Concerted proton-electron transfer in the oxidation of hydrogen-bonded phenols

Non-coordinated and Hydrogen Bonded Phenolate Anions as One-Electron Reducing Agents

In this work, the syntheses of non-coordinated electron-rich phenolate anions via deprotonation of the corresponding alcohols with an extremely powerful perethyl tetraphosphazene base (Schwesinger base) are reported. The application of uncharged phosphazenes renders the selective preparation of anionic phenol-phenolate and phenolate hydrates possible, which allows for the investigation of hydrogen bonding in these species. Hydrogen bonding brings about decreased redox potentials relative to the corresponding non-coordinated phenolate anions. The latter show redox potentials of up to -0.72(1) V vs. SCE, which is comparable to that of zinc metal, thus qualifying their application as organic zinc mimics. We utilized phenolates as reducing agents for the generation of radical anions in addition to the corresponding phenoxyl radicals. A tetracyanoethylene radical anion salt was synthesized and fully characterized as a representative example. We also present the activation of sulfur hexafluoride (SF6 ) with phenolates in a SET reaction, in which the nature of the respective phenolate determines whether simple fluorides or pentafluorosulfanide ([SF5 ]- ) salts are formed.

Laser Access to Quercetin Radicals and Their Repair by Co-antioxidants

We have demonstrated the feasibility and ease of producing quercetin radicals by photoionization with a pulsed 355 nm laser. A conversion efficiency into radicals of 0.4 is routinely achieved throughout the pH range investigated (pH 2-9), and the radical generation is completed within a few ns. No precursor other than the parent compound is needed, and the ionization by-products do not interfere with the further fate of the radicals. With this generation method, we have characterized the quercetin radicals and studied the kinetics of their repairs by co-antioxidants such as ascorbate and 4-aminophenol. Bell-shaped pH dependences of the observed rate constants reflect opposite trends in the availability of the reacting protonation forms of radical and co-antioxidant and even at their maxima mask the much higher true rate constants. Kinetic isotope effects identify the repairs as proton-coupled electron transfers. An examination of which co-antioxidants are capable of repairing the quercetin radicals and which are not confines the bond dissociation energies of quercetin and its monoanion experimentally to 75-77 kcal mol-1 and 72-75 kcal mol-1 , a much narrower interval in the case of the former than previously estimated by theoretical calculations.

Concept, mechanism, and applications of phenolic antioxidants in foods

In this review, the concept of phenolic antioxidants, mechanisms of action, and applications have been reviewed. Phenolic compounds (PCs) acts as an antioxidant by reacting with a variety of free radicals. The mechanism of antioxidant actions involved either by hydrogen atom transfer, transfer of a single electron, sequential proton loss electron transfer, and chelation of transition metals. In foods, the PCs act as antioxidants which are measured with several in vitro spectroscopic methods. The PCs have been found in milk and a wide range of dairy products with sole purposes of color, taste, storage stability, and quality enhancement. The role of PCs in three types of food additives, that is, antimicrobial, antioxidant, and flavoring agents have been critically reviewed. The literature revealed that PCs present in a variety of foods possess several health benefits such as antibacterial, antihyperlipidemic, anticancer, antioxidants, cardioprotective, neuroprotective, and antidiabetic properties. PRACTICAL APPLICATIONS: Phenolic compounds are strong antioxidants and are safer than synthetic antioxidants. The wide occurrence in plant foods warranted continuous review applications. This review, therefore, presented an updated comprehensive overview of the concept, mechanism, and applications of phenolic antioxidants in foods.

Direct observation of light-driven, concerted electron-proton transfer

The phenols 4-methylphenol, 4-methoxyphenol, and N-acetyl-tyrosine form hydrogen-bonded adducts with N-methyl-4, 4'-bipyridinium cation (MQ⁺) in aqueous solution as evidenced by the appearance of low-energy, low-absorptivity features in UV-visible spectra. They are assigned to the known examples of optically induced, concerted electron-proton transfer, photoEPT. The results of ultrafast transient absorption measurements on the assembly MeOPhO-H---MQ⁺ are consistent with concerted EPT by the instantaneous appearance of spectral features for MeOPhO·---H-MQ⁺ in the transient spectra at the first observation time of 0.1 ps. The transient decays to MeOPhO-H---MQ⁺ in 2.5 ps, accompanied by the appearance of oscillations in the decay traces with a period of ∼1 ps, consistent with a vibrational coherence and relaxation from a higher υ(N-H) vibrational level or levels on the timescale for back EPT.

Accelerating slow excited state proton transfer

Visible light excitation of the ligand-bridged assembly [(bpy)(2)Ru(a)(II)(L)Ru(b)(II)(bpy)(OH(2))(4+)] (bpy is 2,2'-bipyridine; L is the bridging ligand, 4-phen-tpy) results in emission from the lowest energy, bridge-based metal-to-ligand charge transfer excited state (L(-•))Ru(b)(III)-OH(2) with an excited-state lifetime of 13 ± 1 ns. Near-diffusion-controlled quenching of the emission occurs with added HPO(4)(2-) and partial quenching by added acetate anion (OAc(-)) in buffered solutions with pH control. A Stern-Volmer analysis of quenching by OAc(-) gave a quenching rate constant of k(q) = 4.1 × 10(8) M(-1) • s(-1) and an estimated pK(a)* value of ~5 ± 1 for the [(bpy)(2)Ru(a)(II)(L(•-))Ru(b)(III)(bpy)(OH(2))(4+)]* excited state. Following proton loss and rapid excited-state decay to give [(bpy)(2)Ru(a)(II)(L)Ru(b)(II)(bpy)(OH)(3+)] in a H(2)PO(4)(-)/HPO(4)(2-) buffer, back proton transfer occurs from H(2)PO(4)(-) to give [(bpy)(2)Ru(a)(II)(L)Ru(b)(bpy)(OH(2))(4+)] with k(PT,2) = 4.4 × 10(8) M(-1) • s(-1). From the intercept of a plot of k(obs) vs. [H(2)PO(4)(-)], k = 2.1 × 10(6) s(-1) for reprotonation by water providing a dramatic illustration of kinetically limiting, slow proton transfer for acids and bases with pK(a) values intermediate between pK(a)(H(3)O(+)) = -1.74 and pK(a)(H(2)O) = 15.7.

On the role of hydrogen bonds in photoinduced electron-transfer dynamics between 9-fluorenone and amine solvents

Using ultrafast fluorescence upconversion and mid-infrared spectroscopy, we explore the role of hydrogen bonds in the photoinduced electron transfer (ET) between 9-fluorenone (FLU) and the solvents trimethylamine (TEA) and dimethylamine (DEA). FLU shows hydrogen-bond dynamics in the methanol solvent upon photoexcitation, and similar effects may be anticipated when using DEA, whereas no hydrogen bonds can occur in TEA. Photoexcitation of the electron-acceptor dye molecule FLU with a 400 nm pump pulse induces ultrafast ET from the amine solvents, which is followed by 100 fs IR probe pulses as well as fluorescence upconversion, monitoring the time evolution of marker bands of the FLU S(1) state and the FLU radical anion, and an overtone band of the amine solvent, marking the transient generation of the amine radical cation. A comparison of the experimentally determined forward charge-separation and backward charge-recombination rates for the FLU-TEA and FLU-DEA reaction systems with the driving-force dependencies calculated for the forward and backward ET rates reveals that additional degrees of freedom determine the ET reaction dynamics for the FLU-DEA system. We suggest that hydrogen bonding between the DEA molecules plays a key role in this behaviour.

Concerted heavy-atom bond cleavage and proton and electron transfers illustrated by proton-assisted reductive cleavage of an O-O bond

Electron transfer may be concerted with proton transfer. It may also be concerted with the cleavage of a bond between heavy atoms. All three events may also be concerted. A model is presented to analyze the kinetics of these all-concerted reactions for homogeneous or electrochemical reduction or oxidation processes. It allows the estimation of the kinetic advantage that derives from the increase of the bond-breaking driving force resulting from the concerted proton transfer. Application of the model to the electrochemical reductive cleavage of the O-O bond of an organic peroxide in the presence of a proximal acid group illustrates the applicability of the model and provides an example demonstrating that electron transfer, heavy-atom bond breaking, and proton transfer may be all concerted. Such analyses are expected to be useful for the invention, analysis, and optimization of reactions involved in contemporary energy challenges as well as for the comprehension of major biochemical processes, a number of which involve electron and proton transfer together with cleavage of bonds between heavy atoms.

Intrinsic reactivity and driving force dependence in concerted proton-electron transfers to water illustrated by phenol oxidation

Three experimental techniques, laser flash photolysis, redox catalysis, and stopped-flow, were used to investigate the variation of the oxidation rate constant of phenol in neat water with the driving force offered by a series of electron acceptors. Taking into account a result previously obtained with a low-driving force electron acceptor thus allowed scanning more than half an electron-volt driving force range. Variation of the rate constant with pH showed the transition between a direct phenol oxidation reaction at low pH, where the rate constant does not vary with pH, and a stepwise reaction involving the prior deprotonation of phenol by OH(-), characterized by a unity-slope variation. Analyses of the direct oxidation kinetics, based on its variation with the driving force and on the determination of H/D isotope effects, ruled out a stepwise mechanism in which electron transfer is followed by the deprotonation of the initial cation radical at the benefit of a pathway in which proton and electron are transferred concertedly. Derivation of the characteristics of counterdiffusion in termolecular reactions allowed showing that the concerted process is under activation control. It is characterized by a remarkably small reorganization energy, in line with the electrochemical counterpart of the reaction, underpinning the very peculiar behavior of water as proton acceptor when it is used as the solvent.

The electrochemical approach to concerted proton--electron transfers in the oxidation of phenols in water

Establishing mechanisms and intrinsic reactivity in the oxidation of phenol with water as the proton acceptor is a fundamental task relevant to many reactions occurring in natural systems. Thanks to the easy measure of the reaction kinetics by the current and the setting of the driving force by the electrode potential, the electrochemical approach is particularly suited to this endeavor. Despite challenging difficulties related to self-inhibition blocking the electrode surface, experimental conditions were established that allowed a reliable analysis of the thermodynamics and mechanisms of the proton-coupled electron-transfer oxidation of phenol to be carried out by means of cyclic voltammetry. The thermodynamic characterization was conducted in buffer media whereas the mechanisms were revealed in unbuffered water. Unambiguous evidence of a concerted proton-electron transfer mechanism, with water as proton acceptor, was thus gathered by simulation of the experimental data with appropriately derived theoretical relationships, leading to the determination of a remarkably large intrinsic rate constant. The same strategy also allowed the quantitative analysis of the competition between the concerted proton-electron transfer pathway and an OH(-)-triggered stepwise pathway (proton transfer followed by electron transfer) at high pHs. Investigation of the passage between unbuffered and buffered media with the example of the PO(4)H(2)(-)/PO(4)H(2-) couple revealed the prevalence of a mechanism involving a proton transfer preceding an electron transfer over a PO(4)H(2-)-triggered concerted process.

Probing concerted proton-electron transfer in phenol-imidazoles

A series of seven substituted 4,6-di-tert-butyl-2-(4,5-diarylimidazolyl)-phenols have been prepared and characterized, along with two related benzimidazole compounds. X-ray crystal structures of all of the compounds show that the phenol and imidazole rings are close to coplanar and are connected by an intramolecular ArOHN hydrogen bond. One-electron oxidation of these compounds occurs with movement of the phenolic proton to the imidazole base by concerted proton-electron transfer (CPET) to yield fairly stable distonic radical cations. These phenol-base compounds are a valuable system in which to examine the key features of CPET. Kinetic measurements of bimolecular CPET oxidations, with E(rxn) between +0.04 and -0.33 V, give rate constants from (6.3 +/- 0.6) x 10(2) to (3.0 +/- 0.6) x 10(6) M(-1) s(-1). There is a good correlation of log(k) with DeltaG degrees , with only one of the 15 rate constants falling more than a factor of 5.2 from the correlation line. Substituents on the imidazole affect the (O-HN) hydrogen bond, as marked by variations in the (1)H NMR and calculated vibrational spectra and geometries. Crystallographic d(ON) values appear to be more strongly affected by crystal packing forces. However, there is almost no correlation of rate constants with any of these measured or computed parameters. Over this range of compounds from the same structural family, the dominant contributor to the differences in rate constant is the driving force DeltaG degrees .