Constitutionally Asymmetric and Chiral [2]Pseudorotaxanes1

Abstract

The self-assembly and characterization of a range of chiral pseudorotaxanes has been explored using chiroptical methods. The syntheses of constitutionally asym. acyclic hydroquinone-contg. polyethers and optically active hydroquinone-contg. acyclic polyethers, bearing a couple of Me or iso-Bu groups related to each other in a C2-sym. manner within the polyether backbone, are described. The combination of the tetracationic cyclophane cyclobis(paraquat-p-phenylene) tetrakis(hexafluorophosphate), possessing a p-electron deficient cavity, and the linear noncentrosym. acyclic polyethers produces [2]pseudorotaxanes that have been characterized by 1H NMR, UV/vis and CD spectroscopies in soln. and X-ray crystallog. in the solid state. The introduction of constitutional asymmetry or chirality gives rise to a no. of different geometries for the [2]pseudorotaxanes in both the soln. and solid states. In particular, CD-spectroscopic measurements on the optically active [2]pseudorotaxanes have shown that - depending on the positions of the C2 sym. related chiral centers in the polyether chains with respect to the hydroquinone rings - the chirality present in the p-electron rich threadlike guest can induce chirality that is assocd. with the supramol. structure as a whole, resulting in a chiral charge-transfer transition involving not only the p-donors in the chiral guests but also the p-acceptors in the achiral host.