Tetrazine Explosives

The synthesis and properties of various 1,2,4,5‐tetrazine explosives and energetic materials are described. These are the nitrate and perchlorate salts of 3,6‐diguanidino‐1,2,4,5‐tetrazine, the nitrate and perchlorate salts of 3,6‐diguanidino‐1,2,4,5‐tetrazine‐1,4‐di‐N‐oxide, 3,6‐bis(1H‐1,2,3,4‐tetrazol‐5‐ylamino)‐1,2,4,5‐tetrazine and its 1,4‐di‐N‐oxide derivative, 3,3′‐azobis(6‐amino‐1,2,4,5‐tetrazine) and its oxidation products.     

Synthesis of high metal‐complexation linear elastomers containing sym‐1,2,4,5‐tetrazine rings

We report the synthesis of polyurethane‐urea elastomers containing 1,2,4,5‐tetrazine covalently reacted within the main chains of the polymers. Our study investigates the synthesis of 3,6‐diamino‐1,2,4,5‐tetrazine (DAT), the polymerization reaction conditions for reacting DAT into the backbone of segmented polyurethane elastomers, and the metal‐complexation capabilities of tetrazine‐containing elastomers with cobalt (II) chloride. Tetrazines are highly colored and electro‐active heterocyclic moieties, which have a very high electron affinity which make them reducible at high to very high potentials. Upon complexation with metals, we observed a strong color shift of the polymers from deep red to blue indicating the binding efficacy for the polymers. We quantified the metal‐complexation capability of the tetrazine elastomers and determined a molar ratio of approximately two metal atoms per tetrazine allowing us to provide a plausible complexation mechanism for the active polymers. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Superbase‐Catalyzed [4+2] Cycloaddition of Acetylenes to 3,6‐Di(pyrrol‐2‐yl)‐1,2,4,5‐tetrazine: A Facile Synthesis of 3,6‐Di(pyrrol‐2‐yl)pyridazines

Acetylenes undergo the [4+2] cycloaddition to 3,6‐di(pyrrol‐2‐yl)‐1,2,4,5‐tetrazine in the potassium hydroxide/dimethyl sulfoxide or potassium tert‐butoxide/dimethyl sulfoxide systems (80 °C, 2.5–4 h) to afford (after extrusion of the nitrogen molecule from the intermediate) 3,6‐di(pyrrol‐2‐yl)pyridazines in up to 73% yield, while under non‐catalytic conditions this reaction does not take place. This unusual result substantially extends the scope of synthetic application and mechanistic diversity of the Diels–Alder reaction. The step‐wise mechanisms involving the formation of [OH/tetrazine]⁻ or [t‐BuO/tetrazine]⁻ anionic intermediate complexes or cycloaddition of tetrazine to the acetylide anion are considered.     

Coordination‐Assisted Bioorthogonal Chemistry: Orthogonal Tetrazine Ligation with Vinylboronic Acid and a Strained Alkene

Bioorthogonal chemistry can be used for the selective modification of biomolecules without interfering with any other functionality that might be present. Recent developments in the field include orthogonal bioorthogonal reactions to modify multiple biomolecules simultaneously. During our research, we observed that the reaction rates for the bioorthogonal inverse‐electron‐demand Diels–Alder (iEDDA) reactions between nonstrained vinylboronic acids (VBAs) and dipyridyl‐s‐tetrazines were exceptionally higher than those between VBAs and tetrazines bearing a methyl or phenyl substituent. As VBAs are mild Lewis acids, we hypothesised that coordination of the pyridyl nitrogen atom to the boronic acid promoted tetrazine ligation. Herein, we explore the molecular basis and scope of VBA–tetrazine ligation in more detail and benefit from its unique reactivity in the simultaneous orthogonal tetrazine labelling of two proteins modified with VBA and norbornene, a widely used strained alkene. We further show that the two orthogonal iEDDA reactions can be performed in living cells by labelling the proteasome by using a nonselective probe equipped with a VBA and a subunit‐selective VBA bearing a norbornene moiety.     

Theoretical Investigation of Several 1,2,3,4‐Tetrazine‐Based High‐Energy Compounds

The enthalpies of formation of six 1,2,3,4‐tetrazine‐based compounds were calculated according to the Density Functional Theory BOP/TNP method and by using homodesmotic reaction designs. Their detonation performances, including detonation velocity and pressure, were predicted in terms of the Stine equations. The 1,2,3,4‐Tetrazine‐based compounds labeled A, B, C, D, and F are powerful high‐energy compounds. The detonation performances of A and B, including detonation velocity, and detonation pressure, are superior to that of the current high‐energy explosive CL‐20. The detonation velocity, detonation pressure, and oxygen balance of 1,2,3,4‐tetrazine related oxo derivatives can be improved by partial oxidation of the nitrogen atoms in the tetrazine ring, but further oxidation causes reduction of the enthalpies and specific impulses of the oxo derivatives. Calculation of the molecular resonance energies indicated that E [C₆N₁₂] and F have more negative values, i.e, the ring strain energies of their configurations are high, whereas the resonance energies of C and D are low, only compound B has a very positive resonance energy. Considering energy and stability, B is a promising compound for practical use with both high energy and low sensitivity.     

An Energetic N‐Oxide and N‐Amino Heterocycle and its Transformation to 1,2,3,4‐Tetrazine‐1‐oxide

This study reports the preparation of 1‐amino‐1,2,3‐triazole‐3‐oxide (DPX2) and its transformation to 1,2,3,4‐tetrazine‐1‐oxide. DPX‐2 provides insight into a novel N‐oxide/N‐amino high‐nitrogen system, being the first energetic material in this class. The ability of this material to undergo a nitrene insertion forming 1,2,3,4‐tetrazine‐1‐oxide was also studied, and evidence for this material, the first non‐benzoannulated 1,2,3,4‐tetrazine‐1‐oxide, is presented. The existence of both of these materials opens new strategies in energetic materials design. DPX2 was characterized chemically (Infrared, Raman, NMR, X‐ray) and as a high explosive in terms of energetic performances (detonation velocity, pressure, etc.) and sensitivities (impact, friction, electrostatic). DPX‐2 was found to possess good thermal stability and moderate sensitivities, indicating the viability of N‐amino N‐oxides as a strategy for the preparation of new energetic materials.     

Study on the Azido‐Tetrazolo Tautomerizations of 3,6‐Bis(azido)‐1,2,4,5‐tetrazine

The azido‐tetrazolo tautomerizations of 3,6‐diazido‐1,2,4,5‐tetrazine (DIAT) in different solvents were investigated with HPLC and ¹³C NMR spectroscopy. 6‐Amino‐tetrazolo[1,5‐b]‐1,2,4,5‐tetrazine (ATTZ) was irreversibly formed as the final product by azido‐cyclization following N₂ elimination from one of the azido substituents at room temperature in DMSO. The structure of ATTZ was characterized by X‐ray crystallography; differential scanning calorimetry (DSC), mass spectrometry, as well as IR and ¹H NMR and ¹³C NMR spectroscopy. The crystal density was found to be 1.272 g cm⁻³. DSC result suggested that ATTZ with the melting point of 84 °C strongly decomposes with explosion at 198 °C, which can be regarded as a primary explosive.     

Click‐Chemistry‐Mediated Rapid Microbubble Capture for Acute Thrombus Ultrasound Molecular Imaging

Bioorthogonal coupling chemistry has been studied as a potentially advantageous approach for molecular imaging because it offers rapid, efficient, and strong binding, which might also benefit stability, production, and chemical conjugation. The inverse‐electron‐demand Diels–Alder reaction between a 1,2,4,5‐tetrazine and trans‐cyclooctene (TCO) is an example of a highly selective and rapid bioorthogonal coupling reaction that has been used successfully to prepare targeted molecular imaging probes. Here we report a fast, reliable, and highly sensitive approach, based on a two‐step pretargeting bioorthogonal approach, to achieving activated‐platelet‐specific CD62p‐targeted thrombus ultrasound molecular imaging. Tetrazine‐modified microbubbles (tetra‐MBs) could be uniquely and rapidly captured by subsequent click chemistry of thrombus tagged with a trans‐cyclooctene‐pretreated CD62p antibody. Moreover, such tetra‐MBs showed great long‐term stability under physiological conditions, thus offering the ability to monitor thrombus changes in real time. We demonstrated for the first time that a bioorthogonal targeting molecular ultrasound imaging strategy based on tetra‐MBs could be a simple but powerful tool for rapid diagnosis of acute thrombosis.     

Biomedical applications of tetrazine cycloadditions

Disease mechanisms are increasingly being resolved at the molecular level. Biomedical success at this scale creates synthetic opportunities for combining specifically designed orthogonal reactions in applications such as imaging, diagnostics, and therapy. For practical reasons, it would be helpful if bioorthogonal coupling reactions proceeded with extremely rapid kinetics (k > 10(3) M(-1) s(-1)) and high specificity. Improving kinetics would minimize both the time and amount of labeling agent required to maintain high coupling yields. In this Account, we discuss our recent efforts to design extremely rapid bioorthogonal coupling reactions between tetrazines and strained alkenes. These selective reactions were first used to covalently couple conjugated tetrazine near-infrared-emitting fluorophores to dienophile-modifed extracellular proteins on living cancer cells. Confocal fluorescence microscopy demonstrated efficient and selective labeling, and control experiments showed minimal background fluorescence. Multistep techniques were optimized to work with nanomolar concentrations of labeling agent over a time scale of minutes: the result was successful real-time imaging of covalent modification. We subsequently discovered fluorogenic probes that increase in fluorescence intensity after the chemical reaction, leading to an improved signal-to-background ratio. Fluorogenic probes were used for intracellular imaging of dienophiles. We further developed strategies to react and image chemotherapeutics, such as trans-cyclooctene taxol analogues, inside living cells. Because the coupling partners are small molecules (<300 Da), they offer unique steric advantages in multistep amplification. We also describe recent success in using tetrazine reactions to label biomarkers on cells with magneto-fluorescent nanoparticles. Two-step protocols that use bioorthogonal chemistry can significantly amplify signals over both one-step labeling procedures as well as two-step procedures that use more sterically hindered biotin-avidin interactions. Nanoparticles can be detected with fluorescence or magnetic resonance techniques. These strategies are now being routinely used on clinical samples for biomarker profiling to predict malignancy and patient outcome. Finally, we discuss recent results with tetrazine reactions used for in vivo molecular imaging applications. Rapid tetrazine cycloadditions allow modular labeling of small molecules with the most commonly used positron emission tomography isotope, (18)F. Additionally, recent work has applied this reaction directly in vivo for the pretargeted imaging of solid tumors. Future work with tetrazine cycloadditions will undoubtedly lead to optimized protocols, improved probes, and additional biomedical applications.     

Tetrazine‐Responsive Self‐immolative Linkers

Molecules that undergo activation or modulation following the addition of benign external small‐molecule chemical stimuli have numerous applications. Here, we report the highly efficient “decaging” of a variety of moieties by activation of a “self‐immolative” linker, by application of water‐soluble and stable tetrazine, including the controlled delivery of doxorubicin in a cellular context.     

The Synthesis and Energetic Properties of 5,7‐Dinitrobenzo‐1,2,3,4‐tetrazine‐1,3‐dioxide (DNBTDO)

Energetic tetrazine‐1,3‐dioxide, 5,7‐dinitrobenzo‐1,2,3,4‐tetrazine‐1,3‐dioxide ( DNBTDO ), was synthesized in 45 % yield. DNBTDO was characterized as an energetic material in terms of performance (V_det 8411 m s⁻¹; p_{C J} 3.3×10¹⁰ Pa at a density of 1.868 g cm⁻³), mechanical sensitivity (impact and friction as a function of grain size), and thermal stability (T_dec 204 °C). DNBTDO exhibits a sensitivity slightly higher than that of RDX , and a performance slightly lower (96 % of RDX ).     

Synthesis and Characterization of 3,3′‐Azobis(6‐Amino‐1,2,4,5‐Tetrazine) DAAT – A New Promising Nitrogen‐Rich Compound

The synthesis of 3,3′‐azobis(6‐amino‐1,2,4,5‐tetrazine) DAAT is described, obtained with an overall yield of 20% after six reaction steps. DAAT is a new high‐nitrogen energetic material with remarkable thermal stability and insensitivity against friction and impact. DAAT decomposes at relatively high temperatures (>250 °C) releasing one of the highest heats of decomposition ever measured by DSC. The decomposition pathway and its products investigated by thermal analysis are described.     

Genetic Encoding of a Bicyclo[6.1.0]nonyne‐Charged Amino Acid Enables Fast Cellular Protein Imaging by Metal‐Free Ligation

Visualizing biomolecules by fluorescent tagging is a powerful method for studying their behaviour and function inside cells. We prepared and genetically encoded an unnatural amino acid (UAA) that features a bicyclononyne moiety. This UAA offered exceptional reactivity in strain‐promoted azide–alkyne cycloadditions. Kinetic measurements revealed that the UAA reacted also remarkably fast in the inverse‐electron‐demand Diels–Alder cycloaddition with tetrazine‐conjugated dyes. Genetic encoding of the new UAA inside mammalian cells and its subsequent selective labeling at low dye concentrations demonstrate the usefulness of the new amino acid for future imaging studies.     

Laser Ignition of DAAF,DHT and DAATO3.5

CO₂ laser ignition experimental results are reported for the high‐nitrogen materials 3,6‐dihydrazino‐1,2,4,5‐tetrazine (DHT), 3,3′‐diamino‐4,4′‐azoxyfurazan (DAAF), and mixed N‐oxides of 3,3′‐azo‐bis(6‐amino‐1,2,4,5‐tetrazine) (DAATO_3.5, where the “3.5” indicates the average oxide content) at a maximum irradiance level of approximately 140 W/cm². Diagnostics include a photodiode, indium antimonide (InSb) IR detector, high speed (HS) video and a CO₂ photodetector. “First light” is measured for DAATO_3.5 and DAAF, however, due to the low visible light emission of the gas phase, thermal runaway, as measured by the InSb, is used as the ignition criterion for DHT. Ignition in the gas phase is captured by the high speed camera. It is observed that an increase in laser irradiance results in an increase in ignition and flame stand‐off distance for DAATO_3.5. The high‐nitrogen material laser ignition results are compared to the common nitramine explosive, octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX). Laser ignition delays for the different high‐nitrogen materials are also compared in the context of Differential Scanning Calorimetry (DSC) data. It is determined that DSC onset temperature, while a rough indicator of ignition delay trends, is not the equivalent of a direct measure of ignition temperature.     

A One‐Pot Synthesis of [1,2,4]Triazino[4,3‐b]‐[1,2,4,5]tetrazines

Reactions of hydrazonoyl halides 6 with either 4‐amino‐2,3‐dihydro‐6‐substituted‐3‐thioxo‐[1,2,4]‐triazin‐5(4H)ones 1 ( 2 ) or 4‐amino‐3‐methylthio‐6‐substituted‐[1,2,4]‐triazin‐5(4H)ones 3 ( 4 ) gave [1,2,4]‐triazino‐[4,3‐b][1,2,4,5]tetrazine derivatives 9 ( 10 ), respectively. The mechanism of the reactions studied is discussed.     

Monodisperse‐porous N‐methyl‐D‐glucamine functionalized poly(vinylbenzyl chloride‐co‐divinylbenzene) beads as boron selective sorbent

To generate a new sorbent with high boron adsorption capacity, we synthesized monodisperse‐porous poly(vinylbenzyl chloride‐co‐divinylbenzene), poly(VBC‐co‐DVB), beads 8.5 μm in size by a new “modified seeded polymerization” technique. By using their chloromethyl functionality, the beads were derivatized by a simple, direct reaction with a boron‐selective ligand, N‐methyl‐D ‐glucamine (NMDG). The selection of poly(VBC‐co‐DVB) beads as a starting material allowed to obtain high boron sensitive‐ligand density on the beads depending on their high chloromethyl content. In the batch adsorption runs performed using NMDG‐attached poly(VBC‐co‐DVB) beads as sorbent, boron removal was efficiently performed in a wide pH range between 4 and 11. Quantitative boron removal was observed with the sorbent concentration of 4 g/L. In the same runs, plateau value of equilibrium adsorption isotherm was obtained as 14 mg boron/g beads. Relatively higher boron adsorption was explained by high ligand density and high specific surface area of the sorbent. Boron adsorption isotherms were analyzed using Langmuir and Freundlich models. In the kinetic runs performed for boron removal, the equilibrium was attained within 10 min at a value of 98%. The fast kinetic behavior was explained by the smaller particle size and enhanced porosity of the new sorbent. Infinite solution volume model and unreacted core model were used to evaluate boron adsorption onto the NMDG‐attached poly(VBC‐co‐DVB) beads. The results indicated that the adsorption process is controlled by the particle‐diffusion step. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Boron‐dependent microstructural evolution,thermal stability,and crystallization of mechanical alloying derived SiBCN

Amorphous boron‐rich SiBCN were prepared by high‐energy ball‐milling of the mixtures of Si, graphite, h‐BN, and inorganic boron, which acted as extra boron source. The solid‐state amorphization, thermal stability, and crystallization of the boron‐rich SiBCN were studied in detail. It was suggested that mechanical alloying can drive solid‐state amorphization but also can be an initiation step for the nucleation of nanocrystals. The amorphous networks of Si‐C, C‐B, C‐C, C‐N, B‐N, and C‐B‐N bonds are detected by XPS; however, solid‐state NMR further confirms the formation of a new chemical environment around B atoms, BC₃. The increases in boron content improve the thermal stability of SiBCN ceramics but weaken their oxidation resistance. Nano‐SiC crystallizes first while BN(C) forms subsequently. Boron promoting SiC crystallization may result from the reduced hindering effects of B‐N‐C nanodomains that retard SiC crystallization.     

Boron removal by liquid‐phase polymer‐based retention technique using poly(glycidyl methacrylate N‐methyl D‐glucamine)

The removal of boron was analyzed by liquid‐phase polymer based retention (LPR) technique using washing and enrichment method. The extracting reagents were water‐soluble polymers (WSPs) containing quaternary ammonium salts and N‐methyl‐D ‐glucamine (NMG) groups. The removal experiments of boron using the washing method were conducted at 1 bar of pressure by varying pH, polymer:boron molar ratio, and concentrations of interfering ions (chloride and sulfate). The results showed higher retention capacity for boron (60%) at pH 10 with the polymer containing NMG group. The optimal polymer:boron molar ratio was 40 : 1. Selectivity experiments showed that the presence of interfering ions did not affect the boron removal capacity. The maximal boron retention capacity was determined by the enrichment method, obtaining a value of 12 mg B/g‐polymer. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

New High‐Nitrogen Materials Based on Nitroguanyl‐Tetrazines: Explosive Properties,Thermal Decomposition and Combustion Studies

This paper describes the explosive sensitivity and performance properties of two novel high‐nitrogen materials, 3,6‐bis‐nitroguanyl‐1,2,4,5‐tetrazine ( 1 , (NQ₂Tz)) and its corresponding bis‐triaminoguanidinium salt ( 2 , (TAG)₂(NQ)₂Tz)). These materials exhibit very low pressure dependence in burning rate. Flash pyrolysis/FTIR spectroscopy was performed, and insight into this interesting burning behavior was obtained. Our studies indicate that 1 and 2 exhibit highly promising energetic materials properties.     

Hydrophilic trans‐Cyclooctenylated Noncanonical Amino Acids for Fast Intracellular Protein Labeling

Introduction of bioorthogonal functionalities (e.g., trans‐cyclooctene‐TCO) into a protein of interest by site‐specific genetic encoding of non‐canonical amino acids (ncAAs) creates uniquely targetable platforms for fluorescent labeling schemes in combination with tetrazine‐functionalized dyes. However, fluorescent labeling of an intracellular protein is usually compromised by high background, arising from the hydrophobicity of ncAAs; this is typically compensated for by hours‐long washout to remove excess ncAAs from the cellular interior. To overcome these problems, we designed, synthesized, and tested new, hydrophilic TCO‐ncAAs. One derivative, DOTCO‐lysine was genetically incorporated into proteins with good yield. The increased hydrophilicity shortened the excess ncAA washout time from hours to minutes, thus permitting rapid labeling and subsequent fluorescence microscopy.