Hydrogen Bonded Semi‐Rigidified Bispyridyl‐Incorporating Aryl Amide Oligomers: Efficient "C"‐Styled Receptors for Aliphatic Ammoniums, a Remarkable Protonation Effect and Chiral Induction

Abstract

The complexing behaviour of two linear compounds 1 and 2 toward the trifluoroacetic acid (TFA) salts of n-dodecylamine, di-n-octylamine, n-dodecyl D-and L-phenylalaninates (3, 4, and D-and L-5) in chloroform has been described. Compounds 1 and 2 consist of two folded amide moieties with two pyridyls at the terminals which are connected directly or with an acetylene linker. The rigidified folded moieties are stabilized by intramolecular hydrogen bonding, while the whole molecules can adopt an "S"-or a "C"-styled conformation depending on the relative orientation of the two rigidified moieties.1 H NMR, UV-Vis, fluorescent and circular dichroism investigations revealed that 1 and 2 could bind primary and secondary ammonium cations with their "C"-styled conformation. In the presence of 1 equiv. of TFA, the binding stability was increased significantly as a result of promoted formation of the "C"-styled conformation due to the monoprotonation of the peripheral pyridyl units in 1 and 2. In contrast, the addition of 2 equiv. of TFA substantially weakened the binding stability because the "S"-styled conformation was favored as a result of protonation of both of their pyridyl units. Moreover, remarkable induced circular dichroisms were also displayed for the complexes of 1 and 2 with chiral D-and L-5.

Molecular Recognition with Linear Molecules as Receptors

“…Therefore, they also exhibited a binding affinity toward the TFA salts of n-dodecylamine, di-n-octylamine, ndodecyl D-and L-phenylalaninates in chloroform (Fig. 38) [103]. Adding 1 equiv of TFA could further enhance the binding, with binding constants being determined from 1000 M -1 to 90000 M -1 , because the acid promoted the formation of a hydrogen bridge between the two pyridines, which facilitated the "C"-styled conformation.…”