294 N-Heterocycles
easy to synthesize and, coupled with their high performance, it is probable that some of these
compounds may eventually replace common high explosives like RDX.
Vast research efforts have been pooled into finding new energeticN-heterocycles over
the past 30 years and, consequently, the number of reported compounds is huge. It is quite
impossible to discuss all the materials reported in this area in the space available. We personally
believe thatN-heterocycles should be the subject of its own book and this may well be the
case in the future. We draw the reader to a number of excellent reviews,^1 which together cover
most of the past and present literature onN-heterocycles.
7.2 5-Membered rings – 1N – pyrroles
Nitro derivatives of pyrrole are not considered practical explosives for two reasons. Firstly, the
heat of formation of the pyrrole ring offers no benefits over standard arylene hydrocarbons.
Secondly, during nitration, pyrroles, like thiophenes and furans, are much more prone to
oxidation and acid-catalyzed ring-opening than arylene hydrocarbons. A common strategy
for the synthesis of highly nitrated pyrroles is to conduct the nitration in stages, the initial
mono-nitration using a mild nonacidic nitrating agent. As more nitro groups are introduced
the pyrrole ring becomes more electron deficient and less prone to oxidation and so allows for
the use of harsher and more acidic nitrating agents for further nitration.
Pagoria and co-workers^2 reported the nitration ofN-tert-butylpyrrole toN-tert-butyl-2,3,4-
trinitropyrrole in 40 % yield over three steps. Stegel and co-workers^3 reported the same synthe-
sis but conducted the nitration in two steps using mixed acid. Hinshaw and co-workers^4 used
N-tert-butyl-2,3,4-trinitropyrrole for the synthesis of 2,3,4,5-tetranitropyrrole in a reaction in-
volving initial deprotection followed by nitration with mixed acid at elevated temperature.
2,3,4,5-Tetranitropyrrole has a perfect oxygen balance but slowly decomposes on storage at
room temperature. Stegel and co-workers^3 also reported the synthesis ofN-methyl-2,3,4,5-
tetranitropyrrole from the nitration ofN-methyl-2,3,4-trinitropyrrole with mixed acid.
Russian chemists have reported the synthesis ofN-alkyl-3,4-dinitropyrroles from the cy-
clization of primary amines, formaldehyde and the potassium salt of 2,3,3-trinitropropanol.^5
7.3 5-Membered rings – 2N
7.3.1 Pyrazoles
Heat of formation and density calculations correlate so well with performance parameter like
detonation velocity that chemists have a good idea of the performance of an energetic material
before its synthesis and testing. The pyrazolo[4,3-c]pyrazoles DNPP (9) and LLM-119 (10)
were predicted^2 to exhibit performances equal to 85 % and 104 % relative to that of HMX.
Shevelev and co-workers^6 first synthesized DNPP (9) from 3,5-dimethylpyrazole. Subse-
quently, Pagoria and co-workers^7 improved the synthesis, obtaining DNPP (9) in 21 % over-
all yield from 2,4-pentanedione (1). An interesting feature of this synthesis is the tandem
decarboxylation–nitration step which occurs on treating (8) with absolute nitric acid at ele-
vated temperature. As predicted from theoretical calculations DNPP (9) is less energetic than
HMX but exhibits higher thermal stability and lower sensitivity to impact. Amination of DNPP