A cavitand stabilizes the Meisenheimer complex of SNAr reactions

Comprehensive Characterization of the Self-Folding Cavitand Dynamics

The conformational equilibria and guest exchange process of a resorcin[4]arene derived self-folding cavitand receptor have been characterized in detail by molecular dynamics simulations (MD) and ¹ H EXSY NMR experiments. A multi-timescale strategy for exploring the fluxional behaviour of this system has been constructed, exploiting conventional MD and accelerated MD (aMD) techniques. The use of aMD allows the reconstruction of the folding/unfolding process of the receptor by sampling high-energy barrier processes unattainable by conventional MD simulations. We obtained MD trajectories sampling events occurring at different timescales from ns to s: 1) rearrangement of the directional hydrogen bond seam stabilizing the receptor, 2) folding/unfolding of the structure transiting partially open intermediates, and 3) guest departure from different folding stages. Most remarkably, reweighing of the biased aMD simulations provided kinetic barriers that are in very good agreement with those determined experimentally by ¹ H NMR. These results constitute the first comprehensive characterization of the complex dynamic features of cavitand receptors. Our approach emerges as a valuable rational design tool for synthetic host-guest systems.

Stabilization of reactive species by supramolecular encapsulation

This review describes and updated overview of the stabilization of reactive species and reaction intermediates by inclusion in nanocavities provided by covalent- and supra-molecular containers.

Supramolecular catalysis. Part 2: artificial enzyme mimics

The design of artificial catalysts able to compete with the catalytic proficiency of enzymes is an intense subject of research. Non-covalent interactions are thought to be involved in several properties of enzymatic catalysis, notably (i) the confinement of the substrates and the active site within a catalytic pocket, (ii) the creation of a hydrophobic pocket in water, (iii) self-replication properties and (iv) allosteric properties. The origins of the enhanced rates and high catalytic selectivities associated with these properties are still a matter of debate. Stabilisation of the transition state and favourable conformations of the active site and the product(s) are probably part of the answer. We present here artificial catalysts and biomacromolecule hybrid catalysts which constitute good models towards the development of truly competitive artificial enzymes.

Self-folding cavitands: structural characterization of the induced-fit model

Deletion of structural elements in cavitand receptors is used to evaluate the thermodynamic and kinetic parameters of binding in these hosts. The use of a bulky guest showcases the importance of host-guest shape adaptations in the molecular recognition process.

Structure-reactivity relationships in a recognition mediated [3+2] dipolar cycloaddition reaction

The [3+2] dipolar cycloaddition between an azide and maleimide can be accelerated by a factor of more than 100 simply by attaching complementary recognition sites to the reactive partners. This rate acceleration derives from the formation of a reactive binary complex between the azide and the maleimide. The variation of the observed rate acceleration with simple structural changes, such as adding additional rotors, should be relatively predictable. However, the application of a simple, rotor-based increment in the systems reported here is insufficient to predict reactivity correctly. Computational studies suggest that the nature of the available reaction pathways within the binary complex formed by the reactants is important in determining the reactivity of a given complex.

Reaction of isonitriles with carboxylic acids in a cavitand: observation of elusive isoimide intermediates

A deep cavitand with an inwardly directed carboxylic acid function reacts with small aliphatic isonitriles to form N-acyl formamides inside the cavity. The unique isolation and stabilization of covalently bound guests within the structured environment of the cavitand allows for observation of the labile O-acyl isoimide intermediate using conventional spectroscopic methods.

A deep cavitand catalyzes the Diels-Alder reaction of bound maleimides

A deep cavitand catalyzes Diels-Alder reactions of bound maleimides via activation of the dienophile by interaction with the organized hydrogen bonding network at the cavitand rim. Rapid in-out exchange of reactant and product allows efficient turnover. The increase in steric bulk of the reaction product lessens its binding affinity, reducing (and in some cases completely eliminating) product inhibition.