Asymmetric Induction Research Articles

Tuning the Geometry and Optical Chirality of Pentatwinned Au Nanoparticles with 5-Fold Rotational Symmetry.

Chirality transfer from chiral molecules to chiral nanomaterials represents an important topic for exploring the origin of chirality in many natural and artificial systems. Moreover, developing a promising class of chiral nanomaterials holds great significance for various applications, including sensing, photonics, catalysis, and biomedicine. Here we demonstrate the geometric control and tunable optical chirality of chiral pentatwinned Au nanoparticles with 5-fold rotational symmetry using the seed-mediated chiral growth method. A distinctive growth pathway and optical chirality are observed using pentatwinned decahedra as seeds, in comparison with the single-crystal Au seeds. By employing different peptides as chiral inducers, pentatwinned Au nanoparticles with two distinct geometric chirality (pentagonal nanostars and pentagonal prisms) are obtained. The intriguing formation and evolution of geometric chirality with the twinned structure are analyzed from a crystallographic perspective upon maneuvering the interplay of chiral molecules, surfactants, and reducing agents. Moreover, the interesting effects of the molecular structure of peptides on tuning the geometric chirality of pentatwinned Au nanoparticles are also explored. Finally, we theoretically and experimentally investigate the far-field and near-field optical properties of chiral pentatwinned Au nanoparticles through numerical simulations and single-particle chiroptical measurements. The ability to tune the geometric chirality in a controlled manner represents an important step toward the development of chiral nanomaterials with increasing architectural complexity for chiroptical applications.

Bimodal Use of Chiral α-Trifluoromethylalanine in Aib Foldamers: Study of the Position Impact Towards the Helical Screw-Sense Preference.

Oligomers of the achiral α-aminoisobutyric acid (Aib) adopt a 310 helical conformation in which the screw-sense preference can be controlled by a single chiral residue. The use of the fluorinated residue α-Trifluoromethylalanine (α-TfmAla) revealed a unique way to both induce and measure the screw-sense preference of such oligomers acting as 19F NMR probe. This work proposes a systematic study of the effect of this fluorinated chiral inducer on the helical screw-sense preference of poly-Aib oligomers. The impact of the position of the fluorinated residue into pentamers (N-terminal, central or C-terminal) as well as the nature of the C-terminal capping of the peptides was thoroughly studied in light of complete structural analysis. A deeper understanding of the fluorine effect was achieved confirming the unique ability of α-TfmAla as a helical screw-sense controller.

Chiral Ni(Ⅱ)-Based coordination polymers with helixes and dinuclear clusters constructed from lactate synthons

Chiral coordination polymers [Ni((R)-ceba) (1,3-dpp) (H2O)0.5] (HU15-R), [Ni((R)-ceba) (1,3-dpp) (H2O)0.5] (HU15–S), [Ni((R)-ceba) (4,4′-dib) (H2O)0.5] (HU16-R) and [Ni((S)-ceba) (4,4′-dib) (H2O)0.5](HU16–S) were prepared from lactate-derived synthons and different N-donor linkers under hydrothermal conditions (H2ceba = 4-(1-carboxyethoxy)benzoic acid, 1,3-dpp = 1,3-di(pyridine-4-yl)propane, 4,4′-dib = 4,4′-di (1H-imidazole-1-yl)-1,1′-biphenyl). Structural analysis confirmed that that all complexes are three-dimensional frameworks containing dinuclear [Ni2(CO2)2(H2O)]2+ clusters. In addition, four types of helical structures were also obtained though chiral transfer and asymmetric induction from chiral synthons among them. (R)-ceba2- anions and Ni(Ⅱ) ions were bridged together to construct right-handed (R)-ceba-Ni chains in HU15-R and HU16-R, while (S)-ceba2- anions were linked by Ni(Ⅱ) ions to build right-handed (S)-ceba-Ni chains in HU15–S and HU16–S. Apart from chiral synthons, achiral linkers 1,3-dpp and 4,4′-dib assembled with Ni(Ⅱ) ions to form single 1,3-dpp-Ni chains in HU15-R and HU15–S and double 4,4′-dib-Ni chains in HU16-R and HU16–S, respectively. Meanwhile, the introductions of N-donor ligands resulted in varied topological structures. HU15-R and HU15–S are 4-connected dia nets, while HU16-R and HU16–S are 8-connected frameworks. Moreover, magnetic experiments indicated that antiferromagnetic interactions exist in these complexes.

Chiral Viologen-Derived Water-Stable Small Band Gap Lead Halides: Synthesis, Characterization, and Optical Properties.

Chiral organic-inorganic metal halides (OIMHs) are attractive for their potential applications in chiral optoelectronics and spintronics, such as circular polarized light emitters, detectors, and chiral-induced spin selectivity. Here, we report three pairs of chiral OIMHs with great water stability constructed from chiral viologens. These OIMHs contain either 1D or 0D structures, however, with small band gaps around 2 eV. Circular dichroism (CD) spectroscopy on transparent thin films of two OIMH pairs showed a wide CD response covering most of the visible light range. Although the chiral center is not directly attached to the pyridinium in these chiral viologens, the chirality is still successfully transferred into both the band gap and the exciton absorption ranges. Liquid and solid CD studies of the chiral viologens further indicate that the chiral induction inside these OIMHs is possibly through chiral crystallization. This work demonstrated the design strategy of water-stable, small band gap chiral OIMHs through chiral viologens. These low-dimensional chiral materials may provide an interesting system to investigate chiral induction, and their broad CD response may enable their potential application as circular photodetectors with a wide detection range.

Synergistic Pd/Cu-Catalyzed 1,5-Double Chiral Inductions.

Much attention has been focused on the catalytic asymmetric creation of single chiral centers or two adjacent stereocenters. However, the asymmetric construction of two nonadjacent stereocenters is of significant importance but is challenging because of the lack of remote chiral induction models. Herein, based on a C═C bond relay strategy, we report a synergistic Pd/Cu-catalyzed 1,5-double chiral induction model. All four stereoisomers of the target products bearing 1,5-nonadjacent stereocenters involving both allenyl axial and central chirality could be obtained divergently by simply changing the combination of two chiral catalysts with different configurations. Control experiments and DFT calculations reveal a novel mechanism involving 1,5-oxidative addition, contra-thermodynamic η3-allyl palladium shift, and conjugate nucleophilic substitution, which play crucial roles in the control of reactivity, regio-, enantio-, and diastereoselectivity. It is expected that this C═C bond relay strategy may provide a general protocol for the asymmetric synthesis of structural motifs bearing two distant stereocenters.

Construction of Si-Stereogenic Silanols by Palladium-Catalyzed Enantioselective C-H Alkenylation.

The construction of silicon-stereogenic silanols via Pd-catalyzed intermolecular C-H alkenylation with the assistance of a commercially available L-pyroglutamic acid has been realized for the first time. Employing oxime ether as the directing group, silicon-stereogenic silanol derivatives could be readily prepared with excellent enantioselectivities, featuring a broad substrate scope and good functional group tolerance. Moreover, parallel kinetic resolution with unsymmetric substrates further highlighted the generality of this protocol. Mechanistic studies indicate that L-pyroglutamic acid could stabilize the Pd catalyst and provide excellent chiral induction. Preliminary computational studies unveil the origin of the enantioselectivity in the C-H bond activation step.

Construction of Si‐Stereogenic Silanols by Palladium‐Catalyzed Enantioselective C−H Alkenylation

AbstractThe construction of silicon‐stereogenic silanols via Pd‐catalyzed intermolecular C−H alkenylation with the assistance of a commercially available L‐pyroglutamic acid has been realized for the first time. Employing oxime ether as the directing group, silicon‐stereogenic silanol derivatives could be readily prepared with excellent enantioselectivities, featuring a broad substrate scope and good functional group tolerance. Moreover, parallel kinetic resolution with unsymmetric substrates further highlighted the generality of this protocol. Mechanistic studies indicate that L‐pyroglutamic acid could stabilize the Pd catalyst and provide excellent chiral induction. Preliminary computational studies unveil the origin of the enantioselectivity in the C−H bond activation step.

Stereoselective Syntheses of Organophosphorus Compounds

The review is devoted to the theoretical and synthetic aspects of the stereochemistry of organophosphorus compounds. Organophosphorus compounds are not only widely exist in biologically active pharmaceuticals and agrochemicals, but also have widespread applications in material science and organic synthesis as ligands for transition metal complexes. One of the mainstreams for the development in this field is the creation of biologically active organophosphorus compounds that are searched and used as drugs or plant-protecting agents, which leads to the elaboration of advanced methods and monitoring, yielding up-to-date approaches to perform synthesis in an environmentally friendly manner. The review consists of two parts. The first part presents methods for the asymmetric synthesis of organophosphorus compounds using asymmetric organocatalysis and metal complex catalysis. In the review is described the nature of the chirality generation in the prebiotic period, the mechanisms of asymmetric induction, and double stereodifferentiation are discussed. The use of these methods for the preparation of chiral phosphorus analogs of natural compounds (phosphono-isonorstatin, phosphono-GABOB, phosphacarnitine, bis-phosphonates, and others) is described. Some data concerning of λ5-phosphanediones as metaphosphate anion analogues are also reported. The second part of the presented review shows examples of the use of these methods for the synthesis of phosphorus analogues of natural compounds—chiral phosphonoamino acids and hydroxyphosphonates: phosphonoaspartic acid, phosphonoglutamic acid, phosphonohomoproline, chiral bis-phosphonates. The reaction of dehydration aromatization with the formation of pho sphono isoindolinones, including isoindolinone bis-phosphonates, has been studied. Some of the synthesized compounds showed biological activity as protein tyrosine phosphatase inhibitors. A phosphonic analogue of iso-norstatine was synthesized. A stereoselective method for the synthesis of tetradecapentaenoic acid derivatives was developed.

The Interplay of Spacer Chirality and Parity in Mesogenic Dimers.

Introducing chirality into soft materials, including liquid crystals (LCs), profoundly impacts their self-organization and physical properties. In this study, we synthesized a novel series of LC dimers with a chiral center as part of their flexible spacer. The dimers were prepared in racemic and enantiomerically pure forms. Their spacer length and parity were varied to investigate the effect of spacer chirality and parity on mesomorphic behavior and on chiral induction in the nematic phase of achiral mesogens. Our results show that the even-membered chiral dimers only have chiral nematic phases. In contrast, the odd-membered dimers display rich mesomorphism, including the intriguing blue phase (BP) and chiral form of the twist-bend nematic phase (N*TB). The observed significant difference in the 3D surface morphology between the racemic and chiral forms of the N*TB phase suggests that the chiral moiety in the spacer promotes a chiral hierarchy. Furthermore, the chiral dimers show a prominent odd-even effect in the helical twisting power in nematic hosts. These findings highlight the importance of the position of the chiral group within the dimeric molecule and provide new insights into how intrinsic chirality in the spacer affects the overall structural chirality.

Enantioselective Phase-Transfer-Catalyzed Synthesis of Spirocyclic Azetidine Oxindoles.

Spiro-3,2'-azetidine oxindoles combine two independently important pharmacophores in an understudied spirocyclic motif that is attractive for medicinal chemistry. Here, the enantioselective synthesis of these structures is achieved in up to 2:98 er through intramolecular C-C bond formation, involving activation of the substrate with a novel SF5-containing chiral cation phase-transfer (PT) catalyst. The products are readily elaborated/deprotected to afford medicinally relevant enantioenriched compounds. Control experiments suggest an interfacial PT mechanism, whereby catalytic asymmetric induction is achieved through the activation of the chloride leaving group.

Toward Bicalutamide Analogues with High Structural Diversity Using Catalytic Asymmetric Oxohydroxylation.

A catalytic enantioselective synthesis of bicalutamide derivatives with promising potentials in prostate cancer treatment has been disclosed. The key intermediates, α-hydroxy-β-keto esters, were efficiently constructed through cinchoninium-mediated asymmetric oxohydroxylation of easily accessible alkenes with potassium permanganate. Good yields and high levels of asymmetric induction are achieved. This method provides a new synthetic route to bicalutamide analogues with high structural diversity, which will beneficially support subsequent structure-activity relationship studies and boost prostate cancer drug development.

Rhodium-Catalyzed Asymmetric Macrocyclization towards Crown Ethers Using Hydroamination of Bis(allenes).

Enantiomerically enriched crown ethers (CE) exhibit strong asymmetric induction in phase transfer catalysis, supramolecular catalysis and molecular recognition processes. Traditional methods have often been used to access these valuable compounds, which limit their diversity and consequently their applicability. Herein, a practical catalytic method is described for the gram scale synthesis of a class of chiral CEs (aza-crown ethers; ACEs) using Rh-catalyzed hydroamination of bis(allenes) with diamines. Using this approach, a wide range of chiral vinyl functionalized CEs with ring sizes ranging from 12 to 36 have been successfully prepared in high yields of up to 92 %, dr of up to >20 : 1 and er of up to >99 : 1. These vinyl substituted CEs allow for further diversification giving facile access to various CE derivatives as well as to their three-dimensional analogues using ring-closing metathesis. Some of these chiral CEs themselves display high potential for use in asymmetric catalysis.

Rhodium‐Catalyzed Asymmetric Macrocyclization towards Crown Ethers Using Hydroamination of Bis(allenes)

AbstractEnantiomerically enriched crown ethers (CE) exhibit strong asymmetric induction in phase transfer catalysis, supramolecular catalysis and molecular recognition processes. Traditional methods have often been used to access these valuable compounds, which limit their diversity and consequently their applicability. Herein, a practical catalytic method is described for the gram scale synthesis of a class of chiral CEs (aza‐crown ethers; ACEs) using Rh‐catalyzed hydroamination of bis(allenes) with diamines. Using this approach, a wide range of chiral vinyl functionalized CEs with ring sizes ranging from 12 to 36 have been successfully prepared in high yields of up to 92 %, dr of up to >20 : 1 and er of up to >99 : 1. These vinyl substituted CEs allow for further diversification giving facile access to various CE derivatives as well as to their three‐dimensional analogues using ring‐closing metathesis. Some of these chiral CEs themselves display high potential for use in asymmetric catalysis.

Synthesis of alkynyl spacer-containing chiral polyguanidines and their noncovalent modification for organocatalyst design

A novel biscarbodiimide compound containing alkynyl spacer has been prepared and successfully used as an electrophilic comonomer in the polyaddition reaction with chiral secondary and primary 1,2-diamines. The resulting chiral polyguanidines have exhibited Brønsted base catalytic activities with asymmetric induction ability, which can be greatly enhanced through noncovalent modification with achiral Brønsted acids, such as phosphoric acid diesters and carboxylic acids. This study reveals for the first time the validity of a strategy incorporating guanidine functionality into polymer main chains followed by their noncovalent modification for organocatalyst design.

Access to Saturated Aza‐Heterocycles using the Borrowing Hydrogen Methodology

AbstractBorrowing hydrogen (BH) has reached a wide attention from the chemistry community as a new opportunity to construct efficiently C−N bond (as well as C−C or C−S bond). This straightforward strategy is usually supported by relevant organometallic catalysts, involved in the dehydrogenation‐reduction processes. This concept highlights specifically the application of the borrowing hydrogen to the preparation of saturated aza‐heterocycles (piperidines, pyrrolidines and piperazines) extensively present in high‐added value molecules of pharmaceutical industry over the last 40 years using homogenous catalysis in particular. A focus will be given to the appropriate metals (Ru, Ir and more recently earth‐abundant metals) used for this transformation. An extension to chiral induction in order to prepare enantiopure N‐heterocycles will be also reported.

Strong Circularly Polarized Phosphorescence of Achiral Pt(II) Metallomesogen Induced by Using a Chiral Co‐Assembly Strategy

AbstractThe high planarity and heavy‐atom effect of Pt(II) metallomesogens promote their stacking in the aggregated state and highly efficient phosphorescence. This makes them promising candidate materials for circularly polarized phosphorescence (CPP) triggered by using a chiral co‐assembly strategy. In this paper, a nematic phase, achiral robot‐like Pt(II) metallomesogen (TFPt) is synthesized, which combines two anchored chiral binaphthyl inducers (R/S1 or R/S2) to construct cholesteric liquid crystal (N*‐LC) co‐assemblies (R/S1)n‐(TFPt)1‐n and (R/S2)n‐(TFPt)1‐n via intermolecular interactions. The resultant N*‐LC chiral co‐assemblies (R1)0.03‐(TFPt)0.97 and (R2)0.03‐(TFPt)0.97 exhibit the strongest CPP emission with high dissymmetry factors (gem) of up to 0.18 and 0.27, respectively, stemming from the formation of regular helical nanofibers on spin‐coated films via co‐assembly process after thermal annealing at 165 °C.

Optically Electroactive Polymer Synthesized in a Liquid Crystal with Cyclosporin A─Circularly Polarized Electron Spin Resonance.

Cyclosporine A (CsA), a naturally derived biomaterial and physiologically active substance, is commonly used as an immunosuppressant. In this study, CsA was revealed to function as a chiral inducer of cholesteric liquid crystals (CLCs) with a high helical twisting power. CsA induced helical structures in 4-cyano-4'-pentylbiphenyl, a synthetic liquid crystal (LC) used for general purposes. Electrochemical polymerization in CLC with CsA was also performed. The polymer prepared in CLC showed electro-optical activity via chiral induction by CsA. Synchrotron X-ray diffraction measurements indicated that the polymer film prepared in the CLC formed in the manner of LC molecular arrangement through molecular form imprinting from the LC order, although the polymer exhibited no liquid crystallinity. The polymer showed structural color and laser light oscillation diffraction derived from its periodic structure. The anisotropy of the circularly polarized electron spin resonance signals for the resulting polymer with respect to the magnetic field was observed.

Synthesis and Chiroptical Properties of Radially Extended Carbazole with Chiral [2.2]Paracyclophane Core

Abstract[2.2]Paracyclophanes ([2.2]PCs) with substituents at specific positions possesses planar chirality and stands as a promising building unit for the development of chiroptical materials associated with polarized light. However, one of the easily accessible starting materials, bromo‐substituted [2.2]PC, shows low reactivity, and effective methods for π‐extension of planar chiral [2.2]PC are limited. Herein, an effective π‐extension method has established for chiral [2.2]PC with four substitutions from 4,7,12,15‐tetrabromo[2.2]PC. This has led to tetrakis(trimethylsilylphenyl)‐[2.2]PC that serves as a versatile building resource for radial‐type chiral π‐conjugated compounds with a [2.2]PC core. In this study, synthesizing compounds are further succeeded with the introduction of carbazole (Cbz4‐[2.2]PC and tBuCbz4‐[2.2]PC) and investigated their chiroptical properties and chiral induction ability in achiral π‐conjugated polymers. A thin film of [Poly[9,9‐dioctylfluorenyl‐2,7‐diyl)‐alt‐(benzo[2,1,3]thiadiazol‐4,7‐diyl)]] (F8BT) with 3wt% Cbz4‐[2.2]PC prepared by spin‐coating exhibits circularly polarized luminescence (CPL) properties with a high dissymmetry factor (|glum| = 0.01), which is much higher than that of Cbz4‐[2.2]PC in CH2Cl2 solution. By contrast, the doped film of F8BT with 3wt% tBuCbz4‐[2.2]PC exhibited only weak CPL properties. In addition, these chiral compounds show facile redox properties owing to the presence of carbazole units. Furthermore, Cbz4‐[2.2]PC undergoes electropolymerization to produce an insoluble thin film on the electrode.

Chirality Induction and Memory of Pillar[4]arene[1]quinone Derivatives in Visible-Light Range.

Pillar[4]arene[1]quinone derivatives (PQXs) were synthesized by the oxidation of pillar[5]arenes, which exhibited notable charge transfer (CT) transitions at approximately 485 nm. Successful chiral resolution of two pairs of enantiomeric conformers was achieved. Despite reduced binding affinity, PQXs demonstrated slower racemization kinetics. Visible-light chiroptical induction with a significant dissymmetry factor was attained by complexing PQXs with a chiral guest. The induced enantiomeric excess could be maintained through competitive binding with an achiral guest, offering a promising strategy for chiral sensing and memory.

Er3+/Yb3+ ions doped chiral inorganic nanostructured BiOCl phosphor as a novel temperature sensing material

AbstractInorganic chiral materials have attracted increasing attention due to their application prospects. In this work, we synthesized the chiral nanostructured bismuth oxychloride co‐doped with Er3+/Yb3+ phosphor (D‐BYEP) by hydrothermal growth with a subsequent air‐annealing strategy using chiral sugar alcohols as the chiral inducer. The D‐BYEP emitted bright red light under the excitation of 980 nm near‐infrared laser. The temperature sensing behavior was evaluated by employing the thermally coupled (TCLs) or non‐thermally coupled (NTCLs) levels of Er3+. The maximum relative sensitivities (SR) obtained are 0.84% K−1 and 1.30% K−1 (298 K), respectively. The results show that the D‐BYEP may be ideal candidates for optical temperature sensors. We anticipate that this new type of inorganic chiral phosphor may have a range of applications in temperature measurement of light sources.