Electron transfer reactions between the superoxide ion and quinones

The electron transfer reactions of the superoxide ion with benzoquinone, trimethylbenzoquinone, and menadione in dimethylformamide were studied. A procedure of the determination of the relative rate constants of these reactions was developed; the reaction of O? ₂ with butyl bromide was chosen as a standard one. The relative rate constants measured at 20,°, 35°, and 50°C were slightly dependent on the quinone structure. The relationship between the free energy ΔF*of the electron transfer reactions and the standard free energy ΔF^o was discussed. This relationship is proposed as ΔF* = αΔF^o + β, where the proportionality coefficient α is equal to 0.04–0.11 for exothermal reactions and to 0.90–0.96 for endothermal reactions.     

酰基过氧化物的化学 VII: 过氧化环丙基甲酰与硝基烷烃阴离子的单电子转移反应

过氧化环丙基甲酰(CPP)能将硝基仲烷烃阴离子氧化成相应的酮. 产物分析和ESR研究揭示该反应经过由硝基烷烃阴离子向CPP的单电子转移引发步骤及随后的一系列自由基反应步骤, 生DN 主要产物酮和典型的自由基副产物, 如双环丙基、硝基烷基二聚体和能被ESR直接检测的双环丙基氮氧自由基等.     

Electron transfer reactions between vinylmagnesium bromide and phenyl-substituted methyl chlorides

The reaction of vinylmagnesium bromide ( 1 ) with trityl chloride ( 2 ) was investigated and the products were identified as triphenylmethane ( 3 ) (81 %), acetylene ( 4 ) (74%), trityl peroxide ( 6 ) (4%), and minute amounts of triphenylpropene ( 7 ) and p-diphenylmethyltetraphenylmethane ( 8 ). The results indicate that 1 possesses reducing property. The reaction mixture showed an ESR spectrum identical with that for trityl radical and the CIDNP of enhanced absorption for 4-H of l-diphenylmethylene-4-trityl-2,5-cyclohexadiene ( 9 ). Trapping experiment with styrene decreased the amount of 3 but had no effect on the formation of 4. The reducing property of 1 is discussed and one electron transfer mechanism for the reaction of 1 with 2 is proposed. S-T₀ mixing between trityl and·MgBr occurred and the coupling of polarized trityl radical with another trityl radical resulted in the formation of polarized dimeric product. The different character between the reaction of 1 and 2 and that of 1 and diphenylmethyl chloride ( 10 ) may be rationalized by the different electron-accepting abilities between 2 and 10. The origin of CIDNP effect shown by the product tetraphenylethane ( 11 ) for the reaction of 1 with 10 is also explained according to Kaptein's sign rule.     

Electron transfer in some nucleophilic reactions

Using kinetic, steric and thermodynamic results it is argued that the rate-determining step in some aliphatic, nucleophilic substitutions is the transfer of an electron.     

Electron transfer reactions to electroactive polymers

In the electrochemical reduction of an electroactive polymer several thousand electrons may be transferred to a single molecule. For polymers with noninteracting centers, recently developed theory indicates that the current—potential response would be identical with that of the corresponding molecule solvation, structural change, and variation in ionic environment, and may be used to probe these factors. A series of model compound of poly(vinylbenzophenone) and of poly(vinylbenzophenone-co-styrene) were prepared and investigated. In this study samples of various constant chain length with differing spacing of electroactive centers were employed. The current—potential measurements corresponded to the ideal theoretical case. This indicates that the behavior of the electroactive groups attached to the polymer chain was not influenced by the macromolecular environment or by neighboring group interaction.     

Electron transfer reactions at gold nanoparticles

It is demonstrated that scanning electrochemical microscopy can be used to investigate the kinetics of electron transfer reactions catalysed by metal nanoparticles supported on an insulating substrate.     

Electron transfer reactions of fluorotyrosyl radicals

The complex Re(bpy)(CO)3CN is an excited state oxidant of tyrosine upon deprotonation of the tyrosyl phenol. A series of Re(bpy-FnY)(CO)3CN complexes ([Re]-FnY: [Re]-Y, [Re]-3-FY, [Re]-3,5-F2Y, [Re]-2,3-F2Y, [Re]-2,3,5-F3Y, [Re]-2,3,6-F3Y, and [Re]-F4Y) have been prepared so as to vary the FnY*/FnY- reduction potential and thus the driving force for electron transfer in this system. Time-resolved emission and nanosecond absorption spectroscopies have been used to measure the rates for charge separation, CS, and charge recombination, CR, for each complex. A driving force analysis reveals that CS is well described by Marcus' theory for ET, is strongly driving force dependent (activated), and occurs in the normal region for ET. CR, on the other hand, is weakly driving force dependent (near activationless) and occurs in the inverted region for ET. These data demonstrate that fluorotyrosines will be powerful probes for unraveling charge transport mechanisms in enzymes that utilize tyrosyl radicals.     

Electron transfer reactions relevant to atom transfer radical polymerization

The continuous development of more active and stable catalysts in atom transfer radical polymerization (ATRP) has increasingly required a thorough knowledge of concurrent electron transfer reactions that can affect catalyst performance. Special attention is provided in this short review to such processes, including disproportionation, most pronounced in Cu-mediated ATRP, the reduction of radicals to carbanions or oxidation to carbocations, and radical coordination to the metal catalyst resulting in the interplay of controlled radical polymerization mechanisms.     

Electron transfer reactions: generalized spin-boson approach

We introduce a mathematically rigorous analysis of a generalized spin-boson system for the treatment of a donor–acceptor (reactant-product) quantum system coupled to a thermal quantum noise. The donor/acceptor probability dynamics describes transport reactions in chemical processes in presence of a noisy environment – such as the electron transfer in a photosynthetic reaction center. Besides being rigorous, our analysis has the advantages over previous ones that (1) we include a general, non energy-conserving system-environment interaction, and that (2) we allow for the donor or acceptor to consist of multiple energy levels lying closely together. We establish explicit expressions for the rates and the efficiency (final donor–acceptor population difference) of the reaction. In particular, we show that the rate increases for a multi-level acceptor, but the efficiency does not.     

酰基过氧化物的化学 VI:过氧化苯甲酰与硝基烷烃阴离子间的单电子转移反应一种合成醛、酮的新反应

在50℃乙腈溶液中,用苯甲酰过氧化物(BPO)能顺利地转化硝基烷烃钠盐为相应的醛和酮.用ESR和IR研究的结果表明反应包含一个从碳阴离子到BPO的电子转移过程.主要产品醛、酮以及苯甲酰亚硝酸酯均来自不稳定的苯甲酸硝基烷基酯的碎裂反应,后者是在离子-自由基对分裂时产生的硝烷基和过氧化苯甲酰基自由基的笼形复合过程中形成的,这些新反应可用于从硝基烷烃合成醛和酮.     

Aromatic fluoro derivatives. LXX. Interaction of pentafluorobenzoyl peroxide with decafluorodiphenyl

The interaction of pentafluorobenzoyl peroxide with decafluorodiphenyl (at 80°C and 200°C) results in the preferential formation of perfluorotetrakisphenyl-tetrahydrodiphenyls (a mixture of isomers). Defluorination of perfluoro-3,3′,5,5′-tetrakisphenyl-1,1′,2,2′-tetrahydrodiphenyl-1,1′ gives rise to perfluoro-3,3′,5,5′-tetrakisphenyldiphenyl-1,1′. The results obtained confirm the proposition about the 1,2-shift of the fluorine atom in the initial σ-complex formed by the addition of the pentafluorophenyl radical to decafluorodiphenyl.     

Electron transfer reactions between copper(II) porphyrin complexes and various oxidizing reagents in acetonitrile

Homogeneous electron transfer reactions of the Cu(II) complexes of 5,10,15,20-tetraphenylporphyrin (TPP) and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) with various oxidizing reagents were spectrophotometrically investigated in acetonitrile. The reaction products were confirmed to be the pi-cation radicals of the corresponding Cu(II)-porphyrin complexes on the basis of the electronic spectra and the redox potentials of the complexes. The rate of the electron transfer reaction between the Cu(II)-porphyrin complex and solvated Cu(2+) was determined as a function of the water concentration under the pseudo first-order conditions where Cu(2+) is in large excess over the Cu(II)-porphyrin complex. The decrease in the pseudo first-order rate constant with increasing the water concentration was attributed to the stepwise displacement of acetonitrile in [Cu(AN)(6)](2+)(AN = acetonitrile) by water, and it was concluded that only the Cu(2+) species fully solvated by acetonitrile, [Cu(AN)(6)](2+), possesses sufficiently high redox potential for the oxidation of Cu(ii)-OEP and Cu(ii)-TPP. The reactions of the Cu(II)-porphyrin complexes with other oxidizing reagents such as [Ni(tacn)(2)](3+)(tacn = 1,4,7-triazacyclononane) and [Ru(bpy)(3)](3+)(bpy = 2,2'-bipyridine) were too fast to be followed by a conventional stopped-flow technique. Marcus cross relation for the outer-sphere electron transfer reaction was used to estimate the rate constants of the electron self-exchange reaction between Cu(II)-porphyrin and its pi-cation radical: log(k/M(-1) s(-1))= 9.5 +/- 0.5 for TPP and log(k/M(-1) s(-1))= 11.1 +/- 0.5 for OEP at 25.0 degrees C. Such large electron self-exchange rate constants are typical for the porphyrin-centered redox reactions for which very small inner- and outer-sphere reorganization energies are required.     

Electron transfer reactions. Reaction of dibenzobarrelenes with potassium

The results of our studies on potassium-induced transformations of several bridgehead-substituted dibenzobarrelenes are presented. The dibenzobarrelenes 1b,c,e,f having methyl, hydroxymethyl, methoxy and phenyl groups, respectively, at the bridgehead position and the bridgehead-dimethyl derivative 1g give, on treatment with potassium in THF, the corresponding anthracenes 8b,c,e-g, dihydrodibenzobarreneses 6b,c,e-g and benzoic acid (5). The dibenzobarrelenes 1b,g give, in addition to 8b,g, 6b,g and 5, the corresponding mono-debenzoylated products 7b,g, whereas the methoxy derivative 1e gives both anthraquinone (9) and an enol derivaiive 11, besides 8e, 6e and 5. In contrast, the reaction of the hydroxy derivative 1d with potassium in THF gives a mixture of 5, anthracene (8a), the anthrone 10 and the anthrol 12. To assess the role of oxygen in these reactions, if any, the reactions of some representative substrates such as 1b,d,e with potassium in THF, saturated with oxygen and with potassium superoxide have been studied. Cyclic voltammetric studies have been carried out to measuee the reduction potentials for both one electron and two electron processes, leading to the generation of the corresponding radical anions and dianion intermediates. The radical anions of 1b-g have also been generated pulse radiolytically in methanol and their spectra show absorption maxima in the regions 310-390 and 400-450 nm.     

Electron transfer reactions of cytochrome c at metal electrodes

The heterogeneous electron transfer reactions of cytochromec occurring at platinum, gold and mercury electrodes are shown to be quasi-reversible. In each case the electrodes have not been modified and the cytochromec samples are native. This work extends previous work and demonstrates that biological molecule electron transfer reactions can be studied at clean metal surfaces to gain fundamental knowledge of the mechanisms of these reactions.     

Electron transfer between colloidal ZnO nanocrystals

Colloidal ZnO nanocrystals capped with dodecylamine and dissolved in toluene can be charged photochemically to give stable solutions in which electrons are present in the conduction bands of the nanocrystals. These conduction-band electrons are readily monitored by EPR spectroscopy, with g* values that correlate with the nanocrystal sizes. Mixing a solution of charged small nanocrystals (e(-)(CB):ZnO-S) with a solution of uncharged large nanocrystals (ZnO-L) caused changes in the EPR spectrum indicative of quantitative electron transfer from small to large nanocrystals. EPR spectra of the reverse reaction, e(-)(CB):ZnO-L + ZnO-S, showed that electrons do not transfer from large to small nanocrystals. Stopped-flow kinetics studies monitoring the change in the UV band-edge absorption showed that reactions of 50 μM nanocrystals were complete within the 5 ms mixing time of the instrument. Similar results were obtained for the reaction of charged nanocrystals with methyl viologen (MV(2+)). These and related results indicate that the electron-transfer reactions of these colloidal nanocrystals are quantitative and very rapid, despite the presence of ~1.5 nm long dodecylamine capping ligands. These soluble ZnO nanocrystals are thus well-defined redox reagents suitable for studies of electron transfer involving semiconductor nanostructures.     

Electron transfer between two stable radicals

Electron spin resonance (ESR) is a sensitive tool to detect the electron-transfer processes in some specified systems from the change of the ESR signals. In this paper, the interaction between poly(N,N-dimethylaniline) (PDMA) and 1,1-diphenyl-2-picrylhydrazyl (DPPH) was investigated by ESR spectra based on the stability of their radicals in ethanol. It is shown that an electron-transfer process occurs from nitrogen atoms of PDMA molecules to those of DPPH molecules which makes PDMA form more stable radical cations and eliminates the radicals on DPPH.     

Electron transfer in reactions of tert-perfluoroalkyl bromides with alkenes and nucleophiles

Tertiary perfluoroalkyl bromides (R_FBr) in nonpolar solvents under mild conditions can be added to the multiple bond of terminal alkenes, alkynes, and butadiene. Slow addition to alkenes at 20°C is accelerated in proton-donating solvents and is catalyzed by readily oxidizable nucleophiles. Bromination of the multiple bond and formation of R_FBr reduction products suggests a radicalchain mechanism initiated by electron transfer to the R_FBr molecule.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2810–2815, December, 1991.     

Electron transfer reactions. Reactions of epoxyketones and benzoylaziridines with potassium

The results of our studies on the reaction of some epoxyketones and aziridines with potassium in tetrahydrofuran (THF) are presented. Treatment of trans-l,3-diphenyl-2,3-epozypropan-1-one (1a.) with, potassium, for example, gives a mixture of acetophenone (9), the chalcone 4a, the dihydrochalcone 14a, the cyclopentene isomers 20a and 21a, the hydroxy acid 10a and benzoic acid 6, whereas the reaction of 1a with potassium, under oxygen saturation, gives a mixture of 4a, 14a, 6, the diketone 17a, the dihydroxycarboxylic acid 7a and the hydroxyfuranone 19a. The reaction of 1a. with potassium superoxide, however gives a mixture of 6, 7a and 19a. Similar results are obtained in the reaction of trans-2,3-epoxy-1-phenyl-3-p-tolylpropan-1-one 1b with potassuum. The reaction of 7-oxa-2,3-dibenzoylbtcyclo[2 2 1]hepta-2,5-diene (22a) with potassium gives a mixture of 6 and o-dibenzoylbenzene (26a), whereas 2,3-dibenzoyl-1,4-dihydro-1,4-diphenyl-1,4-epoxynaphthalene (22b), under analogous conditions, gives a mixture of 6, 1,3-diphenylisobenzofuran (25b), 26a, 2,3-dibenzoyl-1,4-diphenylnapntnelene (26b) and 2-benzoyl-1,4-dtphenylnaphthalene (27b). Treatment of the benzoylaztridines 28a-d with potassium gives the stilbenes, 33a,c, the hydroxy amides, 34,a,c,d, and carboxylic acids 6, 11b, whereas the aziridine 35, on treatment with potassium, gives a mixture of the isoquinoline 37 and the phthalimidine 39. Cyclic voltammetric and pulse radiolysis studies have been carried out, in an attempt to characterize the radical anion intermediates involved in these reactions. Document No. NDRL-3120 from the Notre Dame Radiation Laboratory and No. RRLT-PRU-5 from the Regional Research Laboratory Trivandrum.     

Electron transfer between cytochrome c and copper enzymes

The electron transfer between cytochrome c and ascorbate oxidase or laccase from Coriolus hirsutus was investigated using both an electrochemical and a spectrophotometric method. A quasi-reversible cyclic volammogram of cytochrome c was observed on a gold electrode modified with 4,4′-dithiodipyridine. The addition of laccase resulted in the appearance of a catalytic current due to the regeneration of ferricytochrome c by laccase in the presence of oxygen. The second-order rate constant of the reaction between cytochrome c and laccase is calculated to be 9.2 × 10³ M⁻¹ s⁻¹ in 50 mM phosphate buffer of pH 5.8. The reaction rate with ascorbate oxidase is almost three orders of magnitude slower. The difference in the redox potential is considered to be the driving force of the reaction between cytochrome c and the copper proteins investigated.