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Cite This: J. Org. Chem. 2018, 83, 10505−10509
Synthetic Routes to a Triazole and Tetrazole with Trinitroalkyl Substitution at Nitrogen Thomas M. Klapötke,* Burkhard Krumm,* Thomas Reith, and Cornelia C. Unger Department of Chemistry, Ludwig Maximilian University of Munich, Butenandtstr. 5−13 (D), 81377 Munich, Germany
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ABSTRACT: Two N-substituted trinitroalkyl azoles, one triazole, and one tetrazole were synthesized and isolated via efficient cyclization reactions. Both materials were thoroughly characterized, and their structures were confirmed by X-ray diffraction. The formation of the N-trinitroethyl substituted triazole proceeds unexpectedly via nitrosation of an N-substituted diaminomaleonitrile initially with HNO3 and subsequently confirmed with HNO2. The N-trinitropropyl substituted tetrazole was prepared via a standard cyclization route from trinitropropylammonium chloride with orthoformate and azide.
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or by nitration of dinitromethyl derivatives with nitronium tetrafluoroborate. The N-substitution with longer trinitroalkyl chains, such as trinitroethyl and trinitropropyl units, should provide a better thermal and mechanical stability,12 but none have been described to the best of our knowledge. The low reactivity of deactivated aromatic azoles seems not sufficient enough to react with nitroform or trinitroethanol.13−15 At the same time, trinitromethyl-containing starting materials tend to be rather chemically labile,16,17 particularly against bases and high temperatures, which prevent many ring-closing mechanisms of functional groups.18 In this contribution, pathways to two polyazoles with Nsubstituted trinitroethyl and trinitropropyl moieties are reported and their properties examined. In both cases, a trinitroalkyl-containing precursor was selected to further cyclize to a triazole and a tetrazole (Scheme 1).
he trinitromethyl group is an important building block in the research for new energetic materials with a good availability of the sources nitroform or trinitroethanol.1−3 In combination with already oxygen-rich energetic materials, many potent replacements for the common but harmful3 oxidizer ammonium perchlorate were investigated, such as trinitroethyl nitrocarbamate (A) and bis(trinitroethyl)oxalate (B).1,4 In the ongoing research to replace the secondary explosive RDX, the combination of the trinitromethyl moiety with different azoles results in interesting compounds (Figure 1) (C−F).5−8 In general, either trinitroethanol was reacted
Scheme 1. Synthesis of 4,5-Dicyano-1N-(trinitroethyl)1,2,3-triazole (3) and 1N-Trinitropropyl Tetrazole (5) from the Precursors Diaminomaleonitrile (1) or Trinitropropylammonium Chloride (4) Figure 1. Oxygen-rich materials: trinitroethyl nitrocarbamate (A), bis(trinitroethyl)oxalate (B), 2,4-dinitro-N-(2,2,2-trinitroethyl)-1Himidazol-1-amine (C), 5-nitro-3-trinitromethyl-1H-1,2,4-triazole (D), 3-nitro-1-trinitromethyl-1H-1,2,4-triazol-5-amine (E), and 1(2,2,2-trinitroethylamino)tetrazole (F).
with heterocyclic amines to incorporate the trinitromethyl unit8,9 or the exhaustive nitration of activated methylene groups forms the trinitromethyl moiety, e.g. in azolylacetic acids.6,7,10 However, only very sparse information exists about azoles with nitrogen-substituted trinitroalkyl units.7,10,11 In those few examples, rather sensitive N-trinitromethyl triazoles were obtained either by exhaustive nitration mentioned above © 2018 American Chemical Society
Received: April 27, 2018 Published: August 16, 2018 10505
DOI: 10.1021/acs.joc.8b01072 J. Org. Chem. 2018, 83, 10505−10509
Note
The Journal of Organic Chemistry Diaminomaleonitrile (1) was converted quantitatively into amino-(trinitroethyl)amino)maleonitrile (2) by reaction with formaldehyde and nitroform in aqueous solution. Without further purification, nitration was performed in white fuming nitric acid. Surprisingly, the originally intended nitration at the NH2 group of 2 was not observed, but a cyclized product, which was identified as 4,5-dicyano-1N-(trinitroethyl)-1,2,3triazole (3), was isolated. The substituted maleonitrile 2 turns very sensitive upon thorough drying and may deflagrate spontaneously without external stimulation, which occurred once in our laboratory during storage. Triazole 3 is still very sensitive toward impact (2 J) and moderately sensitive to friction (216 N) but is stable in air at room temperature for at least several months. Primary amines undergo heterocyclization with triethyl orthoformate and sodium azide in acetic acid. This facile synthetic route yields N-substituted tetrazoles in good yields.19 Our precursor, 3,3,3-trinitropropyl amine as the HCl salt (4),4 undergoes the cyclization reaction at 60 °C for 6 h. Simple workup procedures afforded 1N-trinitropropyl tetrazole (5) as an orange-colored solid. The substituted maleonitrile 2 starts to decompose quickly in several deuterated solvents. However, reasonable spectra were obtained from acetonitrile CD3CN. The 1H NMR spectrum shows three resonances at 5.1 ppm (br), 4.65 ppm (d), and 4.08 ppm (t). In the 13C{1H} NMR spectra the expected six resonances are observed. The 14N NMR spectrum shows the signal for the nitro groups at −31 ppm. NMR spectroscopy of the triazole 3 in deuterated acetone reveals a singlet at 7.02 ppm in the 1H NMR spectrum and six resonances in the range between 128.2 and 51.3 ppm in the 13 C{1H} NMR spectrum, including the typically broadened signal for the trinitromethyl carbon atom. The 14N NMR spectrum shows the resonance at −35 ppm for the nitro groups. Tetrazole 5 shows the acidic tetrazole hydrogen resonance at 9.41 ppm and the methylene resonances at 5.00 and 4.20 ppm in the 1H NMR spectrum. In the 13C{1H} NMR spectrum. the four carbon resonances are in the expected range between 144.4 ppm for the azole carbon atom and 32.3 ppm for the methylene group. In the 15N NMR spectrum, the nitrogen atoms of the tetrazole ring and the trinitro moiety are detected at 12.4 (N4), −14.6 (N2/N3), −29.9 (C(NO2)3), −52 (N3/ N4), and −147.5 ppm (N1). Suitable single-crystals for X-ray diffraction of triazole 3 were obtained from aqueous work up by evaporation at ambient temperature. The 4,5-dicyano-1N-(trinitroethyl)-1,2,3-triazole (3) crystallizes in the monoclinic space group P21/n with four molecules per unit cell and a density of 1.72 g cm−3 at 173 K (Figure 2). In the solid state, the triazole ring forms an almost planar system with the two nitrile substituents (torsion angles along the triazole ring less than 1°, N2−N3−C2−C3 179.5°).20 The trinitromethyl unit forms the typical propeller-type structure.21 The molecule contains no classical proton donor; therefore, strong hydrogen bonds are absent. Some weak intermolecular hydrogen bonds can be found between the methylene group as a proton donor and neighboring nitro oxygen atoms or nitrile nitrogen atoms as proton acceptors (C5−H2···O4, d(D−H) = 0.94 Å, d(H···A) = 2.49 Å,
