Resorcinarene

A resorcinarene (also resorcarene or calix[4]resorcinarene) is a macrocycle, or a cyclic oligomer, based on the condensation of resorcinol (1,2-dihydroxybenzene) and an aldehyde. Resorcinarenes are a type of calixarene.

Synthesis
The resorcinarene macrocycle is typically prepared by condensation of resorcinol and an aldehyde in concentrated acid solution.[1][2] Recrystallization typically gives the desired isomer in quite pure form. However, for certain aldehydes, the reaction conditions lead to significant by-products. Therefore, alternative condensation conditions have been developed, including the use of Lewis acid catalysts.



A green chemistry procedure was recently developed using solvent-free conditions: resorcinol, an aldehyde, and p-toluenesulfonic acid are ground together in a mortar and pestle at low temperature.[3] A paste-like solid forms upon sitting for several minutes, which can be washed with water and crystallized to give the desired resorcinarene.

Under each of these conditions, aromatic aldehydes react much faster, but less stereoselectively than aliphatic aldehydes, and the reaction frequently leads to a mixture of desired and undesired products.

Structure
Resorcinarenes can be characterized by a wide upper rim and a narrow lower rim. The upper rim includes eight hydroxyl groups that can participate in hydrogen bonding interactions. Depending on the aldehyde starting material, the lower rim includes four appending groups, usually chosen to give optimal soubility. The resorcin[n]arene nomenclature is analogous to that of calix[n]arenes, in which 'n' represents the number of repeating units in the ring. Pyrogallolarenes are related macrocycles derived from the condensation of pyrogallol (1,2,3-trihydroxybenzene) with an aldehyde.

History
During the late 19th century, Adolf von Baeyer noted that the condensation reaction of resorcinol and benzaldehyde gave a mixture of products that was beyond the characterization methods of that time. Over the following century, the product mixture was more fully characterized by X-ray crystallography and Nuclear Magnetic Resonance and found to include general structure shown above and several other stereoisomers.

Supramolecular Chemistry
Yasuhiro Aoyama's research group at Kyoto University first noted the potential for resorcinarenes to interact with other molecules as a host-guest complex.[4] It was later found that resorcinarenes and pyrogallolarenes self-assemble into to larger supramolecular structures. Both in the crystalline state and in organic solvents, six resorcinarene molecules are known to form hexamers with an internal volume of around one cubic nanometer (nanocapsules) and shapes similar to the Archimedean solids.[5] Hydrogen bonds appear to hold the assembly together. A number of solvent or other molecules reside inside.[6]

The resorcinarene is also the basic structural unit for other molecular recognition scaffolds. A number of chemists, including Nobel-laureate Donald J. Cram, constructed novel molecular structures based on this macrocycle, namely cavitands and carcerands.