Presence Of Proton Donors Research Articles

A Comparative Study of Electrochemical Reduction of Levulinic Acid on Various Electrodes in Organic Solvents.

Studies on the electrochemical hydrogenation (ECH) of levulinic acid (LA) to valeric acid (VA) or γ-valerolactone (GVL) have mainly focused on the electrochemical reduction of LA in acidic aqueous solutions. However, the narrow range of applied potentials has hindered understanding of some mechanistic aspects of LA electrochemical conversion. Earlier, we discovered that employing proton-deficient non-aqueous reaction media provides more comprehensive insights into the mechanism of LA electrochemical reduction. Here, we conducted further investigations into the LA electroreduction process using cyclic voltammetry in various organic solvents on a Pt electrode and on various electrode materials in acetonitrile, both with and without the addition of proton donors. The products of the ECH processes were identified using HPLC. The solvent nature, the presence of proton donors, the electrode material, and the applied potential strongly influence the LA electroreduction process. This study reveals that LA, in the presence proton donors, can undergo electrochemical reduction through different pathways, depending on the difference (ΔE1/2) between the reduction half-wave potential of protons and LA on a certain electrode. When the difference is large, the LA reduction is incomplete and the formation of GVL is observed. Under the close reduction potentials of protons and LA, LA can be completely reduced to VA.

Effect of cathode material on electrochemical reduction of levulinic acid to γ-valerolactone and valeric acid in aqueous and organic media

The electrochemical conversion of biobased levulinic acid (LA) into renewable chemicals and biofuel precursors represents an important and reasonable alternative to the high temperature conventional catalytic processes of great importance for the development of a sustainable and cost-effective biorefinery. The establishment of the mechanism of levulinic acid reduction is a promising strategy in choosing the optimal electrocatalyst for the redox-transformation of biobased substrates. Herein, we report a new approach to study an electrochemical reduction mechanism of levulinic acid using of proton-deficient non-aqueous reaction media. The electrochemical reduction of levulinic acid to γ-valerolactone (GVL) and valeric acid (VA) in aqueous and organic solutions on various electrodes (glassy carbon, graphite, Al, Pb) was studied. The mechanism of LA electrochemical reduction and major reaction products significantly was found to depend on the solvent, the presence of proton donors, the material of cathode, and the magnitude of the applied potential. In an aqueous solution the process proceeded with the formation of valeric acid on all the electrodes studied. In acetonitrile in the presence of protons, the electrochemical reduction of LA proceeded by various mechanisms, both with the participation of atomic hydrogen and the protonated form of LA, and led to the formation of GVL and/or VA. The difference (ΔE1/2) between the reduction half-wave potential of protons and levulinic acid was found to play an important role in the reduction pathway of LA carbonyl group. At a large ΔE1/2, as in the case of the GC electrode, the LA reduction resulted in the GVL formation. LA can be completely reduced to VA by transferring four electrons due to the close reduction potentials of protons and LA (a low ΔE1/2), as on a Pb electrode. The pathway depends on the conditions of the reduction process and can be estimated based on electrochemical data obtained in the study of reaction products in organic media.

Electrocatalytic Oxygen Reduction Reaction Improved By Facilitated Proton Transfer

Electrocatalytic oxygen reduction reaction (ORR) in aqueous media proceeds along a pathway having multiple steps in which multiple surface intermediates are involved (* -> *OO -> *OOH -> *O -> *OH where * = active site of catalyst). Proton supplied from media plays a crucial role on the ORR mechanism. Therefore, pH of media affects the kinetics of ORR significantly. In the same vein, it is expected that the ORR activity is improved in the presence of proton donor (HD) around active sites of catalysts. In this presentation, we demonstrate a possibility that a series of proton donors are involved in protonation of the surface intermediates. The ORR overpotential was improved by 100 mV at pH 13 on transition metal oxide catalysts. The overpotential improvement was intensified as the pH of media decreased. It is suspected that the pH affects deprotonation of the proton donors (HD -> H+ + D-). High pH deprotonate the proton donors so that the effects of proton donors decrease. However, the proton of HD survives in acidic media so that the proton transfer from HD to active site is facilitated. The ORR overpotential gain was as large as 400 mV at pH 4 even if the oxide catalyst was not stable at the acidic pH. At pH 7, the overpotential improvement was estimated at 200 mV.

Electrochemical behavior of cinnamic acid associates on carbositall electrode in dimethylformamide in the presence of external proton donors

The electroreduction (ER) of trans-cinnamic acid (CA) and its associates in an aprotic solvent, dimethylformamide (DMF), in the presence of proton donors: weak (H2O) and moderate (acetic acid AcOH) and also with addition of R4NOH (R = Et; Bu) to the CA solution in DMF is studied by the methods of cyclic voltammetry (CVA) and chronoamperometry on a cathode with the high hydrogen overpotential—carbositall. The electrochemical behavior of AcOH in DMF and in the mixture 16% H2O–DMF is examined. The results obtained show that CA associates in DMF can include AcOH (or H2O) molecules to form sufficiently stable mixed associates. The anionic species that appear at the reduction of such associates decompose with abstraction of the original CA, which results in the appearance of inverted, i.e., cathodic peaks at ca. -2.0 V in the anodic CVA branch. A comparative study of methyl cinnamate is carried out under similar conditions on the carbositall electrode. In contrast to CA, the behavior of methyl cinnamate in DMF in the presence of a proton donor (AcOH) corresponds to the classical ER scheme for α,β-unsaturated esters.

Electrocatalytic CO2 Reduction with Homogeneous Transition Metals: Early Metals

Homogeneous electrocatalysts for CO2 reduction have been explored by many groups. However, the focus has been on systems based on groups 7 or later in the periodic table. Recently, a few examples of catalytic CO2 reduction have been observed at group 6 metals (Cr, Mo, and W) or earlier groups. Group 6 is notable as it is present in biological oxoreductase enzymes such as Formate Dehydroganse, which can convert between CO2 and formate. The electrochemical response of the homoleptic group 6 carbonyls was explored at a glassy carbon electrode. In addition, other “early” metal systems were examined. The complexes were examined under aprotic conditions as well as in the presence of proton donors under argon and carbon dioxide. The results of the work with undergraduate researchers will be discussed. The integration of research and teaching chemistry concepts to undergraduates using electrochemistry will also be discussed.

Self sensitized photooxidation of N-methyl phenothiazine: acidity control of the competition between electron and energy transfer mechanisms

The reaction pathways following electronic excitation of 10-methyl phenothiazine (MPS) in the presence of oxygen have been investigated as a contribution to establish the mechanisms involved in the phototoxic reactions related to phenothiazine drugs. In the context of previously published results, the pathways of oxidation via the radical cation and/or by reactive oxygen species, such as singlet oxygen and superoxide anion, are of particular interest. The effects of polarity of the medium as well as of proton donors on the different reaction pathways, in particular on the formation of reactive oxygen species and the intermediates of the oxidation of 10-methyl phenothiazine, have been investigated. No reaction was observed in non-polar solvents. In polar solvents, both self-sensitized and sensitized singlet oxygen generation lead to the oxidation of MPS and the production of 10-methyl phenothiazine sulfoxide (MPSO) most probably via a zwitterionic persulfoxide. During self-sensitized photooxidation of MPS in the presence of proton donors, such as carboxylic acids, the zwitterionic intermediate is protonated to the corresponding cation that in turn facilitates the reaction with a second molecule of MPS. In the presence of strong acids however, the formation of the radical cation of MPS and of the superoxide anion, by electron transfer from the triplet excited state of MPS to molecular oxygen, competes efficiently with singlet oxygen formation. In this case, the scavenging of the superoxide anion by protons to yield its conjugated acid (hydroperoxyl radical) and the subsequent disproportionation of the latter prevents back electron transfer.

Valence anions of N-acetylproline in the gas phase: computational and anion photoelectron spectroscopic studies.

We report the photoelectron spectrum of anionic N-acetylproline, (N-AcPro)(-), measured with 3.49 eV photons. This spectrum, which consists of a band centered at an electron binding energy of 1.4 eV and a higher energy spectral tail, confirms that N-acetylproline forms a valence anion in the gas phase. The neutrals and anions of N-AcPro were also studied computationally at the B3LYP∕6-31++G(d,p) level. Based on the calculations, we conclude that the photoelectron spectrum is due to anions which originated from proton transfer induced by electron attachment to the π* orbital localized at the acetyl group of N-AcPro. We also characterized the energetics of reaction paths leading to pyrrolidine ring opening in the anionic N-AcPro. These data suggest that electron induced decomposition of peptides/proteins comprising proline strongly depends on the presence of proton donors in the close vicinity to the proline residue.

Electron- and Hydride-Transfer Reactivity of an Isolable Manganese(V)−Oxo Complex

The electron-transfer and hydride-transfer properties of an isolated manganese(V)−oxo complex, (TBP8Cz)Mn(V)(O) (1) (TBP8Cz = octa-tert-butylphenylcorrolazinato) were determined by spectroscopic and kinetic methods. The manganese(V)−oxo complex 1 reacts rapidly with a series of ferrocene derivatives ([Fe(C5H4Me)2], [Fe(C5HMe4)2], and ([Fe(C5Me5)2] = Fc*) to give the direct formation of [(TBP8Cz)Mn(III)(OH)]− ([2-OH]−), a two-electron-reduced product. The stoichiometry of these electron-transfer reactions was found to be (Fc derivative)/1 = 2:1 by spectral titration. The rate constants of electron transfer from ferrocene derivatives to 1 at room temperature in benzonitrile were obtained, and the successful application of Marcus theory allowed for the determination of the reorganization energies (λ) of electron transfer. The λ values of electron transfer from the ferrocene derivatives to 1 are lower than those reported for a manganese(IV)−oxo porphyrin. The presumed one-electron-reduced intermediate, a Mn(IV) complex, was not observed during the reduction of 1. However, a Mn(IV) complex was successfully generated via one-electron oxidation of the Mn(III) precursor complex 2 to give [(TBP8Cz)Mn(IV)]+ (3). Complex 3 exhibits a characteristic absorption band at λ(max) = 722 nm and an EPR spectrum at 15 K with g(max)′ = 4.68, g(mid)′ = 3.28, and g(min)′ = 1.94, with well-resolved 55Mn hyperfine coupling, indicative of a d3 Mn(IV)S = 3/2 ground state. Although electron transfer from [Fe(C5H4Me)2] to 1 is endergonic (uphill), two-electron reduction of 1 is made possible in the presence of proton donors (e.g., CH3CO2H, CF3CH2OH, and CH3OH). In the case of CH3CO2H, saturation behavior for the rate constants of electron transfer (k(et)) versus acid concentration was observed, providing insight into the critical involvement of H+ in the mechanism of electron transfer. Complex 1 was also shown to be competent to oxidize a series of dihydronicotinamide adenine dinucleotide (NADH) analogues via formal hydride transfer to produce the corresponding NAD+ analogues and [2-OH]−. The logarithms of the observed second-order rate constants of hydride transfer (k(H)) from NADH analogues to 1 are linearly correlated with those of hydride transfer from the same series of NADH analogues to p-chloranil.

Integrated Study of the Dinitrobenzene Electroreduction Mechanism by Electroanalytical and Computational Methods

Electroreduction of 1,2-, 1,3-, and 1,4-dinitrobenzenes in DMF has been investigated by a set of experimental (cyclic voltammetry, chronoamperometry, and controlled potential electrolysis) and theoretical methods (digital simulation and quantum chemical calculations). The transformation of only one nitro group is observed in the presence of proton donors. The process selectivity is provided by reactions of radical anions' intermediate products. The key reactions here are protonation of radical anions of nitrosonitrobenzenes and N-O bond cleavage in radical anions of N-(nitrophenyl)-hydroxylamines.

One-Electron Redox Processes in a Cyclic Selenide and a Selenoxide: A Pulse Radiolysis Study

One-electron redox reactions of cyclic selenium compounds, DL-trans-3,4-dihydroxy-1-selenolane (DHS(red)), and DL-trans-3,4-dihydroxy-1-selenolane oxide (DHS(ox)) were carried out in aqueous solutions using nanosecond pulse radiolysis, and the resultant transients were detected by absorption spectroscopy. Both *OH radical and specific one-electron oxidant, Br(2)(*-) radical reacted with DHS(red) to form similar transients absorbing at 480 nm, which has been identified as a dimer radical cation (DHS(red))(2)(*+). Secondary electron transfer reactions of the (DHS(red))(2)(*+) were studied with 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS(2-)) and superoxide (O(2)(*-)) radicals. The bimolecular rate constants for the electron transfer reaction between (DHS(red))(2)(*+) with ABTS(2-) was determined as 2.4 +/- 0.4 x 10(9) M(-1) s(-1). From this reaction, the yield of (DHS(red))(2)(*+) formed on reaction with *OH radical was estimated in the presence of varying phosphate concentrations. (DHS(red))(2)(*+) reacted with O(2)(*-) radical with a bimolecular rate constant of 2.7 +/- 0.1 x 10(9) M(-1) s(-1) at pH 7. From the same reaction, the positive charge on (DHS(red))(2)(*+) was confirmed by the kinetic salt effect. HPLC analysis of the products formed in the reaction of (DHS(red))(2)(*+) with O(2)(*-) radicals showed formation of the selenoxide, DHS(ox). In order to know if a similar mechanism operated during the reduction of DHS(ox), its reactions with e(aq)(-) were studied at pH 7. The rate constant for this reaction was determined as 5.6 +/- 0.9 x 10(9) M(-1) s(-1), and no transient absorption could be observed in the wavelength region from 280 to 700 nm. It is proposed that the radical anion (DHS(ox))(*-) formed by a one-electron reduction would get protonated to form a hydroxyl radical adduct, which in presence of proton donors, would undergo dehydration to form DHS(*+). Evidence for this mechanism was obtained by converting DHS(*+) to (DHS(red))(2)(*+) with the addition of DHS(red) to the same system. Quantum chemical calculations provided supporting evidence for some of the redox reactions.

The role of acids and bases on the electrochemical oxidation of hydroquinone: Hydrogen bonding interactions in acetonitrile

Abstract The electrochemical oxidation of hydroquinone has been investigated in acetonitrile solutions under acidic and basic conditions. In aprotic medium, the oxidation mechanism of pure hydroquinone follows an ECE sequence under a mixed kinetic control by electron and proton transfer. When a strong acid is present in the medium, the stabilization of the radical cation generated in the first oxidation step allows a mechanistic transition from ECE to DISP2. The oxidation mechanism is also sensitive to the presence of weak and strong hydrogen bonding acceptors, such as dimethylsulfoxide and the acetate ion. Under these conditions, the oxidation is less anodic, which can be explained by formation of association complexes in which proton transfer is possible after the electron transfer. It is shown that the oxidation wave observed in typical experiments of quinones in the presence of proton donors is attributed to hydroquinone associated with the conjugated base of the proton donor. Cyclic voltammetry, 1H NMR spectrometry and X-ray diffraction were used to obtain the information here discussed.

Effect of the potential of an external electron donor on C2H2 reduction catalyzed by the nitrogenase active center (FeMoco) isolated from the enzyme

The effect of potential value and chemical properties of an external electron donor on C2H2 reduction catalyzed by nitrogenase active center (cluster [(μ6-N)Fe7MoS9·homocitrate] FeMoco isolated from the enzyme) has been investigated in the presence of proton donors of different acidity. The temperature—reaction rate dependences of these reactions have been studied. It has been shown that the rate-limiting steps of the reactions differ depending on the proton donor used. When thiophenol or water are used as proton donors, and electrochemical step — the electron transfer from cathode to adsorbed catalytic cluster — has been found to be a rate-limiting one. The effective activation energy of ethane formation as a product of four-electron C2H2 reduction is found to be 1.5 times lower than that of ethylene, namely, ∼13 kcal mol−1. When stronger acid, pentafluorothiophenol, is used as a proton donor, the chemical step of intramolecular rearrangement of the catalyst—substrate complex taking place in solution becomes a rate-limiting one. The effective activation energies of both ethylene and ethane become equal to 32 kcal mol−1.

Correlation between pK(a) and reactivity of quinuclidine-based catalysts in the Baylis-Hillman reaction: discovery of quinuclidine as optimum catalyst leading to substantial enhancement of scope.

The reactivity of a variety of quinuclidine-based catalysts in the Baylis-Hillman reaction has been examined, and a straightforward correlation between the basicity of the base and reactivity has been established, without exception. The following order of reactivity was established with pK(a)'s of the conjugate acids (measured in water) given in parentheses: quinuclidine (11.3), 3-hydroxyquinuclidine (9.9), DABCO (8.7), 3-acetoxyquinuclidine (9.3), 3-chloroquinuclidine (8.9), and quinuclidinone (7.2). The higher than expected reactivity of DABCO, based on its pK(a), was analyzed by comparing the relative basicity of DABCO and 3-acetoxyquinuclidine in DMSO. It was found that in aprotic solvent, DABCO was 0.6 pK(a) units more basic than 3-acetoxyquinuclidine, thus establishing a direct link between pK(a) of the amine and its reactivity. In contrast to previous literature work that reported the contrary, quinuclidine, which has the highest pK(a), was found to be the most active catalyst. The reaction profile with quinuclidine showed significant autocatalysis, which suggested that the presence of proton donors might further enhance rates. Thus, a series of additives bearing polar X-H bonds were investigated and it was found that methanol, triethanolamine, formamide, and water all provided additional acceleration. Methanol was found to be optimum, and the powerful combination of quinuclidine with methanol was tested with a host of aldehydes and Michael acceptors. Not only were the reactions more efficient and faster than previously reported, but now new substrates that were previously unreactive could be employed. Notable examples include the use of acetylenic aldehydes and the employment of vinyl sulfones, acrylamides, delta-lactones, and even alpha,beta-unsaturated esters bearing a beta-substituent.

Anions of Transition Metals Carbonyls in Nucleophilic Vinyl Substitution: VII.2* Carbonylates Reaction with 1-Chloro-2-(trifluoromethyl)- and 1-Chloro-2-(perfluoro-tert-butyl)hexafluorocyclopent-1-enes, and with Z and E Isomers of ββ-Chloro-αα,ββ-difluorostyrenes. Halophilic and/or Nucleophilic Reaction mechanism2*2*

Reactions were studied between 1-chloro-2-(trifluoromethyl)-hexafluorocyclopent-1-ene with anions [CpFe(CO)2]-, [Re(CO)5]-, [Mn(CO)5]-, [CpMo(CO)3]-, and [CpW(CO)3]-. The effect of proton donors on the composition of reaction products was established. The reactions with [Re(CO)5]- and [Mn(CO)5]- were shown to proceed quantitatively along nucleophilic route (SN2Vin) resulting in σ-vinyl complexes of rhenium and manganese. The [CpMo(CO)3]- and [CpW(CO)3]- anions turned out to be 104 times less nucleophilic than [Mn(CO)5]-. In this case the main course of the reaction is the metal-halogen exchange with concurrent nucleophilic substitution. In reaction of [CpMo(CO)3]- and [CpW(CO)3]- with 1-chloro-2- (trifluoromethyl)-hexafluorocyclopent-1-ene the σ-vinyl complexes arise along an unusual autocatalytic mechanism: the reaction of carbonyl occurs not with the initial substrate but with pentafluoromethylcyclopent-1-ene that forms from the substrate under the action of a fluoride ion. In an extremely fast reaction of the same substrate with [CpFe(CO)2]K the main process is carbonylate oxidation along unestablished mechanism. The reaction between carbonylates and 1-chloro-2-(perfluoro-tert-butyl)hexafluorocyclopent-1-ene proceeds exclusively as metal-halogen exchange that with [Re(CO)5]- furnishes anionic acyl complex cis[C9F15C(O)Re(CO)4Cl]-. The reaction of Z and E isomers of β-chloro-α,β-difluorostyrenes with [CpFe(CO)2]- in the presence of proton donors confirmed the previously assumed (SN2Vin) mechanism of nucleophilic substitution and also the intermediate formation of alkenyl carbanions in the parallel redox process.

Photochemical and electrochemical behavior of thiophene-S-oxides

The photochemical and electrochemical behavior of thiophene-S-oxides as a class was studied for the first time. It was shown that in both cases deoxygenation of the S—O functionality takes place. The outcome of the photoirradiation is very dependent on the substituent pattern of the starting material. Thiophene-S-oxides show different reduction behaviors in presence and absence of proton donors. In the absence of proton donors the reduction potential of the compounds is dependent on the substituents of the molecules. In the presence of proton donors, the substituents play a less significant role and a number of thiophene-S-oxides were reduced electrochemically to the corresponding thiophenes in presence of a 10-fold excess of benzoic acid. Copyright © 2000 John Wiley & Sons, Ltd.

Some Examples of Phase-Transfer Catalysis Application in Organochlorine Chemistry

Kinetics of phase-transfer catalysed (PTC'd) dehydrochlorination of the α-isomer of hexachlorocyclohexane in the presence of proton donors was investigated. Carboxylic acids and picric acid act as inhibitors. Benzyl alcohol strongly promotes the reaction. The plots of observed rate constants vs phenol and pentachlorophenol concentration have a bell-shaped form that was not observed before. An example of long-chain quats application in the synthesis of the ketamine anaesthetic intermediate by dehydrochlorination in dilute alkali is shown. Another application of phase-transfer catalysis (PTC) in organochlorine chemistry is the aqueous (sodium hypochlorite) chlorination of alkyl aromatic compounds. The results of m-phenoxytoluene chlorination and reaction kinetics in the presence of the polymeric crown are described.

Reactions of Dimethyl Heteroaroylphosphonates with Trialkyl Phosphite in the Presence of Proton Donors

We have previously shown that substituted bcnzoylphosphonates (1; Ar ˭ X-Ph) react with trimethyl phosphite to give anionic intermediates which in the absence of electrophiles decompose to give carbene intermediates (3). The nature of the subsequent reaction products depends on the type and position of the substituents on the aromatic ring.ll1 When proton donors are present the initially formed anionic intermediates (2) give the quasi-phosphonium salts (4) which usually decompose to give the phohphonate-phosphates (5). although for the case of (1; Ar ˭ 4-MeOPh) formation of the phosphonates (6) was observed.

Cyclizations in the homolytic reactions of unsaturated nitriles withtert-butylmercury halides in the presence of proton donors

Cyclizations are observed in the homolytic reactions of t-BuHgI with CH2CHCH2YCH2CN [Y = CH2, O, CMe2, C(CO2Et)2, NCH2CN] and CH2CHCH2CH2YCH2CN [Y = CH2, O, C(CO2Et)2] in Me2SO in the presence of hydriodic acid. Only with Y = C(CO2Et)2 does the adduct radical, t-BuCH2ĊHCH2YCH2CN, undergo facile 5-exo cyclization in the absence of a proton donor. The other 5-exo and all 6-exo cyclizations require substrate protonation to yield t-BuCH2ĊH(CH2)nYCH2CNH+ (n = 1, 2), which cyclizes readily to the iminium radical cation followed by electron transfer with I− or t-BuHgI2− to form the imine as a precursor to the cyclopentanone or cyclohexanone upon hydrolysis. For CH2CHCH2C(CO2Et)2CH2CN the formation of the cyclopentanone is dramatically promoted by NH4I in the dark in the absence of any other acid. In this case, where cyclization of the adduct radical occurs readily without substrate activation, protonation of the cyclized iminyl radical allows the electron transfer with I− or t-BuHgI2− to occur with regeneration of t-Buċ. A similar effect is observed with CH2CHCH2C(CO2Et)2CH2N3 where only a slow reaction is observed upon photolysis with t-BuHgI in the absence of NH4I, although apparently cyclization of t-BuCH2ĊHCH2C(CO2Et)2CH2N3 (with loss of N2) occurs readily. In the presence of NH4I the cyclized aminyl radical can be protonated and the resulting amine radical cation readily reduced by I− or t-BuHgI2− to continue a chain process. With the thioesters CH2CHCH2YCH2C(O)SPh [Y = O, CH2, CMe2, C(CO2Et)2], significant cyclization upon photolysis with t-BuHgX occurred only for Y = C(CO2Et)2. © 1998 John Wiley & Sons, Ltd.

Cyclizations in the homolytic reactions of unsaturated nitriles with tert‐butylmercury halides in the presence of proton donors

Cyclizations are observed in the homolytic reactions of t-BuHgI with CH2CHCH2YCH2CN [Y = CH2, O, CMe2, C(CO2Et)2, NCH2CN] and CH2CHCH2CH2YCH2CN [Y = CH2, O, C(CO2Et)2] in Me2SO in the presence of hydriodic acid. Only with Y = C(CO2Et)2 does the adduct radical, t-BuCH2ĊHCH2YCH2CN, undergo facile 5-exo cyclization in the absence of a proton donor. The other 5-exo and all 6-exo cyclizations require substrate protonation to yield t-BuCH2ĊH(CH2)nYCH2CNH+ (n = 1, 2), which cyclizes readily to the iminium radical cation followed by electron transfer with I− or t-BuHgI2− to form the imine as a precursor to the cyclopentanone or cyclohexanone upon hydrolysis. For CH2CHCH2C(CO2Et)2CH2CN the formation of the cyclopentanone is dramatically promoted by NH4I in the dark in the absence of any other acid. In this case, where cyclization of the adduct radical occurs readily without substrate activation, protonation of the cyclized iminyl radical allows the electron transfer with I− or t-BuHgI2− to occur with regeneration of t-Buċ. A similar effect is observed with CH2CHCH2C(CO2Et)2CH2N3 where only a slow reaction is observed upon photolysis with t-BuHgI in the absence of NH4I, although apparently cyclization of t-BuCH2ĊHCH2C(CO2Et)2CH2N3 (with loss of N2) occurs readily. In the presence of NH4I the cyclized aminyl radical can be protonated and the resulting amine radical cation readily reduced by I− or t-BuHgI2− to continue a chain process. With the thioesters CH2CHCH2YCH2C(O)SPh [Y = O, CH2, CMe2, C(CO2Et)2], significant cyclization upon photolysis with t-BuHgX occurred only for Y = C(CO2Et)2. © 1998 John Wiley & Sons, Ltd.

The β-hydroxyethyl radical as a model system for two-pathway electroreduction in the presence of proton donors

The mechanism of electrode reactions of the β-hydroxyethyl radical has been found experimentally to depend on pH, and a general kinetic scheme of organic radical electroreduction is suggested which includes two parallel pathways of electron transfer, either to adsorbed radical R ads or to its metastable protonated complex [R ads ·BH + ]; the competition between these pathways is determined by the capacity of R ads for complex formation, the concentration of the proton donor, and electron transfer rates to R ads and [R ads ·BH + ].