Sterically hindered free radicals. Part 20. EPR and ENDOR spectroscopy of α-carbonyl radicals

Kubiak, B.; Lehnig, M.; Neumann, Wilhelm P.; Pentling, U.; Zarkadis, Antonios K.

doi:10.1039/p29920001443

Cited by 14 publications

(5 citation statements)

References 27 publications

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“…4B, gray, see Figs. S4, S11, S12 for details), which is in consistent with previously reported data for α-carbonyl radicals (35,36).…”

Section: Radical Addition and Subsequent Set Reductionsupporting

confidence: 91%

Watching a Full Photocatalytic Cycle by Electron Paramagnetic Resonance

Qi,

Suo,

Liu

et al. 2023

Preprint

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Molecular photocatalysis has shown tremendous success in sustainable energy and chemical synthesis. However, visualizing the intricate mechanisms in photocatalysis is a significant and long-standing challenge. By employing our recently developed sensitivity-enhanced time-resolved electron paramagnetic resonance technique, we directly observed all radicals and radical ions involved in the photocatalytic addition of a tertiary amine to tert-butyl acrylate. The full picture of the photocatalytic cycle has been vividly illustrated by the fine structures, chemical kinetics, and dynamic spin polarization of all open-shell intermediates directly observed in this prototypical system. Given the universality of this methodology, we believe it greatly empowers the research paradigm of direct observation in both photocatalysis and radical chemistry.

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“…4B, gray, see Figs. S4, S11, S12 for details), which is in consistent with previously reported data for α-carbonyl radicals (35,36).…”

Section: Radical Addition and Subsequent Set Reductionsupporting

confidence: 91%

Watching a Full Photocatalytic Cycle by Electron Paramagnetic Resonance

Qi,

Suo,

Liu

et al. 2023

Preprint

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“…The acquired spectrum confirmed the formation of the identical radical intermediate assigned as radical adduct R3• (Figure B, green spectrum). This radical exhibits degenerated hyperfine splitting from α- and β-C–H (3 a H = −1.99 mT), as revealed by the simulated steady-state EPR spectrum (Figure B, gray; see Figures S4, S11, S12 for details), which is inconsistent with previously reported data for α-carbonyl radicals. , …”

contrasting

confidence: 88%

“…This radical exhibits degenerated hyperfine splitting from αand β-C−H (3a H = −1.99 mT), as revealed by the simulated steady-state EPR spectrum (Figure 4B, gray; see Figures S4, S11, S12 for details), which is inconsistent with previously reported data for α-carbonyl radicals. 24,25 By monitoring the designated peak corresponding to radical adduct R3• in the PMP/DMAP/1/PC1 reaction system with varying concentrations of acceptor 1, we observed concentration-dependent changes in the kinetic profile of this radical intermediate (Figure 4C). Nonlinear fitting of the acquired kinetic data allowed us to obtain the second-order rate constant of 8.2 × 10 5 M −1 s −1 for the radical addition (Figure 4C, inset; see Figure S14 and Table S3 for details), which is consistent with the magnitudes of the reported rate constants for the addition of α-aminoalkyl radicals to methyl acrylate.…”

mentioning

confidence: 99%

Direct Observation of All Open-Shell Intermediates in a Photocatalytic Cycle

Qi,

Suo,

Liu

et al. 2024

J. Am. Chem. Soc.

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Molecular photocatalysis has shown tremendous success in sustainable energy and chemical synthesis. However, visualizing the transient open-shell intermediates in photocatalysis is a significant and long-standing challenge. By employing our recently developed innovative time-resolved electron paramagnetic resonance technique, we directly observed all radicals and radical ions involved in the photocatalytic addition of pempidine to tert−butyl acrylate. The full picture of the photocatalytic cycle is vividly illustrated by the fine structures, chemical kinetics, and dynamic spin polarization of all open-shell intermediates directly observed in this prototypical system. Given the universality of this methodology, we believe it greatly empowers the research paradigm of direct observation in both photocatalysis and radical chemistry.

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“…A quantitative analysis of the obtained kinetic data by means of linear free energy relationships was performed for the derivatives with meta and para substituents. The ortho-substituted benzofuranones 14 − 16 were omitted because of possible steric hindrance effects . Figure shows the plot of the logarithmic quenching rate constants versus the Hammett substituent constants σ , according to the Hammett equation (log k q = log k 0 + ρσ ), which yielded a satisfactory linear relation ( r = 0.886, n = 13).…”

Section: Resultsmentioning

confidence: 95%

Reaction of Singlet-Excited 2,3-Diazabicyclo[2.2.2]oct-2-ene and tert-Butoxyl Radicals with Aryl-Substituted Benzofuranones

Lundgren

Koner²,

Tinkl³

et al. 2006

J. Org. Chem.

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5,7-Di-tert-butyl-3-aryl-3H-benzofuran-2-ones are lactones with potential antioxidant activity, owing to their abstractable benzylic C-H hydrogens. The fluorescence quenching of the azoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO), an established probe for the hydrogen-donor propensity of chain-breaking antioxidants, was investigated for 16 aryl-substituted benzofuranone derivatives [m,m-(CF3)2, p-CN, m-CN, p-CF3, p-COOCH3, m-CF3, p-Cl, p-F, H, m-CH3, p-CH3, m,p-(CH3)2, p-OCH3, o-CH3, o-CF3, o,m-(CH3)2]. Analysis of the rate data in terms of a linear free energy relationship yielded a reaction constant of rho = +0.35. This implies that n,pi*-excited DBO acts as nucleophilic species. In contrast, hydrogen abstraction of tert-butoxyl radicals from the benzofuranones was accelerated by electron-donating substituents (rho = -0.23), in conformity with the electrophilic character of oxygen-centered alkoxyl radicals. Possible implications for the optimization of the hydrogen-donor propensity of antioxidants through structural variation are discussed.

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Sterically hindered free radicals. Part 20. EPR and ENDOR spectroscopy of α-carbonyl radicals

Cited by 14 publications

References 27 publications

Watching a Full Photocatalytic Cycle by Electron Paramagnetic Resonance

Watching a Full Photocatalytic Cycle by Electron Paramagnetic Resonance

Direct Observation of All Open-Shell Intermediates in a Photocatalytic Cycle

Reaction of Singlet-Excited 2,3-Diazabicyclo[2.2.2]oct-2-ene and tert-Butoxyl Radicals with Aryl-Substituted Benzofuranones

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