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Free Radicals and Radical Ions
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Free Radicals and Radical Ions

A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Organic Chemistry".

Deadline for manuscript submissions: closed (31 August 2014) | Viewed by 176389

Special Issue Editors

Prof. Dr. John C. Walton

E-Mail Website
Guest Editor
EaStCHEM School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, UK
Interests: radical chemistry; organic synthetic methods; free-radical rearrangements; photoredox catalysis; oxime derivatives; EPR spectroscopy; DFT applications; enhanced acidity of radicals
Special Issues, Collections and Topics in MDPI journals
Prof. Dr. Francon Williams

E-Mail Website
Guest Editor
Department of Chemistry, University of Tennessee, Knoxville, TN, USA
Interests: matrix ESR spectroscopy of radical anions and radical cations; quantum tunneling in hydrogen-atom transfer reactions; vibronic coupling in molecular ions; radical cation rearrangements; radiation-induced cationic polymerization

Special Issue Information

Dear Colleagues,

Reactive free radicals and radical ions are some of the most important classes of reactive intermediates in the chemical, polymer, and biological sciences. Also, stable varieties of free radicals are hugely important in conducting and magnetic materials. New radical-based reagents, including those derived from peroxides, boron-based compounds, oxime derivatives, epoxides, metal hydrides, and others, are at the forefront of advances in synthetic chemistry. Novel radical-mediated procedures, including cyclizations, cascades, radical to nucleophile couplings and cyclizations, and radical-regulated catalytic systems with one or two electron donors, are all equally exciting. They all have contributed to a notable flowering of innovative synthetic methodologies.

The involvement of fresh radical-ordered procedures has deeply penetrated polymer science. Furthermore, novel radical-containing magnetic materials and conductors are expanding horizons in materials science. Although relatively unexplored, in comparison with their neutral molecule counterparts, radical ions participate in a broad swath of important processes by virtue of rapid electron-transfer reactions, which can be induced by both chemical catalysis and either photo- or radiation-chemical means. The use of radical ions in organic synthesis continues to develop with the radical-cation modification of the Cope reaction and similar rearrangements, as well as by their participation in the well-established Hofmann-Löffler, Würtz, and Birch reactions. Biochemical applications feature the interplay of simple oxygen–containing species, such as the oxygen radical anion O2─ and its protonated form, HO2. Also, the key role of radical ions in the important emerging technologies of alternative energy sources is illustrated by their involvement in the complementary fields of photovoltaics and light-emitting diodes.

Research articles covering all areas of free radical and radical ion chemistry, such as synthetic advances and applications, mechanistic insights, spectroscopic, structural, computational, and kinetic studies, as well as biochemical and electrochemical applications, are welcomed for inclusion in this Special Issue of Molecules.

Prof. Dr. John C. Walton
Prof. Dr. T. Ffrancon Williams
Guest Editors

Submission

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. Papers will be published continuously (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are refereed through a peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Molecules is an international peer-reviewed Open Access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1800 CHF (Swiss Francs).

Keywords

  • reactive intermediates
  • synthetic methods
  • redox (electron transfer) processes
  • radical and radical cation cyclizations
  • organic and biochemical mechanisms
  • electron donors
  • characterization by electron spin and paramagnetic properties

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Editorial

Jump to: Research, Review

625 KiB  
Editorial
The Games Radicals Play: Special Issue on Free Radicals and Radical Ions
by John C. Walton and Ffrancon Williams
Molecules 2015, 20(2), 2831-2834; https://doi.org/10.3390/molecules20022831 - 09 Feb 2015
Cited by 1 | Viewed by 5393
Abstract
Chemistry and Physics have aptly been described as “most excellent children of Intellect and Art” [1]. Both these “children” engage with many playthings, and molecules rank as one of their first favorites, especially radicals, which are amongst the most lively and exciting. Checking [...] Read more.
Chemistry and Physics have aptly been described as “most excellent children of Intellect and Art” [1]. Both these “children” engage with many playthings, and molecules rank as one of their first favorites, especially radicals, which are amongst the most lively and exciting. Checking out radicals dancing to the music of entropy round their potential energy ballrooms is surely both entertaining and enlightening. Radicals’ old favorite convolutions are noteworthy, but the new styles, modes and arrangements appearing on the scene are even more interesting. Some of these are ephemeral and enjoy only a brief appearance, others are retro-types reappearing in new guises, still others are genuinely new and “go viral” in the scientific world. This Special Issue of Molecules contains the observations and reflections of a select group of chemists and physicists fascinated by this spectacle. It contains an eclectic mix reflecting on new modes and advances as well as on permutations and combinations that revive mature themes. [...] Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)

Research

Jump to: Editorial, Review

826 KiB  
Article
In Situ EPR Studies of Reaction Pathways in Titania Photocatalyst-Promoted Alkylation of Alkenes
by Shona Rhydderch and Russell F. Howe
Molecules 2015, 20(3), 4055-4070; https://doi.org/10.3390/molecules20034055 - 03 Mar 2015
Cited by 5 | Viewed by 6480
Abstract
In situ EPR spectroscopy at cryogenic temperatures has been used to observe and identify paramagnetic species produced when titania is irradiated in the presence of reactants used in the photocatalytic alkylation of maleimide with t-butyl carboxylic acid or phenoxyacetic acid. It is shown [...] Read more.
In situ EPR spectroscopy at cryogenic temperatures has been used to observe and identify paramagnetic species produced when titania is irradiated in the presence of reactants used in the photocatalytic alkylation of maleimide with t-butyl carboxylic acid or phenoxyacetic acid. It is shown that maleimide acts as an acceptor of conduction band electrons. Valence band holes oxidise t-butyl carboxylic acid to the t-butyl radical and phenoxyacetic acid to the phenoxyacetic acid radical cation. In the presence of maleimide, the phenoxymethyl radical is formed from phenoxyacetic acid. The relevance of these observations to the mechanisms of titania photocatalyst-promoted alkylation of alkenes is discussed. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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853 KiB  
Communication
Electron Transfer-Induced Coupling of Haloarenes to Styrenes and 1,1-Diphenylethenes Triggered by Diketopiperazines and Potassium tert-Butoxide
by Eswararao Doni, Shengze Zhou and John A. Murphy
Molecules 2015, 20(2), 1755-1774; https://doi.org/10.3390/molecules20021755 - 22 Jan 2015
Cited by 27 | Viewed by 9011
Abstract
The coupling of haloarenes to styrenes and 1,1-diarylethenes has been achieved with potassium tert-butoxide in the presence of N,N'-dialkyldiketopiperazines. In contrast to previously reported reactions where phenanthroline has been used to mediate the reactions, the use of diketopiperazines can lead [...] Read more.
The coupling of haloarenes to styrenes and 1,1-diarylethenes has been achieved with potassium tert-butoxide in the presence of N,N'-dialkyldiketopiperazines. In contrast to previously reported reactions where phenanthroline has been used to mediate the reactions, the use of diketopiperazines can lead to either 1,1,2-triarylethenes or 1,1,2-triarylethanes, depending on the conditions used. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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1103 KiB  
Article
Transfer Hydrogenation in Open-Shell Nucleotides — A Theoretical Survey
by Florian Achrainer and Hendrik Zipse
Molecules 2014, 19(12), 21489-21505; https://doi.org/10.3390/molecules191221489 - 22 Dec 2014
Cited by 2 | Viewed by 6459
Abstract
The potential of a larger number of sugar models to act as dihydrogen donors in transfer hydrogenation reactions has been quantified through the calculation of hydrogenation energies of the respective oxidized products. Comparison of the calculated energies to hydrogenation energies of nucleobases shows [...] Read more.
The potential of a larger number of sugar models to act as dihydrogen donors in transfer hydrogenation reactions has been quantified through the calculation of hydrogenation energies of the respective oxidized products. Comparison of the calculated energies to hydrogenation energies of nucleobases shows that many sugar fragment radicals can reduce pyrimidine bases such as uracil in a strongly exothermic fashion. The most potent reducing agent is the C3' ribosyl radical. The energetics of intramolecular transfer hydrogenation processes has also been calculated for a number of uridinyl radicals. The largest driving force for such a process is found for the uridin-C3'-yl radical, whose rearrangement to the C2'-oxidized derivative carrying a dihydrouracil is predicted to be exothermic by 61.1 kJ/mol in the gas phase. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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1756 KiB  
Article
Spectral and Kinetic Properties of Radicals Derived from Oxidation of Quinoxalin-2-One and Its Methyl Derivative
by Konrad Skotnicki, Julio R. De la Fuente, Alvaro Cañete and Krzysztof Bobrowski
Molecules 2014, 19(11), 19152-19171; https://doi.org/10.3390/molecules191119152 - 19 Nov 2014
Cited by 7 | Viewed by 8015
Abstract
The kinetics and spectral characteristics of the transients formed in the reactions of •OH and •N3 with quinoxalin-2(1H)-one (Q), its methyl derivative, 3-methylquinoxalin-2(1H)-one (3-MeQ) and pyrazin-2-one (Pyr) were studied by pulse radiolysis in aqueous solutions at [...] Read more.
The kinetics and spectral characteristics of the transients formed in the reactions of •OH and •N3 with quinoxalin-2(1H)-one (Q), its methyl derivative, 3-methylquinoxalin-2(1H)-one (3-MeQ) and pyrazin-2-one (Pyr) were studied by pulse radiolysis in aqueous solutions at pH 7. The transient absorption spectra recorded in the reactions of •OH with Q and 3-MeQ consisted of an absorption band with λmax = 470 nm assigned to the OH-adducts on the benzene ring, and a second band with λmax = 390 nm (for Q) and 370 nm (for 3-MeQ) assigned, inter alia, to the N-centered radicals on a pyrazin-2-one ring. The rate constants of the reactions of •OH with Q and 3-MeQ were found to be in the interval (5.9–9.7) × 109 M–1·s–1 and were assigned to their addition to benzene and pyrazin-2-one rings and H-abstraction from the pyrazin-2-one nitrogen. In turn, the transient absorption spectrum observed in the reaction of •N3 exhibits an absorption band with λmax = 350 nm. This absorption was assigned to the N-centered radical on the Pyr ring formed after deprotonation of the respective radical cation resulting from one-electron oxidation of 3-MeQ. The rate constant of the reaction of •N3 with 3 MeQ was found to be (6.0 ± 0.5) × 109 M–1·s–1. Oxidation of 3-MeQ by •N3 and Pyr by •OH and •N3 confirms earlier spectral assignments. With the rate constant of the •OH radical with Pyr (k = 9.2 ± 0.2) × 109 M–1·s‒1, a primary distribution of the •OH attack was estimated nearly equal between benzene and pyrazin-2-one rings. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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1559 KiB  
Article
Thermal Reactivity of Neutral and Oxidized Ferrocenyl-Substituted Enediynes
by Mehmet Emin Cinar, Guido Morbach and Michael Schmittel
Molecules 2014, 19(11), 18399-18413; https://doi.org/10.3390/molecules191118399 - 12 Nov 2014
Cited by 2 | Viewed by 6052
Abstract
The coupling of two equivalents of ethynylferrocene (2) with one equivalent of 1,2-diiodocyclohexene (1) and 1,2-diiodobenzene (4) using Sonogashira cross-coupling conditions led to 1,2-bis(ferrocenylethynyl)cyclohexene (3) and 1,2-bis(ferrocenylethy­nyl)benzene (5), respectively. At high temperatures enediynes [...] Read more.
The coupling of two equivalents of ethynylferrocene (2) with one equivalent of 1,2-diiodocyclohexene (1) and 1,2-diiodobenzene (4) using Sonogashira cross-coupling conditions led to 1,2-bis(ferrocenylethynyl)cyclohexene (3) and 1,2-bis(ferrocenylethy­nyl)benzene (5), respectively. At high temperatures enediynes 3 and 5 showed exothermic signals in differential scanning calorimetry (DSC) measurements, suggestive of intramolecular diradicaloid ring formation (Bergman (C1−C6) or Schreiner-Pascal (C1−C5) cyclizations). The oxidation of 3 and 5 to the mono-oxidized enediynes 3+ and 5+ decreased the onset temperatures drastically. Equally, 1-ferrocenylethynyl-2-(p-nitro-phenyl)ethynylbenzene (8) displayed a significant decrease in the onset temperature after oxidation to 8+. Because the insoluble nature of the polymeric material formed in the thermolysis of the oxidized enediynes prevented characterization, the origin of this drastic effect was studied by DFT. Contrary to expectations, one-electron oxidation does not lower the barrier for intramolecular cyclization. Rather, the computations suggest that the polymerization is initiated by a bimolecular process. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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1647 KiB  
Article
The Dynamical Behavior of the s-Trioxane Radical Cation—A Low-Temperature EPR and Theoretical Study
by Sergej S. Naumov, Wolfgang Knolle, Sergej P. Naumov, Andreas Pöppl and Igor Janovský
Molecules 2014, 19(11), 17305-17313; https://doi.org/10.3390/molecules191117305 - 28 Oct 2014
Cited by 1 | Viewed by 6462
Abstract
The radical cation of s-trioxane, radiolytically generated in a freon (CF3CCl3) matrix, was studied in the 10–140 K temperature region. Reversible changes of the EPR spectra were observed, arising from both ring puckering and ring inversion through the molecular [...] Read more.
The radical cation of s-trioxane, radiolytically generated in a freon (CF3CCl3) matrix, was studied in the 10–140 K temperature region. Reversible changes of the EPR spectra were observed, arising from both ring puckering and ring inversion through the molecular plane. The ESREXN program based on the Liouville density matrix equation, allowing the treatment of dynamical exchange, has been used to analyze the experimental results. Two limiting conformer structures of the s-trioxane radical cation were taken into account, namely “rigid” half-boat and averaged planar ones, differing strongly in their electron distribution. The spectrum due to the “rigid” half-boat conformer can be observed only at very low (<60 K) temperatures, when the exchange of conformers is very slow. Two transition states for interconversion by puckering and ring-inversion were identified, close in activation energy (2.3 and 3.0 kJ/mol calculated). Since the energy difference is very small, both processes set on at a comparable temperature. In the case of nearly complete equilibration (fast exchange) between six energetically equivalent structures at T > 120 K in CF3CCl3, a septet due to six equivalent protons (hfs splitting constant 5.9 mT) is observed, characteristic of the dynamically averaged planar geometry of the radical cation. DFT quantum chemical calculations and spectral simulation including intramolecular dynamical exchange support the interpretation. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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815 KiB  
Article
Photochemical Aryl Radical Cyclizations to Give (E)-3-Ylideneoxindoles
by Michael Gurry, Ingrid Allart-Simon, Patrick McArdle, Stéphane Gérard, Janos Sapi and Fawaz Aldabbagh
Molecules 2014, 19(10), 15891-15899; https://doi.org/10.3390/molecules191015891 - 30 Sep 2014
Cited by 3 | Viewed by 7456
Abstract
(E)-3-Ylideneoxindoles are prepared in methanol in reasonable to good yields, as adducts of photochemical 5-exo-trig of aryl radicals, in contrast to previously reported analogous radical cyclizations initiated by tris(trimethylsilyl)silane and azo-initiators that gave reduced oxindole adducts. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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825 KiB  
Article
Hydrogenations without Hydrogen: Titania Photocatalyzed Reductions of Maleimides and Aldehydes
by David W. Manley, Luca Buzzetti, Andrew MacKessack-Leitch and John C. Walton
Molecules 2014, 19(9), 15324-15338; https://doi.org/10.3390/molecules190915324 - 24 Sep 2014
Cited by 15 | Viewed by 8402
Abstract
A mild procedure for the reduction of electron-deficient alkenes and carbonyl compounds is described. UVA irradiations of substituted maleimides with dispersions of titania (Aeroxide P25) in methanol/acetonitrile (1:9) solvent under dry anoxic conditions led to hydrogenation and production of the corresponding succinimides. Aromatic [...] Read more.
A mild procedure for the reduction of electron-deficient alkenes and carbonyl compounds is described. UVA irradiations of substituted maleimides with dispersions of titania (Aeroxide P25) in methanol/acetonitrile (1:9) solvent under dry anoxic conditions led to hydrogenation and production of the corresponding succinimides. Aromatic and heteroaromatic aldehydes were reduced to primary alcohols in similar titania photocatalyzed reactions. A mechanism is proposed which involves two proton-coupled electron transfers to the substrates at the titania surface. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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1382 KiB  
Article
Photochemistry and Radical Chemistry under Low Intensity Visible Light Sources: Application to Photopolymerization Reactions
by Jacques Lalevée, Fabrice Morlet-Savary, Céline Dietlin, Bernadette Graff and Jean-Pierre Fouassier
Molecules 2014, 19(9), 15026-15041; https://doi.org/10.3390/molecules190915026 - 18 Sep 2014
Cited by 17 | Viewed by 8578
Abstract
The search for radical initiators able to work under soft conditions is a great challenge, driven by the fact that the use of safe and cheap light sources is very attractive. In the present paper, a review of some recently reported photoinitiating systems [...] Read more.
The search for radical initiators able to work under soft conditions is a great challenge, driven by the fact that the use of safe and cheap light sources is very attractive. In the present paper, a review of some recently reported photoinitiating systems for polymerization under soft conditions is provided. Different approaches based on multi-component systems (e.g., photoredox catalysis) or light harvesting photoinitiators are described and discussed. The chemical mechanisms associated with the production of free radicals usable as initiating species or mediators of cations are reported. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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1656 KiB  
Article
Presolvated Electron Reactions with Methyl Acetoacetate: Electron Localization, Proton-Deuteron Exchange, and H-Atom Abstraction
by Alex Petrovici, Amitava Adhikary, Anil Kumar and Michael D. Sevilla
Molecules 2014, 19(9), 13486-13497; https://doi.org/10.3390/molecules190913486 - 01 Sep 2014
Cited by 10 | Viewed by 7208
Abstract
Radiation-produced electrons initiate various reaction processes that are important to radiation damage to biomolecules. In this work, the site of attachment of the prehydrated electrons with methyl acetoacetate (MAA, CH3-CO-CH2-COOCH3) at 77 K and subsequent reactions of [...] Read more.
Radiation-produced electrons initiate various reaction processes that are important to radiation damage to biomolecules. In this work, the site of attachment of the prehydrated electrons with methyl acetoacetate (MAA, CH3-CO-CH2-COOCH3) at 77 K and subsequent reactions of the anion radical (CH3-CO•−-CH2-COOCH3) in the 77 to ca. 170 K temperature range have been investigated in homogeneous H2O and D2O aqueous glasses by electron spin resonance (ESR) spectroscopy. At 77 K, the prehydrated electron attaches to MAA forming the anion radical in which the electron is delocalized over the two carbonyl groups. This species readily protonates to produce the protonated electron adduct radical CH3-C(•)OH-CH2-COOCH3. The ESR spectrum of CH3-C(•)OH-CH2-COOCH3 in H2O shows line components due to proton hyperfine couplings of the methyl and methylene groups. Whereas, the ESR spectrum of CH3-C(•)OH-CH2-COOCH3 in D2O glass shows only the line components due to proton hyperfine couplings of CH3 group. This is expected since the methylene protons in MAA are readily exchangeable in D2O. On stepwise annealing to higher temperatures (ca. 150 to 170 K), CH3-C(•)OH-CH2-COOCH3 undergoes bimolecular H-atom abstraction from MAA to form the more stable radical, CH3-CO-CH•-COOCH3. Theoretical calculations using density functional theory (DFT) support the radical assignments. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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728 KiB  
Article
Dissociative Electron Transfer to Diphenyl-Substituted Bicyclic Endoperoxides: The Effect of Molecular Structure on the Reactivity of Distonic Radical Anions and Determination of Thermochemical Parameters
by David C. Magri and Mark S. Workentin
Molecules 2014, 19(8), 11999-12010; https://doi.org/10.3390/molecules190811999 - 11 Aug 2014
Cited by 5 | Viewed by 6289
Abstract
The heterogeneous electron transfer reduction of the bicyclic endoperoxide 1,4-diphenyl-2,3-dioxabicyclo[2.2.1]hept-5-ene (4) was investigated in N,N-dimethylformamide at a glassy carbon electrode. The endoperoxide reacts by a concerted dissociative ET mechanism resulting in reduction of the O-O bond with an [...] Read more.
The heterogeneous electron transfer reduction of the bicyclic endoperoxide 1,4-diphenyl-2,3-dioxabicyclo[2.2.1]hept-5-ene (4) was investigated in N,N-dimethylformamide at a glassy carbon electrode. The endoperoxide reacts by a concerted dissociative ET mechanism resulting in reduction of the O-O bond with an observed peak potential of −1.4 V at 0.2 V s−1. The major product (90% yield) resulting from the heterogeneous bulk electrolysis of 4 at −1.4 V with a rotating disk glassy carbon electrode is 1,4-diphenyl-cyclopent-2-ene-cis-1,3-diol with a consumption of 1.73 electrons per mole. In contrast, 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]oct-5-ene (1), undergoes a two-electron reduction mechanism in quantitative yield. This difference in product yield between 1 and 4 is suggestive of a radical-anion mechanism, as observed with 1,4-diphenyl-2,3-dioxabicyclo-[2.2.2] octane (2) and 1,4-diphenyl-2,3-dioxabicyclo[2.2.1]heptane (3). Convolution potential sweep voltammetry is used to determine unknown thermochemical parameters of 4, including the O-O bond dissociation energy and the standard reduction potential and a comparison is made to the previously studied bicyclic endoperoxides 1–3 with respect to the effect of molecular structure on the reactivity of distonic radical anions. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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Review

Jump to: Editorial, Research

1061 KiB  
Review
Strategies for the Synthesis of Yardsticks and Abaci for Nanometre Distance Measurements by Pulsed EPR
by Silvia Valera and Bela E. Bode
Molecules 2014, 19(12), 20227-20256; https://doi.org/10.3390/molecules191220227 - 03 Dec 2014
Cited by 10 | Viewed by 7996
Abstract
Pulsed electron paramagnetic resonance (EPR) techniques have been found to be efficient tools for the elucidation of structure in complex biological systems as they give access to distances in the nanometre range. These measurements can provide additional structural information such as relative orientations, [...] Read more.
Pulsed electron paramagnetic resonance (EPR) techniques have been found to be efficient tools for the elucidation of structure in complex biological systems as they give access to distances in the nanometre range. These measurements can provide additional structural information such as relative orientations, structural flexibility or aggregation states. A wide variety of model systems for calibration and optimisation of pulsed experiments has been synthesised. Their design is based on mimicking biological systems or materials in specific properties such as the distances themselves and the distance distributions. Here, we review selected approaches to the synthesis of chemical systems bearing two or more spin centres, such as nitroxide or trityl radicals, metal ions or combinations thereof and outline their application in pulsed EPR distance measurements. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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677 KiB  
Review
Antioxidant Property of Coffee Components: Assessment of Methods that Define Mechanisms of Action
by Ningjian Liang and David D. Kitts
Molecules 2014, 19(11), 19180-19208; https://doi.org/10.3390/molecules191119180 - 19 Nov 2014
Cited by 369 | Viewed by 31648
Abstract
Coffee is a rich source of dietary antioxidants, and this property, coupled with the fact that coffee is one of the world’s most popular beverages, has led to the understanding that coffee is a major contributor to dietary antioxidant intake. Brewed coffee is [...] Read more.
Coffee is a rich source of dietary antioxidants, and this property, coupled with the fact that coffee is one of the world’s most popular beverages, has led to the understanding that coffee is a major contributor to dietary antioxidant intake. Brewed coffee is a complex food matrix with numerous phytochemical components that have antioxidant activity capable of scavenging free radicals, donating hydrogen and electrons, providing reducing activity and also acting as metal ion pro-oxidant chelators. More recent studies have shown that coffee components can trigger tissue antioxidant gene expression and protect against gastrointestinal oxidative stress. This paper will describe different in vitro, cell-free and cell-based assays that both characterize and compare the antioxidant capacity and mechanism of action of coffee and its bioactive constituents. Moreover, evidence of cellular antioxidant activity and correlated specific genomic events induced by coffee components, which are relevant to antioxidant function in both animal and human studies, will be discussed. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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864 KiB  
Review
Synthetic Applications of Intramolecular Thiol-Ene “Click” Reactions
by Eoin M. Scanlan, Vincent Corcé and Aoife Malone
Molecules 2014, 19(11), 19137-19151; https://doi.org/10.3390/molecules191119137 - 19 Nov 2014
Cited by 39 | Viewed by 12890
Abstract
The intermolecular thiol-ene reaction is emerging as a highly efficient; free-radical mediated “click” process with diverse applications in biofunctionalisation and materials science. The related intramolecular thiol-ene reactions offer significant potential for the preparation of a wide range of sulphur containing heterocycles including synthetic [...] Read more.
The intermolecular thiol-ene reaction is emerging as a highly efficient; free-radical mediated “click” process with diverse applications in biofunctionalisation and materials science. The related intramolecular thiol-ene reactions offer significant potential for the preparation of a wide range of sulphur containing heterocycles including synthetic therapeutics such as cyclic peptides and thiosugars. Herein, we review recent advances in intramolecular thiyl-radical mediated reactions and their applications for synthetic and medicinal chemistry. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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414 KiB  
Review
New Developments in Spin Labels for Pulsed Dipolar EPR
by Alistair J. Fielding, Maria Grazia Concilio, Graham Heaven and Michael A. Hollas
Molecules 2014, 19(10), 16998-17025; https://doi.org/10.3390/molecules191016998 - 23 Oct 2014
Cited by 55 | Viewed by 11640
Abstract
Spin labelling is a chemical technique that enables the integration of a molecule containing an unpaired electron into another framework for study. Given the need to understand the structure, dynamics, and conformational changes of biomacromolecules, spin labelling provides a relatively non-intrusive technique and [...] Read more.
Spin labelling is a chemical technique that enables the integration of a molecule containing an unpaired electron into another framework for study. Given the need to understand the structure, dynamics, and conformational changes of biomacromolecules, spin labelling provides a relatively non-intrusive technique and has certain advantages over X-ray crystallography; which requires high quality crystals. The technique relies on the design of binding probes that target a functional group, for example, the thiol group of a cysteine residue within a protein. The unpaired electron is typically supplied through a nitroxide radical and sterically shielded to preserve stability. Pulsed electron paramagnetic resonance (EPR) techniques allow small magnetic couplings to be measured (e.g., <50 MHz) providing information on single label probes or the dipolar coupling between multiple labels. In particular, distances between spin labels pairs can be derived which has led to many protein/enzymes and nucleotides being studied. Here, we summarise recent examples of spin labels used for pulse EPR that serve to illustrate the contribution of chemistry to advancing discoveries in this field. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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1705 KiB  
Review
Radical Addition to Iminium Ions and Cationic Heterocycles
by Johannes Tauber, Dennis Imbri and Till Opatz
Molecules 2014, 19(10), 16190-16222; https://doi.org/10.3390/molecules191016190 - 10 Oct 2014
Cited by 141 | Viewed by 23502
Abstract
Carbon-centered radicals represent highly useful reactive intermediates in organic synthesis. Their nucleophilic character is reflected by fast additions to electron deficient C=X double bonds as present in iminium ions or cationic heterocycles. This review covers diverse reactions of preformed or in situ-generated [...] Read more.
Carbon-centered radicals represent highly useful reactive intermediates in organic synthesis. Their nucleophilic character is reflected by fast additions to electron deficient C=X double bonds as present in iminium ions or cationic heterocycles. This review covers diverse reactions of preformed or in situ-generated cationic substrates with various types of C-radicals, including alkyl, alkoxyalkyl, trifluoromethyl, aryl, acyl, carbamoyl, and alkoxycarbonyl species. Despite its high reactivity, the strong interaction of the radical’s SOMO with the LUMO of the cation frequently results in a high regioselectivity. Intra- and intermolecular processes such as the Minisci reaction, the Porta reaction, and the Knabe rearrangement will be discussed along with transition metal and photoredox catalysis or electrochemical methods to generate the odd-electron species. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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