Characteristics of the spin-trapping reaction of a free radical derived from AAPH: further development of the ORAC-ESR assay

Nakajima, Akira; Matsuda, Emiko; Masuda, Yukari; Sameshima, Hiroshi; Ikenoue, Tsuyomu

doi:10.1007/s00216-012-6021-8

“…The c 50 values of phenolic compounds with catechol group were CA [ CAT * EP [ RTN, which suggested that the functional group directly combined with the catechol group retarded strongly the elimination of alkoxyl radical by catechol group and the order of the retardation was -C=CH-CH-COOH \ chroman * -CH 2 CH-NH 2 -COOH. As extra free radical, such as hydroxyl radical which was observed in the optical-irradiation system [5], did not produce in the present system, valid c 50 values for phenolic compounds could be obtained. The ratio of maximum c 50 value (CA) to minimum (MAN) was 5500, the ratio of maximum ID 50 for superoxide radical (TRP) to minimum (CAT) was 9200, and the ratio of maximum ID 50 for hydroxyl radical (MAN) to minimum (TRX) was 150.…”

Section: Antioxidant Ability Of Selected Phenolic Compounds and Biosumentioning

confidence: 72%

“…The rate constant of alkoxyl radical formation (Eqs. 3 and 4) is about 10 5 M -1 s -1 [8], and the spin adduct, DMPO/OR should be fairly stable [5]. The spin adduct, DMPO/OR, was only observed in the present system.…”

Section: Evaluation Of Antioxidant Abilitymentioning

confidence: 91%

“…The hyperfine coupling constants (hfcc) due to a nitrogen and a hydrogen atom were, respectively, estimated to be a N = 1.42 and a H = 1.48 mT. On the basis of the hfcc, the major paramagnetic species was assigned as the alkoxyl radical (ÁOR) spin adduct of DMPO (defined as DMPO/OR) [5]. In addition, the concentration of the DMPO/OR radical was evaluated to be ca.…”

Section: Fi-esr Systemmentioning

confidence: 99%

“…To overcome these demerits, the ORAC-electron spin resonance (ESR) assay, which can evaluate the antioxidant ability as the competitive elimination reaction of the alkoxyl radical (ROÁ, R = C(CH 3 ) 2 -C( ? NH 2 Cl -)NH 2 ) derived from AAPH by antioxidants, was developed [4,5]. The optical irradiation in the process of the alkoxyl radical production from AAPH caused the formation of the hydroxyl radical through the electron transfer [5,6], which affected the kinetics study of phenolic compounds.…”

Section: Introductionmentioning

confidence: 99%

“…NH 2 Cl -)NH 2 ) derived from AAPH by antioxidants, was developed [4,5]. The optical irradiation in the process of the alkoxyl radical production from AAPH caused the formation of the hydroxyl radical through the electron transfer [5,6], which affected the kinetics study of phenolic compounds. The optical method for the alkoxyl radical formation from AAPH should not be adopted for the kinetic study of the radical.…”

Section: Introductionmentioning

confidence: 99%

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Quantitative Spin-trapping ESR Investigation of Alkoxyl Radical Derived from AAPH: Development of a Flow-injection Spin-trapping ESR System for the Oxygen Radical Absorbance Capacity Assay

Nakajima

¹

,

Yamaguchi

²

,

Yamashita

³

et al. 2015

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A flow-injection electron spin resonance (ESR) system was developed for the quantitative detection of an alkoxyl radical (ROÁ, R = C(CH 3 ) 2 -C( ? NH 2-Cl -)NH 2 ) derived from AAPH (2,2 0 -azobis(2,4-amidinopropane) dihydrochloride) by thermal decomposition. Optimal measurement conditions for the system were examined, and it was found that the control of the radical formation by quenching in an ice-water vessel should be effective to obtain the stable and accurate results. The system was applied for the estimation of the alkoxyl radical-elimination ability and the oxygen radical absorbance capacity values of selected biosubstances, such as Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), caffeic acid, 4-hydroxycinnamic acid, epinephrine, rutin, (?)-catechin, L-tryptophane, and Dmannitol. As the rate constant for the reaction between alkoxyl radical and substrate, k S , could not be unequivocally determined, the defined value, c 50 = k S /k 1 , (k 1 , the rate constant for the reaction between alkoxyl radical and spin trap) was used as a reaction parameter of substrate for alkoxyl radical. The flow-injection electron spin resonance system using alkoxyl radical elimination gave valid c 50 values for biosubstances, and the system should be a valuable method for the evaluation of the antioxidant ability.

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DMSO‐Enabled Selective Radical O−H Activation of 1,3(4)‐Diols

Zhu

¹

,

Zhang

²

,

Jin

³

et al. 2020

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Control of selectivity is one of the central topics in organic chemistry. Although unprecedented alkoxyl-radicalinduced transformations have drawn a lot of attention, compared to selective CÀH activation, selective radical OÀH activation remains less explored. Herein, we report a novel selective radical O À H activation strategy of diols by combining spatial effects with proton-coupled electron transfer (PCET). It was found that DMSO is an essential reagent that enables the regioselective transformation of diols. Mechanistic studies indicated the existence of the alkoxyl radical and the selective interaction between DMSO and hydroxyl groups. Moreover, the distal C À C cleavage was realized by this selective alkoxylradical-initiation protocol. The selective transformation of hydroxyl (OH) groups in diand polyols is a frequently encountered problem in organic synthesis, in contexts ranging from simple alcohol transformations to the preparation of highly functionalized natural products. [1] Despite the importance of this issue, strategies for the selective O À H activation are rare compared to the fastgrowing studies on selective CÀH activation. [2] The classic methods for selective activation of one hydroxyl in diols involve ionic transformations by steric regulation or based on special hydrocarbon structure. [3] Unlike traditional ionic alcohol transformations involving nucleophilic substitution/ addition, [4] oxidation [5] and elimination [6] of hydroxyls, the alkoxyl radical induced transformations have drawn more attention with the discovery of unprecedented strategies for the generation of alkoxy radicals from alcohol without the need for pre-activation. [7] The OÀH radical processes greatly enlarged the reaction types of alcohols either with alkoxyl radical induced b scission [8] or hydrogen abstraction. [9, 10] However, to the best of our knowledge, the general methods for the alkoxyl radical induced distal CÀC cleavage remains unexplored (Scheme 1 a). Moreover, the realized OÀH radical transformations are limited to the activation of mono-ols, so far, there is a lack of selective radical O À H activation due to the almost identical bond-dissociation energy between two hydroxyls in diols (% 105 kcal mol À1) [11] (Scheme 1 b). To address this problem, we paid our attention to the strategy of proton coupled electron transfer (PCET), which was reported that a Brønsted base and an oxidant can synergistically remove a proton and an electron from the substrate to afford a free radical (Scheme 1 c). [12] Recently, Knowles and co-workers significantly achieved the O À H bond homolysis through PCET. [13] Inspired by these reports, we hypothesized that if we could find a proper Brønsted or Lewis base that can selectively form a hydrogen bond with one hydroxyl in diols, we would have a chance to selectively activate one O À H bond and realize the selective alkoxyl radical transformation of diols (Scheme 1 d). Scheme 1. Selective hydroxyl activation of diols.

show abstract

DMSO‐Enabled Selective Radical O−H Activation of 1,3(4)‐Diols

Zhu

¹

,

Zhang

²

,

Jin

³

et al. 2020

View full text Add to dashboard Cite

Control of selectivity is one of the central topics in organic chemistry. Although unprecedented alkoxyl-radicalinduced transformations have drawn a lot of attention, compared to selective CÀH activation, selective radical OÀH activation remains less explored. Herein, we report a novel selective radical O À H activation strategy of diols by combining spatial effects with proton-coupled electron transfer (PCET). It was found that DMSO is an essential reagent that enables the regioselective transformation of diols. Mechanistic studies indicated the existence of the alkoxyl radical and the selective interaction between DMSO and hydroxyl groups. Moreover, the distal C À C cleavage was realized by this selective alkoxylradical-initiation protocol. The selective transformation of hydroxyl (OH) groups in diand polyols is a frequently encountered problem in organic synthesis, in contexts ranging from simple alcohol transformations to the preparation of highly functionalized natural products. [1] Despite the importance of this issue, strategies for the selective O À H activation are rare compared to the fastgrowing studies on selective CÀH activation. [2] The classic methods for selective activation of one hydroxyl in diols involve ionic transformations by steric regulation or based on special hydrocarbon structure. [3] Unlike traditional ionic alcohol transformations involving nucleophilic substitution/ addition, [4] oxidation [5] and elimination [6] of hydroxyls, the alkoxyl radical induced transformations have drawn more attention with the discovery of unprecedented strategies for the generation of alkoxy radicals from alcohol without the need for pre-activation. [7] The OÀH radical processes greatly enlarged the reaction types of alcohols either with alkoxyl radical induced b scission [8] or hydrogen abstraction. [9, 10] However, to the best of our knowledge, the general methods for the alkoxyl radical induced distal CÀC cleavage remains unexplored (Scheme 1 a). Moreover, the realized OÀH radical transformations are limited to the activation of mono-ols, so far, there is a lack of selective radical O À H activation due to the almost identical bond-dissociation energy between two hydroxyls in diols (% 105 kcal mol À1) [11] (Scheme 1 b). To address this problem, we paid our attention to the strategy of proton coupled electron transfer (PCET), which was reported that a Brønsted base and an oxidant can synergistically remove a proton and an electron from the substrate to afford a free radical (Scheme 1 c). [12] Recently, Knowles and co-workers significantly achieved the O À H bond homolysis through PCET. [13] Inspired by these reports, we hypothesized that if we could find a proper Brønsted or Lewis base that can selectively form a hydrogen bond with one hydroxyl in diols, we would have a chance to selectively activate one O À H bond and realize the selective alkoxyl radical transformation of diols (Scheme 1 d). Scheme 1. Selective hydroxyl activation of diols.

show abstract

Characteristics of the spin-trapping reaction of a free radical derived from AAPH: further development of the ORAC-ESR assay

Cited by 23 publications

References 45 publications

Quantitative Spin-trapping ESR Investigation of Alkoxyl Radical Derived from AAPH: Development of a Flow-injection Spin-trapping ESR System for the Oxygen Radical Absorbance Capacity Assay

Quantitative Spin-trapping ESR Investigation of Alkoxyl Radical Derived from AAPH: Development of a Flow-injection Spin-trapping ESR System for the Oxygen Radical Absorbance Capacity Assay

DMSO‐Enabled Selective Radical O−H Activation of 1,3(4)‐Diols

DMSO‐Enabled Selective Radical O−H Activation of 1,3(4)‐Diols

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