Control of Structural Isomerism in Noncovalent Hydrogen-Bonded Assemblies Using Peripheral Chiral Information

Abstract

The results of a systematic study of the structural isomerism in more than 30 noncovalent hydrogenbonded assemblies are described. These dynamic assemblies, composed of three calix[4]arene dimelamines and six barbiturates/cyanurates, can be present in three isomeric forms with either D 3 , C 3h , or C s symmetry. The isomeric distribution can be readily determined via a combination of 1 H NMR and 13 C NMR spectroscopy. In one case it is shown that the covalent capture of the dynamic assemblies via a ring-closing metathesis (RCM) reaction provides a novel analytical tool to distinguish between the D 3 and C 3h isomeric forms of the assembly. For the D 3 isomer the RCM results in the formation of a cyclic trimer, comprising three dimelamines, whereas for the C 3h isomer a cyclic monomer is formed. Molecular dynamics simulations in chloroform are qualitatively in agreement with the experimental data and reveal that the isomeric distribution is determined by a combination of steric, electronic, and solvation effects. A wide range of isomeric distributions covering all extremes has been found for the studied assemblies. Those with 5,5-disubstituted barbituric acid derivatives exclusively form the D 3 isomer, because steric hindrance between the barbiturate substituents prevents formation of the C 3h and C s isomers. In contrast, assemblies with isocyanuric acid derivatives exhibit increased stability of the C 3h and C s isomers upon increasing the size of the isocyanurate substituent. The outcome of the assembly process can be controlled to a large extent via chiral substituents in the calix[4]arene dimelamines, due to the preferred orientation of the chiral centers.

Chiral exciton coupling of merocyanine dyes within a well defined hydrogen-bonded assembly

“…§ The d.e. is based on comparison of ⌬298 of assembly (M)-23⅐(BAR)6 (95 l⅐mol Ϫ1 ⅐cm Ϫ1 ) with ⌬298 of structurally related quantitatively induced assemblies (95-100 l⅐mol Ϫ1 ⅐cm Ϫ1 ) (19,21). A detailed discussion, including the 1 H NMR spectrum, is provided in the supplemental data, which are published on the PNAS web site, www.pnas.org.…”

“…Evidence for the quantitative formation of assembly 1 3 ⅐(BAR) 6 was obtained from 1 H NMR spectroscopy in CDCl 3 . The spectrum shows the characteristic proton signals at 13.76 and 13.65 ppm for the hydrogen-bonded NH-protons of BAR in assembly 1 3 ⅐(BAR) 6 (19).…”

Ditopic Complexation and Release of Neutral Guest Molecules by a Hydrogen‐Bonded “Endo–Exo” Receptor

“…[17] The encapsulation is accompanied by a change in the symmetry of the host assembly, from D 3 to C 3h symmetry. [18,19] After the encapsulation of 2 a 3 in 1 3 · (DEB) 6 had been achieved (endo complexation), the role of 1 3 ·(DEB) 6 ·2 a 3 as an exo receptor was studied. Addition of three equivalents of dibenzoyl-d-tartaric acid (3 a) to the complex 1 3 ·(DEB) 6 [ 20] The stoichiometry of the complexation of 3 a by 1 3 ·(DEB) 6 ·2 a 3 is not clear from these 1 H NMR studies.…”

“…The splitting of the signals arises from the transfer of chirality from the chiral guest (3 a) to the assembly. [19] This induction of chirality is not complete, and therefore the two possible diastereomers (P and M) are formed. [25] Additional proof for the complexation and selective release of guest molecules from 1 3 ·(DEB) 6 ·2 a 3 ·3 a 3 was obtained from CD spectroscopy (Figure 3).…”

“…[25] Additional proof for the complexation and selective release of guest molecules from 1 3 ·(DEB) 6 ·2 a 3 ·3 a 3 was obtained from CD spectroscopy (Figure 3). Receptor 1 3 ·(DEB) 6 with D 3 symmetry exists as a racemic mixture of P and M enantiomers [19] and is therefore CD inactive. As mentioned earlier, the formation of the complex 1 3 ·(DEB) 6 ·2 a 3 is accompanied by a change in the symmetry of the hydrogen-bonded receptor from D 3 to C 3h .…”