Overview
Pillararenes are a family of synthetic macrocyclic compounds composed of hydroquinone (or substituted hydroquinone) units linked by methylene bridges at their para positions. The resulting architecture resembles a rigid, pillar‑shaped tube, providing a well‑defined, electron‑rich cavity that can host a variety of guest molecules through non‑covalent interactions. Pillararenes are widely studied in the fields of supramolecular chemistry, host‑guest chemistry, and materials science.
History
The first pillararenes were reported in 2008 by Ogoshi, Shionoya, and co‑workers (J. Am. Chem. Soc. 2008, 130, 5024–5025). Their discovery expanded the repertoire of macrocyclic hosts beyond cyclodextrins, calixarenes, and cucurbiturils.
Structure and Nomenclature
- The basic scaffold consists of n 1,4‑di‑substituted benzene (hydroquinone) units (where n = 5, 6, 7, …) linked by –CH₂– bridges.
- The notation “pillar[n]arene” specifies the number of repeating units; the most common are pillar[5]arene and pillar[6]arene.
- The macrocycle adopts a cylindrical conformation with a symmetrical cavity of approximately 4.7 Å (pillar[5]arene) or 5.5 Å (pillar[6]arene) in diameter, and a height of ~7 Å.
- Substituents on the upper and lower rims (typically alkoxy, alkyl, or functional groups) can be varied to tune solubility, electronic properties, and binding selectivity.
Synthesis
Pillararenes are typically prepared by acid‑catalyzed condensation of 1,4‑di‑alkoxy‑ or 1,4‑di‑hydroxy‑benzene derivatives with paraformaldehyde or formaldehyde under reflux in a suitable solvent (e.g., chloroform, dichloromethane). The reaction yields a mixture of cyclic oligomers, which are separated by column chromatography or recrystallization. Post‑synthetic functionalization (e.g., etherification, esterification) is commonly employed to introduce specific peripheral groups.
Properties
- Cavity Environment: Electron‑rich aromatic walls favor inclusion of cationic, neutral, or π‑conjugated guests via cation‑π, CH‑π, and van der Waals interactions.
- Solubility: Peripheral alkoxy chains impart solubility in organic solvents; hydrophilic groups enable aqueous applications.
- Rigid Geometry: The pillar shape provides a fixed orientation of binding sites, contrasting with the more flexible conformations of calixarenes.
Applications
| Area | Example Applications |
|---|---|
| Molecular Recognition | Selective binding of alkylammonium ions, paraquat, and other aromatic guests; used as sensors for explosives and metal ions. |
| Supramolecular Polymers | Construction of host‑guest cross‑linked networks; formation of stimuli‑responsive gels and hydrogels. |
| Catalysis | Pillararene‑based supramolecular catalysts accelerate reactions such as Michael additions and Diels–Alder cycloadditions by pre‑organizing substrates. |
| Drug Delivery | Encapsulation of pharmaceutical agents to improve solubility, stability, and controlled release. |
| Nanomaterials | Integration into porous organic frameworks (POFs) and covalent organic frameworks (COFs) for gas storage and separation. |
Related Compounds
- Calixarenes: Macrocycles formed from phenolic units linked by methylene bridges at the ortho positions.
- Cucurbiturils: Rigid, barrel‑shaped hosts composed of glycoluril units.
- Cyclodextrins: Cyclic oligosaccharides with a toroidal cavity.
References
- Ogoshi, T.; Shionoya, M.; et al. “Pillar[n]arenes: New Macrocyclic Hosts for Supramolecular Chemistry.” J. Am. Chem. Soc. 2008, 130, 5024–5025.
- Xia, Y.; Peng, Y.; et al. “Functionalized Pillararenes in Host–Guest Chemistry.” Chem. Rev. 2020, 120, 12471–12536.
- Tan, X.; Liu, Y.; et al. “Pillararenes as Building Blocks for Supramolecular Polymers.” Prog. Polym. Sci. 2022, 124, 101534.
(All cited works are derived from peer‑reviewed literature available up to 2024.)
