Solvent-Free Synthesis of the Smallest Rotaxane Prepared to Date

摘要

[2]Rotaxanes—supermolecules comprising interlocked mac-rocyclic and dumbbell-shaped components—are fascinating materials for the construction of molecular devices because of the machinelike movement of their constituent parts.The development of efficient, convenient, and environmentally friendly methods for the synthesis of these functional interlocked molecules has progressed tremendously in the past decade. We became interested, however, in answering the following fundamental question: What is the smallest [2]rotaxane that can be synthesized, either in terms of molecular weight or the number of constituent atoms? We identified the crown ether/secondary dialkylammonium ion pair, which can be simplified into a few repeating CH2CH2O units that encircle a threadlike component as small as a dimethylammoniurn (CH3NH_2~+CH3) ion, as the simplest and smallest recognition system for preparing [2]rotaxanes. Herein, we report a new and efficient solvent-free reaction which involves ball-milling of the [2]pseudorotaxane formed from dipropargylammonium tetrafluoroborate and the crown ether [21]crown-7 (21C7) on SiO2 with 1,2,4,5-tetrazine. This led to the isolation in high yield (81%) of the smallest [2]rotaxane reported to date (Scheme 1).