Phenyl-Extended Resorcin[4]arenes: Synthesis and Highly Efficient Iodine Adsorption.

The continuous exploration of new analogs of calixarenes and pillararenes unlocks infinite opportunities in supramolecular chemistry and materials. In this work, we introduce a new class of macrocycle, phenyl-extended resorcin[4]arenes (ExR4), a unique and innovative design that incorporates unsubstituted phenylene moieties into the resorcin[4]arene scaffold. Single-crystal analysis reveals a chair-like conformation for per-methylated ExR4 (Me-ExR4) and a twisted "figure-of-eight" shaped conformation for per-hydroxylated ExR4 (OH-ExR4). Notably, OH-ExR4 demonstrates exceptional adsorption capability toward I3- ions in an aqueous solution, with a rapid kinetic rate of 1.18×10-2 g·mg-1·min-1. Furthermore, OH-ExR4 shows excellent recyclability and potential as a stationary phase in column setups. The discovery of ExR4 opens up new avenues for constructing new macrocycles and inspires further research in functional adsorption materials for water pollutant removal.

Pillararene-Inspired Macrocycles: From Extended Pillar[n]arenes to Geminiarenes.

chemistry since their establishment due to their innate functional features of molecular recognition and complexation. The rapid development of modern supramolecular chemistry has also significantly benefited from creating new macrocycles with distinctive geometries and properties. For instance, pillar[n]arenes (pillarenes), a relatively young generation of star macrocyclic hosts among the well-established ones (e.g., crown ethers, cyclodextrins, cucurbiturils, and calixarenes), promoted a phenomenal research hotspot all over the world in the past decade. Although the synthesis, host-guest properties, and various supramolecular functions of pillarenes have been intensively studied, many objective limitations and challenges still cannot be ignored. For example, high-level pillar[n]arenes (n > 7) usually do not possess applicable large-sized cavities due to structural folding and cannot be synthesized on a large scale because of the uncompetitive cyclization process. Furthermore, two functional groups must be covalently para-connected to each repeating phenylene unit, which severely limits their structural diversity and flexibility. In this context, we have developed a series of pillarene-inspired macrocycles (PIMs) using a versatile and modular synthetic strategy during the past few years, aiming to break through the synthetic limitations in traditional pillarenes and find new opportunities and challenges in supramolecular chemistry and beyond. Specifically, by grafting biphenyl units into the pillarene backbones, extended pillar[n]arenes with rigid and nanometer-sized cavities could be obtained with reasonable synthetic yields by selectively removing hydroxy/alkoxy substitutes on pillarene backbones, leaning pillar[6]arenes and leggero pillar[n]arenes with enhanced structural flexibility and cavity adaptability were obtained. By combining the two types of bridging modes in pillarenes and calixarenes, a smart macrocyclic receptor with two different but interconvertible conformational features, namely geminiarene, was discovered. Benefiting from the synthetic accessibility, facile functionalization, and superior host-guest properties in solution or the solid state, this new family of macrocycles has exhibited a broad range of applications, including but not limited to supramolecular assembly/gelation/polymers, pollutant detection and separation, porous organic polymers, crystalline/amorphous molecular materials, hybrid materials, and controlled drug delivery. Thus, in this Account, we summarize our research efforts on these PIMs. We first present an overview of their design and modular synthesis and a summary of their derivatization strategies. Thereafter, particular attention is paid to their structural features, supramolecular functions, and application exploration. Finally, the remaining challenges and perspectives are outlined for their future development. We hope that this Account and our works can stimulate further advances in synthetic macrocyclic chemistry and supramolecular functional systems, leading to practical applications in various research areas.

Macrocycle-Based Crystalline Supramolecular Assemblies Built with Intermolecular Charge-Transfer Interactions.

Synthetic macrocycles have served as principal tools for supramolecular chemistry, have greatly extended the scope of organic charge transfer (CT) complexes, and have proved to be of great practical value in the solid state during the past few years. In this Minireview, we summarize the research progress on the macrocycle-based crystalline supramolecular assemblies primarily driven by intermolecular CT interactions (a.k.a. macrocycle-based crystalline CT assemblies, MCCAs for short), which are classified by their donor-acceptor (D-A) constituent elements, including simplex macrocyclic hosts, heterogeneous macrocyclic hosts, and host-guest D-A pairs. Particular attention will be focused on their diverse functions and applications, as well as the underlying CT mechanisms from the perspective of crystal engineering. Finally, the remaining challenges and prospects are outlined.

A Water-Soluble Leggero Pillar[5]arene.

The study of aqueous-phase molecular recognition of artificial receptors is one of the frontiers in supramolecular chemistry since most biochemical processes and reactions take place in an aqueous medium and heavily rely on it. In this work, a water-soluble version of leggero pillar[5]arene bearing eight positively charged pyridinium moieties (CWP[5]L) was designed and synthesized, which exhibited good binding affinities with certain aliphatic sulfonate species in aqueous solutions. Significantly, control experiments demonstrate that the guest binding performance of CWP[5]L is superior to its counterpart water-soluble macrocyclic receptor in traditional pillararenes.

Modulating Supramolecular Charge-Transfer Interactions in the Solid State using Compressible Macrocyclic Hosts.

Modulating intermolecular charge-transfer (ICT) interactions between specific donor and acceptor species in host-guest systems is a big challenge and full of research value in supramolecular chemistry and materials science. In this work, a strategy to modulate the supramolecular ICT interactions in the solid state is developed by compressing the binding cavity of a macrocyclic host named perethylated leaning pillar[6]arene (p-EtLP6). The solid-state ICT affinities of p-EtLP6 toward multi-types of electron-deficient planar guests could be significantly enhanced by transforming the macrocyclic backbone from the original para-bridged mode into a hybrid para- and meta-bridged isomeric form (m-EtLP6). X-ray single-crystal structural analyses incorporating theoretical calculation demonstrate that the improved ICT affinities are mainly attributed to the superior host-guest size fit arising from the compressed binding cavity in m-EtLP6 as compared with p-EtLP6.

Bottom-Up Solid-State Molecular Assembly via Guest-Induced Intermolecular Interactions.

The manipulation of molecular motions to construct highly ordered supramolecular architectures from chaos in the solid state is considered to be far more complex and challenging in comparison to that in solution. In this work, a bottom-up molecular assembly approach based on a newly designed skeleton-trimmed pillar[5]arene analogue, namely the permethylated leggero pillar[5]arene MeP[5]L, is developed in the solid state. An amorphous powder of MeP[5]L can take up certain guest vapors to form various ordered linker-containing solid-state molecular assemblies, which can be further used to construct a thermodynamically favored linker-free superstructure upon heating. These approaches are driven by vapor-induced solid-state molecular motions followed by a thermally triggered phase-to-phase transformation. The intermolecular interactions play a crucial role in controlling the molecular arrangements in the resulting assemblies. This research will open new insights into exploring controllable molecular motions and assemblies in the solid state, providing new perspectives in supramolecular chemistry and materials.

Grinding-induced supramolecular charge-transfer assemblies with switchable vapochromism toward haloalkane isomers.

Synthetic macrocycles have proved to be of great application value in functional charge-transfer systems in the solid state in recent years. Here we show a switchable on-off type vapochromic system toward 1-/2-bromoalkane isomers by constructing solid-state charge-transfer complexes between electron-rich perethylated pillar[5]arene and electron-deficient aromatic acceptors including 4-nitrobenzonitrile and 1,4-dinitrobenzene. These charge-transfer complexes with different colors show opposite color changes upon exposure to the vapors of 1-bromoalkanes (fading) and 2-bromoalkanes (deepening). Single-crystal structures incorporating X-ray powder diffraction and spectral analyses demonstrate that this on-off type vapochromic behavior is mainly attributed to the destruction (off) and reconstruction (on) of the charge-transfer interactions between perethylated pillar[5]arene and the acceptors, for which the competitive host-guest binding of 1-bromoalkanes and the solid-state structural transformation triggered by 2-bromoalkanes are respectively responsible. This work provides a simple colorimetric method for distinguishing positional isomers with similar physical and chemical properties.

Guest-induced amorphous-to-crystalline transformation enables sorting of haloalkane isomers with near-perfect selectivity.

The separation of haloalkane isomers with distillation-free strategies is one of the most challenging research topics in fundamental research and also gave high guiding values to practical industrial applications. Here, this contribution provides a previously unidentified solid supramolecular adsorption material based on a leggero pillararene derivative BrP[5]L, which can separate 1-/2-bromoalkane isomers with near-ideal selectivity. Activated solids of BrP[5]L with interesting amorphous and nonporous features could adsorb 1-bromopropane and 1-bromobutane from the corresponding equal volume mixtures of 1-/2-positional isomers with purities of 98.1 and 99.0%, respectively. Single-crystal structures incorporating theoretical calculation reveal that the high selectivity originates from the higher thermostability of 1-bromoalkane-loaded structures compared to its corresponding isomer-loaded structures, which could be further attributed to the perfect size/shape match between BrP[5]L and 1-bromoalkanes. Moreover, control experiments using its counterpart macrocycle of traditional pillararene demonstrate that BrP[5]L has better adsorptive selectivity, benefiting from the intrinsic free-rotation phenylene subunit on its backbone.

Enantioselective Synthesis of Quaternary Carbon Stereocenters by Asymmetric Allylic Alkylation: A Review.

Quaternary stereocenters are of great importance to the three-dimensionality and enhanced properties of new molecules, but the synthetic challenges in creating quaternary stereocenters greatly hinder their wide use in drug discovery, organic material design, and natural product synthesis. The asymmetric allylic alkylation (AAA) of allylic substrates has proven to be a powerful methodology for enantioselective formation of structure skeletons bearing single or more quaternary carbon centers in modern asymmetric organocatalysis. AAA has certain advantages in constructing the tetrasubstituted stereocenters, including but not limited to mild reactive conditions, effective reaction rates, new functional group introduction, and carbon chains length extension. This review outlines the key considerations in the application of AAA reactions and summarizes the recent progress of AAA reactions in the enantioselective synthesis of products containing quaternary stereocenters. Meanwhile, a detailed discussion of the AAA reactions such as ligands, scope of substrates, transformations and the general reaction mechanisms is also provided. We hope this review could stimulate further advances in much broader areas, including organic synthesis, asymmetric catalysis, C-H activation, and symmetrical pharmaceutical chemistry.

Asymmetric Organocatalysis Combined with Palladium Catalysis: Synergistic Effect on Enantioselective Mannich/α-Allylation Sequential Reactions of Pyrazolones in Constructing Vicinal Quaternary Stereocenters.

In this letter, an efficient one-pot asymmetric sequential reaction is achieved by organo/transition metal relay catalysis in constructing two consecutive C-C bonds, which involves enantioselective amino squaramide catalytic Mannich-type addition of pyrazolones to isatin-derived ketimines and a subsequent palladium catalyzed diastereoselective allylic alkylation of pyrazolones with allylic acetates. An array of novel pyrazolone-aminooxindole-propylene structural motifs are obtained in a high level of yield and with excellent enantio- and diastereoselectivity (up to 95% yield, >20:1 dr, >99% ee). This methodology features the formation of vicinal quarternary carbon-stereocenters, and the second all-carbon tetrasubstituted stereogenic center is induced by joint action of the achiral palladium catalysis and the chiral environment generated from the Mannich step. Moreover, the usefulness of this methodology is highlighted by converting the allylic product into the carbonyl compound.