Host-guest Chemistry and Proton-catalyzed Reactivity in a Self-assembled Supramolecular Assembly PDF Download

Author: Michael Dwight Pluth
Publisher:
ISBN:
Category :
Languages : en
Pages : 1088

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Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 1650

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Synthetic supramolecular host assemblies can impart unique reactivity to encapsulated guest molecules. Synthetic host molecules have been developed to carry out complex reactions within their cavities, despite the fact that they lack the type of specifically tailored functional groups normally located in the analogous active sites of enzymes. Over the past decade, the Raymond group has developed a series of self-assembled supramolecules and the Bergman group has developed and studied a number of catalytic transformations. In this Account, we detail recent collaborative work between these two groups, focusing on chemical catalysis stemming from the encapsulation of protonated guests and expanding to acid catalysis in basic solution. We initially investigated the ability of a water-soluble, self-assembled supramolecular host molecule to encapsulate protonated guests in its hydrophobic core. Our study of encapsulated protonated amines revealed rich host-guest chemistry. We established that self-exchange (that is, in-out guest movement) rates of protonated amines were dependent on the steric bulk of the amine rather than its basicity. The host molecule has purely rotational tetrahedral (T) symmetry, so guests with geminal N-methyl groups (and their attendant mirror plane) were effectively desymmetrized; this allowed for the observation and quantification of the barriers for nitrogen inversion followed by bond rotation. Furthermore, small nitrogen heterocycles, such as N-alkylaziridines, N-alkylazetidines, and N-alkylpyrrolidines, were found to be encapsulated as proton-bound homodimers or homotrimers. We further investigated the thermodynamic stabilization of protonated amines, showing that encapsulation makes the amines more basic in the cavity. Encapsulation raises the effective basicity of protonated amines by up to 4.5 pK{sub a} units, a difference almost as large as that between the moderate and strong bases carbonate and hydroxide. The thermodynamic stabilization of protonated guests was translated into chemical catalysis by taking advantage of the potential for accelerating reactions that take place via positively charged transition states, which could be potentially stabilized by encapsulation. Orthoformates, generally stable in neutral or basic solution, were found to be suitable substrates for catalytic hydrolysis by the assembly. Orthoformates small enough to undergo encapsulation were readily hydrolyzed by the assembly in basic solution, with rate acceleration factors up to 3900 compared with those of the corresponding uncatalyzed reactions. Furthering the analogy to enzymes that obey Michaelis-Menten kinetics, we observed competitive inhibition with the inhibitor NPr4, thereby confirming that the interior cavity of the assembly was the active site for catalysis. Mechanistic studies revealed that the assembly is required for catalysis and that the rate-limiting step of the reaction involves proton transfer from hydronium to the encapsulated substrate. Encapsulation in the assembly changes the orthoformate hydrolysis from an A-1 mechanism (in which decomposition of the protonated substrate is the rate-limiting step) to an A-S{sub E}2 mechanism (in which proton transfer is the rate-limiting step). The study of hydrolysis in the assembly was next extended to acetals, which were also catalytically hydrolyzed by the assembly in basic solution. Acetal hydrolysis changed from the A-1 mechanism in solution to an A-2 mechanism inside the assembly, where attack of water on the protonated substrate is rate limiting. This work provides rare examples of assembly-catalyzed reactions that proceed with substantial rate accelerations despite the absence of functional groups in the cavity and with mechanisms fully elucidated by quantitative kinetic studies.

Author: Jack Halpern
Publisher:
ISBN:
Category : Self-assembly (Chemistry)
Languages : en
Pages : 452

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Author: Jeffrey Scott Mugridge
Publisher:
ISBN:
Category :
Languages : en
Pages : 778

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The work described in this thesis examines the noncovalent interactions, dynamics and structure of a self-assembled supramolecular host-guest system. The highly-charged, water soluble supramolecular assembly is able to selectively bind cationic molecules to the host interior and exterior and mediate the physical properties and chemical reactivity of bound guests. Herein, physical organic chemistry in the context of this supramolecular system is used to elucidate some of the fundamental host-guest interactions that underlie guest binding and reactivity. Chapter 1. First, an overview of different noncovalent interactions and their importance to biological systems is presented. Selected examples of synthetic supramolecular host-guest systems are then reviewed, focusing on examples that illustrate how chemists have used noncovalent interactions to construct complex host architectures and affect guest chemical reactivity. Finally, the [Ga4L6]12- supramolecular assembly, which is the topic of Chapters 2 - 5, is introduced and previous work examining the host-guest chemistry of this system is briefly reviewed. Chapter 2. Ortho-substituted benzylphosphonium guest molecules are used to quantitatively probe the steric effects of confinement within the [Ga4L6]12- host. Encapsulated guest bond rotational barriers and tumbling rates are measured in different solvents and at elevated external pressures. These studies reveal that despite the flexibility of the host assembly, guest molecules experience significant steric confinement on the host interior and their bond rotational barriers are increased by up to 6 kcal/mol. Significant solvent and pressure effects on the bond rotational rates are also observed; these suggest that the host cavity is smaller or less flexible in organic than in aqueous solution, and that internal solvent pressure is responsible for the observed changes in ligand framework flexibility. The apparently smaller cavity volumes in organic solvents are further supported by NOE distance measurements, which show shorter average host-guest distances in organic, as compared to aqueous, solutions. Chapter 3. Building on the solvent effects observed in Chapter 2, some additional, qualitative solvent effects are first presented; these show that guest binding and exchange is also sensitive to bulk solvent and that N, N-dimethylformamide can act as weakly bound guest. The second part of this chapter presents a quantitative study of the solvent effects on the thermodynamics and kinetics of guest binding and exchange in the [Ga4L6]12- host. No correlation between the guest binding or exchange parameters across different solvents are observed, illustrating the complexity of host and guest solvation in these highly-charged supramolecular systems. Chapter 4. A brief literature review of isotope effects on guest binding and exchange in supramolecular host-guest systems is first presented. The measurement of kinetic and equilibrium isotope effects on guest binding and exchange in the [Ga4L6]12- assembly are then described. Protiated guests are found to be more strongly bound to both the host interior and exterior, with significantly larger equilibrium isotope effects observed for C-H/D bonds that can participate in cation-[pi] interactions. DFT-level computations reveal that the equilibrium isotope effects arise from changes in low frequency C-H/D vibrational motions upon guest association. Kinetic isotope effects during the guest ejection process are also observed. A general model to explain both equilibrium and kinetic isotope effects, based only on changes in C-H/D vibrational force constants and zero-point energies is presented. The observation of significant isotope effects on both guest binding and exchange demonstrates the remarkable sensitivity of the [Ga4L6]12- host assembly to even the most subtle changes in guest architecture. Chapter 5. Guest molecules encapsulated in [Ga4L6]12- experience dramatic changes in their 1H nuclear magnetic resonance (NMR) chemical shifts due to the unique magnetic environment of the host interior. Gauge-independent atomic orbital NMR chemical shift calculations are carried out and compared with experiment to provide information about host-guest conformation and structure.

Author: Piet W.N.M. van Leeuwen
Publisher: John Wiley & Sons
ISBN: 3527349022
Category : Technology & Engineering
Languages : en
Pages : 708

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Supramolecular Catalysis Provides a timely and detailed overview of the expanding field of supramolecular catalysis The subdiscpline of supramolecular catalysis has expanded in recent years, benefiting from the development of homogeneous catalysis and supramolecular chemistry. Supramolecular catalysis allows chemists to design custom-tailored metal and organic catalysts by devising non-covalent interactions between the various components of the reaction. Edited by two world-renowned researchers, Supramolecular Catalysis: New Directions and Developments summarizes the most significant developments in the dynamic, interdisciplinary field. Contributions from an international panel of more than forty experts address a broad range of topics covering both organic and metal catalysts, including emergent catalysis by self-replicating molecules, switchable catalysis using allosteric effects, supramolecular helical catalysts, and transition metal catalysis in confined spaces. This authoritative and up-to-date volume: Covers ligand-ligand interactions, assembled multi-component catalysts, ligand-substrate interactions, and supramolecular organocatalysis and non-classical interactions Presents recent work on supramolecular catalysis in water, supramolecular allosteric catalysis, and catalysis promoted by discrete cages, capsules, and other confined environments Highlights current research trends and discusses the future of supramolecular catalysis Includes full references and numerous figures, tables, and color illustrations Supramolecular Catalysis: New Directions and Developments is essential reading for catalytic chemists, complex chemists, biochemists, polymer chemists, spectroscopists, and chemists working with organometallics.

Author: Jubaraj Bikash Baruah
Publisher: CRC Press
ISBN: 0429608047
Category : Science
Languages : en
Pages : 226

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Supramolecular catalysis is involved in assimilation or growth of biological products and it has advantages over conventional catalysis in dealing with systems beyond molecules to mimic the biological catalytic processes. Principles and Advances in Supramolecular Catalysis shows how a supramolecular catalytic reaction proceeds and how interactions among molecules provide vessels or specific binding sites to carry out chemical reactions. The utilities of such catalytic reactions in waste, hazard management, medicine, food, etc. are explained in this book. The book focuses on examples to provide a fundamental basis so that, in the future, supramolecular catalytic reactions are utilised in the field of chemical, biological, biophysical sciences and technologies. Features: Discusses fundamental and interdisciplinary aspects of supramolecular catalysis Narrates mechano-chemical and stimuli-guided supramolecular catalytic reactions Divulges the intriguing aspects of self-replications and self-assembling performed through supramolecular catalysis Incorporates supramolecular catalytic reactions of metal-organic frameworks as artificial metalloenzymes

Author: Brian D. Wagner
Publisher: Walter de Gruyter GmbH & Co KG
ISBN: 3110564394
Category : Science
Languages : en
Pages : 355

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Book Description
This textbook addresses the chemical and physicochemical principles of supramolecular host-guest chemistry in solution. It covers the thermodynamics and dynamics of inclusion and highlights several types of organic hosts. Various applications of host-guest chemistry in analytical and environmental chemistry as well as pharmaceutical and chemical industry demonstrate the versatile usability of molecular cages.

Author: 385 Insane Publisher Publishing
Publisher:
ISBN: 9781086331950
Category :
Languages : en
Pages : 122

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This Notebook makes a great present for Christmas, Easter and birthdays.

Author: Dennis H. Leung
Publisher:
ISBN:
Category :
Languages : en
Pages : 494

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Author: Rinaldo Poli
Publisher: Springer
ISBN: 3319502077
Category : Science
Languages : en
Pages : 272

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This book highlights the recent advances and state of the art in the use of functionalized nanostructured environments on catalysis. Nanoconfinements considered include well-defined molecular cages, imprinted self-assembled supramolecules, polymers made by living or controlled polymerization, metallorganic frameworks, carbon nanotubes, mesoporous inorganic solids, and hybrids thereof. Advantages of nanoconfinement of catalysts discussed include higher activities, improved selectivities, catalyst stabilization, cooperativity effects, simplified protocols for cascade syntheses, better catalyst recovery, and recyclability. The multiple applications that these materials offer are revolutionizing industrial sectors such as energy, electronics, sensors, biomedicine, and separation technology.