Hydrogen Atom Abstraction Reactions of Charged Polyaromatic σ-Radicals Related to the Active Intermediates of the Enediyne Antitumor Drugs

Petucci, Chris; Nyman, Marianne C.; Guler, Leo; Kenttämaa, Hilkka I.

doi:10.1021/ja012243c

Cited by 20 publications

(36 citation statements)

References 31 publications

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“…These findings are in agreement with previous gas-phase studies that indicated enhanced reactivity when the EA of the radical site increases. 19,21,33 For the 2-dehydropyridinium cation, the percentage yields of hydrogen atom abstraction were found to be unaffected by pH. This is because of formation of a second reaction intermediate, the 2-pyridyl cation, which complicates the experiment in solution.…”

Section: Resultsmentioning

confidence: 99%

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Direct Comparison of Solution and Gas-Phase Reactions of the Three Distonic Isomers of the Pyridine Radical Cation with Methanol

et al. 2012

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In order to directly compare the reactivity of positively charged carbon-centered aromatic σ-radicals toward methanol in solution and in the gas phase, the 2-, 3-, and 4-dehydropyridinium cations (distonic isomers of the pyridine radical cation) were generated by ultraviolet photolysis of the corresponding iodo-precursors in a mixture of water and methanol at varying pH. The reaction mixtures were analyzed by using liquid chromatography/mass spectrometry. Hydrogen atom abstraction was the only reaction observed for the 3- and 4-dehydropyridinium cations (and –pyridines) in solution. This also was the major reaction observed earlier in the gas phase. Depending on the pH, the hydrogen atom can be abstracted from different molecules (i.e., methanol or water) and from different sites (in methanol) by the 3- and 4-dehydropyridinium cations/-pyridines in solution. In the pH range of 1 – 4, the methyl group of methanol is the main hydrogen atom donor site for both 3- and 4-dehydropyridinium cations (just like in the gas phase). At higher pH, the hydroxyl groups of water and methanol also act as hydrogen atom donors. This finding is rationalized by a greater abundance of the unprotonated radicals that preferentially abstract hydrogen atoms from the polar hydroxyl groups. The percentage yield of hydrogen atom abstraction by these radicals was found to increase with lowering the pH in the pH range of 1.0-3.2. This pH effect is rationalized by polar effects: the lower the pH, the greater the fraction of protonated (more polar) radicals in the solution. This finding is consistent with previous results obtained in the gas phase and suggests that gas-phase studies can be used to predict solution reactivity, but only as long as the same reactive species is studied in both experiments. This was found not to be the case for the 2-iodopyridinium cation. Photolysis of this precursor in solution resulted in the formation of two major addition products, 2-hydroxy- and 2-methoxypyridinium cations, in addition to the hydrogen atom abstraction product. These addition products were not observed in the earlier gas-phase studies on 2-dehydropyridinium cation. Their observation in solution is explained by the formation of another reactive intermediate, the 2-pyridyl cation, upon photolysis of 2-iodopyridinium cation (and 2-iodopyridine). The same intermediate was observed in the gas phase but it was removed before examining the reactions of the desired radical, 2-dehydropyridinium cation (which cannot be done in solution).

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Section: Resultsmentioning

confidence: 99%

“…These radicals were selected since their reactivity has been already examined in the gas phase. 19,21,32,33 …”

Section: Introductionmentioning

confidence: 99%

Direct Comparison of Solution and Gas-Phase Reactions of the Three Distonic Isomers of the Pyridine Radical Cation with Methanol

et al. 2012

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“…Hydrogen atom abstraction reactions were the sole reactions observed when distonic ions 17 - 23 were allowed to react with tetrahydrofuran and 2-methyltetrahydrofuran, while cyano radical and hydrogen cyanide abstraction reactions were observed with tert -butyl isocyanide. 148 The trends observed for the reactivities of these radicals were found not to arise from differences in reaction enthalpy, size of the radical, or the position of the radical site in the aromatic ring system. Analogous to the results discussed above, the reactivity of these radicals directly correlates with the calculated vertical electron affinities (EA v s) of their radical sites.…”

Section: Properties and Reactivitymentioning

confidence: 88%

“…67,68,148 In general, the distonic ions were generated in a dual-cell FT-ICR by subjecting appropriate protonated precursors to sustained off-resonance collision-activated dissociation (SORI-CAD) to cleave either a weak C-I or C-NO 2 bond. Hydrogen atom abstraction reactions were the sole reactions observed when distonic ions 17 - 23 were allowed to react with tetrahydrofuran and 2-methyltetrahydrofuran, while cyano radical and hydrogen cyanide abstraction reactions were observed with tert -butyl isocyanide.…”

Section: Properties and Reactivitymentioning

confidence: 99%

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Properties and Reactivity of Gaseous Distonic Radical Ions with Aryl Radical Sites

et al. 2013

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Reactivity of the 3,4,5‐Tridehydropyridinium Cation—An Aromatic σ,σ,σ‐Triradical

et al. 2008

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Unterschiedliche Angriffspunkte: Studien zur Reaktivität des 3,4,5‐Tridehydropyridiniumkations mithilfe eines Fourier‐Transformations‐Ionencyclotronresonanz‐Massenspektrometers belegen, dass bei radikalischen Reaktionen zuerst über C3 Bindungen gebildet werden, bei nichtradikalischen Reaktionen über C3 oder C4 (siehe Schema). Das stärker Brønsted‐saure isomere Kation 2,4,6‐Tridehydropyridinium verhält sich anders, weil die meta‐Benz‐in‐Positionen weniger reaktiv sind.

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Hydrogen Atom Abstraction Reactions of Charged Polyaromatic σ-Radicals Related to the Active Intermediates of the Enediyne Antitumor Drugs

Cited by 20 publications

References 31 publications

Direct Comparison of Solution and Gas-Phase Reactions of the Three Distonic Isomers of the Pyridine Radical Cation with Methanol

Direct Comparison of Solution and Gas-Phase Reactions of the Three Distonic Isomers of the Pyridine Radical Cation with Methanol

Properties and Reactivity of Gaseous Distonic Radical Ions with Aryl Radical Sites

Reactivity of the 3,4,5‐Tridehydropyridinium Cation—An Aromatic σ,σ,σ‐Triradical

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