Reactivity of tert‐butoxy radicals towards thiols, alkyl sulfides, and alkyl disulfides

Encinas, M. V.; Lissi, E. A.; Majmud, C.; Olea, Andrés F.

doi:10.1002/kin.550210404

Cited by 9 publications

(5 citation statements)

References 34 publications

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“…The rate constant for hydrogen abstraction from the thiol tautomer of 4-NHPT by t -BuO• was estimated to be 3.5 × 10 9 M -1 s -1 . This value is higher than that reported in the case of hexanethiol ( k Q = 6.5 × 10 7 M -1 s -1 ), which can be rationalized in terms of S−H bond strengths, known to be lower for aromatic thiols than for aliphatic thiols . The same reactivity pattern was also reported in the case of aromatic and aliphatic alcohols…”

Section: Resultscontrasting

confidence: 58%

“…To further confirm the hypothesis of homolytic S−H bond cleavage, 4-PyNOS• was generated independently using the known property of tert -butoxyl radicals ( t -BuO•) to abstract hydrogen atoms, , especially from thiols . Flash photolysis (λ exc = 266 nm) of di- tert -butyl peroxide in O 2 -saturated CH 2 Cl 2 (which produces t -BuO•) carried out in the presence of 4-NHPT gave rise to a secondary transient species, the absorption spectrum of which was found to be identical to that produced by direct excitation (λ exc = 308 nm) of the thiol tautomer of 4-NHPT in CH 2 Cl 2 , whereas the same experiment performed in O 2 -saturated DMSO, under conditions where the thione was the predominant tautomer, did not.…”

Section: Resultsmentioning

confidence: 98%

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Selective Photoexcitation of the Thione and Thiol Forms of N-Hydroxypyridine-4(1H)-Thione: A Tautomeric Heteroaromatic System

and

Redmond

1999

J. Am. Chem. Soc.

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The strong solvent-dependence of the tautomeric equilibrium exhibited by N-hydroxypyridine-4(1H)-thione (4-NHPT) and the distinct absorption properties of the thione and thiol tautomers offer the rare opportunity to investigate the photochemistry of each form independently. Laser flash photolysis studies have revealed that triplet state formation was the main photoprocess undergone by the thione tautomer of 4-NHPT in protic solvents (ΦT = 0.90, λexc = 355 nm), whereas pulsed excitation (λexc = 308 nm) of the thiol form in apolar media resulted in a less efficient intersystem crossing (ΦT = 0.19) accompanied by homolytic S−H bond cleavage (ΦS - H = 0.24). The latter process leads to the production of the N-oxy-4-pyridinethiyl radical (4-PyNOS•). In most organic solvents, however, the thione and thiol tautomers of 4-NHPT coexist. Under these conditions, the form absorbing the excitation light was observed to undergo the expected primary photochemistry followed by secondary processes involving reaction with the ground state of the nonexcited tautomer (i.e., electron transfer from the thione triplet state to the thiol form and addition of 4-PyNOS• to the carbon−sulfur double bond of the tautomeric thione). Contrary to the closely related N-hydroxypyridine-2(1H)-thione, 4-NHPT was not found to be a primary photochemical precursor of hydroxyl radicals (•OH) in organic solvents. The results obtained in this work are discussed in terms of structure/photoreactivity and with regard to the reported photobiological effects of N-hydroxypyridinethiones.

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Section: Resultscontrasting

confidence: 58%

Section: Resultsmentioning

confidence: 98%

Selective Photoexcitation of the Thione and Thiol Forms of N-Hydroxypyridine-4(1H)-Thione: A Tautomeric Heteroaromatic System

and

Redmond

1999

J. Am. Chem. Soc.

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“…Lissi and co-workers performed a product study of the reaction of dimethyl sulfide (DMS) with the tert -butoxyl radical ( t -BuO • ) in benzene solution, and showed that the reaction proceeds through hydrogen abstraction with k H = 3.5 × 10 6 M –1 s –1 at T = 37 °C , and accordingly, the k H value measured for the reaction of the former radical with DMS can be conveniently compared with the value measured for the reaction of CumO • with DMSO ( k H = 1.8 × 10 4 M –1 s –1 , at T = 25 °C).…”

Section: Discussionmentioning

confidence: 99%

Reactions of the Cumyloxyl and Benzyloxyl Radicals with Strong Hydrogen Bond Acceptors. Large Enhancements in Hydrogen Abstraction Reactivity Determined by Substrate/Radical Hydrogen Bonding

2012

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A kinetic study on hydrogen abstraction from strong hydrogen bond acceptors such as DMSO, HMPA, and tributylphosphine oxide (TBPO) by the cumyloxyl (CumO(•)) and benzyloxyl (BnO(•)) radicals was carried out in acetonitrile. The reactions with CumO(•) were described in terms of a direct hydrogen abstraction mechanism, in line with the kinetic deuterium isotope effects, k(H)/k(D), of 2.0 and 3.1 measured for reaction of this radical with DMSO/DMSO-d(6) and HMPA/HMPA-d(18). Very large increases in reactivity were observed on going from CumO(•) to BnO(•), as evidenced by k(H)(BnO(•))/k(H)(CumO(•)) ratios of 86, 4.8 × 10(3), and 1.6 × 10(4) for the reactions with HMPA, TBPO, and DMSO, respectively. The k(H)/k(D) of 0.91 and 1.0 measured for the reactions of BnO(•) with DMSO/DMSO-d(6) and HMPA/HMPA-d(18), together with the k(H)(BnO(•))/k(H)(CumO(•)) ratios, were explained on the basis of the formation of a hydrogen-bonded prereaction complex between the benzyloxyl α-C-H and the oxygen atom of the substrates followed by hydrogen abstraction. This is supported by theoretical calculations that show the formation of relatively strong prereaction complexes. These observations confirm that in alkoxyl radical reactions specific hydrogen bond interactions can dramatically influence the hydrogen abstraction reactivity, pointing toward the important role played by structural and electronic effects.

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“…Among the abstracting species, alkoxyl radicals have attracted considerable interest, and several aspects of their hydrogen atom abstraction reactivity have been studied in detail. These include the nature of the substrate and of the abstractable hydrogen atom (C–H − or X–H, where X = O, − N, , S, Si, Ge, or Sn), the role of the solvent, ,− the reaction selectivity, ,− and the possible competition with other reactive pathways. − , …”

Section: Introductionmentioning

confidence: 99%

Hydrogen Atom Abstraction Selectivity in the Reactions of Alkylamines with the Benzyloxyl and Cumyloxyl Radicals. The Importance of Structure and of Substrate Radical Hydrogen Bonding

2011

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A time-resolved kinetic study on the hydrogen abstraction reactions from a series of primary and secondary amines by the cumyloxyl (CumO(•)) and benzyloxyl (BnO(•)) radicals was carried out. The results were compared with those obtained previously for the corresponding reactions with tertiary amines. Very different hydrogen abstraction rate constants (k(H)) and intermolecular selectivities were observed for the reactions of the two radicals. With CumO(•), k(H) was observed to decrease on going from the tertiary to the secondary and primary amines. The lowest k(H) values were measured for the reactions with 2,2,6,6-tetramethylpiperidine (TMP) and tert-octylamine (TOA), substrates that can only undergo N-H abstraction. The opposite behavior was observed for the reactions of BnO(•), where the k(H) values increased in the order tertiary < secondary < primary. The k(H) values for the reactions of BnO(•) were in all cases significantly higher than those measured for the corresponding reactions of CumO(•), and no significant difference in reactivity was observed between structurally related substrates that could undergo exclusive α-C-H and N-H abstraction. This different behavior is evidenced by the k(H)(BnO(•))/k(H)(CumO(•)) ratios that range from 55-85 and 267-673 for secondary and primary alkylamines up to 1182 and 3388 for TMP and TOA. The reactions of CumO(•) were described in all cases as direct hydrogen atom abstractions. With BnO(•) the results were interpreted in terms of the rate-determining formation of a hydrogen-bonded prereaction complex between the radical α-C-H and the amine lone pair wherein hydrogen abstraction occurs. Steric effects and amine HBA ability play a major role, whereas the strength of the substrate α-C-H and N-H bonds involved appears to be relatively unimportant. The implications of these different mechanistic pictures are discussed.

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Reactivity of tert‐butoxy radicals towards thiols, alkyl sulfides, and alkyl disulfides

Cited by 9 publications

References 34 publications

Selective Photoexcitation of the Thione and Thiol Forms of N-Hydroxypyridine-4(1H)-Thione: A Tautomeric Heteroaromatic System

Selective Photoexcitation of the Thione and Thiol Forms of N-Hydroxypyridine-4(1H)-Thione: A Tautomeric Heteroaromatic System

Reactions of the Cumyloxyl and Benzyloxyl Radicals with Strong Hydrogen Bond Acceptors. Large Enhancements in Hydrogen Abstraction Reactivity Determined by Substrate/Radical Hydrogen Bonding

Hydrogen Atom Abstraction Selectivity in the Reactions of Alkylamines with the Benzyloxyl and Cumyloxyl Radicals. The Importance of Structure and of Substrate Radical Hydrogen Bonding

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