Expanding the Pillararene Chemistry: Synthesis and Application of a 10 + 1 Functionalized Pillar[5]arene

Pillar[5]arenes decorated with six-membered-ring aromatics at all the substitution positions

Macrocyclic molecules have characteristic properties different from linear ones, such as high symmetry and guest-inclusion ability. To bring drastic changes to these properties, direct introduction of many substituents is a challenging but effective tool. Herein, we attain direct installation of ten six-membered-ring aromatic π-units into both rims of a pillar[5]arene. In contrast to previous pillar[n]arenes with less hindered five-membered-ring units, which showed conformational complexity and crushed crystal structures, the per-phenyl-substituted pillar[5]arene has a cylinder-shaped crystal structure with a dichloromethane inside the cavity and is obtained as a single pair of D 5-symmetric enantiomers. The average dihedral angles between the core and peripheral benzene rings sharply increase from 38° to 66°. These differences indicate the importance of local steric repulsion on both rims for determining the structures and properties of macrocycles.

Stereoselective Single Step Cyclization to Give Belt-Functionalized Pillar[6]arenes

Two macrocycles were synthesized through cyclization reactions of secondary benzylic alcohols, giving pillar[6]arenes with a methyl substituent at each belt position. These macrocycles form stereoselectively with only the rtctct isomer with alternating up and down orientations of the belt methyl groups definitively identified. Isolated yields were modest (7 and 9%), but the macrocycles are prepared in a single step from either a commercially available alcohol or a very readily prepared precursor. X-ray crystal structures of the macrocycles indicate they have a capsule-like structure, which is far from the conventional pillar shape. Density functional theory calculations reveal that the energy barrier required to obtain the pillar conformation is significantly higher for these belt-functionalized macrocycles than for conventional belt-unfunctionalized pillar[6]arenes.

Per-Arylation of Pillar[n]arenes: An Effective Tool to Modify the Properties of Macrocycles

Installation of various substituents is a reliable and versatile way to alter the properties of macrocyclic molecules, but high-yield and controlled methods are not always available especially for multifold reactions. Herein, we report 10- and 12-fold introduction of aryl substituents onto both rims of cylinder-shaped pillar[n]arenes, which usually have alkoxy substituents slanting to the cylinder axes. Although alkoxy pillar[5]arenes exist as D₅-symmetric enantiomeric pairs, arylated pillar[5]arenes provide crushed single-crystal structures and stereoisomerism including C₂-symmetric conformations depending on the aryl groups. Pillar[n]arenes with 2-benzofuranyl groups display bright fluorescence with quantum yields of 88-90% and no host-guest complexation with electron-deficient molecules in solution due to large deviation from alkoxy compounds. A benzofuran-appended pillar[6]arene instead captures small gaseous molecules in the solid state, probably owing to outside spaces surrounded by aromatic rings.

Solvent-Dependent Self-Assembly of a Pillar[5]arene-Based Poly-Pseudo-Rotaxane Linked and Threaded by Silver(I) Trifluoroacetate: A Double Role

In the supramolecule area, the fabrication of a new concept called polyrotaxanes or poly-pseudo-rotaxanes remains challenging. We herein report the formation of a poly-pseudo-rotaxane in which the same salt-type guest serves both linking and threading in the resulting structure. The combination of A1/A2-thiopyridyl pillar[5]arene (L) and silver(I) trifluoroacetate in CHCl₃/CH₃OH afforded a one-dimensional (1D) poly-pseudo-rotaxane. In this structure, to our surprise, the AgCF₃CO₂ guest not only links the di-armed L ligands via an infinite -L-Ag-L-Ag- arrangement but also threads into a pillar[5]arene cavity in a dimer form, (AgCF₃CO₂)₂. In contrast, the same reaction in CH₂Cl₂/CH₃OH yielded a simple 1D coordination polymer because an included CH₂Cl₂ molecule in the pillar[5]arene cavity prevents the threading of the silver(I) trifluoroacetate guest. Comparative ¹H- and ¹⁹F-NMR studies support the solvent-dependent poly-pseudo-rotaxane formation at a lower concentration of L.

Pillar[n]arene-Mimicking/Assisted/Participated Carbon Nanotube Materials

The recent progress in pillar[n]arene-assisted/participated carbon nanotube hybrid materials were initially summarized and discussed. The molecular structure of pillar[n]arene could serve different roles in the fabrication of attractive carbon nanotube-based materials. Firstly, pillar[n]arene has the ability to provide the structural basis for enlarging the cylindrical pillar-like architecture by forming one-dimensional, rigid, tubular, oligomeric/polymeric structures with aromatic moieties as the linker, or forming spatially "closed", channel-like, flexible structures by perfunctionalizing with peptides and with intramolecular hydrogen bonding. Interestingly, such pillar[n]arene-based carbon nanotube-resembling structures were used as porous materials for the adsorption and separation of gas and toxic pollutants, as well as for artificial water channels and membranes. In addition to the art of organic synthesis, self-assembly based on pillar[n]arene, such as self-assembled amphiphilic molecules, is also used to promote and control the dispersion behavior of carbon nanotubes in solution. Furthermore, functionalized pillar[n]arene derivatives integrated carbon nanotubes to prepare advanced hybrid materials through supramolecular interactions, which could also incorporate various compositions such as Ag and Au nanoparticles for catalysis and sensing.

"Rim-Differentiated" Pillar[6]arenes

A "rim-differentiated" pillar[6]arene (RD-P[6]) was obtained successfully, with the assistance of a dimeric silver trifluoroacetate template, among eight different constitutional isomers in a direct and regioselective manner. The solid-state conformation of this macrocycle could switch from the 1,3,5-alternate to a truly rim-differentiated one upon guest inclusion. This highly symmetric RD-P[6] not only hosts metal-containing molecules inside its cavity, but also can form a pillar[6]arene-C₆₀ adduct through co-crystallization on account of donor-acceptor interactions. The development of synthetic strategies to desymmetrize pillararenes offers new opportunities for engineering complex molecular architectures and organic electronic materials.

Dynamic combinatorial libraries of a dimercapto-pillar[5]arene

We report the synthesis and dynamic covalent chemistry (DCvC) of an A1/A2-dimercapto-functionalized pillar[5]arene (Di-SH-P5). The introduction of thiol moieties into this macrocyclic host makes it an effective building block for making a dynamic combinatorial library (DCL), giving rise to a set of cyclic trimers with intriguing host-guest properties as the dominant constituents.

ortho-Functionalization of Pillar[5]arene: An Approach to Mono-ortho-Alkyl/Aryl-Substituted A1/A2-Dihydroxypillar[5]arene

Despite the fact that the rim and lateral functionalizations of pillar[n]arenes have been well explored, ortho-functionalization has rarely been realized. In this work, we report a facile method of introducing a single functionality ortho to the hydroxyl group in A1/A2-dihydroxypillar[5]arene via a Grignard addition to pillar[4]arene[1]quinone followed by a dienone-phenol rearrangement. The described ortho-alkylation/arylation method allowed formation of various mono ortho-alkyl/aryl-substituted A1/A2-dihydroxypillar[5]arenes previously difficult to obtain.

Fullerene-containing pillar[n]arene hybrid composites

The construction and application of fullerene-containing pillar[n]arene organic–inorganic hybrid composites/systems has been discussed and summarized.

Step-Growth Cyclo-Oligomerization for the Preparation of Functionalized Pillar[6]arenes with Alternating Methylene Bridge Substitutions

Methylene-bridge-substituted pillar[6]arenes (PA[6]) are synthesized by step-growth cyclo-oligomerization. Dimers, trimers, tetramers, and hexamers with substituted methylene bridges are synthesized. Hexamers are converted to PA[6] derivatives with alternating methylene bridge substitutions by ring-closing reactions. PA[6] derivatives are further modified with pyrene groups or carboxylate groups by Suzuki-Miyaura coupling reactions. The modifications render PA[6] fluorescent or water-soluble. A host-guest chemistry study confirms that the water-soluble PA[6] derivative is a high-affinity host toward suitable guests in water.

The recent progress of synergistic supramolecular polymers: preparation, properties and applications

Interactions for forming supramolecular polymers were reviewed together with their unique properties and applications with detailed examples.

Correlating Solution- and Solid-State Structures of Conformationally Flexible Resorcinarenes: Significance of a Sulfonyl Group in Intramolecular Self-Inclusion

The synthesis of tetramethoxyresorcinarene podands bearing p-toluene arms connected by -SO3 - (1) and -CH2 O- (2) linkers is presented herein. In the solid state, the resorcinarene podand 1 forms an intramolecular self-inclusion complex with the pendant p-toluene group of a podand arm, whereas the resorcinarene podand 2 does not show self-inclusion. The conformations of the flexible resorcinarene podands in solution were investigated by variable-temperature experiments using 1D and 2D NMR spectroscopic techniques as well as by computational methods, including a conformational search and subsequent DFT optimisation of representative structures. The 1 H NMR spectra of 1 and 2 at room temperature show a single set of proton signals that are in agreement with C4v symmetry. At low temperatures, the molecules exist as a mixture of boat conformations featuring slow exchange on the NMR timescale. Energy barriers (ΔG≠ 298 ) of 55.5 and 52.0 kJ mol-1 were calculated for the boat-to-boat exchange of 1 and 2, respectively. The results of the ROESY experiments performed at 193 K and computational modelling suggest that in solution the resorcinarene podand 1 adopts a similar conformation to that present in its crystal structure, whereas podand 2 populates a more versatile range of conformations in solution.

Quinoline appended pillar[5]arene (QPA) as Fe3+ sensor and complex of Fe3+ (FeQPA) as a selective sensor for F-, arginine and lysine in the aqueous medium

A quinoline functionalized pillar[5]arene, QPA has been prepared and its interaction with biologically relevant ions and molecules in aqueous solution has been demonstrated. The sensor molecule, QPA has shown selectivity towards Fe³⁺ among eleven metal ions studied. The Fe³⁺ complex of QPA (FeQPA) selectively interacts with F^- among halides by ∼4 fold fluorescence enhancement. Further, FeQPA has shown selectivity towards arginine and lysine among twenty naturally occurring amino acids. The binding of QPA with Fe³⁺ has been confirmed by MALDI-TOF and ¹H NMR titrations.

pH/Redox-Controlled Interaction between Lipid Membranes and Peptide Derivatives with a "Helmet"

How to reduce the cytotoxicity of antitumor peptides to normal cells remains an ongoing challenge. Here, we designed a pH/redox-responsive supramolecular structure (Pep-V⊂P-PEG) composed of a peptide modified with viologen (Pep-V) and polyethylene glycol bearing pillar[5]arene (P-PEG) as a "helmet". By shielding the hydrophobic moiety of the peptide derivative with pillar[5]arene via host-guest interactions, its disruption on normal cells can be effectively reduced. At acidic pH, the supramolecular structure can selectively adsorb onto negatively charged lipid membranes because of electrostatic interactions. Owing to redox responsiveness of the viologen group, Pep-V could be separated from P-PEG after the addition of reductants and inserted into lipid bilayers, which leads to membrane disruption. Cargo leakage of liposome models was investigated to understand Pep-V⊂P-PEG-induced liposomal membrane disruption under different pH values and redox conditions. Results showed that Pep-V⊂P-PEG caused almost no cargo leakage from (1,2-dipalmitoyl-sn-glycerol-3-phosphocholine) liposomes at pH 7.4 but significant leakage from negatively charged (1,2-dipalmitoyl-sn-glycerol-3-phospho-(1-rac-glyerol)) liposomes at pH 5.0 under a reducing environment. Pep-V⊂P-PEG displayed low destructive effects on mimic normal cells and significant disruption to mimic tumor cells when exposed to a reducing environment that is expected to be a potential antitumor agent.

Functionalization at Will of Rim-Differentiated Pillar[5]arenes

The development of an efficient synthetic route toward rim-differentiated C₅-symmetric pillar[5]arenes (P[5]s), whose two rims are decorated with different chemical functionalities, opens up successive transformations of this macrocyclic scaffold. This paper describes a gram-scale synthesis of a C₅-symmetric penta-hydroxy P[5] precursor, and a range of highly efficient reactions that allow functionalizing either rim at will via, e.g., sulfur(VI) fluoride exchange (SuFEx) reactions, esterifications, or Suzuki–Miyaura coupling. Afterward, BBr₃ demethylation activates another rim for similar functionalizations.

A novel supramolecular polymer π-gel based on bis-naphthalimide functionalized-pillar[5]arene for fluorescence detection and separation of aromatic acid isomers

A pillar[5]arene-based supramolecular polymer π-gel, BPN-G, can selectively identify and separate benzoic acid isomers through rationally introduced multi-intermolecular interactions.

Light-driven exchange between extended and contracted lasso-like isomers of a bistable [1]rotaxane

A photoactive hydrogen-bonded lasso having an amide-based [1]rotaxane structure has been constructed from acyclic precursors through a self-templating approach. The stability, structural integrity and switching are described.