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David Avnir - One of the best experts on this subject based on the ideXlab platform.
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disproportIonatIon of dihydroarenes by sol gel entrapped rhcl3 Quaternary Ammonium Ion pair catalysts
Journal of Catalysis, 1996Co-Authors: Ayelet Rosenfeld, Jochanan Blum, David AvnirAbstract:The Ion pairs generated from RhCl3·3H2O and the Quaternary Ammonium salts [(C8H17)3NMe]Cl (Aliquat 336) and [Me3N(CH2)3Si(OMe)3]Cl were physically and chemically entrapped, respectively, in SiO2sol–gel matrices under mild conditIons. The resulting immobilized Ion pairs proved to be stable, leach-proof, and recyclable catalysts for disproportIonatIon of 1,3-cyclohexadiene and several othervic-dihydroarenes. In these reactIons, equimolar quantities of the respective tetrahydro- and fully aromatic compounds were obtained. The entrapped catalysts, in most cases, proved to be more efficient and more selective than their homogeneous analogues. The reactIon rates and conversIons were shown to depend strongly on steric effects of substituents and on the bulkiness of the substrate skeleton. The recorded first order kinetics in the substrates (organic reactants) suggests that the mechanism involves stepwise additIon of two molecules of the dihydroarenes to the rhodium nucleus, and that the additIon of the first substrate molecule is rate limiting.
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comparison between homogeneous and sol gel encapsulated rhodium Quaternary Ammonium Ion pair catalysts
Journal of Molecular Catalysis A-chemical, 1996Co-Authors: Jochanan Blum, Ayelet Rosenfeld, Nava Polak, Osnat Israelson, Herbert Schumann, David AvnirAbstract:The Ion pairs RhCl3/Aliquat 336® and RhCl3/[Me3N(CH2)3Si(OMe)3]Cl were entrapped in SiO2 sol-gel matrices, and used as catalysts for isomerizatIon of allybenzene, disproportIonatIon of 1,3-cyclohexadiene, hydrogenatIon of alkenes, alkynes, arenes, nitriles and nitro-compounds, and for hydroformylatIon of CC double bonds. The immobilized Ion pairs proved to be stable, leach-proof and recyclable. OccasIonally the catalytic efficiency dropped upon recycling, owing to pore blockage but the activity could be restored by treatment with boiling water. The performances of the sol-gel-entrapped Ion pairs were compared with those of the homogeneous RhCl3/Aliquat 336 catalyst. In most cases the immobilized catalysts proved superior to their homogeneous versIon.
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sol gel encapsulated transitIon metal Quaternary Ammonium Ion pairs as highly efficient recyclable catalysts
Journal of The Chemical Society Chemical Communications, 1993Co-Authors: Ayelet Rosenfeld, David Avnir, Jochanan BlumAbstract:Sol-gel entrapped Rh–, Pt–and Co-Ammonium Ion pairs formed by polymerizatIon of (MeO)4Si in the presence of the appropriate metal halides and Quaternary Ammonium salts, are highly stable, efficient and recyclable hydrogen transfer catalysts.
Jochanan Blum - One of the best experts on this subject based on the ideXlab platform.
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disproportIonatIon of dihydroarenes by sol gel entrapped rhcl3 Quaternary Ammonium Ion pair catalysts
Journal of Catalysis, 1996Co-Authors: Ayelet Rosenfeld, Jochanan Blum, David AvnirAbstract:The Ion pairs generated from RhCl3·3H2O and the Quaternary Ammonium salts [(C8H17)3NMe]Cl (Aliquat 336) and [Me3N(CH2)3Si(OMe)3]Cl were physically and chemically entrapped, respectively, in SiO2sol–gel matrices under mild conditIons. The resulting immobilized Ion pairs proved to be stable, leach-proof, and recyclable catalysts for disproportIonatIon of 1,3-cyclohexadiene and several othervic-dihydroarenes. In these reactIons, equimolar quantities of the respective tetrahydro- and fully aromatic compounds were obtained. The entrapped catalysts, in most cases, proved to be more efficient and more selective than their homogeneous analogues. The reactIon rates and conversIons were shown to depend strongly on steric effects of substituents and on the bulkiness of the substrate skeleton. The recorded first order kinetics in the substrates (organic reactants) suggests that the mechanism involves stepwise additIon of two molecules of the dihydroarenes to the rhodium nucleus, and that the additIon of the first substrate molecule is rate limiting.
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comparison between homogeneous and sol gel encapsulated rhodium Quaternary Ammonium Ion pair catalysts
Journal of Molecular Catalysis A-chemical, 1996Co-Authors: Jochanan Blum, Ayelet Rosenfeld, Nava Polak, Osnat Israelson, Herbert Schumann, David AvnirAbstract:The Ion pairs RhCl3/Aliquat 336® and RhCl3/[Me3N(CH2)3Si(OMe)3]Cl were entrapped in SiO2 sol-gel matrices, and used as catalysts for isomerizatIon of allybenzene, disproportIonatIon of 1,3-cyclohexadiene, hydrogenatIon of alkenes, alkynes, arenes, nitriles and nitro-compounds, and for hydroformylatIon of CC double bonds. The immobilized Ion pairs proved to be stable, leach-proof and recyclable. OccasIonally the catalytic efficiency dropped upon recycling, owing to pore blockage but the activity could be restored by treatment with boiling water. The performances of the sol-gel-entrapped Ion pairs were compared with those of the homogeneous RhCl3/Aliquat 336 catalyst. In most cases the immobilized catalysts proved superior to their homogeneous versIon.
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polystyrene supported rhcl3 Quaternary Ammonium Ion pair as a long lived efficient and recyclable catalyst
Tetrahedron Letters, 1994Co-Authors: Merav Settyfichman, Yoel Sasson, Jochanan Blum, Moris S EisenAbstract:Abstract Polymer-supported Ion pair, generated from RhCl 3 and Dowex R 1 anIon exchanger was found to be a highly stable, efficient and recyclable catalyst for isomerizatIon of allylic compounds, disproportIonatIon of cyclic 1,3-dienes and trimerizatIon of alkynes.
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sol gel encapsulated transitIon metal Quaternary Ammonium Ion pairs as highly efficient recyclable catalysts
Journal of The Chemical Society Chemical Communications, 1993Co-Authors: Ayelet Rosenfeld, David Avnir, Jochanan BlumAbstract:Sol-gel entrapped Rh–, Pt–and Co-Ammonium Ion pairs formed by polymerizatIon of (MeO)4Si in the presence of the appropriate metal halides and Quaternary Ammonium salts, are highly stable, efficient and recyclable hydrogen transfer catalysts.
David T. Yue - One of the best experts on this subject based on the ideXlab platform.
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Ultra-deep blockade of Na+ channels by a Quaternary Ammonium Ion: catalysis by a transitIon-intermediate state?
The Journal of physiology, 1993Co-Authors: K J Gingrich, David Beardsley, David T. YueAbstract:1. Individual Na+ channels from isolated guinea-pig ventricular heart cells were studied using the patch-clamp technique. To localize the selectivity regIon of the channels we investigated their blockade by a permanently charged Quaternary Ammonium Ion (QX-314, 2-(triethylamino)-N-(2,6-dimethylphenyl)acetamide, 0-5 mM) that was applied to the cytoplasmic side of the channel. 2. ResolutIon of individual blocking events was enhanced by covalent removal of fast inactivatIon following brief internal exposure to the enzyme papain. The improved resolutIon reveals the existence of two distinct modalities of blockade: reductIon of unitary current, and millisecond interruptIons of current. 3. Both modes of internal block could be potentiated by lowering external Na+ concentratIon. This finding argues that the two corresponding sites of interactIon are both located within the channel pore. 4. Analysis of the voltage dependence of block placed both binding sites deep within the pore, at 70% of the electric field from the cytoplasmic entrance. Combined with recent studies localizing block by external Cd2+, the present results argue that the selectivity regIon of Na+ channels is quite narrow (spanning about 10% of the electric field), and located near the external side of the channel. 5. The manner in which the two blocking processes interact, along with the physical proximity of their binding sites, leads us to propose that the block configuratIon responsible for the reductIon in unitary current serves as a transitIon intermediate that catalyses formatIon of the discrete-block complex.
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ultra deep blockade of na channels by a Quaternary Ammonium Ion catalysis by a transitIon intermediate state
The Journal of Physiology, 1993Co-Authors: K J Gingrich, David Beardsley, David T. YueAbstract:1. Individual Na+ channels from isolated guinea-pig ventricular heart cells were studied using the patch-clamp technique. To localize the selectivity regIon of the channels we investigated their blockade by a permanently charged Quaternary Ammonium Ion (QX-314, 2-(triethylamino)-N-(2,6-dimethylphenyl)acetamide, 0-5 mM) that was applied to the cytoplasmic side of the channel. 2. ResolutIon of individual blocking events was enhanced by covalent removal of fast inactivatIon following brief internal exposure to the enzyme papain. The improved resolutIon reveals the existence of two distinct modalities of blockade: reductIon of unitary current, and millisecond interruptIons of current. 3. Both modes of internal block could be potentiated by lowering external Na+ concentratIon. This finding argues that the two corresponding sites of interactIon are both located within the channel pore. 4. Analysis of the voltage dependence of block placed both binding sites deep within the pore, at 70% of the electric field from the cytoplasmic entrance. Combined with recent studies localizing block by external Cd2+, the present results argue that the selectivity regIon of Na+ channels is quite narrow (spanning about 10% of the electric field), and located near the external side of the channel. 5. The manner in which the two blocking processes interact, along with the physical proximity of their binding sites, leads us to propose that the block configuratIon responsible for the reductIon in unitary current serves as a transitIon intermediate that catalyses formatIon of the discrete-block complex.
Larry M Wolf - One of the best experts on this subject based on the ideXlab platform.
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a systematic investigatIon of Quaternary Ammonium Ions as asymmetric phase transfer catalysts applicatIon of quantitative structure activity selectivity relatIonships
Journal of Organic Chemistry, 2011Co-Authors: Scott E Denmark, Nathan D Gould, Larry M WolfAbstract:Although the synthetic utility of asymmetric phase-transfer catalysis continues to expand, the number of proven catalyst types and design criteria remains limited. At the origin of this scarcity is a lack in understanding of how catalyst structural features affect the rate and enantioselectivity of phase transfer catalyzed reactIons. Described in this paper is the development of quantitative structure−activity relatIonships (QSAR) and −selectivity relatIonships (QSSR) for the alkylatIon of a protected glycine imine with libraries of Quaternary Ammonium Ion catalysts. Catalyst descriptors including Ammonium Ion accessibility, interfacial adsorptIon affinity, and partitIon coefficient were found to correlate meaningfully with catalyst activity. The physical nature of the descriptors was ratIonalized through differing contributIons of the interfacial and extractIon mechanisms to the reactIon under study. The variatIon in the observed enantioselectivity was ratIonalized employing a comparative molecular field...
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A systematic investigatIon of Quaternary Ammonium Ions as asymmetric phase-transfer catalysts. ApplicatIon of quantitative structure activity/selectivity relatIonships.
The Journal of organic chemistry, 2011Co-Authors: Scott E Denmark, Nathan D Gould, Larry M WolfAbstract:Although the synthetic utility of asymmetric phase-transfer catalysis continues to expand, the number of proven catalyst types and design criteria remains limited. At the origin of this scarcity is a lack in understanding of how catalyst structural features affect the rate and enantioselectivity of phase transfer catalyzed reactIons. Described in this paper is the development of quantitative structure−activity relatIonships (QSAR) and −selectivity relatIonships (QSSR) for the alkylatIon of a protected glycine imine with libraries of Quaternary Ammonium Ion catalysts. Catalyst descriptors including Ammonium Ion accessibility, interfacial adsorptIon affinity, and partitIon coefficient were found to correlate meaningfully with catalyst activity. The physical nature of the descriptors was ratIonalized through differing contributIons of the interfacial and extractIon mechanisms to the reactIon under study. The variatIon in the observed enantioselectivity was ratIonalized employing a comparative molecular field...
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A Systematic InvestigatIon of Quaternary Ammonium Ions as Asymmetric Phase-Transfer Catalysts. Synthesis of Catalyst Libraries and EvaluatIon of Catalyst Activity
The Journal of organic chemistry, 2011Co-Authors: Scott E Denmark, Nathan D Gould, Larry M WolfAbstract:Despite over three decades of research into asymmetric phase-transfer catalysis (APTC), a fundamental understanding of the factors that affect the rate and stereoselectivity of this important process are still obscure. This paper describes the initial stages of a long-term program aimed at elucidating the physical organic foundatIons of APTC employing a chemoinformatic analysis of the alkylatIon of a protected glycine imine with libraries of enantiomerically enriched Quaternary Ammonium Ions. The synthesis of the Quaternary Ammonium Ions follows a diversity-oriented approach wherein the tandem inter[4 + 2]/intra[3 + 2] cycloadditIon of nitroalkenes serves as the key transformatIon. A two-part synthetic strategy comprised of (1) preparatIon of enantioenriched scaffolds and (2) development of parallel synthesis procedures is described. The strategy allows for the facile introductIon of four variable groups in the vicinity of a stereogenic Quaternary Ammonium Ion. The Quaternary Ammonium Ions exhibited a wid...
Ayelet Rosenfeld - One of the best experts on this subject based on the ideXlab platform.
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disproportIonatIon of dihydroarenes by sol gel entrapped rhcl3 Quaternary Ammonium Ion pair catalysts
Journal of Catalysis, 1996Co-Authors: Ayelet Rosenfeld, Jochanan Blum, David AvnirAbstract:The Ion pairs generated from RhCl3·3H2O and the Quaternary Ammonium salts [(C8H17)3NMe]Cl (Aliquat 336) and [Me3N(CH2)3Si(OMe)3]Cl were physically and chemically entrapped, respectively, in SiO2sol–gel matrices under mild conditIons. The resulting immobilized Ion pairs proved to be stable, leach-proof, and recyclable catalysts for disproportIonatIon of 1,3-cyclohexadiene and several othervic-dihydroarenes. In these reactIons, equimolar quantities of the respective tetrahydro- and fully aromatic compounds were obtained. The entrapped catalysts, in most cases, proved to be more efficient and more selective than their homogeneous analogues. The reactIon rates and conversIons were shown to depend strongly on steric effects of substituents and on the bulkiness of the substrate skeleton. The recorded first order kinetics in the substrates (organic reactants) suggests that the mechanism involves stepwise additIon of two molecules of the dihydroarenes to the rhodium nucleus, and that the additIon of the first substrate molecule is rate limiting.
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comparison between homogeneous and sol gel encapsulated rhodium Quaternary Ammonium Ion pair catalysts
Journal of Molecular Catalysis A-chemical, 1996Co-Authors: Jochanan Blum, Ayelet Rosenfeld, Nava Polak, Osnat Israelson, Herbert Schumann, David AvnirAbstract:The Ion pairs RhCl3/Aliquat 336® and RhCl3/[Me3N(CH2)3Si(OMe)3]Cl were entrapped in SiO2 sol-gel matrices, and used as catalysts for isomerizatIon of allybenzene, disproportIonatIon of 1,3-cyclohexadiene, hydrogenatIon of alkenes, alkynes, arenes, nitriles and nitro-compounds, and for hydroformylatIon of CC double bonds. The immobilized Ion pairs proved to be stable, leach-proof and recyclable. OccasIonally the catalytic efficiency dropped upon recycling, owing to pore blockage but the activity could be restored by treatment with boiling water. The performances of the sol-gel-entrapped Ion pairs were compared with those of the homogeneous RhCl3/Aliquat 336 catalyst. In most cases the immobilized catalysts proved superior to their homogeneous versIon.
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sol gel encapsulated transitIon metal Quaternary Ammonium Ion pairs as highly efficient recyclable catalysts
Journal of The Chemical Society Chemical Communications, 1993Co-Authors: Ayelet Rosenfeld, David Avnir, Jochanan BlumAbstract:Sol-gel entrapped Rh–, Pt–and Co-Ammonium Ion pairs formed by polymerizatIon of (MeO)4Si in the presence of the appropriate metal halides and Quaternary Ammonium salts, are highly stable, efficient and recyclable hydrogen transfer catalysts.
