Dynamic clipping strategy to synthesize [2]rotaxanes consisting of borate ion-containing crown ether and ammonium ions with kinetic and thermodynamic control

Abstract

Herein, we report a dynamic clipping approach to synthesize (pseudo)[2]rotaxanes. A reversible borate-forming reaction between bis-catechol 1 and boric acid or trimethyl borate in the presence of secondary amines 2 afforded the desired borate ion-containing crown ether and ammonium ion-type [2]rotaxanes in good yields. Nuclear magnetic resonance (NMR) spectroscopy was used to monitor the decomplexation of these (pseudo)[2]rotaxanes, and the results revealed that bulky cyclohexyl groups prevented dethreading of the 23-membered borate crown ether at room temperature but that phenyl groups were not sufficiently bulky. An axle component of [2]rotaxane was exchanged in the presence of a secondary amine, which is an axle component precursor, via a reversible borate-forming reaction. Additionally, in competitive rotaxane-forming reactions involving two amines, acidic ammonium ions from the less basic amine served as an effective template for [2]rotaxane formation under kinetic conditions, and the [2]rotaxane formed from the less acidic ammonium ion was thermodynamically more stable than that formed from the more acidic ammonium ion because of the nature of stabilities of the ammonium ions.