Planar Chirality Research Articles

Pressure-driven, solvation-directed planar chirality switching of cyclophano-pillar[5]arenes (molecular universal joints).

Planar chiral cyclophanopillar[5]arenes with a fused oligo(oxyethylene) or polymethylene subring (MUJs), existing as an equilibrium mixture of subring-included (in) and -excluded (out) conformers, respond to hydrostatic pressure to exhibit dynamic chiroptical property changes, leading to an unprecedented pressure-driven chirality inversion and the largest ever-reported leap of anisotropy (g) factor for the MUJ with a dodecamethylene subring. The pressure susceptivity of MUJs, assessed by the change in g per unit pressure, is a critical function of the size and nature of the subring incorporated and the solvent employed. Mechanistic elucidations reveal that the in–out equilibrium, as the origin of the MUJ's chiroptical property changes, is on a delicate balance of the competitive inclusion of subrings versus solvent molecules as well as the solvation of the excluded subring. The present results further encourage our use of pressure as a unique tool for dynamically manipulating various supramolecular devices/machines.

Asymmetric synthesis, structures, and chiroptical properties of helical cycloparaphenylenes.

Planar chiral carbon nanorings and nanobelts (CNRs and CNBs), the sidewall segment molecules of chiral-type carbon nanotubes (CNTs), have attracted attention owing to their characteristic chiroptical properties. From the appropriate CNTs, axially or planar chiral CNRs and CNBs have been designed and synthesized, but multiply helical sidewall segments were almost unexplored due to the difficulty in simultaneous control of multiple chiralities. In this article, we have succeeded in the perfectly diastereo- and enantiocontrolled catalytic synthesis of a cycloparaphenylene with four helical and two planar chiralities showing good chiroptical responses as chiral organic molecules. The perfectly stereocontrolled multiply helical structure was confirmed by a single-crystal X-ray diffraction analysis. The experimental and theoretical studies established the importance of the highly symmetric multiply helical structure in the cylindrical axis in obtaining good chiroptical responses.

Pre-regulation of the planar chirality of pillar[5]arenes for preparing discrete chiral nanotubes.

Regulating the chirality of macrocyclic host molecules and supramolecular assemblies is crucial because chirality often plays a role in governing the properties of these systems. Herein, we describe pillar[5]arene-based chiral nanotube formation via pre-regulation of the building blocks' chirality, which is different from frequently used post-regulation strategies. The planar chirality of rim-differentiated pillar[5]arenes is initially regulated by chiral awakening and further induction/inversion through stepwise achiral external stimuli. The pre-regulated chiral information is well stored in discrete nanotubes by interacting with a per-alkylamino-substituted pillar[5]arene. Such pre-regulation is more efficient than post-regulating the chirality of nanotubes.

Synthesis, structural characterization, and bonding analysis of two-coordinate copper(I) and silver(I) complexes of pyrrole-based bis(phosphinimine): new metal-pyrrole ring π-interactions.

The reaction between 2,5-bis(diphenylphosphinomethyl)pyrrole and Me3SiN3 gave the new pyrrole-based bis(phosphinimine) L1H in an excellent yield. L1H reacts with [CuCl(COD)]2, AgBF4, or AgOTf to give the corresponding two-coordinate mononuclear ionic complex formulated as [M{(L1H)-κ2N,N}]+[X]- where M = Cu and Ag; X = [CuCl2], BF4 or OTf. Their single crystal X-ray diffraction studies confirmed the two-coordinate geometry formed by the chelate bonding mode of L1H. These 10-membered metalacycles exhibit planar chirality and were also characterized by spectroscopic methods. In addition, in all three structures, there exists a hitherto unknown π-interaction between the pyrrole ring atoms and metal, represented as η2-(Cα-N) in the copper(i) complex, and η3-(Cα-N-Cα') in the silver(i) complexes. These weak interactions were supported by DFT calculations in terms of their electron densities, non-covalent interaction plots and the decrease in the aromaticity of the pyrrole ring.

Crown ether-pillararene hybrid macrocyclic systems.

A combination of Nobel macrocycle-crown ether and star macrocycle-pillararenes together in organic synthesis and material science is significant in obtaining hybrid systems, with rigid/flexible structural architecture, induced planar chirality, a negative cooperative effect and multiple fused cyclic hosts. In this review, we will discuss the synthesis/preparation of crown ether-pillararene hybrid macrocyclic systems by covalent bonds, supramolecular interactions and mechanical bonds, leading to hybrid compounds, supramolecular assemblies and mechanically interlocked molecules. The practical applications of crown ether-containing pillararenes will also be discussed in diverse areas, such as molecular recognition via fused multiple macrocycles and ion channels as well as external stimuli-responsive smart materials. We also call the attention of related researchers towards academic and technical issues about topological structures and applied functions in this fresh new fused macrocyclic field.

Adaptive Chirality of an Achiral Cage: Chirality Transfer, Induction, and Circularly Polarized Luminescence through Aqueous Host–Guest Complexation

Chirality transfer, induction, and circularly polarized luminescence (CPL) using supramolecular hosts, such as macrocycles and cages, have been explored for wide-ranging applications in chiral reco...

Optically active Pt-terpyridyl coordination assemblies derived from planar chiral metallothioligands

Optically active halogenated arenes containing stereogenic benzylic alcohol reacts with [CpRu(CH3CN)3][PF6] to give the major diastereomer with 90% de, in which the metal center is preferentially placed on one face of the arene ring. Thus starting with (S)-1-(2-chlorophenyl)ethanol the metal complex (pS, S)-[CpRu(HO(CH3)CH-C6H4-Cl)][PF6] (pS, S)-1 was obtained in good yield, while the (R)-alcohol provided (pR, R)-[CpRu(HO(CH3)CH-C6H4-Cl)][PF6] (pR, R)-1 metal complex. These compounds display planar and central chiralities. For comparison purposes the analogous racemic compound (rac)-[CpRu(HO(CH3)CH-C6H4-Cl)][PF6] (rac)-1 was also prepared in good yield as well. Subsequent treatment of enantiopure and racemic complexes 1 with NaSH in THF allows halogen displacement to occur at room temperature and generates the related enantiopure neutral metallothioligands of the type (pS, S)-[CpRu(HO(CH3)CH-C6H4-S)], (pS, S)-2, (pR, R)-[CpRu(HO(CH3)CH-C6H4-S)] (pR, R)-2, and (rac)-[CpRu(HO(CH3)CH-C6H4-S)] (rac)-2 (S = sulfur). These metallothioligands react rapidly with [(MeCN)Pt(terpy)][OTf]2 building blocks to give the chiral coordination assemblies [Pt(terpy)-(pS,S)-2][OTf]2 (pS,S)-3, [Pt(terpy)-(pR,R)-3][OTf]2 (pR,R)-3 and the related racemic complex (rac)-3 in which the metal-chromophore is now placed in a chiral environment. The molecular structures of (rac)-3 and the enantiopure heterobimetallic complex (pR,R)-3 were determined and show different arrangements at the supramolecular level depending on whether a homochiral or heterochiral assembly is obtained. The induced circular dichroism curves for the right-handed (pR,R)-3 and left-handed (pS,S)-3 bimetallic complexes displayed mirror image Cotton bands, confirming the enantiomeric relationship between both compounds. Our method provides an entry to the preparation of a wide range of optically pure coordination compounds with potentially important properties.

1,1′-2-Trisubstituted ferrocenyl dibismuthines containing N / O donor pendant arm

New unsymmetrical chiral 1,1′-2-ferrocenyl dibismuthines [(C5H4)(BiPh2)]Fe(2-{Me2NCH(R)}C5H3)(BiPh2) containing 2-[(Me2NCH(R)] where R = Me (1) or H (3) pendant arm on ferrocenyl ring were synthesized. An unsymmetrical 1,1′-ferrocenyl-bismuthine (5) in a low yield and 1,1′-2-ferrocenyl chlorodibismuthine (11) as an impurity were also isolated from the reaction mixture. Furthermore, Monoammonium salt [CpFe{2-(Me3N+CH2)C5H3}(BiPh2)2][I−] (6) and 1,1´,2-vinylferrocenyl dibismuthine derivative (7) were obtained by the reaction of CH3I with (3) and (1) respectively. The nucleophilic substitution reaction of (6) by o-hydroxyacetaphenone, salicylaldehyde, and N-methyl piperazine in acetonitrile gave the new functionalized 1,1′-2-substituted ferrocenyl dibismuthines [(C5H4)(BiPh2)]Fe[{2-(RCH2)C5H3}BiPh2] where R = OC6H4-2-C(O)Me (8), OC6H4-2-C(O)H (9) and N(CH2CH2)2NMe (10) respectively with different N or O containing pendant arm. 1,2-disubstituted ferrocenyl compounds presented in this work present planar chirality. These new compounds were characterized by IR, 1H-NMR, 13C-NMR spectroscopy, and High-Resolution Mass Spectrometry (HRMS). The X-ray structures of 1,1´,2-trisubstituted ferrocenyl dibismuthines (10) and (11) were determined. The molecular structure of (11) shows hypervalent Bi···N bonding in the solid-state. The compound (11) has planar chirality, central chirality, and Bi center-chirality with pseudo-trigonal bipyramidal geometry (see-saw structure) around the chiral Bi center while the other bismuth atom has distorted trigonal pyramidal geometry. The variable temperature 1H-NMR spectra of compound (3) in −40°C to 40°C range in CDCl3 indicates a lack of Bi···N coordination.

Molecular motor-functionalized porphyrin macrocycles

Molecular motors and switches change conformation under the influence of an external stimulus, e.g. light. They can be incorporated into functional systems, allowing the construction of adaptive materials and switchable catalysts. Here, we present two molecular motor-functionalized porphyrin macrocycles for future photo-switchable catalysis. They display helical, planar and point chirality, and are diastereomers, which differ in the relative orientation of the motor and macrocyclic components. Fluorescence, UV-vis, and 1H NMR experiments reveal that the motor-functionalized macrocycles can bind and thread different variants of viologen guests, including a one-side blocked polymeric one of 30 repeat units. The latter feature indicates that the motor systems can find the open end of a polymer chain, thread on it, and move along the chain to eventually bind at the viologen trap, opening possibilities for catalytic writing on single polymer chains via chemical routes.

Sequential Chiral Induction and Regulator‐Assisted Chiral Memory of Pillar[5]arenes

AbstractChirality transfer is widely observed in life processes, and many artificial chiral transfer systems have been developed. In these systems, chiral information is transferred from chiral inducers to chiral acceptors by a direct chiral induction process and a direct chiral memorization process. We have developed ternary nondirect chiral transfer systems based on pillar[5]arenes, in which a third factor was introduced as a regulator. The planar chirality of an acceptor was induced and memorized by a chiral inducer with precise control by a regulator. In the chiral induction period, the feed sequence of the chiral inducer and regulator affected the chiral induction behavior of the chiral acceptor. The chiral memory ability of the acceptor was substantially improved by the combined action of the chiral inducer and regulator.

Sequential Chiral Induction and Regulator-Assisted Chiral Memory of Pillar[5]arenes.

Chirality transfer is widely observed in life processes, and many artificial chiral transfer systems have been developed. In these systems, chiral information is transferred from chiral inducers to chiral acceptors by a direct chiral induction process and a direct chiral memorization process. We have developed ternary nondirect chiral transfer systems based on pillar[5]arenes, in which a third factor was introduced as a regulator. The planar chirality of an acceptor was induced and memorized by a chiral inducer with precise control by a regulator. In the chiral induction period, the feed sequence of the chiral inducer and regulator affected the chiral induction behavior of the chiral acceptor. The chiral memory ability of the acceptor was substantially improved by the combined action of the chiral inducer and regulator.

Control of Axial Chirality by Planar Chirality Based on Optically Active [2.2]Paracyclophane.

Optically active X-shaped molecules based on the planar chiral [2.2]paracyclophane building block were prepared, in which di(methoxy)terphenyl units were stacked on the central benzene rings. At 25 °C, anisolyl rings freely rotate in solution, while in the crystal form, they are fixed by intramolecular CH-π interactions, thereby leading to the expression of the axial chirality, i.e., propeller chirality was exhibited by the planar chiral [2.2]paracyclophane moiety. The X-shaped molecule exhibited good circularly polarized luminescence (CPL) profiles with moderate ΦPL and a large glum value in the order of 10-3 at 25 °C, in solution. In contrast, at -120 °C, dual CPL emission with opposite signs was observed. According to the theoretical studies, the rotary motion of the anisolyl units is suppressed in the excited states, and so emission from two isomers could be observed. These results demonstrate that the axial chirality was controlled by the planar chirality, leading ultimately to propeller chirality.

Cover Feature: Control of Axial Chirality by Planar Chirality Based on Optically Active [2.2]Paracyclophane (Chem. Eur. J. 65/2020)

Cover Feature: Control of Axial Chirality by Planar Chirality Based on Optically Active [2.2]Paracyclophane (Chem. Eur. J. 65/2020)

Formation of Planar Chiral Platinum Triangles via Pillar[5]arene for Circularly Polarized Luminescence.

Chiral metal-organic complexes hold great promise as new functional materials that exhibit unique stereochemical and optical properties. Here, we report the formation of optically pure pillar[5]arene-based platinum chiral metallacycles. By coordination with 60° and 90° Pt(II) acceptors, planar chiral platinum triangles were self-assembled efficiently and characterized by multiple spectroscopic techniques. Optical studies indicated that these metallacycles had chiral properties: pS enantiomers showed a negative Cotton effect, and pR enantiomers exhibited a positive Cotton effect. In addition, these metallacycles also exhibited circularly polarized luminescence.

Enantioselective Synthesis of Polycyclic Aromatic Hydrocarbon (PAH)-Based Planar Chiral Bent Cyclophanes by Rhodium-Catalyzed [2+2+2] Cycloaddition.

The enantioselective synthesis of polycyclic aromatic hydrocarbon (PAH)-based planar chiral cyclophanes was achieved for the first time by the rhodium-catalyzed intramolecular regio- and enantioselective [2+2+2] cycloaddition of tethered diyne-benzofulvenes followed by stepwise oxidative transformations. The thus synthesized planar chiral bent cyclophanes, that possess bent p-terphenyl- and 9-fluorenone-cores, were converted to 9-fluorenol-based ones with excellent ee values of >99 % by diastereoselective 1,2-reduction. These 9-fluorenol-based cyclophanes exhibited high fluorescence quantum yields, which were significantly higher than that of an acyclic reference molecule (78-82 % vs. 48 %). The bending effect on the chiroptical property was also examined, which revealed that the anisotropy factors (gabs values) for electronic circular dichroism (ECD) of these 9-fluorenol-based planar chiral bent cyclophanes increase as the tether length becomes shorter.

Chiral Transient Directing Group Strategies in Asymmetric Synthesis.

The development of novel methodologies for catalytic enantioselective functionalization reactions enabled by chiral transient directing groups is accompanying in a paradigm shift in the field of asymmetric synthesis. In particular, these highly atom- and step-economic enantioinduction processes commonly proceed either via enantioselective C-H functionalization, or via enantioselective hydroarylation of the pro-chiral substrates generating point, axial or planar chirality. The use of the transient directing group strategy in C-H functionalizations precludes the stoichiometric installations and removal of directing groups and enables efficient, more compatible and economical chemical routes. This minireview highlights asymmetric transition-metal-catalyzed methodologies involving chiral transient directing groups together with the scope, utility and future perspective of the field.

Cover Feature: Enantioselective Synthesis of Polycyclic Aromatic Hydrocarbon (PAH)‐Based Planar Chiral Bent Cyclophanes by Rhodium‐Catalyzed [2+2+2] Cycloaddition (Chem. Eur. J. 55/2020)

Cover Feature: Enantioselective Synthesis of Polycyclic Aromatic Hydrocarbon (PAH)‐Based Planar Chiral Bent Cyclophanes by Rhodium‐Catalyzed [2+2+2] Cycloaddition (Chem. Eur. J. 55/2020)

Post-translational formation of strained cyclophanes in bacteria.

Cyclic peptide natural products have served as important drug molecules, with several examples used clinically. Enzymatic or chemical macrocyclization is the key transformation for constructing these chemotypes. Methods to generate new and diverse cyclic peptide scaffolds enabling the modular and predictable synthesis of peptide libraries are desirable in drug discovery platforms. Here we identify a suite of post-translational modifying enzymes from bacteria that install single or multiple strained cyclophane macrocycles. The crosslinking occurs on three-residue motifs that include tryptophan or phenylalanine to form indole- or phenyl-bridged cyclophanes. The macrocycles display restricted rotation of the aromatic ring and induce planar chirality in the asymmetric indole bridge. The biosynthetic gene clusters originate from a broad range of bacteria derived from marine, terrestrial and human microbiomes. Three-residue cyclophane-forming enzymes define a new and significant natural product family and occupy a distinct region in sequence-function space.

Crystallization-Induced Planar Chirality by Asymmetric Ferrocene-Appended Tetraazanaphthacene

Although highly challenging, spontaneous crystallization without racemization can be an innovative strategy for the facile preparation of an enantioselective conglomerate with planar chirality. The...

Chiral Oxazolinyl Hydroxyl Ferrocene Catalysts: Synthesis and Applications in Asymmetric Reactions

AbstractChiral oxazolinyl hydroxyl ferrocenes have been demonstrated as representative catalysts in enantioselective nucleophilic addition reactions in the last few decades. They have received much interest from chemists because of not only the numerous substitution patterns at their sandwich backbones, but also their superior catalytic activity in asymmetric transformations. Recent research has revealed that the planar chirality of these catalysts influences the stereochemistry of asymmetric products and the application scope in enantioselective reactions. Herein, we provide an intensive survey of chiral oxazolinyl hydroxyl ferrocene compounds, including the structures of chiral compounds already published, designed synthetic approaches for ferrocenes with different patterns, and their utilization in the enantioselective preparation of secondary alcohols using organozinc reagents.