A High-Energy Density Compound with High Stability

Letter Cite This: Org. Lett. 2018, 20, 7172−7176

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2,4,4,6,8,8-Hexanitro-2,6-diazaadamantane: A High-Energy Density Compound with High Stability Jian Zhang,‡ Tianjiao Hou,‡ Lin Zhang,* and Jun Luo* School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China

Org. Lett. 2018.20:7172-7176. Downloaded from pubs.acs.org by DURHAM UNIV on 11/16/18. For personal use only.

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ABSTRACT: A novel high-performance energetic compound of the polynitroazaadamantane family, 2,4,4,6,8,8-hexanitro-2,6-diazaadamantane, was designed and synthesized from 1,5-cyclooctadiene by two routes. Based on the experimental and calculated results, it exhibits a surprisingly high density (1.959 g cm−3), high thermal stability (onset decomposition temperature of 235 °C), high positive heat of formation, and excellent detonation properties. These fascinating properties, which are comparable to those of CL-20, show great promise for potential applications as a high-energy density material.

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Cage-like compounds such as cubane, isowurtzitane, and adamantane are very stable owing to their symmetrical and rigid skeleton. The introduction of explosive groups onto their frameworks can make them have the characteristics of high energy, heat resistance, and insensitivity at the same time, which might be one of the best candidates to solve the inherent contradiction. Particularly, adamantane is an ideal fundamental framework because of some attractive advantages: (1) excellent stability owing to the diamand-like structure; (2) high inherent density stemning from the ring tension and rigid structure; and (3) good designability due to ten functionable sites. As a fact, polynitroadamantanes have attracted widespread attention in the field of energetic materials since the first reported 1,3,5,7tetranitroadamantane,15 which exhibits a moderate power output but unbelievably high thermal stability (Tdec. > 360 °C) and excellent impact stability.16 Until now, several polynitroadamantanes such as 1,2,2-trinitroadamantane,17 2,2,4,4tetranitroadamantane, 18 2,2,6,6-tetranitroadamantane, 19 2,2,4,4,6,6-hexanitroadamantane (HNA),20 and 4,4,8,8-tetranitroadamantane-2,6-diyl dinitrate21 have been synthesized. Replacing the carbon atoms on the adamantane skeleton with one or more nitramino groups will give the nitramine derivatives higher density and energy while maintaining the symmetry and good thermal stability.22 However, there are only a few reports on the synthesis of polynitroazaadamantanes. 3,5,7-Trinitro-1-azaadamantane was first synthesized as a precursor of antitumor drugs by Theodor and co-workers in 1971.23 In the late 1990s, Nielsen and co-workers reported the synthesis of 2,4,10-trinitro-2,4,10-triazaadamantane,24 a template compound for the synthesis of 2,4,6,8,9,10-hexanitrohexaazaadamantane (HNHAA)25 that has not yet been synthesized. In 2017, our group reported the synthesis of two

ver the past decades, the development of new highenergy density materials (HEDMs) has attracted significant interest.1 The desirable characteristics of these materials include high density, high detonation velocity and pressure, good thermal stability, and low sensitivity toward impact and friction.2 However, the discovery of high-energy density compounds (HEDCs) is still an ongoing challenge due to the inherent contradiction between detonation performance and stability.3 In order to seek new HEDMs with an ideal balance between performance and molecular stability, extensive effort for synthetic and theoretical investigation has been devoted, and a large amount of HEDCs have been reported.4 For example, N,N′-bis(trinitroethyl)-3,5′-dinitramino-4-(1,2,4triazol-3-yl)furazan,5 N-(2,3,5,6-tetranitro-4H,9H-dipyrazolo[1,5-a:5′,1′-d][1,3,5]triazin-4-yl)nitramide,6 3-nitramino-4-(5nitramino-1,2,4-triazol-3-yl)furazan, 7 and 2,5-bis(trinitromethyl)-1,3,4-oxadiazole8 have been reported to be powerful HEDCs with crystal densities of 1.932, 1.937, 1.920, and 1.92 g cm−3, respectively. However, they still suffer from the problems of poor thermal stability and sensitivity. On the other hand, 5,5′-dinitro-3,3′-bis-1,2,4-triazole-2,2′-dinitromethyl (1.995 g cm −3 ) 9 and dipotassium N,N′-(3,6dinitropyrazolo[4,3-c]pyrazole-1,4-diyl)dinitramidate (2.11 g cm−3)10 are two examples of superhigh density HEDCs with good thermal stability; however, they are extremely sensitive to mechanical stimuli (shock and friction). In other cases, 5-(5amino-1H-1,2,4-triazole-3-yl)-1H-tetrazole,11 4-amino-3,5-bis(4-chloro-3,5-dinitro-pyrazol-1yl)methyl-1,2,4-triazole, 12 5,5′,6,6′-tetranitro-2,2′-bibenzimidazole,13 and bis(4-amino3,5-dinitropyrazolyl)methane14 have excellent thermal stability (Tdec. > 300 °C) but, unfortunately, exhibit poor density and detonation performance. Therefore, the research focus of HEDCs is still to seek an ideal balance between high performance and acceptable molecular stability. © 2018 American Chemical Society

Received: September 29, 2018 Published: November 5, 2018 7172

DOI: 10.1021/acs.orglett.8b03107 Org. Lett. 2018, 20, 7172−7176

Letter

Organic Letters

designed two routes for the preparation of the key intermediate 4. One route would be realized from dione 5 by hydrazonation and Bamford−Stevens elimination. Stoncius and co-workers reported a method to synthesize the 2,6-oxygenated 9azabicyclo[3.3.1]nonanes by diepoxide 6,28 which could be applied to synthesize dione 5. Finally, diepoxide 6 could be achieved from 1,5-cyclooctadiene (8) by the reported method. Alternatively, we presumed that the key precursor 4 could also be prepared from 1,2,5,6-tetrabromocyclooctane (7) by ammonolysis, elimination with liquid NH3, and nitrogen acetylation.29 Similarly, compound 7 could also be synthesized from 8 by the reported method. As outlined in Scheme 1, the synthesis commenced with 5,10-dioxatricyclo[7.1.0.04,6]decane (6), prepared from 1,5-

polynitroazaadamantanes, namely, 2,4,9-trinitro-2,4,9-triazaadamantane-7-yl nitrate26 and 2,4,4,8,8-petanitro-2-azaadamantane.27 Four existing polynitroadamantanes or polynitroazaadamantanes with good explosive characteristics are depicted in Figure 1. Based on the comparison of properties, we envisaged

Scheme 1. Synthesis of Diene 13 by Route 1

Figure 1. Chemical structures of several existing polynitroadamantanes and polynitroazaadamantanes.

cyclooctadiene (8) by epoxidation reaction with oxone and acetone in 79% yield.28 Compound 6 was then converted to 9azabicyclo[3.3.1]nonane-2,6-diol (9) through electrophilic addition with saturated solution of ammonia in methanol.30 Treatment of 9 with Ac2O provided acetamide 10 in 45% yield. The oxidation of the two hydroxyl groups was attempted. To our disappointment, various oxidation methods such as Swern oxidation, Jones oxidation, PCC oxidation, Colin oxidation, DMP oxidation, and NaIO4/RuO2 oxidation were attempted, but no target product 5 could be isolated. The low lipid solubility of the raw materials might be the possible reason.28,31 The acetyl group was therefore replaced with a more soluable tert-butyloxycarbonyl (Boc) group. The reaction of compound 9 with di-tert-butyl dicarbonate in methanol under reflux afforded compound 11 in 94% yield. Stoncius and co-workers synthesized the dione 12 by NaIO 4 /RuO 2 oxidation in 80% yield. Different from the report, dione 12 was synthesized by Swern oxidation in 90% yield. Diene 13 was obtained in 70% yield via condensation of dione 12 with ptoluenesulfonyl hydrazine followed by Bamford−Stevens elimination with n-BuLi. Alternatively, according to the work reported by Nelsen and co-workers,29 another route to synthesize diene 13 was attempted (Scheme 2). First of all, 1,2,5,6-tetrabromocyclooctane (7) was prepared in 60% yield from 1,5-cyclooctadiene (8) by electrophilic addition with bromine. Subsequently, it was converted to 9azabicyclo[3.3.1]nona-3,7-diene (14) by ammonolysis and elimination with excess NH3 in a steel hydrogenation bomb.

that the replacement of 6-carbon in 2,4,4,8,8-pentanitro-2azaadamantane with nitramino functionality to construct a more symmetrical structure with more azo atoms and nitro groups must give the analogue with observably higher detonation performance and better stability. In this paper, the synthesis and thermostability of the designed molecule 2,4,4,6,8,8-hexanitro-2,6-diazaadmanatane were investigated. The calculating detonation performance was compared to some typical energetic compounds with excellent performance. The retrosynthetic analysis for 2,4,4,6,8,8-hexanitro-2,6diazaadamantane (1) is summarized in Figure 2. The target

Figure 2. Retrosynthetic analysis of 1.

Scheme 2. Synthesis of Diene 13 by Route 2

product could be obtained by nitration of the acetamide and oxidative nitration of the ketoxime derived from the precursor 2. We envisioned that dione 2 might be obtained by nitrogen acetylation and hydroxyl oxidation from the corresponding diol 3. Compound 3 could be achieved from diene 4 by epoxidation and electrophilic addition with ammonia. We 7173

DOI: 10.1021/acs.orglett.8b03107 Org. Lett. 2018, 20, 7172−7176

Letter

Organic Letters After the protection of secondary amine with (Boc)2O, diene 13 was achieved smoothly in 19% yield for two steps. As a result, the two routes were both successful. The former route has the advantages of higher yield (39.7%) for over 6 steps and relatively convenient experimental operation. Although the total yield of the latter route was just 11.4% and autoclave condition was needed, it was realized through a much shorter route (only 3 steps), which is more suitable for rapid and scaled-up preparation of diene 13. With diene 13 in hand, the stage was set for the construction of the diazaadamantane skeleton (Scheme 3). Following a Scheme 3. Synthesis of 1

Figure 3. X-ray crystal structure of 1 (ellipsoid view).

similar synthetic strategy as shown in route 1, diepoxide 15 was obtained in 65% yield by epoxidation with mCPBA. After electrophilic addition with a saturated solution of ammonia in methanol, the skeleton of 2,6-diazaadamantane was successfully constructed. Taking the subsequent oxidation of the diol into account, the secondary amine was still protected with (Boc)2O to afford 16 in 86% yield for two steps. To our delight, 16 was then smoothly oxidated by Dess-Martin periodinane to produce the corresponding dione 17 in 85% yield. In addition, some other oxidation methods such as Swern oxidation, TEMPO/NaClO oxidation, and NaIO4/ RuO2 oxidation were applied, and the target product could also be isolated with much lower yields (20%∼40%). The direct nitrolysis of N-Boc was found to be a failure. Referring to our established synthetic strategy for 2,4,4,8,8-pentanitro-2-azaadamantane, the deprotection of N-Boc was furnished in TFA/ CH2Cl2 at room temperature. The derived secondary amine was treated with Ac2O to afford the acetamide 2. Subsequently, treatment of 2 with hydroxylamine hydrochloride in methanol gave the corresponding dioxime derivative. After oxidative nitration of dioxime with N2O5 in reflux CH2Cl2, the gemdinitro product 18 was obtained (18% over 4 steps). Finally, the nitrolysis of N-Ac in HNO3/oleum (20%) at 80 °C for 16 h delivered the target product 2,4,4,6,8,8-hexanitro-2,6diazaadamantane (1) in 82% yield. The crystal structure of compound 1 was depicted in Figure 3. The thermal stability of 1 was determined by differential scanning calorimetry (DSC) and thermogravimetry (TG). The onset decomposition temperature was found to be 235 °C, and the exothermic peak at 276 °C accounts for a rapid decomposition (Figure 4). In short, compound 1 possesses satisfactory thermal stability. The physicochemical properties of 1, including the thermal and detonation properties, were calculated by using the

Figure 4. TG-DSC curves of 1.

Gaussian 09 suite of programs.32 DFT with the B3LYP33 functional and 6-31G* basis set34 was used to optimize the structure. The detonation velocity, detonation pressure, and heat of formation were estimated by the Kamlet−Jacobs formula.35 By comparing physicochemical properties of 1 and 2,4,4,8,8-petanitro-2-azaadamantane (Table 1), we found that Table 1. Physical Properties of Compound 1 and Some Typical Energetic Compounds compound

da (g·cm−3)

ΔHfb (kJ·mol−1)

Dc (m·s−1)

Pd (GPa)

Tdece (°C)

1 RDX β-HMX ε-CL-20

1.959 1.80636 1.90437 2.03538

169.9 86.3 116.1 365.4

9310 8983 9221 9455

40.3 34.9 39.2 45.2

235 210 279 215

a

Density determined from single-crystal X-ray diffraction analysis. Heat of formation. cDetonation velocity. dDetonation pressure. e Decomposition temperature (onset). b

the symmetrical introduction of one nitroamino functionality to replace the 6-carbon can greatly improve the density, thermal stability, and detonation properties, which proved the validity of our original hypothesis. These fascinating properties, which are comparable to those of CL-20,39 show great promise for potential applications as a high-energy density material. In conclusion, a novel high-density energetic compound, 2,4,4,6,8,8-hexanitro-2,6-diazaadamantane, has been designed and successfully synthesized from 1,5-cyclooctadiene by two routes. Its crystal density was determined to be 1.959 g cm−3, 7174

DOI: 10.1021/acs.orglett.8b03107 Org. Lett. 2018, 20, 7172−7176

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which was much higher than known energetic compounds with adamantane and azaadamantane skeletons. In addition, it also exhibits high thermal stability (with an onset decomposition temperature of 235 °C). The calculated detonation velocity (9310 m s−1) and detonation pressure (40.3 GPa) suggest that it has the potential to be a high-performance energetic material. Moreover, the symmetrical cage-like compounds with a polynitro functionality may be ideal energetic materials that can solve the long-standing inherent contradiction between detonation performance and stability.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03107. Detailed experimental procedures and characterization data (PDF) Accession Codes

CCDC 1854396 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Lin Zhang: 0000-0002-4193-8877 Jun Luo: 0000-0002-8093-5345 Author Contributions ‡

Jian Zhang and Tianjiao Hou contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was sponsored by the Qing Lan Project of Jiangsu Province, P. R. China. The authors are grateful to Dr. Yue Zhao from Nanjing University for the single-crystal structure analysis.

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REFERENCES

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