Enantioselective Synthesis of 3-Aryl-3-hydroxypropanoic Esters as Subunits for Chiral Liquid Crystals

Abstract

Chiral liquid crystals (LCs) with their unique optical and mechanical properties are perspective functional soft materials for fundamental science and advanced technological applications. Herein, we introduce the chiral 3-aryl-3-hydroxypropanoic ester moiety as a versatile building block for the preparation of LC compounds. Three chiral subunits differing in the aromatic part were obtained through asymmetric transfer hydrogenation using Ru(II) complexes with ee from 98% to >99%. Chiral LC compounds of diverse topologies were further prepared without deterioration of the ee during the synthesis. The mesomorphic behavior of rod-shaped, bent-shaped flexible dimeric, and polycatenar LCs is consistent with their topology�chiral nematic and smectic phases were identified as well as the rarely observed twist grain boundary A and blue phases. The utilization of synthetic chiral building blocks offers the possibility of fine tuning the intermolecular interactions by subtle changes in the molecular structure as well as the preparation of the corresponding racemic forms. This paves the way for the study of self-organization and the structure−property relationship in chiral soft materials.

The Interplay of Spacer Chirality and Parity in Mesogenic Dimers

“…The targeted molecules were prepared following the convergent synthetic route described in Scheme 1. The synthesis of the starting compound 1 is described in our previous work [41] . Compounds 3 a – f were obtained via esterification of acid 1 and the corresponding alcohols 2 a – f in a very good yield (58 %–98 %).…”

“…The synthesis of the starting compound 1 is described in our previous work. [41] Compounds 3 a-f were obtained via esterification of acid 1 and the corresponding alcohols 2 a-f in a very good yield (58 %-98 %). Deprotection of the benzylic protecting group of compounds 3 af gave the intermediate diols 4 a-f. Esterification of compounds 4 a-f with 4-butoxybenzoic acid provided molecules 5 a-f in 31 %-96 % yields.…”

“…This building block allowed the preparation of LC molecules of various topologies, both in racemic and enantiomerically pure forms. [41] Encouraged by this result, we design a novel series of dimers using 3-phenyl-3-hydroxypropanoic acid that features a chiral center as part of their flexible spacer. The chiral 3-phenyl-3hydroxypropanoic acid subunit has the hydroxyl group at the chiral center, which is less spacious than the methyl group, while the carboxylic group allows upgrading into dimeric molecules with different spacer lengths (Figure 1).…”

“…In a previous report, we described the synthesis of a versatile building block, 3‐aryl‐3‐hydroxypropanoic ester subunit. This building block allowed the preparation of LC molecules of various topologies, both in racemic and enantiomerically pure forms [41] . Encouraged by this result, we design a novel series of dimers using 3‐phenyl‐3‐hydroxypropanoic acid that features a chiral center as part of their flexible spacer.…”