Silver nanoparticles supported on alumina-a highly efficient and selective nanocatalyst for imine reduction

Article abstract of DOI:10.1039/c3dt52499j

Silver nanoparticles supported on alumina were prepared and tested in the catalytic reduction of various imines to primary and secondary amines and were shown to be exceptionally active and chemoselective. Furthermore, the catalytic activity of the prepared nanocatalyst was also tested in the synthesis of secondary amines from primary amines in a tandem reaction protocol (oxidation-imination-reduction) using air and molecular hydrogen as oxidizing and reducing agents, respectively. The reported synthesis is performed under mild reaction conditions, which complies with the demands of modern organic synthesis. Due to the mild reaction conditions and high conversion as well as high selectivity, we consider that the utilization of silver nanoparticles supported on alumina represents an attractive and environmentally friendly alternative to the current synthesis of N-alkyl amines.

Full text of DOI:10.1039/c3dt52499j

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Silver nanoparticles supported on alumina–
a highly efficient and selective nanocatalyst
for imine reduction†
Cite this: Dalton Trans., 2014, 43,
4255
Received 11th September 2013,
Accepted 27th November 2013
Raju Poreddy, Eduardo J. García-Suárez, Anders Riisager and Søren Kegnæs*
DOI: 10.1039/c3dt52499j
www.rsc.org/dalton
1
4
Silver nanoparticles supported on alumina were prepared and hydroamination of alkynes and other methods. Within these
tested in the catalytic reduction of various imines to primary and methods, the homogeneous transition metal-catalyzed oxidative
secondary amines and were shown to be exceptionally active and dehydrogenation of alcohols to carbonyl compounds that can
chemoselective. Furthermore, the catalytic activity of the prepared form an imine by condensation with an amine is an attractive
15–19
nanocatalyst was also tested in the synthesis of secondary amines synthetic approach due to high atom efficiency.
Neverthe-
from primary amines in a tandem reaction protocol (oxidation– less, these homogeneous catalysts suffer from drawbacks such
imination–reduction) using air and molecular hydrogen as oxidiz- as difficulty in recovery and reuse of expensive metals, necessity
ing and reducing agents, respectively. The reported synthesis is of special handling of metal complexes, and the required use
performed under mild reaction conditions, which complies with of co-catalysts such as a base promoter and a stabilizing ligand.
the demands of modern organic synthesis. Due to the mild reac- Hence, the application of efficient, easily recoverable hetero-
tion conditions and high conversion as well as high selectivity, we geneous catalysts which are relatively inexpensive, would clearly
consider that the utilization of silver nanoparticles supported on be advantageous. Taking up these challenges a few hetero-
alumina represents an attractive and environmentally friendly geneous catalysts based on, for instance, molecular sieves,
alternative to the current synthesis of N-alkyl amines.
amorphous silica–alumina, gallosilicates, boron–aluminum,
2
0
and iron phosphate have been reported in the 1990s. More
recently, Shimizu and co-workers reported that besides the cata-
lytic activity of silver clusters, alumina with acid–base surface
sites has a cooperative effect on oxidation of alcohols under
Introduction
In recent years amines and imines have received much attention
due to their wide usability as synthetic intermediates in the pro-
duction of pharmaceuticals, agrochemicals, fine chemicals, bio-
2
1
oxidant-free conditions.
Very recently, Jaenicke and co-
workers reported that alumina entrapped Ag nanoparticles can
1–6
catalyze the N-alkylation of amines with alcohols using the bor-
active compounds, and dye chemicals. In particular, N-alkyl
amines are important pharmacophores in numerous bio-active
compounds due to their physiological activities, and are also
2
2
rowing-hydrogen methodology. However, the essential use of a
co-catalyst or a base promoter (e.g. Cs CO , K PO ) to make the
2
3
3
4
7
dehydrogenation feasible results in low atom efficiency, high
metal waste/by-product production and increased difficulty in
purification. Accordingly, development of new highly efficient
low-cost heterogeneous catalysts that can circumvent these
drawbacks – making the whole process greener – is required.
In this work we report that alumina-supported silver nano-
particles can efficiently catalyze the reduction of imines to
their corresponding amines using hydrogen as the reducing
agent (Scheme 1). Furthermore, the same catalyst was able to
widely used in the area of drug discovery. N-Alkyl amines are
prepared by the reaction of primary amines with organic
8
9
halides, reductive alkylation or hydroamination and hydro-
1
0
aminomethylation reactions. Despite the widespread interest,
these known methods for the formation of secondary amines
are often associated with problems such as high cost of the
starting materials, harsh reaction conditions, poor yields, and/
or low chemical selectivity, along with inevitable environmental
1
1
issues. On the other hand, imines are being synthesized by
acid–base promoted condensation of primary amines and car-
1
2
13
bonyl compounds, oxidative coupling of secondary amines,
Centre for Catalysis and Sustainable Chemistry, Department of Chemistry,
Building 207, Technical University of Denmark, 2800 Kgs, Lyngby, Denmark.
E-mail: skk@kemi.dtu.dk; Tel: (+45) 45252402
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c3dt52499j
Scheme 1 Silver catalyzed chemo-selective reduction of imines to
N-alkylated amines.
1
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Scheme 2 Silver catalyzed two-step synthesis of secondary amines from primary amines.
a
catalyze the two-step synthesis of N-alkylated amines Table 1 Effect of temperature on the reduction of N-benzylidineaniline
without adding any co-catalyst at low temperatures (100 °C)
Temperature
[°C]
Conversion
[%]
Selectivity
[%]
Yield
[%]
(Scheme 2).
Entry
2
An Ag/Al O (S
of Al O = 256 m g−1) catalyst was pre-
2
3
BET
2 3
1
2
3
80
100
120
120
35
90
99
0
95
96
96
0
33
86
95
0
pared by incipient wetness impregnation of the solid support
with an aqueous solution of silver nitrate resulting in 5 wt%
content of silver on the support. The prepared nanocatalyst
was fully characterized by means of X-ray powder diffraction
b
4
a
Reaction conditions: 2 mmol imine, 100 mg of catalyst, H (10 bar,
0% H /N mixture), 6 mL toluene, 24 h, GC yields. Control
2 2
2
(XRPD; ESI Fig. S1†), scanning electron microscopy (SEM; ESI
b
1
2 3
Fig. S1†), energy dispersive X-ray (EDX; ESI Fig. S2†) spec- experiment with only the support (Al O ).
troscopy and transmission electron microscopy (TEM; ESI
Fig. S1†) and Temperature Programmed Desorption (TPD-NH ,
3
Fig. S4†) and Temperature Programmed Reduction (TPR-H2,
Fig. S5†).
Fig. 1 shows a representative TEM image of the silver nano-
particles supported on alumina. Well distributed silver nano-
particles throughout the support material were obtained with
an average diameter in the range of 6.3 ± 1.4 nm.
Firstly, the catalytic activity of the prepared nanocatalyst
was tested in the reduction of N-benzylidineaniline using
2 2
formier gas (10 bar, 10% H /N mixture) as a reducing agent at
different temperatures in order to optimize the reaction temp-
erature. As shown in Table 1, the conversion of imine to the
corresponding amine increased from 35 to 99% when the
temperature was increased from 80 to 120 °C in a 24 hour reac-
tion. Since there was only a minor difference in catalyst per-
formance between 100 and 120 °C the lowest reaction
temperature is always preferred, and it was selected for further
experiments. Furthermore, a control experiment using only the
support in the role of a catalyst to discard its influence in the
catalytic activity was performed in the absence of silver. As
expected no conversion was achieved in our reaction con-
ditions (entry 4, Table 1).
Fig. 2 Effect of hydrogen pressure on the activity of the catalyst in
N-benzylidineaniline reduction after 24 hours at 100 °C.
The effect of hydrogen pressure in the N-benzylidineaniline instead of formier gas (10% H /N mixture); the results are
2
2
reduction was subsequently studied using pure hydrogen shown in Fig. 2. The conversion increased from 49 to 99% by
increasing the hydrogen pressure from 1 to 4 bars, whereas no
significant changes were observed when the pressure was
increased further to 6 bars. Thus 4 bars were considered to be
an appropriate pressure for the reaction.
The selectivity towards N-benzylaniline increased as well
with hydrogen pressure, and a maximum selectivity and yield
of 97% were reached with 4 or 6 bars of hydrogen. At lower
pressures of hydrogen (1 or 2 bars) the reduction of N-benzyli-
dineaniline was significantly slower, thus allowing the gen-
eration of side products formed by, e.g. retro-condensation to
benzaldehyde and aniline followed by the reduction of benz-
Fig. 1 Representative TEM image of the 5 wt% silver–alumina nano-
aldehyde to benzyl alcohol (Scheme 3; ESI Fig. S3†), thus
catalyst (magnified image inset) and corresponding histogram with
mean particle diameter and standard deviation.
decreasing the selectivity.
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Scheme 3 Retro-condensation of imine to its amine and transient formation of aldehyde which is reduced to its corresponding alcohol.
Table 2 Reduction of imines to N-alkyl anilines by hydrogen
a
Conversion
[%]
Selectivity
[%]
Yield
[%]
Entry
1
Product
100
96
96
b
2
100
100
70
95
98
99
95
98
70
3
4
Fig. 3 Conversion of N-benzylidineaniline with time over silver nano-
particles supported on alumina.
5
6
93
90
97
99
90
90
After optimization of two important parameters in the cata-
lytic reaction such as reaction temperature (100 °C) and hydro-
gen pressure (4 bars) the objective was shifted to study the
reaction progress as a function of time. Even though 96% of
the N-benzylidineaniline is converted to its reduction product
in about 16 h (Fig. 3), the reaction was run for 24 h to get
sheer conversion of the substrate with 96% selectivity.
7
8
100
100
96
97
96
97
Once the reaction parameters were optimized, the prepared
nanocatalyst was tested with different substrates in order to
explore the expediency. Accordingly, structurally diverse aro- (6 mL), 100 °C, 24 h, GC yields. Recycled catalyst.
a
Imine (2 mmol), Ag/Al
O
2 3
; 5 wt% Ag (100 mg), H
2
(4 bar), toluene
b
matic (activated) as well as one sample of aliphatic (non-
activated) imines were examined towards the formation of
the corresponding amines. The results are shown in Table 2.
Retro-condensation of the imines to their corresponding
All the examined substrates were converted to their corres- amines and aldehydes with subsequent reduction to the alco-
ponding amines with good to excellent conversion and with a hols was also here noticed by GC analysis (ESI, Fig. S3†). This
remarkably high selectivity (>99%), thus demonstrating the minor side reaction seemed unavoidable and could possibly
versatility of the prepared nanocatalyst in the reduction of be facilitated by trace amounts of moisture present within the
both the activated and non-activated imines to their corres- particles of the catalyst.
ponding amines with no substantial difference in performance
after 24 h of reaction. However, the conversion of (E)-N,1-bis- recovery from the reaction medium and reuse. The prepared
4-methoxyphenyl)methanimine (entry 4) was significantly lower nanocatalyst could easily be separated from the reaction
The important features of heterogeneous catalysts are their
(
under the given reaction conditions. The decrease in conver- mixture by simple filtration and it was reused in the hydrogen-
sion or slow rate of reaction may be attributed to the steric ation reaction of N-benzylidineaniline (Table 2, entry 2). Upon
hindrance, or electronic effects caused by substituents on the second use, the catalytic performance remained unchanged
substrates (entries 4, 5, and 6).
with yields of 96–95%, showing the apparent stability of the
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Scheme 4 Synthesis of N-benzylaniline via oxidation/condensation/reduction sequence using supported silver nanoparticles as a heterogeneous
catalyst.
prepared catalyst in our reaction conditions. Furthermore, reached. Since water is the only formed by-product and no extra
in order to prove that there was no leaching occurring during the added bases are required under our reaction conditions, the
reaction, the filtered solution was tested in the catalytic reac- prepared nanocatalyst is supposed to be an attractive alternative
tion without exhibiting any activity under the same reaction green route to the previously reported method in the synthesis
conditions. This clearly supported the heterogeneity of the of secondary amines. Further investigations are on going in
reaction. In addition, the absence of silver leaching was also order to improve the recyclability of the catalytic system.
confirmed by ICP-AES analysis of the filtered solution. These
findings clearly ruled out the contribution of leached silver in
the imine reduction reaction.
Acknowledgements
The highly selective reduction of imines to their corres-
We gratefully acknowledge the support of the Danish Council
ponding amines by the silver nanoparticles enticed us to
for Independent Research, grant no. 10-093717 and grant no.
further examine the synthesis of secondary amines via a two-
12-127580.
step (1) oxidative dehydrogenation-condensation and (2)
reduction reaction (Scheme 4).
The overall reaction yield was found to be 59% with a
selectivity of 99% towards the desired product. In the first
step, involving oxidation of the alcohol to the aldehyde and
further condensation with the aniline, the conversion and
selectivity were higher than 98% and 99%, respectively. On the
other hand, only a modest 60% conversion with 99% selecti-
vity was attained in the second step. We speculate that the low
conversion reached in the reduction step could be explained in
terms of irreversible chemisorption of the hydride ion on the
surface active sites of the silver clusters, as well as due to the
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