Rhodium-terpyridine Catalyzed Transfer Hydrogenation of Aromatic Nitro Compounds in Water

Article abstract of DOI:10.1002/asia.202100321

A rhodium terpyridine complex catalyzed transfer hydrogenation of nitroarenes to anilines with i-PrOH as hydrogen source and water as solvent has been developed. The catalytic system can work at a substrate/catalyst (S/C) ratio of 2000, with a turnover frequency (TOF) up to 3360 h^?1, which represents one of the most active catalytic transfer hydrogenation systems for nitroarene reduction. The catalytic system is operationally simple and the protocol could be scaled up to 20 gram scale. The water-soluble catalyst bearing a carboxyl group could be recycled 15 times without significant loss of activity.

Full text of DOI:10.1002/asia.202100321

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Rhodium-terpyridine Catalyzed Transfer Hydrogenation of
Aromatic Nitro Compounds in Water
Authors: Yuxuan Liu, Wang Miao, Weijun Tang, Dong Xue, Jianliang
Xiao, Chao Wang, and Changzhi Li
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To be cited as: Chem. Asian J. 10.1002/asia.202100321
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Rhodium-terpyridine Catalyzed Transfer Hydrogenation of
Aromatic Nitro Compounds in Water
Yuxuan Liu,[a][b] Wang Miao, Weijun Tang, Dong Xue, Jianliang Xiao, Chao Wang,* Changzhi
[a]
[a]
[a]
[c]
[a]
Li*[b]
[
a]
Y. Liu, W. Miao, Prof. W. Tang, Prof. D. Xue, Prof. J. Xiao, Prof. C. Wang
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering
Shaanxi Normal University
Xi’an, 710119, China
E-mail: c.wang@snnu.edu.cn
Y. Liu, Prof. C. Li
CAS Key Laboratory of Science and Technology on Applied Catalysis
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Dalian, 116023, China
[
b]
c]
E-mail: licz@dicp.ac.cn
Prof. J. Xiao
[
Department of Chemistry
University of Liverpool
Liverpool, L69 7ZD, United Kingdom
Supporting information for this article is available on the WWW.
Abstract: Rhodium terpyridine complexes catalyzed transfer
hydrogenation of nitroarenes to anilines with i-PrOH as hydrogen
source and water as solvent has been developed. The catalytic
system can work at a substrate/catalyst (S/C) ratio of 2000, with a
to 12000 h-1 at a substrate/catalyst (S/C) ratio of 400 using
hydrazine as reducing agent for transfer hydrogenation of
nitrobenzene in a continuous flow system.[22a] Despite the
progress, the development of simple, green, efficient and practical
catalytic transfer hydrogenation systems for nitroarene reduction
is still highly desirable.
-
1
turnover frequency (TOF) up to 3360 h , which represents one of the
most active catalytic transfer hydrogenation systems for nitroarene
reduction. The catalytic system is operationally simple and the
protocol could be scaled up to 20 gram scale. The water-soluble
catalyst bearing a carboxyl group could be recycled 15 times without
significant loss of activity.
Herein, we report a new transfer hydrogenation system for
the reduction of nitro compounds to the corresponding anilines
with i-PrOH as hydrogen source catalyzed by simple Rh
terpyridine complexes in water. The catalytic system can work at
a substrate/catalyst (S/C) ratio of 2000, with a turnover frequency
-
1
(
TOF) up to 3360 h , providing one of the most active transfer
Aromatic amines are an important class of compounds, which are
used as intermediates in the synthesis of numerous fine
chemicals.[1] There are a wide variety of methods available to
[7c, 22a]
hydrogenation systems for nitro reduction.
The robustness
of the protocol is demonstrated with a 20-gram scale reaction and
the water-soluble catalyst can be recycled 15 times without
significant loss of activity.
[
2]
obtain amine compounds, including hydrogen-borrowing ,
[
3]
[1, 4]
[3b, 5]
reductive amination , N-arylation , reduction of imines
or
Terpyridine metal complexes are oxidatively and thermally
robust[23], and their use as catalysts for organic synthesis are
relatively few.[24] Crabtree and co-workers reported that Ru and Ir
terpyridine complexes could catalyze borrowing hydrogen
[
6]
[7]
nitriles , and the reduction of nitro compounds , etc. Amongst
them, the reduction of nitro compounds is perhaps the most
widely utilized method in industrial processes. Traditional non-
catalytic reduction processes (that consume Fe/HCl[8] or
[25]
reactions between alcohols. Our recent work showed that Rh
[
9]
Sn/HCl ) generate large amounts of undesirable waste, which is
not environmentally acceptable nowadays. The catalytic
hydrogenation of nitro aromatics to anilines is a favorable process,
terpyridine complexes are an effective catalyst for
dehydrogenation of alcohols[26], dehydrogenative coupling
[27]
reactions , and aqueous redox-neutral depolymerization of
and a number of transition metal catalysts (based on Pt[10], Pd[11]
,
[28]
lignin in water via a hydrogen transfer process. The hydrogen-
[
12]
[13]
[14]
Au
,
Ni
,
Co
etc.) have been employed for the
transfer ability of Rh terpyridine complexes for alcohols “in
water”[29] renders them possible catalysts for transfer
hydrogenation reaction, which would be more economical and
environmental benefits with water as solvent. Indeed, they are
found to be suitable catalysts for transfer hydrogenation of
nitroarenes into anilines in water with i-PrOH as hydrogen source
in this work. Initial results showed that in the presence of 0.25
mol% complex 1a and 0.1 equivalent of Na CO in water at 100 °C
2 3
for 10 minutes, nitroarene 2a was completely converted, providing
three dominant products, viz. 3a (azoxybenzene, 37% yield), 4a
transformation. However, this protocol is not easy to operate and
usually requires harsh reaction conditions (such as high hydrogen
pressure and high temperature). Apart from the use of hydrogen
gas, several other reducing agents such as silanes[15], boranes[16]
and sodium borohydride[17] have also been used in combination
with a number of different metal catalysts. In addition, CO/H
O[18]
,
2
is also reported as hydrogen source for nitroarene reduction.
Alternatively, transfer hydrogenation of nitro compounds has
appeared as a very attractive method, due to the simple
experimental setups and easy operation procedure. Progress has
(
azobenzene, 10% yield), and 5a (aniline, 50% yield) (Table 1,
been made with i-PrOH[19], ethanol[20], formic acid[21], and
entry 1). Catalyst with an electron-rich phenyl substituent
provided the best activity amongst the catalysts tested (1a, Table
hydrazine[
22]
as hydrogen sources. Notably, Kappe and co-
workers reported an example with a turnover frequency (TOF) up
1
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COMMUNICATION
1
,
entries 1-6). Eletron-rich catalyst would form
a
Rh-H
soluble complex 1f as catalyst (5c-5j, 5g). Upon full conversion
for these substrates, the corresponding aromatic amines were
obtained in good isolated yields (78%-97% for 5a-5q). However,
substrates with electron-withdrawing substituents (such as
carbonyl or CN) (Table 2, 5r, 5s) failed to give high yields of the
desired products even reacting for longer time with higher catalyst
loading. To our delight, nitrocyclohexane could be transfer
hydrogenated to cyclohexylamine with high yield in the system (up
to 94% yield based on normalization method) (Table 2, 5t).
intermediate with high hydricity, which will be benificial for
reduction. The higher activities observed for catalysts with a
hydrophobicity aryl group suggest that the reaction could proceed
faster in an “on water”[30] manner. Switching the base from
Na
improved (Table 1, entry 7). Raising the reaction temperature to
20 °C further increased the selectivity of 5a (91% yield in 10
minutes, Table 1, entry 8). Impressively, a turnover frequency
TOF) of 3360 h−1 was observed when the reaction was performed
2 3 3 4
CO to K PO , the selectivity towards product 5a was
1
(
at 120 °C for 6 minutes (Table 1, entry 9). Full conversion of 2a,
with almost exclusive formation of 5a (up to 96% yield determined
Table 2. The scope of nitro compoundsa
by GC) was achieved with 1a as catalyst in the presence of K
3
PO
4
(
0.1 equiv) and i-PrOH (20 equiv) in water at 120 °C for 3 h (Table
1, entry 10 and SI). The reaction could be conduct at a S/C of
2000 with high yield of 5a (Table 1, entry 11).
Table 1. Rh catalyzed transfer hydrogenation of nitrobenzene in watera
a
Reaction conditions: 2a-2s (1 mmol), 1a (0.0025 mmol), K
3 4
PO (0.1 mmol), i-
b
PrOH (20 equiv), H
2
O (1 mL), 120 oC, 3 h, under Ar, isolated yield. 1a (0.01
c
d
e
mmol). 1f (0.01 mmol), i-PrOH (3 mL), no water. 1f (0.02 mmol). 1f (0.01
mmol). 1a (0.01 mmol), i-PrOH (3 mL), no water. 1f (0.01 mmol), Na CO (1
2 3
f
g
equiv).
Experiments were then carried out to demonstrate the
practical usefulness of the catalytic system. 1f was chosen as the
catalyst in these studies as it is water-soluble in the presence of
a base, which makes it easy to be separated from the products
and possible for catalyst recycle. Firstly, gram-scale reactions
were carried out (Scheme 1). Starting with 20.30 g of 2a, the
desired product 5a could be easily isolated from the reaction
mixture through simple extraction, with a yield of 80% (12.28 g).
Noting that 5p is an important precursor of lidocaine (typical
anesthetic), the reduction of 2p was conducted. Pleasingly,
despite its steric hindrance, 20.39 g of 2p was converted to 16.00
g of 5p with isolated yield up to 98% after extraction. ICP-OES
analysis of the large scale reaction products showed that only a
trace amount of rhodium was leached (5.15 ppm in 5a, 6.65 ppm
in 5p).
a
2 3
Reaction conditions: 2a (1 mmol), catalyst (0.0025 mmol), Na CO (0.1 mmol),
b
o
i-PrOH (20 equiv), H
determined by GC-FID with biphenyl as internal standard.
TOF = 3360 h
mmol), i-PrOH (20 equiv), H
2
O (1 mL), 100 C, 10 mins, under Ar. Yields were
c
K
3
PO
4
(0.1 mmol). d
PO (0.25
−
1 e
.
2a (2.5 mmol), 1a (0.00125 mmol, S/C = 2000), K
3
4
o
2
O (2 mL), 120 C, 96 h, under Ar.
With the optimal reaction condition identified, we then
explored the scope of aromatic nitro compounds to probe the
versatility of the catalyst (Table 2). It should be noted that the
isolated yield of 5a (82%) is lower than GC yield (96%) because
it is easy to volatilize. It is found that electron-donating
substituents at ortho-, meta- or para-positions are all tolerated
(
5a-5q), but some of substrates worked better with the water-
2
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COMMUNICATION
Based on previous studies on nitro reduction[19b-e, 21b, 31], the
intermediates observed in our reaction and our previous studies
on Rh terpyridine complex catalyzed hydrogen transfer
[
28]
reactions , possible reaction pathways for the reduction of
nitroarenes are proposed in Scheme 2. Under basic conditions,
the chloride in the Rh catalyst might first be replaced by a
hydroxide to form complex I, which might be the real catalyst.
Complex I could then deprotonate i-PrOH to form an intermediate
II, followed by β-hydride elimination to produce acetone and an
active species of Rh hydride. The Rh hydride could then reduce
the nitrobenzene via two possible reaction pathways. The first
(
5
red dashed box) involves direct transfer hydrogenation of 2a to
a via the intermediary of nitrosoarene III and hydroxylamine IV.
The second (blue dashed box) is a condensation route involving
the coupling of nitrosoarene III with hydroxylamine IV to afford
azoxy compound 3a, which is finally reduced to 5a via 4a and
intermediate V.
Scheme 1 Gram scale reactions catalyzed by 1f in water. See the SI for details.
Catalyst recyclability is a key factor that determines the
application potential of a catalytic process. The recyclability of 1f
was further investigated with 2a as substrate (See the SI for
details). As 1f is water-soluble in the presence of a base, and the
product 5a has low solubility in water, 5a was easily extracted with
petroleum ether after the reaction. The catalyst 1f remained in the
aqueous phase, which was directly used for the reduction of
freshly added 2a. As shown in Figure 1, 1f could be reused at
least 15 times without significant decreases in catalytic activity
and selectivity. To the best of our knowledge, this is the highest
recycle times ever reported for homogeneous catalytic transfer
hydrogenation of nitroarenes in water (Figure 1).
Scheme 2. Plausible reaction pathway for the transfer hydrogenation of 2a.
In order to determine which route the catalytic system follows,
the time course of product formation for the transfer
hydrogenation of 2a was monitored. As shown in Figure 2, most
of 2a is selectively transformed into 3a, 4a and 5a within 6 minutes
under the standard conditions. The amount of 3a and 4a
decreased after about 4 minutes, which suggests that 3a and 4a
might serve as intermediates in this reaction. Several more control
experiments were carried out. First, the reduction of 3a to 5a was
probed under the standard conditions (Scheme 3, equation 1).
Indeed, 3a (77% conversion) could be transformed into 4a (41%)
Figure 1. Reuse of the catalyst 1f in water. See the SI for detailed reaction
conditions.
3
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COMMUNICATION
and 5a (35%) in 1 h under the standard conditions. Moreover, 4a
could be converted to 5a in 1 h under the standard conditions
results further support the intermediary of 3a and 4a and a
possible condensation pathway.
(
51% conversion, 49% yield, Scheme 3, equation 2). These
The possibility of a direct pathway[19b-e, 21b, 31] was also
examined, although intermediate IV was not observed during the
reaction. Interestingly, the hydroxylamine IV, the intermediate for
the direct route, could also be converted into 5a (83% yield) with
a small amount of 3a (8% yield) and 4a (8% yield) in 1 h under
the standard conditions, with an even faster rate than that of 3a
and 4a (Scheme 3, equation 3). The time course for transfer
hydrogenation of IV was then monitored. As shown in Figure 3,
intermediate IV was indeed reduced to 5a in a faster rate. 3a and
4a were also detected during the reduction of IV. The formation
of 3a might be due to a reversible dehydrogenation and
hydrogenation process between intermediates IV and III, followed
by the condensation of III and IV to form 3a.[19d, 31a] Taken the
above results together, the reaction might proceed via both the
direct and the condensation pathways, with the direct pathway
being the dominant one.
In conclusion, we have developed a catalytic system for
transfer hydrogenation of aromatic nitro compounds with simple
Rh terpyridine complexes in water. This robust catalytic system
can work at a S/C ratio of 2000, at 20 gram scale, with a TOF up
-1
Figure 2. Time course of product formation for transfer hydrogenation of 2a.
to 3360 h . The water-soluble catalyst is easy to be separated
from the product by simple phase separation and reused 15 times
without significant loss of catalytic activity. Mechanistic studies
suggest that the transfer hydrogenation may proceed via both the
direct route and the condensation route, with the direct pathway
being the dominant one. Being easy to operate and scale up, the
reaction provides a green and practical method for the reduction
of nitroarenes to obtain aromatic amines.
Acknowledgements
This research was supported by the National Key R&D Program
of China (2019YFC1905300, 2019YFC1905302), the 2017 Royal
Society International Collaboration Award (IC170044, UK), the
National Natural Science Foundation of China (21773145,
2
1473109, 21690083, 21878288), Projects for the Academic
Leaders and Academic Backbones, Shaanxi Normal University
16QNGG008).
Scheme 3. The reduction of intermediates.
(
Conflict of Interest
Some of this research has been included in a Chinese patent
(
patent application number: CN201911234123.0).
Keywords: Rhodium • terpyridine • transfer hydrogenation •
nitroarenes • anilines
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4
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A green, efficient and recyclable catalytic system for transfer hydrogenation of nitro arenes to aromatic amines has been developed.
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