Showing papers on "Radical ion published in 2011"

Synthesis and Characterization of a Neutral Tricoordinate Organoboron Isoelectronic with Amines

Abstract: Amines and boranes are the archetypical Lewis bases and acids, respectively. The former can readily undergo one-electron oxidation to give radical cations, whereas the latter are easily reduced to afford radical anions. Here, we report the synthesis of a neutral tricoordinate boron derivative, which acts as a Lewis base and undergoes one-electron oxidation into the corresponding radical cation. These compounds can be regarded as the parent borylene (H-B:) and borinylium (H-B(+.)), respectively, stabilized by two cyclic (alkyl)(amino)carbenes. Ab initio calculations show that the highest occupied molecular orbital of the borane as well as the singly occupied molecular orbital of the radical cation are essentially a pair and a single electron, respectively, in the p(π) orbital of boron.

Radical Cation Diels–Alder Cycloadditions by Visible Light Photocatalysis

Abstract: Ruthenium(II) polypyridyl complexes promote the efficient radical cation Diels–Alder cycloaddition of electron-rich dienophiles upon irradiation with visible light. These reactions enable facile [4 + 2] cycloadditions that would be electronically mismatched under thermal conditions. Key to the success of this methodology is the availability of ligand-modified ruthenium complexes that enable rational tuning of the electrochemical properties of the catalyst without significantly perturbing the overall photophysical properties of the system.

Metal-free intermolecular oxidative C-N bond formation via tandem C-H and N-H bond functionalization.

Abstract: The development of a novel intermolecular oxidative amination reaction, a synthetic transformation that involves the simultaneous functionalization of both a N–H and C–H bond, is described. The process, which is mediated by an I(III) oxidant and contains no metal catalysts, provides a rapid and green method for synthesizing protected anilines from simple arenes and phthalimide. Mechanistic investigations indicate that the reaction proceeds via nucleophilic attack of the phthalimide on an aromatic radical cation, as opposed to the electrophilic aromatic amination that has been reported for other I(III) amination reactions. The application of this new reaction to the synthesis of a variety of substituted aniline derivatives is demonstrated.

Selective photocatalytic aerobic bromination with hydrogen bromide via an electron-transfer state of 9-mesityl-10-methylacridinium ion†

Abstract: Photocatalytic bromination of aromatic hydrocarbons by molecular oxygen with hydrogen bromide occurs efficiently to produce monobrominated products selectively using 9-mesityl-10-methylacridinium ion (Acr+–Mes) as a photocatalyst under visible light irradiation. Both the product yield and selectivity for the bromination of 1,3,5-trimethoxybenzene were 100% with a quantum yield of 4.8%. The photocatalytic turnover number is 900 based on the initial concentration of Acr+–Mes. The reactive radical intermediates involved in the photocatalytic cycle have been successfully detected by laser flash photolysis measurements. The photocatalytic bromination is initiated by photoinduced electron transfer from the mesitylene moiety to the singlet excited state of acridinium ion, which results in formation of the electron-transfer state of Acr+–Mes (Acr˙–Mes˙+), followed by electron transfer from aromatic hydrocarbons to the mesitylene radical cation moiety and electron transfer from the acridinyl radical moiety to O2. The resulting radical cations of aromatic hydrocarbons react with Br− to produce the corresponding monobrominated products selectively.

Activation of oxygen on MgO: O2*- radical ion formation on thin, metal-supported MgO(001) films.

Abstract: The spontaneous activation of molecular oxygen forming an O2·- radical upon adsorption on a 4 monolayer thick MgO(001) film is demonstrated by EPR spectroscopy and DFT calculations.

Electron-Transfer-Induced Intermolecular [2 + 2] Cycloaddition Reactions Based on the Aromatic “Redox Tag” Strategy

Abstract: Novel electron-transfer-induced intermolecular [2 + 2] cycloaddition reactions between an aliphatic cyclic enol ether and several unactivated olefins have been demonstrated on the basis of the aromatic “redox tag” strategy. The aromatic “redox tag” was oxidized during the formation of the cyclobutane ring, affording the relatively long-lived aromatic radical cation, which was then reduced to complete the overall reaction that constructed the corresponding [2 + 2] cycloadducts. The aromatic “redox tag” was also found to facilitate electron-transfer-induced cycloreversion reactions of cyclobutane rings.

Reactions of simple and peptidic alpha-carboxylate radical anions with dioxygen in the gas phase

Abstract: α-Carboxylate radical anions are potential reactive intermediates in the free radical oxidation of biological molecules (e.g., fatty acids, peptides and proteins). We have synthesised well-defined α-carboxylate radical anions in the gas phase by UV laser photolysis of halogenated precursors in an ion-trap mass spectrometer. Reactions of isolated acetate (˙CH2CO2−) and 1-carboxylatobutyl (CH3CH2CH2˙CHCO2−) radical anions with dioxygen yield carbonate (CO3˙−) radical anions and this chemistry is shown to be a hallmark of oxidation in simple and alkyl-substituted cross-conjugated species. Previous solution phase studies have shown that Cα-radicals in peptides, formed from free radical damage, combine with dioxygen to form peroxyl radicals that subsequently decompose into imine and keto acid products. Here, we demonstrate that a novel alternative pathway exists for two α-carboxylate Cα-radical anions: the acetylglycinate radical anion (CH3C(O)NH˙CHCO2−) and the model peptide radical anion, YGGFG˙−. Reaction of these radical anions with dioxygen results in concerted loss of carbon dioxide and hydroxyl radical. The reaction of the acetylglycinate radical anion with dioxygen reveals a two-stage process involving a slow, followed by a fast kinetic regime. Computational modelling suggests the reversible formation of the Cα peroxyl radical facilitates proton transfer from the amide to the carboxylate group, a process reminiscent of, but distinctive from, classical proton-transfer catalysis. Interestingly, inclusion of this isomerization step in the RRKM/ME modelling of a G3SX level potential energy surface enables recapitulation of the experimentally observed two-stage kinetics.

Fluorescence of Radical Ions in Liquid Solution: Wurster's Blue as a Case Study

Abstract: The fluorescence lifetime of the radical cation of N,N,N',N'-tetramethyl-p-phenylenediamine (Wurster's blue) decreases from 260 ps at 82 K to 200 fs at room temperature. Calculations indicate a small barrier between the excited-state minimum (D1 min) and a conical intersection (CI) of the excited and ground state potentials. The intersection is reached within 200 fs upon torsion of one of the C—N bonds.

Direct observation of the β-carotene reaction with hydroxyl radical.

Abstract: Hydroxyl radical reacts readily with β-carotene following submicrosecond laser photolysis of N-hydroxypyridine-2(1H)-thione (N-HPT) as a "photo-Fenton" reagent generating hydroxyl and thiyl radicals in acetonitrile:tetrahydrofuran (4:1, v/v) solution. On the basis of spectral evidence, and supported by kinetic considerations and thermodynamic calculations, a short-lived transient species, detected by time-resolved absorption spectroscopy with an absorption maximum at ∼750 nm and a lifetime of ∼150 ns at 25 °C under anaerobic conditions, is suggested to be the long-sought neutral β-carotene radical formed by hydrogen-atom abstraction. The transient spectrum is different from the spectra of the β-carotene radical cation (∼1000 nm absorption maximum with a millisecond lifetime), the β-carotene radical adducts (∼520 nm, several microsecond lifetime), the β-carotene radical cation ion pair (∼750 nm, several hundred microsecond lifetime), and the β-carotene radical anion (∼880 nm, a few tens of microsecond lifetime). In parallel, β-carotene reacts with the thiyl radical to yield a sulfur radical adduct with absorption maximum at ∼520 nm with a lifetime of 3.0 μs. For astaxanthin and canthaxanthin, the reaction with the thiyl radical dominates and the neutral radical is hardly formed in agreement with the less reducing properties of these 4,4'-diketo carotenoids without the reactive 4,4'-hydrogens.

Structure and reactivity of the cysteine methyl ester radical cation

Abstract: The structure and reactivity of the cysteine methyl ester radical cation, CysOMe(center dot+), have been examined in the gas phase using a combination of experiment and density functional theory (DFT) calculations. CysOMe(center dot+) undergoes rapid ion molecule reactions with dimethyl disulfide, ally! bromide, and allyl iodide, but is unreactive towards allyl chloride. These reactions proceed by radical atom or group transfer and are consistent with CysOMe(center dot+) possessing structure 1, in which the radical site is located on the sulfur atom and the amino group is protonated. This contrasts with DFT calculations that predict a captodative structure 2, in which the radical site is positioned on the a carbon and the carbonyl group is protonated, and that is more stable than 1 by 13.0 kJ mol(-1). To resolve this apparent discrepancy the gas-phase IR spectrum of CysOMe(center dot+) was experimentally determined and compared with the theoretically predicted IR spectra of a range of isomers. An excellent match was obtained for 1. DFT calculations highlight that although 1 is thermodynamically less stable than 2, it is kinetically stable with respect to rearrangement.

Gas-phase ion-molecule reactions using regioselectively generated radical cations to model oxidative damage and probe radical sites in peptides.

Abstract: Collision induced dissociation (CID) of sodiated peptide derivatives containing a nitrate ester functionality was used to regiospecifically generate three isomeric radicals of the model peptide Bz-Ala-Gly-OMe corresponding to radicals formed at: Cα of the alanine residue [4+Na]+; Cα of the glycine residue [5+Na]+; and the side chain of alanine [6+Na]+. The ion-molecule reactions of these peptide radicals were examined to model oxidative damage to peptides and to probe whether the radical sites maintain their integrity or whether they isomerise via intramolecular hydrogen atom transfer (HAT). Only [6+Na]+ is reactive towards O2, forming the peroxyl radical [7+Na]+, which loses O2, HO˙ and HO2˙ under CID. The radical ion [7 + Na]+ abstracts a hydrogen atom from 4-fluorothiophenol to form the hydroperoxide [8+Na]+, which upon CID fragments via the combined loss of HO˙ and CH2O. In contrast, all three of the isomeric sodiated radicals react with NO˙ and NO2˙ to form adducts. CID of the NO adducts only regenerates the radicals viaNO˙ loss, thus providing no structural information. In contrast, CID of the NO2 adducts gives rise to a range of product ions and the spectra are different for each of the three adducts, suggesting that the isomeric radicals [4+Na]+, [5+Na]+ and [6+Na]+ are produced as discrete species. Finally, CID of the NO2 adducts was used to probe the rearrangement of the radicals [4+Na]+, [5+Na]+ and [6+Na]+ prior to their reaction with NO2˙: [6 + Na]+ rearranges to a mixture of [4+Na]+ and [5+Na]+ while [5+Na]+ rearranges to [4+Na]+.

Mimicking the Silicon Surface: Reactivity of Silyl Radical Cations toward Nucleophiles

Abstract: Radical cations of selected low molecular-weight silicon model compounds were obtained by photoinduced electron transfer. These radical cations react readily with a variety of nucleophiles, regularly used in monolayer fabrication onto hydrogen-terminated silicon. From time-resolved kinetics, it was concluded that the reactions proceed via a bimolecular nucleophilic attack to the radical cation. A secondary kinetic isotope effect indicated that the central Si−H bond is not cleaved in the rate-determining step. Apart from substitution products, also hydrosilylation products were identified in the product mixtures. Observation of the substitution products, combined with the kinetic data, point to an bimolecular reaction mechanism involving Si−Si bond cleavage. The products of this nucleophilic substitution can initiate radical chain reactions leading to hydrosilylation products, which can independently also be initiated by dissociation of the radical cations. Application of these data to the attachment of or...

Spin-trapping reactions of a novel gauchetype radical trapper G-CYPMPO

Abstract: Chemical reactions of a novel gauchetype spin trap, G-CYPMPO (sc-5-(5,5-dimethyl-2-oxo-1,3,2-dioxaphosphinan-2-yl)-5-methy-1-pyrroline N-oxide, O1-P1-C6-N1 torsion angle = 52.8°), with reactive oxygen species were examined by pulse radiolysis technique with 35 MeV electron beam and by electron spin resonance spectroscopy after (60)Co γ-ray irradiation. The spin-trapping reaction rate constants of G-CYPMPO toward the hydroxyl radical and the hydrated electron were estimated to be (4.2 ± 0.1) × 10(9) and (11.8 ± 0.2) × 10(9) M(-1)s(-1), respectively. Half-lives of the spin adducts, hydroxyl radical, and perhydroxyl radical adducted G-CYPMPO were estimated to be ∼35 and ∼90 min, respectively. A comparison of the results with earlier reports using different radical sources suggests that the purity of the solution and/or the radical generation technique may influence the stability of the spin adducts.

Synthesis, Characterization, and Photoinduced Energy and Electron Transfer in a Supramolecular Tetrakis (Ruthenium(II) Phthalocyanine) Perylenediimide Pentad

Abstract: Metal coordination was probed as a versatile approach for designing a novel electron donor/acceptor hybrid [PDIpy(4){Ru(CO)Pc}(4)] (1), in which four pyridines placed at the bay region of a perylenediimides (PDIpy(4)) coordinate with four ruthenium phthalocyanine units [Ru(CO)Pc]. This structural motif was expected to promote strong electronic coupling between the electron donors and the electron acceptor, a hypothesis that was confirmed in a full-fledged physicochemical investigation focusing on the ground and excited state reactivities. As far as the ground state is concerned, absorption and electrochemical assays indeed reveal a notable redistribution of electron density, that is, from the electron-donating [Ru(CO)Pc] to the electron-accepting PDIpy(4). The most important thing to note in this context is that both the [Ru(CO)Pc] oxidation and the PDIpy(4) reduction are rendered more difficult in 1 than in the individual building blocks. Likewise, in the excited state, strong electronic communication is the inception for a rapid charge-transfer process in photoexcited 1. Regardless of exciting [Ru(CO)Pc] or PDIpy(4), spectral characteristics of the [RuPc] radical cation (broad absorptive features from 425 to 600 nm with a maximum at 575 nm, as well as a band centered at 725 nm) and of the PDI radical anion (780 nm maximum) emerge. The correspondingly formed radical ion pair state lasts for up to several hundred picoseconds in toluene, for example. On the other hand, employing more polar solvents, such as dichloromethane, destabilizes the radical ion pair state.

Light harvesting phthalocyanine/subphthalocyanine system: intermolecular electron-transfer and energy-transfer reactions via the triplet subphthalocyanine

Abstract: Photoinduced electron transfer from the electron-donating zinc tetra-tert-butylphthalocyanine, ZnTBPc, to the electron-accepting dodecafluorosubphthalocyanine, SubPcF12, in the polar benzonitrile has been investigated with nanosecond laser photolysis method. The examined ZnTBPc/SubPcF12 mixture absorbs the light in a wide section of the UV/vis/NIR spectra. Owing to the particular electronic properties of both entities, such combination seems to be perfectly suited for the study of intermolecular electron-transfer process in the polar solvents via the triplet-excited state of SubPcF12. Upon excitation of SubPcF12 with 570 nm laser light in polar benzonitrile (es = 25.2), the electron transfer from ZnTBPc to the triplet-excited state of SubPcF12 was confirmed by observing the transient absorption bands of ZnTBPc radical cation and SubPcF12 radical anion in the visible and near-IR region. On addition of an appropriate electron acceptor with excellent electron-accepting properties, namely dicyanoperylene-3,4,9,10-bis(dicarboximide) (PDICN2), the anion radical of SubPcF12 transfers to the PDICN2 yielding the PDICN2 radical anion. These observations confirm the photosensitized electron-transfer/electron-mediating cycle of ZnTBPc/SubPcF12/PDICN2 system. In non-polar toluene (es = 2.2), the energy-transfer process from the triplet-excited state of SubPcF12 to the low-lying triplet state of ZnTBPc was confirmed by the consecutive appearance of the triplet ZnTBPc.

Charge transfer events in semiconducting single-wall carbon nanotubes.

Abstract: Electron-donating ferrocene units have been attached to SWNTs, with different degrees of functionalization. By means of a complementary series of novel spectroscopic techniques (i.e., steady-state and time-resolved), we have documented that mutual interactions between semiconducting SWNT and the covalently attached electron donor (i.e., ferrocene) lead, in the event of photoexcitation, to the formation of radical ion pairs. In the accordingly formed radical ion pairs, oxidation of ferrocene and reduction of SWNT were confirmed by spectroelectrochemistry. It is, however, shown that only a few semiconducting SWNTs [i.e., (9,4), (8,6), (8,7), and (9,7)] are susceptible to photoinduced electron transfer processes. These results are of relevant importance for the development of SWNT-based photovoltaics.

Effect of the N-terminal basic residue on facile Cα-C bond cleavages of aromatic-containing peptide radical cations.

Abstract: Fragmentation of radical cationic peptides [R(G)n−2X(G)7−n]˙+ and [R(G)m−2XG]˙+ (X = Phe or Tyr; m = 2–5; n = 2–7) leads selectively to an+ product ions through in situ Cα–C peptide backbone cleavage at the aromatic amino acid residues. In contrast, substituting the arginine residue with a less-basic lysine residue, forming [K(G)n−2X(G)7−n]˙+ (X = Phe or Tyr; n = 2–7) analogs, generates abundant b–y product ions; no site-selective Cα–C peptide bond cleavage was observed. Studying the prototypical radical cationic tripeptides [RFG]˙+ and [KFG]˙+ using low-energy collision-induced dissociation and density functional theory, we have examined the influence of the basicity of the N-terminal amino acid residue on the competition between the isomerization and dissociation channels, particularly the selective Cα–C bond cleavage via β-hydrogen atom migration. The dissociation barriers for the formation of a2+ ions from [RFG]˙+ and [KFG]˙+via their β-radical isomers are comparable (33.1 and 35.0 kcal mol−1, respectively); the dissociation barrier for the charge-induced formation of the [b2 − H]˙+ radical cation from [RFG]˙+via its α-radical isomer (39.8 kcal mol−1) was considerably higher than that from [KFG]˙+ (27.2 kcal mol−1). Thus, the basic arginine residue sequesters the mobile proton to promote the charge-remote selective Cα–C bond cleavage by energetically hindering the competing charge-induced pathways.

Competitive Adsorption and Interaction of Benzene and Oxygen on Cu/HZSM5 Zeolites

Abstract: The competitive adsorption of benzene and oxygen on Cu/HZSM5 zeolites has been studied by FTIR and EPR to understand the gas-phase oxidation of benzene to phenol using molecular oxygen. Benzene proved to be the stronger adsorbate on copper, leading to a poisoning of the catalyst. Phenol, water, and deep combustion products could be detected. The evolution of the benzene radical cation was observed in situ, while benzene and oxygen were adsorbed on the sample either simultaneously or alternatingly. At the same time, the total spin density of the sample and thus the oxidation state of copper were monitored. A new reaction path based on the interplay of the zeolitic hydroxyl groups and the exchanged copper ions was proposed. The benzene resides inside the zeolites channel intersections, but due to benzene molecules blocking the channels, the products are hardly able to diffuse out and thus have to be extracted.

A Fully Conjugated TTF–π–TCAQ System: Synthesis, Structure, and Electronic Properties

Abstract: The synthesis of the first fully conjugated tetrathiafulvalene-tetracyano-p-quinodimethane ((TTF)-TCNQ)-type system has been carried out by means of a Julia-Kocienski olefination reaction. In particular, a tetracyanoanthraquinodimethane (TCAQ) formyl derivative and two new sulfonylmethyl-exTTFs (exTTF = 2-[9-(1,3-dithiol-2-ylidene)anthracen-10(9H)-ylidene]-1,3-dithiole)--prepared as new building blocks--were linked. A variety of experimental conditions reveal that the use of sodium hexamethyldisilazane (NaHMDS) as base in THF afforded the E olefins with excellent stereoselectivity. Theoretical calculations at the B3LYP/6-31G** level point to highly distorted exTTF and TCAQ that form an almost planar stilbene unit between them. Although calculations predicted appreciable electronic communication between the donor and the acceptor, cyclic voltammetric studies did not substantiate this effect. It was only in photophysical assays that the electronic communication emerged in the form of a charge-transfer (CT) absorption and emission. Once photoexcited (i.e., the locally excited state or excited charge-transfer state), an ultrafast, subpicosecond charge separation leads to a radical ion pair state in which the spectroscopic features of the radical cation of exTTF as well as the radical anion of TCAQ are discernable. The radical ion pair is metastable and undergoes a fast ((1.0±0.2) ps) charge recombination to reconstitute the electronic ground state. Such ultrafast charge separation and recombination processes come as a consequence of the very short vinyl linkage between the two electroactive units.