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Hydride Transfer (hydride + transfer)
Selected AbstractsChemInform Abstract: Copper-Catalyzed Hydride Transfer from LiAlH4 for the Formation of Alkylidenecyclopropane Derivatives.CHEMINFORM, Issue 21 2009Samah Simaan Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Iron-Catalyzed Hydrogenation, Hydride Transfer, and Hydrosilylation: An Alternative to Precious-Metal Complexes?CHEMSUSCHEM CHEMISTRY AND SUSTAINABILITY, ENERGY & MATERIALS, Issue 6 2008Sylvain Gaillard Dr. The dawn of a new iron age? Iron is one of the least expensive and non-toxic metals, however, its chemistry has remained less studied than that of precious metals. Recent advances in reduction chemistry using iron complexes as catalysts are reviewed and the great potential of this cheap but chic metal is illustrated in this Highlight. [source] Synthetic and Mechanistic Aspects of Acid-Catalyzed Disproportionation of Dialkyl Diarylmethyl Ethers: A Combined Experimental and Theoretical StudyEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 25 2009Margherita Barbero Abstract The disproportionation reactions of various dialkyl diarylmethyl ethers have been carried out in the presence of a catalytic amount (10 mol-%) of o -benzenedisulfonimide as a Brønsted acid catalyst; the reaction conditions were mild, and the yields of the diarylmethane target products were good. The catalyst was easily recovered and purified, ready to be used in further reactions. The theoretical study confirmed that the reaction proceeds in two steps: The formation of a carbocation from the protonated ether followed by hydride transfer. Although the hydride transfer is the rate-determining step, it is the stability of the carbocation that determines the reaction rate and therefore the yields. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Thermodynamic and kinetic analysis of the isolated FAD domain of rat neuronal nitric oxide synthase altered in the region of the FAD shielding residue Phe1395FEBS JOURNAL, Issue 12 2004Adrian J. Dunford In rat neuronal nitric oxide synthase, Phe1395 is positioned over the FAD isoalloxazine ring. This is replaced by Trp676 in human cytochrome P450 reductase, a tryptophan in related diflavin reductases (e.g. methionine synthase reductase and novel reductase 1), and tyrosine in plant ferredoxin-NADP+ reductase. Trp676 in human cytochrome P450 reductase is conformationally mobile, and plays a key role in enzyme reduction. Mutagenesis of Trp676 to alanine results in a functional NADH-dependent reductase. Herein, we describe studies of rat neuronal nitric oxide synthase FAD domains, in which the aromatic shielding residue Phe1395 is replaced by tryptophan, alanine and serine. In steady-state assays the F1395A and F1395S domains have a greater preference for NADH compared with F1395W and wild-type. Stopped-flow studies indicate flavin reduction by NADH is significantly faster with F1395S and F1395A domains, suggesting that this contributes to altered preference in coenzyme specificity. Unlike cytochrome P450 reductase, the switch in coenzyme specificity is not attributed to differential binding of NADPH and NADH, but probably results from improved geometry for hydride transfer in the F1395S, and F1395A,NADH complexes. Potentiometry indicates that the substitutions do not significantly perturb thermodynamic properties of the FAD, although considerable changes in electronic absorption properties are observed in oxidized F1395A and F1395S, consistent with changes in hydrophobicity of the flavin environment. In wild-type and F1395W FAD domains, prolonged incubation with NADPH results in development of the neutral blue semiquinone FAD species. This reaction is suppressed in the mutant FAD domains lacking the shielding aromatic residue. [source] Catalytic Deoxygenation of 1,2-Propanediol to Give n -PropanolADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 5 2009Marcel Schlaf Abstract Deoxygenation of 1,2-propanediol (1.0,M in sulfolane) catalyzed by bis(dicarbonyl)(,-hydrido)(pentamethylcyclopentadiene)ruthenium trifluoromethanesulfonate ({[Cp*Ru(CO)2]2(,-H)}+OTf,) (0.5 mol%) at 110,°C under hydrogen (750 psi) in the presence of trifluoromethanesulfonic acid (HOTf) (60,mM) gives n -propanol as the major product, indicating high selectivity for deoxygenation of the internal hydroxy group over the terminal hydroxy group of the diol. The deoxygenation of 1,2-propanediol is strongly influenced by the concentration of acid, giving faster rates and proceeding to higher conversions as the concentration of HOTf is increased. Propionaldehyde was observed as an intermediate, being formed through acid-catalyzed dehydration of 1,2-propanediol. This aldehyde is hydrogenated to n -propanol through an ionic pathway involving protonation of the aldehyde, followed by hydride transfer from the neutral hydride, dicarbonyl(pentamethylcyclopentadiene)ruthenium hydride [Cp*Ru(CO)2H]. The proposed mechanism for the deoxygenation/hydrogenation reaction involves formation of a highly acidic dihydrogen complex [Cp*Ru(CO)2(,2 -H2)]+ OTf,. [source] Origin of Enantioselectivity in the Organocatalytic Reductive Amination of ,-Branched AldehydesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 4 2009Tommaso Marcelli Abstract The reason for enantioselectivity in the reductive amination of ,-branched aldehydes was investigated. The relative energies of all the diastereomeric transition states for hydride transfer of a suitable computational model were calculated at the B3LYP/6-311+G(2d,2p) level of theory. Our calculations successfully reproduce and rationalize the experimentally observed stereochemical outcome of the reaction. [source] Direct hydride transfer in the reaction mechanism of quinoprotein alcohol dehydrogenases: a quantum mechanical investigationJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 15 2001A. Jongejan Abstract Oxidation of alcohols by direct hydride transfer to the pyrroloquinoline quinone (PQQ) cofactor of quinoprotein alcohol dehydrogenases has been studied using ab initio quantum mechanical methods. Energies and geometries were calculated at the 6-31G(d,p) level of theory. Comparison of the results obtained for PQQ and several derivatives with available structural and spectroscopic data served to judge the feasibility of the calculations. The role of calcium in the enzymatic reaction mechanism has been investigated. Transition state searches have been conducted at the semiempirical and STO-3G(d) level of theory. It is concluded that hydride transfer from the C,-position of the substrate alcohol (or aldehyde) directly to the C(5) carbon of PQQ is energetically feasible. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1732,1749, 2001 [source] Insights into the mechanisms of flavoprotein oxidases from kinetic isotope effects,JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS, Issue 11-12 2007Paul F. Fitzpatrick Abstract Deuterium, solvent, and 15N kinetic isotope effects have been used to probe the mechanisms by which flavoproteins oxidize carbon,oxygen and carbon,nitrogen bonds in amines, hydroxy acids, and alcohols. For the amine oxidases D -amino acid oxidase, N -methyltryptophan oxidase, and tryptophan monooxygenase, D -serine, sarcosine, and alanine are slow substrates for which CH bond cleavage is fully rate limiting. Inverse isotope effects for each of 0.992,0.996 are consistent with a common mechanism involving hydride transfer from the uncharged amine. Computational analyses of possible mechanisms support this conclusion. Deuterium and solvent isotope effects with wild-type and mutant variants of the lactate dehydrogenase flavocytochrome b2 show that OH and CH bond cleavage are not concerted, but become so in the Y254F enzyme. This is consistent with a highly asynchronous reaction in which OH bond cleavage precedes hydride transfer. The results of Hammett analyses and solvent and deuterium isotope effects support a similar mechanism for alcohol oxidase. Copyright © 2007 John Wiley & Sons, Ltd. [source] Reactivity of isobutane in fluorosulfonic based superacids,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 12 2002Alain Goeppert Abstract The behavior of isobutane in DSO3F containing various amounts of SbF5 has been studied in relation to the acid strength of the superacid system. In contrast to the DF,SbF5 system, H/D exchange occurs only in the weakest superacid via deprotonation of the t -butyl ion intermediate formed by an oxidative process. Kinetic isotope effect determination shows that the slow step is hydride transfer. At higher acidity, the increasing stability of this intermediate impedes isotopic exchange. Copyright © 2002 John Wiley & Sons, Ltd. [source] Determination of enzyme mechanisms by molecular dynamics: Studies on quinoproteins, methanol dehydrogenase, and soluble glucose dehydrogenasePROTEIN SCIENCE, Issue 8 2004Swarnalatha Y. Reddy Abstract Molecular dynamics (MD) simulations have been carried out to study the enzymatic mechanisms of quinoproteins, methanol dehydrogenase (MDH), and soluble glucose dehydrogenase (sGDH). The mechanisms of reduction of the orthoquinone cofactor (PQQ) of MDH and sGDH involve concerted base-catalyzed proton abstraction from the hydroxyl moiety of methanol or from the 1-hydroxyl of glucose, and hydride equivalent transfer from the substrate to the quinone carbonyl carbon C5 of PQQ. The products of methanol and glucose oxidation are formaldehyde and glucolactone, respectively. The immediate product of PQQ reduction, PQQH, [,HC5(O,) ,C4( = O) ,] and PQQH [,HC5(OH) ,C4( = O) ,] converts to the hydroquinone PQQH2 [,C5(OH) = C4(OH) ,]. The main focus is on MD structures of MDH , PQQ , methanol, MDH , PQQH,, MDH , PQQH, sGDH , PQQ , glucose, sGDH , PQQH, (glucolactone, and sGDH , PQQH. The reaction PQQ , PQQH, occurs with Glu 171,CO2, and His 144,Im as the base species in MDH and sGDH, respectively. The general-base-catalyzed hydroxyl proton abstraction from substrate concerted with hydride transfer to the C5 of PQQ is assisted by hydrogen-bonding to the C5 = O by Wat1 and Arg 324 in MDH and by Wat89 and Arg 228 in sGDH. Asp 297,COOH would act as a proton donor for the reaction PQQH, , PQQH, if formed by transfer of the proton from Glu 171,COOH to Asp 297,CO2, in MDH. For PQQH , PQQH2, migration of H5 to the C4 oxygen may be assisted by a weak base like water (either by crystal water Wat97 or bulk solvent, hydrogen-bonded to Glu 171,CO2, in MDH and by Wat89 in sGDH). [source] Ion chemistry of chloroethanes in air at atmospheric pressureRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 20 2001Anna Nicoletti Ion chemistry at atmospheric pressure is of major relevance to novel methods for the abatement of volatile organic compounds (VOCs) that employ non-thermal plasmas. For this reason, positive and negative APCI (atmospheric pressure chemical ionization) mass spectra of all six di-, tri- and tetrachloroethanes diluted in air (500,1500,ppm) at atmospheric pressure were investigated at 30,°C and at 300,°C. Spectral changes due to collisional activation of the ions achieved by increasing ,V, the potential difference between sampling and skimmer cones, are informative of structures and ion-molecule reactions. Positive ion chemistry of the chloroethanes (M) can, in general, be ascribed to C-C and C-Cl cleavages of the molecular ion, M+·, never detected but likely formed via exothermic charge exchange from primary ions of the APCI plasma. Exceptions to this characteristic pattern were observed for 1,1-dichloroethane and 1,1,2,2-tetrachloroethane, which give [M,,,H]+ and [M,,,HCl]+· species, respectively. It is suggested that both such species are due to ionization via hydride transfer. Upon increasing ,V, the [M,,,HCl]+· ion formed from 1,1,2,2-tetrachloroethane undergoes the same fragmentation and ion-molecule reactions previously reported for trichloroethene. A nucleophilic reaction of water within the [C2H4Cl+](H2O)n ionic complexes to displace HCl is postulated to account for the [C2H5O+](H2O)m species observed in the positive APCI spectra of the dichloroethanes. Negative ion spectra are, for all investigated chloroethanes, dominated by Cl, and its ion-neutral complexes with one, two and, in some cases, three molecules of the neutral precursor and/or water. Another common feature is the formation of species (X,)(M)n where X, is a background ion of the APCI plasma, namely O2,,O3, and, in some cases, (NO)2,. Peculiar to 1,1,1-trichloroethane are species attributed to Cl, complexes with phosgene, (Cl,)(Cl2C=O)n(n,=,1,2). Such complexes, which were not observed for either the isomeric 1,1,2-trichloroethane or for the tetrachloroethanes, are of interest as oxidation intermediates in the corona-induced decomposition process. No conclusions can be drawn in the case of the dichloroethanes, since, for these compounds, the ions (Cl,)(Cl2C=O)n and (Cl,)(M)n happen to be isobaric. Copyright © 2001 John Wiley & Sons, Ltd. [source] Structural characterization of tartrate dehydrogenase: a versatile enzyme catalyzing multiple reactionsACTA CRYSTALLOGRAPHICA SECTION D, Issue 6 2010Radhika Malik The first structure of an NAD-dependent tartrate dehydrogenase (TDH) has been solved to 2,Å resolution by single anomalous diffraction (SAD) phasing as a complex with the intermediate analog oxalate, Mg2+ and NADH. This TDH structure from Pseudomonas putida has a similar overall fold and domain organization to other structurally characterized members of the hydroxy-acid dehydrogenase family. However, there are considerable differences between TDH and these functionally related enzymes in the regions connecting the core secondary structure and in the relative positioning of important loops and helices. The active site in these complexes is highly ordered, allowing the identification of the substrate-binding and cofactor-binding groups and the ligands to the metal ions. Residues from the adjacent subunit are involved in both the substrate and divalent metal ion binding sites, establishing a dimer as the functional unit and providing structural support for an alternating-site reaction mechanism. The divalent metal ion plays a prominent role in substrate binding and orientation, together with several active-site arginines. Functional groups from both subunits form the cofactor-binding site and the ammonium ion aids in the orientation of the nicotinamide ring of the cofactor. A lysyl amino group (Lys192) is the base responsible for the water-mediated proton abstraction from the C2 hydroxyl group of the substrate that begins the catalytic reaction, followed by hydride transfer to NAD. A tyrosyl hydroxyl group (Tyr141) functions as a general acid to protonate the enolate intermediate. Each substrate undergoes the initial hydride transfer, but differences in substrate orientation are proposed to account for the different reactions catalyzed by TDH. [source] Gas-Phase Reactivity of Metavanadate [VO3], towards Methanol and Ethanol: Experiment and TheoryCHEMISTRY - A EUROPEAN JOURNAL, Issue 31 2007Tom Waters Dr. Abstract The gas-phase reactivity of the metavanadate anion [VO3], towards methanol and ethanol was examined by a combination of ion,molecule reaction and isotope labelling experiments in a quadrupole ion-trap mass spectrometer. The experimental data were interpreted with the aid of density functional theory calculations. [VO3], dehydrated methanol to eliminate water and form [VO2(,2 -OCH2)],, which features an [,2 - C,O -OCH2]2, ligand formed by formal removal of two protons from methanol and which is isoelectronic with peroxide. [VO3], reacted with ethanol in an analogous manner to form [VO2(,2 -OCHCH3)],, as well as by loss of ethene to form [VO2(OH)2],. The calculations predicted that important intermediates in these reactions were the hydroxo alkoxo anions [VO2(OH)(OCH2R)], (R: H, CH3). These were predicted to undergo intramolecular hydrogen-atom transfer to form [VO(OH)2(,1 -OCHR)], followed by ,1 - O,,2 - C,O rearrangements to form [VO(OH)2(,2 -OCHR)],. The latter reacted further to eliminate water and generate the product [VO2(,2 -OCHR)],. This major product observed for [VO3], is markedly different from that observed previously for [NbO3], containing the heavier Group,5 congener niobium. In that case, the major product of the reaction was an ion of stoichiometry [Nb, O3, H2], arising from the formal dehydrogenation of methanol to formaldehyde. The origin of this difference was examined theoretically and attributed to the intermediate alkoxo anion [NbO2(OH)(OCH3)], preferring hydride transfer to form [HNbO2(OH)], with loss of formaldehyde. This contrasts with the hydrogen-atom-transfer pathway observed for [VO2(OH)(OCH3)],. [source] Synthetic Scope and Mechanistic Studies of Ru(OH)x/Al2O3 -Catalyzed Heterogeneous Hydrogen-Transfer ReactionsCHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2005Kazuya Yamaguchi Dr. Abstract Three kinds of hydrogen-transfer reactions, namely racemization of chiral secondary alcohols, reduction of carbonyl compounds to alcohols using 2-propanol as a hydrogen donor, and isomerization of allylic alcohols to saturated ketones, are efficiently promoted by the easily prepared and inexpensive supported ruthenium catalyst Ru(OH)x/Al2O3. A wide variety of substrates, such as aromatic, aliphatic, and heterocyclic alcohols or carbonyl compounds, can be converted into the desired products, under anaerobic conditions, in moderate to excellent yields and without the need for additives such as bases. A larger scale, solvent-free reaction is also demonstrated: the isomerization of 1-octen-3-ol with a substrate/catalyst ratio of 20,000/1 shows a very high turnover frequency (TOF) of 18,400 h,1, with a turnover number (TON) that reaches 17,200. The catalysis for these reactions is intrinsically heterogeneous in nature, and the Ru(OH)x/Al2O3 recovered after the reactions can be reused without appreciable loss of catalytic performance. The reaction mechanism of the present Ru(OH)x/Al2O3 -catalyzed hydrogen-transfer reactions were examined with monodeuterated substrates. After the racemization of (S)-1-deuterio-1-phenylethanol in the presence of acetophenone was complete, the deuterium content at the ,-position of the corresponding racemic alcohol was 91,%, whereas no deuterium was incorporated into the ,-position during the racemization of (S)-1-phenylethanol-OD. These results show that direct carbon-to-carbon hydrogen transfer occurs via a metal monohydride for the racemization of chiral secondary alcohols and reduction of carbonyl compounds to alcohols. For the isomerization, the ,-deuterium of 3-deuterio-1-octen-3-ol was selectively relocated at the ,-position of the corresponding ketones (99,% D at the ,-position), suggesting the involvement of a 1,4-addition of ruthenium monohydride species to the ,,,-unsaturated ketone intermediate. The ruthenium monohydride species and the ,,,-unsaturated ketone would be formed through alcoholate formation/,-elimination. Kinetic studies and kinetic isotope effects show that the RuH bond cleavage (hydride transfer) is included in the rate-determining step. [source] Facile Oxidation of Leucomethylene Blue and Dihydroflavins by Artemisinins: Relationship with Flavoenzyme Function and Antimalarial Mechanism of ActionCHEMMEDCHEM, Issue 8 2010Richard Abstract The antimalarial drug methylene blue (MB) affects the redox behaviour of parasite flavin-dependent disulfide reductases such as glutathione reductase (GR) that control oxidative stress in the malaria parasite. The reduced flavin adenine dinucleotide cofactor FADH2 initiates reduction to leucomethylene blue (LMB), which is oxidised by oxygen to generate reactive oxygen species (ROS) and MB. MB then acts as a subversive substrate for NADPH normally required to regenerate FADH2 for enzyme function. The synergism between MB and the peroxidic antimalarial artemisinin derivative artesunate suggests that artemisinins have a complementary mode of action. We find that artemisinins are transformed by LMB generated from MB and ascorbic acid (AA) or N -benzyldihydronicotinamide (BNAH) in,situ in aqueous buffer at physiological pH into single electron transfer (SET) rearrangement products or two-electron reduction products, the latter of which dominates with BNAH. Neither AA nor BNAH alone affects the artemisinins. The AA,MB SET reactions are enhanced under aerobic conditions, and the major products obtained here are structurally closely related to one such product already reported to form in an intracellular medium. A ketyl arising via SET with the artemisinin is invoked to explain their formation. Dihydroflavins generated from riboflavin (RF) and FAD by pretreatment with sodium dithionite are rapidly oxidised by artemisinin to the parent flavins. When catalytic amounts of RF, FAD, and other flavins are reduced in,situ by excess BNAH or NAD(P)H in the presence of the artemisinins in the aqueous buffer, they are rapidly oxidised to the parent flavins with concomitant formation of two-electron reduction products from the artemisinins; regeneration of the reduced flavin by excess reductant maintains a catalytic cycle until the artemisinin is consumed. In preliminary experiments, we show that NADPH consumption in yeast GR with redox behaviour similar to that of parasite GR is enhanced by artemisinins, especially under aerobic conditions. Recombinant human GR is not affected. Artemisinins thus may act as antimalarial drugs by perturbing the redox balance within the malaria parasite, both by oxidising FADH2 in parasite GR or other parasite flavoenzymes, and by initiating autoxidation of the dihydroflavin by oxygen with generation of ROS. Reduction of the artemisinin is proposed to occur via hydride transfer from LMB or the dihydroflavin to O1 of the peroxide. This hitherto unrecorded reactivity profile conforms with known structure,activity relationships of artemisinins, is consistent with their known ability to generate ROS in,vivo, and explains the synergism between artemisinins and redox-active antimalarial drugs such as MB and doxorubicin. As the artemisinins appear to be relatively inert towards human GR, a putative model that accounts for the selective potency of artemisinins towards the malaria parasite also becomes apparent. Decisively, ferrous iron or carbon-centered free radicals cannot be involved, and the reactivity described herein reconciles disparate observations that are incompatible with the ferrous iron,carbon radical hypothesis for antimalarial mechanism of action. Finally, the urgent enquiry into the emerging resistance of the malaria parasite to artemisinins may now in one part address the possibilities either of structural changes taking place in parasite flavoenzymes that render the flavin cofactor less accessible to artemisinins or of an enhancement in the ability to use intra-erythrocytic human disulfide reductases required for maintenance of parasite redox balance. [source] | |||||||||||||||||||