Highly Transparent and Colorless Self‐Healing Polyacrylate Coatings Based on Diels–Alder Chemistry

The efficient integration of reversible polymer networks into acrylate‐based polymeric materials is of peculiar interest for the development of coatings that combine high transparency with self‐healing ability. In this work, reversible networks are obtained by reacting a series of linear copolymers of furfuryl methacrylate with aliphatic bismaleimides through Diels–Alder (DA) reaction between furan and maleimide moieties. Owing to dynamic crosslinking, the obtained coatings exhibit thermal reversibility, as determined by differential scanning calorimetry and dissolution experiments. Furthermore, upon heating over the retro‐DA temperature, an excellent recovery of mechanically induced surface damages proves successful thermal remendability. Compared to previous reports on DA‐based acrylate networks, the presented thermally responsive coatings exhibit outstanding transparency and absence of color, as a result of an accurate choice of suitable monomeric precursors. In addition, a pronounced hydrophobic behavior and excellent adhesive properties make the proposed material particularly suitable for optical applications.     

Facile Preparation of Polydimethylsiloxane Elastomer with Self‐Healing Property and Remoldability Based on Diels–Alder Chemistry

The polydimethylisioxane elastomer based on Diels–Alder (DA) chemistry is successfully prepared by directly crosslinking bis(3‐aminopropyl)‐terminated polydimethylsiloxane with the bisepoxide containing two DA bonds in one molecule via epoxy‐amine reaction. The elastomer prepared based on DA chemistry exhibits good mechanical property, high self‐healing, and remolded efficiencies. The as‐prepared elastomer can be stretched to over 400% and its tensile strength can reach 0.80 MPa. The self‐healing efficiency and remolded efficiency are up to 93% and 95%, respectively. This work provides a simple and efficient way to fabricate the self‐healing and remolded polydimethylsiloxane elastomer with good mechanical properties. The as‐prepared elastomer has a promising potential in artificial muscles, protective coatings, and intelligent flexible electronics.     

Super‐Tough,Self‐Healing Polyurethane Based on Diels‐Alder Bonds and Dynamic Zinc–Ligand Interactions

Self‐healing polymer materials have attracted extensive attention and have been explored due to their ability of crack repairing in materials. This paper aims to develop a novel polyurethane‐based material with high self‐healing efficiency and excellent mechanical properties under 80 °C on the basis of reversible Diels–Alder bonds as well as zinc–ligand structure (DA‐ZN‐PU). By integrating DA bonds and zinc–ligand structure, as‐prepared DA‐ZN‐PU samples reach the maximum tensile strength as much as 28.45 MPa. After self‐healing, the tensile strength is 25.85 MPa, leading to the high self‐healing efficiency of 90.8%. In addition, by introducing carbonyl iron powder (CIP), a new polyurethane containing carbonyl iron powder (DA‐ZN‐CIP‐PU) can be achieved, exhibiting microwave‐assisted self‐healing property. And the self‐healing efficiency can be reached to 92.6% in 3 min. Due to high self‐healing efficiency and excellent mechanical properties of the prepared novel polyurethane, it has application attributes in crack repair of functional composite materials.     

Synthesis and characterization of epoxy with improved thermal remendability based on Diels‐Alder reaction

To produce an epoxy resin with high intrinsic self‐healing efficiency, furfurylglycidyl ether (FGE) was synthesized following a two‐step route. It carried one furan and one epoxide on each of its molecules. Having been cured using N,N′‐(4,4′‐diphenylmethane)bismaleimide and methylhexahydrophthalic anhydride, FGE was then polymerized with two types of intermonomer linkages. That is, thermally reversible Diels–Alder (DA) bonds from the reaction between furan and maleimide groups, and thermally irreversible bonds from the reaction between epoxide and anhydride groups. These two types of bonds provide the polymer with thermal remendability and load‐bearing capacity, respectively. Compared with N,N‐diglycidylfurfurylamine, which was previously developed by the authors and has a similar structure to FGE but with fewer furan rings, FGE can react with maleimide with lower activation energy and the DA bonds formed exhibit higher reversibility. Consequently, improved crack healability of the cured FGE characterized by nearly full recovery of fracture toughness was revealed using double cleavage drilled compression tests. Copyright © 2010 Society of Chemical Industry     

Niobium‐Catalyzed Highly Enantioselective Aza‐Diels–Alder Reactions

Niobium‐based chiral Lewis acid was found to be highly effective catalyst for aza‐Diels–Alder reactions of imines with Danishefsky‘s dienes. The reactions proceed in high yield with high enantioselectivity for both aromatic and aliphatic imines. The developed methodology was applied to total synthesis of (+)‐anabasine.     

Epoxy‐amine based thermoresponsive networks designed by Diels–Alder reactions

The synthesis of Diels–Alder (DA) adducts from stoichiometric quantities of a new multi‐maleimide dienophile and epoxy‐amine type oligomers bearing furan group units on their side chains was investigated. Precursors of the DA reaction were first synthesized and their functionalities were determined by ¹H NMR and gel permeation chromatography/SEC analysis. TGA and DSC were used to characterize their thermal properties. In this study, the effect of the multi‐furan diene functionality on the network density was analyzed. Rheological analysis was used to highlight the thermal reversibility of the DA reaction and to calculate the average molar weight between crosslinks. The results showed that network density could be regulated or modulated by varying the functionality of the diene. Copyright © 2012 Society of Chemical Industry     

Thermally reversible and self‐healing novolac epoxy resins based on Diels–Alder chemistry

The modified novolac epoxy resins with furan pendant groups were prepared by novolac epoxy resin and furfuryl alcohol and then crosslinked by bifunctional maleimide via Diels–Alder (DA) chemistry to obtain the thermally reversible and self‐healing novolac epoxy resins. The as‐prepared crosslinked novolac epoxy resins were characterized by FT‐IR, NMR, TGA, and DMA. The results indicate that the novel crosslinked novolac epoxy resins present higher storage modulus (2.37 GPa at 30°C) and excellent thermal stability (348°C at 5% mass loss). Furthermore, the thermal reversible and self‐healing properties were studied in detail by DSC, SEM, thermal re‐solution, and gel–solution–gel transition experiments. All the results reveal that the crosslinked novolac epoxy resins based on DA reaction can be used as smart material for the practical application of electronic packaging and structural materials. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42167.     

Efficient Asymmetric Synthesis of 2,6‐Disubstituted 2H‐Dihydropyrans via a Catalytic Hetero‐Diels–Alder/Allylboration Sequence

[4+2] Cycloaddition of (E)‐3‐borylacrolein 1 with ethyl vinyl ether, catalysed by chromium complex (1R,2S) or (1S,2R) 2 , led to the corresponding cycloadducts with high diastereo‐ and enantioselectivities. Further reaction with aldehydes offers an attractive asymmetric route to synthetically useful substituted 3,4‐dihydro‐2H‐pyrans.     

Retro‐Diels–Alder Reactions of Masked Cyclopentadienones Catalyzed by Heterogeneous Brønsted Acids

The zeolite H‐Beta catalyzes the retro‐Diels–Alder reaction of a range of cyclopentadiene cyclo‐adducts at moderate temperatures and ambient pressure, in the presence of an active dienophile. The active catalyst was identified and optimum reaction conditions established after screening a range of zeolites in the retro‐Diels–Alder reaction of the cyclopentadiene adduct of cyclopentenone. Our results suggest that retro‐Diels–Alder reactions of tricyclodecadienones are catalyzed by Brønsted acids and the high catalytic performance of H‐Beta catalysts can be ascribed to the optimal balance between the number of acid sites and their strength as well as to the accessibility of these sites. The methodology was then applied to a series of alkyl derivatives of cyclopentadienylcyclopentenones to provide a viable alternative synthetic route to 4‐alkylcyclopentenones and the versatility of the approach was demonstrated by the successful cycloreversion of N‐cyclohexyl‐2‐azanorborn‐5‐ene.     

Switchable cholesterol recognition system with Diels–Alder reaction using molecular imprinting technique on self‐assembled monolayer

Molecular imprinting was conducted using a method to form a polymeric coating on a self‐assembled monolayer (SAM). This molecular imprinting system was able to obtain an on–off function using the Diels–Alder reaction. Cholesterol was used as a target compound and the SAM was formed using thiol compound on a gold plate. Epoxy resin was used to form the polymeric coating. Covalent bonding method was used in molecular imprinting. The formed molecular imprinting system recognized cholesterol better than analogs of cholesterol. Epoxy resin curing agent with furan group was synthesized to form the molecular imprinting system. Using the Diels–Alder reaction between the furan group and phenylmaleimide, this molecular imprinting system offers a switchable function. © 2019 Society of Chemical Industry     

Chiral Phosphoric Acid‐Catalyzed Enantioselective Three‐ Component Aza‐Diels–Alder Reactions of Aminopyrroles and Aminopyrazoles

A highly stereoselective three‐component Povarov reaction, catalyzed by (R)‐ and (S)‐BINOL hydrogen phosphate, was achieved for the first time with aminopyrroles and aminopyrazoles as 2‐azadiene precursors. A variety of aldehydes, enecarbamates, amino‐substituted azines participated in the reaction to afford the tetrahydropyrrolopyridines and tetrahydropyrazolopyridines in good yields with excellent diatereo‐ and enantioselectivities. A stereochemical model is proposed to account for the observed absolute stereochemistry.

     

Copolymerization of a bisanthracene compound with bismaleimides by Diels–Alder cycloaddition

Four new polymers have been synthesized via succesive Diels–Alder cycloadditions of a bisdiene compound bearing two anthracene groups and four bisdienophiles, all containing bismaleimide functions. The polycondensation reaction was performed in N,N‐dimethylacetamide at 120 °C leading to polymers soluble in polar solvents having molecular weights lower than 6000 g mol^?1. © 2001 Society of Chemical Industry     

A New Class of Enantioselective Catalytic 2‐Pyrone Diels–Alder Cycloadditions

A highly enantioselective catalytic Diels–Alder (DA) cycloaddition of 2H‐pyran‐2,5‐diones (synthon of 5‐hydroxy‐2‐pyrones) has been developed with a Cinchona‐derived thiourea as the catalyst. The conditions were optimized by using 0.2 equiv. of the catalyst and 0.1 equiv. of formic acid in 2‐propanol at room temperature, which afforded the DA products in yields of up to 90% (exo/endo=5.5:1, 98% ee) with trans‐β‐nitrostyrene derivatives as the dienophiles. The structure/activity relationships of the bifunctional catalyst and the effects of the steric, electronic and hydrogen‐bonding properties of the dienophiles have been studied.     

Higher‐Order Human Telomeric G‐Quadruplex DNA Metalloenzymes Enhance Enantioselectivity in the Diels–Alder Reaction

Short human telomeric (HT) DNA sequences form single G‐quadruplex (G₄) units and exhibit structure‐based stereocontrol for a series of reactions. However, for more biologically relevant higher‐order HT G₄‐DNAs (beyond a single G₄ unit), the catalytic performances are unknown. Here, we found that higher‐order HT G₄‐DNA copper metalloenzymes (two or three G₄ units) afford remarkably higher enantioselectivity (>90 % ee) and a five‐ to sixfold rate increase, compared to a single G₄ unit, for the Diels–Alder reaction. Electron paramagnetic resonance (EPR) and enzymatic kinetic studies revealed that the distinct catalytic function between single and higher‐order G₄‐DNA copper metalloenzymes can be attributed to different Cu^II coordination environments and substrate specificity. Our finding suggests that, like protein enzymes and ribozymes, higher‐order structural organization is crucial for G₄‐DNA‐based catalysis.     

Mechanistic Insights into Transition Metal‐Catalysed Oxidation of a Hydroxamic Acid with in situ Diels–Alder Trapping of the Acyl Nitroso Derivative

New insights into the mechanism for the transition metal‐mediated oxidation of hydroxamic acids to give intermediate acyl nitroso species, with subsequent hetero‐Diels–Alder trapping are presented. The activation of triphenylphosphine‐ligated ruthenium‐salen complexes is examined, and evidence is presented for the ruthenium‐oxo species which are involved in the oxidative process of the hydroxamic acid. The observation of the lack of asymmetric induction involved in the intermolecular cycloaddition process involving the intermediate acyl nitrsoso species is explained, with the aid of comparing the ruthenium‐salen‐based systems with nitrosotoluene, and copper(I)/copper(II) BINAP‐based catalysis of nitrosopyridine complexes. This study demonstrates the importance of secondary coordination to achieve asymmetric induction in nitroso‐Diels–Alder reactions.     

Kinetic and thermodynamic measurements for the facile property prediction of diels–alder‐conjugated material behavior

The Diels–Alder (DA) reaction between maleimide and furan moieties possessing various substitutions was performed as a means for developing predictive capabilities for temperature and conversion‐dependent material properties in networks comprised of DA moieties. Using HNMR spectroscopy, the reactions of maleimide‐ and furan‐containing molecules further functionalized with carboxylic acids were monitored to ascertain the impact that substitutional changes had on the thermodynamic and kinetic behavior of the DA reaction. The reaction rate and equilibrium conversion of the furan and maleimide increased when the carboxylic acid functional group directly connected to the furan ring was moved from the two to the three position. When an aliphatic two‐carbon spacer was used, such that the π‐electrons of the carboxylic acid and furan were no longer conjugated, the reaction rate increased further. We also report the reactivity effect on the distance between the carboxylic acid functional group and the maleimide, which yielded little impact on the reaction rate but exhibited increased equilibrium conversion with increasing distance. Additionally, the impact on the kinetic and thermodynamic properties of coupling the carboxylic acid to another molecule, tert‐butyl glycine, was also determined. When the carboxylic acid was coupled to an amine, the DA reaction between the furan and maleimide was generally found to have similar kinetic and thermodynamic behavior as compared to their uncoupled, carboxylic acid equivalents. Thus, the characterized and tabulated kinetic and thermodynamic data presented herein enables the prediction of a broad set of temperature‐dependent chemical and material properties. Finally, we discuss practical limitations and nuances of the DA reaction, such as the potential for the maleimide to ring‐open in aqueous media via hydrolysis. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

Synthesis and thermal behavior of some anthracene‐based copolymers obtained by Diels–Alder cycloaddition reactions

New polymer structures have been synthesized via Diels–Alder cycloaddition of bisdiene compounds bearing two anthracene groups and different bisdienophiles, all containing bismaleimide or biscitraconimide functions. The monomers and polymers were characterized by FTIR, UV, and ¹H NMR techniques and compared with two models having a cycloadduct structure. The polymers were studied by thermogravimetric analyses. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Bio‐orthogonal Fluorescent Labelling of Biopolymers through Inverse‐Electron‐Demand Diels–Alder Reactions

Bio‐orthogonal labelling schemes based on inverse‐electron‐demand Diels–Alder (IEDDA) cycloaddition have attracted much attention in chemical biology recently. The appealing features of this reaction, such as the fast reaction kinetics, fully bio‐orthogonal nature and high selectivity, have helped chemical biologists gain deeper understanding of biochemical processes at the molecular level. Listing the components and discussing the possibilities and limitations of these reagents, we provide a recent snapshot of the field of IEDDA‐based biomolecular manipulation with special focus on fluorescent modulation approaches through the use of bio‐orthogonalized building blocks. At the end, we discuss challenges that need to be addressed for further developments in order to overcome recent limitations and to enable researchers to answer biomolecular questions in more detail.