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Fig. 1 TEM images: (a) before the reaction, (b) after the reaction.
moieties also remained unaffected with regard to their stereo-
chemistry under these conditions (Table 1, entries 23 and 24).
The fused heterocyclic rings such as quinoline, indoline, dioxane,
benzimidazole and cyclic imide also remained protected under
the reaction conditions (Table 1, entries 20 and 26–29).
It is suggested that the reaction proceeds through –NQO,
–NHOH, –NQN(O)– as transient intermediates to provide
the NH2 product as we observe the formation of these
transient species in the HRMS spectra of reaction mixture at
an intermediate stage during monitoring the progress of the
reduction (see ESIw).
In general, the reductions are very clean, giving the amines in
high yields. No intermediate product, such as hydroxylamine or
hydrazine was found after the reaction was over. The products
are obtained very pure just by extraction with ethyl acetate in
majority of the reactions. In a few cases column chromatography
was used to obtain the pure product.
Significantly, after the reaction the spherical Fe(0) nano-
particles were transformed into Fe3O4 nanotubes as shown by
the TEM image (Fig. 1b). This type of change in shape and
size of nanoparticles over an organic transformation is
interesting and is being investigated in more detail for possible
applications.
In summary, we have introduced a novel protocol for
reduction of aromatic nitro compounds to amines by a target
specific reducing agent involving Fe(0) nanoparticles in water.
The simple operation, use of inexpensive and benign iron NPs
as reducing species, no requirement of conventional co-reducing
agent, mild reaction conditions of room temperature and
aqueous medium, short reaction time, high yields of amines
and, above all, the unique chemoselectivity over a wide range
of functional groups make this procedure an obvious choice
for reduction of aromatic nitro compounds.
We acknowledge the financial support of J. C. Bose fellowship
to one of us (BCR) from DST, New Delhi (SR/S2/JCB-11/2008).
R.D. and S.A. thank CSIR for their fellowships. We acknowledge
support of Nanoscience Project at IACS funded by DST. We
thank Dr Narayan Pradhan, Dr Sugata Ray and Dr Tapan
K. Paine of our institute for helpful discussions.
Notes and references
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c
7984 Chem. Commun., 2012, 48, 7982–7984
This journal is The Royal Society of Chemistry 2012