Achiral Compounds Research Articles

First‐Row Transition Metals Complexes with Fused Oxazolidine (FOX) Ligands

AbstractIn this comprehensive study, we report the syntheses and crystal structures of 6 fused oxazolidine (FOX) bicycles and 22 complexes with FOX ligands. The structures are compared to the few FOX complexes found in literature. For our investigations, we developed synthetic strategies to selectively synthesize chiral or achiral FOX bicycles with pyridyl substituents. The coordination chemistry of the chiral and achiral FOX compounds with first‐row transition metals is discussed in an in‐depth analysis of the crystal structures. We found that the binding modes of the ligands are dependent on the configuration of the FOX backbone and on the steric demand of the substituents which were introduced to the pyridyl groups. The configuration of the FOX ligands is retained during the coordination of metal(II) ions such as Mn2+, Fe2+, Co2+, Ni2+, Cu2+. Reactions of these metal ions with chiral FOX ligands lead to the formation of chiral complexes. In contrast, stronger Lewis acidic metal(III) ions, such as Fe3+ or Al3+, induce an isomerization of the chiral FOX ligands to their achiral diastereomers.

Broadband reflection cholesteric liquid crystal film fabricated by near-infrared photothermal response technology

ABSTRACT A polyvinyl alcohol–modified copper sulphide (PVA-CuS) nanoparticlephotothermal film with modulated photothermal response properties is fabricated. Aliquid crystal cell with modulated photothermal conversion function is fabricated by using the glass substrate with this photothermal film as one side of this liquid crystal cell. In the formula of cholesteric liquid crystal (CLC), there is achiral compound with temperature-dependent helical twisting power and aultraviolet (UV)-polymerisable monomer. Therefore, under the irradiation of the near-infrared laser, the temperature of the sample gradually increases due to the photothermal conversion function of the liquid crystal cell, which causes the pitch of the CLC to gradually increase. Meanwhile, the liquid crystallinity polymerisable monomer polymerises under the irradiation of UV light to form apolymer network, and fix the pitch at each temperature section, then form anon-uniform distribution of the pitch to achieve broadband reflection. The influence of heating speed and UV light intensity is investigated under different modulation modes of near-infrared laser.

Spontaneous Deracemizations.

Spontaneous deracemizations is a challenging, multidisciplinary subject in current chirality research. In the absence of any chiral inductors, an achiral substance or a racemic composition is driven into an enantioenriched or even homochiral state through a selective energy input, e.g., chemical potential, photoirradiation, mechanical grinding, ultrasound waves, thermal gradients, etc. The most prominent examples of such transformations are the Soai reaction and the Viedma deracemization. In this review, we track the most recent developments in this topic and recall that many other deracemizations have been reported for solutions from mesophases to conglomerate crystallizations. A compiled set of simply available achiral organic, inorganic, organometallic, and MOF compounds, yielding conglomerate crystals, should give the impetus to realize new experiments on spontaneous deracemizations. Taking into account thermodynamic constraints, modeling efforts have shown that structural features alone are not sufficient to describe spontaneous deracemizations. As a guideline of this review, particular attention is paid to the physicochemical origin and symmetry requirements of such processes.

1H NMR and conformational analysis of diastereotopic methylene protons in achiral flexible molecules

Unlike diastereotopic -CH2- protons from chiral compounds, the diastereotopic protons from achiral compounds may not be easily predictable. For molecules with flexible chain this task is even more problematic. Additionally, the assignment from the experimental 1H NMR spectrum of chemical shifts and coupling constants is very frequently uncertain. Nevertheless, computationally, for both chiral and achiral molecules the -CH2- protons diastereotopic nature is evident as well as NMR parameters assignments. In the present paper we examine the NMR parameters calculated as a sum of Boltzmann weighted average of the lowest conformations that fits a window less than 10 kJ/mole. A Spartan20 six steps protocol was chosen to select the lower energy conformations. The NMR conformationally averaged parameters obtained for 2-ethyl-2-hyroxybutanoic acid (EHB) and citric acid (CA), both achiral molecules, and the chiral malic acid (MA) fit acceptable well with the experimental data, although the calculation is done for gas phase (conformations dominated by intermolecular hydrogen bonding) and the experiment was carried out in D2O (conformations controlled by intermolecular hydrogen bonding of highly decorated molecules with OH and COOH that can interact with the solvent). In addition, in the present paper we have examined EHB, CA and MA the lowest energy conformer geometry, Atom in Molecules (AIM) and Natural Bond Orbital protocols to identify the intramolecular hydrogen bonds.

Effect of Halogen Bonding on Chiral Assemblies of Achiral Sulfonamide Molecules in the Crystalline Phase

We investigated the relationship between the proportion of achiral compounds that crystallize as chiral crystals and how halogen bonds affect the molecular assemblies in the crystals. In addition, ...

Success or Failure of Chiral Crystallization of Similar Heterocyclic Compounds.

Single crystals of two achiral and planar heterocyclic compounds, C9H8H3O(CA1) and C8H5NO2 (CA4), recrystallized from ethanol, were characterized by single crystal X-ray analysis, respectively, and chiral crystallization was observed only for CA1 as P212121 (# 19), whereas it was not observed for CA4 P21/c (# 14). In CA1, as a monohydrate, the hydrogen bonds were pronounced around the water of crystallization (O4), and the planar cyclic sites were arranged in parallel to slightly tilted positions. On the other hand, an anhydride CA4 formed a dimer by hydrogen bonds between adjacent molecules in the crystal, which were aggregated by van der Waals forces and placed in parallel planar cyclic sites.

Construction of a liquid crystalline double helix supramolecular structure and its electro-responsive behaviour

ABSTRACT A chiral urea compound, N,N’-bis(4-(3,4,5-trialkoxyphenylmethyl)phenyl)urea (1a), possessing a long core and six chiral alkyl chains ((S)-citronellyl groups) was designed to construct a double helix supramolecular structure in a liquid crystal phase. As a result, the urea compounds exhibited a hexagonal columnar liquid crystal phase, and the molecules were self-organised into a double helix structure by intermolecular interactions (dipole-dipole interactions, hydrogen bonds, and π-π interactions). Furthermore, the compound’s responsive behaviour to an applied external electric field was investigated, and its phase transition behaviour, column alignment, columnar structures, and electro-responsiveness were compared with those of an achiral urea compound (1b) possessing six n-decyl groups to investigate the effect on introducing the chirality. These results indicated that higher-order structures such as double helices are effective in suppressing thermal motion and delaying polarisation relaxation.

Transition Metal-Catalyzed α-Position Carbon–Carbon Bond Formations of Carbonyl Derivatives

α-Functionalization of carbonyl compounds in organic synthesis has traditionally been accomplished via classical enolate chemistry. As α-functionalized carbonyl moieties are ubiquitous in biologically and pharmaceutically valuable molecules, catalytic α-alkylations have been extensively studied, yielding a plethora of practical and efficient methodologies. Moreover, stereoselective carbon–carbon bond formation at the α-position of achiral carbonyl compounds has been achieved by using various transition metal–chiral ligand complexes. This review describes recent advances—in the last 20 years and especially focusing on the last 10 years—in transition metal-catalyzed α-alkylations of carbonyl compounds, such as aldehydes, ketones, imines, esters, and amides and in efficient carbon–carbon bond formations. Active catalytic species and ligand design are discussed, and mechanistic insights are presented. In addition, recently developed photo-redox catalytic systems for α-alkylations are described as a versatile synthetic tool for the synthesis of chiral carbonyl-bearing molecules.

Generation of Axially Polar Ferroelectricity in a Columnar Liquid Crystal Phase by Introducing Chirality

AbstractAxially polar ferroelectric columnar liquid crystals (AP‐FCLCs) are materials in which the molecules are self‐organized into axially polar columns and exhibit switching and holding of their polar directions. If the polar directions along the columnar axis can be controlled by using a nanosized electrode, AP‐FCLCs have the potential to realize ultrahigh‐density memory devices. Though switching polarities in columnar liquid crystal phases have been studied by many scientists, it remains difficult to realize bistable switching. In this study, it is found that replacement of the normal alkyl groups (decyl groups) with chiral groups ((S)‐citronellyl groups) in an N,N’‐bis(3,4,5‐trialkoxyphenyl)urea realizes perfect ferroelectricity in the rectangular columnar liquid crystal phase, and it is confirmed that the phase is an AP‐FCLC phase. The differences in the properties and molecular packing structures between the achiral and chiral urea compounds are investigated. Furthermore, the mechanism for achieving ferroelectricity by introduction of the chiral alkyl chains is postulated.

Impact of Chirality on Hydrogen-Bonded Supramolecular Assemblies and Photoconductivity of Diketopyrrolopyrrole Derivatives.

Hydrogen bonds can efficiently guide the self-assembly of organic materials, enabling to tune the properties of the aggregation processes. In the case of π-conjugated materials, several parameters such as temperature, concentration and solvent can be used to modify the aggregation state while tuning the optoelectronic properties. Chirality can be included within the impacting parameters due to the differences in molecular packing. Here, chiral and achiral thiophene-capped diketopyrrolopyrrole derivatives were designed and synthesized containing amide bonds, with the aim to study the interplay between chiral assemblies and their stabilization through hydrogen-bonding. Differences in aggregation properties were observed with spectroscopy and microscopy, and a contactless microwave-based technique was used to study their intrinsic charge carrier mobility. The positive role of hydrogen-bonding has been highlighted and the differences between chiral and achiral compounds have been elucidated.

Inhibition of bacterial growth and galactosyltransferase activity of WbwC by α, ω-bis(3-alkyl-1H-imidazolium)alkane salts: Effect of varying carbon content

A series of compounds was designed and synthesized having two imidazolium rings separated by a polymethylene spacer and having alkyl substituents on each of the imidazolium rings. The compounds were assayed for their effects on the activity of galactosyltransferase WbwC, and also on the growth of Gram-negative and Gram-positive bacteria, as well as human cells. The inhibition observed on enzyme activities and cell growth was dependent on the total number of carbons in the spacer and the alkyl substituents on the imidazolium rings. These readily synthesized, achiral compounds have potential as antimicrobial and antiseptic agents.

Crystallization of chiral molecular compounds: what can be learned from the Cambridge Structural Database?

A detailed study on chiral compound structures found in the Cambridge Structural Database (CSD) is presented. Solvates, salts and co-crystals have intentionally been excluded, in order to focus on the most basic structures of single enantiomers, scalemates and racemates. Similarity between the latter and structures of achiral monomolecular compounds has been established and utilized to arrive at important conclusions about crystallization of chiral compounds. For example, the fundamental phenomenon of conglomerate formation and, in particular, their frequency of occurrence is addressed. In addition, rarely occurring kryptoracemates and scalemic compounds (anomalous racemates) are discussed. Finally, an extended search of enantiomer solid solutions in the CSD is performed to show that there are up to 1800 instances most probably hiding among the deposited crystal structures, while only a couple of dozen have been previously known and studied.

Isolation, Absolute Configuration, and Biological Activities of Chebulic Acid and Brevifolincarboxylic Acid Derivatives from Euphorbia hirta.

Twenty new chebulic acid and brevifolincarboxylic acid derivatives, including eight optically pure or achiral compounds (1-7 and 14) and six pairs of enantiomers (8a/8b-13a/13b), along with nine known analogues (15-23), were isolated from an EtOH extract of the aerial parts of Euphorbia hirta. The absolute configurations of the new compounds were assigned based on single-crystal X-ray diffraction analysis and comparison of the experimental and calculated ECD data. Racemic or scalemic mixtures of 8-13 were isolated, and their enantiomers were analyzed by chiral-phase HPLC-ECD measurements. Compound 12 possesses an unprecedented 2H-cyclopenta[de]chromene-2,5(4H)-dione scaffold. Compounds 12, 20, and 23 displayed moderate inhibitory effects against lipopolysaccharide-induced nitric oxide production in BV-2 microglial cells, while all the isolates exhibited significant DPPH radical scavenging activities with EC50 values of 2.2-15.8 μM.

Stereochemical Rules Govern the Soft Self-Assembly of Achiral Compounds: Understanding the Heliconical Liquid-Crystalline Phases of Bent-Core Mesogens.

A series of bent‐shaped 4‐cyanoresorcinol bisterephthalates is reported. Some of these achiral compounds spontaneously form a short‐pitch heliconical lamellar liquid‐crystalline phase with incommensurate 3‐layer pitch and the helix axis parallel to the layer normal. It is observed at the paraelectric‐(anti)ferroelectric transition, if it coincides with the transition from random to uniform tilt and with the transition from anticlinic to synclinic tilt correlation of the molecules in the layers of the developing tilted smectic phase. For compounds with long chains the heliconical phase is only field‐induced, but once formed it is stable in a distinct temperature range, even after switching off the field. The presence of the helix changes the phase properties and the switching mechanism from the naturally preferred rotation around the molecular long axis, which reverses the chirality, to a precession on a cone, which retains the chirality. These observations are explained by diastereomeric relations between two coexisting modes of superstructural chirality. One is the layer chirality, resulting from the combination of tilt and polar order, and the other one is the helical twist evolving between the layers. At lower temperature the helical structure is replaced by a non‐tilted and ferreoelectric switching lamellar phase, providing an alternative non‐chiral way for the transition from anticlinic to synclinic tilt.

Pyrimidine-based ferroelectric mixtures – The influence of oligophenyl based chiral doping system

In searching for new functional self-assembling materials a multicomponent ferroelectric liquid crystalline mixtures with features appropriate for utilization in photonic and electro-optic devices have been designed. Designed mixtures are composed of bicyclic pyrimidine achiral compounds and chiral dopants with an oligophenyl rigid core and two homochiral terminal chains. Basic mesomorphic, electro-optical and structural properties have been carefully studied and discussed. For several mixtures stable enantiotropic ferroelectric phase with very good alignment and desired functional features in surface stabilized geometry have been found. Based on these properties and computational as well as conformational analysis of chiral dopants, the structure-property relations have been established and analysed.

Stereochemistry and Antioxidant Activity of 1,3‐Diol Derivatives of Diacetylcurcumin‐4H: A Joint NMR, X‐Ray, and Biological Approach

AbstractThe stereochemistry of two new 1,3‐diol curcuminoid derivatives, ((3R*,5R*)‐3,5‐dihydroxyheptane‐1,7‐diyl) bis (2‐methoxy‐4,1‐phenylene) diacetate namely rac‐6, its acetylated derivative, (3R*,5R*)‐1,7‐bis(4‐acetoxy‐3‐methoxyphenyl) heptane‐3,5‐diyl diacetate (rac‐7) and the diastereomeric model compound 1,3‐bis(4‐methoxyphenyl)‐1,3‐propanediol (rac‐2) were investigated through their X‐ray crystal structures and nuclear magnetic resonance spectra. Achiral compounds diacetylcurcumin (4) and diacetylcurcumin‐4H (5), crystallized in the Sohncke space group P21 allowed its unambiguous stereochemical assignment. The unit cell of tetraacetylated compound rac‐7 consists of a co‐crystal of two conformers and constitute a rare case of conformational isomorphism composed by TTTTTT and TTTTGT conformers in a 1:1 stoichiometric proportions. The antioxidant activity of each pair of derivatives was investigated and a significant difference in activity was found for tetraacetylated pairs rac‐7/(3R,5 S)‐1,7‐bis(4‐acetoxy‐3‐methoxyphenyl) heptane‐3,5‐diyl diacetate (meso‐7) and cyclic sulfites (((4R*,6R*)‐2‐oxido‐1,3,2‐dioxathiane‐4,6‐diyl) bis (ethane‐2,1‐diyl)) bis (2‐methoxy‐4,1‐phenylene) diacetate (rac‐8)/(((4 S,6R)‐2‐oxido‐1,3,2‐dioxathiane‐4,6‐diyl) bis (ethane‐2,1‐diyl)) bis (2‐methoxy‐4,1‐phenylene) diacetate (meso‐8). The present study addresses the stereochemistry and biological activity of these types of compounds.

Imidazolium ionic liquid-enhanced poly(quinine)-modified silica as a new multi-mode chromatographic stationary phase for separation of achiral and chiral compounds

In this work, a novel imidazolium ionic liquid-functionalized poly(quinine)-modified silica stationary phase (Sil-PQn-MIm) was successfully synthesized via surface radical chain transfer and nucleophilic substitution reaction. The modified silica was confirmed by series of characterizations including Fourier transform infrared spectroscopy (FT-IR), elemental analysis (EA), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). The multi-mode chromatographic performances of the Sil-PQn-MIm column were investigated by anion-exchange mode for separation of aromatic acid samples, hydrophilic interaction mode for separation of nucleosides/nucleobases and sulfanilamides, and reversed-phase mode for separation of alkylbenzenes, benzene and polycyclic aromatic hydrocarbons (PAHs), and the Tanaka test mixtures, respectively. As expected, compared to the Sil-PQn column only with quinine as functional group, the Sil-PQn-MIm column further modified by imidazolium ionic liquid possessed higher separation performance, especially for the separation of nucleosides/nucleobases. The effects including buffer concentration, pH, organic solvent content and column temperature on chromatographic performance were studied, which proved that multiple interactions including electrostatic, hydrophobic and hydrophilic interactions can be simultaneously existed between the stationary phase and the analytes. In addition, reproducibility and efficiency of the Sil-PQn-MIm column were also investigated, the results illustrated that the stationary phase have good enough reproducibility (RSDs 0.15%–0.72%, n = 7) and high efficiency (plates per meter, ~90000 plates/m). In conclusion, the prepared stationary phase with multiple-mode retention capabilities could realize separation for various types of samples by optimizing chromatographic conditions, even for some chiral compounds.

Chirality Induction through Nano‐Phase Separation: Alternating Network Gyroid Phase by Thermotropic Self‐Assembly of X‐Shaped Bolapolyphiles

AbstractThe single gyroid phase as well as the alternating double network gyroid, composed of two alternating single gyroid networks, hold a significant place in ordered nanoscale morphologies for their potential applications as photonic crystals, metamaterials and templates for porous ceramics and metals. Here, we report the first alternating network cubic liquid crystals. They form through self‐assembly of X‐shaped polyphiles, where glycerol‐capped terphenyl rods lie on the gyroid surface while semiperfluorinated and aliphatic side‐chains fill their respective separate channel networks. This new self‐assembly mode can be considered as a two‐color symmetry‐broken double gyroid morphology, providing a tailored way to fabricate novel chiral structures with sub‐10 nm periodicities using achiral compounds.

Chirality Induction through Nano-Phase Separation: Alternating Network Gyroid Phase by Thermotropic Self-Assembly of X-Shaped Bolapolyphiles.

The single gyroid phase as well as the alternating double network gyroid, composed of two alternating single gyroid networks, hold a significant place in ordered nanoscale morphologies for their potential applications as photonic crystals, metamaterials and templates for porous ceramics and metals. Here, we report the first alternating network cubic liquid crystals. They form through self‐assembly of X‐shaped polyphiles, where glycerol‐capped terphenyl rods lie on the gyroid surface while semiperfluorinated and aliphatic side‐chains fill their respective separate channel networks. This new self‐assembly mode can be considered as a two‐color symmetry‐broken double gyroid morphology, providing a tailored way to fabricate novel chiral structures with sub‐10 nm periodicities using achiral compounds.

Artificial Neural Network based Modelling and Simulation to Identify new candidates for hosting Skyrmions

Abstract The premise of this work is to identify new candidates for hosting exotic topological excitations known as skyrmions. The tool used in this research is artificial neural network. The strategy is the statistical analysis of chiral structures contained in the ICSD database, with the additional information about the group number of constituent elements in the periodic table. The model described in this study predicts ideal chiral crystal and proposes a new chiral crystal design direction. Skyrmions are topologically protected materials with an unusual spin composition that are structurally asymmetric. This research introduces a deep learning method for L1 and L2 regularization with solution to a loss minimization problem for identifying the chiral crystals in skyrmion material of thin films. This paper introduces an approach to building a probabilistic classifier and an Artificial Neural Network(ANN) from a true or false chirality dataset consisting of chiral and achiral compounds with elements of the type ‘A’ and ‘B’. A quantitative indicator is demonstrated for the accuracy of chiral crystal formation. Through contrasting with probabilistic classifier method, the viability of the ANN approach is checked comprehensively. We present deep learning algorithm with Artificial Neural Network(ANN)modeling and simulations. Through out the manuscript we employ statistical techniques as well as deep learning strategies to identify chirality property for chiral crystal design. This research work opens the way to build sophisticated crystal design software methodology.