A Homochiral Poly(2-oxazoline)-based Membrane for Efficient Enantioselective Separation

Chiral separation membranes have shown great potential for the efficient separation of racemic mixtures into enantiopure components for many applications, such as in the food and pharmaceutical industries; however, scalable fabrication of membranes with both high enantioselectivity and flux remains a challenge. Herein, enantiopure S-poly(2,4-dimethyl-2-oxazoline) (S-PdMeOx) macromonomers were synthesized and used to prepare a new type of enantioselective membrane consisting of a chiral S-PdMeOx network scaffolded by graphene oxide (GO) nanosheets. The S-PdMeOx-based membrane showed a near-quantitative enantiomeric excess (ee) (98.3±1.7 %) of S-(−)-limonene over R-(+)-limonene and a flux of 0.32 mmol m⁻² h⁻¹. This work demonstrates the potential of homochiral poly(2,4-disubstituted-2-oxazoline)s in chiral discrimination and provides a new route to the development of highly efficient enantioselective membranes using synthetic homochiral polymer networks.     

Chiral recognition in self-complexes of tetrahydroimidazo[4,5-d]imidazole derivatives: from dimers to heptamers

The chiral discrimination in the self-association of chiral 1,3a,4,6a-tetrahydroimidazo[4,5-d]imidazoles has been studied using density functional theory methods. Clusters from dimers to heptamers have been considered. The heterochiral dimers (RR:SS or SS:RR) are more stable than the homochiral ones (RR:RR or SS:SS) with energy differences up to 17.5 kJ/mol. Besides, in larger clusters the presence of two adjacent homochiral molecules impose an energetic penalty when compared to alternated chiral systems (RR:SS:RR:SS...). The differences in interaction energy within the dimers of the different derivatives have been analyzed based on the atomic energy partition carried out within the atoms in molecules framework. The mechanism of proton transfer in the homo- and heterochiral dimers shows large transition-state barriers except in those cases in which a third additional molecule is involved in the transfer. The optical rotatory power of several clusters of the parent compound have been calculated and rationalized based on the number of homochiral interactions and the number of monomers of each enantiomer within the complexes.     

Controlled generation of acentric and homochiral coordination compounds from a versatile asymmetric ligand 4-(1H-1,2,4-triazol-3-yl)-4H-1,2,4-triazole

Luminescent acentric and homochiral coordination compounds, which possess new topological networks, multiple helical structures and NLO properties, have been generated through the control of a versatile asymmetric ligand as well as inorganic anions, and this suggests a new protocol for preparing acentric and chiral materials.     

Homochiral Zn and Cd coordination polymers containing amino acid-tetrazole ligands

The reactions of (S)-3-cyanophenylalanine with NaN(3) in the presence of H(2)O and Lewis acids (ZnCl(2) and CdCl(2)) afford two unprecedented 3D homochiral networks, mono[(S)-5-(3-tetrazoyl)-phenylalaninato]zinc(II) (1) and mono[(S)-5-(3-tetrazoyl)-phenylalaninato]cadmium(II) monoaqua(II) (2), respectively. The two compounds are isostructural with noninterpenetrated SrAl(2) topology. The structure of these coordination polymers gives new insight into Sharpless' reaction of chiral 5-substituted 1H-tetrazole and homochiral supramolecular array constructions.     

Synthesis of complementary double-stranded helical oligomers through chiral and achiral amidinium-carboxylate salt bridges and chiral amplification in their double-helix formation

A series of complementary molecular strands from 2-mer to 5-mer that are composed of m-terphenyl units bearing chiral/achiral amidine or achiral carboxyl groups linked via Pt(II) acetylide complexes were synthesized by sequential stepwise reactions, and their chiroptical properties on the double-helix formation were investigated by circular dichroism (CD) and (1)H NMR spectroscopies. In CHCl(3), the "all-chiral" amidine strands consisting of (R)- or (S)-amidine units formed preferred-handed double helices with the complementary achiral carboxylic acid strands through the amidinium-carboxylate salt bridges, resulting in characteristic induced CDs in the Pt(II) acetylide complex regions, indicating that the chiral substituents on the amidine units biased a helical sense preference. The Cotton effect patterns and intensities were highly dependent on the molecular lengths. The complementary double-helix formation was also explored using the chiral/achiral amidine strands with different sequences in which a chiral amidine unit was introduced at the center (center-chiral) or a terminus (edge-chiral) of the amidine strands. The effect of the sequences of the chiral and achiral amidine units on the amplification of chirality (the "sergeants and soldiers" effect) in the double-helix formation was investigated by comparing the CD intensities with those of the corresponding all-chiral amidine double helices with the same molecular lengths. Variable-temperature CD experiments of the all-chiral and chiral/achiral amidine duplexes demonstrated that the Pt(II)-linked complementary duplexes are dynamic and their chiroptical properties including the chirality transfer from the chiral amidine unit to the achiral amidine ones are significantly affected by the molecular lengths, sequences, and temperatures. On the basis of the above results together with molecular dynamics simulation results, key structural features of the Pt(II)-linked oligomer duplexes and the effect of the chiral/achiral amidine sequences on the amplification of chirality are discussed.     

Spirobicyclic diamines. Part 3: Synthesis and metal complexation of proline-derived [4.4]-spirodiamines

The syntheses of racemic and homochiral [4.4]-spirolactams, starting from l-proline, in good yields are described. Reduction of the lactam carbonyl group of spirolactams, containing chiral substituents on the lactam nitrogen, with lithium aluminium hydride, gives a series of homochiral [4.4]-spirodiamines. The crystal structure of one of these spirodiamines on complexation with zinc chloride was obtained. Interestingly it showed a hydrogen-bonded dimeric structure, where the monomers are diastereoisomeric diamines.     

Configurational rules governing the stereochemical control of chiral initiators in ring-opening polymerizations of heterocyclic monomers

Configuration rules were established for the polymerization of cyclic monomers with chiral initiators. The latter recognize in the monomer an asymmetric carbon of homochiral configuration. The chirality of the initiator is defined by the “local” asymmetry of the ligand and by the overall asymmetry of the particular arrangement of catalytic aggregates. Two types of process called “homosteric” and “antisteric,” which apply to the polymerization of resolvable monomers and achiral monomers of meso structure, are defined. These configurational rules have a general character and are verified in many examples of differently substituted three- or four-membered cyclic monomers (oxiranes, thiiranes, β-lactones). Stereoelective polymerization appears to be an original method for the determination of the absolute configuration of some monomers of unknown chirality.     

Spontaneous Symmetry Breaking in the Formation of Supramolecular Polymers: Implications for the Origin of Biological Homochirality

Aqueous solutions of the achiral, monomeric, nucleobase mimics (2,4,6‐triaminopyrimidine, TAP, and a cyanuric acid derivative, CyCo6) spontaneously assemble into macroscopic homochiral domains of supramolecular polymers. These assemblies exhibit a high degree of chiral amplification. Addition of a small quantity of one handedness of a chiral derivative of CyCo6 generates exclusively homochiral structures. This system exhibits the highest reported degree of chiral amplification for dynamic helical polymers or supramolecular helices. Significantly, homochiral polymers comprised of hexameric rosettes with structural features that resemble nucleic acids are formed from mixtures of cyanuric acid (Cy) and ribonucleotides (l‐, d ‐pTARC) that arise spontaneously from the reaction of TAP with the sugars. These findings support the hypothesis that nucleic acid homochirality was a result of symmetry breaking at the supramolecular polymer level.     

Twisted perylene stereodimers reveal chiral molecular assembly codes

Unique perylene diastereomeric linear and cyclic dimers were synthesized from twisted perylene monomers, revealing that pi-stacking stereoisomerism imparted specific intermolecular self-assembly and intramolecular folding. Only the homochiral twisted tetrachloroperylene monomers cyclized via a cooperative reaction, forming the homochiral diastereomers. The heterochiral tetrachloroperylene monomers proceeded through a stepwise reaction and yielded a linear heterochiral dimer, which equilibrated with the linear homochiral dimers. The linear homochiral dimers cyclized to produce the same cyclic homochiral diastereomers. These results demonstrated that homochiral and heterochiral self-assemblies were two distinct molecular codes, directing two specific chemical pathways. The homochiral cyclic dimers remain isomerically pure at -20 degrees C but can be interconverted to the heterochiral cyclic dimer meso compound at room temperature. The diastereomers were readily separated by HPLC. While driven by solvophobic forces, foldable linear dimers synthesized from the same twisted monomers using phosphoramidite chemistry folded into homodimer and heterodimer, confirming the inherent molecular codes, which were dictated by the perylene chirality, ultimately gauged the weak pi-stack forces, and directed self-assembly and folding.     

手性催化剂负载的一种新方法: 自负载策略

较系统地介绍了手性催化剂负载的一种新方法(即“自负载”策略)及其在非均相不对称催化反应中应用的最新进展. 与传统的负载模式不同, “自负载”策略中利用含双或多官能团的配体与金属通过自组装形成的有机-无机聚合物做为催化剂, 因此不需使用任何载体. “自负载”手性催化剂在若干非均相不对称催化反应中显示了优秀的催化活性和对映选择性并且能够简单回收再利用, 为手性催化剂的负载化提供了一个新的策略.     

Broadband reflection in polymer stabilized cholesteric liquid crystal cells with chiral monomers derived from cholesterol

In this study, a series of achiral monomers and chiral monomers of different flexible spacer chains based on cholesteryl moiety were synthesized. Polymer stabilized cholesteric liquid crystal (PSCLC) cells were then created by incorporation of the polymer networks. The influence of the nature of the monomers and the spacer length of chiral monomers on the reflectance properties of PSCLC was investigated as well as the polymerization condition. The results strongly suggest that the chirality of the polymer networks plays an integral role in the observed reflection spectra, and the chiral polymer networks with chiral centers separated well from the polymer backbone induce a greater change in the bulk helix pitch, and produce the broader reflection band in these LC composites. In addition, the temperature dependence of the pitch of the composites before and after polymerization was investigated. To broaden the reflection band further, the experimental processes of thermally induced pitch variation simultaneously with a UV crosslinking reaction of the composites were presented. The morphology of the polymer network in the composites was studied by scanning electron microscopy (SEM). Copyright © 2008 John Wiley & Sons, Ltd.     

Manipulating the twist sense of helical nanofilaments of bent-core liquid crystals using rod-shaped,chiral mesogenic dopants

In some liquid crystal (LC) mixtures of bent-core host molecules that form helical nanofilaments (HNFs) and chiral, rod-shaped molecular guests, the spontaneous chirality of the HNFs is not influenced by the guest handedness. In other mixtures, the filaments become homochiral, responding to the handedness of the guest. We show that the important distinction between these two behaviours is the solubility of the guest material in the HNF phase. In our experiments, chiral LC mesogens doped into the HNF phase result in an enantiomeric imbalance and sometimes change the phase sequence on cooling from the isotropic melt.     

Reversing the Handedness of Self‐Assembled Porous Molecular Networks through the Number of Identical Chiral Centres

Scanning tunnelling microscope observations at the 1‐phenyloctane/graphite interface reveal how chiral structural information at the molecular level is transferred and expressed structurally at the 2D supramolecular level for a porous system. The chirality of self‐assembled molecular networks formed by chiral dehydrobenzo[12]annulene (cDBA) derivatives having three chiral chains and three achiral chains, alternatingly, is compared with those of cDBAs having six chiral chains reported previously. While for all cDBAs homochiral surfaces are formed, their handedness is not simply a reflection of the absolute configuration of the stereogenic centres. Both the number of stereogenic centres as well as the length of the achiral chains determine the supramolecular handedness, providing a deep insight into the supramolecular chirality induction mechanisms at play. Moreover, these cDBAs act to induce chirality in porous networks formed by achiral DBAs.     

Chiral recognition in diaziridine clusters and the problem of racemization waves

Theoretical calculations (B3LYP/6-31+G**) of chiral clusters of diaziridines have been carried out. Five configurations of chiral and nonchiral clusters with up to eight monomers have been considered. The proton transfer within the neutral and protonated clusters has been studied as a possible source of racemization waves. The optical rotatory power (ORP) has been calculated for the neutral and protonated homochiral clusters. The results show that the clusters with alternated chiral molecules are the preferred ones and that the proton transfer proceeds with low energetic barriers in the protonated systems. The ORP results are very dependent on the shape of the clusters and the neutral or protonated state of them.     

手性金属-有机骨架[Cu_3(HL)_2(L)_2(bpy)_3]·4H_2O用于高效液相色谱拆分手性化合物

手性金属-有机骨架材料(MOFs)作为一种新型多孔材料,由于具有比表面积大、结构多样、孔尺寸可调和化学稳定性良好等特点,而备受色谱分离领域的关注。该文以(1R,2R)-1,2-环己烷二甲酸(H2L)和4,4'-联吡啶(bpy)为配体与铜离子反应,通过溶剂热法合成了一种具有二维手性网状结构的手性MOF[Cu_3(HL)_2(L)_2(bpy)_3]·4H_2O。将该手性MOF作为手性固定相制备了高效液相色谱柱。为了考察MOF[Cu_3(HL)_2(L)_2(bpy)_3]·4H_2O的手性识别能力,在正相色谱(流动相:正己烷-异丙醇)条件下,对一系列外消旋化合物在手性MOF柱上进行了拆分。该手性MOF柱对醇类、酮类、酸类、环氧化合物和醚类等10种手性化合物表现出较好的拆分效果。对该手性MOF柱的重现性、稳定性作了评价,考察了进样量对分离效果的影响,结果表明该手性MOF柱具有较好的重现性和稳定性。     

Competitive H-bonding in bicyclic bis-lactams: self-assembly in nanotubes or in unbalanced chiral threefold interpenetrated diamondoid network

The first example of an H-bonded diamondoid network formed by a chiral molecule is reported. Diamondoid networks in the crystal structure of the described bicyclic bis-lactam are homochiral and threefold interpenetrated, two of them being of one chirality and the third of the other. Thus, imbalanced chiral packing with ee 33% is observed. A slight change in the structural formula leads to an entirely different crystal structure with nanosize channels.     

Novel glass-forming liquid crystals. II. Systems containing 1-phenyl-2-(6-cyanonaphth-2-yl)ethyne as a high optical birefringence moiety

A new series of glass-forming molecular materials have been synthesized that incorporate 1-phenyl-2-(6-cyanonaphth-2-yl)ethyne as the high optical birefringence nematogenic moiety and a diphenylacetylenic group as part of the chiral moiety containing (S)-(-)-1-phenylethanol. To facilitate the preparation of chiral nematic systems, the key intermediate, i.e. 1,3,5-cyclohex-anetricarboxylic anhydride chloride, was synthesized and characterized. The thermotropic and optical properties of all products and the blends prepared therefrom were characterized by polarizing optical microscopy, differential scanning calorimetry, and UV-VIS-NIR spectrophotometry. It was found that all the resultant molecular materials readily form glasses with a T_g of around 60°C while showing no residual crystallinity below T_g or tendency toward crystallization by heating above T_g. Furthermore, binary blends showed a decreasing degree of miscibility at an increasing extent of structural dissimilarity with the all-chiral/all-nematic systems exhibiting significant phase separation at a chiral mole fraction of 0.16.     

Chiral Amplification of Oligopeptides in the Polymerization of α‐Amino Acid N‐Carboxyanhydrides in Water

This article is concerned with the chiral amplification of oligopeptides formed in the polymerization of chiral, nonracemic mixtures of the N‐carboxyanhydride (NCA) of Leu and Glu in aqueous solution. Labeling (deuteration) of one enantiomer and reversed‐phase and normal‐phase high‐performance liquid chromatography mass spectrometry (RP‐ and NP‐HPLC/MS, respectively) were used to determine the product distribution, both with respect to oligopeptide chain length and stereoisomer distribution. Starting the polymerization with an enantiomeric excess (ee) of 20% of the L ‐enantiomer (L ‐amino acid/D ‐amino acid 6 : 4) leads to an ee of 73% at the level of the homochiral enantiomeric (Leu)₅, and of 71% at the level of the homochiral enantiomeric (Glu)₇. For the Leu system and in the presence of a solid support (quartz), the ee reached values of up to 100%. We argue that such amplification processes could be relevant for the chemical evolution towards single‐handedness.     

Homo- and heterochiral alkylzinc fencholates: linear or nonlinear effects in dialkylzinc additions to benzaldehyde

Scalemic mixtures of chiral anisyl fenchols with different ortho-substituents (X) in the anisyl moieties [X = H (1), Me (2), SiMe3 (3) and tBu (4)] are employed as pre-catalysts in enantioselective additions of diethylzinc to benzaldehyde. While a remarkable asymmetric depletion is apparent for X = H and Me, a linear relationship between the enantiomeric purity of the chiral source and the product 1-phenylpropanol is observed for X = SiMe3 and tBu. X-ray single crystal analyses show that racemic methylzinc fencholates obtained from 1 (X = H) and 2 (X = Me) yield homochiral dimeric complexes, while for 3 (X = SiMe3) and 4 (X = tBu) the heterochiral dimeric alkylzinc structures are formed. The enantiopure fenchols 1-4 all yield homochiral dimeric methylzinc complexes. Computed relative energies of homo- and heterochiral fencholate dimers with X = H and Me reveal an intrinsic preference for the formation of the homochiral dimers, consistent with the observed negative NLE. In contrast, similar stabilities are computed for homo- and heterochiral complexes of ligands 3 (X = SiMe3) and 4 (X = tBu), in agreement with the absence of a nonlinear effect for bulky ortho-subsituents.     

Chirality-controlled and solvent-templated catenation isomerism in metal-organic frameworks

A family of highly porous homochiral, racemic, and meso metal-organic frameworks (MOFs) were synthesized based on a new elongated tetra-carboxylate ligand and the copper paddle-wheel building units. These MOFs exhibited remarkable catenation isomerism that is controlled by both chirality of the bridging ligand and the size of solvent molecules. The ability to manipulate framework interpenetration is key to future synthesis of mesoporous homochiral MOFs which hold great promise in heterogeneous asymmetric catalysis and chiral separations.