Guest-mediated chirality transfer in the host–guest complexes of an atropisomeric perylene bisimide cyclophane host12. Issue 7 (5th March 2019) - Sapotta, Meike
Abstract : Chirality transfer upon preferential binding of homochiral guests to one stereoisomer of a conformationally equilibrated atropisomeric cyclophane is reported. Abstract : The dynamic equilibrium of stereoisomers of a cyclophane, which is comprised of two atropisomeric perylene bisimide (PBI) subunits, and its host–guest binding with an achiral and a series of chiral guests have been studied. Temperature-dependent 1 H NMR spectroscopic studies have shown that at higher temperature all three stereoisomers (( M, M ) and ( P, P ) enantiomers and ( M, P )/( P, M ) mesomers) of the cyclophane are in a dynamic equilibrium as only one set of proton signals was observed. However, at a lower temperature (260 K) two sets of signals in a ratio of 2 : 1, instead of the theoretically expected 1 : 1 ratio, were observed for the diastereomers ( M, M )/( P, P ) and ( M, P )/( P, M ) of the cyclophane presumably due to the chiral recognition of the enantiomeric ( M, M )/( P, P ) conformers. The 1 H NMR studies further revealed that the achiral guest perylene mostly selects the ( M, M )/( P, P ) enantiomeric pair of the host for encapsulation. The induction of chirality in the host–guest complexes of the cyclophane with chiral guests has been investigated by circular dichroism (CD) spectroscopy. A distinct CD effect was observed upon addition of chiral guests to the solution of a racemic cyclophane host, indicating a pronounced preference of the chiral guests for one of the two homochiral stereoisomers, ( M, M ) and ( P, P ), for encapsulation. Kinetic studies by time-dependent NMR spectroscopy did not lead to conclusive results to assign the recognition process of chiral guests to either the induced fit or the conformational selection model.