Fe2O3-supported nano-gold catalyzed one-pot synthesis of N-alkylated anilines from nitroarenes and alcohols

Article abstract of DOI:10.1039/c1cc11057h

Here, we show the one-step synthesis of N-alkylated anilines from nitrobenzenes and alcohols catalyzed by nano-gold catalyst. The yields to N-alkylated anilines were ～90% under mild conditions. The mechanism of this reaction was explored. It shows promise for clean and simple synthesis of N-alkylated anilines.

Full text of DOI:10.1039/c1cc11057h

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Fe2O3-supported nano-gold catalyzed one-pot synthesis of N-alkylated
anilines from nitroarenes and alcoholsw
Qiling Peng, Yan Zhang, Feng Shi* and Youquan Deng
Received 23rd February 2011, Accepted 21st April 2011
DOI: 10.1039/c1cc11057h
Here, we show the one-step synthesis of N-alkylated anilines from
nitrobenzenes and alcohols catalyzed by nano-gold catalyst. The
yields to N-alkylated anilines were B90% under mild conditions.
The mechanism of this reaction was explored. It shows promise for
clean and simple synthesis of N-alkylated anilines.
Scheme 1 An illustration for the synthesis of anilines.
Aromatic amines are extensively found in biologically active
catalyst could be used in this reaction without additional organic
natural products, pharmaceuticals, dyes and ligands for
ligand and base.
transition-metal-catalyzed reactions.1 Much effort has been spent
The unique activity of supported nano-gold catalyst has been
reported in many reactions.10 Recently, it was found that nano-
for more than a century on methods to prepare aromatic amines.
gold could be a good catalyst in selective oxidation11 and selective
In industry, nevertheless, the catalytic hydrogenation of nitro-
arenes serves as the key technology.2 Meanwhile, the anilines need
hydrogenation reactions.12 Inspired by these interesting results,
to be further functionalized to get more valuable compounds.
we speculate that supported nano-gold may be applied in the
Besides the well-known non-catalytic N-alkylations of anilines
reductive benzylation of nitrobenzene with alcohol, in which the
with alkyl halides in the presence of stoichiometric amounts of
key steps are the oxidation of alcohol and the reduction of imine
inorganic bases,3 various catalytic reactions have been developed
intermediate.13 Herein, we want to present our new results on the
because the use of alkyl halides is undesirable from an environ-
alkylation of nitrobenzene derivatives catalyzed by iron oxide
mental point of view and causes handling problems as well as
immobilized nano-gold catalyst, Scheme 1. N-monobenzylated
or N,N0-dibenzylated aniline could be obtained by tuning the
significant waste generation. In the last decades, modern
transition-metal-catalyzed processes, i.e. reaction of amine with
amines or aryl halides,4 reductive amination,5 hydroaminations6
A series of metal oxide immobilized nano-gold catalysts were
reaction conditions finely.
and hydroaminomethylations,7 etc., were successfully employed
prepared by a co-precipitation method. For the physicochemical
and elegant results were obtained. Excepting the methods
properties of the catalysts, see Table S2. The BET surface area of
all the catalysts was B100 m2 gꢀ1. HR-TEM, XPS and XRD
mentioned above, the N-alkylation of anilines with alcohol, in
which water was produced as the only byproduct, could be a
characterization confirmed the formation of metal oxide
good choice for the development of an environmentally benign
immobilized nano-catalyst. For a typical sample, i.e. 9.1 wt%
method for N-functionalized anilines synthesis. In the former
Au/Fe2O3, the particle size of the nano-gold was B3 nm and the
reports, extensive attention has been focused upon Ni, Ru, Ir, Pd,
crystal lattice Au (200) on the iron oxide could be observed clearly
Au, Cu, and Fe as effective catalysts.8 However, the usage of
(2002 JCPDS 65-2870), Fig. 1, Fig. S2–4.
organic ligand and additional base as cocatalyst makes the
The results for the benzylation of nitrobenzene with benzyl
catalyst system complicated and not practical. Moreover, the
alcohol using different immobilized nano-catalysts are shown
anilines need to be firstly synthesized through the reduction of
in Table 1. Only N-benzylideneaniline was observed if using
nitrobenzenes.2 Recently, it was reported that N-alkylated anilines
could be synthesized in one step using nitrobenzenes as starting
materials with alcohol as alkylation reagent.9 However, it also
needs the usage of complicated organic ligand and additional base
as cocatalyst. It would be highly desirable if a heterogeneous
Centre for Green Chemistry and Catalysis, Lanzhou Institute of
Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000,
China. E-mail: fshi@licp.cas.cn; Tel: +86-9318277088
w Electronic supplementary information (ESI) available: Experimental
details on the catalyst preparation, reaction procedure and product
characterization. See DOI: 10.1039/c1cc11057h
Fig. 1 HR-TEM picture of 9.1 wt% Au/Fe2O3.
c
6476 Chem. Commun., 2011, 47, 6476–6478
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Table 1 N-benzylaniline synthesis from nitrobenzenea
Au
mol
(%)
Conv.
(%)b
Sel.
(%)c
P.S.
(nm)d
Entry
Catalyst
1
2
3
4
5
6
7
8
Fe2O3
—
51
>98
>98
>98
>98
89
97
95
63
59
0
—
1.8 wt% Au/Fe2O3
4.3 wt% Au/Fe2O3
6.3 wt% Au/Fe2O3
9.1 wt% Au/Fe2O3
4.6 wt% Au/CeO2
2.8 wt% Au/Co3O4
2.5 wt% Au/NiO
3.6 wt% Pd/Fe2O3
2.9 wt% Ru/Fe2O3
11.5 wt% Au/Fe2O3
0.18%
0.44%
0.64%
0.92%
0.47%
0.28%
0.25%
0.37%
0.29%
1.17%
25
53
66
96
6
6
60
5
B3
B3
B3
B3
e
—
B6
B4
e
9
10
11f
—
e
8
78
—
B4.5
>99
a
5 mmol (0.615 g) nitrobenzene, 50 mmol (5.400 g) benzyl alcohol,
b
0.1 g catalyst, 160 1C, 8 h, 20 mL pressure tube, Ar. Conversion of
nitrobenzene determined by GC-FID. Selectivity to N-benzylaniline
c
determined by GC-FID. The major byproduct was N-benzylideneaniline.
d
e
Gold particle size determined by TEM. Difficult to be determined.
Catalyst from Prof. Haruta’s group. It was prepared by a co-precipitation
f
method and calcined at 400 1C. Prior to usage, the catalyst was activated
under flowing air (20 mL minꢀ1) at 300 1C for 1 h. The gold particle was
estimated by HR-TEM measurement.
Scheme 2 Results of the reductive alkylation of nitrobenzenes.
Au/Fe2O3, Scheme 2. For the reaction of nitrobenzene and benzyl
alcohol, 87% yield was obtained. If performing the reaction with
2-pyridine methanol as alkylation reagent, 92% yield could also
be gotten. That means the presence of a heterocyclic group
doesn’t affect the catalytic activity of the nano-gold. Then, the
reaction of 4-chloronitrobenzene with benzyl alcohol, 2-methyl-
benzyl alcohol, 4-isopropylbenzyl alcohol and 2-pyridine
methanol gave 81–92% yields. Interestingly, 99% yield was
achieved using 2-phenylnitrobenzene and benzyl alcohol as
starting materials. Importantly, the reaction could also progress
with aliphatic alcohol as starting material. The yield to N-octyl-4-
methoxyaniline was 89%.
hematite as the catalyst simply although 51% nitrobenzene was
reduced, Entry 1. The introduction of gold could remarkably
improve the formation of N-benzylaniline, i.e. the conversion of
N-benzylideneaniline to N-benzylaniline, Entry 2. By increasing
the loading of gold from 1.8 wt% to 9.1 wt%, an almost
stoichiometric amount of N-benzylaniline could be produced,
Entries 3–5. The conversion was >98% and the selectivity to
N-benzylaniline is 96% using catalyst 9.1 wt% Au/Fe2O3,
Entry 5. The supports play an extremely important role in order
to achieve good activity. By applying catalysts 4.6 wt% Au/CeO2
and 2.8 wt% Au/Co3O4, the selectivities to N-benzylaniline are all
less than 10%, Entries 6–8. Also, a relatively good result was
obtained if 2.5 wt% Au/NiO was used, Entry 8. The iron oxide
immobilized Pd and Ru catalysts were also prepared and checked
here, Entries 9–10. However, there is almost no improvement if
comparing the result using iron oxide solely. Therefore, it is
possibly a unique catalytic activity of iron oxide immobilized
nano-gold to convert nitrobenzene into N-benzylaniline. Another
iron oxide supported nano-gold catalyst, i.e. 11.5 wt%
Au/a-Fe2O3 calcined at 400 1C, which was independently
prepared by Haruta et al., was also measured under the same
reaction conditions, Entry 11. The conversion of nitrobenzene
was almost 100% and the selectivity to N-benzylaniline was 78%.
This difference in selectivity might be attributed to the difference
in the size of Au particles and calcination temperature. When
9.1 wt% Au/Fe2O3 was reused for the second run, unfortunately,
the major product was imine intermediate, i.e. >80%. ICP
analysis showed 4.4 wt% Au was maintained in the used sample
and XRD characterization suggested the severe aggregating of
nano-gold during the reaction, Fig. S2b, which should be the
reasons for the deactivation of the catalyst. Although there is no
direct evidence, it could be imagined that the formation of water
during the reaction is one of the possible reasons for the
aggregation of nano-gold particles.
Thus, our nano-gold catalyst could activate both aromatic and
aliphatic alcohols. Also, the product from aliphatic alcohol was
different from those from aromatic alcohol. As was mentioned
above, the major product with aromatic alcohol as starting
material was benzaldehyde. For aliphatic alcohol, i.e. 1-octanol,
octyl octanoate and coupling products from octanal were
detectable by GC-MS. However, this nano-gold catalyst is not
active for the amination of methanol and secondary alcohols such
as isopropanol and phenethyl alcohol. Only B20% aniline was
formed in the presence of methanol and no reaction occurred with
isopropanol and phenethyl alcohol as starting materials.
It is worth noting that N,N-di-substituted anilines with
different functional groups could be synthesized just by
varying the amount of catalyst and the ratio between nitro-
benzene and alcohol. The yields to N,N-dibenzylaniline,
N,N-dibenzyl-4-methylaniline, N,N-dibenzyl-2-methylaniline,
N,N-dibenzyl-4-methoxyaniline, N,N-di-4-isopropylbenzylaniline,
N,N-di-4-chlorobenzylaniline and N,N-dibenzyl-4-chloro-
aniline reached 85–96% respectively. Therefore, we can
synthesize anilines with different structure with this simple
nano-gold catalyst system.
Further on, the reactions of different nitrobenzene and
alcohol derivatives were employed with catalyst 9.1 wt%
c
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Chem. Commun., 2011, 47, 6476–6478 6477

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The iron oxide supported nano-gold catalyst could also be
used in the synthesis of imine. As an example, N-benzylidene-
4-methoxyaniline could be obtained if reducing the amount of
catalyst simply.
operationable. The improvement of the catalyst and reaction
is now underway in our laboratory.
This work was financially supported by the National
Natural Science Foundation of China (21073208) and the
‘‘Hundred Talents Program’’ of the Chinese Academy of
Sciences. We thank Prof. M. Haruta for offering the catalyst
sample.
Moreover, this catalyst offers a method for the synthesis of
N,N-disubstituted amine from N-substituted amine. As a
model reaction, N-benzyl-N-4-methoxybenzylaniline could be
synthesized with 97% yield.
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c
6478 Chem. Commun., 2011, 47, 6476–6478
This journal is The Royal Society of Chemistry 2011