Oxidation of arylamines, arylhydroxylamines and other derivatives 1491,2,4- and 1,2,3-trinitrobenzenes were formed from the isomeric 2,4- and 2,6-dinitroanilines
respectively.^148 In a similar way, 3,4,5-,^149 2,3,5-,^150 and 2,3,6-^151 trinitrotoluenes were pre-
pared from the isomeric dinitrotoluidines. Holleman^152 used a similar route for the conversion
of picramide (53) to 1,2,3,5-tetranitrobenzene (54) in 69 % yield. Such examples illustrate why
nitration via diazotisation is a useful route to polynitroarylenes – products not available from
the direct nitration of aromatic substrates can be synthesized.
4.7 Oxidation of arylamines, arylhydroxylamines and other derivatives
4.7.1 Oxidation of arylamines and their derivatives
The oxidation of an aromatic amino group to a nitro group is an attractive strategy for the
synthesis of polynitroarylenes. Successive introduction of nitro groups into an aromatic ring
reduces electron density and makes further electrophilic nitration more difficult. The amino
group is a strongly activating group that will allow the polynitration of arylamines under mild
nitrating conditions. The directing and activating properties of the amino group, coupled with
its ability to be oxidized to a nitro group, has allowed the synthesis of many highly nitrated
arylenes, and particularly those with unusual substitution patterns which cannot be prepared
from direct nitration.
The reactivity of an arylamine to an electrophilic oxidant mainly depends on its nucle-
ophilicity, which in turn, is loosely related to basicity. The basicity of an amino group is
related to whether the other substituents on the aromatic ring are electron releasing or electron
withdrawing. Electron-withdrawing groups, like the nitro group, reduce amine basicity and
make oxidation more difficult. In general, the weaker the basicity of the amino group the more
powerful the oxidant needed for amino to nitro group conversion. Of equal importance is a sub-
strate’s susceptibility to oxidative degradation. Many polynitroarylamines can be subjected to
really quite harsh oxidizing conditions without noticeable degradation. Best results are usually
achieved using a balancing act between the substrate and strength of oxidant used, so avoiding
overoxidation but ensuring a high enough reactivity to effect substrate to product conversion in a
reasonable time. Slow oxidations may allow side-reactions to predominate and lead to substrate
degradation. If fused ring or aromatic methyl groups are present this is a potential problem.
Peroxyacids are the most widely used class of oxidant for aromatic amino to nitro group
conversion and include: peroxydisulfuric, peroxymonosulfuric, peroxyacetic, peroxytrifluo-
roacetic and peroxymaleic acids. The oxidizing potential of the peroxyacid is, as a rule, pro-
portional to the strength of the parent deoxy-acid. Dimethyldioxirane (DMDO) and ozone have
also found use for amino to nitro group conversion.
4.7.1.1 Peroxydisulfuric acid
2 H 2 SO 4 + H 2 O 2 H 2 S 2 O 8 + 2 H 2 O (Eq. 4.5)H 2 O + SO 3 H 2 SO 4 (Eq. 4.6)Figure 4.23