calixarene

Calixarenes are a family of macrocyclic compounds composed of phenol units linked by methylene bridges at the para positions, producing a cup- or calyx-shaped cavity. They are named for the cup-like structure they form and are built from repeating phenolic units, most commonly four in calix[4]arene, but also six, eight, and higher numbers in other members. The number in brackets denotes the count of phenolic units. In solution, calixarenes can adopt several conformations, including cone, partial-cone, 1,3-alternate, and 1,2-alternate; the cone form is particularly common for calix[4]arene and defines a distinct lower rim and upper rim that can be selectively functionalized.

Synthesis of calixarenes typically involves base-catalyzed condensation of para-substituted phenols with formaldehyde under controlled conditions. The reaction proceeds via methylene-bridged linkages and is influenced by factors such as solvent, concentration, and the use of templates or directing substituents to favor macrocyclization. Template-directed methods, including the use of metal ions, can stabilize specific conformations and improve yields for certain derivatives.

Functionalization occurs at the lower rim (phenolic oxygens) or at the upper rim and methylene bridges, enabling tuning of cavity size, rigidity, solubility, and binding properties. Derivatization yields water-soluble sulfonated calixarenes, alkylated variants, and other substituted forms, allowing selective binding of ions or neutral guests.

Calixarenes are widely used in host–guest chemistry as receptors for cations, anions, and organic molecules. They find applications in extraction and separation, sensing and fluorescence assays, phase-transfer catalysis, and as ligands or scaffolds in catalysis and materials science due to their modular, tuneable cavities and rim functionalities.