Green synthesis and catalytic properties of palladium nanoparticles for the direct reductive amination of aldehydes and hydrogenation of unsaturated ketones

Article abstract of DOI:10.1039/c4nj01440e

This paper reports on the synthesis and use of palladium nanoparticles as heterogeneous catalysts for the reductive amination of aldehydes and hydrogenation of unsaturated ketones. This method has the advantages of high yields, simple methodology and easy work up. The catalyst can be recovered and reused several times without significant loss of catalytic activity. This journal is

Full text of DOI:10.1039/c4nj01440e

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Green synthesis and catalytic properties of
palladium nanoparticles for the direct reductive
amination of aldehydes and hydrogenation of
unsaturated ketones
Cite this:DOI: 10.1039/c4nj01440e
Mahmoud Nasrollahzadeh
Received (in Montpellier, France)
2
6th August 2014,
Accepted 8th September 2014
This paper reports on the synthesis and use of palladium nanoparticles as heterogeneous catalysts for
the reductive amination of aldehydes and hydrogenation of unsaturated ketones. This method has the
advantages of high yields, simple methodology and easy work up. The catalyst can be recovered and
reused several times without significant loss of catalytic activity.
DOI: 10.1039/c4nj01440e
www.rsc.org/njc
4
separation from the reaction mixture and recyclability. However,
Introduction
there is an example on catalyst recovery by distillation of a reaction
5
Amines are valuable intermediates in the synthesis of pharma- mixture. Moreover, the homogeneous palladium catalysts tend to
4
ceutically important compounds. There are several methods for lose their catalytic activity because of Pd metal aggregation. These
the preparation of amines. Among various methods for the problems are of environmental and economic concern in large-scale
6
synthesis of amines, direct reductive amination of carbonyl syntheses. To overcome this problem, palladium nanoparticles are
compounds is very attractive in organic synthesis because generally dispersed on polymer or inorganic supports which makes
7
ketones and aldehydes can be transformed directly into the the system heterogeneous.
corresponding primary or secondary amines, with the most
The green synthesis techniques are generally synthetic
common reducing agents such as sodium cyanoborohydride routes that utilize relatively nontoxic solvents such as water,
and sodium triacetoxyborohydride or molecular hydrogen in biological extracts, biological systems and microwave assisted
1
8
the presence of a platinum-triad catalyst. The use of sodium synthesis. Green synthesis of NPs has several advantages over
cyanoborohydride and sodium triacetoxyborohydride has as chemical synthesis, such as simplicity, cost effectiveness as well as
drawback the formation of toxic byproducts (HCN and NaCN) compatibility for biomedical and pharmaceutical applications.
or the use of corrosive acetic acid.
Typically the gum consists of volatile oil, water soluble gum
The field of nanotechnology is an active area of research in (polysaccharides), lipophilic terpenes and insoluble matter.
modern materials science. In the present century, nanotechnology Polysaccharides are abundant in neutral sugars and are composed
9
has drawn attention in various fields such as electronics, chemical, of galactose, arabinose, xylose and D-glucuronic acid. There is a
2
biotechnology, bioanalytics and other industries. Metal nano- considerable interest in the use of naturally occurring polymers,
particles have been used widely in recent years due to their unique in particular those of polysaccharides as supports to achieve
electronic, optical, mechanical, magnetic and chemical properties high-performance and environmentally friendly catalysts that
3
10
which differ greatly from the bulk substances. These special and are chemically stable and are biodegradable.
unique properties could be attributed to their small dimensions
and large surface areas.
Recently our group described the synthesis of palladium(0)
nanoparticles as heterogeneous catalysts by Sour Cherry tree gum
In nanotechnology, palladium nanoparticles are the most (stabilizing/reducing agent) and their applications in Sonogashira
11
promising ones. Palladium nanoparticles (Pd NPs) have been coupling reactions. Due to the importance of amines and hydro-
widely used during the past few years as homogeneous or genation of olefinic double bonds in the synthesis of pharmaceuticals
heterogeneous catalysts in organic synthesis. Generally, the and functional materials and in continuation of our efforts to develop
12
homogeneous catalysis suffers from some difficulties in catalyst environmentally friendly synthetic methodologies, we report
convenient methodologies for the reductive amination of aldehydes
and hydrogenation of unsaturated ketones in the presence of gum
Department of Chemistry, Faculty of Science, University of Qom, PO Box 37185-359,
stabilized palladium nanoparticles as recyclable heterogeneous
catalysts that are air and moisture stable and highly active.
Qom, Iran. E-mail: mahmoudnasr81@gmail.com; Fax: +98 25 32103595;
Tel: +98 25 32850953
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Result and discussion
Pd nanoparticles (Pd NPs) were characterized using powder
1
1
XRD, SEM, UV-vis and TEM.
Pd nanoparticles stabilized on the surface of Sour Cherry tree
gum form a dark brown colored solution in water. The absence
II
of absorption bands for the yellow colored Pd solution (lmax
1
1
415 nm) confirms the complete reduction of Pd ions.
The crystalline structure of the catalyst was confirmed using
powder XRD (X-ray powder diffraction) measurements. The X-ray
diffraction pattern of Pd NPs is shown in Fig. 1. The X-ray
diffraction pattern of Pd nanoparticles encapsulated with Sour
Cherry tree gum exhibits five peaks at 20.01, 28.81, 40.01, 46.01,
5
0.51 and 66.71, which could be attributed to Pd NPs synthesized
1
1
in the presence of a stabilizing agent.
The size and shape of the products were examined by scanning
electron microscopy (SEM). Fig. 2 shows typical large-scale SEM
images of the as-produced palladium nanosphere. The products
are of spherical morphology and have very narrow diameter
distributions.
Fig. 3 shows the transmission electron microscopy (TEM)
image of Pd nanoparticles. The sizes of Pd nanoparticles are in
the range of 2.5 to 15 nm.
Fig. 2 SEM images of Pd nanoparticles.
Evaluation of the catalytic activity of Pd NPs through the
reductive amination of aldehydes
The catalytic behaviour of the Pd NPs was studied for the
reductive amination of aldehydes.
For optimization of the reaction conditions, we chose the
reaction of benzaldehyde with nitrobenzene in the presence of
1.3 mol% of catalyst as the model reaction, and the effects of
the solvent were examined. As shown in Table 1, the reaction
was influenced significantly by the solvent employed and based
on the reaction yields and environmental consideration the
best result was obtained in the case of ethanol (Table 1, entry 6).
The optimum conditions were found to be 1.0 mmol of Fig. 3 TEM image of Pd nanoparticles.
benzaldehyde, 1.2 mmol of nitrobenzene and 1.3 mol% of
catalyst in EtOH (5.0 mL) at room temperature under a H
atmosphere for 6 h. Increasing the amount of catalyst showed Table 1 Optimization of reaction conditions under a H
2
a
2
atmosphere
no substantial improvement in the yield (entry 7). An increase
b
Entry
Catalyst (mol%)
Base
Yield (%)
1
2
3
4
5
6
7
8
9
1.3
1.3
1.3
1.3
1.3
1.3
2.0
1.3
1.3
s-BuOH
i-BuOH
n-BuOH
i-PrOH
MeOH
EtOH
EtOH
EtOH
EtOH
38
51
81
75
92
92
92
c
78
d
32
a
Reaction conditions: 1.0 equiv. of benzaldehyde, 1.2 equiv. of nitro-
benzene, 1.3 mol% of catalyst and 5.0 mL of solvent, room temperature,
b
c
d
6
h. Isolated yield. At 40 1C. At 60 1C.
in the temperature from room temperature to 40 1C or 60 1C
afforded the product in lower yield (Table 1, entries 8 and 9).
Optimization of the experimental conditions led to one-pot
reaction with 92% yield (Table 1). These conditions (two step
Fig. 1 XRD pattern of Pd nanoparticles.
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Table 2 Reductive amination of different aldehydes with nitrobenzene
a
2
under a H atmosphere
b
Entry
Aldehyde
Time (h)
Yield (%)
b
1
2
6
6
92 (88)
90
85
Scheme 1 Reaction steps and intermediates proposed in the reductive
amination of aldehydes.
3
6
4
5
7
8
92
90
method) involve the following procedure: the initial quantitative
reduction of nitrobenzene with hydrogen (1 atm) in the presence of
Pd NPs (step 1); the addition of benzaldehyde and the subsequent
hydrogenation of the resulting imine under one atmosphere
6
7
8
6
81
86
(Scheme 1). Further investigations on the mechanism of reaction
are in progress in our laboratory.
As reported by Sreedhar and coworkers, when the reductive
amination of benzaldehyde with nitrobenzene was performed
in the presence of traditional heterogeneous catalysts, such as
8
9
8
6
91
93
Pd–SiO
2 2
and Pd–TiO , the nitrobenzene was reduced to the
1
0
6
6
96
98
corresponding amine moderately, but the hydrogenation of
1
3
11
in situ generated imine did not proceed even after 24 h. On
the other hand, using 10% Pd/C as catalyst, the same reaction
a
b
1
3
Yields are after work-up. Yield after the 4th cycle.
gave, after 6 h, only 20% yield of the product. The enhanced
activity of Pd NP catalysts over other supported palladium
catalysts screened results from the high surface area and strong
Then, the generality of the Pd NPs was examined by extending
hydrogen-trapping property of palladium nanoparticles compared the benzaldehyde reaction to a series of different nitroarenes. In
14
to bulk palladium.
all cases, the experimental results show that the all the reactions
Upon optimization of reaction conditions, the scope of the proceeded smoothly under optimized conditions to produce the
protocol was subsequently extended to a range of substituted respective products in good yields (Table 3). As shown in Tables
benzaldehyde and nitroarenes as substrates (Table 2). The 2 and 3, the steric hindrance of the substituent did not influence
electron-neutral, electron-rich and electron-poor benzaldehyde the product yield in the reductive amination of aldehydes using
reacted with nitrobenzene very well to generate the corres-
ponding products in good to excellent yields under the stan-
dard reaction conditions.
Table 3 Reductive amination of different aldehydes with nitrobenzene
2
under a H atmosphere
Aliphatic aldehydes were also used for the one-pot reductive
amination with Pd NPs. The reaction gave satisfactory yields for
aliphatic aldehydes (Table 2, entries 8–11).
The conversion was estimated by GC. In all cases, the
conversion of reactions is very high (490%). It was observed
that in all of these reactions there was no formation of any
byproducts. Aliphatic aldehydes gave excellent yield of the
desired products because the imine intermediate formed is
unstable and is readily converted into the product by hydro-
genation. In fact, the aliphatic imine intermediates are readily
hydrogenated to the product, and the hydrogenation of the
carbonyl group is not competitive with the reduction of
the nitro or imine groups. On the other hand, in the case of
aromatic aldehydes, though the imine intermediate formed
is quite stable, it can undergo backward reaction to form
aldehyde and amine, resulting in a competition between hydro-
genation and backward reaction that leads to a decrease
in yield.
a
Entry
Nitrobenzene
Time (h)
Yield (%)
1
2
3
4
6
6
6
6
81
89
89
77
5
6
6
85
90
6
a
Yields are after work-up.
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Evaluation of the catalytic activity of Pd NPs through the
hydrogenation of unsaturated ketones
Due to the importance of hydrogenation of olefinic double bonds in
the synthesis of natural products, pharmaceuticals, and functional
materials, we next turned our attention to applying Pd NPs to the
hydrogenation of unsaturated ketones. The desired products are
obtained in good yields.
Scheme 2 Selective reductive amination of aldehyde.
the Pd nanoparticles (Table 2, entries 3 and 6 and Table 3,
entries 5 and 6).
We initially selected hydrogenation of (E)-chalcone as a
model reaction. Among the various solvents (H O, MeOH,
As shown in Scheme 2, the reported method is highly selective
for the preparation of amines from aldehydes, and the reductive
amination of aldehydes in the presence of ketones, by this method,
gives only secondary amines of the aldehydes.
2
PhMe and EtOH) tested in the presence of Pd NPs, EtOH led
to significant conversion. The best result was obtained with
chalcone (1.0 mmol), Pd NPs (1.3 mol%) and EtOH (5 mL) at
room temperature under a hydrogen atmosphere, which gave
the product in an excellent yield (93%). We have examined the
hydrogenation of different unsaturated compounds on the
surface of palladium nanoparticles to afford the saturated
products in good to excellent yields and the results are presented
in Table 4. The enhanced activity of Pd NPs over other supported
palladium catalysts screened results from the high surface area and
strong hydrogen-trapping property of palladium nanoparticles
compared to bulk palladium.
Generally, aromatic ketones are poor substrates for reductive
15
amination protocols. Acetophenone as a ketone was also used for
the one-pot reductive amination with Pd NPs. Scheme 3 shows the
results obtained for reactions carried out using acetophenone with
nitrobenzene at room temperature and 80 1C. The result is, however,
interesting because acetophenone has shown to be an inert or a
1
5
difficult substrate in some reported reductive amination reactions.
Compared with the other literature studies on the reductive
16
amination of aldehydes, the notable features of our method are:
In hydrogenation of unsaturated compounds, we do believe
that the selective reduction of the CQC double bond takes
place at room temperature. The catalyst is versatile and highly
selective. The selectivity of the reactions is very high. The
reaction is believed to occur by selective addition of molecular
hydrogen, generated in situ, on the carbon–carbon double
bond. The molecular hydrogen reacts efficiently with the carbon–
carbon double bond of unsaturated compounds to yield the
corresponding saturated products in high yields.
ꢀ
The reaction system is simple;
The reaction was performed under mild conditions at
ꢀ
room temperature;
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Elimination of toxic reagents and homogeneous catalysts;
Wide substrate scope and generality;
Toxic and expensive ligands are not needed;
The use of ethanol as a green solvent;
The yields of the products are high;
2
The procedure is ‘‘green’’ and easy, requiring H as a clean
Table 4 shows that Pd NPs can catalyze chemoselective
reagent and avoiding any excess of hazardous reducing agent
borohydrides, hydrazine);
The use of plant extract as an economic and effective
reduction of CQC in the presence of the NO group (entry 9).
(
2
Reduction of NO did not occur even after the reaction time was
ꢀ
2
extended to 8 h. Our control experiments with this substrate
showed that hydrogenation with Pd/C (under same reaction
conditions with respect to time and Pd content in the catalyst)
led to reduction of both nitro and CQC moieties (entry 10 in
Table 4).
alternative represents an interesting, fast and clean synthetic
route for the large scale synthesis of Pd nanoparticles;
ꢀ
The catalyst is versatile and highly selective;
No surfactant, capping agent, and/or template were used
ꢀ
in this method for the synthesis of the Pd NPs;
For the further investigation of the selectivity of the reaction,
we attempted to reduce acetophenone under the same conditions
in ethanol (Table 4, entry 11). No further hydrogenation of aceto-
phenone was observed in ethanol even within 20 h, which
confirmed the selectivity of this method.
ꢀ
The reactions are relatively more environmentally friendly
with easy and efficient recyclability of the catalyst;
According to the UV-vis results, the synthesized Pd NPs by
ꢀ
this method are quite stable and can be kept under inert
atmosphere for several months.
After removal of the solvent and purification, the products were
characterized by melting points, elemental analysis (CHN), IR,
H NMR and C NMR. All of the products are known and were
identified by comparison of their physical and spectral data with
those of authentic samples. All melting points were compared
satisfactorily with those reported in the literature.
These advantages make the present method to be considered as
a convenient alternative method for the reductive amination of
aldehydes. These advantages make this methodology attractive for
the development of large-scale industrial synthesis.
1
13
Catalyst recyclability
The work-up procedure is straightforward due to the hetero-
geneous nature of the catalyst. To evaluate the reusability of Pd
NPs, the catalyst after completion of the reaction was separated
(Table 2, entry 1) and was washed with ethanol, dried in
Scheme 3
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a
2
Table 4 Hydrogenation of different compounds under a H atmosphere
b
Entry
1
Substrate
Product
Time (h)
6
Yield (%)
94
91
90
2
3
6
5
4
5
6
7
6
6
6
5
90
93
92
94
8
9
7
6
82
92
c
1
0
1
6
91
1
No reaction
20
0
a
b
c
Reaction conditions: 1.0 equiv. of the substrate, 1.3 mol% of the catalyst and 5.0 mL of EtOH, room temperature. Isolated yield. Control
experiment using 10% Pd/C.
vacuum and the recycled catalyst was saved for the next reac- These results further confirmed the high recyclability of the
tion. The recycled catalyst could be reused four times with no catalyst.
loss of activity. Thus, the nanocatalyst is stable during the
reductive amination of aldehydes. The reusability of the catalyst
was also studied for the hydrogenation of unsaturated ketones
Conclusion
under the present reaction conditions. The catalytic activity did In summary, we have established that the Pd NPs are a highly
not decrease considerably after four catalytic cycles (Fig. 4). efficient, stable and recyclable catalyst for the reductive amination
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of palladium nanoparticles. Then the colored solution of palla-
dium nanoparticles was centrifuged at 6000 rpm for 30 min to
completely disperse.
Typical procedure for the reductive amination of different
aldehydes with nitroarenes
Under a hydrogen atmosphere, 1.3 mol% of gum-Pd as a
catalyst was suspended in ethanol (3 mL) and stirred for
2
0 min at room temperature. Then 1.0 mmol of the different
aldehydes and 1.2 mmol of nitroarenes dissolved in ethanol
2 mL) were added. The reaction mixture was stirred at room
(
temperature for the specified period of time. The progress of
the reaction was monitored by TLC. After completion of
the reaction, the reaction mixture was filtered and the solvent
Fig. 4 Reusability of Pd NPs for the hydrogenation of (E)-chalcone.
of aldehydes and hydrogenation of unsaturated ketones. The was evaporated under reduced pressure and the residue was
present method has the advantages of readily available starting purified by column chromatography on silica gel to give the
materials, straightforward and easy work-up procedures, elimina- desired product. All the products are known compounds and
tion of homogeneous catalysts, mild reaction conditions, high the spectral data and melting points were identical to those
1
7
yields, tolerance for a wide variety of functionality, and excellent reported in the literature.
reusability of the catalyst.
General procedure for the hydrogenation of different
unsaturated compounds
Experimental
Under hydrogen atmosphere, 1.3 mol% of gum-Pd as catalyst
was suspended in ethanol (3 mL) and stirred for 20 min at room
temperature. Then 1.0 mmol of the reactant dissolved in
ethanol (2 mL) was added and the mixture was stirred at room
temperature for appropriate time. After completion of the
reaction (as monitored by TLC), the catalyst was separated
from the reaction mixture and the solvent was removed under
reduced pressure, and the residue was subjected to column
chromatography to afford pure products. All the products are
known compounds and the spectral data and melting points
High-purity chemical reagents were purchased from the Merck and
Aldrich chemical companies. All materials were of commercial
reagent grade. Melting points were determined in open capillaries
using a BUCHI 510 melting point apparatus and are uncorrected.
1
13
H NMR and C NMR spectra were recorded on a Bruker Avance
DRX-400 spectrometer at 400 and 100 MHz, respectively. FT-IR
spectra were recorded on a Nicolet 370 FT/IR spectrometer
(
Thermo Nicolet, USA) using pressed KBr pellets. The element
analyses (C, H, N) were obtained from a Carlo ERBA Model EA
108 analyzer carried out on Perkin-Elmer 240c analyzer. X-ray
1
8
were identical to those reported in the literature.
1
diffraction (XRD) measurements were carried out using a Philips
powder diffractometer type PW 1373 goniometer (Cu Ka =
À1
Acknowledgements
1
1
.5406 Å). The scanning rate was 21 min in the 2y range from
0 to 801. Scanning electron microscopy (SEM) was performed
We gratefully acknowledge the Iranian Nano Council and the
University of Qom for the support of this work.
on a Cam scan MV2300. Transmission electron microscopy
TEM) was recorded on a JEOL JSM 100CX. XRD was recorded
(
on a Bruker D8 XRD instrument SWAX.
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