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Vesa Nevalainen - One of the best experts on this subject based on the ideXlab platform.
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Quantum chemical modeling of Chiral Catalysis. Part 19. Strain and stability - oxazadiboretanes potentially involved in the enantioselective reduction of ketones promoted by Chiral oxazaborolidines
Tetrahedron-asymmetry, 1994Co-Authors: Vesa NevalainenAbstract:Abstract The relative stabilities of 2,7-dioxa-5-aza-1,6-diborabicyclo[3.2.0 1,5 ]heptane (e.g. 4′a ) and 2,10-dioxa-8-aza-1,9-diboratricyclo[6.2.0 1,8 .0 4,8 ]decane derivatives (e.g. 4′b ) were studied by means of ab initio MO (RHF) methods. The stabilities were assessed in the light of energies of reactions leading to the opening of the oxazadiboretane ring system of the derivatives. Opening reactions of the tricyclo derivatives giving rise to the regeneration of the catalyst were found to require ≈ 20 kJ mol −1 (MP2/6-31G//6-31G) more energy than those of the corresponding bicyclo derivatives. Cleavages of the tricyclo systems leading to the rupture of both the oxazaborolidine and oxazadiboretane rings were found to require ≈ 30 kJ mol −1 (MP2/6-31G//6-31G) more energy than those of the bicyclo ones. The stability of oxazadiboretane adducts increases with the increasing angle strain related to the partial BN π-bond of the parent oxazaborolidines. The role of electron correlation in the description of the stability of oxazadiboretanes (relative to the parent amino- and alkoxyboranes) was found to be more significant than that of polarization functions.
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Quantum chemical modeling of Chiral Catalysis. Part 17. On the diboranc derivatives of Chiral oxazaborolidines used as catalysts in the enantioselective reduction of ketones
Tetrahedron-asymmetry, 1994Co-Authors: Vesa NevalainenAbstract:Abstract Relative stabilities of isomers of borane (H 3 B) diadducts of oxazaborolidines ( 1 ) were investigated by means of ab initio MO (RHF) methods [1,3,2-oxazaborolidine ( 1 ′) as a model of 1 ]. Three isomers ( 4 ′, 5 ′ and 6 ′, all hydride-bridged) were found to be (51, 24 and 69 kJ mol −1 ; MP2/6-31G*//6–31G* more stable than the borane N,O -diadduct ( 3 ′) of 1 ′. The most stable isomer ( 6 ′) could be described as a system in which two bridges (“-H 2 B-H-BH 2 -” and “-H 2 B-”) connect the O - and N -ends of the 2-aminoethoxy moiety. The other two isomers ( 4 ′ and 5 ′) could be both considered as (amino)diborane derivatives. In the former ( 4 ′) the diborane system is fused to the BN bond of borane O -adduct of 1 ′ (one hydride bridge) whereas in the latter the BH bond of the boron of the oxazaborolidine ring of borane N -adduct of 1 ′ is involved in the diborane structure (two hydride bridges). A new plausible regeneration pathway for the CBS reduction was proposed.
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Quantum chemical modeling of Chiral Catalysis. Part 13. On the role of borane O-adducts in the enantioselective reduction of ketones catalyzed by Chiral oxazaborolidines
Tetrahedron-asymmetry, 1993Co-Authors: Vesa NevalainenAbstract:Abstract Plausible reactions of Lewis bases (ketones and ethers) with borane O-adducts of Chiral oxazaborolidines used as catalysts in the enantioselective reduction of ketones were investigated by means of ab initio MO methods. Properties of the O-adducts were found to be different from those of the corresponding N-adducts. The O-adducts were not able to form complexes with ketones and ethers similar to those of the corresponding N-adducts proposed to be essential for the performance of oxazaborolidines as Chiral catalysts in the enantioselective reduction of ketones.
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Quantum chemical modeling of Chiral Catalysis. Part 11. Isomerism in borane adducts of Chiral oxazaborolidines used as catalysts in the enantioselective reduction of ketones
Tetrahedron-asymmetry, 1993Co-Authors: Vesa NevalainenAbstract:Abstract The relative stability of isomers of borane adducts of 1,3,2-oxazaborolidines was investigated by means of ab initio molecular orbital methods. One open-chain isomer was found to be more stable than the borane N-adduct proposed to be one of the key-intermediates of the catalytic enantioselective reduction of ketones. Borane N-adduct of 1,3,2-oxazaborolidine was found to be 19 kJ mol−1 (6-31G*//6-31G*) more stable than the corresponding O-adduct. Geometry of the ring oxygen of the O-adduct appeared out to be practically planar.
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Quantum chemical modeling of Chiral Catalysis. Part 9. On the aggregation reactions of borane and Chiral oxazaborolidines used as catalysts in the enantioselective reduction of ketones
Tetrahedron-asymmetry, 1993Co-Authors: Vesa NevalainenAbstract:Abstract Aggregation reactions of borane and oxazaborolidine type of Chiral catalysts were investigated by means of ab initio molecular orbital methods. Aggregates consisting of borane and two catalyst molecules were studied by using 1,3,2-oxazaborolidine as a model of the catalyst and those of two boranes and two catalyst molecules by using H2NB(H)OH. The behaviour of oxazaborolidines, as they react with intermediates of the CBS reduction, was found to resemble that of ketones or Lewis basic solvents (the polar B'N bond of oxazaborolidine behaves as the CO bond of ketone; nitrogen of oxazaborolidine behaves as a Lewis base).
Francisco Zaera - One of the best experts on this subject based on the ideXlab platform.
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The Physico-chemical Properties of Cinchona Alkaloids Responsible for their Unique Performance in Chiral Catalysis
Topics in Catalysis, 2008Co-Authors: Larry Mink, David S. Sholl, Zhen Ma, Ryan A. Olsen, Joanna N. James, Leonard J. Mueller, Francisco ZaeraAbstract:The physico-chemical properties of cinchona alkaloids have been characterized in connection to their use for catalytic enantioselective conversions. Adding to the previous identification of their active site at the nitrogen atom in the quinuclidine ring and the Chiral environment provided by the carbon centers of the neighboring alcohol linker, an argument is made here for the importance of the adoption of certain rotational conformations by those cinchona alkaloids in optimizing their Chiral promotion. Because Catalysis with cinchona alkaloids involves a liquid phase, there is a dynamic conformation isomerization process controlled by a number of factors having to do with the exact structure of the cinchona as well as with the nature of the solvent used and, in the case of heterogeneous Catalysis, the presence of a solid surface. Solvents of intermediate polarity have been found to be the best for dissolving the cinchona, for establishing rapid adsorption equilibria with metal surfaces, and for promoting Chiral Catalysis. Protonation also leads to a dramatic change in performance, locking the cinchona molecule in a specific conformation held in place by the counter anion of the acid used, and modifying the chemical and biological activity of the system. Comparative studies with several cinchona indicate that molecular groups attached to peripheral positions also exert a great influence on the conformational and adsorption behavior of these molecules.
Larry Mink - One of the best experts on this subject based on the ideXlab platform.
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The Physico-chemical Properties of Cinchona Alkaloids Responsible for their Unique Performance in Chiral Catalysis
Topics in Catalysis, 2008Co-Authors: Larry Mink, David S. Sholl, Zhen Ma, Ryan A. Olsen, Joanna N. James, Leonard J. Mueller, Francisco ZaeraAbstract:The physico-chemical properties of cinchona alkaloids have been characterized in connection to their use for catalytic enantioselective conversions. Adding to the previous identification of their active site at the nitrogen atom in the quinuclidine ring and the Chiral environment provided by the carbon centers of the neighboring alcohol linker, an argument is made here for the importance of the adoption of certain rotational conformations by those cinchona alkaloids in optimizing their Chiral promotion. Because Catalysis with cinchona alkaloids involves a liquid phase, there is a dynamic conformation isomerization process controlled by a number of factors having to do with the exact structure of the cinchona as well as with the nature of the solvent used and, in the case of heterogeneous Catalysis, the presence of a solid surface. Solvents of intermediate polarity have been found to be the best for dissolving the cinchona, for establishing rapid adsorption equilibria with metal surfaces, and for promoting Chiral Catalysis. Protonation also leads to a dramatic change in performance, locking the cinchona molecule in a specific conformation held in place by the counter anion of the acid used, and modifying the chemical and biological activity of the system. Comparative studies with several cinchona indicate that molecular groups attached to peripheral positions also exert a great influence on the conformational and adsorption behavior of these molecules.
Yitzhak Mastai - One of the best experts on this subject based on the ideXlab platform.
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Amino acid-based ionic liquids as precursors for the synthesis of Chiral nanoporous carbons
Nanoscale Advances, 2019Co-Authors: Sapir Shekef Aloni, Milena Perovic, Michal Weitman, Reut Cohen, Martin Oschatz, Yitzhak MastaiAbstract:The synthesis of Chiral nanoporous carbons based on Chiral ionic liquids (CILs) of amino acids as precursors is described. Such unique precursors for the carbonization of CILs yield Chiral carbonaceous materials with high surface area (≈620 m2 g−1). The enantioselectivities of the porous carbons are examined by advanced techniques such as selective adsorption of enantiomers using cyclic voltammetry, isothermal titration calorimetry, and mass spectrometry. These techniques demonstrate the Chiral nature and high enantioselectivity of the Chiral carbon materials. Overall, we believe that the novel approach presented here can contribute significantly to the development of new Chiral carbon materials that will find important applications in Chiral chemistry, such as in Chiral Catalysis and separation and in Chiral sensors. From a scientific point of view, the approach and results reported here can significantly deepen our understanding of Chirality at the nanoscale and of the structure and nature of Chiral nonporous materials and surfaces.
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Atomic layer deposition of enantioselective thin film of alumina on Chiral self-assembled-monolayer
Surface Science, 2014Co-Authors: Hagay Moshe, Gila Levi, Daniel Sharon, Yitzhak MastaiAbstract:Abstract In this paper, we describe the synthesis of new Chiral nanosized surfaces based on Chiral self-assembled monolayers coated with metal oxide (Al 2 O 3 ) nanolayers. In this new type of nanosized Chiral surface, the Al 2 O 3 nanolayers enable the protection of the Chiral self-assembled monolayers while preserving their enantioselective nature. The Chiral nature of the SAMs/Al 2 O 3 films was characterized by a variety of techniques, such as, quartz crystal microbalance, circular dichroism (CD) spectroscopy and Chiral crystallization. The proposed methodology for the preparation of nanoscale Chiral surfaces described in this article could open up opportunities in other fields of chemistry, such as Chiral Catalysis.
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Chiral Thin Films of Metal Oxide
Chemistry: A European Journal, 2013Co-Authors: Hagay Moshe, Maarten Vanbel, Ventsislav K. Valev, Thierry Verbiest, David Dressler, Yitzhak MastaiAbstract:In this paper, we describe for the first time the synthesis of new Chiral nanosized metal oxide surfaces based on Chiral self-assembled monolayers (SAMs) coated with metal oxide (TiO2) nanolayers. In this new type of nanosize Chiral surface, the metal oxide nanolayers enable the protection of the Chiral self-assembled monolayers while preserving their enantioselective nature. The Chiral nature of the SAM/TiO2 films was characterized by variety of unique techniques, such as second-harmonic generation circular dichroism (SHG-CD), quartz crystal microbalance, and Chiral adsorption measurements with circular dichroism spectroscopy. The Chiral resolution abilities of the SAMs coated with metal oxide (TiO2) nanolayers were investigated in the crystallization of a racemic mixture of threonine and glutamic acid. Our proposed methodology for the preparation of nanoscale Chiral surfaces described in this article could open up opportunities in other fields of chemistry, such as Chiral Catalysis. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
David S. Sholl - One of the best experts on this subject based on the ideXlab platform.
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The Physico-chemical Properties of Cinchona Alkaloids Responsible for their Unique Performance in Chiral Catalysis
Topics in Catalysis, 2008Co-Authors: Larry Mink, David S. Sholl, Zhen Ma, Ryan A. Olsen, Joanna N. James, Leonard J. Mueller, Francisco ZaeraAbstract:The physico-chemical properties of cinchona alkaloids have been characterized in connection to their use for catalytic enantioselective conversions. Adding to the previous identification of their active site at the nitrogen atom in the quinuclidine ring and the Chiral environment provided by the carbon centers of the neighboring alcohol linker, an argument is made here for the importance of the adoption of certain rotational conformations by those cinchona alkaloids in optimizing their Chiral promotion. Because Catalysis with cinchona alkaloids involves a liquid phase, there is a dynamic conformation isomerization process controlled by a number of factors having to do with the exact structure of the cinchona as well as with the nature of the solvent used and, in the case of heterogeneous Catalysis, the presence of a solid surface. Solvents of intermediate polarity have been found to be the best for dissolving the cinchona, for establishing rapid adsorption equilibria with metal surfaces, and for promoting Chiral Catalysis. Protonation also leads to a dramatic change in performance, locking the cinchona molecule in a specific conformation held in place by the counter anion of the acid used, and modifying the chemical and biological activity of the system. Comparative studies with several cinchona indicate that molecular groups attached to peripheral positions also exert a great influence on the conformational and adsorption behavior of these molecules.
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Final Technical Report for DOE Grant DE-FG02-03ER15473 “Molecular Level Design of Heterogeneous Chiral Catalysis”
2007Co-Authors: David S. Sholl, Andrew J. GellmanAbstract:The production of enantiomerically pure Chiral compounds is of great importance in the pharmaceutical industry. Although processes involving Chiral Catalysis and separations involving solid surfaces are known, the molecular-scale details of these processes are not well understood. This lack of understanding strongly limits the development of new Chiral processes. Our collaborative research effort examines several intertwined aspects of Chirality and enantioselectivity at catalytically active metal surfaces. At Carnegie Mellon, our efforts focus on the development of Chirally imprinted metal powders as materials for Chiral columns and the experimental and theoretical study of small Chiral molecules adsorbed on well-characterized metal surfaces, both aChiral and Chiral. These efforts are being performed in close collaboration with our team members at the University of California Riverside and the University of Wisconsin Milwaukee.