- One to Find Them All: A General Route to Ni(I)-Phenolate Species
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The past 20 years have seen an extensive implementation of nickel in homogeneous catalysis through the development of unique reactivity not easily achievable by using noble transition metals. Many catalytic cycles propose Ni(I) complexes as potential reac
- Bismuto, Alessandro,Finkelstein, Patrick,Jeschke, Gunnar,Müller, Patrick,Morandi, Bill,Trapp, Nils
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supporting information
p. 10642 - 10648
(2021/07/31)
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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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- A Reactive, Photogenerated High-Spin (S = 2) FeIV(O) Complex via O2Activation
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Addition of dioxygen at low temperature to the non-heme ferrous complex FeII(Me3TACN)((OSiPh2)2O) (1) in 2-MeTHF produces a peroxo-bridged diferric complex Fe2III(μ-O2)(Me3TACN)2((OSiPh2)2O)2 (2), which was characterized by UV-vis, resonance Raman, and va
- Albert, Therese,Bill, Eckhard,Dey, Aniruddha,Goldberg, David P.,Gordon, Jesse B.,Mo?nne-Loccoz, Pierre,Sabuncu, Sinan,Siegler, Maxime A.
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p. 21637 - 21647
(2022/01/03)
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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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- Concerted proton-electron transfer oxidation of phenols and hydrocarbons by a high-valent nickel complex
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The high-valent nickel(iii) complex Ni(pyalk)2+ (2) was prepared by oxidation of a nickel(ii) complex, Ni(pyalk)2 (1) (pyalk = 2-pyridyl-2-propanoate). 2 and derivatives were fully characterized by mass spectrometry and X-ray crystallography. Electron paramagnetic resonance spectroscopy and X-ray photoelectron spectroscopy confirm that the oxidation is metal-centered. 2 was found to react with a variety of phenolic and hydrocarbon substrates. A linear correlation between the measured rate constant and the substrate bond dissociation enthalpy (BDE) was found for both phenolic and hydrocarbon substrates. Large H/D kinetic isotope effects were also observed for both sets of substrates. These results suggest that 2 reacts through concerted proton-electron transfer (CPET). Analysis of measured thermodynamic parameters allows us to calculate a bond dissociation free energy (BDFE) of ~91 kcal mol-1 for the O-H bond of the bound pyalk ligand. These findings may shed light onto CPET steps in oxidative catalysis and have implications for ligand design in catalytic systems.
- Fisher, Katherine J.,Feuer, Margalit L.,Lant, Hannah M. C.,Mercado, Brandon Q.,Crabtree, Robert H.,Brudvig, Gary W.
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p. 1683 - 1690
(2020/02/25)
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- Functional models of nonheme diiron enzymes: Reactivity of the μ-oxo-μ-1,2-peroxo-diiron(iii) intermediate in electrophilic and nucleophilic reactions
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The reactivity of the previously reported peroxo-adduct [FeIII2(μ-O)(μ-1,2-O2)(IndH)2(solv)2]2+ (1) (IndH = 1,3-bis(2-pyridyl-imino)isoindoline) has been investigated in nucleophilic (e.g., deformylation of alkyl and aryl alkyl aldehydes) and electrophilic (e.g. oxidation of phenols) stoichiometric reactions as biomimics of ribonucleotide reductase (RNR-R2) and aldehyde deformylating oxygenase (ADO) enzymes. Based on detailed kinetic and mechanistic studies, we have found further evidence for the ambiphilic behaviour of the peroxo intermediates proposed for diferric oxidoreductase enzymes.
- Kripli, Balázs,Szávuly, Miklós,Csendes, Flóra Viktória,Kaizer, József
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supporting information
p. 1742 - 1746
(2020/02/20)
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- A designed second-sphere hydrogen-bond interaction that critically influences the O-O bond activation for heterolytic cleavage in ferric iron-porphyrin complexes
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Heme hydroperoxidases catalyze the oxidation of substrates by H2O2. The catalytic cycle involves the formation of a highly oxidizing species known as Compound I, resulting from the two-electron oxidation of the ferric heme in the active site of the resting enzyme. This high-valent intermediate is formed upon facile heterolysis of the O-O bond in the initial FeIII-OOH complex. Heterolysis is assisted by the histidine and arginine residues present in the heme distal cavity. This chemistry has not been successfully modeled in synthetic systems up to now. In this work, we have used a series of iron(iii) porphyrin complexes (FeIIIL2(Br), FeIIIL3(Br) and FeIIIMPh(Br)) with covalently attached pendent basic groups (pyridine and primary amine) mimicking the histidine and arginine residues in the distal-pocket of natural heme enzymes. The presence of pendent basic groups, capable of 2nd sphere hydrogen bonding interactions, leads to almost 1000-fold enhancement in the rate of Compound I formation from peracids relative to analogous complexes without these residues. The short-lived Compound I intermediate formed at cryogenic temperatures could be detected using UV-vis electronic absorption spectroscopy and also trapped to be unequivocally identified by 9 GHz EPR spectroscopy at 4 K. The broad (2000 G) and axial EPR spectrum of an exchange-coupled oxoferryl-porphyrin radical species, [FeIVO Por+] with geff⊥ = 3.80 and geff‖ = 1.99, was observed upon a reaction of the FeIIIL3(Br) porphyrin complex with m-CPBA. The characterization of the reactivity of the FeIII porphyrin complexes with a substrate in the presence of an oxidant like m-CPBA by UV-vis electronic absorption spectroscopy showed that they are capable of oxidizing two equivalents of inorganic and organic substrate(s) like ferrocene, 2,4,6-tritertiary butyl phenol and o-phenylenediamine. These oxidations are catalytic with a turnover number (TON) as high as 350. Density Functional Theory (DFT) calculations show that the mechanism of O-O bond activation by 2nd sphere hydrogen bonding interaction from these pendent basic groups, which are protonated by a peracid, involves polarization of the O-O σ-bond, leading to lowering of the O-O σ?-orbital allowing enhanced back bonding from the iron center. These results demonstrate how inclusion of 2nd sphere hydrogen bonding interaction can play a critical role in O-O bond heterolysis.
- Bhunia, Sarmistha,Dey, Abhishek,Dey, Somdatta Ghosh,Ivancich, Anabella,Rana, Atanu
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p. 2681 - 2695
(2020/03/23)
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- Hydrogen Atom Transfer Oxidation by a Gold-Hydroxide Complex
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AuIII-oxygen adducts have been implicated as intermediates in homogeneous and heterogeneous Au oxidation catalysis, but their reactivity is under-explored. Complex 1, ([AuIII(OH)(terpy)](ClO4)2, (terpy = 2,2′:6′,2-terpyridine), readily oxidized substrates bearing C-H and O-H bonds. Kinetic analysis revealed that the oxidation occurred through a hydrogen atom transfer (HAT) mechanism. Stable radicals were detected and quantified as products of almost quantitative HAT oxidation of alcohols by 1. Our findings highlight the possible role of AuIII-oxygen adducts in oxidation catalysis and the capability of late transition metal-oxygen adducts to perform proton coupled electron transfer.
- Lovisari, Marta,McDonald, Aidan R.
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supporting information
(2020/03/13)
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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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- Structure, Spectroscopy, and Reactivity of a Mononuclear Copper Hydroxide Complex in Three Molecular Oxidation States
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Structural, spectroscopic, and reactivity studies are presented for an electron transfer series of copper hydroxide complexes supported by a tridentate redox-active ligand. Single crystal X-ray crystallography shows that the mononuclear [CuOH]1+ core is stabilized via intramolecular H-bonds between the H-donors of the ligand and the hydroxide anion when the ligand is in its trianionic form. This complex undergoes two reversible oxidation processes that produce two metastable "high-valent"CuOH species, which can be generated by addition of stoichiometric amounts of 1e- oxidants. These CuOH species are characterized by an array of spectroscopic techniques including UV-vis absorption, electron paramagnetic resonance (EPR), and X-ray absorption spectroscopies (XAS), which together indicate that all redox couples are ligand-localized. The reactivity of the complexes in their higher oxidation states toward substrates with modest O-H bond dissociation energies (e.g., 4-substitued-2,6-di-tert-butylphenols) indicates that these complexes act as 2H+/2e- oxidants, differing from the 1H+/1e- reactivity of well-studied [CuOH]2+ systems.
- Garcia-Bosch, Isaac,Lancaster, Kyle M.,Macmillan, Samantha N.,Rajabimoghadam, Khashayar,Siegler, Maxime A.,Wu, Tong
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supporting information
p. 12265 - 12276
(2020/08/06)
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- Detection of reactive intermediates in manganese(III) porphyrin catalyzed oxidation reaction using 2,4,6-tri-tert-butylphenol as probe substrate
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The 5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)-porphine-Mn(III) chloride (F20TPPMn(III)Cl) catalyzed oxidation of 2,4,6-tri-tert-butylphenol (TTBP) by pentafluoroiodosylbenzene (C6F5IO) in dichloromethane (CH2
- Rani, Bhawna,Singh, Amit,Shrivastava, Rahul,Agarwala, Arunava
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- Stable Iron Porphyrin Intramolecularly Coordinated by Alcoholate Anion: Synthesis and Evaluation of Axial Ligand Effect of Alcoholate on Spectroscopy and Catalytic Activity
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We synthesized intramolecularly aliphatic alcoholate-coordinated iron porphyrins (1a, 1b) that retain their axial coordination in the presence of another ligand or oxidant. The electron-donative character of alcoholate was less than that of thiolate, and
- Shirakawa, Yoshinori,Yano, Yuuki,Niwa, Yuki,Inabe, Kanako,Umezawa, Naoki,Kato, Nobuki,Hisamatsu, Yosuke,Higuchi, Tsunehiko
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p. 4268 - 4274
(2019/03/29)
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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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- A cobalt(ii) iminoiodane complex and its scandium adduct: Mechanistic promiscuity in hydrogen atom abstraction reactions
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In addition to oxometal [Mn+O] and imidometal [Mn+NR] units, transient metal-iodosylarene [M(n-2)+-OIPh] and metal-iminoiodane [M(n-2)+-N(R)IPh] adducts are often invoked as a possible second oxidant responsible for the oxo and imido group transfer reactivity. Although a few metal-iodosylarene adducts have been recently isolated and/or spectroscopically characterized, metal-iminoiodane adducts have remained elusive. Herein, we provide UV-Vis, EPR, NMR, XAS and DFT evidence supporting the formation of a metal-iminoiodane complex 2 and its scandium adduct 2-Sc. 2 and 2-Sc are reactive toward substrates in the hydrogen-atom and nitrene transfer reactions, which confirm their potential as active oxidants in metal-catalyzed oxidative transformations. Oxidation of para-substituted 2,6-di-tert-butylphenols by 2 and 2-Sc can occur by both coupled and uncoupled proton and electron transfer mechanisms; the exact mechanism depends on the nature of the para substituent.
- Kundu, Subrata,Chernev, Petko,Engelmann, Xenia,Chung, Chan Siu,Dau, Holger,Bill, Eckhard,England, Jason,Nam, Wonwoo,Ray, Kallol
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supporting information
p. 14538 - 14543
(2016/09/28)
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- Reactivity of a Nickel(II) Bis(amidate) Complex with meta-Chloroperbenzoic Acid: Formation of a Potent Oxidizing Species
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Herein, we report the formation of a highly reactive nickel-oxygen species that has been trapped following reaction of a NiII precursor bearing a macrocyclic bis(amidate) ligand with meta-chloroperbenzoic acid (HmCPBA). This compound is only detectable at temperatures below 250 K and is much more reactive toward organic substrates (i.e., CH bonds, CC bonds, and sulfides) than previously reported well-defined nickel-oxygen species. Remarkably, this species is formed by heterolytic OO bond cleavage of a Ni-HmCPBA precursor, which is concluded from experimental and computational data. On the basis of spectroscopy and DFT calculations, this reactive species is proposed to be a NiIII-oxyl compound. A highly reactive nickel-oxygen species has been spectroscopically trapped after heterolytic OO bond cleavage during the reaction of a NiII precursor with meta-chloroperbenzoic acid (HmCPBA). This species is used to carry out the oxidation of different substrates, such as olefins, sulfides, and CH bonds (see scheme).
- Corona, Teresa,Pfaff, Florian F.,Acu?a-Parés, Ferran,Draksharapu, Apparao,Whiteoak, Christopher J.,Martin-Diaconescu, Vlad,Lloret-Fillol, Julio,Browne, Wesley R.,Ray, Kallol,Company, Anna
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supporting information
p. 15029 - 15038
(2015/10/20)
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- Conformational Change with Steric Interactions Affects the Inner Sphere Component of Concerted Proton-Electron Transfer in a Pyridyl-Appended Radical Cation System
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Proton-coupled electron transfer (PCET) model systems combine one-electron oxidants and bases to generate net hydrogen atom acceptors. We have generated two persistent pyridyl-appended radical cations: 10-(pyrid-2-yl)-10H-phenothiazinium (PPT?+) and 3-(pyrid-2-yl)-10-methyl-10H-phenothiazinium (MPTP?+). EPR spectra and corresponding calculations indicate phenothiazinium radical cations with minimal spin on the pyridine nitrogen. Addition of hindered phenols causes the radical cations to decay, and protonated products and the corresponding phenoxyl radicals to form. The ΔG° values for the formation of intermediates (determined through cyclic voltammetry and pKa measurements) rule out a stepwise mechanism, and kinetic isotope effects support concerted proton-electron transfer (CPET) as the mechanism. Calculations indicate that the reaction of PPT?+ + tBu3PhOH undergoes a significant conformational change with steric interactions on the diabatic surface while maintaining the hydrogen bond; in contrast, MPTP?+ + tBu3PhOH maintains its conformation throughout the reaction. This difference is reflected in both experiment and calculations with (Formula presented.) (Formula presented.). Experimental results with 2,6-di-tert-butyl-4-methoxyphenol are similar. Hence, despite the structural similarity between the compounds, differences in the inner sphere component for CPET affect the kinetics.
- Welker, Evan A.,Tiley, Brittney L.,Sasaran, Crina M.,Zuchero, Matthew A.,Tong, Wing-Sze,Vettleson, Melissa J.,Richards, Robert A.,Geruntho, Jonathan J.,Stoll, Stefan,Wolbach, Jeffrey P.,Rhile, Ian J.
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p. 8705 - 8712
(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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- 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)
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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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p. 9925 - 9937
(2014/08/05)
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- Nitric oxide reactivity of [2Fe-2S] clusters leading to H2S generation
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The crosstalk between two biologically important signaling molecules, nitric oxide (NO) and hydrogen sulfide (H2S), proceeds via elusive mechanism(s). Herein we report the formation of H2S by the action of NO on synthetic [2Fe-2S] clusters when the reaction environment is capable of providing a formal H? (e-/H+). Nitrosylation of (NEt4)2[Fe2S 2(SPh)4] (1) in the presence of PhSH or tBu3PhOH results in the formation of (NEt 4)[Fe(NO)2(SPh)2] (2) and H2S with the concomitant generation of PhSSPh or tBu3PhO ?. The amount of H2S generated is dependent on the electronic environment of the [2Fe-2S] cluster as well as the type of H ? donor. Employment of clusters with electron-donating groups or H? donors from thiols leads to a larger amount of H 2S evolution. The 1/NO reaction in the presence of PhSH exhibits biphasic decay kinetics with no deuterium kinetic isotope effect upon PhSD substitution. However, the rates of decay increase significantly with the use of 4-MeO-PhSH or 4-Me-PhSH in place of PhSH. These results provide the first chemical evidence to suggest that [Fe-S] clusters are likely to be a site for the crosstalk between NO and H2S in biology.
- Tran, Camly T.,Williard, Paul G.,Kim, Eunsuk
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supporting information
p. 11874 - 11877
(2014/12/10)
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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.
-
supporting information
p. 12221 - 12224
(2014/11/08)
-
- Incorporation of ring nitrogens into diphenylamine antioxidants: Striking a balance between reactivity and stability
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The incorporation of nitrogen atoms into the aryl rings of conventional diphenylamine antioxidants enables the preparation of readily accessible, air-stable analogues, several of which have temperature-independent radical-trapping activities up to 200-fold greater than those of typical commercial diphenylamines. Amazingly, the nitrogen atoms raise the oxidation potentials of the amines without greatly changing their radical-trapping (H-atom transfer) reactivity.
- Hanthorn, Jason J.,Valgimigli, Luca,Pratt, Derek A.
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supporting information; experimental part
p. 8306 - 8309
(2012/07/13)
-
- Selective hydroxylation of alkanes catalyzed by iron(IV)corrole
-
The complex meso-tris(pentafluorophenyl)corrolatoiron(IV)chloride [(F15TPC)FeCl] emerged as efficient catalyst in hydroxylating alkanes at room temperature. Cyclohexane and adamantane have been oxidized to the corresponding alcohols using m-chloroperbenzoic acid (m-CPBA) as terminal oxidant. Cyclohexane has been converted to cyclohexanol in 50% yield with 100% selectivity. Adamantane has also been hydroxylated up to 75% overall yield under identical reaction condition. Significantly high regioselectivity in adamantane oxidation has been observed. The reactive intermediates have been quantitatively trapped by 2,4,6-tri-t-butylphenol (TTBP). Kinetic analysis of the (F15TPC)FeCl-catalyzed oxidation of TTBP has found consistent with rapid reaction of organic substrate with an intermediate formed in the first and rate-determining step.
- Biswas, Achintesh Narayan,Das, Purak,Agarwala, Arunava,Bandyopadhyay, Debkumar,Bandyopadhyay, Pinaki
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scheme or table
p. 94 - 98
(2010/10/01)
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- Heme-copper-dioxygen complexes: Toward understanding ligand-environmental effects on the coordination geometry, electronic structure, and reactivity
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The nature of the ligand is an important aspect of controlling the structure and reactivity in coordination chemistry. In connection with our study of heme-copper-oxygen reactivity relevant to cytochrome c oxidase dioxygen-reduction chemistry, we compare the molecular and electronic structures of two high-spin heme-peroxo-copper [FeIIIO2 2-CuII]+ complexes containing N4 tetradentate (1) or N3 tridentate (2) copper ligands. Combining previously reported and new resonance Raman and EXAFS data coupled to density functional theory calculations, we report a geometric structure and more complete electronic description of the high-spin heme-peroxo-copper complexes 1 and 2, which establish μ-(O22-) side-on to the Fe III and end-on to CuII (μ-η2: η1) binding for the complex 1 but side-on/side-on (μ-η2:η2) μ-peroxo coordination for the complex 2. We also compare and summarize the differences and similarities of these two complexes in their reactivity toward CO, PPh3, acid, and phenols. The comparison of a new X-ray structure of μ-oxo complex 2a with the previously reported 1a X-ray structure, two thermal decomposition products respectively of 2 and 1, reveals a considerable difference in the Fe-O-Cu angle between the two μ-oxo complexes (∠Fe-O-Cu = 178.2° in 1a and ∠Fe-O-Cu = 149.5° in 2a). The reaction of 2 with 1 equiv of an exogenous nitrogen-donor axial base leads to the formation of a distinctive low-temperature-stable, low-spin heme-dioxygen-copper complex (2b), but under the same conditions, the addition of an axial base to 1 leads to the dissociation of the heme-peroxo-copper assembly and the release of O 2. 2b reacts with phenols performing H-atom (e- + H +) abstraction resulting in O-O bond cleavage and the formation of high-valent ferryl [FeIV=O] complex (2c). The nature of 2c was confirmed by a comparison of its spectroscopic features and reactivity with those of an independently prepared ferryl complex. The phenoxyl radical generated by the H-atom abstraction was either (1) directly detected by electron paramagnetic resonance spectroscopy using phenols that produce stable radicals or (2) indirectly detected by the coupling product of two phenoxyl radicals.
- Halime, Zakaria,Kieber-Emmons, Matthew T.,Qayyum, Munzarin F.,Mondal, Biplab,Gandhi, Thirumanavelan,Puiu, Simona C.,Chufan, Eduardo E.,Sarjeant, Amy A. N.,Hodgson, Keith O.,Hedman, Britt,Solomon, Edward I.,Karlin, Kenneth D.
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scheme or table
p. 3629 - 3645
(2010/06/18)
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- Organochalcogen substituents in phenolic antioxidants
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Little is known about the ED/EW character of organochalcogen substituents and their contribution to the O-H bond dissociation enthalpy (BDE) in phenolic compounds. A series of ortho- and para-(S,Se,Te)R-substituted phenols were prepared and investigated by EPR, IR, and computational methods. Substituents lowered the O-H BDE by >3 kcal/mol in the para position, while the ortho-effect was modest due to hydrogen bonding (~3 kcal/mol) to the O-H group.
- Amorati, Riccardo,Pedulli, Gian Franco,Valgimigli, Luca,Johansson, Henrik,Engman, Lars
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scheme or table
p. 2326 - 2329
(2010/07/20)
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- 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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scheme or table
p. 13435 - 13440
(2010/06/11)
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- Proton-coupled electron transfer of ruthenium(III)-Pterin complexes: A mechanistic insight
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Ruthenium(II) complexes having pterins of redox-active heteroaromatic coenzymes as ligands were demonstrated to perform multistep proton transfer (PT), electron transfer (ET), and proton-coupled electron transfer (PCET) processes. Thermodynamic parameters
- Miyazaki, Soushi,Kojima, Takahiko,Mayer, James M.,Fukuzumi, Shunichi
-
experimental part
p. 11615 - 11624
(2011/02/26)
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- Novel reactivity of side-on (disulfido)dicopper complexes supported by bi- and tridentate nitrogen donors: Impact of axial coordination
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(Figure Presented) What a difference a ligand makes: A comparison of the reactivity of the two complexes shown (Cu green, S yellow, N blue, O red, F pink, C gray) has revealed new redox processes previously unseen for copper-sulfur complexes. Furthermore, intriguing supporting-ligand effects on the properties of (disulfido)dicopper cores are uncovered.
- Bar-Nahum, Itsik,York, John T.,Young Jr., Victor G.,Tolman, William B.
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p. 533 - 536
(2008/09/21)
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- Electron transfer between protonated and unprotonated phenoxyl radicals
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(Chemical Equation Presented) The reaction of phenoxyl radicals with acids is investigated. 2,4,6-Tri-tert-butylphenoxyl radical (13), a persistent radical, deteriorates in MeOH/PhH in the presence of an acid yielding 4-methoxycyclohexa-2,5-dienone 18a and the parent phenol (14). The reaction is facilitated by a strong acid. Treatment of 2,6-di-tert-butyl-4-methylphenoxyl radical (2), a short-lived radical, generated by dissociation of its dimer, with an acid in MeOH provides 4-methoxycyclohexa-2,5-dienone 4 and the products from disproportionation of 2 including the parent phenol (3). A strong acid in a high concentration favors the formation of 4 while the yield of 3 is always kept high. Oxidation of the parent phenol (33) with PbO2 to generate transient 2,6-di-tert-butylphenoxyl radical (35) in AcOH/H2O containing an added acid provides eventually p-benzoquinone 39 and 4,4′-diphenoquinone 42, the product from dimerization of 35. A strong acid in a high concentration favors the formation of 39. These results suggest that a phenoxyl radical is protonated by an acid and electron transfer takes place from another phenoxyl radical to the protonated phenoxyl radical, thus generating the phenoxyl cation, which can add an oxygen nucleophile, and the phenol (eq 5). The electron transfer is a fast reaction.
- Omura, Kanji
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p. 858 - 867
(2008/09/19)
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- Extreme rate acceleration by axial thiolate coordination on the isomerization of endoperoxide catalyzed by iron porphyrin
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(Chemical Equation Presented) A coordinated effort: The isomerization mechanism of prostaglandin H2 (PGH2), which is catalytically isomerized to prostacyclin or thromboxane A2 by cytochrome P450s, was investigated using a hemethiolate complex and an endoperoxide. Isomerization of endoperoxides proceeded very rapidly with this complex, whereas imidazole- or chloride-ligated heme had slight or no catalytic activity (see scheme).
- Yamane, Takehiro,Makino, Kohei,Umezawa, Naoki,Kato, Nobuki,Higuchi, Tsunehiko
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supporting information; experimental part
p. 6438 - 6440
(2009/03/11)
-
- Hydrogen atom transfer reactions of imido manganese(V) corroie: One reaction with two mechanistic pathways
-
Hydrogen atom transfer (HAT) reactions of (tpfc)MnNTs have been investigated (tpfc = 5,10,-15-tris(pentafluorophenyl)corrole and Ts = p-toluenesulfonate). 9,10-Dihydroanthracene and 1,4-dihydrobenzene reduce (tpfc)MnNTs via HAT with second-order rate constants 0.16 ± 0.03 and 0.17 ± 0.01 M-1 s-1, respectively, at 22°C. The products are the respective arenes, TsNH2 and (tpfc)MnIII. Conversion of (tpfc)MnNTs to (tpfc)Mn by reaction with dihydroanthracene exhibits isosbestic behavior, and formation of 9,9′,10,10′- tetrahydrobianthracene is not observed, suggesting that the intermediate anthracene radical rebounds in a second fast step without accumulation of a MnIV intermediate. The imido complex (tpfc)-MnVNTs abstracts a hydrogen atom from phenols as well. For example, 2,6-di-tert-butyl phenol is oxidized to the corresponding phenoxyl radical with a second-order rate constant of 0.32 ± 0.02 M-1 s-1 at 22°C. The other products from imido manganese(V) are TsNH2 and the trivalent manganese corrole. Unlike reaction with dihydroarenes, when phenols are used isosbestic behavior is not observed, and formation of (tpfc)-Mn IV(NHTs) is confirmed by EPR spectroscopy. A Hammett plot for various p-substituted 2,6-di-tert-butyl phenols yields a V-shaped dependence on σ, with electron-donating substituents exhibiting the expected negative ρ while electron-withdrawing substituents fall above the linear fit (i.e., positive ρ). Similarly, a bond dissociation enthalpy (BDE) correlation places electron-withdrawing substituents above the well-defined negative slope found for the electron-donating substituents. Thus two mechanisms are established for HAT reactions in this system, namely, concerted proton - electron transfer and proton-gated electron transfer in which proton transfer is followed by electron transfer.
- Zdilla, Michael J.,Dexheimer, Jennifer L.,Abu-Omar, Mahdi M.
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p. 11505 - 11511
(2008/03/14)
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- Kinetic and thermodynamic parameters for the equilibrium reactions of phenols with the dpph. radical
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The kinetics and energetics of the reversible reaction of phenols with the dpph. radical have been studied; steric shielding of the divalent N by the o-NO2 in dpph. seems to be the main cause of the entropic barriers of this reaction. The Royal Society of Chemistry 2006.
- Foti, Mario C.,Daquino, Carmelo
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p. 3252 - 3254
(2008/10/09)
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- Alkyl substituted phenoxyl decay in a hydrogen transfer equilibrium
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The kinetics of radical decay in the equilibrium: 2,4.6-tri-tert- butylphenoxyl radical 1 + 2,6-di-tert-butyl-4-methylphenol 2 = 2.4,6-tri-tert-butylphenol 3 + 2,6-di-tert-butyl-4-methylphenoxyl radical 4 was studied at 298 and 273 K by means of EPR spectroscopy. At 298 K second order prevails, whereas at 273 K the best fit was order 3/2. The extinction of 4 takes place in two steps: dimerization followed by disproportionation of the dimer, but the stable radical 1 enters in crossed dimerization with 4 to yield nonradical products. The mechanism ensures a constant |4|/|1| ratio along the decay. 2004 Wiley Periodicals, Inc.
- Coronel, Marta E. J.
-
-
- Abnormal solvent effects on hydrogen atom abstractions. 1. The reactions of phenols with 2,2-diphenyl-1-picrylhydrazyl (dpph?) in alcohols
-
Rate constants, kArOH/dpph?,s, for hydrogen atom abstraction from 13 hindered and nonhindered phenols by the diphenylpicrylhydrazyl radical, dpph?, have been determined in n-heptane and a number of alcoholic and nonalcoholic, hydrogen-bond accepting solvents. Abnormally enhanced values of kArOH/dpph?,s have been observed in alcohols. It is proposed that this is due to partial ionization of the phenols and a very fast electron transfer from phenoxide anion to dpph?. The popular assessment of the antioxidant activities of phenols with dpph? in alcohol solvents will generally lead to an overestimation of their activities.
- Litwinienko, Grzegorz,Ingold
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p. 3433 - 3438
(2007/10/03)
-
- Comparison of electron spin relaxation times measured by Carr-Purcell-Meiboom-Gill and two-pulse spin-echo sequences
-
Electron spin relaxation times obtained by two-pulse spin-echo and Carr-Purcell-Meiboom-Gill (CPMG) experiments were compared for samples with: (i) low concentrations of nuclear spins, (ii) higher concentrations of nuclear spins and low concentrations of
- Harbridge, James R.,Eaton, Sandra S.,Eaton, Gareth R.
-
-
- A new method for the generation of stable phenoxyl radicals by the reaction of [(Me3Si)2N]2E (E = Ge, Sn) with sterically hindered phenols
-
An unusual reaction of diaminogermylene and diaminostannylene with sterically hindered phenols which leads to the formation of stable phenoxyl radicals in high concentrations was found. The reaction mechanism was proposed. Amides [(Me3Si)2N]2E (E = Ge, Sn) were investigated electrochemically.
- Orlov,Moiseeva,Butin,Egorov
-
p. 1961 - 1962
(2007/10/03)
-
- On the role of nitrogen monoxide (nitric oxide) in the nitration of a tyrosine derivative and model compounds
-
The nitration of tyrosine derivatives with nitrogen monoxide (nitric oxide) occurs only in the presence of dioxygen, and the hypothesized mechanism involves nitrogen dioxide (.NO2). For better understanding of the reaction mechanism, the nitration of model compounds - such as 1- and 2-naphthols and their corresponding 2- and 1-nitroso derivatives with nitrogen monoxide in the presence and in the absence of dioxygen was studied. The results described here show that tyrosine and naphthols do not undergo nitrosation when they react with .NO, and so nitrosation of tyrosine in biological systems is highly unlikely. In addition, the oxidation of nitrosonaphthols ? isonitrosonaphthols by nitric oxide and its derivatives to the corresponding nitro derivatives does not involve the oxoammonium ion, as reported previously. The mechanistic proposals are supported mainly by ESR investigation and electrochemical data. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.
- Giorgini, Elisabetta,Petrucci, Rita,Astolfi, Paola,Mason, Ronald P.,Greci, Lucedio
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p. 4011 - 4017
(2007/10/03)
-
- Nanosecond generation of tyrosyl radicals via laser-initiated decaging of oxalate-modified amino acids
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We describe a general method for the unimolecular photochemical generation of tyrosyl radicals from a diaryl oxalate ester platform on the nanosecond time scale. Symmetric and asymmetric tyrosine oxalate esters have been prepared in gram quantities. Direc
- Chang, Michelle C.Y.,Miller, Scott E.,Carpenter, Scott D.,Stubbe, JoAnne,Nocera, Daniel G.
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p. 6820 - 6822
(2007/10/03)
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- Reactivity of tris(trimethylsilyl)silane toward diarylaminyl radicals
-
Absolute rate constants for the reaction of tri-tert-butylphenoxyl radical (ArO·) with (TMS)3SiH were measured spectrophotometrically in the temperature range 321-383 K. Rate constants for the hydrogen abstraction from (TMS)3SiH by diarylaminyl radicals of type (4-X-C6H4)2N· were determined by using a method in which the corresponding amines catalyze the reaction of ArO· with (TMS)3SiH. At 364.2 K, rate constants·are in the range of 2-50 M-1 s-1 for X = H, CH3, CH3O, and Br, whereas the corresponding value for ArO· is 3 orders of magnitude lower. A common feature of these reactions is the low preexponential factor [log(A/M-1 s-1) of 4.4 and 5.2 for ArO· and Ph2N·, respectively], which reflects high steric demand in the transition state. A semiempirical approach based on intersecting parabolas suggests that the observed reactivity is mainly related to the enthalpy of the reaction and allowed to estimate activation energies for the reaction of (4-X-C6H4)2N· and ArO· radicals with a variety of silicon hydrides.
- Varlamov,Denisov,Chatgilialoglu
-
p. 6317 - 6322
(2007/10/03)
-
- Effect of Solvation on the Bond Dissociation Energies of Phenolic Antioxidants
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The effect of solvent on the bond dissociation energies (BDEs) of the oxygen-hydrogen bond in substituted phenolic antioxidants has been investigated by means of an EPR technique.On changing the solvent from benzene to tert-butanol the BDE's were found to increase by ca. 2.2 kcal/mol for phenols without ortho substituents, by ca. 1 kcal/mol for 2,6-dimethyl substituted phenols while in 2,6-di-tert-butyl phenols they seem to be substantially unaffected.This behaviour has been interpreted by admitting that the BDE increase observed in tert-butanol is essentially due to the solvation of the hydroxylic hydrogen which stabilises the phenol, leaving the energy of the phenoxyl radical unaltered.Thus, solvation effects are expected to be large with unhindered phenols and relatively unimportant in phenols containing bulky substituents in the proximity of the OH group.
- Pedrielli, Pamela,Pedulli, Gian Franco
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p. 509 - 512
(2007/10/03)
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- On the Structures of the Intermediates from Reversible Coupling between Hindered Phenoxy Radicals
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Hindered phenoxy radicals 1 and 2 are found to undergo reversible, C-C rather than C-O cross-coupling, and give bis(cyclohexadienone)s 14 and 17.These primary products are not isolable but are recovered as phenolic cyclohexadienones 15 and 18, respectively, after treatment with Et3N or as biphenols 16 and 12, respectively, after treatment with TFA.The other products obtained after treating the reaction mixture with Et3N or TFA are phenol 5 and 4,4'-diphenoquinone 13 alone.Dienones 14 and 17 are interconvertible with each other via dissociation into the parent radicals, and 14 appears to be thermodynamically more stable than 17.Phenoxy radical 1 and other, less hindered 2,6-dialkylphenoxy radicals 24 also form intermediates of reversible cross-coupling.Treatment with TFA of the mixtures containing the intermediates provides 2,4'-biphenols 25 preferentially.
- Omura, Kanji
-
p. 6901 - 6910
(2007/10/02)
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- Negative activation energies and compensation effects for the reactions of diarylanimyl radicals with phenols
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The temperature dependence of the rate constants of the reaction of 4,4'-disubstituted diphenylaminyl radicals (ArAr'N.) with phenols has been studied by a laser pulse photolysis technique.The linear relationships between activation energies E1 (negative in the majority of cases) and logarithms of pre-exponential factors logA1 have been established.These results coincide with the two-parametric correlations of logk1 with ?+ constants of substituents in phenol and aminyl radicals under isothermal conditions.The reaction rate constants decrease considerably when toluene is used instead of n-decane. - Key words: aminyl radicals, phenols, negative temperature coefficient, isoparametric correlations.
- Varlamov, V. T.,Denisov, N. N.,Nadtochenko, V. A.
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p. 2282 - 2286
(2007/10/03)
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- Bond Dissociation Enthalpy of α-Tocopherol and Other Phenolic Antioxidants
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The equilibrium constants, K1, for the reaction between galvinoxyl and a series of phenolic antioxidants have been determined by means of EPR spectroscopy.With aroxyl radicals decaying at appreciable rates, K1 was obtained by performing kinetic analyses of the time dependence of the concentrations of the equilibrating radicals after mixing the reactants.In two cases the temperature dependence of K1 was also studied and the entropy change for the equilibration reaction was determined.Bond dissociation enthalpies, DH, of the ArO-H bond of the examined phenols were calculated by comparison with the known value of 2,4,6-tri-tert-butylphenol (81.24 kcal mol-1).A larger than expected DH value was found for probucol (81.03 kcal mol-1) and an explanation of this behavior was given in terms of the preferred conformation adopted by the para alkylthio group.The DH value of α-tocopherol (78.93 kcal mol-1) was found to be very close to that of the phenolic precursor of galvinoxyl (78.80 kcal mol-1) and somewhat larger than that of 2,6-di-tert-butyl-4-methoxyphenyl (77.61 kcal mol-1).
- Lucarini, Marco,Pedulli, Gian Franco,Cipollone, Marta
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p. 5063 - 5070
(2007/10/02)
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- CONVENIENT UNIMOLECULAR SOURCES OF ARYLOXYL RADICALS.III - PHOTOLYSIS OF BIS(ARYLOXY)PHOSPHINE AZIDES
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Photolysis of sterically hindered bis(aryloxy)phosphine azides in solution, frozen matrix, or neat solid states leads to facile unimolecular production of aryloxyl radicals.
- Kalgutkar, Rajdeep,Ionkin, Alexey S.,Quin, Louis D.,Lahti, Paul M.
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p. 3889 - 3892
(2007/10/02)
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- Reactions of Nitric Oxide with Phenolic Antioxidants and Phenoxyl Radicals
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By EPR, NMR, and TLC methods it was possible to show that nitric oxide (.NO) reacts with five different methyl- or tert-butyl-substituted phenols including α-tocopherol to produce the phenoxyl radical which subsequently couples reversibly with excess .NO.
- Janzen, Edward G.,Wilcox, Allan L.,Manoharan, Vinothane
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p. 3597 - 3599
(2007/10/02)
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- Synthesis and Photolysis of N-(Phenoxycarbonyloxy)-2-thiopyridone Derivatives. A New Unimolecular Route to Quantitative Generation of Phenoxyl Radicals
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N-(Phenoxycarbonyloxy)-2-thiopyridone derivatives were prepared by the reaction of lithium phenoxides with phosgene followed by the reaction with 2-mercaptopyridine N-oxide and photolyzed to give quantitative yield of phenoxyl radicals.
- Togo, Yoshifumi,Nakamura, Nobuo,Iwamura, Hiizu
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p. 1201 - 1204
(2007/10/02)
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- Mechanism of Reactions of Hydrogen Peroxide and Hydroperoxides with Iron(III) Porphyrins. Effects of Hydroperoxide Structure on Kinetics
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The buffer-catalyzed and uncatalyzed reactions of various alkyl hydroperoxides and hydrogen peroxide with chelated protohemin chloride have been studied.The rates of the uncatalyzed reactions show dependence on structure as the catalyzed reactions.The rat
- Traylor, Teddy G.,Ciccone, Joseph P.
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p. 8413 - 8420
(2007/10/02)
-
- CONVENIENT UNIMOLECULAR SOURCES OF ARYLOXYL RADICALS I-ARYLOXYOXALYL CHLORIDES
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UV photolysis of readily synthesized aryloxyoxalyl chloride half-esters provides a new unimolecular source of aryloxyl radicals, as shown by ESR and UV-vis spectroscopy.
- Modarelli, David A.,Rossitto, Frank C.,Lahti, Paul M.
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p. 4473 - 4476
(2007/10/02)
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- KINETIC STUDY OF THE REACTION OF BENZOYL PEROXIDE WITH PHENOLIC COMPOUNDS. A TECHNIQUE FOR THE EVALUATION OF PHENOLIC O-H BOND DISSOCIATION ENERGIES
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From kinetic data concerning the reaction of benzoyl peroxide (POB) with several phenolic antioxidants (ArOH): POB + ArOH -> C6H5CO2. + C6H5CO2H + ArO., it is shown that differences in the phenolic O-H bond dissociation energies may be obtained.In the particular case of α-tocopherol (α-TH), a stabilization energy defined with respect to phenol: D(C6H5O-H) - D(α-T-H) = 10 kcal.mol-1 has been estimated in heptanol as solvent.It would correspond for α-TH to an O-H bond dissociation energy around 78 kcal.mol-1.
- Rousseau-Richard, Claire,Richard, Claude,Martin, Rene
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p. 2057 - 2066
(2007/10/02)
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- CONVENIENT UNIMOLECULAR SOURCES OF ARYLOXYL RADICALS II-ARYLOXYOXALYL tert-BUTYLPEROXIDES
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UV photolysis and mild thermolysis of aryloxyoxalyl tert-butylperoxides provides a new unimolecular source of aryloxyl radicals, as shown by ESR and UV-vis spectroscopy.
- Modarelli, David A.,Rossitto, Frank C.,Lahti, Paul M.
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p. 4477 - 4480
(2007/10/02)
-