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cyano, and carboxylic acid functional groups, giving the corre-
sponding anilines 3–8b in excellent yields within short reac-
tion times ranging from 45 min to 2 h (entries 3–8). Interesting-
ly, in the case of substrate 9a having a p-benzyloxy group, the
Pd nanocatalyst was found to selectively reduce the nitro
group without giving rise to any debenzylated products, and
consequently aniline 9b could be obtained in 96% yield after
0
Scheme 1. Pd –AmP–MCF-catalyzed one-pot sequential transformation to
access monoalkylated amine 1c from nitroarenes 1a.
8
h (entry 9). By performing the hydrogenation for 4 h under
slightly diluted conditions in a 1:1 mixture of EtOAc/MeOH, it
was also possible to quantitatively reduce 4-nitrobenzenesulfo-
namide 10a into 4-aminobenzenesulfonamide 10b, which
constitute an important fragment in several antimicrobial
rapidly reduced by the Pd nanocatalyst into the desired mono-
alkylated amine in 85% yield (Scheme 1).
[
13]
agents (entry 10).
ortho positions, as exemplified with 2-nitro-1,1’-biphenyl 11 a,
-nitrophenol 12a, 1,3-dimethyl-2-nitrobenzene 13a, and 2-
Nitroarenes substituted in one or both
Recyclability, kinetic, and stability studies
2
To provide for an initial assessment on the reusability of the Pd
nanocatalyst, a recycling study was conducted in which the hy-
drogenation of nitroarene 1a was investigated over five cycles
in EtOAc for 1 h. In conformity with our previous work on the
nitro-4-(trifluoromethyl)aniline 14a, could all be converted to
the corresponding aniline products 11 b–14b in 95–99% yield
(entry 11–14). Among these substrates, the doubly ortho-me-
0
[11]
thylated 13a gave the slowest reaction and needed 5 h to
reach completion, demonstrating the importance of unhin-
dered access of the nitro functionality to the catalyst surface.
Consequently, the hydrogenation of the bulky 1-nitronaptha-
lene 15a was also found to proceed slowly, requiring the use
of elevated catalyst loadings and longer reaction times to give
satisfactory yields of 15b (entry 15). To our delight, the catalyt-
ic protocol also proved effective in the hydrogenation of both
heterocyclic and aliphatic nitro compounds, allowing for the
preparation of amines 16b–20b in excellent yields (Table 2, en-
tries 16–20). In the case of the heterocyclic substrates 17a–
Pd –AmP–MCF, an excellent recyclability was observed also
for this transformation because the catalyst afforded amine 1b
quantitatively over all cycles. Furthermore, to gain insights into
how the rate of the reaction was affected upon consecutive
reuse of the Pd nanocatalyst, a kinetic study was conducted
on the first and fifth cycle, in which the conversion of 1a was
monitored over time (see the Supporting Information, Fig-
ure S2). By comparing the slope of the two curves during the
0
first 30 min, it could be concluded that the recycled Pd –AmP–
MCF maintained approximately 88% of the original activity,
demonstrating its high stability under the present reaction
conditions. Another interesting observation that can be made
upon inspection of Figure S2 is that the kinetic profile belong-
ing to the reaction of the unused catalyst seems to remain
linear up to 95% conversion. This behavior is uncommon for
catalytic reactions, which normally display an exponential de-
cline in reaction rate over time as the substrate concentration
gets lower. A possible explanation for the zero-order kinetics is
that the nitro substrate binds strongly to the catalyst surface,
and in this way the catalyst may become saturated. The rate of
the reaction is thus proportional to the substrate bound to the
catalyst, and this amount would then become constant and in-
dependent of the substrate concentration between 0 and 95%
19a, the slower rate of reduction can be ascribed to a combi-
nation of large steric bulk and inhibition of the Pd nanocatalyst
by the additional heteroatom functions.
Unfortunately, the Pd nanocatalyst proved to be incapable
of selectively reducing the nitro group of substrates containing
olefin, acetylene, bromo, or formyl substituents under the opti-
mized reaction conditions (results not shown in Table 2). In the
hydrogenations of 4-nitrophenylacetylene and 3-nitrostyrene,
the carbon–carbon multiple bonds were found to undergo
a faster reduction than the nitro functionality and consequent-
ly the corresponding ethyl nitroarenes were formed almost ex-
1
clusively after 5–10 min as determined by H NMR spectrosco-
[
14]
1
py. For the reaction of 1-bromo-4-nitrobenzene, nitro group
reduction and debromination were observed to occur simulta-
neously, and thus it was only possible to obtain aniline 2b se-
lectively. The selective formation of aminobenzaldehydes in ad-
equate yields by this catalytic protocol was prevented by a fast
condensation reaction between amine- and formyl-containing
products, which resulted in a complex mixture of polymeric
byproducts. However, we identified that this latter reactivity
could be exploited to yield monoalkylated amine products as
previously demonstrated by Sreedhar et al. using gum acacia
conversion. By following the reactions by H NMR spectroscopy
over time, we could in almost all cases observe that the un-
reacted nitro compound and the product amine constituted
the major species in solution during the entire course of the
reaction, whereas the corresponding nitroso and hydroxyla-
mine intermediates were typically present in trace amounts at
most. This observation suggests that these intermediates are
not readily released into solution by the catalyst, and once
formed they are most likely quickly reduced all the way to the
aniline product.
[
15]
0
0
stabilized Pd nanoparticles. To demonstrate that our Pd –
AmP–MCF could also function as a catalyst for this transforma-
tion, a reaction was set up in which nitroarene 1a was first re-
duced into the corresponding aniline 1b by using the standard
conditions, which was followed by addition of pentanal
Further evidence for the robustness of the Pd –AmP–MCF
was obtained from TEM analyses of catalyst recovered from the
fifth cycle. As depicted in Figure S1, the majority of the Pd
nanoparticles were still found to be in the 1.5–3.0 nm size range
and only a small degree of agglomeration into larger clusters
could be seen. The retained Pd nanoparticle size indicates that
(1.3 equiv) to form the imine condensation product that was
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ChemCatChem 2014, 6, 3153 – 3159 3156