Fe@Pd/C: An efficient magnetically separable catalyst for direct reductive amination of carbonyl compounds using environment friendly molecular hydrogen in aqueous reaction medium

Article abstract of DOI:10.1016/j.cattod.2014.07.057

Direct reductive amination of carbonyl compounds with variety of amines has been reported by using magnetically separable Fe@Pd/C as an efficient catalyst in aqueous reaction medium. The developed methodology offers synthesis of various secondary and tertiary amines using different aldehydes and ketones with amines giving excellent yield. Moreover, the magnetically separable Fe@Pd/C catalyst was easily separated from reaction mixture and can be recycled for further reaction.

Full text of DOI:10.1016/j.cattod.2014.07.057

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Fe@Pd/C: An efficient magnetically separable catalyst for direct
reductive amination of carbonyl compounds using environment
friendly molecular hydrogen in aqueous reaction medium
Nilesh M. Patil, Bhalchandra M. Bhanage∗
Department of Chemistry, Institute of Chemical Technology, N. Parekh Marg, Matunga, Mumbai 400 019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Direct reductive amination of carbonyl compounds with variety of amines has been reported by using
magnetically separable Fe@Pd/C as an efficient catalyst in aqueous reaction medium. The developed
methodology offers synthesis of various secondary and tertiary amines using different aldehydes and
ketones with amines giving excellent yield. Moreover, the magnetically separable Fe@Pd/C catalyst was
easily separated from reaction mixture and can be recycled for further reaction.
Received 29 March 2014
Received in revised form 15 July 2014
Accepted 30 July 2014
Available online xxx
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Direct reductive amination
Aqueous media
Magnetically separable
Core shell
Fe@Pd/C catalyst
1. Introduction
Recent development in direct reductive amination includes
the use of gold catalyst in the presence of silane [9],
Amines are highly versatile building blocks for various organic
substrates and have wide applications in the pharmaceutical,
agrochemical, chemical industries, materials science, and biotech-
nology [1–3]. Direct reductive amination of aldehydes and ketones
with primary or secondary amines is one of the powerful and
widely used C N bond formation reactions for the synthesis of
substituted amines in synthetic organic chemistry. In the last two
decades, many research groups reported versatile strategies for
the synthesis of substituted amines by the direct reductive ami-
nation method. To attain this conversion, use of various boron
based hydride reagents has been developed, including NaBH4 [4],
zinc borohydride [5], sodium cyanoborane [6], pyridine-BH3 [7],
InCl3/Et3SiH [8]. Nevertheless, above mentioned methods requires
stoichiometric metal hydride source, cyanoborohydride reagents
are extremely poisonous, which leads contamination of products
with toxic by-products, besides some hydride source pyridine-BH3
is highly unstable to heat and therefore need to take extreme care
while handling.
poly(methylhydrosiloxane) PMHS [10] and palladium based cat-
alysts [11] were also reported. Direct reductive amination under
transfer hydrogenation method has been investigated by Hu
et al. and Xiao group [12]. However, the above mentioned
methods have some drawbacks such as hazardous by-products,
requires an acidic condition, harsh reaction condition, use of
organic solvents and homogenous catalytic system which lead
to problem of catalyst–product separation from the reaction
mixture. Considering these drawbacks efforts were taken to
develop improved heterogeneous recyclable catalytic system for
direct reductive amination using environmental-friendly molec-
ular hydrogen as a clean hydrogen source under mild reaction
condition.
Nowadays, the use of water as a reaction medium has attracted
considerable attention since water is an inexpensive, safe, readily
available and non toxic solvent hence provides remarkable advan-
tages over organic solvent [13]. Recently we have reported aqueous
biphasic catalytic system for direct reductive amination of car-
bonyl compounds using polymer supported palladium complex
[14]. Other reports from group of Ajjou [15a], Ogo et al. [15b] and
Xiao et al. [15c] were also explored water as a reaction medium.
Therefore, the hydrogenation of organic molecules in a water phase
offers a simple solution for recycling the catalyst, and improves the
products/catalyst separation.
∗
Corresponding author. Tel.: +91 22 3361 1111; fax: +91 22 24145614.
E-mail addresses: bm.bhanage@ictmumbai.edu.in, bm.bhanage@gmail.com
(B.M. Bhanage).
http://dx.doi.org/10.1016/j.cattod.2014.07.057
0920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: N.M. Patil, B.M. Bhanage, Fe@Pd/C: An efficient magnetically separable catalyst for direct reductive
amination of carbonyl compounds using environment friendly molecular hydrogen in aqueous reaction medium, Catal. Today (2014),
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Recently, bimetallic transition metal core–shell nanoparticles
(NPs) with magnetic properties have attracted great attention in
catalysis, organometallic chemistry, biotechnology and magnetic
resonance imaging. These magnetic nanoparticles were easily sep-
arated by using an external magnetic field, therefore offers the
advantage of recyclability [16]. Palladium is a versatile catalyst
and its nanoparticles have been widely used for various organic
transformations as NPs have a high surface area and excellent cat-
alytic activity. Generally the catalytic reaction take place on the
surface of palladium nanoparticles and remaining part of palladium
is inactive especially the core [17]. Therefore, it is challenging to
use entire palladium atom to decrease Pd loading without affect-
ing its catalytic activity. Herein, the concept of core shell has been
introduced to overcome these difficulties and gaining the attention
toward bi-metallic catalytic system [18]. In this core shell, palla-
dium is substituted by other transition metals like Ni, Fe and Co
which significantly improves catalytic activity due to the electronic
and structural rearrangement of the two metals [19]. Recently, Li
et al. have also explored magnetically separable Fe@Pd/C catalyst
for Suzuki reaction in aqueous medium [20], this catalytic system
offers several advantages like reuse of precious palladium metal by
using magnet, avoids possibility of metal contamination in prod-
ucts and shows high catalytic activity in aqueous reaction medium.
Hence, these results encourage us to explore Fe@Pd/C catalyst for
direct reductive amination reaction. On continuing the exploration
of new catalytic systems for direct reductive amination [21] herein,
we report core–shell Fe@Pd/C as a versatile catalyst for direct
reductive amination of carbonyl compounds with primary and sec-
ondary amines using environmental-friendly molecular hydrogen
in aqueous reaction media.
Fig. 1. XRD patterns of Fe@Pd/C.
using a TESCAN MIRA 3 model to study the morphology of prepared
catalyst.
2.3. Typical procedure for direct reductive amination of
aldehydes with amines
To a 100 mL autoclave were added aldehyde (2.5 mmol) and
amine (2.5 mmol) results the formation of imine, then catalyst
(1 mol%), and water (15 mL) were added and reactor was closed.
The reactor was then purged 3 times with nitrogen and finally reac-
tion mixture was pressurized to 30 bar of hydrogen pressure. The
reactor was heated to 80 ◦C and stirred for 8 h at 500 rpm. After
completion, the reactor was cooled to room temperature and the
remaining hydrogen gas was carefully removed. The catalyst was
separated from reaction mixture by using magnet. The product
from aqueous phase was extracted with ethyl acetate. The ethyl
acetate layer was evaporated in vacuum by rotary vapour to obtain
the product. All the products are well known in literature and were
confirmed by GC (Perkin Elmer, Clarus 400) (BP-10 GC column,
30 m × 0.32 mm ID, film thickness 0.25 mm) and GCMS (Shimadzu
GC-MS QP 2010) (Scheme 1).
2. Experimental
All the chemicals and metal precursor were procured from
reputed firm and was used without further purification. The cat-
alyst Fe@Pd/C was prepared according to the reported procedure
in the literature and was characterized with various analytical tech-
niques [20].
2.1. Catalyst preparation
In a round bottom flask containing active carbon (200 mg) was
added to nitric acid (2 M, 100 mL) and then heated at 80 ◦C for 3 h
in order to remove impurities. The Fe@Pd core–shell structured
nanoparticles were prepared with amorphous Fe with molar ratio
of Pd:Fe = 1:20. In a round bottom flask, 100 mg dried active car-
bon and 289.6 mg of FeCl3 were dispersed in 30 mL water. The
resulting suspension was kept in sonicator bath for 20 min. Then
aqueous solution of NaBH4 (676.2 mg in 90 mL water) was added
dropwise into reaction mixture to reduce Fe3+ to Fe0. The reaction
mixture was then shaken vigourously till all hydrogen gas releases
completely. The Fe/C NPs were then collected using magnet and
washed with water and ethanol to remove excess of NaBH4. In next
step Fe/C was suspended in water, to this suspension aqueous solu-
tion of PdCl2 (0.09 mmol) was added dropwise and stirred for 1 h.
The atoms in outer-layer of the Fe core were sacrificed to reduce
Pd2+ to Pd0. Finally, the catalyst was dried in a vacuum at 50 ◦C for
overnight.
3. Results and discussion
3.1. Catalyst characterization
The XRD pattern of Fe@Pd/C can be indexed with respective to 2Â
of iron oxide, iron and palladium as shown in (Fig. 1). The Peaks in X-
ray diffraction pattern of iron and iron oxide indicate the presence
of magnetic properties of the catalyst. The XRD pattern of palladium
clearly indicates the diffraction peaks occurs at 2Â values of 40.1◦,
46.6◦, and 68.0◦ can be assingned to the (1 1 1), (2 0 0) and (2 2 0)
crystal planes, respectively.
The scannig electron microscope (FEG-SEM) image of of Fe/C NPs
(Fig. 2a) showed that iron core supported on carbon which indicates
the presence of small pores of Fe/C on the surface. In high resolu-
tion FEG-SEM image also shows that activated carbon is uniform
in pores (Fig. 2b). Fig. 2c and d shows small particles of metallic
palladium sitiuated on iron core which reveals outer layer of iron
core–shell atoms was sacrificed with palladium. This was also con-
firmed by EDS mapping of Fe@Pd/C, the EDS mapping shows the
presence of Fe, Pd and carbon.
2.2. Characterization method
The X-ray diffractometer of catalyst was obtained on a Shimadzu
Lab XRD-6100 with the scan speed of 2◦/min over a range from 20◦
to 80◦ using the copper target with ꢀ = 1.54 A. Field emission gun-
In energy dispesive X-ray elemental mapping (Fig. 3) for
Fe@Pd/C shows that presence of Fe (green), Pd (red) and carbon
˚
scanning electron microscopy (FEG-SEM) images were obtained
Please cite this article in press as: N.M. Patil, B.M. Bhanage, Fe@Pd/C: An efficient magnetically separable catalyst for direct reductive
amination of carbonyl compounds using environment friendly molecular hydrogen in aqueous reaction medium, Catal. Today (2014),
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Scheme 1. Direct reductive amination of carbonyl compounds with amines in aqueous medium.
Fig. 2. FEG-SEM images of Fe/C (a and b) and Fe@Pd/C (c and d).
(blue) species coexist in the material. In EDS mapping (Fig. 3) it
was observed that the concentration of Pd (red) is lower than the Fe
(green). The heterogeneous Pd2+ reduce on iron core shell at time of
development of substitution of palladium. Thus, the obatined result
reveals the formation of core-shell structure of Fe@Pd/C.
products as compared to commercially available Pd/C, Ru/C cat-
alysts. Whereas, using Fe/C NPs as a catalyst, trace of product
formation was observed in aqueous medium (entries 1–4). Then,
we checked the effect of catalyst loading ranging from 0.25 mol%
to 1 mol%. It was observed that, there is gradual increase in cata-
lyst concentration from 0.25 mol% to 1 mol%, the increase in yield
of desired product was observed (entries 5–7).
Next, we studied the effect of hydrogen pressure; it was
observed that increasing hydrogen pressure from 10 to 30 bar,
increases the yield of desired product (entries 8–10). Further we
have also studied the effect of temperature and time. The reac-
tions were carried out at different temperature ranging from 40 ◦C
to 80 ◦C, it was observed that 80 ◦C is the optimum tempera-
ture to get maximum yield (entries 11–13). Time study reveals
that 8 h are sufficient to give excellent yield of desired product
(entries 14–16). Therefore, the final optimized reaction parameters
3.2. Study of optimization parameters for direct reductive
amination in aqueous media
We checked the activity of Fe@Pd/C as a choice of catalyst
for direct reductive amination of benzaldehyde with aniline in
aqueous media as a model reaction. Various reaction parameters
such as catalyst screening, catalyst loading, effect of hydro-
gen pressure, reaction temperature and time were investigated
and obtained results are summerized in (Table 1). Among the
screened calalysts, Fe@Pd/C furnish excellent yield of the desired
Please cite this article in press as: N.M. Patil, B.M. Bhanage, Fe@Pd/C: An efficient magnetically separable catalyst for direct reductive
amination of carbonyl compounds using environment friendly molecular hydrogen in aqueous reaction medium, Catal. Today (2014),
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Table 1
Effect of reaction parameters on direct reductive amination in aqueous medium.a
Entry
Catalyst
Catalyst loading (mol%)
H2(bar)
Temperature (◦C)
Time (h)
Yield (%)b
Catalyst screening
1
2
3
4
Pd/C
Ru/C
Fe/C NPs
Fe@Pd/C
1
1
1
1
30
30
30
30
80
80
80
80
8
8
8
8
Trace
22
Trace
96
Catalyst loading
5
6
7
Fe@Pd/C
Fe@Pd/C
Fe@Pd/C
0.25
0.5
1
30
30
30
80
80
80
8
8
8
34
62
96
Effect of hydrogen pressure (bar)
8
9
10
Fe@Pd/C
Fe@Pd/C
Fe@Pd/C
1
1
1
10
20
30
80
80
80
8
8
8
32
59
96
Effect of temperature
11
12
13
Fe@Pd/C
Fe@Pd/C
Fe@Pd/C
1
1
1
30
30
30
40
60
80
8
8
8
18
54
96
Effect of time
14
15
16
Fe@Pd/C
Fe@Pd/C
Fe@Pd/C
1
1
1
30
30
30
80
80
80
4
6
8
36
78
96
a
Reaction conditions: benzaldehyde (2.5 mmol), aniline (2.5 mmol), water (15 mL), catalyst, hydrogen gas.
Yields based on GC analysis.
b
Naphthalen-2-amine also reacted smoothly with benzaldehyde
giving good yield of desire product (entry 10). Next, we treated
various benzyl amine derivatives and pharmaceutically important
1-phenylethanamine with benzaldehyde furnishing excellent yield
of desired product (entries 11–13).
Further we tested our protocol for cyclic and aliphatic amines
such as cyclohexyl amine, cyclopropyl amine and butyl amine
reacts with benzaldehyde giving desired secondary amines in
excellent yields (entries 14–16). The secondary amines such as
morpholine and n-methyl benzyl amines were screened with
benzaldehyde resulting in the formation of tertiary amines in
good yields (entries 17, 18). Noteworthy to mention that cyclo-
hexanone was also reacted with primary and secondary amines
such as aniline and morpholine giving excellent yields of the
corresponding product (entry 19, 20). Moreover, we have also
screened heterocyclic amine such as 2-amino pyridine providing
good yield of corresponding secondary amine (entry 21). Whereas,
employing cinnamaldehyde with aniline furnished corresponding
secondary amine with complete reduction of olefinic bond (entry
22). Acetaldehyde also provide appreciable yield with aniline giving
desired product (entry 23).
Thus, the present methodology was found to be broadly appli-
cable for direct reductive amination of various structurally and
electronically diverse aldehydes and ketone with primary, sec-
ondary and heterocyclic amines providing good to excellent yield
of corresponding secondary and tertiary amines.
Fig. 3. Energy dispesive X-ray elemental mapping of Fe@Pd/C. (For interpretation
of the references to color in the text of this figure citation, the reader is referred to
the web version of this article.)
were benzaldehyde (2.5 mmol), aniline (2.5 mmol), water (15 mL),
Fe@Pd/C catalyst (1 mol%), H2 pressure (30 bar), temperaure (80 ◦C)
and time (8 h).
3.3. Recyclability study of Fe@Pd/C catalyst for direct reductive
amination in water
With these optimized reaction conditions; we explored the
developed protocol for the synthesis of wide variety of substituted
amines by direct reductive amination in aqueous media (Table 2).
The reaction of benzaldehyde with aniline gave corresponding
secondary amine in excellent yield (entry 1). It was observed
that both electron donating or withdrawing groups on aldehyde
were well tolerated under optimized condition furnishing good
to excellent yield of secondary amines, (entries 2–5). We found
that various substituted aromatic amines react smoothly with
benzaldehyde giving desired product in good yield (entries 6–9).
To make developed protocol more economical, recyclability of
the Fe@Pd/C catalyst was studied for direct reductive amination
in water. After complition of reaction, the reaction mixture was
cooled to room temperature, then Fe@Pd/C catalyst was seperated
by using magnet. The catalyst was then washed with water fol-
lowed by ethanol to remove traces of organic compounds, dried in
vaccum and used for further reaction (Fig. 4).
The catalyst could be reused up to four consecutive cycle with-
out affecting its catalyitc acttivity for direct reductive amination
(Fig. 5).
Please cite this article in press as: N.M. Patil, B.M. Bhanage, Fe@Pd/C: An efficient magnetically separable catalyst for direct reductive
amination of carbonyl compounds using environment friendly molecular hydrogen in aqueous reaction medium, Catal. Today (2014),
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Table 2
Direct reductive amination of carbonyl compound with primary and secondary amines using Fe@Pd/C as catalyst in aqueous medium.a
Entry
Aldehyde
Amine
Product
Yield (%)b
1
96
2
3
4
5
92
84
85
78
6
7
87
91
8
81
9
72
79
10
Please cite this article in press as: N.M. Patil, B.M. Bhanage, Fe@Pd/C: An efficient magnetically separable catalyst for direct reductive
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Table 2 (Continued)
Entry
Aldehyde
Amine
Product
Yield (%)b
11
95
12
89
13
14
86
92
15
16
17
18
19
78
89
90
81
94
20
21
92
80
Please cite this article in press as: N.M. Patil, B.M. Bhanage, Fe@Pd/C: An efficient magnetically separable catalyst for direct reductive
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Table 2 (Continued)
Entry
Aldehyde
Amine
Product
Yield (%)b
22
91
23
82
a
Reaction conditions: aldehyde (2.5 mmol), amine (2.5 mmol), Fe@Pd/C (1 mol%), Water (15 mL), H2 press. (30 bar), temp. (80 ◦C), time (8 h).
Yield based on GC analysis.
b
Fig. 4. (a) Catalyst suspension before applying magnet, (b) after applying magnet.
4. Conclusion
100
80
60
40
20
0
93
91
88
86
In conclusion, we have developed simple, efficient, greener
protocol for the synthesis of higher substituted amines by direct
reductive amination of carbonyl compounds with various amines
in aqueous media. The prepared Fe@Pd/C catalyst is highly sta-
ble and shows excellent activity in water for direct reductive
amination reaction. The catalyst (Fe@Pd/C) was easily separated
from reaction mixture by magnet and could be reused up to
four successive cycles without losing its catalytic activity. The
developed protocol is more advantageous due to use of water as
environmentally benign solvent and effective catalyst recyclabil-
ity.
1
2
3
Recycle run
4
Acknowledgments
Fig. 5. Catalyst recyclability study for direct reductive amination in aqueous media.a
aReaction condition: benzaldehyde (2.5 mmol), aniline (2.5 mmol), water (15 mL),
Fe@Pd/C (1 mol%), H2 pressure (30 bar), temperaure (80 ◦C), time (8 h).
The authors (N.M. Patil) are grateful to UGC-SAP (Univer-
sity Grant Commission, India) for research fellowship and also
acknowledge for Nano Mission Project (No.SR/NM/NS-1097/2011)
supported by Department of Science and Technology, Govt. of India
for providing financial support.
Please cite this article in press as: N.M. Patil, B.M. Bhanage, Fe@Pd/C: An efficient magnetically separable catalyst for direct reductive
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Please cite this article in press as: N.M. Patil, B.M. Bhanage, Fe@Pd/C: An efficient magnetically separable catalyst for direct reductive
amination of carbonyl compounds using environment friendly molecular hydrogen in aqueous reaction medium, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.07.057