154 Synthetic Routes to AromaticC-Nitro Compounds
Azo and azoxy compounds are potential by-products of peroxyacetic acid oxidations, partic-
ularly when the rate of oxidation is slow.^164 ,^171 Competitive condensation reactions are usually
avoided by using an excess of oxidant and also avoiding the presence of excess acid which
retards amine oxidation. The latter is sometimes suppressed by using a sodium bicarbonate
buffer.^171 ,^172
4.7.1.5 Peroxytrifluoroacetic acid
Trifluoroacetic acid (CF 3 CO 3 H) sits between peroxyacetic acid and peroxydisulfuric acid in
oxidizing potential. Anhydrous solutions of peroxytrifluoroacetic acid in methylene chloride
can be prepared by the addition of 90 %+hydrogen peroxide to a solution of trifluoroacetic
anhydride in methylene chloride containing a trace of sulfuric acid.^167 Solutions of peroxytri-
fluoroacetic acid prepared from less concentrated hydrogen peroxide solution or trifluoroacetic
acid are less reactive to arylamines.
Emmons^167 found anhydrous solutions of peroxytrifluoroacetic acid in methylene chloride
particularly clean and efficient for the oxidation of nitroanilines and phenylenediamines to the
corresponding dinitrobenzenes, and for the conversion of dinitrotoluidines and dinitroanilines
to the corresponding trinitrotoluenes and trinitrobenzenes respectively (Table 4.2, Entries 5a,
6a and 7). Blucher and co-workers,^173 and Novikov and co-workers,^168 have also reported
the synthesis of polynitroarylenes via this method. Peroxytrifluoroacetic acid fails to convert
weakly basic amines to their nitro compounds; trinitroanilines, like picramide, are not converted
to tetranitrobenzenes.^167 Peroxytrifluoroacetic acid also has a high potential for overoxidation
with substrates containing strong electron-donating groups or condensed rings.^167 However, the
formation of azo and azoxy condensation products is generally not a problem with peroxytri-
fluoroacetic acid and many products are obtained in excellent yield and in a high state of purity.
4.7.1.6 Peroxymaleic acid
Peroxymaleic acid (HO 2 CHC=CHCO 3 H) is between peroxytrifluoroacetic acid and peroxy-
acetic acid in terms of oxidant strength. Peroxymaleic acid is preparedin situfrom the reaction
of 90 % hydrogen peroxide with maleic anhydride in methylene chloride.^166 Like peroxytriflu-
oroacetic acid, it degrades anilines containing strong electron-donating groups but it is more
tolerant of arylamine substrates containing condensed rings. Peroxymaleic acid efficiently
oxidizes nitroanilines to the corresponding dinitrobenzenes (Table 4.2, Entry 4).
4.7.1.7 Dimethyldioxirane (DMDO)
NH 2O 2 N NO 2NO 2O 2 N NO 2(CH 3 ) 2 CO, 94 %30OO2Figure 4.27