Direct measurement of the rate of hydrogen-atom exchange between a phenol and its phenoxy radical

Arick, Martin R.; Weissman, S. I.

doi:10.1021/ja01008a047

Cited by 12 publications

(13 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…To our knowledge, there are no direct experiments reported so far for the title reaction, although several experimental studies are available for analogous reactions in solution. [42][43][44][45][46] For example, Arick and Weissman [44] studied the hydrogen transfer of the 2,4,6tri-tert-butylphenol complex with its phenoxyl radical in carbon tetrachloride solution. Their reported value of activation energy, 14 kcal/mol, is rather high compared to the present result for the phenoxyl/phenol couple in the gas phase (we recall that the barrier height in Figure 10 needs to be subtracted by the binding energy of about 10 kcal/mol in order to compare with the activation energy).…”

Section: Relevance With Experimental Resultsmentioning

confidence: 99%

Proton‐coupled electron transfer of the phenoxyl/phenol couple: Effect of Hartree‐Fock exchange on transition structures

2011

View full text Add to dashboard Cite

Proton-coupled electron transfer (PCET) and hydrogen atom transfer (HAT) reactions of the phenoxyl/phenol couple are studied theoretically by using wave function theory (WFT) as well as DFT methods. At the complete active space self-consistent field (CASSCF) level, geometry optimization is found to give two transition states (TSs); one is the PCET type with two benzene rings being nearly coplanar, and the other is the HAT type with two benzene rings taking a stacking structure. Geometry optimization at the (semilocal) DFT level, on the other hand, is found to give only one transition state (i.e., the PCET-type one) and fail to obtain the stacking TS structure. By comparing various levels of theories (including long-range corrected DFT functionals), we demonstrate that the Hartree-Fock exchange at long range plays a critical role in obtaining the sufficient stacking stabilization of the present open-shell system, and that the sole addition of empirical dispersion correction to semilocal DFT functionals may not be adequate for describing such a stacking interaction. Next, we investigate the solvent effect on the PCET and HAT TS thus obtained using the reference interaction site model self-consistent field (RISM-SCF) method. The results suggest that the free energy barrier increases with increasing polarity of the solvent, and that the solvent effects are stronger for the PCET TS than the stacking HAT TS pathway. The reason for this is discussed based on the dipole moment of different TS structures in solution.

show abstract

Section: Relevance With Experimental Resultsmentioning

confidence: 99%

Proton‐coupled electron transfer of the phenoxyl/phenol couple: Effect of Hartree‐Fock exchange on transition structures

2011

View full text Add to dashboard Cite

show abstract

“…An inner-sphere mechanism involving a ferric 2,4,6-tri-tert-butylphenoxide complex is unlikely, given that there are no structurally characterized octahedral metal complexes that include a 2,6-disubstituted tertbutylphenoxide ligand. 87,88 The observed kinetic isotope effect (KIE) of 2.0 ( 0.3 for the reduction of 2 by TTBP is large for a secondary isotope effect, particularly given that the KIE for TTBP proton self-exchange is 1.24, 89 and suggests that C-H bond cleavage may be involved in the RDS. In addition, the observation of large primary KIEs for cyclohexadiene, dihydroanthracene (DHAn), and toluene implicates C-H cleavage in the RDS for the reduction of 2 by hydrocarbons.…”

Section: Discussionmentioning

confidence: 99%

C−H Bond Activation by a Ferric Methoxide Complex: Modeling the Rate-Determining Step in the Mechanism of Lipoxygenase

2001

View full text Add to dashboard Cite

Lipoxygenases are mononuclear non-heme iron enzymes that regio- and stereospecifcally convert 1,4-pentadiene subunit-containing fatty acids into alkyl peroxides. The rate-determining step is generally accepted to be hydrogen atom abstraction from the pentadiene subunit of the substrate by an active ferric hydroxide species to give a ferrous water species and an organic radical. Reported here are the synthesis and characterization of a ferric model complex, [Fe(III)(PY5)(OMe)](OTf)(2), that reacts with organic substrates in a manner similar to the proposed enzymatic mechanism. The ligand PY5 (2,6-bis(bis(2-pyridyl)methoxymethane)pyridine) was developed to simulate the histidine-dominated coordination sphere of mammalian lipoxygenases. The overall monoanionic coordination provided by the endogenous ligands of lipoxygenase confers a strong Lewis acidic character to the active ferric site with an accordingly positive reduction potential. Incorporation of ferrous iron into PY5 and subsequent oxidation yields a stable ferric methoxide species that structurally and chemically resembles the proposed enzymatic ferric hydroxide species. Reactivity with a number of hydrocarbons possessing weak C-H bonds, including a derivative of the enzymatic substrate linoleic acid, scales best with the substrates' bond dissociation energies, rather than pK(a)'s, suggesting a hydrogen atom abstraction mechanism. Thermodynamic analysis of [Fe(III)(PY5)(OMe)](OTf)(2) and the ferrous end-product [Fe(II)(PY5)(MeOH)](OTf)(2) estimates the strength of the O-H bond in the metal bound methanol in the latter to be 83.5 +/- 2.0 kcal mol(-1). The attenuation of this bond relative to free methanol is largely due to the high reduction potential of the ferric site, suggesting that the analogously high reduction potential of the ferric site in LO is what allows the enzyme to perform its unique oxidation chemistry. Comparison of [Fe(III)(PY5)(OMe)](OTf)(2) to other coordination complexes capable of hydrogen atom abstraction shows that, although a strong correlation exists between the thermodynamic driving force of reaction and the rate of reaction, other factors appear to further modulate the reactivity.

show abstract

“…b Rate constant per hydrogen. c CCl 4 solvent; ref . d Reference 29. e k SE (3,6- t Bu 2 catechol/3,6- t Bu 2 orthosemiquinone radical) in CCl 4 solvent, ref .…”

Section: Discussionmentioning

confidence: 99%

Electron and Hydrogen-Atom Self-Exchange Reactions of Iron and Cobalt Coordination Complexes

et al. 2003

View full text Add to dashboard Cite

Reported here are self-exchange reactions between iron 2,2'-bi(tetrahydro)pyrimidine (H(2)bip) complexes and between cobalt 2,2'-biimidazoline (H(2)bim) complexes. The (1)H NMR resonances of [Fe(II)(H(2)bip)(3)](2+) are broadened upon addition of [Fe(III)(H(2)bip)(3)](3+), indicating that electron self-exchange occurs with k(Fe,e)(-) = (1.1 +/- 0.2) x 10(5) M(-1) s(-1) at 298 K in CD(3)CN. Similar studies of [Fe(II)(H(2)bip)(3)](2+) plus [Fe(III)(Hbip)(H(2)bip)(2)](2+) indicate that hydrogen-atom self-exchange (proton-coupled electron transfer) occurs with k(Fe,H.) = (1.1 +/- 0.2) x 10(4) M(-1) s(-1) under the same conditions. Both self-exchange reactions are faster at lower temperatures, showing small negative enthalpies of activation: DeltaH++(e(-)) = -2.1 +/- 0.5 kcal mol(-1) (288-320 K) and DeltaH++(H.) = -1.5 +/- 0.5 kcal mol(-1) (260-300 K). This behavior is concluded to be due to the faster reaction of the low-spin states of the iron complexes, which are depopulated as the temperature is raised. Below about 290 K, rate constants for electron self-exchange show the more normal decrease with temperature. There is a modest kinetic isotope effect on H-atom self-exchange of 1.6 +/- 0.5 at 298 K that is close to that seen previously for the fully high-spin iron biimidazoline complexes.(12) The difference in the measured activation parameters, E(a)(D) - E(a)(H), is -1.2 +/- 0.8 kcal mol(-1), appears to be inconsistent with a semiclassical view of the isotope effect, and suggests extensive tunneling. Reactions of [Co(H(2)bim)(3)](2+)-d(24) with [Co(H(2)bim)(3)](3+) or [Co(Hbim)(H(2)bim)(2)](2+) occur with scrambling of ligands indicating inner-sphere processes. The self-exchange rate constant for outer-sphere electron transfer between [Co(H(2)bim)(3)](2+) and [Co(H(2)bim)(3)](3+) is estimated to be 10(-)(6) M(-1) s(-1) by application of the Marcus cross relation. Similar application of the cross relation to H-atom transfer reactions indicates that self-exchange between [Co(H(2)bim)(3)](2+) and [Co(Hbim)(H(2)bim)(2)](2+) is also slow, < or =10(-3) M(-1) s(-1). The slow self-exchange rates for the cobalt complexes are apparently due to their interconverting high-spin [Co(II)(H(2)bim)(3)](2+) with low-spin Co(III) derivatives.

show abstract

Direct measurement of the rate of hydrogen-atom exchange between a phenol and its phenoxy radical

Cited by 12 publications

References 0 publications

Proton‐coupled electron transfer of the phenoxyl/phenol couple: Effect of Hartree‐Fock exchange on transition structures

Proton‐coupled electron transfer of the phenoxyl/phenol couple: Effect of Hartree‐Fock exchange on transition structures

C−H Bond Activation by a Ferric Methoxide Complex: Modeling the Rate-Determining Step in the Mechanism of Lipoxygenase

Electron and Hydrogen-Atom Self-Exchange Reactions of Iron and Cobalt Coordination Complexes

Contact Info

Product

Resources

About