Supramolecular alternating copolymers with highly efficient fluorescence resonance energy transfer

This article reports alternating supramolecular copolymerization of two naphthalene-diimide (NDI)-derived building blocks (NDI-1 and NDI-2) under thermodynamic control. Both monomers contain a central NDI chromophore, attached to a hydrocarbon-chain and a carboxylic-acid group. The NDI core in NDI-2 is symmetrically substituted with two butane-thiol groups, which makes it distinct from NDI-1. In decane, a 1 : 1 mixture of NDI-1 and NDI-2 shows spontaneous gelation and a typical fibrillar network, unlike the behavior of either of the components individually. The solvent-dependent UV/vis spectrum of the mixed sample in decane shows bathochromically shifted sharp absorption bands and a sharp emission band (holds a mirror-image relationship) with a significantly small Stokes shift compared to those in CHCl3, indicating J-aggregation. In contrast, the aggregated spectra of the individual monomers show broad structureless features, suggesting ill-defined aggregates. Cooling curves derived from the temperature-dependent UV/vis spectroscopy studies revealed early nucleation and a signature of well-defined cooperative polymerization for the mixed sample, unlike either of the individual components. Molecular dynamics simulations predicted the greatest dimer formation tendency for the NDI-1 + NDI-2 (1 : 1), followed by pure NDI-1 and NDI-2. Theoretical studies further revealed a partial positive charge in the NDI ring of NDI-1 when compared to NDI-2, promoting the alternating stacking propensity, which is also favored by the steric factor as NDI-2 is core-substituted with alkyl thiols. Such theoretical predictions fully corroborate with the experimental results showing 1 : 1 stoichiometry (from Job's plot) of the two monomers, indicating alternate stacking sequences in the H-bonded (syn-syn catemer type) supramolecular copolymer. Such alternating supramolecular copolymers showed highly efficient (>93%) fluorescence resonance energy transfer (FRET).

Cellulose levulinate ester as a robust building block for the synthesis of fully biobased functional cellulose esters

Facile modification of cellulose or cellulosic derivatives is one of the important strategies to prepare materials with targeted properties, multifunctionality, thus extending their applications in various fields. Cellulose levulinate ester (CLE) has the structural advantage of acetyl propyl ketone moiety pendant, on which fully biobased cellulose levulinate ester derivatives (CLEDs) have been successfully designed and prepared via aldol condensation reaction of CLE with lignin-derived phenolic aldehydes catalyzed by DL-proline. The structure of CLEDs are featured by a phenolic α,β-unsaturated ketone structure, thus endowing them with good UV absorption properties, excellent antioxidant activity, fluorescence properties and satisfactory biocompatibility. The utility of this aldol reaction strategy, together with the facile tunable substitution degree of cellulose levulinate ester and the diversity of aldehydes, can provide potentially a large spectrum of structurally diverse functionalized cellulosic polymers and create new avenues to advanced polymeric architectures.

Micelle and Nanotape Formation of Benzene Tricarboxamide Analogues with Selective Cancer Cell Cytotoxicity

Analogues of benzene-1,3,5-tricarboxamide bearing combinations of different alkyl chains (dodecyl to octadecyl) and ester-linked PEG (polyethylene glycol) chains are shown to self-assemble into either micelles or nanotapes in aqueous solution, depending on the architecture (number of alkyl vs PEG chains). The cytotoxicity to cells is selectively greater for breast cancer cells than fibroblast controls in a dose-dependent manner. The compounds show strong stability, retaining their self-assembled structures at low pH (relevant to acidic tumor conditions) and in buffer and cell culture media.

Switchable aqueous catalytic systems for organic transformations

In living organisms, enzyme catalysis takes place in aqueous media with extraordinary spatiotemporal control and precision. The mechanistic knowledge of enzyme catalysis and related approaches of creating a suitable microenvironment for efficient chemical transformations have been an important source of inspiration for the design of biomimetic artificial catalysts. However, in "nature-like" environments, it has proven difficult for artificial catalysts to promote effective chemical transformations. Besides, control over reaction rate and selectivity are important for smart application purposes. These can be achieved via incorporation of stimuli-responsive features into the structure of smart catalytic systems. Here, we summarize such catalytic systems whose activity can be switched 'on' or 'off' by the application of stimuli in aqueous environments. We describe the switchable catalytic systems capable of performing organic transformations with classification in accordance to the stimulating agent. Switchable catalytic activity in aqueous environments provides new possibilities for the development of smart materials for biomedicine and chemical biology. Moreover, engineering of aqueous catalytic systems can be expected to grow in the coming years with a further broadening of its application to diverse fields.

Chirality-directed hydrogel assembly and interactions with enantiomers of an active pharmaceutical ingredient

Enantiomers of the low-molecular-weight gelator (LMWG) DBS-CONHNH₂, based on D- or L- 1,3 : 2,4-dibenzylidenesorbitol (DBS), were synthesised. Enantiomeric gels are equivalent, but when mixtures of enantiomers are used, although gels still form, they are weaker than homochiral gels. Nanoscale chirality is lost on adding even a small proportion of the opposite enantiomer - homochiral assembly underpins effective gelation. Enantiomeric gels encapsulate the two enantiomers of anti-inflammatory drug naproxen, with thermal & mechanical differences between diastereomeric systems. We hence demonstrate the importance of chirality in DBS assembly and its interactions with chiral additives.

A cucurbit[8]uril-stabilized 3D charge transfer supramolecular polymer with a remarkable confinement effect for enhanced photocatalytic proton reduction and thioether oxidation

A water-soluble porous supramolecular polymer is assembled through a CB[8]-based 2 + 2 host–guest binding motif, which can greatly increase the efficiency of photocatalysis.

Anisotropic nanomaterials for asymmetric synthesis

The production of enantiopure chemicals is an essential part of modern chemical industry. Hence, the emergence of asymmetric catalysis led to dramatic changes in the procedures of chemical synthesis, and now it provides the most advantageous and economically executable solution for large-scale production of chiral chemicals. In recent years, nanostructures have emerged as potential materials for asymmetric synthesis. Indeed, on the one hand, nanomaterials offer great opportunities as catalysts in asymmetric catalysis, due to their tunable absorption, chirality, and unique energy transfer properties; on the other hand, the advantages of the larger surface area, increased number of unsaturated coordination centres, and more accessible active sites open prospects for catalyst encapsulation, partial or complete, in a nanoscale cavity, pore, pocket, or channel leading to alteration of the chemical reactivity through spatial confinement. This review focuses on anisotropic nanomaterials and considers the state-of-the-art progress in asymmetric synthesis catalysed by 1D, 2D and 3D nanostructures. The discussion comprises three main sections according to the nanostructure dimensionality. We analyze recent advances in materials and structure development, discuss the functional role of the nanomaterials in asymmetric synthesis, chirality, confinement effects, and reported enantioselectivity. Finally, the new opportunities and challenges of anisotropic 1D, 2D, and 3D nanomaterials in asymmetric synthesis, as well as the future prospects and current trends of the design and applications of these materials are analyzed in the Conclusions and outlook section.

Unraveling the Complexity of Supramolecular Copolymerization Dictated by Triazine-Benzene Interactions

Supramolecular copolymers formed by the noncovalent synthesis of multiple components expand the complexity of functional molecular systems. However, varying the composition and microstructure of copolymers through tuning the interactions between building blocks remains a challenge. Here, we report a remarkable discovery of the temperature-dependent supramolecular copolymerization of the two chiral monomers 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)tribenzamide (S-T) and 4,4′,4″-(benzene-1,3,5-triyl)tribenzamide (S-B). We first demonstrate in the homopolymerization of the two individual monomers that a subtle change from the central triazine to benzene in the chemical structure of the monomers significantly affects the properties of the resulting homopolymers in solution. Homopolymers formed by S-T exhibit enhanced stability in comparison to S-B. More importantly, through a combination of spectroscopic analysis and theoretical simulation, we reveal the complex process of copolymerization: S-T aggregates into homopolymers at elevated temperature, and upon slow cooling S-B gradually intercalates into the copolymers, to finally give copolymers with almost 80% alternating bonds at 10 °C. The formation of the predominantly alternating copolymers is plausibly contributed by preferred heterointeractions between triazine and benzene cores in S-T and S-B, respectively, at lower temperatures. Overall, this work unravels the complexity of a supramolecular copolymerization process where an intermediate heterointeraction (higher than one homointeraction and lower than the other homointeraction) presents and proposes a general method to elucidate the microstructures of copolymers responsive to temperature changes.

Water compatible supramolecular polymers: recent progress

Water compatible supramolecular polymers (WCSPs) combine aqueous compatibility with the reversibility and environmental responsiveness of supramolecular polymers. WCSPs have seen application across a number of fields, including stimuli-responsive materials, healable materials, and drug delivery, and are attracting increasing attention from the design, synthesis, and materials perspectives. In this review, we summarize the chemistry of WCSPs from 2016 to mid-2021. For the sake of discussion, we divide WCSPs into five categories based on the core supramolecular approaches at play, namely hydrogen-bonding arrays, electrostatic interactions, large π-conjugated subunits, host-guest interactions, and peptide-based systems, respectively. We discuss both synthesis and polymer structure, as well as the underlying design expectations. The goal of this overview is to deepen our understanding of the strategies that have been exploited to prepare WCSPs, as well as their properties and uses. Thus, a section devoted to potential applications is included in this review.

Benzene tricarboxamide derivatives with lipid and ethylene glycol chains self-assemble into distinct nanostructures driven by molecular packing

The self-assembly in aqueous solution of benzene-1,3,5-tricarboxamide (BTA) bearing one alkyl chain and two PEG (polyethylene glycol) chains or two alkyl chains and one PEG chain yields completely distinct nanostructures. Two series of derivatives were synthesized and extensively characterized and electron microscopy and small-angle X-ray scattering (SAXS) reveal micelle structures for derivatives with one alkyl and two PEG chains, but nanotapes and nanoribbons for the series with two alkyl and one PEG chain.

Experimental and computational diagnosis of the fluxional nature of a benzene-1,3,5-tricarboxamide-based hydrogen-bonded dimer

Precise characterization of the hydrogen bond network present in discrete self-assemblies of benzene-1,3,5-tricarboxamide monomers derived from amino-esters (ester BTAs) is crucial for the construction of elaborated functional co-assemblies. For all ester BTA dimeric structures previously reported, ester carbonyls in the side chain acted as hydrogen bond acceptors, yielding well-defined dimers stabilized by six hydrogen bonds. The ester BTA monomer derived from glycine (BTA Gly) shows a markedly different self-assembly behaviour. We report herein a combined experimental and computational investigation aimed at determining the nature of the dimeric species formed by BTA Gly. Two distinct dimeric structures were characterized by single-crystal X-ray diffraction measurements. Likewise, a range of spectroscopic and scattering techniques as well as molecular modelling were employed to diagnose the nature of dynamic dimeric structures in toluene. Our results unambiguously establish that both ester and amide carbonyls are involved in the hydrogen bond network of the discrete dimeric species formed by BTA Gly. The participation of roughly 4.5 ester carbonyls and 1.5 amide carbonyls per dimer as determined by FT-IR spectroscopy implies that several conformations coexist in solution. Moreover, NMR analysis and modelling data reveal rapid interconversion between these different conformers leading to a symmetric structure on the NMR timescale. Rapid hydrogen bond shuffling between conformers having three (three), two (four), one (five) and zero (six) amide carbonyl groups (ester carbonyl groups, respectively) as hydrogen bond acceptors is proposed to explain the magnetic equivalence of the amide N-H on the NMR timescale. When compared to other ester BTA derivatives in which only ester carbonyls act as hydrogen bond acceptors, the fluxional behaviour of the hydrogen-bonded dimers of BTA Gly likely originates from a larger range of energetically favorable conformations accessible through rotation of the BTA side chains.

Ultrasonication-Assisted Synthesis of a d-Glucosamine-Based β-CD Inclusion Complex and Its Application as an Aqueous Heterogeneous Organocatalytic System

For the first time, an inclusion complex has been crafted between a carbohydrate-based molecule and a β-cyclodextrin (CD) hydrophobic cavity for asymmetric catalytic applications. This novel d-glucosamine-based inclusion compound has been synthesized in high yields using an innovative and proficient acoustic cavitation technology and well characterized using various techniques, such as infrared spectroscopy, nuclear magnetic resonance spectroscopy, ultraviolet-visible spectroscopy, thermogravimetric analysis, scanning electron microscopy, and other spectroscopic techniques. It was observed that the inclusion of a d-glucosamine derivative into the hydrophobic cavity of β-CD increased its surface area and thermal stability. This catalytic system worked well in water for the direct aldol reaction to afford the products in excellent yields with high diastereo- and enantioselectivities.

Catalytic Gels for a Prebiotically Relevant Asymmetric Aldol Reaction in Water: From Organocatalyst Design to Hydrogel Discovery and Back Again

This paper reports an investigation into organocatalytic hydrogels as prebiotically relevant systems. Gels are interesting prebiotic reaction media, combining heterogeneous and homogeneous characteristics with a structurally organized active "solid-like" catalyst separated from the surrounding environment, yet in intimate contact with the solution phase and readily accessible via "liquid-like" diffusion. A simple self-assembling glutamine amide derivative 1 was initially found to catalyze a model aldol reaction between cyclohexanone and 4-nitrobenzaldehyde, but it did not maintain its gel structure during reaction. In this study, it was observed that compound 1 could react directly with the benzaldehyde to form a hydrogel in situ based on Schiff base 2 as a low-molecular-weight gelator (LMWG). This new dynamic gel is a rare example of a two-component self-assembled LMWG hydrogel and was fully characterized. It was demonstrated that glutamine amide 1 could select an optimal aldehyde component and preferentially assemble from mixtures. In the hunt for an organocatalyst, reductive conditions were applied to the Schiff base to yield secondary amine 3, which is also a highly effective hydrogelator at very low loadings with a high degree of nanoscale order. Most importantly, the hydrogel based on 3 catalyzed the prebiotically relevant aldol dimerization of glycolaldehyde to give threose and erythrose. In buffered conditions, this reaction gave excellent conversions, good diastereoselectivity, and some enantioselectivity. Catalysis using the hydrogel of 3 was much better than that using non-assembled 3-demonstrating a clear benefit of self-assembly. The results suggest that hydrogels offer a potential strategy by which prebiotic reactions can be promoted using simple, prebiotically plausible LMWGs that can selectively self-organize from complex mixtures. Such processes may have been of prebiotic importance.

Visible-light photooxidation in water by 1O2-generating supramolecular hydrogels

An aqueous photocatalytic system exploits photophysical properties arising from the formation of supramolecular hydrogels, with properties and assembly modulated by the amino acids appended to an organic chromophore.

Chiral Gating for Size- and Shape-Selective Asymmetric Catalysis

A poor or mediocre stereoselectivity is a key roadblock for a chiral catalyst to find practical adoptions. We report a facile method to create a tunable chiral space near a chiral catalyst to augment its selectivity. The space was created rationally through templated polymerization within cross-linked micelles, using readily available amino acid derivatives. It provided gated entrance of reactants to the catalyst, enabling a mediocre prolinamide to catalyze aldol condensation in water with excellent yields and ee, in a size- and shape-selective manner.

The pathway and kinetics of hierarchical assembly of ionic oligomers into a lyotropic columnar phase

Ionic benzene-1,3,5-tricarboxamide (BTA) molecules can self-assemble into hollow cylinders which further arrange into columnar phases in water. In situ investigations suggest a multi-step pathway of supramolecular assembly via formation of dispersed molecular aggregates followed by a less ordered intermediate phase before the equilibrium columnar phase is formed. The pathway and kinetics of the formation of lyotropic LC phases through hierarchical supramolecular assembly are similar to non-classical crystallization, in line with an emerging holistic view on crystallization and self-assembly.

Light-Responsive Arylazopyrazole Gelators: From Organic to Aqueous Media and from Supramolecular to Dynamic Covalent Chemistry

Versatile photoresponsive gels based on tripodal low molecular weight gelators (LMWGs) are reported. A cyclohexane-1,3,5-tricarboxamide (CTA) core provides face-to-face hydrogen bonding and a planar conformation, inducing the self-assembly of supramolecular polymers. The CTA core was substituted with three arylazopyrazole (AAP) arms. AAP is a molecular photoswitch that isomerizes reversibly under alternating UV and green light irradiation. The E isomer of AAP is planar, favoring the self-assembly, whereas the Z isomer has a twisted structure, leading to a disassembly of the supramolecular polymers. By using tailor-made molecular design of the tripodal gelator, light-responsive organogels and hydrogels were obtained. Additionally, in the case of the hydrogels, AAP was coupled to the core through hydrazones, so that the hydrogelator and, hence, the photoresponsive hydrogel could also be assembled and disassembled by using dynamic covalent chemistry.

Extra hydrogen bonding interactions by peripheral indole groups stabilize benzene-1,3,5-tricarboxamide helical assemblies

Benzene-1,3,5-tricarboxamide monomers derived from alkyl esters of tryptophan (BTA Trp) self-assemble into helices with an inner threefold hydrogen bond network surrounded by a second network involving the indole N-H groups. As a consequence of this extra stabilization of its helical assemblies, BTA Trp forms more viscous solutions than a range of ester and alkyl BTAs.

Impact of the water-compatible periphery on the dynamic and structural properties of benzene-1,3,5-tricarboxamide based amphiphiles

The consequences of using saccharides versus tetra(ethyleneglycol) chains as water-compatible moieties on the morphology and dynamics of supramolecular polymers in aqueous solutions are investigated. The saccharides form many H-bonds with other saccharides within the polymer and with water, increasing the hydration of the fiber and changing its dynamics.

Norbornene chaotropic salts as low molecular mass ionic organogelators (LMIOGs)

Phenylalanine functionalised norbornene (9:Na) functions as a potent, low molecular-mass (MW = 333 Da) ionic organogelator with a minimum gelating concentration of 0.5 wt% in THF, i-PrOH, 1,4-dioxane and n-BuOH. Fibrous crystals form in the gel and X-ray crystallography identified a cation mediated helical assembly process controlled by the chirality of the phenylalanine. In addition to excellent gelating properties 9:Na readily forms aqueous biphasic and triphasic systems.

Efficient sampling of reversible cross-linking polymers: Self-assembly of single-chain polymeric nanoparticles

We present a new simulation technique to study systems of polymers functionalized by reactive sites that bind/unbind forming reversible linkages. Functionalized polymers feature self-assembly and responsive properties that are unmatched by the systems lacking selective interactions. The scales at which the functional properties of these materials emerge are difficult to model, especially in the reversible regime where such properties result from many binding/unbinding events. This difficulty is related to large entropic barriers associated with the formation of intra-molecular loops. In this work, we present a simulation scheme that sidesteps configurational costs by dedicated Monte Carlo moves capable of binding/unbinding reactive sites in a single step. Cross-linking reactions are implemented by trial moves that reconstruct chain sections attempting, at the same time, a dimerization reaction between pairs of reactive sites. The model is parametrized by the reaction equilibrium constant of the reactive species free in solution. This quantity can be obtained by means of experiments or atomistic/quantum simulations. We use the proposed methodology to study the self-assembly of single-chain polymeric nanoparticles, starting from flexible precursors carrying regularly or randomly distributed reactive sites. We focus on understanding differences in the morphology of chain nanoparticles when linkages are reversible as compared to the well-studied case of irreversible reactions. Intriguingly, we find that the size of regularly functionalized chains, in good solvent conditions, is non-monotonous as a function of the degree of functionalization. We clarify how this result follows from excluded volume interactions and is peculiar of reversible linkages and regular functionalizations.

Coordination chemistry of flexible benzene-1,3,5-tricarboxamide derived carboxylates; notable structural resilience and vaguely familiar packing motifs

Flexible benzene-1,3,5-tricarboxamides (BTAs), organic species well-known for their tendencies to form functional soft-materials by virtue of their complementary hydrogen bonding, are explored as structurally reinforcing supramolecular building blocks in porous coordination polymers. We report the synthesis and characterisation of two related, carboxylate-terminated BTA derivatives, and the structure and functionality of their polymeric Cd(ii) complexes. The polycarboxylate ligand benzene-1,3,5-tricarboxamide tris(phenylacetic acid) H3L1 was prepared, and the analogous trimethyl benzene-1,3,5-tricarboxamide tris acetate Me3L2 was prepared and its single crystal structure elucidated. On reaction with cadmium nitrate in a DMF/H2O mixture, each BTA compound yielded coordination polymer species with columnar packing motifs comparable to the familiar BTA triple helix seen in purely organic systems. In the case of Me3L2, this transformation was achieved through a convenient in situ ester hydrolysis. Complex 1 is a 2-dimensional layered material containing tubular intralayer pores, in which amide-amide hydrogen bonding is a notable structural feature. In contrast, the structure of 2 contains no amide-amide hydrogen bonding, and instead a columnar arrangement of ligand species is linked by trinuclear Cd(ii) cluster nodes into a densely packed three-dimensional framework. The crystal structures revealed both materials exhibited significant solvent-accessible volume, and this was probed with thermal analysis and CO2 and N2 adsorption experiments; complex 2 showed negligible gas uptake, while compound 1 possesses an unusually high CO2 capacity for a two-dimensional material with intralayer porosity and surprising structural resilience to guest exchange, evacuation and exposure to air.

Polymer ionic liquid network: a highly effective reusable catalyst for one-pot synthesis of heterocyclic compounds

Significant efforts have been devoted to developing immobilized chiral catalysts with high activity, selectivity, and stability. In this present study, a new heterogeneous proline catalyst system was prepared based on strong noncovalent interactions between polymer ionic liquid (PIL) and l-proline. First, pyridine PILs, which can complex with l-proline monomers through noncovalent interactions, were synthesized using reversible addition–fragmentation chain transfer (RAFT) polymerization. The polymer network-supported chiral catalysts were obtained following further free radical polymerization. Different structures were formed in response to different ratios of PIL and chiral monomer, as well as different PIL anions, in the reactions. The new formed layer structures and synergic effects of PIL resulted in heterogeneous catalysts with high catalytic activity and enantioselectivity, thus endowing them with better catalytic performance for the one-pot synthesis of heterocyclic compounds compared to homogeneous catalytic systems. These catalytic systems were able to be reused and recycled five times with no discernible loss in catalytic activity and enantioselectivity. l-Proline was efficiently loaded onto the polymer network simply based on supramolecular interactions, providing a novel method of synthesizing high performance supported catalysts for organic reactions.

Significant efforts have been devoted to developing immobilized chiral catalysts with high activity, selectivity, and stability.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Tuning the nature and stability of self-assemblies formed by ester benzene 1,3,5-tricarboxamides: the crucial role played by the substituents

As the benzene 1,3,5-tricarboxamide (BTA) moiety is commonly used as the central assembling unit for the construction of functionalized supramolecular architectures, strategies to tailor the nature and stability of BTA assemblies are needed. The assembly properties of a library of structurally simple BTAs derived from amino dodecyl esters (ester BTAs, 13 members) have been studied, either in the bulk or in cyclohexane solutions, by means of a series of analytical methods (NMR, DSC, POM, FT-IR, UV-Vis, CD, ITC, high-sensitivity DSC, SANS). Two types of hydrogen-bonded species have been identified and characterized: the expected amide-bonded helical rods (or stacks) that are structurally similar to those formed by BTAs with simple alkyl side chains (alkyl BTAs), and ester-bonded dimers in which the BTAs are connected by means of hydrogen bonds linking the amide N-H and the ester C[double bond, length as m-dash]O. MM/MD calculations coupled with simulations of CD spectra allow for the precise determination of the molecular arrangement and of the hydrogen bond pattern of these dimers. Our study points out the crucial influence of the substituent attached on the amino-ester α-carbon on the relative stability of the rod-like versus dimeric assemblies. By varying this substituent, one can precisely tune the nature of the dominant hydrogen-bonded species (stacks or dimers) in the neat compounds and in cyclohexane over a wide range of temperatures and concentrations. In the neat BTAs, stacks are stable up to 213 °C and dimers above 180 °C whilst in cyclohexane stacks form at c* > 3 × 10^-5 M at 20 °C and dimers are stable up to 80 °C at 7 × 10^-6 M. Ester BTAs that assemble into stacks form a liquid-crystalline phase and yield gels or viscous solutions in cyclohexane, demonstrating the importance of controlling the structure of these assemblies. Our systematic study of these structurally similar ester BTAs also allows for a better understanding of how a single atom or moiety can impact the nature and stability of BTA aggregates, which is of importance for the future development of functionalized BTA supramolecular polymers.

Correlation between the Selectivity and the Structure of an Asymmetric Catalyst Built on a Chirally Amplified Supramolecular Helical Scaffold

For the first time, supramolecular helical rods composed of an achiral metal complex and a complementary enantiopure monomer provided a good level of enantioinduction in asymmetric catalysis. Mixtures containing an achiral ligand monomer (BTA(PPh2), 2 mol %) and an enantiopure ligand-free comonomer (ester BTA, 2.5 mol %), both possessing a complementary benzene-1,3,5-tricarboxamide (BTA) central unit, were investigated in combination with [Rh(cod)2]BArF (1 mol %) in the asymmetric hydrogenation of dimethyl itaconate. Notably, efficient chirality transfer occurs within the hydrogen-bonded coassemblies formed by BTA Ile and the intrinsically achiral catalytic rhodium catalyst, providing the hydrogenation product with up to 85% ee. The effect of the relative content of BTA Ile as compared to the ligand was investigated. The amount of chiral comonomer can be decreased down to one-fourth of that of the ligand without deteriorating the enantioselectivity of the reaction, while the enantioselectivity decreases for mixtures containing high amounts of BTA Ile. The nonlinear relationship between the amount of chiral comonomer and the enantioselectivity indicates that chirality amplification effects are at work in this catalytic system. Also, right-handed helical rods are formed upon co-assembly of the achiral rhodium complex of BTA(PPh2) and the enantiopure comonomer BTA Ile as confirmed by various spectroscopic and scattering techniques. Remarkably, the major enantiomer and the selectivity of the catalytic reaction are related to the handedness and the net helicity of the coassemblies, respectively. Further development of this class of catalysts built on chirally amplified helical scaffolds should contribute to the design of asymmetric catalysts operating with low amounts of chiral entities.

Temperature-responsive hairy particle-supported proline for direct asymmetric aldol reaction in water

In this paper, three kinds of hairy particles with different brush structures were prepared and evaluated as chiral catalysts in the direct asymmetric aldol reaction.