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Hydrogen-transfer Reactions (hydrogen-transfer + reaction)

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Chemistry100%

Selected Abstracts

ChemInform Abstract: Auto-Tandem Catalysis in the Synthesis of Substituted Quinolines from Aldimines and Electron-Rich Olefins: Cascade Povarov,Hydrogen-Transfer Reaction.

CHEMINFORM, Issue 8 2009
Naoya Shindoh
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]

ChemInform Abstract: Acceleration of Ruthenium(II)- and Rhodium(I)-Catalyzed Hydrogen-Transfer Reaction by Rare Earth Metal Triflates.

CHEMINFORM, Issue 10 2002
Hirofumi Matsunaga
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 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]

Highly Dispersed Ruthenium Hydroxide Supported on Titanium Oxide Effective for Liquid-Phase Hydrogen-Transfer Reactions

CHEMISTRY - A EUROPEAN JOURNAL, Issue 36 2008
Kazuya Yamaguchi Dr.
Abstract Supported ruthenium hydroxide catalysts (Ru(OH)x/support) were prepared with three different TiO2 supports (anatase TiO2 (TiO2(A), BET surface area: 316,m2,g,1), anatase TiO2 (TiO2(B), 73,m2,g,1), and rutile TiO2 (TiO2(C), 3.2,m2,g,1)), as well as an Al2O3 support (160,m2,g,1). Characterizations with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), and X-ray absorption fine structure (XAFS) showed the presence of monomeric ruthenium(III) hydroxide and polymeric ruthenium(III) hydroxide species. Judging from the coordination numbers of the nearest-neighbor Ru atoms and the intensities of the ESR signals, the amount of monomeric hydroxide species increased in the order of Ru(OH)xhydrogen-transfer reactions, such as racemization of chiral secondary alcohols and the reduction of carbonyl compounds and allylic alcohols. The catalytic activities of Ru(OH)x/TiO2(A) for these hydrogen-transfer reactions were at least one order of magnitude higher than those of previously reported heterogeneous catalysts, such as Ru(OH)x/Al2O3. These catalyses were truly heterogeneous, and the catalysts recovered after the reactions could be reused several times without loss of catalytic performance. The reaction rates monotonically increased with an increase in the amount of monomeric ruthenium hydroxide species, which suggests that the monomeric species are effective for these hydrogen-transfer reactions. [source]

Synthetic Scope and Mechanistic Studies of Ru(OH)x/Al2O3 -Catalyzed Heterogeneous Hydrogen-Transfer Reactions

CHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2005
Kazuya 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]

Highly Enantioselective Ruthenium-Catalyzed Reduction of Ketones Employing Readily Available Peptide Ligands

CHEMISTRY - A EUROPEAN JOURNAL, Issue 1 2004
Anders Bøgevig Dr.
Abstract Highly efficient and selective catalysts for the asymmetric reduction of aryl alkyl ketones under hydrogen-transfer conditions (2-propanol) were obtained by combining a novel class of pseudo-dipeptide ligands with [{RuCl2(p- cymene)}2]. A library of 36 dipeptide-like ligands was prepared from N -Boc-protected ,-amino acids and the enantiomers of 2-amino-1-phenylethanol and 1-amino-2-propanol. The catalyst library was evaluated with the reduction of acetophenone and excellent enantioselectivity of 1-phenylethanol was obtained with several of the novel catalysts. A ligand based on the combination of N -Boc- L -alanine and (S)-1-amino-2-propanol (ligand A - (S)- 4) was found to be particular effective. When the situ formed ruthenium complex of this ligand was employed as the catalyst in the hydrogen-transfer reaction of various aryl alkyl ketones, the corresponding alcohol products were achieved in excellent enantioselectivity (up to 98,% ee). [source]

Graphite-Supported Gold Nanoparticles as Efficient Catalyst for Aerobic Oxidation of Benzylic Amines to Imines and N -Substituted 1,2,3,4-Tetrahydroisoquinolines to Amides: Synthetic Applications and Mechanistic Study

CHEMISTRY - AN ASIAN JOURNAL, Issue 10 2009
Man-Ho So
Abstract Selective oxidation of amines using oxygen as terminal oxidant is an important area in green chemistry. In this work, we describe the use of graphite-supported gold nanoparticles (AuNPs/C) to catalyze aerobic oxidation of cyclic and acyclic benzylic amines to the corresponding imines with moderate-to-excellent substrate conversions (43,100,%) and product yields (66,99,%) (19,examples). Oxidation of N -substituted 1,2,3,4-tetrahydroisoquinolines in the presence of aqueous NaHCO3 solution gave the corresponding amides in good yields (83,93,%) with high selectivity (up to amide/enamide=93:4) (6,examples). The same protocol can be applied to the synthesis of benzimidazoles from the reaction of o -phenylenediamines with benzaldehydes under aerobic conditions (8,examples). By simple centrifugation, AuNPs/C can be recovered and reused for ten consecutive runs for the oxidation of dibenzylamine to N -benzylidene(phenyl)methanamine without significant loss of catalytic activity and selectivity. This protocol "AuNPs/C+O2" can be scaled to the gram scale, and 8.9,g (84,% isolated yield) of 3,4-dihydroisoquinoline can be obtained from the oxidation of 10,g 1,2,3,4-tetrahydroisoquinoline in a one-pot reaction. Based on the results of kinetic studies, radical traps experiment, and Hammett plot, a mechanism involving the hydrogen-transfer reaction from amine to metal and oxidation of M-H is proposed. [source]

Highly Dispersed Ruthenium Hydroxide Supported on Titanium Oxide Effective for Liquid-Phase Hydrogen-Transfer Reactions

CHEMISTRY - A EUROPEAN JOURNAL, Issue 36 2008
Kazuya Yamaguchi Dr.
Abstract Supported ruthenium hydroxide catalysts (Ru(OH)x/support) were prepared with three different TiO2 supports (anatase TiO2 (TiO2(A), BET surface area: 316,m2,g,1), anatase TiO2 (TiO2(B), 73,m2,g,1), and rutile TiO2 (TiO2(C), 3.2,m2,g,1)), as well as an Al2O3 support (160,m2,g,1). Characterizations with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), and X-ray absorption fine structure (XAFS) showed the presence of monomeric ruthenium(III) hydroxide and polymeric ruthenium(III) hydroxide species. Judging from the coordination numbers of the nearest-neighbor Ru atoms and the intensities of the ESR signals, the amount of monomeric hydroxide species increased in the order of Ru(OH)xhydrogen-transfer reactions, such as racemization of chiral secondary alcohols and the reduction of carbonyl compounds and allylic alcohols. The catalytic activities of Ru(OH)x/TiO2(A) for these hydrogen-transfer reactions were at least one order of magnitude higher than those of previously reported heterogeneous catalysts, such as Ru(OH)x/Al2O3. These catalyses were truly heterogeneous, and the catalysts recovered after the reactions could be reused several times without loss of catalytic performance. The reaction rates monotonically increased with an increase in the amount of monomeric ruthenium hydroxide species, which suggests that the monomeric species are effective for these hydrogen-transfer reactions. [source]

Synthetic Scope and Mechanistic Studies of Ru(OH)x/Al2O3 -Catalyzed Heterogeneous Hydrogen-Transfer Reactions

CHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2005
Kazuya 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]

1,4-Butanediol as a Reducing Agent in Transfer Hydrogenation Reactions

CHEMISTRY - AN ASIAN JOURNAL, Issue 3 2010
Hannah
Abstract 1,4-Butanediol is able to deliver two equivalents of H2 in hydrogen-transfer reactions to ketones, imines, and alkenes. Unlike simple alcohols, which establish equilibrium in the reduction of ketones, 1,4-butanediol acts essentially irreversibly owing to the formation of butyrolactone, which acts as a thermodynamic sink. It is therefore not necessary to use 1,4-butanediol in great excess in order to achieve reduction reactions. In addition, allylic alcohols are reduced to saturated alcohols through an isomerization/reduction sequence using a ruthenium catalyst with 1,4-butanediol as the reducing agent. Imines and alkenes are also reduced under similar conditions. [source]

Ab Initio Group Contribution Method for Activation Energies of Hydrogen Abstraction Reactions

CHEMPHYSCHEM, Issue 1 2006
Mark Saeys Prof.
Abstract The group contribution method for activation energies is applied to hydrogen abstraction reactions. To this end an ab initio database was constructed, which consisted of activation energies calculated with the ab initio CBS-QB3 method for a limited set of well-chosen homologous reactions. CBS-QB3 is shown to predict reaction rate coefficients within a factor of 2,4 and Arrhenius activation energies within 3,5 kJ,mol,1of experimental data. Activation energies in the set of homologous reactions vary over 156 kJ,mol,1with the structure of the abstracting radical and over 94 kJ,mol,1with the structure of the abstracted hydrocarbon. The parameters required for the group contribution method, the so-called standard activation group additivity values, were determined from this database. To test the accuracy of the group contribution method, a large set of 88 additional activation energies were calculated from first principles and compared with the predictions from the group contribution method. It was found that the group contribution method yields accurate activation energies for hydrogen-transfer reactions between hydrogen molecules, alkylic hydrocarbons, and vinylic hydrocarbons, with the largest deviations being less than 6 kJ,mol,1. For reactions between allylic and propargylic hydrocarbons, the transition state is believed to be stabilized by resonance effects, thus requiring the introduction of an appropriate correction term to obtain a reliable prediction of the activation energy for this subclass of hydrogen abstraction reactions. [source]