Structure and stability of acylnitroxyls

Jenkins, Terence C.; Perkins, M. J.; Siew, Nancy P. Y.

doi:10.1039/c39750000880

Cited by 23 publications

(4 citation statements)

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“…(NNR 4-7) undergo reversible changes in the same range of pH [8][9][10][11]. At the same time, the ESR spectra of 10-12, with a methoxy group at C-4, did not change over the pH range 1-14.…”

Section: Resultsmentioning

confidence: 98%

ESR and ¹H NMR studies of a new class of nitroxyl, nitronylnitroxyl and iminonitroxyl radicals

Khramtsov

Weiner

Gogolev

et al. 1986

Magnetic Reson in Chemistry

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A new synthesis of nitronylnitroxyl (NNR), iminonitroxyl (INR) and nitroxyl radicals (NR) is suggested, involving oxidation of 4H-imidazol di-N-oxides with 0, or PbO, in the presence of nucleophilic reagents. This method allowed the preparation of radicals which could not be synthesized by known procedures. These radicals were studied by ESR and ' H NMR spectroscopy. The hyperfine interaction constants of the NNR were measured and assigned (in some cases radicals containing 15N were used). The influence of protonation and deprotonation of functional groups (OH, NH,) in the NNR on their ESR spectra was investigated and the pK values of these radicals were measured by ESR.Differences between the ESR spectra of two isomeric INR were found, and the structures of these radicals were established by ' H NMR. Changes in spin density delocalization in 3-imidazoline 3-oxide nitroxyl radicals were studied, and the spin density was found to be 'pushed out' to the peripheral fragments of these radicals on increasing the number of methoxy groups in the a-position to the N-0' fragment. Hindered rotation around the a-bond of a bulky snbstituent in the a-position to the N-0' fragment was detected by ESR. The activation energy of this hindered rotation was AE = 30 f 1.7 kJ mo1-l.

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Section: Resultsmentioning

confidence: 98%

ESR and ¹H NMR studies of a new class of nitroxyl, nitronylnitroxyl and iminonitroxyl radicals

Khramtsov

Weiner

Gogolev

et al. 1986

Magnetic Reson in Chemistry

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“…One can notice that a hyperfine coupling constant, a N , of the PINO radical (4.23 G) is quite small, in comparison with that of other nitroxyl radicals. − The π-spin density, ρ N , at nitrogen can be calculated by eq 5: Table shows the reported hyperfine coupling constants and calculated spin density of selected nitroxyl radicals, rate constants of hydrogen abstraction reactions, and BDEs of the corresponding hydroxylamines.…”

Section: Resultsmentioning

confidence: 99%

Kinetic Study of the Phthalimide N-Oxyl (PINO) Radical in Acetic Acid. Hydrogen Abstraction from C−H Bonds and Evaluation of O−H Bond Dissociation Energy of N-Hydroxyphthalimide

2003

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The reactions of the phthalimide N-oxyl (PINO) radical with several hydrocarbons having different C−H bond dissociation energies (BDEs) were investigated in HOAc at 25 °C. The slope of the Evans−Polanyi plot is 0.38, which indicates that the reactions are mildly exothermic or almost thermoneutral. This finding is supported by the O−H BDE of N-hydroxyphthalimide (NHPI), 375 ± 10 kJ mol-1, obtained by means of a thermodynamic cycle. The observed kinetic isotope effects (KIEs) are related to the reaction free-energy changes, which can be explained by a Marcus-type equation. The comparison of the PINO radical reactions with analogous hydrogen atom abstraction reactions of the dibromide radical implies that HBr2• reactions are more exothermic than the PINO radical reactions. This factor is put forth to explain the different KIEs.

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“…For the different nitroxides, a strong influence of the N substituent on the BDE was observed, and delocalization of the free electron pair at nitrogen into the substituents leads to a higher O−H BDE. For TEMPO-H (A), a BDE of 69.6 kcal/mol 210 was reported, and hydroxyl amines derived from amidoxyl radicals 211,212 show higher BDEs. For example, a BDE of 77.2 kcal/mol was calculated for the N-benzoyl-N-tert-butylhydroxylamine (C2).…”

Section: Physical Properties and Reactivitymentioning

confidence: 99%

Organic Synthesis Using Nitroxides

Leifert

Studer

2023

Chem. Rev.

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Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R 1 R 2 N−O • . The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R 1 and R 2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R 1 R 2 N�O + ) or reduced to anions (R 1 R 2 N−O − ), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C−O bonds, allowing for the thermal generation of C radicals through reversible C−O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

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Structure and stability of acylnitroxyls

Cited by 23 publications

References 0 publications

ESR and ¹H NMR studies of a new class of nitroxyl, nitronylnitroxyl and iminonitroxyl radicals

ESR and ¹H NMR studies of a new class of nitroxyl, nitronylnitroxyl and iminonitroxyl radicals

Kinetic Study of the Phthalimide N-Oxyl (PINO) Radical in Acetic Acid. Hydrogen Abstraction from C−H Bonds and Evaluation of O−H Bond Dissociation Energy of N-Hydroxyphthalimide

Organic Synthesis Using Nitroxides

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Structure and stability of acylnitroxyls

Cited by 23 publications

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ESR and 1H NMR studies of a new class of nitroxyl, nitronylnitroxyl and iminonitroxyl radicals

ESR and 1H NMR studies of a new class of nitroxyl, nitronylnitroxyl and iminonitroxyl radicals

Kinetic Study of the Phthalimide N-Oxyl (PINO) Radical in Acetic Acid. Hydrogen Abstraction from C−H Bonds and Evaluation of O−H Bond Dissociation Energy of N-Hydroxyphthalimide

Organic Synthesis Using Nitroxides

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ESR and ¹H NMR studies of a new class of nitroxyl, nitronylnitroxyl and iminonitroxyl radicals

ESR and ¹H NMR studies of a new class of nitroxyl, nitronylnitroxyl and iminonitroxyl radicals