Organic Chemistry of Explosives

Cubanes 73

respectively. Fractional recrystallization of this isomeric mixture from diethyl ether affords the

1,3,5,7-isomer (35) in 31 % yield.

Eaton and co-workers^10 ,^11 used cubane-1,3,5,7-tetracarboxylic acid chloride (35) as a pre-

cursor to 1,3,5,7-tetranitrocubane (39) (TNC). Treatment of cubane-1,3,5,7-tetracarboxylic

acid chloride (35) with trimethylsilyl azide gave the dangerously explosive tetraacylazide,

which generates the tetraisocyanate (38) on heating in chloroform at reflux. Isolation of the

tetraacylazide (38) from solution is a dangerous operation and limits the scale of this reaction.

Acylazide formation is generally much safer if conducted in dilute solution, although reac-

tions are then too slow to be useful. Eaton^10 found that acylazide isolation was not necessary if

azide formation was preformed with trimethylsilyl azide in the presence of a catalytic amount

of 2,6-di-tert-butylpyridine hydrochloride. Under these conditions Curtius rearrangement to

the tetraisocyanate (38) can be conductedin situ. Oxidation of the tetraisocyanate (38) with

dimethyldioxirane in wet acetone proceeds smoothly to give 1,3,5,7-tetranitrocubane (39) in

47 % yield overall yield from the acid chloride (35).^10 ,^11 1,3,5,7-Tetranitrocubane has a density

of 1.814 g/cm^3 , is highly energetic, and doesn’t melt until 270◦C.

NCO

OCN

NCO

OCN

NO 2

NO 2

O 2 N

O 2 N

NO 2

O 2 N NO 2

O 2 N

NO 2

O 2 N

O 2 N

NO 2

O 2 N

NO 2

O 2 N

O 2 N

O 2 N

NO 2

NO 2

COCl

ClOC

COCl

ClOC 1. TMSN 3 , CHCl 3

38 39

(CH 3 ) 2 CO, H 2 O

47 % from 35

4041

35

2. CHCl 3 , Heat
   
   
   1. LiN(TMS) 2 ,
      CH 2 Cl 2 , -78 C
   
   
   2. NOCl, -78 C
   
   
   3. O 3 , -78 C
      45—55 % (3 steps)
      
      
      1. 4 eq NaN(TMS) 2 ,
         THF, -78 C
      
      
      2. N 2 O 4 (excess),
         pentane, -125 C
      
      
      3. HNO 3 , Et 2 O, -30 C
         74 % (3 steps)
      
      
      
      
   
   
   
   

OO

Figure 2.12

Highly nitrated derivatives of cubane are prepared by treating the anion(s) of 1,3,5,7-

tetranitrocubane (39) with dinitrogen tetroxide in frozen THF.^10 ,^14 Anion formation requires

the use of a relatively strong base, namely, sodium hexamethyldisilazide, a consequence of

proton removal from a carbonβto the nitro group. This is not normally possible, but in this

special case three electron-withdrawing nitro groups flank eachβ-proton and the strained

cubane core causes deviation from sp^3 hybridization towards sp^2 character. Treating 1,3,5,7-

tetranitrocubane (39) with four equivalents of base, followed by reaction with frozen N 2 O 4

in tetrahydrofuran–pentane, gives heptanitrocubane (40) in 74 % yield. Further nitration to

octanitrocubane (41) (ONC) required the anion of (40) to be treated with nitrosyl chloride

followed by oxidation with ozone.

Octanitrocubane (ONC) has a density of 1.979 g/cm^3 , a calculated^15 heat of formation of

594 kJ/mol, and a decomposition temperature above 200◦C. The explosive performance of

octanitrocubane (41) from theoretical calculations is predicted to be extremely high. The most

recent theoretical estimate14b,^16 of VOD is 9900 m/s, making this compound one of the

most powerful explosives synthesized to date. Surprisingly, the density of heptanitrocubane