Nucleophilic Reactions Using Alkali Metal Fluorides Activated by Crown Ethers and Derivatives

We review crown ether-facilitated nucleophilic reactions using metal salts, presenting the studies using kinetic measurements and quantum chemical methods. We focus on the mechanistic features, specifically on the contact ion-pair (CIP) mechanism of metal salts for nucleophilic processes promoted by crown ethers and derivatives. Experimental verification of the CIP form of the metal salt CsF complexed with [18-Crown-6] by H-NMR spectroscopy is described. The use of chiral crown ethers and derivatives for enantioselective nucleophilic processes is also discussed.

Hierarchical Self-Assembly of Triphilic Main-Chain-Type Semifluorinated Alternating Graft Copolymers in Aqueous Solution

Fluorinated copolymers can self-assemble in solution and form micelles with rare properties due to the peculiar behavior of fluorinated groups. However, the process description of the self-assembly is still largely phenomenological and difficult to explain due to the tendency of the fluorinated segments to segregate from both the hydrophilic and lipophilic segments, which can result in various morphologies. Herein, the controlled formation of ellipsoidal micelles, disklike micelles, and sheets by hierarchical self-assembly of triphilic main-chain-type semifluorinated alternating graft copolymers (AB)_n A-g-mOEG is presented (where A represents unit of α,ω-diiodoperfluoroalkane, B represents the unit of α,ω-unconjugated diene, and mOEG represents methoxy oligo(ethylene glycol)), which are synthesized by step transfer-addition and radical-termination (START) polymerization and azide-alkyne click chemistry. Furthermore, the possible self-assembly mechanism of these micron-level aggregates is proposed, which is ascribed to the hierarchical self-assembly, crowding effect of hydrophilic chains and the interfacial tension between the fluoroalkane and alkane segments. This study can provide a facile and highly efficient approach to the synthesis of main-chain-type fluorinated graft copolymers and expand the research field for the solution self-assembly of fluorinated copolymers.

Alkali metal ion binding using cyclic polyketones

Cyclic oligoketones composed of 3,3-dimethylpentane-2,4-diones showed crown ether-like alkali metal ion binding behavior with association constants up to 1.7 × 10⁴ M^-1 in chloroform/acetonitrile (v/v, 9/1). The binding properties have been used for catalysis in the Finkelstein reaction in a low-polarity solvent. Furthermore, novel ion-binding hosts were generated by terminal functionalization of linear polyketones.

Thermodynamic and electrochemical study of tailor-made crown ethers for redox-switchable (pseudo)rotaxanes

Crown ethers are common building blocks in supramolecular chemistry and are frequently applied as cation sensors or as subunits in synthetic molecular machines. Developing switchable and specifically designed crown ethers enables the implementation of function into molecular assemblies. Seven tailor-made redox-active crown ethers incorporating tetrathiafulvalene (TTF) or naphthalene diimide (NDI) as redox-switchable building blocks are described with regard to their potential to form redox-switchable rotaxanes. A combination of isothermal titration calorimetry and voltammetric techniques reveals correlations between the binding energies and redox-switching properties of the corresponding pseudorotaxanes with secondary ammonium ions. For two different weakly coordinating anions, a surprising relation between the enthalpic and entropic binding contributions of the pseudorotaxanes was discovered. These findings were applied to the synthesis of an NDI-[2]rotaxane, which retains similar spectroelectrochemical properties compared to the corresponding free macrocycle. The detailed understanding of the thermodynamic and electrochemical properties of the tailor-made crown ethers lays the foundation for the construction of new types of molecular redox switches with emergent properties.

Entropy-Driven Reversible Agglomeration of Crown Ether Capped Gold Nanoparticles

It is shown that plasmonic gold nanoparticles functionalised with a thiolated 18-crown-6 ligand shell agglomerate spontaneously from aqueous dispersion at elevated temperatures. This process takes place over a narrow temperature range, is accompanied by a colour change from red to purple-blue and is fully reversible. Moreover, the temperature at which it occurs can be adjusted by the degree of complexation of the crown ether moiety with appropriate cations. More complexation leads to higher transition temperatures. The process has been studied by UV/Vis spectroscopy, electron microscopy, dynamic light scattering and zeta potential measurements. A thermodynamic rationale is provided to suggest an entropy-driven endothermic agglomeration process based on attractive hydrophobic interactions of the complexed crowns that are competing against electrostatic repulsion of the charged ligand shells.

N-Heterocyclic olefins as efficient phase-transfer catalysts for base-promoted alkylation reactions

N-Heterocyclic olefins (NHOs) chemically transfer the base from solid phase and promote alkylation reaction on a wide range of substrates.

Factors that influence competitive intermolecular solvation of protonated groups in peptides and proteins in the gas phase

The ability of 18-crown-6 (18C6) to form noncovalent complexes with cationic groups in the gas phase has been leveraged in numerous, largely orthogonal mass spectrometry-based applications. Although the fundamental interaction between 18C6 and a charged group in the gas phase is quite strong, the strength of attachment of 18C6 to large molecules is more difficult to predict because intramolecular binding of the cation can be competitive. Herein, we demonstrate in experiments with model peptides that 18C6 adducts are not strongly attached to flexible molecules with numerous potential hydrogen bonding sites. 18C6 adduct stability is increased if intramolecular charge complexation is inhibited by sterics or competitive binding. It is demonstrated with molecular mechanics that significant structural changes occur upon loss of 18C6 in model peptides. Examination of the loss of 18C6 adducts from proteins following collisional activation reveals that lower charge states lose the most 18C6. The degree of 18C6 adduct stability may reflect the degree of structural reorganization that occurs following collisional activation, suggesting that lower charge states represent structures that are not similar to gas phase idealized states. In this regard, 18C6 may serve the function of protecting solution phase protein structure. Collisional activation of holomyoglobin with 18C6 adducts attached reveals that heme loss occurs primarily after 18C6 loss, further supporting the notion that 18C6 protects native structure by solvating charged sites.

Perfluoroalkylated 4,13-Diaza-18-Crown-6 Ethers: Synthesis, Phase-Transfer Catalysis, and Recycling Studies

A series of N,N'-dialkyl-4,13-diaza-18-crown-6 lariat ethers possessing two C8H17 (2), (CH2)3C8F17 (3), (CH2)3C10F21 (4), and (CH2)2C8F17 (5) side arms were synthesized in good yields by N-alkylation of 4,13-diaza-18-crown-6. Potassium picrate could be extracted from an aqueous solution into an organic phase by all of the perfluoroalkylated macrocycles demonstrating their potential to be used as phase-transfer catalysts, and preliminary studies on a classical nucleophilic substitution established that they each gave higher catalytic activities under solid-liquid than under liquid-liquid phase-transfer conditions. The light fluorous macrocycles gave similar, if not better, catalytic activity compared to the parent, non-fluorinated phase-transfer catalyst 2 under solid-liquid conditions in conventional organic solvents in both an aliphatic and an aromatic nucleophilic substitution. N,N'-Bis(1H,1H,2H,2H,3H,3H-perfluoroundecyl)-4,13-diaza-18-crown-6 (3) was recycled six times in the iodide displacement reaction of 1-bromooctane and four times in the fluoride displacement reaction of 2,4-dinitrochlorobenzene using fluorous solid-phase extraction without any loss in activity.