- A Structurally Characterized Nonheme Cobalt-Hydroperoxo Complex Derived from Its Superoxo Intermediate via Hydrogen Atom Abstraction
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Bubbling O2 into a THF solution of CoII(BDPP) (1) at -90 °C generates an O2 adduct, Co(BDPP)(O2) (3). The resonance Raman and EPR investigations reveal that 3 contains a low spin cobalt(III) ion bound to a superoxo ligand. Significantly, at -90 °C, 3 can react with 2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPOH) to form a structurally characterized cobalt(III)-hydroperoxo complex, CoIII(BDPP)(OOH) (4) and TEMPO?. Our findings show that cobalt(III)-superoxo species are capable of performing hydrogen atom abstraction processes. Such a stepwise O2-activating process helps to rationalize cobalt-catalyzed aerobic oxidations and sheds light on the possible mechanism of action for Co-bleomycin.
- Wang, Chun-Chieh,Chang, Hao-Ching,Lai, Yei-Chen,Fang, Huayi,Li, Chieh-Chin,Hsu, Hung-Kai,Li, Zong-Yan,Lin, Tien-Sung,Kuo, Ting-Shen,Neese, Frank,Ye, Shengfa,Chiang, Yun-Wei,Tsai, Ming-Li,Liaw, Wen-Feng,Lee, Way-Zen
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Read Online
- Mechanism of electrochemical oxidation of 1-chloro-2,2,6,6- tetramethylpiperidine
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In contrast to 2,2,6,6-tetramethylpiperidine and other aliphatic amines, at the electrochemical oxidation of 1-chloro-2,2,6,6-tetramethylpiperidine a sufficiently stable cation-radical is formed. Its formation is confirmed by the data of cyclic voltammetry and electron paramagnetic resonance. Further transformation of the cation-radical leads to the formation of 2,2,6,6-tetramethylpiperidin-1-oxyl.
- Kagan,Yanilkin,Morozov,Nastapova,Zhukova,Kashparov,Kashparova
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Read Online
- An Iron(III) Superoxide Corrole from Iron(II) and Dioxygen
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A new structurally characterized ferrous corrole [FeII(ttppc)]? (1) binds one equivalent of dioxygen to form [FeIII(O2?.)(ttppc)]? (2). This complex exhibits a 16/18O2-isotope sensitive ν(O-O) stretch at 1128 cm?1 concomitantly with a single ν(Fe-O2) at 555 cm?1, indicating it is an η1-superoxo (“end-on”) iron(III) complex. Complex 2 is the first well characterized Fe-O2 corrole, and mediates the following biologically relevant oxidation reactions: dioxygenation of an indole derivative, and H-atom abstraction from an activated O?H bond.
- Albert, Therese,Goldberg, David P.,Mo?nne-Loccoz, Pierre,Sacramento, Jireh Joy D.,Siegler, Maxime
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- Method for preparing hindered amine nitroxide free radical compound by alkaline heterogeneous catalysis system
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The method comprises the following steps: dissolving a hindered amine compound in an organic solvent; adjusting pH by a carbonate aqueous solution; reacting with an aqueous hydrogen peroxide solution; and generating a hindered amine nitroxide free radical compound (IV). (V) Or (VI). The method is high in universality, and the hindered amine nitroxide free radical compound with various structures is prepared. The method is high in catalytic activity, short in reaction time, high in yield, simple in preparation process and convenient to operate; a high-purity target product can be obtained through simple phase separation, drying and concentration in the post-treatment process; meanwhile, the aqueous solution system and ethyl acetate can be recycled. Small by-products.
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Paragraph 0050-0053; 0058-0059
(2021/09/26)
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- Singlet Oxygen Generation from a Water-Soluble Hypervalent Iodine(V) Reagent AIBX and H2O2: An Access to Artemisinin
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Herein, we report an efficient method for the chemical generation of 1O2 by treatment of H2O2 with AIBX, a highly water-soluble, bench-stable, recyclable hypervalent iodine(V) reagent developed by our group. The generation of 1O2 was confirmed by the following results: (1) capture of 1O2 with the sodium salt of anthracene-9,10-bis(ethanesulfonate) produced the corresponding endoperoxide and (2) TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) produced by the oxidation of 2,2,6,6-tetramethylpiperidine with 1O2 generated using the AIBX/H2O2 system was detected by electron spin resonance spectroscopy. To illustrate the potential utility of this method for organic synthesis, we used the AIBX/H2O2 system to perform typical reactions of 1O2: [2 + 2]/[4 + 2] cycloadditions, Schenck ene reactions, and heteroatom oxidation reactions, which afforded the corresponding products in high yields. Moreover, we used the method to synthesize the antimalarial drug artemisinin. Finally, we demonstrated that AIBX could be regenerated after the reaction by means of a workup involving extraction and removal of water to obtain a precursor of AIBX, which could then be re-oxidized.
- Hu, Ze-Nan,Shen, Hui-Jie,Zhang, Chi
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- Reaction of hydroxyl radical with arenes in solution—On the importance of benzylic hydrogen abstraction
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The regioselectivity of hydroxyl radical reactions with alkylarenes was investigated using a nuclear magnetic resonance (NMR)-based methodology capable of trapping and quantifying addition and hydrogen abstraction products of the initial elementary step of the oxidation process. Abstraction products are relatively minor components of the product mixtures (15–30 mol%), depending on the magnitude of the overall rate coefficient and the number of available hydrogens. The relative reactivity of addition at a given position on the ring depends on its relation to the methyl substituents on the hydrocarbons under study. The reactivity enhancements for disubstituted and trisubstituted rings are approximately additive under the conditions of this study.
- Waggoner, Abygail R.,Abdulrahman, Yahya,Iverson, Alexis J.,Gibson, Ethan P.,Buckles, Mark A.,Poole, James S.
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- The effect of viscosity on the coupling and hydrogen-abstraction reaction between transient and persistent radicals
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The effect of viscosity on the radical termination reaction between a transient radical and a persistent radical undergoing a coupling reaction (Coup) or hydrogen abstraction (Abst) was examined. In a non-viscous solvent, such as benzene (bulk viscosity bulk 99% Coup/Abst selectivity, but Coup/Abst decreased as the viscosity increased (89/11 in PEG400 at 25 °C [bulk = 84 mPa s]). While bulk viscosity is a good parameter to predict the Coup/Abst selectivity in each solvent, microviscosity is the more general parameter. Poly(methyl methacrylate) (PMMA)-end radicals had a more significant viscosity effect than polystyrene (PSt)-end radicals, and the Coup/Abst ratio of the former dropped to 50/50 in highly viscous media (bulk = 3980 mPa s), while the latter maintained high Coup/ Abst selectivity (84/16). These results, together with the low thermal stability of dormant PMMA-TEMPO species compared with that of PSt-TEMPO species, are attributed to the limitation of the nitroxide-mediated radical polymerization of MMA. While both organotellurium and bromine compounds were used as precursors of radicals, the former was superior to the latter for the clean generation of radical species.
- Li, Xiaopei,Kato, Tatsuhisa,Nakamura, Yasuyuki,Yamago, Shigeru
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supporting information
p. 966 - 972
(2021/04/29)
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- Transformation of Formazanate at Nickel(II) Centers to Give a Singly Reduced Nickel Complex with Azoiminate Radical Ligands and Its Reactivity toward Dioxygen
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The heteroleptic (formazanato)nickel bromide complex LNi(μ-Br)2NiL [LH = Mes-NH-N═C(p-tol)-N═N-Mes] has been prepared by deprotonation of LH with NaH followed by reaction with NiBr2(dme). Treatment of this complex with KC8led to transformation of the formazanate into azoiminate ligands via N-N bond cleavage and the simultaneous release of aniline. At the same time, the potentially resulting intermediate complex L′2Ni [L′ = HN═C(p-tol)-N═N-Mes] was reduced by one additional electron, which is delocalized across the π system and the metal center. The resulting reduced complex [L′2Ni]K(18-c-6) has aS=1/2ground state and a square-planar structure. It reacts with dioxygen via one-electron oxidation to give the complex L′2Ni, and the formation of superoxide was detected spectroscopically. If oxidizable substrates are present during this process, these are oxygenated/oxidized. Triphenylphosphine is converted to phosphine oxide, and hydrogen atoms are abstracted from TEMPO-H and phenols. In the case of cyclohexene, autoxidations are triggered, leading to the typical radical-chain-derived products of cyclohexene.
- Ar, Deniz,Kilpatrick, Alexander F. R.,Cula, Beatrice,Herwig, Christian,Limberg, Christian
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supporting information
p. 13844 - 13853
(2021/05/04)
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- Controlling the Reactivity of a Metal-Hydroxo Adduct with a Hydrogen Bond
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The enzymes manganese lipoxygenase (MnLOX) and manganese superoxide dismutase (MnSOD) utilize mononuclear Mn centers to effect their catalytic reactions. In the oxidized MnIIIstate, the active site of each enzyme contains a hydroxo ligand, and X-ray crystal structures imply a hydrogen bond between this hydroxo ligand and aciscarboxylate ligand. While hydrogen bonding is a common feature of enzyme active sites, the importance of this particular hydroxo-carboxylate interaction is relatively unexplored. In this present study, we examined a pair of MnIII-hydroxo complexes that differ by a single functional group. One of these complexes, [MnIII(OH)(PaPy2N)]+, contains a naphthyridinyl moiety capable of forming an intramolecular hydrogen bond with the hydroxo ligand. The second complex, [MnIII(OH)(PaPy2Q)]+, contains a quinolinyl moiety that does not permit any intramolecular hydrogen bonding. Spectroscopic characterization of these complexes supports a common structure, but with perturbations to [MnIII(OH)(PaPy2N)]+, consistent with a hydrogen bond. Kinetic studies using a variety of substrates with activated O-H bonds, revealed that [MnIII(OH)(PaPy2N)]+is far more reactive than [MnIII(OH)(PaPy2Q)]+, with rate enhancements of 15-100-fold. A detailed analysis of the thermodynamic contributions to these reactions using DFT computations reveals that the former complex is significantly more basic. This increased basicity counteracts the more negative reduction potential of this complex, leading to a stronger O-H BDFE in the [MnII(OH2)(PaPy2N)]+product. Thus, the differences in reactivity between [MnIII(OH)(PaPy2Q)]+and [MnIII(OH)(PaPy2N)]+can be understood on the basis of thermodynamic considerations, which are strongly influenced by the ability of the latter complex to form an intramolecular hydrogen bond.
- Day, Victor W.,Hessefort, Logan,Jackson, Timothy A.,Opalade, Adedamola A.
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supporting information
p. 15159 - 15175
(2021/09/29)
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- Combining Structural with Functional Model Properties in Iron Synthetic Analogue Complexes for the Active Site in Rabbit Lipoxygenase
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Iron complexes that model the structural and functional properties of the active iron site in rabbit lipoxygenase are described. The ligand sphere of the mononuclear pseudo-octahedral cis-(carboxylato)(hydroxo)iron(III) complex, which is completed by a tetraazamacrocyclic ligand, reproduces the first coordination shell of the active site in the enzyme. In addition, two corresponding iron(II) complexes are presented that differ in the coordination of a water molecule. In their structural and electronic properties, both the (hydroxo)iron(III) and the (aqua)iron(II) complex reflect well the only two essential states found in the enzymatic mechanism of peroxidation of polyunsaturated fatty acids. Furthermore, the ferric complex is shown to undergo hydrogen atom abstraction reactions with O-H and C-H bonds of suitable substrates, and the bond dissociation free energy of the coordinated water ligand of the ferrous complex is determined to be 72.4 kcal·mol-1. Theoretical investigations of the reactivity support a concerted proton-coupled electron transfer mechanism in close analogy to the initial step in the enzymatic mechanism. The propensity of the (hydroxo)iron(III) complex to undergo H atom abstraction reactions is the basis for its catalytic function in the aerobic peroxidation of 2,4,6-tri(tert-butyl)phenol and its role as a radical initiator in the reaction of dihydroanthracene with oxygen.
- Bonck, Thorsten,De Waal Malefijt, Matina Elo?se,Dobbelaar, Emiel,Kelm, Harald,Klein, Johannes E. M. N.,Krüger, Hans-J?rg,Rauber, Christian,Schmitz, Markus
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supporting information
p. 13145 - 13155
(2021/09/03)
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- A Thioether-Ligated Cupric Superoxide Model with Hydrogen Atom Abstraction Reactivity
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The central role of cupric superoxide intermediates proposed in hormone and neurotransmitter biosynthesis by noncoupled binuclear copper monooxygenases like dopamine-β-monooxygenase has drawn significant attention to the unusual methionine ligation of the CuM ( CuB ) active site characteristic of this class of enzymes. The copper-sulfur interaction has proven critical for turnover, raising still-unresolved questions concerning Nature's selection of an oxidizable Met residue to facilitate C-H oxygenation. We describe herein a model for CuM, [(TMGN3S)CuI]+ ([1]+), and its O2-bound analog [(TMGN3S)CuII(O2?-)]+ ([1·O2]+). The latter is the first reported cupric superoxide with an experimentally proven Cu-S bond which also possesses demonstrated hydrogen atom abstraction (HAA) reactivity. Introduction of O2 to a precooled solution of the cuprous precursor [1]B(C6F5)4 (-135 °C, 2-methyltetrahydrofuran (2-MeTHF)) reversibly forms [1·O2]B(C6F5)4 (UV/vis spectroscopy: λmax 442, 642, 742 nm). Resonance Raman studies (413 nm) using 16O2 [18O2] corroborated the identity of [1·O2]+ by revealing Cu-O (446 [425] cm-1) and O-O (1105 [1042] cm-1) stretches, and extended X-ray absorption fine structure (EXAFS) spectroscopy showed a Cu-S interatomic distance of 2.55 ?. HAA reactivity between [1·O2]+ and TEMPO-H proceeds rapidly (1.28 × 10-1 M-1 s-1, -135 °C, 2-MeTHF) with a primary kinetic isotope effect of kH/kD = 5.4. Comparisons of the O2-binding behavior and redox activity of [1]+ vs [2]+, the latter a close analog of [1]+ but with all N atom ligation (i.e., N3S vs N4), are presented.
- Bhadra, Mayukh,Transue, Wesley J.,Lim, Hyeongtaek,Cowley, Ryan E.,Lee, Jung Yoon C.,Siegler, Maxime A.,Josephs, Patrick,Henkel, Gerald,Lerch, Markus,Schindler, Siegfried,Neuba, Adam,Hodgson, Keith O.,Hedman, Britt,Solomon, Edward I.,Karlin, Kenneth D.
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supporting information
p. 3707 - 3713
(2021/04/06)
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- Establishing plasmon contribution to chemical reactions: alkoxyamines as a thermal probe
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The nature of plasmon interaction with organic molecules is a subject of fierce discussion about thermal and non-thermal effects. Despite the abundance of physical methods for evaluating the plasmonic effects, chemical insight has not been reported yet. In this contribution, we propose a chemical insight into the plasmon effect on reaction kinetics using alkoxyamines as an organic probe through their homolysis, leading to the generation of nitroxide radicals. Alkoxyamines (TEMPO- and SG1-substituted) with well-studied homolysis behavior are covalently attached to spherical Au nanoparticles. We evaluate the kinetic parameters of homolysis of alkoxyamines attached on a plasmon-active surface under heating and irradiation at a wavelength of plasmon resonance. The estimation of kinetic parameters from experiments with different probes (Au-TEMPO,Au-SG1,Au-SG1-TEMPO) allows revealing the apparent differences associated with the non-thermal contribution of plasmon activation. Moreover, our findings underline the dependency of kinetic parameters on the structure of organic molecules, which highlights the necessity to consider the nature of organic transformations and molecular structure in plasmon catalysis.
- Guselnikova, Olga,Audran, Gérard,Joly, Jean-Patrick,Trelin, Andrii,Tretyakov, Evgeny V.,Svorcik, Vaclav,Lyutakov, Oleksiy,Marque, Sylvain R. A.,Postnikov, Pavel
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p. 4154 - 4161
(2021/04/06)
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- Oxidative C-S Bond Cleavage of Benzyl Thiols Enabled by Visible-Light-Mediated Silver(II) Complexes
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The oxidative cleavage reaction of the C-S bond using singlet oxygen is challenging because of its uncontrollable nature. We have developed a novel method for the singlet-oxygen-mediated selective C-S bond cleavage reaction using silver(II)-ligand complexes. Visible-light-induced silver catalysis enables the controlled oxidative cleavage of benzyl thiols to afford carbonyl compounds, such as aldehydes or ketones, which are important synthetic components.
- Hong, Boseok,Aganda, Kim Christopher C.,Lee, Anna
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supporting information
p. 4395 - 4399
(2020/06/05)
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- Visible-Light-Induced Cysteine-Specific Bioconjugation: Biocompatible Thiol–Ene Click Chemistry
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Bioconjugation methods using visible-light photocatalysis have emerged as powerful synthetic tools for the selective modification of biomolecules under mild reaction conditions. However, the number of photochemical transformations that allow successful protein bioconjugation is still limited because of the need for stringent reaction conditions. Herein, we report that a newly developed water-compatible fluorescent photosensitizer QPEG can be used for visible-light-induced cysteine-specific bioconjugation for the installation of QPEG by exploiting its intrinsic photosensitizing ability to activate the S?H bond of cysteine. The slightly modified QCAT enables the effective photocatalytic cysteine-specific conjugation of biologically relevant groups. The superior reactivity and cysteine selectivity of this methodology was further corroborated by traceless bioconjugation with a series of complex peptides and proteins under biocompatible conditions.
- Choi, Hangyeol,Hong, Sungwoo,Jang, Jaebong,Kim, Myojeong
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supporting information
p. 22514 - 22522
(2020/10/15)
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- Correction to "electrochemically Determined O-H Bond Dissociation Free Energies of NiO Electrodes Predict Proton-Coupled Electron Transfer Reactivity" (Journal of the American Chemical Society (2019)141: 38 (14971-14975)Doi: 10.1021/jacs.9b07923)
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The aqueous CG value used to calculate the bond dissociation free energy (BDFE) values reported in the published Communication was incorrect due to a sign error in its derivation. This systematic error does not affect the conclusions of the study, as all of the aqueous BDFE values shift together. The correct aqueous CG,H2O value is 52.8 kcal mol?1, as reported by Connelly, Wiedner, and Appel.1 We thank Drs. Wiedner and Appel for helpful discussions regarding this correction. We report here revised equations, tables, and schemes with BDFE values adjusted for the correct aqueous CG,H2O term. Pages 14971 and 14972. Equation 1 has been modified to report the correct aqueous CG term, and eqs 4 and 5, which give BDFE values for NiII(OH)2 and NiIIIO(OH), have also been adjusted accordingly. The revised equations are shown below: BDFE(X?H) = 23.06E(pH 0) + 52.8 kcal mol?1 (1) = } =} ? ? Ni O(OH)/Ni (OH) E 0.99 0.03 V BDFE 75.6 1.0 kcal mol III II 2 1 (4) = } = } ? ? Ni O /Ni O(OH) E 1.36 0.02 V BDFE 84.2 1.0 kcal mol IV 2 III 1 (5) Revised BDFE values for the PCET substrates discussed in the original text are given in Table 1. Page 14973. The BDFE ranges discussed in the original publication were adjusted in a similar manner. Thermodynamically favorable reactions at NiIIIO(OH) are predicted for substrates with X?H BDFE less than 75 kcal mol?1 (and were observed for substrates with X?H BDFE ranging from 61 to 73 kcal mol?1). Thermodynamically unfavorable reactivity is predicted (and was observed) for substrates with X?H BDFE greater than 76 kcal mol?1. The observed equilibrium reactivity with 2,4,6-tBu3PhOH is consistent with both the substrate and NiII(OH)2 having an O?H BDFE of ?75.5 kcal mol?1. The number line in Scheme 1 has been adjusted to reflect the corrected BDFE values, and the revised scheme is shown below. [Formula presented] Supporting Information. The BDFE values reported in Tables S1 and S5 were also adjusted for the correct aqueous CG value. The corrected tables are provided in the complete, revised Supporting Information file.
- Wise, Catherine F.,Mayer, James M.
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supporting information
p. 12544 - 12545
(2020/07/14)
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- Doubly N-Confused Calix[6]phyrin Bis-Organopalladium Complexes: Photostable Triplet Sensitizers for Singlet Oxygen Generation
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Triplet photosensitizers that generate singlet oxygen efficiently are attractive for applications such as photodynamic therapy (PDT). Extending the absorption band to a near-infrared (NIR) region (700 nm≈) with reasonable photostability is one of the major demands in the rational design of such sensitizers. We herein prepared a series of mono- and bis-palladium complexes (1-Pd-H2, 2-Pd-H2, 1-Pd-Pd, and 2-Pd-Pd) based on modified calix[6]phyrins as photosensitizers for singlet oxygen generation. These palladium complexes showed intense absorption profiles in the visible-to-NIR region (500–750 nm) depending on the number of central metals. Upon photoirradiation in the presence of 1,5-dihydroxynaphthalene (DHN) as a substrate for reactive oxygen species, the bis-palladium complexes generated singlet oxygen with high efficiency and excellent photostability. Singlet oxygen generation was confirmed from the characteristic spectral feature of the spin trapped complex in the EPR spectrum and the intact 1O2 emission at 1270 nm.
- Pushpanandan, Poornenth,Won, Dong-Hoon,Mori, Shigeki,Yasutake, Yuhsuke,Fukatsu, Susumu,Ishida, Masatoshi,Furuta, Hiroyuki
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p. 1729 - 1736
(2019/01/29)
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- Structure and Reactivity of (μ-Oxo)dimanganese(III,III) and Mononuclear Hydroxomanganese(III) Adducts Supported by Derivatives of an Amide-Containing Pentadentate Ligand
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Mononuclear MnIII-hydroxo and dinuclear (μ-oxo)dimanganese(III,III) complexes were prepared using derivatives of the pentadentate, amide-containing dpaq ligand (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate). Each of these ligand derivatives (referred to as dpaq5R) contained a substituent R (where R = OMe, Cl, and NO2) at the 5-position of the quinolinyl group. Generation of the MnIII complexes was achieved by either O2 oxidation of MnII precursors (for [MnII(dpaq5OMe)]+ and [MnII(dpaq5Cl)]+ or PhIO oxidation (for [MnII(dpaq5NO2)]+). For each oxidized complex, 1H NMR experiments provided evidence of a water-dependent equilibrium between paramagnetic [MnIII(OH)(dpaq5R)]+ and an antiferromagnetically coupled [MnIIIMnIII(μ-O)(dpaq5R)2]2+ species in acetonitrile, with the addition of water favoring the MnIII-hydroxo species. This conversion could also be monitored by electronic absorption spectroscopy. Solid-state X-ray crystal structures for each [MnIIIMnIII(μ-O)(dpaq5R)2](OTf)2 complex revealed a nearly linear Mn-O-Mn core (angle of ca. 177°), with short Mn-O distances near 1.79 ?, and a Mn···Mn separation of 3.58 ?. X-ray crystallographic information was also obtained for the mononuclear [MnIII(OH)(dpaq5Cl)](OTf) complex, which has a short Mn-O(H) distance of 1.810(2) ?. The influence of the 5-substituted quinolinyl moiety on the electronic properties of the [MnIII(OH)(dpaq5R)]+ complexes was demonstrated through shifts in a number of 1H NMR resonances, as well as a steady increase in the MnIII/II cyclic voltammetry peak potential in the order [MnIII(OH)(dpaq5OMe)]+ III(OH)(dpaq)]+ III(OH)(dpaq5Cl)]+ III(OH)(dpaq5NO2)]+. These changes in oxidizing power of the MnIII-hydroxo adducts translated to only modest rate enhancements for TEMPOH oxidation by the [MnIII(OH)(dpaq5R)]+ complexes, with the most reactive [MnIII(OH)(dpaq5N2)]+ complex showing a second-order rate constant only 9-fold larger than that of the least reactive [MnIII(OH)(dpaq5OMe)]+ complex. These modest rate changes were understood on the basis of density functional theory (DFT)-computed pKa values for the corresponding [MnII(OH2)(dpaq5R)]+ complexes. Collectively, the experimental and DFT results reveal that the 5-substituted quinolinyl groups have an inverse influence on electron and proton affinity for the MnIII-hydroxo unit.
- Rice, Derek B.,Munasinghe, Aruna,Grotemeyer, Elizabeth N.,Burr, Andrew D.,Day, Victor W.,Jackson, Timothy A.
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p. 622 - 636
(2019/01/11)
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- TEMPO-Mediated Catalysis of the Sterically Hindered Hydrogen Atom Transfer Reaction between (C5Ph5)Cr(CO)3H and a Trityl Radical
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We have demonstrated the ability of TEMPO to catalyze H· transfer from (C5Ph5)Cr(CO)3H to a trityl radical (tris(p-tert-butylphenyl)methyl radical). We have measured the rate constant and activation parameters for the direct reaction, and for each step in the catalytic process: H· transfer from (C5Ph5)Cr(CO)3H to TEMPO and H· transfer from TEMPO-H to the trityl radical. We have compared the measured rate constants with the differences in bond strength, and with the changes in the Global Electrophilicity Index determined with high accuracy for each radical using state of the art quantum chemical methods. We conclude that neither is a major factor in determining the rates of these H· transfer reactions and that the effectiveness of TEMPO as a catalyst is largely the result of its relative lack of steric congestion compared to the trityl radical.
- Gunasekara, Thilina,Abramo, Graham P.,Hansen, Andreas,Neugebauer, Hagen,Bursch, Markus,Grimme, Stefan,Norton, Jack R.
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supporting information
p. 1882 - 1886
(2019/02/14)
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- Copper/TEMPO Redox Redux: Analysis of PCET Oxidation of TEMPOH by Copper(II) and the Reaction of TEMPO with Copper(I)
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Copper salts and organic aminoxyls, such as TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl), are versatile catalysts for aerobic alcohol oxidation. Previous reports in the literature contain conflicting proposals concerning the redox interactions that take place between copper(I) and copper(II) salts with the aminoxyl and hydroxylamine species, TEMPO and TEMPOH, respectively. Here, we reinvestigate these reactions in an effort to resolve the conflicting claims in the literature. Under anaerobic conditions, CuIIX2 salts [X = acetate (OAc), trifluoroacetate (TFA), and triflate (OTf)] are shown to promote the rapid proton-coupled oxidation of TEMPOH to TEMPO: CuIIX2 + TEMPOH → CuIX + TEMPO + HX. In the reaction with acetate, however, slow reoxidation of CuIOAc occurs. This process requires both TEMPO and HOAc and coincides with the reduction of TEMPO to 2,2,6,6-tetramethylpiperidine. Analogous reactivity is not observed with trifluoroacetate and triflate species. Overall, the facility of the proton-coupled oxidation of TEMPOH by CuII salts suggests that this process could contribute to catalyst regeneration under aerobic oxidation conditions.
- Ryan, Michael C.,Whitmire, Lauren D.,McCann, Scott D.,Stahl, Shannon S.
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supporting information
p. 10194 - 10200
(2019/08/26)
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- A Drastic Effect of TEMPO in Zinc-Catalyzed Stannylation of Terminal Alkynes with Hydrostannanes via Dehydrogenation and Oxidative Dehydrogenation
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With a system consisting of a catalytic zinc Lewis acid, pyridine, and TEMPO in a nitrile medium, terminal alkynes coupled with HSnBu3, providing alkynylstannanes with structural diversity. The resulting alkynylstannane, without being isolated, could be directly used for Pd- and Cu-catalyzed transformations to deliver internal alkynes and more intricate tin-atom-containing molecules. Mechanistic studies indicated that TEMPOSnBu3 formed in situ from TEMPO and HSnBu3 works to stannylate the terminal alkyne in collaboration with the zinc catalyst, and that both of dehydrogenation and oxidative dehydrogenation processes are uniquely involved in a single reaction. (Figure presented.).
- Kai, Yuichi,Oku, Shinya,Sakurai, Kyoko,Tani, Tomohiro,Tsuchimoto, Teruhisa
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supporting information
(2019/08/21)
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- Photochemical generation of the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical from caged nitroxides by near-infrared two-photon irradiation and its cytocidal effect on lung cancer cells
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Novel caged nitroxides (nitroxide donors) with near-infrared two-photon (TP) responsive character, 2,2,6,6-tetramethyl-1-(1-(2-(4-nitrophenyl)benzofuran-6-yl)ethoxy)piperidine (2a) and its regioisomer 2b, were designed and synthesized. The one-photon (OP) (365 ± 10 nm) and TP (710–760 nm) triggered release (i.e., uncaging) of the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical under air atmosphere were discovered. The quantum yields for the release of the TEMPO radical were 2.5% (2a) and 0.8% (2b) in benzene at ≈1% conversion of 2, and 13.1% (2a) and 12.8% (2b) in DMSO at ≈1% conversion of 2. The TP uncaging efficiencies were determined to be 1.1 GM at 740 nm for 2a and 0.22 GM at 730 nm for 2b in benzene. The cytocidal effect of compound 2a on lung cancer cells under photolysis conditions was also assessed to test the efficacy as anticancer agents. In a medium containing 100 μg mL?1 of 2a exposed to light, the number of living cells decreased significantly compared to the unexposed counterparts (65.8% vs 85.5%).
- Yamada, Ayato,Abe, Manabu,Nishimura, Yoshinobu,Ishizaka, Shoji,Namba, Masashi,Nakashima, Taku,Shimoji, Kiyofumi,Hattori, Noboru
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supporting information
p. 863 - 873
(2019/04/17)
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- Highly Efficient and Selective Photooxidation of Sulfur Mustard Simulant by a Triazolobenzothiadiazole-Moiety-Functionalized Metal-Organic Framework in Air
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A photoactive triazolobenzothiadiazole (TBTD)-conjugated terphenyldicarboxylate (TPDC) linker was introduced into a porous and robust UiO-68 isoreticular zirconium metal-organic framework (denoted as UiO-68-TBTD) by the de novo synthetic approach of mixed TPDC struts. Under blue-light-emitting-diode irradiation, UiO-68-TBTD can serve as a heterogeneous photocatalyst for the highly efficient and selective oxidation of a sulfur mustard simulant (2-chloroethyl ethyl sulfide) to the corresponding much less toxic sulfoxide product, with a half-life of only 3 min in the open air atmosphere.
- Zhang, Wen-Qiang,Cheng, Ke,Zhang, He,Li, Qiu-Yan,Ma, Zheng,Wang, Zixuan,Sheng, Jialing,Li, Yinwei,Zhao, Xinsheng,Wang, Xiao-Jun
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supporting information
p. 4230 - 4233
(2018/04/23)
-
- A smart photosensitizer based on a red emitting solution processable porous polymer: Generation of reactive oxygen species
-
A novel approach for the fabrication of a solution processable conjugated porous organic polymer (CzBDP) involving a flexible core composed of carbazole and boron dipyrromethene was developed. The red emitting soluble polymer was found to be an excellent probe for the generation of both singlet oxygen and superoxide anion radicals under visible light irradiation.
- Bandyopadhyay, Sujoy,Kundu, Subhankar,Giri, Arkaprabha,Patra, Abhijit
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supporting information
p. 9123 - 9126
(2018/08/23)
-
- Method for efficiently preparing 2,2,6,6-tetramethyl piperidine oxide
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The invention discloses a method for efficiently preparing a 2,2,6,6-tetramethyl piperidine oxide, and belongs to the technical field of the chemical material synthesis. The method comprises the following steps: using a compound of magnesium hydrate and kieselguhr as a catalyst, performing an oxidation reaction in the presence of hydrogen peroxide, and filtering, extracting by using ethyl acetateor propyl acetate, after distilling and removing a solvent, to obtain a product. A purity of the product is up to about 99.5%, and a yield of the reaction is improved from 80% in the prior art to 97%.Problems existing in the prior art that the pollution is serious and the yield is low and the like are solved.
- -
-
Paragraph 0029; 0032-0037; 0044-0061
(2018/11/03)
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- Nickel(I) Aryl Species: Synthesis, Properties, and Catalytic Activity
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In this work, Ni(I) aryl species that are directly relevant to cross-coupling have been synthesized. Transmetalation of (dppf)NiIX (dppf = 1,1′-bis(diphenylphosphino)-ferrocene, X = Cl, Br) with aryl Grignard reagents or aryl boronic acids in t
- Mohadjer Beromi, Megan,Banerjee, Gourab,Brudvig, Gary W.,Hazari, Nilay,Mercado, Brandon Q.
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p. 2526 - 2533
(2018/03/13)
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- The special role of B(C6F5)3 in the single electron reduction of quinones by radicals
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In the presence of two molar equiv. of B(C6F5)3p-benzoquinone reacts with persistent radicals TEMPO, trityl or decamethylferrocene by single electron transfer to give doubly O-borylated benzosemiquinone radical anions with
- Tao, Xin,Daniliuc, Constantin G.,Knitsch, Robert,Hansen, Michael Ryan,Eckert, Hellmut,Lübbesmeyer, Maximilian,Studer, Armido,Kehr, Gerald,Erker, Gerhard
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p. 8011 - 8018
(2018/11/03)
-
- Separating Proton and Electron Transfer Effects in Three-Component Concerted Proton-Coupled Electron Transfer Reactions
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Multiple-site concerted proton-electron transfer (MS-CPET) reactions were studied in a three-component system. 1-Hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH) was oxidized to the stable radical TEMPO by electron transfer to ferrocenium oxidants coupled to proton transfer to various pyridine bases. These MS-CPET reactions contrast with the usual reactivity of TEMPOH by hydrogen atom transfer (HAT) to a single e-/H+ acceptor. The three-component reactions proceed by pre-equilibrium formation of a hydrogen-bonded adduct between TEMPOH and the pyridine base, and the adduct is then oxidized by the ferrocenium in a bimolecular MS-CPET step. The second-order rate constants, measured using stopped-flow kinetic techniques, spanned 4 orders of magnitude. An advantage of this system is that the MS-CPET driving force could be independently varied by changing either the pKa of the base or the reduction potential (E°) of the oxidant. Changes in ΔG°MS-CPET from either source had the same effect on the MS-CPET rate constants, and a combined Br?nsted plot of ln(kMS-CPET) vs ln(Keq) was linear with a slope of 0.46. These results imply a synchronous concerted mechanism, in which the proton and electron transfer components of the CPET process make equal contributions to the rate constants. The only outliers to the Br?nsted correlation are the reactions with sterically hindered pyridines, which apparently hinder the close approach of proton donor and acceptor that facilitates MS-CPET. These three-component reactions are compared with a related HAT reaction of TEMPOH, with the 2,4,6-tri-tert-butylphenoxyl radical. The MS-CPET and HAT oxidations of TEMPOH at the same driving force occurred with similar rate constants. While this is an imperfect comparison, the data suggest that the separation of the proton and electron to different reagents does not significantly inhibit the proton-coupled electron transfer process.
- Morris, Wesley D.,Mayer, James M.
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supporting information
p. 10312 - 10319
(2017/08/09)
-
- Photoinduced, Copper-Catalyzed Alkylation of Amines: A Mechanistic Study of the Cross-Coupling of Carbazole with Alkyl Bromides
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We have recently reported that a variety of couplings of nitrogen, sulfur, oxygen, and carbon nucleophiles with organic halides can be achieved under mild conditions (-40 to 30 °C) through the use of light and a copper catalyst. Insight into the various mechanisms by which these reactions proceed may enhance our understanding of chemical reactivity and facilitate the development of new methods. In this report, we apply an array of tools (EPR, NMR, transient absorption, and UV-vis spectroscopy; ESI-MS; X-ray crystallography; DFT calculations; reactivity, stereochemical, and product studies) to investigate the photoinduced, copper-catalyzed coupling of carbazole with alkyl bromides. Our observations are consistent with pathways wherein both an excited state of the copper(I) carbazolide complex ([CuI(carb)2]-) and an excited state of the nucleophile (Li(carb)) can serve as photoreductants of the alkyl bromide. The catalytically dominant pathway proceeds from the excited state of Li(carb), generating a carbazyl radical and an alkyl radical. The cross-coupling of these radicals is catalyzed by copper via an out-of-cage mechanism in which [CuI(carb)2]- and [CuII(carb)3]- (carb = carbazolide), both of which have been identified under coupling conditions, are key intermediates, and [CuII(carb)3]- serves as the persistent radical that is responsible for predominant cross-coupling. This study underscores the versatility of copper(II) complexes in engaging with radical intermediates that are generated by disparate pathways, en route to targeted bond constructions.
- Ahn, Jun Myun,Ratani, Tanvi S.,Hannoun, Kareem I.,Fu, Gregory C.,Peters, Jonas C.
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supporting information
p. 12716 - 12723
(2017/09/25)
-
- Thermodynamics of a μ-oxo Dicopper(II) Complex for Hydrogen Atom Abstraction
-
The mono-μ-hydroxo complex {[Cu(tmpa)]2-(μ-OH)}3+ (1) can undergo reversible deprotonation at -30 °C to yield {[Cu(tmpa)]2-(μ-O)}2+ (2). This species is basic with a pKa of 24.3. 2 is competent for concerted proton-electron transfer from TEMPOH, but is an intrinsically poor hydrogen atom abstractor (BDFE(OH) of 77.2 kcal/mol) based on kinetic and thermodynamic analyses. Nonetheless, DFT calculations experimentally calibrated against 2 reveal that [Cu2O]2+ is likely thermodynamically viable in copper-dependent methane monoxygenase enzymes.
- Ali, Ghazanfar,Vannatta, Peter E.,Ramirez, David A.,Light, Kenneth M.,Kieber-Emmons, Matthew T.
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supporting information
p. 18448 - 18451
(2018/01/08)
-
- Synergistic copper-TEMPO catalysis of intermolecular vicinal diamination of styrenes
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A copper-catalyzed, 2,2,6,6-tetramethyl piperidine N-oxy radical-assisted intermolecular diamination of styrenes with N-fluorobenzenesulfonimide has been developed. The current protocol proved amenable to a diverse array of styrenes via cascade radical addition to readily afford synthetically useful aromatic vicinal diamines with exclusive diastereoselectivity.
- Weng, Shiue-Shien,Hsieh, Kun-Yi,Zeng, Zih-Jian,Zhang, Jia-Wei
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supporting information
p. 670 - 673
(2017/01/25)
-
- Zn(II) porphyrin based nano-/microscale metal-organic frameworks: Morphology dependent sensitization and photocatalytic oxathiolane deprotection
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With Zn(ii) porphyrin as a building block, nano-/microscale MOFs with different morphologies were synthesized as sensitizers for O2 (forming 1O2). The sensitization efficiency was observed to enhance remarkably with the decrease of morphological dimension of the sample. With high sensitization ability, the one-dimensional MOF could selectively photocatalyze the deprotection of oxathiolanes to ketones.
- Zhao, Fang-Yao,Li, Wen-Juan,Guo, An,Chang, Lan,Li, Yue,Ruan, Wen-Juan
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p. 26199 - 26202
(2016/03/25)
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- Steric and Electronic Influence on Proton-Coupled Electron-Transfer Reactivity of a Mononuclear Mn(III)-Hydroxo Complex
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A mononuclear hydroxomanganese(III) complex was synthesized utilizing the N5 amide-containing ligand 2-[bis(pyridin-2-ylmethyl)]amino-N-2-methyl-quinolin-8-yl-acetamidate (dpaq2Me). This complex is similar to previously reported [MnIII(OH)(dpaqH)]+ [Inorg. Chem. 2014, 53, 7622-7634] but contains a methyl group adjacent to the hydroxo moiety. This α-methylquinoline group in [MnIII(OH)(dpaq2Me)]+ gives rise to a 0.1 ? elongation in the Mn-N(quinoline) distance relative to [MnIII(OH)(dpaqH)]+. Similar bond elongation is observed in the corresponding Mn(II) complex. In MeCN, [MnIII(OH)(dpaq2Me)]+ reacts rapidly with 2,2′,6,6′-tetramethylpiperidine-1-ol (TEMPOH) at -35 °C by a concerted proton-electron transfer (CPET) mechanism (second-order rate constant k2 of 3.9(3) M-1 s-1). Using enthalpies and entropies of activation from variable-temperature studies of TEMPOH oxidation by [MnIII(OH)(dpaq2Me)]+ (ΔH? = 5.7(3) kcal-1 M-1 ΔS? = -41(1) cal M-1 K-1), it was determined that [MnIII(OH)(dpaq2Me)]+ oxidizes TEMPOH ~240 times faster than [MnIII(OH)(dpaqH)]+. The [MnIII(OH)(dpaq2Me)]+ complex is also capable of oxidizing the stronger O-H and C-H bonds of 2,4,6-tri-tert-butylphenol and xanthene, respectively. However, for these reactions [MnIII(OH)(dpaq2Me)]+ displays, at best, modest rate enhancement relative to [MnIII(OH)(dpaqH)]+. A combination of density function theory (DFT) and cyclic voltammetry studies establish an increase in the MnIII/MnII reduction potential of [MnIII(OH)(dpaq2Me)]+ relative to [MnIII(OH)(dpaqH)]+, which gives rise to a larger driving force for CPET for the former complex. Thus, more favorable thermodynamics for [MnIII(OH)(dpaq2Me)]+ can account for the dramatic increase in rate with TEMPOH. For the more sterically encumbered substrates, DFT computations suggest that this effect is mitigated by unfavorable steric interactions between the substrate and the α-methylquinoline group of the dpaq2Me ligand. The DFT calculations, which reproduce the experimental activation free energies quite well, provide the first examination of the transition-state structure of mononuclear MnIII(OH) species during a CPET reaction.
- Rice, Derek B.,Wijeratne, Gayan B.,Burr, Andrew D.,Parham, Joshua D.,Day, Victor W.,Jackson, Timothy A.
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p. 8110 - 8120
(2016/08/24)
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- Kinetics and thermodynamics of reversible disproportionation-comproportionation in redox triad oxoammonium cations - Nitroxyl radicals - Hydroxylamines
-
Kinetics and equilibrium of the acid-catalyzed disproportionation of cyclic nitroxyl radicals R2NO· to oxoammonium cations R2NO+ and hydroxylamines R2NOH is defined by redox and acid-base properties of these compounds. In a recent work (J. Phys. Org. Chem. 2014, 27, 114-120), we showed that the kinetic stability of R2NO· in acidic media depends on the basicity of the nitroxyl group. Here, we examined the kinetics of the reverse comproportionation reaction of R2NO+ and R2NOH to R2NO· and found that increasing in -I-effects of substituents greatly reduces the overall equilibrium constant of the reaction K4. This occurs because of both the increase of acidity constants of hydroxyammonium cations K3H+ and the difference between the reduction potentials of oxoammonium cations ER2NO+/R2NO· and nitroxyl radicals ER2NO·/R2NOH. pH dependences of reduction potentials of nitroxyl radicals to hydroxylamines E1/3σ and bond dissociation energies D(O-H) for hydroxylamines R2NOH inwater were determined. For a wide variety of piperidine- and pyrrolidine-1-oxyls values of pK3H+ and ER2NO+/R2NO· correlate with each other, as well aswith the equilibriumconstants K4 and the inductive substituent constants ωI. The correlations obtained allowprediction of the acid-base and redox characteristics of redox triads R2NO·-R2NO+-R2NOH.
- Sen, Vasily D.,Tikhonov, Ivan V.,Borodin, Leonid I.,Pliss, Evgeny M.,Golubev, Valery A.,Syroeshkin, Mikhail A.,Rusakov, Alexander I.
-
-
- Generation, Characterization, and Reactivity of a CuII-Alkylperoxide/Anilino Radical Complex: Insight into the O-O Bond Cleavage Mechanism
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The reaction of [CuI(TIPT3tren) (CH3CN)]ClO4 (1) and cumene hydroperoxide (C6H5C(CH3)2OOH, ROOH) at -60°C in CH2Cl2 gave a CuII-alkylperoxide/anilino radical complex 2, the formation of which was confirmed by UV-vis, resonance Raman, EPR, and CSI-mass spectroscopy. The mechanism of formation of 2, as well as its reactivity, has been explored.
- Paria, Sayantan,Ohta, Takehiro,Morimoto, Yuma,Ogura, Takashi,Sugimoto, Hideki,Fujieda, Nobutaka,Goto, Kei,Asano, Kaori,Suzuki, Takeyuki,Itoh, Shinobu
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p. 10870 - 10873
(2015/09/15)
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- O-H bond oxidation by a monomeric MnIII-OMe complex
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Manganese-containing, mid-valent oxidants (MnIII-OR) that mediate proton-coupled electron-transfer (PCET) reactions are central to a variety of crucial enzymatic processes. The Mn-dependent enzyme lipoxygenase is such an example, where a MnIII-OH unit activates fatty acid substrates for peroxidation by an initial PCET. This present work describes the quantitative generation of the MnIII-OMe complex, [MnIII(OMe)(dpaq)]+ (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) via dioxygen activation by [MnII(dpaq)]+ in methanol at 25 °C. The X-ray diffraction structure of [MnIII(OMe)(dpaq)]+ exhibits a Mn-OMe group, with a Mn-O distance of 1.825(4) ?, that is trans to the amide functionality of the dpaq ligand. The [MnIII(OMe)(dpaq)]+ complex is quite stable in solution, with a half-life of 26 days in MeCN at 25 °C. [MnIII(OMe)(dpaq)]+ can activate phenolic O-H bonds with bond dissociation free energies (BDFEs) of less than 79 kcal mol-1 and reacts with the weak O-H bond of TEMPOH (TEMPOH = 2,2′-6,6′-tetramethylpiperidine-1-ol) with a hydrogen/deuterium kinetic isotope effect (H/D KIE) of 1.8 in MeCN at 25 °C. This isotope effect, together with other experimental evidence, is suggestive of a concerted proton-electron transfer (CPET) mechanism for O-H bond oxidation by [MnIII(OMe)(dpaq)]+. A kinetic and thermodynamic comparison of the O-H bond oxidation reactivity of [MnIII(OMe)(dpaq)]+ to other MIII-OR oxidants is presented as an aid to gain more insight into the PCET reactivity of mid-valent oxidants. In contrast to high-valent counterparts, the limited examples of MIII-OR oxidants exhibit smaller H/D KIEs and show weaker dependence of their oxidation rates on the driving force of the PCET reaction with O-H bonds. This journal is
- Wijeratne, Gayan B.,Day, Victor W.,Jackson, Timothy A.
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p. 3295 - 3306
(2015/03/05)
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- Cacalol and cacalol acetate as photoproducers of singlet oxygen and as free radical scavengers, evaluated by EPR spectroscopy and TBARS
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Photodynamic therapy (PDT) is an emerging cancer treatment based on the production of singlet oxygen (1O2) upon illumination of a photosensitizer in the presence of oxygen. Antioxidants are primarily reducing agents prone to scavenge reactive species in one way or another. Cacalol (C) and cacalol acetate (CA) were examined and compared regarding to their capacity to produce singlet oxygen and as scavengers of free radicals. Their role as singlet oxygen photoproducers under UV-vis light irradiation was examined by electron paramagnetic resonance (EPR) using 2,2,6,6-tetramethyl-piperidine (TEMP) as spin-trapping material. The quantum yield to produce 1O2 was found to be 0.4 ± 0.05 for CA and 0.13 ± 0.05 for C. Their properties as scavengers of hydroxyl (OH), nitrogen-centered (2,2-diphenyl-1-picryhydrazyl radical, DPPH) and organic radicals (R and ROO) were evaluated using EPR and the thiobarbituric reactive substances (TBARS) method. C and CA differed in their abilities to trap DPPH. By contrast, both compounds showed similar activity to trap OH, R and ROO. A relationship between the redox potentials of the compounds and their activity as scavengers of DPPH was observed. The producing/inhibiting properties showed by C and CA make them interesting options for new therapeutic applications to treat tumors and other diseases.
- Gomez-Vidales, Virginia,Granados-Oliveros, Gilma,Nieto-Camacho, Antonio,Reyes-Solis, Mirna,Jimenez-Estrada, Manuel
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p. 1371 - 1377
(2014/01/06)
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- Scope and limitations of the TEMPO/EPR method for singlet oxygen detection: The misleading role of electron transfer
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For many biological and biomedical studies, it is essential to detect the production of 1O2 and quantify its production yield. Among the available methods, detection of the characteristic 1270-nm phosphorescence of singlet oxygen by time-resolved near-infrared (TRNIR) emission constitutes the most direct and unambiguous approach. An alternative indirect method is electron paramagnetic resonance (EPR) in combination with a singlet oxygen probe. This is based on the detection of the TEMPO free radical formed after oxidation of TEMP (2,2,6,6-tetramethylpiperidine) by singlet oxygen. Although the TEMPO/EPR method has been widely employed, it can produce misleading data. This is demonstrated by the present study, in which the quantum yields of singlet oxygen formation obtained by TRNIR emission and by the TEMPO/EPR method are compared for a set of well-known photosensitizers. The results reveal that the TEMPO/EPR method leads to significant overestimation of singlet oxygen yield when the singlet or triplet excited state of the photosensitizer is efficiently quenched by TEMP, acting as electron donor. In such case, generation of the TEMP+ radical cation, followed by deprotonation and reaction with molecular oxygen, gives rise to an EPR-detectable TEMPO signal that is not associated with singlet oxygen production. This knowledge is essential for an appropriate and error-free application of the TEMPO/EPR method in chemical, biological, and medical studies.
- Nardi, Giacomo,Manet, Ilse,Monti, Sandra,Miranda, Miguel A.,Lhiaubet-Vallet, Virginie
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supporting information
p. 64 - 70
(2015/02/19)
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- Photo-induced proton-coupled electron transfer reactions of acridine orange: Comprehensive spectral and kinetics analysis
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The triplet excited state of acridine orange (3*AO) undergoes a proton-coupled electron transfer (PCET) reaction with tri-tert-butylphenol (ttbPhOH) in acetonitrile. Each of the reaction components possesses a spectroscopic signature, providing a rare opportunity to monitor the individual proton transfer, electron transfer, and H ?-transfer components in parallel via transient absorption spectroscopy. This enhanced optical tracking, along with excited-state thermochemical analysis, facilitates assignment of the mechanism of excited-state PCET reactivity. 3*AO is quenched via concerted proton-electron transfer (CPET) from ttbPhOH to form acridine radical (AOH?) and ttbPhO? (kCPET = 3.7 × 108 M-1 s-1, KIE = 1.3). Subsequently, AOH? reduces the phenoxyl radical (kET = 5.5 × 109 M-1 s-1), forming AOH + and ttbPhO-, followed by proton transfer (kPT = 1.0 × 109 M-1 s-1) to regenerate the starting reactants.
- Eisenhart, Thomas T.,Dempsey, Jillian L.
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p. 12221 - 12224
(2014/11/08)
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- Mechanistic insights into the oxidation of substituted phenols via hydrogen atom abstraction by a cupric-superoxo complex
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To obtain mechanistic insights into the inherent reactivity patterns for copper(I)-O2 adducts, a new cupric-superoxo complex [(DMM-tmpa)CuII(O2?-)]+ (2) [DMM-tmpa = tris((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)amine] has been synthesized and studied in phenol oxidation-oxygenation reactions. Compound 2 is characterized by UV-vis, resonance Raman, and EPR spectroscopies. Its reactions with a series of para-substituted 2,6-di-tert-butylphenols (p-X-DTBPs) afford 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) in up to 50% yields. Significant deuterium kinetic isotope effects and a positive correlation of second-order rate constants (k2) compared to rate constants for p-X-DTBPs plus cumylperoxyl radical reactions indicate a mechanism that involves rate-limiting hydrogen atom transfer (HAT). A weak correlation of (kBT/e) ln k 2 versus Eox of p-X-DTBP indicates that the HAT reactions proceed via a partial transfer of charge rather than a complete transfer of charge in the electron transfer/proton transfer pathway. Product analyses, 18O-labeling experiments, and separate reactivity employing the 2,4,6-tri-tert-butylphenoxyl radical provide further mechanistic insights. After initial HAT, a second molar equiv of 2 couples to the phenoxyl radical initially formed, giving a CuII-OO-(ArO') intermediate, which proceeds in the case of p-OR-DTBP substrates via a two-electron oxidation reaction involving hydrolysis steps which liberate H2O2 and the corresponding alcohol. By contrast, four-electron oxygenation (O-O cleavage) mainly occurs for p-R-DTBP which gives 18O-labeled DTBQ and elimination of the R group.
- Lee, Jung Yoon,Peterson, Ryan L.,Ohkubo, Kei,Garcia-Bosch, Isaac,Himes, Richard A.,Woertink, Julia,Moore, Cathy D.,Solomon, Edward I.,Fukuzumi, Shunichi,Karlin, Kenneth D.
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supporting information
p. 9925 - 9937
(2014/08/05)
-
- Saturation kinetics in phenolic O-H bond oxidation by a mononuclear Mn(III)-OH complex derived from dioxygen
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The mononuclear hydroxomanganese(III) complex, [MnIII(OH)(dpaq)] +, which is supported by the amide-containing N5 ligand dpaq (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) was generated by treatment of the manganese(II) species, [MnII(dpaq)] (OTf), with dioxygen in acetonitrile solution at 25 °C. This oxygenation reaction proceeds with essentially quantitative yield (greater than 98% isolated yield) and represents a rare example of an O2-mediated oxidation of a manganese(II) complex to generate a single product. The X-ray diffraction structure of [MnIII(OH)(dpaq)]+ reveals a short Mn-OH distance of 1.806(13) A, with the hydroxo moiety trans to the amide function of the dpaq ligand. No shielding of the hydroxo group is observed in the solid-state structure. Nonetheless, [MnIII(OH)(dpaq)]+ is remarkably stable, decreasing in concentration by only 10% when stored in MeCN at 25 °C for 1 week. The [MnIII(OH)(dpaq)]+ complex participates in proton-coupled electron transfer reactions with substrates with relatively weak O-H and C-H bonds. For example, [Mn III(OH)(dpaq)]+ oxidizes TEMPOH (TEMPOH = 2,2′-6,6′-tetramethylpiperidine-1-ol), which has a bond dissociation free energy (BDFE) of 66.5 kcal/mol, in MeCN at 25 °C. The hydrogen/deuterium kinetic isotope effect of 1.8 observed for this reaction implies a concerted proton-electron transfer pathway. The [Mn III(OH)(dpaq)]+ complex also oxidizes xanthene (C-H BDFE of 73.3 kcal/mol in dimethylsulfoxide) and phenols, such as 2,4,6-tri-t- butylphenol, with BDFEs of less than 79 kcal/mol. Saturation kinetics were observed for phenol oxidation, implying an initial equilibrium prior to the rate-determining step. On the basis of a collective body of evidence, the equilibrium step is attributed to the formation of a hydrogen-bonding complex between [MnIII(OH)(dpaq)]+ and the phenol substrates.
- Wijeratne, Gayan B.,Corzine, Briana,Day, Victor W.,Jackson, Timothy A.
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supporting information
p. 7622 - 7634
(2014/08/05)
-
- Unveiling chemical reactivity and structural transformation of two-dimensional layered nanocrystals
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Two-dimensional (2D) layered nanostructures are emerging fast due to their exceptional materials properties. While the importance of physical approaches (e.g., guest intercalation and exfoliation) of 2D layered nanomaterials has been recognized, an understanding of basic chemical reactions of these materials, especially in nanoscale regime, is obscure. Here, we show how chemical stimuli can influence the fate of reaction pathways of 2D layered nanocrystals. Depending on the chemical characteristics (Lewis acid (1O 2) or base (H2O)) of external stimuli, TiS2 nanocrystal is respectively transformed to either a TiO2 nanodisc through a "compositional metathesis" or a TiO2 toroid through multistage "edge-selective structural transformation" processes. These chemical reactions can serve as the new design concept for functional 2D layered nanostructures. For example, TiS2(disc)- TiO2(shell) nanocrystal constitutes a high performance type II heterojunction which not only a wide range solar energy coverage (~80%) with near-infrared absorption edge, but also possesses enhanced electron transfer property.
- Han, Jae Hyo,Lee, Sujeong,Yoo, Dongwon,Lee, Jae-Hyun,Jeong, Sohee,Kim, Jin-Gyu,Cheon, Jinwoo
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p. 3736 - 3739
(2013/04/23)
-
- Redox properties of a mononuclear copper(II)-superoxide complex
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Redox properties of a mononuclear copper(II) superoxide complex, (L)Cu II-OO?, supported by a tridentate ligand (L = 1-(2-phenethyl)-5-[2-(2-pyridyl)ethyl]-1,5-diazacyclooctane) have been examined as a model compound of the putative reactive intermediate of peptidylglycine α-hydroxylating monooxygenase (PHM) and dopamine β-monooxygenase (DβM) (Kunishita et al. J. Am. Chem. Soc. 2009, 131, 2788-2789; Inorg. Chem. 2012, 51, 9465-9480). On the basis of the reactivity toward a series of one-electron reductants, the reduction potential of (L)CuII-OO ? was estimated to be 0.19 ± 0.07 V vs SCE in acetone at 298 K (cf. Tahsini et al. Chem. - Eur. J. 2012, 18, 1084-1093). In the reaction of TEMPO-H (2,2,6,6-tetramethylpiperidine-N-hydroxide), a simple HAT (hydrogen atom transfer) reaction took place to give the corresponding hydroperoxide complex LCuII-OOH, whereas the reaction with phenol derivatives (XArOH) gave the corresponding phenolate adducts, LCu II-OXAr, presumably via an acid-base reaction between the superoxide ligand and the phenols. The reaction of (L)CuII-OO ? with a series of triphenylphosphine derivatives gave the corresponding triphenylphosphine oxides via an electrophilic ionic substitution mechanism with a Hammett ρ value as -4.3, whereas the reaction with thioanisole (sulfide) only gave a copper(I) complex. These reactivities of (L)CuII-OO? are different from those of a similar end-on superoxide copper(II) complex supported by a tetradentate TMG 3tren ligand (1,1,1-Tris{2-[N2-(1,1,3,3- tetramethylguanidino)]ethyl}amine (Maiti et al. Angew. Chem., Int. Ed. 2008, 47, 82-85).
- Tano, Tetsuro,Okubo, Yuri,Kunishita, Atsushi,Kubo, Minoru,Sugimoto, Hideki,Fujieda, Nobutaka,Ogura, Takashi,Itoh, Shinobu
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p. 10431 - 10437
(2013/10/01)
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- Quantitation of Li2O2 stored in Li-O2 batteries based on its reaction with an oxoammonium salt
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Precise knowledge of the discharge and charge reactions within Li-O 2 batteries is an important aspect of developing highly efficient, rechargeable Li-O2 cells. We describe an analytical method capable of determining the quantity of
- Hase, Yoko,Ito, Emi,Shiga, Tohru,Mizuno, Fuminori,Nishikoori, Hidetaka,Iba, Hideki,Takechi, Kensuke
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supporting information
p. 8389 - 8391
(2013/09/23)
-
- Kinetics and mechanism of benzyl chloride reaction with zinc in dimethylacetamide
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Oxidative dissolution of zinc in the system of benzyl chloride- dimethylacetamide was investigated. The reaction stereochemistry as well as intermediates and reaction products formed were studied. The kinetic and thermodynamic parameters of the process were measured. The process was shown to follow the Langmuir-Hinshelwood mechanism with the formation of benzyl radicals and mono-solvated organozinc compound on the zinc surface. The components of mixture are adsorbed at various sites of the zinc surface, while recombination and the isomerization of the benzyl radicals occurs in solution. Pleiades Publishing, Ltd., 2012.
- Egorov,Matyukhova,Dashkova
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p. 1686 - 1699
(2013/02/23)
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- Electrochemical oxidation of 1-chloro(bromo)-2,2,6,6-tetramethylpiperidines
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Electrochemical oxidation of 1-haloamines of the 2,2,6,6- tetramethylpiperidine series results in the formation of relatively stable radical-cations, detected by the methods of cyclic voltammetry and ESR spectroscopy. The final products of electrochemical oxidation of these haloamines are stable nitroxyl radicals. Pleiades Publishing, Ltd., 2011.
- Kagan,Yanilkin,Nastapova,Morozov,Zhukova,Kashparov,Kashparova
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experimental part
p. 2151 - 2156
(2012/03/12)
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- Potential phototoxicity of rosuvastatin mediated by its dihydrophenanthrene-like photoproduct
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In this work, rosuvastatin has been used to gain insight into the molecular basis of statin photosensitization. This lipid-lowering drug, also known as "superstatin", contains a 2-vinylbiphenyl-like moiety and has been previously described to decompose under solar irradiation, yielding stable dihydrophenanthrene analogues. During photophysical characterization of rosuvastatin, only a long-lived transient at ca. 550 nm was observed and assigned to the primary photocyclization intermediate. Thus, the absence of detectable triplet-triplet absorption and the low yield of fluorescence rules out the role of the parent drug as an efficient sensitizer. In this context, the attention has been placed on the rosuvastatin main photoproduct (ppRSV). Indeed, the photobehavior of this dihydrophenanthrene-like compound presents the essential components needed for an efficient biomolecule photosensitizer i.e. (i) a high intersystem crossing quantum yield (ΦISC = 0.8), (ii) a triplet excited state energy of ca. 67 kcal mol-1, and (iii) a quantum yield of singlet oxygen formation (ΦΔ) of 0.3. Furthermore, laser flash photolysis studies revealed a triplet-triplet energy transfer from the triplet excited state of ppRSV to thymidine, leading to the formation of cyclobutane thymidine dimers, an important type of DNA lesion. Finally, tryptophan has been used as a probe to investigate the type I and/or type II character of ppRSV-mediated oxidation. In this way, both an electron transfer process giving rise to the tryptophanyl radical and a singlet oxygen mediated oxidation were observed. On the basis of the obtained results, rosuvastatin, through its major photoproduct ppRSV, should be considered as a potential sensitizer.
- Nardi, Giacomo,Lhiaubet-Vallet, Virginie,Leandro-Garcia, Paula,Miranda, Miguel Angel
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experimental part
p. 1779 - 1785
(2012/05/04)
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- Tuning of the thermochemical and kinetic properties of ascorbate by its local environment: Solution chemistry and biochemical implications
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Ascorbate (vitamin C) is a ubiquitous biological cofactor. While its aqueous solution chemistry has long been studied, many in vivo reactions of ascorbate occur in enzyme active sites or at membrane interfaces, which have varying local environments. This report shows that the rate and driving force of oxidations of two ascorbate derivatives by the TEMPO radical (2,2′,6,6′-tetramethylpiperidin-1-oxyl) in acetonitrile are very sensitive to the presence of various additives. These reactions proceed by the transfer of a proton and an electron (a hydrogen atom), as is typical of biological ascorbate reactions. The measured rate and equilibrium constants vary substantially with added water or other polar solutes in acetonitrile solutions, indicating large shifts in the reducing power of ascorbate. The correlation of rate and equilibrium constants indicates that this effect has a thermochemical origin rather than being a purely kinetic effect. This contrasts with previous examples of solvent effects on hydrogen atom transfer reactions. Potential biological implications of this apparently unique effect are discussed.
- Warren, Jeffrey J.,Mayer, James M.
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supporting information; experimental part
p. 7784 - 7793
(2010/08/06)
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- Predicting organic hydrogen atom transfer rate constants using the Marcus cross relation
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Chemical reactions that involve net hydrogen atom transfer (HAT) are ubiquitous in chemistry and biology, from the action of antioxidants to industrial and metalloenzyme catalysis. This report develops and validates a procedure to predict rate constants for HAT reactions of oxyl radicals (RO ?) in various media. Our procedure uses the Marcus cross relation (CR) and includes adjustments for solvent hydrogen-bonding effects on both the kinetics and thermodynamics of the reactions. Kinetic solvent effects (KSEs) are included by using Ingold's model, and thermodynamic solvent effects are accounted for by using an empirical model developed by Abraham. These adjustments areshown to be critical to the success of our combined model, referred to as the CR/KSE model. As an initial test of the CR/KSE model we measured self-exchange and cross rate constants in different solvents for reactions of the 2,4,6-tri-tert-butylphenoxyl radical and the hydroxylamine 2,2′-6,6′-tetramethylpiperidin-1-ol. Excellent agreement is observed between the calculated and directly determined cross rate constants. We then extend the model to over 30 known HAT reactions of oxyl radicals with OH or CH bonds, including biologically relevant reactions of ascorbate, peroxyl radicals, and α-tocopherol. The CR/KSE model shows remarkable predictive power, predicting rate constants to within a factor of 5 for almost all of the surveyed HAT reactions.
- Warren, Jeffrey J.,Mayer, James M.
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scheme or table
p. 5282 - 5287
(2010/09/10)
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- TEMPO reacts with oxygen-centered radicals under acidic conditions
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In the presence of organic acids in organic media, 2,2,6,6- tetramethylpiperidine-N-oxyl (TEMPO) reacts with peroxyl radicals at nearly diffusion-controlled rates by proton-coupled electron transfer from the protonated nitroxide. The Royal Society of Chemistry 2010.
- Amorati, Riccardo,Pedulli, Gian Franco,Pratt, Derek A.,Valgimigli, Luca
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supporting information; experimental part
p. 5139 - 5141
(2010/10/21)
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- Silica- and polymer-supported platinum(II) polypyridyl complexes: Synthesis and application in photosensitized oxidation of alkenes
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Square-planar polypyridine platinum(II) complexes have been introduced into a silica/polymer matrix by covalent ligand modification. The photophysical properties of the supported matrices are well retained as their model complexes, and the quantum yields for singlet oxygen (1O2) generation are comparable with that of TPP (tetraphenylporphyrin) under similar conditions. A preliminary application in photosensitized oxidation indicates the silica/polymer-supported matrices are promising, which can be reused without loss of reactivity by a simple filtration. Moreover, the polymer-supported matrix exhibits excellent compatibility in various solvents. The Royal Society of Chemistry 2009.
- Feng, Ke,Peng, Ming-Li,Wang, Deng-Hui,Zhang, Li-Ping,Tung, Chen-Ho,Wu, Li-Zhu
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supporting information; scheme or table
p. 9794 - 9799
(2010/03/04)
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- Nitroxyl radical plus hydroxylamine pseudo self-exchange reactions: Tunneling in hydrogen atom transfer
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Bimolecular rate constants have been measured for reactions that involve hydrogen atom transfer (HAT) from hydroxylamines to nitroxyl radicals, using the stable radicals TEMPO· (2,2,6,6-tetramethylpiperidine-1-oxyl radical), 4-oxo-TEMPO·/
- Wu, Adam,Mader, Elizabeth A.,Datta, Ayan,Hrovat, David A.,Borden, Weston Thatcher,Mayer, James M.
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supporting information; scheme or table
p. 11985 - 11997
(2009/12/08)
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