- Direct Comparison of the reactivity of model complexes for compounds 0, I, and II in oxygenation, hydrogen-abstraction, and hydride-transfer processes
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The iron(III) meso-tetramesitylporphyrin complex is a good biomimetic to study the catalytic reactions of cytochrome P450. All of the three most discussed reactive intermediates concerning P450 catalysis (namely, Cpd 0, Cpd I, and Cpd II) can be selective
- Fertinger, Christoph,Hessenaue-Ilicheva, Natalya,Franke, Alicja,Van Eldik, Rudi
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supporting information; scheme or table
p. 13435 - 13440
(2010/06/11)
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- C-H activation by a mononuclear manganese(III) hydroxide complex: Synthesis and characterization of a manganese-lipoxygenase mimic?
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Lipoxygenases are mononuclear non-heme metalloenzymes that regio- and stereospecifically convert 1,4-pentadiene subunit-containing fatty acids into alkyl peroxides. The rate-determining step is generally accepted to be hydrogen atom abstraction from the p
- Goldsmith, Christian R.,Cole, Adam P.,Stack, T. Daniel P.
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p. 9904 - 9912
(2007/10/03)
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- Kinetic study of the hydrogen abstraction reaction of the benzotriazole-N-oxyl radical (BTNO) with H-donor substrates
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The aminoxyl radical (>N-O.) BTNO (benzotriazole-N-oxyl) has been generated by the oxidation of 1-hydroxybenzotriazole (HBT; >N-OH) with a CeIV salt in MeCN. BTNO presents a broad absorption band with λmax 474 nm and e 184
- Brandi, Paolo,Galli, Carlo,Gentili, Patrizia
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p. 9521 - 9528
(2007/10/03)
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- Generation and reactivity toward oxygen of carbon-centered radicals containing indane, indene, and fluorenyl moieties
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Resonance-stabilized radicals containing indane, indene, and fluorenyl moieties exhibit attenuated reactivity toward oxygen. Rate constants of ~105 M-1 s-1 were observed for the most stabilized radicals. The dependence of kox (rate constant for radical trapping by oxygen) on the corresponding bond dissociation energies revealed that stereoelectronic effects are more important than steric effects in determining the low radical reactivity with oxygen. Scavenging by the nitroxide TEMPO was also examined, and revealed that in this case steric effects are more important than in the case of oxygen. The rate constants for the hydrogen abstraction by cumyloxyl and tert-butoxyl radicals generated thermally and photochemically have been determined in benzene, and were in the range of ca. (1-13) × 106 M-1 s-1, showing that benzylic stabilization has a modest effect on substrate reactivity as a hydrogen donor toward alkoxyl radicals.
- Font-Sanchis, Enrique,Aliaga, Carolina,Bejan, Elena V.,Cornejo, Raecca,Scaiano
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p. 3199 - 3204
(2007/10/03)
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- Kinetics of the reaction of the TEMPO radical with alkylarenes
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The kinetics of the reaction of the stable radical 2,2,6,6- tetramethylpiperidine-N-oxyl (TEMPO) with a series of alkylarenes containing primary and secondary benzyl C-H bonds was studied by ESR, and the reaction rate constants were determined. The scheme
- Opeida,Matvienko,Bakurova,Voloshkin
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p. 900 - 904
(2007/10/03)
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- Hydrocarbon oxidation by bis-μ-oxo manganese dimers: Electron transfer, hydride transfer, and hydrogen atom transfer mechanisms
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Described here are oxidations of alkylaromatic compounds by dimanganese μ-xo and μ-hydroxo dimers [(phen)2MnIV (μ-O)2MnIV(phen)2]4+ ([Mn2(O)2]4+), [(phen)2MnIV (μ-O)2MnIII(phen)2]3+ ([Mn2(O)2]3+), and [(phen)2MnIII (μ-O)(μ-OH)MnIII(phen)2]3+ ([Mn2(O)(OH)]3+). Dihydroanthracene, xanthene, and fluorene are oxidized by [Mn2(O)2]3+ to give anthracene, bixanthenyl, and bifluorenyl, respectively. The manganese product is the bis(hydroxide) dimer, [(phen)2MnIII (μ-OH)2Mn(phen)2]3+ ([Mn2(OH)2]3+). Global analysis of the UV/vis spectral kinetic data shows a consecutive reaction with buildup and decay of [Mn2(O)(OH)]3+ as an intermediate. The kinetics and products indicate a mechanism of hydrogen atom transfers from the substrates to oxo groups of [Mn2(O)2]3+ and [Mn2(O)(OH)]3+. [Mn2(O)2]4+ is a much stronger oxidant, converting toluene to tolyl-phenylmethanes and naphthalene to binaphthyl. Kinetic and mechanistic data indicate a mechanism of initial preequilibrium electron transfer for p-methoxytoluene and naphthalenes because, for instance, the reactions are inhibited by addition of [Mn2(O)2]3+. The oxidation of toluene by [Mn2(O)2]4+, however, is not inhibited by [Mn2(O)2]3+. Oxidation of a mixture of C6H5CH3 and C6H5CD3 shows a kinetic isotope effect of 4.3 ± 0.8, consistent with C-H bond cleavage in the rate-determining step. The data indicate a mechanism of initial hydride transfer from toluene to [Mn2(O)2]4+. Thus, oxidations by manganese oxo dimers occur by three different mechanisms: hydrogen atom transfer, electron transfer, and hydride transfer. The thermodynamics of e-, H?, and H- transfers have been determined from redox potential and pKa measurements. For a particular oxidant and a particular substrate, the choice of mechanism is influenced both by the thermochemistry and by the intrinsic barriers. Rate constants for hydrogen atom abstraction by [Mn2(O)2]3+ and [Mn2(O)(OH)]3+ are consistent with their 79 and 75 kcal mol-1 affinities for H?. In the oxidation of p-methoxytoluene by [Mn2(O)2]4+, hydride transfer is thermochemically 24 kcal mol-1 more facile than electron transfer; yet the latter mechanism is preferred. Thus, electron transfer has a substantially smaller intrinsic barrier than does hydride transfer in this system.
- Larsen, Anna S.,Wang, Kun,Lockwood, Mark A.,Rice, Gordon L.,Won, Tae-Jin,Lovell, Scott,Sadilek, Martin,Turecek, Frantisek,Mayer, James M.
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p. 10112 - 10123
(2007/10/03)
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- The reaction of cyclopentadienylidine, fluorenylidene and tetrachlorocyclopentadienylidene with alcohols. A laser flash photolysis study
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Rate constants of reaction of cyclopentadienylidene, fluorenylidene and tetrachlorocyclopentadienylidene with alcohols and other quenchers were determined by laser flash photolysis methods. The rate constants of reaction of cyclopentadienylidene and fluorenylidene with various alcohols were determined and found to increase with increasing alcohol acidity. Alcohols as a group reacted faster with cyclopentadienylidene and fluorenylidene than likely ylide formers such as pyridine, ethyl acetate and tetrahydrofuran. Bronsted plots of the reaction of cyclopentadienylidene and fluorenylidene with alcohols are linear with slopes of 0·061 and 0·082, respectively. In the case of tetrachlorocyclopentadienylidene, an ylide type of reaction mechanism with alcohols is indicated. Tetrachlorocyclopentadienylidene reacts most rapidly with the least acidic alcohol studied and this carbene reacts more rapidly with tetramethylurea, pyridine and tetrahydrofuran than with methanol.
- Olson, David R.,Platz, Matthew S.
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p. 759 - 769
(2007/10/03)
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- Flash photolysis study of a Friedel-Crafts alkylation. Reaction of the photogenerated 9-fluorenyl cation with aromatic compounds
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A combination of flash photolysis and product analysis is employed to investigate the reaction of aromatic compounds (ArH) with the 9-fluorenyl cation (Fl+) photogenerated from 9-fluorenol in 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP).The availability of the photochemical route to Fl+ means that the reaction of benzylic-type cation with ArH can be directly followed by flash photolysis.An additional feature with electron-rich ArH is that the cyclohexadienyl cation is observed to grow as Fl+ decays.Thus both cationic intermediates of a Friedel-Crafts alkylation are observed in the same experiment.The formation of the cyclohexadienyl cation is demonstrated to be reversible, or at least quasi-reversible, with the kinetic analysis furnishing absolute rate constants for the formation of this cation as well as for its loss of H+ and Fl+.Values of kH:kD for benzene:benzene and toluene:toluene are ca. 1.5 and demonstrate that Fl+ addition is at least partly reversible with these compounds as well.The Hammett ρ+ value obtained for a series of the less electron-rich ArH is -8, indicative of a transition state with considerable cyclohexadienyl cation character.Anisole shows a negative deviation from from Hammett correlation line, explained by the addition of Fl+ to ArH becoming encounter-controlled.This behaviour is dramatically illustrated in a comparison of data for Fl+ and Br2.For the less electron-rich ArH, rate constants for the two electrophiles are parallel.However, from m-xylene through pentamethylbenzene, the rate with Fl+ is unchanged, while the rate with Br2 increases over 1000-fold.The concept of encounter control with Fl+ is strongly supported by the absolute rate constants, which for the electron-rich ArH are all in the range 1-2 E9 dm3 mol-1 s-1, a magnitude typical of diffusion-controlled reactions.The electron-rich ArH also show no intermolecular selectivity since their reactions are encounter-controlled, but have a high intramolecular selectivity.It is suggested that a factor influencing the latter is the reversibility of formation of the cyclohexadienyl cation from the encounter complex.
- McClelland, Robert A.,Cozens, Frances L.,Li, Jianhui,Steenken, Steen
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p. 1531 - 1544
(2007/10/03)
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- Rate Constants for Termination and TEMPO Trapping of Some Resonance Stabilized Hydroaromatic Radicals in the Liquid Phase
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The rate constants for the termination reaction (2k1) of some resonance stabilized carbon centered radicals (SR.) derived from hydroaromatics (Sr. + SR. -> P) have been determined at 294 +/- 2 K by laser flash photolysis with UV-vis detection.The radicals were generated by hydrogen atom abstraction by t-BuO-radicals from the corresponding hydrocarbon (SRH + t-BuO. -> SR. + t-BuOH. k4).The extinction coefficients (e) of the SR., essential to calculate 2k1, were obtained using a relative kinetic technique.The change in 2k1 for the radicals derived from 1,4-cyclohexadiene, fluorine, 9,10-dihydroanthracene, diphenylmethane, tetralin, indan, indene, and phenol appeared to be modest; a range of 2k1 = 2-10 x 1E9 M-1 s-1 in mixtures of benzene and di-tert-butyl peroxide was observed.Most of the rate constants are near the diffusion controlled limit.In contrast, quenching the radicals with a persistent radical, 2,2,5,5-tetramethylpiperidin-1-oxyl (TEMPO), resulted in a larger variation of -1 s-1.The strength of the N-O bond formed in the latter process may have an important contribution to the observed rate constant.
- Arends, I.W.C.E.,Mulder, P.,Clark, K.B.,Wayner, D.D.M.
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p. 8182 - 8189
(2007/10/02)
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- Laser flash photolysis of 9-diazofluorene in low-temperature glasses
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The laser flash photolysis of 9-diazofluorene was investigated in several viscous organic glasses at low temperature. The data indicate that triplet fluorenylidene reacts with "soft warm" glasses by classical H atom abstraction, but the mechanism changes to quantum mechanical tunneling in colder and more rigid matrices.
- Ruzicka, Jan,Leyva, Elisa,Platz, Matthew S.
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p. 897 - 905
(2007/10/02)
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- INDIRECT OBSERVATION OF SPIN POLARIZATION IN TRIPLET FLUORENYLIDENE AT ROOM TEMPRTATURE
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The spin polarization of triplet fluorenylidene has been observed indirectly at room temperature via CIDEP spectroscopy.The observed absorptive polarization is in agreement with direct observations of the spin polarization of diphenylmethylene at low temperature.
- Jenks, William S.,Turro, Nicholas J.
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p. 4469 - 4472
(2007/10/02)
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- A Kinetic Study of the Photosolvolysis of 9-Fluorenol
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Excitation of 9-fluorenol in methanol and methanol-water mixtures leads to both heterolytic and homolytic cleavage via the singlet excited state.The partitioning between homolysis and heterolysis is controlled by solvent composition, the rate constant for cation formation being a function of the solvent dielectric constant.The rate constants of intersystem crossing, fluorescence, and singlet-derived radical formation are linear functions of the mole fraction of water.
- Gaillard, Elizabeth,Fox, Marye Anne,Wan, Peter
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p. 2180 - 2186
(2007/10/02)
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- Mobility in Single Crystals Studied by Optical Nuclear Polarization
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By optical excitation of molecular triplet states along with selective population of their spin sublevels nuclear spin polarization can be generated up to a level far above the Boltzmann equilibrium.This effect known as Optical Nuclear Polarization (ONP) has been applied for the investigation of dynamical processes in solid state matrices in the following three ways: (i) The efficiency of the ONP process and its dependency on an external magnetic field is affected by the fluctuation of local fields which is correlated to molecular motions. (ii) By pulsed irradiation of light and radio-frequencies one can achieve a temporal resolution of the polarization process; in this way the time evolution of the spin order of the molecular states involved in the optical excitation cycle can be mapped out. (iii) The high nuclear polarization makes time resolved NMR measurements on solid state systems feasible which cannot be studied by conventional techniques because of unsufficient sensitivity, e.g. guest molecules in low concentrations or isotopes with small natural abundance.The three methods, which cover together a time range between 1E-12 and 1E3 seconds are illustrated by experiments on various organic crystals. - Crystals / Photochemistry / Spectroscopy, Nuclear Magnetic Resonance
- Allgeier, J.,Buntkowsky, G.,Hentrich, S.,Nack, M.,Vieth, H.-M.
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p. 1281 - 1285
(2007/10/02)
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- THE REACTIONS OF FLUORENYLIDENE WITH HETEROATOMIC NUCLEOPHILES
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The reaction of fluorenylidene with heteroatomic nucleophiles gives good yields of addition products.The rates of these reactions were determined using laser spectrophotometric techniques.Kinetic and product isotope effects were measured.The results of this investigation show that the reaction of the carbene with the nucleophile is not a one-step process.The implication of these conclusions to the spin selective reactions of carbenes and their utility as photoaffinity labels is discussed.
- Zupancic, Joseph J.,Grasse, Peter B.,Lapin, Stephen C.,Schuster, Gary B.
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p. 1471 - 1478
(2007/10/02)
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- CHEMICAL AND PHYSICAL PROPERTIES OF FLUORENYLIDENE: EQUILIBRATION OF THE SINGLET AND TRIPLET CARBENES.
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Irradiation of 9-diazofluorene, with a pulsed laser on a picosecond or nanosecond time scale at room temperature, or at 10 K in a glassy matrix, gives detectable fluorenylidene. This species is shown to exist, under most experimental conditions, as a rapi
- Grasse,Brauer,Zupancic,Kaufmann,Schuster
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p. 6833 - 6845
(2007/10/02)
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- A Kinetic Study of the Reactions of Carbonyl Ylides Formed by the Addition of Fluorenylidene to Ketones
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Fluorenylidene addds to aliphatic ketones to give carbonyl ylide intermediates.With acetone, for example, laser flash photolysis experiments showed that the rate constant for this reaction was 1 x 107 M-1 s-1 in acetonitrile solvent.The resulting carbonyl ylide had an absorption spectrum with λmax = 640 nm in the absence of quenchers underwent ring closure to the corresponding oxirane.The lifetimes, τy, for the ring closure reaction are described by the equation -log(τy/s) = (13.26 +/- 0.11) - (10.96 +/- 0.12) / θ where θ = 2.3RT kcal mol-1.The spectra of the carbonyl ylides could be quenched with rate constants of circa 107 M-1 s-1 by electron-deficient olefins or oxygen.
- Wong, P. C.,Griller, D.,Scaiano, J. C.
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p. 6631 - 6635
(2007/10/02)
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