Three-dimensional aromaticity in an antiaromatic cyclophane

Ryo Nozawa,Dongho Kim,Jinseok Kim,Heike Fliegl,Anna Lamping,Soji Shimizu,Juwon Oh,Ichiro Hisaki,Tim Kowalczyk,Hiroshi Shinokubo,Yemei Wang

doi:10.1038/s41467-019-11467-4

Abstract

Understanding of interactions among molecules is essential to elucidate the binding of pharmaceuticals on receptors, the mechanism of protein folding and self-assembling of organic molecules. While interactions between two aromatic molecules have been examined extensively, little is known about the interactions between two antiaromatic molecules. Theoretical investigations have predicted that antiaromatic molecules should be stabilized when they stack with each other by attractive intermolecular interactions. Here, we report the synthesis of a cyclophane, in which two antiaromatic porphyrin moieties adopt a stacked face-to-face geometry with a distance shorter than the sum of the van der Waals radii of the atoms involved. The aromaticity in this cyclophane has been examined experimentally and theoretically. This cyclophane exhibits three-dimensional spatial current channels between the two subunits, which corroborates the existence of attractive interactions between two antiaromatic π-systems.

Highlights

Understanding of interactions among molecules is essential to elucidate the binding of pharmaceuticals on receptors, the mechanism of protein folding and self-assembling of organic molecules
In sharp contrast to cyclophanes composed of aromatic subunits (AR-cyclophanes), theoretical calculations have predicted that the corresponding cyclophanes containing two face-to-face antiaromatic π-systems (AN-cyclophanes) should be expected to be stabilized through mutual orbital interactions between the stacked antiaromatic subunits (Fig. 1b)[3]
The bond length alternation (BLA) in cyclophane 5 is significantly smaller than that in monomer 6, indicating the a AR-cyclophane composed of aromatic -systems

Summary

Introduction

Understanding of interactions among molecules is essential to elucidate the binding of pharmaceuticals on receptors, the mechanism of protein folding and self-assembling of organic molecules. Theoretical investigations have predicted that antiaromatic molecules should be stabilized when they stack with each other by attractive intermolecular interactions. We report the synthesis of a cyclophane, in which two antiaromatic porphyrin moieties adopt a stacked face-to-face geometry with a distance shorter than the sum of the van der Waals radii of the atoms involved. The aromaticity in this cyclophane has been examined experimentally and theoretically. This cyclophane exhibits three-dimensional spatial current channels between the two subunits, which corroborates the existence of attractive interactions between two antiaromatic π-systems. We have synthesized a tethered norcorrole dimer 2 (Fig. 2b), which predominantly adopts a stacked structure in the solid state[21], and we have discovered that the antiaromatic character of 2 significantly decreases in the stacked state

Methods

Results

Conclusion