scispace - formally typeset
Search or ask a question

Showing papers on "Homolysis published in 2008"


Journal ArticleDOI
TL;DR: It is concluded that a hypothetical outer-sphere electron transfer (OSET) in ATRP should occur via concerted dissociative electron transfer rather than a two-step process with radical anion intermediates, implying that the ISET mechanism is preferred.
Abstract: High-level ab initio molecular orbital calculations are used to study the thermodynamics and electrochemistry relevant to the mechanism of atom transfer radical polymerization (ATRP). Homolytic bond dissociation energies (BDEs) and standard reduction potentials (SRPs) are reported for a series of alkyl halides (R−X; R = CH2CN, CH(CH3)CN, C(CH3)2CN, CH2COOC2H5, CH(CH3)COOCH3, C(CH3)2COOCH3, C(CH3)2COOC2H5, CH2Ph, CH(CH3)Ph, CH(CH3)Cl, CH(CH3)OCOCH3, CH(Ph)COOCH3, SO2Ph, Ph; X = Cl, Br, I) both in the gas phase and in two common organic solvents, acetonitrile and dimethylformamide. The SRPs of the corresponding alkyl radicals, R•, are also examined. The computational results are in a very good agreement with the experimental data. For all alkyl halides examined, it is found that, in the solution phase, one-electron reduction results in the fragmentation of the R−X bond to the corresponding alkyl radical and halide anion; hence it may be concluded that a hypothetical outer-sphere electron transfer (OSET) in ...

248 citations


Journal ArticleDOI
TL;DR: In this paper, the representative arylamine hole transport materials undergo chemical transformations in operating organic light-emitting diode (OLED) devices, and the subsequent chemical reactions are likely to yield long-lived, stabilized free radicals capable of acting as deep traps.
Abstract: We report that the representative arylamine hole transport materials undergo chemical transformations in operating organic light-emitting diode (OLED) devices. Although the underlying chemical mechanisms are too complex to be completely elucidated, structures of several identified degradation products point at dissociations of relatively weak carbon-nitrogen and carbon-carbon bonds in arylamine molecules as the initiating step. Considering the photochemical reactivities, the bond dissociation reactions of arylamines occur by the homolysis of the lowest singlet excited states formed by recombining charge carriers in the operating OLED device. The subsequent chemical reactions are likely to yield long-lived, stabilized free radicals capable of acting as deep traps—nonradiative recombination centers and fluorescence quenchers. Their presence in the hole transport layer results in irreversible hole trapping and manifests as a positive fixed charge. The extent and localization of chemical transformations in se...

153 citations


Book ChapterDOI
TL;DR: The detailed mechanism of nitration of tyrosine, a vital aromatic amino acid, is delineated, showing the difference in the nitration yield between the addition of authentic peroxynitrite and its continuous generation by NO and O2- radicals.
Abstract: In biological systems, nitric oxide (NO) combines rapidly with superoxide (O2-) to form peroxynitrite ion (ONOO-), a substance that has been implicated as a culprit in many diseases. Peroxynitrite ion is essentially stable, but its protonated form (ONOOH, pKa = 6.5 to 6.8) decomposes rapidly via homolysis of the O-O bond to form about 28% free NO2 and OH radicals. At physiological pH and in the presence of large amounts of bicarbonate, ONOO- reacts with CO2 to produce about 33% NO2 and carbonate ion radicals (CO3-) in the bulk of the solution. The quantitative role of OH/CO3(-) and NO2 radicals during the decomposition of peroxynitrite (ONOOH/ONOO-) under physiological conditions is described in detail. Specifically, the effect of the peroxynitrite dosage rate on the yield and distribution of the final products is demonstrated. By way of an example, the detailed mechanism of nitration of tyrosine, a vital aromatic amino acid, is delineated, showing the difference in the nitration yield between the addition of authentic peroxynitrite and its continuous generation by NO and O2- radicals.

132 citations


Journal ArticleDOI
TL;DR: In this article, the influence of Cγ-OH on pyrolytic cleavage mechanism of β-ether-linkage in lignin dimer was studied in N2 with Cγdeoxy-type dimers, which have various p-substituents (H, Cl, OCH3) at their Cβ-phenoxy groups.

111 citations


Journal ArticleDOI
TL;DR: In this paper, the SET-LRP of methyl acrylate initiated with the alkyl chlorides methyl-2-chloropropionate (MCP) and chloroform (CHCl3) and catalyzed by Cu(0)/Me6-TREN/CuCl2 in DMSO at 25 °C.
Abstract: Single-electron transfer living radical polymerization (SET-LRP) proceeds by an outer-sphere single-electron transfer mechanism that induces a heterolytic bond cleavage of the initiating and propagating R-X (where X = Cl, Br, and I) species. Therefore, unlike the homolytic bond cleavage mechanism claimed for ATRP, SET-LRP is expected to show a small dependence of the nature of the halide group on the apparent rate constant of activation. This means the R-X with X = Cl, Br, and I must all be efficient initiators for SET-LRP and no chain transfer must be observed in the case of initiators with X = Br and I. Here, we report the SET-LRP of methyl acrylate initiated with the alkyl chlorides methyl-2-chloropropionate (MCP) and chloroform (CHCl3) and catalyzed by Cu(0)/Me6-TREN/CuCl2 in DMSO at 25 °C. A combination of kinetic and structural analysis was used to elucidate the MCP and CHCl3 initiating behavior under SET-LRP conditions, and to demonstrate the very small dependence of the SET-LRP apparent rate constant of propagation on X while providing polymers with well defined architecture. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4917–4926, 2008

74 citations


Journal ArticleDOI
TL;DR: It is demonstrated that H2O2 cannot be deprotonated to yield Fe(III)OOH, and hence the surrogate reaction starts from the FeHOOH complex, and the role of the water cluster in the distal pocket in creating "function" for the enzyme is highlighted.
Abstract: Heme degradation by heme oxygenase (HO) enzymes is important in maintaining iron homeostasis and prevention of oxidative stress, etc. In response to mechanistic uncertainties, we performed quantum mechanical/molecular mechanical investigations of the heme hydroxylation by HO, in the native route and with the oxygen surrogate donor H2O2. It is demonstrated that H2O2 cannot be deprotonated to yield Fe(III)OOH, and hence the surrogate reaction starts from the FeHOOH complex. The calculations show that, when starting from either Fe(III)OOH or Fe(III)HOOH, the fully concerted mechanism involving O-O bond breakage and O-C(meso) bond formation is highly disfavored. The low-energy mechanism involves a nonsynchronous, effectively concerted pathway, in which the active species undergoes first O-O bond homolysis followed by a barrier-free (small with Fe(III)HOOH) hydroxyl radical attack on the meso position of the porphyrin. During the reaction of Fe(III)HOOH, formation of the Por+*FeIV=O species, compound I, competes with heme hydroxylation, thereby reducing the efficiency of the surrogate route. All these conclusions are in accord with experimental findings (Chu, G. C.; Katakura, K.; Zhang, X.; Yoshida, T.; Ikeda-Saito, M. J. Biol. Chem. 1999, 274, 21319). The study highlights the role of the water cluster in the distal pocket in creating "function" for the enzyme; this cluster affects the O-O cleavage and the O-Cmeso formation, but more so it is responsible for the orientation of the hydroxyl radical and for the observed alpha-meso regioselectivity of hydroxylation (Ortiz de Montellano, P. R. Acc. Chem. Res. 1998, 31, 543). Differences/similarities with P450 and HRP are discussed.

66 citations


Journal ArticleDOI
TL;DR: In this paper, pyrolytic cleavage mechanisms of lignin-ether linkages were studied with some dimers and trimers which have various p-substituted Cα-phenoxy groups (-H, -OCH3, -Cl or -COCH3).
Abstract: Abstract Pyrolytic cleavage mechanisms of lignin-ether linkages were studied with some dimers and trimers which have various p-substituted Cα-phenoxy groups (-H, -OCH3, -Cl or -COCH3). Pyrolysis of these model compounds provides phenols and isoeugenol type products. To determine whether the reactions mechanisms are heterolytic or homolytic, the reactivities were compared based on Hammett's substituent constant (σ p ) and the ΔBDE parameter, namely the bond dissociation energy (BDE) reduction. The α-ether-linkages in phenolic forms are cleaved in a heterolytic mechanism, while in non-phenolic forms the α-ether linkages are cleaved homolytically. Cleavage of these α-ether linkages is the rate-determining step for the scission of the Cβ-O bond in trimers. The β-ether-linkages in the non-phenolic trimers are cleaved through the β-scission type reaction from the benzyl radical intermediates. On the other hand, quinone methide formation through heterolytic cleavage of the α-ether linkages is the key step for following homolysis of the Cβ-O bonds in the phenolic trimers. Electron attracting character of the quinone methide structure reduces the BDE of the Cβ-O bond.

62 citations


Journal ArticleDOI
TL;DR: Discovery of a relatively efficient radical 5-endo-dig closure, accompanied by a C-C bond formation, provides further support to stereoelectronic considerations at the heart of the Baldwin rules and fills one of the last remaining gaps in the arsenal of radical cyclizations.
Abstract: Despite being predicted to be stereoelectronically favorable by the Baldwin rules, efficient formation of a C−C bond through a 5-endo-dig radical cyclization remained unknown for more than 40 years This work reports a remarkable increase in the efficiency of this process upon β-Ts substitution, which led to the development of an expedient approach to densely functionalized cyclic 1,3-dienes Good qualitative agreement between the increased efficiency and stereoselectivity for the 5-endo-dig cyclization of Ts-substituted vinyl radicals and the results of density functional theory analysis further confirms the utility of computational methods in the design of new radical processes Although reactions of Br atoms generated through photochemical Ts−Br bond homolysis lead to the formation of cyclic dibromide side products, the yields of target bromosulfones in the photochemically induced reactions can be increased by recycling the dibromide byproduct into the target bromosulfones through a sequence of additio

61 citations


Journal ArticleDOI
TL;DR: The one-electron reactivity of transition-metal complexes and its relevance in CRP is interested in.
Abstract: In the past decade, controlled/living radical polymerization (CRP) processes have seen a considerable surge of interest owing, in part, to their relevance to the accessibility of a variety of well-defined polymer structures (e.g. predetermined molecular mass, narrow molecular weight distribution). We have been interested in the one-electron reactivity of transition-metal complexes and its relevance in CRP. One way in which transition-metal complexes can be used to control radical polymerization is through a reversible deactivation. The growing radical chain is trapped by formation of a metal–carbon bond to yield a metal-capped polymer chain, which is a dormant organometallic species (Figure 1). We refer to this particular control mechanism as “organometallic radical polymerization” (OMRP). One of the outstanding challenges in this area is the possibility to control the polymerization of less reactive monomers (e.g. vinyl chloride, vinylidene dichloride, vinyl acetate), for which activation is made difficult by the relatively strong bonds established with common radical traps. Reasonable control for the radical propagation of poly(vinyl acetate) (PVAc) has been achieved on the basis of another control mechanism (degenerative transfer, DT, based on the use of xanthates or dithiocarbamates). 4] Results obtained by atom transfer radical polymerization (ATRP) have not been nearly as good, while good control was recently achieved in the presence of [Co(acac)2] (Mw/Mn as low as 1.1; acac = acetylacetonate). 9] Recent studies have shown that this process occurs either by DT or by OMRP, depending on the presence of additional ligands such as pyridine or water. 11]

60 citations


Journal ArticleDOI
TL;DR: Recently, this work provided unambiguous proof that R2Zn compounds, or their adducts with Lewis bases, have a marked tendency to undergo oxidation of only one alkyl group under controlled conditions with subsequent formation of RZnOOR or RznOR species, and simultaneously structurally characterized the first examples of zincAlkylperoxides.
Abstract: Interest in the reaction of alkylzinc complexes with O2 has persisted for over 150 years since the pioneering studies by Frankland. The nature of the products, however, has been the subject of intense controversy. The widely accepted freeradical chain-reaction mechanism for these reactions, as found in the vast majority of text books, assumes the initiation by adventitious alkyl radicals (RC) followed by a cascade of fast reactions with little opportunity for the detection of intermediates. Recently, however, we provided unambiguous proof that R2Zn compounds, or their adducts with Lewis bases, have a marked tendency to undergo oxidation of only one alkyl group under controlled conditions with subsequent formation of RZnOOR or RZnOR species, and simultaneously structurally characterized the first examples of zinc alkylperoxides. We also proposed a plausible hypothesis for the mechanism of the reaction of alkylzinc complexes with O2. [5,6] In recent years there has been an increased interest in various radical additions initiated by the R2Zn/O2 system, especially regarding organic substrates which contain donor sites capable of forming the Lewis acid/base adducts with R2Zn that are actually involved in the reaction with O2. [7] This latter fact is usually ignored, and another assumption made in this field, which seems irrefutable, involves an alkyl radical RC (generated through the oxygenation reaction) acting as the chain carrier. Moreover, the most effective initiation systems involve Me2Zn, [7f,g] which, according to recent findings, can be selectively transformed into MeZnOMe without the generation of free MeC radicals. In light of this fact, as well as the lack of structurally characterized ZnOOMe species, it seemed reasonable to wonder how the oxygenated products participate in radical reactions. To gain a more in-depth view of both the role of the supporting ligands and the character of the radical species formed in the reactions of Me2Zn with O2, we have turned our attention to a-diimines, which have been widely used in fundamental coordination chemistry as noninnocent ligands. In the field of zinc chemistry, van Koten et al. have extensively studied the reactions of R2Zn compounds with 1,4-diazabutadiene (R-DAB) ligands and have demonstrated convincingly that this reaction system smoothly forms both paramagnetic and diamagnetic species. Herein we report the synthesis and structural characterization of a novel zinc oxo(methylperoxide) cubane along with the MeOC radical entrapped product, the formation of which involves ZnO OMe bond homolysis. Previous studies have demonstrated that the treatment of Me2Zn with tBu-DAB in diethyl ether at ambient temperature results in the formation of [Me2Zn(tBu-DAB)] (1). [11] According to the authors, the four-coordinate adduct 1 is rather stable under these reaction conditions and only upon heating above 35 8C does it undergo an inner-sphere single electron transfer to give the radical pair [MeZn(tBu-DABC)(MeC)], which subsequently dimerizes to the C C coupled dinuclear compound [{MeZn(tBu-DABC)}2] (2 ; Scheme 1). In light of these findings, we decided to modify the reaction system slightly and, in the first instance, stirred equimolar amounts of tBu-DAB andMe2Zn in toluene at 22 8C for 7 h. [12]

59 citations


Journal ArticleDOI
TL;DR: The [M + Ag](+) ions of polystyrene (PS) oligomers are formed by matrix-assisted laser desorption/ionization, and their fragmentation characteristics are determined by tandem mass spectrometry experiments in a quadrupole/time-of-flight mass spectromaeter.
Abstract: The [M + Ag]+ ions of polystyrene (PS) oligomers are formed by matrix-assisted laser desorption/ionization, and their fragmentation characteristics are determined by tandem mass spectrometry experiments in a quadrupole/time-of-flight mass spectrometer. Collisionally activated dissociation (CAD) of [M + Ag]+ starts with random homolytic CC bond cleavages in the PS chain, which generate radical ions carrying either the initiating (an•, bn•) or the terminating (yn•, zn•) chain end and primary (an•, yn•) or benzylic (bn•, zn•) radical centers. The fragments ultimately observed arise by consecutive, radical-induced dissociations. The primary radical ions mainly decompose by monomer evaporation and, to a lesser extent, by β-H• loss. The benzylic radical ions primarily decompose by 1,5-H rearrangement (backbiting) followed by β C−C bond scissions; this pathway leads to either closed-shell fragments with CH2 end groups, internal fragments with 2−3 repeat units, or truncated benzylic bn•/zn• radical ions that can ...

Journal ArticleDOI
TL;DR: This study enables us to generalize the mechanisms of O-O activation, elucidated so far by QM/MM calculations, for other heme enzymes, e.g., cytochrome P450cam, horseradish peroxidase (HRP), nitric oxide synthase (NOS), and heme oxygenase (HO).
Abstract: The formation of Compound I (Cpd I), the active species of the enzyme chloroperoxidase (CPO), was studied using QM/MM calculation. Starting from the substrate complex with hydrogen peroxide, FeIII−HOOH, we examined two alternative mechanisms on the three lowest spin-state surfaces. The calculations showed that the preferred pathway involves heterolytic O−O cleavage that proceeds via the iron hydroperoxide species, i.e., Compound 0 (Cpd 0), on the doublet-state surface. This process is effectively concerted, with a barrier of 12.4 kcal/mol, and is catalyzed by protonation of the distal OH group of Cpd 0. By comparison, the path that involves a direct O−O cleavage from FeIII−HOOH is less favored. A proton coupled electron transfer (PCET) feature was found to play an important role in the mechanism nascent from Cpd 0. Initially, the O−O cleavage progresses in a homolytic sense, but as soon as the proton is transferred to the distal OH, it triggers an electron transfer from the heme-oxo moiety to form water a...

Journal ArticleDOI
TL;DR: The amide-iminyl radical moiety unprecedentedly displaces methyl, methoxy, and fluorine radicals from an aromatic carbon atom in a tin-free radical cascade cyclization process, a seminal reaction in the field of radical chemistry.
Abstract: Amide-iminyl radicals are versatile and efficient intermediates in cascade radical cyclizations of N-acylcyanamides. They are easily trapped by alkenes or (hetero-)aromatic rings and cyclize into a series of new heterocyclic compounds which bear a pyrroloquinazoline moiety. As an illustration of the synthetic importance of these compounds, the total synthesis of the natural antitumor compound luotonin A was achieved through a tin-free radical cascade cyclization process. Not only do amide-iminyl radicals lead to new tetracyclic heterocycles but these nitrogen-centered radical species also react in aromatic homolytic substitutions. Indeed, the amide-iminyl radical moiety unprecedentedly displaces methyl, methoxy, and fluorine radicals from an aromatic carbon atom. This seminal reaction in the field of radical chemistry has been developed experimentally and its mechanism has additionally been investigated by a theoretical study.

Journal ArticleDOI
TL;DR: The Ag(+) adducts of polystyrene (PS) oligomers with different sizes and initiating (alpha) or terminating (omega) end groups mainly decompose via free radical chemistry pathways upon collisionally activated dissociation.
Abstract: The Ag+ adducts of polystyrene (PS) oligomers with different sizes (6−19 repeat units) and initiating (α) or terminating (ω) end groups mainly decompose via free radical chemistry pathways upon collisionally activated dissociation. This reactivity is observed for ions formed by matrix-assisted laser desorption/ionization as well as electrospray ionization. With end groups lacking weak bonds (robust end groups), dissociation starts with random homolytic C−C bond cleavages along the PS chain, which lead to primary and benzylic radical ions containing either of the chain ends. The primary radical ions mainly depolymerize by successive β C−C bond scissions. For the benzylic radical ions, two major pathways are in competition, namely, depolymerization by successive β C−C bond scissions and backbiting via 1,5-H rearrangement followed by β C−C bond scissions. The extent of backbiting decreases with internal energy. With short PS chains, the primary radical ions also undergo backbiting involving 1,4- and 1,6-H re...

Journal ArticleDOI
TL;DR: In this article, it was shown that the ATDDFT has much too low excitation energies at long bond length in (dissociating) electron pair bonds, and this easily escapes attention due to the occurrence of the problems at slightly longer distances than the equilibrium geometry.

Journal ArticleDOI
TL;DR: The unimolecular degradation of alkali-metal cationized polyacrylates with the repeat unit CH2CH(COOR) and a variety of ester pendants has been examined by tandem mass spectrometry to identify the fragmentation patterns resulting from collisionally activated dissociation.
Abstract: The unimolecular degradation of alkali-metal cationized polyacrylates with the repeat unit CH2CH(COOR) and a variety of ester pendants has been examined by tandem mass spectrometry. The fragmentation patterns resulting from collisionally activated dissociation depend sensitively on the size of the ester alkyl substituent (R). With small alkyl groups, as in poly(methyl acrylate), lithiated or sodiated oligomers (M) decompose via free-radical chemistry, initiated by random homolytic C-C bond cleavages along the polymer chain. The radical ions formed this way dissociate further by backbiting rearrangements and β scissions to yield a distribution of terminal fragments with one of the original end groups and internal fragments with 2–3 repeat units. If the ester alkyl group bears three or more carbon atoms, cleavages within the ester moieties become the predominant decomposition channel. This distinct reactivity is observed if R = t-butyl, n-butyl, or the mesogenic group (CH2)11-O-C6H4-C6H4-CN. The [M+alkali metal]+ ions of the latter polyacrylates dissociate largely by charge-remote 1,5-H rearrangements that convert COOR to COOH groups by expulsion of 1-alkenes. The acid groups may displace an alcohol unit from a neighboring ester pendant to form a cyclic anhydride, unless hindered by steric effects. Using atom transfer radical polymerization, hyperbranched polyacrylates were prepared carrying ester groups both within and between the branches. Unique alkenes and alcohols are cleaved from ester groups at the branching points, enabling determination of the branching architecture.

Journal ArticleDOI
TL;DR: In this article, the synthesis of bicyclic N-heterocycles using aromatic homolytic substitution was studied, in which the initiator or breakdown products from the inhibitor are responsible for the H-abstraction step.

Journal ArticleDOI
TL;DR: Under physiological conditions the adduct is mainly deprotonated, and therefore nitroxides can detoxify thiyl radicals, and the proposed mechanism can account for the protective effect of nitroxide against reactive oxygen- and nitrogen-derived species in the presence of thiols.
Abstract: Cyclic nitroxides effectively protect cells, tissues, isolated organs, and laboratory animals from radical-induced damage. The present study focuses on the kinetics and mechanisms of the reactions of piperidine and pyrrolidine nitroxides with thiyl radicals, which are involved in free radical "repair" equilibria, but being strong oxidants can also produce cell damage. Thiyl radicals derived from glutathione, cysteine, and penicillamine were generated in water by pulse radiolysis, and the rate constants of their reactions with 2,2,6,6-tetramethylpiperidine-1-oxyl (TPO), 4-OH-TPO, and 3-carbamoyl-proxyl were determined to be (5-7) x 10 (8) M (-1) s (-1) at pH 5-7, independent of the structure of the nitroxide and the thiyl radical. It is suggested that the reaction of nitroxide (>NO (*)) with thiyl radical (RS (*)) yields an unstable adduct (>NOSR). The deprotonated form of this adduct decomposes via heterolysis of the N-O bond, yielding the respective amine (>NH) and sulfinic acid (RS(O)OH). The protonated form of the adduct decomposes via homolysis of the N-O bond, forming the aminium radical (>NH (*+)) and sulfinyl radical (RSO (*)), which by subsequent reactions involving thiol and nitroxide produce the respective amine and sulfonic acid (RS(O) 2OH). Nitroxides that are oxidized to the respective oxoammonium cations (>N (+)O) are recovered in the presence of NADH but not in the presence of thiols. This suggests that the reaction of >N (+)O with thiols yields the respective amine. Two alternative mechanisms are suggested, where >N (+)O reacts with thiolate (RS (-)) directly generating the adduct >NOSR or indirectly forming >NO (*) and RS (*), which subsequently together yield the adduct >NOSR. Under physiological conditions the adduct is mainly deprotonated, and therefore nitroxides can detoxify thiyl radicals. The proposed mechanism can account for the protective effect of nitroxides against reactive oxygen- and nitrogen-derived species in the presence of thiols.

Journal ArticleDOI
TL;DR: The detonation data of 1,3, 3,5,7,7-hexanitro-1,5-diazacyclooctane (HNDZ) and TNBDFAPP show that they meet the requirements for HEDMs.

Journal ArticleDOI
TL;DR: Electrospray ionization tandem mass spectrometry has been used to characterize the microstructure of a nitroxide-mediated poly(ethylene oxide)/polystyrene block copolymer, called SG1-capped PEO-b-PS, suggesting that rearrangement reactions do not compete effectively with dissociations of the odd-electron fragment ions.

Journal ArticleDOI
TL;DR: Mechanistic pathways for the aromatic hydroxylation by [CuII(L1)(TMAO)(O)](-) (L1=hippuric acid, TMAO=trimethylamine N-oxide), derived from the O--N bond homolysis of its precursor, were explored by using hybrid density functional theory and highly correlated ab initio methods.
Abstract: Mechanistic pathways for the aromatic hydroxylation by [CuII(L1)(TMAO)(O)](-) (L1=hippuric acid, TMAO=trimethylamine N-oxide), derived from the O--N bond homolysis of its [CuII(L1)(TMAO)2] precursor, were explored by using hybrid density functional theory (B3LYP) and highly correlated ab initio methods (QCISD and CCSD). Published experimental studies suggest that the catalytic reaction is triggered by a terminal copper-oxo species, and a detailed study of electronic structures, bonding, and energetics of the corresponding electromers is presented. Two pathways, a stepwise and a concerted reaction, were considered for the hydroxylation process. The results reveal a clear preference for the concerted pathway, in which the terminal oxygen atom directly attacks the carbon atom of the benzene ring, leading to the ortho-selectively hydroxylated product. Solvent effects were probed by using the PCM and CPCM solvation models, and the PCM model was found to perform better in the present case. Excellent agreement between the experimental and computational results was found, in particular also for changes in reactivity with derivatives of L1.

Journal ArticleDOI
TL;DR: Results imply different mechanisms for the Criegee rearrangement steps of intradiol and extradiol catechol dioxygenases: a direct 1,2-alkenyl migration for extradiol cleavage and an O-O homolytic cleavage mechanism for intradiola cleavage.
Abstract: Three mechanistic probes were used to investigate whether the Criegee rearrangement step of catechol 1,2-dioxygenase (CatA) from Acinetobacter sp. proceeds via a direct 1,2-acyl migration, via homolytic O-O cleavage, or via a benzene oxide-oxepin rearrangement. Incubation of CatA with 3-chloroperoxybenzoic acid led to the formation of a 9:1 mixture of 2-chlorophenol and 3-chlorophenol, via a mechanism involving O-O homolytic cleavage. Incubation of CatA with 2-hydroperoxy-2-methylcyclohexanone led to formation of 5,6-diketoheptan-1-ol, also consistent with an O-O homolytic cleavage mechanism, and not consistent with a direct 1,2-acyl migration. No reaction product was isolated from incubation of CatA with 6-hydroxymethyl-6-methylcyclohexa-2,4-dienone, an analogue that is able to undergo the benzene oxide-oxepin rearrangement, but not able to undergo O-O homolytic cleavage. In contrast, incubation of extradiol dioxygenase MhpB from Escherichia coli with 6-hydroxymethyl-6-methylcyclohexa-2,4-dienone led to the formation of a 2-tropolone ring expansion product, consistent with a direct 1,2-alkenyl migration for extradiol cleavage. Taken together, the results imply different mechanisms for the Criegee rearrangement steps of intradiol and extradiol catechol dioxygenases: a direct 1,2-alkenyl migration for extradiol cleavage and an O-O homolytic cleavage mechanism for intradiol cleavage.

Journal ArticleDOI
TL;DR: In this paper, the decomposition of 4-nitrophenyl 2-(2,2,6,6-tetramethylpiperidine-1-yloxy)-2-methylpropionate (la) and 4-nippyl 2.2.6.6 (2.3.6)-propanoate (2a) has been studied by 1 H NMR in the presence and in the absence of scavenger (thiophenol PhSH).
Abstract: In a recent article, we have showed that the nitroxide mediated polymerization of methyl methacrylate was possible up to 80% conversion for reasonable masses M n = 60,000 g mol -1 when 2,2-diphenyl-3-phenylimino-2,3-dihydroindol-1-yloxyl nitroxide (DPAIO) was used as control agent. We have claimed that the success of this experiment relied on the absence of H-transfer reaction both in the alkoxyamine and between alkyl and nitroxyl radical. In this article, the decomposition of 4-nitrophenyl 2-(2,2,6,6-tetramethylpiperidine-1-yloxy)-2-methylpropionate (la) and 4-nitrophenyl 2-(2,2-diphenyl-3-phenylimino-2,3-dihydroindol-1-yloxy)-2-methylpropanoate (2a) has been studied by 1 H NMR in the presence and in the absence (persistent radical effect condition) of scavenger (thiophenol PhSH). At temperature lower than the one used for polymerization, fast and quantitative H-transfer reaction was observed for la whereas no H-transfer reaction was observed for 2a. The scavenging technique proved for the first time that the H-transfer was an intermolecular process for la. However, the slow side-reaction of N-OC bond homolysis, which did not impede the control of the polymerization but may exert a detrimental effect on the livingness, was observed and quantified for 2a.

Journal ArticleDOI
TL;DR: In this paper, the photophysical properties of a series of 1,8-naphthalimide photoacid generators were studied by steady state fluorescence and phosphorescence spectroscopy.
Abstract: The photophysical properties of a series of 1,8-naphthalimide photoacid generators were studied by steady state fluorescence and phosphorescence spectroscopy. Emission and excitation anisotropies, triplet quantum yields in polar and nonpolar solvent and photoacid generation were evaluated. The singlet excited state exhibits a low polarity and is strongly deactivated by an efficient intersystem crossing process. In protic solvent, a homolytic singlet cleavage of the N-O bond occurs and leads to the acid production. The existence of a triplet state close to the singlet state was clearly evidenced. The presence of close singlet excited states is supported by fluorescence anisotropy and picosecond laser spectroscopy experiments. Results of DFT calculations well confirm the experimental contentions and yield important information about the cleavage process involved in such compounds.

Journal ArticleDOI
TL;DR: Based on the observed changes in chemical compositions of fluorescent and phosphorescent carbazole-based OLEDs during operation, a free-radical mechanism of operational degradation is proposed in this paper, where the relationship between the excited-state energy and the weakest bond dissociation energy of OLED materials is of the fundamental importance for the operational stability of OLED devices.
Abstract: — Based on the observed changes in chemical compositions of fluorescent and phosphorescent carbazole-based OLEDs during operation, a free-radical mechanism of operational degradation is proposed. Chemical analysis and identification of low molecular weight and oligomeric products, device physics, photochemistry, and electron paramagnetic resonance (EPR) studies point to the excited-state homolytic-bond dissociation followed by radical additions as key mechanism steps. Comparable bond dissociation energy and singlet excited-state energy result in a relatively fast degradation process of carbazole-based OLEDs. OLED operation leads to the accumulation of solid-matrix-trapped long-lived π-radical species in their charged or neutral forms, acting as non-radiative recombination centers and luminescence quenchers. The proposed free-radicals-mediated degradation mechanism could be a common degradation mechanism affecting a wide range of OLED compositions and structures. In the framework of this mechanism, the relationship between the excited-state energy and the weakest bond dissociation energy of OLED materials is of the fundamental importance for the operational stability of OLED devices.

Journal ArticleDOI
TL;DR: In this article, the authors used the density functional theory method to accurately calculate the homolytic Co−C bond dissociation enthalpy (BDE), which is central to the understanding of organocobalt-mediated reactions in the areas of bioinorganic chemistry and transition-metal catalysis.


Journal ArticleDOI
TL;DR: It is shown that protein cysteine thiyl radicals (CysS*) can reversibly abstract hydrogen atoms from the alpha C-H bonds of selected amino acids in a protein (insulin).
Abstract: The selective oxidative modification of proteins can have significant consequences for structure and function. Here, we show that protein cysteine thiyl radicals (CysS•) can reversibly abstract hydrogen atoms from the αC−H bonds of selected amino acids in a protein (insulin). CysS• were generated photolytically through homolysis of cystine and through photoionization of an aromatic residue, followed by one-electron reduction of cystine. Intramolecular hydrogen transfer was monitored through the covalent incorporation of deuterium into specific amino residues. Of 51 insulin amino residues, only six incorporated significant levels of deuterium: Leu(B6), Gly(B8), Ser(B9), Val(B18), Gly(B20), and Cys(A20). All these amino acids are located at the beginning/end or outside of α-helices and β-sheets, in accordance with theory, which predicts that specifically the αC−H bonds of amino acids in these secondary structures have higher homolytic C−H bond dissociation energies compared to the αC−H bonds of amino acids ...

Journal ArticleDOI
TL;DR: Zhao et al. as mentioned in this paper reviewed two problems for which quantum calculations could readily provide a great deal of helpful information, but for which that potential has not been fully realized, and suggested that statistical rate theory will be insufficient to understand the yields of RONO2.

Journal ArticleDOI
TL;DR: In this article, the effect of the penultimate unit on the C-ON bond homolysis rate was studied for model alkoxyamines, and it was shown that the k d for fragments containing penultimate and antepenultimate units were nicely predicted by the tri-parametric relationship developed in Macromolecules 2005, 38, 2638 and Eur. Chem. Org. 2006, 7, 1755 (log k d = -14.33 +15.06 x σ RS + 20.00x σ I + 6.96 x υ
Abstract: The effect of the penultimate unit on the C-ON bond homolysis rate constant k d was studied for model alkoxyamines. It was shown that the k d for fragments containing penultimate and antepenultimate units were nicely predicted by the tri-parametric relationship developed in Macromolecules 2005, 38, 2638 and Eur. J. Org. Chem. 2006, 7, 1755 (log k d = -14.33 +15.06 x σ RS + 20.00 x σ I + 6.96 x υ). A striking effect was observed only for a tert-butyl-like group as the penultimate unit.