The Effects of Substituents on the Geometry of π–π Interactions

Abstract

We have designed and utilized a simple molecular recognition system to study the substituent effects in aromatic interactions. Recently, we showed that 3- and 3,5-disubstituted benzoyl leucine diethyl amides with aromatic rings of varying electronic character organized into homochiral dimers in the solid state through a parallel displaced π-π interaction and two hydrogen bonds, but no such homochiral dimerization was observed for the unsubstituted case. This phenomenon supports the hypothesis that substituents stabilize π-π interactions regardless of their electronic character. To further investigate the origin of substituent effects for π-π interactions, we synthesized and crystallized a series of 4-substituted benzoyl leucine diethyl amides. Surprisingly, only two of the 4-substituted compounds formed homochiral dimers. A comparison among the 4-substituted compounds that crystallized as homochiral dimers and their 3-substituted counterparts revealed that there are differences in regard to the geometry of the aromatic rings with respect to each other, which depend on the electronic nature and location of the substituent. The crystal structures of the homochiral dimers that showed evidence of direct, local interactions between the substituents on the aromatic rings also displayed nonequivalent dihedral angles in the individual monomers. The crystallographic data suggests that such "flexing" may be the result of the individual molecules orienting themselves to maximize the local dipole interactions on the respective aromatic rings. The results presented here can potentially have broad applicability towards the development of molecular recognition systems that involve aromatic interactions.

Exploiting the interactions of aromatic units for folding and assembly in aqueous environments

“…29-31 , Snyder et al . 32 , Lee et al . 33 , and Grimme 34 have also indicated the importance of the interactions of the substituents around the periphery of aromatic molecules.…”

Competition Between π–π and CH/π Interactions: A Comparison of the Structural and Electronic Properties of Alkoxy‐Substituted 1,8‐Bis((propyloxyphenyl)ethynyl)naphthalenes

“…Based on the prior experimental results reported by Carey et al. for 3,5‐substituted arenes,13b, c it is surprising that 8 does not adopt a geometry that is even more aligned for π‐stacking than that of 4 ; apparently, the proximity of the arenes causes C‐H/π interactions to draw the arenes out of plane and dominate the conformation of the molecule. Hence, we performed electronic structure computations to understand this difference.…”

Induction of Λ-helicity in a zinc complex with an alanine-appended aminopyridine ligand

“…In the analysis of these torsion angles, it should be borne in mind that alanine residues in BADGAR and in (1) are of opposite chirality, so the torsion angles are expected to have opposite signs. Additionally, all parameters of the stacking interactions encountered in the entire crystal structure of (1) and intradimer stacking interactions in BADGAR are collected in Table 4, with the parameters of interaction defined by Snyder and co-workers (Snyder et al, 2012b). Also, all hydrogen bonds found in the crystal structure of (1) (which include the van Staveren intradimer hydrogen bond) and the Herrick intradimer hydrogen bonds in BADGAR are collected in Table 5.…”