Direct reductive coupling of nitroarenes and alcohols catalysed by Co-N-C/CNT@AC

Article abstract of DOI:10.1039/c8gc03818j

A non-noble heterogeneous catalyst-a Co, N and C composite encapsulated carbon nanotube grown in situ on the surface of activated carbon (Co-N-C/CNT@AC)-was fabricated via the pyrolysis-reduction process. Co-N-C/CNT@AC catalyzed the reductive coupling of structurally diverse nitroarenes and alcohols to imines and secondary amines under exogenous base- and solvent-free conditions. BET, TEM, SEM, XRD and XPS characterization showed that the exceptional catalytic property of Co-N-C/CNT@AC could be explained by its nanostructure, Co-N and basic N species. Our protocol had advantages of low cost, environment-friendliness, and good applicability. Furthermore, a reaction route of nitroarenes, amines and alcohols as the starting materials was proposed to improve the atom economy of the reductive coupling of nitroarenes with alcohols.

Full text of DOI:10.1039/c8gc03818j

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Direct Reductive Coupling of Nitroarenes and Alcohols Catalysed
by Co-N-C/CNT@AC
Received 00th January 20xx,
Accepted 00th January 20xx
Di Liu,* Ping Yang, Hao Zhang, Minjie Liu, Wenfei Zhang, Dongmei Xu, and Jun Gao
DOI: 10.1039/x0xx00000x
A non-noble heterogeneous catalyst - Co, N and C composite encapsulated carbon nanotube grown in-situ on the surface
of activated carbon (Co-N-C/CNT@AC) was fabricated via the pyrolysis - reduction process. The Co-N-C/CNT@AC catalyzed
the reductive coupling of structurally diverse nitroarenes and alcohols to imines and secondary amines under exogenous
base- and solvent- free conditions. BET, TEM, SEM, XRD and XPS characterizations displayed that the exceptional catalytic
property of Co-N-C/CNT@AC could be explained by its nanostructure, Co-N and basic N species. Our protocol had
advantages of low cost, environment-friendliness, and good applicability. Furthermore, a reaction route of nitroarenes,
amines and alcohols as the starting materials was proposed to improve the atom economy of the reductive coupling of
nitroarenes with alcohols.
www.rsc.org/
2 3 2
Au/TiO ·3H
-VS 27, Ru hydroxide 28, RuCl O 29 and Au-Pd/Ru-
3
0
Introduction
Pd were reported. These catalytic systems achieved the N-
alkylation of nitroarenes with alcohols, whereas there were
different levels of drawbacks or limitations as follows: i) the
costs of these catalysts were high on account of the use of
Carbon-nitrogen bonding is a very important transformation in
organic chemistry since nitrogen-containing compounds, such
as amines and imines, are the basic building blocks in the
1
0, 22, 23, 29
noble metal, ii) excess exogenous base (K
2
CO
3
, KOH
1
production of numerous fine chemicals . The formation of C-N
2
8
9, 26
21
20
O 24), organic
,
Cs
2
CO
3
, t-BuOK , KHCO
3
, LiOH ·H
2
2
, 3
or C=N bonds can be conducted by hydroamination
,
1
1, 17, 22
12
10, 23, 25,
solvent (benzotrifluoride
7, 28
, acetonitrile , toluene
4
5, 6
hydroaminomethylation , self-coupling of primary amine
2
chlorobenzene 20, mesitylene 18, 30, xylene
19
,
and/or
7, 8
and reductive amination
. Recently, the direct reductive
1
8, 29, glycerol
10, 22, 23, 30
hydrogen source (H
2
) were required, iii)
coupling of nitroarenes with alcohols by using a hydrogen-
borrowing strategy (also known as a hydrogen autotransfer
most catalytic systems presented a narrow range of
application, only a few cases were applied to synthesize
different classes of amines such as imines, secondary amines
9
process) to synthesis of amines was reported . In this one-pot
multistep transformation, the alcohol also serves as hydrogen
source for nitro reduction except for the role of alkylating
agent, avoiding the use of additional hydrogen donor. And
alcohols and nitroarenes have the advantages of low-cost,
1
0, 21, 22
and tertiary amines
. In terms of economy and
environmental friendliness, it would be highly desirable to
develop an inexpensive, recyclable and universal catalytic
system without solvent, exogenous base and hydrogen source
for the reductive coupling of nitroarenes with alcohols.
1
0
good accessibility and stable chemical property . Thus, the
direct reductive coupling of nitroarenes with alcohols has
aroused significant attention.
Based on our previous research 31, 32, herein we developed a
non-noble heterogeneous catalyst- Co, N and C composite
encapsulated multiwalled carbon nanotubes (CNT) grown in-
situ on the surface of activated carbon, which was easily
obtained by the successive pyrolysis and hydrogen reduction
process of the mixture of the precursors. Benefiting from the
composition and nanostructure of the catalyst, imines and
secondary amines are efficiently synthesized through the
reductive coupling of nitroarenes with alcohols in the absence
of solvent, exogenous base and hydrogen source.
For the reductive coupling of nitroarenes with alcohols,
1
1
12, 13
14
photocatalysts such as Cd
X
Zn1-X
S
, CdS/TiO
2
16
, TiO
2
,
1
5
17
Pd/TiO
2
, Pd/H1.07Ti1.73
O
4
·H
2
O nanosheets and CdIn
2
S
4
have been studied owing to the characteristics of the mild
reaction condition of the photocatalysis. Besides, thermal
catalysis was also employed in this transformation. Zanardi et
9
al successfully prepared imines from the direct reductive
coupling of nitroarenes and alcohols over an Ir-Pd catalyst.
1
0, 18, 19
20-22
23
Next, Ag-MCP-1
, Ru complex
, Au/Ag-Mo-NR
,
,
Pd/DNA 24, Pd/hydrotalcite 25, Ir-Au dimetallic complexes
26
Results and discussion
College of Chemical and Environmental Engineering, Shandong University of
Science and Technology, Qingdao, 266590, P. R. China; E-mail:
ld002037132@163.com
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Table 1 Coupling reaction of nitrobenzene with benzyl alcohol to N-Benzylideneaniline a
NO2
OH catalyst
N
3
+
1
2
Conversion of 1
yield of 3
(%)
Entry
Catalyst
Condition
b
c
(%)
1
2
3
Co@AC
Co-N-C/CNT@AC
Co-N-C@AC
0.3g/140°C /solvent-free
0.3g/140°C /solvent-free
0.3g/140°C /solvent-free
0.3g/140°C /solvent-free
0.3g/140°C /solvent-free
0.3g/140°C /p-xylene
6
1.7
68
52
1.5
42
47
22
88
97
96
72
58
1.5
43
51
22
92
98
98
4
N-C@AC
d
5
Co-N-C/CNT@AC
Co-N-C/CNT@AC
Co-N-C/CNT@AC
Co-N-C/CNT@AC
Co-N-C/CNT@AC
Co-N-C/CNT@AC
6
7
8
9
0.3g/140°C /DMF
0.3g/160°C /solvent-free
0.4g/160°C /solvent-free
0.5g/160°C /solvent-free
1
0
a
Reaction conditions: nitrobenzene (5 mmol), benzyl alcohol (15 mmol), N
Determined by GC.
open-air atmosphere.
2
atmosphere, 18h.
b, c
d
In the beginning of our studies, the optimization of the scatter the CNTs in the Co-N-C/CNT@AC. To reveal the
catalytic systems and reaction conditions was first carried out. underlying factors of the catalytic property difference between
The imination of nitrobenzene and benzyl alcohol with 1:3 these two catalysts, N adsorption-desorption method was
2
molar ratio was selected as the model reaction. Based on the performed to characterize the textural properties of Co@AC
intention of searching inexpensive catalyst, we attempted to and Co-N-C/CNT@AC.
prepare a non-noble metal Co@AC to catalyze this reaction,
whereas the Co@AC catalyst exhibited poor catalytic
performance (entry 1, Table 1). And nitrogen doping was
implemented to further improve the catalytic activity of
Co@AC by introducting DCD as N source. Amazingly, the
obtained sample exhibited a much better catalytic activity in
comparison with Co@AC (entry 2, Table 1). SEM images of the
sample displayed the existence of carbon nanotubes (CNTs)
400
Co@AC
3
2
1
00
00
00
0
Co-N-C/CNT@AC
(Figure 1). CNTs were formed in-situ and attached on the
surface of AC during the pyrolysis of DCD under catalysis of
cobalt species 3
3,
34
. AC as support was able to effectively
0
.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (p / p°)
Figure 2 N
2
adsorption/desorption isotherms of catalysts.
Table 2 The textural properties of Co@AC and Co-N-C/CNT@AC catalysts.
Pore volume
Mean pore size
2
Sample
S
BET (m /g)
3
(cm /g)
(nm)
1.21
1.07
Co-N-C/CNT@AC
Co@AC
846.6
1184.0
0.51
0.63
Figure 1 SEM images of Co-N-C/CNT@AC catalyst.
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It was seen from Figure 2 that the isotherms of Co@AC and (220) crystalline planes of the face-centered cubic metallic
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5
DOI: 10.1039/C8GC03818J
Co-N-C/CNT@AC catalysts all displayed a type-I sorption cobalt (JCPDS 15-0806) respectively, which manifested the
0
isotherm with H
4
-type hysteresis loop, the high uptake at low formation of graphite carbon and Co during the pyrolysis of
relative pressure suggested the developed micropore structure the precursor. The difference was that the intensity of
0
of these two catalysts. The specific surface area and pore diffraction peaks of Co in Co@AC was stronger than that of
volume of Co-N-C/CNT@AC were lower than that of Co@AC Co-N-C/CNT@AC catalyst, demonstrating the smaller grain size
0
(
Table 2). The reason for this was probably that the pyrolysis of cobalt particles in Co-N-C/CNT@AC catalyst. These were
products of DCD partly filled or covered pore organization of supported by TEM images, as shown in Figure 4. The formation
0
AC. In general, catalysts with low specific surface area of Co was confirmed by two fringes spacing of 0.205 nm and
exhibited weaker catalytic activity because of the insufficient 0.182 nm corresponding to the (111) and (200) plane of Co0
exposure of active sites. However, Co-N-C/CNT@AC possessed (Figure 4b, 4c, inset). The CNTs of Co-N-C/CNT@AC presented
higher catalytic activity, which indicated that the other factors bamboo-like structures with different lengths varying from
dominated its catalytic behavior. Subsequently, the crystalline several hundreds of nanometers to several micrometers and
0
structures and the chemical valence states of Co@AC and Co- diameters of 10-40 nm. Most Co nanoparticles were
0
N-C/CNT@AC catalysts were displayed by XRD.
encapsulated in the CNTs, and Co nanoparticles were tightly
wrapped by a few graphitic layers with an interlayer spacing of
0
4
.339 nm corresponding to the (002) plane of graphite (Figure
b). In addition, Co-N-C@AC catalyst was prepared for
comparison by a similar method with urea as N source. There
were no CNTs in Co-N-C@AC from TEM image (Figure 4d). Co
particles were present in larger size and concentrated state
relative to Co-N-C/CNT@AC, which was also judged based on
the intensity of diffraction peaks in Figure 3. Apparently, CNTs
were helpful to hinder the Co nanoparticles growth. And Co-N-
C@AC exhibited a moderate catalytic activity in these three
catalysts (entry 3, Table 1). According to the above analysis, it
was found that besides smaller size and higher dispersion of
cobalt particles, N doping could significantly improve catalytic
performance. Thus, XPS was carried out to gain more details
on N doping affecting the activities of as-prepared catalysts.
Figure 3 XRD patterns of Co@AC, Co-N-C@AC and Co-N-C/CNT@AC.
a)
b)Co-N-C/CNT@AC Co2p
2+
C 1s
Co
3+
Co
Co-N-C/CNT@AC
Co 2p
Co-N
Satellite
Satellite
Co
O 1s
N 1s
0
Co
Co@AC
0
200
400
600
800
1000
770
780
790
800
810
Binding Energy / eV
Binding Energy / eV
c)Co-N-C/CNT@AC N1s
Co-N
d)Co@AC Co2p
Co
2+
3+
Co
Satellite
Satellite
graphitic N
pyrrolic N
pyridinic N
0
Co
3
92 394 396 398 400 402 404 406 408 410
770
780
790
800
810
Binding Energy / eV
Binding Energy / eV
Figure 4 TEM images of Co-N-C/CNT@AC (a, b), Co@AC (c) and Co-N-C@AC (d).
As shown in Figure 3, XRD patterns of Co@AC and Co-N-
C/CNT@AC exhibited the diffraction peaks at 2θ = 24.0°, 43.5°
signed to the crystal surface (002), (101) of graphite-type
carbon 32 and 44.2°, 51.5°, 75.8° indexed to the (111), (200),
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dehydrogenation of alcohols, transfer hydrogenation of
f)Co@AC C1s
C=C
e)Co-N-C/CNT@AC C1s
C=C
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9,
DOI: 10.1039/C8GC03818J
nitroarenes and the condensation of amines with aldehydes
3
8
,
and previous studies had revealed that transition metal
3
9,
C-C
C-O O-C=O
nitrides had a high hydrogenation/ dehydrogenation activity
40. To sum up, we could identify that Co-N species were the
C=N
C-N
O-C=O
main active phase for oxidation-reduction of nitroarenes with
alcohols. Based on this, if calculating in terms of Co-N species
without regard to the inactive Co and Co-O species, the TON
280
285
290
295
300
280
285
290
295
Binding Energy / eV
Binding Energy / eV
0
of imine was 46 (entry 9, Table 1. the content of Co-N species
from XPS deconvolution).
In addition, there were three peaks with binding energies of
Figure 5 XPS spectra of Co@AC and Co-N-C/CNT@AC (a); XPS spectrum of Co-N-
C/CNT@AC with the Co 2p (b), N 1s (c) and C 1s (e); XPS spectrum of Co@AC with the
Co 2p (d) and C 1s (f).
3
98.5, 400.1 and 401.2 eV assigned to pyridinic groups,
XPS survey data was displayed in Figure 5. The elements-Co, C pyrrolic groups and graphitic-N severally except for the peak of
and O were identified in the Co@AC and Co-N-C/CNT@AC Co-N in the high-resolution N 1s spectrum of Co-N-C/CNT@AC
3
4, 37
from Figure 5a. The occurrence of O most likely arised from (Figure 5c)
, and the XPS of C 1s could be divided into four
the partial oxidation of metallic Co when the catalysts exposed peaks at 284.7, 285.7, 286.6 and 289.8eV belonging to C=C,
4
1, 42
to air. Especially, no N 1s signal was detected due to the C=N, C-N and O-C=O
. These denoted the presence of basic
4
3
extremely low content of N in the Co@AC, and N content in N species in the Co-N-C/CNT@AC . For Co@AC catalyst, no
Co-N-C/CNT@AC catalyst was 7.83 at%, confirming the XPS peak of N was also found in four resolving peaks centered
successful doping of nitrogen. In order to check the chemical at 284.7 eV (C=C), 285.3 eV (C-C), 286.3 eV (C-O) and 289.8 (O-
4
1, 44
state of elements, XPS spectra of Co@AC and Co-N-C/CNT@AC C=O) eV of the C 1s spectrum (Figure 5f)
. Some former
were carefully analyzed. The Co 2p3/2 spectrum of Co-N- researches showed that basic N species were active in catalytic
4
2, 45, 46
C/CNT@AC catalyst could be resolved to four peaks at 778.7 oxidation of alcohols
of benzyl alcohol to benzaldehyde reached 8.6% although N-
Co-O, Co-N and the satellite peak of Co-O, respectively (Figure C@AC almost had no activity in the reductive coupling of
nitroarenes with alcohols (entry 4, Table 1). Moreover, no
N was found in the deconvoluted result of N1s spectrum intermediate aniline was detected in the experiment of both
N-C@AC and Co-N-C/CNT@AC as catalyst, indicating that basic
species in Co-N-C/CNT@AC catalyst. While only similar three N species also effectively promoted the coupling of amines
with aldehydes, which was supported by Morales report 47
Therefore, it was reasonable to conclude that these basic N
demonstrating the inexistence of Co-N species in Co@AC (Fig. species assisted Co-N species in catalysing the reductive
d). The Co loading in Co@AC and Co-N-C/CNT@AC coupling of nitroarenes with alcohols in our catalytic system so
. We also found that the conversion
0
eV, 780.6 eV, 782.6 eV, and 786.2 eV, corresponding to Co ,
3
6
5
b) . Meanwhile, the fitting peak at 399.2 eV assigned to Co-
3
4
(
Figure 5c) . These corroborated the presence of Co-N
0
.
peaks at 778.7eV, 781.3eV and 786.2eV representing Co and
3
6, 37
Co-O were observed in Co 2p3/2 spectrum of Co@AC
,
5
determined by ICP is 9.5% and 8.3%, respectively. Base on this, that the use of exogenous base was avoided.
the turnover numbers (TONs) here were calculated by the In sum, the superior catalytic performance of Co-N-C/CNT@AC
moles of imine produced per mole of active metal in the whole catalyst benefited from its Co-N, nanostructure and basic N
catalyst. For Co@AC catalyst, the TON value of imine in terms species.
of metal Co was only 0.18 in 18 h (entry 1, Table 1). These Next, the model reaction was performed over Co-N-
0
C/CNT@AC under air atmosphere, and low conversion of
results indicated that Co and Co-O species possessed very
poor catalytic activities. Whereas, the TON of imine in terms of nitrobenzene was afforded (entry 5, Table 1). Effect of solvents
metal Co for Co-N-C/CNT@AC was 8 (entry 2, Table 1), which was also investigated, the desired product could be obtained
was much larger than that of Co@AC. Meanwhile, the metal- in solvents. Nevertheless, the best yield was still achieved
free N-C@AC catalyst gave only 1.5% yield of imine (entry 4, under solvent-free condition (entries 6-7, Table 1). In the
Table 1) manifesting that basic N species alone did not catalyze optimized experiments of the amount of catalyst, it was found
the reductive coupling of nitroarenes with alcohols. Also, it that the 0.4g dosage of Co-N-C/CNT@AC was sufficient to
was clear that the direct reductive coupling of nitroarenes with promote the reaction under the given reaction condition
alcohols
involved
the
following
tandem
steps: (entries 8-10, Table 1).
a
Table 3 Reductive imination of nitrobenzenes with alcohols catalyzed by Co-N-C/CNT@AC
NO2
N
3
R2
Co-N-C/CNT@AC
+
R2
OH
N2, 160℃
R1
R1
1
2
Entry
Substrate 1
Substrate 2
Time (h)
Product 3
Conversion
Selectivity
4
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c
(%)
(
98
98
%)b
View Article Online
DOI: 10.1039/C8GC03818J
NO2
NO2
OH
1
2
18
99
N
OH
24
N
94
97
Cl
Cl
NO2
NO2
OH
3
28
N
100
H3C
H3C
OH
4
5
6
20
30
36
N
100
98
90
98
98
H3CO
H3CO
CH3
NO2
OH
OH
N
N
H3C
NO2
97
CH3
CH3
NO2
CH3
OH
OH
7
8
9
42
40
36
91
95
96
97
96
98
N
CH3
Br
NO2
N
Br
Cl
Cl
NO2
OH
OH
N
N
F
NO2
1
0
1
22
28
100
75
99
22
F
NO2
OH
N
1
NO2
NO2
N
1
1
2
3
24
24
21
12
44
10
OH
N
OH
a
b
c
Reaction conditions: nitrobenzenes (5 mmol), alcohols (15 mmol), Co-N-C/CNT@AC (0.4 g), N
Conversion was calculated based on substrate 1.
Selectivity was calculated based on products 3.
2
atmosphere, 160°C, solvent-free.
With the optimized reaction conditions, we explored the alcohols were lower than that of benzyl alcohol (entries 1-4,
universality of our procedure for the synthesis of imines by a Table 3). Furthermore, the reactions of regioisomers of
wide range of structurally diverse nitrobenzenes and alcohols methylnitrobenzene with benzyl alcohols were investigated, it
as starting materials. The results were summarized in Table 3. was found that the yields were dependent on the substituted
Firstly, the nitrobenzenes reacted smoothly with both position on the benzene ring with the order as follows: para >
electron-donating (-Me, -OMe) and electron-withdrawing meta > ortho isomer, manifesting that the reaction was
groups (-Cl) substituted benzyl alcohols, giving the imine sensitive to the steric hindrance (entries 5-7, Table 3). In the
products with high conversion and selectivity (entries 2-4, next set of experiments, nitrobenzenes with substituents (-F, -
Table 3). Whereas the reactivities of substituted benzyl Cl, -Br) and benzyl alcohol reacted, furnishing the
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corresponding imines in 91-99% yields, and the reactivity of product N-benzylidene-aniline (entries 11, TaVbielwe Ar3tic)l.e OTnlhinee
the electron-withdrawing groups substituted benzyl alcohol reaction of aliphatic alcohols with nitDroObI:e1n0z.1e0n3e9/Cw8GeCre038a1ls8oJ
with nitrobenzene decreased in the order of -F > -Cl > -Br, examined, a considerable amount of intermediate anilines
which revealed the presence of an electron-withdrawing were detected, so the yields of imines were unsatisfactory
inductive effect (entries 8-10, Table 3). For the reaction of allyl (entries 12, 13, Table 3).
alcohol with nitrobenzene, selectivity of allyl imine was very
low on Co-N-C/CNT@AC since carbon-carbon double bond of
allyl alcohol was easily oxidized leading to the formation of by-
Table 4 Reductive amination of nitrobenzenes with alcohols catalyzed by Co-N-C/CNT@AC
a
R2
NO2
OH
Co-N-C/CNT@AC
N
NH
+
+
+
R1
R1
N
N , 160℃
R1
R2
2
R2
R2
R2
R1
1
2
3
4
5
Yield (%)b
Product 4
Entry
1
Substrate 1
Substrate 2
Time (h)
38
Product 3
7
Product 5
NO2
OH
82
11
NO2
NO2
OH
OH
2
3
4
5
6
44
46
46
48
48
8
14
16
4
83
85
74
84
86
9
H3C
1
H3CO
NO2
NO2
OH
OH
OH
10
12
10
H3C
Cl
NO2
4
F
a
b
Reaction conditions: nitrobenzenes (2 mmol), alcohols (24 mmol), Co-N-C/CNT@AC (0.24 g), N
GC Yield was calculated based on nitrobenzene.
2
atmosphere, 160°C, solvent-free.
On the strength of the one-pot synthesis of imines from the
reductive coupling of nitrobenzenes with alcohols, it would be
a useful extension if the imines could be hydrogenated to
provide the corresponding secondary amines. Much to our joy,
this transformation was successfully achieved by increasing
alcohols/nitrobenzenes molar ratio. The nitrobenzenes were
completely converted and structurally varied secondary
amines were obtained with 74-86% yields (Table 4). Note that
in this transformation course, the secondary amines were
partly alkylated to afford the corresponding tertiary amines (1-
1
2%). So, it was reasonably believed that our protocol could be
applied to synthesize tertiary amines by strengthing reaction
conditions.
6
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involves a condensation reaction leading to azo compound,
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DOI: 10.1039/C8GC03818J
which is then hydrogenated/dehydrated, finally generating the
2
2, 26
anilines (B)
. By tracing the reaction with GC-MS, very small
amounts of azobenzene and/or aniline could be detected in
some synthetic reaction. And we also performed the reaction
starting from azobenzene in the presence of benzyl alcohol
under reaction condition, imine was obtained with high yield.
This fact implied that transfer hydrogenation of nitrobenzenes
to anilines was via a condensation process of nitrobenzene
over Co-N-C/CNT@AC catalyst. Ⅲ) the coupling of amines with
4
7
aldehydes gave imines promoted by basic N species , and
imines could be further hydrogenated to secondary amines.
Ⅳ
) the secondary amines were alkylated to the corresponding
tertiary amines.
Figure 6 Reusability of the Co-N-C/CNT@AC catalyst. Reaction conditions: nitrobenzene
10 mmol), benzyl alcohol (30 mmol), Co-N-C/CNT@AC (0.8 g), N atmosphere, 160°C,
solvent-free, GC yields.
(
2
Moreover, we explored the reusability of Co-N-C/CNT@AC
catalyst by a simple filtration from the reaction mixture. Prior
to reuse in the next run, the catalyst needed to be refluxed at
8
0°C for 30min and sonicated in ethanol orderly, then followed
the reactivation in H atmosphere at 375°C for 1 h. In the
2
reusability test, a slight activity loss after five rounds was
found (97-92%), which demonstrated the excellent thermal
and chemical stability of Co-N-C/CNT@AC catalyst (Figure 6).
Further, we performed XRD and TEM characterization for the
spent catalyst after 2 cycles. No significant change was
observed from XRD patterns, and CNTs could still remain
except for minor morphology change compared to fresh
catalyst (Figure 4a vs. Figure 7).
Scheme 1 A tentative mechanism for the direct alkylation of nitroarenes.
It's worth pointing out that large amounts of aldehydes were
produced by dehydrogenation of alcohols in the reductive
amination of nitroarenes with alcohols, which increased the
difficulty of product separation on the one hand and did not
meet the requirement of atomic economy on the other hand
although aldehydes were valuable products. Based on our
effort in the economic synthesis of N-substituted amine, we
tried to aminate excess alcohols by introducing amines. For
this, a model reaction was made via nitrobenzene, aniline and
benzyl alcohol with 1: 2: 3 molar ratio as the starting materials
for the synthesis of imine (Scheme 2). The result exhibited that
all reactants were almost quantitatively reacted to afford the
desired imine with 96% yield, and only trace amount of
benzaldehyde was detected. This methodology provided an
alternative for the atom-economic reductive amination
reaction of nitroarenes with alcohols.
Figure 7 XRD and TEM image of spent Co–N–C/CNT@AC catalyst.
To examine the practical applicability of our protocol, the
imination of nitrobenzene and benzyl alcohol was conducted
on a 0.2 mol nitrobenzene scale under the above optimized
conditions. The reaction proceeded for 24h and gave imine
with 91% GC yield.
NO2
NH2
OH
Catalyst
N
Based on our experiment results and previous literature
reports, a tentative reaction mechanism to rationalize this
transformation was depicted in Scheme 1. The direct reductive
coupling of nitroarenes with alcohols involved the following
four steps: Ⅰ) the dehydrogenation of alcohols afforded
aldehydes, and released proton and hydride which migrated to
+
2
+
3
3
Reaction conditions: nitrobenzene (5 mmol), aniline (10mmol), benzyl
alcohol (15mmol) and catalyst (0.8 g), 160 °C, N atmosphere, 30h.
2
Scheme 2 Coupling reaction of nitrobenzene, aniline and benzyl alcohol to N-
Benzylideneaniline
4
8,
the substrate under the action of Co-N and basic N species
.
4
9
Ⅱ) transfer hydrogenation of nitrobenzenes. Two possible
Experimental
Materials
Activated Carbon (Norit RX 3 Extra) was obtained from Cabot
Corporation. Other chemicals were purchased from Sinopharm
reaction pathways were widely accepted for this step. One is
the direct reduction of nitrobenzenes to anilines undergoing
hydroxylamine
intermediates
via
a
sequential
hydrogenation/dehydration process (A). The second route
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Journal Name
Chemical Reagent Co., Ltd. and Aladdin Reagent Co. Ltd. detector and Agilent GC-MS 7890. The conversions, yields and
View Article Online
DOI: 10.1039/C8GC03818J
(
Shanghai, China). Beyond that, all chemicals were of analytical selectivities were calculated by the following equations.
grade or higher and were used without further purification.
Preparation of the Catalyst
Conversion (%) = [(C
Selectivity (%) = [C /(C
O (2.34 g) were Yield (%) = C /C × 100
dissolved in deionized water (120 mL), activated Carbon Here, C represented the initial concentration of the reactants;
powder (AC, 5g) was added to the above solution. Then, the and C are the concentrations of the residual reactants and
0
-C
r
)/C
-C )] × 100
r
0
] × 100
p
0
Dicyandiamide (DCD, 10 g) and Co(OAc)
2
·4H
2
p
0
0
C
r
p
mixture was continually stirred at 60 °C until the solution the corresponding amines, respectively.
completely evaporated. The resulting solid sample was
calcined in the following consecutive steps: 1) The solid sample
Conclusions
was heated to 550°C with a rate of 5 °C/min under flowing N
2
and held at 550°C for 1 h. 2) The solid sample was further
raised to 700°C (3°C/min) and kept the temperature for 30 min
A non-noble nanostructured Co-N-C/CNT@AC catalyst was
fabricated via the successive pyrolysis and reduction of the
mixture of the precursors. The imines and secondary amines
were synthesized successfully through the route of the
reductive coupling of nitroarenes and alcohols over Co-N-
C/CNT@AC under exogenous base- and solvent- free
conditions. And the reusability and amplification experiments
confirmed the decent applicability of Co-N-C/CNT@AC. In
short, our catalytic system had the merits of cost effectiveness,
wide universality, environmental benignity and good
reproducibility. More work is under way to further improve
catalytic performance, explore reaction mechanism and
expand the scope of application of our catalyst.
in N
75°C. 3) The material was reduced for 2h under flowing high-
purity H at 375°C, following natural cooling to 200°C. 4) The
sample was passivated in N flow from 200°C to room
2
flow. Then the temperature was naturally lowered to
3
2
2
temperature to obtain final products, which was named as Co-
N-C/CNT@AC.
Co@AC and N-C@AC catalyst were prepared based on the
2 2
above method in the absence of DCD and Co(OAc) ·4H O,
respectively. In addition, Co-N-C@AC catalyst without CNT was
prepared for comparison by a similar method using urea as N
source.
Characterization techniques
Powder X-ray diffraction patterns (XRD) of the catalysts were
carried out on a Rigaku diffractometer (UItima IV, 3 kW) by Cu Conflicts of interest
2
Ka radiation (40 kV, 30 mA, 0.1543 nm). The N adsorption-
There are no conflicts to declare.
desorption isotherms were measured at 77 K using a
Micromeritics ASAP 2020 instrument. The specific surface area
was calculated using the Brunanuer-Emmett-Teller (BET)
method, the pore volume and pore size distributions were
obtained using the Barrett-Joyner-Halenda (BJH) method. The
morphologies and structures of the catalysts were observed by
scanning electron microscope (SEM, Hitachi S-4800) and
transmission electron microscopy (TEM, FEI Tecnai G2 F20 and
JEOL 2100F). The X-ray photoelectron spectroscopy (XPS) was
performed on an ESCALAB 250Xi Spectrometer for excitation
to characterize the chemical compositions or elemental
structure type of the catalysts surface. The contents of
elements were measured using inductively coupled plasma-
atomic emission spectrometry (ICP-AES, Shimadzu ICPE-9000).
General procedure for the coupling reaction of nitroarenes with
alcohols
Acknowledgements
This work was supported by grants from the National Natural
Science Foundation of China (No. 21878178).
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DOI: 10.1039/C8GC03818J
Imines and amines were synthesized on Co-N-C/CNT@AC by the coupling of
nitroarenes and alcohols under base- and solvent- free conditions.