Preferential Crystallization of a Helicene–Viologen Hybrid – An Efficient Method to Resolve [5]Helquat Enantiomers on a 20 g Scale

Abstract

The bistriflate salt of racemic [5]helquat 1 was found to form a conglomerate. Based on this finding, the preferential crystallization of this chiral helicene–viologen hybrid was developed to deliver pure enantiomers on a 20 g scale (10.5 g of each enantiomer). To the best of our knowledge, this is the largest amount of nonracemic helicene‐like compound obtained by preferential crystallization to date. The absolute configuration of the P enantiomer was confirmed by X‐ray crystal structure analysis. The chromatography‐free synthesis of racemic 1 on a multigram scale (30 g) is also presented as an entry point to this resolution study.

Chiroptical Redox Switching of Tetra‐Cationic Derivatives of Azoniahelicenes

“…Preparation of helquats used the synthetic roots already published . The separation of enantiomers was described in previous communications . Racemization barrier of the helquat 4 is 115.5 kJ/mol at 66 °C, which corresponds to racemization half‐life 8 h 50 min at this temperature.…”

“…Intensive research of spiral compounds dealt mainly with uncharged or di‐cationic compounds. Relatively unexplored is the field of helically chiral N‐heterohelicenia …”

Dutch Resolution of a configurationally stable [5]helquat

“…In the course of our studies of helquats and other chiral cations, we benefited from unexpectedly high success rate of racemate resolutions via diastereomeric salts with dibenzoyltartrate anions (L‐DBT ‐ in Figure ) . However, there are some racemic helquats, we have not been able to resolve with this anion .…”

Synthesis, Properties, and Two‐Dimensional Adsorption Characteristics of [6]Hexahelicene‐7‐carboxylic acid

“…Interest has also been shown in the two‐dimensional crystallization of helicenes . Multigram syntheses of helicenes remain a challenge, although in recent years synthetic methodologies to prepare larger amounts have been described . We are particularly interested in the basic hexahelicene skeleton provided with a substituent, which can be used as a handle that allows further manipulation.…”

“…Since it is known from the literature that several helicenes, including tetra‐ to nonahelicene, as well as some heterohelicenes, crystallize as conglomerates—enantiomorphic crystals in non‐centrosymmetric space groups—this could offer perspective for new methods for resolution of conglomerates . This hope was fueled by a recent publication from the group of Teplý, who screened helicenes that contain a pyridine ring to find a salt that crystallizes as a conglomerate and which might be separated into enantiomers by preferential crystallization. To make further use of the unique properties of the helicenes, it would be useful to prepare a helicene that contains a functional group, which allows further derivatization or immobilization.…”