Deracemization of Carbohelicenes by a Chiral Perylene Bisimide Cyclophane Template Catalyst

Manuel Weh,Ana‐Maria Krause,Jessica Rühe,Benedikt Herbert,Frank Würthner

doi:10.1002/ange.202104591

Abstract

AbstractDeracemization describes the conversion of a racemic mixture of a chiral molecule into an enantioenriched mixture or an enantiopure compound without structural modifications. Herein, we report an inherently chiral perylene bisimide (PBI) cyclophane whose chiral pocket is capable of transforming a racemic mixture of [5]‐helicene into an enantioenriched mixture with an enantiomeric excess of 66 %. UV/Vis and fluorescence titration studies reveal this cyclophane host composed of two helically twisted PBI dyes has high binding affinities for the respective homochiral carbohelicene guests, with outstanding binding constants of up to 3.9×1010 m−1 for [4]‐helicene. 2D NMR studies and single‐crystal X‐ray analysis demonstrate that the observed strong and enantioselective binding of homochiral carbohelicenes and the successful template‐catalyzed deracemization of [5]‐helicene can be explained by the enzyme‐like perfect shape complementarity of the macrocyclic supramolecular host.

Highlights

The field of supramolecular chemistry started with studies on macrocyclic receptors for the molecular recognition of cations and neutral molecules.[1,2] For the latter, cyclophanes in particular enjoyed, and continue to enjoy, great popularity because they can provide suitable binding pockets to surround guest molecules and afford both binding strength and selectivity by shape complementarity.[3]
To the best of our knowledge, despite the long history of research on the deracemization of organic molecules[11] by dynamic kinetic resolutions via inclusion complexes[12] and chromatography on chiral phases,[13] deracemization by a templating chiral cyclophane host has not yet been demonstrated. Toward this goal we considered our recently introduced perylene bisimide (PBI) based cyclophane hosts as promising candidates
Similar to previously described examples,[8,9] chirality transfer from chiral guest molecules to achiral PBI cyclophanes could be observed by CD spectroscopy.[17]

Summary

Introduction

The field of supramolecular chemistry started with studies on macrocyclic receptors for the molecular recognition of cations and neutral molecules.[1,2] For the latter, cyclophanes in particular enjoyed, and continue to enjoy, great popularity because they can provide suitable binding pockets to surround guest molecules and afford both binding strength and selectivity by shape complementarity.[3]. Similar titration experiments for 1-MM (and 1-PP) with phenanthrene and [4]-helicene afforded averaged binding constants of Ka = 2.3 104 mÀ1 and Ka = 2.7 106 mÀ1 in chloroform at 22 8C, respectively (Figures S29–S32).

Results

Conclusion