Simple reversible fixation of a magnetic catalyst in a continuous flow system: Ultrafast reduction of nitroarenes and subsequent reductive amination using ammonia borane

Article abstract of DOI:10.1039/c9cy02021g

Continuous reductive amination of aldehydes with nitroarenes over a Pd-Pt-Fe₃O₄ catalyst was performed. We used NH₃BH₃ as not only a hydrogen source for nitro reduction, but also a reductant for imine reduction. Secondary aromatic amines were obtained in the continuous flow reaction in good to excellent yields.

Full text of DOI:10.1039/c9cy02021g

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Byun, S. M. Kim, H. J. Kim, C. Lei, D. Y. Kang, A. Cho, B. M. Kim and J. K. Park, Catal. Sci. Technol., 2019,
DOI: 10.1039/C9CY02021G.
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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/C9CY02021G
Catalysis Science & Technology
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Simple reversible fixation of magnetic catalyst in continuous flow
system: Ultrafast reduction of nitroarenes and subsequent
reductive amination using ammonia borane
Received 00th November20xx,
Accepted 00th January 20xx
Hong Won Kim,+a Sangmoon Byun,+b Seong Min Kim,+a Ha Joon Kim,a Cao Lei,a Dong Yun Kang,a
Ahra Cho,b B. Moon Kimb* and Jin Kyoon Parka*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A continuous reductive amination of aldehydes with nitroarenes (dehydrogenation) followed by the reduction of nitroarenes to
over a Pd-Pt-Fe3O4 catalyst was performed. We used NH3BH3 as not amines (hydrogenation), which occurs within
min.33
5
only a hydrogen source for nitro reduction but also a reductant for Furthermore, the magnetic catalyst was recycled over 300 times
imine reduction. Secondary aromatic amines were obtained in the with easy retrieval through the use of an external magnet. To
continuous flow reaction in good to excellent yields.
further improve this reaction for practical purposes, we began
the pursuit of a versatile immobilisation strategy for this
catalyst for application in a continuous process.
Continuous flow reaction technology has become an ideal
tool for organic transformations with distinctively
advantageous characteristics such as the detection of transient
reactive species,1,2 safe usage of hazardous or explosive
reagents,3-8 multi-step capability of important compounds, and
easy scale-up with no further process development.9-15 As one
successful example among many organic reactions, chemists
have developed continuous hydrogenation to overcome the
limited scale-up and slow H2 diffusion rates in batch-type
reactions.16-23 In most cases, a direct connection to a high-
pressure tank, special mixing setups such as H-Cube or a gas-
permeable membrane for the saturation of a solvent with H2 is
involved;24-27 safety equipment for the prevention of potential
explosion and an expensive in situ hydrogen generator are
required. Towards avoiding the aforementioned drawbacks, in
situ hydrogen generation from stable H2 precursors, such as
ammonium formate, hydrazine and metal hydrides, has been
highly recommended as an ideal alternative.28 In addition to
them, NH3BH3 recently gathered great attention not only as a
hydrogen storage but also a hydrogen precursor for easy
laboratory manipulation due to its high solvolytic stability, quick
H2 evolution through contact with a transition metal, and its
own intrinsic reduction power.29-32
Neodymium magnets have been known to be the strongest
type of commercially available permanent magnet. They have
very diverse shapes and low prices and are made from alloys of
neodymium, iron and boron.34,35 Therefore, many attempts to
utilise neodymium magnet are known in flow setups, such as
continuous catalyst recirculation in microfluidic system,36
catalyst agitation in a glass column,37 and catalyst fixation in a
micro device.38 An induced magnet using a Helmholtz coil with
an alternating current is also reported as an alternative to
permanent magnet.39 We have devised a new method for the
simple fixation of the active magnetic nanoflakes in tubes using
neodymium magnets as shown in Figure 1.40 The tubes
containing dispersed nanoflakes are placed in a strip of U-
shaped staples with neodymium magnets, which is then
covered with another strip and more magnets. The nanoflakes
could be retrieved easily by removing the neodymium magnets
(reversible fixation). With this robust method for nanoflake
fixation, we began to devise a practical strategy to combine
three reactions in flow.
Recently, we developed highly active and recyclable magnetic
bimetallic nanoflakes for the conversion of NH3BH3 to H2 gas
a. Department of Chemistry and Chemistry Institute of Functional Materials, Pusan
National University, Busan 46241, Korea. E-mail: pjkyoon@pusan.ac.kr
b. Department of Chemistry, Seoul National University, Seoul 08826, Korea.
E-mail: kimbm@snu.ac.kr
+ These authors contributed equally.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
Figure 1. Reversible fixation of magnetic nanoflakes to tubes with external magnets.
J. Name., 2013, 00, 1-3 | 1
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Journal Name
manner at room temperature with a retention timVeieowfAlreticslse Othnlainne
1 min (bottom, Scheme 2).
H
DOI: 10.1039/C9CY02021G
NO2
NH2
H2 from NH3BH3
N
Aldehyde
H+
NH3BH3
R
Pd-Pt-Fe3O4
R
R
R
Table 1. Optimisation of nitro reduction conditions.a
Scheme 1. Two roles of ammonia borane (hydrogen source and imine reductant)
Neodymium magnet
Pd-Pt-Fe3O4
Following our previous successful nitro group reduction,33 we
targeted a continuous three-step process combining nitro
group reduction, imine formation, and imine reduction (so
called reductive amination) utilizing ammonia borane not only
as a hydrogen precursor but also a reductant (Scheme 1).
NH3BH3 has been known to be applicable to reductive
amination in the presence of a Lewis acid.41 Initially, this
continuous approach may seem simple, but it is challenging
because NH3BH3 can reduce both aldehydes and imines.
Consequently, in batch-type reductive amination reactions,
NH3BH3 should be added after the completion of imine
formation. For successful execution of the three successive
steps, fast induction of imine formation ahead of unwanted
aldehyde reduction, taking advantage of the flow reaction
technique, is essential; this cannot be realised in a batch-type
reaction. In our strategy, a main stream of excess NH3BH3 and a
nitroarene compound will be passed through a bed of a
transition metal catalyst for nitro reduction, and then excess
NH3BH3 will reduce the imine formed in situ from the reaction
between already-reduced amine and a stream of aldehyde
BPR
Neodymium magnet
Nitrobenzene
NH3BH3 in MeOH
Aniline
NH3BH3
(equiv)
Flow rate
(mL/min)
Conversionb
(%)
Yieldc
(%)
Entry
1d,e
2d
3d,f
4d
5d
6
3.0
3.0
3.0
3.0
3.0
3.0
2.5
2.0
1.5
1.0
0.2
0.2
0.2
0.4
0.8
0.4
0.4
0.4
0.4
0.4
86
>99
99
70
>99
89
>99
>99
>99
>99
>99
>99
94
>99
97
>99
>99
>99
97
7
8
9
10
84
a Tube was filled with 80 mg of Pd-Pt-Fe3O4 and reaction was carried out under the
following conditions: room temperature, 40 psi, nitrobenzene (1.0 equiv, 0.2 M)
and NH3BH3 (concentration varied to adjust its stoichiometry, for example 0.4 M
for 2.0 equiv) in MeOH. Gas chromatography (GC) conversion. GC yield
determined using 2-isopropylnaphthalene as an internal standard.
nitrobenzene. e 30 mg of Pd-Pt-Fe3O4 was used. f Without BPR.
connected through
a T-shaped tubing connector. The
installation of various back pressure regulators (BPRs) (i.e., for
pressure control) could be the ideal option for reaction
optimisation.
b
c
d
0.1 M
We began the screening of the nitroarene reduction with Pd-
Pt-Fe3O4 and 3 equiv of NH3BH3 with varying flow rates (Table
1). In the initial attempt, 30 mg of Pd-Pt-Fe3O4 was used, but
this amount was not sufficient for the completion of the
reaction (entry 1). However, product 2a was obtained in 99%
yield when the catalyst loading was increased to 80 mg (entry
2). Next, when the reaction was performed without using a BPR,
a slight decrease in the yield (89%) was observed at the same
flow rate (entry 3). Upon doubling the flow rate from 0.2 to 0.4
mL/min, the yield increased to 99% (entry 4). Increasing the
flow rate to 0.8 mL/min led to a slight decrease in the yield
(entry 5). When the reaction concentration was increased to 0.2
M, a 99% yield was achieved (entry 6). Then, we attempted to
reduce the amount of NH3BH3. Reduction from 3 to 2 equiv did
not affect the product yield (entries 6–8). However, decreasing
the amount of NH3BH3 further resulted in 97% and 84% yields
(entries 9 and 10, respectively).
H
H2, Au/Al2O3
N
R
50-90 oC, 50 bar
2015, Nuzhdin
R
H
H2, FeNi/C
NO2
CHO
N
R
+
100-150 oC, 10 bar
R
2015, Esposito
R
R
H
N
NH3BH3, Pd-Pt-Fe3O4
RT, 2.8 bar (40 psi)
R
This work
R
Scheme 2. Two roles of ammonia borane and continuous flow reductive aminations of
aldehydes with nitroarenes.
Reductive amination of aldehydes with nitroarenes over a
Au/Al2O3 catalyst in a continuous flow reactor has been
reported; it was performed under a high pressure (50 bar) at
80–90 °C and afforded the desired products along with decent
amounts of unreacted amine and imine (top, Scheme 2).42
Continuous reductive aminations using H2 catalysed by an Fe/Ni
alloy are also known but require harsh conditions (100–150 °C,
10 bar) and give low yields (middle, Scheme 2).43 Other seminal
direct reductive aminations were reported as well using amine
and carbonyl compounds.44-46 Herein, we describe reductive
amination between nitroarenes and aldehydes in a continuous
Me
NH2
Me
NH2
NH2
NH2
NH2
Me
HO
Me
2a
2b
2c
2d
2e
>99%
97%
96%
91%
92%
(120 h-1)
(116 h-1)
(115 h-1)
(109 h-1)
(110 h-1)
NH2
F
Cl
NH2
NH2
NH2
NH2
H2N
F
MeO
O
2 | J. Name., 2012, 00, 1-3
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a
2f
2g
2h
2i
2j
Reaction conditions: unless otherwise noted, all reactions were performed at
View Article Online
95%
98%
98%
98%
92%
room temperature and 40 psi using a 0.5 m residence element (6 s residence time).
DOI: 10.1039/C9CY02021G
(114 h-1)
(118 h-1)
(118 h-1)
(118 h-1)
(110 h-1)
Syringe A: PhNO2 (1.0 equiv, 0.2 M) and NH3BH3 (concentration varied to adjust
stoichiometry, for example 1 M for 5.0 equiv) in MeOH with Pd-Pt-Fe3O4 (80 mg,
0.04 mmol). Syringe B: PhCHO (concentration varied to adjust stoichiometry, for
example 0.7 M for 3.5 equiv) and acetic acid (10% of the total volume of solution
H2N
NH2
NH2
NC
b
c
B) in MeOH. Flow rate of A + flow rate of B. GC yield determined using 2-
2k
2l
2m
d
91%
92%
97%
isopropylnaphthalene as an internal standard. Amount of acetic acid was 5% of
the total volume of solution B. e Length of residence element was 1 m (residence
time of 8 s). f 0.1 M nitrobenzene.
(109 h-1)
(110 h-1)
(116 h-1)
Scheme 3. Scope of nitro reduction reaction. Reaction conditions: room temperature, 40
psi, substrate (1.0 equiv, 0.2 M), Pd-Pt-Fe3O4 (80 mg, 0.04 mmol), ammonia borane (2.0
equiv, 0.4 M), MeOH, 0.4 mL/min flow rate. Isolated yields are shown and TOFs are in
the parenthesis. The reaction mixture was collected for 12 min for the yield
determination.
Next, we attempted to combine the reductive amination with
the nitroarene reduction for the synthesis of secondary amines
(Table 2). A key idea for the reductive amination is the
utilisation of leftover NH3BH3 for imine reduction. Specifically,
excess NH3BH3 and a reduced amine injected by syringe A will
meet a second stream containing benzaldehyde and acetic acid
as an additive from syringe B at the junction of a T-shaped
connector, where the desired imine formation followed by
imine reduction should occur rather than aldehyde reduction.
We varied the amounts of NH3BH3, aldehyde, and acetic acid
and the flow rate of each to ensure complete reductive
amination. The entire reaction was optimised at room
temperature and 40 psi. We initiated our studies by screening
the amount of acetic acid used in the one-pot reductive
amination of benzaldehyde with nitrobenzene. When the
amount was 5% of the total volume of solution B, a moderate
yield of 55% of desired amine 3a was obtained (entry 1). An
improved yield of 86% was achieved upon increasing the
amount to 10% of the total volume of solution B (entry 2).
Increasing the flow rates from both syringes, with a slightly
faster flow from syringe B, led to a quantitative yield of the
reductive amination product (99%, entry 3); similar results were
obtained using less benzaldehyde (3.5 equiv, entry 4). However,
further reduction of the amount of aldehyde resulted in
incomplete conversion. A complete imine formation should
require an excess amount of aldehyde, which reveals the
restriction of our substrate scope. Next, the effect of the
ammonia borane amount was examined. Although complete
conversion could still be achieved with 5 equiv of ammonia
borane (entry 6), a slight decrease in yield was observed when
4.5 equiv was used (entry 7). Increasing the flow rate, which
reduces the residence time, was not beneficial to the yield.
Amine 3a was obtained in only 91% yield (entry 8). This is
presumably because a longer reaction time may be required for
imine formation than for reduction. This assumption was
confirmed by the detection of unreacted amine in the crude
reaction mixture through GC analysis. Based on this observation,
we doubled the length of the tube, or in other words, increased
the retention time, and were able to obtain reduction product
3a in 99% yield (entry 9). Finally, a 0.1 M solution of
nitrobenzene was also found to be effective (entry 10). We
chose the conditions in entry 6 as the optimal conditions to
expand the scope further.
In order to investigate the generality of the reduction, we
tested the reactivities of a variety of nitroarenes under the
optimal conditions (entry 8, Table 1). As shown in Scheme 3, the
reduction method turned out to be effective regardless of
electronic and reduction-sensitive functionalities. From
nitrobenzene substituted with a methyl group at either the
ortho- or para-position, the corresponding anilines (2b and 2c)
were obtained in quantitative yield, while nitrobenzene
substituted with two methyl groups at the meta-positions was
also tolerated, albeit providing the corresponding aniline (2d) in
a slightly lower yield (91%). Even nitrobenzene with a free
hydroxyl group (2e) or amine (2f) showed excellent reactivity
without any significant decrease in yield. Substrates with
electron-withdrawing groups at the ortho-, para-, and meta-
positions also gave the desired amines (2g–2j) in excellent yields
(92–98%), indicating the mildness and selectiveness of the
present catalyst system. Moreover, the reaction is not
significantly impeded for nitroarenes with cyanomethyl
substituent (2k) or multiple aryl rings (2l and 2m).
Table 2. Optimisation of reductive amination.a
Neodymium magnet
Pd-Pt-Fe3O4
A
Neodymium magnet
PhNO2 + NH3BH3
in MeOH
B
BPR
PhCHO + acetic acid 10% (v/v)
in MeOH
b
c
NH3BH3
(equiv)
PhCHO
(equiv)
Flow rate
Yield
(%)
Entry
(mL/min)
0.20 + 0.20
0.20 + 0.20
0.25 + 0.35
0.25 + 0.35
0.25 + 0.35
0.25 + 0.35
0.25 + 0.35
0.35 + 0.45
0.35 + 0.45
0.25 + 0.35
d
1
6.0
6.0
6.0
6.0
6.0
5.0
4.5
5.0
5.0
5.0
4.0
4.0
4.0
3.5
3.0
3.5
3.5
3.5
3.5
3.5
55
86
2
3
4
5
6
7
8
>99
>99
82
>99
89
H
N
H
N
91
H
N
H
N
e
9
>99
>99
F
f
10
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3a
3b
3c
3d
isolated in excellent yields (up to 94%). Notably, VtiheweAsrtticalenOdnalirnde
DOI: 10.1039/C9CY02021G
reaction conditions were compatible with cyano groups in
96%
93%
86%
92%
(72 h-1)
(70 h-1)
(65 h-1)
(69 h-1)
aldehyde component; secondary amine 3q was afforded in an
impressive yield (89%). In addition, amines were prepared
smoothly from aliphatic, polyaryl, and heteroaryl aldehydes
with equal efficiency (3r–3x). It is noteworthy that the sterically
bulky pivalaldehyde was readily reacted to provide 3v. Lastly,
we attempted the reductive amination of nitroalkanes with
aliphatic aldehydes. Unfortunately, however, we observed only
nitro and aldehyde reductions presumably because the imine
formation must not be fast enough.
H
N
H
N
H
H
N
N
NC
N
F
Cl
3e
91%
3f
3g
3h
95%
82%
52%
(68 h-1)
(71 h-1)
(62 h-1)
(39 h-1)
H
N
H
N
H
N
H
N
HO
3i
3j
3k
3l
90%
87%
93%
91%
(68 h-1)
(65 h-1)
(70 h-1)
(68 h-1)
OMe
Cl
H
N
H
N
H
N
H
N
OMe
Cl
3m
3n
3o
3p
94%
94%
94%
90%
(71 h-1)
(71 h-1)
(71 h-1)
(68 h-1)
CN
H
N
H
N
H
N
H
N
n-Propyl
n-decyl
3q
3r
3s
3t
89%
92%
82%
85%
(67 h-1)
(69 h-1)
(62 h-1)
(64 h-1)
H
N
H
N
H
N
H
N
S
Figure 2. Results of long-term effectiveness experiment.
3u
3v
3w
88%
3x
81%
83%
83%
(61 h-1)
(62 h-1)
(66 h-1)
(62 h-1)
After successfully evaluating the generality of the scope with
various nitroarenes and aldehydes, we became interested in
verifying the effectiveness of the catalyst system in our T-
shaped flow setup. Accordingly, we conducted the reductive
amination for 24 hours under the optimal conditions (entry 6,
Table 2) using nitrobenzene and benzaldehyde, and the hourly
yields are presented in Figure 2. Notably, the catalytic activity in
the present flow process was maintained even after 24 h with
an average yield of 93%.47 This gram-scale reaction provided
66.6 mmol (12.2 g, TON = 1.67 x 103) of the secondary amine
without much effort.
Scheme 4. Scope of reductive amination reaction. Reaction conditions: unless otherwise
noted, all reactions were performed at room temperature and 40 psi using a 0.5 m
residence element (6 s residence time). Syringe A: nitro substrate (1.0 equiv, 0.2 M) and
NH3BH3 (5.0 equiv, 1.0 M) in MeOH with Pd-Pt-Fe3O4 (80 mg, 0.04 mmol), 0.25 mL/min
flow rate. Syringe B: aldehyde substrate (3.5 equiv, 0.7 M) and acetic acid (10% of the
total volume of solution B) in MeOH, 0.35 mL/min flow rate. Isolated yields are shown
and TOFs are in the parenthesis. The reaction mixture was collected for 12 min for the
yield determination. TOF of Pd-Pt was calculated based on the final yield.
Then, we ran the continuous three-step process with a broad
range of nitroarenes and benzaldehydes with various
substituents (Scheme 4). Nitrobenzene with one and two
electron-donating methyl groups gave desired amines 3b and 3c
in 93% and 86% yields, respectively. Nitrobenzene with halogen
substituents, at either the para- or ortho-positions, also
Conclusions
In conclusion, we developed a rapid and continuous three-
underwent the present reaction, yielding the corresponding step process of nitro reduction, imine formation, and imine
products in comparable yields (up to 95%, 3d–f). Similar to the reduction, which enables direct and facile preparation of
arene reduction of 2k, the one-pot reductive amination of 4- various secondary amines under relatively mild conditions. The
cyanomethylnitrobenzene proceeded smoothly without reactions were carried out at ambient temperature with the use
affecting the nitrile functionality, affording amine 3g in 82% of ammonia borane as a hydrogen source. Because the
yield. The reaction with 3-nitropyridine gave product 3h in a previously reported reductive aminations have obstacles
moderate yield (52%), while nitronaphthalene was transformed associated with them, this continuous flow technique is much
with good efficiency (3i, 90% yield). Adequate reactivity was more practical on the basis of the relative ease with which it
also observed for 4-nitrophenol (3j, 87% yield). To further test delivers the desired amines, even on the gram scale.
the versatility of this reaction, we also examined the effect of
substituents on the aryl aldehyde. Aryl aldehydes featuring
electron-donating substituents (methyl or methoxy) at either Conflicts of interest
the ortho- or para-positions readily afforded the respective
There are no conflicts to declare
amination products in comparable yields (3k–n, 91–94%).
Substrates with chloro substituents were also effective in the
present reaction and corresponding products 3o and 3p were
4 | J. Name., 2012, 00, 1-3
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Acknowledgements
DOI: 10.1039/C9CY02021G
This study was supported financially by the National Research
Foundation of Korea (NRF) funded by the Korean government
(2016R1D1A1B03933754, 2019R1A2C2004902) and the Nano
Material Development Program (2015M3A7B4070612) through the
NRF funded by the Ministry of Education, Science and Technology
(MEST).
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DOI: 10.1039/C9CY02021G
Neodymium magnet
Pd-Pt-Fe3O4
A
Neodymium magnet
PhNO2 + NH3BH3
in MeOH
B
BPR
PhCHO + acetic acid 10% (v/v)
in MeOH
A continuous reductive amination was
performed using NH3BH3 through
reversible magnetic bimetallic fixation
under room temperature.