Hunting for the Intermolecular Diels-Alderase

ConspectusThe Diels-Alder reaction is well known as a concerted [4 + 2] cycloaddition governed by the Woodward-Hoffmann rules. Since Prof. Otto Diels and his student Kurt Alder initially reported the intermolecular [4 + 2] cycloaddition between cyclopentadiene and quinone in 1928, it has been recognized as one of the most powerful chemical transformations to build C-C bonds and construct cyclic structures. This named reaction has been widely used in synthesizing natural products and drug molecules. Driven by the synthetic importance of the Diels-Alder reaction, identifying the enzyme that stereoselectively catalyzes the Diels-Alder reaction has become an intriguing research area in natural product biosynthesis and biocatalysis. With significant progress in sequencing and bioinformatics, dozens of Diels-Alderases have been characterized in microbial natural product biosynthesis. However, few are evolutionally dedicated to catalyzing an intermolecular Diels-Alder reaction with a concerted mechanism.This Account summarizes our endeavors to hunt for the naturally occurring intermolecular Diels-Alderase from plants. Our research journey started from the biomimetic syntheses of D-A-type terpenoids and flavonoids, showing that plants use both nonenzymatic and enzymatic intermolecular [4 + 2] cycloadditions to create complex molecules. Inspired by the biomimetic syntheses, we identify an intermolecular Diels-Alderase hidden in the biosynthetic pathway of mulberry Diels-Alder-type cycloadducts using a biosynthetic intermediate probe-based target identification strategy. This enzyme, MaDA, is an endo-selective Diels-Alderase and is then functionally characterized as a standalone intermolecular Diels-Alderase with a concerted but asynchronous mechanism. We also discover the exo-selective intermolecular Diels-Alderases in Morus plants. Both the endo- and exo-selective Diels-Alderases feature a broad substrate scope, but their mechanisms for controlling the endo/exo pathway are different. These unique intermolecular Diels-Alderases phylogenetically form a subgroup of FAD-dependent enzymes that can be found only in moraceous plants, explaining why this type of [4 + 2] cycloadduct is unique to moraceous plants. Further studies of the evolutionary mechanism reveal that an FAD-dependent oxidocyclase could acquire the Diels-Alderase activity via four critical amino acid mutations and then gradually lose its original oxidative activity to become a standalone Diels-Alderase during the natural evolution. Based on these insights, we designed new Diels-Alderases and achieved the diversity-oriented chemoenzymatic synthesis of D-A products using either naturally occurring or engineered Diels-Alderases.Overall, this Account describes our decade-long efforts to discover the intermolecular Diels-Alderases in Morus plants, particularly highlighting the importance of biomimetic synthesis and chemical proteomics in discovering new intermolecular Diels-Alderases from plants. Meanwhile, this Account also covers the evolutionary and catalytic mechanism study of intermolecular Diels-Alderases that may provide new insights into how to discover and design new Diels-Alderases as powerful biocatalysts for organic synthesis.

Synthesis of [60]- and [70]Fullerene-Fused Tetrahydroquinoxaline Derivatives by Oxidative [4 + 2] Cycloaddition with Unusual Reactivity and Regioselectivity

The oxidative [4 + 2] reaction of o-phenylenediamine-derived disulfonamides with fullerene C₆₀ and C₇₀ is reported, in which electron-deficient reactants showed high reactivity. The reaction of C₇₀ exhibited unusual regioselectivity, yielding a [5,6]-adduct as the major product, which was characterized by ¹H, ¹³C NMR and single-crystal X-ray diffraction. DFT calculations revealed the reaction is an inverse-electron-demand Diels-Alder (IEDDA) reaction, and the [5,6]-adduct of C₇₀ is a kinetic product.

Reversible functionalization and exfoliation of graphite by a Diels-Alder reaction with furfuryl amine

Furfuryl amine-functionalized few-layered graphene was prepared via a mechanochemical process by a [4 + 2] cycloaddition under solvent-free conditions. By employing ball milling, active sites are merged mostly at the edge of the graphene sheets which makes them prone to Diels–Alder click reactions (D–A) in the presence of a diene precursor. Consequently, one-pot grafting with furfuryl amine onto the graphene sheets, exfoliates pristine graphite resulting in functionalized few-layered graphene which is soluble in organic solvents. Thereafter, the cleavage of the bonds in the adduct can occur by exposure to an external stimulus like temperature, to initiate a retro-Diels–Alder reaction. The success of the thermoreversible functionalization of the few-layered graphene was confirmed by Raman spectroscopy, TGA, XPS, EDX, contact angle and XRD analysis. The morphology of the samples was investigated by scanning electron microscopy and AFM. The latter was utilized to estimate graphene thickness. The results showed that functionalization proceeded under nitrogen with dry ball milling and mild temperatures efficiently.

Furfuryl amine-functionalized few-layered graphene was prepared via a mechanochemical process by a [4 + 2] cycloaddition under solvent-free conditions.

Fabrication and characterization of impact-resistant core-spun yarn fabrics with a hydroxylated fullerene-strengthened shear thickening fluid

Shear thickening fluid (STF) is investigated to strength soft armor; however, its impact resistance still does not meet practical needs. In this work, a small amount of hydroxylated fullerene (C₆₀) was mixed with STF to improve the thickening ratio. First, furfuryl alcohol (FA) was grafted onto C₆₀ through a Diels–Alder (D–A) reaction to improve the dispersity of C₆₀ in the STF. Sheath-core composite fibers (polyketone (PK) as the sheath and STF as the core) were then fabricated by coaxial electrospinning. Finally, composite fibers containing STF and C₆₀ were wrapped on the surface of aramid yarns to fabricate a core-spun yarn. Under impact, these core-spun yarns manifested the characteristics of aramid fibers and the thickening advantages of the STF, solving problems of the hygroscopicity, migration, and leakage of STF. In addition, the content of STF was also greatly increased. The spike punching resistance of the core-spun yarn fabric is about 2.8 times that of the aramid fabric (AF) under the same area density. Impact-resistant core-spun yarn fabrics could provide a new direction for the development of soft armor.

Shear thickening fluid (STF) is investigated to strength soft armor; however, its impact resistance still does not meet practical needs.

Modulating the reactivity of polymer with pendant ester groups by methylation reaction for preparing functional polymers

Chemical reaction triggered the reactivity of polymeric esters.

Functionalization of Single-Walled Carbon Nanotubes with End-Capped Polystyrene via a Single-Step Diels-Alder Cycloaddition

A facile, single-step, [4+2] Diels-Alder cycloaddition reaction for the surface functionalization of single-walled carbon nanotubes (SWNTs) with end-capped polystyrene chains is presented. The thermal cycloaddition reaction took place at high temperature (~230 °C) without any catalyst between the sp2 network of carbon nanotubes, which acted as dienophile, and the diphenylethylene cyclobutene (DPE-CB) terminal group of the polystyrene chain. Anionic polymerization was employed for the synthesis of the polystyrene macromolecule, and successful and quantitative end-capping reaction with the DPE-CB molecule was confirmed by matrix-assisted laser desorption/ionization time of flight mass spectroscopy. Thermogravimetric analysis revealed the wt % of the grafted macromolecule on the CNT surface as well as the grafting density of the polymer chains on the SWNTs (0.027 chains nm-2). Direct evidence for the surface functionalization and the presence of thin polystyrene film was obtained by transmission electron microscopy (TEM) and by atomic force microscopy (AFM).

Low-Temperature Intramolecular [4+2] Cycloaddition of Allenes with Arenes for the Synthesis of Diene Ligands

The intramolecular [4+2] cycloaddition between arenes and allenes first reported by Himbert gives rapid access to rigid polycyclic scaffolds. Herein, we report a one-pot oxyalkynylation/cycloaddition reaction proceeding under mild conditions (23-90 °C) and providing complex polycyclic architectures with high efficiency, and atom and step economy. The bicyclo[2.2.2]octadiene products were obtained with a wide variety of useful functional groups and were successfully applied as chiral ligands for metal catalysis. Computational studies gave a first rationalization of the low activation energy for the cycloaddition based on counter-intuitive favorable dispersive interactions in the transition state.

Construction of activated carbon-supported B3N3 doped carbon as metal-free catalyst for dehydrochlorination of 1,2-dichloroethane to produce vinyl chloride

Metal-free catalysts synthesized by impregnating activated carbons with B₃N₃-containing arylacetylene resin showed good catalytic performance for industrial dehydrochlorination of 1,2-dichloroethane to produce vinyl chloride monomer.

Preparation of graphene via wet ball milling and in situ reversible modification with the Diels–Alder reaction

Functionalized graphene (G-MA) was prepared by a facile wet ball milling strategy, which achieved exfoliation and functionalization of graphite simultaneously.

Construction of Layered B3N3-Doped Graphene Sheets from an Acetylenic Compound Containing B3N3 by a Semisynthetic Strategy

The structural modification of graphene at the atomic level is crucial for electrochemical applications. Doping heteroatoms to modify the structure of graphene has widely been adopted. However, the construction and controllable doping of heteroatom-doped graphene remains a challenge. Herein, a novel semisynthetic method is developed to synthesize a borazine (B₃N₃)-containing acetylenic compound as a precursor, and a series of B₃N₃-doped few-layered graphene nanosheets are prepared after annealing at different temperatures. To form graphene sheets, the in situ-forming MgBrCl salt is used as an intercalation agent to enlarge the mutual distance between molecules, which can inhibit the unwanted cross-linking reaction. Nanosheets with different thicknesses of 2.5, 3.5, and 4.1 nm can be obtained at annealing temperatures of 1500, 1200, and 1000 °C, respectively. The results demonstrate that the B and N atoms are co-doped in the graphene by the structure of B₃N₃, and the doping site can be changed with different annealing temperatures. The optical gap of graphene can be successfully opened by doping with B₃N₃, and the resultant material can be potentially utilized as a catalyst and semiconductor material. Furthermore, this new semisynthetic strategy will offer the opportunity to fabricate more carbon materials via controllable heteroatom doping.

The journey of self-healing and shape memory polyurethanes from bench to translational research

In this critical review, we have enlisted a comprehensive summary of different approaches that have been used over the past decade to synthesize self-healing polyurethanes including “close then heal” and “shape memory assisted self-healing” concept.

Addition of azomethine ylides to carbon-encapsulated iron nanoparticles

Carbon-encapsulated iron nanoparticles have been covalently functionalized using the Prato reaction.

Ultrasound-promoted direct functionalization of multi-walled carbon nanotubes in water via Diels-Alder "click chemistry"

A facile and environmentally friendly strategy for grafting polymers onto the surface of multi-walled carbon nanotubes (CNTs) was demonstrated by Diels-Alder "click chemistry". Firstly, the copolymers of poly(styrene-alt-maleic anhydride) (PSM) were prepared by the reversible addition-fragmentation chain transfer (RAFT) polymerization and subsequently functionalized with furfuryl amine to introduce anchoring groups. The copolymers were then grafted on CNTs via the Diels-Alder reaction in water through a conventional heating-stirring route and ultrasound-assisted method. The obtained nanocomposite materials were characterized by thermogravimetric analysis, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and transmission electron microscopy. The results indicated that the reaction rate under ultrasound irradiation was accelerated about 12 times than the one under the conventional heating-stirring condition without losing the grafting efficiency. The direct functionalization of CNTs formed a stably dispersed solution in water, promising a green and effective method for industrial process.

Dynamically Cross-linked Elastomer Hybrids with Light-Induced Rapid and Efficient Self-Healing Ability and Reprogrammable Shape Memory Behavior

Pristine carbon nanotubes (CNTs) were activated to exhibit Diels-Alder (DA) reactivity in a polymer matrix, which was modified with monomers containing furan groups. The DA-active polymer matrix was transferred into a dynamic reversible cross-linked inorganic-organic network via a Diels-Alder reaction with CNTs, where pristine CNTs were used as dienophile chemicals and furan-modified SBS acted as the macromolecular diene. In this system, the mechanical properties as well as resilience and solvent resistance were greatly improved even with the presence of only 1 wt % CNTs. Meanwhile, the hybrids retained recyclability and exhibited some smart behaviors, including self-healing and reprogrammable shape memory properties. Furthermore, due to the photothermal effect of CNTs, a retro-Diels-Alder (rDA) reaction was activated under laser irradiation, and healing of a crack on the hybrid surface was demonstrated in approximately 10 s with almost complete recovery of the mechanical properties. Such fast and efficient self-healing performance provides a new concept in designing self-healing nanocomposites with tunable structures and mechanical properties. Furthermore, the DA and rDA reactions could be combined to reprogram the shape memory behavior under laser irradiation or thermal treatment, wherein the temporary shape of the sample could be transferred to a permanent shape via the rDA reaction at high temperature.

Chemically reprogrammable metal organic frameworks (MOFs) based on Diels–Alder chemistry

We pioneer a new class of reprogrammable MOFs able to switch their interlattice chemistry via a facile Diels–Alder based cycloreversion process.

Reactivity of Single-Walled Carbon Nanotubes in the Diels-Alder Cycloaddition Reaction: Distortion-Interaction Analysis along the Reaction Pathway

Diels-Alder cycloaddition is one of the most powerful tools for the functionalization of single-walled carbon nanotubes (SWCNTs). Density functional theory at the B3-LYP level of theory has been used to investigate the reactivity of different-diameter SWCNTs (4-9,5) in Diels-Alder reactions with 1,3-butadiene; the reactivity was found to decrease with increasing SWCNT diameter. Distortion/interaction analysis along the whole reaction pathway was found to be a better way to explore the reactivity of this type of reaction. The difference in interaction energy along the reaction pathway is larger than that of the corresponding distortion energy. However, the distortion energy plots for these reactions show the same trend. Therefore, the formation of the transition state can be determined from the interaction energy. A lower interaction energy leads to an earlier transition state, which indicates a lower activation energy. The computational results also indicate that the original distortion of the SWCNTs leads to an increase in the reactivity of the SWCNTs.

Light-driven nitrile imine-mediated tetrazole-ene cycloaddition as a versatile platform for fullerene conjugation

An efficient methodology for modular fullerene functionalization via the photo-induced nitrile imine-mediated tetrazole-ene cycloaddition (NITEC) is introduced. The versatility and platform character of the method is illustrated by the light-driven reaction of fullerenes with small molecule, polymeric and surface-immobilized tetrazoles. The efficient fullerene conjugation is evidenced via mass spectrometric techniques.

Recent advances in organic–inorganic well-defined hybrid polymers using controlled living radical polymerization techniques

Controlled living radical polymerizations, such as ATRP and RAFT polymerization, could be utilized for the preparation of well-defined organic–inorganic hybrid polymers based on POSS, PDMS, silica nanoparticles, graphene, CNTs and fullerene.

The role of curvature in Diels–Alder functionalization of carbon-based materials

We have estimated theoretically the impact of curvature on the free energies of activation and reaction associated with Diels–Alder reactions on carbon-based materials.

Lewis Acid Assisted Diels-Alder Reaction with Regio- and Stereoselectivity: Anti-1,4-Adducts with Rigid Scaffolds and Their Application in Explosives Sensing

Unusual anti-1,4-adducts of anthracene derivatives and anti-adducts of inert arenes with rigid scaffolds have been obtained via AlCl3-assisted Diels-Alder reaction in good to excellent yields under mild conditions. Further derivation of 1,4-adducts gave π-conjugated polymers which could act as sensors of explosive species. This highly efficient synthesis method provides versatile approaches to solid-state emissive π-conjugated polymers.

Unusual regio- and stereo-selectivity in Diels-Alder reactions between bulky N-phenylmaleimides and anthracene derivatives

Unusual regio- and stereo-selectivity in Diels-Alder (D-A) reactions were achieved between bulky N-phenylmaleimides and anthracene derivatives. Using multiple substituents with steric hindrance on both diene and dienophile, a noticeable shift toward 1,4-addition was successfully obtained. The substrate scope in this reaction was broad and the highest yield of anti-1,4-adducts was over 90%. Novel structures of anti-1,4-adducts were confirmed by single crystal X-ray diffraction analysis. This study not only provides the first reported method of synthesizing anti-1,4-adducts and achieving otherwise unattainable regio- and stereo-selectivity, but also elucidates the importance of combining the steric effects of two reactants to shift products toward 1,4-adducts. Moreover, the resulting 1,4-adducts could be further functionalized through their halogen groups via carbon-carbon coupling reactions.

Phosphonated furan-functionalized poly(ethylene oxide)s using orthogonal click chemistries: synthesis and Diels–Alder reactivity

The synthesis and the reactivity in Diels–Alder and retro Diels–Alder thermoreversible reactions of new phosphonate- and phosphonic acid-terminated furan-functionalized PEO are reported.

Dynamic crosslinked poly(styrene-block-butadiene-block-styrene) via Diels–Alder chemistry: an ideal method to improve solvent resistance and mechanical properties without losing its thermal plastic behavior

Poly(styrene-block-butadiene-block-styrene) (SBS) is a typical example of thermal plastic elastomers (TPE).

Graft polymer growth using tandem photoinduced photoinitiator-free CuAAC/ATRP

In this work, we describe the use of a one-pot, photoinduced but photoinitiator-free combined copper-catalyzed azide–alkyne cycloaddition (CuAAC) and atom-transfer radical polymerization (ATRP) protocol to provide a graft copolymer of polystyrene-g-poly(methyl methacrylate) (PS-g-PMMA) in good conversion and polydispersity.

Facile synthesis of silica nanoparticles grafted with quaternized linear, comblike and toothbrushlike copolymers

The alkoxysilane–hydroxyl coupling reaction, quaternization and RAFT polymerization were combined to synthesize three types of quaternized copolymers grafted silica with thermo-dependent surface wettability.

Nitrosocarbonyl Hetero-Diels-Alder Cycloaddition: A New Tool for Conjugation

It is demonstrated that nitrosocarbonyl hetero-Diels-Alder chemistry is an efficient and versatile reaction that can be applied in macromolecular synthesis. Polyethylene glycol functionalized with a hydroxamic acid moiety undergoes facile coupling with cyclopentadiene-terminated polystyrene, through a copper-catalyzed as well as thermal hetero-Diels-Alder reaction. The mild and orthogonal methods used to carry out this reaction make it an attractive method for the synthesis of block copolymers. The resulting block copolymers were analyzed and characterized using GPC and NMR. The product materials could be subjected to thermal retro [4 + 2] cycloaddition, allowing for the liberation of the individual polymer chains and subsequent recycling of the diene-terminated polymers.

π-conjugated polymer-fullerene covalent hybrids via ambient conditions Diels-Alder ligation

The established ability of graphitic carbon-nanomaterials to undergo ambient condition Diels-Alder reactions with cyclopentadienyl (Cp) groups is herein employed to prepare fullerene-polythiophene covalent hybrids with improved electron transfer and film forming characteristics. A novel precisely designed polythiophene (M n 9.8 kD, Đ 1.4) with 17 mol% of Cp-groups bearing repeat unit is prepared via Grignard metathesis polymerization. The UV/Vis absorption and fluorescence (λex 450 nm) characteristics of polythiophene with pendant Cp-groups (λmax 447 nm, λe-max 576 nm) are comparable to the reference poly(3-hexylthiophene) (λmax 450 nm, λe-max 576 nm). The novel polythiophene with pendant Cp-groups is capable of producing solvent-stable free-standing polythiophene films, and non-solvent assisted self-assemblies resulting in solvent-stable nanoporous-microstructures. (1) H-NMR spectroscopy reveals an efficient reaction of the pendant Cp-groups with C60 . The UV/Vis spectroscopic analyses of solution and thin films of the covalent and physical hybrids disclose closer donor-acceptor packing in the case of covalent hybrids. AFM images evidence that the covalent hybrids form smooth films with finer lamellar-organization. The effect is particularly remarkable in the case of poorly soluble C60 . A significant enhancement in photo-voltage is observed for all devices constituted of covalent hybrids, highlighting novel avenues to developing efficient electron donor-acceptor combinations for light harvesting systems.