Hydrogen-Free Cobalt-Rhodium Heterobimetallic Nanoparticle-Catalyzed Reductive Amination of Aldehydes and Ketones with Amines and Nitroarenes in the Presence of Carbon Monoxide and Water

Article abstract of DOI:10.1021/acscatal.5b01198

Cobalt-rhodium heterobimetallic nanoparticle-catalyzed reductive amination of aldehydes and ketones with amines in the presence of 5 atm carbon monoxide without an external hydrogen source has been developed. Water added and generated in situ produces hydrogen via a water-gas-shift reaction. The reaction can be extended to the tandem reduction of aldehydes and ketones with nitroarenes. The catalytic system is stable under the reaction conditions and could be reused eight times without losing any catalytic activity. (Chemical Equation Presented).

Full text of DOI:10.1021/acscatal.5b01198

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ACS Catalysis
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Hydrogen-Free Cobalt-Rhodium Heterobimetallic Nanoparticle-
Catalyzed Reductive Amination of Aldehydes and Ketones with
Amines and Nitroarenes in the Presence of Carbon Monoxide and
Water
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Jang Won Park and Young Keun Chung*
Department of Chemistry, College of Natural Sciences, Seoul National University, Seoul 151-747, Korea
ABSTRACT: Cobalt-rhodium heterobimetallic nanoparticle-catalyzed reductive amination of aldehydes and ketones
with amines in the presence of 5 atm carbon monoxide without an external hydrogen source has been developed.
Water added and generated in situ produces hydrogen via a water-gas-shift reaction. The reaction can be extended to
the tandem reduction of aldehydes and ketones with nitroarenes. The catalytic system is stable under the reaction
conditions and could be reused eight times without losing any catalytic activity.
KEYWORDS : heterogeneous catalysis, amination, reduction, carbon monoxide, water
Homogeneous transition metal complexes-catalyzed di-
rect reductive amination procedures are well delveloped.1
Two types of reducing agents are employed for direct re-
ductive amination of aldehyde with amines;2 based on
metal-catalyzed hydrogenation and hydride reducing
agents (Scheme 1a).3 Recently, alternatives to the use of
hydrogen or hydride have been reported.4 Chusov and
List5 recently described a Rh-catalyzed reductive amina-
tion of aldehydes with aryl amines (Scheme 1b). Their
amination utilizes the existing hydrogen atoms of the
amine substrates and high pressures of carbon monoxide
o
(20 ~100 bar) as the reductant in THF at 120 ~ 140 C. A
method of reduction without an external hydrogen source
is a highly desirable process. Inspired by the work, we
envisioned in situ generated water as a hydrogen source.
If a reductive amination of aldehydes with amines was
carried out in the presence of carbon monoxide, the liber-
ated water in the condensation reaction might react with
carbon monoxide to give hydrogen and carbon dioxide in
the presence of a water-gas-shift reaction catalyst. Then
the generated hydrogen molecule can act as a reducing
agent, resulting in producing amines. Several decades ago,
Iqbal reported the use of carbon monoxide and water in
the reduction of a nitro group in nitroarene compounds.6
Scheme 1. Reductive Amination Reactions3,5 / Hydro-
genation10
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The emergence of transition metal nanoparticles has led
Table 1. Optimization of the Reaction Conditions
to an explosive growth in catalysis.7 Transition metal
nanoparticles are potentially attractive catalysts due to
their high catalytic activities and synergistic effects.
Recently, the use of heterobimetallic nanoparticles as
catalysts has attracted much attention because their
catalytic performance is generally superior to that of a
single nanoparticle by itself and there is potential to
create new types of catalysts for reactions which may not
be achieved by monometallic catalyst.8 We reported that
cobalt/rhodium heterobimetallic nanoparticles (Co2Rh2,
derived from Co2Rh2(CO)12) immobilized on charcoal
(Co2Rh2/C) were quite useful catalysts in carbonylation9
and/or hydrogenation reactions (Scheme 1c).10 In the hope
of finding new catalytic amination system, we decided to
use Co2Rh2/C as catalyst in the reductive amination
between aldehydes (or ketones) and amines in the
presence of carbon monoxide without any external
hydrogen source. We found that the catalytic system was
quite effective in the reductive amination of aldehydes or
ketones with amines in the presence of 5 atm CO in wet
THF at 100 oC for 12 h (Scheme 1d). We herein
communicate our preliminary results. Recently, Franke
1
2
3
4
5
6
7
8
Yield (%)a
H2O
Temp
(oC)
Time
(h)
Entry
(mL)
3aa
65
52
0
4aa
20
20
98
0
9
1
2
0
100
100
100
100
85
15
6
6
6
6
6
6
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6
3
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0
3b
0
4
0.3
0.3
0.3
0.3
0.3
0.03
0.03
0.03
0.03
0.03
94
0
5
98
62
17
6
90
36
80
54
98
97
17
7
95
8c
100
100
120
100
100
100
44
0
9
10
11d
12
13e
0
and
Beller
reported11
Ru-catalyzed
6
6
6
80
75
97
hydroaminomethylation of olefins with an amine using a
water-gas shift reaction (40 bar CO used). The Co2Rh2/C-
catalyzed reductive amination could be conducted at
substantially lesser pressure than the one previously
employed under rhodium and ruthenium catalysis.5,12
23
0
a
b
c
Isolated yield. In the presence of molecular sieves (3Å).
3
mol% Co2Rh2/C used. d Under 3 atm CO. e In toluene.
Initially, the reaction of benzaldehyde (1a) with 4-
methoxyaniline (2a) to afford the amination product, N-
benzyl-4-methoxyaniline (3aa), was chosen as the model
reaction in the presence of Co2Rh2/C (5 mol%) in THF at
influences the hydrogenation of 4aa (entry 4 vs 9).The
reaction was highly sensitive to the reaction temperature-
and the amount of catalyst used (entry 4 - 7). Lowering
o
100 C (Scheme 2). After workup, 3aa was isolated in 65%
o
o
the reaction temperature from 100 C to 85 C completely
blocked the formation of an amine. Moreover, the reac-
tion was highly sensitive to the amount of Co2Rh2/C used
(entry 4 vs 8). Decreasing the amount of catalyst from 5
mol% to 3 mol% led to the formation of a mixture of 3aa
and 4aa in 54 % and 44% yield, respectively. The best
yield (97%) of 3 was observed when the reaction carried
out in the presence of 5 mol% of Co2Rh2/C in a mixture-
solvent of water and THF (0.03 mL/3.0 mL) at 100oC for 6
h. When the reaction was carried out at 120oC (entry 10),
the reaction time could be shortened with a high yield
(95%). The reaction was highly sensitive to the CO pres-
sure and reaction time. Under 3 atm of CO, a mixture of
3aa and 4aa was formed in 17% and 80% yields, respec-
tively (entry 11). For 3 h of reaction time, a mixture of 3aa
and 4aa was obtained in 23% and 75% yields, respectively
(entry 12). When the same reaction was carried out in
toluene, 4aa was obtained as the sole product (entry 13).
Therefore, the optimum reaction conditions were as fol-
lows: 5 mol% Co2Rh2/C in 0.03 mL H2O and 3.0 mL THF
at 100oC for 6 h of reaction time. In general, strict anhy-
drous conditions are favorable in the transition metal-
catalyzed reductive amination and it is operationally
troublesome to keep anhydrous conditions during a reac-
tion. However, the present operationally simple catalytic
yield with a concomitant formation of an imine, N-(4-
methoxyphenyl)-1-phenylmethanimine, 4aa, in 20% yield.
Scheme 2. Initial observation
Encouraged by the above observation, we decided to op-
timize the reaction conditions for the amination of 1a
with 2a under 5 atm CO (Table 1). Decreasing the reaction
time from 12 h to 6 h led to formation of a mixture of 3aa
and 4aa in 52% and 20%, respectively (entry 2). In the
presence of molecular sieves (3Å), no amine product was
isolated. Instead, 4aa was isolated in 98% yield (entry 3).
This observation suggested that water was needed for
hydrogenation of 4aa. Water generated in the formation
of 4aa would react with carbon monoxide by means of the
water–gas shift reaction to produce molecular hydrogen
and carbon dioxide. Thus, we varied the amount of water
in the reaction mixture (water/THF, from 0.3 mL/3.0 mL
to 0.03 mL/3.0 mL) to know how the water concentration
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and secondary amines as well as aliphatic and aromatic
system was quite effective for the amination in the pres-
1
2
3
4
5
6
7
8
ence of water, using the solvent (THF) without purifica-
tion.
aldehydes and ketones. Both electron-donating (OMe,
OH, and Me) and –withdrawing (F, Cl, Br, CF3) groups on
the aromatic substituents of the aldehydes and amines are
tolerated. Functional groups such as hydroxyl and halo
groups are tolerant in the reaction. However, a vinyl
group is completely hydrogenated (3ag). The reductive
amination reaction shows excellent generality for a varie-
ty of aldehydes and primary and secondary amines with
very good to excellent yields. The lowest yield (3ak, 82%)
was observed with 4-fluoroaniline. Generally, the use of
an aryl aldehyde or aryl amine gave a relatively higher
yield than that of an aliphatic aldehyde (3ma, 84%), ke-
tone (3la, 86%), or amine (3af, 85%) used. When 2 equiv
of an aldehyde was used, a tertiary-amine with two sub-
stituents from the aldehyde was isolated in excellent
yields (3a’a; 95%: 3a’e; 94%). When a secondary amine
with two different substituents was reacted with another
aldehyde, an unsymmetrical tertiary amine having three
different substituents was isolated in high yields. Reaction
Table 2. Amination of Aldehydes and Ketones with
Aminesa
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3aa, 97%
94%g
3ac, 93%b
3ab, 98%
3af, 85%c
3ad,
98%c
3ae, 95%c
of
N-benzyl-4-methoxyaniline
(3aa)
with
4-
3a'e,
3a'a, 95%d
94%d
methoxybenzaldehyde (1b) afforded N-benzyl-4-methoxy-
N-phenylaniline (3bl) in 88% yield. Thus, the catalytic
reaction process developed in this study provided an easy
way to make secondary and tertiary amines from various
aldehydes and amines. In addition, the reaction can be
conducted on a gram-scale (2.0 g of 3aa, 94% yield).
3da, 95%
3ba, 98%
3ea, 89%
3ha, 90%
3ca, 95%
3ga, 93%
3ah, 95%
3ak, 82%
3fa, 97%
3ag, 87%e
Scheme 3. Synthesis of Isoindolin-1-ones
The amination of aldehydes can be extended to the reac-
tions of 2-formylbenzoic acid. Reaction of 2-
formylbenzoic acid with anilines afforded excellent yields
of isoindolin-1-ones (4) (Scheme 3). The highest yield
(98%) was observed with 4-methoxyaniline.
3aj, 91%
3ia, 96%
3la, 86%c
3ai, 98%
3bl, 88%f
Generally, aromatic amines are produced by a catalytic
reduction of nitroarenes.13 Therefore, the use of ni-
troarenes instead of aromatic amines in the reductive
amination of aldehydes is highly desirable because it does
not need prior reduction of the respective nitroarenes.
Hydrogenation of nitroarenes over catalyst in the pres-
ence of CO and water has long history.14 However, the
tandem reductive amination is relatively rare.15 It is still
highly desirable to develop an effective catalyst for this
transformation. Thus, we decided to study the use of ni-
troarenes as an amine source in the amination of alde-
hydes and ketones. The optimum reaction conditions
were as follows: 5 mol% Co2Rh2/C in 0.15 mL H2O and 3.0
3ja, 96%c
3ma, 84%c
3ka, 96%
a
Reaction conditions: aldehyde (1.0 mmol), amine (1.0 mmol), H2O
(0.03 mL), CO (5 atm), and Co2Rh2 catalyst (5 mol%, 90 mg) in 3 mL
THF at 100 oC for 6 h, Isolated yield. b 6 h and 130 oC. c 12 h and 130oC.
d
o
f
aldehyde (2.0 mmol) used, 14 h, 130 C. e 4-vinylaniline used. 14 h
and 100 oC. g gram scale (2.0 g of 3aa)
With the optimized reaction conditions in hands, the
substrate scope of the reaction was examined (Table 2).
The amination reaction found to be effective for all rele-
vant substrates, including aromatic and aliphatic primary
o
mL THF at 120 C for 24 h (for screening the reaction
conditions, see SI). The results were summarized in Table
3.
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Table 3. Amination of Aldehydes and Ketones with
Nitroarenes.a
1
2
3
4
15
74
Aldehyde or
Ketone
Nitroarene
Product
Yield
(%)b
3ja
Entry
16
88
5
6
7
8
9
NO2
3ka
1
92
90
90
76
75
93
91
3ab
OMe
N
17
18
93
90
10
11
12
13
14
15
16
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19
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5
2
3
4
5
6
7
3ah
3ai
6
a
Reaction conditions: aldehyde (0.5 mmol), nitroarene (0.5 mmol),
H2O (0.15 mL), CO (5 atm) and Co2Rh2 catalyst (5 mol%, 45 mg) in 3
mL THF at 120 oC for 24 h. b Isolated yield.
All the yields observed with nitroarenes (entries 1 ~ 16)
were slightly lower than those observed with amines.
However, as shown in the Table 2, the reactions of ben-
zaldehyde with various nitroarenes except with 4-fluoro-
and 4-chloroaniline still afforded N-benzyl-anilines in
excellent yields. With 4-fluoro and 4-chloroanilines (en-
tries 4 and 5), the corresponding secondary amines were
isolated in 75% and 76% yield, respectively. Interestingly,
the reaction was tolerant of an ester group (entry 7). 1-
Nitronaphthalene also afforded an excellent yield (90%)
of the corresponding amine (entry 8). Benzaldehydes hav-
ing an electron-withdrawing group afforded slightly lower
yields than those with an electron-donating group (en-
tries 9 - 11 vs 12 - 14). Aliphatic ketones such as acetone
and cyclohexanone also afforded the corresponding
amines in high yields (entries 15 and 16).However, acetone
(74%) was not a good substrate as cyclohexanone (88%).
Reaction of 1-methyoxy-4-nitrobenzene with 2,5-
hexandione afforded 1-(4-methoxyphenyl)-2,5-dimethyl-
1H-pyrrole (5), in 94% yield (entry 17). An intramolecular
reaction of 3-(2-nitrophenyl)acrylaldehyde afforded quin-
olone (6) in 92% yield (entry 18). For entries 17 and 18, the
reduction of nitro group was observed, but the reductive
amination product was not formed. The amination of
aldehydes and ketones with nitroarenes were quite effec-
tive under our reaction conditions. However, when 1-
nitrohexane was used as a nitrogen source, no amination
was observed.
3aj
3ak
3aa
O
O
N
H
3al
8
9
90
89
90
92
87
80
3am
3da
3ba
10
11
When a reaction of benzaldehyde with 1-methoxy-4-
nitromethoxybenzene was conducted in the presence of D2O,
deuterated amine was observed (Scheme 4). Thus, sequential
reactions of hydrogenation and amination occurred under
our reaction conditions.
3ca
3ea
3fa
3ga
12
13
14
84
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investigations of the present catalytic system to other
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reactions are ongoing in our laboratory.
ASSOCIATED CONTENT
Scheme 4. Deuterium Labelling Experiment
Supporting Information.
General experimental procedure and characterization of all
compounds are provided. This material is available free of
charge via the Internet at http://pubs.acs.org
When we used hydrogen itself instead of carbon monoxide
under identical conditions, the expecting product was ob-
tained with a comparable yield (Scheme 5).
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AUTHOR INFORMATION
Corresponding Author
* E-mail; ykchung@snu.ac.kr
Scheme 5. Reductive Amination with Hydrogen Gas
Notes
The authors declare no competing financial interest.
The reusability of Co2Rh2/C was also examined for the
amination of 1a with 2a (Table 4). After reaction, the cata-
lyst was filtered from the reaction mixture, dried in vacu-
um, and reused for the further catalytic reaction. The cat-
alytic system is stable under the reaction conditions. The
catalyst maintained its high level of activity even after
being reused eight times (97%, 95%, 94%, 92%, 92%, 93%,
90%, and 92%, respectively); the maximum reusability
has not been tested.
ACKNOWLEDGMENT
This work was supported by a National Research Foundation
of Korea (NRF) grant funded by the Korean govern-
ment (2014R1A5A1011165 and 2007-0093864). JWP is a recipi-
ent of BK21 plus Fellowship.
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Table 4. Reuse of Co2Rh2/C Catalyst for Reductive
Amination of 1a with 2a a
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Entry
Catalyst
Yield (%)b
1
Co2Rh2 5 mol%
97
95
94
92
92
93
90
92
2
3
4
5
6
7
8
Recovered from #1
Recovered from #2
Recovered from #3
Recovered from #4
Recovered from #5
Recovered from #6
Recovered from #7
a1a (1.0 mmol), 2a (1.0 mmol), H2O (0.03 mL), CO (5 atm), and
o
b
Co2Rh2 catalyst (5 mol%, 90 mg) in 3 mL THF at 100 C for 6 h. I-
solated yield.
In conclusion, we have developed the first Co2Rh2 nano-
particles/charcoal-catalyzed reductive amination of alde-
hydes and ketones with amines using a water-gas-shift
reaction instead of hydrogen. Advantageously, no compli-
cated ligands or additional acid or base is needed. The
reaction can be extended to the tandem reduction of al-
dehydes and ketones with nitroarenes. The experimental
simplicity and the reusability are especially attractive and
should encourage the use of this catalytic system among
synthetic chemists and in industrial application. Further
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