Nanoporous palladium catalyzed one-pot synthesis N-alkyl amines by hydrogen transfer reaction under mild conditions

Full text of DOI:10.1016/j.inoche.2020.108143

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Short communication
Nanoporous palladium catalyzed one-pot synthesis N-alkyl amines by hydro-
gen transfer reaction under mild conditions
Yang Cao, Zhiwen Li, Chao Hou, Xiaoqun Cao
PII:
S1387-7003(20)30733-4
DOI:
https://doi.org/10.1016/j.inoche.2020.108143
INOCHE 108143
Reference:
To appear in:
Inorganic Chemistry Communications
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Revised Date:
Accepted Date:
16 February 2020
19 July 2020
19 July 2020
Please cite this article as: Y. Cao, Z. Li, C. Hou, X. Cao, Nanoporous palladium catalyzed one-pot synthesis N-
alkyl amines by hydrogen transfer reaction under mild conditions, Inorganic Chemistry Communications (2020),
doi: https://doi.org/10.1016/j.inoche.2020.108143
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Short communication
Nanoporous palladium catalyzed one-pot synthesis N-alkyl amines by
hydrogen transfer reaction under mild conditions
Yang Cao, Zhiwen Li*, Chao Hou, Xiaoqun Cao
School of Chemistry and Pharmaceutical Engineering, Shandong First Medical
University ＆ Shandong Academy of Medical Science, Taian 271016, PR China
* Corresponding authors.
E-mail addresses: lzwaizmandpan@126.com
The one-pot strategy has been extensively used in organic synthesis. When using this
strategy, choosing the appropriate substrate and developing an effective design
decreases the reaction steps, increases efficiency, and allows the reaction to be greener
and more economical [1]. A suitable catalyst allows the system to operate smoothly and
under mild conditions [2-4]. The hydrogen transfer reaction is an effective approach to
promote the overall strategy [5].
Due to their fundamental role in biologically active natural products,
pharmaceuticals, dyes, and various organic molecules, amines have attracted increased
research attention [6]. As common intermediates, N-alkyl amines (secondary amines)
have been extensively studied among amine compounds. There are many different
methods to synthesize N-alkyl amines [7-10]. However, expensive organic halides are
conventionally used as alkylating reagents, the preparation methods are
environmentally unfriendly, and secondary and tertiary amine separation has
limitations [11, 12]. The hydrogenation of imines has proven to be a mild method,
wherein the raw material is obtained by coupling aromatic aldehydes and aromatic
amines [13, 14]. On this basis, if nitroarenes (the main precursor of aromatic amines
[15, 16]) are used as the raw material instead of aromatic amines, this hydrogen-
transfer-based approach would be a valuable, atom-economic, environmentally
attractive, one-pot, multistep transformation [17-19]. This system involves two
hydrogenation processes: reduction of nitroarenes and hydrogenation of imines. The
hydrogen transfer reaction with a suitable hydrogen source may be a good choice for

the one-pot reductive N-alkylation of nitroarenes. Nevertheless, the specific condition
that promotes the optimal performance of the system still needs to be determined.
Selection of a suitable hydrogen source and catalyst plays an important role in the
hydrogen transfer reaction. In a previous study, the oxidation of hydrosilane by ROH
(water or alcohol) [20, 21] was conducted to produce active hydrogen during the
reaction. This silane-based hydrogen source is widely used not only in the hydrogen
transfer reaction but also in the production of molecular hydrogen on demand. With
nanoporous gold (np-Au) as the catalyst, semihydrogenation of terminal and internal
alkynes was realized using a np-Au/hydrosilane/water system [22]. Although the
reduction of imines or ketimines could also be realized using this system [23, 24], due
to the low hydrogenation activity of gold, the reaction also needed high temperature.
The raw materials were aromatic amines rather than nitroarenes. Optimizing the system
is essential to further develop a silane-based hydrogen source. Generally, palladium
catalysts have excellent hydrogenation activities, while the reduction of nitroarenes is
promoted by nanoporous Pd (np-Pd) with hydrosilane/alcohol as the hydrogen source
[25]. We speculated that the np-Pd/hydrosilane/alcohol system could be a good
combination for the one-pot synthesis of N-alkyl amines by reductive N-alkylation of
nitroarenes under mild conditions. In this system (Scheme 1), nitroarenes with aromatic
aldehydes were used as the raw material, and the oxidation of hydrosilane supplied
active hydrogen. Nitroarenes were reduced to aromatic amines, which were coupled
with aromatic aldehydes to obtain imines. The corresponding N-alkyl amines were
obtained after the hydrogenation of imines.
Scheme 1. Schematic representation of one-pot synthesis of N-alkyl amines by
reductive N-alkylation of nitroarenes with silane/alcohol as the hydrogen source.

In the present work, we demonstrate that one-pot synthesis of N-alkyl amines is
realized by the hydrogen transfer reaction with the np-Pd/hydrosilane/alcohol system
under mild conditions (room temperature and atmospheric pressure). Nitroarenes,
aromatic amines, and imines could be the raw materials for the synthesis of N-alkyl
amines. When nitroarenes and aromatic aldehydes were used as substrates, a wide range
of N-alkylation amines could be obtained in high yields. This was a new attempt to use
hydrosilanes/alcohol as the hydrogen source for the continuous hydrogenation system.
The catalyst was prepared by dealloying an aluminum-palladium alloy (Al75Pd25,
at.%; for the preparation of the alloy, see ESI 1. General). Aluminum in the alloy was
leached by sodium hydroxide (NaOH 2.5 M) at room temperature, and after 24 h,
nanoporous Pd was obtained. Scanning electron microscopy (SEM) was used to
observe the morphologies of np-Pd samples. As shown in Fig. 1 (a), a three-dimensional
bicontinuous nanoporous structure was observed, and the ligament size was below 10
nm. The Brunner-Emmet-Teller (BET) measurements results showed that the specific
surface area of the catalyst was 30.6 m2/g, and the Barrett-Joyner-Halenda (BJH) results
showed the nano-pore diameter was approximately 5 nm (see ESI Fig. S3).
Fig. 1 (a) SEM image of dealloyed np-Pd before catalytic reaction; (b) SEM image of
dealloyed np-Pd after being used eight times.
The hydrogenation of the carbon-nitrogen double bonds (C=N) was first tested. The
imine N-(4-methoxybenzylidene)aniline (4-CH3-O-Ph-CH=N-Ph, 1a) was chosen as
the raw material, with np-Pd as the catalyst. Triethylsilane (Et3SiH) and methanol
(MeOH) acted as the hydrogen source, while methanol was used as the solvent. During

the reaction, minimal gas was generated, and the active hydrogen reacted with C=N.
After 2.5 h, the corresponding N-alkylation amine N-(4-methoxybenzyl)aniline (4-
CH3-O-Ph-CH2-NH-Ph, 2a) was obtained in high yield at room temperature and
atmospheric pressure (Entry 1, Table 1). Minimal aniline (2%) was found by HPLC,
which indicated that the carbon-nitrogen double bond (C=N) may break during the
reaction. The main product and oxidation product of silane were purified by column
chromatography. These results illustrated that the np-Pd/hydrosilane/alcohol system
could meet the requirement for imine reduction.
Table 1. Reduction of N-(4-methoxybenzylidene)aniline (4-CH3-O-Ph-CH=N-Ph, 1a)
with different hydrosilane/alcohol hydrogen sources.
Entry a)
Silane
Et3SiH
ROH
MeOH
EtOH
Time/h
Yield/%
97
1
2
3
6
6
3
6
4
6
Et3SiH
96
3
Et3SiH
iPrOH
MeOH
MeOH
MeOH
MeOH
77
4
(EtO)3SiH
(Me2SiH)2O
PhMe2SiH
Ph2MeSiH
95
5 b)
6
87
93
7
92
a) Reactions were performed using the catalyst (0.1 mmol), 1a (1.0 mmol), silanes (3.0
mmol), and ROH (4.0 mL) at room temperature. b) silanes (1.5 mmol)
To optimize the hydrogen source, different combinations of hydrosilane and alcohol
were tested. The reduction was complete with ethanol but the reaction time was long
(Entry 2, Table 1). Due to the low reaction rate between isopropyl alcohol (iPrOH) and
Et3SiH under the reaction conditions, the hydrogenation was slow with 77% yield.
Meanwhile, the C=N bond breaking reaction increased, and the yield of aniline reached
10% (Entry 3, Table 1). When triethoxysilane ((EtO)3SiH) was used with a long
reaction time, the yield reached 87% (Entry 4, Table 1). Additionally, 1,1,3,3-

tetramethyldisiloxane ((Me2SiH)2O), which contains two Si-H bonds in one molecule,
had qualitative efficacy similar to that of triethylsilane (Entry 5, Table 1). When
aromatic
silanes
such
as
dimethylphenylsilane
(PhMe2SiH)
and
dimethylphenylmethoxylsilane (Ph2MeSiH) were used in the system, the reaction
proceeded smoothly. Due to the steric hindrance of the benzene ring the yield was
slightly decreased (Entries 6 and 7, Table 1). Therefore, the Et3SiH/MeOH component
with low steric hindrance showed high reactivity in this system and was a suitable
hydrogen source combination for the following reactions.
To further understand the hydrogen transfer process, the effect of reactant quantity
was studied. First, the influence of imines was tested. As shown in Fig. S2a, the reaction
rate was linearly dependent on the concentration of imines. As shown in Fig. S2b, no
evident changes in the reaction rate were observed with the dosage of Et3SiH increased
from 2.0 mmol to 5.0 mmol. It can be speculated that the hydrogen-producing reaction
was fast on the nanoporous palladium surface, and the hydrogen source provided
enough hydrogen for the reduction reaction. The activation of the C=N bond and
hydrogenation was the rate-limiting step. Different imines as substituents were tested
(see ESI Table S1). Through these results, we speculated that the substituent on the
carbon side of benzene was crucial to the hydrogenation of the C=N bond, which is
similar to the findings of a previous study [26].
Then, aromatic amines instead of imines were used as raw materials. In our design,
imines could be obtained smoothly by the thermodynamically feasible reaction between
aniline (Ph-NH2, 3a) and 4-anisaldehyde (4-CH3O-Ph-CHO, 4a). Imine 1a could be
reduced to the corresponding secondary amine 2a. As shown in Table 2, when equal
dosages of 3a and 4a were added into methanol with Et3SiH simultaneously (Method
A), the yield was only 64%. When 3a and 4a were reacted without hydrosilanes, the
yield of imines was above 95%. This means that the reduction of 4a to 4-anisyl alcohol
inhibited the production of imines and negatively affected the yield. When excess 4a
was added, the yield of 2a increased as the ratio of 4a increased (Entries 2 and 3, Table
2). This illustrates that excess 4a caused the coupling between 3a and 4a to proceed
smoothly. In another Method (B), 3a and 4a were added first, and Et3SiH was added

after the imines were generated completely (after approximately 2.5 h). Compared with
Method A, under this condition, imines were obtained without interference and were
then hydrogenated. With Method B, the yield could reach 90%, and with increased
dosage of 4a, the yield increased to 94%. However, when the ratio increased from
1:1.25 to 1:1.5, the yield did not increase significantly.
Table 2. Synthesis of N-(4-methoxybenzyl)aniline from aniline and 4-anisaldehyde.
Entry
Ratio (3a:4a)
1:1
Method
Time/h
Yield/%
64
1
2
3
4
5
6
A
A
A
B
B
B
5
5
5
6
6
6
1:1.25
1:1.5
91
92
1:1
90
1:1.25
1:1.5
94
94
a) Reactions were performed using the catalyst (0.1 mmol), aniline (1.0 mmol), 4-
anisaldehyde (1.0 mmol ~1.5 mmol), Et3SiH (3.0 mmol), and MeOH (4.0 mL) at room
temperature. In Method A, Et3SiH was added simultaneously with other reactants; in
Method B, Et3SiH was added after 2.5 h.
Different aromatic amines and aromatic aldehydes (1:1.25) were then investigated as
raw materials. As shown in Table S2, when substituted aromatic aldehydes (2-HO-, 4-
CH3-) were used, the reactions took place smoothly, and the yield of the corresponding
N-alkylation amine was high. In particular, the yield from Method B was above 96%
(Entries 2 and 4, Table S2). When aniline (3a) and benzaldehyde (4d) were used, the
yields were 50% and 79% with Methods A and B, respectively (Entries 5 and 6, Table
S2). The yield of substituted aromatic amine 3b was low even with Method B (Entries
7 and 8, Table S2). Using the substituted aromatic aldehyde 4c led to improved yields
(Entries 9 and 10, Table S2). It is worth noting that the substituent group on the aromatic
amine may have led to difficulties in the formation of the carbon-nitrogen double bond

(C=N) during the reaction.
On this basis, the synthesis of N-alkyl amines was carried out using nitroarenes as raw
materials. When nitrobenzene (5a) and 4-anisaldehyde (4a) were added simultaneously,
the yield of N-(4-methoxybenzyl)aniline (2a) was only 52% (Entry 1, Table 3).
Although the conversion of nitrobenzene into aniline was close to 100%, 4-
anisaldehyde was also reduced rapidly. After such reduction, the amount of 4a that
should react with 3a to obtain imines would be low. The results showed that the catalyst
had similar selectivity for nitro and aldehyde groups, thus the nitroarenes should be
reduced primarily. The reduction of imine 1a was tested with 4-anisaldehyde (4a).
Although 4-anisaldehyde could be reduced, when 1a was present, the main product was
N-alkyl amines (Entry 2, Table 3). This result proved that the C=N bond was primarily
reduced, while the reduction of C=O was inhibited in this system. When 3a and 4a were
both present, the imine could be generated rapidly with excess 4a. Although 4-
anisaldehyde was reduced, 2a was also obtained in high yield (Entry 2, Table 3). On
these bases, when nitroarenes were used, Et3SiH was added two separate times. The
first was when nitroarenes were added to reduce -NO2, and aromatic aldehydes were
subsequently added to the system. The second was after imines appeared to reduce the
N=C bond, and the target product was obtained. Under this condition, the yield of 2a
could reach 94% (Entry 4, Table 3).
Table 3. The reaction between 4-anisaldehyde and different reactant.
Yield of Yield of Yield of Yield of
Entry
1
Reactant
Time/h
6
3a /%
1a /%
2a /%
6a /%
5a
39
9
52
50

2
3
1a
3a
5a
3
5
7
2
/
/
97
91
94
5
4
1
20
20
4b)
4
a) Reactions were performed using the catalyst (0.1 mmol), 4-anisaldehyde 4a (1.25
mmol), MeOH (4.0 mL), N-reactant (1.0 mmol), Et3SiH (8.0 mmol). b) 5a (1.0 mmol)
with Et3SiH (5.0 mmol) for 2h, then 4a (1.25 mmol) was added for 2 h. Lastly, Et3SiH
(3.0 mmol) was added again.
Different nitroarenes and aromatic aldehydes were used to synthesize N-alkyl amines
under the condition that Et3SiH was added twice. As shown in Table 5, a wide range of
N-alkylation amines were obtained in high yields. No substituted nitrobenzene and
benzaldehyde proceeded smoothly, and the yield of 2b was similar to that when aniline
was used (Entry 1, Table 4). Then, aromatic aldehydes with different substituents were
tested, and not only electron-donating groups (methyl, methoxyl, and hydroxy) but also
electron-withdrawing groups (nitrile) on aromatic aldehydes could achieve the reaction.
For the reaction between 2-, 3-, and 4-anisaldehyde (the same substituent but different
positions) and nitrobenzene, the yields were all above 85%. Moreover, 4-anisaldehyde
(94%) showed the highest activity, and the yield of 3-anisaldehyde (85%) was lowest
(Entries 2-4, Table 4). Similar results were obtained for 2-, 3-, and 4-tolualdehyde. For
this reaction, 3-tolualdehyde showed the lowest activity, while 2-tolualdehyde had the
highest yield (Entries 5-7, Table 4). When salicylaldehyde was used, the yield reached
92% (Entry 8, Table 4). For 4-cyanobenzaldehyde, the electron-withdrawing group did
not affect the activity, and the unsaturated nitrile bond was not hydrogenated (Entry 9,
Table 4). When substituted nitroarenes were reacted with the unsubstituted
benzaldehyde, the yield was 48% (Entry 10, Table 4). However, increasing the ratio of
benzaldehyde would lead to the yield reaching 70% (Entry 11, Table 4). The yield
increased significantly when substituted nitroarenes were reacted with substituted
benzaldehyde. Different aromatic aldehydes could make the reaction with substituted
nitroarenes proceed smoothly (Entries 12-15, Table 4).
Table 4. Synthesis of N-alkyl amines from different nitroarenes and aromatic aldehydes.

Entry
1
R1
H-
R2
H-
Product
Time/h Yield/%
6
7
7
84
94
85
2
3
H-
H-
4-CH3O-
3-CH3O-
4
H-
2-CH3O-
6
92
5
6
7
H-
H-
H-
4-CH3-
3-CH3-
2-CH3-
6
6
5
92
86
95
8
H-
2-HO
5
92
9
H-
4-CN-
H-
7
98
48
70
85
10
3-CH3-
3-CH3-
3-CH3-
5.5
5.5
8
11b)
12
H-
4-CH3-

13
14
15
3-CH3-
3-CH3-
3-CH3-
4-CH3O-
3-CH3-
2-CH3-
8
8
8
89
78
80
a) Reactions were performed using the catalyst (0.1 mmol), nitroarenes 2 (1.0 mmol),
Et3SiH (5.0 mmol), and MeOH (4.0 mL) at room temperature for 2 h. Then aromatic
aldehydes 4 (1.25 mmol) was added for 2 h. Lastly, Et3SiH (3.0 mmol) was added again.
b) Aromatic aldehydes 4 (2.0 mmol) were added.
To explain the stability of np-Pd, we carried out the following experiments. In the
reduction of nitrobenzene, after Et3SiH was added to the reaction system for 1 h, the
np-Pd was separated from the system by filtration. Furthermore, the conversion of
nitrobenzene was measured to reach 60% by high performance liquid chromatography
analysis (HPLC analysis). The filtrate remained without the catalyst for an additional 1
h, and further consumption of nitrobenzene amounted to 3%. The second experiment
was conducted in another system; the reduction of nitrobenzene was completed, and N-
(4-methoxybenzylidene)aniline was obtained. After Et3SiH was added to the reaction
system for 1.5 h, the catalyst was filtered from the reaction mixture. In this experiment,
the yield of N-(4-methoxybenzyl)aniline (2a) was measured to be 55% by HPLC
analysis. The filtrate remained without the catalyst for an additional 1.5 h, and further
yield of N-(4-methoxybenzyl)aniline amounted to 2%. Then, the solution without the
catalyst was analyzed by inductivity coupled plasma atomic emission spectroscopy
(ICP-AES). The concentration of palladium in the filtrate was lower than the detection
limit (< 0.02 ppm). According to a previous study [20], these results showed that the
leaching of palladium was minimal, and the majority of the catalytic activity originated
from the heterogeneous nanoporous palladium.

Scheme 2. Stability studies of nanoporous palladium.
The cycling experiment was used to further test the recycling capacity and reusability
of the catalyst. The catalyst was separated by filtration after the reaction. After rinsing
with acetone without other purification, the dried catalyst was reused. The catalyst
could be reused at least eight times with the yield maintained above 90% (Scheme 3).
As shown in Fig. 1b, although ligament size coarsened, the catalyst recovered after
eight times of recycling also retained a three-dimensional bicontinuous nanoporous
structure. To better understand the coarsening phenomenon, BET and BJH calculations
were performed. The BET results showed that the specific surface area of the used
catalyst was 25.4 m2/g which was decreased slightly from 30.6 m2/g. Moreover, the
BJH results showed that the pore diameter was also approximately 5 nm (see ESI Fig.
S2).
Scheme 3. Reusability studies of np-Pd. Reactions were performed using the catalyst
(0.1 mmol), 5a (1.0 mmol), Et3SiH (5.0 mmol), and MeOH (4.0 mL) at room
temperature for 2 h. Then, 4a (1.25 mmol) was added for 2 h. Lastly, Et3SiH (3.0 mmol)
was added again.
In summary, the one-pot synthesis of N-alkyl amines was realized with nanoporous
palladium as the sustainable catalyst and silane/alcohol as the hydrogen source. Imines,
aromatic amines, and nitroarenes could be used as raw materials. A series of N-alkyl
amines was obtained in considerable yield under mild conditions. Np-Pd could be

reused at least eight times. The results described here provide further evidence that
nanoporous Pd is a suitable choice to promote and develop a clean and efficient reaction.
Further studies to extend the scope of the np-Pd catalytic hydrogen transfer system are
in progress in our laboratory.
Acknowledgements
Financial support from the Academic promotion programme of Shandong First
Medical University (2019QL008), Natural Science Foundation of Shandong Province
(ZR2016BB09) and the National Students’ Innovation and Entrepreneurship Training
Program (201610439174).
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Highlights
Np-Pd/hydrosilanes/alcohol system was used for one-pot hydrogen transfer reaction.
One-pot synthesis N-alkyl amines was realized with green hydrogen source.
Stability, recycling capacity and reusability of catalyst was proved.
One-pot synthesis N-alkyl amines was realized by using nanoporous palladium as
sustainable, heterogeneous catalyst and silane/alcohol as hydrogen source.
Keywords: Nanoporous palladium, Hydrogen transfer reaction, One-pot reaction, N-
alkyl amines
Yang Cao: Data curation. Zhiwen Li: Conceptualization, Methodology,
Investigation, Writing-Original draft preparation, Writing-Reviewing and
Editing. Cao Hou: Investigation. Xiaoqun Cao: Project administration.

Declaration of interests
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.
☐The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests: