Catechol[4]arene: The Missing Chiral Member of the Calix[4]arene Family

Enantioselective Synthesis of Inherently Chiral Tetraazacalix[4]aromatics from a Chiral Phosphoric Acid-Catalyzed Intramolecular SNAr Reaction

Inherently chiral calixarenes and heteracalixaromatics (ICCHeC) have been drawing great attention, because of their unique 3D stereochemical structures and tantalizing applications. The majority of ICCHeC arises from the cyclic arrays of different aromatic segments. Examples of ICCHeC generated from various substituents on methylene and heteroatom linkages are extremely rare. Here, we report the enantioselective synthesis of tetraazacalix[1]arene[1]pyridine[2]triazines from a chiral phosphoric acid-catalyzed SNAr reaction. The inherent chirality of the resulting 1,3-alternate heteracalix[4]aromatics stems from the variation of only one substituent on the bridging nitrogen atom. Enantiomers were very stable, and they did not undergo racemization at an elevated temperature. This study opens a new avenue to design and construct novel and functional ICCHeC that may find useful applications in supramolecular science.

Recent Applications of Pillararene-Inspired Water-Soluble Hosts

Pillararenes and their derivatives have emerged in supramolecular chemistry as unique macrocycles for applications in host-guest chemistry, materials science and biomimetics. Many variations have been conceived and synthesized in recent years and in this review, we relate progress in water-soluble versions: leaning towerarenes, extended-pillararenes, biphenarenes, helicarenes and octopusarenes. These are applied in targeted drug delivery, selective uptake and release of aromatic guests, fabrication of gold/silver and mesoporous silica nanoparticles, cell imaging, pollutant separation, biomedicine (e. g. biofilm disruptors, taste masking containers, neuromuscular blockers, antidotes for macromolecular biotoxin) and enantioselective recognition. It is intended that this review will be helpful for research in synthetic macrocyclic chemistry and supramolecular functional systems, leading to practical applications in various research areas.

Chiral synthetic hosts for efficient enantioselective molecular recognition. Design principles and synthetic aspects

Discrimination of enantiomeric substrate molecules is one of the fundamental properties of biological hosts. Replicating enantioselective molecular recognition with synthetic receptors is a topic of interest with implications in diverse applications such as bioinspired enantioselective catalysis, enantiomer separation, or sensing. In this review, five different systems reported in the literature are discussed, and their performance and versatility are analyzed. A recently reported host featuring a flexible scaffold challenges the long-established view that a high degree of preorganization in combination with strongly directional non-covalent interactions is required for efficient enantiodiscrimination. The review is complemented with an analysis of the synthetic effort required for each of the hosts presented.

Recent advances in novel chiral macrocyclic arenes

Chiral macrocyclic arenes possess confined three-dimensional asymmetric cavities, electron-rich structures, chiral luminescence properties and excellent enantioselective recognition properties and have become a frontier and hotspot of macrocyclic chemistry and supramolecular chemistry. In recent years, there has been growing interest in the development of novel chiral macrocyclic arenes, which have found applications in various research areas. In this review, the construction, properties and functional applications of novel chiral macrocyclic arenes in enantioselective recognition, chiral sorting and construction of chiral luminescent materials according to their chiral types, including central, axial, planar, and inherent chiralities, are summarized. It is expected that this review will be helpful for research on supramolecular chemistry and for promoting the development of synthetic chemistry, materials chemistry and biochemistry.

Chiral Macrocycles for Enantioselective Recognition

The efficient synthesis of chiral macrocycles with highly enantioselective recognition remains a challenge. We have addressed this issue by synthesizing a pair of chiral macrocycles, namely, R/S-BINOL[2], achieving total isolated yields of up to 62% through a two-step reaction sequence. These macrocycles are readily purified by column chromatography over silica gel without the need for chiral separation, thus streamlining the overall synthesis. R/S-BINOL[2] demonstrated enantioselective recognition toward chiral ammonium salts, with enantioselectivity (KS/KR) values reaching up to 13.2, although less favorable separations were seen for other substrates. R/S-BINOL[2] also displays blue circularly polarized luminescence with a |glum| value of up to 2.2 × 10-3. The R/S-BINOL[2] macrocycles of this study are attractive as chiral hosts in that they both display enantioselective guest recognition and benefit from a concise, high-yielding synthesis. As such, they may have a role to play in chiral separations.

Quinoacridane[4]arenes─Very Large Conformationally Restricted Macrocycles

Phenol-based macrocycles play a fundamental role in supramolecular chemistry, but their size has been rather limited. Here we report a novel class of very large, bowl-shaped macrocycles with a diameter of 21.8 Å. These quinoacridane[4]arenes are 150% larger than the current record holders, the acridane[4]arenes, and three times the size of resorcin[4]arene. We expect the quinoacridane[4]arenes to be a useful platform for the construction of molecular containers.

De novo synthesis of inherently chiral heteracalix[4]aromatics from enantioselective macrocyclization enabled by chiral phosphoric acid-catalyzed intramolecular SNAr reaction

We report herein the synthesis of highly enantiopure inherently chiral N3,O-calix[2]arene[2]triazines from enantioselective macrocyclization enabled by chiral phosphoric acid-catalyzed intramolecular nucleophilic aromatic substitution reaction. In contrast to documented examples, the inherent chirality of the acquired compounds arises from one heteroatom difference in the linking positions of heteracalix[4](het)arenes.

Synthesis and Host-Guest Chemistry of Chiral Spirobifluorene-Based Macrocycles Soluble in Basic Aqueous Solution

Synthesis and host-guest chemistry of water-soluble (pH 12.5) chiral spirobifluorene-based macrocycles 2-[n] were carried out. Cationic guests, such as quaternary ammonium salts, were accommodated well in the hosts. Cp2Co+ was especially strongly bound in 2-[4] (Ka of up to 3.0 × 105 M-1). Enantioselective recognition with (l)-carnitine was also achieved.

Recent advances in the synthesis and applications of macrocyclic arenes

Macrocyclic arenes including calixarenes, resorcinarenes, cyclotriveratrylene, pillararenes and so on have emerged as highly attractive synthetic macrocyclic hosts due to their unique structures, facile functionalization, and broad range of applications. In recent years, there has been growing interest in the development of novel macrocyclic arenes composed of various aromatic building blocks bridged by methylene groups, which have found applications in various research areas. Consequently, the development of novel macrocyclic arenes has become a frontier and hot topic in supramolecular and macrocyclic chemistry. In this review, we feature the recent advances in the synthesis and applications of novel macrocyclic arenes that have emerged in the last decade. The general synthetic strategies employed for these macrocyclic arenes are systematically summarized, and their wide applications in molecular recognition and assemblies, molecular machines, biomedical science and functional materials are highlighted.

Design of surface nanostructures for chirality sensing based on quartz crystal microbalance

Quartz crystal microbalance (QCM) has been widely used for various sensing applications, including chirality detection due to the high sensitivity to nanogram or picogram mass changes, fast response, real-time detection, easy operation, suitability in different media, and low experimental cost. The sensing performance of QCM is dependent on the surface design of the recognition layers. Various strategies have been employed for studying the relationship between the structural features and the specific detection of chiral isomers. This review provides an overview of the construction of chiral sensing layers by various nanostructures and materials in the QCM system, which include organic molecules, supermolecular assemblies, inorganic nanostructures, and metal surfaces. The sensing mechanisms based on these surface nanostructures and the related potentials for chiral detection by the QCM system are also summarized.

Megalo-Cavitands: Synthesis of Acridane[4]arenes and Formation of Large, Deep Cavitands for Selective C70 Uptake

Deep cavitands, concave molecular containers, represent an important supramolecular host class that has been explored for a variety of applications ranging from sensing, switching, purification and adsorption to catalysis. A major limitation in the field has been the cavitand volume that is restricted by the size of the structural platform utilized (diameter approx. 7 Å). We here report the synthesis of a novel, unprecedentedly large structural platform, named acridane[4]arene (diameter approx. 14 Å), suitable for the construction of cavitands with volumes of up to 814 Å3 . These megalo-cavitands serve as size-selective hosts for fullerenes with mM to sub-μM binding affinity for C60 and C70 . Furthermore, the selective binding of fullerene C70 in the presence of C60 was demonstrated.

Macrocyclic host molecules with aromatic building blocks: the state of the art and progress

Macrocyclic host molecules play the central role in host-guest chemistry and supramolecular chemistry. The highly structural symmetry of macrocyclic host molecules can meet people's pursuit of aesthetics in molecular design, and generally means a balance of design, synthesis, properties and applications. For macrocyclic host molecules with highly symmetrical structures, building blocks, which could be described as repeat units as well, are the most fundamental elements for molecular design. The structural features and recognition ability of macrocyclic host molecules are determined by the building blocks and their connection patterns. Using different building blocks, different macrocyclic host molecules could be designed and synthesized. With decades of developments of host-guest chemistry and supramolecular chemistry, diverse macrocyclic host molecules with different building blocks have been designed and synthesized. Aromatic building blocks are a big family among the various building blocks used in constructing macrocyclic host molecules. In this feature article, the recent developments of macrocyclic host molecules with aromatic building blocks were summarized and discussed.

Advances in Chirality Sensing with Macrocyclic Molecules

The construction of chemical sensors that can distinguish molecular chirality has attracted increasing attention in recent years due to the significance of chiral organic molecules and the importance of detecting their absolute configuration and chiroptical purity. The supramolecular chirality sensing strategy has shown promising potential due to its advantages of high throughput, sensitivity, and fast chirality detection. This review focuses on chirality sensors based on macrocyclic compounds. Macrocyclic chirality sensors usually have inherent complexing ability towards certain chiral guests, which combined with the signal output components, could offer many unique advantages/properties compared to traditional chiral sensors. Chirality sensing based on macrocyclic sensors has shown rapid progress in recent years. This review summarizes recent advances in chirality sensing based on both achiral and chiral macrocyclic compounds, especially newly emerged macrocyclic molecules.

Thioderivatives of Resorcin[4]arene and Pyrogallol[4]arene: Are Thiols Tolerated in the Self-Assembly Process?

Three novel thiol bearing resorcin[4]arene and pyrogallol[4]arene derivatives were synthesized. Their properties were studied with regards to self-assembly, disulfide chemistry, and Brønsted acid catalysis. This work demonstrates that (1) one aromatic thiol on the resorcin[4]arene framework is tolerated in the self-assembly process to a hexameric hydrogen bond-based capsule, (2) thio-derivatized resorcin[4]arene analogs can be covalently linked through disulfides, and (3) the increased acidity of aromatic thio-substituent is not sufficient to replace HCl as cocatalyst for capsule catalyzed terpene cyclizations.

Resorcin[4]arene-based multidentate phosphate ligands with superior binding affinity for nanocrystal surfaces

We designed and synthesized two resorcin[4]arene scaffolds with four phosphate binding groups. The ligands effectively bind in at least a tridentate fashion at low surface coverage. The superior binding affinity is demonstrated using solution NMR spectroscopy and exceeds that of single phosphonates.

Enantioselective synthesis of chiral porphyrin macrocyclic hosts and kinetic enantiorecognition of viologen guests

The construction of macromolecular hosts that are able to thread chiral guests in a stereoselective fashion is a big challenge. We herein describe the asymmetric synthesis of two enantiomeric C 2-symmetric porphyrin macrocyclic hosts that thread and bind different viologen guests. Time-resolved fluorescence studies show that these hosts display a factor 3 kinetic preference (ΔΔG ‡ on = 3 kJ mol-1) for threading onto the different enantiomers of a viologen guest appended with bulky chiral 1-phenylethoxy termini. A smaller kinetic selectivity (ΔΔG ‡ on = 1 kJ mol-1) is observed for viologens equipped with small chiral sec-butoxy termini. Kinetic selectivity is absent when the C 2-symmetric hosts are threaded onto chiral viologens appended with chiral tails in which the chiral moieties are located in the centers of the chains, rather than at the chain termini. The reason is that the termini of the latter guests, which engage in the initial stages of the threading process (entron effect), cannot discriminate because they are achiral, in contrast to the chiral termini of the former guests. Finally, our experiments show that the threading and de-threading rates are balanced in such a way that the observed binding constants are highly similar for all the investigated host-guest complexes, i.e. there is no thermodynamic selectivity.