Au/Ag-Mo nano-rods catalyzed reductive coupling of nitrobenzenes and alcohols using glycerol as the hydrogen source

Article abstract of DOI:10.1039/c2cc34178f

A highly efficient Au/Ag-Mo nano-rods catalyst was prepared for the one-pot synthesis of imine and amine using equal molar ratio of nitrobenzene and alcohol as starting materials, and bio-based glycerol as the hydrogen source. The reaction mechanism of the nitrobenzene reduction, amine and aldehyde coupling, and imine reduction was explored.

Full text of DOI:10.1039/c2cc34178f

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COMMUNICATION
Au/Ag–Mo nano-rods catalyzed reductive coupling of nitrobenzenes and
alcohols using glycerol as the hydrogen sourcew
Xinjiang Cui,ab Chengming Zhang,ab Feng Shi*a and Youquan Denga
Received 11th June 2012, Accepted 2nd August 2012
DOI: 10.1039/c2cc34178f
A highly efficient Au/Ag–Mo nano-rods catalyst was prepared
for the one-pot synthesis of imine and amine using equal molar
ratio of nitrobenzene and alcohol as starting materials, and
bio-based glycerol as the hydrogen source. The reaction mechanism
of the nitrobenzene reduction, amine and aldehyde coupling, and
imine reduction was explored.
atom-economic and environmentally friendly. Through the
borrowing hydrogen process, N-alkyl amines and imines could
be obtained by tuning the transition metal complex, ligands and
reaction conditions. Except the development of homogenous
catalysts for the coupling of amines and alcohols,9 the synthesis
of N-alkyl amines and imines by borrowing hydrogen methods
was also realized with supported catalysts.10 Furthermore, as the
major raw materials of amine synthesis, the use of nitrobenzenes
as a nitrogen source has attracted special attentions because
nitrobenzenes are cheap and readily available.11 However, in
order to realize this reaction, the use of large excess of alcohols is
absolutely necessary to reduce the nitrobenzenes and the ratio of
alcohols to nitrobenzenes is in the range of 6 : 1 to 10 : 1.11
Herein, we present the one-pot synthesis of imines and
amines using equal molar ratio of nitrobenzenes and alcohols
as starting materials and bio-based glycerol as the hydrogen
source. In order to realize this one-pot coupling reaction, an
Ag–Mo nano-rods supported nano-gold catalyst (Ag–Mo-NR)
was prepared, and nice results were obtained.
The past studies have demonstrated that the supported gold
nanoparticles showed superior catalytic activity in various reactions
because small gold nanoparticles possess abundant low-coordinated
atoms and high d10 energy, which results in the formation of active
sites.1 Now supported nano-gold catalysts have become one of the
choices in fine chemical synthesis.2 Recently, supported gold nano-
particles have emerged as active catalysts for the synthesis of basic
chemicals by selective oxidation of bio-based glycerol.3 These results
are very important because it allows the transformation of bio-mass
into fine chemicals, which might be one of the sustainable ways for
fine chemical synthesis. In these processes, oxygen is commonly
used to trap the hydrogen from glycerol, and water was formed as
the byproduct. From another point of view, glycerol can be used as
a hydrogen source to participate in the valuable reducing process.
As far as we know, reports on this objective are still scarce.
The Ag–Mo-NR was prepared by a modified hydrothermal
method, and the Au/Ag–Mo-NR catalyst was synthesized by
the deposition of nano-Au particles on Ag–Mo-NR (see the
Experimental section). The Au seeding nanoparticles were
prepared via the reduction of HAuCl4Á3H2O by CO in
BMImBF4 ionic liquid.12 After reduction, suitable amount
of water was added and the gold nanoparticles were extracted
by the n-tetradecylamine containing benzene solution.
The gold nanoparticles were deposited on the Ag–Mo-NR
and the as-synthesized Au/Ag–Mo-NR was washed with
ethanol to remove the organic compounds. Further on, the
Au/Ag–Mo-NR catalyst was extensively characterized. According
to the XRD characterization, there is no observable diffraction
pattern of gold species, which suggested that the gold species
were highly dispersed on the Ag–Mo nano-rods, Fig. S1
(ESIw). From the scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) images, it could be
seen that the diameter of Ag–Mo-NR was B100 nm and
the particle size of the gold nanoparticles, which were well
dispersed on the wall of Ag–Mo-NR, were 5–8 nm, Fig. 1a–c
and Fig. S2 (ESIw). Interestingly, only the crystal lattice of the
Au (111) plane is observable in the HRTEM image and the big
gold particles are composed of smaller gold particles, Fig. 1d.
According to the ICP-AES analysis, the gold loading of
Au/Ag–Mo-NR was 0.3 wt%.
N-Alkyl amines and imines are building blocks for the
synthesis of polymers, dyes, pharmaceuticals and bio-active
compounds.4 For the preparation of N-alkyl amines, the most
traditional procedure is the reaction of amines with organic
halides but this method often suffers from environmental
problems, high cost of the starting materials and low selectivity
to the desired products.5 In addition, the N-alkyl amines can also
be prepared by hydroamination and hydroaminomethylation
reactions.6 Imines are often synthesized by acid–base promoted
condensation of amines and aldehydes or ketones, oxidative
coupling of secondary amines,7 hydroamination of alkynes and
other methods.8 In the last decades, the transition-metal catalyzed
direct coupling of amines and alcohols using borrowing hydrogen
methodology has attracted extensive attentions because it is
a Centre for Green Chemistry and Catalysis, Lanzhou Institute of
Chemical Physics, CAS Middle Tian Road, 18, Lanzhou, China.
E-mail: fshi@licp.cas.cn; Fax: +86-931-8277088
b Graduate School of the Chinese Academy of Sciences, Beijing,
100049, China
w Electronic supplementary information (ESI) available: Catalyst
preparation procedure and characterization details. See DOI:
10.1039/c2cc34178f
c
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Chem. Commun., 2012, 48, 9391–9393 9391

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Table 1 Reductive imination of alcohols with nitrobenzenesa
a
1 mmol benzyl alcohol, 1 mmol nitrobenzene, 40 mg catalyst, 10 mol%
K2CO3, 1.0 g glycerol, 0.5 mL toluene, 120 1C, 24 h, GC yields.
reacted smoothly with p-methyl nitrobenzene to give the corre-
sponding imines in 86–92% yields, while the electron-withdrawing
group, i.e. chlorine, on the benzene ring decreased the reactivity.
Au/Ag–Mo-NR showed high activity in the tandem imination
reactions of aliphatic alcohols, which are more difficult to be
activated than aromatic alcohols. For example, 2-phenylethanol
was successfully activated to generate the corresponding imine
with 90% yield. Other aliphatic alcohols such as cyclohexyl-
methanol, dodecan-1-ol and octan-1-ol could also react with
p-methyl nitrobenzene to produce imines with excellent yields.
In the reaction of benzyl alcohol and p-methyl nitrobenzene,
N-benzylidene-4-methylaniline was formed with >98% selectivity
and 2% N-benzyl-4-methylaniline was observed as a side product.
We were inspired by this result that it may be possible to synthesize
secondary amines by this method, too. To our delight, the
N-substituted aniline was successfully produced under refluxing
xylene, Table 2. Good results were obtained for the coupling
reactions of nitrobenzene, 1-nitronaphthalene and activated nitro-
benzenes with benzyl alcohols, such as p-methyl nitrobenzene,
p-methoxy nitrobenzene and p-chloro nitrobenzene. Moreover, as
expected, activated benzyl alcohols, such as 4-isopropyl benzyl
alcohol, 3-methyl benzyl alcohol and 4-methylthio benzyl alcohol,
all reacted smoothly with p-methyl nitrobenzene to give the
corresponding amines in high yields. It was noteworthy that this
catalyst was found to maintain high activity in the coupling of
aliphatic alcohols and nitrobenzenes, too.
Fig. 1 (a) SEM image of Au/Ag–Mo-NR, (b) TEM image of Au/
Ag–Mo-NR, (c) TEM image of Au/Ag–Mo-NR and (d) HRTEM
image of Au/Ag–Mo-NR.
Subsequently, the catalytic activity of Au/Ag–Mo-NR in
the imination of alcohols with nitrobenzenes was explored
using benzyl alcohol and p-methyl nitrobenzene with 1 to
1 molar ratio as the model substrates, Table S1 (ESIw).
Clearly, the tandem reaction hardly proceeded in the absence
of the Au/Ag–Mo-NR catalyst and base, entries 1–2. Then,
the influence of bases such as K2CO3, Na2CO3, NaOH and
KOH, and solvents such as toluene, THF, DMF and 1,4-dioxane,
on the efficiency of the reaction was studied, entries 3–10. It was
shown that 55% conversion of p-methyl nitrobenzene and 96%
selectivity to N-benzylidene-4-methylaniline were obtained using
K2CO3 as the additive and toluene as the solvent. The application
of other bases and solvents resulted in lower conversion of the
nitrobenzene and lower selectivity to imine. Complete conversion
of p-methyl nitrobenzene to the desired product is observed with
prolonged reaction time and the selectivity to the aimed product
was 98%, entry 11. Under the optimized reaction conditions,
other catalysts were tested for the one-pot tandem synthesis of
N-benzylidene-4-methylaniline, too. If Ag or Mo materials
including Ag2O, MoO3 and Ag2MoO4 were used as catalysts,
6–73% conversion of p-methyl nitrobenzene was obtained,
entries 12–15. In the presence of Au–TiO2 and Au–Fe2O3, the
conversion of p-methyl nitrobenzene was lower than 50%, and
the undesired formation of benzyl benzoate was observed,
entries 16 and 17.
Table 2 Reductive amination of alcohols with nitrobenzenesa
In order to explore the scope and limitations of the Au/
Ag–Mo-NR catalyst system, a variety of structurally diverse
primary alcohols and nitrobenzenes were examined towards
the formation of the corresponding imines, as shown in
Table 1. The transformation of nitrobenzenes, which contain
electron-withdrawing and electron-donating substituents on
the aryl ring, was effectively proceeded to give the corre-
sponding imines with up to 98% yield. Nice results were also
obtained by applying 1-nitronaphthalene as the starting
material with 76% yield. Notably, the transformation of
various alcohols with p-methyl nitrobenzene could also be
realized. Benzyl alcohols with electron-donating groups
a
1 mmol benzyl alcohol, 1 mmol nitrobenzene, 40 mg catalyst, 10 mol%
K2CO3, 1.0 g glycerol, 0.5 mL xylene, 150 1C, 24 h, GC yields.
c
9392 Chem. Commun., 2012, 48, 9391–9393
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A plausible reaction mechanism was revealed and further
studies are underway.
We thank the National Natural Science Foundation of
China (21073208) for financial support.
Notes and references
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Scheme 1 Reduction of p-Me nitrobenzene.
Scheme 2 The reduction of imine to N-alkyl aniline by glycerol.
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Scheme 3 The plausible mechanism of the synthesis of N-alkyl ani-
line using glycerol as the reducing agent.
The catalyst is easily separated by filtration and its recyclability
was investigated in the coupling reaction of benzyl alcohol and
p-methyl nitrobenzene to synthesize the corresponding imine. To
our delight, the catalytic activity of Au/Ag–Mo-NR remained
almost unchanged after 5 runs and no gold leaching occurred
according to ICP-AES measurement. Furthermore, the filtered
solution did not exhibit any activity under the same reaction
conditions and the gold leached into the filtrate was out of the
detection limit. These findings ruled out the contribution of the
leached gold in the coupling reaction.
The investigation of the reaction mechanism should be
interesting. By tracing the coupling reaction of p-methyl nitro-
benzene and benzyl alcohol with GC-MS, p-methyl aniline was
observed at the initial stage and a certain amount was kept
during the whole process. However, we cannot observe the
oxidation product of glycerol by GC-MS. So, the reaction mixture
was characterized by LC-MS, which suggested the formation of
oxidized glycerol, i.e. glycerosone or glyceraldehydes, Fig. S3
1
(ESIw). According to the H NMR spectra, Fig. S4 (ESIw), the
aldehyde protons of benzaldehye and glyceraldehydes could be
observed. Thus, benzylalcohol and glycerol were all responsible for
the reduction of p-methyl nitrobenzene at the initial stage of the
reaction and the oxidation of glycerol might occur on the terminal
hydroxyl group, Scheme 1. The following step is the formation
of N-benzylidene-4-methylaniline and its further reduction to
N-benzyl-p-methyl aniline by glycerol, Scheme 2. The reaction
mechanism could be clarified as given in Scheme 3.
In summary, we have developed a highly efficient hetero-
geneous catalyst for the tandem reaction of alcohols and
nitrobenzenes to generate N-alkyl amines and imines using
glycerol as the hydrogen source. Good to excellent yields of
N-alkyl amines and imines were achieved under mild condi-
tions with 1 to 1 molar ratio of alcohols and nitrobenzenes.
12 S. Guo, F. Shi, Y. L. Gu, J. Yang and Y. Q. Deng, Chem. Lett.,
2005, 830.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9391–9393 9393