The Entrapment of Chiral Guests with Gated Baskets: Can a Kinetic Discrimination of Enantiomers Be Governed through Gating?

Abstract

The capacity of gated hosts for controlling a kinetic discrimination between stereoisomers is yet to be understood. To conduct corresponding studies, however, one needs to develop chiral, but modular and gated hosts. Accordingly, we used computational (RI-BP86/TZVP//RI-BP86/SV(P)) and experimental (NMR/CD/UV/Vis spectroscopy) methods to examine the transfer of chirality in gated baskets. We found that placing stereocenters of the same kind at the rim (R(1) =CH3, so-called bottom) and/or top amide positions (R(2) =sec-butyl) would direct the helical arrangement of the gates into a P or M propeller-like orientation. With the assistance of (1)H NMR spectroscopy, we quantified the intrinsic (thermodynamic) and constrictive (kinetic) binding affinities of (R)- and (S)-1,2-dibromopropane 5 toward baskets (S3b/P)-2, (S3t/M)-3, and (S3bt/P)-4. Interestingly, each basket has a low ( ≤1.3 kcal mol(-1)), but comparable (de<10%) affinity for entrapping enantiomeric (R/S)-5. In terms of the kinetics, basket (S3b/P)-2, with a set of S stereocenters at the bottom and P arrangement of the gates, would capture (R)-5 at a faster rate (kin(R)/kin(S) =2.0±0.2). Basket (S3t/M)-3, with a set of S centers at the top and M arrangement of the gates, however, trapped (S)-5 at a faster rate (kin(R)/kin(S) =0.30±0.05). In light of these findings, basket (S3bt/P)-4, with a set of S stereocenters installed at both top and bottom sites along with a P disposition of the gates, was found to have a lower ability to differentiate between enantiomeric (R/S)-5 (kin(R)/kin(S) =0.8). Evidently, the two sets of stereocenters in this "hybrid" host acted concurrently, each with the opposite effect on the entrapment kinetics. Gated baskets are hereby established to be a prototype for quantifying the kinetic discrimination of enantiomers through gating and elucidating the electronic/steric effects on the process.