Synthesis and characterization of multicyclic oxadiazoles and 1-hydroxytetrazoles as energetic materials

Improvement of Thermal Stability and Sensitivity of Furazan/Furoxan via Ring Closure Generation of Vicinal Amino-Nitro Structure

Energetic compounds that display high thermal stability and insensitivity properties are essential for applications in mining, gas drilling, etc. In this work, a novel 4-nitro-5-aminoisoxazole energetic moiety oriented to enhance thermal stability and decrease the sensitivity of furazan/furoxan analogues was constructed. The generation of a vicinal amino-nitro structure can be effectively realized in one step by a green and mild ring-closing reaction of nitroacetonitrile potassium salt with chloroxime in aqueous solution. Seven new energetic compounds with good thermal stability (Td: 155.8∼270.3 °C) and low sensitivity (IS: >40 J, FS: 96∼192 N) were synthesized. The nitro-furazan and isoxazole combined product exhibits the advantages of both skeletons, with energetic properties (Dv = 8350 m s-1, P = 30.1 GPa) that may find value in insensitive energetic materials. This strategy promises to achieve a balance between energy levels and stability of furazan/furoxan analogues and offers a new way for the design and synthesis of highly thermally stable compounds that meet special applications.

Investigation into a Conformationally Locked (Z)-Azidoxime

High nitrogen compounds find wide use in the development of new propellants and explosives as well as pharmaceutical chemistry as bioisosteres, bacterial stains, and antifungal agents. A class of underexplored high-nitrogen materials includes azidoximes and their 1-hydroxytetrazole isomers. Azidoximes possess an energetic azide group and are quite sensitive to impact, spark, and friction. Therefore, these materials are generated in situ and cyclized under mild acidic conditions to their 1-hydroxytetrazole isomers. Recently, we synthesized a novel 1,2,4-triazine-derived azidoxime; however, upon subjecting this material to established cyclization conditions, no reaction was observed, even after prolonged reaction times with heating. Additional 1,2,4-triazine-derived azidoximes also displayed a similar lack of reactivities. This observation led us to probe the reactivity of these materials with both a DFT investigation and crystallographically based electrostatic potential mapping. In all, the lack of reactivity toward cyclization was found to be due to an inability of 1,2,4-triazine-based azidoximes to isomerize into the reactive (E)-conformation, requiring an activation energy of 26.4 kcal mol-1.

Chemical Descriptors for a Large-Scale Study on Drop-Weight Impact Sensitivity of High Explosives

The drop-weight impact test is an experiment that has been used for nearly 80 years to evaluate handling sensitivity of high explosives. Although the results of this test are known to have large statistical uncertainties, it is one of the most common tests due to its accessibility and modest material requirements. In this paper, we compile a large data set of drop-weight impact sensitivity test results (mainly performed at Los Alamos National Laboratory), along with a compendium of molecular and chemical descriptors for the explosives under test. These data consist of over 500 unique explosives, over 1000 repeat tests, and over 100 descriptors, for a total of about 1500 observations. We use random forest methods to estimate a model of explosive handling sensitivity as a function of chemical and molecular properties of the explosives under test. Our model predicts well across a wide range of explosive types, spanning a broad range of explosive performance and sensitivity. We find that properties related to explosive performance, such as heat of explosion, oxygen balance, and functional group, are highly predictive of explosive handling sensitivity. Yet, models that omit many of these properties still perform well. Our results suggest that there is not one or even several factors that explain explosive handling sensitivity, but that there are many complex, interrelated effects at play.

Enhancement of Energetic Performance through the Construction of Trinitromethyl Substituted β-Bis(1,2,4-oxadiazole)

Halogen-free substitutions of ammonium perchlorate (AP), which meet the requirements of high density, high performance, and acceptable stability, have been a subject of scientific research for many years. In this work, by regrouping atoms in trinitromethyl substituted bis(1,2,4-oxadiazole), a novel oxidizer, trinitromethyl substituted β-bis(1,2,4-oxadiazole) (3), was designed and synthesized. It possesses an improved density (ρ = 2.002 g cm^-3 at 170 K, ρ = 1.942 g cm^-3 at 298 K) and thermal stability (T_d = 142.8 °C) in comparison to its regioisomer 5,5'-bis(trinitromethyl)-3,3'-bi(1,2,4-oxadiazole) (E). The properties for 3 were studied by both experimental and theoretical methods. The high positive oxygen balance of +7.3% and high specific impulse of 250 s also make compound 3 a promising candidate for an energetic oxidizer.

Synthetic Strategies Toward Nitrogen-Rich Energetic Compounds Via the Reaction Characteristics of Cyanofurazan/Furoxan

The structural units of amino-/cyano-substituted furazans and furoxans played significant roles in the synthesis of nitrogen-rich energetic compounds. This account focused on the synthetic strategies toward nitrogen-rich energetic compounds through the transformations based on cyanofurazan/furoxan structures, including 3-amino-4-cyanofurazan, 4-amino-3-cyano furoxan, 3,4-dicyanofurazan, and 3,4-dicyanofuroxan. The synthetic strategies toward seven kinds of nitrogen-rich energetic compounds, such as azo (azoxy)-bridged, ether-bridged, methylene-bridged, hybrid furazan/furoxan-tetrazole–based, tandem furoxan–based, hybrid furazan-isofurazan–based, hybrid furoxan-isoxazole–based and fused framework–based energetic compounds were fully reviewed, with the corresponding reaction mechanisms toward the nitrogen-rich aromatic frameworks and examples of using the frameworks to create high energetic substances highlighted and discussed. The energetic properties of typical nitrogen-rich energetic compounds had also been compared and summarized.

Solvent-free synthesis of 3,5-isoxazoles via 1,3-dipolar cycloaddition of terminal alkynes and hydroxyimidoyl chlorides over Cu/Al2O3 surface under ball-milling conditions

Scalable, solvent-free synthesis of 3,5-isoxazoles under ball-milling conditions has been developed. The proposed methodology allows the synthesis of 3,5-isoxazoles in moderate to excellent yields from terminal alkynes and hydroxyimidoyl chlorides, using a recyclable Cu/Al₂O₃ nanocomposite catalyst. Furthermore, the proposed conditions are reproducible to a 1.0-gram scale without further milling time variations.

A practical and scalable mechanochemical 1,3-dipolar cycloaddition between hydroxyimidoyl chlorides and terminal alkynes catalyzed by Cu/Al₂O₃ allows a quick access to 3,5-isoxazole derivatives.

In Search of Small Molecules That Selectively Inhibit MBOAT4

Ghrelin is a 28-residue peptide hormone produced by stomach P/D1 cells located in oxyntic glands of the fundus mucosa. Post-translational octanoylation of its Ser-3 residue, catalyzed by MBOAT4 (aka ghrelin O-acyl transferase (GOAT)), is essential for the binding of the hormone to its receptor in target tissues. Physiological roles of acyl ghrelin include the regulation of food intake, growth hormone secretion from the pituitary, and inhibition of insulin secretion from the pancreas. Here, we describe a medicinal chemistry campaign that led to the identification of small lipopeptidomimetics that inhibit GOAT in vitro. These molecules compete directly for substrate binding. We further describe the synthesis of heterocyclic inhibitors that compete at the acyl coenzyme A binding site.

5-Nitrotetrazol and 1,2,4-Oxadiazole Methylene-Bridged Energetic Compounds: Synthesis, Crystal Structures and Performances

A new structural type for melt cast materials was designed by linking nitrotetrazole ring with 1,2,4-oxadiazole through a N-CH₂-C bridge for the first time. Three N-CH₂-C linkage bridged energetic compounds, including 3-((5-nitro-2H-tetrazol-2-yl) methyl)-1,2,4-oxadiazole (NTOM), 3-((5-nitro-2H-tetrazol-2-yl)methyl)-5-(trifluoromethyl)-1,2,4 -oxadiazole (NTOF) and 3-((5-nitro-2H-tetrazol-2-yl)methyl)-5-amine-1,2,4-oxadiazole (NTOA), were designed and synthesized through a two-step reaction by using 2-(5-nitro-2H-tetrazole -2-yl)acetonitrile as the starting material. The synthesized compounds were fully characterized by NMR (¹H, ¹³C), IR spectroscopy and elemental analysis. The single crystals of NTOM, NTOF and NTOA were successfully obtained and investigated by single-crystal X-ray diffraction. The thermal stabilities of these compounds were evaluated by DSC-TG measurements, and their apparent activation energies were calculated by Kissinger and Ozawa methods. The crystal densities of the three compounds were between 1.66 g/cm³ (NTOA) and 1.87 g/cm³ (NTOF). The impact and friction sensitivities were measured by standard BAM fall-hammer techniques, and their detonation performances were computed using the EXPLO 5 (v. 6.04) program. The detonation velocities of the three compounds are between 7271 m/s (NTOF) and 7909 m/s (NTOM). The impact sensitivities are >40 J, and the friction sensitivities are >360 N. NTOM, NTOF and NTOA are thermally stable, with decomposition points > 240 °C. The melting points of NTOM and NTOF are 82.6 °C and 71.7 °C, respectively. Hence, they possess potential to be used as melt cast materials with good thermal stabilities and better detonation performances than TNT.

Synthesis and Characterization of the Potential Melt-Castable Explosive 3-(1,2,4-Oxadiazolyl)-5-Nitratomethyl Isoxazole

The synthesis of 3-(1,2,4-oxadiazolyl)-5-nitratomethyl isoxazole (C₆ H₄ N₄ O₅ ), its physical properties, and its theoretical performances are described. This energetic material was found to have a melting point range of 76.6-79.2 °C, and a thermal onset decomposition temperature of 184.5 °C. These thermal features put this material into the standalone melt-castable explosive class. The material was found to have TNT performance, and was found to be insensitive to impact, friction, and electrostatic discharge, despite having a nitric ester functionality. A critical reaction in making this molecule was the desymmetrization of diaminoglyoxime. The optimization of this transformation is described. Previous reports of this desymmetrization were found to be inaccurate, as the desymmetrization reaction produces a co-crystal of mono- and bi-1,2,4-oxadiazole products.

Effect of multiple oxadiazole rings with nitro and nitramino functionalities on energetic properties: computational analysis of the structure–property relationship

The oxadiazole ring is an essential nitrogen-containing heterocycle and a valuable building block that is used in developing numerous functionalized energetic compounds.

3-R-4-(5-Methyleneazide-1,2,4-oxadiazol-3-yl)furazan and its ionic salts as low-sensitivity and high-detonation energetic materials

3-R-4-(5-Methyleneazide-1,2,4-oxadiazol-3-yl)furazan compounds as low-sensitivity and high-detonation energetic materials.

Novel energetic metal-organic frameworks assembled from the energetic combination of furazan and tetrazole

Two novel 2D energetic metal-organic frameworks (MOFs), namely, [Pb(BTF)(H2O)2]n (1) and [Ba(BTF)(H2O)4]n (2), that possess the combined advantages of tetrazole and furazan rings were successfully synthesized. Their crystal structures were determined by single-crystal X-ray diffraction and fully characterized by elemental analysis and FT-IR spectroscopy. Their thermal stability and sensitivity were also investigated. The MOFs have good thermal stability (Tdec > 250 °C) and are insensitive to impact (IS > 25 J) and friction (FS > 360 N), and also have good oxygen balance (Ω > -20%). Crystal structure analyses reveal that the two compounds have densities up to 3.382 g cm-3 and 2.336 g cm-3, respectively, and excellent physicochemical properties. Tetrazole and furazan rings as ligands can commendably increase the densities and oxygen balance of energetic MOFs, thereby enhancing their detonation performance. We anticipate that this work will open a new direction for the development of energetic MOFs. Moreover, (1) exhibits outstanding catalytic performance for ammonium perchlorate (AP). When the supramolecular complex was added in 10 wt% amount, the high-temperature decomposition peak of AP advanced by 95 °C and the reaction rates enhanced by lowering the activation energy.

Nitro compounds as the core structures of promising energetic materials and versatile reagents for organic synthesis

This review addresses some promising areas of chemistry of nitro compounds extensively developed in recent years in Russia (particularly at the N.D.Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences) and worldwide. The most important results in the synthesis of novel energetic N-, C- and O-nitro compounds are summarized. New environmentally friendly approaches to the preparation of known compounds of this series, used as components of energetic compositions, are considered. Methods for selective transformations of various nitro compounds to valuable products of organic synthesis, primarily biologically active products and their precursors, are systematically analyzed.

The bibliography includes 446 references.

A novel energetic framework combining the advantages of furazan and triazole: a design for high-performance insensitive explosives

Combining energetic furazan and triazole rings to achieve a series of high performance and insensitive energetic compounds.

Synthesis and Characterization of 3-(5-(Fluorodinitromethyl)-1H-1,2,4-triazol-3-yl)-4-nitrofurazan: A Novel Promising Energetic Component of Boron-based Fuels for Rocket Ramjet Engines

The synthesis of a new energetic 1,2,4-triazole compound bearing nitrofurazanyl and fluorodinitromethyl units, which may find use as a component for rocket ramjet engines (RRE), is described. The target product was prepared in a four-step process applying oxidation/nitration/decarboxylation/fluorination reactions and is fully characterized. Its density and structural features were uniquely determined by X-ray analysis. It is shown that replacing HMX with the compound of this study in boron-based fuels gives an increase in energy.

1,2,4-Oxadiazole-Bridged Polynitropyrazole Energetic Materials with Enhanced Thermal Stability and Low Sensitivity

A family of mono or di(1,2,4-oxadiazole)-bridged polynitropyrazole derivatives with C-nitro/N-nitro functionalities is reported. All compounds were fully characterized by IR, NMR (¹ H, ¹³ C), elemental analysis and differential scanning calorimetry (DSC). The solid-state structure features were further investigated with X-ray diffraction. Of these, compounds 3,5-bis(3,4-dinitro-1H-pyrazol-5-yl)-1,2,4-oxadiazole (3 a) and 5,5'-bis(3,4-dinitro-1H-pyrazol-5-yl)-3,3'-bi(1,2,4-oxadiazole) (3 b) possess high thermal stability (3 a: T_dec =274 °C; 3 b: T_dec =272 °C), sensitivity (IS >30 J, FS >360 N) and comparable detonation properties (3 a: D_v =8741 m s^-1 , P=34.0 GPa; 3 b: D_v =8685 m s^-1 , P=33.4 Gpa) to RDX. In addition, 3,5-bis(4-nitro-1H-pyrazol-3-yl)-1,2,4-oxadiazole (4 a) and 5,5'-bis(4-nitro-1H-pyrazol-3-yl)-3,3'-bi(1,2,4-oxadiazole) (4 b) have high decomposition temperature (4 a: T_dec =314 °C; 4 b: T_dec =317 °C), low sensitivity (IS >40 J; FS>360 N) and superior detonation performances (4 a: D_v =8027 m s^-1 , P=26.4 GPa; 4 b: D_v =7991 m s^-1 , P=25.2 Gpa) than conventional heat-resistant explosive hexanitrostilbene (HNS: T_dec =318 °C; IS=5 J; FS=240 N; D_v =7612 m s^-1 , P=24.3 GPa), thus suggesting their potential application as heat-resistant explosives.

Aminonitrones as highly reactive bifunctional synthons. An expedient one-pot route to 5-amino-1,2,4-triazoles and 5-amino-1,2,4-oxadiazoles – potential antimicrobials targeting multi-drug resistant bacteria

A four-component one-pot reaction proceeds very rapidly under mild conditions and gives the heterocyclic systems in good yields.

Polycyclic N-oxides: high performing, low sensitivity energetic materials

Polycyclic N-oxides were developed based on the heterocycles 1,2,4,5-tetrazine and 4H,8H-difurazano[3,4-b:3′,4′-e]pyrazine.

1,2,4-Oxadiazole-derived polynitro energetic compounds with sensitivity reduced by a methylene bridge

This is a new family of energetic compounds combining a high-energy polynitro group with an insensitive methylene linkage and thermostable 1,2,4-oxadiazole skeleton.

Mild Synthesis of Substituted 1,2,5-Oxadiazoles Using 1,1'-Carbonyldiimidazole as a Dehydrating Agent

1,1'-Carbonyldiimidazole was found to induce the formation of a variety of 3,4-disubstituted 1,2,5-oxadiazoles (furazans) from the corresponding bisoximes at ambient temperature. This method enables these inherently energetic compounds to be prepared at temperatures well below their decomposition points and with improved functional group compatibility relative to prior methods. Conditions were developed that allowed for the first high-yielding synthesis of chlorofurazans from their amino counterparts, enabling the mild synthetic manipulation of these heterocycles.

Pyrazole-Tetrazole Hybrid with Trinitromethyl, Fluorodinitromethyl, or (Difluoroamino)dinitromethyl Groups: High-Performance Energetic Materials

High-nitrogen-content compounds have attracted great scientific interest and technological importance because of their unique energy content, and they find diverse applications in many fields of science and technology. Understanding of structure-property relationship trends and how to modify them is of paramount importance for their further improvement. Herein, the installation of oxygen-rich modules, C(NO₂ )₃ , C(NO₂ )₂ F, or C(NO₂ )₂ NF₂ , into an endothermic framework, that is, the combination of a nitropyrazole unit and tetrazole ring, is used as a way to design novel energetic compounds. Density, oxygen balance, and enthalpy of formation are enhanced by the presence of these oxygen-containing units. The structures of all compounds were confirmed by XRD. For crystal packing analysis, it is proposed to use new criterion, Δ_OED , that can serve as a measure of the tightness of molecular packing upon crystal formation. Overall, the materials show promising detonation and propulsion parameters.

Effect of counter-ion on packing and crystal density of 5,5'-(3,3'-bi[1,2,4-oxa-diazole]-5,5'-di-yl)bis-(1H-tetra-zol-1-olate) with five different cations

In energetic materials, the crystal density is an important parameter that affects the performance of the material. When making ionic energetic materials, the choice of counter-ion can have detrimental or beneficial effects on the packing, and therefore the density, of the resulting energetic crystal. Presented herein are a series of five ionic energetic crystals, all containing the dianion 5,5'-(3,3'-bi[1,2,4-oxa-diazole]-5,5'-di-yl)bis-(1H-tetra-zol-1-olate), with the following cations: hydrazinium (1) (2N2H5+·C6N12O42-), hydroxyl-ammonium (2) 2NH4O+·C6N12O42- [Pagoria et al.. (2017). Chem. Heterocycl. Compd, 53, 760-778; included for comparison], di-methyl-ammonium (3) (2C2H8N+·C6N12O42-), 5-amino-1H-tetra-zol-4-ium (4) (2CH4N5+·C6N12O42-·4H2O), and amino-guanidinium (5) (2CH7N4+·C6N12O42-). Both the supra-molecular inter-actions and the sterics of the cation play a role in the density of the resulting crystals, which range from 1.544 to 1.873 Mg m-1. In 5, the tetra-zolate ring is disordered over two positions [occupancy ratio 0.907 (5):0.093 (5)] due to a 180° rotation in the terminal tetra-zole rings.

Nitramino-functionalized tetracyclic oxadiazoles as energetic materials with high performance and high stability: crystal structures and energetic properties

The combination of 1,2,4-oxadiazole and nitramino-1,2,5-oxadiazole is a promising strategy for designing energetic materials with high performance and high stability.