Cyclic Host Liquids for Facile and High-Yield Synthesis of [2]Rotaxanes

We developed cyclic host liquids (CHLs) as a new type of solvent. The CHLs are a nonvolatile liquid over a wide temperature range, are biocompatible and recyclable, have high thermal stability, and are miscible with many organic solvents. Compared with typical complexation systems, the CHL system is extremely efficient for maintaining host-guest complexation because an additional solvent is not required. Based on the efficient host-guest complexation in the CHL system, we demonstrated synthesis of [2]rotaxanes in pillar[5]arene-based CHL. High yields were obtained for [2]rotaxanes capped by cationization (yield 91%) and Huisgen reaction (yield 88%) between the axle and the stopper components in the CHL system, while the association constants between the axles and wheels were quite low (10-15 W-1) in CDCl3. The CHL system provides a new powerful approach for synthesis of mechanically interlocked molecules (MIMs) even with unfavorable statistical combinations of host-guest complexes.