Donor/Acceptor‐Substituted Chiral Molecular Clips – Synthesis and Host–Guest Complex Formation

Abstract

AbstractThe synthesis, separation, and characterization of some substituted stereoisomeric dimethylene‐bridged molecular clips bearing donor or acceptor groups at the tips of the naphthalene sidewalls and two acetoxy, hydroxy, or methoxy groups at the central benzene spacer unit are reported. The host–guest complex formation was studied for these substituted molecular clips as host molecules with 1,2,4,5‐tetracyanobenzene (TCNB), N‐methyl‐p‐(methoxycarbonyl)pyridinium iodide (Kosower's salt, KS), and N‐methylnicotinamideiodide (NMNA) as guest molecules. The binding constants, Ka, and the complexation‐induced 1H NMR shifts of the guest signals, Δδmax, obtained by NMR titration experiments, are compared with those reported for the parent diacetoxybenzene, hydroquinone, or dimethoxybenzene clips. The diacetoxybenzene clip, bearing donor pyrrolidinyl groups at the tips of both naphthalene sidewalls, forms the most stable complexes with TCNB and KS, overwhelming the corresponding complexes of the parent clip and the clips bearing one nitro or methoxycarbonyl group at the tip of one naphthalene sidewall. The clips bearing two acceptor groups (two nitro or methoxycarbonyl groups) at the tips of both naphthalene sidewalls do not form any complex with TCNB, KS, or NMNA within the limits of NMR detection. The large complexation‐induced 1H NMR shifts of the guest signals provide good evidence that in each complex the guest molecule is clipped between the naphthalene sidewalls of the host molecule by attractive aromatic π–π and CH–π interactions, as suggested by force‐field calculations. This structural assignment of the complexes is further confirmed by a single‐crystal structure of the KS complex of the mono‐nitro‐substituted clip, which resembles the complex structure of the parent clip with KS. The good correlation between the clip's electrostatic potential surface (EPS; calculated by DFT for the donor‐ or acceptor‐substituted molecular clips) and the host–guest complex stability confirms the assumption that in chloroform solution the host–guest binding (resulting from attractive aromatic π–π and CH–π interactions) is largely electrostatic in nature, whereas the EPS values do not correlate with the binding constants found in methanol solution, indicating that additional binding forces (resulting for example from solvophobic effects) contribute to the host–guest binding. The separated optically active diacetoxybenzene clips substituted by one or two methoxycarbonyl groups at the naphthalene sidewalls are a good starting point for future studies of chiral molecular recognition and organic catalysis.