Hydrogen Atom Acceptor Research Articles

Conformational Change with Steric Interactions Affects the Inner Sphere Component of Concerted Proton-Electron Transfer in a Pyridyl-Appended Radical Cation System.

Proton-coupled electron transfer (PCET) model systems combine one-electron oxidants and bases to generate net hydrogen atom acceptors. We have generated two persistent pyridyl-appended radical cations: 10-(pyrid-2-yl)-10H-phenothiazinium (PPT•+) and 3-(pyrid-2-yl)-10-methyl-10H-phenothiazinium (MPTP•+). EPR spectra and corresponding calculations indicate phenothiazinium radical cations with minimal spin on the pyridine nitrogen. Addition of hindered phenols causes the radical cations to decay, and protonated products and the corresponding phenoxyl radicals to form. The ΔG° values for the formation of intermediates (determined through cyclic voltammetry and pKa measurements) rule out a stepwise mechanism, and kinetic isotope effects support concerted proton–electron transfer (CPET) as the mechanism. Calculations indicate that the reaction of PPT•+ + tBu3PhOH undergoes a significant conformational change with steric interactions on the diabatic surface while maintaining the hydrogen bond; in contrast, MPTP•+ + tBu3PhOH maintains its conformation throughout the reaction. This difference is reflected in both experiment and calculations with ΔG(⧧)MPTP•+ < ΔG(⧧)PPT•+ despite ΔG°MPTP•+ > ΔG°PPT•+. Experimental results with 2,6-di-tert-butyl-4-methoxyphenol are similar. Hence, despite the structural similarity between the compounds, differences in the inner sphere component for CPET affect the kinetics.

Active Site Dynamical Effects in the Hydrogen Transfer Rate-limiting Step in the Catalysis of Linoleic Acid by Soybean Lipoxygenase-1 (SLO-1): Primary and Secondary Isotope Contributions.

Using ab initio molecular dynamics (AIMD) simulations that facilitate the treatment of rare events, we probe the active site participation in the rate-determining hydrogen transfer step in the catalytic oxidation of linoleic acid by soybean lipoxygenase-1 (SLO-1). The role of two different active site components is probed. (a) On the hydrogen atom acceptor side of the active site, the hydrogen bonding propensity between the acceptor side hydroxyl group, which is bound to the iron cofactor, and the backbone carboxyl group of isoleucine (residue number 839) is studied toward its role in promoting the hydrogen transfer event. Primary and secondary (H/D) isotope effects are also probed and a definite correlation with subtle secondary H/D isotope effects is found. With increasing average nuclear kinetic energy, the increase in transfer probability is enhanced due to the presence of the hydrogen bond between the backbone carbonyl of I839 and the acceptor oxygen. Further increase in average nuclear kinetic energy reduces the strength of this secondary hydrogen bond which leads to a deterioration in hydrogen transfer rates and finally embrances an Arrhenius-like behavior. (b) On the hydrogen atom donor side, the coupling between vibrational modes predominantly localized on the donor-side linoleic acid group and the reactive mode is probed. There appears to be a qualitative difference in the coupling between modes that belong to linoleic acid and the hydrogen transfer mode, for hydrogen and deuterium transfer. For example, the donor side secondary hydrogen atom is much more labile (by nearly a factor of 5) during deuterium transfer as compared to the case for hydrogen transfer. This appears to indicate a greater coupling between the modes belonging to the linoleic acid scaffold and the deuterium transfer mode and also provides a new rationalization for the abnormal (nonclassical) secondary isotope effect results obtained by Knapp, Rickert, and Klinman in J. Am. Chem. Soc. , 2002 , 124 , 3865 . To substantiate our findings noted in point a above, we have suggested an I839 → A839 or I839 → V839 mutation. This will modify the bulkiness of hydrogen the bonding residue, allowing greater flexibility in the secondary hydrogen bond formation highlighted above and adversely affecting the reaction rate.

Photoinduced PCET in ruthenium-phenol systems: thermodynamic equivalence of uni- and bidirectional reactions.

Six termolecular reaction systems comprised of Ru(4,4′-bis(trifluoromethyl)-2,2′-bipyridine)32+, phenols with different para substituents, and pyridine in acetonitrile undergo proton-coupled electron transfer (PCET) upon photoexcitation of the metal complex. Five of these six phenols are found to release in concerted fashion an electron to the ruthenium photooxidant and a proton to the pyridine base. The kinetics for this concerted bidirectional PCET process and its relationship to the reaction free energy were compared to the driving-force dependence of reaction kinetics for unidirectional concerted proton–electron transfer (CPET) between the same phenols and Ru(2,2′-bipyrazine)32+, a combined electron/proton acceptor. The results strongly support the concept of thermodynamic equivalence between separated electron/proton acceptors and single-reagent hydrogen-atom acceptors. A key feature of the explored systems is the similarity between molecules employed for bi- and unidirectional CPET.

Determination of Thermodynamic Affinities of Various Polar Olefins as Hydride, Hydrogen Atom, and Electron Acceptors in Acetonitrile

A series of 69 polar olefins with various typical structures (X) were synthesized and the thermodynamic affinities (defined in terms of the molar enthalpy changes or the standard redox potentials in this work) of the polar olefins obtaining hydride anions, hydrogen atoms, and electrons, the thermodynamic affinities of the radical anions of the polar olefins (X(•-)) obtaining protons and hydrogen atoms, and the thermodynamic affinities of the hydrogen adducts of the polar olefins (XH(•)) obtaining electrons in acetonitrile were determined using titration calorimetry and electrochemical methods. The pure C═C π-bond heterolytic and homolytic dissociation energies of the polar olefins (X) in acetonitrile and the pure C═C π-bond homolytic dissociation energies of the radical anions of the polar olefins (X(•-)) in acetonitrile were estimated. The remote substituent effects on the six thermodynamic affinities of the polar olefins and their related reaction intermediates were examined using the Hammett linear free-energy relationships; the results show that the Hammett linear free-energy relationships all hold in the six chemical and electrochemical processes. The information disclosed in this work could not only supply a gap of the chemical thermodynamics of olefins as one class of very important organic unsaturated compounds but also strongly promote the fast development of the chemistry and applications of olefins.

Ferrocenyl-Labeled Sugar Amino Acids: Conformation and Properties

Novel organometallic sugar amino acid conjugates 1–5 have been prepared by amide coupling of O-protected N-acetylmuramic acid and iso-muramic acid (2-[3-amino-2,5-dihydroxy-6-(hydroxymethyl)oxan-4-yl]oxypropanoic acid) with 1-aminoferrocene, 1-aminoferrocene-1′-carboxylic acid (H-Fca-OH), or 1,1′-diaminoferrocene, respectively. The influence of the ferrocenyl moiety and presence of additional remote potential hydrogen atom acceptors and donors at the ferrocenyl core on the conformation and lipophilicity is investigated by TLC, IR, NMR, and CD spectroscopic methods augmented by density functional calculations. Furthermore, the redox potential of the ferrocene/ferrocenium couple is tuned by the electron-withdrawing and -donating nature of the substituents at the ferrocenyl label.

Spin-forbidden hydrogen atom transfer reactions in a cobalt biimidazoline system

Described here are hydrogen atom transfer (HAT) reactions from high-spin cobalt(II) tris(2,2′-bi-2-imidazoline) (CoIIIIH22bim) to the hydrogen atom acceptors, 2,2,6,6-tetramethyl-1-piperidinyl-oxyl (TEMPO), 2,4,6-tri-tert-butylphenoxyl radical (tBu3ArO˙), and benzoquinone (BQ). The cobalt product is the oxidized and deprotonated, low-spin cobalt(III) complex (CoIIIIIIHbim), and the organic products are TEMPOH, tBu3ArOH, or hydroquinone, respectively. These reactions are formally spin forbidden because the spin state of the reactants is different from that of the products. For instance, quartet CoIIIIH22bim plus doublet RO˙ can have a triplet or quintet ground state, while the CoIIIIIIHbim + ROH product state is a singlet. Kinetics measured in the forward and reverse directions and thermochemical measurements provide a detailed picture of the reactions. The reactions are quite slow: the reaction of 10 mM CoIIIIH22bim with excess TEMPO requires roughly a day at ambient temperatures to reach equilibrium. This is 3400 times slower than the related reaction of the iron analogue FeIIIIH22bim, which is 2 kcal mol−1 more uphill. Mechanistic analyses show that the TEMPO reaction occurs by hydrogen atom transfer (HAT), and this is likely for the tBu3ArO˙ and BQ reactions as well. This is an unusually well defined spin-forbidden HAT system, which serves as a model for more complex multi-spin state HAT processes such as those suggested to occur in cytochrome P450 and metal-oxo model systems. In principle, HAT could occur before, after, or concerted with spin change. Computational studies indicate a reaction mechanism involving pre-equilibrium spin state interconversion of quartet 44CoIIIIH22bim to its doublet excited state 22CoIIIIH22bim, followed by spin-allowed HAT to the organic acceptor. This mechanism is consistent with the available kinetic, thermochemical and spectroscopic measurements. It indicates that the slow rates are due to the large change in geometry between CoIIIIH22bim and CoIIIIIIHbim, rather than any inherent difficulty in changing spin state. The implications of these results for other spin-forbidden or ‘two-state’ HAT processes are discussed.

STABILIZATION OF RAW DIENIC SYNTHETIC RUBBER POLYMERS

Abstract The mechanism of rubber polymer autoxidation is briefly reviewed with an emphasis and an appreciation for what these uninhibited free radical reactions can do to polymer properties and quality. Significant differences between hydrocarbon and rubber oxidation are discussed. Differences in polymer substructure are described which affect the mode of oxidation as well as the choice of stabilizer to protect these materials. An explanation of color formation and color prevention for 2,6-dialkyl-4-methylphenols is provided which also includes a discussion of how their oxidized transformation products interact with the rubber polymer. Additional factors that affect polymer quality include proxidative effects of the stabilizer, oxygen diffusion, and the formation of gel. Studies which varied the steric bulk of the alkyl groups in the 2,6 position were conducted to optimize the antioxidant activity of this system in its function in the hydrogen atom donor—hydrogen atom acceptor mechanism of stabilization. A description of the preparation and activity impact of the secondary antioxidant synergist Wingstay® SN-1 is included. Finally, the discovery of highly active 2,6-di(alkylthiomethyl) substituted phenols is described which led to the discovery of Wingstay® K and its contained synergist methylene-bis(dodecylsulfide). This material is a highly persistent auto synergistic stabilizer which is outstanding for the protection of emulsion-styrene butadiene rubber when used 50/50 with Wingstay® 29. When used by itself in polybutadiene rubber polymers, it is highly effective against the effects of oxidation and also for the prevention of gel formation and Mooney viscosity changes.

Ionic liquids as benign catalysts for the carbonylation of amines to formamides

1-Butyl-3-methylimidazolium hydrogen carbonate ([BMIm][HCO 3]), prepared from the reaction of [BMIm]Cl with K 2CO 3 in methanol, exhibits high activity for the carbonylation of amines to produce formamides. Computational calculation results on the carbonylation reaction of methylamine implies that such high activity of [BMIm][HCO 3] could be ascribed to the bi-functional actions of [HCO 3] − as a hydrogen atom acceptor and a donor.

Hydrogen bond and halogen bond inside the carbon nanotube

The hydrogen bond and halogen bond inside the open-ended single-walled carbon nanotubes have been investigated theoretically employing the newly developed density functional M06 with the suitable basis set and the natural bond orbital analysis. Comparing with the hydrogen or halogen bond in the gas phase, we find that the strength of the hydrogen or halogen bond inside the carbon nanotube will become weaker if there is a larger intramolecular electron-density transfer from the electron-rich region of the hydrogen or halogen atom donor to the antibonding orbital of the X-H or X-Hal bond involved in the formation of the hydrogen or halogen bond and will become stronger if there is a larger intermolecular electron-density transfer from the electron-rich region of the hydrogen or halogen atom acceptor to the antibonding orbital of the X-H or X-Hal bond. According to the analysis of the molecular electrostatic potential of the carbon nanotube, the driving force for the electron-density transfer is found to be the negative electric field formed in the carbon nanotube inner phase. Our results also show that the X-H bond involved in the formation of the hydrogen bond and the X-Hal bond involved in the formation of the halogen bond are all elongated when encapsulating the hydrogen bond and halogen bond within the carbon nanotube, so the carbon nanotube confinement may change the blue-shifting hydrogen bond and the blue-shifting halogen bond into the red-shifting hydrogen bond and the red-shifting halogen bond. The possibility to replace the all electron nanotube-confined calculation by the simple polarizable continuum model is also evaluated.

Concerted Proton−Electron Transfer in a Ruthenium Terpyridyl-Benzoate System with a Large Separation between the Redox and Basic Sites

To understand how the separation between the electron and proton-accepting sites affects proton-coupled electron transfer (PCET) reactivity, we have prepared ruthenium complexes with 4'-(4-carboxyphenyl)terpyridine ligands, and studied reactivity with hydrogen atom donors (H-X). Ru(II)(pydic)(tpy-PhCOOH) (Ru(II)PhCOOH), was synthesized in one pot from [(p-cymene)RuCl(2)](2), sodium 4'-(4-carboxyphenyl)-2,2':6',2''-terpyridine ([Na(+)]tpy-PhCOO(-)), and disodium pyridine-2,6-dicarboxylate (Na(2)pydic). Ru(II)PhCOOH plus (n)Bu(4)NOH in DMF yields the deprotonated Ru(II) complex, (n)Bu(4)N[Ru(II)(pydic)(tpy-PhCOO)] (Ru(II)PhCOO(-)). The Ru(III) complex (Ru(III)PhCOO) has been isolated by one-electron oxidation of Ru(II)PhCOO(-) with triarylaminium radical cations (NAr(3)(*+)). The bond dissociation free energy (BDFE) of the O-H bond in Ru(II)PhCOOH is calculated from pK(a) and E(1/2) measurements as 87 kcal mol(-1), making Ru(III)PhCOO a strong hydrogen atom acceptor. There are 10 bonds and ca. 11.2 A separating the metal from the carboxylate basic site in Ru(III)PhCOO. Even with this separation, Ru(III)PhCOO oxidizes the hydrogen atom donor TEMPOH (BDFE = 66.5 kcal mol(-1), DeltaG(o)(rxn) = -21 kcal mol(-1)) by removal of an electron and a proton to form Ru(II)PhCOOH and TEMPO radical in a concerted proton-electron transfer (CPET) process. The second order rate constant for this reaction is (1.1 +/- 0.1) x 10(5) M(-1) s(-1) with k(H)/k(D) = 2.1 +/- 0.2, similar to the observed kinetics in an analogous system without the phenyl spacer, Ru(III)(pydic)(tpy-COO(-)) (Ru(III)COO). In contrast, hydrogen transfer from 2,6-di-tert-butyl-p-methoxyphenol [(t)Bu(2)(OMe)ArOH] to Ru(III)PhCOO is several orders of magnitude slower than the analogous reaction with Ru(III)COO.

Electron Delocalization Mediates the Metal-Dependent Capacity for CH/π Interactions of Acetylacetonato Chelates

CH/pi interactions between the coordinated acetylacetonato ligand and phenyl rings were analyzed in the crystal structures from the Cambridge Structural Database and by quantum chemical calculations. The acetylacetonato ligand may engage in two types of interactions: it can be hydrogen atom donor or acceptor. The analysis of crystal structures and calculations show that interactions with the acetylacetonato ligand acting as hydrogen atom donor depend on the metal in an acetylacetonato chelate ring; the chelate rings with soft metals make stronger interactions. The same trend was not observed in the interactions where the acetylacetonato chelate ring acts as the hydrogen atom acceptor.

Bimolecular Reactions of Radical Generation Involving Nitrogen-Containing Compounds with Multiple Bonds

Retrodisproportionation reactions R"C=NR"" + HY → R"\({\text{C}}^ \cdot\)NHR"" + \({\text{Y}}^ \cdot\), RC≡N + HY → R\({\text{C}}^ \cdot\)=NH + \({\text{Y}}^ \cdot\), RN=NR + HY → R\({\text{N}}^ \cdot\)NHR + \({\text{Y}}^ \cdot\), RNO + HY → R\({\text{N}}^ \cdot\)OH + \({\text{Y}}^ \cdot\), and RNO2 + HY → RN(\({\text{O}}^ \cdot\))OH + \({\text{Y}}^ \cdot\) (where HY represents hydrocarbons, alcohols, phenols, amines, thiophenols, etc.) are considered as sources of radical generation. The enthalpies of these reactions are calculated. The parabolic model is used to calculate the rate constants for 254 such reactions. Various HY compounds acting as hydrogen-atom donors are compared with nitrogen-containing compounds acting as hydrogen-atom acceptors. The PhN(O) and PhNO2 compounds exhibit the highest activity among the studied acceptors.

Reactivity of ozone as a hydrogen-atom acceptor in reactions with antioxidants

Atom abstraction is shown to be the main reaction of ozone with phenols, aromatic amines and hydroxylamines in non-polar solvents. These reactions were analysed by using the parabolic model of the transition state of free-radical reactions. Activation energies and rate constants of ozone reactions with 144 phenols, sterically hindered phenols, aromatic amines and hydroxylamines were calculated. The results are in agreement with experimental measurements. The mechanism of ozone reaction with antioxidants is discussed.

Chemical model studies on the monoamine oxidase-B catalyzed oxidation of 4-substituted 1-methyl-1,2,3,6-tetrahydropyridines

The MAO-B catalyzed α-carbon oxidation of amines has been proposed to proceed via either a single electron transfer (SET) or hydrogen atom transfer (HAT) pathway. In an attempt to distinguish between these pathways, we have examined the α-carbon oxidation of a series of 4-substituted 1-methyl-1,2,3,6-tetrahydropyridine derivatives, compounds which are MAO-B substrates, employing chemical models of the SET pathway [using the PF 6 − salt of Fe +3 (1,10-phenanthroline) 3 as the electron acceptor] and HAT pathway (using the tert-butoxyl radical as the hydrogen atom acceptor). The rates of oxidation and deuterium isotope effects observed with these compounds were similar with the two model reactions. Consequently, unlike their utility in modeling the related cytochrome P450 catalyzed α-carbon oxidation of N, N-dimethylaniline derivatives, it appears that these reagents will not distinguish between the proposed pathways.

Kinetics of the interaction of ketyl and semiquinone radicals with dioxygen. Concerted electron and proton transfer involving ketyl and semiquinone radicals

Kinetics of the interaction of ketyl and neutral semiquinone radicals with dioxygen was studied by the flash photolysis technique. The reactivity of neutral semiquinone radicals in the transfer of a hydrogen atom to O2 is lower than that of ketyl radicals and increases as the reduction ability of the radicals increases, which give evidence for the charge transfer from the radicals to O2 in the transition state of the reaction. The deuterium kinetic isotope effect of the reaction (up to 2.6) suggests considerable weakening of the O−H bond of the seminquinone radical in the transition state. A cyclic structure of the transition state similar to that in the reactions of ketyl radicals with hydrogen atom acceptors is proposed. In aprotic volvents, solvation has essentially no effect on the reactivity of neutral anthrasemiquinone radicals up to solvent nucleophilicityB≈240. In solvents with higher nucleophilicity and in protic solvents, their reactivity drops sharply. Hydrogen atom transfer reactions involving ketyl and neutral semiquinone radicals are shown to involve concerted electron and proton transfers, and to have transition states in which the partial transfer of an electron and a proton from the ketyl or semiquinone radical to an acceptor occurs.

On the possibility of synthetic use of radicals generated by electroinduced hydrogen abstraction

Radicals are produced by indirect reduction of a compound which cleaves to a good hydrogen atom acceptor; this acceptor abstracts a hydrogen atom from a donor, and the radical thus formed may couple with the mediator or react in other ways. Thus, on reduction of 1,4-dicyanobenzene in the presence of iodobenzene and cyclohexane, 4-cyclohexylbenzonitrile was formed in fair yield. This type of reaction requires that certain conditions are met, and the possibilities of such reactions are discussed with regard to the rates of the competing reactions.

Reduction of NAD + and NADP + and reductive cleavage of NADH and NADPH yielding NAD . and NADP .

The reductive cleavage of NADH (and/or NADPH) was studied by means of controlled-potential electrolysis at mercury and platinum cathodes. The reduction products were characterized using high performance liquid chromatography (HPLC). NAD 2 dimers were found (30% of NADH converted) after electrolysis at the platinum electrode at −1.0 V (vs. SCE) in a buffered medium. Chemisorbed NAD . resulted after the reduction of 0.5–2.0 mmol/1 NADH on the mercury electrode. Homogeneous reduction of NADH with dithionite yielded NAD + after air oxidation of the original products. In order to identify the responses of NAD ., the reversibility of the redox reaction NAD +/NAD . (and NADP +/ NADP .) was studied by means of reverse pulse polarography (RPP) in a buffered neutral medium. The large potential difference between NAD + reduction (at approximately −1.0 V) and NAD . oxidation was explained by the chemisorption of NAD . at the mercury electrode. Reductive cleavage of the C(4)-H bond in the dihydronicotinamide moiety of NADH occurred under conditions identical to those of NAD + reduction. In the coenzyme action, the same electron-transfer reaction to NAD + or to NADH could produce either a hydrogen atom acceptor NAD . (from NAD + ) or a H . donor from NADH.

ChemInform Abstract: Tetramethylallene and 2,4‐Dimethyl‐1,3‐pentadiene as Hydrogen Atom Acceptors in Reactions with HMn(CO)5 and HCo(CO)4.

AbstractReactions of tetramethylallene (I) with the titlc metal hydrides proceed by Initial hydrogen atom transfer, providim the first examples of such reactions of nonconjugated alkenes.

Tetramethylallene and 2,4-dimethyl-1,3-pentadiene as hydrogen-atom acceptors in reactions with HMn(CO)5 and HCo(CO)4

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTTetramethylallene and 2,4-dimethyl-1,3-pentadiene as hydrogen-atom acceptors in reactions with HMn(CO)5 and HCo(CO)4John F. Garst, T. Michael. Bockman, and Rajnish. BatlawCite this: J. Am. Chem. Soc. 1986, 108, 7, 1689–1691Publication Date (Print):April 1, 1986Publication History Published online1 May 2002Published inissue 1 April 1986https://doi.org/10.1021/ja00267a048RIGHTS & PERMISSIONSArticle Views270Altmetric-Citations23LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (390 KB) Get e-Alerts Get e-Alerts