Synthesis, Structure and Functionalization of Pillar[n]arenes
Polygons are beautiful structures with excellent symmetry. In 2008, we reported a new class of pillar-shaped macrocyclic hosts, known as “pillar[n]arenes”. Based on the highly symmetric pillars that constitute the Parthenon in Athens, we named this type of paracyclophane ‘‘pillar[n]arenes’’. In the 11 years since thediscovery of pillar[n]arenes in 2008, numerous publications reporting the use of pillar[n]arenes have been published (>600 papers), most of them in the last 5 years, which indicates that pillar[n]arene chemistry is a very hot topic in supramolecular chemistry.
Synthesis
Pillar[n]arenes (n = 5, 6) can be prepared selectively depending on the nature of the solvent used for the cyclisation reaction. For example, pillar[5]arenes, which are cyclic pentamers, can be synthesised in high yields using 1,2-dichloroethane as a solvent, which acts as a suitable guest molecule for pillar[5]arenes.10 In contrast, pillar[6]arenes, which are cyclic hexamers, can be synthesised in high yields when chlorocyclohexane is used as a solvent because the solvent molecules represent a good size matching for the cavity of pillar[6]arenes. The solvent molecules therefore work as templates to facilitate the production of a particular pillar[n]arene homologue in high yield. The regents typically used to prepare pillar[5]arenes are commercially available and inexpensive, and the products themselves can be readily isolated using a simple recrystallisation procedure. The ease with which these materials can be synthesised is exemplified by the fact that a pillar[5]arene system bearing 10 methoxy groups has been sold commercially by Tokyo Chemical Industry (TCI) since 2014.
Structure
The most important point regarding the chemical structures of pillar[n]arenes is the position of the methylene bridges connecting the units. Pillar[n]arenes are composed of 1,4-dialkoxybenzene units, which are connected by methylene bridges at their 2- and 5-positions. In contrast, in calix[n]arenes, the phenol units are linked by methylene bridges at their 2- and 6-positions. As a result of the different methylene bridge positions, the shapes of pillar[n]arenes are completely different from those of calix[n]arenes. Calix[n]arenes have open-ended, non-symmetric calix-shaped structures, which is why they were named calix[n]arenes. In contrast, pillar[n]arenes have completely symmetric cylindrical structures, as a result of the linkages at the 2- and 5-positions. Based on the highly symmetric pillars that constitute the Parthenon in Athens, we named this type of paracyclophane ‘‘pillar[n]arenes’’.
Functionalization
The alkoxy groups on both rims of pillar[n]arenes can be converted to highly reactive phenol groups by de-protection using BBr3. When an excess amount of BBr3 was used, the all alkoxy groups were converted to phenol groups. On the other hand, by adjusting amount of BBr3 and reaction time, it was possible to synthesize pillar[n]arene in which only one was a phenol group. In the case of pillar[n]arenes having two or more phenol groups have many conformers, thus synthesis of multi-reactive pillar[n]arenes is difficult target. As a new method for synthesizing pillar[n]arenes having two or more phenol groups, we developed oxidation/reduction approach. Oxidation/eeduciton of one di-alkoxybenzene units resulted in the formation of two reactive pillar[n]arenes in which one unit has two phenol groups. By utilizing the high reactivity of phenol groups, we are able to produce pillar[n]arenes with various substituents.
