Chemoselective synthesis of imine and secondary amine from nitrobenzene and benzaldehyde by Ni3Sn2 alloy catalyst supported on TiO2

Article abstract of DOI:10.1016/j.mcat.2021.111503

Ni₃Sn₂/TiO₂ alloy catalyst prepared by hydrothermal method was applied for the direct one-pot reductive imination of benzaldehyde with nitrobenzene in the presence of H₂ gas as a reducing agent. A desired imine, benzylideneaniline, was produced in an excellent yield of 90% without the formation of any by-products. This is due to the chemoselective molecular recognition of Ni₃Sn₂ alloy nanoparticles supported on TiO₂ for the nitro group over the carbonyl group. By prolonging the reaction time, the desired imine was converted into the corresponding secondary amine, N-phenylbenzylamine, with a remarkably high yield of 80% through the catalytic hydrogenation of the C[dbnd]N bond. Finally, we demonstrated that it was easy to reuse the catalyst up to five times.

Full text of DOI:10.1016/j.mcat.2021.111503

Molecular Catalysis 505 (2021) 111503
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Chemoselective synthesis of imine and secondary amine from nitrobenzene
and benzaldehyde by Ni3Sn2 alloy catalyst supported on TiO2
,
,
Nobutaka Yamanakaa b, Takayoshi Haraa, Nobuyuki Ichikunia, Shogo Shimazua *
a Department of Applied Chemistry and Biotechnology, Graduate School of Science and Engineering, Chiba University, 1-33 Yayoi, Chiba 263-8522, Japan
b Department of Applied Chemistry, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
Ni3Sn2/TiO2 alloy catalyst prepared by hydrothermal method was applied for the direct one-pot reductive
imination of benzaldehyde with nitrobenzene in the presence of H2 gas as a reducing agent. A desired imine,
benzylideneaniline, was produced in an excellent yield of 90% without the formation of any by-products. This is
due to the chemoselective molecular recognition of Ni3Sn2 alloy nanoparticles supported on TiO2 for the nitro
group over the carbonyl group. By prolonging the reaction time, the desired imine was converted into the
Ni3Sn2/TiO2 alloy catalyst
Nitrobenzene
Benzaldehyde
Imine
Secondary amine
corresponding secondary amine, N-phenylbenzylamine, with a remarkably high yield of 80% through the cat-
–
alytic hydrogenation of the C N bond. Finally, we demonstrated that it was easy to reuse the catalyst up to five
–
times.
1. Introduction
catalysts.
For the cascade reductive imination to be successful, not carbonyl
Nitrogen-containing compounds such as primary, secondary, and
tertiary amines, imines, and azo compounds have attracted enormous
attention for a long history [1–4]. In particular, imines are highly
desirable due to their wide applications as electrophilic reagents for
diverse organic reactions such as reductions, additions, condensations,
compounds but nitroarenes should be reduced first. Then, the coupling
between in situ formed primary amines and carbonyl compounds occurs
rapidly without the aid of any catalyst and reaches equilibrium (Scheme
1). From our continuing works on Ni3Sn2 alloy catalysis, we discovered
that the highly dispersed Ni3Sn2 alloy nanoparticles on TiO2 support
were efficient for the chemoselective hydrogenation of a broad range of
functionalized nitroarenes [20]. Notably, other reactive groups such as
olefin, chloride, ketone, and ester groups remain unaffected before
almost complete exhaustion of the nitro group. Due to the prominent
efficiency of Ni3Sn2/TiO2 alloy catalyst for the chemoselective nitro
hydrogenation, we envisioned that the Ni3Sn2/TiO2 alloy-catalyzed
strategy could offer a new efficient protocol for the direct one-pot
reductive imination of carbonyl compounds with nitroarenes.
–
and cycloadditions [5–7]. The formation of the C N bond involved in
–
the synthesis of imines is one of the most important areas in synthetic
organic chemistry [6,8–12].
Traditionally, imines have been synthesized from the dehydrative
condensation between primary amines and carbonyl compounds in the
presence of an acid catalyst, the self-condensation of primary amines
upon oxidation, and the oxidation of secondary amines [7,13–17]. The
one-pot conversion of alcohols with primary amines has also been
developed for producing imines although a large excess of oxidants was
required [7,18]. Compared with these methods using primary amines,
one-pot reductive imination of carbonyl compounds directly from
nitroarenes is conceived to be especially attractive from the viewpoint of
green sustainable chemistry because it does not require prior reduction
of nitroarenes [1,19]. In spite of the great progress being made in the
selective imine formation, there are scarcely any available reports
dealing with the direct one-pot reductive imination of carbonyl com-
pounds with nitroarenes by non-noble metal-based heterogeneous
Secondary amines, which are important building blocks used for the
synthesis of various functional organic molecules such as polymers,
additives, dyes, pharmaceuticals, herbicides, surfactants, and bioactive
molecules, can be obtained via the one-pot reductive imination of
carbonyl compounds with nitroarenes in the presence of H2 gas as a
reducing agent (Scheme 1) [1,21–23]. To the best of our knowledge, the
direct synthesis of secondary amines from carbonyl compounds and
nitroarenes has been performed mainly by noble metal-based hetero-
geneous catalysts. In this context, our developed non-noble metal-based
* Corresponding author.
E-mail address: shimazu@faculty.chiba-u.jp (S. Shimazu).
https://doi.org/10.1016/j.mcat.2021.111503
Received 2 February 2021; Received in revised form 18 February 2021; Accepted 21 February 2021
Available online 30 March 2021
2468-8231/© 2021 Elsevier B.V. All rights reserved.

N. Yamanaka et al.
Molecular Catalysis 505 (2021) 111503
alloy catalyst Ni3Sn2/TiO2 was applied for the direct one-pot reductive
amination of carbonyl compounds with nitroarenes.
With Ni3Sn2/TiO2 alloy catalyst, we aimed to evaluate the catalytic
property for the one-pot reaction of benzaldehyde with nitrobenzene in
different reaction solvents under H2 1.0 MPa at 423 K (Table 1). The
physicochemical properties such as the particle size of Ni3Sn2 alloy, H2
and CO uptakes, BET surface area, and average pore volume are shown
in our previous report [20]. Initially, 1,4-dioxane solvent was employed
(Table 1, entries 6 and 7). The favored hydrogenation of nitrobenzene
over benzaldehyde proceeded, providing the desired imine benzylide-
neaniline (1) as a consequence of the condensation between in situ
formed aniline and benzaldehyde. No formation of the by-product
benzyl alcohol (3) was observed during the early stage of the reaction.
This is due to the chemoselective molecular recognition of Ni3Sn2 alloy
nanoparticles supported on TiO2. The imine yield was 37% at the
benzaldehyde conversion of 40% after 12 h. The product distribution
differed depending on the reaction time. After 24 h, the other desired
secondary amine N-phenylbenzylamine (2) through the catalytic hy-
2. Experimental section
2.1. Materials
All chemicals were commercially available and used as received
without further purification. NiCl2, SnCl2∙2H2O, NaOH, TiO2, nitro-
benzene, benzaldehyde, aniline, benzyl alcohol, naphthalene, 1,4-
dioxane, p-xylene, o-xylene, mesitylene, methanol (MeOH), ethanol
(EtOH), tetrahydrofuran (THF), triethylamine (NEt3), and N,N-dime-
thylacetamide (DMA) were supplied from Wako. Benzylideneaniline
and N-phenylbenzylamine were purchased from TCI.
2.2. Catalyst preparation
–
drogenation of the C N bond in benzylideneaniline was obtained in a
–
moderate yield of 50%, along with a large amount of benzyl alcohol.
These results indicate that 1,4-dioxane solvent was not suitable for the
one-pot synthesis of the nitrogen-containing compounds.
Ni3Sn2 alloy catalyst supported on TiO2 were prepared by hydro-
thermal method in accordance with the procedure reported earlier [20].
Briefly, Ni(II) and Sn(IV) precursors were mixed at Ni:Sn molar ratio of
1.5 at room temperature. TiO2 was then added into the mixture as a
support and stirred for 1 h at room temperature. The resulting solution
was hydrothermally treated at 423 K for 24 h, filtered, washed with
distilled water, and dried under vacuum at room temperature overnight.
Finally, the recovered powder was reduced under H2 atmosphere at
673 K for 1 h to obtain 10 wt% Ni3Sn2/TiO2 alloy catalyst.
The reaction solvent has been reported to strongly affect the catalytic
activity and product distribution [24]. Therefore, the effect of the sol-
vent on the catalytic performance of Ni3Sn2/TiO2 alloy catalyst for the
one-pot reaction was investigated to explore the best solvent under
identical reaction conditions (H2 1.0 MPa, 423 K). The results are
summarized in Table 1. The use of protic polar solvents such as MeOH
2.3. Catalytic test
Table 1
Effect of solvent on the catalytic performance of Ni3Sn2/TiO2 alloy catalyst for
the direct one-pot reaction of benzaldehyde with nitrobenzenea.
The one-pot reaction of benzaldehyde with nitrobenzene was carried
out in a high-pressure autoclave with a pressure gauge, a magnetic
stirrer, and an oil bath. Typically, Ni3Sn2/TiO2 alloy catalyst (0.10 g),
nitrobenzene (1.0 mmol), benzaldehyde (1.0 mmol), naphthalene
(0.3 mmol) as an internal standard material, and mesitylene solvent
(5.0 mL) were placed into a glass reaction tube and stirred at room
temperature for a selected time. After the autoclave was sealed, pure H2
gas was introduced in order to remove air from the system and kept at a
desired pressure, and then the reaction system was heated to a given
temperature. After the reaction, the reaction mixture was taken out of
the reaction system and analyzed by gas chromatography. The products
were identified by gas chromatography (GC-8A, Shimadzu, using a
flame ionization detector) equipped with a flexible quartz capillary
column coated with Silicon OV-17. The conversion and yields of the
products were calculated using an internal standard method.
Yield (%)c
Entry
Solvent
Conv. (%)b
Sel. (%)d
1
2
3
1
MeOH
EtOH
100
100
100
25
0
58
52
17
0
40
37
57
0
58
57
34
88
90
93
55
91
76
100
100
95
95
2
5
3
DMA
17
22
52
37
5
4
THF
5
NEt3
58
0
0
6
1,4-Dioxane
40
0
0
7e
8
100
90
50
0
45
0
o-Xylene
Mesitylene
p-Xylene
82
21
64
90
74
78
9e
10
11e
12
13e
100
64
54
0
25
0
90
0
0
78
0
0
82
0
0
a
Reaction conditions: nitrobenzene, 1.0 mmol; benzaldehyde, 1.0 mmol;
Ni3Sn2/TiO2 alloy catalyst, 0.10 g; solvent, 5.0 mL; naphthalene, 0.30 mmol;
H2, 1.0 MPa; reaction temperature, 423 K; reaction time, 12 h. The conversion
and yields were determined by GC using an internal standard technique.
3. Results and discussion
3.1. Effect of solvent
b
c
Conversion of benzaldehyde.
Yields of entries 1, 2, and 3 represent benzylideneaniline, N-phenylbenzyl-
In this work, we began our study by using nitrobenzene and benz-
aldehyde as starting materials. The desired products are imine inter-
mediate benzylideneaniline (1) and over-hydrogenated secondary
amine N-phenybenzylamine (2). The by-product is benzyl alcohol (3).
amine, and benzyl alcohol, respectively, as shown in Scheme 1.
d
Selectivity to benzylideneaniline and N-phenylbenzylamine.
e
Reaction time, 24 h.
Scheme 1. Synthetic pathway of nitrogen-containing compounds starting from nitrobenzene and benzaldehyde.
2

N. Yamanaka et al.
Molecular Catalysis 505 (2021) 111503
and EtOH produced the highest catalytic activity but had a negative
impact on the selectivity to the imine and secondary amine (Table 1,
entries 1 and 2). Some researchers have proposed that a strong inter-
action between protic polar solvents (e.g., C1-C4 primary alcohols) and
2-butanone by hydrogen bonding dramatically lowers the activation
energy barrier and significantly enhances the catalytic activity [25,26].
The catalytic activity of Ni3Sn2/TiO2 alloy catalyst in aprotic polar DMA
solvent, was next to protic polar solvents (Table 1, entry 3). The use of
other aprotic polar solvents such as THF, NEt3, and 1,4-dioxane led to
low-to- moderate catalytic activity (Table 1, entries 4–6). This is prob-
ably due to the strong adsorption of the solvent molecules on the catalyst
surface blocking the accessibility of the active Ni sites [26–28].
Non-polar solvents such as p-xylene, o-xylene, and mesitylene showed a
lower catalytic activity but provided the imine in higher yields without a
hint of benzyl alcohol formation, when compared with aprotic polar
DMA solvent (Table 1, entries 8, 10, and 12). It can be concluded that the
catalytic activity is higher in a more polar solvent with the following
order: protic polar solvent > aprotic polar solvent > non-polar solvent.
The direct one-pot reaction of benzaldehyde with nitrobenzene in the
non-polar solvents steadily proceeded with increasing the reaction time
(Table 1, entries 9, 11, and 13). Although the complete conversion of
benzaldehyde was achieved using o-xylene after 24 h, the yield of the
imine largely decreased and the secondary amine was obtained in 54%
yield together with the by-product (Table 1, entry 9). In contrast, se-
lective formation of the imine was accomplished by employing mesity-
lene and p-xylene (Table 1, entries 11 and 13). The benzaldehyde
conversions were 90% and 82% less than 100%, respectively. On the
basis of these results, it can be assumed that the formation of the desired
secondary amine N-phenylbenzylamine and the by-product benzyl
alcohol is inhibited before the complete exhaustion of benzaldehyde.
Mesitylene yielded the desired imine compound almost quantitatively
and therefore was shown to be the best solvent for the Ni3Sn2/TiO2
alloy-catalyzed reductive imination of benzaldehyde with nitrobenzene.
Fig. 1 presents the time profile for the Ni3Sn2/TiO2 alloy-catalyzed
one-pot reaction of benzaldehyde with nitrobenzene in mesitylene. At
a reaction time of 6 h, the in situ formed aniline (◆) was condensed with
benzaldehyde, giving rise to only the desired imine product (▴). The
yield of the imine (▴) steeply increased with increasing the reaction time
along with a small amount of aniline (◆), and it reached 90% after 24 h.
The formation of the corresponding secondary amine (●) was observed
at 30 h when the benzaldehyde conversion was almost full, more pre-
cisely, 98%. The yield of the secondary amine (●) dramatically
increased to a remarkably high yield of 80% at only 6 h. The control of
the reaction time in mesitylene can allow an efficient one-pot synthesis
of the imine and secondary amine. Notably, the yield of the secondary
amine (●) was slightly lower than that of the parent imine (▴). This is
because in situ formed H2O resulted in recovery of aniline (◆) and
benzaldehyde as a consequence of the imine hydrolysis [29].
3.2. Effect of the addition of molecular sieve 3 Å
The reverse transformation of the imine benzylideneaniline into
benzaldehyde and aniline in the presence of the in situ produced water
prompted us to add molecular sieve 3 Å into the catalytic system for the
removal. The time profile is illustrated in Fig. 2. In comparison with
Fig. 1, it has been proven that the addition of the molecular sieve plays a
significant role in accelerating the imine formation step during the
present one-pot reaction. However, the reverse reaction into aniline and
benzaldehyde proceeded regardless of the presence of the molecular
sieve. A similar yield of the imine and secondary amine to that of the
catalytic system without the molecular sieve was obtained at shorter
reaction times of 19 h and 30 h, respectively.
3.3. Effect of H2 pressure
Optimization of the reaction conditions was carried out for the one-
pot reaction of benzaldehyde with nitrobenzene using Ni3Sn2/TiO2 alloy
catalyst. First, the effect of H2 pressure was investigated at 423 K in
mesitylene. The results are given in Table 2. A lower H2 pressure of
0.6 MPa decreased the yield of the imine (Table 2, entry 1). This is
because the hydrogenation of nitrobenzene to aniline is sensitive to the
initial H2 pressure. The results presented in Table 2, entries 1 and 2 show
that the kinetic behavior of Ni3Sn2/TiO2 alloy catalyst during this pro-
cess enables the selective formation of the imine before the complete
exhaustion of benzaldehyde. To further improve the catalytic efficiency,
we performed the one-pot reaction under a higher H2 pressure of
2.0 MPa (Table 2, entry 3). In this situation, the imine was converted
into the corresponding secondary amine in a moderate yield, accom-
panied by the unwanted side product because the benzaldehyde con-
version reached 100%.
3.4. Effect of reaction temperature
Next, the effect of reaction temperature was studied under H2
1.0 MPa in mesitylene. The results are listed in Table 3. A relatively high
Fig. 1. Time profile for the one-pot reaction of benzaldehyde with nitroben-
zene by Ni3Sn2/TiO2 alloy catalyst in mesitylene. Reaction conditions: nitro-
benzene, 1.0 mmol; benzaldehyde, 1.0 mmol; Ni3Sn2/TiO2 alloy catalyst,
0.10 g; mesitylene, 5.0 mL; naphthalene, 0.30 mmol; H2, 1.0 MPa; reaction
temperature, 423 K.
Fig. 2. Time profile for the one-pot reaction of benzaldehyde with nitroben-
zene by Ni3Sn2/TiO2 alloy catalyst in mesitylene. Reaction conditions: nitro-
benzene, 1.0 mmol; benzaldehyde, 1.0 mmol; Ni3Sn2/TiO2 alloy catalyst,
0.10 g; mesitylene, 5.0 mL; naphthalene, 0.30 mmol; H2, 1.0 MPa; reaction
temperature, 423 K; molecular sieve 3 Å, 0.10 g.
3

N. Yamanaka et al.
Molecular Catalysis 505 (2021) 111503
Table 2
Table 4
Effect of H2 pressure on the catalytic performance of Ni3Sn2/TiO2 alloy catalyst
Reusability test of Ni3Sn2/TiO2 alloy catalyst for the one-pot reaction of benz-
for the one-pot reaction of benzaldehyde with nitrobenzenea.
aldehyde with nitrobenzenea.
Run
Yield (%)c
1
2
3
4
5
Entry
H2 pressure (MPa)
Conv. (%)b
Sel. (%)d
1
2
3
Conv. (%)b
90
85
80
78
75
Sel. (%)c
100
100
100
100
100
1
2
3
0.6
1.0
2.0
73
68
90
20
0
0
93
a
90
0
0
100
74
Reaction conditions: nitrobenzene, 1.0 mmol; benzaldehyde, 1.0 mmol;
Ni3Sn2/TiO2 alloy catalyst, 0.10 g; mesitylene, 5.0 mL; naphthalene, 0.30 mmol;
H2, 1.0 MPa; reaction temperature, 423 K; reaction time, 24 h. The conversion
was determined by GC using an internal standard technique.
100
54
22
a
Reaction conditions: nitrobenzene, 1.0 mmol; benzaldehyde, 1.0 mmol;
Ni3Sn2/TiO2 alloy catalyst, 0.10 g; mesitylene, 5.0 mL; naphthalene, 0.30 mmol;
reaction temperature, 423 K; reaction time, 24 h. The conversion and yields
were determined by GC using an internal standard technique.
b
c
Conversion of benzaldehyde.
Selectivity to benzylideneaniline.
b
Conversion of benzaldehyde.
c
Yields of entries 1, 2, and 3 represent benzylideneaniline, N-phenylbenzyl-
amine, and benzyl alcohol, respectively, as shown in Scheme 1.
d
Selectivity to benzylideneaniline and N-phenylbenzylamine.
Table 3
Effect of reaction temperature on the catalytic performance of Ni3Sn2/TiO2 alloy
catalyst for the one-pot reaction of benzaldehyde with nitrobenzenea.
Yield (%)c
Sel.
Entry
Temp. (K)
Conv. (%)b
(%)d
1
2
3
1
2
3
403
423
443
81
75
90
54
0
0
93
90
0
0
100
76
100
22
20
a
Reaction conditions: nitrobenzene, 1.0 mmol; benzaldehyde, 1.0 mmol;
Ni3Sn2/TiO2 alloy catalyst, 0.10 g; mesitylene, 5.0 mL; naphthalene, 0.30 mmol;
H2, 1.0 MPa; reaction time, 24 h. The conversion and yields were determined by
GC using an internal standard technique.
b
Conversion of benzaldehyde.
c
Yields of entries 1, 2, and 3 represent benzylideneaniline, N-phenylbenzyl-
amine, and benzyl alcohol, respectively, as shown in Scheme 1.
d
Selectivity to benzylideneaniline and N-phenylbenzylamine.
Fig. 3. XRD patterns of (a) the fresh and (b) spent Ni3Sn2/TiO2 alloy catalysts.
yield of 75% for the imine was obtained at 403 K. The increase of the
reaction temperature to 443 K increased the benzaldehyde conversion to
100%. This is why the formation of the secondary amine and benzyl
alcohol occurred.
materials into the desired imine benzylideneaniline in an excellent yield
of 90% after 24 h, without the formation of the by-product benzyl
alcohol. The catalyst efficiency is ascribed to the high chemoselectivity
for the nitro group over the carbonyl group. Prolonging the reaction
time to 36 h produced the corresponding secondary amine N-phenyl-
benzylamine with a remarkably high yield of 80% through the catalytic
3.5. Reusability test
Using the optimized reaction conditions, the reusability of Ni3Sn2/
TiO2 alloy catalyst was investigated for the one-pot reaction of benzal-
dehyde with nitrobenzene, and the result is shown in Table 4. After the
reaction, the catalyst was separated from the reaction solution by
centrifugation, thoroughly washed with acetone, dried in vacuum
overnight, and then reused for the next run under the same reaction
conditions. Ni3Sn2/TiO2 alloy catalyst can be reused at least five times
for the one-pot reaction. The selectivity of the imine remained constant
at 100% during a period of five successive runs. A slight decay in the
catalytic activity was observed after the first run, likely due to the
oxidation of the catalyst surface [30]. However, XRD analysis showed
that the spent catalyst had no appreciable change relative to the fresh
catalyst (Fig. 3). The XRD patterns of the fresh and spent catalysts
exhibited three characteristic peaks at 2θ = 30.8◦, 42.5◦, and 44.2◦,
which correspond to the Ni3Sn2 (101), Ni3Sn2 (102), and Ni3Sn2 (110)
diffraction peaks, respectively [31].
–
hydrogenation of the C N bond. The addition of the molecular sieve
–
into the catalytic system has a positive effect on the reaction time to
produce the imine and secondary amine. In summary, the one-pot
reductive imination and amination of benzaldehyde with nitrobenzene
was accomplished using Ni3Sn2/TiO2 alloy catalyst under the optimized
reaction conditions (H2 1.0 MPa, 423 K). This is the first time that the
two nitrogen-containing compounds have been synthesized over a non-
noble metal catalyst using an easily available and environmentally
benign hydrogen source.
CRediT authorship contribution statement
Nobutaka Yamanaka: Conceptualization, Methodology, Valida-
tion, Writing - original draft. Takayoshi Hara: Data curation.
Nobuyuki Ichikuni: Data curation. Shogo Shimazu: Supervision,
Writing - review & editing, Conceptualization.
4. Conclusions
Declaration of Competing Interest
We have developed a simple, eco-friendly, and cost-effective cata-
lytic system for the direct one-pot reductive imination and amination of
benzaldehyde with nitrobenzene. The relationship between the catalytic
activity and solvent properties used was revealed. In non-polar mesity-
lene solvent, Ni3Sn2/TiO2 alloy catalyst transformed the starting
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
4

N. Yamanaka et al.
Molecular Catalysis 505 (2021) 111503
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