One-pot, chemoselective synthesis of secondary amines from aryl nitriles using a PdPt-Fe3O4nanoparticle catalyst

Article abstract of DOI:10.1039/d0cy00630k

We have developed a new catalytic method for the one-pot, cascade synthesis of unsymmetrical secondary amines via the reductive amination of aryl nitriles with nitroalkanes using a PdPt-Fe3O4 nanoparticle (NP) catalyst. The use of a bimetallic catalyst resulted in enhanced reactivity and selectivity compared to that of either monometallic Pd-Fe3O4 or the Pt-Fe3O4 NP catalyst. Using this bimetallic catalytic system, we were successful in the synthesis of various unsymmetrical secondary amines under mild conditions. However, aryl nitriles containing an electron-donating substituent were rather resistant to the reductive amination, and when hexafluoroisopropanol (HFIP) was used as a co-solvent, the reaction selectivity and yield for unsymmetrical secondary amines increased dramatically. Using the catalyst system, one-pot, gram-scale synthesis of indole was possible from 2-nitrophenylacetonitrile. Due to the magnetic property of the Fe3O4 support, the bimetallic catalyst could easily be recycled using an external magnet at least four times.

Full text of DOI:10.1039/d0cy00630k

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ISSN 2044-4761
PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
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DOI: 10.1039/D0CY00630K
ARTICLE
One-pot, chemoselective synthesis of secondary amines from aryl
nitriles using PdPt–Fe3O4 nanoparticle catalyst†
Received 00th January 20xx,
Accepted 00th January 20xx
Jin Hee Cho,‡a Sangmoon Byun,‡b Ahra Cho,a and B. Moon Kim*a
DOI: 10.1039/x0xx00000x
We have developed a new catalytic method for the one-pot, cascade synthesis of unsymmetrical secondary amines via the
reductive amination of aryl nitriles with nitroalkanes using a PdPt–Fe3O4 nanoparticles (NPs) catalyst. The use of a bimetallic
catalyst resulted in enhanced reactivity and selectivity compared to that of either monometallic Pd–Fe3O4 or Pt–Fe3O4 NPs
catalyst. Using this bimetallic catalytic system, we were successful in the synthesis of various unsymmetrical secondary
amines under mild conditions. However, aryl nitriles containing an electron-donating substituent were rather resistant to
the reductive amination, and when hexafluoroisopropanol (HFIP) was used as a co-solvent, the reaction selectivity and yield
for unsymmetrical secondary amines increased dramatically. Using the catalyst system, one-pot, gram-scale synthesis of
indole was possible from 2-nitrophenylacetonitrile. Due to the magnetic property of the Fe3O4 support, the bimetallic
catalyst could easily be recyled using an external magnet at least four times.
Br
Introduction
Cl
NH2
H
H
N
N
H
N
Secondary amines are crucial intermediates in the synthesis of
natural products, drug molecules, and industrial materials.1
Among them, N-alkylated unsymmetrical secondary amines
play particularly important roles in pharmaceutical industry.2
They are embedded as structurally important units in drug
molecules. Some selected examples of drug molecules
containing secondary N-alkylamines such as Cinacalcet,3
Br
F3
C
OH
Clobenzorex
Ambroxol
Cinacalcet
H
N
H
N
O
N
N
H
H
N
CF3
N
CF3
N
Cl
Clobenzorex,4 Ambroxol,5 Fluoxetine, and Protriptyline7 and
6
Fluoxetine
Protriptyline
Pexidartinib
Pexidartinib8 are listed in Fig. 1. Conventional methods of
synthesizing secondary amines include direct mono-N-
alkylation,9 alkylative amination,10 and reductive amination of
carbonyl compounds.11 However, these methods have
limitations such as over-alkylation, employment of expensive
starting materials, low selectivity, or harsh reaction conditions.
Among many methods of preparing secondary amines, catalytic
hydrogenation of nitriles is an efficient one-step approach,
which represents an alternative way to remove the need to use
alkyl halides or carbonyl compounds.12 However, a key problem
with the nitrile hydrogenation is its selectivity, since a mixture
of primary, secondary, tertiary amines and imines can be
Fig. 1 Selective examples of drug molecules embedding N-alkylated secondary
amines.
H2
H2
(a)
R
CN
R
R
NH2
R
NH
NH2
-NH3
H2
H2
R
N
R
R
N
R
R
N
R
H
R
NH
R
R' NH2
-NH3
H2
H2
R'
R'
(b)
R
CN
R
N
R
N
R
NH
H
Scheme 1 (a) Probable reaction process for hydrogenation of nitrile. (b) Reaction
pathway of reductive amination of nitrile with amine for unsymmetrical secondary
amine.
a. Department of Chemistry, College of Natural Sciences, Seoul National University,
1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea.
b. The Research Institute of Basic Sciences, Seoul National University, 1 Gwanak-ro,
Gwanak-gu, Seoul, 08826, Republic of Korea.
* Corresponding author E-mail: kimbm@snu.ac.kr.
‡These authors contributed equally
†Electronic Supplementary Information (ESI) available: Catalyst characterization,
supplementary Figures, Tables, and 1H and 13C NMR spectra. See
DOI: 10.1039/x0xx00000x
formed (Scheme 1a). Many research groups have shown that
the selectivity for hydrogenation of nitriles can be achieved
using transition metal catalysts under various reaction
conditions.12a Although metal-catalyzed formation of secondary
amines from nitriles has been studied to a much lesser extent
than that of primary amines,13 there have been some successful
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reports on the reactions employing Pd,14 Rh,15 Ru,16 Pt,17 PtMo alloyed PdPt NPs would have a distinctive synergiVsiteiwcAertficfleecOtnloinne
alloy,18 and Pd-FeCu NP19 catalysts.
the hydrogenation of nitriles.
DOI: 10.1039/D0CY00630K
For the formation of unsymmetrical secondary amines, the
Herein, we report the first use of a bimetallic PdPt–Fe3O4
reductive amination of nitriles with amines in the presence of catalyst in the one-pot reductive amination of aryl nitriles with
homogeneous or heterogeneous catalysts has also been nitroalkanes toward the synthesis of various unsymmetrical
investigated (Scheme 1b). As a successful example of the secondary amines. Unique in this study is that we used nitro
homogeneous catalysis systems, Liu et al. reported the compounds instead of amines, and studied the formation of
preparation of unsymmetrical secondary amines from nitriles secondary amines by simultaneously reducing nitroalkanes and
and amines using a cobalt catalyst and ammonia borane in aryl nitriles. To the best of our knowledge, this is the first
HFIP.20 Recently, Punji et al. reported that (Xantphos)CoCl2 example of the direct use of aryl nitriles and nitroalkanes
catalyzed reductive amination of nitriles with diethylamine toward the synthesis of unsymmetrical secondary amines
borane could produce unsymmetrical secondary amines.21 As without employing high pressure and high temperature.
an approach utilizing a heterogenous catalysis system, Sajiki et
al. reported Pd/C catalyzed hydrogenation of aliphatic nitriles in
the presence of an amine or a nitro compound to obtain
unsymmetrical secondary amines.22 However, aromatic nitriles
Results and discussion
were not applicable to this catalytic system since benzonitrile
was quickly reduced to benzylamine under their reaction
condition. In another case, Pt nanowire catalyst has been used
for the selective hydrogenation of aromatic nitriles in the
presence of 1-pentanamine to yield unsymmetrical secondary
amines under 1-4 bar pressure of hydrogen.17 Williams et al.
reported the synthesis of secondary amines through reductive
amination from nitriles using Pt/C catalyst in a continuous flow
multichannel microreactor.23
Catalyst characterization (PdPt–Fe3O4 NPs)
The bimetallic PdPt–Fe3O4 catalyst was prepared via a one-pot
hydrothermal method from palladium(II) chloride (PdCl2) and
potassium(II) tetrachloroplatinate (K2PtCl4) in the presence of
Fe3O4 NPs in polyvinylpyrrolidone (PVP) and ethylene glycol
(EG). The deposition of PdPt alloy NPs resulted onto the surface
of the Fe3O4 NPs. High resolution transmission electron
microscopy (HR-TEM), scanning electron microscopy (SEM),
energy dispersive X-ray spectroscopy (EDS) analyses of the
PdPt–Fe3O4 NPs confirmed well-defined morphology (Fig. 2 and
S1-S3). Mapping images from SEM-EDS showed that the Pd
Despite the aforementioned catalysts’ successful
applications, heterogeneous systems still require high pressure
of hydrogen or high temperatures for efficient synthesis of
23
unsymmetrical secondary amines from nitriles.17,
In
particular, it is difficult to control reactivity and selectivity of
nitrile reduction in heterogeneous catalysis system. Therefore,
the development of heterogeneous catalytic system with high
activity and selectivity remains to be a challenge. Given the
importance of various amine synthesis in industrial application,
it is crucial to develop efficient heterogeneous catalytic system
that can produce secondary amines selectively from the
hydrogenation of nitriles under the mild conditions.
(a)
Bimetallic NPs have shown remarkable achievement as a
catalyst in organic reactions. They often present unique
synergistic catalytic activity when compared to the
corresponding monometallic NPs.24 In this regard, we have
reported extremely efficient catalytic reduction of
nitroarenes,25 silylation of aryl halides,26 and continuous flow
hydrogenation for reductive amination27 using a magnetically
recyclable PdPt–Fe3O4 catalyst. This bimetallic PdPt–Fe3O4
catalyst exhibited enhanced catalytic activity when compared
to either monometallic Pd–Fe3O4 or Pt–Fe3O4 in the reactions
mentioned above. Moreover, the catalyst could be easily
recovered and reused with the assistance of an external magnet
for 250 cycles in the nitroarene reduction and 20 cycles in the
aryl silylation reaction, without the need for filtration or
centrifugation. In addition, researchers have used bimetallic
PdPt nanoparticles to enhance the reactivity of the catalyst by
synthesizing various types of core-shell,28 alloy, 29 and nanotube.
(b)
(c)
(d)
(e)
30
Fig. 2 (a) SEM-EDS image of PdPt–Fe3O4 NPs; (b) and (c) HR-TEM image
of PdPt–Fe3O4 NPs; The XPS data of PdPt–Fe3O4 NPs(d) Pd 3d region and
(e) Pt 4f region.
Both Pd and Pt catalysts have been utilized for the
hydrogenation of nitriles,14,17 however, no report exists on the
use of PdPt alloy catalyst system. We envisioned that the use of
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/D0CY00630K
2 mol% PdPt-Fe3O4
CN
N
H
NaBH4, MeOH
8 h, rt
98%
(GC yield)
Scheme 2 The formation of dibenzylamine from benzonitrile
CN
H2
catalyst
NH
NH2
NO2
1a'
1a
2a
2a'
H2, catalyst
NH2
H2
N
H
N
NH2
catalyst
3b
3a
NH
Fig. 3 STEM-EDS mapping image of alloyed PdPt NPs.
1a'
N
H2
N
H
NH2
and Pt atoms were well distributed on the surface of Fe3O4 NPs
in the same region (Fig. 2a). To examine the physical properties
of PdPt NPs more closely, we explored high angle annular dark
field (HAADF) scanning transmission electron microscopy
(STEM) and bright field (BF)-STEM (Fig. 3 and S4-S5). We
confirmed that a homogeneous contrast of nanoparticles in
STEM images are clearly observable due to the different mass
contrast (Z) of Pd and Pt (Fig. 3). The brighter area of the STEM
catalyst
3d
3c
Scheme 3 The possible reaction pathways for the one-pot reductive
amination using benzonitrile and 1-nitrohexane.
image correspond to the heavier atom (Pt) and the darker area converted to dibenzylamine in the presence of the bimetallic
correspond to the lighter atom (Pd).31 In this regard, we catalyst in 98% yield with NaBH4 in MeOH (Scheme 2, Table S1).
confirmed that Pd and Pt are randomly alloyed and bimetallic Under these conditions, the formation of N-benzylidene-
PdPt NPs have homogeneous structure with an average size of benzylamine and benzylamine in 1% yield each was observed
5.5 nm. (Fig. 3 and S6). To further confirm the alloy structure, with no tribenzylamine formation. We compared this result
we conducted EDS line scan analysis on single nanoparticle. As with those of monometallic Pd– or Pt–Fe3O4 catalysts. The yield
shown in the Fig. S9, the EDS line scan profile shows the both of dibenzylamine in Pd–Fe3O4 catalyzed reaction was 60%.
intensity of Pd and Pt in the same region. Therefore, we However, reactions using Pt–Fe3O4 catalyst produced only N-
concluded that PdPt NPs are not core-shell type, but rather benzylidenebenzylamine as the major product (54%) with no
homogeneous structure. We examined X-ray photoelectron hint of dibenzylamine formation. Under the same conditions,
spectroscopy (XPS) to determine the electronic conditions and the bimetallic PdPt catalyst showed much better conversion
oxidation states of Pd and Pt (Fig. 2d-e and S8). Two Pd 3d peaks and selectivity than those of monometallic Pd– or Pt–Fe3O4
were identified at 340.2 eV and 334.9 eV, respectively. These catalyst. Compared to the previously reported heterogeneous
two binding energies correspond to a Pd(0) species. The binding catalysts,14a, 15a, 17-19 the bimetallic PdPt–Fe3O4 catalyst showed
energies of Pt 4f7/2 and 4f5/2 peaks were detected at 70.8 and very high selectivity, without high temperature and high
74.1 eV’s, respectively, which indicated Pt(0) species. When we pressure (Table S2).
compared the XPS data of bimetallic PdPt-Fe3O4 NPs with
monometallic NPs (Pd– and Pt–Fe3O4) , Pt 4f2/7 peak (70.8 eV) One-pot reductive amination of benzonitrile with 1-
slightly shifted toward a lower binding energy in the PdPt–Fe3O4 nitrohexane
NPs compared with that of the monometallic Pt–Fe3O4 NPs We then investigated one-pot cascade reaction of benzonitrile
(71.0 eV). Inductively coupled plasma atomic emission in the presence of 1-nitrohexane. In order for this reaction to be
spectroscopic (ICP-AES) analysis showed that PdPt–Fe3O4 NPs successful, three reductive steps should occur in an appropriate
consist of 3.94 wt% palladium and 6.06 wt% platinum (Fig. S7). sequence (Scheme 3). Initially, the nitro group in compound 2a
should be rapidly reduced to the amine 2a’ followed by
Hydrogenation of benzonitrile with PdPt–Fe3O4 NPs
hydrogenation of nitrile 1a to give unsubstituted imine 1a′,
We first evaluated the catalytic activity of the PdPt–Fe3O4 NPs which forms an unsymmetrical secondary imine 3a upon
in the reduction of benzonitrile. Benzonitrile was selectively
reaction with the n-hexylamine (2a’). Finally, imine 3a should be
reduced to the desired secondary amine 3b. To determine the
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optimal reaction conditions, we ran the catalytic reaction using NP catalyst, the initial hydrogenation of benzonVitierwileArtidcliedOnnlionet
NaBH4 as the stoichiometric reductant in methanol for 8 h at progress at all (Table 1, entry 7). From tDhOeIr: e10a.c10ti3o9n/De0mCYp0l0o6y3in0Kg
room temperature. First, we examined the catalytic activity of PdPt–CeO2 NPs catalyst, the conversion of benzonitrile was
the Fe3O4 support and found that Fe3O4 did not catalyze the 40%, however, N-hexyl-1-phenylmethanimine and N-
reduction (Table 1, entry 1). We then compared the catalytic benzylidene-benzylamine were also obtained in 30% and 8%,
activity of the bimetallic PdPt–Fe3O4 catalyst with either respectively (Table 1, entry 8). The PdPt–TiO2 catalyzed
monometallic Pd–Fe3O4 or Pt–Fe3O4 catalysts. When the reaction provided the desired product in 37% yield (Table 1,
reaction was performed with 4 mol% Pd–Fe3O4, the desired entry 9). Among the supports examined, Fe3O4 showed the best
product was obtained in 32% yield along with dibenzylamine in results for the reduction of benzonitrile and the intermediate
13% yield. However, no imines were detected (Table 1, entry 2). imine.
Interestingly,
our
previous
studies
on
When
4
mol% Pt–Fe3O4 was used, N-hexyl-1-phenyl- hydroxymethylfurfural (HMF) oxidation32 and reductive
methanimine (3a) was obtained as a major product instead of amination33 have consistently revealed the enhanced catalytic
the desired product (Table 1, entry 3). When a reaction was activity of metal NP catalysts on Fe3O4 compared to other
performed using a combination of 2 mol% Pd–Fe3O4 and 2 mol% supports.
Pt–Fe3O4, the yield of the desired product increased
Since the production of a symmetrical diamine is always a
slightly(36%) compared to those obtained with either Pd–Fe3O4 competing reaction in the synthesis of unsymmetrical
or Pt–Fe3O4 (Table 1, entry 4). Interestingly, the highest yield secondary amines, we examined the ratio of benzonitrile and 1-
was obtained with the bimetallic alloy catalyst, PdPt–Fe3O4 nitrohexane to improve the yield and selectivity for the
(Table 1, entry 5). This indicates that high conversions for both unsymmetrical secondary amines (Table S3). As NaBH4 is
benzonitrile and imine reduction can be achieved with the required as a reducing agent to reduce both nitrohexane and
bimetallic catalyst. When the catalyst loading was reduced to 1 benzonitrile, we searched for an optimum amount of NaBH4.
mol% PdPt–Fe3O4, the yield of the desired product was 28% and When the benzonitrile/1-nitroheaxne ratio was 1.5 and 2 equiv
recovered benzonitrile and large portions of the intermediate of NaBH4 was employed, the yield of the desired unsymmetrical
imines (3a and 3c) remained in the product mixture (Table 1, secondary amine was highest (86%) (Table S3, entry 1).
entry 6).
To investigate the effects of Fe3O4 support, we compared
the catalytic activity of PdPt on different supports such as C,
Effect of metal ratio of bimetallic PdxPty–Fe3O4 catalyst
CeO2, and TiO2. Catalysts with these supports were synthesized To establish the effect of the alloy metal composition, we
by the one-pot hydrothermal method as in the case of PdPt– compared the catalytic activities of PdxPty–Fe3O4 using different
Fe3O4 NPs (Fig. S10-S12). In the reaction employing PdPt–C
proportions of Pd and Pt (x/y = 2.3:1, 1:0.97, and 1:1.7), under
otherwise the same reaction conditions (Table S4 and Fig. S14).
Interestingly, the reactions performed using the catalysts
containing a large proportion of the Pd component (Pd–Fe3O4
and Pd2.3Pt1–Fe3O4) showed high reactivity in the imine
reduction steps. In the case of Pd2.3Pt1–Fe3O4, the desired amine
product was produced in 78% yield and the yields of the side
product imines were low. On the other hand, the reactions
performed with catalysts containing a large proportion of the Pt
component (Pt– Fe3O4 and Pd1 Pt1.7–Fe3O4) did not produce the
desired product at all due to the poor reactivity in the imine
reduction step. In the reaction using the Pd1Pt1.7–Fe3O4 catalyst,
the predominant production of N-hexyl-1-phenylmethanimine
(55%) was observed. The Pd1Pt0.97–Fe3O4 catalyst showed both
the best selectivity and reactivity, furnishing the highest yield
(86%) of the desired product. Overall, the alloyed PdPt NP
catalyst increased the reactivity toward the reduction of the
nitrile when compared to the monometallic catalysts. In
addition, the higher the proportion of Pd, the more favorable
the imine reduction reaction becomes. Among the reactions
employing catalysts with different metal compositions, the
system using an almost equal ratio of Pd/Pt (1:0.97) gave the
highest turnover number (TON) (Table S4).
Table 1 Investigation of the various catalysts used for the synthesis of unsymmetrical
amines via a cascade reductive amination of benzonitrile and 1-nitrohexanea
N
N
3a
H
CN
2 mol% PdPt-Fe3O4
3b
NO2
NaBH4
8 h, rt
, MeOH
N
H
N
‘
3c
3d
Conversion of
Selectivityb
(3a:3b:3c:3d)
-
0 : 32 : 0 : 13
39 : 0 : 7 : 0
Entry
Catalyst
Fe3O4
benzonitrileb
1
2
3
0
45
48
4 mol% Pd–Fe3O4
4 mol% Pt–Fe3O4
2 mol% Pd–Fe3O4 +
2 mol% Pt–Fe3O4
2 mol% PdPt– Fe3O4
1 mol% PdPt–Fe3O4
2 mol% PdPt–C
4
47
1 : 36 : 1 : 9
5
6
7
8
9
100
90
0
40
54
4 : 62 : 1 : 27
42 : 28 : 10 : 9
-
30 : 0 : 8 : 0
0 : 37 : 0 : 17
2 mol% PdPt–CeO2
2 mol% PdPt–TiO2
a Reaction conditions: benzonitrile (0.3 mmol),1-nitrohexane (0.3 mmol), MeOH (1.8 mL),
NaBH4 (0.6 mmol), room temperature, 8 h. b Yields were determined via GC analysis using
anisole as an internal standard.
Substrate scope
With the optimized reaction conditions for the production of
secondary amines in hand, we investigated the substrate scope
4 | J. Name., 2012, 00, 1-3
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of the reaction with a variety of aryl nitriles and nitroalkanes reaction with o-tolunitrile did not furnish the desired amine
View Article Online
(Scheme 4). Reductive amination of benzonitrile with various 1- presumably due to steric hindranceD. OWI: 1h0e.1n0394/D-t0eCrYt-0b0u63ty0lK-
benzonitrile was used, a 60% yield of the desired amine 3k was
2 mol% PdPt-Fe3O4
CN
produced. The reactions using 2-, 3-, and 4-fluorobenzonitriles
R
N
R
NO2
H
with 1-nitrohexane afforded their corresponding un-
symmetrical amine products 3l–n in 62, 60, and 81% yield,
respectively. The reaction of naphthalene-2-carbonitrile with 1-
nitrohexane gave the desired unsymmetrical secondary amine
3o in 87% yield. When 4-methoxybenzonitrile was used as a
starting material in the reductive amination reaction with 1-
nitropropane, the conversion of the nitrile was low and the
reaction stopped at the imine stage. Further reduction of the
unsymmetrical secondary imine to the amine did not proceed
under the previously optimized reaction conditions. Then we
screened various acidic additives to increase the reactivity of
the intermediate imine to favor the desired secondary amine
(Table S5). Among the conditions examined, the highest yield
was obtained for the desired N-(4-methoxybenzyl)-propan-1-
amine product with the use of HFIP as a co-solvent (Table S6).
Recently, HFIP has been a popular solvent in various reactions
due to its unique properties.34 Since HFIP is a protic solvent
exhibiting strong hydrogen-bonding capability, it is used to
activate various organic functional groups, including
carbonyls,35 imines,36 and C-H bonds.37 Using HFIP as a co-
solvent, we examined the reductive amination of benzonitriles
substituted with an electron-donating group (i.e. methoxy and
ethoxy groups) with 1-nitroalkanes, which were reluctant to
proceed further than the intermediate imine under our
standard reaction conditions. With the use of HFIP, the catalytic
reactions proceeded smoothly and the results can be seen in
Scheme 4. The reactions of 4-methoxybenzonitrile with 1-
nitropropane or 1-nitrohexane afforded the desired amines 3p
and 3q in 80 and 71% yields, respectively. In addition, the
reductive amination of 3-methoxybenzonitrile with 1-
nitropropane and 1-nitrohexane furnished secondary amines 3r
and 3s in 66% and 67% yield, respectively. The reaction of 4-
ethoxybenzonitrile gave the corresponding secondary amine 3t
in 73% yield. It was gratifying to accomplish an efficient
synthesis of indole 3u via the intramolecular reductive
amination of 2-nitrophenylacetonitrile. It is of particular note
that the synthesis of the indole did not proceed in methanol,
however, with the addition of HFIP as a co-solvent an excellent
yield (93%) of indole was obtained at 50 C.
NaBH4
Solevnt
R
R
2
1
3
Solvent: MeOHa,d
N
N
H
N
H
H
3a
3e
8 h, 86%
3f
8 h, 80%
8 h, 76%
N
H
N
H
N
H
3g
3i
3h
14 h, 84%
8 h, 80%
8 h, 75%
N
H
N
H
t-Bu
3k
18 h, 60%
3j
12 h, 72%
N
N
H
H
F
3l
3m
F
18 h, 60%
24 h, 62%
N
H
N
H
F
3o
3n
18 h, 81%
8 h, 87%
Solvent: MeOH:HFIP(2:1)b,d
N
N
H
N
H
H
MeO
MeO
OMe
3r
3p
3q
24 h, 80%
24 h, 71%
24 h, 66%
N
H
N
H
N
H
EtO
3t
OMe
3s
3uc
24 h, 73%
24 h, 67%
24 h, 93%
Scheme 4. The PdPt–Fe3O4 catalyzed reductive amination of different
a
nitriles with nitroalkanes. Reaction conditions: nitrile (1.5 mmol),
nitroalkane (1.0 mmol), PdPt–Fe3O4 (2.0 mol% based on Pd), NaBH4 (2.0
mmol), methanol (6 mL), RT. b Reaction conditions: nitrile (1.5 mmol),
nitroalkane (1.0 mmol), PdPt–Fe3O4 catalyst (2.0 mol% based on Pd),
NaBH4 (3.0 mmol), methanol (4 mL) and HFIP (2 mL), RT. c 50 C. d Yields
of isolated products.
In addition to the one-pot hydrogenation of nitriles in the
existence of a nitro compound, we anticipated that direct use
of an amine as a nucleophile would also proceed to produce
secondary amines under the same reaction conditions with the
PdPt–Fe3O4 catalyst. We carried out the reactions of a variety of
nitriles in the presence of various primary amines and obtained
good to excellent yields of the desired secondary amine
products, as shown in Scheme 5. Under the optimized reaction
condition (Table S7), reaction of benzonitrile with 1-hexylamine
or cyclohexylamine produced the desired amine 5a or 5b, in
80% and 64% yields, respectively. When the reactions employing 4-
methylbenzylamine, 4-tert-butylbenzylamine and 4-methoxy-
benzylamine were carried out with benzonitrile in the presence of
the bimetallic catalyst, 81, 78 and 92% yields of the corresponding
nitroalkanes (n = 3-6) gave the desired secondary amines (3g,
3f, 3e and 3a) in 75, 76, 86, and 80% yields, respectively. The
reaction with nitrocyclopentane gave the corresponding
product 3h in 80% yield. The duration of the reaction was
adjusted for each aryl nitrile derivative since a mixture of amine
and imine was formed after 8 h. The reactions performed using
m- and p-tolunitrile proceeded to give the corresponding
amines 3i and 3j in 84 and 72% yield, respectively. However, the
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Journal Name
2 mol% PdPt-Fe3O4 NPs
CN
products (5c-e) were obtained, respectively. The reductive amination
of p-tolunitrile with 4-methoxy-benzylamine proceeded smoothly to
N
View Article Online
NO2
DOI: 10.1039H/D0CY00630K
NaBH4, MeOH
12 h, rt
1
2a
1a
10 mmol scalea
yield the unsymmetrical secondary amine 5f (87%).
73% yieldc
CN
2 mol% PdPt-Fe3O4
R
N
R
NH2
H
2 mol% PdPt-Fe3O4 NPs
R
NaBH4
MeOH
CN
NO2
5a,b
N
H
1
NaBH4
4
MeOH:HFIP (2:1)
3u
24 h, 50 o
C
10 mmol scaleb
79% yieldc (94% yieldd)
N
N
H
N
H
H
Scheme 6. Gram-scale synthesis of secondary amine using PdPt-
5a
5b
8 h, 64%
5c
8 h, 81%
a
Fe3O4 NPs. Reaction conditions: benzonitrile (15 mmol), 1-nitrohexane (10
8 h, 80%
b
mmol), PdPt–Fe3O4 (2.0 mol%), NaBH4 (20 mmol), methanol (60 mL), RT.
N
H
Reaction condition: 2-nitrophenylaceotnitrile (10 mmol), PdPt–Fe3O4 (2.0
mol%), NaBH4 (50 mmol), methanol (40 mL) and HFIP (20 mL).c Isolated yield.
d Determined by 1H NMR analysis.
N
H
OMe
5e
8 h, 92%
5d
8 h, 78%
To demonstrate the practicality and scalability of the catalytic
process, we carried out the gram-scale synthesis of secondary
amines using bimetallic PdPt–Fe3O4 NPs catalyst. We performed
the scale-up experiment using benzonitrile and 1-nitrohexane
as starting materials under the standard condition (Scheme 6).
The reaction gave 1.44 g (73% isolated yield) of N-benzylhexan-
1-amine (2a). In addition, a large-scale synthesize of indole from
2-nitrophenylacetonitrile was also performed. We confirmed
N
H
N
H
OMe
F
OMe
5f
5g
16 h, 81%
16 h, 87%
N
H
N
H
F
5h
18 h, 80%
5i
1
that the indole was produced in 94% yield thorough H NMR
12 h, 73%
analysis. After isolation, 0.93 g (79% yield) of indole was
obtained. Thus, we demonstrated that the bimetallic PdPt-
Fe3O4 NPs catalyst could be successfully applied to large-scale
synthesis of amines.
Scheme 5. The PdPt–Fe3O4 catalyzed reductive amination of nitrile
derivatives with various amine compounds for the synthesis of
a
unsymmetrical secondary amines. Reaction conditions: nitrile (0.5
mmol), amine (0.75 mmol), PdPt–Fe3O4 (2.0 mol% based on Pd), NaBH4
(1.0 mmol), methanol (3 mL), RT. b Isolated yield.
Reusability Test
One of the attractive features of a heterogeneous catalyst with
Fe3O4 is the reusability due to its superparamagnetic property.38
There was a report on recycled catalyst on the formation of
unsymmetrical secondary amine from the reductive amination
of nitrile in the presence of a primary amine,17 however,
recycling on the formation of unsymmetrical secondary amines
from nitriles and nitroalkanes has not been documented. When
a reaction was complete, the bimetallic catalyst can be easily
separated using an external magnet. To evaluate the
recyclability of the catalyst, we recovered and reused the PdPt–
Fe3O4 catalyst with fresh benzonitrile and 1-nitrohexane (Fig.
S15). The catalyst showed considerable activity up to four cycles.
However, the yield dropped sharply after the 5th run. The ICP-
AES, XPS, and HR-TEM data obtained for the recovered catalyst
from the 5th reaction showed considerable agglomeration of the
PdPt NPs, which presumably caused the reduced catalytic
activity (Fig. S7, S8, and S16).
Reaction of 4-fluorobenzonitrile with either 4-methoxy-
benzylamine or 1-hexylamine gave the corresponding product
5g or 5h in 81 and 80% yields, respectively. The reaction of
naphthalene-2-carbonitrile with 1-hexylamine afforded the
desired amine 5i in 73% yield. However, when aniline was used
as a nucleophile, only dibenzylamine was produced as a sole
product. This is presumably because aniline’s nucleophilicity is
considerably lower than that of its competitor, benzylamine.
Finally, we compared reaction conditions of synthesizing
unsymmetrical secondary amines with the bimetallic PdPt–
Fe3O4 catalyst system and those using previously reported
heterogeneous catalysts (Table S8). Previously Pd and Pt
monometallic catalysts have been used, 17, 22-23 however, the
use of aryl nitrile has not been explored to the synthesis of
unsymmetrical secondary amines with heterogeneous Pd
catalysts, and high temperature and pressure were required for
the synthesize the desired amines in the case of Pt catalysts. Use
of bimetallic PdPt–Fe3O4 catalyst ensured successful synthesis Conclusion
of unsymmetrical secondary amines without high temperature
and hydrogen pressure. These results are summarized in Table
S8.
In summary, we developed a bimetallic PdPt–Fe3O4 catalyst
system for unsymmetrical secondary amine synthesis via one-
pot cascade hydrogenation of aryl nitriles with nitroalkanes or
alkylamines. This catalytic system provides a straightforward
Gram-scale synthesis
6 | J. Name., 2012, 00, 1-3
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way to synthesize secondary amines from nitriles under mild round-bottom flask. Then, the heated solution of PdCl2, K2PtCl4
View Article Online
DOI: 10.1039/D0CY00630K
conditions. With the use of the catalyst, benzonitrile was and PVP was added dropwise to the Fe3O4 suspension with
converted to dibenzylamine in 98% yield with PdPt–Fe3O4 NPs syringe pump. Resulting mixture was heated at 100 oC for 24 h.
and NaBH4 in methanol. The PdPt bimetallic catalyst was proven The product was washed with ethanol. Finally, PdPt-Fe3O4
to be superior to its corresponding monometallic Pd and Pt (0.250 g) was obtained from drying on a rotary evaporator for
catalysts in terms of both its reactivity and selectivity. Especially, 30 min at 50 oC.
the employment of PdPt NPs has shown to be effective in
producing unsymmetrical secondary amines from the reactions Synthesis of Pd–Fe3O4 NPs
of aryl nitriles and nitroalkanes. When monometallic Pt–Fe3O4
NPs were used, the major product of the nitrile reduction was
Palladium(II) chloride (PdCl2, 0.102 mg) and polyvinyl-
pyrrolidone (PVP) (Mw∼10,000, 1.2 g) were placed in ethylene
imine. However, when Pt was alloyed with Pd to form the
glycol (26 mL). This solution was thoroughly mixed using a
bimetallic PdPt–Fe3O4, the desired amines were obtained due
sonicator for 10 min and heated for 1 hour at 100 °C. Fe3O4
to their synergetic effect. Solvent proved to be critical in the
(0.300g) NPs in EG (100 mL) was prepared in a 250 mL round
nitrile and imine reduction steps particularly for nitriles
bottom flask. The preheated PdCl2 mixture was added dropwise
possessing an electron-donating group since the conversion
with syringe pump to Fe3O4 suspension then the mixture was
rate and the selectivity dramatically increased when HFIP was
stirred for 24 h at 100 oC. The resulting Pd–Fe3O4 was washed in
used as a co-solvent. Compared to the bimetallic PdPt NPs
the same way as in the preparation of PdPt–Fe3O4. Finally, Pd-
prepared on other supports such as C, CeO2 and TiO2, the PdPt
Fe3O4 (0.270 g) was obtained from drying on a rotary evaporator
on Fe3O4 exhibited the best reactivity and selectivity. Also, the
for 30 min at 50 oC.
almost equal ratio of the Pd and Pt metals in the catalyst system
proved to be critical for the optimal reduction performance.
Synthesis of Pt–Fe3O4 NPs
Particularly, through the use of the bimetallic PdPt–Fe3O4
Potassium(II) tetrachloroplatinate (K2PtCl4,0.080g) and poly-
catalyst one-pot synthesis of indole in 93% yield was possible.
vinylpyrrolidone (PVP) (Mw ∼10,000, 0.150 g) were placed in 9
mL of EG in a 50 mL round-bottom flask. This mixture was
sonicated for 10 min and stirred for 1 hour at 100 oC. Meanwhile,
commercially available Fe3O4 (0.100 g) was placed in 20 mL of
Experimental
EG in a two-necked 100 mL round-bottom flask. Then, prepared
platinum precursor solution was added dropwise. Stirring of the
Materials
mixture was continued for 24 h at 100 oC. The resulting Pt–Fe3O4
was washed in the same way as in the preparation of PdPt–
Fe3O4. Finally, Pt–Fe3O4 (0.240 g) was obtained from drying on a
rotary evaporator for 30 min at 50 oC.
All commercially available chemicals were used as received
without further purification. Palladium chloride (99% purity)
and potassium tetrachloroplatinate (99.9 % purity) were
purchased from Alfa-Aesar. Polyvinylpyrrolidone (PVP, Mw ∼
10,000) was purchased from Sigma-Aldrich. Iron oxide
nanoparticle (Fe3O4 NPs) was purchased from DK nanomaterials.
Synthesis of PdxPty–Fe3O4 NPs
PdxPty–Fe3O4 NPs were synthesized in the same method as for
PdPt-Fe3O4 with changing quantities of Pd and Pt metals. For the
synthesis of Pd2Pt1–Fe3O4 NPs, PdCl2 (0.068 g) and K2PtCl4
(0.080 g) together with PVP (Mw ~10000, 0.300 g) and Iron
oxide (0.100g) were used. In the case of Pd1Pt-Fe3O4 NPs, PdCl2
(0 .034 g) and K2PtCl4 (0.160 g) together with PVP (Mw ~10,000,
0.300 g) and iron oxide (0.100 g) were used.
Characterization methods
All products were confirmed through comparison with
authentic compounds and quantified through gas chromate-
graphy/mass spectrometry (GC-Mass) analyses (Hewlett
Packard 5890 Gas Chromatograph and Agilent 5793 Mass
Selective Detector) with the use of anisole as an internal
standard. All Transmission Electron Microscopy (TEM) images
were performed using JEM-3010 microscope at an accelerating
voltage of 300 kV. The Energy Disperse Spectroscopy (EDS) map
sum spectrum pattern was acquired using Oxford Instruments
X-Maxn (software: Aztex). Sonication was performed in 120 W
ultrasonic bath (Branson, B-3210).
Synthesis of PdPt–C NPs, PdPt–CeO2 NPs, and PdPt–TiO2 NPs
Catalysts with different supports (C, CeO2, and TiO2) were
synthesized in the same method as for PdPt-Fe3O4 with
changing supports. For the synthesis of PdPt–C NPs, PdCl2
(0.034 g) and K2PtCl4 (0.080 g) together with PVP (Mw ~10000,
0.300 g) and activated carbon (DARCO®, -100 mesh particle size,
0.100 g) were used. In the case of PdPt–CeO2 NPs, PdCl2 (0.034
g) and K2PtCl4 (0.080 g) together with PVP (Mw ~10000, 0.300
g) and cerium(IV) oxide (< 50 nm particle size, 0.100 g) were
used. When synthesizing PdPt–TiO2 NPs, we used PdCl2 (0.034
g) and K2PtCl4 (0.080 g) together with PVP (Mw ~10000, 0.300
g) and titanium (IV) oxide (<100 nm particle size, 0.100 g).
Synthesis of PdPt–Fe3O4 NPs
Palladium(II) chloride (PdCl2, 0.102g), potassium tetra-
chloroplatinate (K2PtCl4, 0.240 g) and polyvinylpyrrolidone
(PVP) (Mw ∼10,000, 1.2 g) were placed in 26 mL of EG in a 100
mL round-bottom flask. This mixture was sonicated for 10 min
and stirred for 1 hour at 100 °C. Meanwhile, Fe3O4 (100 nm,
0.300 g) was placed in EG (100mL) in a two-necked 250 mL
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B.M.K. thanks the Mid-career Researcher Program (NRF-
View Article Online
DOI: 10.1039/D0CY00630K
A general procedure for reductive amination of nitriles with
nitroalkanes.
2019R1A2C1004173) for an NRF grant funded by MEST.
PdPt–Fe3O4 NPs (2 mol %) catalyst, a nitrile (R-CN, 1.5 mmol), a
nitroalkane (R-NO2, 1 mmol) and sodium borohydride (NaBH4, 2
mmol) were introduced in a 55 mL glass vial. Methanol (6 mL)
was added to the vial and the mixture was stirred at room
temperature for a specified period. After the reaction was
terminated, the catalyst was recovered with an external magnet.
The resulting mixture was diluted with 10 mL dichloromethane.
The mixture was placed into a separatory funnel and the organic
layer was washed with water. It was dried (MgSO4) and
concentrated to give a crude mixture, which was purified by
flash column chromatography on silica gel with DCM/methanol
to give the desired product.
Notes and references
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Procedure for gram-scale synthesis
The PdPt–Fe3O4 catalyst (2 mol%), 1-nitrohexane (10 mmol),
benzonitrile (15 mmol) and methanol (60 mL) were introduced
in round-bottom pressure flask (500 mL) with a magnetic stirrer
bar. The reaction mixture was sonicated for 1 min. Then sodium
borohydride (NaBH4, 20 mmol) was added to the mixture and
the flask was closed with a Teflon plug. The mixture was stirred
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for 1 min at 0 C and then stirring was continued for 12 h at
room temperature. After completion of the reaction, the
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magnet. The solvent was evaporated from rotary evaporator
and the resulting reaction mixture was treated with water (50
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method of synthesizing indole (3u) was carried out in the same
way as above. 2-Nitrophenylaceotnitrile (10 mmol), PdPt–Fe3O4
(2.0 mol%) in methanol (40 mL) and HFIP (20 mL) was placed in
round-bottom pressure flask. Then, sodium borohydride
(NaBH4, 50 mmol) was added to the mixture in the same method
as described above. The reaction was stirred at 50 °C for 24 h.
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using a magnet and solvent was removed from a rotary
evaporator. The mixture was quenched with water (50 mL) and
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DOI: 10.1039/D0CY00630K