Reduction of Di-tert-Butyl Peroxide: Evidence for Nonadiabatic Dissociative Electron Transfer

Ligand-to-Metal Charge Transfer (LMCT) Catalysis: Harnessing Simple Cerium Catalysts for Selective Functionalization of Inert C-H and C-C Bonds

ConspectusChemists have long pursued harnessing light energy and photoexcitation processes for synthetic transformations. Ligand-to-metal charge transfer (LMCT) in high-valent metal complexes often triggers bond homolysis, generating oxidized ligand-centered radicals and reduced metal centers. While photoinduced oxidative activations can be enabled, this process, typically seen as photochemical decomposition, remains underexplored in catalytic applications. To mitigate decomposition during LMCT excitation, we developed a catalytic cycle integrating in situ coordination, LMCT, and ligand homolysis to activate ligated alcohols transiently into alkoxy radicals. This catalytic approach leverages Ce(IV) LMCT excitation and highly reactive alkoxy radical intermediates for selective functionalizations of C(sp3)-H and C(sp3)-C(sp3) bonds under mild conditions. In this Account, we discuss these advancements, highlighting the practical utility of cost-effective cerium salts as catalysts and their potential to develop innovative transformations, addressing long-standing synthetic challenges.Selective functionalization of chemically inert C(sp3)-H bonds has long posed a significant challenge. We first detail our research using LMCT-enabled alkoxy radical-mediated hydrogen atom transfer (HAT) processes for selective C(sp3)-H functionalizations. Using readily available CeCl3, we established a general protocol for employing free alcohols in the Barton reaction. By integrating LMCT and HAT catalysis, we introduced a selective photocatalytic strategy for functionalizing feedstock alkanes, converting gaseous hydrocarbons into valuable products. Employing simple cerium salts like Ce(OTf)3 and CeCl3, we achieved selective C-H amination of methane and ethane at ambient temperature, achieving turnover numbers of 2900 and 9700, respectively. This catalytic manifold has been further exploited to address the site-selectivity challenge in the C-H functionalization of linear alkanes. The use of methanol as a cocatalyst enabled preferential functionalization of the most electron-rich sites, achieving a high intrinsic selectivity over 12:1 of secondary vs primary sites in pentane and hexane.Next, we discuss the catalytic utilization of alkoxy-radical-mediated β-scission, a frequently encountered side reaction in HAT transformations, for selective cleavage and functionalization of C-C bonds. The versatility of the LMCT catalytic platform facilitates the generation of alkoxy radicals from various free alcohols. In our initial demonstration of LMCT-enabled C(sp3)-C(sp3) bond activation, we developed a cerium-catalyzed ring-opening and amination of cycloalkanols, providing an effective protocol for cleaving unstrained C-C bonds. This strategy has been successfully applied to various radical cross-coupling processes, leading to innovative transformations such as ring expansions of cycloalkanols, dehydroxymethylative alkylation, amination, alkenylation, and ring expansions of cyclic ketones. These results highlight the synthetic potential of employing LMCT-mediated β-scission and ubiquitous C-C bonds as unconventional functional handles for generating molecular complexity.Lastly, we delve into our mechanistic investigations. Beyond the catalytic application of Ce(IV) LMCT in various transformations, we have undertaken comprehensive mechanistic studies. These investigations encompass characterization of Ce(IV) alkoxide complexes to elucidate their structures, evaluation of their photoactivity and selectivity in radical generation, and elucidation of kinetic pathways associated with transient LMCT excited states. Our research has revealed ultrafast bond homolysis, back electron transfer, and the selectivity of heteroleptic complexes in homolysis, providing crucial insights for advancing LMCT catalysis.

Reactions of lipid hydroperoxides and how they may contribute to ferroptosis sensitivity

The accumulation of lipid hydroperoxides (LOOHs) has long been associated with numerous pathologies and has more recently been shown to drive a specific type of cell death known as ferroptosis. In competition with their detoxification by glutathione peroxidases, LOOHs can react with both one-electron reductants and one-electron oxidants to afford radicals that initiate lipid peroxidation (LPO) chain reactions leading to more LOOH. These radicals can alternatively undergo a variety of (primarily unimolecular) reactions leading to electrophilic species that destabilize the membrane and/or react with cellular nucleophiles. While some reaction mechanisms leading to lipid-derived electrophiles have been known for some time, others have only recently been elucidated. Since LOOH (and related peroxides, LOOL) undergo these various reactions at different rates to afford distinct product distributions specific to their structures, not all LOOHs (and LOOLs) should be equivalently problematic for the cell - be it in their propensity to initiate further LPO or fragment to electrophiles, drive membrane permeabilization and eventual cell death. Herein we briefly review the fates of LOOH and discuss how they may contribute to the modulation of cell sensitivity to ferroptosis by different lipids.

Identification of Alkoxy Radicals as Hydrogen Atom Transfer Agents in Ce-Catalyzed C-H Functionalization

The intermediacy of alkoxy radicals in cerium-catalyzed C-H functionalization via H-atom abstraction has been unambiguously confirmed. Catalytically relevant Ce(IV)-alkoxide complexes have been synthesized and characterized by X-ray diffraction. Operando electron paramagnetic resonance and transient absorption spectroscopy experiments on isolated pentachloro Ce(IV) alkoxides identified alkoxy radicals as the sole heteroatom-centered radical species generated via ligand-to-metal charge transfer (LMCT) excitation. Alkoxy-radical-mediated hydrogen atom transfer (HAT) has been verified via kinetic analysis, density functional theory (DFT) calculations, and reactions under strictly chloride-free conditions. These experimental findings unambiguously establish the critical role of alkoxy radicals in Ce-LMCT catalysis and definitively preclude the involvement of chlorine radical. This study has also reinforced the necessity of a high relative ratio of alcohol vs Ce for the selective alkoxy-radical-mediated HAT, as seemingly trivial changes in the relative ratio of alcohol vs Ce can lead to drastically different mechanistic pathways. Importantly, the previously proposed chlorine radical-alcohol complex, postulated to explain alkoxy-radical-enabled selectivities in this system, has been examined under scrutiny and ruled out by regioselectivity studies, transient absorption experiments, and high-level calculations. Moreover, the peculiar selectivity of alkoxy radical generation in the LMCT homolysis of Ce(IV) heteroleptic complexes has been analyzed and back-electron transfer (BET) may have regulated the efficiency and selectivity for the formation of ligand-centered radicals.

Basicity as a Thermodynamic Descriptor of Carbanions Reactivity with Carbon Dioxide: Application to the Carboxylation of α,β-Unsaturated Ketones

The utilization of carbon dioxide as a raw material represents nowadays an appealing strategy in the renewable energy, organic synthesis, and green chemistry fields. Besides reduction strategies, carbon dioxide can be exploited as a single-carbon-atom building block through its fixation into organic scaffolds with the formation of new C-C bonds (carboxylation processes). In this case, activation of the organic substrate is commonly required, upon formation of a carbanion C^-, being sufficiently reactive toward the addition of CO₂. However, the prediction of the reactivity of C^- with CO₂ is often problematic with the process being possibly associated with unfavorable thermodynamics. In this contribution, we present a thermodynamic analysis combined with density functional theory calculations on 50 organic molecules enabling the achievement of a linear correlation of the standard free energy (ΔG⁰) of the carboxylation reaction with the basicity of the carbanion C^-, expressed as the pK_a of the CH/C^- couple. The analysis identifies a threshold pK_a of ca 36 (in CH₃CN) for the CH/C^- couple, above which the ΔG⁰ of the carboxylation reaction is negative and indicative of a favorable process. We then apply the model to a real case involving electrochemical carboxylation of flavone and chalcone as model compounds of α,β-unsaturated ketones. Carboxylation occurs in the β-position from the doubly reduced dianion intermediates of flavone and chalcone (calculated ΔG⁰ of carboxylation in β = -12.8 and -20.0 Kcalmol^-1 for flavone and chalcone, respectively, associated with pK_a values for the conjugate acids of 50.6 and 51.8, respectively). Conversely, the one-electron reduced radical anions are not reactive toward carboxylation (ΔG⁰ > +20 Kcalmol^-1 for both substrates, in either α or β position, consistent with pK_a of the conjugate acids < 18.5). For all the possible intermediates, the plot of calculated ΔG⁰ of carboxylation vs. pK_a is consistent with the linear correlation model developed. The application of the ΔG⁰ vs. pK_a correlation is finally discussed for alternative reaction mechanisms and for carboxylation of other C=C and C=O double bonds. These results offer a new mechanistic tool for the interpretation of the reactivity of CO₂ with organic intermediates.

Electrochemical Investigation of Redox Processes of Labile Cu(II)/Cu(I)-Cl Complexes by Scanning Electrochemical Microscopy

The studies reported here provide detailed information on the kinetics and reaction mechanism determined by the scanning electrochemical microscopy (SECM) for Cu(II)/Cu(I) redox reactions in a system with excess chloride. In positive feedback mode, the tip voltammograms obtained consist of two partially overlapping reduction waves, not one quasi-reversible process as was always thought. The reduced species of Cu^IICl₄^2- and [Cu^IICl₃(H₂O)]^- could coexist during the time scale of SECM measurements, due to the slowness of the ligand substitution reaction of [Cu^IICl₃(H₂O)]^- to Cu^IICl₄^2-. The stepwise EC reactions of Cu^IICl₄^2- have experienced the same electron transfer kinetics as do outer-sphere reactants, while the reactions of [Cu^IICl₃(H₂O)]^- followed concerted EC pathways and much lower rate constants can be obtained, mainly due to the nonadiabatic character of electron transfer (k⁰_Pt/k⁰_Au = 1.625). The findings of this study can offer valuable insights for analyzing the electrode kinetics of labile complexes of metals in aqueous solutions.

The Persistent Radical Effect in Organic Synthesis

Radical-radical couplings are mostly nearly diffusion-controlled processes. Therefore, the selective cross-coupling of two different radicals is challenging and not a synthetically valuable transformation. However, if the radicals have different lifetimes and if they are generated at equal rates, cross-coupling will become the dominant process. This high cross-selectivity is based on a kinetic phenomenon called the persistent radical effect (PRE). In this Review, an explanation of the PRE supported by simulations of simple model systems is provided. Radical stabilities are discussed within the context of their lifetimes, and various examples of PRE-mediated radical-radical couplings in synthesis are summarized. It is shown that the PRE is not restricted to the coupling of a persistent with a transient radical. If one coupling partner is longer-lived than the other transient radical, the PRE operates and high cross-selectivity is achieved. This important point expands the scope of PRE-mediated radical chemistry. The Review is divided into two parts, namely 1) the coupling of persistent or longer-lived organic radicals and 2) "radical-metal crossover reactions"; here, metal-centered radical species and more generally longer-lived transition-metal complexes that are able to react with radicals are discussed-a field that has flourished recently.

Critical Factors in Determining the Heterolytic versus Homolytic Bond Cleavage of Terminal Oxidants by Iron(III) Porphyrin Complexes

Heterolytic versus homolytic cleavage of the metal-bound terminal oxidant is the key for determining the nature of reactive intermediates in metalloenzymes and metal catalyzed oxygenation reactions. Here, we study the bond cleavage process of hypochlorite by iron(III) porphyrin complexes having 4-methoxy-2,6-dimethylphenyl (1), 2,4,6-trimethylphenyl (2), 4-fluoro-2,6-dimethylphenyl (3), 2-chloro-6-methylphenyl (4), 2,6-dichlorophenyl (5), and 2,4,6-trichlorophenyl (6) groups at the meso position. Oxoiron(IV) porphyrin π-cation radical complexes (CompI) are characterized from the reactions of 1-4 with tetra- n-butylammonium hypochlorite (TBA-OCl) in dichloromethane at -80 °C, while oxoiron(IV) porphyrin complexes (CompII) are characterized for 5 and 6 under the same conditions. For all of 1-6, we find the formation of an epoxidation product in good yields from the catalytic reactions with TBA-OCl, suggesting heterolytic cleavages of the O-Cl bonds. CompI of 5 and 6 are reduced to the corresponding CompII by both chloride and hypochlorite, while CompI of 1-4 are not. The reduction reactions with hypochlorite are much faster than those with chloride. These results provide a mechanism where the O-Cl bond of the iron-bound hypochlorite is cleaved heterolytically to form CompI for all of 1-6, but the subsequent reduction reaction with remaining hypochlorite affords CompII for 5 and 6. The E(OCl^•/OCl^-) value is the boundary to discriminate the identity of the final product: CompI or CompII. Thermodynamic analysis based on the redox potential is successfully applied for explaining the bond cleavage processes of the hypochlorite, hydroperoxide, and tert-butyl peroxide complexes.

Recent progress in mild Csp3–H bond dehydrogenative or (mono-) oxidative functionalization

This review summarizes recent advances in mild and green dehydrogenative and mono-oxidative Csp³–H bond functionalization reactions, considering both new approaches and the re-elaboration of known methodologies.

Breaking bonds with electrons: stepwise and concerted reductive cleavage of C–S, C–Se and Se–CN bonds in phenacylthiocyanates and phenacylselenocyanates

The mechanistic aspects of the electrochemical reduction of phenacylthio- and selenocyanates have been studied.

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

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.

Electron transfer catalysis with monolayer protected Au₂₅ clusters

Au₂₅L₁₈ (L = S(CH₂)₂Ph) clusters were prepared and characterized. The resulting monodisperse clusters were reacted with bis(pentafluorobenzoyl) peroxide in dichloromethane to form Au₂₅L₁₈⁺ quantitatively. The kinetics and thermodynamics of the corresponding electron transfer (ET) reactions were characterized via electrochemistry and thermochemical calculations. Au₂₅L₁₈⁺ was used in homogeneous redox catalysis experiments with a series of sym-substituted benzoyl peroxides, including the above peroxide, bis(para-cyanobenzoyl) peroxide, dibenzoyl peroxide, and bis(para-methoxybenzoyl) peroxide. Peroxide dissociative ET was catalyzed using both the Au₂₅L₁₈/Au₂₅L₁₈⁻ and the Au₂₅L₁₈⁺/Au₂₅L₁₈ redox couples as redox mediators. Simulation of the CV curves led to determination of the ET rate constant (k(ET)) values for concerted dissociative ET to the peroxides. The ET free energy ΔG° could be estimated for all donor-acceptor combinations, leading to observation of a nice activation-driving force (log k(ET)vs.ΔG°) relationship. Comparison with the k(ET) obtained using a ferrocene-type donor with a formal potential similar to that of Au₂₅L₁₈/Au₂₅L₁₈⁻ showed that the presence of the capping monolayer affects the ET rate rather significantly, which is attributed to the intrinsic nonadiabaticity of peroxide acceptors.

Determination of the antimalaria drug artemether in pharmaceutical preparations by differential pulse polarography

A differential pulse polarographic method has been developed for the determination of artemether in its pharmaceutical formulations. The polarographic behaviour of artemether was examined in various buffer systems over the pH range 3.0-10.0. In phosphate buffer pH 5.5/methanol solution (7:3, v/v) the differential pulse polarograms displayed reproducible peaks at Ep-0.01 V versus Ag/AgCl. Under these conditions strict linearity between artemether concentration and peak height was observed in 3.4x10(-7)-3.0x10(-5)mol/L concentration range (R=0.9998). The detection limit was calculated to be 32 ng/mL. The polarographic method was applied to the determination of the content of artemether in tablets and capsules by using the standard addition method. The analysis of tablets containing 20mg artemether showed a mean value of 19.73 mg with a relative standard deviation (R.S.D.) of +/-1.01%. A content of 39.74 mg artemether was found in 40 mg capsules with a relative standard deviation of +/-0.53%. The polarographic method is characterised to be cheap, precise and not time-consuming and can therefore be used for routine analysis of artemether in its pharmaceutical preparations.

Effect of structural change in viologen acceptors on the rate of single electron transfer from tributylphosphine

The "flexible" 3 and "rigid" cyclic viologens 4, diquarternary salts of 2,2'-bipyridine and 1,10-phenanthroline, respectively, were treated with tributylphosphine (1) in acetonitrile containing a large amount of methanol under an argon atmosphere. A single electron transfer (SET) easily occurred from the latter to the former, the SET to 4 being 10(5)-10(6) times faster than the SET to 3. The reorganization energy lambda for the latter SET is thought to be larger than that for the former SET, because 3 undergoes a structural change upon the one-electron reduction to its radical cation, whereas the one-electron reduction of 4 takes place without a structural change. Taking into account the difference in lambda, and also taking into account the bond formation energy brought about by the follow-up reaction of the phosphine radical cation 1*(+) with methanol, the observed kinetics were well interpreted in terms of the Marcus theory.

Regioselective S—O vs. C—O bond cleavage in sulfenate ester radical anions

The electron transfer (ET) reduction of benzyl benzenesulfenate ester (1) and tert-butyl benzenesulfenate ester (2) was investigated using electrochemical techniques. Analysis of the cyclic voltammetry of each compound suggests that the ET reduction proceeds via a stepwise dissociative mechanism. The voltammograms of 1 are similar to those of diaryl disulfides and it was found through controlled potential electrolysis (CPE) product studies that ET reduction leads to S—O bond cleavage. The voltammograms of 2 are dramatically different with a sharper dissociative wave occurring at a more negative peak potential. CPE experiments indicate products that result from ET leading to C—O bond cleavage in this case. DFT calculations of the singly occupied molecular orbitals (SOMOs) of 1 and 2 were performed and offer a rationale for the different reactivity of the two radical anions.Key words: sulfenate esters, dissociative electron transfer, electrochemistry, radical anions.

Sticky Dissociative Electron Transfer to Polychloroacetamides. In-Cage Ion−Dipole Interaction Control through the Dipole Moment and Intramolecular Hydrogen Bond

The reductive cleavage of chloro- and polychloroacetamides in N,N-dimethylformamide gives new insights into the nature of the in-cage ion radical cluster formed upon dissociative electron transfer. Within the family of compounds investigated, the electrochemical reduction leads to the successive expulsion of chloride ions. At each stage the electron transfer is concerted with the breaking of the C-Cl bond and acts as the rate-determining step. The reduction further leads to the formation of the corresponding carbanion with the injection of a second electron, which is in turn protonated by a weak acid added to the solution. From the joint use of cyclic voltammetric data, the sticky dissociative electron-transfer model and quantum ab initio calculations, the interaction energies within the cluster fragments (*R, Cl-) resulting from the first electron transfer to the parent RCl molecule are obtained. It is shown that the stability of these adducts, which should be viewed as an essentially electrostatic radical-ion pair, is mainly controlled by the intensity of the dipole moment of the remaining radical part and may eventually be strengthened by the formation of an intramolecular hydrogen bond, as is the case with 2-chloroacetamide.

Dehydrogenation versus oxygenation in two-electron and four-electron reduction of dioxygen by 9-alkyl-10-methyl-9,10-dihydroacridines catalyzed by monomeric cobalt porphyrins and cofacial dicobalt porphyrins in the presence of perchloric acid

Dehydrogenation of 10-methyl-9,10-dihydroacridine (AcrH(2)) by dioxygen (O(2)) proceeds efficiently, accompanied by the two-electron and four-electron reduction of O(2) to produce H(2)O(2) and H(2)O, which are effectively catalyzed by monomeric cobalt porphyrins and cofacial dicobalt porphyrins in the presence of perchloric acid (HClO(4)) in acetonitrile (MeCN) and benzonitrile (PhCN), respectively. The cobalt porphyrin catalyzed two-electron reduction of O(2) also occurs efficiently by 9-alkyl-10-methyl-9,10-dihydroacridines (AcrHR; R = Me, Et, and CH(2)COOEt) to yield 9-alkyl-10-methylacridinium ion (AcrR+) and H(2)O(2). In the case of R = Bu(t) and CMe(2)COOMe, however, the catalytic two-electron and four-electron reduction of O(2) by AcrHR results in oxygenation of the alkyl group of AcrHR rather than dehydrogenation to yield 10-methylacridinium ion (AcrH+) and the oxygenated products of the alkyl groups, i.e., the corresponding hydroperoxides (ROOH) and the alcohol (ROH), respectively. The catalytic mechanisms of the dehydrogenation vs the oxygenation of AcrHR in the two-electron and four-electron reduction of O(2), catalyzed by monomeric cobalt porphyrins and cofacial dicobalt porphyrins, respectively, are discussed in relation to the C(9)-H or C(9)-C bond cleavage of AcrHR radical cations produced in the electron-transfer oxidation of AcrHR.

Photosensitized Reactions of Oxime Ethers: A Steady-State and Laser Flash Photolysis Study

The mechanistic aspects of the photosensitized reactions of a series of oxime ethers were studied by steady-state (product studies) and laser flash photolysis methods. Nanosecond laser flash photolysis studies have shown that chloranil-sensitized reactions of the oxime ethers result in the formation of the corresponding radical cations. The radical cation species react with nucleophiles such as MeOH by clean second-order kinetics with rate constants of (0.7-1.4) x 10(6) M(-1) s(-1). Only a small steric effect is observed in these reactions, which is taken as an indication that the reaction center is not the O-alkyl moiety, but rather somewhere else in the molecule. Product studies in a polar nonnucleophilic solvent (MeCN) revealed that in order for the oxime ether radical cation to react more readily, alpha-protons must be available on the alkyl group. The O-methyl (1), O-ethyl (2), and O-benzyl (3) acetophenone oximes all reacted readily to give acetophenone oxime as the major product (as well as an aldehyde derived from the O-alkyl group), whereas O-tert-butyl acetophenone oxime (4) did not. The product formation can be explained by a mechanism that involves electron transfer followed by proton transfer (alpha to the oxygen) and subsequent beta-cleavage. When using 3 in MeOH, a change in the product formation is observed, the most important difference being the presence of benzyl alcohol rather than benzaldehyde as the major product. On the basis of the data from LFP and steady-state experiments, it is suggested that the competing mechanism under these conditions involves electron transfer, followed by a nucleophilic attack on the nitrogen, a MeOH-assisted [1,3]-proton transfer, and subsequent loss of benzyl alcohol. This mechanism is supported by DFT (B3LYP/6-31G) and AM1 calculations.

Successive removal of chloride ions from organic polychloride pollutants. Mechanisms of reductive electrochemical elimination in aliphatic gem-polychlorides, alpha,beta-polychloroalkenes, and alpha,beta-polychloroalkanes in mildly protic medium

The factors that control the successive reductive expulsion of chloride ions from aliphatic gem-polychlorides are investigated, taking as examples the electrochemical reduction of polychloromethanes and polychloroacetonitriles in N,N-dimethylformamide. At each elimination stage, the reaction involves, as a rate-determining step, the transfer of one electron concerted with the cleavage of the carbon-chloride bond. The second step is an immediate electron transfer to the ensuing radical, taking place at a potential more positive than the potential at which the first electron transfer occurs. The carbanion thus formed is sufficiently basic to be protonated by any trace weak acid present in the reaction medium. The three successive elimination steps require increasingly negative potentials. Application of the "sticky" dissociative electron transfer model allows one to quantitatively unravel the factors that control the energetics of the successive reductive expulsion of chloride ions. The large potential gaps between each stage stem primarily from large differences in the dissociative standard potentials. They are also strongly affected by two cumulative intrinsic activation barrier factors, namely, the bond dissociation energy of the substrate that decreases with the number of chlorine atoms and the interaction between chloride ion and the radical that increases in the same direction. In the case of alpha,beta-polychloroethanes (Cl(3)C-CCl(3), Cl(2)HC-CCl(3), Cl(2)HC-CHCl(2), ClH(2)C-CHCl(2)) too, the first step is a dissociative electron transfer with sizable ion-radical interactions in the product cluster. Likewise, a second electron transfer immediately leads to the carbanion, which however prefers to expel a second chloride ion, leading to the corresponding olefin, than to be protonated to the hydrogenolysis product. The ion-radical interaction in the product cluster plays a major role in the control of the reduction potential. The reduction of the alpha,beta-polychloroethenes (Cl(2)C=CCl(2), ClHC=CCl(2), ClHC=CHCl) follows a similar 2e(-)-2Cl(-) reaction sequence, leading then to the corresponding alkynes. However, unlike the polychloroethane case, the expulsion of the first chloride ion follows a stepwise electron transfer/bond cleavage mechanism. The reduction potential is thus essentially governed by the thermodynamics of the anion radical formation.

Mechanisms of Hydrogen-, Oxygen-, and Electron-Transfer Reactions of Cumylperoxyl Radical

Rates of hydrogen-transfer reactions from a series of para-substituted N,N-dimethylanilines to cumylperoxyl radical and oxygen-transfer reactions from cumylperoxyl radical to a series of sulfides and phosphines have been determined in propionitrile (EtCN) and pentane at low temperatures by use of ESR. The observed rate constants exhibit first-order and second-order dependence with respect to concentrations of N,N-dimethylanilines. This indicates that the hydrogen- and oxygen-transfer reactions proceed via 1:1 charge-transfer (CT) complexes formed between the substrates and cumylperoxyl radical. The primary kinetic isotope effects are determined by comparing the rates of N,N-dimethylanilines and the corresponding N,N-bis(trideuteriomethyl)anilines. The isotope effect profiles are quite different from those reported for the P-450 model oxidation of the same series of substrates. Rates of electron-transfer reactions from ferrocene derivatives to cumylperoxyl radical have also been determined by use of ESR. The catalytic effects of Sc(OTf)(3) (OTf = triflate) on the electron-transfer reactions are compared with those of Sc(OTf)(3) on the hydrogen- and oxygen-transfer reactions. Such comparison provides strong evidence that the hydrogen- and oxygen- transfer reactions of cumylperoxyl radical proceed via a one-step hydrogen atom and oxygen atom transfer rather than via an electron transfer from substrates to cumylperoxyl radical.

Metallophthalocyanines photosensitize the breakdown of (hydro)peroxides in solution to yield hydroxyl or alkoxyl and peroxyl free radicals via different interaction pathways

Interactions of organic peroxides (R'OOR) and hydroperoxides (R'OOH), including H2O2, with excited triplet and singlet state metallophthalocyanines (MPc, M = Zn, Al) have been studied by T-T absorption decay and fluorescence quenching. The ensuing photochemical processes result in decomposition of (hydro)peroxides as assessed by photo-EPR (electron paramagnetic resonance) and spin trapping. In argon-saturated apolar solutions and low MPc concentrations, alkoxyl free radicals (*OR) were identified as the primary products of (hydro)peroxide breakdown. Similarly, photosensitized decomposition of symmetric disulfides results in the formation of sulfur-centered radicals. In air-free aqueous solutions, ROOH photosensitization always gave rise to a mixture of hydroxyl and peroxyl radical (*OOR) adducts in varying molar ratios. At high MPc concentrations, both in polar and in apolar solutions, the most abundant products of ROOH decomposition were identified as *OOR. This indicates a change in the predominant interaction pathway, most likely mediated by MPc exciplexes and involving H-atom abstraction from ROOH by MPc-cation radicals. The prevalence of MPc singlet vs. triplet state interactions was confirmed by the much higher singlet quenching rate constants (log kq up to 9.5; vs. log kT < or = 4.5). In contrast to the triplet quenching, singlet quenching rates were found to depend on the (hydro)peroxide structure, following closely the trend of varying *OR yields for different substrates. Thermodynamic calculations were performed to correlate experimental results with models for electronic energy and charge transfer processes in agreement with the Marcus theory (Rhem and Weller approximation) and Savéant's model for a concerted dissociative electron transfer mechanism.

Dissociative electron transfer to haloacetonitriles. An example of the dependency of in-cage ion-radical interactions upon the leaving group

The reductive cleavage of the haloacetonitriles (Cl, Br, I) in DMF provides additional examples of the formation of a fragment cluster upon dissociative electron transfer, which is able to survive in this polar solvent thanks to the electron-withdrawing character of the cyano group. The remarkable sensitivity of the activation energy to small changes of the interaction energies allows, with help of the "sticky" dissociative electron-transfer model, the precise determination of interaction energies down to a few millielectronvolts from the cyclic voltammetric data. The interaction energy rapidly decreases from Cl to Br and to I, correlated with the increase of the halide radius. These observations add to the previously gathered evidence to confirm the existence of such interactions and to highlight their electrostatic character. This is further corroborated by the quantum chemical computation of the potential energy profiles, which exhibit a long-distance energy minimum. This revisiting of the notion of sigma-ion radicals and of their status in a polar medium makes them appear as an electrostatic radical-ion pair rather than covalently bound molecules. Their stability is a function of the Lewis acid-base properties of both the radical and the leaving ion and is strongly influenced by the nature of the solvent.

Insights into the free-energy dependence of intramolecular dissociative electron transfers

To study the relationship between rate and driving force of intramolecular dissociative electron transfers, a series of donor-spacer-acceptor (D-Sp-A) systems has been devised and synthesized. cis-1,4-Cyclohexanedyil and a perester functional group were kept constant as the spacer and acceptor, respectively. By changing the aryl substituents of the phthalimide moiety, which served as the donor, the driving force could be varied by 0.74 eV. X-ray diffraction crystallography and ab initio conformational calculations pointed to D-Sp-A molecules having the cis-(cyclohexane) equatorial(phthalimido)-axial(perester) conformation and the same D/A orientation. The intramolecular dissociative electron-transfer process was studied by electrochemical means in N,N-dimethylformamide, in comparison with thermodynamic and kinetic information obtained with models of the acceptor and the donor. The intramolecular process consists of the electron transfer from the electrochemically generated phthalimide-moiety radical anion to the peroxide functional group. The electrochemical analysis provided clear evidence of a concerted dissociative electron-transfer mechanism, leading to the cleavage of the O-O bond. Support for this mechanism was obtained by ab initio MO calculations, which provided information about the LUMO of the acceptor and the SOMO of the donor. The intramolecular rate constants were determined and compared with the corresponding intermolecular values, the latter data being obtained by using the model molecules. As long as the effective location of the centroid of the donor SOMO does not vary significantly by changing the aryl substituent(s), the intramolecular dissociative electron transfer obeys the same main rules already highlighted for the corresponding intermolecular process. On the other hand, introduction of a nitro group drags the SOMO away from the acceptor, and consequently, the intramolecular rate drops by as much as 1.6 orders of magnitude from the expected value. Therefore, a larger solvent reorganization than for intermolecular electron transfers and the effective D/A distance and thus electronic coupling must be taken into account for quantitative predictions of intramolecular rates.

Theoretical and electrochemical analysis of dissociative electron transfers proceeding through formation of loose radical anion species: reduction of symmetrical and unsymmetrical disulfides

The dissociative reduction of a series of symmetrical (RSSR, R = H, Me, t-Bu, Ph) and unsymmetrical disulfides (RSSR', R = H, R' = Me and R = Ph, R' = Me, t-Bu) was studied theoretically, by MO ab initio calculations and, for five of them, also experimentally, by convolution voltammetry in N,N-dimethylformamide. The reduction is dissociative but proceeds by a stepwise mechanism entailing the formation of the radical anion species. The electrochemical data led to estimated large intrinsic barriers, in agreement with an unusually large structural modification undergone by the disulfide molecules upon electron transfer. The theoretical results refer to MP2/3-21G*//MP2/3-21G*, MP2/3-21*G*//MP2/3-21G*, CBS-4M, and G2(MP2), the latter approach being used only for the molecules of small molecular complexity. A loose radical-anion intermediate was localized and the dissociation pattern for the relevant bonds analyzed. For all compounds, the best fragmentation pathway in solution is cleavage of the S-S bond. In addition, S-S bond elongation is the major structural modification undergone by the disulfide molecule on its way to the radical anion and eventually to the fragmentation products. The calculated energy of activation for the initial electron transfer was estimated from the crossing of the energy profiles of the neutral molecule and its radical anion (in the form of Morse-like potentials) as a function of the S-S bond length coordinate. The inner intrinsic barrier obtained in this way is in good agreement with that determined by convolution voltammetry, once the solvent effect is taken into account.

Thermodynamics, kinetics, and dynamics of the two alternative aniomesolytic fragmentations of C-O bonds: an electrochemical and theoretical study

Fragmentation reactions of radical anions (mesolytic cleavages) of cyanobenzyl alkyl ethers (intramolecular dissociative electron transfer, heterolytic cleavages) have been studied electrochemically. The intrinsic barriers for the processes have been established from the experimental thermodynamic and kinetic parameters. These values are more than 3 kcal/mol lower as an average than the related homolytic mesolytic fragmentations of radical anions of 4-cyanophenyl ethers. In the particular case of isomers 4-cyanobenzyl phenyl ether and 4-cyanophenyl benzyl ether, the difference in intrinsic barriers amounts to 5.5 kcal/mol, and this produces an energetic crossing where the thermodynamically more favorable process (homolytic) is the kinetically slower one. The fundamental reasons for this behavior have been established by means of theoretical calculations within the density functional theory framework, showing that, in this case, the factors that determine the kinetics are clearly different (mainly present in the transition state) from those that determine the thermodynamics and they are not related to the regioconservation of the spin density ("spin regioconservation principle"). Our theoretical results reproduce quite well the experimental energetic difference of barriers and demonstrate the main structural origin of the difference.

Stabilities of ion/radical adducts in the liquid phase as derived from the dependence of electrochemical cleavage reactivities upon solvent

The idea that significant ion/radical interactions should vary with solvent if they do exist in the liquid phase was pursued by an investigation of the dissociative electron-transfer reactivity of carbon tetrachloride and 4-cyanobenzyl chloride in four different solvents, 1,2-dichloroethane, N,N-dimethylformamide, ethanol, and formamide, by means of their cyclic voltammetric responses. Modification of the conventional dissociative electron transfer theory to take account of an interaction between fragments in the ion/radical pair resulting from the dissociative electron reaction allows a satisfactory fitting of the experimental data leading to the determination of the interaction energy. There is an approximate correlation between the interaction energies in the ion/radical pair and the solvation free energies of the leaving anion, Cl(-). The interaction is maximal in 1,2-dichloroethane, which is both the least polar and the least able to solvate Cl(-). The interaction is smaller in the polar solvents, albeit distinctly measurable. The two protic solvents, ethanol and formamide, which are the most able to solvate Cl(-), give rise to similar interaction energies. The interaction is definitely stronger in N,N-dimethylformamide, which has a lesser ability to solvate Cl(-) than the two other polar solvents. The existence of significant ion/radical interactions in polar media is thus confirmed and a route to their determination opened.

A numerical modelling of voltammetric reduction of substituted iodobenzenes reaction series. A relationship between reductions in the consecutive-mode multistep system and a multicomponent system. Determination of the potential variation of the elementary charge transfer coefficient

The cyclic voltammetry reduction process of the reaction series of substituted iodobenzenes X-C6H4-I where X = H, p-Cl, p-Br, p-I, p-CH3, m-CF3 was investigated in 0.3 M TBAP in DMF. A numerical model of the process consistent with the ECE mechanism of mono-iodobenzenes reduction and consecutive ECE-ECE reduction of p-diiodobenzene was applied. On the basis of alpha(i) vs. E(p,i) dependence, the value of delta a(i)/delta E was found to be 0.37 +/- 0.07 for first electron transfer (Eq. (10)). The ECE-ECE system seems to be an another example of elementary alpha kinetic discrimination between two identical two-electron processes analogous to that described in a previous paper (Sanecki, P., Kaczmarski, K. (1999). J. Electroanal. Chem. 471, 14 and erratum to it (2001)). A method of simultaneous treatment of the substrate and all electroactive intermediates, i.e. the transformation of any experimental consecutive CV reduction curves (e.g. ECE or ECE-ECE) into curves corresponding to reduction of the multi-component systems is presented and discussed.

Intramolecular, intermolecular, and heterogeneous nonadiabatic dissociative electron transfer to peresters

The electron transfer to peresters was studied by electrochemical means in N,N-dimethylformamide. The reduction was carried out by three independent methods: (i) heterogeneously, by using glassy carbon electrodes, (ii) homogeneously, by using electrogenerated radical anions as the donors, and (iii) intramolecularly, by using purposely synthesized donor-spacer-acceptor (D-Sp-A) systems. Convolution analysis of the heterogeneous data led to results in excellent agreement with the dissociative electron transfer theory. The homogeneous redox catalysis data also confirmed the reduction mechanism. The cyclic voltammetries of the D-Sp-A molecules could be simulated, leading to determination of the corresponding intramolecular dissociative rate constants. Analysis of the results showed that, regardless of the way by which the acceptor is reduced, the investigated dissociative electron transfers are strongly nonadiabatic and, particularly, that the experimental rates are several orders of magnitude smaller than the adiabatic limit. A possible mechanism responsible for the observed behavior is discussed.