Mapping the Binding Motifs of Deprotonated Monounsaturated Fatty Acids and Their Corresponding Methyl Esters within Supramolecular Capsules

Abstract

A suite of NMR techniques revealed that a cavitand (1) formed 2:1 host-guest complexes with a range of monounsaturated fatty carboxylates and their corresponding methyl esters. All of the carboxylates bound to the capsule in a J-shaped motif with the carboxylate at the equatorial region of the dimeric capsule, and the reverse turn of the chain and the methyl terminal in each polar region of the host. Guest exchange was slow on the NMR time scale, while tumbling was slow or close to the NMR time scale depending on the position and stereochemistry of the double bond. In contrast, the methyl esters were found to bind in three motifs depending on the position and stereochemistry of the double bond. Thus, the esters were observed to bind in a J-shaped, U-shaped (the turn in the guest occupying a polar region and the two termini competing for occupancy of the other pole), or a reverse J-shaped motif (ester moiety and turn each occupying a pole and the methyl terminal located near the equator). Relative binding constant (K) determinations revealed that the affinity for the capsule was dependent on the position and stereochemistry of the double bond.

Supramolecular Strategies for Controlling Reactivity within Confined Nanospaces

“…As the packing is more compressed, coiled helical conformations with more contact to the walls of hosts are found to predominate . For long chain alkanes or fatty acids, bent binding motifs (unlikely high‐energy conformations with a reverse turn located within the chain) are often observed when they are sequestrated . This can result in close proximity of terminal reactive functional groups through an enforced orientation of the guest.…”

Confined molecules: experiment meets theory in small spaces

“…The water-soluble capsules discovered by Gibb (Jordan and Gibb, 2015), who mapped the contortions of guests in his capsules (Wang and Gibb, 2017) and applied them as protecting groups (Liu et al ., 2010). Atoms buried deep in the cavitand are hidden from reagents while atoms near the middle of capsules can be exposed to reagents in the solvent.…”

Relative hydrophilicities of cis and trans formamides

“…The competition of the cis,trans isomers by this method is direct since both are present in the same solution and, in the case of the diformamides, in the same molecule. The shape and volume of the cavity can impose selectivity of its own (22), but the cis and trans formamides isomers are not expected to differ in size and shape enough to bias the competition. The possibility of hydrogen-bonded, dimeric structures exists in media of low polarity, but the competitive aqueous solvent (outside) and the limited size of the cavitand (inside) prevents selfassociation of these guests.…”