www.asiachem.news December 2021 | 73 report, Kim and coworkers discovered other cucurbit[n]uril homologs, i.e., cucurbit[5-8]urils.8 These were obtained by tuning the reaction conditions. Their development triggered the expansion of cucurbit[n]uril chemistry because, apart from cucurbit[6]uril, cucurbit[n]uril homologs have relatively high solubilities in common solvents. However, these host molecules were reported until 2000, which suggests that the discovery of new classes of macrocyclic hosts that can be used widely by chemists is challenging. In 2008, we unexpectedly discovered new type of pillar-shaped macrocyclic host molecules, i.e., pillar[n]arenes (Figure 1a).10,11 Figure 1f lists a number of publications that report pillar[n]arenes. Until 2011, only a limited number of researchers, including ourselves, worked in this field. However, from 2012, global interest in pillar[n]arenes began to grow, and this triggered further expansion of pillar[n]arene chemistry. Since 2018, which is the tenth anniversary of pillar[n]arene chemistry, the number of publications per year has exceeded 200. This shows that pillar[n]arene chemistry is expanding. The success of pillar[n]arene chemistry has been recognized by chemists as a new route to novel fields of chemistry and this has instigated the development of new macrocyclic host molecules. Pillar[n]arenes are therefore recognized as game-changing molecules in supramolecular chemistry in the early 21st century. How were pillar[n]arenes discovered? The discovery of pillar[n]arenes was accidental as were the discoveries of other famous macrocyclic hosts. Pillar[n]arenes were first obtained as an unexpected product when we synthesized phenolic polymers. When I started my academic career as an assistant professor at Kanazawa University, one of the projects in the laboratory was the synthesis of new types of phenolic polymers. I started my research on the synthesis of new phenolic polymers by designing phenolic monomers with my students. We tried to synthesize new phenolic polymers by reacting 1,4-dimethoxybenzene with paraformaldehylde in the presence of a Lewis acid. We screened the polymerization conditions and obtained phenolic polymers with high molecular weights by changing the reaction conditions such as the types of Lewis acids and solvents. Unexpectedly, when 1,2-dichloroethane was used as the solvent, the product was not a polymer but a particular oligomer (Figure 2a). Figure 2 Solvent effect on macrocyclic formation. (a) 1,2-dichloethane and (c) chlorocylohexane acted as templates for pillar[5]arenes and pillar[6]arenes synthesis, respectively. (b) Chloroform did not act as a template for the particular pillar[n] arene formation. X-ray crystallographic analysis showed that the obtained product was a cyclic pentamer composed of five 1,4-dimethoxybenzene units connected by methylene bridges at the 2,5-position (Figure 1a, acrocyclic Hosts Supramolecular Assemblies By Tomoki Ogoshi https://doi.org/10.51167/acm00026
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