Crystallization of Chiral para-n-Alkylphenyl Glycerol Ethers: Phase Diversity and Impressive Predominance of Homochiral Guaifenesin-Like Supramolecular Motif

Abstract

When studying the crystallization in a monotonously varying series of chiral glycerol ethers para-Alk-C 6 H 4 -OCH 2 CH(OH)CH 2 OH [Alk = Me (1), Et (2), n-Pr (3), and n-Bu (4)] it was established that all except one member form stable crystalline homo-or heterochiral α-phase, depending on the enantiomeric composition (racemic or close to enantiopure) of the feed medium. Propyl derivative 3 falls out of the series and invariably forms stable homochiral crystals. Regardless of the enantiomeric composition, the last two members of the set can form a stable (4) or metastable (3) liquid-crystalline β-phase. For the entire set, regardless of enantiomeric composition, the formation of metastable γphase was registered in supercooled melts; according to a number of evidence, the phase is close to an ideal solid solution of enantiomers. It is shown by X-ray diffraction analysis that in crystals of 1−4 samples with high enantiomeric purity, as well as in racemic 3 and 4 samples, a common homochiral guaifenesinlike supramolecular motif is realized. Some considerations are given on the reasons for this behavior.

Chirality, Gelation Ability and Crystal Structure: Together or Apart? Alkyl Phenyl Ethers of Glycerol as Simple LMWGs

“…A sophisticated way of constructing it with the participation of all four independent molecules present in the asymmetric unit is illustrated in Figure 5 in Ref. [39]. In Table 1, this motif is designated as BL-4.…”

“…In 22 samples of compounds 1-14 studied by single crystal X-ray diffraction, seven different types of supramolecular crystal-forming motifs were realized, including four bilayer ones, of which only two were realized in the crystals of supramolecular gelators. The bilayer motifs BL-3 and BL-4, realized in rac-1, rac-13 and rac-2 crystals, were described in detail by us earlier [35,39]. Being organized by glide planes, which is typical for racemic crystals, such bilayers are linked by a two-dimensional network of intermolecular hydrogen bonds •••O-H•••O-H•••, which gives them a rigid character.…”

“…However, it is unlikely that the driving forces responsible for the formation of both mesophases are of the same nature. Otherwise, it is not clear why, in the melts of compounds 3-9, shown in Scheme 1, liquid crystal phases are registered, which have similar properties for both enantiopure and racemic samples [38,39]. That said, supramolecular gelators in this set are only enantiopure samples of ethers 4-7.…”

“…Enantiopure diols (R)-2-9 and (R)-14 were obtained from (R)-3-chloropropane-1,2-diol (98% ee) and the appropriately substituted phenol by analogy to the published procedures for the racemic samples [38]. The single crystals of the compounds (R)-2-4 were investigated by X-ray diffraction by us earlier [39]. The single crystals of the compounds (R)-5-9 and rac-14, first investigated by X-ray diffraction in this paper, were prepared by slow evaporation of solutions of the corresponding samples in a mixture of n-hexane and ethyl acetate (8:2).…”

Recognition in the Domain of Molecular Chirality: From Noncovalent Interactions to Separation of Enantiomers

“…In some cases, a condition for conglomerate formation may be the growth of homochiral motifs such as chains or helices. 891 However, this feature not always leads to spontaneous resolution. 892 Recently, Gerasimova et al reported a doubly enantiophobic behavior during the crystallization of rac-1-benzyl-3-chloro-4-[(4-chlorophenyl)sulfanyl]-5-hydroxy-1,5-dihydro-2H-pyrrol-2-one (53), observing the formation of two conglomerates with different crystal structures (Figure 73).…”