Fully Aromatic High Performance Thermoset via Sydnone–Alkyne Cycloaddition

Vibrational analyses of the reaction of oxymyoglobin with NO using a photolabile caged NO donor at cryogenic temperatures

The NO dioxygenation reaction catalyzed by heme-containing globin proteins is a crucial aerobic detoxification pathway. Accordingly, the second order reaction of NO with oxymyoglobin and oxyhemoglobin has been the focus of a large number of kinetic and spectroscopic studies. Stopped-flow and rapid-freeze-quench (RFQ) measurements have provided evidence for the formation of a Fe(III)-nitrato complex with millisecond lifetime prior to release of the nitrate product, but the temporal resolution of these techniques is insufficient for the characterization of precursor species. Most mechanistic models assume the formation of an initial Fe(III)-peroxynitrite species prior to homolytic cleavage of the OO bond and recombination of the resulting NO2 and Fe(IV)=O species. Here we report vibrational spectroscopy measurements for the reaction of oxymyoglobin with a photolabile caged NO donor at cryogenic temperatures. We show that this approach offers efficient formation and trapping of the Fe(III)-nitrato, enzyme-product, complex at 180 K. Resonance Raman spectra of the Fe(III)-nitrato complex trapped via RFQ in the liquid phase and photolabile NO release at cryogenic temperatures are indistinguishable, demonstrating the complementarity of these approaches. Caged NO is released by irradiation <180 K but diffusion into the heme pocket is fully inhibited. Our data provide no evidence for Fe(III)-peroxynitrite of Fe(IV)=O species, supporting low activation energies for the NO to nitrate conversion at the oxymyoglobin reaction site. Photorelease of NO at cryogenic temperatures allows monitoring of the reaction by transmittance FTIR which provides valuable quantitative information and promising prospects for the detection of protein sidechain reorganization events in NO-reacting metalloenzymes.

Sydnone: Synthesis, Reactivity and Biological Activities

Sydnones are among the most well-known mesoionic compounds. Since their synthesis in 1935 by Earl and Mecknay, numerous researches have shown that the chemical behavior, physical and biological properties of sydnones make them the most useful compounds in organic chemistry. Sydnones undergo thermal 1,3-dipolar cycloaddition reaction with dipolarophiles (alkynes or alkenes) to give exclusively derivatives containing a pyrazole moiety exhibiting numerous applications, such as pharmaceuticals and agrochemicals. However, the sydnone cycloaddition reaction with alkynes requires harsh conditions, like high temperatures and long reaction times, giving poor regioselectivity to the resulting products. To overcome these constraints, new reactions named CuSAC (Copper- Catalyzed Sydnone-Alkyne Cycloaddition) and SPSAC (Strain-Promoted Sydnone- Alkyne Cycloaddition) have been developed, leading to pyrazoles with interesting constant kinetics.

Recyclable Tough Thermoset with Imide-Hexahydrotriazine Structure

Developing sustainable thermosets is regarded as a major approach to cope with environmental pollution and resource waste, but it still remains a challenge to design recyclable thermosets with exceptional comprehensive...

Effect of curing reaction types on the structures and properties of acetylene-containing thermosets: towards optimization of curing procedure

High-temperature thermosets are usually prepared from resins containing alkynyl groups, and their properties depend much upon the curing process containing various types of curing reactions. However, how the curing process affects the properties remains unclear due to the complicated curing reactions. We used molecular dynamics simulations to investigate the effect of curing reaction types, including cyclotrimerization, Diels-Alder reaction, and radical reaction, on the structures and properties of imide oligomers terminated with alkynyl groups. The results show that the cycloadditions such as cyclotrimerization and Diels-Alder reaction endow the thermosets with rigid structures and high moduli. Compared with the cycloadditions, the radical reaction enables the formation of flexible cured structures, which can enhance the toughness of thermosets. The differences in thermal and mechanical properties caused by different curing types were elucidated by the relaxation processes of fragments in these cured systems and were explained by the variation of torsion energy in different curing forms. As this work aims to optimize the curing procedure to obtain high-performance resins with desired properties, different curing procedures were finally designed according to the theoretical studies, and the obtained cured polymers show significant differences in the properties from different curing ways. The results can guide the preparation of desired thermosetting resins by tuning the curing procedure.

Elucidating the role of acetylene in ortho-phthalimide functional benzoxazines: design, synthesis, and structure–property investigations

Novel ortho-phthalimide-benzoxazines containing acetylene have been designed and their corresponding thermosets exhibit excellent thermal stability although the expected benzoxazole cyclization at a much higher temperature did not take place.

Exploiting the Site Selectivity of Perfluoropyridine for Facile Access to Densified Polyarylene Networks for Carbon-Rich Materials

Fluorinated molecules containing reactive functionalities are of great interest to the materials community as these compounds can be used to prepare fluorinated polymers with desirable physical and electronic properties. Despite their potential, many of these compounds are limited by their synthesis which generally requires transition-metal-catalyzed coupling reactions or harsh fluorinating conditions. Perfluoroheteroaromatic compounds provide a unique solution to this problem as compounds such as perfluoropyridine can undergo S_NAr reactions with a wide range of simple nucleophiles in a controlled and regioselective manner. Herein we report the transition-metal-free synthesis of a pool of highly soluble high aromatic content (HAC) perfluoropyridine-based thermosetting precursors and compounds of interest which can be easily obtained from readily available chemical precursors using simple nucleophilic chemistries. These thermally active monomers cure readily, in 350-400 °C temperature ranges, into highly densified polyaryelene networks and demonstrate decomposition temperatures well above 400 °C and high char yields at 900 °C, making these promising materials for high-temperature applications as well as templates for carbon-based nanomaterials.

Easily Processable Thermosets with Outstanding Performance via Smart Twisted Small-Molecule Benzoxazines

High-performance aromatic polymers have excellent thermal, mechanical, and electrical properties and are lightweight, but it is highly challenging to deliver outstanding performances while still maintaining good processability of the precursors. Here, a new family of small-molecule benzoxazine resins with ortho-maleimide functionality is reported, which strikes an exceptional balance between the processability and performance. The excellent processability is attributed to the twisted molecular configurations of ortho-maleimide-substituted benzoxazines, which prevent intermolecular packing in the resin systems. The new benzoxazines can polymerize through multiple routes, which eliminate the twisted structures and create highly cross-linked polymer networks. The resulting new polymers are found to possess fascinating properties such as a high thermal stability (no T_g can be detected before 400 °C), excellent flame retardancy (a heat release capacity of 42.5 J g^-1 K^-1 ), and low dielectric constants (2.62-2.30 in the frequency range of 1 Hz to 10 MHz). The combined processability and versatility highlight the potential of smart benzoxazines in the preparation of high-performance thermosets, with important new applications that may span aerospace, transportation, and electronic packaging materials.

Stoichiometric imbalance-promoted step-growth polymerization based on self-accelerating 1,3-dipolar cycloaddition click reactions

A stoichiometric imbalance-promoted step growth polymerization method was developed based on self-accelerating 1,3-dipolar cycloaddition click reactions of Sondheimer diyne and varied 1,3-dipoles, such as diazo, sydnone, and nitrone groups.

Serendipitous Formation of 2H-Pyrazolo[3,4-d]pyridazin-7(6H)-ones from 3-Arylsydnones

Fused nitrogen heterocyclesnamely, pyrazolo[3,4-d]pyridazin-7(6H)-ones have been obtained by exploiting the 1,3-dipolar nature of N-arylsydnones, from hydrazones of 3-aryl-4-acetylsydnones via the Vilsmeier-Haack strategy. Facile intramolecular nucleophilic addition followed by CO₂ elimination under reflux or upon microwave irradiation was presented. Plausible mechanisms for the formation of the title compounds are proffered. Structure confirmatory evidence came from single-crystal X-ray crystallography.

[3 + 2]-Cycloaddition reaction of sydnones with alkynes

This review covers all known examples of [3 + 2]-cycloaddition between sydnones and both terminal as well as internal alkynes/cycloalkynes taken from literature since its discovery by Huisgen in 1962 up to the current date. Except enumeration of synthetic applications it also covers mechanistic studies, catalysis, effects of substituents and reaction conditions influencing reaction rate and regioselectivity.

Origins of halogen effects in bioorthogonal sydnone cycloadditions

Halogen substituents increase sydnone cycloaddition reactivities substantially. Fluoro-sydnones are superior to bromo- and chloro-sydnones, and can achieve extremely high second-order rate constants with strained alkynes. Computational studies have revealed the fluorine substituent increases the reactivity of sydnone mainly by lowering its distortion energy.

Sydnone-alkyne cycloaddition: applications in synthesis and bioconjugation

Sydnones are among the most popular mesoionic compounds studied so far for cycloaddition reactions. However, despite their good chemical stability and versatility, only a limited number of research groups have worked on their chemistry and use in organic synthesis. This feature article aims at providing an overview of the most recent developments in sydnone-alkyne cycloadditions, with particular attention on the strategies that allow us to achieving high regiocontrol and milder reaction conditions. The recent discovery that this dipole is able to undergo click and biorthogonal reactions with cycloalkynes may stimulate renewed interest from the scientific community. Given the high potential and flexibility of this family of mesoionics, we believe that major developments are to be expected both in terms of organic synthetic methodologies and biorthogonal chemistry applications in the field of chemical biology.

Synthesis and characterization of amphiphilic miktoarm star polymers based on sydnone-maleimide double cycloaddition

The synthesis of miktoarm star polymers based on sydnone-maleimide double cycloaddition (SMDC) via three approaches.

Recyclable heat-resisting polymer poly(ether azaindole ketone)-H+via hydrogen bonding crosslinking

A novel recyclable hydrogen bonding crosslinked polymer PEAINKH⁺ was prepared, which possesses outstanding thermal stability.

Facile synthesis of recyclable Zn(ii)-metallosupramolecular polymers and the visual detection of tensile strength and glass transition temperature

A series of recyclable crosslinked Zn(ii)-metallosupramolecular coordination polymers are successfully achieved, of which tensile strength and T_g could be visually detected.

Influence of Endo- and Exocyclic Heteroatoms on Stabilities and 1,3-Dipolar Cycloaddition Reactivities of Mesoionic Azomethine Ylides and Imines

The geometries, stabilities, and 1,3-dipolar cycloaddition reactivities of 24 mesoionic azomethine ylides and imines were investigated using density functional theory calculations at the M06-2X/6-311+G-(d,p)/M06-2X/6-31G-(d) level. The computed structures highlight how the commonly used "aromatic" resonance form should be replaced by two more accurate resonance structures. Stabilities of the dipoles were assessed by various homodesmotic schemes and are consistent with these compounds being nonaromatic. The activation free energies with ethylene or acetylene range from 11.8 to 36.6 kcal/mol. Within each dipole type, the predicted cycloaddition reactivities correlate with the reaction energies and the resonance stabilization energies provided by the various substituents. Endocyclic (X) heteroatoms increase the reactivity of the 1,3-dipoles in the order of O > NH ≅ S, whereas exocyclic (Y) substituents increase it in the order of CH₂ > NH > O > S. Distortion/interaction analysis indicated that the difference in reactivity between differently substituted 1,3-dipoles is driven by distortion, whereas the difference between azomethine ylides and imines is related to lower interaction energies of imines with the dipolarophiles.