Formation of [2]rotaxanes by encircling [20], [21] and [22]crown ethers onto the dibenzylammonium dumbbell

Abstract

The ability of a dibenzylammonium dumbbell to form [2]rotaxanes with less than [24]crown ethers has been investigated in order to correlate the spatial requirement of the ammonium moiety in the dibenzylammonium ion with the size of the encircling crown ether. The investigation hinges on template-directed ring-closing olefin metathesis using 2nd generation Grubbs’ catalyst, known as the clipping approach. A series of acyclic diolefin polyethers were independently subjected to the ring-closing metathesis in the presence of dibenzylammonium hexafluorophosphate. In this study, we obtained nine threaded molecules involving dibenzylammonium ion and crown ethers having less than 24 atoms. Three of the nine threaded molecules exhibited pseudorotaxane character; all the three pseudorotaxanes generated were 1 : 1 complexes between dibenzylammonium ion and three different [23]crown ethers, with reasonably high association constant values. The remaining six threaded molecules incorporating unsaturated and saturated [20], [21] and [22]crown ethers onto the ammonium moiety of the dibenzylammonium ion were [2]rotaxanes, where the benzene rings acted as stoppers. The smallest crown ether encompassing the dibenzylammonium dumbbell was found to be an unsaturated [20]crown ether. These threaded and interlocked species are well defined and well characterized by ¹H NMR, ¹³C NMR and HRMS. In addition, single crystals suitable for crystallographic analysis were obtained and characterized, not only confirming the threaded structures but also substantiating our claim.