Zwitterionic-surfactant-stabilized palladium nanoparticles as catalysts in the hydrogen transfer reductive amination of benzaldehydes

Article abstract of DOI:10.1021/jo5000362

Palladium nanoparticles (NPs) stabilized by a zwitterionic surfactant are revealed here to be good catalysts for the reductive amination of benzaldehydes using formate salts as hydrogen donors in aqueous isopropanol. In terms of environmental impact and e

Full text of DOI:10.1021/jo5000362

Article
pubs.acs.org/joc
Zwitterionic-Surfactant-Stabilized Palladium Nanoparticles as
Catalysts in the Hydrogen Transfer Reductive Amination of
Benzaldehydes
Emma E. Drinkel, Roberta R. Campedelli, Alex M. Manfredi, Haidi D. Fiedler, and Faruk Nome*
́
Departamento de Química, Universidade Federal de Santa Catarina, INCT-Catalysis, Florianopolis, Santa Catarina 88040-900, Brazil
S
* Supporting Information
ABSTRACT: Palladium nanoparticles (NPs) stabilized by a zwitterionic surfactant
are revealed here to be good catalysts for the reductive amination of benzaldehydes
using formate salts as hydrogen donors in aqueous isopropanol. In terms of
environmental impact and economy, metallic NPs offer several advantages over
homogeneous and traditional heterogeneous catalysts. NPs usually display greater
activity due to the increased metal surface area and sometimes exhibit enhanced
selectivity. Thus, it is possible to use very low loadings of expensive metal. The
methodology eliminates the use of a hydrogen gas atmosphere or toxic or expensive
reagents. A range of aromatic aldehydes were converted to benzylamines when reacted
with primary and secondary amines in the presence of the Pd NPs, which also displayed good activity when supported on
alumina. In every case, the Pd NPs could be easily recovered and reused up to three more times, and at the end of the process,
the product was metal-free.
with metal catalysts and nontoxic hydrogen transfer agents has
previously been reported to bring about reductive amination.8
With increasing concern for the environment, chemists are
constantly looking for ways to reduce the impact of chemical
processes. One way in which this can be achieved, in addition
to controlling the side products produced, is by eliminating the
use of organic solvents and instead carrying out reactions in
environmentally benign solvents such as water. Another
approach is to reduce the amount of metal required in catalytic
processes, cutting the quantity of heavy metal waste at the end
of reactions. Metallic nanoparticles have a much greater surface
area of metal available compared with other heterogeneous
catalysts and thus are potentially more active and require less
metal to be used in catalysis. As with other heterogeneous
catalysts, they also offer the advantage of being usable in more
than one catalytic cycle. The use of palladium nanoparticles in
reductive amination is known, but to the best of our knowledge
only under an atmosphere of hydrogen.9 Transfer hydro-
genation reductive amination has been carried out using Pd/
C10 and nickel nanoparticles8b,c as the catalytic species, but in
neither case was the catalyst reused for further cycles.
INTRODUCTION
■
The formation of C−N bonds is an important transformation,
as amines are often found in naturally occurring molecules and
have a wide range of application in pharmaceuticals and
agrochemicals. Other methods of synthesizing amines include
the alkylation of ammonia, reduction of other nitrogen-
containing functional groups, and Buchwald−Hartwig cou-
pling.1 Reductive amination is a particularly useful method.2 It
is very adaptable because of the availability of a great range of
amines and aldehydes, usually requires only mild conditions,
and can be carried out in one pot without the need to isolate
the intermediate imine (Figure 1). Previous reports on this
Figure 1. Reductive amination.
reaction have focused on the use of metal hydrides,3
cyanoborohydride4 and other borohydride compounds,5 and
organosilanes6 as the initiating species. These reagents are often
used in stoichiometric quantities, thus increasing the yields of
sometimes toxic side products and making the reaction less
environmentally sound.
Other reports have demonstrated the use of metallic catalysts
in combination with a hydrogen atmosphere.7 The use of a
hydrogen atmosphere also poses safety risks due to the use of
high pressures of a flammable gas. Therefore, the use of
hydrogen transfer agents such as formic acid or isopropanol is
preferred. The side products (carbon dioxide or acetone,
respectively) are nontoxic and easily removed, and high
pressures of gas are not required. Transfer hydrogenation
RESULTS AND DISCUSSION
■
Palladium nanoparticles stabilized by the zwitterionic surfactant
molecules ImS3-12 and ImS3-1411 have previously been
reported by our group as excellent catalysts for hydrogenation
reactions12 (Figure 2). Palladium nanoparticles stabilized by
ImS3-14 were made by the same method as previously
Received: January 8, 2014
Published: February 19, 2014
© 2014 American Chemical Society
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acid to sodium formate was found to be optimal (entry 5). The
reaction was not tested in the absence of surfactant, since the
palladium precipitated as a black solid.
Increasing the temperature to 70 °C increased the yield
without causing decomposition of the nanoparticles (Table 1,
entry 6). A reaction temperature of 90 °C, despite giving a
good yield, caused decomposition of the nanoparticles, which
could no longer be used in further catalytic cycles (entry 8).
Comparison of entries 6 and 7 shows that the addition of 200
μL of isopropanol increased the yield by a further 11%, possibly
by helping with phase transfer. Finally, the same palladium
nanoparticles supported on alumina (entry 9) were less active
than unsupported ones for this reaction.
The optimized conditions were then used to investigate the
scope of the reaction catalyzed by the ImS3-14-stabilized
nanoparticles (Table 2). Benzaldehydes with electron-donating
groups gave considerably better yields than those with electron-
withdrawing groups, as demonstrated by the poor yields for 4-
nitrobenzaldehyde and 2-chlorobenzaldehyde (0 and 35%,
respectively; entries 3 and 5). Furthermore, comparison of
entries 2 and 7 reveals that the position of the electron-
donating group is also important. The ortho-substituted
anisaldehyde resulted in a yield 14% lower than the para-
substituted analogue. It is possible that the mechanism of the
reaction passes through a positively charged intermediate that is
stabilized by electron donation from the aryl groups.
Figure 2. Zwitterionic surfactant molecules.
described for ImS3-12-stabilized nanoparticles.12a No presence
of Pd black was detected by eye at the end of the synthesis, and
the images obtained by transmission electron microscopy
(TEM) confirmed the formation of palladium nanoparticles
(Figure 3). The TEM images showed the nanoparticles to be
well-dispersed with diameters of 3.4 1.2 nm.
At the end of the reaction there was no visible evidence of
decomposition of the palladium nanoparticles, and the aqueous
solution in which they were present was therefore separated
from the organic phase in order to test their recyclability. As is
clear from Table 3, the Pd/ImS3-14 nanoparticles could be
successfully used four times in the reaction shown in Table 2,
entry 2 without considerable loss in activity. Even after the
fourth use, no Pd black was observed in the reaction tube. This
shows that the catalyst was stable, with no or minimal leaching
of Pd into the product (see below). The first and second uses of
the nanoparticles gave about the same amount of product.
However, in further uses, they gradually became less active.
This is probably because the nanoparticles aggregated with
continued use, therefore decreasing the surface area and
reducing their efficiency.
Following the results with aniline, other amines were tested
using the conditions described previously with Pd/ImS3-14.
However, little or no product was usually obtained. Moreover,
decomposition of the catalyst was usually observed at the end
of the reaction time. It was hoped that Pd/ImS3-14
nanoparticles supported on alumina, which have already been
reported as excellent catalysts in the selective hydrogenation of
biodiesel,12b would be more stable and able to overcome these
problems. Initial tests using the same conditions as in entry 9 of
Table 1 resulted in poor yields of product. However changing
the base to NEt3, eliminating the excess of formic acid, and
using isopropanol as the solvent (the conditions shown in
Scheme 2) drastically improved the outcome. With these new
Figure 3. TEM image of ImS3-14-stabilized palladium nanoparticles.
To extend their utility, the ImS3-14-stabilized palladium
nanoparticles were employed in the reductive amination of
benzaldehydes (Scheme 1). In order to eliminate the use of
flammable hydrogen gas as the reducing agent, the reaction was
attempted using formate salts as the hydrogen donors. Table 1
shows the preliminary results obtained when the palladium
nanoparticles were tested in the reaction shown in Scheme 1.
Notably, the palladium loading was very low, with just 0.54
μmol used to catalyze the reactions of 1 mmol of reactants. In
initial efforts, formic acid was used with an equal concentration
of sodium hydroxide as the base (entry 1), but only traces of
product were seen by GC−MS at the end of the reaction. It was
subsequently found that using a large excess of formic acid
relative to the base was beneficial to the reaction (entries 2 and
3). However, as entry 4 shows, the total absence of base
resulted in no product. Interestingly, the nature of the base did
not affect the yield greatly, and sodium formate was
subsequently used as the preferred base. A 1:1 ratio of formic
Scheme 1. Palladium-Nanoparticle-Catalyzed Reductive Amination of Benzaldehyde with Aniline at 70°C
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Table 1. Preliminary Tests of ImS3-14-Stabilized Palladium Nanoparticles in the Reductive Amination Reaction Shown in
Scheme 1
a
e
Entry
Catalyst
Base
T (°C)
Yield (%)
TON
b
1
2
3
4
5
6
Pd/ImS3-14
Pd/ImS3-14
Pd/ImS3-14
Pd/ImS3-14
Pd/ImS3-14
Pd/ImS3-14
Pd/ImS3-14
Pd/ImS3-14
Pd/ImS3-14@Al2O3
Pd/C
NaOH (5 equiv)
60
60
60
60
60
70
70
90
70
70
trace
37
35
−
54
66
77
93
52
39
−
685
NaOH (0.1 equiv)
(NaHCO3 0.1 equiv)
−
NaCO2H (5 equiv)
NaCO2H (5 equiv)
NaCO2H (5 equiv)
NaCO2H (5 equiv)
NaCO2H (5 equiv)
NaCO2H (5 equiv)
648
−
1000
1222
1426
1722
963
c
7
cd
,
8
9
c
10
722
a
Reaction conditions: 1 mmol of benzaldehyde, 2 mmol of aniline, 0.54 μmol of Pd, 5 mmol of formic acid, 2 mL of H2O, 6 h. Isolated yields by
b
c
d
column chromatography. Amounts are equivalents with respect to benzaldehyde. 200 μL of iPrOH was added. Pd black was observed at the end
e
of the reaction. Turnover number (mol of product/mol of Pd).
Table 2. Reductive Amination with Pd/ImS3-14
Reaction conditions: 1 mmol of aldehyde, 2 mmol of aniline, 0.54 μmol of Pd, 5 mmol of formic acid, 5 mmol of sodium formate, 200 μL of iPrOH,
a
b
2 mL of H2O, 6 h, 70 °C. Isolated yields by column chromatography. Turnover number (mol of product/mol of Pd). Values in parentheses are
corrected turnover numbers (by means of the magic-number approach) considering only the exposed atoms on the nanoparticle surface (35%).
with morpholine was still very good (entry 3). In these
reactions, the loading of palladium required to achieve these
results was again very low (0.56 μmol), and no Pd black was
observed at the end of the reaction.
Table 3. Reuse of Pd/ImS3-14 in the Reductive Amination
of p-Tolualdehyde with Aniline
Use
1
2
3
4
a
Yield (%)
88
87
75
63
In the case of Pd/ImS3-14@Al2O3, it was seen that the
catalyst gave better results with respect to recyclability when the
temperature of the reaction was lowered to 50 °C. Table 5
shows the yields obtained over four uses of Pd/ImS3-14@
Al2O3 in the reductive amination of p-tolualdehyde with
piperidine. As can be seen, the first three uses of Pd/ImS3-14@
Al2O3 resulted in essentially the same amount of product when
the reaction was carried out at 50 °C. The fourth cycle led to a
decrease in yield by about 10%, but no decomposition of the
Reaction conditions for reuse: 1 mmol of 4-tolualdehyde, 1 mmol of
a
aniline, 1 mmol of formic acid, 200 μL of iPrOH, 6 h, 70 °C. Isolated
yields by column chromatography.
conditions, Pd/ImS3-14@Al2O3 could successfully catalyze the
reductive amination of p-tolualdehyde with cyclic secondary
amines (Table 4). Excellent yields of over 90% were obtained
with piperazine and piperidine (entries 1 and 2). Although
slightly lower, the yield of product recovered from the reaction
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Scheme 2. Reductive Amination of p-Tolualdehyde with Cyclic Amines Catalyzed by Supported Palladium Nanoparticles
Table 4. Reductive Amination with Pd/ImS3-14@Al2O3
Reaction conditions: 1 mmol of p-tolualdehyde, 1 mmol of amine, 5 mmol of triethylamine, 5 mmol of formic acid, 0.56 μmol of Pd, 500 μL of
a
b
iPrOH, 6 h, 60 °C. Isolated yields by column chromatography. Turnover numbers (mol of product/mol of Pd). Values in parentheses are the
corrected turnover numbers (by means of the magic-number approach) considering only the exposed atoms on the nanoparticle surface (35%).
Table 5. Reuse of Pd/ImS3-14@Al2O3 in the Reductive
Amination of p-Tolualdehyde with Piperidine
Use
1
2
3
4
a
Yield (%)
87
86
87
77
Reaction conditions for reuse: 1 mmol of p-tolualdehyde, 1 mmol of
amine, 1 mmol of triethylamine, 1 mmol of formic acid, 200 μL of
a
iPrOH, 6 h, 50 °C. Isolated yields by column chromatography.
nanoparticles was observed postreaction. As suggested above,
the nanoparticles probably lose activity as they begin to
aggregate over more uses.
Figure 4. Graph showing the progress of the reductive amination
■
After the good outcome of the recyclability tests, another
experiment was conducted to ensure that Pd did not leach into
the organic phase during the course of the reaction. The
reaction between benzaldehyde and aniline was set up as usual
using the optimized conditions from Table 1 in two separate
reaction tubes. After 1 h, the aqueous phase containing the
nanoparticles was removed from one of the reactions and
replaced by an equal volume of an aqueous solution containing
all of the reagents except the nanoparticles. Both reactions were
monitored by GC−MS, and the results are shown in Figure 4.
As can be seen, when the catalyst was removed after 1 h, the
reaction did not continue. This proves that the nanoparticles do
not leach into the product, confirming that the ImS3-14-
stabilized nanoparticles can be quantitatively recovered and
reused.
Finally, the determination of Pd was carried out by direct
analysis of the organic phase using energy-dispersive X-ray
fluorescence (EDXRF), following the Pd Kα1 signal. With this
method, the detection limit (DL) for Pd is 0.6 μg/mL.13 The
analysis of the organic phase (products) showed that Pd could
not be detected, indicating that the Pd concentration was lower
than the detection limit of the equipment and that the final
product was metal-free.
reactions of benzaldehyde with aniline: ( ) with Pd nanoparticles
○
throughout; ( ) with Pd nanoparticles removed after 1 h of reaction.
CONCLUSIONS
■
We have shown that zwitterionic-surfactant-stabilized palladium
nanoparticles, which have previously proven to be excellent
catalysts in hydrogenation reactions, are also good catalysts for
transfer hydrogenation reductive amination. Yields of up to
88% could be achieved using the unsupported nanoparticles in
water, and yields of up to 96% were obtained using the
supported nanoparticles in the reductive amination of p-
tolualdehyde with secondary amines. In both cases, the
nanoparticles could be used up to four times without critical
loss of activity, and no observable precipitation of Pd black was
seen. Notably, the loading of Pd required to catalyze the
reaction is extremely low compared with standard homoge-
neous and heterogeneous catalysts: only 0.54−0.56 μmol was
required to obtain the results shown.
The use of nanoparticles such as these to catalyze reactions is
of great interest for green chemistry, not only because they can
be employed in exceptionally low loadings but also because
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mmol), 200 μL of isopropanol, 101 μL of benzaldehyde, and 91 μL of
aniline (1 equiv, 1 mmol), and the reaction tube was again heated to
70 °C and left for 6 h. After this time, the product was isolated as
described before.
they are in a different phase than the product and thus can be
easily separated for reuse without contamination of the product.
Furthermore, reactions can be employed using more environ-
mentally sound solvents such as water or alcohol. Work
continues in our laboratory to further extend the utility of these
zwitterionic-surfactant-stabilized palladium nanoparticles in
other green catalytic processes.
General Procedure for the Reductive Amination of p-
Tolualdehyde with Other Amines. A reaction tube was charged
with 695 μL of NEt3 (5 equiv, 5 mmol), and 188 μL of formic acid (5
equiv, 5 mmol) was slowly added with stirring. Subsequently, 500 μL
of isopropanol, 118 μL of p-tolualdehyde (1 equiv, 1 mmol), and 1
mmol of the appropriate amine were added. Finally, the Pd/ImS3-
14@Al2O3 nanoparticles (10 mg, 0.56 μmol of Pd) were added, and
the reaction tube was then sealed with a rubber septum and heated to
the stated temperature for 6 h. After this time, the reaction tube was
allowed to cool, and 1 mL of petroleum ether was added to the
mixture to form two phases, with the products in the organic phase
above and the nanoparticles in the lower phase. The organic phase was
removed, and the addition of petroleum ether was repeated twice more
to remove all of the product and remaining starting material. The
product could be isolated by column chromatography using 4% ethyl
acetate in petroleum ether as the eluent.
EXPERIMENTAL SECTION
■
General. Sodium hydride, imidazole, 1-bromododecane, isopropa-
nol, K2PdCl4, and NaBH4 were of analytical grade and were used
without further purification. Organic solvents were carefully dried
using molecular sieves (type 3A or 4A), and synthetic reactions were
carried out under strictly anhydrous conditions under argon.
Chemicals and inorganic salts were of the highest purity available
and were used as purchased. 1H NMR spectra were recorded on a 400
or 200 MHz NMR spectrometer with sodium 3-(trimethylsilyl)-
propionate (TSP) as an internal reference. Chemical shifts (δ) are
reported in parts per million.
EDXRF Analysis. The EDXRF measurements were made in a
temperature-controlled room (23 1 °C). Before analysis, instrument
calibration and a stability check were performed, and the detection
limit (DL) for Pd was 0.6 μg/mL. Samples (5 mL) were placed in an
X-Cell (with a diameter of 40 mm) and covered with a 5 μm thick
polypropylene film, special for XRF analyses. The measurements were
carried out using a Pd X-ray tube operated with Cu filter at 50 kV and
250 μm. The acquisition time was 150 s (measurement time per region
using helium), and the X-rays to excite the sample were produced
using a 50 W (50 kV, 2 mA) VF50 X-ray tube. The tube and generator
are capable of operating at voltages ranging from 10 to 50 kV and
currents from 1 to 2000 μA, providing that the maximum power of 50
W is not exceeded.
Synthesis of 3-(1-Tetradecyl-3-imidazolyl)propanesulfonate
(ImS3-14). The surfactant 3-(1-tetradecyl-3-imidazolyl)-
propanesulfonate (ImS3-14) was described previously.11 The syn-
thesis involved N-alkylation of imidazole followed by reaction with 1,3-
propanesultone. The zwitterionic ImS3-14 surfactant was charac-
terized by 1H NMR (400 MHz) in CDCl3 (δ relative to TMS): 10.26
(s, 1H), 8.03 (s, 1H), 7.68 (s, 1H), 4.87 (t, 2H), 4.53 (t, 2H), 3.04 (t,
2H), 2.55 (t, 2H), 1.99 (m, 2H), 1.33 (22H, m), 0.94 (t, 3H). The
signals are fully consistent with those reported previously.11
Synthesis of Palladium Nanoparticles. The nanoparticles were
synthesized as described previously.12 To a solution of ImS3-14 (1.75
mmol, 676.5 mg), K2PdCl4 (0.52 mmol, 171.4 mg), and NaCl (14
mmol, 818.3 mg) in doubly distilled water (175 mL) was added 17.5
mL of a freshly prepared 0.3 M aqueous solution of NaBH4 with
vigorous stirring. The solution became black immediately and was
stirred for an additional 24 h at room temperature. The nanoparticles
were stable and could be kept for more than a year without any Pd
black precipitate being observed.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR data, chromatograms, and mass spectra. This material
is available free of charge via the Internet at http://pubs.acs.org.
Synthesis of Palladium/ImS3-14@Al2O3.12b Basic aluminum
oxide (2.58 g) was stirred with 50 mL of the palladium nanoparticle
solution prepared as described above. The uptake of the nanoparticles
was monitored by UV−vis spectroscopy of the supernatant, and after 7
days the nanoparticles were quantitatively adsorbed. The Pd/ImS3-
14@Al2O3 nanoparticles were centrifuged at 4000 rpm, washed twice
with 20 mL of doubly distilled water, and allowed to dry in air at 60 °C
to give a pale-gray powder. The theoretical palladium content based on
total adsorption of the nanoparticles on the aluminum oxide support
was 0.56 mol %. A 10 mg sample of the prepared Pd/ImS3-14@Al2O3
nanoparticles was mixed with aqua regia, and this solution was diluted
and analyzed by inductively coupled plasma optical emission
AUTHOR INFORMATION
Corresponding Author
*Tel: +55 48 3721-6849. Fax: +55 48 3721-6850. E-mail: faruk.
nome@ufsc.br.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to INCT-Catal
and CAPES for financial support.
■
́
ise, PRONEX, FAPESC, CNPq,
spectroscopy (ICP-OES). This gave a value of 0.57
0.02 mol %
for the Pd content, which is in excellent agreement with the value
calculated assuming 100% adsorption.
REFERENCES
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General Procedure for the Reductive Amination of
Benzaldehyde with Aniline. A reaction tube was charged with 2
mL of sodium formate aqueous solution (2.5 M), to which were added
200 μL of isopropanol, 189 μL of formic acid (5 equiv, 5 mmol), 101
μL of benzaldehyde (1 equiv, 1 mmol), and 182 μL of aniline (2 equiv,
2 mmol). Finally, 200 μL of Pd/ImS3-14 aqueous solution (2.7 mM,
0.54 μmol of Pd) was added, and the reaction was sealed with a rubber
septum and heated to 70 °C in an oil bath for 6 h. After this time, the
reaction tube was removed from the oil bath and allowed to cool.
Petroleum ether (1 mL) was added to the reaction mixture to extract
the product. The Pd nanoparticles remained in a separate phase, and
the organic layer could be decanted off carefully. This was repeated
twice more to remove all of the product and remaining reactant. The
product could be isolated by column chromatography using 4% ethyl
acetate in petroleum ether as the eluent. The nanoparticles could then
be used again to catalyze another cycle of the same reaction. To the
recovered nanoparticles were added 38 μL of formic acid (1 equiv, 1
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