Construction of Persistent Phenoxyl Radical with Intramolecular Hydrogen Bonding

Maki, Toshihide; Araki, Yoshihiro; Ishida, Yukihiro; Onomura, Osamu; Matsumura, Yoshihiro

doi:10.1021/ja002453+

Cited by 113 publications

(98 citation statements)

References 31 publications

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“…Cyclic voltammograms of these compounds in anaerobic acetonitrile are chemically reversible with peak separations larger than the theoretical value of 59 mV (SI Fig. 10 in the SI Appendix), as have been observed with other hydrogen-bonded phenols (20,(36)(37)(38)(39). Reduction potentials are taken as the average of cathodic and anodic peak potentials (E 1/2 ) and vary from 0.42 to 0.66 V ( )] to yield the hydrogen-bonded phenol-base ArOH---im can be separated into the free energies of the O-H⅐ ⅐ ⅐N hydrogen bond (⌬G HB°) and the free-energy change for deprotonation to yield the hypothetical phenol-base lacking the hydrogen bond (HOArim), ⌬G a,nonHB°.…”

Section: Synthesis and Characterization Of Phenol-imidazolessupporting

confidence: 52%

Probing concerted proton–electron transfer in phenol–imidazoles

Markle

Rhile

DiPasquale

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

123

188

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A series of seven substituted 4,6-di-tert-butyl-2-(4,5-diarylimidazolyl)-phenols have been prepared and characterized, along with two related benzimidazole compounds. X-ray crystal structures of all of the compounds show that the phenol and imidazole rings are close to coplanar and are connected by an intramolecular ArOH⅐ ⅐ ⅐N hydrogen bond. One-electron oxidation of these compounds occurs with movement of the phenolic proton to the imidazole base by concerted proton-electron transfer (CPET) to yield fairly stable distonic radical cations. These phenol-base compounds are a valuable system in which to examine the key features of CPET. Kinetic measurements of bimolecular CPET oxidations, with E rxn between ؉0.04 and ؊0.33 V, give rate constants from (6.3 ؎ 0.6) ؋ 10 2 to (3.0 ؎ 0.6) ؋ 10 6 M ؊1 s ؊1 . There is a good correlation of log(k) with ⌬G°, with only one of the 15 rate constants falling more than a factor of 5.2 from the correlation line. Substituents on the imidazole affect the (O-H⅐ ⅐ ⅐N) hydrogen bond, as marked by variations in the 1 H NMR and calculated vibrational spectra and geometries. Crystallographic dO⅐ ⅐ ⅐N values appear to be more strongly affected by crystal packing forces. However, there is almost no correlation of rate constants with any of these measured or computed parameters. Over this range of compounds from the same structural family, the dominant contributor to the differences in rate constant is the driving force ⌬G°.Marcus Theory ͉ oxyl radicals ͉ proton-coupled ͉ ROS ͉ tyrosyl radicals R eactive oxygen species (ROS) exhibit a wide range of reactivity, from the extraordinarily potent hydroxyl radical to much more inert aryloxyl and ascorbyl radicals. These species are sometimes categorized by their redox potentials at pH 7, but most reactions of ROS do not proceed by simple electron transfer (ET). Typically, ROS react by proton-coupled electron transfer (PCET), as in the disproportionation of hydrogen peroxide to dioxygen and water and the interconversions of tyrosyl radicals and tyrosine residues. Understanding the detailed mechanisms of these and other PCET

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Section: Synthesis and Characterization Of Phenol-imidazolessupporting

confidence: 52%

Probing concerted proton–electron transfer in phenol–imidazoles

Markle

Rhile

DiPasquale

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

123

188

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“…For instance, o,p-tBu-protected phenol compounds that are intramolecularly H-bonded to a nitrogen base (from amino, imidazole or pyridine groups) have proved to undergo a (quasi)reversible proton-coupled electron transfer (PCET) oxidation process to H-bonded phenoxyl radicals of the type R À OC···H À + N. [4][5][6][7][8][9][10][11][12][13][14] In these systems, the phenol oxidation process occurs at a much lower potential than that of a non-H-bonded phenol, which could, at first sight, be a consequence of H-bonding effects in the reduced and oxidised forms. However, Mayer and co-workers have elegantly demonstrated that this decrease in redox potential results predominantly from the driving force for proton movement in a concerted PCET (i.e.…”

Section: Introductionmentioning

confidence: 99%

“…4 ], in CH 3 CN at 298 K, exhibits a fully reversible one-electron oxidation process attributed to the formation of the corresponding phenoxyl radical species (Scheme 1, Figure 5). This oxidation process occurs at E 1/2 = À0.119, À0.161 and À0.423 V versus ferrocene/ferrocenium (Fc/Fc À are comparable to that of the previously reported 2-amido-4,6-di-tert-butylphenol recorded in water/ CH 3 CN under basic conditions (pH > 12).…”

mentioning

confidence: 99%

Persistent Hydrogen‐Bonded and Non‐Hydrogen‐Bonded Phenoxyl Radicals

Wanke¹,

Bénisvy

Kuznetsov³

et al. 2011

Chemistry A European J

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The production of stable phenoxyl radicals is undoubtedly a synthetic chemical challenge. Yet it is a useful way to gain information on the properties of the biological tyrosyl radicals. Recently, several persistent phenoxyl radicals have been reported, but only limited synthetic variations could be achieved. Herein, we show that the amide-o-substituted phenoxyl radical (i.e. with a salicylamide backbone) can be synthesised in a stable manner, thereby permitting easy synthetic modifications to be made through the amide bond. To study the effect of H-bonding on the properties of the phenolate/phenoxyl radical redox couple, simple H-bonded and non-H-bonded o,p-tBu-protected salicylamidate compounds have been prepared. Their redox properties were examined by cyclic voltammetry and showed a fully reversible one-electron oxidation process to the corresponding phenoxyl radical species. Remarkably, the redox potential appears to be correlated, at least partially, with H-bond strength, as relatively large differences (ca. 300 mV) in the redox potential between H-bonded and non-H-bonded phenolate salts are observed. The corresponding phenoxyl radicals produced electrochemically are persistent at room temperature for at least an hour; their UV/Vis and EPR characterisation is consistent with that of phenoxyl radicals, which makes them excellent models of biological tyrosyl radicals. The analyses of the experimental data coupled with theoretical calculations indicate that both the deviation from planarity of the amide function and intramolecular H-bonding influence the oxidation potential of the phenolate. The latter H-bonding effect appears to be predominantly exerted on the phenolate and not (or only a little) on the phenoxyl radical. Thus, in these systems the H-bonding energy involved in the phenoxyl radical appears to be relatively small.

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“…A model compound for hydrogen bonded phenol and its electrochemical properties was utilized to understand proton coupled electron transfer, which is considered as one of the key biological processes involving phenols. 5) Furthermore, the photocleavable property of acylphenol derivatives is applied for semi-quantitative probes for laser desorption ionization mass spectrometry. 6,7) In addition, there are large numbers of synthetic phenol derivatives with various pharmacological applications.…”

mentioning

confidence: 99%

“…[5][6][7] On the course of our study, we investigated the antibacterial activities for our chemicals. In this report, two new nitrobenzyl-oxy-phenol derivatives were synthesized, their structures were confirmed based on their spectral data and their antibacterial activities determined.…”

mentioning

confidence: 99%

Synthesis and Antimicrobial Activity of Nitrobenzyl-oxy-phenol Derivatives

Mohamed

Maki

Shah

et al. 2016

Biological & Pharmaceutical Bulletin

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Two hydroquinone derivatives were prepared and their antimicrobial activity evaluated. Their minimum inhibitory concentrations (MICs) were determined using a broth dilution method. Gentamycin and ciprofloxacin were used as reference antibiotics. The antimicrobial activity of 4-(benzyloxy)phenol (monobenzone) was also evaluated based on its structural similarity to the new compounds; activity was comparable to that of 3,5-dimethyl-4-((4-nitrobenzyl)oxy)phenol (4a). 2,3,5-Trimethyl-4-((4-nitrobenzyl)oxy)phenol (4b) exhibited the best antibacterial activity against both clinical isolates and type strain of Moraxella catarrhalis (M. catarrhalis), with a MIC value of 11 µM, comparable to ciprofloxacin 9 µM.

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Construction of Persistent Phenoxyl Radical with Intramolecular Hydrogen Bonding

Cited by 113 publications

References 31 publications

Probing concerted proton–electron transfer in phenol–imidazoles

Probing concerted proton–electron transfer in phenol–imidazoles

Persistent Hydrogen‐Bonded and Non‐Hydrogen‐Bonded Phenoxyl Radicals

Synthesis and Antimicrobial Activity of Nitrobenzyl-oxy-phenol Derivatives

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