Organic Chemistry of Explosives

296 N-Heterocycles

Poullain and co-workers^10 synthesized N-substituted-3,5-diamino-4-nitropyrazoles by

treating substituted pyrimidines withN-alkylhydrazines. 3,5-Diamino-4-nitropyrazole (14)

has been synthesized by treating the pyrimidine (13) with hydrazine hydrate. This methodol-

ogy is limited in scope but the products are useful intermediates for the synthesis of insensitive

high explosives.

N

N

H

NH 2

N

N

H

O 2 NONO 2 2 NO 2 N

N

N

H

N 3

NaNO 2 (excess)

dil. H 2 SO 4

1. NaNO 2 , H 2 SO 4 (aq)


2. NaN 3 (aq)

Figure 7.4

A common route to nitropyrazoles involves the diazotization of the corresponding amino

derivatives in the presence of excess sodium nitrite.^11 Diazotization in the presence of sodium

azide allows the introduction of the azido ‘explosophore’.^11

N

N

H

N

N NO 2

N

N

H

O 2 N

AcOH, 25 °C

70 %

HNO 3 , Ac 2 O H 2 SO 4 , 0 °C

80 %

Figure 7.5

Initial nitration of pyrazole derivatives with nitric acid in acetic or trifluoroacetic anhydrides

leads toN-nitropyrazoles, which rearrange to theC-nitrated product on stirring in concentrated

sulfuric acid at subambient temperature. This N→C nitro group rearrangement often occurs

in situwhen pyrazoles are nitrated with mixed acid.

7.3.2 Imidazoles

The direct nitration of imidazole with acidic reagents is difficult due to facile nitrogen proto-

nation (pKaH∼7). Nitration of imidazoles proceeds in the 4- and 5-positions with the amidine

2-position being quite inert. Imidazole can be directly nitrated to 4,5-dinitroimidazole but

no further.^12 2,4,5-Trinitroimidazole (TNI) can be prepared from the successive nitration of

2-nitroimidazole; the latter synthesized from the diazotization of 2-aminoimidazole in the pres-

ence of excess sodium nitrite and a copper salt.^12 The nitrative cleavage of polyiodoimidazoles

also provides a route to polynitroimidazoles.^12 ,^13

NN

O 2 N NO 2

NO 2

NHN NH 4

NO 2

O 2 N

16

(ANTI)

15

(2,4-DNI)

Figure 7.6

2,4-Dinitroimidazole (2,4-DNI) (15) is readily prepared from the nitration of 2-

nitroimidazole.^12 ,^14 2,4-DNI exhibits moderate performance and is regarded as a shock in-

sensitive explosive. The relatively low cost and facile synthesis makes 2,4-DNI a realistic