Journal of Advances in Science and Technology (ISSN No. 0971-9563)

Journal of Advances in Science and Technology (ISSN No. 0971-9563)

Vol 14 (1), June 2014,

1. Introduction

Oneofthemostinteresting energetic molecules developed inrecent yearsisthe polycyclicnitramine2,4,6,8,10,12-hexa- nitro-2,4,6,8,10,12-hexaazaisowurtzitane(HNIW) orCL-20. In view ofitssuperiorperformance,HNIWcanbe regardedas anextgenerationhigh-energymaterial.Itshighdensity and heat offormationexceed thecorresponding values of cyclo-tetramethylenetetranitramine (HMX, high meltingexplosive). ItcanbeusedassuitablecandidateforreplacingHMX1,2. Thesynthesisand physicalcharacteristicsofCL-20have alreadybeen reported[3–6].Thefirststepinthe synthesisof HNIW(CL-20)involvescreationofthe basic cagestructure throughcondensationofglyoxalwithbenzylamine,leading tothe formation ofhexabenzyl hexaazaisowurtzitane(HBIW). ConversionofHBIWtoCL-20posesamajorchallenge.Sever- alattemptsledtothe emergenceofalternateroutes ofreductive debenzylation2b,3. Reductive debenzylationof

HBIWbyhydrogenationinthe presenceof

excessofacetic anhydrideresultsinformation of4,10-dibenzyl-2,6,8,12-tetra-acetyl-2,4,6,8,10,12-hexaazaisowurtzitane (TADBIW).Thegeneralmethodforthe synthesis ofCL-20isbased onTADBIW. Theremaining benzylgroups canberemoved either byformylation,which yields 2,6,8,12-tetraacetyl-4,10-diformyl-2,4,6,8,10,12-hexaazaisowurtzitane (TADFIW) or by nitrosation, leading to 2,6,8,12-tetraacetyl-4,10-dinitroso-2,4,6,8,10,12-hexaazaisowurtzitane(TADNIW). Both TADFIWand TADNIW can be easilyconvertedto CL-20bynitration withvariousreagents3–6.However,most ofthe syntheses reportedhave one ormore ofthe followingdrawbacks: (i) low yields, (ii) long reaction times, (iii) the use of large amountof reagent, (iv)the use of toxic or expensive reagents, (v)tedious work-up,(vi)harshreactioncondition,(vii) requirementofcertainreaction conditions. Thus,anefficient,inexpensive, simple,non-pollutingand one step methodfor thesynthesisofCL-20isyettobefound.

Recently,application ofeco-friendly applicable industrialsystem hasreceivedconsiderableinterest. Alongthis line, the use ofsolidacidssuch ashydrogensulfate salts[7,8], whichare green,inexpensive,andnon-toxichasreceived increased attention forvarioustypesoforganictransformations due totheirsimplicity,efficiencyand selectivitycombined withtheirgreen characteristics. Considering the high importance ofsolidacidsinorganicsynthesis,melaminium- tris(hydrogen sulfate) (MHS)was synthesized by the reactionofmelamine withsulfuricacidatroom temperature in a straightforwardprocedure. Itisanticipatedthat MHS is abletoactasefficientreagentinreactions that need acidicreagentstospeed up.

Ourinterest inthe study ofthe explosive and thermal characteristics of CL-20[9–15] prompted us to develop asimple,andefficientprocedureforthedirectsynthesisvia the one-stepreaction of 2,6,8,12-tetraacetyl-4,10-dibenzyl-2,4,6,8,10,12-hexaazaisowurtzitane (TADBIW) and HNO3 in the presenceofMHSasan efficient,green, and cheap re- agent (Scheme1).

Scheme1. ConversionofTADBIWtoCL-20usingMHS/HNO3.

2. Results and Discussion

2.1 EffectofVariousNitrating systems

Firstly,the reaction ofTADBIWwithinthe presenceofdif- ferent acidic reagentssuch as NaHSO4·H2O, tricholoroiso- cyanuric acid (TCCA), P2O5-SiO2,LaCl3,silicasulfuric acid, and various nitrating agents such asNaNO3 orHNO3was tested(Table1).Ascanbe seen fromTable 1,higher yields and shorter reaction times were obtainedwhen melamini- um-tris-(hydrogensulfate) was used as reagent (Table1, entry12).Theresultsalsoshowed that NaNO3isnot aneffectivenitrating agent (Table1,entry11).

Table3. EffectofvarioussolventsforsynthesisofCL-20.

Entry Solvent Time[h] Yield[%]

1 DMSO 5 –a)

2 DMF 5 –a)

3 EtOAc 5 sluggish

4 CHCl3 5 sluggish

5 CH3CN 5 sluggish

6 THF 5 sluggish

7 H2O 5 sluggish

8 free 2 80

a)Mixtureofproducts.

2.2 EffectofMHS,HNO3,Solvent, and Temperature

Inthe nextstep, the reaction ofTADBIWwithMHSwasexamined using variousamountsofHNO3(Table 2).Asobviousfrom Table2, the optimal amount ofMHSwas 0.75mmol.Further increasing the amountofMHSdidnot improve the results.Tostudythe effectofnitricacidonreaction time and productyield,experiments with various amountsof nitric acid were carried out. Ascanbe seen fromTable2,longer reaction times and loweryieldsofthe reaction were obtainedwhenthe amountofacidwasreduced tolessthan 2mL(Table2). Otherwise,increasingthe HNO3 amountfrom 2mL to 2.5mLdid not significantly change theyield.

3. ExperimentalSection

HNO3,melamine, and NaNO3 were obtainedfrom Aldrich, Merck,Alfa-Aesarandusedwithout furtherpurification.

3.1 Synthesis ofMelaminium-tris-(hydrogensulfate)(MHS)

A50mL flaskcharged with sulfuricacid (2.5g,98%)was put into an ice-bath, and melamine (1.1g,8.3mmol) was added inportions over a period of10min. Theresulting mixture was stirred for 30min. Afterwards CHCl3(15mL) wasadded toit,and stirred for10min.Theresulting solid wasfiltered,washed withCHCl3,and dried togiveMHSas awhitesolid.

3.2 SynthesisofCL-20fromTADBIWusing MHS/HNO3

A mixture of TADBIW (1mmol, 0.516g), fuming HNO3 (2mL)and MHS(1.7mmol, 0.75g) werestirred at 80oC. Aftercompletionofthe reaction, as monitoredwith TLC, the reaction mixture was diluted with CHCl3 and filtered. The filtrate was washed with water and concentrated undervacuum.Thecrudeproduct wasadsorbed ontosilica and purifiedbyshort columnchromatographyonsilica gel with EtOAc/n-hexane (1:4). The product was isolated in80%yield(0.35g).1HNMR(300MHz,[D6]acetone):δ=8.05 (s,2H),8.19 (s,4H).13CNMR(75MHz,[D6]acetone):

δ=71.6,75.1 ppm.

4. Conclusion

Ahighlyefficientand simplemethodforthepreparationof CL-20wasdeveloped.Thesynthesis isperformedinthe one-potreactionofTADBIWwithHNO3inthepresenceofmelaminium-tris-(hydrogensulfate)(MHS)assolidacid.Thismethod has many advantages, such as efficiency, simple work-up, shortreaction times, relativelymildreaction conditions, low cost,andcleanproductionoftheproductsinhighyields.

References

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