Selective Acceptorless Dehydrogenation of Primary Amines to Imines by Core–Shell Cobalt Nanoparticles

Article abstract of DOI:10.1002/anie.201915526

Core–shell nanocatalysts are attractive due to their versatility and stability. Here, we describe cobalt nanoparticles encapsulated within graphitic shells prepared via the pyrolysis of a cationic poly-ionic liquid (PIL) with a cobalt(II) chloride anion. The resulting material has a core–shell structure that displays excellent activity and selectivity in the self-dehydrogenation and hetero-dehydrogenation of primary amines to their corresponding imines. Furthermore, the catalyst exhibits excellent activity in the synthesis of secondary imines from substrates with various reducible functional groups (C=C, C≡C and C≡N) and amino acid derivatives.

Full text of DOI:10.1002/anie.201915526

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Selective Acceptorless Dehydrogenation of Primary Amines to
Imines by Core-shell Cobalt Nanoparticles
Authors: Paul J. Dyson, Xinjiang Cui, Wu Li, Kathrin Junge, Zhaofu Fei,
and Matthias Beller
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201915526
Angew. Chem. 10.1002/ange.201915526
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Selective Acceptorless Dehydrogenation of Primary Amines to
Imines by Core-shell Cobalt Nanoparticles
Xinjiang Cui,[a],+ Wu Li,[b],+ Kathrin Junge,[b] Zhaofu Fei, Matthias Beller* and Paul J. Dyson.*
[a]
[b]
[a]
[
a]
Dr. X. Cui, Dr. Z. Fei, Prof. P. J. Dyson.
Institute of Chemical Sciences and Engineering, École Polytechnique Fedérale de Lausanne (EPFL), 1015 Lausanne, Switzerland.
E-mail: paul.dyson@epfl.ch
[
[
b]
+]
Dr. W. Li, Dr. K. Junge, Prof. M. Beller.
Leibniz-Institute for Catalysis, Albert Einstein Str. 29a, 18059 Rostock, Germany. E-mail: matthias.beller@catalysis.de
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: Core-shell nanocatalysts are attractive due to their
versatility and stability. Here, we describe cobalt nanoparticles
encapsulated within graphitic shells prepared via the pyrolysis of a
cationic poly-ionic liquid (PIL) with a cobalt(II) chloride anion. The
resulting material has a core-shell structure that displays excellent
activity and selectivity in the self-dehydrogenation and hetero-
dehydrogenation of primary amines to their corresponding imines.
Furthermore, the catalyst exhibits excellent activity in the synthesis of
secondary imines from substrates with various reducible functional
groups (C=C, C≡C and C≡N) and amino acid derivatives.
The acceptorless dehydrogenation of primary amines by
heterogeneous catalysts is becoming increasingly attractive
because these catalysts tend to be more stable, are easily
recycled and are amenable to scale-up for industrial applications.
A few heterogeneous systems based on noble metals have been
developed for this process. For example, Pd nanoparticles
supported on basic hydrotalcites allow the selective synthesis of
secondary imines via acceptorless dehydrogenation.[19] In
addition, a non-noble metal iron-based catalyst was used to
prepare
several
primary
amines
via
acceptorless
dehydrogenation, which is active only in the presence of a strong
base (t-BuOK).[20] Despite these improvements, the efficient
synthesis of imines by acceptorless dehydrogenation of primary
amines, especially for hetero-couplings and couplings in the
presence of additional reducible groups, remains problematic.
Thus, heterogeneous catalysts based on a non-noble metal that
Introduction
Imines are key intermediates in the synthesis of fine chemicals,
pharmaceuticals and natural products.[1-4] Generally, their
synthesis involves the direct condensation of an amine and a
carbonyl compound.[5, 6] Although imines can also be obtained by
oxidative dehydrogenation (OD) of primary amines,[7-12] undesired
aldehydes and nitriles are also formed as side-products, leading
to low selectivities (Scheme 1).[10, 13] Consequently, it would be
advantageous to eliminate the use of oxidants for this process.
Recently, the acceptorless dehydrogenation (AD) of primary
amines to imines was shown to be an attractive method for their
preparation as fewer side-products are obtained under more inert
efficiently
catalyze
the
oxidant-free
and
base-free
dehydrogenation of primary amines to secondary imines would
fulfil an unmet need in organic synthesis.
conditions (with the release of H
thermodynamically challenging,
2
).[14, 15] As this reaction is more
the acceptorless
dehydrogenation of primary amines has received less attention
compared to that of N-heterocycles, with the later reaction having
been widely explored for hydrogen storage applications.[16-18]
Acceptorless dehydrogenation of primary amines is usually
achieved with molecular catalysts mainly based on Ru.[14, 15]
However, the aldimine intermediate may undergo a second
dehydrogenation to afford a nitrile, which competes with imine
formation via a transamination step, resulting in low selectivity of
the desired imines. For example, a Ru complex with a P, N-ligand
catalyzes the dehydrogenation of benzylamine to imines in only
Scheme 1. Previous and current work on the dehydrogenation of primary
amines to imines.
The unique nature of core-shell nano-structures often leads to
5
4% yield because of the formation of the nitrile product.[14]
improved physical and chemical properties compared to other
nanocatalysts, leading to new applications.[
21-25]
N-doping
1
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
strategies represent an effective method for the preparation of
selective and active catalysts with numerous advantages
the core region, whereas C, N and O are distributed throughout
the entire particle, confirming the core-shell structure. The
demonstrated in photo-, electro- and redox-catalysis.[13, 26-31] Here, composition of the nanostructures were further investigated by
we describe the preparation, characterization and catalytic
evaluation of cobalt-based nanoparticles immobilized within a N-
doped carbon support. The optimal non-noble metal
energy-dispersive X-ray (EDX) spectroscopy, with N, O and Co
detected (see Figure 1e). In contrast, a Co/C-800 catalyst
prepared in the absence of the PIL was aggregated with
nanoparticles > 200 nm (Figure. S5) that do not possess a core
shell, indicating that the formation of the core-shell structure
inhibits nanoparticle aggregation.
heterogeneous
system
catalyzes
the
acceptorless
dehydrogenation of primary amines to secondary imines with
unprecedented activities and selectivities (Scheme 1). To the best
of our knowledge, non-noble metal-based heterogeneous
catalysts have not been reported for the oxidant-free and base-
free dehydrogenation of primary amines to imines.
Results and discussion
The route used to prepare the Co-N/C catalyst is illustrated in
Scheme 2, and commences with the synthesis of an imidazolium-
based poly-ionic liquid (PIL) with a cobalt(II) chloride counter
anion, followed by its immobilization and pyrolysis. In the first step,
1
,3-bis(4-vinylbenzyl)imidazolium chloride is obtained according
[32]
to literature method.
The 1,3-bis(4-vinylbenzyl)imidazolium
chloride monomer is then reacted with cobalt(II) chloride to afford
the monomer containing the CoCl 2-
anion, which is then
polymerized in the presence of 2,2′-Azobis(2-methylpropionitrile)
AIBN, an initiator). With the addition of carbon support, the
4
(
resulting slurry was stirred, then dried and pyrolyzed under an
argon atmosphere. The most active catalyst, termed Co-N/C-800
was obtained following pyrolysis at 800 °C for 2 h. Other catalysts
obtained at different pyrolysis temperatures, i.e. 600, 700 and
9
00 °C were also evaluated. Although this method was expected
to promote the formation of a N-doped shell during pyrolysis, to
the best of our knowledge, N-graphene shells have not previously
been obtained from PIL templates.
Scheme 2. Preparation of the core-shell cobalt catalyst.
The Co-N/C catalysts were initially characterized by transmission
electron microscopy (TEM). As shown in Figures S1-S3, the size
of the cobalt nanoparticles increases slightly as the pyrolysis
o
temperature is raised. Following pyrolysis at 800 C for 2 h, cobalt
nanoparticles ranging from 20-50 nm are observed on the surface
of the carbon support (Figure S1). The high-resolution TEM (HR-
TEM) image shows that the nanoparticles are protected by
graphene-like shells (Figures 1a-1c), comprising typically 3-12
layers with a thickness of 1.2-3.0 nm. Presumably, carbonization
of the nitrogen-containing PIL leads to the formation of nitrogen-
doped graphene-like shells. Nanoscale element mapping
Figure 1. Characterization of the Co-N/C-800 catalyst: (a) Bright field TEM
image; (b) HR-TEM image; (c) HAADF-STEM image; TEM-EDS mapping
images: (d) overlap; (e) EDX spectrum; (f) Co; (g) C; (h) O; (i) N.
ICP-OES (Inductively coupled plasma - optical emission
spectrometry) and XPS (X-ray photoelectron spectroscopy)
analysis show that the carbon content increases with the pyrolysis
(
Figures 1d and 1f-1i) reveals that the cobalt is concentrated in
2
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RESEARCH ARTICLE
temperature, whereas the content of N and Co decreases (Tables
S1 and S2). The content of the Co drops from 2.7 to 1.8% as the
pyrolysis temperature increases from 600 to 800 °C and the N
decreases from 4.10 to 2.43%. However, upon increasing the
temperature to 900 °C, the content of Co and N decrease sharply
to 0.35 and 0.81%, respectively. The peaks in the N 1s spectrum
at 398.6, 400.5 and 401.6 eV may be tentatively assigned to
pyridinic, pyrrolic and graphitic N on the basis of their respective
binding energies (Figure 2b).[33, 34] The percentage of pyridine-
type nitrogen decreases as the pyrolysis temperature increases
whereas the pyrrolic and graphitic N-species increase with the
pyrolysis temperature (Table S3). In addition, Co(0), Co-O and
Co-N species are present, evidenced by the binding energies at
whereas the peak corresponding to the Co-N species is very low,
consistent with the XPS data. As shown in Figure 2d, the
diffraction peak at 44.5° could be due to the (111) plane of Co(0)
in the Co/C-800 catalyst, close in value to the Co-N peak. Notably,
the pyrolysis temperature and the PIL exert a significant influence
on the formation of active sites.[33]
The catalytic dehydrogenation of 4-methoxybenzylamine (1a)
was chosen as benchmark reaction to evaluate and optimize
catalytic performance (Table 1). The catalyst prepared at 600 °C
is the least active of the Co-N/C catalysts affording N-(4-
methoxybenzylidene)-4-methoxybenzylamine (2a) in 71% yield
(
Table 1, entry 1). The activity of the Co-N/C-700 and Co-N/C-800
are considerably higher with almost quantitative yields of
corresponding imine obtained in toluene for the latter (Table 1,
entries 2-3).
7
79.1, 780.4, and 783.1 eV after pyrolysis at 800 °C (Figure 2a
and Table S4). The presence of Co-O bonds might be due to the
retention of oxygen in the carbon support or the surface Co atoms
being partially oxidized when the catalysts are exposed to air after
pyrolysis. Upon increasing the pyrolysis temperature from 600 to
Table 1. Optimization of the self-dehydrogenation of 4-methoxybenzylamine. [a]
8
00 °C, the content of Co-O bonds decreases from 72 to 48%
whereas the Co-N bonds increase from 19 to 31% (Table S4). In
addition, the percentage of Co(0) increases from 9 to 22% as the
pyrolysis temperature is increased from 600 to 800 °C (Table S4).
At 900 °C the contents of Co(0) and Co-N species decreases to 9
and 15%, respectively. Taken together, in the Co-N/C-800
catalyst the main active sites are likely to be based on CoNx units.
Entry
Cat
Solvent
t (h)
Yield
(
Sel(%)
TON
%)[
b]
1
2
3
4
5
6
7
8
9
Co-N/C-600
Co-N/C-700
Co-N/C-800
Co/C-800
Toluene
Toluene
Toluene
Toluene
Toluene
Xylene
12
12
12
12
12
12
12
12
12
71
93
99
21
85
86
75
81
84
99
99
99
72
98
98
99
99
99
118.3
155.0
165.0
35
Co-N/C-900
Co-N/C-800
Co-N/C-800
Co-N/C-800
Co-N/C-800
141.7
143.3
125.0
135.0
140.0
Dioxane
Water
Butyl
digol
Figure 2. XPS spectra for the (a) Co 2p and (b) N 1s of the Co-N/C-800 catalyst.
(
c) XRD patterns of the Co-N/C-600, Co-N/C-700, Co-N/C-800 and Co-N/C-900
1
1
1
0
1
2
Co-N/C-800
Co-N/C-800
Co-N/C-800
Co-N/C-800
Toluene
Toluene
Toluene
Toluene
Toluene
6
67
84
98
98
81
99
99
99
99
99
111.7
140.0
163.3
163.3
135.0
catalysts. (d) Partial XRD patterns of the Co-N/C-600, Co-N/C-700, Co-N/C-800,
Co-N/C-900 and Co/C-800 materials.
10
16
24
12
XRD analysis was used to probe the structure of the catalysts
see Figure 2c), with diffraction peaks observed at 36.7 and 44.1°
(
in all the catalysts, which are close to the (101) and (111) planes
of CoOx and CoNx units, respectively[33]. Stronger and sharper
diffraction peaks are observed in the Co-N/C-800 catalyst,
implying higher ordering and crystallinity in this material. In the
Co-N/C-900 catalyst, the peak corresponding to the Co-O
component is of higher intensity than in the other samples,
13
14
Co-N/Al
800
2 3
O -
3
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RESEARCH ARTICLE
15
Co-N/SiO
00
2
-
Toluene
12
67
99
111.7
8
16
17
18
19
20
21
22
CoCl
CoCl
CoCl
Pd/C
Pt/C
2
2
2
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
12
12
12
12
12
12
12
22
29
34
65
52
39
56
99
99
99
67
53
41
99
36.7
48.3
56.7
108.3
86.7
65.0
93.4
+ C
+ N/C
Ru/C
Co-N/C-
[
c
800
[
a] Reaction conditions: catalyst (0.6 mol% of Co relative to the substrate), 4-
methoxybenzylamine (0.2 mmol), solvent (1 mL), N
2
atmosphere, 120 oC. [b]
GC yield. TON = mol product/mol Co. [c] The catalyst was treated under air.
In comparison, the Co/C-800 control catalyst without core-shell
structure affords the desired product in only 21% yield (Table 1,
entry 4), with the secondary amine obtained in 8 % yield,
indicating that the core-shell structure in Co-N/C-800 inhibits the
over-reduction of C=N bonds leading to high selectivity. Note that
the Co-N/C-900 catalyst is less active than the Co-N/C-800
catalyst (Table 1, entry 5), possibly due to the presence of fewer
Co-N sites. To confirm this hypothesis, the catalyst was further
treated at 250 oC for 2 h under air, with the resulting catalyst
displaying an even lower yield (Table 1, entry 22). ICP-OES
analysis showed the N content decreased sharply (Table S1).
These results suggest that the selectivity of the dehydrogenation
may be attributed to the core-shell structure, whereas the Co-N
units act as the active sites enhancing the reaction rate.
Figure 3. Reaction profile for dehydrogenative coupling of 4-
methoxybenzylamine to afford the imine (C=N), secondary amine (C-N) and
nitrile (C≡N) using a 0.6 mol% loading of the Co-N/C-800 catalyst (top). In-situ
1
H-NMR spectra of the self-dehydrogenation of 4-methoxybenzylamine using a
1
.0 mol % loading of the Co-N/C-800 catalyst under the optimized reaction
Next, different solvents including xylene, dioxane, water and
butyl digol were evaluated in the model reaction at 120 C, giving
conditions in tolunene-d8 (bottom).
o
yields in the range 75 – 86% (Table 1, entries 6-9) with 99%
selectivity, i.e. lower than the yield obtained in toluene. As
expected, the yield is lower at shorter reaction times (Table 1,
entries 10 and 11), whereas the N-alkylated amine is no longer
observed following prolonged reaction times (Table 1, entries 12
and 13), showing that hydrogenation of C=N bond cannot take
place, which leads to the high selectivity observed. Low catalytic
From the reaction profile as shown in Figure 3, in which nitrile
formation is not observed, it would appear that the transamination
step is much faster than the second dehydrogenation of the
aldimine intermediate, which differs to the reactivity observed for
[35, 36]
homogeneous catalysts.
After 12 h, the imine was obtained
in 96% yield. Importantly, there is no evidence for the formation of
the N-alkylated secondary amine and tertiary amine, resulting
from the further hydrogenation of the imine using the released
hydrogen, which are detected in reactions employing other
heterogeneous[19] and homogeneous catalysts.[37] The imine is
activity was obtained using Co-N/Al
supported catalysts (Table 1, entries 14 and 15). The catalytic
activity of CoCl in the presence of the carbon and N-doped
2 3 2
O -800and Co-N/SiO -800 as
2
carbon was also very low (Table 1, entries 16 – 18). Remarkably,
the performance of the Co-N/C-800 catalyst is even superior to
that of commercial noble metal catalysts (Pd/C, Pt/C and Ru/C),
due to the formation of N-alkylated amine with these catalysts
leading to 41-67% of selectivity (Table 1, entries 19-21).
stable beyond 12 h, implying the reverse hydrogenation is not
possible.
4
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RESEARCH ARTICLE
3
4
24
12
91
97
(1c)
(2c)
(1d)
(2d)
5
16
94
(
1e)
1f)
(2e)
6
7
28
24
91(83)
97
(2f)
(
Figure 4. Recycling of Co-N/C-800 catalyst in the dehydrogenation of 1a.
Reaction conditions: 1a (0.2 mmol), toluene (1 mL), N atmosphere, 120 °C, 16
2
h.
(1g)
(2g)
1
The reaction was studied in situ by H-NMR spectroscopy, and
over time the signals corresponding to the 4-methoxybenzylamine
8
9
48
48
89
96
-
CH
to the -CH
the proton signal at 8.13 ppm corresponding to the HC=N proton
c in Figure 3) also increases with time. Note that release of H
2
group (a in Figure 3) decrease and the peaks corresponding
(1h)
2
group(b in Figure 3) of the product increase. Moreover,
(2h)
(
2
was detected by gas chromatography (Figure S8). The stability
and recyclability of the Co-N/C-800 catalyst were also established.
After each reaction, the Co-N/C-800 catalyst was recovered by
centrifugation and the product quantified. The catalytic
transformation of 1a to 2a could be repeated multiple times
(
2i)
(
1i)
1j)
10
48
97(90)
(
2j)
(
Figure 4). High selectivity was retained although the conversion
(
decreased from 99% to 90% over six batches. XPS analysis
revealed that the amount of Co-N bonds decreased to 26% after
the recycling, leading to the loss of the conversion (Figure S7),
further confirming the role of these bonds (see above). However,
the core-shell structure remains intact and hence the selectivity of
the Co-N/C-800 catalyst remains unchanged (Figure S4).
11
12
13
14
30
30
24
24
95
(
2k)
(1k)
46[c]
(1l)
Table 2. Co-N/C-800 catalyzed self-dehydrogenation of primary amines.[a]
(2l)
88
86
(2m)
(1m)
Entry
1
Substrate
Product
t
(
Yield
(%)[
b]
h)
(2n)
12
99(91)
(1n)
(
2a)
2b)
1
5
48
87(78)
(
1a)
(1o)
(2o)
2
15
93(85)
[
a] Reaction conditions: catalyst (0.6 mol% of Co to substrate), amine (0.2
mmol), toluene (1 mL), N2 atmosphere, 120
Isolated yields. [c] H-NMR spectroscopic yield.
o
C, H2 yield in parentheses. [b]
(
1
(1b)
5
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RESEARCH ARTICLE
Since the Co-N/C-800 catalyst displayed the best activity in the
acceptorless dehydrogenation of 4-methoxybenzylamine (1a) to
the corresponding imine (2a), the scope of the reaction was
studied using this catalyst under the optimized conditions. As
shown in Table 2, 3-methoxybenzylamine (1b) was converted to
the corresponding imine in 93% yield (Table 2, entry 2). However,
4
5
24
18
84
94
(3d)
(4d)
2
-methoxybenzylamine (1c), which has a more sterically hindered
(3e)
(4e)
functional group, is converted to the corresponding imine in 91%
yield with a reaction time to 24 h (Table 2, entry 3). As expected,
in the case of 3,4-dimethoxybenzylamine, a high yield was
achieved using the Co-N/C-800 catalyst (Table 2, entry 4).
Substituted benzylamines with electron-donating ester, methyl or
isopropyl groups at the para-position, were converted to the
corresponding imines in high yields by extending the reaction time
6
7
18
20
95
67
(
3f)
(4f)
(
(
Table 2, entries 5-7). Although benzylamine (1h) and substrates
1i, 1j) with electron-withdrawing functional groups (F and Cl) are
(3g)
less reactive, high yields can still be achieved by prolonging the
reaction time to 48 without the substrates undergoing
(4g)
h
dehalogenation (Table 2, entries 8-10). In contrast, 2-
thiophenemethylamine (1k) was dehydrogenated to the
corresponding imine in 95% yield (Table 2, entry 11). However, a
yield of only 46% was observed for the dehydrogenation of 2-
picolyamine 1l (Table 2, entry 12). Notably, bio-based primary
amines such as furfurylamine (1m) and 5-methyl furfurylamine
8
20
78
(3h)
(4h)
(
1n) were transformed to the corresponding imines in 88 and 86%
yield, respectively (Table 2, entries 13 and 14). Remarkably, 2o
was obtained by the self-dehydrogenation of n-octylamine in 87%
yield without the formation of a nitrile or N-alkylated amine (Table
9
20
75
, entry 15).[10, 13] This methodology offers a straightforward
(3i)
2
(4i)
approach to aromatic and aliphatic imines under the oxidant- and
base-free conditions.
10
20
79
Table 3, Hetero-dehydrogenation of primary amines using the Co-N/C-800
catalyst.
(3j)
[
a]
(4j)
[
a] Reaction conditions: catalyst (0.6 mol% of Co to substrate), 4-
methoxybenzylamine (0.2 mmol), 3a-3j (0.3 mmol), toluene (1 mL),
atmosphere, 120 C. [b] H NMR spectroscopic yields based on 1a.
N
2
o
1
-
Entry
Substrate
Product
t
Yield
(h)
[
b]
After demonstrating the activity of the catalyst towards a broad
range of primary amines, the catalyst was used to explore the
catalytic acceptorless hetero-coupling of different amines (Table
3). The catalytic dehydrogenation of 4-methoxybenzylamine (1a)
with other amines was initially explored. Using 1-
phenylethylamine (3a) and 2-phenylethylamine (3b), the reaction
proceeds with complete consumption of 4-methoxybenzylamine,
affording the hetero-coupled products in 74 and 84% yield,
respectively (Table 3, entries 1-2). Note that excess amines are
needed to afford the desired products in high yield. Moreover,
after hetero-dehydrogenation, 4c and 4d were successfully
prepared in 84% and 97% yields, respectively, from the
corresponding aliphatic amines (Table 3, entries 3 - 4). High yields
were also achieved using cyclohexanamine (3e) and
cyclopentanamine (3f) in the presence of the Co-N/C-800 catalyst
1
24
74
(
3a)
(
4a)
2
3
24
24
84
97
(3b)
(4b)
(3c)
(4c)
(
Table 3, entries 5-6). N-arylimines were obtained in high yield
6
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RESEARCH ARTICLE
when using anilines as reactants, irrespective of whether they
have electron-withdrawing (F or I) or electron-donating (methyl)
substituents, illustrating the utility of anilines in the hetero-
dehydrogenation reaction (Table 3, entries 7-10). Note that the 1a
can be fully converted in all cases in the presence of Co-N/C-800,
however the self-dehydrogenation of 1a was not avoided leading
to the lower yields of the desired hetero-dehydrogenated imines.
Applications of amino acids derivatives that can be derived from
biomass α-hydroxyl acids are particularly attractive.[38-41]
Consequently, we investigated the hetero-dehydrogenation of
primary amines with amino acids derivatives. As shown in
Scheme 4, in the presence of 1.5 equivalents of hexylamine or
octylamine, methyl 2-amino-2-phenylacetate was converted to
the corresponding products (7a/b) in 66% and 57% yield,
respectively.
Scheme 3. Dehydrogenation of primary amines with reducible groups using the
Scheme 5. Further reactions of imines. Reaction conditions: a) catalyst (0.6
mol% of Co to substrate, toluene, 48 h. b) Allyltrimethylsilane (0.6 mmol), imine
(0.5 mmol), 4 Å molecular sieves (150 mg), TBAF (5 mol%) and THF (2 mL), 70
Co-N/C-800 catalyst. Reaction conditions:catalyst (0.6 mol% of Co to substrate),
atmosphere, 120 oC. [a] n-
octylamine (0.3 mmol), [b] H-NMR spectroscopic yield.
primary amine (0.2 mmol), toluene (1 mL), N
2
1
o
C, 18 h, yield based on 3g. a´) As shown in Table 2. b´) Imine (0.5 mmol),
o
alkyne (0.75 mmol), Cu(OTf)
2
(1 mol %), Na
2
SO
4
, toluene (2 mL), 100 C, 24 h.
yield based on 1h.
As molecular hydrogen is released during the reaction, we
decided to investigate whether other reducible functional groups
would be compatible with the catalyst. As shown in Figure S9,
self-dehydrogenation of 4-methoxyl benzylamine was achieved in
high yield in the presence of compounds with reducible groups,
demonstrating the tolerance to reducible functional groups.
Consequently, we explored the application of the Co-N/C-800
catalyst in the synthesis of several imines bearing reducible
groups (Scheme 3). Notably, imines containing C=C bonds can
be obtained selectively by the self-dehydrogenation of amines.
Imines bearing C≡C and C≡N groups (5b and 5c) can also be
prepared via self-coupling without the reduction of these groups.
Using 4-nitroaniline, hetero-coupling of the desired product imine
The synthetic utility of secondary imines was also probed under
practical conditions. As shown in Scheme 5, N-benzylideneaniline
was successfully transformed into the corresponding product (8a)
in 65ꢀ% yield following reaction with allyltrimethylsilane. Cross-
coupling of N-benzylidenebenzylamine and 1-pentyne was also
achieved in 70% yield to afford 8b.
Conclusions
Herein we described a new method for the synthesis of
nanostructured nitrogen-doped graphene-layer coated cobalt
nanoparticle catalysts immobilized on a carbon support. The
resulting core-shell catalysts are easily obtained in a simple and
scalable way via pyrolysis of a PIL with an appropriate metal
(
5d) with n-octylamine can be achieved. To the best of our
knowledge, such dehydrogenations have not been described
previously and, therefore, this method is powerful for the further
functionalization of substituted imines.
halide anion, in this case CoCl 2-. The optimized catalyst was
4
used in the selective self-dehydrogenation of primary amines to
secondary imines, in the hetero-coupling of different amines to the
corresponding imines. The utility of the catalyst is highlighted by
the synthesis of imines with reducible groups and amino acids
derivatives.
Acknowledgements
We thank the EPFL, Swiss National Science Foundation and the
Swiss Competence Center for Energy Research (SCCER) for
Heat and Electricity Storage for financial support. We also thank
Pierre Mettraux and Lagrange Thomas for analytical support.
Scheme 4. Dehydrogenation of primary amines with amino-acid derivatives.
Reaction conditions: catalyst (1.0 mol% of Co to substrate), phenylglycine
2
methyl ether (0.2 mmol), amine (0.3 mmol), toluene (1 mL), N atmosphere,
o
1
1
20 C, 18 h, H-NMR spectroscopic yield.
Keywords: Heterogeneous catalysis • Dehydrogenation• Cobalt
•
Amines • Imines
7
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Angewandte Chemie International Edition
10.1002/anie.201915526
RESEARCH ARTICLE
The self-dehydrogenation and hetero-dehydrogenation of primary amines to their corresponding imines was achieved employing
carbon supported cobalt nanoparticles coated with nitrogen-doped graphene shell. The catalyst is obtained using a facile and scalable
procedure via pyrolysis of a polymer ionic liquid with a CoCl 2-
these reactions.
counter-anion on a carbon support and shows unprecedented activity in
4
1
0
This article is protected by copyright. All rights reserved.