Synthesis of 2,4,6,8,10,12Hexanitro2,4,6,8,10 ... - Wiley Online Library

Short Communication DOI: 10.1002/prep.201300034

Synthesis of 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12hexaazaisowurtzitane Using Melaminium-tris(hydrogensulfate) by a Simple One-Pot Nitration Procedure Yadollah Bayat,[a] Mohammad A. Zolfigol,[b] Ardeshir Khazaei,[b] Mohamad Mokhlesi,*[b] Masoume Daraei,[b] Amin Heydari Nezhad Tehrani,[a] and Golamabbas Chehardoli[c]

Abstract: A highly efficient protocol for the synthesis of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane

(CL-20) in the presence of melaminium-tris(hydrogen sulfate) as a solid acid is described.

Keywords: Melaminium-tris(hydrogen sulfate) · CL-20 · One-pot procedure

1 Introduction One of the most interesting energetic molecules developed in recent years is the polycyclic nitramine 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (HNIW) or CL-20. In view of its superior performance, HNIW can be regarded as a next generation high-energy material. Its high density and heat of formation exceed the corresponding values of cyclotetramethylenetetranitramine (HMX, high melting explosive). It can be used as suitable candidate for replacing HMX [1, 2]. The synthesis and physical characteristics of CL-20 have already been reported [3–6]. The first step in the synthesis of HNIW (CL-20) involves creation of the basic cage structure through condensation of glyoxal with benzylamine, leading to the formation of hexabenzylhexaazaisowurtzitane (HBIW). Conversion of HBIW to CL-20 poses a major challenge. Several attempts led to the emergence of alternate routes of reductive debenzylation [2b, 3]. Reductive debenzylation of HBIW by hydrogenation in the presence of excess of acetic anhydride results in formation of 4,10-dibenzyl-2,6,8,12-tetraacetyl-2,4,6,8,10,12-hexaazaisowurtzitane (TADBIW). The general method for the synthesis of CL-20 is based on TADBIW. The remaining benzyl groups can be removed either by formylation, which yields 2,6,8,12-tetraacetyl-4,10-diformyl2,4,6,8,10,12-hexaazaisowurtzitane (TADFIW) or by nitrosation, leading to 2,6,8,12-tetraacetyl-4,10-dinitroso2,4,6,8,10,12-hexaazaisowurtzitane (TADNIW). Both TADFIW and TADNIW can be easily converted to CL-20 by nitration with various reagents [3–6]. However, most of the syntheses reported have one or more of the following drawbacks: (i) low yields, (ii) long reaction times, (iii) the use of large amount of reagent, (iv) the use of toxic or expensive reagents, (v) tedious work-up, (vi) harsh reaction condition, (vii) requirement of certain reaction conditions. Thus, an efficient,

Propellants Explos. Pyrotech. 2013, 38, 745 – 747

inexpensive, simple, non-polluting and one step method for the synthesis of CL-20 is yet to be found. Recently, application of eco-friendly applicable industrial system has received considerable interest. Along this line, the use of solid acids such as hydrogen sulfate salts [7, 8], which are green, inexpensive, and non-toxic has received increased attention for various types of organic transformations due to their simplicity, efficiency and selectivity combined with their green characteristics. Considering the high importance of solid acids in organic synthesis, melaminiumtris(hydrogen sulfate) (MHS) was synthesized by the reaction of melamine with sulfuric acid at room temperature in a straightforward procedure. It is anticipated that MHS is able to act as efficient reagent in reactions that need acidic reagents to speed up. Our interest in the study of the explosive and thermal characteristics of CL-20 [9–15] prompted us to develop a simple, and efficient procedure for the direct synthesis via the one-step reaction of 2,6,8,12-tetraacetyl-4,10-dibenzyl2,4,6,8,10,12-hexaazaisowurtzitane (TADBIW) and HNO3 in the presence of MHS as an efficient, green, and cheap reagent (Scheme 1). [a] Y. Bayat, A. Heydari Nezhad Tehrani Faculty of Material and Manufacturing, Technologies Malek Ashtar University of Technology Tehran, Iran [b] M. A. Zolfigol, A. Khazaei, M. Mokhlesi, M. Daraei Faculty of Chemistry Bu-Ali Sina University Hamedan, 6517838683, Iran *e-mail: [email protected] [c] G. a. Chehardoli School of Pharmacy Hamedan University of Medical Sciences 65178 Hamedan, Iran

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Short Communication

Y. Bayat et al. Table 2. Optimization of amount of MHS and HNO3 for the synthesis of CL-20a).

Scheme 1. Conversion of TADBIW to CL-20 using MHS/HNO3.

2 Results and Discussion

Firstly, the reaction of TADBIW with in the presence of different acidic reagents such as NaHSO4·H2O, tricholoroisocyanuric acid (TCCA), P2O5-SiO2, LaCl3, silica sulfuric acid, and various nitrating agents such as NaNO3 or HNO3 was tested (Table 1). As can be seen from Table 1, higher yields and shorter reaction times were obtained when melaminium-tris-(hydrogensulfate) was used as reagent (Table 1, entry 12). The results also showed that NaNO3 is not an effective nitrating agent (Table 1, entry 11). 2.2 Effect of MHS, HNO3, Solvent, and Temperature

In the next step, the reaction of TADBIW with MHS was examined using various amounts of HNO3 (Table 2). As obvious from Table 2, the optimal amount of MHS was 0.75 mmol. Further increasing the amount of MHS did not improve the results. To study the effect of nitric acid on reaction time and product yield, experiments with various amounts of nitric acid were carried out. As can be seen from Table 2, longer reaction times and lower yields of the reaction were obtained when the amount of acid was reduced to less than 2 mL (Table 2). Otherwise, increasing the HNO3 amount from 2 mL to 2.5 mL did not significantly change the yield.

Table 1. Synthesis of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane using different systemsa). Entry

Nitrating system

Yieldb) [%]

1 2 3 4 5 6 7 8 9 10 11 12

NaHSO4·H2O/NaNO3 NaHSO4·H2O/HNO3 TCCA/NaNO3 TCCA/HNO3 P2O5-SiO2/NaNO3 P2O5-SiO2/HNO3 LaCl3/NaNO3 LaCl3/HNO3 SSAc)/NaNO3 SSAc)/HNO3 MHS/NaNO3 MHS/HNO3

trace 20 N. R.d) N. R.d) N. R.d) < 10 N. R.d) N. R.d) N. R.d) N. R.d) N. R.d) 80

a) Reaction conditions: TADBIW (1 mmol), acidic agent, nitrating agent, 808, solvent free, stirring. b) Isolated yields. c) Silica sulfuric acid. d) No reaction.

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MHS [g]

HNO3 [mL]

Yieldb) [%]

1 2 3 4 5 6 7

0.75 0.75 0.75 0.75 0.5 1 1.5

1 1.5 2 2.5 2 2 2

25 52 80 81 33c) 80 82

a) Reaction conditions: TADBIW (1 mmol, 0.516 g), Temperature: 808, solvent free, time: 2 h. b) Isolated yields. c) Time: 4 h.

2.1 Effect of Various Nitrating Systems

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Entry

Table 3. Effect of various solvents for synthesis of CL-20. Entry

Solvent

Time [h]

Yield [%]

1 2 3 4 5 6 7 8

DMSO DMF EtOAc CHCl3 CH3CN THF H2O free

5 5 5 5 5 5 5 2

–a) –a) sluggish sluggish sluggish sluggish sluggish 80

a) Mixture of products.

Table 4. Effect of temperature for synthesis of CL-20. Entry

Temperature [8C]

Time/h

Yielda) [%]

1 2 3 4

25 50 80 100

5 5 2 5

N. R.b) 30 80 81

a) Isolated yields. b) No reaction.

The solvent has a significant effect on the reaction. Thus, it was investigated with similar concentrations of the reactants in DMSO, DMF, CH3CN, CHCl3, EtOAc, THF, and H2O. In the presence of DMSO and DMF, respectively, a mixture of the products was obtained and in the presence of CH3CN, CHCl3, EtOAc, THF, and H2O the reactions were sluggish. Thus, the reaction was carried out under solvent free conditions. The results listed in Table 3 suggested that in the absence of solvent, higher yields and shorter reaction times for the synthesis of CL-20 were obtained. Investigation of the influence of temperature showed that the optimum reaction temperature was 80 8C. Further increasing the reaction temperature did not improve the results (Table 4).

3 Experimental Section HNO3, melamine, and NaNO3 were obtained from Aldrich, Merck, Alfa-Aesar and used without further purification.



2,4,6,8,10,12-Hexanitro2,4,6,8,10,12-hexaazaisowurtzitane in a One-pot Nitration Procedure

3.1 Synthesis of Melaminium-tris-(hydrogensulfate) (MHS)

A 50 mL flask charged with sulfuric acid (2.5 g, 98 %) was put into an ice-bath, and melamine (1.1 g, 8.3 mmol) was added in portions over a period of 10 min. The resulting mixture was stirred for 30 min. Afterwards CHCl3 (15 mL) was added to it, and stirred for 10 min. The resulting solid was filtered, washed with CHCl3, and dried to give MHS as a white solid.

[4] [5]

[6]

3.2 Synthesis of CL-20 from TADBIW using MHS/HNO3

A mixture of TADBIW (1 mmol, 0.516 g), fuming HNO3 (2 mL) and MHS (1.7 mmol, 0.75 g) were stirred at 80 8C. After completion of the reaction, as monitored with TLC, the reaction mixture was diluted with CHCl3 and filtered. The filtrate was washed with water and concentrated under vacuum. The crude product was adsorbed onto silica and purified by short column chromatography on silica gel with EtOAc/n-hexane (1/4). The product was isolated in 80 % yield (0.35 g). 1H NMR (300 MHz, [D6]acetone): d = 8.05 (s, 2 H), 8.19 (s, 4 H). 13C NMR (75 MHz, [D6]aceton): d = 71.6, 75.1 ppm.

4 Conclusion A highly efficient and simple method for the preparation of CL-20 was developed. The synthesis is performed in the onepot reaction of TADBIW with HNO3 in the presence of melaminium-tris-(hydrogensulfate) (MHS) as solid acid. This method has many advantages, such as efficiency, simple work-up, short reaction times, relatively mild reaction conditions, low cost, and clean production of the products in high yields.

[7]

[8]

[9]

[10]

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Received: April 2, 2013 Revised: June 29, 2013 Published online: December 2, 2013

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