Pretzelanes with Planar Chirality and Guest Recognition Capabilities

Abstract

Developing efficient approaches to synthesize robust large-sized macrocycles with a guest recognition ability represents one of the major focuses in the field of host–guest chemistry. Here, we obtained a series of macrocycles each containing no less than 100 nonhydrogen atoms in high or close to quantitative yield by leveraging catenation as the ligation reaction in the ring closure step. Each of these macrocycles, in the form of pretzelanes, consists of a catenane core whose two interlocked macrocycles are interconnected by a bridging unit. Their pseudoquantitative yields arise from the marriage of dynamic nature of the hydrazone bonds that allows error correction and a hydrophobic effect that drives catenation. Within each pretzelane, the bridging unit breaks planar symmetry, imparting intrinsic planar chirality. When the bridge is sufficiently rigid, it restricts the intramolecular pirouetting of one macrocycle relative to the other, preventing racemization of the enantiomers. Moreover, the rigid bridging and catenane components create a preorganized cavity capable of accommodating various hydrophobic guests. When an anthracene derivative is encapsulated within the pretzelane cavity featuring a viologen bridge, the latter can quench the excited state of the guest, protecting it from UV-stimulated oxidation to 9,10-anthraquinone.