Chemistry of the <i>t</i>-Butoxyl Radical:  Evidence that Most Hydrogen Abstractions from Carbon are Entropy-Controlled

Finn, M G; Friedline, Robert; Suleman, N. Kamrudin; Wohl, Christopher J.; Tanko, James M.

doi:10.1021/ja0493493

Cited by 162 publications

(180 citation statements)

References 67 publications

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“…3B). The sensitivity of the observed rates to the substrate C−H BDE for APO-II is similar to that of APO-I reacting with strong C−H bond substrates (29), suggesting that APO-II has a nearly symmetrical transition state for hydrogen atom transfer for these weaker C−H bonds (47).…”

Section: Kinetic Characterization Of Apo-ii Toward C−h and O−h Substrmentioning

confidence: 63%

Heme-thiolate ferryl of aromatic peroxygenase is basic and reactive

Wang

Ullrich

Hofrichter

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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A kinetic and spectroscopic characterization of the ferryl intermediate (APO-II) from APO, the heme-thiolate peroxygenase from Agrocybe aegerita, is described. APO-II was generated by reaction of the ferric enzyme with metachloroperoxybenzoic acid in the presence of nitroxyl radicals and detected with the use of rapidmixing stopped-flow UV-visible (UV-vis) spectroscopy. The nitroxyl radicals served as selective reductants of APO-I, reacting only slowly with APO-II. APO-II displayed a split Soret UV-vis spectrum (370 nm and 428 nm) characteristic of thiolate ligation. Rapid-mixing, pHjump spectrophotometry revealed a basic pK a of 10.0 for the Fe IV −O−H of APO-II, indicating that APO-II is protonated under typical turnover conditions. Kinetic characterization showed that APO-II is unusually reactive toward a panel of benzylic C−H and phenolic substrates, with second-order rate constants for C−H and O−H bond scission in the range of 10-10 7 M −1 ·s −1 . Our results demonstrate the important role of the axial cysteine ligand in increasing the proton affinity of the ferryl oxygen of APO intermediates, thus providing additional driving force for C−H and O−H bond scission.

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Section: Kinetic Characterization Of Apo-ii Toward C−h and O−h Substrmentioning

confidence: 63%

Heme-thiolate ferryl of aromatic peroxygenase is basic and reactive

Wang

Ullrich

Hofrichter

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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“…[21] Diamondoidyl radicals: The correlation of the relative stabilities of hydrocarbon radicals with the selectivities in CÀH substitution reactions is often crude because hydrogen-atom abstractions are mostly entropy-controlled. [17,35] We also computed all possible isomeric tertiary radicals derived from compounds 1-7; the B3LYP/6-31G* DH 298 values vary only in the range of 1 kcal mol…”

mentioning

confidence: 99%

Functionalized Nanodiamonds Part I. An Experimental Assessment of Diamantane and Computational Predictions for Higher Diamondoids

Fokin

Tkachenko

Gunchenko

et al. 2005

Chemistry A European J

123

150

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The structures, strain energies, and enthalpies of formation of diamantane 1, triamantane 2, isomeric tetramantanes 3-5, T(d)-pentamantane 6, and D(3d)-hexamantane 7, and the structures of their respective radicals, cations, as well as radical cations, were computed at the B3LYP/6-31G* level of theory. For the most symmetrical hydrocarbons, the relative strain (per carbon atom) decreases from the lower to the higher diamondoids. The relative stabilities of isomeric diamondoidyl radicals vary only within small limits, while the stabilities of the diamondoidyl cations increase with cage size and depend strongly on the geometric position of the charge. Positive charge located close to the geometrical center of the molecule is stabilized by 2-5 kcal mol(-1). In contrast, diamondoid radical cations preferentially form highly delocalized structures with elongated peripheral C-H bonds. The effective spin/charge delocalization lowers the ionization potentials of diamondoids significantly (down to 176.9 kcal mol(-1) for 7). The reactivity of 1 was extensively studied experimentally. Whereas reactions with carbon-centered radicals (Hal)(3)C(*) (Hal=halogen) lead to mixtures of all possible tertiary and secondary halodiamantanes, uncharged electrophiles (dimethyldioxirane, m-chloroperbenzoic acid, and CrO(2)Cl(2)) give much higher tertiary versus secondary selectivities. Medial bridgehead substitution dominates in the reactions with strong electrophiles (Br(2), 100 % HNO(3)), whereas with strong single-electron transfer (SET) acceptors (photoexcited 1,2,4,5-tetracyanobenzene) apical C(4)-H bridgehead substitution is preferred. For diamondoids that form well-defined radical cations (such as 1 and 4-7), exceptionally high selectivities are expected upon oxidation with outer-sphere SET reagents.

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“…Methyl trimethylacetate (6) and butyl isovalerate (9) were chosen to represent poly(tert-butyl acrylate) and poly(n-butyl acrylate), respectively. Hydrocarbons 2,2-dimethylbutane (7) and trans-3-hexene (8) Benzophenone (ca 4 mg) was dissolved in 6 mL of an approximately 90 mM solution of model compound (1)(2)(3)(4)(5)(6)(7)(8)(9) in CD 3 CN. The solution was transferred to a quartz tube capable of being inserted into a standard NMR tube, and inert gas was bubbled through the solution for 5 min to remove oxygen.…”

Section: Methodsmentioning

confidence: 99%

¹H NMR Study of Hydrogen Abstraction in Model Compound Mimics of Polymers

Lancaster

Smilowitz

Turro

et al. 2013

Photochem & Photobiology

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Small molecules representing common synthetic polymers were subjected to photochemically induced hydrogen abstraction by benzophenone. Reactions were monitored using 1 H NMR to query the factors that influence preferential abstraction of protons in unique chemical environments. Differences in bond dissociation energies do not fully explain the observed hydrogen abstraction preferences. To that end, we identify contributions to abstraction from prereactive complexation, radical stability, steric effects and charge transfer effects. Using representative small molecule model compounds in lieu of polymeric materials is a novel approach to understanding photochemical reaction in polymers; however, it cannot probe the contributions of macromolecular effects-e.g. polymer rigidity or side chain and backbone mobility-to preferential hydrogen abstraction.

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Chemistry of the t-Butoxyl Radical: Evidence that Most Hydrogen Abstractions from Carbon are Entropy-Controlled

Cited by 162 publications

References 67 publications

Heme-thiolate ferryl of aromatic peroxygenase is basic and reactive

Heme-thiolate ferryl of aromatic peroxygenase is basic and reactive

Functionalized Nanodiamonds Part I. An Experimental Assessment of Diamantane and Computational Predictions for Higher Diamondoids

¹H NMR Study of Hydrogen Abstraction in Model Compound Mimics of Polymers

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Chemistry of the t-Butoxyl Radical: Evidence that Most Hydrogen Abstractions from Carbon are Entropy-Controlled

Cited by 162 publications

References 67 publications

Heme-thiolate ferryl of aromatic peroxygenase is basic and reactive

Heme-thiolate ferryl of aromatic peroxygenase is basic and reactive

Functionalized Nanodiamonds Part I. An Experimental Assessment of Diamantane and Computational Predictions for Higher Diamondoids

1H NMR Study of Hydrogen Abstraction in Model Compound Mimics of Polymers

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Product

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¹H NMR Study of Hydrogen Abstraction in Model Compound Mimics of Polymers