Adaptive Planar Chirality of Pillar[5]arenes Invertible by <i>n</i>-Alkane Lengths
Keisuke Adachi, Shixin Fa, Keisuke Wada, Kenichi Kato, Shunsuke Ohtani, Yuuya Nagata, Shigehisa Akine, Tomoki Ogoshi
Abstract
Chirality of host molecules can be induced and/or inverted by the guest molecules. However, the adapting chirality of hosts to the length of n -alkanes remains a great challenge because n -alkanes are neutral, achiral, and linear molecules, resulting in a weak interaction with most compounds. Herein, we report a system with chirality adapted to n -alkane lengths, using a pillar[5]arene-based macrocyclic host, S -Br, which contains five stereogenic carbons and five terminal bromine atoms on each rim. The electron-rich cavity of S -Br could include n -alkanes and the planar-chiral isomers sensitively inverted in response to the lengths of the complexed n -alkanes. The inclusion of a short n -alkane such as n -pentane made S -Br more inclined to be in the pS -form, whereas the inclusion of long n -alkanes such as n -heptane made the pR -form more favorable. The difference in the stability of the isomers was supported by the crystal structures and the theoretical calculations. Furthermore, temperature drives the adaptive chirality of S -Br with n -alkanes. An n -alkane with middle length, n -hexane, showed the dominance of the pR -form of S -Br at a higher temperature, whereas the pS -form was shown at a lower temperature.