H-atom Transfer Reactions Research Articles

An evolved artificial radical cyclase enables the construction of bicyclic terpenoid scaffolds via an H-atom transfer pathway.

While natural terpenoid cyclases generate complex terpenoid structures via cationic mechanisms, alternative radical cyclization pathways are underexplored. The metal-catalysed H-atom transfer reaction (M-HAT) offers an attractive means for hydrofunctionalizing olefins, providing access to terpenoid-like structures. Artificial metalloenzymes offer a promising strategy for introducing M-HAT reactivity into a protein scaffold. Here we report our efforts towards engineering an artificial radical cyclase (ARCase), resulting from anchoring a biotinylated [Co(Schiff-base)] cofactor within an engineered chimeric streptavidin. After two rounds of directed evolution, a double mutant catalyses a radical cyclization to afford bicyclic products with a cis-5-6-fused ring structure and up to 97% enantiomeric excess. The involvement of a histidine ligation to the Co cofactor is confirmed by crystallography. A time course experiment reveals a cascade reaction catalysed by the ARCase, combining a radical cyclization with a conjugate reduction. The ARCase exhibits tolerance towards variations in the dienone substrate, highlighting its potential to access terpenoid scaffolds.

106-fold faster C-H bond hydroxylation by a CoIII,IV2(µ-O)2 complex [via a CoIII2(µ-O)(µ-OH) intermediate] versus its FeIIIFeIV analog.

The hydroxylation of C-H bonds can be carried out by the high-valent CoIII,IV2(µ-O)2 complex 2a supported by the tetradentate tris(2-pyridylmethyl)amine ligand via a CoIII2(µ-O)(µ-OH) intermediate (3a). Complex 3a can be independently generated either by H-atom transfer (HAT) in the reaction of 2a with phenols as the H-atom donor or protonation of its conjugate base, the CoIII2(µ-O)2 complex 1a. Resonance Raman spectra of these three complexes reveal oxygen-isotope-sensitive vibrations at 560 to 590 cm-1 associated with the symmetric Co-O-Co stretching mode of the Co2O2 diamond core. Together with a Co•••Co distance of 2.78(2) Å previously identified for 1a and 2a by Extended X-ray Absorption Fine Structure (EXAFS) analysis, these results provide solid evidence for their "diamond core" structural assignments. The independent generation of 3a allows us to investigate HAT reactions of 2a with phenols in detail, measure the redox potential and pKa of the system, and calculate the O-H bond strength (DO-H) of 3a to shed light on the C-H bond activation reactivity of 2a. Complex 3a is found to be able to transfer its hydroxyl ligand onto the trityl radical to form the hydroxylated product, representing a direct experimental observation of such a reaction by a dinuclear cobalt complex. Surprisingly, reactivity comparisons reveal 2a to be 106-fold more reactive in oxidizing hydrocarbon C-H bonds than corresponding FeIII,IV2(µ-O)2 and MnIII,IV2(µ-O)2 analogs, an unexpected outcome that raises the prospects for using CoIII,IV2(µ-O)2 species to oxidize alkane C-H bonds.

Development of a sample cell for Radio Frequency µSR studies of metal nanoparticle systems with surface-adsorbed reactants in mesoporous hosts

We have recently begun an investigation of paramagnetic (free-radical) final states formed on metal nanoparticles by muonium (Mu) reactivity with surface-adsorbed molecules. The nanoparticles are incorporated into mesoporous silica, facilitating specific reaction steps in the silica host that involve H-atom transfer reactions important to studies in heterogeneous catalysis. Radio frequency (RF) methods are an essential tool for characterising final state species in these systems, and a non-metallic sample cell is essential for the RF field to penetrate the sample. Unfortunately, several significant problems were encountered during initial experiments using a cell made from PEEK polymer, the most serious being a temporal instability in the signals likely due to reactant molecules adsorbing on the PEEK. This paper discusses the problems encountered using the PEEK cell, and then considers the development of a ceramic cell designed to give better reproducibility in the measurements. The success of this new cell is demonstrated both through off-line tests and by muon measurements, including a series of TF 2G Mu spin precession measurements verifying the temporal stability of the experimental setup. Finally, an RF cavity was fashioned, and RF measurements made for muons stopped in bare silica, with signals from both diamagnetic and paramagnetic muon states clearly seen.

Surface ligand length influences kinetics of H-atom uptake in polyoxovanadate-alkoxide clusters.

The uptake of hydrogen atoms (H-atoms) at reducible metal oxide nanocrystal surfaces has implications in catalysis and energy storage. However, it is often difficult to gain insight into the physicochemical factors that dictate the thermodynamics and kinetics of H-atom transfer to the surface of these assemblies. Recently, our research group has demonstrated the formation of oxygen-atom (O-atom) defects in polyoxovanadate-alkoxide (POV-alkoxide) clusters via conversion of surface oxido moieties to aquo ligands, which can be accomplished via addition of two H-atom equivalents. Here, we present the dependence of O-atom defect formation via H-atom transfer at the surface of vanadium oxide clusters on the length of surface alkoxide ligands. Analysis of H-atom transfer reactions to low-valent POV-alkoxide clusters [V6O7(OR)12]1- (R = Me, Et, nPr, nBu) reveals that the length of primary alkoxide surface ligands does not significantly influence the thermodynamics of these processes. However, surface ligand length has a significant impact on the kinetics of these PCET reactions. Indeed, the methoxide-bridged cluster, [V6O7(OMe)12]1- reacts ∼20 times faster than the other derivatives evaluated. Interestingly, as the aliphatic linkages are increased in size from -C2H5 to -C4H9, reaction rates remain consistent, suggesting restricted access to available ligand conformers as a result of the incompatibility of the aliphatic ligands and acetonitrile may buffer further changes to the rate of reaction.

ANTIOXIDANT POTENTIAL OF EIGHT PHENOLIC ACIDS USING HIGH-PERFORMANCE DENSITY FUNCTIONAL THEORY

Phenolic acids are one of the major fractions identified in the extract of several vegetable seed oils which present interesting antioxidant properties. A density functional theory (DFT) study on the antioxidant potential of eight phenolic acids including gallic, vanillic, isovanillic, ferulic, caffeic, b-coumaric, cinnamic, and chlorogenic acids, is presented in this paper. The bond dissociation enthalpies (BDEs) of C–H and O–H bonds, the proton affinities (PA) and the ionization energies (IEs) were calculated in detail by using the LC-wPBE functional coupled with the 6-311++G(d,p) basis set. The standard Gibbs free energies (DrG0) for the scavenging reactions towards HOO· radical were calculated. In addition, the kinetics of H-atom transfer reaction was evaluated. As a result, the chosen long-range corrected DFT LC-wPBE functional is shown as the highly reliable computational approaches in calculating geometrical properties as well as the thermochemical parameters by comparison with CCSD(T)/aug-cc-pVDZ results. The differences of bond length are about 0.1 Angstroms, while the ones of BDE are only from 0.1 to 0.6 kcal/mol. Furthermore, gallic, caffeic, ferulic and chlorogenic acids represent as the most reactive antioxidants in the reaction with HOO· radical in water occurring via H transfer process with the negative DrG0 ranging from -3.3 kcal/mol for caffeic to -5.9 kcal/mol for ferulic compounds. Kinetic calculations in the gas phase based on transition state theory (TST) for the studied compound confirm that chlorogenic acid is shown as the most reactive antioxidant via HAT process with the lowest activation free energy (i.e. 17.9 kcal/mol) and the highest reaction rate (i.e. 4.20 × 10-19 cm3/molecule/s).

Surface chemistry of reduced graphene oxide: H-atom transfer reactions

In the present work, the surface chemistry of reduced graphene oxide, modified with hydrocarbon, hydroxyl, aldehyde and carboxyl groups is studied by means of computational chemistry. The simulations by ab-initio molecular dynamics show that the reactions depend on the proximity of chemical groups and possibilities for H-atom transfer and gas evolution. Defects in close proximity can also promote certain reactions, especially in case of good leaving groups as products (CO2, H2O, CH3OH, CO). Parts of the surface, rich in sp2 carbon atoms (i.e. regular graphene surface) can significantly decrease the scission energy of CC bonds from the leaving groups, compared to gas phase ethane molecule or to the corresponding CC- bonds located at the edge (or defect) of the two-dimensional carbon material. This effect is caused by the unpaired electron, formed after bond dissociation, joining the global π system and restoring of the regular graphene structure. Decarboxylation reactions are found to be energetically favorable at both edge and surface. Generally, reactions at the edge are found to be disfavored energetically if they involve the participation of a C atom from the surface of the two-dimensional carbon material.

H-atom transfer reaction of photoinduced excited triplet duroquinone with tryptophan and tyrosine in acetonitrile-water and ethylene glycol-water homogeneous solutions

Laser flash photolysis was used to investigate the photoinduced reactions of excited triplet bioquinone molecule duroquinone (DQ) with tryptophan (Trp) and tyrosine (Tyr) in acetonitrile-water (MeCN-H2O) and ethylene glycol-water (EG-H2O) solutions. The reaction mechanisms were analyzed and the reaction rate constants were measured based on Stern-Volmer equation. The H-atom transfer reaction from Trp (Tyr) to 3DQ* is dominant after the formation of 3DQ* during the laser photolysis. For DQ and Trp in MeCN-H2O and EG-H2O solutions, 3DQ* captures H-atom from Trp to generate duroquinone neutral radical DQH•, carbon-centered tryptophan neutral radical Trp•/NH and nitrogen-centered tryptophan neutral radical Trp/N•. For DQ and Tyr in MeCN-H2O and EG-H2O solutions, 3DQ* captures H-atom from Tyr to generate duroquinone neutral radical DQH• and tyrosine neutral radical Tyr/O•. The H-atom transfer reaction rate constant of 3DQ* with Trp (Tyr) is on the level of 109 L·mol−1·s−1, nearly controlled by diffusion. The reaction rate constant of 3DQ* with Trp (Tyr) in MeCN/H2O solution is larger than that in EG/H2O solution, which agrees with Stokes-Einstein relationship qualitatively.

Thermo-kinetic theoretical studies on pyrolysis of dimethoxymethane fuel additive

Many recent studies have been considered dimethoxymethane (DMM) as a potent fuel additive which reduces soot production and nitrogen oxides (NOx) emissions of diesel and biodiesel fuel. This study reports thermo-kinetic investigation of DMM pyrolysis in temperature range 300–2000 K using M06-2X and ωB97XD density functional methods and modest cost ab initio/complete basis set-quadratic Becke3 (CBS-QB3) procedure. The energy profile has been constructed using possible unimolecular H-atom transfer reactions (R1–R4) and simple bond fission reactions (R5–R11). Rate constant calculations were conducted at high-pressure (HP) limit and in the fall-off regimes using classical transition state (TST) and the statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theories, respectively. Based on CBS-QB3 energies, branching ratios analysis reveals unimportance of all H-atom transfer reactions at T ≥ 400 K, while the homolytic bond cleavage reaction R5 which produces CH3 and gauche– OCH2OCH3 fragments is considered the main dominated decomposition channel with high competition from R6 (CH3 and trans– CH3OCH2O) and R7 (CH3O and CH2OCH3) channels especially at elevated temperature.

Theoretical investigation on the contribution of HO[rad], SO4[rad]- and CO3[rad]- radicals to the degradation of phenacetin in water: Mechanisms, kinetics, and toxicity evaluation

Indirect oxidation induced by reactive free radicals, such as hydroxyl radical (HO), sulfate radical (SO4-) and carbonate radical (CO3-), plays an important or even crucial role in the degradation of micropollutants. Thus, the coadjutant degradation of phenacetin (PNT) by HO, SO4- and CO3-, as well as the synergistic effect of O2 on HO and HO2 were studied through mechanism, kinetics and toxicity evaluation. The results showed that the degradation of PNT was mainly caused by radical adduct formation (RAF) reaction (69% for Г, the same as below) and H atom transfer (HAT) reaction (31%) of HO. For the two inorganic anionic radicals, SO4- initiated PNT degradation by sequential radical addition-elimination (SRAE; 55%), HAT (28%) and single electron transfer (SET; 17%) reactions, while only by HAT reaction for CO3-. The total initial reaction rate constants of PNT by three radicals were in the order: SO4- > HO > CO3-. The kinetics of PNT degradation simulated by Kintecus program showed that UV/persulfate could degrade target compound more effectively than UV/H2O2 in ultrapure water. In the subsequent reaction of PNT with O2, HO and HO2, the formation of mono/di/tri-hydroxyl substitutions and unsaturated aldehydes/ketones/alcohols were confirmed. The results of toxicity assessment showed that the acute and chronic toxicity of most products to fish increased and to daphnia decreased, and acute toxicity to green algae decreased while chronic toxicity increased.

Photoprotective properties of natural antioxidant flavonoids: A DFT and TD‐DFT study on acridone derivatives

AbstractAntioxidant and photoprotective properties of two natural acridone derivatives namely Paratrimerin C (1) and Citrusinine‐I (2) have been studied by DFT and TD‐DFT at the M05‐2X/6‐311++G(3df, 3p)//M05‐2X/6‐31+G(d) level of theory. Three common mechanisms characterized for the antioxidant properties including H‐atom transfer (HAT) and proton transfer (PT) towards HOO●/HO● radicals and single electron transfer (SET) were investigated in the gas phase, water and pentyle thanoate. The DFT results revealed that both compounds can efficiently scavenge HOO● and HO● radicals via HAT mechanism in all media. Particularly, the HAT reactions of 2 with HO● radical in water are the most favored reaction (∆H ‐37.7 kcal/mol). In addition, the efficient UV‐absorption ability of the studied compound was elucidated by TD‐DFT. All compounds can absorb UV radiations in the range of 200‐335 nm, for which the easiest excitations are at 334‐332 nm and the strongest absorptions were found at 234‐227 nm, for 1 and 2, respectively. The HOMO to LUMO and HOMO‐3 to LUMO (π‐π*) transitions are assigned for the corresponding UV‐absorption.

Identification of an Emitting Metastable State of p-Fluorophenol-Ammonia 1:2 Complex by Laser-Induced Fluorescence Spectroscopy.

We have demonstrated here, for the first time to our knowledge, the formation of an emitting metastable species upon lowest electronic excitation (S1) of a hydrogen-bonded 1:2 complex of para-fluorophenol (pFP) with ammonia (NH3), which is known to be one of the smallest reactive complexes to undergo excited state H-atom transfer (HAT) reaction to produce •NH4(NH3) radical fragment. The emission spectrum of the species is characterized to be red-shifted, broad, and structureless. From the viewpoint of energy balance, an excited state proton transfer (ESPT) is unfavorable, but according to predicted electronic structure parameters, the metastable state species could be stabilized by charge transfer (CT) interaction at the hydrogen-bonded geometry of the complex. We propose that this species could act as an intermediate to the HAT process in the excited state. The observation of such a state could be valuable to understand the complex dynamics of similar events in biologically relevant systems.

Theoretical, Experimental, and Modeling Study of the Reaction of Hydrogen Atoms with 1- and 2-Pentene.

Alkyl radicals are prominent in combustion chemistry as they are formed by hydrocarbon decomposition or from a radical attack on hydrocarbons. Accurate determinations of the thermochemistry and kinetics of their unimolecular isomerization and decomposition reactions and related addition reactions of alkenes are therefore important in simulating the combustion chemistry of virtually all hydrocarbon fuels. In this work, a comprehensive potential energy surface (PES) for Ḣ-atom addition to and abstraction from 1- and 2-pentene, and the subsequent C-C and C-H β-scission reactions, and H-atom transfer reactions has been considered. Thermochemical values for the species on the Ċ5H11 PES were calculated as a function of temperature (298-2000 K), with enthalpies of formation determined using a network of isodesmic reactions. High-pressure limiting and pressure-dependent rate constants were calculated using the Rice-Ramsperger-Kassel-Marcus theory coupled with a one-dimensional master equation. As a validation of our theoretical results, hydrogen atomic resonance absorption spectrometry experiments were performed on the Ḣ-atom addition and abstraction reactions of 1- and 2-pentene. By incorporating our calculations into a detailed chemical kinetic model (AramcoMech 3.0), excellent agreement with these experiments is observed. The theoretical results are further validated via a comprehensive series of simulations of literature data. Our a priori model is found to reproduce important absolute species concentrations and product ratios reported therein.

Generation and Reactivity of a One-Electron-Oxidized Manganese(V) Imido Complex with a Tetraamido Macrocyclic Ligand.

The synthesis and X-ray structure of a new manganese(V) mesitylimido complex with a tetraamido macrocyclic ligand (TAML), [MnV (TAML)(N-Mes)]- (1), are reported. Compound 1 is oxidized by [(p-BrC6 H4 )3 N]+. [SbCl6 ]- and the resulting MnVI species readily undergoes H-atom transfer and nitrene transfer reactions.

Hydrogen on Cobalt Phosphide.

Cobalt phosphide (CoP) is one of the most promising earth-abundant replacements for noble metal catalysts for the hydrogen evolution reaction (HER). Critical to HER is the binding of H atoms. While theoretical studies have computed preferred sites and energetics of hydrogen bound to transition metal phosphide surfaces, direct experimental studies are scarce. Herein, we describe measurements of stoichiometry and thermochemistry for hydrogen bound to CoP. We studied both mesoscale CoP particles, exhibiting phosphide surfaces after an acidic pretreatment, and colloidal CoP nanoparticles. Treatment with H2 introduced large amounts of reactive hydrogen to CoP, ca. 0.2 H per CoP unit, and on the order of one H per Co or P surface atom. This was quantified using alkyne hydrogenation and H-atom transfer reactions with phenoxy radicals. Reactive H atoms were even present on the as-prepared materials. On the basis of the reactivity of CoP with various molecular hydrogen donating and accepting reagents, the distribution of binding free energies for H atoms on CoP was estimated to be roughly 51-66 kcal mol-1 (ΔG°H ≅ 0 to -0.7 eV vs H2). Operando X-ray absorption spectroscopy gave preliminary indications about the structure of hydrogenated CoP, showing a slight lattice expansion and no significant change of the effective nuclear charge of Co under H2-flow. These results provide a new picture of catalytically active CoP, with a substantial amount of reactive H atoms. This is likely of fundamental relevance for its catalytic and electrocatalytic properties. Additionally, the approach developed here provides a roadmap to examine hydrogen on other materials.

Photoinduced hydrogen-transfer reactions in pyridine-water clusters: Insights from excited-state electronic-structure calculations

Recent experiments have shown that photoexcited pyridine can abstract a hydrogen atom from a water molecule in pyridine-water clusters containing at least four water molecules. To explain these findings, we explored the electron-driven proton-transfer reaction from water to pyridine in pyridine-(H2O)n, n = 1–4, complexes with ab initio methods. It is shown that the photoinduced electron/proton transfer reaction is energetically possible for all clusters. The calculations reveal that the hydrogen bond between pyridine and the adjacent water molecule is weakened (strengthened) in the 1nπ∗ (1ππ∗) excited state, which is unfavorable (favorable) for the H-atom transfer reaction. For pyridine-(H2O)n clusters with n = 1–3, the steepest descent path leads to a local minimum of 1nπ∗ character, while for the pyridine-(H2O)4 cluster, this path leads to a local minimum of 1ππ∗ character. The transition state calculations show the presence of this 1ππ∗ minimum substantially reduces the barrier height for the H-atom transfer reaction. These results provide a tentative explanation of the experimental observations.

Hydrogen Atom Transfer Reactions of Mononuclear Nonheme Metal-Oxygen Intermediates.

Molecular oxygen (O2), the greenest oxidant, is kinetically stable in the oxidation of organic substrates due to its triplet ground state. In nature, O2 is reduced by two electrons with two protons to produce hydrogen peroxide (H2O2) and by four electrons with four protons to produce water (H2O) by oxidase and oxygenase metalloenzymes. In the process of the two-electron/two-proton and four-electron/four-proton reduction of O2 by metalloenzymes and their model compounds, metal-oxygen intermediates, such as metal-superoxido, -peroxido, -hydroperoxido, and -oxido species, are generated depending on the numbers of electrons and protons involved in the O2 activation reactions. The one-electron reduction of metal-oxygen intermediates is coupled with the binding of one proton. Such a hydrogen atom transfer (HAT) is defined as proton-coupled electron transfer (PCET), and there is a mechanistic dichotomy whether HAT occurs via a concerted PCET pathway or stepwise pathways [i.e., electron transfer followed by proton transfer (ET/PT) or proton transfer followed by electron transfer (PT/ET)]. The metal-oxygen intermediates formed are oxidants that can abstract a hydrogen atom (H-atom) from substrate C-H bonds. The H-atom abstraction from substrate C-H bonds by the metal-oxygen intermediates can also occur via a concerted PCET or stepwise PCET pathways. In the PCET reactions, a proton can be provided not only by the substrate itself but also by an acid that is added to a reaction solution. This Account describes the reactivities of metal-oxygen intermediates, such as metal-superoxido, -peroxido, -hydroperoxido, and -oxido complexes, in HAT reactions, focusing on the mechanisms of PCET reactions of metal-oxygen intermediates and on the mechanistic dichotomy of concerted versus stepwise pathways. Recent developments in the reactivity studies of Cr-, Fe-, and Cu-superoxido complexes in H-atom and hydride transfer reactions are discussed. Reactivities of an iron(III)-hydroperoxido complex and an iron(III)-peroxido complex binding redox-inactive metal ions are also summarized briefly. Mononuclear nonheme iron(IV)- and manganese(IV)-oxido complexes have shown high reactivities in HAT reactions, and their chemistry in PCET reactions is discussed intensively. Acid-catalyzed HAT reactions of metal-oxygen intermediates are also discussed to demonstrate a unified driving force dependence of logarithm of the rate constants of acid-catalyzed oxidation of various substrates by an iron(IV)-oxido complex and that of PCET from one-electron donors to the iron(IV)-oxido complex. PCET reactions of metal-oxygen intermediates are shown to proceed via a concerted pathway (one-step HAT) or a stepwise ET/PT pathway depending on the ET and PCET driving forces (-Δ G). The boundary conditions between concerted versus stepwise PCET pathways are clarified to demonstrate a switchover of the mechanisms only by changing the reaction temperature in the boundary conditions. This Account summarizes recent developments in the HAT reactions by synthetic mononuclear nonheme metal-oxygen intermediates over the past 10 years.

Recent Insights on Hydrogen Atom Transfer in the Inhibition of Hydrocarbon Autoxidation

Autoxidation, the free radical chain reaction that nominally inserts O2 into hydrocarbons to give peroxides, is primarily responsible for the degradation of all organic materials. Peroxyl radicals propagate autoxidation mainly by abstraction of labile H-atoms from the hydrocarbons, whereas radical-trapping antioxidants (RTAs) inhibit autoxidation by donating an H-atom to the peroxyl radical to give a nonpropagating radical. As such, a detailed understanding of the kinetics and thermodynamics of H-atom transfer (HAT) reactions to peroxyl radicals, and the effects of sterics, electronics, and medium thereupon, is key to understanding the mechanisms and products of autoxidation and the ability of RTAs to inhibit it. Due to their relatively weak O-H and N-H bonds, phenols and aromatic amines have long been utilized as RTAs, but only phenols have been extensively optimized to maximize their reactivity. Amines offer greater structural variability owing to their trivalent central nitrogen atom. Simply linking the two aromatic rings of a diarylamine to afford a phenoxazine offers profound differences in HAT reactivity: 1000-times greater than diphenylamine and 10-fold more reactive than α-tocopherol, Nature's optimized phenolic RTA. Thus, phenoxazines are an exciting scaffold for RTA development. Indeed, we have recently shown that ring substitution of phenoxazine or 2,4-diazaphenoxazine can yield compounds that undergo barrierless HAT reactions with peroxyl radicals. Amines also have the distinct advantage that they can react with peroxyl radicals to yield nitroxides, which can inhibit autoxidation in a catalytic manner utilizing the substrate itself as the stoichiometric reductant. Herein we provide an account of our recent efforts to understand how they manage this feat, which have revealed at least four mechanisms depending on the specific reaction conditions (i.e., saturated hydrocarbons at elevated temperatures, unsaturated hydrocarbons, acidic media, aqueous media/lipid dispersions). We also reiterate how their impressive RTA activity translates from solution to mammalian cell culture, wherein we have demonstrated that diarylamines and their derived nitroxides are potent inhibitors of ferroptosis, a recently characterized form of cell death associated with lipid peroxidation (autoxidation). In addition to phenols and amines, organosulfur compounds have long been used as antioxidants. The prevailing view has been that they undergo ionic reactions with product peroxides, preventing the initiation of further chain reactions. In recent years, we have found that many organosulfur compounds exhibit very good RTA activity. In particular, sulfenic acids (RSOH) and hydropersulfides (RSSH) are found to be among the best HAT agents known, particularly to peroxyl radicals where secondary orbital interactions are found to play a significant role. Consequently, oxidation of the sulfenic acid to a sulfinic acid greatly diminishes its HAT reactivity to peroxyls. Polysulfides and their oxides also undergo direct reactions with peroxyl radicals, thereby inhibiting autoxidation, but do so by homolytic substitution reactions. These insights suggest that the RTA activity of organosulfur compounds may be as important to the inhibition of hydrocarbon autoxidation, if not more so, than their ionic reactions.

Is Vitamin A an Antioxidant or a Pro-oxidant?

Antioxidant efficiency of all-trans-retinol has been studied on the basis of characteristic thermochemical properties using density functional theory. The influence of the solvent polarity has also been evaluated. It is found that retinol may act in parallel as an effective antioxidant via H atom donating as well as a pro-oxidant in yielding reactive hydroxyl radical. In fact, the lowest values of bond dissociation enthalpy were found at the C18-H and C18-OH positions. Retinol was also determined to be a good electron donor but bad acceptor in the single electron transfer (ET) reaction with hydroperoxyl (HOO•) radical. In addition, potential energy surfaces of H atom transfer (HAT) and radical adduct formation (RAF) reactions between retinol and HOO• radical were also investigated in the gas phase and in the solvent. The results demonstrated that the RAF mechanism was generally more predominant than ET and HAT ones. The most favored radical addition position was found at the C2═C3 double bond in the cyclohexenyl ring. Moreover, the radical scavenging reactivity via RAF reactions was strongly exergonic and thermodynamically feasible while the ET one was endergonic. Natural bond orbital analysis showed that the lone pairs of electrons on the oxygen atom of the HOO• radical were donated to the unoccupied antibonding orbital of transferred H atom in HAT reactions. In contrast, in the case of RAF reactions, strong interactions between 2p orbitals on oxygen atoms of the radical and the π orbital of the double bond on the retinol molecule were recognized. The results obtained in this work were in agreement with previous experimental observations.

Tuning the reactivity of mononuclear nonheme manganese(iv)-oxo complexes by triflic acid

Triflic acid (HOTf)-bound nonheme Mn(IV)-oxo complexes, [(L)MnIV(O)]2+-(HOTf)2 (L = N4Py and Bn-TPEN; N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) and Bn-TPEN = N-benzyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine), were synthesized by adding HOTf to the solutions of the [(L)MnIV(O)]2+ complexes and were characterized by various spectroscopies. The one-electron reduction potentials of the MnIV(O) complexes exhibited a significant positive shift upon binding of HOTf. The driving force dependence of electron transfer (ET) from electron donors to the MnIV(O) and MnIV(O)-(HOTf)2 complexes were examined and evaluated in light of the Marcus theory of ET to determine the reorganization energies of ET. The smaller reorganization energies and much more positive reduction potentials of the [(L)MnIV(O)]2+-(HOTf)2 complexes resulted in much enhanced oxidation capacity towards one-electron reductants and para-X-substituted-thioanisoles. The reactivities of the Mn(IV)-oxo complexes were markedly enhanced by binding of HOTf, such as a 6.4 × 105-fold increase in the oxygen atom transfer (OAT) reaction (i.e., sulfoxidation). Such a remarkable acceleration in the OAT reaction results from the enhancement of ET from para-X-substituted-thioanisoles to the MnIV(O) complexes as revealed by the unified ET driving force dependence of the rate constants of OAT and ET reactions of [(L)MnIV(O)]2+-(HOTf)2. In contrast, deceleration was observed in the rate of H-atom transfer (HAT) reaction of [(L)MnIV(O)]2+-(HOTf)2 complexes with 1,4-cyclohexadiene as compared with those of the [(L)MnIV(O)]2+ complexes. Thus, the binding of two HOTf molecules to the MnIV(O) moiety resulted in remarkable acceleration of the ET rate when the ET is thermodynamically feasible. When the ET reaction is highly endergonic, the rate of the HAT reaction is decelerated due to the steric effect of the counter anion of HOTf.

Discovery of low energy pathways to metal-mediated B[double bond, length as m-dash]N bond reduction guided by computation and experiment.

This manuscript describes a combination of DFT calculations and experiments to assess the reduction of borazines (B-N heterocycles) by η6-coordination to Cr(CO)3 or [Mn(CO)3]+ fragments. The energy requirements for borazine reduction are established as well as the extent to which coordination of borazine to a transition metal influences hydride affinity, basicity, and subsequent reduction steps at the coordinated borazine molecule. Borazine binding to M(CO)3 fragments decreases the thermodynamic hydricity by >30 kcal mol-1, allowing it to easily accept a hydride. These hydricity criteria were used to guide the selection of appropriate reagents for borazine dearomatization. Reduction was achieved with an H2-derived hydride source, and importantly, a pathway which proceeds through a single electron reduction and H-atom transfer reaction, mediated by anthraquinone was uncovered. The latter transformation was also carried out electrochemically, at relatively positive potentials by comparison to all prior reports, thus establishing an important proof of concept for any future electrochemical B[double bond, length as m-dash]N bond reduction.