Planar Chirality Research Articles

Design and synthesis of a new family of planar and central chiral ferrocenyl phosphine ligands.

A family of planar chiral indene-fused ferrocenes were prepared through an intramolecular asymmetric C-H arylation in excellent yields (up to 99%) with excellent enantioselectivities (up to 99% ee). They were thereafter successfully transformed to chiral ferrocenyl phosphines, featuring both planar and central chiralities, in good yields (up to 83%) and excellent diastereoselectivities (up to 99% de) through highly diastereoselective phosphination. This protocol offers a general method for planar and central chiral ferrocenyl phosphines. The potential applications of the newly developed ligands were demonstrated by a Pd-catalyzed enantioselective allylic alkylation reaction, in which high enantioselectivity (92% ee) and good yield (89%) were obtained.

Paracyclophane-based bipolar near-ultraviolet emitters showing advanced circularly polarized luminescent properties

<p>As linking unit, [2.2]paracyclophane (PCP) has been inserted in between electron-donating groups (<em>i.e.</em> carbazole and 10<em>H</em>-phenoxazine) and electron-accepting groups (<em>i.e.</em> triphenyl-1,3,5-triazine and diphenylsulfone) to prepare a series of linear bipolar racemic near-ultraviolet (NUV) emitters owing to the planar chirality associated with the PCP. In toluene, the racemic bipolar PCP-based emitters can show emission wavelength in the range of 400-444 nm. Critically, very small Commission Internationale de L��Eclairage (CIE) chromaticity coordinate of y value of 0.03 has been achieved for the photoluminescent spectra of these bipolar racemic NUV emitters in solution. The PCP-based bipolar emitters can show efficient NUV emission with photoluminescent quantum yields (PLQY) of <em>ca.</em> 0.5 in doped mCP film. In addition, the enantiomers of the NUV emitters have been obtained to show obvious circular dichroism (CD) behavior and circularly polarized luminescence (CPL). The absorption asymmetric factor (<em>g</em><sub>abs</sub>) of <em>ca.</em> 4.4��10<sup>-4</sup> and photoluminescent asymmetric factor (<em>g</em><sub>PL</sub>) of <em>ca.</em> 1.5��10<sup>-3</sup> can be achieved by the PCP-based NUV enantiomers. After doped in the emission layer of organic light-emitting diodes (OLEDs), efficient NUV and deep blue-emitting OLEDs have been constructed. The NUV OLEDs based on the racemic emitters can achieve forward-viewing maximum external quantum efficiency (<em>��</em><sub>ext</sub>) of 5.03% and electroluminescence (EL) peak at 404 nm with CIE<sub>y</sub> of 0.05, while the OLEDs with deep blue emission can show <em>��</em><sub>ext</sub> of 4.12% and EL peak at 412 nm with CIE<sub>y</sub> of 0.08. Furthermore, the NUV chiral enantiomers can bring <em>��</em><sub>ext</sub> of 5.25% and EL peak at 404 nm with CIE<sub>y</sub> of 0.05, while the deep blue chiral enantiomers can give <em>��</em><sub>ext</sub> of 4.40% and EL peak at 412 nm with CIE<sub>y</sub> of 0.08. Clearly, these encouraging EL data show the great potential of these linear PCP-based bipolar emitters in the field of OLEDs.</p>

Post‐Mechanical Bond Formation Desymmetrization Approach to Obtain Mechanically Planar Chiral Rotaxanes

AbstractThe preparation of chiral mechanically interlocked molecules has experienced a great development in the last decade, showing a wide range of applications, including catalysis and molecular machinery. New types of chirality have emerged as a consequence of the mechanical bond, such as mechanically planar chirality, which can exist even when both components are achiral. The mechanically planar chirality in such interlocked structures is usually created during the assembly of the rotaxane architecture. Recently, Tian, Zhu and coworkers decoupled the creation of mechanically planar chirality and assembly of the mechanical bond by using a desymmetrizing Suzuki‐Miyaura reaction of a series of mechanically prochiral rotaxanes. This strategy afforded the target chiral interlocked products showing up to 96.5 : 3.5 enantiomeric ratio.

Stereoselective Construction of Chiral Linear [3]Catenanes and [2]Catenanes.

We have successfully constructed a chiral linear [3]catenane stereoselectively by coordination-driven self-assembly using a ditopic monodentate ligand containing l-valine residues with a binuclear half-sandwich organometallic rhodium(III) unit. Furthermore, by increasing the steric hindrance of the amino acid residues in the ligand, a chiral [2]catenane was obtained, which can be regarded as the factor catenane of the chiral linear [3]catenane from a topological viewpoint. Notably, the resulting molecular catenanes all exhibit complex coconformational mechanical helical chirality and planar chirality ascribed to the point chirality of the ligands. Linear [3]catenanes and [2]catenanes with the opposite chirality can be obtained by using ligands containing the corresponding d-amino acid residues, which have been confirmed by single-crystal X-ray diffraction, NMR, mass spectrometry, and circular dichroism spectroscopy.

Simplified Synthesis of an Air-Stable Copper-Complexed Josiphos Ligand via Ugi's Amine: Complete Preparation and Analysis from Ferrocene.

Ligands containing ferrocene backbones often feature both planar chirality and asymmetric centers, making them attractive options for asymmetric catalysis. Ugi's amine is a ubiquitous ferrocene-based chiral building block that can be functionalized to form a variety of tunable Josiphos ligands; however, few sources lay out the route from start to finish. Starting from ferrocene, we compile a synthetic route to an air- and moisture-stable copper(I)-Josiphos complex via enantiopure Ugi's amine, providing a potential one-stop shop for the synthesis of a wide range of Josiphos ligands.

Improving the kinetic resolution of rac-2-(diphenylthiophosphinoferrocene) methanol catalyzed by Thermomyces lanuginosus lipase immobilized on immobead-150

The organometallic 2-(diphenylthiophosphinoferrocene)methanol (1) is a ferrocene derivative that exhibits planar chirality. In this work, we report the lipase mediated kinetic resolution (KR) of rac-1, via acylation reaction, as a more sustainable alternative to produce the chiral compounds. The study resulted the enantiopure remaining alcohol (Rp)-1 (ees = >99%) and its enantioenriched acetate (Sp)-2 (eep = 98%) with ideal conversion (c = 50%) and enantioselectivity (E > 200) values when the KR was catalyzed by the lipase from Thermomyces lanuginosus immobilized on immobead-150 (enzyme:substrate ratio 1:1 (w/w); vinyl acetate as acylating agent; toluene as solvent; 35°C; 48 h). Results from molecular docking study confirmed the enantiopreference of the lipase for acetylating the (Sp)-alcohol, yielding (Sp)-2 as product. Additionally, the cytotoxicity of rac-1 and its enantiomer (Rp)-1 was investigated against SNB-19 (Astrocytoma), HCT-116 (Human Colon Carcinoma), PC3 (Prostate Carcinoma), HL60 (Promyelocytic Leukemia), and non-tumor HACAT (Human keratinocytes) cell lines. Both compounds showed very low cytotoxicity and no selectivity.

Confused-Prism[5]arene: a Conformationally Adaptive Host by Stereoselective Opening of the 1,4-Bridged Naphthalene Flap.

The confused-prism[5]arene macrocycle (c-PrS[5]Me ) shows conformational adaptive behavior in the presence of ammonium guests. Upon guest inclusion, the 1,4-bridged naphthalene flap reverses its planar chirality from pS to pR (with reference to the pS(pR)4 enantiomer). Stereoselective directional threading is also observed in the presence of directional axles, in which up/down stereoisomers of homochiral (pR)5 -c-PrS[5]Me pseudorotaxanes are formed.

Preparation of planar chiral oxazolinyl hydroxyl ferrocene silyl ethers and their desilylation derivatives

Featured by abundant derivation and powerful catalytic activities, chiral catalysts and ligands with oxazolinyl ferrocenes core play a vital role in the area of enantioselective transformations. Despite great researches on phosphine ligands with oxazolinyl ferrocene silyl ether core have been broadly explored, the example of N,O-ferrocene compounds with identical core is rare. We herein disclosed the practical approaches to the preparation of planar chiral oxazolinyl hydroxyl ferrocene silyl ethers. Several available methods were employed for the establishment of the configuration on planar chirality, which considered as key parameter for the subsequent relative products. Furthermore, an array of N,O,O-tridentate ferrocenes was obtained through desilylation treated with TBAF. The absolute stereochemical configuration of the synthesized ferrocene derivatives was determined by X-ray diffraction.

Brønsted Acid versus Phase-Transfer Catalysis in the Enantioselective Transannular Aminohalogenation of Enesultams.

We have studied the enantioselective transannular aminohalogenation reaction of unsaturated medium-sized cyclic benzosulfonamides by using both chiral Brønsted acid and phase-transfer catalysis. Under optimized conditions, a variety of bicyclic adducts can be obtained with good yields and high enantioselectivities. The mechanism of the reaction was also studied by using computational tools; we observed that the reaction involves the participation of a conformer of the nine-membered cyclic substrate with planar chirality in which the stereochemical outcome is controlled by the relative reactivity of the two pseudorotational enantiomers when interacting with the chiral catalyst.

Enantioselective Synthesis of Both Axially and Planar Chiral Ferrocenes via Axial-to-Planar Diastereoinduction.

Ferrocenes with planar chirality have emerged as an important class of scaffolds for ligands in asymmetric catalysis; however, ferrocene molecules with polychiral structures have not been well explored. Herein, both axially and planar chiral ferrocenes were synthesized via palladium/chiral norbornene cooperative catalysis and axial-to-planar diastereoinduction. In this work, chiral norbornene was used to stereoselectively control the aromatic axial chirality, and further selectivity induced C(sp2)-H activation for ferrocene planar chirality. Based on density functional theory calculations, the catalytic model of chiral norbornene with the substrate and the axial-to-planar diastereoinduction process were confirmed.

Design and Synthesis of Pyrrolidinyl Ferrocene-Containing Ligands and Their Application in Highly Enantioselective Rhodium-Catalyzed Olefin Hydrogenation.

Herein, we report the design and synthesis of a series of chiral pyrrolidine-substituted ferrocene-derived ligands. The proficiency of this novel structural motif was demonstrated in the Rh-catalyzed asymmetric hydrogenation of dehydroamino acid esters and α-aryl enamides. The products were obtained with full conversions and excellent levels of enantioselectivities of up to >99.9% ee and 97.7% ee, respectively, using a BINOL-substituted phosphine-phosphoaramidite ligand which possesses planar, central, and axial chirality elements.

Mechanically planar chiral rotaxanes through catalytic desymmetrization

<h2>Summary</h2> Establishing mechanical chirality has remained a substantial synthetic hurdle despite the fast-growing applications of interlocked architectures in supramolecular chemistry. Here, we report the development of a catalyst-controlled desymmetrization strategy that decouples the creation of mechanically planar chirality and assembly of interlocked bonds. Employing a novel class of anionic chiral phosphine ligands, the Pd-catalyzed Suzuki-Miyaura reactions enable enantioselective access to various mechanically planar chiral rotaxanes. This study demonstrates that precise catalytic stereocontrol over mechanical chirality is achievable by engaging distal ionic substrate-catalyst interactions, despite the noncovalent nature of the interlocking stereogenic elements.

A Quantum-Guided Molecular Mechanics Force Field for the Ferrocene Scaffold.

Ferrocene derivatives have a wide range of applications, including as ligands in asymmetric catalysis, due to their chemical stability, rigid backbone, steric bulk, and ability to encode stereochemical information via planar chirality. Unfortunately, few of the available molecular mechanics force fields incorporate parameters for the accurate study of this important building block. Here, we present a MM3* force field for ferrocenyl ligands, which was generated using the quantum-guided molecular mechanics (Q2MM) method. Detailed validation by comparison to DFT calculations and crystal structures demonstrates the accuracy of the parameters and uncovers the physical origin of deviations through excess energy analysis. Combining the ferrocene force field with a force field for Pd-allyl complexes and comparing the crystal structures shows the compatibility with previously developed MM3* force fields. Finally, the ferrocene force field was combined with a previously published transition-state force field to predict the stereochemical outcomes of the aminations of Pd-allyl complexes with different amines and different chiral ferrocenyl ligands, with an R2 of ∼0.91 over 10 examples.

Self‐Assembly of Macrocyclic Triangles into Helicity‐Opposite Nanotwists by Competitive Planar over Point Chirality

AbstractWhile helix has elegant biomimetic structures and functionalities, it still remains a big question how the nanoscale helicity evolved from the molecular chiral building blocks across length scales. Herein, macrocyclic triangles composed of achiral edges and chiral vertices were rationally designed, in which the planar chirality emerged due to the restriction of edge rotation by intermolecular stackings and led to a unique chiral self‐assembly. In contrast to the solution systems where the chiroptical property is exclusively dominated by the point chiral vertices, the emerged planar chirality was found to control the chiral self‐assembly, resulting the nanotwist with the handedness determined by the planar chirality. Our work unveiled the self‐assembly behaviors of macrocyclic conformers for the first time and provided a deep understanding on the macrocyclic chirality evolution including the excited‐state chirality.

Self-Assembly of Macrocyclic Triangles into Helicity-Opposite Nanotwists by Competitive Planar over Point Chirality.

While helix has elegant biomimetic structures and functionalities, it still remains a big question how the nanoscale helicity evolved from the molecular chiral building blocks across length scales. Herein, macrocyclic triangles composed of achiral edges and chiral vertices were rationally designed, in which the planar chirality emerged due to the restriction of edge rotation by intermolecular stackings and led to a unique chiral self-assembly. In contrast to the solution systems where the chiroptical property is exclusively dominated by the point chiral vertices, the emerged planar chirality was found to control the chiral self-assembly, resulting the nanotwist with the handedness determined by the planar chirality. Our work unveiled the self-assembly behaviors of macrocyclic conformers for the first time and provided a deep understanding on the macrocyclic chirality evolution including the excited-state chirality.

Ferrocene derivatives with planar chirality and their enantioseparation by liquid-phase techniques.

In the last decade, planar chiral ferrocenes have attracted a growing interest in several fields, particularly in asymmetric catalysis, medicinal chemistry, chiroptical spectroscopy and electrochemistry. In this frame, the access to pure or enriched enantiomers of planar chiral ferrocenes has become essential, relying on the availability of efficient asymmetric synthesis procedures and enantioseparation methods. Despite this, in enantioseparation science, these metallocenes were not comprehensively explored, and very few systematic analytical studies were reported in this field so far. On the other hand, enantioselective high-performance liquid chromatography has been frequently used by organic and organometallic chemists in order to measure the enantiomeric purity of planar chiral ferrocenes prepared by asymmetric synthesis. On these bases, this review aims to provide the reader with a comprehensive overview on the enantioseparation of planar chiral ferrocenes by discussing liquid-phase enantioseparation methods developed over time, integrating this main topic with the most relevant aspects of ferrocene chemistry. Thus, the main structural features of ferrocenes and the methods to model this class of metallocenes will be briefly summarized. In addition, planar chiral ferrocenes of applicative interest as well as the limits of asymmetric synthesis for the preparation of some classes of planar chiral ferrocenes will also be discussed with the aim to orient analytical scientists towards 'hot topics' and issues which are still open for accessing enantiomers of ferrocenes featured by planar chirality.

Eight new arnebinol B-based meroterpenoids with planar chirality in the constrained 6/10/5 tricyclic skeleton from Arnebia euchroma and their cytotoxicities

Eight new arnebinol B-based meroterpenoids ((−)-1, 2, 3, (−)-5, and 7–10) with a constrained 6/10/5 tricyclic backbone were isolated from the roots of Arnebia euchroma. The planar and steric structures of these new compounds were unambiguously elucidated by extensive spectroscopic analyses, X-ray diffraction crystallography, and ECD calculations. The predominant relative orientation between H-7 and the Z double bond with a methyl substituent in the rigid 10-membered carbocycle, along with the planar chirality of the Z double bond was analyzed and discussed for the first time. The illustration of the planar chirality derived from the Z double bond should be paid great importance during the structure elucidation on these homologous meroterpenoids. All the isolated meroterpenoids were screened for their cytotoxicities against the HCT-8, PANC-1, HGC-27, HepG2, and PC9 cell lines, and compounds (+)-5 and (−)-5 exhibited the most potent cytotoxicity.

Stereoretentive Regio- and Enantioselective Allylation of Isoxazolinones by a Planar Chiral Palladacycle Catalyst.

The catalytic allylic substitution is one of the most important tools in asymmetric synthesis to form C-C bonds in an enantioselective way. While high efficiency was previously accomplished in terms of enantio- and regiocontrol using different catalyst types, a strong general limitation is a very pronounced preference for the formation of allylic substitution products with (E)-configured C=C double bonds. Herein, we report that with a planar chiral palladacycle catalyst a diastereospecific reaction outcome is achieved using isoxazolinones and allylic imidates as substrates, thus maintaining the C=C double bond geometry of the allylic substrates in the highly enantioenriched products. DFT calculations show that the reactions proceed via an SN 2 mechanism and not via π-allyl Pd complexes. Crucial for the high control is the stabilization of the allylic fragment in the SN 2 transition state by π-interactions with the phenyl substituents of the pentaphenylferrocenyl catalyst core.

Intense Molar Circular Dichroism in Fully Conjugated All-Carbon Macrocyclic 1,3-Butadiyne Linked pseudo-meta [2.2]Paracyclophanes.

The synthetic access to macrocyclic molecular topologies with interesting photophysical properties has greatly improved thanks to the successful implementation of organic and inorganic corner units. Based on recent reports, we realized that pseudo-meta [2.2]paracyclophanes (PCPs) might serve as optimal corner units for constructing 3D functional materials, owing to their efficient electronic communication, angled substituents and planar chirality. Herein, we report the synthesis, characterization and optical properties of four novel all-carbon enantiopure macrocycles bearing three to six pseudo-meta PCPs linked by 1,3-butadiyne units. The macrocycles were obtained by a single step from enantiopure, literature-known dialkyne pseudo-meta PCP and were unambiguously identified and characterized by state of the art spectroscopic methods and in part even by x-ray crystallography. By comparing the optical properties to relevant reference compounds, it is shown that the pseudo-meta PCP subunit effectively elongates the conjugated system throughout the macrocyclic backbone, such that already the smallest macrocycle consisting of only three subunits reaches a polymer-like conjugation length. Additionally, it is shown that the chiral pseudo-meta PCPs induce a remarkable chiroptical response in the respective macrocycles, reaching unprecedented high molar circular dichroism values for all-carbon macrocycles of up to 1307 L mol-1 cm-1 .

Assembly of planar chiral superlattices from achiral building blocks

The spontaneous assembly of chiral structures from building blocks that lack chirality is fundamentally important for colloidal chemistry and has implications for the formation of advanced optical materials. Here, we find that purified achiral gold tetrahedron-shaped nanoparticles assemble into two-dimensional superlattices that exhibit planar chirality under a balance of repulsive electrostatic and attractive van der Waals and depletion forces. A model accounting for these interactions shows that the growth of planar structures is kinetically preferred over similar three-dimensional products, explaining their selective formation. Exploration and mapping of different packing symmetries demonstrates that the hexagonal chiral phase forms exclusively because of geometric constraints imposed by the presence of constituent tetrahedra with sharp tips. A formation mechanism is proposed in which the chiral phase nucleates from within a related 2D achiral phase by clockwise or counterclockwise rotation of tetrahedra about their central axis. These results lay the scientific foundation for the high-throughput assembly of planar chiral metamaterials.