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Dynamic Imine Chemistry
Abstract
Formation of an imine--from an amine and an aldehyde--is a reversible reaction which operates under thermodynamic control such that the formation of kinetically competitive intermediates are, in the fullness of time, replaced by the thermodynamically most stable product(s). For this fundamental reason, the imine bond has emerged as an extraordinarily diverse and useful one in the hands of synthetic chemists. Imine bond formation is one of a handful of reactions which define a discipline known as dynamic covalent chemistry (DCC), which is now employed widely in the construction of exotic molecules and extended structures on account of the inherent 'proof-reading' and 'error-checking' associated with these reversible reactions. While both supramolecular chemistry and DCC operate under the regime of reversibility, DCC has the added advantage of constructing robust molecules on account of the formation of covalent bonds rather than fragile supermolecules resulting from noncovalent bonding interactions. On the other hand, these products tend to require more time to form--sometimes days or even months--but their formation can often be catalysed. In this manner, highly symmetrical molecules and extended structures can be prepared from relatively simple precursors. When DCC is utilised in conjunction with template-directed protocols--which rely on the use of noncovalent bonding interactions between molecular building blocks in order to preorganise them into certain relative geometries as a prelude to the formation of covalent bonds under equilibrium control--an additional level of control of structure and topology arises which offers a disarmingly simple way of constructing mechanically-interlocked molecules, such as rotaxanes, catenanes, Borromean rings, and Solomon knots. This tutorial review focuses on the use of dynamic imine bonds in the construction of compounds and products formed with and without the aid of additional templates. While synthesis under thermodynamic control is giving the field of chemical topology a new lease of life, it is also providing access to an endless array of new materials that are, in many circumstances, simply not accessible using more traditional synthetic methodologies where kinetic control rules the roost. One of the most endearing qualities of chemistry is its ability to reinvent itself in order to create its own object, as Berthelot first pointed out a century and a half ago.
... 1−3 One promising avenue is to prepare novel dynamic covalent polymer networks (DCPN), which typically can be chemically recycled (repaired, reshaped, and reprocessed) via dynamic chemistries such as transimination, transesterification, andtranscarbamoylation etc. 4 The reversible reaction between amines and ketones/ aldehydes is one of the most common reactions to construct robust molecules by the formation of covalent imine bonds in organic chemistry. 5 The imine bonds can break and reform simultaneously under triggers like heat, pressure, excessive free amines, catalysts, or a solvent via imine metathesis and exchange reactions. 4−6 In terms of the DCPN developed by dynamic imine bonds, vanillin is a widely investigated bioderived feedstock 7,8 due to the aldehyde group that allows for imine condensation, the capability of monomer recovery, and the retaining of highperformance properties. ...
... Imine chemistry is one of the few synthetic strategies to construct robust molecules with chemical recyclability due to the formation of dynamic covalent bonds. 4,5 Lignin is a widely used bioderived precursor for the synthesis of high-performance polymers or DCPN due to the abundant functional groups. 34 Herein, lignin-OAm and lignin-PDMS polyimines are successfully synthesized via a catalyst-free Schiff-base reaction between ketones in the modified lignin and amines in OAm and PDMS (Figure 1). ...
... [22][23][24] It is believed that the imine or azomethine group (>C=N-) is important for the bio screening of Schiff bases. [25][26][27] The present study mainly deals with the in vitro and in silico studies of five Schiff bases against these hazardous bacteria. ...
... With their distinct reactivity and diverse structures, the imine functional group serves as a versatile and crucial component, facilitating the construction of intricate molecular architectures and functional materials. 31,32 Owing to the electron-rich nature of the azomethine nitrogen atom through which they coordinate with metal ions, Schiff bases are known as one of the strongest chelators. Schiff bases possess remarkable pharmacological activities such as antibacterial, antifungal, and anticancer activities due to the presence of C]N moiety. ...
Chromones are well known as fundamental structural elements found in numerous natural compounds and medicinal substances. The Schiff bases of chromones have a much wider range of pharmacological applications such as antitumor, antioxidant, anti-HIV, antifungal, anti-inflammatory, and antimicrobial properties. A lot of research has been carried out on chromone-based copper(ii) Schiff-base complexes owing to their role in the organometallic domain and promise as potential bioactive cores. This review article is centered on copper(ii) Schiff-base complexes derived from chromones, highlighting their diverse range of pharmacological applications documented in the past decade, as well as the future research opportunities they offer.
... Most of these bond exchange mechanisms are activated by specific catalysts, which imposes constraints on their application and impedes their widespread adoption [13]. Catalyst-free solutions such as polyimines synthesised from aldehydes or ketones and primary amines have been investigated due to their facile heat-triggered imine exchange reactions that also enable their chemical recycling through hydrolysis or transamination reactions [14,15]. ...
We investigated the recycling process of carbon fibre-reinforced polyimine vitrimer composites and compared composites made from virgin and recycled fibres. The vitrimer matrix consisted of a two-component polyimine-type vitrimer system, and as reinforcing materials, we used nonwoven felt and unidirectional carbon fibre. Various diethylenetriamine (DETA) and xylene solvent ratios were examined to find the optimal dissolution conditions. The 20:80 DETA–xylene ratio provided efficient dissolution, and the elevated temperature (80 °C) significantly accelerated the process. Scaling up to larger composite structures was demonstrated. Scanning electron microscopy (SEM) confirmed effective matrix removal, with minimal residue on carbon fibre surfaces and good adhesion in recycled composites. The recycled nonwoven composite exhibited a decreased glass transition temperature due to the residual solvents in the matrix, while the UD composite showed a slight increase. Dynamic mechanical analysis on the recycled composite showed an increased storage modulus for nonwoven composites at room temperature and greater resistance to deformation at elevated temperatures for the UD composites. Interlaminar shear tests indicated slightly reduced adhesion strength in the reprocessed composites. Overall, this study demonstrates the feasibility of recycling vitrimer composites, emphasising the need for further optimisation to ensure environmental and economic sustainability while mitigating residual solvent and matrix effects.
... We chose imine bonds 24 as a dynamic linker between small molecular components to construct foldamer-based receptors based on complexation-driven equilibrium shifting. Imine bonds have been widely used in the development of functional supramolecular assemblies 25,26 in dynamic covalent chemistry. ...
The development of synthetic receptors capable of selectively binding guests with diverse structures and multiple functional groups poses a significant challenge. Here, we present the efficient assembly of foldamer-based receptors for monosaccharides, utilising the principles of complexation-induced equilibrium shifting and adaptive folding. Diimine 4 can be quantitatively assembled from smaller components when d-galactose is added as a guest among monosaccharides we examined. During this assembly, dual complexation-induced equilibrium shifts toward both the formation of diimine 4 and the conversion of d-galactose into α-d-galactofuranose are observed. Diimine 6 is quantitatively assembled in the presence of two different guests, methyl β-d-glucopyranoside and methyl β-d-galactopyranoside, resulting in the formation of two dimeric complexes: (6-MP)2⊃(methyl β-d-glucopyranoside)2 and (6-MM)2⊃(methyl β-d-galactopyranoside∙2H2O)2, respectively. These two complexes exhibit distinct folding structures with domain-swapping cavities depending on the bound guest and temperature. Interestingly, (6-MM)2⊃(methyl β-d-galactopyranoside∙2H2O)2 is exclusively formed at lower temperatures, while (6-MP)2⊃(methyl β-d-glucopyranoside)2 is only formed at higher temperatures.
... Under neutral conditions, they are regarded to be kinetically inert, even in pure water. However, they are known to be subject to all three reactions of imines: (a) exchange, (b) metathesis and (c) hydrolysis [13]. ...
A new Schiff base compound of N′-[(2,6-dichlorophenyl)methylidene]-2-{[3-(trifluoro-methyl)phenyl] amino}benzohydrazide was synthesized and characterized through various spectroscopic techniques, including infrared,1H NMR, 13C NMR spectroscopy and X-ray diffraction. Experimental results collected by XRD were compared with theoretical results obtained from Density functional theory method. Hirshfeld surface analysis was used to obtain three-dimension molecular surface and two-dimension fingerprint plots to illustrate the intermolecular bonding. Theoretical calculations provide valuable insights into both global and local chemical activity, as well as the properties of molecules and chemicals, including their nucleophilic and electrophilic nature. The DFT method at B3LYP/6-311++G(d,p) basis set was employed to study the optimized structure and geometric parameters, as well as to explore the frontier molecular orbitals, global reactive parameters, Mullikan population analaysis, Natural bond orbital and molecular electrostatic potential characteristics which cannot be obtained by experimental methods. Additionally, electrophilicity based charge transfer study was carried out with DNA bases to determine the direction of charge transfer. Finally, an investigation was carried out using molecular docking analysis to examine the binding energies of the title compound with PDB ID: 2QDJ protein target. The analysis yielded significant insights into the possible interactions, offering valuable findings in the process.
... As imine bonds can be conveniently cleaved under acidic conditions, 47,48 we subjected P(FIm)-TREN to a solution of MeTHF and 1 M HCl (v : v = 2 : 8) at 50°C for 24 hours with continuous stirring (Scheme 1C). Within several hours, the bulk material started to break. ...
Over the past few decades, thermosetting plastics have emerged as indispensable materials in both industrial applications and our daily lives, primarily due to their exceptional thermal and mechanical properties resulting from their covalently crosslinked structures. Nevertheless, conventional thermosets face a significant environmental challenge due to their inability to be recycled and reliance on the petroleum resources. Consequently, there is an urgent need to develop innovative, biobased thermosetting materials that are smartly designed to enable efficient chemical recycling, thus contributing to the realization of a circular plastic economy. Here, we present synthesis of a biobased di-furfural monomer and its polymerization with mixtures of various biobased multi-functional amines to construct a library of polyimines. These polyimine thermosets displayed tailor-made thermal and mechanical properties, featuring a wide range of glass transition temperatures from 8 °C to 60 °C and tensile strength spanning from 6.5 to 77.8 MPa. They also demonstrated high char yields, reaching 57% at 800 °C. Notably, these novel polyimines exhibit high bio-content (in the range of 78% to 90%) and closed-loop recyclability under mildly acidic and energy-efficient conditions. This unique property enables the recovery of monomers on demand with high yields and purity. The findings presented in this work represent a valuable contribution in the field of biobased thermosetting polymers with circular economy potential, offering new possibilities for sustainable material design.
... [1d,f,3] This is especially true for imine systems, which, formed by the reversible condensation between an amine and an aldehyde, display many ideal dynamic covalent bond characteristics. [5] To address stability problems hampering the isolation of larger nanostructures, intramolecular hydrogen bonds of the hydroxyl protons of a salicylic aldehyde, also called "imine clip", are sometimes employed to impede hydrolysation. [1e,6] This enabled the synthesis of a shape-persistent exofunctionalised nanocage by Mastalerz in high yields, [6g] followed by a large series of salicylimine-based cages with exciting properties. ...
We present the multiple post‐modification of organic macrocycles and cages, introducing functional groups into two‐ and three‐dimensional supramolecular scaffolds bearing fluorine substituents, which opens up new possibilities in multi‐step supramolecular chemistry employing the vast chemical space of readily available isocyanates. The mechanism and scope of the reaction that proceeds after isocyanate addition to the benzylamine motif via an azadefluorination cyclisation (ADFC) were investigated using DFT calculations, and a series of aromatic isocyanates with different electronic properties were tested. The compounds show excellent chemical stability and were fully characterised. They can be used for subsequent cross‐coupling reactions, and ADFC can be used directly to generate cross‐linked membranes from macrocycles or cages when using ditopic isocyanates. Single‐crystal X‐ray (SC‐XRD) analysis shows the proof of the formation of the desired supramolecular entity together with the connectivity predicted by calculations and from ¹⁹F NMR shifts, allowing the late‐stage functionalisation of self‐assembled macrocycles and cages by ADFC.
Synthesizing uniform functional covalent organic framework (COF) microspheres is the prerequisite of applying COFs as novel stationary phases for liquid chromatography. However, the synthesis of functionalized COF microspheres is challenging due to the difficulty in maintaining microspheric morphology when conferring functions. Here, a facile and universal “self‐limited dynamic linker exchange” strategy is developed to achieve surface functionalization of uniform COF microspheres. Six different types of COF microspheres are constructed, showing the universality and superiority of the strategy. The library of COF microspheres’ stationary phases can be further enriched on demand by varying different functional building blocks. The “self‐limited dynamic linker exchange” is attributed to the result of a delicate balance of reaction thermodynamics and molecular diffusion energy barrier. As a demonstration, the chiral functional COF microspheres are used as stationary phases of chiral chromatography and realized effective enantioseparation.
Dynamic covalent polymers (DCPs) that strike a balance between high performance and rapid reconfiguration have been a challenging task. For this purpose, a solution is proposed in the form of a new dynamic covalent supramolecular motif—guanidine urea structure (GUAs). GUAs contain complex and diverse chemical structures as well as unique bonding characteristics, allowing guanidine urea supramolecular polymers to demonstrate advanced physical properties. Noncovalent interaction aggregates (NIAs) have been confirmed to form in GUA‐DCPs through multistage H‐bonding and π‐π stacking, resulting in an extremely high Young's modulus of 14 GPa, suggesting remarkable mechanical strength. Additionally, guanamine urea linkages in GUAs, a new type of dynamic covalent bond, provide resins with excellent malleability and reprocessability. Guanamine urea metathesis is validated using small molecule model compounds, and the temperature dependent infrared and rheological behavior of GUA‐DCPs following the dissociative exchange mechanism. Moreover, the inherent photodynamic antibacterial properties are extensively verified by antibacterial experiments. Even after undergoing three reprocessing cycles, the antibacterial rate of GUA‐DCPs remains above 99% after 24 h, highlighting their long‐lasting antibacterial effectiveness. GUA‐DCPs with dynamic nature, tuneable composition, and unique combination of properties make them promising candidates for various technological advancements.
Imination reactions in water represent a challenge not only because of the high propensity of imines to be hydrolysed but also as a result of the competing hydrate formation through H2O addition to the aldehyde. In the present work we report a successful approach that allows for favouring imitation reactions while silencing hydrate formation. Such remarkable reactivity and selectivity can be attained by fine-tuning the electronic and steric structural features of the ortho-substituents of the carbonyl groups. It resulted from studying the structure–reactivity relationships in a series of condensation reactions between different amines and aldehydes, comparing the results to the ones obtained in the presence of the biologically-relevant pyridoxal phosphate (PLP). The key role of negatively-charged and sterically-crowding units (i.e., sulfonate groups) in disfavouring hydrate formation was corroborated by DFT and steric-hindrance calculations. Furthermore, the best-performing aldehyde leads to higher imine yields, selectivity and stability than those of PLP itself, allowing for the inhibition of a PLP-dependent enzyme (transaminase) through dynamic aldimine exchange. These results will increase the applicability of imine-based dynamic covalent chemistry (DCvC) under physiological conditions and will pave the way for the design of new carbonyl derivatives that might be used in the dynamic modification of biomolecules.
The use of N-(2-carboxyphenyl)salicylideneimine (saphHCOOH) and N-(4-chloro-carboxyphenyl)salicylideneimine (4ClsaphHCOOH) for the synthesis of hexanuclear {ZnII4MIII2} (M = Cr, Fe) complexes is described. [Zn4Fe2(saphCOO)6(NO3)2(EtOH)2] (1), [Zn4Cr2(saphCOO)6(NO3)2(H2O)2] (2) and [Zn4Fe2(4ClsaphCOO)6(NO3)2(EtOH)2] (3), as 4CH2Cl2.2EtOH...
Synthesis of conjugated compounds with unusual shape-persistent structures remains a challenge. Herein, utilizing thermodynamically reversible intermolecular Friedel–Crafts alkylation, a dynamic covalent chemistry (DCC) reaction, we facilely synthesized a figure-eight shaped macrocycle FEM and cage molecules CATPA/CACz. X-ray crystallographic analysis confirmed the chemical geometries of tetracation FEM⁴⁺(PF6⁻)4 and hexacation CACz⁶⁺(SbF6⁻)6. FEM and CATPA displayed higher photoluminescence quantum yield in solid states compared to that in solution, whereas CACz gave the reverse result. DFT calculations showed that fluorescence-related frontier molecular orbital profiles are mainly localized on their arms consisting of a p-quinodimethane (p-QDM) unit and two benzene rings of triphenylamine or carbazole. Owing to their space-confined structures, variable-temperature ¹H NMR measurements showed that FEM, CATPA and FEM⁴⁺ have intramolecular restricted motion of phenyl rings on their chromophore arms. Accordingly, FEM and CATPA with flexible triphenylamine subunits displayed aggregation-induced emission behavior (AIE), whereas CACz with a rigid carbazole subunits structure showed no AIE behavior.
Nanodrug delivery systems (NDDS) continue to be explored as novel strategies enhance therapy outcomes and combat microbial resistance. The need for the formulation of smart drug delivery systems for targeting infection sites calls for the engineering of responsive chemical designs such as dynamic covalent bonds (DCBs). Stimuli response due to DCBs incorporated into nanosystems are emerging as an alternative way to target infection sites, thus enhancing the delivery of antibacterial agents. This leads to the eradication of bacterial infections and the reduction of antimicrobial resistance. Incorporating DCBs on the backbone of the nanoparticles endows the systems with several properties, including self-healing, controlled disassembly, and stimuli responsiveness, which are beneficial in the delivery and release of the antimicrobial at the infection site. This review provides a comprehensive and current overview of conventional DCBs-based nanosystems, stimuli-responsive DCBs-based nanosystems, and targeted DCBs-based nanosystems that have been reported in the literature for antibacterial delivery. The review emphasizes the DCBs used in their design, the nanomaterials constructed, the drug release-triggering stimuli, and the antibacterial efficacy of the reported DCBs-based nanosystems. Additionally, the review underlines future strategies that can be used to improve the potential of DCBs-based nanosystems to treat bacterial infections and overcome antibacterial resistance.
Shape‐persistent organic cages are an intriguing class of molecular porous materials. Through hierarchical molecular design, size and shape of the intrinsic molecular voids are controlled by dynamic covalent chemistry, while pore structure and topology are governed by noncovalent alignment in the solid state. However, the predictable and reliable crystallization of organic cages is still challenging since long‐range superstructures are solely based on weak and rather unidirectional supramolecular interactions. In this tutorial review, we provide a general classification of porous solid‐state materials and discuss specific design principles regarding the dynamic covalent reactions, the small‐molecule building blocks and solid‐state engineering. Furthermore, we introduce the most important analytical techniques for porous materials with a special focus on organic cages.
Two anionic tetrahedral cages were self-assembled as the only observable products in weakly basic water via imine condensation. The success of high-yielding formation of the cages in water relies on...
The dynamic nature of calamitic liquid crystals is exploited to perform isothermal phase transitions driven by dynamic covalent chemistry. For this purpose, nematic (N) arrays based on aldehyde 1 were treated with different amines (A‐E) in an on‐surface process, which resulted in different isothermal phase transitions. These phase transformations were caused by in‐situ imination reactions and are dependent on the nature of the added amine. Transitions from the N to crystal (1A, 1E), isotropic (1B), and smectic (Sm) (1C, 1D) phases were achieved, while the resulting materials feature thermotropic liquid crystal behavior. A sequential transformation from the N 1 to the Sm 1C and then to the N 1B was achieved by coupling an imination to a transimination processes and adjusting the temperature. All of these processes were well characterized by microscopic, spectroscopic, and X‐ray techniques, unlocking not only the constitutional but also the structural aspects of the phase transitions. This work provides new insights into designing constitutionally and structurally adaptable liquid crystal systems, paving the way toward the conception of programable evolutive pathways and adaptive materials.
The dynamic nature of calamitic liquid crystals is exploited to perform isothermal phase transitions driven by dynamic covalent chemistry. For this purpose, nematic (N) arrays based on aldehyde 1 were treated with different amines (A–E) in an on‐surface process, which resulted in different isothermal phase transitions. These phase transformations were caused by in situ imination reactions and are dependent on the nature of the added amine. Transitions from the N to crystal (1A, 1E), isotropic (1B), and smectic (Sm) (1C, 1D) phases were achieved, while the resulting materials feature thermotropic liquid crystal behavior. A sequential transformation from the N 1 to the Sm 1C and then to the N 1B was achieved by coupling an imination to a transimination processes and adjusting the temperature. All of these processes were well characterized by microscopic, spectroscopic, and X‐ray techniques, unlocking not only the constitutional but also the structural aspects of the phase transitions. This work provides new insights into designing constitutionally and structurally adaptable liquid crystal systems, paving the way toward the conception of programable evolutive pathways and adaptive materials.
Click chemistry has reached its maturity as the weapon of choice for the irreversible ligation of molecular fragments, with over 20 years of research resulting in the development or improvement of highly efficient kinetically controlled conjugation reactions. Nevertheless, traditional click reactions can be disadvantageous not only in terms of efficiency (side products, slow kinetics, air/water tolerance, etc.), but also because they completely avoid the possibility to reversibly produce and control bound/unbound states. Recently, non‐covalent click chemistry has appeared as a more efficient alternative, in particular by using host‐guest self‐assembled systems of high thermodynamic stability and kinetic lability. This review discusses the implementation of molecular switches in the development of such non‐covalent ligation processes, resulting in what we have termed stimuli‐responsive click chemistry, in which the bound/unbound constitutional states of the system can be favored by external stimulation, in particular using host‐guest complexes. As we exemplify with handpicked selected examples, these supramolecular systems are well suited for the development of human‐controlled molecular conjugation, by coupling thermodynamically regulated processes with appropriate temporally resolved extrinsic control mechanisms, thus mimicking nature and advancing our efforts to develop a more function‐oriented chemical synthesis.
Two‐dimensional conjugated polymers (2DCPs) possess extended in‐plane π‐conjugated lattice and out‐of‐plane π‐π stacking, which results in enhanced electronic performance and potentially unique band structures. These properties, along with pre‐designability, well‐defined channels, easy post‐modification, and order structure attract extensive attention from material science to organic electronics. In this review, we summarize the recent advance in the interfacial synthesis and conductivity tuning strategies of 2DCP thin films, as well as their application in organic electronics. Furthermore, we show that, by combining topology structure design and targeted conductivity adjustment, researchers have fabricated 2DCP thin films with pre‐designed active groups, highly ordered structures and enhanced conductivity. These films exhibit great potential for various thin‐film organic electronics, such as organic transistors, memristors, electrochromism, chemiresistors, and photodetectors. Finally, we discuss the future research directions and perspectives of 2DCPs in terms of the interfacial synthetic design and structure engineering for the fabrication of fully conjugated 2DCP thin films, as well as the functional manipulation of conductivity to advance their applications in future organic electronics.
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Additive manufacturing/3D printing is praised as revolutionary by many because it enables the decentralized and on‐demand manufacturing of complex shapes. With the ongoing rapid development and increased availability of these technologies, it has become crucial to develop novel materials with unique functionalities for 3D‐printed components. Advances in self‐healing materials have resulted in structures capable of recovering from physical damage through material remendability. By applying self‐healing solutions to 3D printing, researchers are attempting to overcome current barriers such as low mechanical properties, material waste, and printability issues, among others. This review summarizes the current state of research on self‐healing materials used for 3D printing and presents recent progress in the 3D printing of self‐healing components. The particular focus is on the use of intrinsic polymer self‐healing, which is prominent in the published literature. The self‐healing mechanisms of the proposed material systems as well as their composition are highlighted, and their intended application with respect to 3D printing is focused on. The scientific and technological challenges involved in the implementation of self‐healing materials and approaches and the potential functional 3D printing applications of these new materials are also discussed.
Biomaterials with features that allow them to heal themselves when damaged by mechanical, thermal, or other mechanisms and restore their original sets of properties autonomously and automatically without any external intervention have been the pinnacle of regenerative medicine. Self-healing hydrogels are of particular interest, especially in tissue engineering, and this healing capacity is derived from either reversible chemical or physical bonds or a combination of both. In addition, self-healing hydrogels with shear-thinning properties can potentially be used in bioprinting for the fabrication of tissue engineering constructs. The development and advancements in 3D-printed self-healing hydrogels will allow researchers to enhance the flexibility and performance of soft tissue engineering scaffolds in a significant manner. Self-healing hydrogels, when fabrications using extrusion-based 3D printing approach, can recover their mechanical integrity and heal autonomously once it is extruded through the nozzle. In addition, studies have been carried out to examine the relationship between hydrogel ink properties and printability using extrusion-based printers to achieve both adequate printability and high cell viability.
Dynamic Covalent Chemistry, consisting in the use of dynamic covalent bonds (DCBs) to create complex objects by working at the thermodynamic equilibrium, has undeniable advantages for the preparation of conjugated systems with tailored optoelectronic properties for organic electronics. Chemists can combine simple building blocks with simple functional groups of appropriate geometry, structure and stoichiometry to build multidimensional conjugated architectures. Dynamic covalent reactions are often precious metal‐free, generate little to no side products and are very efficient. DCBs however afford sensitive materials, and consequently a balance needs to be found between the ease of synthesis, the stability and the performances. The dynamicity of the target materials hence can considerably reduce their applicability in organic electronics where strongly stable materials are needed, but opens the door to stimuli‐responsive behaviour and recyclability. A way to overcome dynamicity issues is to lock DCBs, but often at the cost of decreased conjugation. In this review, we will highlight how DCBs are employed to prepare functional optoelectronically active materials, such as discrete molecules, polymers or covalent organic frameworks, applied in fields ranging from organic light‐emitting diodes and solar cells to organic batteries and transistors. We will also discuss their limitations, benefits and the current challenges to overcome.
Imine-based vitrimers were prepared from synthesized diimine-dimethacrylate monomer derived from biobased vanillin. First, a methacrylate derivative starting from vanillin was synthesized. The diimine derivative was synthesized by condensation of the aldehyde groups from two vanillin methacrylate units with the amine groups of hexamethylenediamine (HMDA). The synthesized product was used in formulations containing ethylene glycol phenyl ether methacrylate (EGPMA) as a reactive diluent for the customization of final material properties and cured by exposure to ultraviolet (UV)-light using suitable radical photoinitiators or else with temperature using a radical thermal initiator. Materials with glass transition temperatures (Tgs) ranging from 70 to 90 °C were prepared, showing good thermal stability and mechanical and thermomechanical properties. The evaluation of their vitrimeric characteristics revealed that all materials achieved a stress-relaxation factor (σ = 0.37σ0) in less than 130 s at 160 °C, with photocured materials exhibiting faster relaxation rates. The catalytic effect of phosphine oxide groups in imine metathesis has also been evidenced. All prepared materials could be mechanically recycled and completely solubilized in a two-step degradation process, putting evidence of their potential use for carbon fiber-reinforced composites (CFRCs). In addition, they demonstrated promising self-repairing abilities. Finally, as a proof of concept, it was established that these formulations could be effectively processed using a Digital Light Processing three-dimensional (3D) Printer (DLP), resulting in the fabrication of complex shapes with high resolution.
Traditional polybenzoxazine thermosets cannot be reprocessed or recycled due to the permanent crosslinked networks. The dynamic exchangeable characteristics of imine bonds can impart the networks with reprocessabilities and recyclabilities. This study reported a weldable, reprocessable, and water‐resistant polybenzoxazine vitrimer (C‐ABZ) crosslinked by dynamic imine bonds. It was synthesized through a condensation reaction between an aldehyde‐containing benzoxazine oligomer (O‐ABZ) and 1,12‐dodecanediamine. The resulting C‐ABZ was able to be welded and reprocessed due to the dynamic exchange of imine bonds. The tensile strengths of the welded C‐ABZ and the reprocessed C‐ABZ after three cycles of hot‐pressing were 76.7, 81.3, 70.8, and 58.1 Mpa, with corresponding tensile strength recovery ratios of 74.1%, 78.6%, 68.4%, and 56.1%, respectively. Furthermore, the polybenzoxazine backbone significantly improved the water resistance of the imine bonds. After immersing in water for 30 days at room temperature, the weight gain of C‐ABZ was less than 1% with corresponding tensile strength and tensile strength retention ratio of 59.5 Mpa and 57.5%, respectively. Although the heat resistance of C‐ABZ decreased slightly with increased hot‐pressing cycles, a glass transition temperature (Tg, tanδ) of 150 °C was retained after the third hot‐pressing. Overall, these findings demonstrate that the C‐ABZ possesses excellent comprehensive performances.
Developing cross-linked polymers with both low dielectric constant and degradability for microelectronics and sustainability remains a challenge. Herein, a new fluoro-containing dialdehyde monomer (VF) derived from vanillin was synthesized, which was then cross-linked with a tris (2-aminoethyl) amine (TREN) and different diamines (4, 4′-methylenebis (cyclohexylamine) (PACM), 4, 4′-diaminodiphenylmethane (DDM) and 1, 6-hexamethylenediamine (HMDA)) to give three polyimines thermosets, namely VFT-P, VFT-D and VTF-H, respectively, showing controlled comprehensive properties through regulation of imine cross-linked structures. Properties-function relationship of these polyimines was systematically studied and focused mainly on revealing the degradation process and the mechanism of the degradability of polyimines. The results indicated that among these polyimines, VFT-P exhibited superior conventional properties and acid resistance due to its appropriate reactivity, cross-linked density, rigid non-conjugated Schiff base structures, and high free volume. The storage modulus, glass transition temperature, transmittance, and tensile strength of VFT-P polyimines could reach 3.40 GPa, 134 °C, > 97% (at 500–800 nm) and 62.47 ± 3.65 MPa, respectively. Besides, a low dielectric constant of 2.96 at 100 MHz and superior degradability was achieved. This study introduces a facile and effective design strategy to achieve the compatibility of target properties and also offers a potentially sustainable alternative to conventional cross-linked dielectric polymers.
Graphical abstract
Towards understanding the structural requirements for extending the anodic reversibility, a series of conjugated azomethine triads end-capped with amides were prepared.
Covalent organic polymers (COP) were synthesized by polymerizing 5,10,15-tris(4-aminophenyl)corrole (TAPC) and terephthalaldehyde in the presence of acetic acid. Yolk–shells and filled structures were obtained after ripening of COP synthesized with low and high acetic acid/TAPC, respectively. The low cross-linking degree of COP synthesized by low acetic acid/TAPC resulted in a yolk–shell structure due to the easy dissolution of the interior. The Co-doped yolk–shell showed higher electrocatalytic activity for the oxygen reduction reaction than the filled structure.
Developing dynamic chemistry for polymeric materials offers chemical solutions to solve key problems associated with current plastics. Mechanical performance and dynamic function are equally important in material design because the former determines the application scope and the latter enables chemical recycling and hence sustainability. However, it is a long‐term challenge to balance the subtle trade‐off between mechanical robustness and dynamic properties in a single material. The rise of dynamic chemistry, including supramolecular and dynamic covalent chemistry, provides many opportunities and versatile molecular tools for designing constitutionally dynamic materials that can adapt, repair, and recycle. Facing the growing social need for developing advanced sustainable materials without compromising properties, recent progress showing how the toolbox of dynamic chemistry can be explored to enable high‐performance sustainable materials by molecular engineering strategies is discussed here. The state of the art and recent milestones are summarized and discussed, followed by an outlook toward future opportunities and challenges present in this field.
We present the multiple post‐modification of organic macrocycles and cages, introducing functional groups into two‐ and three‐dimensional supramolecular scaffolds bearing fluorine substituents, which opens up new possibilities in multi‐step supramolecular chemistry employing the vast chemical space of readily available isocyanates. The mechanism and scope of the reaction that proceeds after isocyanate addition to the benzylamine motif via an azadefluorination cyclisation (ADFC) were investigated using DFT calculations, and a series of aromatic isocyanates with different electronic properties were tested. The compounds show excellent chemical stability and were fully characterised. They can be used for subsequent cross‐coupling reactions, and ADFC can be used directly to generate cross‐linked membranes from macrocycles or cages when using ditopic isocyanates. Single‐crystal X‐ray (SC‐XRD) analysis shows the proof of the formation of the desired supramolecular entity together with the connectivity predicted by calculations and from 19F NMR shifts, allowing the late‐stage functionalisation of self‐assembled macrocycles and cages by ADFC.
We disclose the features of a category of reversible nucleophilic aromatic substitutions in view of their significance and generality in dynamic aromatic chemistry. Exchange of sulfur components surrounding arenes and heteroarenes may occur at 25°C, in a process that one may call a "sulfur dance”. These SNAr systems present their own features, apart from common reversible reactions utilized in dynamic chemistry (DCC). By varying conditions, covalent dynamics may operate to provide libraries of thiaarenes with some selectivity, or conversion of a hexa(thio)benzene asterisk into another one. The reversible nature of SNAr is confirmed by three methods: a convergence of the products distribution in reversible SNAr systems, a related product redistribution between two per(thio)benzenes by using a thiolate promoter, and from kinetic/thermodynamic data. A four‐component dynamic covalent system further illustrates the thermodynamically‐driven formation of a thiacalix[2]arene[2]pyrimidine by sulfur component exchanges. This work stimulates the implementation of reversible SNAr in aromatic chemistry and in DCC.
Dpp-imines are classic model substrates for synthetic method studies. Here, we disclose their powerful use as achiral coligands in metal-catalyzed reactions. It is highly interesting to find that the Dpp-imine can not only act as powerful ligand to create excellent chiral pockets with magnesium complexes but also, more importantly, this coligand can dramatically enhance the catalytic ability of the metal catalyst. The underlying reaction mechanism was extensively explored by conducting a series of experiments, including 31P NMR studies of the coordination complex between the Dpp-imine coligand and magnesium complexes, ESI capture results, multiple control experiments, studies and comparison of different coligands, 1H NMR studies on the relationship between the substrate and Dpp-imine coligand, as well as the relationship between the substrate and the full complexes. Furthermore, DFT calculation provided valuable insights in the role of the imine additive and demonstrated that adding the Dpp-imine coligand in the magnesium catalyst can switch the deprotonation/nucleophilic addition steps from a stepwise mechanism to a concerted process during the oxa–cyclization reaction. The crucial factors responsible for the excellent enantioselectivity and enhanced reaction efficiency brought by Dpp-imine have been extracted from the calculation model. These mechanistic experiments and DFT calculation data clearly disclose and prove the powerful and interesting functions of the Dpp-imine coligand, which also direct a novel application of this type of active imine as useful ligands in metal-catalyzed asymmetric reactions.
Stimuli‐responsive fluorescent hydrogels are three‐dimensional networked polymeric materials with tunable luminescence and dynamic properties, which play an important role as a water‐rich soft material in the fields of information encryption, bionic actuation, bioimaging, environmental monitoring, and luminescent materials. Compared with conventional hydrogels, their unique luminescent properties allow the visualization of microscopic dynamics within the polymer network. By rational inclusion of dynamic motifs, such as photoswitches, AIEgens, lanthanide complexes, and host–guest complexes, these materials are endowed with tunability of emission, shape, and phase in time and space in response to environmental effectors. In this review, we summarize the fabrication strategies that are mainly used by recently reported stimuli‐responsive fluorescent hydrogels and the applications of these materials.
A dumbbell-shaped dialkylammonium ion (see scheme) templates the formation, about its NH2⁺ center, of a crown ether like macrocycle under thermodynamic control from dialdehyde and diamine precursors. The imine-containing [2]rotaxane that ensues was converted by reduction into a kinetically stable interlocked molecule, which has been characterized in the solid state by X-ray crystallography in both free base and salt forms.
A new class of tetrahedral metal–organic capsules that can incorporate up to twelve different externally-directed amine residues is reported, allowing for very large dynamic libraries to be formed from mixtures of amines. Selectivity is observed both externally—more electron-rich amines are incorporated in favour of electron-poor amines—and internally—PF6− is bound in preference to CF3SO3− or BF4−.
Stereochemistry—in both its static and dynamic variants—has progressed apace now for more than a century to incorporate all aspects of covalent, coordinative, and noncovalent bonding at levels of structure which encompass constitution, configuration, and conformation. The advent of the mechanical bond in more recent times is now providing opportunities for the emergence of new stereochemical tenets and concepts, some of which bear close analogies with those of days gone by in chemistry. Since terminology helps to define and disseminate a discipline, we advocate that the term “mechanostereochemistry” be used to describe the chemistry of molecules with mechanical bonds.
Asymmetric catalysts, either chemical or biological, effect the reactions of enantiomeric substrates at unequal rates, allowing for the kinetic resolution of racemates. The asymmetric reaction of chirally labile compounds capable of undergoing in situ racemization can, in principle, afford a single stereoisomer in 100% ee and in 100% yield. The second-order reaction provides a powerful tool for the stereoselective synthesis of chiral compounds, as exemplified by, among others, the biochemical hydrolysis of hydantoins or oxazolinones and microbial reductions or BINAP-Ru(II) catalyzed hydrogenation of certain alpha-substituted beta-keto esters. Such transformations are characterized by the presence of parallel reactions interrelated by the stereoinversion of the enantiomeric substrates. The efficiency is decisively affected by the kinetic parameters, particularly the relative rates of the stereoinversion and reaction as well as the intrinsic stereochemical parameters of the catalyst and substrate. Such stereoselective reactions via dynamic kinetic resolution are expressed mathematically and the stereochemical profiles are displayed graphically. The validity of this basic approach has been verified by the correlation to the experimental results.
Two different approaches are described for the creation of supramolecular systems potentially capable of recognition and catalysis. Using the design approach, we have been able to accelerate and influence two different Diels-Alder reactions within the cavities of porphyrin dimers and trimers; this is templating from the outside inwards. The selection approach is a synthetic chemical attempt to capture some of the key evolutionary features of biological systems: dynamic combinatorial chemistry is used to create equilibrating mixtures of potential receptors, and then a template is used to select and amplify the desired system. Five potential reactions for such dynamic chemistry are discussed: base-catalyzed transes- terification, hydrazone exchange, disulfide exchange, alkene metathesis, and Pd-catalyzed allyl exchange, and preliminary templating results (inside outwards) are presented.
The development and fabrication of mechanical devices powered by artificial molecular machines is one of the contemporary goals of nanoscience. Before this goal can be realized, however, we must learn how to control the coupling/uncoupling to the environment of individual switchable molecules, and also how to integrate these bistable molecules into organized, hierarchical assemblies that can perform significant work on their immediate environment at nano-, micro- and macroscopic levels. In this tutorial review, we seek to draw an all-important distinction between artificial molecular switches which are now ten a penny-or a dime a dozen-in the chemical literature and artificial molecular machines which are few and far between despite the ubiquitous presence of their naturally occurring counterparts in living systems. At the single molecule level, a prevailing perspective as to how machine-like characteristics may be achieved focuses on harnessing, rather than competing with, the ineluctable effects of thermal noise. At the macroscopic level, one of the major challenges inherent to the construction of machine-like assemblies lies in our ability to control the spatial ordering of switchable molecules-e.g., into linear chains and then into muscle-like bundles-and to influence the cross-talk between their switching kinetics. In this regard, situations where all the bistable molecules switch synchronously appear desirable for maximizing mechanical power generated. On the other hand, when the bistable molecules switch "out of phase," the assemblies could develop intricate spatial or spatiotemporal patterns. Assembling and controlling synergistically artificial molecular machines housed in highly interactive and robust architectural domains heralds a game-changer for chemical synthesis and a defining moment for nanofabrication.
Surface chemistry is an important field of research, especially for the study and design of (bio)nanostructures in which nearly every atom lies at an interface. Here we show that dynamic covalent chemistry is an efficient tool for functionalizing surfaces in such a way that their interfacial properties can be varied controllably in space and time. Modulation of pH is used to tune the fast, selective and reversible attachment of functional amines (with different pK(a) values) to an aldehyde-coated surface. To illustrate the potential of this technique, we developed dynamic self-assembled monolayers ('DynaSAMs'), which enable the hierarchical construction of mixed gradients comprising either small functional molecules or proteins. Control of the (bio)chemical composition at any point on the surface potentially provides a simple bottom-up method to access numerous surface patterns with a broad range of functionalities.
Whereas combinatorial chemistry is based on extensive libraries of prefabricated molecules, dynamic combinatorial chemistry (DCC) implements the reversible connection of sets of basic components to give access to virtual combinatorial libraries (VCLs), whose constituents comprise all possible combinations that may potentially be generated. The constituent(s) actually expressed among all those accessible is(are) expected to be that(those) presenting the strongest interaction with the target, that is, the highest receptor/substrate molecular recognition. The overall process is thus instructed (target-driven), combinatorial, and dynamic. It bypasses the need to actually synthesize the constituents of a combinatorial library by letting the target perform the assembly of the optimal partner. It comprizes both molecular and supramolecular events. The basic features of the DCC/VCL approach are presented together with its implementation in different fields and the perspectives it offers in a variety of areas of science and technology, such as the discovery of biologically active substances, of novel materials, of efficient catalysts, and so forth. Finally, it participates in the progressive development of an adaptive chemistry.
A new strategy has been developed for synthesizing catenanes; it is based on a generalized template effect, as shown in Figure 1. The first example of a novel class of molecules, the , has been obtained in good yield : it contains copper (I) and macrocyclic phenanthroline derivatives.
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Fullerene tweezers which have a rigid acetylenic spacers were synthesized and their inclusion abilities for C-60 were examined by UV spectroscopy. From the spectra, the association constants were determined using Benesi-Hildebrand method. Fullerene tweezers with a 1,4-bis(4-oxycarbonylphenyl)butadiyne spacer showed the highest association constant for C-60 (39300+/-250 dm(3)mol(-1) in benzene).
It has become clear over the past two decades that, in order to create functional synthetic nanoscale structures, the chemist must exploit a fundamental understanding of the self-assembly of large-scale biological structures, which exist and function at and beyond the nanoscale. This mode of construction of nanoscale structures and nanosystems represents the so-called ‘bottom-up’ or ‘engineering-up’ approach to fabrication. Significant progress has been made in the development of nanoscience by transferring concepts found in the biological world into the chemical arena. The development of simple chemical systems that are capable of instructing their own organisation into large aggregates of molecules through their mutual recognition properties has been central to this success. By utilising a diverse array of intermolecular interactions as the information source for assembly processes, chemists have successfully applied biological concepts in the construction
of complex nanoscale structures and superstructures with a variety of forms and functions. More recently, the utility of assembly processes has been extended through the realisation that recognition processes can be used to select a single structure from a library of equilibrating structures. These developments open the way for the design and implementation of artificial assembly processes that are capable of adapting themselves to the local environment in which they are conducted.
The key feature of enzymic catalysis is recognition of the transition state. Synthesis of designed systems rarely leads to successful catalysts as the rules for conformation and intermolecular interactions are to imperfectly understood. This review describes several current ‘selection’ approaches to the generation of systems that can recognise transitionstate analogues. Examples covered include catalytic antibodies, ribozymes, imprinted polymers. Combinatorial chemistry, and thermodynamic templating. All have the potential to yeild effective catalysts without prior design of every detail.
Cyclocholates are efficiently and rapidly synthesised from suitable monomers by transesterification under thermodynamic, equilibrating conditions using a potassium methoxide–crown ether complex.
The absolute configuration of chiral C3-cyclotriveratrylene derivatives has been determined unambigously as follows. First, the absolute configuration of 2-(2-ethoxy-4-hydroxymethylphenoxy)propionic acid (5b) was established as R-(+) by chemical correlations and circular dichroism measurements. Upon treatment with perchloric acid, compound (+)-(5b) leads to the title compound (–)-(3), isolated in 14% yield, m.p. 127 °C, [α]D25–16.1°(CHCl3). The crystal and molecular structure of (–)-(3) have been determined by single crystal X-ray analysis: orthorhombic, a= 23.428, b= 22.165, c= 15.145 Å space group P212121, Z= 8. The M absolute configuration has been ascribed to (–)-(3), on the basis of internal comparison with groups –CH(Me)CO2Me whose absolute stereochemistry is derived from that of (+)-(5b). Seven chiral C3-cyclotriveratrylene derivatives have been chemically related to (M)-(–)-(3). The absolute configurations so established are identical with those deduced independently from an exciton analysis of circular dichroism spectra.
The taming of cyclobutadiene was accomplished in the interior of the hemicarcerand 1. Cyclobutadiene is a stable compound with a singlet ground state when it is synthesized in the interior of 1. In order to synthesize the incarcerated cyclobutadiene and to characterize its structure, one bimolecular, three photochemical, and two thermal reactions were carried out in the interior of 1. The authors consider it both realistic and useful to regard the internal phase of carcerands and hemicarcerands as a new state of matter.
The emergence of the mechanical bond during the past 25 years is giving chemistry a fillip in more ways than one. While its arrival on the scene is already impacting materials science and molecular nanotechnology, it is providing a new lease of life to chemical synthesis where mechanical bond formation occurs as a consequence of the all-important templation orchestrated by molecular recognition and self-assembly. The way in which covalent bond formation activates noncovalent bonding interactions, switching on molecular recognition that leads to self-assembly, and the template-directed synthesis of mechanically interlocked molecules—of which the so-called catenanes and rotaxanes may be regarded as the prototypes—has introduced a level of integration into chemical synthesis that has not previously been attained jointly at the supramolecular and molecular levels. The challenge now is to carry this level of integration during molecular synthesis beyond relatively small molecules into the realms of precisely functionalized extended molecular structures and superstructures that perform functions in a collective manner as the key sources of instruction, activation, and performance in multi-component integrated circuits and devices. These forays into organic chemistry by a scientific nomad are traced through thick and thin from the Athens of the North to the Windy City by Lake Michigan with interludes on the edge of the Canadian Shield beside Lake Ontario, in the Socialist Republic of South Yorkshire, on the Plains of Cheshire beside the Wirral, in the Midlands in the Heartland of Albion, and in the City of Angels beside the Peaceful Sea.
The long-standing challenge of designing and constructing new crystalline solid-state materials from molecular building blocks is just beginning to be addressed with success. A conceptual approach that requires the use of secondary building units to direct the assembly of ordered frameworks epitomizes this process: we call this approach reticular synthesis. This chemistry has yielded materials designed to have predetermined structures, compositions and properties. In particular, highly porous frameworks held together by strong metal–oxygen–carbon bonds and with exceptionally large surface area and capacity for gas storage have been prepared and their pore metrics systematically varied and functionalized.
By utilizing the dynamic nature of imine bonds, it is possible to construct [2]rotaxanes from a ring and a preformed dumbbell under thermodynamic control. These dynamic [2]rotaxanes, which exhibit reversible supramolecular-like behavior in the presence of appropriate catalysts, can be “fixed” by reduction of their imine bonds.
Thermodynamic control operates in the synthesis of a [2]rotaxane based upon the dibenzylammonium ion/crown ether recognition motif. When dibenzo[24]crown-8 is added to an acetonitrile solution containing a diimine dumbbell-like component, the dynamic nature of the system (i.e., imine hydrolysis/reformation) offers the ring component access to the NH2+ center, allowing the self-assembly of the corresponding “dynamic” [2]rotaxane to occur. The “fixing” of this [2]rotaxane can be achieved upon reduction of the imine bonds, affording a kinetically inert [2]rotaxane.
The phenomenon of "epoxide migration," noted heretofore only in the sugar series, has been shown to occur with simple α,β-epoxy alcohols. In general, this conversion of one epoxy alcohol into another was effected readily in 0.5 N aqueous sodium hydroxide at 25°. A tertiary alcohol underwent 92% conversion to primary. On the other hand, rearrangement of secondary to primary and tertiary to secondary alcohols appeared to be strongly influenced by steric factors.
We report the synthesis of a family of new macrocyclic hydrazone-based anion receptors. Formed from the reaction between isophthalaldehyde and a helical amino acid-disubstituted ferrocene dihydrazide, these macrocycles contain from one to eight ferrocene moieties. The isolation of the four smallest of the macrocycles and their characterisation by UV-Vis, CD and NMR spectroscopy is described. The conformation of the macrocycles is explored, particularly with reference to the formation of a helical intramolecularly hydrogen-bonded structure. An investigation of the use of these macrocycles as anion-receptors shows that they are all effective hosts; the larger macrocycles show the highest affinities for anions. Studies using NMR spectroscopy suggest that the anion-recognition results primarily from the formation of multiple hydrogen bonds between the anions and the electropositive N-H protons of the acylhydrazones.
Dynamic covalent chemistry relates to chemical reactions carried out reversibly under conditions of equilibrium control. The reversible nature of the reactions introduces the prospects of “error checking” and “proof-reading” into synthetic processes where dynamic covalent chemistry operates. Since the formation of products occurs under thermodynamic control, product distributions depend only on the relative stabilities of the final products. In kinetically controlled reactions, however, it is the free energy differences between the transition states leading to the products that determines their relative proportions. Supramolecular chemistry has had a huge impact on synthesis at two levels: one is noncovalent synthesis, or strict self-assembly, and the other is supramolecular assistance to molecular synthesis, also referred to as self-assembly followed by covalent modification. Noncovalent synthesis has given us access to finite supermolecules and infinite supramolecular arrays. Supramolecular assistance to covalent synthesis has been exploited in the construction of more-complex systems, such as interlocked molecular compounds (for example, catenanes and rotaxanes) as well as container molecules (molecular capsules). The appealing prospect of also synthesizing these types of compounds with complex molecular architectures using reversible covalent bond forming chemistry has led to the development of dynamic covalent chemistry. Historically, dynamic covalent chemistry has played a central role in the development of conformational analysis by opening up the possibility to be able to equilibrate configurational isomers, sometimes with base (for example, esters) and sometimes with acid (for example, acetals). These stereochemical “balancing acts” revealed another major advantage that dynamic covalent chemistry offers the chemist, which is not so easily accessible in the kinetically controlled regime: the ability to re-adjust the product distribution of a reaction, even once the initial products have been formed, by changing the reaction's environment (for example, concentration, temperature, presence or absence of a template). This highly transparent, yet tremendously subtle, characteristic of dynamic covalent chemistry has led to key discoveries in polymer chemistry. In this review, some recent examples where dynamic covalent chemistry has been demonstrated are shown to emphasise the basic concepts of this area of science.
The present review is aimed at illustrating to the reader the state of the art on the construction
of artificial chiral supramolecular assemblies held together by means of weak intermolecular non-covalent
forces in solution except metal-ligand coordination. Chiral supramolecules are self-assembled finite
aggregates characterized by the presence of chiral residues or by asymmetric arrangement of components.
These assemblies have proven to be powerful tools to explore chirality and its effects in host-guest
interactions and represents important steps toward asuperior control in asymmetric non-covalent
synthesis, which is still apeculiar prerogative of natural systems. The introduction serves
as aquick overview of weak (non-covalent) interactions refreshing concepts like cooperativity
and multivalency that are crucial to molecular assembly, as well as providing selected examples of
supramolecular chiral systems found in nature. Four main strategies have been so far employed for
the preparation of chiral supramolecular assemblies: these are discussed on the bases of the nature
of the monomeric components employed. Intrinsically chiral scaffolds have been used both in the enantiopure
or racemic mixture form to assemble into chiral aggregates. Asecond approach instead makes
use of achiral building blocks to give racemic chiral supramolecules, and astep further consists
of the enrichment of these species in one enantiomeric form exploiting the chiral memory effect. The
last approach is based on encapsulation of chiral guests, which allows reciprocal sensing of chiral
species confined in close proximity within an achiral host.
Early coordination-driven self-assembly paradigms and more complex and discrete 2D and 3D supramolecular ensembles are reviewed. Work in this field focused on the development of rational methodologies for the self-assembly of predesigned systems along with their characterization. Multinuclear, high-resolution NMR and electrospray mass spectrometry are the primary and essential tools for proper characterization along with X-ray and more recently synchrotron X-ray methods. The most recent and arguably interesting applications have been in catalysis, use as microreactors and biological applications. Raymond and Bergman have exploited the cavities of self-assembled 3D cages for enzyme-like catalysis. Coordination-driven self-assembly will continue to be an active area of research and an important component of supramolecular chemistry and nanoscience.
The external variables and intrinsic factors that influence the recognition or discrimination of supramolecularly interacting chemical species in solution are examined. The external variables that compromise the self-sorting of mixtures of thermodynamically equilibrated H-bonding species may not necessarily apply to self-sorted mixtures involving other such as stronger noncovalent interactions. The presence of bulky substituents in some of the building blocks can prevent their interactions with sterically hindered species or eventually facilitate the exclusive selection of unhindered components. Complementarity plays a particularly important role in the stabilization of hydrogen-bonded assemblies to the point that it facilitates the approximation of the functional groups involved. Homologous molecules with longer bridges showed a lower degree of self-recognition but higher binding constants, which was rationalized in terms of an induced-fit planarization process upon π-π stacking.
Keep it simple: The reaction of n aldehydes with n amines leads to a dynamic library of [n×n] imines. Slow distillation of the library amplifies the most volatile imine, whereby the most volatile aldehyde and amine are extracted from all other library components. Iterative repetition of the process enabled the self-sorting of the dynamic library into n mechanically separated imines, which were obtained in high yield.
Condensation of 2,5-diethoxyterephthalohydrazide with 1,3,5-triformylbenzene or 1,3,5-tris(4-formylphenyl)benzene yields two new covalent organic frameworks, COF-42 and COF-43, in which the organic building units are linked through hydrazone bonds to form extended two-dimensional porous frameworks. Both materials are highly crystalline, display excellent chemical and thermal stability, and are permanently porous. These new COFs expand the scope of possibilities for this emerging class of porous materials.
The role of covalent, coordinative, and supramolecular interactions utilized by chemists and biochemists, when they are assembling molecular knots and links, are presented. The Trefoil Knot comprises a simple overhand knot where the two ends have been connected and can exist in left-handed and right-handed forms. A molecular Trefoil Knot displays inherent topological chirality and any representation of its graph cannot be deformed in 3D space to its mirror image, and exists as two enantiomers. A significant increase in knot yields is achieved by replacing the polymethylene linker with a meta-phenylene bridge, a change of design that resulted in the quantitative assembly of the precursor double helical complex. Alkene metathesis is particularly suited to macrocyclizations involving Cu(I)-templated species as it is a thermodynamically controlled reaction. It also avoids the need for addition of destabilizing bases, such as NaH, which are required when alkylation is the final step in the reaction sequence.
Two intramolecularly hydrogen-bonded arylhydrazone (aryl = phenyl or naphthyl) molecular switches have been synthesized, and their full and reversible switching between the E and Z configurations have been demonstrated. These chemically controlled configurational rotary switches exist primarily as the E isomer at equilibrium and can be switched to the protonated Z configuration (Z-H(+)) by the addition of trifluoroacetic acid. The protonation of the pyridine moiety in the switch induces a rotation around the hydrazone C=N double bond, leading to isomerization. Treating Z-H(+) with base (K(2)CO(3)) yields a mixture of E and "metastable" Z isomers. The latter thermally equilibrates to reinstate the initial isomer ratio. The rate of the Z → E isomerization process showed small changes as a function of solvent polarity, indicating that the isomerization might be going through the inversion mechanism (nonpolar transition state). However, the plot of the logarithm of the rate constant k vs the Dimroth parameter (E(T)) gave a linear fit, demonstrating the involvement of a polar transition state (rotation mechanism). These two seemingly contradicting kinetic data were not enough to determine whether the isomerization mechanism goes through the rotation or inversion pathways. The highly negative entropy values obtained for both the forward (E → Z-H(+)) and backward (Z → E) processes strongly suggest that the isomerization involves a polarized transition state that is highly organized (possibly involving a high degree of solvent organization), and hence it proceeds via a rotation mechanism as opposed to inversion. Computations of the Z ↔ E isomerization using density functional theory (DFT) at the M06/cc-pVTZ level and natural bond orbital (NBO) wave function analyses have shown that the favorable isomerization mechanism in these hydrogen-bonded systems is hydrazone-azo tautomerization followed by rotation around a C-N single bond, as opposed to the more common rotation mechanism around the C=N double bond.
A modular approach for the synthesis of cage structures is described. Reactions of [(arene)RuCl(2)](2) [arene = p-cymene, 1,3,5-C(6)H(3)Me(3), 1,3,5-C(6)H(3)(i-Pr)(3)] with formyl-substituted 3-hydroxy-2-pyridone ligands provide trinuclear metallamacrocycles with pendant aldehyde groups. Subsequent condensation reactions with di- and triamines give molecular cages with 3, 6, or 12 Ru centers in a diastereoselective and chemoselective (self-sorting) fashion. Some of the cages can also be prepared in one-pot reactions by mixing [(arene)RuCl(2)](2) with the pyridone ligand and the amine in the presence of base. The cages were comprehensively analyzed by X-ray crystallography. The diameter of the largest dodecanuclear complex is ∼3 nm; the cavity sizes range from 290 to 740 Å(3). An amine exchange process with ethylenediamine allows the clean conversion of a dodecanuclear cage into a hexanuclear cage without disruption of the metallamacrocyclic structures.
Under the direction of intramolecular three-center hydrogen bonding, two cyclotriveratrylene (CTV)-based capsules are assembled quantitatively from C(3)-symmetric CTV precursors by forming three imine bonds from arylamide-derived foldamer segments. (1)H and (13)C NMR and UV/vis experiments reveal that the capsules strongly encapsulate C(60) and C(70) in discrete solvents.
Several dynamic hexaimine cryptophanes, that are built up from two triformylcyclotribenzylene cavitands and three diamino linkers and spontaneously assemble in water in the presence of a suitable templating guest, are reported. X-ray structure, kinetics and thermodynamics of assembly and molecular recognition properties are discussed.
We describe the use of dynamic combinatorial chemistry to discover a new series of linear hydrazone-based receptors that bind multiple dihydrogen phosphate ions. Through the use of a template-driven, selection-based approach to receptor synthesis, dynamic combinatorial chemistry allows for the identification of unexpected host structures and binding motifs. Notably, we observed the unprecedented selection of these linear receptors in preference to competing macrocyclic hosts. Furthermore, linear receptors containing up to nine building blocks and three different building blocks were amplified in the dynamic combinatorial library. The receptors were formed using a dihydrazide building block based on an amino acid-disubstituted ferrocene scaffold. A detailed study of the linear pentamer revealed that it forms a helical ditopic receptor that employs four acylhydrazone hydrogen-bond donor motifs to cooperatively bind two dihydrogen phosphate ions.
The main strategy for constructing porous solids from discrete organic molecules is crystal engineering, which involves forming regular crystalline arrays. Here, we present a chemical approach for desymmetrizing organic cages by dynamic covalent scrambling reactions. This leads to molecules with a distribution of shapes which cannot pack effectively and, hence, do not crystallize, creating porosity in the amorphous solid. The porous properties can be fine tuned by varying the ratio of reagents in the scrambling reaction, and this allows the preparation of materials with high gas selectivities. The molecular engineering of porous amorphous solids complements crystal engineering strategies and may have advantages in some applications, for example, in the compatibilization of functionalities that do not readily cocrystallize.
The dynamic covalent synthesis, structure and conformational dynamics of a chiral polyimine nanocapsule 1a are reported. Reaction of four tetraformyl cavitands and eight H(2)N(CH(2))(2)NH(2) yields quantitatively 1a, which has a compact, asymmetrically folded, pseudo-C(2)-symmetric structure, as determined by X-ray crystallography, and encapsulates four CHCl(3) and three CH(3)OH guests in the solid state. In solution, 1a enantiomerizes by passing over a barrier of ΔG(298)(double dagger) = 21.5 ± 0.7 kcal mol(-1) via a refolding process.
Hold the protons! A hydrazone-based rotary switch underwent E/Z isomerization upon treatment with Zn2+ through coordination-coupled proton transfer by a mechanism inspired by biological processes that take place in the reaction centers of photosynthetic bacteria and in cation-diffusion facilitators. The process is fully reversible, as the initial Econfiguration can be reinstated by treatment of the zinc complex with cyanide.
Five donor–acceptor oligorotaxanes made up of dumbbells composed of tetraethylene glycol chains, interspersed
with three and five 1,5-dioxynaphthalene units, and terminated by 2,6-diisopropylphenoxy stoppers, have been prepared by the threading of discrete numbers of cyclobis(paraquat-p-phenylene) rings, followed by a
kinetically controlled stoppering protocol that relies on click chemistry. The well-known copper(I)-catalyzed
alkyne–azide cycloaddition between azide functions placed at the ends of the polyether chains and alkyne-bearing
stopper precursors was employed during the final kinetically controlled template-directed synthesis of the five oligorotaxanes, which were characterized subsequently by ^1H NMR spectroscopy at low temperature (233 K) in
deuterated acetonitrile. The secondary structures, as well as the conformations, of the five oligorotaxanes were unraveled by spectroscopic comparison with the dumbbell and ring components. By focusing attention on the changes in
chemical shifts of some key probe protons, obtained from a wide range of low-temperature spectra, a picture emerges of a high degree of folding within the thread protons of the dumbbells of four of the five oligorotaxanes—the fifth oligorotaxane represents a control compound in effect—
brought about by a combination of C-H···O and π–π stacking interactions between the p-electron-deficient bipyridinium
units in the rings and the π-electron-rich 1,5-dioxynaphthalene units and polyether chains in the
dumbbells. The secondary structures of a foldamer-like nature have received further support from a solid-state superstructure of a related [3]pseudorotaxane and density functional calculations performed thereon.
Dynamic libraries of [n × n] imine components spontaneously simplify during a slow oxidation reaction to produce only n discrete products. The selectivity of this self-sorting process is a consequence of different oxidation rates for various imines, while the dynamic nature of the library enables self-sorting to proceed with high efficiency.
A hydrazone-based rotary switch, having a quinolinyl stator and a pyridine ring as part of the rotor, can be induced using pH to undergo a four-step switching sequence. This process yields three stable isomers and a fourth "metastable" one that can all be addressed separately based on the sequence of acid and base added. The switching process proceeds via conformational and/or configurational changes, allowing the molecule to rotate around two different axles.