Anion and ion-pair receptor chemistry: highlights from 2000 and 2001

Supramolecular chemistry in water is a constantly growing research area because noncovalent interactions in aqueous media are important for obtaining a better understanding and control of the major processes in nature. This Review offers an overview of recent advances in the area of water-soluble synthetic receptors as well as self-assembly and molecular recognition in water, through consideration of the functionalities that are used to increase the water solubility, as well as the supramolecular interactions and approaches used for effective recognition of a guest and self-assembly in water. The special features and applications of supramolecular entities in aqueous media are also described.

Supramolecular Chemistry in Water

Anion–arene adducts: C–H hydrogen bonding, anion–π interaction, and carbon bonding motifs

Neutral anion receptors (LH) form stable 1 : 1 H-bond [LH⋯X]− complexes with carboxylates, halides and phosphate (X−). Some of the [LH⋯X]− complexes, in presence of an excess of X−, release an HX fragment, with formation of [HX2]− and the deprotonated receptor L−. The tendency towards deprotonation increases with the acidity of the receptor and with the stability of the [HX2]− self-complex. Thus, the more acidic thiourea containing receptor deprotonates in the presence all the investigated anions except chloride, whereas the less acidic urea containing receptor undergoes deprotonation only in the presence of fluoride, due to the high stability of [HF2]−.

Urea vs. thiourea in anion recognition

This paper refines the nature of the interactions between electron-deficient arenes and halide anions. Conclusions are based on (i) new crystal structures containing alkali halide salts with 1,2,4,5-tetracyanobenzene (TCB) and 18-crown-6, (ii) evaluation of crystal structures found in the Cambridge Structural Database, and (iii) MP2/aug-cc-pVDZ calculations of F-, Cl-, and Br- complexes with TCB, 1,3,5-tricyanobenzene, triazine, and hexafluorobenzene. When the halide lies above the plane of the π system, the results establish that three distinctly different types of complexes are possible:  strongly covalent σ complexes, weakly covalent donor−π-acceptor complexes, and noncovalent anion−π complexes. When aryl C−H groups are present, a fourth type of interaction leads to C−H · · · X- hydrogen bonding. Characterization of the different geometries encountered with the four possible binding motifs provides criteria needed to design host architectures containing electron-deficient arenes.

Structural criteria for the design of anion receptors: the interaction of halides with electron-deficient arenes.

Synthetic receptors for carboxylic acids and carboxylates

A series of acyclic thiourea derivatives, designed to create a cleft with four hydrogen bond donors suitable for carboxylate recognition, have been prepared, and their ability to bind to N-protected amino acid carboxylate salts has been investigated. The crystal structure of one of the thioureas has been determined showing that it forms a hydrogen bonded centrosymmetric dimer in the solid-state, in a conformation appropriate for the desired binding of carboxylates. The thioureas show good discrimination between different amino acids and those thioureas incorporating chiral moieties show moderate enantioselectivity for a range of amino acid derivatives.

Enantioselective amino acid recognition using acyclic thiourea receptors

The development of enantioselective receptors continues to be a challenging endeavor for supramolecular chemists, and enantioselective recognition of biologically relevant molecules in competitive solvents is particularly demanding.[1] Although numerous receptors have been developed for dicarboxylic acids and dicarboxylates,[2] only a few enantioselective receptors for chiral dicarboxylic acids (in the neutral diprotonated form) have been described,[3] and very few examples of enantioselective receptors for chiral dicarboxylates have been reported.[4] We recently described an acyclic monothiourea receptor 1a, which bound a range of N-protected amino acid carboxylate salts with modest enantioselectivity.[5] Building on this work, we have now prepared macrocyclic receptor 2, which features two thiourea moieties flanked by carboxypyridines and separated by a chiral diamine. The receptor was designed to produce a chiral pocket for dicarboxylates by forming up to eight hydrogen-bonding interactions with the carboxylate oxygen atoms and intramolecular hydrogen bonding with the pyridine unit to help preorganize[6] the receptor (Scheme 1). Binding measurements, by NMR titration and isothermal

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A highly enantioselective receptor for N-protected glutamate and anomalous solvent-dependent binding properties

This invention recites isoxazoline substituted derivatives of Formula (1) stereoisomers thereof, geometric isomers thereof, veterinarily acceptable salts 5 thereof, compositions thereof, and their use as a parasiticide in animals. The variables A, W, R 1a, R 1b, R 1c, R 2, R 3, R 5, and n are as described herein.

Isoxazoline derivatives as antiparasitic agents

The present invention provides pyrrolo{2,3-d}pyrimidine compounds, their use as Janus Kinase (JAK) inhibitors, pharmaceutical compositions containing this compounds, and methods for the preparation of these compounds.

Graham M. Kyne

Papers

Synthetic receptors for carboxylic acids and carboxylates

Enantioselective amino acid recognition using acyclic thiourea receptors

A highly enantioselective receptor for N-protected glutamate and anomalous solvent-dependent binding properties

Isoxazoline derivatives as antiparasitic agents

Pyrrolo [ 2, 3 - d] pyrimidine urea compounds as jak inhibitors