Reactivity within a confined self-assembled nanospace

Koblenz, Tehila S.; Wassenaar, Jeroen; Reek, Joost N. H.

doi:10.1039/b614961h

Cited by 614 publications

(321 citation statements)

References 57 publications

(76 reference statements)

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“…To emulate this mode of catalysis, many research groups have designed synthetic hosts capable of binding and directing the reactivity of guest molecules. [5][6][7][8][9][10][11][12][13] Upon encapsulation in either a synthetic host or an enzyme active site, the environment surrounding a guest molecule drastically differs from that of the bulk solution. Steric constraints, functional group positioning, and sequestration from other molecules can enforce reactivity and selectivity that is impossible when simpler catalysts are employed.…”

Section: Introductionmentioning

confidence: 99%

Aza Cope Rearrangement of Propargyl Enammonium Cations Catalyzed By a Self-Assembled “Nanozyme”

et al. 2008

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Section: Introductionmentioning

confidence: 99%

Aza Cope Rearrangement of Propargyl Enammonium Cations Catalyzed By a Self-Assembled “Nanozyme”

et al. 2008

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“…1,2 The complexity of the final molecular structures obtained through self-assembly has evolved considerably since the pioneering work by Lehn, Cram, Pedersen and Breslow and now includes supramolecules able to promote both stoichiometric and catalytic reactions. [2][3][4][5][6][7][8][9] The interior cavities, generally protected from bulk solution, provide a unique environment for encapsulated guest molecules and can stabilize otherwise unstable species or lead to enhanced reactivity and selectivity.…”

Section: Introductionmentioning

confidence: 99%

Proton-Mediated Chemistry and Catalysis in a Self-Assembled Supramolecular Host

2009

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Synthetic supramolecular assemblies able to encapsulate guest molecules often impart unique reactivity on the encapsulated guests. Although much less complex than the active sites of enzymes, synthetic host molecules have been developed that can carry out complex reactions within their cavities.Over the last decade, the Raymond group has developed a series of self-assembled supramolecules able to encapsulate small guest molecules. This Account details recent collaborative work between the Raymond and Bergman 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 self-assembled supramolecular host molecule to encapsulate protonated guests. Our study of encapsulated protonated amines revealed a rich array of host-guest chemistry. Guest exchange studies established that the rates of encapsulated protonated amines were dependent on the steric bulk of the amine rather than its basicity. The purely-rotational T symmetry of the host molecule effectively desymmetrized the geminal N-methyl groups and allowed for the observation and quantification of the barriers for nitrogen inversion followed by bond rotation. In further screening of amine guests, small nitrogen heterocycles such as N-alkylaziridines, -azetidines and -pyrrolidines were found to be encapsulated as proton-bound homodimers or homotrimers. Expanding 3 the study of protonated amines to investigate the thermodynamic stabilization of protonated amines showed that encapsulation makes the encapsulated amines more basic and raises the effective basicity of the protonated amines by up to 4.5 pK a units.The thermodynamic stabilization of protonated guests was translated into chemical catalysis by applying the stabilization of protonated species to reactions proceeding through monocationic protonated intermediates. Orthoformates, which are 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 observed rate accelerations of up to 3900 when compared to the uncatalyzed reaction. Furthering the analogy to enzymes that obey Michaelis-Menten kinetics, competitive inhibition was demonstrated with the inhibitor NPr 4 + , thereby confirming that the interior cavity of the assembly was the active site for catalysis. Mechanistic studies revealed that the assembly is intimately required for catalysis and that the rate limiting step of the reaction is proton-transfer from hydronium to the encapsulated substrate.Encapsulation in the assembly changes the orthoformate hydrolysis mechanism from an A-1, in which decomposition of the protonated substrate is the rate limiting step, to an A-S E 2 mechanism in which proton transfer is the rate limiting step. The study of hydrolysis reactions in the assembly was next extended to acetals, which were also catalytically hydrolyzed ...

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“…While the development of synthetic host-guest systems has not reached the level of enzyme specificity, the characteristics of each synthetic assembly, such as the size, shape, charge, and functional group availability, greatly influence the guest-binding characteristics and have led to remarkable and often unexpected reactivity. 9,[16][17][18][19][20][21][22][23] For example, the increased local concentration upon encapsulation of substrates for bimolecular reactions has been exploited for enhanced reactivity inside of synthetic hosts. By pre-organizing substrates, supramolecular assemblies are able to catalyze cycloadditions or pericyclic reactions such as Diels-Alder reactions.…”

Section: Introductionmentioning

confidence: 99%

Spin Equilibria in Monomeric Manganocenes: Solid-State Magnetic and EXAFS Studies

et al. 2009

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A water-soluble self-assembled supramolecular host molecule catalyzes the hydrolysis of orthoformates in basic solution. Comparison of the rate constants of the catalyzed and uncatalyzed reactions for hydrolysis displays rate accelerations of up to 3900 for tri-n-propyl orthoformate. Kinetic analysis shows that the mechanism of hydrolysis with the supramolecular host obeys the Michaelis-Menten model.Mechanistic studies, including 13 C-labeling experiments, revealed that the resting state of the catalytic system is the neutral substrate encapsulated in the host. Activation parameters for the k cat step of the reaction revealed that upon encapsulation in the assembly, the entropy of activation becomes more negative in contrast to the uncatalyzed reaction. Furthermore, solvent isotope effects reveal a normal k(H 2 O)/k(D 2 O) = 1.6, confirming that proton transfer is occurring in the transition state and is rate limiting in the catalyzed reaction. In comparison to the uncatalyzed reaction, which operates by an A-1 mechanism in which the decomposition of the protonated substrate is rate limiting, the encapsulated reaction proceeds through an A-S E 2 mechanism in which proton transfer from protonated water to the substrate is rate limiting.

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Reactivity within a confined self-assembled nanospace

Cited by 614 publications

References 57 publications

Aza Cope Rearrangement of Propargyl Enammonium Cations Catalyzed By a Self-Assembled “Nanozyme”

Aza Cope Rearrangement of Propargyl Enammonium Cations Catalyzed By a Self-Assembled “Nanozyme”

Proton-Mediated Chemistry and Catalysis in a Self-Assembled Supramolecular Host

Spin Equilibria in Monomeric Manganocenes: Solid-State Magnetic and EXAFS Studies

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