Controlling Dynamic Stereoisomerism in Gated Molecular Baskets.

摘要

Our research group is focused on developing novel cavitands – gated molecular baskets and understanding their mechanism of action. These dynamic hosts comprise a bowl-shaped platform with a set of three aromatic gates revolving at the rim of the cavity and thereby controlling the encapsulation kinetics. We have shown that gated baskets are capable of promoting selective entrapment of guests on the basis of their size and affinity to populate the inner space, promoting conformational changes via stabilization of a transition state, and catalyzing chemical reactions. We have foreseen an opportunity for developing a family of chiral and gated C3 symmetric hosts to explore the relationship between “static” and “dynamic” chirality as well as chiral guest encapsulation. We designed three families of chiral cavitands to address these interesting problems.;Molecular baskets can fold to enclose the cavity in two ways; using intramolecular hydrogen bonding or ligand to metal chelation. We initially installed a chiral center (R or S) at the “hinge” position in pyridine-based gated baskets that fold by chelation to Ag(I) cation and examined the effect of such “static” chirality on adjacent pyridine gates arranged in either right (P) or left-handed (M) helical sense. Importantly, we found that there was an effective transfer of chirality in this particular environment, with R chiral center enforcing M-like orientation of the gates, confirmed by 1H NMR, CD and computational studies. Is the effect predictable and useful for designing other functional hosts? In order to determine if this effect is predictable and useful for designing other functional hosts, we investigated the coordination of another series of chiral baskets with quinoline-based gates to Cu(I)/Cu(II) cations. Importantly, the anticipated transfer of chirality was also established in this particular system with a, surprisingly, C3 symmetric basket placing the Cu(II) cation in a square-pyramidal ligand field. Finally, we initiated a study with chiral baskets capable of forming a seam of intramolecular hydrogen bonds at the rim. In this study we developed a series of baskets that have chirality introduced as part of the “hinge”, as part of the amido groups for establishing the hydrogen bonding, or both, chirality introduced on “hinges” as well as part of the amido functionality. Our results suggested that these baskets arrange their gates in a chiral fashion and would possibly allow the resolution of enantiomeric guests. We found that the stereogenic center of the same kind (S) would direct the helical arrangement in the opposite directions when installed at different positions in a gated molecular basket.