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Hydrogen Transfer (hydrogen + transfer)
Terms modified by Hydrogen Transfer Selected AbstractsRuthenium Complexes Containing Chiral N-Donor Ligands as Catalysts in Acetophenone Hydrogen Transfer , New Amino Effect on EnantioselectivityEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 21 2005Montserrat Gómez Abstract New p -cymene ruthenium species containing chiral amino alcohols (1,3), primary (4,7) and secondary (8, 9) amino-oxazolines, were tested as catalysts in the hydrogen transfer of acetophenone, using 2-propanol as the hydrogen source. A remarkable effect on the enantioselectivity, but also on the activity, was observed depending on the amino-type oxazoline, Ru/8 and Ru/9 being low active and nonselective catalytic systems, in contrast to their primary counterpart Ru/5. Complexes containing amino-oxazolines (10,12) were prepared and fully characterized, both in solution and in solid state. The X-ray structure was determined for (SRu,RC)- 10. The diastereomeric ratios observed for complexes 10 and 11 were determined by 1H NMR and confirmed by means of structural modeling (semi-empirical PM3(tm) level). DFT theoretical calculations for the transition states involved in the hydrogen transfer process proved the important differences in their relative populations, which could justify the enantioselectivity divergences observed between primary and secondary amino-oxazoline ruthenium systems. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Synthesis of Postulated Molecular Probes: Stereoselective Free-Radical-Mediated C-Glycosylation in Tandem with Hydrogen Transfer.CHEMINFORM, Issue 24 2005Yvan Guindon Abstract For Abstract see ChemInform Abstract in Full Text. [source] An Unusual Intramolecular Hydrogen Transfer During the Hydrolysis of a Chloroacetoxy Ester.CHEMINFORM, Issue 7 2005James R. Hanson Abstract For Abstract see ChemInform Abstract in Full Text. [source] Exocellular electron transfer in anaerobic microbial communitiesENVIRONMENTAL MICROBIOLOGY, Issue 3 2006Alfons J. M. Stams Summary Exocellular electron transfer plays an important role in anaerobic microbial communities that degrade organic matter. Interspecies hydrogen transfer between microorganisms is the driving force for complete biodegradation in methanogenic environments. Many organic compounds are degraded by obligatory syntrophic consortia of proton-reducing acetogenic bacteria and hydrogen-consuming methanogenic archaea. Anaerobic microorganisms that use insoluble electron acceptors for growth, such as iron- and manganese-oxide as well as inert graphite electrodes in microbial fuel cells, also transfer electrons exocellularly. Soluble compounds, like humic substances, quinones, phenazines and riboflavin, can function as exocellular electron mediators enhancing this type of anaerobic respiration. However, direct electron transfer by cell,cell contact is important as well. This review addresses the mechanisms of exocellular electron transfer in anaerobic microbial communities. There are fundamental differences but also similarities between electron transfer to another microorganism or to an insoluble electron acceptor. The physical separation of the electron donor and electron acceptor metabolism allows energy conservation in compounds as methane and hydrogen or as electricity. Furthermore, this separation is essential in the donation or acceptance of electrons in some environmental technological processes, e.g. soil remediation, wastewater purification and corrosion. [source] Coexistence of a sulphate-reducing Desulfovibrio species and the dehalorespiring Desulfitobacterium frappieri TCE1 in defined chemostat cultures grown with various combinations of sulphate and tetrachloroetheneENVIRONMENTAL MICROBIOLOGY, Issue 2 2001Oliver Drzyzga A two-member co-culture consisting of the dehalorespiring Desulfitobacterium frappieri TCE1 and the sulphate-reducing Desulfovibrio sp. strain SULF1 was obtained via anaerobic enrichment from soil contaminated with tetrachloroethene (PCE). In this co-culture, PCE dechlorination to cis -dichloroethene was due to the activity of the dehalorespiring bacterium only. Chemostat experiments with lactate as the primary electron donor for both strains along with varying sulphate and PCE concentrations showed that the sulphate-reducing strain outnumbered the dehalogenating strain at relatively high ratios of sulphate/PCE. Stable co-cultures with both organisms present at similar cell densities were observed when both electron acceptors were supplied in the reservoir medium in nearly equimolar amounts. In the presence of low sulphate/PCE ratios, the Desulfitobacterium sp. became the numerically dominant strain within the chemostat co-culture. Surprisingly, in the absence of sulphate, strain SULF1 did not disappear completely from the co-culture despite the fact that there was no electron acceptor provided with the medium to be used by this sulphate reducer. Therefore, we propose a syntrophic association between the sulphate-reducing and the dehalorespiring bacteria via interspecies hydrogen transfer. The sulphate reducer was able to sustain growth in the chemostat co-culture by fermenting lactate and using the dehalogenating bacterium as a ,biological electron acceptor'. This is the first report describing growth of a sulphate-reducing bacterium in a defined two-member continuous culture by syntrophically coupling the electron and hydrogen transfer to a dehalorespiring bacterium. [source] Dehydrogenation of Hydridoirida-,-diketones in Methanol: The Selective Formation of Mono- and Dinuclear Acyl ComplexesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 20 2010Roberto Ciganda Abstract The hydridoirida-,-diketone [IrH{(PPh2(o -C6H4CO))2H}Cl] (1) reacts with diimines (NN) or with pyridine (py) in refluxing methanol to undergo dehydrogenation. The reactions afford selectively the cis -acyl, trans -phosphane isomers of the cationic [Ir(PPh2(o -C6H4CO))2(NN)]+ {NN = 2,2,-bipyridine (2); R,N=C(CH3),C(CH3)=N,R, [R = R, = NH2 (3); R = R, = OH (4); R = OH, R, = NH2 (5)]} or neutral [IrCl(PPh2(o -C6H4CO))2(py)] (6) derivatives. The reactions are faster for ligands containing amino substituents. Refluxing 1 in MeOH affords the formation of an equimolar mixture of dimercationic species [Ir2(,-Cl)(,-PPh2(o -C6H4CO))2(PPh2(o -C6H4CO))2]+ (7a and 7b) containing two acyls and a chloride as bridging groups. The isomers could be separated by fractional precipitation. Compound [3]Cl, containing amino substituents in the imino functionalities, catalyses the hydrogen transfer from 2-propanol to cyclohexanone to afford cyclohexanol. All the complexes were fully characterised spectroscopically. Single crystal X-ray diffraction analysis was performed on complexes 6 and [7b]ClO4. [source] Ruthenium Complexes Containing Chiral N-Donor Ligands as Catalysts in Acetophenone Hydrogen Transfer , New Amino Effect on EnantioselectivityEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 21 2005Montserrat Gómez Abstract New p -cymene ruthenium species containing chiral amino alcohols (1,3), primary (4,7) and secondary (8, 9) amino-oxazolines, were tested as catalysts in the hydrogen transfer of acetophenone, using 2-propanol as the hydrogen source. A remarkable effect on the enantioselectivity, but also on the activity, was observed depending on the amino-type oxazoline, Ru/8 and Ru/9 being low active and nonselective catalytic systems, in contrast to their primary counterpart Ru/5. Complexes containing amino-oxazolines (10,12) were prepared and fully characterized, both in solution and in solid state. The X-ray structure was determined for (SRu,RC)- 10. The diastereomeric ratios observed for complexes 10 and 11 were determined by 1H NMR and confirmed by means of structural modeling (semi-empirical PM3(tm) level). DFT theoretical calculations for the transition states involved in the hydrogen transfer process proved the important differences in their relative populations, which could justify the enantioselectivity divergences observed between primary and secondary amino-oxazoline ruthenium systems. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Alcohols for the ,-Alkylation of Methyl Ketones and Indirect Aza-Wittig Reaction Promoted by Nickel Nanoparticles,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 29 2008Francisco Alonso Abstract Nickel nanoparticles have been found to activate primary alcohols used for the ,-alkylation of ketones or in indirect aza-Wittig reactions. These processes involve hydrogen transfer from the alcohol to the intermediate ,,,-unsaturated ketone or imine, respectively. All these reactions are carried out in the absence of any ligand, hydrogen acceptor or base under mild reaction conditions. For the first time nickel is employed as a potential alternative to noble-metal-based catalysts in both reactions. A reaction mechanism is proposed on the basis of some deuteration experiments. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Kinetic modeling of catalytic conversion of methylcyclohexane over USY zeolites: Adsorption and reaction phenomenaAICHE JOURNAL, Issue 6 2009Mustafa Al-Sabawi Abstract Catalytic conversion of cycloparaffins is a complex process involving competing reaction steps. To understand this process, FCC experiments using methylcyclohexane (MCH) on USY zeolite catalysts were carried out in the mini-fluidized CREC riser simulator. Runs were developed under relevant FCC process conditions in terms of partial pressures of MCH, temperatures (450,550°C), contact times (3,7 s), catalyst-oil mass ratios (5), and using fluidized catalysts. MCH overall conversions ranged between 4 to 16 wt %, with slightly higher conversions obtained using the larger zeolite crystallites. Moreover, it was found that MCH undergoes ring opening, protolytic cracking, isomerization, hydrogen transfer and transalkylation. A heterogeneous kinetic model for MCH conversion including thermal effects, adsorption and intrinsic catalytic reaction phenomena was established. Adsorption and kinetic parameters were determined, including the heat of adsorption (,40 kJ/mol), as well as thermal and primary catalytic intrinsic activation energies, which were in the range of 43,69 kJ/mol, and 50,74 kJ/mol, respectively. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Heavy atom motions and tunneling in hydrogen transfer reactions: the importance of the pre-tunneling stateJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 7 2010Hans-Heinrich Limbach Abstract Arrhenius curves of selected hydrogen transfer reactions in organic molecules and enzymes are reviewed with the focus on systems exhibiting temperature-independent kinetic isotope effects. The latter can be rationalized in terms of a ,pre-tunneling state' which is formed from the reactants by heavy atom motions and which represents a suitable molecular configuration for tunneling to occur. Within the Bell,Limbach tunneling model, formation of the pre-tunneling state dominates the Arrhenius curves of the H and the D transfer even at higher temperatures if a large energy Em is required to reach the pre-tunneling state. Tunneling from higher vibrational levels and the over-barrier reaction via the transition state which lead to temperature-dependent kinetic isotope effects dominate the Arrhenius curves only if Em is small compared to the energy of the transition state. Using published data on several hydrogen transfer systems, the type of motions leading to the pre-tunneling state is explored. Among the phenomena which lead to large energies of the pre-tunneling state are (i) cleavage of hydrogen bonds or coordination bonds of the donor or acceptor atoms to molecules or molecular groups in order to allow the formation of the pre-tunneling state, (ii) the occurrence of an energetic intermediate on the reaction pathway within which tunneling takes place, and (iii) major reorganization of a molecular skeleton, requiring the excitation of specific vibrations in order to reach the pre-tunneling state. This model suggests a solution to the puzzle of Kwart's findings of temperature-independent kinetic isotope effects for hydrogen transfer in small organic molecules. Copyright © 2010 John Wiley & Sons, Ltd. [source] Gas-phase fragmentation of half- and first-generation polyamidoamine dendrimers by electrospray mass spectrometry using a quadrupole ion trapRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 3 2008Thomas J.-C. Polyamidoamine (PAMAM) dendrimers are nanopolymers that can bind with biomolecules such as DNA, drugs or proteins. In order to study these complexes, we first fragmented half- and first-generation PAMAM, G0.5 and G1, respectively, using a quadrupole ion trap (QIT) equipped with an electrospray ionisation source. For both G0.5 and G1 we observed a series of impurities that only can stem from synthesis defects and that are principally due to missing branches and intramolecular cyclisations. Fragmentations of G1 showed regularity in the product ions. These ions result from the loss of 60,Da, obtained by an intramolecular cyclisation, and from the loss of 114,Da, obtained by a four-centred hydrogen transfer or a retro-Michael reaction. The fragmentations stemmed either from competitive or from consecutive reactions, even though resonant fragmentation QIT was used. It is shown that the principal fragmentation reaction is a retro-Michael rearrangement for both G1 and G0.5. In addition, by fragmenting totally deuterated [G1-d28]Na+ we were able to establish fragmentation pathways. Copyright © 2008 John Wiley & Sons, Ltd. [source] Stereospecificity for the hydrogen transfer of pyridoxal enzyme reactionsTHE CHEMICAL RECORD, Issue 5 2001Kenji Soda Abstract We have studied the stereospecificities of various pyridoxal 5,-phosphate dependent enzymes for the hydrogen transfer between the C-4, of a bound coenzyme and the C-2 of a substrate in the transamination catalyzed by the enzymes. Prior to our studies, pyridoxal enzymes so far studied were reported to catalyze the hydrogen transfer only on the si -face of the planar imine intermediate formed from substrate and coenzyme. This finding had been considered as the evidence that pyridoxal enzymes have evolved divergently from a common ancestral protein, because identity in the stereospecificity reflects the similarity in the active-site structure, in particular in the geometrical relationship between the coenzyme and the active site base participating in the hydrogen transfer. However, we found that d -amino acid aminotransferase, branched-chain l -amino acid aminotransferase, and 4-amino-4-deoxychorismate lyase catalyze the re -face specific hydrogen transfer, and that amino acid racemases catalyze the nonstereospecific hydrogen transfer. These findings suggest the convergent evolution of pyridoxal enzymes. Crystallographical studies have shown that the stereospecificity reflects the active-site structure of the enzymes, and that the enzymes with the same fold exhibit the same stereospecificity. The active site structure with the catalytic base being situated on the specific face of the cofactor has been conserved during the evolution among the pyridoxal enzymes of the same family. © 2001 John Wiley & Sons, Inc. and The Japan Chemical Journal Forum Chem Rec 1:373,384, 2001 [source] Gas-phase hydrodechlorination of chlorobenzenes over silica-supported palladium and palladium,ytterbium,APPLIED ORGANOMETALLIC CHEMISTRY, Issue 6-7 2003Satyakrishna Jujjuri Abstract A 5% w/w palladium loading on silica has been achieved via impregnation of the support with Pd(C2H3O2)2 and { (DMF)10Yb2[Pd(CN)4]3} , precursors to deliver monometallic (Pd/SiO2) and bimetallic (Yb,Pd/SiO2) catalyst systems respectively. The catalytic action of each has been assessed in the continuous gas-phase hydrodechlorination (HDC) of chlorobenzene (CB) and 1,2-dichlorobenzene (1,2-DCB) (T = 423 K, inlet chlorine/palladium mol ratio of 5 × 103 h,1) and the hydrogenation of benzene (T = 423 K, inlet C6H6/palladium mol ratio of 35 h,1). Activation of both catalysts delivered similar palladium crystallite size distributions with an average palladium diameter of 5,6 nm where the ytterbium component (in Yb,Pd/SiO2) was present as a thin surface coating. The Pd,Yb bimetallic exhibited significantly higher HDC and hydrogenation activities, the former manifested by significantly greater fractional dechlorinations and benzene selectivities/yields. Yb/SiO2 proved inactive in terms of promoting hydrogen scission or addition and the promotional effect of ytterbium in Yb,Pd/SiO2 is discussed in terms of electron donation and hydrogen transfer via surface YbH2. Under identical reaction conditions, a lower HDC activity was recorded for 1,2-DCB compared with CB, a response that is attributed to steric constraints allied to the deactivating effect of the second chlorine substituent. Both Pd/SiO2 and Yb,Pd/SiO2 exhibited a decline in HDC activity with time-on-stream, but the bimetallic was significantly more resistant to deactivation. Copyright © 2003 John Wiley & Sons, Ltd. [source] Distinct Hydroxy-Radical-Induced Damage of 3,-Uridine Monophosphate in RNA: A Theoretical StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 10 2009bo Zhang Dr. Abstract Cutting ties: Strand scission and base release in hydroxy-radical adducts of 3,-uridine monophosphate (UMP) have been explored by using density functional theory. The presence of the ribose 2,-OH group and the resultant formation of low-barrier hydrogen bonds with oxygen atoms of the 3,-phosphate linkage are highly important for hydrogen transfer and the subsequent bond-breakage reactions (see picture). RNA strand scission and base release in 3,-uridine monophosphate (UMP), induced by OH radical addition to uracil, is studied at the DFT B3LYP/6-31+G(d,p) level in the gas phase and in solution. In particular, the mechanism of hydrogen-atom transfer subsequent to radical formation, from C2, on the sugar to the C6 site on the base, is explored. The barriers of (C2,-)H2,a abstraction by the C6 radical site range from 11.2 to 20.0,kcal,mol,1 in the gas phase and 14.1 to 21.0,kcal,mol,1 in aqueous solution, indicating that the local surrounding governs the hydrogen-abstraction reaction in a stereoselective way. The calculated N1C1, (N1,glycosidic bond) and ,-phosphate bond strengths show that homolytic and heterolytic bond-breaking processes are largely favored in each case, respectively. The barrier for ,-phosphate bond rupture is approximately 3.2,4.0,kcal,mol,1 and is preferred by 8,12,kcal,mol,1 over N1,glycosidic bond cleavage in both the gas phase and solution. The ,-phosphate bond-rupture reactions are exothermal in the gas phase and solution, whereas N1C1, bond-rupture reactions require both solvation and thermal corrections at 298,K to be energetically favored. The presence of the ribose 2,-OH group and its formation of low-barrier hydrogen bonds with oxygen atoms of the 3,-phosphate linkage are highly important for hydrogen transfer and the subsequent bond-breakage reactions. [source] The Role of Fluorine in the Stereoselective Tandem Aza-Michael Addition to Acrylamide Acceptors: An Experimental and Theoretical Mechanistic StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 30 2007Santos Fustero Prof. Abstract Aza-Michael additions of ,-amino esters to fluorinated acceptors take place in a highly stereoselective manner, to give partially modified ,[NHCH2]retropeptides incorporating a hydrolytically stable trifluoroalanine mimic. The reaction mechanism has been investigated experimentally and theoretically, in order to explain the effect of the trifluoromethyl group on the reactivity and the origins of the experimentally observed stereocontrol. The reaction is a two-step process, involving a tandem aza-Michael addition followed by a stereoselective hydrogen transfer. Both steps are base-catalyzed. The high level of stereocontrol is the result of a combination of electrostatic interactions and steric effects. Las adiciones aza-Michael de ,-amino ésteres a aceptores fluorados tienen lugar de un modo altamente estereoselectivo, para dar lugar a ,[NHCH2]retropéptidos parcialmente modificados que incorporan un mimético de trifluoroalanina, estable frente a la hidrólisis. El mecanismo de la reacción ha sido investigado experimental y teóricamente, a fin de explicar el efecto del grupo trifluorometilo en la reactividad y en los orígenes de estereocontrol observado experimentalmente. La reacción es un proceso en dos pasos, que implican una adición aza-Michael seguida de una transferencia estereoselectiva de hidrógeno. Ambos pasos están catalizados por una base. El origen del alto grado de estereocontrol radica en una combinación de interacciones electrostáticas y efectos estéricos. [source] Can [M(H)2(H2)(PXP)] Pincer Complexes (M=Fe, Ru, Os; X=N, O, S) Serve as Catalyst Lead Structures for NH3 Synthesis from N2 and H2?CHEMISTRY - A EUROPEAN JOURNAL, Issue 23 2007Markus Hölscher Dr. Abstract The potential of pincer complexes [M(H)2(H2)(PXP)] (M=Fe, Ru, Os; X=N, O, S) to coordinate, activate, and thus catalyze the reaction of N2 with classical or nonclassical hydrogen centers present at the metal center, with the aim of forming NH3 with H2 as the only other reagent, was explored by means of DF (density functional) calculations. Screening of various complexes for their ability to perform initial hydrogen transfer to coordinated N2 showed ruthenium pincer complexes to be more promising than the corresponding iron and osmium analogues. The ligand backbone influences the reaction dramatically: the presence of pyridine and thioether groups as backbones in the ligand result in inactive catalysts, whereas ether groups such as ,-pyran and furan enable the reaction and result in unprecedented low activation barriers (23.7 and 22.1,kcal,mol,1, respectively), low enough to be interesting for practical application. Catalytic cycles were calculated for [Ru(H)2(H2)(POP)] catalysts (POP=2,5-bis(dimethylphosphanylmethyl)furan and 2,6-bis(dimethylphosphanylmethyl)-,-pyran). The height of activation barriers for the furan system is somewhat more advantageous. Formation of inactive metal nitrides has not been observed. SCRF calculations were used to introduce solvent (toluene) effects. The Gibbs free energies of activation of the numerous single reaction steps do not change significantly when solvent is included. The reaction steps associated with the formation of the active catalyst from precursors [M(H)2(H2)(PXP)] were also calculated. The otherwise inactive pyridine ligand system allows for the generation of the active catalyst species, whereas the ether ligand systems show activation barriers that could prohibit practical application. Consequently the generation of the active catalyst species needs to be addressed in further studies. [source] ,-Carbonyl Substituent Effect on the Lifetimes of Triplet 1,4-Biradicals from Norrish-Type-II ReactionsCHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2006Xichen Cai Dr. Abstract Triplet 1,4-biradicals were generated by Norrish-Type-II hydrogen transfer from ,-heteroatom-substituted ,-branched butyrophenones 1,6 and detected by laser flash absorption measurements. For three oxy-substituted compounds 2,4 (R,=OH, OCOMe, OCOOEt) comparable lifetimes were determined in acetonitrile (roughly 1.5 ,s). In benzene, divergent trends were observed: for the hydroxy compound 2 a lower lifetime of 790 ns was determined, whereas for 3 and 4 the lifetimes increased to 4.9 ,s. Photolyses of the ,-amino-substituted compounds 1 and 6 resulted in transient species with significant lower lifetimes (for 1 160 ns in benzene and 450 ns in acetonitrile; for 6 <100 ns in both solvents). The mesyloxy substrate 5 undergoes rapid CO bond cleavage upon photolysis and no transient triplet species were detected. Computational (UB3,LYP/6,31G* and natural don orbital (NBO) analyses) results supported the assumption of a negative hyperconjugative interaction strongly stabilizing ,-oxy-substituted over ,-amino-substituted radicals. [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] Theoretical studies on transimination of vitamin B6 analogsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2002Antoni Salvà Abstract The transimination reaction of vitamin B6 analogs has been studied by semiempirical quantum mechanics calculations (PM3 Hamiltonian) on a model reactive system consisting of the Schiff base made from a pyridoxal-5'-phosphate (PLP) model compound, methylamine and two auxiliary water molecules. The mechanism of this reaction has been theoretically established through the description of the energy and the geometric parameters of the intermediates and transition-state structures in the gas phase and in water solution by means of the use of two different solvation models. The description of the reaction pathway for transimination through the structures that have resulted from the calculations supports the existence of a geminal diamine as the main intermediate. The reaction also involves the occurrence of internal hydrogen transfers and water-mediated hydrogen transfers as rate-limiting steps. This study highlights the key role of water molecules not only in the formation of selected hydrogen bonds but also as true reactives. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 [source] | ||||||||||||||||||||||