Showing papers on "Homolysis published in 2007"
TL;DR: The monooxygenase activity of the oxygen carrier protein hemocyanin has been explored for the first time by employing urea as an additive in the reaction system and the ortho-hydroxylation of phenols to catechols has been demonstrated to involve the same ionic mechanism.
Abstract: The molecular mechanism of the monooxygenase (phenolase) activity of type 3 copper proteins has been examined in detail both in the model systems and in the enzymatic systems. The reaction of a side-on peroxo dicopper(II) model compound (A) and neutral phenols proceeds via a proton-coupled electron-transfer (PCET) mechanism to generate phenoxyl radical species, which collapse each other to give the corresponding C–C coupling dimer products. In this reaction, a bis(μ-oxo)dicopper(III) complex (B) generated by O–O bond homolysis of A is suggested to be a real active species. On the other hand, the reaction of lithium phenolates (deprotonated form of phenols) with the same side-on peroxo dicopper(II) complex proceeds via an electrophilic aromatic substitution mechanism to give the oxygenated products (catechols). The mechanistic difference between these two systems has been discussed on the basis of the Marcus theory of electron transfer and Hammett analysis. Mechanistic details of the monooxygenase activity...
193 citations
TL;DR: In this article, a mechanistic comparison of the ATRP and SET-LRP is presented, and it is shown that the activation of the initiator and of the propagating dormant species is faster than of the homolytic inner-sphere electron-transfer process responsible for ATRP.
Abstract: A mechanistic comparison of the ATRP and SET-LRP is presented. Subsequently, simulation of kinetic experiments demonstrated that, in the heterolytic outer-sphere single-electron transfer process responsible for the SET-LRP, the activation of the initiator and of the propagating dormant species is faster than of the homolytic inner-sphere electron-transfer process responsible for ATRP. In addition, simulation experiments suggested that in both polymerizations the rate of deactivation is similar. In SET-LRP, the Cu(II)X2/L deactivator is created by the disproportionation of Cu(I)X/L inactive species, while in ATRP its concentration is mediated by the bimolecular termination. The combination of higher rate of activation with the creation of deactivator via disproportionation provides, via SET-LRP, an ultrafast synthesis of polymers with very narrow molecular weight distribution at room temperature. SET-LRP is mediated by a catalytic amount of Cu(0), and under suitable conditions, bimolecular termination is virtually absent. Kinetic and simulation experiments have also demonstrated that the amount of water available in commercial solvents and monomers is sufficient to induce the disproportionation of Cu(I)X/L into Cu(0) and Cu(II)X2/L and, subsequently, to change the polymerization mechanism from ATRP to SET-LRP. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1835–1847, 2007.
125 citations
TL;DR: By properly adjusting the steric demands of the catalysts and the substrates the first examples of reversible epoxide openings were designed and provide a structural basis for the understanding of the factors determining the regioselectivity of ring opening.
Abstract: The mechanism of titanocene-mediated epoxide opening was studied by a combination of voltammetric, kinetic, computational, and synthetic methods. With the aid of electrochemical investigations the nature of a number of Ti(III) complexes in solution was established. In particular, the distribution of monomeric and dimeric Ti(III) species was found to be strongly affected by the exact steric conditions. The overall rate constants of the reductive epoxide opening were determined for the first time. These data were employed as the basis for computational studies of the structure and energies of the epoxide−titanocene complexes, the transition states of epoxide opening, and the β-titanoxy radicals formed. The results obtained provide a structural basis for the understanding of the factors determining the regioselectivity of ring opening and match the experimentally determined values. By employing substituted titanocenes even more selective epoxide openings could be realized. Moreover, by properly adjusting the...
122 citations
TL;DR: Light is shed on the pathway that might lead to a TNT explosion and on the temperature in which it becomes exergonic, and the results appear to correlate closely with the experimentally derived shock wave detonation time.
Abstract: The widespread and long-term use of TNT has led to extensive study of its thermal and explosive properties. Although much research on the thermolysis of TNT and polynitro organic compounds has been undertaken, the kinetics and mechanism of the initiation and propagation reactions and their dependence on the temperature and pressure are unclear. Here, we report a comprehensive computational DFT investigation of the unimolecular adiabatic (thermal) decomposition of TNT. On the basis of previous experimental observations, we have postulated three possible pathways for TNT decomposition, keeping the aromatic ring intact, and calculated them at room temperature (298 K), 800, 900, 1500, 1700, and 2000 K and at the detonation temperature of 3500 K. Our calculations suggest that at relatively low temperatures, reaction of the methyl substituent on the ring (C-H alpha attack), leading to the formation of 2,4-dinitro-anthranil, is both kinetically and thermodynamically the most favorable pathway, while homolysis of the C-NO(2) bond is endergonic and kinetically less favorable. At approximately 1250-1500 K, the situation changes, and the C-NO(2) homolysis pathway dominates TNT decomposition. Rearrangement of the NO(2) moiety to ONO followed by O-NO homolysis is a thermodynamically more favorable pathway than the C-NO(2) homolysis pathway at room temperature and is the most exergonic pathway at high temperatures; however, at all temperatures, the C-NO(2) --> C-ONO rearrangement-homolysis pathway is kinetically unfavorable as compared to the other two pathways. The computational temperature analysis we have performed sheds light on the pathway that might lead to a TNT explosion and on the temperature in which it becomes exergonic. The results appear to correlate closely with the experimentally derived shock wave detonation time (100-200 fs) for which only the C-NO(2) homolysis pathway is kinetically accessible.
116 citations
TL;DR: Fullerene skeleton modification has been investigated through selective cleavage of the fullerene carbon-carbon bonds under mild conditions through cage-opening reactions and a domino mechanism was proposed for this reaction.
Abstract: Fullerene skeleton modification has been investigated through selective cleavage of the fullerene carbon−carbon bonds under mild conditions. Several cage-opened fullerene derivatives including three [59]fullerenones with an 18-membered-ring orifice and one [59]fullerenone with a 19-membered-ring orifice have been prepared starting from the fullerene mixed peroxide 1, C60(OOtBu)6. The prepositioned tert-butyl peroxy groups in 1 serve as excellent oxygen sources for formation of hydroxyl and carbonyl groups. The cage-opening reactions were initiated by photoinduced homolysis of the tBu−O bond, followed by sequential ring expansion steps. A key step of the ring expansion reactions is the oxidation of adjacent fullerene hydroxyl and amino groups by diacetoxyliodobenzene (DIB). Aminolysis of a cage-opened fullerene derivative containing an anhydride moiety resulted in multiple bond cleavage in one step. A domino mechanism was proposed for this reaction. Decarboxylation led to elimination of one carbon atom fro...
106 citations
TL;DR: In this article, a quantum-chemical calculation of the homolytic and heterolytic bond dissociation energies of the model compounds of the monomer and dimer is reported.
Abstract: A quantum-chemical calculation of the homolytic and heterolytic bond dissociation energies of the model compounds of the monomer and dimer is reported. These model compounds include the dormant chloride, bromide, and iodide species for representative activated and nonactivated monomers containing electron-withdrawing groups as well as for a nonactivated monomer containing an electron-donor group. Two examples of sulfonyl and N-halide initiators are also reported. The homolytic inner-sphere electron-transfer bond dissociation is known as atom transfer and is responsible for the activation step in ATRP. The heterolytic outer sphere single electron transfer bond dissociation is responsible for the activation step in single electron transfer mediated living radical polymerization (SET-LRP). The results of this study demonstrated much lower bond dissociation energies for the outer sphere single electron transfer processes. These results explain the higher rate constant of activation, the higher apparent rate constant of propagation, and the lower polymerization temperature for both activated and nonactivated monomers containing electron-withdrawing groups in SET-LRP.
104 citations
TL;DR: Light is shed on the exceptional catalytic performance of the Hangman metalloporphyrin complexes and provides tangible benchmarks for using proton-coupled multielectron reactions to catalyze O-O bond-breaking and bond-making reactions.
Abstract: Hangman metalloporphyrin complexes poise an acid−base group over a redox-active metal center and in doing so allow the “pull” effect of the secondary coordination environment of the heme cofactor of hydroperoxidase enzymes to be modeled. Stopped-flow investigations have been performed to decipher the influence of a proton-donor group on O−O bond activation. Low-temperature reactions of tetramesitylporphyrin (TMP) and Hangman iron complexes containing acid (HPX−CO2H) and methyl ester (HPX−CO2Me) functional groups with peroxyacids generate high-valent FeO active sites. Reactions of peroxyacids with (TMP)FeIII(OH) and methyl ester Hangman (HPX−CO2Me)FeIII(OH) give both O−O heterolysis and homolysis products, Compound I (Cpd I) and Compound II (Cpd II), respectively. However, only the former is observed when the hanging group is the acid, (HPX−CO2H)FeIII(OH), because odd-electron homolytic O−O bond cleavage is inhibited. This proton-controlled, 2e- (heterolysis) vs 1e- (homolysis) redox specificity sheds ligh...
83 citations
TL;DR: Diaminophosphonyl radicals whose reactivity was unknown react smoothly with olefins and a radical thiophosphinoylation of triple bonds has been uncovered.
72 citations
56 citations
TL;DR: 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.
Abstract: 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 σ-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 π-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.
55 citations
TL;DR: In the enzyme-bound state, the RP generated upon homolysis appears to be stabilized against the extent of geminate recombination required to observe a pre-steady-state magnetic field effect (MFE).
Abstract: The apparent conflict between literature evidence for (i) radical pair (RP) stabilization in adenosylcobalamin (AdoCbl)-dependent enzymes and (ii) the manifestation of magnetic field sensitivity due to appreciable geminate recombination of the RP has been reconciled by pre-steady-state magnetic field effect (MFE) investigations with ethanolamine ammonia lyase (EAL). We have shown previous stopped-flow MFE studies to be insensitive to magnetically induced changes in the net forward rate of C-Co homolytic bond cleavage. Subsequently, we observed a magnetic-dependence in the continuous-wave C-Co photolysis of free AdoCbl in 75% glycerol but have not done so in the thermal homolysis of this bond in the enzyme-bound cofactor in the presence of substrate. Consequently, in the enzyme-bound state, the RP generated upon homolysis appears to be stabilized against the extent of geminate recombination required to observe an MFE. These findings have strong implications for the mechanism of RP stabilization and the unprecedented catalytic power of this important class of cobalamin-dependent enzymes.
TL;DR: The reactions of a series of epoxynitriles and epoxyketones induced by titanocene chloride have been studied and the role of Ti(III) is triple: a radical initiator (homolytic cleavage of oxirane), a Lewis acid (coordination to CN or C=O), and a terminator (reduction of iminyl or alkoxyl radicals).
Abstract: The reactions of a series of epoxynitriles and epoxyketones induced by titanocene chloride have been studied. The kinetics of the decyanogenation of β,γ-epoxynitriles with Ti(III) corresponds to a radical reaction (k25 ≈ 106 s-1), as demonstrated by competition experiments with H-transfer from 1,4-cyclohexadiene (1,4-CHD) or PhSH or conjugate addition to acrylonitrile. The 5-exo cyclization onto nitrile induced by Ti(III) is a radical reaction (k25 ≈ 107 s-1) as seen in competition experiments with H-transfer from PhSH or the titanocene−water complex. The iminyl or alkoxyl radicals generated by 5-exo cyclization onto nitriles or ketones only undergo a reduction with Ti(III). This reaction overwhelms any alternative process, such as tandem cyclization onto alkenes or β-scission. Iminyl radicals generated by 4-exo cyclizations onto nitriles undergo reduction with Ti(III) and β-scission reaction in a ratio of 96:4 when the α-substituent is CN. Alkoxyl radicals from 4-exo cyclizations onto ketone carbonyls un...
TL;DR: Temperature-dependent rate measurements suggest a common reaction trajectory for each of these reactions and verify previous theoretical estimates of a ca.
Abstract: Low-spin mononuclear (alkylperoxo)iron(III) complexes decompose by peroxide O−O bond homolysis to form iron(IV) species. We examined the kinetics of previously reported homolysis reactions for (alkylperoxo)iron(III) intermediates supported by TPA (tris(2-pyridylmethyl)amine) in CH3CN solution and promoted by pyridine N-oxide, and by BPMCN (N,N-bis(2-pyridylmethyl)-N,N-dimethyl-trans-1,2-diaminocyclohexane) in its cis-β configuration in CH3CN and CH2Cl2, as well as for the previously unreported chemistry of TPA and 5-Me3TPA intermediates in acetone. Each of these reactions forms an oxoiron(IV) complex, except for the β-BPMCN reaction in CH2Cl2 that yields a novel (hydroxo)(alkylperoxo)iron(IV) product. Temperature-dependent rate measurements suggest a common reaction trajectory for each of these reactions and verify previous theoretical estimates of a ca. 60 kJ/mol enthalpic barrier to homolysis. However, both the tetradentate supporting ligand and exogenous ligands in the sixth octahedral coordination sit...
TL;DR: A method for a photochemically induced mineralization of CCl4 is described in which use is made of reductive radicals, and carbon dioxide radical anion, CO2(-), an intermediate in the mineralization pathway of methanol, is shown to initiate the mineralized phase by reductive dechlorination.
TL;DR: This paper has three goals: to provide a first step in understanding the atomic basis of the role of magnesium in facilitating the dissociation of the P-O bond in phosphorylated biochemical fuel molecules, to compare second-order Møller-Plesset perturbation theory results with those obtained at the more economical density functional theory (DFT) level, and to examine the calculation of atomic total energies from atomic kinetic energies within a Kohn-Sham implemention of DFT.
Abstract: This paper has three goals: (1) to provide a first step in understanding the atomic basis of the role of magnesium in facilitating the dissociation of the P-O bond in phosphorylated biochemical fuel molecules (such as ATP or GTP), (2) to compare second-order Moller-Plesset perturbation theory (MP2) results with those obtained at the more economical density functional theory (DFT) level for a future study of larger more realistic models of ATP/GTP, and (3) to examine the calculation of atomic total energies from atomic kinetic energies within a Kohn-Sham implemention of DFT, as compared to ab initio methods. A newly described method based on the quantum theory of atoms in molecules (QTAIM), which is termed the "atomic partitioning of the bond dissociation energy" (APBDE), is applied to a simple model of phosphorylated biological molecules (HPO42-). The APBDE approach is applied in the presence and in the absence of magnesium. It is found that the P-O(H) bond in the magnesium complex is shorter, exhibits a higher stretching frequency, and has a higher electron density at the bond critical point than in the magnesium-free hydrogen phosphate anion. Though these data would seem to suggest a stronger P-O(H) bond in the magnesium complex compared to the magnesium-free case, the homolytic breaking of the P-O(H) bond in the complex is found to be easier, i.e., has a lower BDE. This effect is the result of the balance of several atomic contributions to the BDE induced by the magnesium cation, which stabilizes the dissociation product more than it stabilizes the intact model molecule.
TL;DR: In this article, the authors focus on the issue of facile metal-carbon bond homolysis, especially following acrylate insertion, using examples from their own work and suggest ways to circumvent these issues, for example forcing 1,2-insertion by imposing steric crowding at the metal.
TL;DR: In the gas-phase reactions of halonitro- and dinitrophenide anions with X (X = F, Cl, Br, NO2) and NO2 groups in ortho or para position to each other with selected C-H acids, products of the SNAr-type reaction are formed.
TL;DR: In this paper, the selective oxidation of 2,3,6-trimethylphenol (TMP) and 2-methyl-1-naphthol (MNL) with H2O2 catalyzed by titanium single-site catalysts, TiO2-SiO2 aerogel and mesostructured hydrothermally stable titanium-silicate, Ti-MMM-2, have been studied by means of EPR spectroscopic technique with spin traps.
Abstract: The selective oxidation of 2,3,6-trimethylphenol (TMP) and 2-methyl-1-naphthol (MNL) with H2O2 catalyzed by titanium single-site catalysts, TiO2–SiO2 aerogel and mesostructured hydrothermally stable titanium-silicate, Ti-MMM-2, have been studied by means of EPR spectroscopic technique with spin traps. The formation of phenoxyl (naphthoxyl) and hydroxyl radical intermediates during the oxidation process have been detected using 3,5-dibromo-4-nitrosobenzene-sulfonic acid (DBNBS) and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) spin traps, respectively. The character of the EPR spectra of the DBNBS adducts strongly depends on the nature of the organic substrate and the reaction temperature. A compilation of the EPR and by-product studies strongly supports a homolytic oxidation mechanism. Study by DR-UV–vis spectroscopy has confirmed the previously suggested chemical adsorption of phenol on the titanium center.
TL;DR: O-isotope-labeling studies confirmed that the oxygen in the CO molecule that is eliminated from the anion of meta-hydroxybenzaldehyde originates from either the aldehydic or the phenolic group.
Abstract: Collision-induced dissociation (CID) mass spectra of anions derived from several hydroxyphenyl carbaldehydes and ketones were recorded and mechanistically rationalized. For example, the spectrum of m/z 121 ion of deprotonated ortho-hydroxybenzaldehyde shows an intense peak at m/z 93 for a loss of carbon monoxide attributable to an ortho-effect mediated by a charge-directed heterolytic fragmentation mechanism. In contrast, the m/z 121 ion derived from meta and para isomers undergoes a charge-remote homolytic cleavage to eliminate an *H and form a distonic anion radical, which eventually loses CO to produce a peak at m/z 92. In fact, for the para isomer, this two-step homolytic mechanism is the most dominant fragmentation pathway. The spectrum of the meta isomer on the other hand, shows two predominant peaks at m/z 92 and 93 representing both homolytic and heterolytic fragmentations, respectively. (18)O-isotope-labeling studies confirmed that the oxygen in the CO molecule that is eliminated from the anion of meta-hydroxybenzaldehyde originates from either the aldehydic or the phenolic group. In contrast, anions of ortho-hydroxybenzaldehyde and 2-hydroxy-1-naphthaldehyde, both of which show two consecutive CO eliminations, specifically lose the carbonyl oxygen first, followed by that of the phenolic group. Anions from 2-hydroxyphenyl alkyl ketones lose a ketene by a hydrogen transfer predominantly from the alpha position. Interestingly, a very significant charge-remote 1,4-elimination of a H(2) molecule was observed from the anion derived from 2,4-dihydroxybenzaldehyde. For this mechanism to operate, a labile hydrogen atom should be available on the hydroxyl group adjacent to the carbaldehyde functionality.
TL;DR: The computations suggest that H2O2 decomposition follows a homolytic route with intermediate formation of an iron(IV) oxo radical cation species (L.+FeIV==O) that resembles Compound I of natural iron porphyrin systems.
Abstract: Non-heme iron(III) complexes of 14-membered tetraaza macrocycles have previously been found to catalytically decompose hydrogen peroxide to water and molecular oxygen, like the native enzyme catalase. Here the mechanism of this reaction is theoretically investigated by DFT calculations at the (U)B3LYP/6-31G* level, with focus on the reactivity of the possible spin states of the FeIII complexes. The computations suggest that H2O2 decomposition follows a homolytic route with intermediate formation of an iron(IV) oxo radical cation species (L.+FeIV==O) that resembles Compound I of natural iron porphyrin systems. Along the whole catalytic cycle, no significant energetic differences were found for the reaction proceeding on the doublet (S=1/2) or on the quartet (S=3/2) hypersurface, with the single exception of the rate-determining O--O bond cleavage of the first associated hydrogen peroxide molecule, for which reaction via the doublet state is preferred. The sextet (S=5/2) state of the FeIII complexes appears to be unreactive in catalase-like reactions.
TL;DR: The photochemistry of isomeric methoxyphenyl chlorides and phosphates has been examined in different solvents (and in the presence of benzene) and found to involve the triplet state as mentioned in this paper.
Abstract: The photochemistry of isomeric methoxyphenyl chlorides and phosphates has been examined in different solvents (and in the presence of benzene) and found to involve the triplet state. With the chlorides, C−Cl bond homolysis occurs in cyclohexane and is superseded by heterolysis in polar media, while the phosphate group is detached (heterolytically) only in polar solvents. Under such conditions, the isomeric triplet methoxyphenyl cations are the first formed intermediates from both precursors, but intersystem crossing (isc) to the singlets can take place. Solvent addition (forming the acetanilide in MeCN, the ethers in alcohols, overall a SN1 solvolysis) is a diagnostic reaction for the singlet cation, as reduction and trapping by benzene are for the corresponding triplet. Solvolysis is most important with the meta isomer, for which the singlet is calculated (UB3LYP/6-31g(d)) to be the ground state of the cation (ΔE = 4 kcal/mol) and isc is efficient (kisc ca. 1 × 108 s-1), and occurs to some extent with th...
TL;DR: The reactions of small saturated hydrocarbons by gaseous nickel cations NiX+ are investigated by means of electrospray ionization mass spectrometry and heterolytic cleavage of secondary carbon--hydrogen bonds is clearly preferred compared to that of primary ones, as revealed by deuterium labeling studies.
Abstract: The reactions of small saturated hydrocarbons by gaseous nickel cations NiX + (X=F, Cl, Br, I) are investigated by means of electrospray ionization mass spectrometry. The halide cations are obtained from solutions of the corresponding Ni II salts in water or methanol as solvents. NiF + is the only Ni II halide complex that brings about thermal activation of methane. The branching ratios of the observed reactions with C 2 H 6 , C 3 H 8 , and nC 4 H 10 are shifted systematically by changing the nature of both the ligand X and the substrate RH. In the elimination of HX (X=F, Cl, Br, I), the formal oxidation state of the metal ion appears to be conserved, and the importance of this reaction channel decreases in going from NiF + to NiI + . A reversed trend is observed in the losses of small closed-shell neutral molecules, that is, H 2 , CH 4 and C 2 H 6 , which dominate the gas-phase ion chemistry of NiI + /RH couples. Additionally, inner-sphere electron-transfer reactions take place for a few systems, that is, the delivery of hydride or methanide ions from the hydrocarbon to NiX + in the course of which the hydrocarbon is converted to a carbenium ion and the cationic metal complex gives rise to a neutral RNiX molecule (R=H, CH 3 ). This process gains importance with decreasing atomic number of the halides and with increasing the size of the alkane. Thus, it constitutes the major pathway in the reactions of NiF + with propane and n-butane, whereas it is not observed for any of the NiI + /RH couples investigated. Concerning the regioselectivity of the reactions with propane and n-butane, heterolytic cleavage of secondary carbon-hydrogen bonds is clearly preferred compared to that of primary ones, as revealed by deuterium labeling studies. For the NiF + /C 3 H 8 couple, the selectivity of the hydride transfer is as large as 360 in favor of the secondary positions. Though smaller, large preferences for the activation of secondary C-H bonds are also operative in homolytic bond activation of RH (R= nC 3 H 7 , nC 4 H 9 ).
TL;DR: The photocleavage mechanism of N -trifluoromethylsulfonyloxy-1,8-naphthalimide usable as a photoacid generator is reported in this paper.
TL;DR: In this article, the authors showed that the head-to-tail dimer 1-(9-fluorenylidene)-4-(trimethylsilyl)-2-[(trimmethylene)methylene]spiro[cyclobutane-3,9′-diyl)-1-chloroallene (12, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35,
TL;DR: In this article, the use of 2,2,6,6-tetramethylpiperidinyl-oxy derivatives for scorch suppression, cure control, and functionalization in peroxide crosslinking of polyethylene was described.
Abstract: This paper describes the use of 2,2,6,6-tetramethylpiperidinyl-oxy (TEMPO) derivatives for scorch suppression, cure control, and functionalization in peroxide crosslinking of polyethylene. When 4-hydroxy 2,2,6,6-tetramethylpiperidin-1-oxyl was used for scorch suppression, there was often a loss in ultimate degree of crosslinking. In contrast, with bis(1-oxyl-2,2,6,6-tetramethylpiperidine-4-yl)sebacate, both scorch suppression and ultimate degree of crosslinking were enhanced. A model study in hexadecane showed that TEMPO radicals terminate with carbon-centered radicals formed as a consequence of peroxide homolysis and propagation steps. This termination occurs preferentially over peroxide-initiated crosslinking and results in TEMPO-grafted polymer. In addition to polymer radical formation, several additional reaction pathways are available following thermal homolysis of the peroxide, including unimolecular disassociation of the peroxy radical to yield a methyl radical and a ketone, and proton extraction from one of several substrates by the peroxy radical to yield an alkyl radical and an alcohol. This study reveals that the reaction rate is limited by the rate of peroxide homolysis, and proceeds to statistical products with little or no preference for any specific species. The implication is that choice of peroxide is a dominant controlling factor over whether the TEMPO derivatives are ultimately grafted to the polymer or are bound to small alkyl radicals. POLYM. ENG. SCI., 47:50–61, 2007. a 2006 Society of Plastics Engineers
TL;DR: Based on the current calculations, the bond scission may be occurring through a homolytic reaction between aliphatic carbon atoms, which may have implications in the reactivity of lignin especially when subjected to processing methods that place large mechanical forces on the structure.
01 Jan 2007
TL;DR: In this article, the use of cyclic voltammetry (CV) for quantitative measurements of bond energies in organometallic complexes is discussed and comprehensive lists of derived bond energy data are presented.
Abstract: Thermochemical cycles that incorporate electrode potential data have emerged as a powerful approach to extract bond energy data that are frequently not directly available by other experimental methods. Cyclic voltammetry (CV) is the most commonly used technique; its transient nature offers the advantage that bonding energetics can be investigated even for short-lived species. The reliability of the extracted bond energies depends on the accuracy of the electrode potential data that are used. The CV technique is briefly discussed with focus on simple but very important issues that frequently lead to inconsistencies in published electrode potential data. The use of thermochemical cycles for quantitative measurements of bond energies in organometallic complexes is discussed and comprehensive lists of derived bond energy data are presented. Particular emphasis is given to metal–hydride bond strengths with respect to dissociation of the hydride ligand as a proton (M–H acidity, pKa), as a hydrogen atom (M–H homolytic bond dissociation energies (BDEs)), or as a hydride (M–H hydricity). Furthermore, determinations of M–M bond energies and homolytic and heterolytic bond energies of C–H bonds in π-coordinated and σ-coordinated ligands are included. The consequences of one-electron redox processes on homolytic and heterolytic M–H, M–X, and ligand C–H bond strengths are quantified. Square schemes that yield thermodynamic data for redox-induced structural change or isomerization processes are briefly discussed. Finally, recent, selected applications where the thermodynamic data have played an interesting role in reactivity studies are highlighted.
TL;DR: A significant reduction in the secondary kinetic isotope effect is consistent with motions of the 5'-hydrogen atoms being coupled in the transition state to the motion of the hydrogen undergoing transfer, in a reaction that involves a large degree of quantum tunneling.
Abstract: Glutamate mutase is one of a group of adenosylcobalamin1 (AdoCbl, coenzyme B12) dependent enzymes that catalyze unusual carbon skeleton isomerisations. These rearrangements formally involve a 1,2 hydrogen atom migration and proceed through a mechanism involving carbon-based free radical intermediates (1-6). The initial steps of these reactions involve homolysis of the reactive cobalt-carbon bond of the coenzyme to form cob(II)alamin and 5’-deoxyadenosyl radical. The adenosyl radical then abstracts the migrating hydrogen from the substrate to form 5-deoxyadenosine and substrate radical (or protein radical in the case of AdoCbl-dependent ribonucleotide reductase). These steps have been studied in some detail for several B12 enzymes and in each case homolysis and hydrogen abstraction are found to be kinetically coupled (7-10), as evidenced by the appearance of a kinetic isotope effect on cobalt-carbon bond homolysis when the enzymes are reacted with deuterated substrates.
This observation implies that adenosyl radical can only be present in very low concentrations as a high energy intermediate that does not accumulate on the enzyme. Arguments have been advanced for a formally concerted mechanism for homolysis and hydrogen abstraction (11), although this is now considered to be less likely (12). In many cases these deuterium isotope effects are unusually large, in the range of 40−30, and the most likely explanation for this is that hydrogen transfer involves a large degree of quantum tunneling (13, 14).
We have recently described rapid quench experiments (15) to measure the α-secondary tritium isotope effects associated with the formation of 5’-deoxyadensosine under pre-steady state conditions, as illustrated in Figure 1. AdoCbl, tritium-labeled in the 5’-position, was used to measure both the secondary equilibrium and kinetic isotope effects on the formation of 5’-dA. The measurements were made at very short time intervals between 10 and 100 ms so that loss of tritium from the 5’-position was minimal. Both the kinetic and equilibrium isotope effects were found to be large and inverse, kH/kT = 0.76 ± 0.02, KH/KT = 0.72 ± 0.04. These results indicate that the 5’-C-H bonds become significantly stiffer in going from AdoCbl to 5’-dA, even though the 5’-carbon remains formally sp3 hybridized, and are consistent with the hydrogen transfer, as opposed cobalt-carbon bond cleavage, being the slower step. Classically, the large inverse kinetic isotope effect would be interpreted as indicating a late transition state. However, if quantum tunneling is occurring in the hydrogen transfer step, this conclusion cannot be drawn (16).
Figure 1
Top: Isomerization reaction catalyzed by glutamate mutase, with the migrating hydrogen atom circled. Bottom: Secondary isotope effects associated with the homolysis of AdoCbl and the formation 5’-dA when hologlutamate mutase is reacted with deuterated ...
To investigate whether quantum tunneling is likely to be important in the mechanism of glutamate mutase, we have repeated the secondary α-tritium isotope effect measurements for the formation of 5’-dA using deuterated glutamate to trigger homolysis of AdoCbl. This introduces a primary deuterium isotope effect at the 5’-carbon, which should not change the secondary isotope effect if this arises purely semi-classically (16). However, we observe that the secondary kinetic isotope effect is significantly reduced in magnitude, which implies that the motions of the primary and secondary hydrogen atoms are coupled together in the transition state and that the reaction involves a significant degree of quantum tunneling.
TL;DR: The G3 results are an indicator of benzylic BDE in p-YC6H4CH2-F and p-YP4H4 CH2-H, via the principle of vinylogy, demonstrated by correlating MSE of the allylic compounds with physical properties of theirbenzylic analogues.
Abstract: The effect of remote substituents on bond dissociation energies (BDE) is examined by investigating allylic C−F and C−H BDE, as influenced by Y substituents in trans-YCHCHCH2−F and trans-YCHCHCH2−H. Theoretical calculations at the full G3 level model chemistry are reported. The interplay of stabilization energies of the parent molecules (MSE) and of the radicals formed by homolytic bond cleavage (RSE) and their effect on BDE are established. MSE values of allyl fluorides yield an excellent linear free energy relationship with the electron-donating or -withdrawing ability of Y and decrease by 4.2 kcal mol-1 from Y = (CH3)2N to O2N. RSE values do not follow a consistent pattern and are of the order of 1−2 kcal mol-1. A decrease of 4.1 kcal mol-1 is found in BDE[C−F] from Y = CH3O to NC. BDE[YCHCHCH2−H] generally increases with decreasing electron-donating ability of Y for electron-donating groups and does not follow a consistent pattern with electron-withdrawing groups, the largest change being an increase o...