Racemic Materials Research Articles

A chiral bipolar host for efficient solution-processed circularly polarized OLEDs via a chirality energy transfer process.

Chiral bipolar hosts of (R/S)-L-2mCPCN are synthesized, which show high singlet/triplet energy levels and clear circularly polarized luminescence. Employing racemic phosphorescent and TADF materials as emitting guest molecules, solution-processable CP-OLEDs based on such chiral hosts are obtained with an EQEmax of 10.7% and |gEL| values of 5.0 × 10-3.

Conformational, Structural, and Chiroptical Properties of an Overcrowded Triply Fused Carbo[7]helicene.

Recently, the synthesis of the racemate of an overcrowded triply fused carbo[7]helicene of formula C66H36 with three carbo[7]helicenes fused within a central six-membered ring was described. This molecule was found to embed an extremely contorted central six-membered ring and two negative curvatures. We report herein the resolution of the corresponding enantiomers and their conformational, structural, photophysical, and chiroptical properties. The racemization of the triply fused carbo[7]helicene was determined to proceed at a rate of krac = 8.06 × 10-4 s-1 at 175 °C in ortho-dichlorobenzene, corresponding to a barrier to enantiomerization ΔGenant‡ = 140.4 kJ·mol-1, a value significantly lower than for pristine carbo[7]helicene. Interestingly, the crystalline structures of the racemic and enantiopure materials show some differences regarding the molecular geometry, with an increased negative curvature in the latter cases. This unusual curved delocalized π-conjugated system afforded notably green fluorescence at room temperature and far-red phosphorescence at low temperature. Finally, electronic circular dichroism and circularly polarized luminescence responses of the enantiopure compounds have been measured and showed very close absorption and emission dissymmetry factors, gabs and glum, respectively, of ca. 2.6 × 10-3, indicating a similar chiral rigid geometry for both ground and excited states.

Targeting Oxide Concentration in Tantalum Oxide-Fluoride Anions

The design of both molecular and non-molecular solid materials with specific properties fundamentally relies on the controlled synthesis of crystals with desired functional groups, bonding motifs, polarity, chirality, and more. To this end, fluoride and oxide-fluoride anions have been utilized as basic building units (BBUs) in the synthesis of noncentrosymmetric racemic materials for their ability to create polar axes that facilitate the breaking of an inversion center as demonstrated in a series of compounds with [MF6]2- anions (M = Ti, Zr, Hf). Targeting an analog with a [TaOF5]2- anion, the phase space of (CuO, Ta2O5)/bpy/HF(aq)/H2O (bpy = 2,2′-bipyridine) was investigated and three new compounds with Cu-bpy cations and Ta-fluoride or Ta-oxyfluoride anions were synthesized: [Cu(bpy)2][TaF6], [Cu(bpy)2][Ta2OF10], and [Cu(bpy)F(H2O)2]2[TaF7]∙H2O with the anions [TaF6]-, [Ta2OF10]2-, and [TaF7]2-, respectively. The formation of these anions was found to be a product of both the concentration of hydrofluoric acid in solution and the ratio of metal-oxide starting materials to ligand. This work contributes to the understanding of mixed anion formation in the solid state.

Redox-enabled direct stereoconvergent heteroarylation of simple alcohols

The direct transformation of racemic feedstock materials to valuable enantiopure compounds is of significant importance for sustainable chemical synthesis. Toward this goal, the radical mechanism has proven uniquely effective in stereoconvergent carbon-carbon bond forming reactions. Here we report a mechanistically distinct redox-enabled strategy for an efficient enantioconvergent coupling of pyrroles with simple racemic secondary alcohols. In such processes, chirality is removed from the substrate via dehydrogenation and reinstalled in the catalytic reduction of a key stabilized cationic intermediate. This strategy provides significant advantage of utilizing simple pyrroles to react with feedstock alcohols without the need for leaving group incorporation. This overall redox-neutral transformation is also highly economical with no additional reagent nor waste generation other than water. In our studies, oxime-derived iridacycle complexes are introduced, which cooperate with a chiral phosphoric acid to enable heteroarylation of alcohols, accessing a wide range of valuable substituted pyrroles in high yield and enantioselectivity.

α-Boryl Organometallic Reagents in Catalytic Asymmetric Synthesis.

Recent years have witnessed an increase in the popularity of α-boryl organometallic reagents as versatile nucleophiles in asymmetric synthesis. These compounds have been adopted in chemo- and stereoselective coupling reactions with a number of different electrophiles. The resulting enantioenriched boronic esters can be applied in stereospecific carbon-carbon or carbon-heteroatom bond construction reactions, enabling a two-step strategy for the construction of complex structures with high efficiency and functional group compatibility. Due to these reasons, tremendous effort has been devoted to the preparation of enantiomerically enriched α-boryl organometallic reagents and to the development of stereoselective reactions of related racemic or prochiral materials. In this review, we describe the enantio- or diastereoselective reactions that involve α-boryl organometallic reagents as starting materials or products and we showcase their synthetic utility.

Effective control of optical purity by chiral HPLC separation for ester-based liquid crystalline materials forming anticlinic smectic phases

ABSTRACT Currently, effective control of optical purity by chiral separation of (S) and (R) enantiomers remains a relevant and highlighted task when designing new chiral organic materials in general, especially for self-assembling materials possessing synclinic and anticlinic smectic phases. An efficient methodology for accomplishing this task was developed and verified with a series of chiral fluorinated liquid-crystalline materials with lateral substitution on the molecular core. The self-assembling behaviour of new racemic materials was established. Upon cooling from the isotropic phase, all materials possessed orthogonal, synclinic and anticlinic smectic mesophases. The materials were available with four fluorine substitution patterns and in racemic and pure (R) and (S) enantiomer forms. Chiral high-performance liquid chromatography was accomplished using polysaccharide-based chiral stationary phases. All separations were performed in normal chromatographic mode, and the baseline separation of all enantiomer pairs was achieved using chiral stationary phases based on derivatised amylose and cellulose. The enantiomer elution order for racemic mixtures was verified by comparing their retention times with those of the respective pure (R) and (S) enantiomers. Interestingly, two materials demonstrated an unexpected switch in enantiomer elution order. This is very important information that is useful, for instance, for potential utilisation in preparative scale chiral chromatography.

The Chiral Twist-Bend Nematic Phase (N*TB ).

The twist-bend nematic, NTB , phase has been observed for chiral materials in which chirality is introduced through a branched 2-methylbutyl terminal tail. The chiral twist-bend nematic phase, N*TB , is completely miscible with the NTB phase of the standard achiral material, CB6OCB. The N*TB phase exhibits optical textures with lower birefringence than those observed for the achiral NTB phase, suggesting an additional mechanism of averaging molecular orientations. The N*-N*TB transition temperatures for the chiral materials are higher than the NTB -N transition temperatures seen for the corresponding racemic materials. This suggests the double degeneracy of helical twist sense in the phase is removed by the intrinsic molecular chirality. A square lattice pattern is observed in the N* phase over a temperature range of several degrees above the N*TB -N phase transition, which may be attributed to a non-monotonic dependence of the bend elastic constant.

Alkyne-Tagged Analogue of Jaspine B: New Tool for Identifying Jaspine B Mode of Action.

The first biologically relevant clickable probe related to the antitumor marine lipid jaspine B is reported. The concise synthetic route to both enantiomers relied on the supercritical fluid chromatography (SFC) enantiomeric resolution of racemic materials. The eutomeric dextrogyre derivative represents the first jaspine B analogue with enhanced cytotoxicity with IC50 down to 30 nm. These enantiomeric probes revealed a chiralitydependent cytoplasmic imaging of U2OS cancer cells by in situ click labeling.

Insight into structural influences on the optical properties and heteroenantiomeric self-assembly of racemic C6-unsubstituted tetrahydropyrimidines with strong aggregation-induced emission

Racemic C-6 unsubstituted tetrahydropyrimidines (THPs), the products of an efficient five-component reaction (5CR), are novel fluorophores with strong aggregation-induced emission (AIE). Here, the influences of molecular structures on the optical properties and heteroenantiomeric self-assembly of THPs have been investigated in details via the design and synthesis of 66 THPs with different substituents R1−R4, the analysis of 22 X-ray diffraction single crystals of 16 THPs and the theoretically calculated HOMOs/LUMOs based on the conformations in single crystals. All THPs with different R1−R4 in n-hexane solutions showed no emission and had similar absorbance wavelengths (λab) (315–320 nm) except R2 = R4 = alkyl (λab = 293 nm). However, upon aggregation, 64 THPs with aromatic R2 and R4 were emissive and had very different solid-state fluorescence quantum yields (ΦF) (3–100%), excitation and emission wavelengths (340–409 nm and 436–492 nm, respectively), except two non-emissive THPs with aliphatic R2 and R4. It was also found that the influences of all R1−R4 depended on their combinations, and the solid-state ΦF values of THPs were greatly enhanced by simply changing the combinations of R1−R4 with larger R1/smaller R3 or smaller R1/larger R3 combining suitable R2 and R4 optimal. 20 THPs with ΦF > 80% were mainly from various aryls except 4-OHPh and rigid naphthalenyl as R3 combining with suitable R1 (Et/Me), R2 and R4 (Ph/4-Cl/4-CF3/3-CF3/3-BrPh). The R- and S-enantiomer alignments of THPs in the 22 crystals were classified into four molecular packing modes (MPMs), whose formation mainly depended on R3 on chiral carbon. The through-space conjugation existing in the HOMOs and LUMOs of THPs with aryl R2 and R4 also correlated with R3: aromatic R3 and R4 when R3 = 4-CNPh, or only R4 when R3 = other aryls participating in conjugation through space. The mechanisms of structural effects on the solid-state fluorescence properties and heteroenantiomeric self-assembly of THPs are discussed based on the obtained experimental results and our previous work on AIE mechanism. The simple and efficient 5CR built a large library of diverse compounds, the relationship between substituent on chiral carbon and heteroenantiomeric self-assembly, the strong and useful AIE characteristics, and moreover, the firstly reported efficient method of enhancing solid-state fluorescence via simply changing the combinations of substituents will be expected to make racemic THPs potential materials and attract great interest in the development and applications of new interesting racemic fluorophores.

Chiroptical Resolution and Thermal Switching of Chirality in Conjugated Polymer Luminescence via Selective Reflection using a Double‐Layered Cell of Chiral Nematic Liquid Crystal

An optically resolvable and thermally chiral‐switchable device for circularly polarized luminescence (CPL) is first constructed using a light‐emitting conjugated polymer film and a double‐layered chiral nematic liquid crystal (N*‐LC) cell. The double‐layered N*‐LC cell with opposite handedness at each layer is fabricated by adding each of two types of N*‐LCs into each of the cells, and the N*‐LCs consist of nematic LCs and chiral dopants with opposite chirality and different mole concentrations. The selective reflection band due to the N*‐LC is thermally shifted so that the band wavelength is close to the luminescence band of the racemic conjugated polymer, such as disubstituted polyacetylene (diPA), yielding CPL with opposite handedness and high dissymmetry factor values (|glum|) of 1.1–1.6 at low and high temperatures. The double‐layered N*‐LC cell bearing the temperature‐controlled selective reflection is useful for generating CPLs from racemic fluorescent materials and for allowing thermal chirality‐switching in CPLs, which present new possibilities for optoelectronic and photochemical applications.

Spin crossover properties of enantiomers, co-enantiomers, racemates, and co-racemates.

Through multi-component self-assembly of chiral phenylethylamine, 1-alkyl-2-imidazolecarboxaldehyde and iron(ii) ions, two couples of enantiomeric iron(ii) complexes , , and with the formula of fac-Λ or Δ-[Fe(L)3](2+)(L = R or S-1-phenyl-N-(1-alkyl-1H-imidazol-2-ylmethylene)ethanamine) have been designed and synthesized as building blocks. Further binary cocrystallization of the prefabricated enantiomers enabled us to construct spin crossover co-enantiomers and , racemates and , and co-racemate . Compared with in a high spin state and with spin crossover at 291 K, the co-enantiomers exhibited gradual spin crossover at a higher temperature of 301 K, and the racemic alloys showed hysteresis loops induced by desolvation above room temperature. It was demonstrated that molecular chirality could be used effectively for stereochemical engineering of spin crossover materials. In addition, crystal packing, intramolecular π-π stacking, intermolecular C-Hπ interactions and solvent effects were elucidated to be responsible for the distinct spin crossover properties. This collective structural and magnetic study not only enriched the spin crossover library, but also provided a full comparison of optically pure, homochiral, and racemic materials with similar molecular structures.

Rh(I)-Catalyzed Ring-Opening of Hetaryne−Furan Diels−Alder Adducts: Rapid Access to Stereochemically Defined Heterocyclic Scaffolds

Probing the nucleophilic ring-opening of various bicyclic [2.2.1] hetaryne-furan Diels-Alder adducts revealed that efficient reactivity could be observed with heteroatom nucleophiles by using a cationic Rh(I) complex in combination with a chiral Josiphos-type phosphine ligand. Remarkably, this catalyst system was not impeded by the incorporation of a heteroatom into the substrate. Racemic materials afforded separable mixtures of enantioenriched regioisomers, indicating that strong reagent control is operative.

Synthesis of chiral calix[4]arenes bearing aminonaphthol moieties and their use in the enantiomeric recognition of carboxylic acids

Two armed chiral calix[4]arenes 8-16 functionalized at the lower rim with chiral aminonaphthol units have been prepared and the structures of these receptors characterized by FTIR, (1)H, and (13)C, DEPT and COSY NMR spectroscopy and elemental analysis. The enantioselective recognition of these receptors with various carboxylic acids has been studied by (1)H NMR and UV/Vis spectroscopy. The receptors exhibited different chiral recognition abilities towards the enantiomers of racemic materials and formed 2 : 1 or 1 : 1 complexes between host and guest. It was also demonstrated that chiral calix[4]arenes 9 and 16 could be used as chiral NMR solvating agents to determine the enantiomeric purity of mandelic acid.

Comparative Study of Homochiral and Racemic Chiral Metal-Organic Frameworks Built from Camphoric Acid

The study of the crystallization of homochiral or racemic chiral framework materials from enantiopure or racemic ligands is an important step for the rational design of crystalline homochiral materials. The self-assembly of extended frameworks in the presence of enantiopure or racemic ligands is regulated by the differing symmetry requirements of the ligand. By employing both enantiopure and racemic ligands as cross-linking ligands, we have prepared three homochiral and three racemic chiral compounds and their synthesis and structural and topological properties are presented here. It is shown that depending on the chemical compositions and framework topologies, the effect of the ligand handedness on the resulting crystal structure can vary greatly. The ligand handedness can have negligible effect on the crystallization, which leads to an interesting example of two pairs of isostructural achiral and homochiral structures with two types of six-connected nets. In another example reported here, the racemic ch...

アキラルな分子から光学活性分子の合成キラル：結晶場の利用

Synthesis of optical active compounds from achiral or racemic materials is one of the most impressive studies in organic chemistry. Here we provide two new technique of generation of optical activity without anyl outside chiral source. One is the breaking of chiral symmetry of axially chiral materials via racemization-preferential crystallization method. Another involves the asymmetric reaction using the frozen chirality generated by chiral crystallization of achiral materials.

Tandem enantioselective organo- and biocatalysis: a direct entry for the synthesis of enantiomerically pure aldols

Direct proline-catalyzed aldol reactions were linked in sequence with lipase-catalyzed kinetic resolutions to afford enantiomerically pure (>99% ee) aldol adducts in higher conversion than a standard resolution of racemic materials. The combination of organocatalytic aldol reactions and enzymatic kinetic resolutions provided exclusively either the ( R)- or ( S)-enantiomer of the aldol adducts even though an ( R)-selective lipase catalyzed the kinetic resolutions. Furthermore, the one-pot tandem reactions are inexpensive, are operationally simple, circumvents the use of organic solvent and are environmentally benign.

Chirality Issues and Interesting Properties of Achiral and Racemic Materials with Bent Molecular Structures

Spontaneous symmetry breaking in achiral and racemic materials is reviewed in terms of the so-called banana-shaped liquid crystal materials. Some novel results are reported in the area of conventional liquid crystal phases generated by achiral, bent-shaped molecular architectures where the smectic C phase is seen to consist of domains of opposite handedness in equal proportion. Moreover, an analogous racemic material was found to generate a liquid crystal phase that showed only one handedness. Other materials based on the same bent-core architecture were found to generate a nematic phase that also consists of domains of opposite handedness in equal proportion. This nematic phase was also found not to show any areas of homeotropic alignment.

Lipase-catalyzed resolution of p-menthan-3-ols monoterpenes: preparation of the enantiomer-enriched forms of menthol, isopulegol, trans- and cis-piperitol, and cis-isopiperitenol

A study on the enzymic resolution of the most common p-menthan-3-ol monoterpene isomers is described. Enantioenriched alcohols 1, 5, 10, 11 and 12 are obtained by means of the lipase-mediated kinetic acetylation of the corresponding racemic materials. The stereochemical aspects of the enzymic process have been investigated. We found that the structural features of the starting p-menthan-3-ol as well as the kind of lipase used, impacted strongly on the enantioselectivity of the resolution. The potentialities of this approach for preparative purposes are discussed.

Highly selective hydrolytic kinetic resolution of terminal epoxides catalyzed by chiral (salen)Co(III) complexes. Practical synthesis of enantioenriched terminal epoxides and 1,2-diols.

The hydrolytic kinetic resolution (HKR) of terminal epoxides catalyzed by chiral (salen)Co(III) complex 1 x OAc affords both recovered unreacted epoxide and 1,2-diol product in highly enantioenriched form. As such, the HKR provides general access to useful, highly enantioenriched chiral building blocks that are otherwise difficult to access, from inexpensive racemic materials. The reaction has several appealing features from a practical standpoint, including the use of H(2)O as a reactant and low loadings (0.2-2.0 mol %) of a recyclable, commercially available catalyst. In addition, the HKR displays extraordinary scope, as a wide assortment of sterically and electronically varied epoxides can be resolved to > or = 99% ee. The corresponding 1,2-diols were produced in good-to-high enantiomeric excess using 0.45 equiv of H(2)O. Useful and general protocols are provided for the isolation of highly enantioenriched epoxides and diols, as well as for catalyst recovery and recycling. Selectivity factors (k(rel)) were determined for the HKR reactions by measuring the product ee at ca. 20% conversion. In nearly all cases, k(rel) values for the HKR exceed 50, and in several cases are well in excess of 200.

The synthesis of 1-hydroxy phosphonates of high enantiomeric excess using sequential asymmetric reactions: titanium alkoxide-catalyzed PC bond formation and kinetic resolution

Titanium alkoxide-catalyzed asymmetric phosphonylation of aldehydes yields hydroxy phosphonates in moderate to good enantiomeric excess (e.e.s ∼70%). The hydroxy phosphonates were acetylated and the acetates were subjected to enzyme-catalyzed kinetic resolution. The non-racemic acetates 2 (predominantly ( R)-enantiomer) were hydrolyzed with an ( R)-enantiomer-selective lipase, resulting predominantly in the hydrolysis of the ( R)-isomer (at 85% conversion) to give the alcohols 3 with high e.e. Alternatively, hydrolysis of the minor enantiomeric ( S)-acetate to approximately 20% conversion left the enriched ( R)-configured acetate with improved e.e. (>90%). The moderate enantioselectivities obtained in the catalytic PC bond formation are enhanced during the enzymatic hydrolysis. Furthermore, availability of the non-racemic phosphonates permits the use of less selective enzymes, resulting in higher yields in comparison with the standard resolution of racemic materials.