- Mechanistic Dichotomy in Proton-Coupled Electron-Transfer Reactions of Phenols with a Copper Superoxide Complex
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The kinetics and mechanism(s) of the reactions of [K(Krypt)][LCuO2] (Krypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, L = a bis(arylcarboxamido)pyridine ligand) with 2,2,6,6-tetramethylpiperdine-N-hydroxide (TEMPOH) and the para-substituted phenols XArOH (X = para substituent NO2, CF3, Cl, H, Me, tBu, OMe, or NMe2) at low temperatures were studied. The reaction with TEMPOH occurs rapidly (k = 35.4 ± 0.3 M-1 s-1) by second-order kinetics to yield TEMPOa€￠ and [LCuOOH]a on the basis of electron paramagnetic resonance spectroscopy, the production of H2O2 upon treatment with protic acid, and independent preparation from reaction of [NBu4][LCuOH] with H2O2 (Keq = 0.022 ± 0.007 for the reverse reaction). The reactions with XArOH also follow second-order kinetics, and analysis of the variation of the k values as a function of phenol properties (Hammett σ parameter, O-H bond dissociation free energy, pKa, E1/2) revealed a change in mechanism across the series, from proton transfer/electron transfer for X = NO2, CF3, Cl to concerted-proton/electron transfer (or hydrogen-atom transfer) for X = OMe, NMe2 (data for X = H, Me, tBu are intermediate between the extremes). Thermodynamic analysis and comparisons to previous results for LCuOH, a different copper-oxygen intermediate with the same supporting ligand, and literature for other [CuO2]+ complexes reveal significant differences in proton-coupled electron-transfer mechanisms that have implications for understanding oxidation catalysis by copper-containing enzymes and abiological catalysts.
- Bailey, Wilson D.,Dhar, Debanjan,Cramblitt, Anna C.,Tolman, William B.
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- Interaction of Li+ with Phenoxide Ions in Acetonitrile
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The charge transfer bands of a betaine dye, 2,6-diphenyl-4-(2,4,6-triphenylpyridino)phenolate (1), and p-nitrophenoxide in solutions in acetonitrile are blue shifted in the presence of various concentrations of LiI.The shape of these bands changes with Li
- Binder, David A.,Kreevoy, Maurice M.
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- Lewis Acidity Scale of Diaryliodonium Ions toward Oxygen, Nitrogen, and Halogen Lewis Bases
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Equilibrium constants for the associations of 17 diaryliodonium salts Ar2I+X- with 11 different Lewis bases (halide ions, carboxylates, p-nitrophenolate, amines, and tris(p-anisyl)phosphine) have been investigated by titrations followed by photometric or conductometric methods as well as by isothermal titration calorimetry (ITC) in acetonitrile at 20 °C. The resulting set of equilibrium constants KI covers 6 orders of magnitude and can be expressed by the linear free-energy relationship lg KI = sI LAI + LBI, which characterizes iodonium ions by the Lewis acidity parameter LAI, as well as the iodonium-specific affinities of Lewis bases by the Lewis basicity parameter LBI and the susceptibility sI. Least squares minimization with the definition LAI = 0 for Ph2I+ and sI = 1.00 for the benzoate ion provides Lewis acidities LAI for 17 iodonium ions and Lewis basicities LBI and sI for 10 Lewis bases. The lack of a general correlation between the Lewis basicities LBI (with respect to Ar2I+) and LB (with respect to Ar2CH+) indicates that different factors control the thermodynamics of Lewis adduct formation for iodonium ions and carbenium ions. Analysis of temperature-dependent equilibrium measurements as well as ITC experiments reveal a large entropic contribution to the observed Gibbs reaction energies for the Lewis adduct formations from iodonium ions and Lewis bases originating from solvation effects. The kinetics of the benzoate transfer from the bis(4-dimethylamino)-substituted benzhydryl benzoate Ar2CH-OBz to the phenyl(perfluorophenyl)iodonium ion was found to follow a first-order rate law. The first-order rate constant kobs was not affected by the concentration of Ph(C6F5)I+ indicating that the benzoate release from Ar2CH-OBz proceeds via an unassisted SN1-type mechanism followed by interception of the released benzoate ions by Ph(C6F5)I+ ions.
- Legault, Claude Y.,Mayer, Robert J.,Mayr, Herbert,Ofial, Armin R.
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supporting information
(2020/03/13)
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- Ambident Reactivity of Phenolate Anions Revisited: A Quantitative Approach to Phenolate Reactivities
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Prompted by the observation that the regioselectivities of phenolate reactions (C versus O attack) are opposite to the predictions by the principle of hard and soft acids and bases, we performed a comprehensive experimental and computational investigation of phenolate reactivities. Rate and equilibrium constants for the reactions of various phenolate ions with benzhydrylium ions (Aryl2CH+) and structurally related quinone methides have been determined photometrically in polar aprotic solvents. Quantum chemical calculations at the SMD(MeCN)/M06-2X/6-31+G(d,p) level confirmed that O attack is generally favored under kinetically controlled conditions, whereas C attack is favored under thermodynamically controlled conditions. Exceptions are diffusion-limited reactions with strong electrophiles, which give mixtures of products arising from O and C attack, as well as reactions with metal alkoxides in nonpolar solvents, where oxygen attack is blocked by strong ion pairing. The Lewis basicity (LB) and nucleophilicity (N, sN) parameters of phenolates determined in this work can be used to predict whether their reactions with electrophiles are kinetically or thermodynamically controlled and whether the rates are activation- or diffusion-limited. Comparison of the measured rate constants for the reactions of phenolates with carbocations with the Gibbs energies for single-electron transfer manifests that these reactions proceed via polar mechanisms.
- Mayer, Robert J.,Breugst, Martin,Hampel, Nathalie,Ofial, Armin R.,Mayr, Herbert
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p. 8837 - 8858
(2019/07/08)
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- A colourimetric calix[4lpyrrole-4-nitrophenolate based anion sensor
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The intense yellow colour of the 4-nitrophenolate anion 2, in MeCN or CH2Cl2 solution, dissipates upon complex formation with meso-octamethylcalix[4lpyrrole 1; the complex may be used as a colourimetric sensor for halide anions, such
- Gale, Philip A.,Twyman, Lance J.,Handlin, Cristin I.,Sessler, Jonathan L.
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p. 1851 - 1852
(2007/10/03)
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- Solvent effects on reactions of hydroxide ion with phosphorus (V) esters. A quantitative treatment
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Second-order rate constants of reactions of HO- with phosphate, phosphinate and thiophosphinate esters, (PhO)2PO.OC6H4NO2-p, Ph2PO.OC6H4NO2-p, Ph2PO.SPh, Ph2PO.SC6H4NO2-p and Ph2PO.SEt, go through minima with decreasing water content of H2O-MeCN or H2O-t-BuOH. The rate decrease is due to stabilization of the non-ionic ester on addition of organic solvent to H2O. This inhibition is partially offset by stabilization of the anionic transition states and in the drier solvents partial desolvation of HO- increases rates.
- Bunton, Clifford A.,Gillitt, Nicholas D.,Kumar, Anurag
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p. 145 - 151
(2007/10/03)
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- Proximate Charge Effects. 6. Anion-Cation-Zwitterion Triplets in Solution
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The reaction of p-nitrophenol with tetra-n-butylammonium taurinate in 95.3 mol percent dioxane-water was found to give an anion-cation-zwitterion triplet which then dissociates to a zwitterion and to an ion pair.The equilibrium constants for the formation of this triplet, Kf, and for dissociation, Kd, were found to be Kf = 3.57 L1/2 mol-1/2 and Kd = 1.73 x 10-7 mol L-1.
- Haberfield, Paul,Cincotta, Joseph J.
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p. 4188 - 4192
(2007/10/02)
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- ACID BASE REACTIONS BETWEEN ACRIDINE ORANGE AND SUBSTITUTED PHENOLS IN BENZONITRILE. PART I. THERMODYNAMICS OF ASSOCIATION AND IONIC EQUILIBRIA.
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Proton transfer reactions of the five substituted phenols 3-Cl-4-nitrophenol CNP, 4-nitrophenol NP, 3-methyl-4-nitrophenol MNP, 4-Cl-3,5-dimethylphenol CMP, and 3,5-dimethylphenol DMP with acridine orange, and the formation of hydrogen bonded complexes AHA** minus of the anions A** minus of CNP, NP, and MNP with the respective phenols AH in benzonitrile solution were investigated by means of absorption spectroscopy. The formation enthalpies of the hydrogen bonded anions AHA** minus are much larger than DELTA H//1, so that DELTA H//2 is mainly determined by this association. On the other hand there is no significant difference between DELTA H//2 and DELTA H//3; thus the hydrogen bond formed between a phenol and a complexed anion AHA** minus is very weak. Substituent effects on the acidity of phenols are about twice as large in benzonitrile as in aqueous solution.
- Suettinger,Strohbusch
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p. 744 - 750
(2007/10/02)
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