Chiral Monomer Research Articles

The Chiral Puzzle of Life

Abstract Biological molecules chose one of two structurally chiral systems which are related by reflection in a mirror. It is proposed that this choice was made, causally, by cosmic rays, which are known to play a major role in mutagenesis. It is shown that magnetically polarized cosmic rays that dominate at ground level today can impose a small, but persistent, chiral bias in the rate at which they induce structural changes in simple, chiral monomers that are the building blocks of biopolymers. A much larger effect should be present with helical biopolymers, in particular, those that may have been the progenitors of ribonucleic acid and deoxyribonucleic acid. It is shown that the interaction can be both electrostatic, just involving the molecular electric field, and electromagnetic, also involving a magnetic field. It is argued that this bias can lead to the emergence of a single, chiral life form over an evolutionary timescale. If this mechanism dominates, then the handedness of living systems should be universal. Experiments are proposed to assess the efficacy of this process.

Design and synthesis of cinchona-based chiral hyperbranched polymers and their application in asymmetric reactions

Cinchona-based chiral hyperbranched polymers (HBPs) were designed and successfully synthesized via the Mizoroki-Heck (MH) coupling reaction. AB2 and A2B-type chiral monomers were prepared from cinchona squaramide derivatives, where A (vinyl) reacted only with B (iodophenyl) under MH reaction conditions. The chiral HBPs obtained by the MH polymerization contained cinchona squaramide moieties and demonstrated excellent diastereoselectivity and enantioselectivity in asymmetric Michael addition reactions of methyl 2-oxocyclopentanecarboxylate and trans-β-nitrostyrene. These newly designed HBPs were structurally robust and could be reused for further reaction without losing their catalytic activity.

Synthesis and properties of cholesteric liquid crystal elastomers with selective reflection centred on D(+)-camphoric acid

ABSTRACT In this study, a series of cholesteric liquid crystal elastomers (ZSD series) was synthesised by hydrosilylation reaction among monomer ML1 containing cholesteryl group, monomer ML2 as the crosslinking agent, monomer ML3 containing D(+)-camphoric acid group. The chemical structures and mesogenic properties of monomers and elastomers were studied by conventional testing methods. By characterising the liquid crystal elastomers, it was found that they all showed colourful Grandjean texture, good elasticity and high thermal stability. For the ZSD series, the values of Tg first decreased and then increased and the values of Ti always decreased with the change of the content of chiral liquid crystal monomers, and all elastomers had the wide mesophase temperature ranges (ΔT). ZSD1-ZSD3 had selective reflection in the visible light region, while the observation for ZSD4-ZSD7 could only be observed under the appropriate external pressure. The effect of the novel chiral monomer (ML3) on the properties of elastomers had been investigated.

Photodynamic Control of the Chain Length in Supramolecular Polymers: Switching an Intercalator into a Chain Capper.

Supramolecular systems are intrinsically dynamic and sensitive to changes in molecular structure and external conditions. Because of these unique properties, strategies to control polymer length, composition, comonomer sequence, and morphology have to be developed for sufficient control over supramolecular copolymerizations. We designed photoresponsive, mono acyl hydrazone functionalized benzene-1,3,5-tricarboxamide (m-BTA) monomers that play a dual role in the coassembly with achiral alkyl BTAs (a-BTA). In the E isomer form, the chiral m-BTA monomers intercalate into stacks of a-BTA and dictate the chirality of the helices. Photoisomerization to the Z isomer transforms the intercalator into a chain capper, allowing dynamic shortening of chain length in the supramolecular aggregates. We combine optical spectroscopy and light-scattering experiments with theoretical modeling to show the reversible decrease in length when switching from the E to Z isomer of m-BTA in the copolymer with inert a-BTA. With a mass-balance thermodynamic model, we gain additional insights into the composition of copolymers and length distributions of the species over a broad range of concentrations and mixing ratios of a-BTA/m-BTA. Moreover, the model was used to predict the impact of an additive (chain capper and intercalator) on the chain length over a range of concentrations, showing a remarkable amplification of efficiency at high concentrations. By employing a stimuli-responsive comonomer in a mostly inert polymer, we can cooperatively amplify the effect of the switching and obtain photocontrol of polymer length. Moreover, this dynamic decrease in chain length causes a macroscopic gel-to-sol phase transformation of the copolymer gel, although 99.4% of the organogel is inert to the light stimulus.

Chiral molecularly imprinted polymeric stir bar sorptive extraction for naproxen enantiomer detection in PPCPs

Chiral micropollutant analysis in pharmaceuticals and personal care products (PPCPs) is interesting but challenging. We firstly developed a series of chiral molecularly imprinted polymeric (CMIP) stir bar sorptive extraction coatings by combining a chiral template with chiral functional monomers via a click reaction for naproxen enantiomer analysis in PPCPs. Heterochiral selectivity was observed in the molecule recognition of the CMIP coatings, which demonstrated good adsorption capability for the chiral template and its structurally similar chiral compounds. The coatings also exhibited excellent enrichment capability for chiral analytes in an aqueous matrix. The surface morphology and pore structure of the CMIP coatings were characterized. The molecular interactions between the chiral template and chiral functional monomer were investigated through UV–vis spectroscopy and theoretical calculations to prove the effective interactions existing in the heterochiral MIPs. The CMIP coatings were used to enrich naproxen enantiomers in chiral drug and environmental water samples, and satisfactory recoveries (83.98 %–118.88 %) with a relative standard deviation of 3.49 %–13.08 % were achieved. The heterochiral imprinted coating-based method provided a sensitive, selective, and effective enrichment strategy for chiral micropollutant analysis in PPCPs. This technique is critical for chiral molecule recognition and enantiomer analysis in complex samples.

Helical Assemblies of One-Dimensional Supramolecular Polymers Composed of Helical Macromolecules: Generation of Circularly Polarized Light Using an Infinitesimal Chiral Source.

We report the synthesis of one-dimensional supramolecular polymers composed of one-handed helical macromolecules bearing fluorescent pendant groups and the generation of circularly polarized light on the basis of hierarchical chiral amplification starting from a tiny amount of chiral substituent. Copolymerization of benzo[1,2-b:4,5-b']dithiophene-appended achiral/chiral isocyanides (99:1, mol/mol) with a solid-state photoluminescence feature afforded submicrometer supramolecular fibers, in which almost perfect single-handed helical polyisocyanides were noncovalently connected end to end. The resulting helical supramolecular polymers were further helically assembled to form a cholesteric liquid crystal film with an intense circularly polarized luminescence (CPL) signal. Surprisingly, the supramolecular system containing only 0.01 mol % of the chiral monomer unit also emitted the observable circularly polarized light owing to multiple chiral amplification from an infinitesimal point chirality to helical chirality and then to supramolecular chirality. Furthermore, chiral information was efficiently transferred from the helically assembled supramolecular system containing 1 mol % of the chiral unit to achiral dye molecules blended in the film, allowing full-color tunable induced CPL with luminescence dissymmetry factors greater than 1.0 × 10-2. This unprecedentedly strong chiral amplification enables the creation of helical supramolecular polymers and chirally assembled systems with various chiral functions based solely on an infinitesimal chiral source.

Recent Advances in Metal-Mediated Stereoselective Ring-Opening Polymerization of Functional Cyclic Esters towards Well-Defined Poly(hydroxy acid)s: From Stereoselectivity to Sequence-Control.

Poly(hydroxy acid)s are a family of biocompatible and (bio)degradable polyesters with various outcomes in different domains of application. To date, poly(hydroxy acid)s are best prepared by ring-opening polymerization (ROP) of the corresponding cyclic esters. Using racemic chiral monomers featuring side-chain groups enables to access, providing a stereoselective catalyst/initiator system is implemented, stereoregular functional polymers, thereby improving their physico-chemical properties, and ultimately, widening their range of uses. Here, we highlight a few important advances in metal-mediated stereoselective ROP of cyclic esters towards the synthesis of (functional) stereoregular poly(hydroxy acid)s that have recently been disclosed, emphasizing on (functional) β- and γ-lactones, diolide and O-carboxyanhydride (OCA) monomers and yttrium-based catalysis. Fine-tuning of the substituents flanked on the catalyst ligand enables reaching poly(hydroxy acid)s with syndiotactic and also isotactic microstructures. The stereocontrol mechanisms at work and their probable origin, relying on steric but also electronic factors imparted in particular by the ligand substituents, are discussed. Taking advantage of such stereoselective ROPs, original copoly(hydroxy acid)s with gradient or alternated patterns then become accessible from the use of mixtures of chemically different, oppositely configured enantiopure monomers.

The Effect of the Applied Temperature on the Maximum Refleactive Wavelength in Chiral Liquid Crystal Polymers

The liqud crystals (LCs) monomers 4-(Undecenoic-1-yloxybenzoyloxy)-4’-cyanobenzene e M1 and 5-{4-[4-(4-undecyloxy) biphenyl] -oxycarbonyl} pentanoic acid cholesteryl ester M2 are synthesized. The liquid crystals polymer was synthesized by nematic and chiral LC monomers. The property of the LC materials are detected by POM and DSC. The bandgaps of the LC polymer are influenced by the change of temperature. The effect of temperature on the reflective wavelength is decreased from 513nm to 394nm with temperature increased. The chiral LC polymer is a one dimension photonic materials. The optical and thermal property of the CLCs polymer is controlled by the out field, and widely used for display, smart windows and chiral liquid crystal laser etc.

Linear [3]Spirobifluorenylene: An S-Shaped Molecular Geometry of p-Oligophenyls.

An S-shaped p-oligophenyl derivative was synthesized, as exemplified by linear [3]spirobifluorenylene (R,P,R/S,M,S)-1, which consisted of two p-sexiphenyls tied at the spiro-carbons of three spirobifluorenes. The direct linking of spirobifluorenes results in the formation of strained p-oligophenyl chains, which support each other to maintain the strained structure. This curved structure was successfully constructed by Suzuki-Miyaura cross-coupling of the corresponding spirobifluorene building blocks. Chiral (R,P,R)-1 and (S,M,S)-1 were also synthesized using chiral monomers, and their chiroptical properties were investigated by CD and CPL spectroscopy. The benzene rings in (R,P,R/S,M,S)-1 are distorted with the largest bend angle of 11.6°. The strain energy of (S,M,S)-1 was estimated to be 88.4 kcal/mol, which lies between those for [6] and [7]CPP. Through-space orbital interactions (spiroconjugation) were evaluated by TD-DFT calculations, and the results suggest the contribution of a 15 nm red-shift in (S,M,S)-1 in comparison with the single strand molecule 10. In addition, spiroconjugation is discussed in terms of molecular orbitals, and the splitting of both HOMO and LUMO was revealed by the calculations. The LUMO splitting is particularly interesting, since it is in contrast to the degenerate LUMOs in a simple spirobifluorene. Desymmetrization based on the distortion of the spirobifluorene moieties induces the LUMO splitting.

In-situ photopolymerized polyhedral oligomeric silsesquioxane-derived monolithic capillary columns with quinidine functionality for enantioseparation by nano-liquid chromatography.

The successful fabrication of monolithic capillary columns for enantiomer separations was achieved within vinylized fused silica capillaries via fast "one-pot" photo-initiated free radical polymerization reaction. A mixture consisting of polyhedral oligomeric silsesquioxane, O-[2-(methacryloyloxy)ethylcarbamoyl]-10,11-dihydroquinidine was copolymerized in the presence of n-butanol, ethylene glycol and photo-initiator 2,2-dimethoxy-2-phenylacetophenone. The morphology of the resultant polymeric hybrid inorganic-organic material and its permeability as well as porosity can be controlled by adjusting the composition of the monomers and binary porogenic solvent. The chromatographic characteristics of the columns have been investigated. Separation factors of N-acetyl-phenylalanine (Ac-Phe) and dichlorprop dropped with decrease of chiral functional monomer. Permeability was better when the macroporogen ethyleneglycol was present at higher concentrations during the polymerization. In general, the chiral compounds were well separated (dichlorprop: α = 1.53, Rs up to 4.14; Ac-Phe: α = 1.36, Rs up to 2.69) by nano-HPLC with an optimized enantioselective monolithic capillary column which can be prepared within a few minutes.

Visible-Light-Controlled Reaction-Separation for Asymmetric Sulfoxidation in Water with Photoresponsive Metallomicelles

Metallomicellar catalysis has gained increasing attention in recent years since it made water-incompatible organometallic catalysis highly efficient in water. However, a convenient approach to combine activity and recovery of metallomicelles from aqueous systems has remained challenging to date. Herein, we developed a series of nitrospiropyran-containing metallomicelles which possessed visible-light-responsivity, to realize the visible-light-controlled reaction and separation for asymmetric sulfoxidation in water. A series of nitrospiropyran-containing diblock chiral salen TiIV copolymers were thus prepared by copolymerizing photosensitive nitrospiropyran derivative with hydrophobic chiral salen TiIV monomer via free-radical polymerization. Characterization results suggested their visible-light-controlled self-assembly behavior in water due to the negative photochromism of nitrospiropyran block. The resultant light-responsive metallomicelles induced a micellar catalysis approach in the dark for asymmetric...

Regio- and Stereospecific Cyclopolymerization of α,ω-Diynes by Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes.

Both solvent-free and acetonitrile-containing cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes of the general formula [Mo(NR')(CHCMe2 R)(NHC)(X)+ A- ] (R' = 2,6-Cl2 -C6 H3 , tBu, 2-CF3 -C6 H4 , 2-tBu-C6 H4 , 2,6-iPr2 -C6 H3 , 2,6-Me2 -C6 H3 ; R = Me, Ph; NHC = 1,3-dimesitylimidazol-2-ylidene (IMes), 1,3-di-iPr-imidazol-2-ylidene (IPr), 1,3,5-triphenyl-1,3,4-triazol-2-ylidene); X = CF3 SO3 , C6 F5 O, OCH(CF3 )2 , OC(CF3 )3 , pyrrolide, C6 F5 COO, 2,6-(CF3 )2 -C6 H3 COO; A- = B(ArF )4 - , Al(OC(CF3 )3 )4 - ), have been investigated for their propensity to cyclopolymerize 4,4-disubstituted 1,6-heptadiynes. All metal complexes contain a stereogenic (chiral) metal center, which accounts for the high reactivity and high regioselectivity of insertion (>99%) that are observed for all metal complexes, leading to highly conjugated, α-insertion-derived polyenes that are based on a highly regular polymer backbone and that show absorption maxima close to 600 nm. With the chiral monomer 4-(ethoxycarbonyl)-4-(1S,2R,5S)-(-)-menthoxycarbonyl-1,6-heptadiyne, high syndiospecifity (>99% syndiotactic) is observed. A mechanism explaining the high regio- and stereoselectivity is presented. Thus, α-addition of the monomers proceeds chain-end-controlled trans to the NHC and is preferred over β-addition through intramolecular Mo-O chelation. Insertion of the monomers entails double inversion at the stereogenic metal center in the course of one complete monomer insertion.

Chiral Conflict as a Method to Create Stimuli-Responsive Materials Based on Dynamic Helical Polymers.

A new multi-sensor material based on helical copolymers showing the chiral conflict effect have been prepared. It can successfully detect and identify diverse metal cations in solution. The design of this material has taken into account not only the opposite helical senses induced by the two chiral monomers in the copolymer, but also their dynamic behavior. The induced helical sense can thus be enhanced, diminished, or inverted by interaction with different stimuli (that is, metal ions). Thus, depending on both the copolymer compositions (such as monomer ratios and absolute configurations) and the nature of the metal ion, the response of these dynamic helical copolymers to adopt a single-handed P or M helix is unique, making it possible not only to detect their presence, but also to identify them individually. New multi-sensors materials based and inspired on this effect should arise in the future choosing appropriate monomers and stimuli.

Chiral Conflict as a Method to Create Stimuli‐Responsive Materials Based on Dynamic Helical Polymers

AbstractA new multi‐sensor material based on helical copolymers showing the chiral conflict effect have been prepared. It can successfully detect and identify diverse metal cations in solution. The design of this material has taken into account not only the opposite helical senses induced by the two chiral monomers in the copolymer, but also their dynamic behavior. The induced helical sense can thus be enhanced, diminished, or inverted by interaction with different stimuli (that is, metal ions). Thus, depending on both the copolymer compositions (such as monomer ratios and absolute configurations) and the nature of the metal ion, the response of these dynamic helical copolymers to adopt a single‐handed P or M helix is unique, making it possible not only to detect their presence, but also to identify them individually. New multi‐sensors materials based and inspired on this effect should arise in the future choosing appropriate monomers and stimuli.

N-Substituted Benzene-1-Urea-3,5-Biscarboxamide (BUBA): Easily Accessible C2 -Symmetric Monomers for the Construction of Reversible and Chirally Amplified Helical Assemblies.

Non-C3 -symmetric supramolecular helices are gaining interest for the design of hierarchical assemblies, for the compartmentalisation or the self-assembly of polymer chains and for application in asymmetric catalysis. Herein, N-substituted benzene-1-urea-3,5-biscarboxamide (BUBA) monomers, which consist of one urea and two carbon-connected amide functions linked to an aromatic ring, are introduced as an easily accessible class of C2 -symmetric supramolecular synthons. In apolar solvents, BUBA monomers assemble into long helical assemblies by means of hydrogen-bonding and aromatic interactions, as assessed by several analytical techniques. To probe the influence of the urea function, BUBA and related benzene-1,3,5-tricarboxamide (BTA) helical polymers have been compared, in terms of their thermodynamics of formation, stability, reversibility and chiral amplification properties. Similar to BTA, BUBA monomers form long helices reversibly through a highly cooperative mechanism and the helicity of their assemblies is governed by chiral amplification effects. However, precise quantification of their properties reveals that BUBA monomers assemble in a more cooperative manner. Also, chiral amplification operates to a higher extent in BUBA helices, as probed by both sergeants-and-soldiers and majority-rules experiments. Compatibility between urea and amide functions also allows the formation of co-assemblies that incorporate both BUBA and BTA monomers. Importantly, a small amount of chiral BUBA monomers in these co-assemblies is sufficient to obtain single-handed helices; thus paving the way towards the development of functional supramolecular helices.

P‐148: Chiral Polymer Network of Polymer‐stabilized Blue Phase Liquid Crystals

The effect of chiral polymer network in polymer‐stabilized blue phase liquid crystals (PS‐BPLCs) was investigated. Compare with the conventional polymer network, the chiral PS‐BPLCs may induce and stabilize the blue phase with less proportion of chiral dopant resulting in a higher Kerr constant. The hysteresis and residual birefringence may be improved by introducing a chiral monomer with more rigid structure. And the response time is little effected by the chiral polymer network.

Synthesis and characterization of (R)-miniPEG-containing chiral γ-peptide nucleic acids using the Fmoc strategy

Diethylene glycol (miniPEG)-containing chiral γPNA is considered to be one of the best PNA derivatives. Its preparation is mainly based on the Boc strategy for solid phase peptide synthesis (SPPS), requiring the repeated use of trifluoroacetic acid TFA, which is not suitable for the in situ synthesis of PNA arrays and some other applications under mild conditions. Herein, Fmoc/Cbz orthogonal protected miniPEG-containing chiral γPNA monomers were synthesized, and a 15mer γPNA was prepared using the Fmoc strategy under mild conditions.

Unexpectedly Strong Chiral Amplification of Chiral/Achiral and Chiral/Chiral Copolymers of Biphenylylacetylenes and Further Enhancement/Inversion and Memory of the Macromolecular Helicity.

We report an unexpectedly strong amplification of the macromolecular helicity in dynamic helical copolymers of chiral/achiral and chiral/chiral ( R/ S) biphenylylacetylenes in which the chiral residues are remote from the biphenyl pendants and further from the main chains. The copolymers consisting of 20 mol % chiral monomers and chiral monomers of 20% enantiomeric excess (ee) showed a full induced circular dichroism as intense as that of the chiral homopolymer. In contrast, an analogous poly(phenylacetylene) bearing the identical chiral residue (100% ee) showed no circular dichroism in the polymer backbone, indicating the critical role of the biphenyl moieties in the observed high chiral amplification. As anticipated, the helix-sense excesses of the copolymer backbones composed of a small amount of chiral units (<20 mol %) and chiral units of low ee (<20%) were reduced. Interestingly, however, the macromolecular helicity of the copolymers was further drastically enhanced as a greater excess of a one-handed helix or inverted upon noncovalent interaction with nonracemic alcohols and subsequently retained (memorized) after complete removal of the chiral alcohol. Even in a polymer consisting of completely racemic repeating units, one-handed right- and left-handed helices could almost be induced and memorized. These unique hierarchical amplifications and memory of the macromolecular helicity in the copolymers by the covalent and further noncovalent chiral interactions are quantitatively explained on the basis of a linear Ising model.

Facile Synthesis of Helical Rod–Coil Block Polymers by the Combination of ATRP and Pd(II)‐Initiated Isocyanides Polymerizations

AbstractA bifunctional initiator containing propargyl bromoisobutyrate and alkyne‐Pd(II) (PBB‐Pd(II)) is designed and synthesized. The propargyl bromoisobutyrate unit of PBB‐Pd(II) can initiate the atom transfer radical polymerization (ATRP) of vinyl monomers, while the Pd(II) complex can initiate the polymerization of phenyl isocyanides. Both the ATRP and Pd(II)‐mediated isocyanide polymerization are proceeded in living/controlled manner. Thus, combining the two living polymerizations, a series of well‐defined block copolymers bearing rod poly(phenyl isocyanide)s and coil poly(acrylate) segments can be facilely prepared in high yield with tunable composition, controlled molar masses (Mns), and narrow molar mass distributions (Mw/Mns). What is more, benefiting from this synthetic strategy, well‐defined core cross‐linked star polymers are readily synthesized. Optically active block copolymers and star polymers can be facilely obtained by using chiral isocyanide monomers due to the formation of predominated one‐handed helix. The chiral star polymers show excellent enantioselective recognition ability in enantioselective crystallization of racemic compounds.

The Chiral Trimer and a Metastable Chiral Dimer of Achiral Hexafluoroisopropanol: A Multi-Messenger Study.

1,1,1,3,3,3-hexafluoro-propan-2-ol aggregates preferentially into an achiral dimer of achiral monomers, but the trimer is found to prefer three metastable chiral monomer units arranged into a strained OH⋅⋅⋅O hydrogen-bonded ring, which is reinforced by secondary CH⋅⋅⋅FC interactions. This is shown by a combination of infrared, microwave, and Raman spectroscopy in supersonic jet expansions and supported by high-level quantum chemical calculations. It involves an activation of the monomers by >15 kJ mol-1 , clearly driven by the much stronger hydrogen-bond interaction available to the gauche and even more to the cis monomer units.