Chiral Elements Research Articles

BINOL as a chiral element in mechanically interlocked molecules.

In this minireview we present the use of the axially chiral 1,1'-binaphthyl-2,2'-diol (BINOL) unit as a stereogenic element in mechanically interlocked molecules (MIMs). We describe the synthesis and properties of such BINOL-based chiral MIMs, together with their use in further diastereoselective modifications, their application in asymmetric catalysis, and their use in stereoselective chemosensing. Given the growing importance of mechanically interlocked molecules and the key advantages of the privileged chiral BINOL backbone, we believe that this research area will continue to grow and deliver many useful applications in the future.

Transfer of Axial Chirality to the Nanoscale Endows Carbon Nanodots with Circularly Polarized Luminescence

AbstractWe report the synthesis, purification and characterization of chiral carbon nanodots starting from atropoisomeric precursors. The obtained atropoisomeric carbon nanodots are soluble in organic solvents and have good thermal stability, which are desirable features for technological applications. The synthetic protocol is robust, as it supports a number of variations in terms of molecular doping agents. Remarkably, the combination of axially chiral precursors and 1,4‐benzoquinone as doping agent results in green‐emissive carbon dots displaying circularly polarized luminescence. Dissymmetry factors of |3.5|×10−4 are obtained in solution, without the need of any additional element of chirality. Introducing axial chirality expands the strategies available to tailor the properties of carbon nanodots, paving the way for carbon nanoparticles that combine good processability in organic solvents with engineered advanced chiroptical properties.

Transfer of Axial Chirality to the Nanoscale Endows Carbon Nanodots with Circularly Polarized Luminescence.

We report the synthesis, purification and characterization of chiral carbon nanodots starting from atropoisomeric precursors. The obtained atropoisomeric carbon nanodots are soluble in organic solvents and have good thermal stability, which are desirable features for technological applications. The synthetic protocol is robust, as it supports a number of variations in terms of molecular doping agents. Remarkably, the combination of axially chiral precursors and 1,4‐benzoquinone as doping agent results in green‐emissive carbon dots displaying circularly polarized luminescence. Dissymmetry factors of |3.5|×10−4 are obtained in solution, without the need of any additional element of chirality. Introducing axial chirality expands the strategies available to tailor the properties of carbon nanodots, paving the way for carbon nanoparticles that combine good processability in organic solvents with engineered advanced chiroptical properties.

Functional Features of a Meta-Interferometer with Spatially Separated and Independently Electrically Controlled Metastructures Based on Chiral and Dipole Electrical Conductive Elements

Functional Features of a Meta-Interferometer with Spatially Separated and Independently Electrically Controlled Metastructures Based on Chiral and Dipole Electrical Conductive Elements

Catalytic Asymmetric Construction of Axially and Centrally Chiral Heterobiaryls by Minisci Reaction

Axially chiral biaryls and heterobiaryls constitute the most represented subclass of atropisomers with prevalence in natural products, bioactive compounds, privileged chiral ligand/catalysts, and optically pure materials. Despite many ionic protocols for their construction, radical-based variants represent another highly desirable and intriguing strategy but are far less developed. Moreover, efficient synthesis of axially chiral heterobiaryl molecules, especially ones having multiple heteroatoms and other types of chiral elements, through radical routes remains extremely limited. We herein disclose the first catalytic asymmetric, metal-free construction of axially and centrally chiral heterobiaryls by Minisci reaction of 5-arylpyrimidines and α-amino acid-derived redox-active esters. This is enabled by the use of 4CzIPN as an organic photoredox catalyst in conjunction with a chiral phosphoric acid catalyst. The reaction achieved a variety of interesting 5-arylpyrimidines featuring the union of an axially chiral heterobiaryl and a centrally chiral α-branched amine with generally excellent regio-, diastereo-, and enantioselectivity (up to 82% yield; >19:1 dr; >99% ee). This finding also builds up a new platform for the development of desymmetrization methods via radical-involved atroposelective functionalization at heteroarene of prochiral heterobiaryls.

Helical Chiral N-Heterocyclic Carbene Ligands in Enantioselective Gold Catalysis.

The first chiral helicene-NHC gold(I) complexes efficient in enantioselective catalysis were prepared. The L-shaped chiral ligand is composed of an imidazo[1,5-a]pyridin-3-ylidene (IPy) scaffold laterally substituted by a configurationally stable [5]-helicenoid unit. The chiral information was introduced in a key post-functionalization step of a NHC-gold(I) complex bearing a symmetrical anionic fluoreno[5]helicene substituent, leading to a racemic mixture of complexes featuring three correlated elements of chirality, namely central, axial and helical chirality. After HPLC enantiomeric resolution, X-ray crystallography and theoretical calculations enabled structural and stereochemical characterization of these configurationally stable NHC-gold(I) complexes. The high potential in asymmetric catalysis is demonstrated in the benchmark cycloisomerization of N-tethered 1,6-enynes with up to 95 : 5 er.

Cooperative Stereoinduction in Asymmetric Photocatalysis.

Stereoinduction in complex organic reactions often involves the influence of multiple stereocontrol elements. The interaction among these can often result in the observation of significant cooperative effects that afford different rates and selectivities between the matched and mismatched sets of stereodifferentiating chiral elements. The elucidation of matched/mismatched effects in ground-state chemical reactions was a critically important theme in the maturation of modern stereocontrolled synthesis. The development of robust methods for the control of photochemical reactions, however, is a relatively recent development, and similar cooperative stereocontrolling effects in excited-state enantioselective photoreactions have not previously been documented. Herein, we describe a tandem chiral photocatalyst/Brønsted acid strategy for highly enantioselective [2 + 2] photocycloadditions of vinylpyridines. Importantly, the matched and mismatched chiral catalyst pairs exhibit different reaction rates and enantioselectivities across a range of coupling partners. We observe no evidence of ground-state interactions between the catalysts and conclude that these effects arise from their cooperative behavior in a transient excited-state assembly. These results suggest that similar matched/mismatched effects might be important in other classes of enantioselective dual-catalytic photochemical reactions.

Organocatalytic asymmetric synthesis of bioactive hexahydropyrrolo[2,3-b]indole-containing tetrasubstituted allenes bearing multiple chiral elements

An organocatalytic asymmetric construction of hexahydropyrrolo[2,3-b]indole-containing tetrasubstituted allene scaffolds bearing both axial chirality and central chirality has been established via a cascade 1,8-addition/dearomatization-cyclization reaction of para-aminophenyl propargylic alcohols with tryptamines in the presence of chiral phosphoric acid (CPA), thus affording a wide range of such tetrasubstituted allenes bearing multiple chiral elements in generally good yields (up to 94%) with high diastereo- and enantioselectivities (up to 95:5 dr, 95% ee). In addition, the evaluation on the cytotoxicity of some selected products indicated that this class of chiral tetrasubstituted allenes could inhibit the growth of the pancreatic cancer cells to some extent. This work not only solved the challenging issues in enantioselective construction of tetrasubstituted allenes via alkynyl (aza)-para-quinone methides, but also represents the first example of using alkyne derivatives as electrophiles in organocatalytic asymmetric dearomatizations of tryptamines.

Laser Ablated Nanocrystalline Diamond Membrane for Infrared Applications.

We are reporting on laser microstructuring of thin nanocrystalline diamond membranes, for the first time. To demonstrate the possibility of microstructuring, we fabricated a diamond membrane, of 9 m thickness, with a two-dimensional periodic array of closely located chiral elements. We describe the fabrication technique and present the results of the measurements of the infrared transmission spectra of the fabricated membrane. We theoretically studied the reflection, transmission, and absorption spectra of a model structure that approximates the fabricated chiral metamembrane. We show that the metamembrane supports quasiguided modes, which appear in the optical spectra due to grating-assisted diffraction of the guided modes to the far field. Due to the C symmetry, the structure demonstrates circular dichroism in transmission. The developed technique can find applications in infrared photonics since diamond is transparent at wavelengths >6 m and has record values of hardness. It paves the way for creation of new-generation infrared filters for circular polarization.

Rational Design of Axially Chiral Styrene‐Based Organocatalysts and Their Application in Catalytic Asymmetric (2+4) Cyclizations

AbstractA new class of axially chiral styrene‐based thiourea tertiary amine catalysts, which have unique characteristics such as an efficient synthetic route, multiple chiral elements, and multiple activating groups, has been rationally designed. These new chiral catalysts have proven to be efficient organocatalysts, enabling the chemo‐, diastereo‐, and enantioselective (2+4) cyclization of 2‐benzothiazolimines with homophthalic anhydrides in good yields (up to 96 %) with excellent stereoselectivities (all >95:5 dr, up to 98 % ee). More importantly, theoretical calculations elucidated the important role of an axially chiral styrene moiety in controlling both the reactivity and enantioselectivity. This work not only represents the first design of styrene‐based chiral thiourea tertiary amine catalysts and the first catalytic asymmetric (2+4) cyclization of 2‐benzothiazolimines, but also gives an in‐depth understanding of axially chiral styrene‐based organocatalysts.

Rational Design of Axially Chiral Styrene-Based Organocatalysts and Their Application in Catalytic Asymmetric (2+4) Cyclizations.

A new class of axially chiral styrene-based thiourea tertiary amine catalysts, which have unique characteristics such as an efficient synthetic route, multiple chiral elements, and multiple activating groups, has been rationally designed. These new chiral catalysts have proven to be efficient organocatalysts, enabling the chemo-, diastereo-, and enantioselective (2+4) cyclization of 2-benzothiazolimines with homophthalic anhydrides in good yields (up to 96 %) with excellent stereoselectivities (all >95:5 dr, up to 98 % ee). More importantly, theoretical calculations elucidated the important role of an axially chiral styrene moiety in controlling both the reactivity and enantioselectivity. This work not only represents the first design of styrene-based chiral thiourea tertiary amine catalysts and the first catalytic asymmetric (2+4) cyclization of 2-benzothiazolimines, but also gives an in-depth understanding of axially chiral styrene-based organocatalysts.

Penicisteckins A-F, Isochroman-Derived Atropisomeric Dimers from Penicillium steckii HNNU-5B18.

Penicisteckins A-D (1-4), two pairs of atropodiastereomeric biaryl-type hetero- and homodimeric bis-isochromans with 7,5'- and 7,7'-linkages and a pair of atropodiastereomeric 2-(isochroman-5-yl)-1,4-benzoquinone derivatives [penicisteckins E (5) and F (6)], were isolated from the Penicillium steckii HNNU-5B18. Their structures including the absolute configuration were determined by extensive spectroscopic and single-crystal X-ray diffraction analysis and TDDFT-ECD calculations. Both the bis-isochromans and the isochroman/1,4-benzoquinone conjugates represent novel biaryl scaffolds containing both central and axial chirality elements. The monomer anserinone B (8) exhibited potent antibacterial activities against Staphylococcus aureus ATCC 29213 and methicillin-resistant Staphylococcus aureus with minimal inhibition concentration values ranging from 2 to 8 μg mL-1. Plausible biosynthetic pathways of 1-6 are proposed, which suggest how the absolute configurations of the isolates were established during the biosynthetic scheme.

Tunable metasurface realizing dynamic chiroptical responses in infrared band

The chiral metasurfaces composed of chiral element structures have attracted much attention because of their interesting optical response. However, with the rapid development of chiral metasurface research, people gradually realize that the traditional chiral metasurfaces lacks tunability in practical application. Therefore, it is the latest research direction to design the tunable / reconfigurable chiral metasurfaces. In this work, a double split-ring metasurface consist of metal/phase change material (PCM) Ge2Sb2Se4Te1(GSS4T1)/metal/dielectric multilayer stack is designed, asymmetric transmission (AT) and circular dichroism (CD) dynamic control are achieved in the infrared band with a large tuning range of more than 50 THz by switching between the amorphous and crystalline states of the Ge2Sb2Se4Te1. The design in this work is of great significance for realizing the GSST-enabled tunable polarization control in the infrared band.

Engineering of inorganic nanostructures with hierarchy of chiral geometries at multiple scales

Engineering of inorganic nanostructures with hierarchy of chiral geometries at multiple scales

Metal-to-Metal Distance Modulation by Ligand Design: A Case Study of Structure-Property Correlation in Planar Chiral Cyclophanyl Metal Complexes.

Multinuclear metal complexes have seen tremendous progress in synthetic advances, their versatile structural features, and emerging applications. Here, we conceptualize Metal‐to‐Metal distance modulation in cyclophanyl metal complexes by bridging ligand design employing the co‐facially stacked cyclophanyl‐derived pseudo‐geminal, ‐ortho, ‐meta, and ‐para constitutional isomers grafted with N‐, O‐, and P‐ containing chelates that allow the installation of diverse (hetero)metallic moieties in a distance‐defined and spatially‐oriented relation to one another. Metal‐to‐Metal distance modulation and innate transannular “through‐space” π–π electronic interactions via the co‐facially stacked benzene rings in cyclophanyl‐derived complexes as well as their specific stereochemical structural features (element of planar chirality) are crucial factors that contribute to the tuning of structure‐property relationships, which stand at the very center from the perspective of cooperative effects in catalysis as well as emerging material applications.

Interaction of electromagnetic wave and metamaterial with inductive type chiral inclusions

The principle of calculation of a plate from a metamaterial with inductive type chiral inclusions is submitted. It is shown that distribution of an electromagnetic wave to such substance can be investigated with the help of introduction of a chiral parameter and on the basis of a detailed method of calculation. By comparison of two methods the dependence of chiral parameter from frequency of electromagnetic radiation falling on a plate is found. With the help of a detailed method the nonlinear differential equation for potential on the chiral plate is found. It is shown that this equation has solutions as traveling solitary and standing waves but not traveling sine waves. The analysis of the received solutions of the nonlinear equation is carried out. Transition from the multiwave solution to the solution as standing waves is graphically shown at reduction ofdistance between the chiral elements.

Chirality Nanosensor with Direct Electric Readout by Coupling of Nanofloret Localized Plasmons with Electronic Transport.

The detection of enantiopurity for small sample quantities is crucial, particularly in the pharmaceutical industry; however, existing methodologies rely on specific chiral recognition elements, or complex optical systems, limiting its utility. A nanoscale chirality sensor, for continuously monitoring molecular chirality using an electric circuit readout, is presented. This device design represents an alternative real-time scalable approach for chiral recognition of small quantity samples (less than 103 adsorbed molecules). The active device component relies on a gold nanofloret hybrid structure, i.e., a high aspect ratio semiconductor-metal hybrid nanosystem in which a SiGe nanowire tip is selectively decorated with a gold metallic cap. The tip mechanically touches a counter electrode to generate a nanojunction, and upon exposure to molecules, a metal-molecule-metal junction is formed. Adsorption of chiral molecules at the gold tip induces chirality in the localized plasmonic resonance at the electrode-tip junction and manifests in an enantiospecific current response.

Mechanically Interlocked Chiral Self-Templated [2]Catenanes from 2,6-Bis(1,2,3-triazol-4-yl)pyridine (btp) Ligands.

We report the efficient self‐templated formation of optically active 2,6‐bis(1,2,3‐triazol‐4‐yl)pyridine (btp) derived homocircuit [2]catenane enantiomers. This represents the first example of the enantiopure formation of chiral btp homocircuit [2]catenanes from starting materials consisting of a classical chiral element; X‐ray diffraction crystallography enabled the structural characterization of the [2]catenane. The self‐assembly reaction was monitored closely in solution facilitating the characterization of the pseudo‐rotaxane reaction intermediate prior to mechanically interlocking the pre‐organised system via ring‐closing metathesis.

Assignment of enantiomorphs for the chiral allotrope β-Mn by diffraction methods.

The assignment of enantiomorphs by diffraction methods shows fundamental differences for x-rays and electrons. This is particularly evident for the chiral allotrope of β-Mn. While it is not possible to determine the sense of chirality of β-Mn with established x-ray diffraction methods, Kikuchi pattern simulation of the enantiomorphs reveals differences, if dynamical electron diffraction is considered. Quantitative comparison between experimental and simulated Kikuchi patterns allows the spatially resolved assignment of the enantiomorph in polycrystalline materials of β-Mn, as well as the structurally strongly related phase Pt2Cu3B. On the basis of enantiomorph distribution maps, crystals were extracted from enantiopure domains by micropreparation techniques. The x-ray diffraction analyses confirm the assignment of the Kikuchi pattern evaluations for Pt2Cu3B and do not allow to distinguish between the enantiomorphs of β-Mn.

Multigram‐Scale Kinetic Resolution of 4‐Acetyl[2.2]Paracyclophane via Ru‐Catalyzed Enantioselective Hydrogenation: Accessing [2.2]Paracyclophanes with Planar and Central Chirality

Abstract[2.2]Paracyclophane (PCP) derivatives have been promising platforms to study the element of planar chirality and through‐space electronic communications in π‐stacked molecular systems. To access enantiomerically pure derivatives thereof, a kinetic resolution of 4‐acetyl[2.2]‐PCP employing a ruthenium‐catalyzed enantioselective hydrogenation process was developed. This method can be performed on a multigram‐scale and gives access to enantiomerically pure derivatives with planar and central chirality of (Rp)‐4‐acetyl‐PCP (≥97% ee, 43%) and (Sp,S)‐PCP derivatives (≥97% ee, 46%), which are useful intermediates for the synthesis of sterically demanding PCP‐based ligand/catalyst systems and chiral synthons for engineering cyclophane‐based chiroptical materials.magnified image