Nitramine rearrangement 147
attempted isolation of numerous nitramines from the nitration of aromatic amines has led to the
isolation of the diazophenol as the sole product; the bis-nitramine (44) is found to decompose
spontaneously at 25◦C in the solid state and in solution to give the diazophenol (45).^142
NO 2 NO 2
NO 2 NO 2
NHNO 2
O 2 NHN
O
NO 2 O
NO 2
N 2
N 2
25 °C
44 45
Figure 4.19
Diazophenol formation is most competitive when a nitramine substrate contains an electron-
withdrawing nitro grouporthoto the nitro group being displaced and hencemetato the
nitramine functionality, assumedly because that site is then activated towards nucleophilic aro-
matic substitution.^144 Heating nitramines in inert chlorinated solvents also favours diazophenol
formation but this is suppressed by using urea or sulfamic acid as additives.
The course of a reaction involvingN-nitration of an aromatic amine is very dependent on
both the substrate and the reaction conditions, i.e. nitrating agent, temperature, work-up etc. The
nitration of 3-amino-2,5-dinitrotoluene with mixed acid in glacial acetic acid at 0◦C yields the
corresponding diazophenol with no trace of the intermediate nitramine.^145 In contrast, nitration
of the same substrate at ambient temperature with mixed acid composed of 90 % nitric acid
and 96 % sulfuric acid yields the corresponding nitramine.^145
CH 3
NO 2
NH 2
46
O 2 N
CH 3
CH 3
NHNO 2
47
NHNO 2
49
NO 2
NO 2
NO 2
O 2 N NO 2
O 2 N
O 2 N
CH 3
NH 2
50
NO 2
O 2 N NO 2
O 2 N
CH 3
N 2
48
O
O 2 N NO 2
HNO 3 , H 2 SO 4
in AcOH, 0 °C
78 %
HNO 3 ,
H 2 SO 4 , 20 °C
100 %
CH 2 Cl 2 , reflux
81 %
H 2 SO 4 , 0 °C
35 %
H 2 SO 4 , 20 °C
78 %
PhOMe,
Figure 4.20
In some cases the Bamberger rearrangement of an arylnitramine can be low yielding and
may not be the best route to a product. For example, 4-amino-2,3,5,6-tetranitrotoluene (50)
is prepared in low yield via the acid-catalyzed rearrangement of the nitramine (47), the latter
formed from the controlled nitration of 4-amino-2,6-dinitrotoluene (46) with mixed acid in
glacial acid or with nitronium tetrafluoroborate in methylene chloride.^145 Alternatively, the
same starting material, 4-amino-2,6-dinitrotoluene (46), can be fully nitrated to 4-amino-
N,2,3,5,6-pentanitrotoluene (49), again with mixed acid, and the nitramino group subsequently
cleaved with sulfuric acid in anisole.^145 This latter route provides the product, 4-amino-2,3,5,6-
tetranitrotoluene (50), in a significantly higher overall yield.