Locking the Dynamic Axial Chirality of Biphenyl Crown Ethers through Threading

This paper describes the syntheses of [2]rotaxanes comprising 23- and 26-membered biphenyl crown ethers as the macrocyclic components and secondary ammonium ions as the dumbbell-shaped components, and the locking of the dynamic axial chirality of the biphenyl moieties in these structures. Chiral high-performance liquid chromatography (HPLC) revealed that our [2]rotaxane featuring the 26-membered crown ether racemized at room temperature, but the racemization of the [2]rotaxane featuring the 23-membered crown ether did not proceed at room temperature over a period of three days. After separation of the enantiomers of the [2]rotaxane incorporating the 23-membered crown ether through chiral HPLC, we studied its racemization at elevated temperature. The rate of stereoinversion in dimethylsulfoxide (a polar solvent) was faster than that ino-dichlorobenzene (a nonpolar solvent), and herein we discuss these kinetic parameters.