Commercially Available CuO Catalyzed Hydrogenation of Nitroarenes Using Ammonia Borane as a Hydrogen Source

Article abstract of DOI:10.1002/cctc.201902391

Tandem ammonia borane dehydrogenation and nitroarenes hydrogenation has been reported as a novel strategy for the preparation of aromatic amines. However, the practical application of this strategy is subjected to the high-cost and tedious preparation of supported noble metal nanocatalysts. The commercially available CuO powder is herein demonstrated to be a robust catalyst for hydrogenation of nitroarenes using ammonia borane as a hydrogen source under mild conditions. Numerous amines (even sterically hindered, halogenated, and diamines) could be obtained through this method. This monometallic catalyst is characteristic of support-free, excellent chemoselectivity, low-cost, and high recyclability, which will favor its future utilization in preparative reduction chemistry. Mechanistic studies are also carried out to clarify that diazene and azoxybenzene are key intermediates of this heterogeneous reduction.

Full text of DOI:10.1002/cctc.201902391

Accepted Article
Title: Commercially Available CuO Catalyzed Hydrogenation of
Nitroarenes Using Ammonia Borane as a Hydrogen Source
Authors: Jialei Du, Jie Chen, Hehuan Xia, Yiwei Zhao, Fang Wang,
Hong Liu, Weijia Zhou, and Bin Wang
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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To be cited as: ChemCatChem 10.1002/cctc.201902391
Link to VoR: http://dx.doi.org/10.1002/cctc.201902391
A Journal of
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ChemCatChem
10.1002/cctc.201902391
COMMUNICATION
Commercially Available CuO Catalyzed Hydrogenation of
Nitroarenes Using Ammonia Borane as a Hydrogen Source
[
a]
[b]
[a]
[c]
[a]
[a,c]
[a]
Jialei Du, Jie Chen, Hehuan Xia, Yiwei Zhao, Fang Wang, Hong Liu, Weijia Zhou,* and Bin
[b]
Wang*
[
a]
J. Du, H. Xia, F. Wang, Prof. H. Liu, Prof. W. Zhou
Collaborative Innovation Center of Technology and Equipment for Biological Diagnosis and Therapy in Universities of Shandong
Institute for Advanced Interdisciplinary Research
University of Jinan
Jinan 250022, Shandong Province, P. R. China
E-mail: ifc_zhouwj@ujn.edu.cn
[
b]
c]
J. Chen, Prof. B. Wang
School of Chemistry and Chemical Engineering
University of Jinan
Jinan 250022, Shandong Province, P. R. China
E-mail: chm_wangb@ujn.edu.cn
Y. Zhao, Prof. H. Liu
[
State Key Laboratory of Crystal Materials
Shandong University
Jinan 250100, Shandong Province, P. R. China
Supporting information for this article is given via a link at the end of the document.
[
15]
Abstract: Tandem ammonia borane dehydrogenation and
nitroarenes hydrogenation has been reported as a novel strategy for
the preparation of aromatic amines. However, the practical
application of this strategy is subjected to the high-cost and tedious
preparation of supported noble metal nanocatalysts. The
commercially available CuO powder is herein demonstrated to be a
robust catalyst for hydrogenation of nitroarenes using ammonia
borane as a hydrogen source under mild conditions. Numerous
amines (even sterically hindered, halogenated, and diamines) could
be obtained through this method. This monometallic catalyst is
characteristic of support-free, excellent chemoselectivity, low-cost,
and high recyclability, which will favor its future utilization in
preparative reduction chemistry. Mechanistic studies are also carried
out to clarify that diazene and azoxybenzene are key intermediates
of this heterogeneous reduction.
(solid at room temperature), safety, and nontoxicity.
Additionally, bulk quantities of AB are commercially available
and affordable. Hydrogen evolution from AB can be expediently
performed via hydrolysis in the presence of transition metals
+
–
3 3 2 4 2 2
(NH ·BH + 2H O → NH + BO + 3H ↑), and one-pot tandem
AB dehydrogenation and unsaturated compound hydrogenation
provides a viable strategy for the manufacture of valuable
molecules. As a metal-free reductant, AB has been directly used
[
16]
[17]
[17]
to initiate the reduction of imines,
ketones,
aldehydes,
[
18]
and even polarized olefins. Although the reducing ability of AB
is not strong enough on its own to initiate the direct
hydrogenation of other unsaturated functional groups, the use of
metal catalysts with this reagent resolves this challenge. There
are numerous reports describing the use of AB as the hydrogen
donor for the catalytic transfer hydrogenation of a diverse range
[
19]
[20]
of unsaturated compounds, including alkenes,
alkynes,
[
21]
[22]
[23]
[24]
nitriles,
enoates,
imines,
β-enamino esters,
Aromatic amines serve as key intermediates for the production
of a wide range of high value-added materials, including dyes,
pharmaceuticals, polymers, agrochemicals, etc. The increasing
demand for functionalized amines has continuously driven the
development of green and economical synthetic methods for
their scalable production. The sustainable reduction of aromatic
nitro compounds represents one of the most straightforward,
[25]
[26]
quinoxalines, and (E)-azo-compounds.
Significant progress has been made for the one-pot transfer
[27-39]
hydrogenation of nitro compounds to the target amines.
Multiple reports have described nanoparticle-type catalysts that
exhibit good activity, selectivity, and reusability for
hydrogenation reactions; however, most of them are composed
[27-32]
of at least two types of transition metals,
metals.
and even noble
[
1]
industrially applicable approaches to synthesize anilines.
Although remarkable progress has been made in the direct
hydrogenation of nitroarenes using pressurized as
the transfer
[30-34,38,39]
In general, these ultra-small metal nanoparticles
[28,29]
are loaded onto special solid supports, such as graphene,
H
2
a
[30]
[31]
MOFs,
reduced
graphene
oxide,
nitrogen-doped
or N-doped porous carbon. For example,
[
2-8]
hydrogen source with non-noble metal catalysts,
[32]
[34]
[35]
graphene, TiO
2
,
hydrogenation could realize safe reductions under mild
conditions without specialized reaction setups. Therefore,
Sun and co-workers reported the chemoselective reduction of 3-
nitrostyrene to 3-vinylaniline within 1.5 h using exquisitely
[
9]
[10]
[11]
[12]
NaBH
silane,
4
[
,
HCOONH
and H PO
3 3
4
,
HCOOH,
hydrazine hydrate,
[36]
designed Cu nanoparticles anchored on WO2.72 nanorods.
13]
[14]
have emerged as alternative hydrogen
Although these are significant advances, the requirement for
supports and noble metals makes these catalysts less
applicable to the industrial scale preparation of aromatic amines.
Accordingly, there remains a need for the development of low-
cost, readily available, and highly selective catalysts for the
synthesis of amines via the reduction of nitro compounds. While
storage materials for reduction of nitro compounds.
Unlike the above hydrogen sources, ammonia borane
(
3 3
NH ·BH , denoted as AB hereafter) has been generally
considered as a practical reservoir for hydrogen storage and
transport, due to its high hydrogen capacity (19.6 wt%), stability
1
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ChemCatChem
10.1002/cctc.201902391
COMMUNICATION
AB has the potential to fulfill these requirements, there are
currently no reports on nitro reduction by unsupported
monometallic catalysts using AB as the hydrogen donor.
for this transformation (entries 2–3). Unexpectedly, common
2
Cu O also proved to be efficient for the reduction of
nitrobenzene (91%, entry 4); this differs from a previous report
[
37]
that used AB as the reducing agent,
activity of Cu O. Replacing CuO with other commercial Cu-
based materials, including Cu powder, Cu(OAc) and
Cu(bpy)(OAc) (bpy = 2,2’-bipyridine), led to extremely low yields
entries 5–7). Other reductants, such as BH -THF complex,
NaBH , hydrazine hydrate, HCOOH, and H gas, showed inferior
indicating the superior
2
2
,
2
(
3
4
2
reducing abilities toward nitroarene hydrogenation (entries 8–12).
Using EtOH or pure water as the solvent, the reduction
proceeded smoothly and afforded impressive yields (entries 13
Scheme 1. Practical tandem catalysis for hydrogenation of nitroarenes by AB
over commercially available CuO and the key intermediates trapped.
and 17); however, other aprotic solvents, such as CH
and acetone, did not lead to any observable conversion (entries
4–16). When the loading of AB or CuO was decreased, the
3
CN, THF,
1
Herein, we report the first use of commercially available CuO
as catalyst for the selective hydrogenation of various functional
aryl nitro compounds using AB as a hydrogen source (Scheme
yields of aniline also decreased (entries 18–20).
Upon completing this investigation, the optimal conditions
were established as follows: open system at 50 C, AB (3 equiv.)
as the hydrogen source, CuO (5 mol%) as the catalyst, MeOH (6
mL) as the solvent, and nitroarene (1 mmol) as the substrate.
Notably, these conditions allow for a scalable reaction, and a
successful gram-scale reduction of nitrobenzene (10 mmol) to
aniline was achieved within 1 h in excellent yield (0.84 g, 90%).
o
1). The activity and selectivity of this commercialized catalyst is
comparable to that of the previously reported noble metal
nanocatalysts, thus revealing the promising application of this
strategy in the community of synthetic chemistry. Moreover, the
condensation pathway for the hydrogenation was proposed
based on the two intermediates (e.g., azoxybenzene and
azobenzene) identified in mechanistic studies.
a
Scheme 2. Substrate scope for CuO-catalyzed nitroarene hydrogenation.
a
Table 1. Optimization of hydrogenation reaction conditions.
e
Entry
Catalyst
CuO
CuO
—
Hydrogen Source
Solvent
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
Yield (%)
1
2
3
4
5
6
7
8
NH
3
·BH
3
93
0
—
NH
NH
NH
NH
NH
3
3
3
3
3
·BH
3
3
3
3
3
0
2
Cu O
·BH
·BH
·BH
·BH
91
5
Cu
Cu(OAc)
2
7
Cu(bpy)(OAc)
2
0
CuO
BH
NaBH
·xH
HCOOH
3
-THF
0
9
CuO
4
10
0
b
10
CuO
N
2
H
4
2
O
c
11
CuO
0
d
12
CuO
H
2
0
13
CuO
NH
NH
NH
NH
NH
NH
NH
NH
3
·BH
3
·BH
3
·BH
3
·BH
3
·BH
3
·BH
3
·BH
3
·BH
3
3
3
3
3
3
3
3
91
0
14
15
16
17
CuO
3
CH CN
CuO
THF
0
CuO
acetone
0
CuO
H
2
O
90
62
35
56
f
18
CuO
MeOH
MeOH
MeOH
g
19
CuO
h
20
CuO
a
Reaction conditions: nitrobenzene (1 mmol), catalyst (5 mol%),
o
b
hydrogen source (3 equiv.), solvent (6 mL), 50
of hydrazine hydrate (
was 0.5 mL. The pressure of H
of AB was used. One equiv. of AB was used. 2.5 mol% CuO was used.
C, 20 min. The volume
c
N
2
H
4
·xH
2
O) was 0.5 mL. The volume of HCOOH
d
e
f
was 1 atm. Isolated yields. Two equiv.
a
2
General reaction conditions: nitroarene (1 mmol), AB (3 equiv.), CuO (5
g
h
o
mol%), MeOH (6 mL), 20 min, 50 C. The reaction time was prolonged to 1 h
(
b), 1.5 h (c), or 3 h (d); all other conditions were maintained.
The reduction of nitrobenzene was chosen as the model
reaction for optimizing the reaction conditions (Table 1). In an
open system heated to 50 C, the reaction of AB (3 equiv.), CuO
Having established the optimized reaction conditions, we next
o
investigated the substrate scope of this transformation to probe
the generality and limitations of the reaction (Scheme 2).
Various electron-donating groups, including Me (2, 3), iPr (4),
(
5 mol%), and nitrobenzene (1 mmol) in methanol (6 ml) led to
the production of aniline (1) in 93% yield (entry 1). Control
experiments indicated that both CuO and AB are indispensable
2 2
NH (6–8), MeO (9, 10), OH (20, 21, 27), MeS (22), and CH OH
2
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ChemCatChem
10.1002/cctc.201902391
COMMUNICATION
(
(
38), and electron-withdrawing substituents, such as halogens
11–14), CF (16, 17), and CO Me (26), were compatible with
A
B
3
2
the reaction system, delivering the aniline products in excellent
yields. This hydrogenation proved to be insensitive towards
steric encumbrance in the vicinity of the targeted nitro group (1–
4, 6–8). Gratifyingly, the reductions were successfully carried out
when reducible functional groups existed in the substrates,
including halogens (10–15), amides (18, 19), oximes (23, 24),
esters (25–30), alkenyl (29, 30), nitriles (40, 41), and imines (42),
furnishing the desired products in moderate to excellent yields
and chemoselectivities. However, nitroarenes substituted with
aldehydes or ketones were not tolerated under the standard
reaction conditions, owing to the reduction of these
functionalities by AB (see Scheme S1 in the Supporting
C
D
[17]
Information), which is consistent with previous reports.
Halogen-substituted nitroarenes represent challenging
a
substrate since hydrodehalogenation can occur as a side
reaction. However, halo substituents, including F, Cl, and Br,
were well tolerated in this procedure (10–13, 15, 17, 21), and
even an iodo-substituted nitroarene could be converted into its
corresponding aniline in moderate yield (14). Notably, the
Figure 1. SEM images of CuO catalyst before (A) and after (B) the 5th round
of reaction. XRD (C) and XPS (D) patterns of CuO catalyst before and after
the reaction. XRD patterns of Cu (Cu#, JCPDS 04-0836) and CuO (CuO*,
JCPDS 45-0937) are included for comparison.
2
aromatic compound bearing two –NO groups could be reduced
to its related diamine (31). Some substituted nitropyridines could
also participate in the hydrogenation reaction, affording the
corresponding amines in excellent yields (33–35). In addition,
nitronaphthalene, 2-nitroquinoline, 4-(4-nitrobenzyl)pyridine, 5-
nitro-1H-indole, and 1-(4-nitrophenyl)ethan-1-ol, could also
Scanning electron microscopy (SEM, Figure 1A, B) and
transmission electron microscopy (TEM, Figure S2) of CuO
before and after its use in the recycling reactions exhibited that
function in this transformation， giving the target products (5, 32, the morphology of the catalyst changed from block-shaped to
3
6, 37). Although several nitroarene substrates were not suitable
spongy with larger specific surface area following its repeated
use. As shown in Figure S2D, clearer lattice fringes were
observed with a d-spacing of 0.21 nm characteristic of the (111)
crystal plane of Cu, implying that copper precursors can
potentially be reduced by AB, owing to the strong reducibility of
the M–H species and hydrogen atoms generated during AB
for this catalytic system (Figure S1), the above experimental
results showed that this catalytic protocol displays a preeminent
substrate scope and application potential.
Table 2. Catalytic recyclability of CuO in the hydrogenation of nitrobenzene by
AB.
[
27]
hydrolysis.
All the fresh and used CuO samples were also
characterized by X-ray diffraction (XRD) and X-ray photoelectron
spectroscopy (XPS) to investigate the change in their
compositions (Figure 1C, D). After the reaction, the XRD pattern
of catalyst can be ascribed to the Cu(0) phase with a cubic
Cycle
1
2
3
4
5
Yield (%)
90
93
91
90
92
[40]
structure (Figure 1C), and the peaks of Cu 2p3/2 and Cu 2p1/2
After completing the reaction scope,
characterization of the catalytic activity of CuO was performed.
The turnover frequency for CuO in the hydrogenation of 4-
a
detailed
were observed with binding energies located at 932.2 and 951.9
[40,41]
eV (Figure 1D),
thus demonstrating that CuO was indeed
reduced to Cu(0). On the basis of the above observations, we
were able to propose that the reduction of block-shaped CuO
powder by ammonia borane would generate the spongy
structure with larger surface area consisting of Cu(0)
nanoparticles (Figure 1B), which could significantly increase the
number of active sites and catalytic activity (Table 2). In contrast,
the non-nanoscale Cu powder (Figure S3) resulted in low
efficiency as well as limited reaction sites (Table 1, entry 5).
–1
nitrophenol was determined to be 0.017 s , which is
comparable to reported nanocatalysts (Table S1). The
heterogeneity of CuO in the hydrogenation reaction was
investigated using a filtration test after a 10 min reaction period.
Following removal of the catalyst, no further conversion of
nitrobenzene was observed in the reaction, even after 2 h under
the optimized conditions. The importance of the heterogeneity of
CuO in the hydrogenation reaction is further supported by the
failure of the soluble Cu(II) salt and complex to catalyze this
reaction (Table 1, entries 6–7), The stability and recyclability of
CuO was confirmed following its reuse in five consecutive cycles
(
Table 2). No significant degradation in activity or selectivity was
observed during this evaluation.
3
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ChemCatChem
10.1002/cctc.201902391
COMMUNICATION
nitroarenes was scalable, and the CuO catalyst was recyclable.
Compared with other published catalytic systems, our CuO/AB
reduction reaction possesses several advantages, including
easily accessible and low-cost, excellent functional group
compatibility, and no requirements for solid supports and
additional noble metals. Preliminary mechanistic studies
identified diazene and azoxybenzene as intermediates of the
catalyzed reduction. We envision that this CuO/AB strategy will
function as a promising and industrially applicable alternative to
currently employed catalytic reduction systems.
(
(
(
a)
b)
c)
▲
(d)
#
(e)
*
(
(
(
f)
♦
g)
h)
#
#
reaction solution
CHCl
3
♦
♦
▲
#
Acknowledgements
▲
▲
♦
▲
*
*
▲
*
We acknowledge financial support of this work from the National
Natural Science Foundation of China (21771087 to B.W and
Figure 2. Intermediates observed during the CuO-catalyzed hydrogenation of
nitrobenzene using AB. H NMR spectra of the reaction solution (h) and the
21703080 to J.C), the NSF of Shandong Province
1
(
ZR2018BB035 to J.D, ZR2017MB007 to B.W and
authentic samples (a-g) in CDCl
3
as the solvent. Symbols in (h) indicate peaks
corresponding to those observed in the standards.
ZR2017BB010 to J.C), Taishan Scholar Program of Shandong
Province (tsqn201812078 to B.W).
The observation that H
the reduction of nitrobenzene (Table 1, entry 12) suggests that
the in situ-generated H from AB hydrolysis can not be activated
and split under the ambient conditions employed in this reaction.
In addition, H gas generated from AB hydrolysis can rapidly
escape from solution, particularly in the open system employed
in this work, suggesting that the H is not responsible for the
reduction of nitrobenzene. Therefore, it is likely that the catalyst
is involved in facilitating the efficient generation and transfer of
an active hydrogen species (e.g., H· or hydride) from AB. Using
2
could not be used in place of AB for
Keywords: copper oxide • hydrogenation • ammonia borane •
nitroarene reduction • aniline
2
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the nitro group (Scheme 3).
Aromatic amines are essential intermediates across a wide
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dehydrogenation and nitroarene hydrogenation under mild
conditions using conventional CuO powder as the catalyst.
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noteworthy that the CuO-catalyzed tandem hydrogenation of
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COMMUNICATION
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ChemCatChem
10.1002/cctc.201902391
COMMUNICATION
Entry for the Table of Contents
A synthetically useful strategy for tandem ammonia borane dehydrogenation and nitroarene hydrogenation was readily accomplished
by commercially available CuO under mild conditions. Mechanistic studies are also carried out to clarify the plausible hydrogenation
mechanism.
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This article is protected by copyright. All rights reserved.