Chiral Building Units Research Articles

High-Fidelity Supramolecular Chirality Transportation Enabled Through Chalcogen Bonding.

The formation of asymmetric microenvironments in proteins benefits from precise transportation of chirality across multiple levels through weak bonds in the folding and assembly process, which inspires the rational design and fabrication of artificial chiral materials. Herein, the chalcogen bonding-directed precise transportation of supramolecular chirality toward multiple levels is reported to aid the fabrication of chiroptical materials. Benzochalcogenadiazole (O, S, Se) motifs are conjugated to amino acid residues, and the solid-state assemblies afforded selective supramolecular chirality with handedness depending on the kinds of chalcogen atoms and amino acids. The chalcogen-N sequence assisted by hydrogen bonding synergistically allows for the complexation with pyrene conjugated aryl carboxylic acids to give macroscopic helical structures with active circular dichroism and circularly polarized luminescence, of which handedness is precisely determined by the pristine chiral species. Then the further application of chiral benzochalcogenadiazole motifs as seeds in directing handedness of benzamide via symmetry breaking is realized. Behaving as the dopants, embedding into the matrix of benzophenone induces the room temperature phosphorescence, whereby the thermal chiroptical switch is fabricated with color-tunable phosphorescent circularly polarized luminescence. This work utilized benzochalcogenadiazole-based chiral building units to accomplish precise transportation of supramolecular chirality in coassemblies with high-fidelity, achieving flexible manipulation of chiroptical properties and macroscopic helical sense.

Stepwise Amplification of Circularly Polarized Luminescence in Chiral Metal Cluster Ensembles.

Chiral metal-organic frameworks(MOFs)are usually endowed by chiral linkers and/or guests. The strategy using chiral secondary building units in MOFsfor solving the trade-off of circularly polarized luminescence (CPL)-active materials, high photoluminescence quantum yields (PLQYs) and high dissymmetry factors (|glum |) has not been demonstrated. This work directionally assembles predesigned chiral silver clusters with ACQ linkers through reticular chemistry. The nanoscale chirality of the cluster transmits through MOF's framework, where the linkers are arranged in a quasi-parallel manner and are efficiently isolated and rigidified. Consequently, this backbone of chiral cluster-based MOFs demonstrates superb CPL, high PLQYs of 50.3%, and |glum | of 1.2 × 10-2 . Crystallographic analyses and DFT calculations show the quasi-parallel arrangement manners of emitting linkers leading to a large angle between the electric and magnetic transition dipole moments, boosting CPL response. As compared, an ion-pair-direct assembly without interactions between linkers induces one-ninth |glum | and one-sixth PLQY values, further highlighting the merits of directional arrangement in reticular nets. In addition, a prototype CPL switching fabricated by a chiral framework is controlled through alternating ultraviolet and visible light. This work is expected to inspire the development of reticular chemistry for high-performance chiroptical materials.

Design of MOFs with Absolute Structures: A Case Study

AbstractMetal‐organic frameworks (MOFs) with absolute structures are of particular interest as the asymmetry of their crystal structures enables wide applications from asymmetric catalysis to enantioselective separation. Previous reports on the synthetic strategies toward MOFs with absolute structures always required the use of chiral precursors or chiral templates, however, it is not always easy to obtain enantiopure starting materials. Here in, we report a new non‐centrosymmetric MOF, MOF‐829, synthesized from aluminium salt and achiral organic linker. Further comparisons are made between MOF‐829 and a reported Al‐based chiral MOF (MOF‐520) that is composed of the same metal ion and linker, and similarly synthesized without using chiral compounds. The configurations of the building units, the absolute structures of both MOFs, and their topologies were investigated in detail. We found that (i) topology is one of the determining factors in the formation of non‐centrosymmetric MOFs; (ii) the formation of chiral MOFs further requires the directionality of chiral linkers but more importantly chiral secondary building units (SBUs), which can be realized by fine tuning of the synthetic conditions. We envision that both synthetic exploration of chiral SBUs and the design of non‐centrosymmetric topologies will open a new direction in the design of MOFs with absolute structures.

Multi-Modal Chiral Superstructures in Self-Assembled Anthracene-Terminal Amino Acids with Predictable and Adjustable Chiroptical Activities and Color Evolution.

Deep understanding of structure-property relationship between packing of chiral building units and their chiroptical behaviors would significantly facilitate the rational design and fabrication of the emerging chiroptical materials such as circularly polarized luminescence (CPL) emissive materials. In this paper, we unveil the universal existence of supramolecular tilt helical superstructures in self-assembled π-conjugated amino acid derivatives. A series of coded amino acid methyl esters were conjugated to anthracene segments at N-terminus, which afforded 21 and 31 symmetry supramolecular tilt chirality in solid-states. Helical assemblies enabled diversified Cotton effects and CPL performances, which were in accordance with the tilted chirality between anthracene segments. Such correlation shows fine universality, whereby the chiroptical prediction could be realized. Furthermore, on top of charge-transfer complexation, manipulation of CPL emission colors and handedness were realized.

Multi‐Modal Chiral Superstructures in Self‐Assembled Anthracene‐Terminal Amino Acids with Predictable and Adjustable Chiroptical Activities and Color Evolution

AbstractDeep understanding of structure–property relationship between packing of chiral building units and their chiroptical behaviors would significantly facilitate the rational design and fabrication of the emerging chiroptical materials such as circularly polarized luminescence (CPL) emissive materials. In this paper, we unveil the universal existence of supramolecular tilt helical superstructures in self‐assembled π‐conjugated amino acid derivatives. A series of coded amino acid methyl esters were conjugated to anthracene segments at N‐terminus, which afforded 21 and 31 symmetry supramolecular tilt chirality in solid‐states. Helical assemblies enabled diversified Cotton effects and CPL performances, which were in accordance with the tilted chirality between anthracene segments. Such correlation shows fine universality, whereby the chiroptical prediction could be realized. Furthermore, on top of charge‐transfer complexation, manipulation of CPL emission colors and handedness were realized.

Cyclohexylammonium sulfanilate: A simple representative of the chiral materials containing only achiral building units

Chiral building units must pack into chiral crystal structures. However, chiral structures can also be induced from achiral building units. In this letter, the later well-known phenomenon is illustrated with the simple synthesis of a new chiral compound from commonly used achiral organic molecules. Thus, the new compound cyclohexylammonium sulfanilate was synthesized at room temperature and characterized by single crystal X-ray diffraction, spectroscopy (UV/Visible and FT-IR), and thermal analysis. The compound is chiral and crystallizes in orthorhombic system with space group P212121. Noncentrosymmetry was confirmed by Second Harmonic Generation (SHG) measurements using solid potassium dihydrogenphosphate (KDP) as reference. The organic ions which form this chiral compound namely cyclohexylammonium (CyNH3+) and sulfanilate (NH2PhSO3−) are both low-cost achiral building units. A statistical investigation of previously reported structures shows that NH2PhSO3− stabilized by noncentrosymmetric cations more likely lead to noncentrosymmetric materials.

Chiral porous metal–organic frameworks from chiral building units with different metrics

Two chiral MOFs from the common Zn4O(COO)6 SBUs and the congeners of chiral ligands were synthesized and characterized. The overall structures are differentiated by the ligand dimensions. These MOFs obtained from the flexible enantiopure struts exhibited severe chirality attenuation during the assembly.

Preservation of Chirality and Polarity between Chiral and Polar Building Units in the Solid State

The new lamellar phases [Zn(2,2'-bpy)(2)(H(2)O)(2)](ZrF(6))·3H(2)O (I) and [Ni(2,2'-bpy)(3)](MoO(2)F(4))·5H(2)O (II) (bpy = bipyridine), which are built from a chiral cation and respectively an inherently nonpolar and a polar anion, provide two contrasting structures with respect to chirality and polarity in the solid state. Each nonpolar layer of I contains enantiomers of both handednesses; conversely, each layer of II contains only a Δ or Λ enantiomer and polar anions oriented along the b or -b axes. A comparison with previously reported structures reveals which combinations and interactions between chiral and polar basic building units can preserve elements of polarity and chirality in the solid state.

Novel fluorinated indium phosphate with a double-layers structure: Assembled from racemic chiral building units

Abstract Assisted by 4,4′-bipyridine (bipy), a new fluorinated indium phosphate, In4F5(HPO4)2(PO4)2(enH)(enH2), has been hydrothermally prepared employing ethylenediamine (en) as structure directing agent (SDA). Single-crystal X-ray diffraction analysis reveals that the compound crystallizes in the monoclinic system with the space group P21/c (No. 14) having cell parameters a = 10.5896(7) A, b = 10.4575(5) A, c = 10.2637 A, β = 99.339(4)°, V = 1121.54(11) A3, and Z = 2 with R1 = 0.0404. It possesses an unique double-layer structure, which features a racemic mixture of chiral building units. The two chiral building units, one left-handed and one right-handed, alternate to construct a racemic sheet with 8-rings. Pairs of these sheets are bridged together by oxygen and fluorine atoms to form the double layers. The mono- and diprotonated en molecules are located between the double layers to neutralize the negative charge of the inorganic framework. Further characterization of the compound was performed by X-ray powder diffraction, ICP, and TGA analyses.

A coordination π–π framework exhibits spontaneous magnetization

A pi-pi scaffolding framework that was assembled as a one-dimensional chain structure comprising alternating Delta- and Lambda-iron(II) chiral building units, [Fe(II)(Delta)Fe(II)(Lambda)(ox)2(phen)2]n, reveals spontaneous magnetization that gives rise to pronounced hysteresis loops below 10 K.

Chiral J-Aggregates Formed by Achiral Cyanine Dyes

The self-assembly of helical supramolecular structures from chiral building units is a basic principle of biological materials. The mesoscopic structure of a chiral molecular aggregate, which formed spontaneously from a nonchiral J-aggregating cyanine dye in aqueous solution, is presented. In single crystals (as shown in the picture) a coexistance of planar molecules with left- and right-handed twisted conformers of the same dye are found; the latter may act as templates to build up the helical superstructures.

Chiral polyesters with azobenzene moieties in the main chain, synthesis and evaluation of nonlinear optical properties

The synthesis, characterization and evaluation of nonlinear optical properties of a new series of chiral copolyesters containing push-pull electronic azobenzene mesogenic groups in the main chain are reported. The copolyesters showed liquid crystalline behaviour with high glass transition temperatures (∼200 °C), stable liquid crystalline mesophases (up to 360 °C) and high second harmonic generation (SHG) efficiency. The second order nonlinear susceptibility values were measured as a function of the percentage of the chiral building unit and the temporal stability was found to be significant over a long duration of time.

Chiral Benzoquinones as a New Class of Ligands for Asymmetric Catalysis: Synthesis and Application to the Palladium(II)-Catalyzed 1,4-Dialkoxylation of 1,3-Dienes

Chiral C2-symmetric 2,5-bisamide hydroquinone ligands, with β-amino alcohols as chiral building units, were synthesized in excellent overall yields. The ligands gave up to 54.4% ee in the palladium-catalyzed 1,4-dialkoxylation of 1,3-dienes. These findings demonstrate the potential of asymmetric induction utilizing chiral benzoquinones as ligands in palladium(II) catalysis, albeit with modest enantiomeric excesses. Weakly coordinating hydroxyl groups of the ligand appear to be crucial for the success of the reaction. Mechanistic aspects and the origin of enantioselectivity are also discussed.

Optically active chlorohydrins as chiral C3 and C4 building units: Microbial resolution and synthetic applications

Production of highly optically active C3 and C4 chlorohydrins was developed by using the bacteria stereoselectively dehalogenating and assimilating the racemic substrate: Pseudomonas sp. and Alcaligenes sp. These bacteria stereoselectively assimilate (RS)-2,3-dichloro-1-propanol (DCP) and (RS)-3-chloro-1,2-propanediol (CPD) followed by microbial preparation of (R)- and (S)-DCP, and (R)- and (S)-CPD with >99% ee. A novel dehalogenating enzyme, halohydrin dehydro-dehalogenase from one of the above strains, Alcaligenes sp. DS-S-7G, was applicable for preparation of optically active 1,2-diols with 60–99% ee. Moreover, microbial resolution of C4 chlorohydrins with whole cells of Pseudomonas sp. was carried out. This resolution reaction using the resting cells gave (R)- and (S)-4-chloro-3-hydroxybutyrate (CHB) and (S)-4-chloro-3-hydroxybutyronitrile (BN) with >98% ee. In the case of the resting cells of Enterobacter sp., both (R)-CHB (>99% ee) and (S)-3-hydroxy-γ-butyrolactone (95% ee) with excellent yield were obtained. Also, some typical synthetic applications using the above chiral C3 and C4 synthons were introduced: ferroelectric liquid crystals, optically active β-blockers, and other chiral pharmaceuticals. Chirality 10:682–692, 1998. © 1998 Wiley-Liss, Inc.

ChemInform Abstract: A Novel Generation of Optically Active Ethyl 4‐Chloro‐3‐ hydroxybutyrate as a C4 Chiral Building Unit Using Microbial Dechlorination.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Lipase catalysed resolution of ( R)- and ( S)-1-trimethylsilyl-1-alkyn-3-ols: useful intermediates for the synthesis of optically active γ-lactones

Various ( R)- and ( S)-1-trimethylsilyl-1-alkyn-3-ols, chiral building units useful for the synthesis of biologically active compounds, have been efficiently resolved by enantioselective acetylation mediated by immobilized lipase PS. The resolution is applied to the synthesis of ( R)- and ( S)-5-octyl-2-(5H)-furanones.

A novel generation of optically active ethyl 4-chloro-3-hydroxybutyrate as a C4 chiral building unit using microbial dechlorination

A novel procedure for the generation of optically active ethyl 4-chloro-3-hydroxybutyrate using bacterial cells was developed. Ethyl ( S)-4-chloro-3-hydroxybutyrate was prepared by Pseudomonas sp. OS-K-29, which stereoselectively assimilates 2,3-dichloro-1-propanol. The reaction was based on its kinetic dehalogenation for both enantiomers using the resting cells. The obtained 4-chloro-3-hydroxybutyrate had high enantiomeric excess of >98 % with a yield of 33 % at the microbial resolution step. Moreover, several C4 compounds having the 4-chloro-3-hydroxyl function were also resolved and gave good enantiomeric purities (>95 %ee). Ethyl ( R)-4-chloro-3-hydroxybutyrate was also obtained with high enantiomeric purity (>98 %ee) using the cells of Pseudomonas sp DS-K-NR818.

新しいキラル素子としてのＬ−クェブラキトール

It is highly important to find a novel chiral building unit for the synthesis of enantiopure natural products of biological interest and also for developing novel asymmetric reactions.L-Quebrachitol (1L- (-) -2-O-methyl-chiro-inositol) is a naturally occurring optically active cyclitol obtained from an exudate of the rubber tree. By developing novel method for the selective demethylation of methyl ether in the presence of cyclohexylidene acetal in L-quebrachitol moieties, total syntheses of myo-inositol 1-phosphate, (+) -conduritol B, and cyclophellitol were achieved.It was found that chiral cyclitols derived from L-quebrachitol were excellent chiral auxiliaries. For instance, diastereoselective reduction and addition to α-keto ester attached to chiral cyclitol derivatives took place highly stereoselectively. Diels-Alder reactions and [3+2] cycloaddition of nitrile oxide to acryloyl ester derived from the chiral cyclitol also proceeded with high diastereoselectivity.

Strategies for the chemoenzymatic preparation of optically active 1-alkyn-3-ols

A series of ( R)- and ( S)-1-alkyn-3-ols, chiral building units for the synthesis of leukotrienes and pheromones, were prepared via enantioselective hydrolysis of their racemic esters. While the majority of biocatalysts employed (lipases, fermenting or freeze-dried microorganisms) failed in discriminating between enantiomers, lyophilized cells of baker's yeast ( Saccharomyces cerevisiae Hansen) gave ( S)-1-alkyn-3-ols and their corresponding ( R)-esters with > 90% e.e.

Organometallics in prostaglandin synthesis

An extremely short way to prostaglandins has been opened by combining the newly devised organometallic methodologies. Convergent, one-pot creation of the prostanoid framework is achieved by organocopper conjugate addition of the S-configurated ω-side-chain unit to (R)-4-trialkylsiloxy-2-cyclopentenone followed by the organotin-aided trapping of the enolate intermediate by α-side-chain alkyl iodides. Prostaglandin E 2 can be prepared in only three steps from the chiral building units. The protected 5,6-didehydro-PGE 2 derivatives thus obtained serve as common intermediates for the synthesis of a variety of naturally occurring prostaglandins including prostacyclin. This approach is also useful for the controlled synthesis of isocarbacyclin.