The Experts below are selected from a list of 15171 Experts worldwide ranked by ideXlab platform
Dean F Toste - One of the best experts on this subject based on the ideXlab platform.
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a supramolecular microenvironment strategy for Transition Metal Catalysis
Science, 2015Co-Authors: David M Kaphan, Mark D Levin, Robert G Bergman, Kenneth N Raymond, Dean F TosteAbstract:A self-assembled supramolecular complex is reported to catalyze alkyl-alkyl reductive elimination from high-valent Transition Metal complexes [such as gold(III) and platinum(IV)], the central bond-forming elementary step in many catalytic processes. The catalytic microenvironment of the supramolecular assembly acts as a functional enzyme mimic, applying the concepts of enzymatic Catalysis to a reactivity manifold not represented in biology. Kinetic experiments delineate a Michaelis-Menten-type mechanism, with measured rate accelerations (k(cat)/k(uncat)) up to 1.9 × 10(7) (here k(cat) and k(uncat) are the Michaelis-Menten enzymatic rate constant and observed uncatalyzed rate constant, respectively). This modality has further been incorporated into a dual catalytic cross-coupling reaction, which requires both the supramolecular microenvironment catalyst and the Transition Metal catalyst operating in concert to achieve efficient turnover.
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enantioselective 1 1 arylborylation of alkenes merging chiral anion phase transfer with pd Catalysis
Journal of the American Chemical Society, 2015Co-Authors: Hosea M Nelson, Brett D Williams, Javier Miro, Dean F TosteAbstract:A palladium-catalyzed three-component coupling of α-olefins, aryldiazonium salts, and bis(pinacolato)diboron affords direct access to chiral benzylic boronic esters. This process is rendered highly enantioselective using an unprecedented example of cooperative chiral anion phase transfer and Transition-Metal Catalysis.
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stable gold iii catalysts by oxidative addition of a carbon carbon bond
Nature, 2015Co-Authors: Takahiro Horibe, Christian Borch Jacobsen, Dean F TosteAbstract:Low-valent late Transition-Metal Catalysis has become indispensable to chemical synthesis, but homogeneous high-valent Transition-Metal Catalysis is underdeveloped, mainly owing to the reactivity of high-valent Transition-Metal complexes and the challenges associated with synthesizing them. Here we report a carbon-carbon bond cleavage at ambient conditions by a Au(i) complex that generates a stable Au(iii) cationic complex. In contrast to the well-established soft and carbophilic Au(i) catalyst, this Au(iii) complex exhibits hard, oxophilic Lewis acidity. For example, we observed catalytic activation of α,β-unsaturated aldehydes towards selective conjugate additions as well as activation of an unsaturated aldehyde-allene for a [2 + 2] cycloaddition reaction. The origin of the regioselectivity and catalytic activity was elucidated by X-ray crystallographic analysis of an isolated Au(iii)-activated cinnamaldehyde intermediate. The concepts revealed suggest a strategy for accessing high-valent Transition-Metal Catalysis from readily available precursors.
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a supramolecular approach to combining enzymatic and Transition Metal Catalysis
Nature Chemistry, 2013Co-Authors: Jane Z Wang, Robert G Bergman, Kenneth N Raymond, Kristen N Clary, Dean F TosteAbstract:Combinations of enzymatic and chemo-Catalysis can result in powerful synthetic transformations. Here, encapsulation of Au(I) or Ru(II) within a supramolecular assembly prevents diffusion of the organoMetallic complexes into solution where they can compromise the activity of an enzyme. This strategy has been applied to tandem reactions employing supramolecular host–guest complexes and enzymes in the Catalysis of organic transformations.
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a powerful chiral counterion strategy for asymmetric Transition Metal Catalysis
Science, 2007Co-Authors: Gregory L Hamilton, Eunjoo Kang, Dean F TosteAbstract:Traditionally, Transition Metal–catalyzed enantioselective transformations rely on chiral ligands tightly bound to the Metal to induce asymmetric product distributions. Here we report high enantioselectivities conferred by a chiral counterion in a Metal-catalyzed reaction. Two different transformations catalyzed by cationic gold(I) complexes generated products in 90 to 99% enantiomeric excess with the use of chiral binaphthol–derived phosphate anions. Furthermore, we show that the chiral counterion can be combined additively with chiral ligands to enable an asymmetric transformation that cannot be achieved by either method alone. This concept of relaying chiral information via an ion pair should be applicable to a vast number of Metal-mediated processes.
Frank Glorius - One of the best experts on this subject based on the ideXlab platform.
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privileged chiral n heterocyclic carbene ligands for asymmetric Transition Metal Catalysis
Chemical Society Reviews, 2017Co-Authors: Daniel Janssenmuller, Christoph Schlepphorst, Frank GloriusAbstract:Chiral ligands play a central role in enantioselective Transition-Metal Catalysis. The success of achiral N-heterocyclic carbenes (NHCs) as stable electron-rich neutral ligands in homogeneous Catalysis led to the development of a manifold of chiral NHCs as stereodirecting ancillary ligands for various enantioselective transformations. Due to the modular design of NHCs and the ease of access to their azolium salt precursors, tailor-made NHCs are readily available. Many chiral NHC scaffolds have been synthesised and tested in Catalysis. Herein, we highlight only those NHC structures which have enabled high degrees of enantioselectivity in Transition-Metal Catalysis. Following a brief introduction to the field of chiral NHCs, this tutorial review introduces different categories of chiral NHCs and provides a guide to the structural fine-tuning of ligand requirements and stereochemical models.
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n heterocyclic carbenes in Transition Metal Catalysis
2007Co-Authors: Frank GloriusAbstract:N-Heterocyclic Carbenes in Catalysis-An Introduction.- N-Heterocyclic Carbenes in Catalysis-An Introduction.- N-Heterocyclic Carbenes as Ligands for High-Oxidation-State Metal Complexes and Oxidation Catalysis.- N-Heterocyclic Carbenes as Ligands for High-Oxidation-State Metal Complexes and Oxidation Catalysis.- Palladium-catalyzed Reactions Using NHC Ligands.- Palladium-catalyzed Reactions Using NHC Ligands.- Routes to N-Heterocyclic Carbene Complexes.- Routes to N-Heterocyclic Carbene Complexes.- Chiral N-Heterocyclic Carbenes as Stereodirecting Ligands in Asymmetric Catalysis.- Chiral N-Heterocyclic Carbenes as Stereodirecting Ligands in Asymmetric Catalysis.- Transition Metal-Catalyzed Reactions Using N-Heterocyclic Carbene Ligands (Besides Pd- and Ru-Catalyzed Reactions).- Transition Metal-Catalyzed Reactions Using N-Heterocyclic Carbene Ligands (Besides Pd- and Ru-Catalyzed Reactions).- N-Heterocyclic Carbenes as Ligands for Olefin Metathesis Catalysts.- N-Heterocyclic Carbenes as Ligands for Olefin Metathesis Catalysts.
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sterically demanding bioxazoline derived n heterocyclic carbene ligands with restricted flexibility for Catalysis
Journal of the American Chemical Society, 2004Co-Authors: Gereon Altenhoff, Richard Goddard, Christian W Lehmann, Frank GloriusAbstract:A unique family of N-heterocyclic carbenes derived from bioxazolines (IBiox) suitable for application in Transition-Metal Catalysis is described. The ligands are electron rich, sterically demanding, and have restricted flexibility. Their usefulness has been demonstrated in the Suzuki-Miyaura cross-coupling of sterically hindered aryl chlorides and boronic acids. For the first time, tetraortho-substituted biaryls with methyl and larger ortho-substituents have been synthesized from aryl chlorides using the Suzuki-Miyaura method.
Joost N H Reek - One of the best experts on this subject based on the ideXlab platform.
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Transition Metal Catalysis in confined spaces
ChemInform, 2015Co-Authors: Stefan H A M Leenders, Rafael Gramagedoria, Bas De Bruin, Joost N H ReekAbstract:Transition Metal Catalysis plays an important role in both industry and in academia where selectivity, activity and stability are crucial parameters to control. Next to changing the structure of the ligand, introducing a confined space as a second coordination sphere around a Metal catalyst has recently been shown to be a viable method to induce new selectivity and activity in Transition Metal Catalysis. In this review we focus on supramolecular strategies to encapsulate Transition Metal complexes with the aim of controlling the selectivity via the second coordination sphere. As we will discuss, catalyst confinement can result in selective processes that are impossible or difficult to achieve by traditional methods. We will describe the template-ligand approach as well as the host–guest approach to arrive at such supramolecular systems and discuss how the performance of the catalyst is enhanced by confining it in a molecular container.
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supramolecular control of selectivity in Transition Metal Catalysis through substrate preorganization
Chemical Science, 2014Co-Authors: Pawel Dydio, Joost N H ReekAbstract:Supramolecular chemistry exploits multiple weak intermolecular interactions to assemble nano-sized molecular architectures, providing new possibilities for (Transition Metal) catalyst development. In this Perspective we focus on the application of such weak (directional) interactions between a substrate molecule and a (bifunctional) catalyst for structural preorganization prior to the catalytic reaction. As we discuss, such effects together with the confinement properties of the nano-space of the ‘active sites’ play a crucial role for the exceptional selectivities and activities of natural enzymes. We will elaborate on the application of such supramolecular strategy to the more traditional Transition-Metal Catalysis, and we will compare it with the traditional substrate preorganization methods. Subsequently, literature examples of such bifunctional catalyst systems will be described in which the function of weak interactions was carefully designed a priori, as well as, the serendipitously found catalysts in which the presence of supramolecular effects was recognized post factum. The discussed examples demonstrate the power of the strategy for the control of selectivity in various types of Metal catalyzed reactions, and the observation of the serendipitous findings can help to generate new leads for more efficient catalyst design.
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supramolecular self assembled ligands in asymmetric Transition Metal Catalysis
Israel Journal of Chemistry, 2012Co-Authors: Rosalba Bellini, Jarl Ivar Van Der Vlugt, Joost N H ReekAbstract:The design of novel chiral ligands is at the core of asymmetric Catalysis. The catalytic characteristics of a Transition Metal catalyst such as activity, selectivity and stability can be fine-tuned by optimization of the steric and electronic properties of the coordinating ligands. In asymmetric transformations, catalyst optimization still relies to a large extent on trial-and-error and educated guesses. New strategies based on combinatorial screening and high-throughput experimentation have been introduced for the design and optimization of new ligands and catalytic systems. Supramolecular bidentate ligands that form by self-assembly of building blocks are particularly suited for this combinatorial approach as the potential number of catalysts grows exponentially with the number of building blocks synthesized. Catalytic systems based on supramolecular interactions have proven to be highly advantageous in creating large ligand libraries for high-throughput screening, which allows optimization of activity and selectivity for a variety of reactions. In this review we describe the progress in this field.
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new directions in supramolecular Transition Metal Catalysis
Organic and Biomolecular Chemistry, 2005Co-Authors: Matthew J Wilkinson, Piet W N M Van Leeuwen, Joost N H ReekAbstract:Supramolecular chemistry has grown into a major scientific field over the last thirty years and has fueled numerous developments at the interfaces with biology and physics, clearly demonstrating its potential at a multidisciplinary level. Simultaneously, organoMetallic chemistry and Transition Metal Catalysis have matured in an incredible manner, broadening the pallet of tools available for chemical conversions. The interface between supramolecular chemistry and Transition Metal Catalysis has received surprisingly little attention. It provides, however, novel and elegant strategies that could lead to new tools in the search for effective catalysts, as well as the possiblity of novel conversions induced by Metal centres that are in unusual environments. This perspective describes new approaches to Transition Metal catalyst development that evolve from a combination of supramolecular strategies and rational ligand design, which may offer Transition Metal catalysts for future applications.
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Transition Metal Catalysis Using Functionalized Dendrimers.
Angewandte Chemie, 2001Co-Authors: G. Eric Oosterom, Joost N H Reek, Paul C. J. Kamer, Piet W. N. M. Van LeeuwenAbstract:: Dendrimers are well-defined hyperbranched macromolecules with characteristic globular structures for the larger systems. These novel polymers have inspired many chemists to develop new materials and several applications have been explored, Catalysis being one of them. The recent impressive strides in synthetic procedures increased the accessibility of functionalized dendrimers, resulting in a rapid development of dendrimer chemistry. The position of the catalytic site(s) as well as the spatial separation of the catalysts appears to be of crucial importance. Dendrimers that are functionalized with Transition Metals in the core potentially can mimic the properties of enzymes, their efficient natural counterparts, whereas the surface-functionalized systems have been proposed to fill the gap between homogeneous and heterogeneous Catalysis. This might yield superior catalysts with novel properties, that is, special reactivity or stability. Both the core and periphery strategies lead to catalysts that are sufficiently larger than most substrates and products, thus separation by modern membrane separation techniques can be applied. These novel homogeneous catalysts can be used in continuous membrane reactors, which will have major advantages particularly for reactions that benefit from low substrate concentrations or suffer from side reactions of the product. Here we review the recent progress and breakthroughs made with these promising novel Transition Metal functionalized dendrimers that are used as catalysts, and we will discuss the architectural concepts that have been applied.
Robert H Crabtree - One of the best experts on this subject based on the ideXlab platform.
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multifunctional ligands in Transition Metal Catalysis
New Journal of Chemistry, 2011Co-Authors: Robert H CrabtreeAbstract:Sophisticated ligands are now being designed that do far more than just fulfil their traditional spectator roles by binding to the Metal and providing a sterically-defined binding pocket for the substrate in homogeneous Transition Metal Catalysis. This Focus review emphasizes selected cases in which ligands carry additional functional groups that change the properties of the ligand as a result of an external stimulus or undergo catalytically-relevant ligand-based reactivity. These include proton responsive ligands capable of gaining or losing one or more protons, ligands having a hydrogen bonding function, electroresponsive ligands capable of gaining or losing one or more electrons, and photoresponsive ligands capable of undergoing a useful change of properties upon irradiation. Molecular recognition ligands and proton coupled electron transfer (PCET) are briefly discussed.
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molecular recognition in homogeneous Transition Metal Catalysis a biomimetic strategy for high selectivity
Chemical Communications, 2008Co-Authors: Siddhartha Das, Gary W Brudvig, Robert H CrabtreeAbstract:Traditional methods for selectivity control in homogeneous Transition Metal Catalysis either employ steric effects in a binding pocket or chelate control. In a supramolecular strategy, encapsulation of the substrate can provide useful shape and size selectivity. A fully developed molecular recognition strategy involving hydrogen bonding or solvophobic forces has given almost completely regioselective functionalization of remote, unactivated C–H bonds.
Don M. Coltart - One of the best experts on this subject based on the ideXlab platform.
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asymmetric induction in hydroacylation by cooperative iminium ion Transition Metal Catalysis
Organic Letters, 2016Co-Authors: Ettore J. Rastelli, Ngoc Truong, Don M. ColtartAbstract:A new strategy for the rhodium-catalyzed enantioselective hydroacylation is described. This has been achieved through the merger of iminium ion Catalysis and Transition-Metal Catalysis such that asymmetric induction derives from a readily accessible, inexpensive chiral nonracemic secondary amine catalyst rather than a chiral nonracemic phosphine as is typical of conventional asymmetric hydroacylation methods.