Combination of 1,2,4-Oxadiazole and 1,2,5-Oxadiazole Moieties for the Generation of High-Performance Energetic Materials

A comprehensive review of recent advances in the biological activities of 1,2,4-oxadiazoles

Nitrogen heterocycles play an essential role in medication development. The 1,2,4-oxadiazole heterocycle has been extensively studied, yielding a large variety of molecules with varied biological functions. The 1,2,4-oxadiazole shows bioisosteric equivalency with ester and amide moieties. In recent years, the 1,2,4-oxadiazole nucleus has received a lot of attention in medicinal chemistry. It was thought to be a pharmacophore component in the production of biologically intriguing drugs. This review presents a comprehensive overview of the recent achievements in the biological activities of 1,2,4-oxadiazoles as potential antimicrobial, anticancer, anti-inflammatory, neuroprotective, and antidiabetic agents. The structure-activity relationship and mechanisms of action are also reviewed.

Construction of Coplanar Bicyclic Backbones for 1,2,4-Triazole-1,2,4-Oxadiazole-Derived Energetic Materials

Combining different nitrogen-rich heterocycles into a molecule can fine-tune its energetic performance and physical properties as well as its safety for use in energetic materials. Here, 1,2,4-oxadiazole was incorporated into 1,2,4-triazole to construct new energetic backbones. 3-(5-Amino-1H-1,2,4-triazol-3-yl)-1,2,4-oxadiazol-5-amine (5) was designed and synthesized. Nitramino-functionalized N-(5-(5-amino-1,2,4-oxadiazol-3-yl)-3H-1,2,4-triazol-3-yl)nitramide (6) and N-(5-(5-(nitramino)-1,2,4-oxadiazol-3-yl)-3H-1,2,4-triazol-3-yl)nitramide (7) were also obtained, and two series of corresponding nitrogen-rich salts were prepared, leading to the creation of new energetic compounds. All derivatives were fully characterized, and five of them were further confirmed by X-ray diffraction. The theoretical calculations, energetic performance, safety, and the main decomposition gaseous products of 1,2,4-triazole-1,2,4-oxadiazole-derived energetic materials were studied. Compound 7 and its dihydroxylammonium salt (7 c) exhibited prominent detonation performance comparable to that of RDX while possessing satisfying thermal stabilities and mechanical sensitivities.

Novel 1,2,4-Oxadiazole Derivatives in Drug Discovery

Five-membered 1,2,4-oxadiazole heterocyclic ring has received considerable attentionbecause of its unique bioisosteric properties and an unusually wide spectrum of biological activities.Thus, it is a perfect framework for the novel drug development. After a century since the1,2,4-oxadiazole have been discovered, the uncommon potential attracted medicinal chemists'attention, leading to the discovery of a few presently accessible drugs containing 1,2,4-oxadiazoleunit. It is worth noting that the interest in a 1,2,4-oxadiazoles' biological application has been doubledin the last fifteen years. Herein, after a concise historical introduction, we present a comprehensiveoverview of the recent achievements in the synthesis of 1,2,4-oxadiazole-based compounds and themajor advances in their biological applications in the period of the last five years as well as briefremarks on prospects for further development.

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.

Design of Energetic Materials Based on Asymmetric Oxadiazole

A new family of asymmetric oxadiazole based energetic compounds were designed. Their electronic structures, heats of formation, detonation properties and stabilities were investigated by density functional theory. The results show that all the designed compounds have high positive heats of formation ranging from 115.4 to 2122.2 kJ mol-1. -N- bridge/-N3 groups played an important role in improving heats of formation while -O- bridge/-NF2 group made more contributions to the densities of the designed compounds. Detonation properties show that some compounds have equal or higher detonation velocities than RDX, while some other have higher detonation pressures than RDX. All the designed compounds have better impact sensitivities than those of RDX and HMX and meet the criterion of thermal stability. Finally, some of the compounds were screened as the candidates of high energy density compounds with superior detonation properties and stabilities to that of HMX and their electronic properties were investigated.

Isomers of Dinitropyrazoles: Synthesis, Comparison and Tuning of their Physicochemical Properties

Three isomeric dinitropyrazoles (DNPs) were synthesized starting from readily available 1H-pyrazole by slightly improved methods than described in the literature. 3,4-Dinitropyrazole (3), 1,3-dinitropyrazole (4), and 3,5-dinitropyrazole (5) were obtained and compared to each other with respect to thermal stability, crystallography, sensitivity and energetic performance. Two isomers (3 and 4) show high densities (1.79 and 1.76 g cm^-3 ) and interesting thermal behavior as melt-castable materials (3: T_melt. =71 °C, T_dec. =285 °C; 5: T_melt. = 68 °C, T_dec. =171 °C). Furthermore, eight salts (sodium, potassium, ammonium, hydrazinium, hydroxylammonium, guanidinium, aminoguanidinium and 3,6,7-triamino-[1,2,4]triazolo[4,3-b][1,2,4]triazole (TATOT) of 3 and 5 were synthesized in order to tune performance and sensitivity values. These compounds were characterized using ¹ H, ¹³ C, ¹⁴ N, ¹⁵ N NMR and IR spectroscopy as well as mass spectrometry, elemental analysis and thermal analysis through differential scanning calorimetry. Crystal structures of 14 compounds were obtained (3-7, 10-12 and 15-20) by low-temperature single crystal X-ray diffraction. Impact, friction and electrostatic discharge (ESD) values were also determined by standard methods. The sensitivity values range between 8.5 and 40 J for impact and 240 N and 360 N for friction and show mainly insensitive character. The energetic performances were determined using recalculated X-ray densities, heats of formation and the EXPLO5 code and support the energetic character of the title compounds. The calculated energetic performances (V_D : 6245-8610 m s^-1 ; p_CJ : 14.1-30.8 GPa) were compared to RDX ((O₂ NNCH₂ )₃ ).

A novel 3D energetic MOF of high energy content: synthesis and superior explosive performance of a Pb(ii) compound with 5,5'-bistetrazole-1,1'-diolate

The development of high-performance insensitive energetic materials is important because of the increasing demand for these materials in military and civilian applications. A novel 3D energetic metal-organic framework (MOF) of exceptionally high energy content, [Pb(BTO)(H2O)]n, was synthesized and structurally characterized by single crystal X-ray diffraction, featuring a three-dimensional parallelogram porous framework, where BTO represents 5,5'-bistetrazole-1,1'-diolate. The thermal stability and energetic properties were determined, exhibiting good thermostability (Td = 309.0 °C), excellent detonation pressure (P) of 53.06 GPa, a detonation velocity (D) of 9.204 km s(-1), and acceptable sensitivity to confirmed impact (IS = 7.5 J). Notably, the complex possesses unprecedented superior density than the reported energetic MOFs. The results highlight this new MOF as a potential energetic material.