Pillararene

Ogoshi and Kanai decided naming the new family of host macrocycles "pillararene", since they are cylindrical or pillar-like in shape and composed of aromatic or "arene" moieties.1 Chemists often refer to them as "pillarenes"[3] orally as this is easier to pronounce and remember.

It is pertinent to mention that Rathore and Kochi [4] first observed pillararenes in 1995, but were unable to characterize, and reported as a polymer-like material being formed along with diarylmethanes during the transformation of bis(methoxymethyl)-p-hydroquinone ether.

Due to the close proximity of many electron-rich hydroquinones, the cavity of pillararenes are able to form strong association complexes with electron-poor species.

DMpillar[5]arene has five planes of chirality which are "in sync" in the lowest energy conformational isomer due to unfavorable steric interactions between the methoxy groups of neighboring 1,4-dimethoxy-phenylene units.

[15] A practical and effective trifluoromethanesulfonic acid (TfOH)-catalyzed cyclooligomerization strategy was also developed for the synthesis of functionalized pillar[n]arenes and copillar[5]arenes from 1,4-dialkoxybenzenes with paraformaldehyde under mild reaction conditions, and the reaction mechanism of solution-phase catalytic synthesis of pillararenes was investigated by room-temperature X-band ESR spectroscopy, mass spectroscopy, NMR and control experiments, suggesting a free radical process initially and a Friedel–Crafts alkylation process during the consequent coupling and ring-closure stage.

Ogoshi and coworkers reported [18] the synthesis of a pillar[6]arene with 1,4-Bis(methylcyclohexyl ether)phenylene units in an 87% yield by using chlorocyclohexane as the solvent.

Because encapsulation within the cavity of carboxylated-pillar[6]arene quenches the fluorescence of proflavine, this gives rise to "on" and "off' states to the dye which may have application in bio-diagnostics.

Pillar[5]arene shows promise in overcoming the development of antimicrobial resistance and biofilm formation as well as reducing the toxicity of bacterial endotoxins (lipopolysaccharides).

Chemical structure of pillar[5]arene
Enantiomers of Pillar[5]arene
Assignment of the absolute stereochemical designation of one hydroquinone unit of planar chiral DMpillar[5]arene. The pilot atom is shown in purple, and the three atoms used to assign the absolute stereochemistry are labeled and shown as spheres.
A molecular model of carboxylated-pillar[7]arene with the anti-Alzheimer drug memantine, showing the total encapsulation of the drug within the macrocycle's cavity. The host–guest complex is stabilized by hydrophobic effects within the cavity, hydrogen bonding, and electrostatic interactions.