A nitrogen-based chiral catenane for enantioenriching photocatalytic aerobic oxidation

Abstract

Tröger's base analogs (TBAs) present a pair of uniquely bent tetracyclic scaffolds that possess two stereogenic amine groups with identical handedness. This work employs TBAs as chiral building blocks in the design of electron-rich (electron-donating) macrocycles, namely enantiopure Trögerophanes. These chiral donor macrocycles are then mechanically interlocked with an achiral electron-deficient (electron-acceptor) macrocycle—namely, cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺)—that carries four positive charges. These electron donor and acceptor components are individually colorless but once mechanically interlocked to one another they generate a charge-transfer (CT) absorption band centered at around 525 nm turning the resulting chiral 2-catenanes purple in color. Strikingly, the circular dichroism spectra of the two opposing enantiomers of this 2-catenane show mirroring patterns and a Cotton effect in the region that corresponds to the CT mainly from 445 to 625 nm. This phenomenon attests to the transfer of chirality from the chiral donor Trögerophane to the achiral acceptor CBPQT⁴⁺ macrocycle and the amplification of chiroptical properties of the interlocked molecule whose structure is almost entirely rigid. Moreover, this work demonstrates the discriminatory interactions of the nitrogen-based stereocenters of Tröger's base with chiral acids. This discriminatory property enables the investigated chiral 2-catenane showing a moderate enantioenrichment while interacting with an amino acid and oxidizing it in a competitive photocatalytic aerobic oxidation process fueled by the energy of visible light absorbed within the CT region.