Pillararenes are macrocycles composed of arene units linked in a para-fashion. In this way they resemble the cucurbiturils and calixarenes that play an important part in host-guest chemistry.
Groups involved in this type of research are that of Cao/Meier (DOI) with pillar-6-arenes and pillar-5-quinones, that of Feihe Huang with pillar-5-arene copolymers (DOI), pillar-5-arenes with isobutyl substituents (DOI) and rotaxane-like supramolecular polymers (DOI) and that of Ogoshi/Nakamoto with pillar-5-arenes with EtO and MeO side groups (DOI) , with ionic liquids as guest (DOI), with pillar-5-arenes as rotaxane components (DOI), with alkyl substituted pillar-5-arenes (DOI), with molecular dynamics (DOI) and with a viologen salt as quest (DOI).
The last group mentioned is sitting on gold because they created their pillararene simply by adding together 1,4-dimethoxybenzene and formaldehyde (dichloroethane, boron trifluoride etherate, rt) when this sort of reaction is supposed to yield just linear oligomers. In a most recent publication the group reported for this reaction an unusual 71% yield (DOI). In their original 2008 publication the yield was just 22% but it was discovered that less is more in formaldehyde (not 10 equivalents but just three) and that the optimum reaction time is not 30 minutes but just three. Deprotection of the methoxy groups (tribromoborane) gives the water soluble decahydroxylpillarene. X-ray analysis reveals flipping of two of the arene units for this compound.