Enantioselective Synthesis of Mechanically Planar Chiral Rotaxanes by an Asymmetric Povarov Reaction-Enabled...

Mechanically planar chiral (MPC) rotaxanes are a type of chiral mechanically interlocked molecules (MIMs) characterized by their unique chiral structures and dynamic motion properties, which have shown significant potential in various chiral science research fields. However, the exploration of efficient catalytic enantioselective methods for synthesizing these chiral molecules is still largely underexplored. We here report an efficient method for asymmetric synthesis of MPC rotaxanes through an enantioselective desymmetrization strategy. By employing the chiral phosphoric acid-catalyzed asymmetric Povarov reaction, the symmetry of prochiral rotaxanes with a rotationally symmetric macrocycle was broken, which was followed by a one-pot oxidative aromatization step to yield various MPC rotaxanes with good yields and high enantioselectivities. Moreover, in cases where the one-pot procedure did not provide satisfactory enantioselectivities, a stepwise protocol was employed to produce both enantiomers of MPC rotaxanes with excellent enantioselectivities, highlighting the practicability of this method. Extensive studies were performed to elucidate the origin of mechanical planar chirality within this method, which was postulated to arise from the synergistic interplay between the excellent enantioselectivity in generating point stereogenicity during the asymmetric Povarov reaction and the preferential selection of the mechanical geometric conformer of substrates during this process.