Organic Chemistry of Explosives

202 Synthetic Routes toN-Nitro

It is observed that the nitration of the amino nitrogens of both (41) and (45) is considerably

more facile than the urea nitrogens, and this is generally observed with other compounds of

similar structure;^31 theN-nitration of amides and ureas is discussed in Section 5.5. It is also

apparent that nitronium tetrafluoroborate is milder and more selective under these conditions

compared to other highly acidic nitrating agents. A mixture of acetic anhydride and fuming ni-

tric acid is a common reagent used forN-nitration. This reagent is relatively mild and excellent

results are frequently observed. A solution of trifluoroacetic anhydride and nitric acid is a more

powerful nitrating agent and frequently used for difficult substrates. A solution of 20–30 %

dinitrogen pentoxide in absolute nitric acid is a powerfulN-nitrating agent and mainly finds

use for nitrolysis reactions (Section 5.6). Using the most powerful nitrating agent is not always

the best strategy for anN-nitration. This can give a lower yield than a less powerful nitrating

agent due to side-reactions such as competing acid-catalyzed ring-opening. This is particularly

noticeable during the nitrolysis of someN-protected heterocyclic amines (Section 5.6).

The synthesis and properties of nitramines derived from strained and bicyclic amines are

discussed in more detail in Chapter 6. Such compounds often exhibit high performance resulting

from high crystal densities and/or high heats of formation due to internal strain.

5.3.2 Nitration with nonacidic reagents

Unlike the direct nitration of amines under acidic conditions, nucleophilic nitration is an

excellent route to both primary and secondary nitramines. In these reactions the amine or the

conjugate base of the amine is used to attack a source of NO 2. This source may be a nitrogen

oxide, nitronium salt, cyanohydrin nitrate, alkyl nitrate ester or any other similar source of

nitronium ion.

5.3.2.1 Alkyl nitrates

ArNH 2 Na Ar

ONa

O

N N ArNHNO 2 (Eq. 5.6)

2. EtONO 2

ArNH H+

EtONO 2

48 49

1. Na, Et 2 O

Figure 5.19

Angeli^32 pioneered the concept of nucleophilic amine nitration byN-nitrating aniline with

ethyl nitrate in the presence of potassium or sodium metal. In this reaction a complex anion

(48) of ethyl nitrate and the deprotonated amine decomposes to the metal salt of the nitramine

(49) which can be freed and the nitramine isolated by acidification at 0◦C (Equation 5.6). It

should be noted that the acidification of aromatic nitramines or their metal salts with aqueous

acid can cause rearrangement to the ring-nitrated product. This problem can be prevented by

first isolating the nitramine as its barium salt, followed by suspending the salt in water and

aspirating with carbon dioxide to release the free nitramine.^33

RNH 2 RNHNO 2 (Eq. 5.7)

n-BuLi EtONO 2 H+

OLi

O

RNH Li R NN

-78 °C

Figure 5.20