2 = CH3C6H4 + NO2) whereas the second one is a more complex process initiated by C−H alpha attack and leading to the formation of anthranil and water (C6H4C(H)ON + H2O). For each compound, the initial step of this last channel is the rate limiting one, the Gibbs activation energy of all systems being very close, that is all in the 40−44 kcal/mol range. More important variations have been observed for the C−NO2 homolysis Gibbs activation energies (46−60 kcal/mol). These variations have been related to electron donor−acceptor properties of substituents by considering signifi...
TL;DR: Key factors and residues in promoting the enzymatic reaction rate have been discussed in detail and the calculations have shown that the Co-C bond dissociation energy is very much reduced in the protein, compared to that in the gas phase.
Abstract: The considerable protein effect on the homolytic Co-C bond cleavage to form the 5'-deoxyadenosyl (Ado) radical and cob(II)alamin and the subsequent hydrogen transfer from the methylmalonyl-CoA substrate to the Ado radical in the methylmalonyl-CoA mutase (MMCM) have been extensively studied by DFT and ONIOM(DFT/MM) methods. Several quantum models have been used to systematically study the protein effect. The calculations have shown that the Co-C bond dissociation energy is very much reduced in the protein, compared to that in the gas phase. The large protein effect can be decomposed into the cage effect, the effect of coenzyme geometrical distortion, and the protein MM effect. The largest contributor is the MM effect, which mainly consists of the interaction of the QM part of the coenzyme with the MM part of the coenzyme and the surrounding residues. In particular, Glu370 plays an important role in the Co-C bond cleavage process. These effects tremendously enhance the stability of the Co-C bond cleavage state in the protein. The initial Co-C bond cleavage and the subsequent hydrogen transfer were found to occur in a stepwise manner in the protein, although the concerted pathway for the Co-C bond cleavage coupled with the hydrogen transfer is more favored in the gas phase. The assumed concerted transition state in the protein has more deformation of the coenzyme and the substrate and has less interaction with the protein than the stepwise route. Key factors and residues in promoting the enzymatic reaction rate have been discussed in detail.
TL;DR: A general and efficient method for the synthesis of cyclic sulfinates and sulfinamides based on intramolecular homolytic substitution (S(H)i) at the sulfur atom by aryl or alkyl radicals is described.
Abstract: A general and efficient method for the synthesis of cyclic sulfinates and sulfinamides based on intramolecular homolytic substitution (SHi) at the sulfur atom by aryl or alkyl radicals is described. Both alkyl and benzofused compounds can be accessed directly from easily prepared acyclic precursors. Enantiomerically enriched sulfur-based heterocycles were formed through an SHi process with inversion of configuration at the sulfur atom. Cyclization of prochiral radicals proceeded with varying stereochemical outcomes, depending on the size of the incoming radical. 2-Pyridyl and 2-quinolyl radicals led to biaryl compounds, which result from attack onto the ortho position of the arylsulfinate rather than a thiophilic substitution.
TL;DR: To date, homolytic MO ORbond cleavage to give the corresponding metal carboxylatespecies has not been considered as a mode of decomposition of both zinc and other metal alkylperoxides.
Abstract: To date, the formation of alkyl perox-ide (path a, Scheme 1), alkoxide (path b), and oxo spe-cies (path c) in the oxygenation of Zn R species has beenwell-documented. To our knowledge, homolytic MO ORbond cleavage to give the corresponding metal carboxylatespecies (path d, scheme 1) has not been considered as a modeof decomposition of both zinc and other metal alkylperoxides.
TL;DR: For the adsorption of H(2) molecules at O vacancy sites it is found that the homolytic process leads to energetically more favorable structures than heterolytic Adsorption and hydride formation.
Abstract: Oxygen vacancies on ZnO(0001¯) have been proposed to be the catalytically active sites for methanol synthesis on pure ZnO. The charge state and thus the chemical reactivity of such vacancies on this polar O-terminated basal plane of ZnO is expected to be intimately connected to the degree of its hydroxylation in view of its Tasker type(3) unstable character. Here, the interplay between hydrogen adsorption and the thermodynamic stability of O vacancies in various charge states, corresponding formally to F++, F+, F0, F−, and F−− centers, is investigated using electronic structure calculations. Assuming thermodynamic equilibrium of the defective surface with a hydrogen containing gas phase the thermodynamically most stable O vacancy type is determined as a function of temperature and pressure. For the adsorption of H2 molecules at O vacancy sites it is found that the homolytic process leads to energetically more favorable structures than heterolytic adsorption and hydride formation. By homolytic adsorption a...
TL;DR: In this paper, an extensive family of thermochemical data is presented for a series of complexes derived from Cp*Mo(μ-S)2(μSMe)(μ-SH)MoCp* and Cp *Mo(mS) 2(mSH)2MoCP* and these data include electrochemical potentials, pKa values, homolytic solution bond dissociation free energies (SBDFEs), and hydride donor abilities in acetonitrile.
Abstract: An extensive family of thermochemical data is presented for a series of complexes derived from Cp*Mo(μ-S)2(μ-SMe)(μ-SH)MoCp* and Cp*Mo(μ-S)2(μ-SH)2MoCp*. These data include electrochemical potentials, pKa values, homolytic solution bond dissociation free energies (SBDFEs), and hydride donor abilities in acetonitrile. Thermochemical data ranged from +0.6 to −2.0 V vs FeCp2+/o for electrochemical potentials, 5 to 31 for pKa values, 43 to 68 kcal/mol for homolytic SBDFEs, and 44 to 84 kcal/mol for hydride donor abilities. The observed values for these thermodynamic parameters are comparable to those of many transition metal hydrides, which is consistent with the many parallels in the chemistry of these two classes of compounds. The extensive set of thermochemical data is presented in free energy landscapes as a useful approach to visualizing and understanding the relative stabilities of all of the species under varying conditions of pH and H2 overpressure. In addition to the previously studied homogeneous re...
TL;DR: Two new models for cytochrome P450, in which the thiolate axial ligand is replaced by a RSO(3)(-) group, form oxo-iron(IV) porphyrin pi-cation radicals as sole oxidation products in "peroxo shunt" reactions independent of the nature of the employed solvent (polar or non-polar).
Abstract: Two new models for cytochrome P 450 in which the thiolate axial ligand is replaced by a RSO3- group, form oxo-iron (IV) porphyrin pi-cation radicals as sole oxidn. products in "peroxo shunt" reactions independent of the nature of the employed solvent (polar or non-polar) and electronic nature of the porphyrin rings. Although the properties of the solvent and push-pull effects from the porphyrin rings do not affect the mode of the O-O bond cleavage (heterolytic or homolytic) in these models, they strongly affect the rate and mechanism of each reaction step leading to the formation of the high-valent iron intermediates. This article reports the results of mechanistic studies involving the measurements of the rate of oxo-iron (IV) porphyrin pi-cation radical formation from the enzyme mimics of P 450 for different oxidant concn., temp. and pressure in selected org. solvents. Extn. of the appropriate rate consts. and activation parameters for the reactions studied enable a detailed discussion of the effects of solvent and electronic nature of the porphyrin rings on the position of the first pre-equil. involving formation of the acylperoxo-iron (III) porphyrin intermediate, as well as on the rate of heterolytic O-O bond cleavage leading to the formation of the high-valent iron species. Furthermore, an unusual effect of solvent on the kinetics of oxo-iron (IV) porphyrin pi-cation radical formation in methanol is demonstrated and discussed in the present work.
TL;DR: The decomposition of peroxymonocarbonate (HCO(4)(-)) has been investigated by flow-injection chemiluminescence (CL) method and it was suggested that the bond rearrangement within radicals yield superoxide ion (O(2)(*-)).
TL;DR: The calculated difference in energy of the addition and abstraction reactions indicates that any Al(12)X-based hydrogen storage technology that involves predissociation of H(2) will be limited by the competing processes, and can contribute to the design of novel solid-state materials made of doped Al clusters, which may ultimately be suitable for catalytic processes.
Abstract: We have investigated axial interactions of H2 with Al 12X(X = Mg, Al, and Si) clusters and found that homolytic dissociation leading to Al12XH and H atom proceeds without a barrier but is an extremely endothermic process. The calculated difference in energy of the addition and abstraction reactions indicates that any Al12X-based hydrogen storage technology that involves predissociation of H2 will be limited by the competing processes. We have also discovered that while there is a modest barrier for dissociation of H2 on a single Al12Mg cluster to give the dihydride, the process occurs spontaneously between two closely spaced Al12Mg clusters, resulting in the formation of two Al12MgH species. Doping of the cluster with an electropositive atom (Mg) enables the transfer of electron density to the Al cage, which enhances H2 dissociation. The information gained can contribute to the design of novel solid-state materials made of doped Al clusters, which may ultimately be suitable for catalytic processes.
TL;DR: In this article, a series of imidazoline-based alkoxyamines containing either isobutyrate-2-yl or 1-phenylethyl alkyl fragments were studied.
Abstract: The H-atom transfer reaction was studied for a series of imidazoline, imidazolidine, and pyrrolidine-based alkoxyamines containing either isobutyrate-2-yl or 1-phenylethyl alkyl fragments. The CO bond homolysis rate constants and activation energies were determined by 1H NMR product analysis as a function of temperature. Inter- and intramolecular H-atom transfer reactions were distinguished by examination of alkoxyamine thermolysis products in the absence and in the presence of a radical scavenger (thiophenol or deuterated styrene). A correlation between the structure of the nitroxyl fragment of alkoxyamine and the H-transfer reaction was found. The high steric demands of the substituents on the nitroxyl part of the isobutyrate-2-yl alkoxyamine decrease both types of reaction. For alkoxyamines containing the 1-phenylethoxyamines, neither inter- nor intramolecular H-atom transfer was observed. Controlled polymerization of methylmethacrylate initiated with imidazoline-based alkoxyamine was observed, although the polymer obtained was not living. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6579–6595, 2009
TL;DR: In this paper, the authors described nitroxide-mediated radical polymerization of styrene and n-butyl acrylate by using four sterically highly hindered 6-membered cyclic nitroxides.
Abstract: The paper describes nitroxide-mediated radical polymerization of styrene and n-butyl acrylate by using four sterically highly hindered 6-membered cyclic nitroxides. The syntheses of the corresponding alkoxyamine initiators are described, and also rate constants for C−O bond homolysis of these systems are discussed. It is shown that rate constants of alkoxyamines can readily be determined by 1H NMR experiments. Polymerization results obtained are compared with data previously achieved with highly efficient nitroxides. Since this class of nitroxides is readily accessible, the 1-tert-butyl-3,3,5,5-tetraalkyl-2-piperazinon-4-oxyl radicals bearing alkyl groups that are ethyl or larger n-alkyl groups are probably the most efficient cyclic 6-membered nitroxides known to date for mediating NMP.
TL;DR: Intermediates trapped at cryogenic temperatures and characterized by using resonance Raman spectroscopy under single-turnover conditions indicate that an initial bridging peroxide intermediate undergoes homolytic OO bond cleavage generating a trans heme/nonheme bis-ferryl intermediate.
Abstract: O2 reactivity of a functional NOR model is investigated by using electrochemistry and spectroscopy. The electrochemical measurements using interdigitated electrodes show very high selectivity for 4e O2 reduction with minimal production of partially reduced oxygen species (PROS) under both fast and slow electron flux. Intermediates trapped at cryogenic temperatures and characterized by using resonance Raman spectroscopy under single-turnover conditions indicate that an initial bridging peroxide intermediate undergoes homolytic OO bond cleavage generating a trans heme/nonheme bis-ferryl intermediate. This bis ferryl species can oxygenate 2 equivalents of a reactive substrate.
TL;DR: The structure of the transient 4-thia-L-lysine radical is analogous to that of the first substrate-related radical in the putative isomerization mechanism, and it supports the hypothetical isomerized mechanism.
Abstract: Lysine 5,6-aminomutase (5,6-LAM1) participates in the fermentation of l- or d-lysine as carbon and nitrogen sources in anaerobic bacteria (1). Anaerobic fermentation of l-lysine proceeds efficiently as in Figure 1, starting with conversion to l-β-lysine by 2,3-LAM, a SAM and PLP-dependent enzyme. 5,6-LAM then converts l-β-lysine into l-3,5-DAH, a molecule poised for dehydrogenation and β-oxidation. Fermentation of d-lysine in Figure 1 begins with conversion to d-2,5-DAH by 5,6-LAM and proceeds to the formation of acetate and butyrate (1).
Figure 1
Metabolism of lysine in anaerobic bacteria.
5,6-LAM is an adenosylcobalamin- and PLP-dependent enzyme that catalyzes the interconversion of d- or l-lysine with d- or l-2,5-DAH or of l-β-lysine with l-3,5-DAH (1-8). The mechanism of action of 2,3-LAM is well worked out, and the structure of the enzyme is fully compatible with the spectroscopic and chemical evidence supporting the mechanism (9,10). The 2,3-LAM mechanism inspires the hypothetical chemical mechanism for 5,6-LAM shown in Scheme 1 (2,4,9), wherein the 5′-deoxyadenosyl radical from adenosylcobalamin initiates the chemistry by abstracting a C5(H) from lysine to generate the substrate-related radical 2, which is bound as the Ne-aldimine to PLP. Radical isomerization analogous to that in 2,3-LAM leads through the aziridincarbinyl intermediate 3 to the product-related radical 4, which is quenched by hydrogen transfer from 5′-deoxyadenosine. In contrast to 2,3-LAM, little experimental evidence bearing on the mechanism of action of 5,6-LAM is available, apart from the mediation of hydrogen transfer by the 5′-deoxyadenosyl moiety of adenosylcobalamin (7). The X-ray crystal structure of 5,6-LAM raises questions regarding coordination in the actions of PLP and adenosylcobalamin (11).
Scheme 1
5,6-LAM is a heterotetrameric protein composed of α- and β-subunits (αβ)2. In the available structure, illustrated in Figure 2 with cobalamin, 5′-deoxyadenosine and PLP as ligands, the α-subunit incorporates a TIM barrel and the β-subunit a Rossman domain. Adenosylobalamin binds in a base-off mode, with most interactions to the β-subunit, which projects the 5′-deoxyadenosyl moiety toward the β–barrel of the α–subunit. The major binding contacts of PLP are to the α–subunit, but the β-subunit binds the carboxaldehyde group of PLP as an internal aldimine with Lysβ144 (4,11). The 24 A separation between 5′-deoxyadenosine and PLP in the structure is too great to represent an active conformation that would allow a substrate to interact chemically with both adenosylcobalamin and PLP.
Figure 2
Structure of 5,6-LAM and relative locations of adenosylcobalamin and PLP.
Spectroscopic experiments show that other adenosylcobalamin-dependent enzymes facilitate the transient and reversible homolytic cleavage of the Co—C5′ bond in adenosylcobalamin to form cob(II)alamin. The resultant 5′-deoxyadenosyl radical initiates catalysis by abstracting a hydrogen atom from the cognate substrate (12-14). Limited evidence for homolytic scission of the Co—C5′ bond is available for 5,6-LAM. Cob(II)alamin is not observable as an intermediate in the steady state with any substrate. The only reported cleavages of the Co—C5′ bond by 5,6-LAM are the formation of cob(III)alamin during suicide inactivation of the enzyme by substrates (2), and the EPR spectroscopic observation of cob(II)alamin in a reaction with the substrate analog 4-thia-l-lysine (15).
EPR spectroscopy is employed in research on the mechanisms of enzymes catalyzing radical reactions, allowing structural assignments to intermediates that are detectable by EPR (16-20). No radical can be detected in the reactions of 5,6-LAM with the natural substrates d-lysine, l-lysine, or l-β-lysine. In this report, we present the results of studies of the reaction of 5,6-LAM with 4-thia-d- and 4-thia-l-lysine. These molecules are structurally similar to d- and l-lysine but have special chemical properties that facilitate the spectroscopic observation of radicals related in structure to possible catalytic intermediates. We also present spectrophotometric evidence for the reaction of 4-thia-d- and 4-thia-l-lysine in cleaving the Co—C5′ bond of adenosylcobalamin to cob(II)alamin and 5′-deoxyadenosine. The results support catalysis of amino group migration by way of the radical mechanism in Scheme 1.
TL;DR: The sequence of organo-cobalt species formed during the induction period for the (TMP)Co-R mediated LRP of MA indicates that homolytic dissociation is a prominent pathway for the interconversion oforgano- cobalt complexes which contrasts with the corresponding vinyl acetate (VAc) system where associative radical exchange totally dominates these processes.
Abstract: Observation of the formation and interconversion of organo-cobalt complexes ((TMP)Co-R) is used to reveal mechanistic features in the living radical polymerization (LRP) of methyl acrylate (MA) mediated by cobalt porphyrins. Both dissociative and associative exchange of radicals in solution with organo-cobalt complexes contribute to controlling the radical polymerization. The sequence of organo-cobalt species formed during the induction period for the (TMP)Co-R mediated LRP of MA indicates that homolytic dissociation is a prominent pathway for the interconversion of organo-cobalt complexes which contrasts with the corresponding vinyl acetate (VAc) system where associative radical exchange totally dominates these processes. The dissociation equilibrium constant (Kd(333 K)) for organo-cobalt complexes formed in methyl acrylate polymerization ((TMP)Co-CH(CO2CH3)CH2P) was estimated as 1.15 × 10−10 from analysis of the polymerization kinetics and 1H NMR. The ratio of the rate constants (333 K) for the cyanoiso...
TL;DR: O− radical anions were observed over a partially hydroxylated MgO surface after illumination by monochromatic light with λ = 280 and 303 nm as discussed by the authors.
Abstract: O− radical anions were observed over a partially hydroxylated MgO surface after illumination by monochromatic light with λ = 280 and 303 nm. As all corner oxygen atoms are covered with adsorbed hyd...
TL;DR: A variety of theoretical methods have been used to study the gas-phase acidity of benzene and its eleven fluorine-substituted derivatives: fluorobenzene, three isomers of diffluorobenezene, trifluorobenzenes, tetrafluorbenzene and 1,2,3,4,5-pentafluorogenene.
Journal Article•
TL;DR: In this article, the effect of amines and water on detonation characteristics of nitromethane (Nm) is discussed from the point of view of the published knowledge about the study of initiation reactivity of this nitro paraffin.
Abstract: A well-known effect of amines, and also of water, on detonation characteristics of nitromethane (Nm) is discussed from the point of view of the published knowledge about the study of initiation reactivity of this nitro paraffin. It is documented that bimolecular and higher interactions during the initiation of Nm are impossible. The most widespread concepts of the primary steps of this initiation, i.e. formation of aci-Nm anion [CH2=NO2] by intermolecular hydrogen transfer in the neat Nm submitted to shock and formation of this anion by action of an amine, have been scrutinized by the dFT B3lYP/cc-pVTZ+ method and evaluated as thermodynamically disadvantageous. Also the 1,3-intramolecular hydrogen shift in the Nm molecule was characterized as a higher-barrier process. Two favorable primary mechanisms of fission in the NM initiation and development 120 S. Zeman, T. Atalar, Z. Friedl, Xue-Hai Ju of its detonation were investigated by dFT B3lYP/cc-pVTZ+ calculations: homolysis of C–NO2 bond in the neat Nm and homolysis of N–OH bond in its aci-form. The second mentioned pathway was found to be thermodynamically the most preferable mechanism of fission. Consequently, a detonation wave of Nm with admixture of amine or water has a considerably reduced reaction zone length in comparison with the detonation of neat Nm. The B3lYP/6-311++G(d,p) calculations of transition states revealed that an admixture of methylamine and/or water influences the conversion of nitromethane to its aci-form, and this effect is more feasible in the case of methylamine rather than water.