Molecules

Editorial

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Open AccessEditorial

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 - 9 Feb 2015

Cited by 1 | Viewed by 5631

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

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826 KiB

Open AccessArticle

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 - 3 Mar 2015

Cited by 6 | Viewed by 6854

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

Open AccessCommunication

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 28 | Viewed by 9603

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

Open AccessArticle

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 6789

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

Open AccessArticle

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 8432

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 λ_max = 470 nm assigned to the OH-adducts on the benzene ring, and a second band with λ_max = 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 λ_max = 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

Open AccessArticle

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 6326

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

Open AccessArticle

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 6810

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

Open AccessArticle

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 7856

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

Open AccessArticle

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 8856

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

Open AccessArticle

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 8988

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

Open AccessArticle

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 - 1 Sep 2014

Cited by 11 | Viewed by 7571

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 speciesreadily protonates to produce the protonated electron adduct radical CH₃-C(•)OH-CH₂-COOCH_3. 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 CH₃group. 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-COOCH₃undergoes 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

Open AccessArticle

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 6 | Viewed by 6613

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

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Open AccessReview

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 - 3 Dec 2014

Cited by 10 | Viewed by 8429

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

Open AccessReview

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 419 | Viewed by 35091

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

Open AccessReview

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 42 | Viewed by 13679

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

Open AccessReview

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 58 | Viewed by 12303

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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Open AccessReview

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 152 | Viewed by 24862

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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