J. Appl. Cryst.の発刊に際して

In the wide area of supramolecular chemistry, cucurbit[n]urils (CBn) present themselves as a young family of molecular containers, able to form stable complexes with various guests, including drug molecules, amino acids and peptides, saccharides, dyes, hydrocarbons, perfluorinated hydrocarbons, and even high molecular weight guests such as proteins (e.g., human insulin). Since the discovery of the first CBn, CB6, the field has seen tremendous growth with respect to the synthesis of new homologues and derivatives, the discovery of record binding affinities of guest molecules in their hydrophobic cavity, and associated applications ranging from sensing to drug delivery. In this review, we discuss in detail the fundamental properties of CBn homologues and their cyclic derivatives with a focus on their synthesis and their applications in catalysis.

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Cucurbiturils: from synthesis to high-affinity binding and catalysis

Mechanochemical solvent-free reactions by milling, grinding or other types of mechanical action have emerged as a viable alternative to solution chemistry. Mechanochemistry offers not only a possibility to eliminate the need for bulk solvent use, and reduce the generation of waste, but it also unlocks the door to a different reaction environment in which synthetic strategies, reactions and molecules previously not accessible in solution, can be achieved. This Minireview examines the potential of mechanochemistry in chemical and materials synthesis, by providing a cross-section of the recent developments in using ball milling for the formation of molecules and materials based on covalent and coordination bonds.

Mechanochemistry for Synthesis

Financial support of the Spanish Ministry of Science and
Innovation (CTQ2012-38543-C03) and Generalitat Valenciana
(PROMETEO/2012/020) is acknowledged. V.M.-C. thanks the
Spanish Ministry of Science and Education for a predoctoral
fellowship (FPU AP2007-02562) and Generalitat Valenciana for
a VALi+d postdoctoral fellowship (APOSTD/2013/041).
M.D.P. thanks the Spanish Ministry of Education for a
postdoctoral fellowship and UGC India for a Startup Grant.

Macrocyclization Reactions: The Importance of Conformational, Configurational, and Template-Induced Preorganization

Halogen bonding, the noncovalent interaction based on electrophilic halogen substituents, features very interesting properties, as illustrated by numerous applications continuously emerging in rece...

Catalysis of Organic Reactions through Halogen Bonding

New chiral cyclohexylhemicucurbit[6]uril.

A general platform for the synthesis of various chiral halogen-bond (XB) donors based on the triazole core and the characterisation of factors that influence the strength of the halogen bond in the solid state and in solution are reported. The characterisation of XB donors in the solid state by X-ray crystallography and in solution by 1H NMR titration can be used to aid the design of new XB donors. We describe the first example of a XB between iodotriazoles and thioureas in solution. In addition, the enantiodiscrimination of acceptors in solution through halogen-bond participation is described.

Synthesis and Characterisation of Chiral Triazole-Based Halogen-Bond Donors: Halogen Bonds in the Solid State and in Solution.

Synthesis of 9,11-secosterols intermediates

A novel eight-membered macrocycle of the hemicucurbit[n]uril family, chiral (all-R)-cyclohexanohemicucurbit[8]uril (cycHC[8]) binds anions in a purely protic solvent with remarkable selectivity. The cycHC[8] portals open and close to fully encapsulate anions in a 1 : 1 ratio, resembling a molecular Pac-Man™. Comprehensive gas, solution and solid phase studies prove that the binding is governed by the size, shape and charge distribution of the bound anion. Gas phase studies show an order of SbF6− ≈ PF6− > ReO4− > ClO4− > SCN− > BF4− > HSO4− > CF3SO3− for anion complexation strength. An extensive crystallographic study reveals the preferred orientations of the anions within the octahedral cavity of cycHC[8] and highlights the importance of the size- and shape-matching between the anion and the receptor cavity. The solution studies show the strongest binding of the ideally fitting SbF6− anion, with an association constant of 2.5 × 105 M−1 in pure methanol. The symmetric, receptor cavity-matching charge distribution of the anions results in drastically stronger binding than in the case of anions with asymmetric charge distribution. Isothermal titration calorimetry (ITC) reveals the complexation to be exothermic and enthalpy-driven. The DFT calculations and VT-NMR studies confirmed that the complexation proceeds through a pre-complex formation while the exchange of methanol solvent with the anion is the rate-limiting step. The octameric cycHC[8] offers a unique example of template-controlled design of an electroneutral host for binding large anions in a competitive polar solvent.

https://pubs.rsc.org/en/content/articlepdf/2017/sc/c6sc05058a

Chiral hemicucurbit[8]uril as an anion receptor: selectivity to size, shape and charge distribution.

Self-organization is one of the most intriguing phenomena of chemical matter. While the self-assembly of macrocycles and cages in dilute solutions has been extensively studied, it remains poorly understood in solvent-free environments. Provided here is the first example of using anionic templates to achieve selective assembly of differently-sized macrocycles in a solvent-free system. Using acid-catalyzed synthesis of cyclohexanohemicucurbiturils as a model, size-controlled, quantitative synthesis of 6- or 8-membered macrocycles by spontaneous anion-directed reorganization of mechanochemically-made oligomers in the solid state is demonstrated.

Riina Aav

Papers

New chiral cyclohexylhemicucurbit[6]uril.

Synthesis and Characterisation of Chiral Triazole-Based Halogen-Bond Donors: Halogen Bonds in the Solid State and in Solution.

Synthesis of 9,11-secosterols intermediates

Chiral hemicucurbit[8]uril as an anion receptor: selectivity to size, shape and charge distribution.

Size-Control by Anion Templating in Mechanochemical Synthesis of Hemicucurbiturils in the Solid State.