Selective Reduction of Nitroarenes with Silanes Catalyzed by Nickel N-Heterocyclic Carbene Complexes

Article abstract of DOI:10.1002/cctc.201700218

An efficient catalytic system for the reduction of nitroarenes to amines was developed using a well-defined nickel–NHC (N-heterocyclic carbene) complex as catalyst and phenylsilane as reducing agent. The excellent activity of the catalyst provides access to anilines containing a wide array of reactive functionalities at 20 °C, and without using any base or additive. Notably, the catalytic system allows the reduction of 5,10,15,20-tetra-(nitrophenyl)porphyrin (TNPP) and Cu^II β-nitroporphyrin to the corresponding aminoporphyrins.

Full text of DOI:10.1002/cctc.201700218

Accepted Article
Title: Selective Reduction of Nitroarenes with Silanes Catalysed by
Nickel N-Heterocyclic Complexes
Authors: Beatriz Royo, Rita Lopes, and Mariette M Pereira
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ChemCatChem
10.1002/cctc.201700218
FULL PAPER
Selective Reduction of Nitroarenes with Silanes Catalysed by
Nickel N-Heterocyclic Carbenes
[a]
[b]
[a]
Rita Lopes, Mariette M. Pereira, Beatriz Royo*
Abstract: An efficient catalytic system for the reduction of
nitroarenes to amines was developed using a well-defined nickel-
NHC complex as catalyst and phenylsilane as reducing agent. The
excellent activity of the catalyst provides access to anilines
containing a wide array of reactive functionalities at 20 °C, and
without using any base or additive. Notably, the catalytic system
allows the reduction of 5,10,15,20-tetra-(nitrophenyl)porphyrin
complex.[10] Pursuing our interest in developing catalytic systems
with iron[11] and nickel NHCs,[5] we became interested in
exploring the activity of half-sandwich Ni-NHC complexes in the
selective reduction of nitroarenes with silanes.
(
TNPP) and Cu(II) -nitroporphyrin to the corresponding
Ph
Ni
aminoporphyrins.
OBut
N
N
O
OH
HSiR3
Introduction
R
R
During the last decade, there has been much interest in the
development of new catalysts based on inexpensive, non-toxic
and abundant first-row transition metals.[1] Among them, nickel is
an attractive metal for catalysis, since it can adopt flexible
number of oxidation states, Ni(0), Ni(I), Ni(II), Ni(III), Ni(IV)
Scheme 1. Half-sandwich nickel-NHC complex catalysing the reduction of
carbonyl groups reported by our group.
[5a]
offering
unique
reactivity
pathways.[2]
In
particular,
Functionalised anilines are valuable building blocks for the
organometallic nickel complexes bearing N-heterocyclic carbene
ligands (NHCs) have become a fascinating class of compounds
for homogeneous catalysis. The rapid evolution of the Ni-NHC
chemistry has led to a large number of applications in the field of
catalysis, including C-C, C-N, C-S bond forming reactions, and
also oxidation reactions.[3,4] In contrast, reduction of functional
groups catalysed by well-defined Ni-NHC complexes is poorly
synthesis
agrochemicals.
of
dyes,
pigments,
pharmaceutical
and
[12]
One of the most general methods for the
synthesis of anilines is by catalytic hydrogenation of nitroarenes.
Well-established hydrogenation methods for industrial–scale
production are based on the use of heterogeneous catalysts,
generally based on precious metals, under high temperature and
gas.[13] However, selective hydrogenation of
high pressure of H
2
developed.
Recently,
we[5a]
and
Chetcuti´s
group[6]
the nitro group in the presence of other reducible groups under
mild conditions remains a challenging task. An alternative
method to direct hydrogenation is the use of hydrosilanes as
reducing agents, which avoids the use of hazardous pressurized
independently disclosed two closely related half-sandwich Ni(II)-
NHC complexes capable to perform the reduction of carbonyl
compounds using silane as the stoichiometric reductant
(
Scheme 1). More recently, the group of Montgomery[7]
2
H gas. At laboratory scale, the catalytic reduction of nitroarenes
developed an interesting system for the hydrosilylation of
carbonyl groups, and Sortais, Ritleng and co-workers extended
the use of half-sandwich Ni(II)-NHC complexes to the reduction
of imines.[8] Apart from hydrosilylation reactions, reduction of
imines through hydrogen transfer processes has been achieved
through hydrosilylation processes is an interesting approach
since hydrosilanes are readily available and easy to handle.
Currently, methods for hydrosilylation of nitroarenes have mainly
developed using precious metals as catalysts,[15] and few
[14]
catalytic systems using cheap metals are described in the
in
good
yields
by
using
a
combination
of
[16,17]
literature.
9]
Ni(0)/IMes.HCl/Et
2
CHONa,[ and electrocatalytic conversion of
Herein, we present the use of well-defined Ni-NHC complexes
as catalysts for the reduction of a set of nitroarenes, including
the value added nitroporphyrins, using phenylsilane as reducing
agent under mild conditions.
CO
2
to CO was performed using a NHC-isoquinoline nickel
[
[
a]
b]
R. Lopes, Dr. B. Royo
ITQB NOVA, Instituto de Tecnologia Química e Biológica António
Xavier
Av. da República, 2780-157 Oeiras, Portugal
E-mail: broyo@itqb.unl.pt
Prof. Dr. M. M. Pereira
Results and Discussion
[5a]
The catalytic activity of half-sandwich Ni-NHC complexes 1
CQC, Department of Chemistry
University of Coimbra, Rua Larga, 3004-535, Coimbra, Portugal
[5b]
and 2 (Scheme 2) in the reduction of nitroarenes was tested
using the primary silane PhSiH
-nitrobenzene as a model substrate. Initially, the reaction was
carried out in toluene at 60 °C in the presence of 2 mol% of
catalyst and using a PhSiH :substrate ratio of 5:1. Under these
3
as reducing agent and 1-chloro-
4
3
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ChemCatChem
10.1002/cctc.201700218
FULL PAPER
conditions, a remarkable effect of the wingtips substituents of
reaction took place in the absence of catalyst (Table 1, entries
the NHC ligands in complexes 1 and 2, [5] was observed (Table 1, 10 and 11). Complex 2 displayed slightly higher activity than the
entries 1 and 3).
2 2
nickel complex Ni(aNHC) Cl bearing an abnormal NHC ligand
recently reported by Mandal and co-workers, which needed
higher temperature (90 °C) to afford 78% yield of the
corresponding amine.17
Ph
Ni
Ph
Ni
Table 1. Reduction of 1-chloro-4-nitrobenzene with PhSiH
3
using 1 and 2[a]
N
I
N
I
N
NO2
NH2
N
[
Ni]
PhSiH3
Loading
1
Cl
2
Cl
Yield (%)[b]
Scheme 2. Nickel NHC complexes 1 and 2.[5]
Entry
Catalyst
Solvent
toluene
toluene
toluene
toluene
toluene
MeCN
THF
T (°C)
60
T (h)
1
2
3
4
5
6
7
8
9
10
1
2
3
62
72
95
83
69
25
34
27
85
0
Complex 2, possessing a larger N-chain, is more active than 1,
affording 95% yield of 4-chloro-aniline in just 3 h. The reaction
profile is depicted in Figure 1. As shown in this profile, complex
1
10
2
20
15
3
2
60
2
displayed higher reaction rate than 1. Progressively, the
2
10
5
20
15
15
3
reaction rate increases to reach selective conversion to 4-
chloroaniline in 3 h, while the reaction catalysed by complex 1
reached just 62% of aniline in 3 h.
2
20
2
2
60
2
2
60
3
Complex 1
Complex 2
2
2
CH
2
Cl
2
60
3
1
00
2
2
neat
60
3
8
6
4
2
0
0
0
0
0
none
none
-----
-----
toluene
toluene
60
3
11
20
15
0
[
(
a] Reaction conditions: substrate (1 mmol), catalyst, PhSiH
0.4 mL). [b] Isolated yield after column chromatography.
3
(5 mmol), solvent
0
1
2
3
4
5
6
7
8
9
t (h)
Then, we decided to investigate the catalytic activity of 2 with
various nitroarenes, and we found that the reductions proceed
with high yields and selectivities when other functional groups
such as halogen and cyano are present. Notably, halogenated
nitroarenes (Br, F, I) were chemoselectively reduced to the
corresponding anilines; no dehalogenated product was observed
in any case. When the reactions are performed at 60 °C using 2
mol% of 2, high yields are obtained in 3 h (Method A). The
reactions can be performed at 20 °C using higher catalyst
loading (10 mol%) and longer reaction times (15 h), Method B.
Results are summarised in Table 2.
Figure 1. Time-dependent profile of the reaction of 1-chloro-4-nitrobenzene
with phenylsilane using complexes 1 and 2. Reaction conditions: 1-chloro-4-
nitrobenzene (1 mmol), catalyst (2 mol%), PhSiH
0 °C.
3
(5 mmol), toluene (0.4 mL),
6
We observed that the catalytic reduction of 1-chloro-4-
nitrobenzene could be conducted at 20 °C by increasing the
catalyst loading to 10 mol%. High yield (83%) of 4-chloroaniline
was obtained in 15 h using 10 mol% of 2 (Table 1, entry 4).
When the amount of 2 is reduced to 5 mol%, a 69 % yield of 4-
chloroaniline was obtained under those conditions (Table 1,
entry 5). Solvent had a significant influence on the reaction in
terms of reactivity. Replacing toluene with acetonitrile, THF or
using 2.[a]
3
Table 2. Reduction of nitrobenzenes to amines with PhSiH
Yield (%)[b]
CH
2
Cl
2
produced a significant drop in conversion (Table 1,
Entry
1
Substrate
NO2
Product
Method
entries 6-8). The reaction can be performed under neat
conditions affording high yield (85%) of the corresponding amine
NH2
NH2
A
B
87
91
(
Table 1, entry 9). Full conversion of 1-chloro-4-nitrobenzene to
NO2
4
6
-chloroaniline was obtained with a primary silane (PhSiH
0 °C using 2 as catalyst (Table 1, entry 3). However, secondary
SiH ), alkyl silanes and alkoxysilanes, such as
SiH and (EtO) SiH showed no reactivity. As expected, no
3
) at
2
silanes (Ph
Et
2
2
3
3
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ChemCatChem
10.1002/cctc.201700218
FULL PAPER
NO2
NH2
NH2
NO2
NO2
NH2
3
A
B
79
91
5
6
67
MeS
O
MeS
O
F
F
NH2
NO2
4
88
F
F
OMe
NO2
NH2
NH2
NH2
OMe
5
A
B
A
70
80
88
NO2
NH2
7
81
Br
Br
Br
NO2
MeO
MeO
6
NO2
NH2
8
9
81
77
Br
NO2
7
NO2
NH2
I
I
NO2
NH2
8
B
90
[a] Reaction conditions: Method B: substrate (1 mmol), 2 (10 mol%), PhSiH
5 mmol), toluene (0.4 mL), 20 °C, 15 h [b] Isolated yield.
3
(
I
I
NO2
NH2
NH2
9
A
B
74
82
Finally, another relevant example to demonstrate the scope of
this new catalytic system, is the highly efficient reduction of
NC
NC
NO2
10
5
,10,15,20-tetra-(nitrophenyl)porphyrin (TNPP, I) and Cu(II) -
nitroporphyrin (III), Scheme 3. In a typical reaction, to a toluene
solution of nitro-porphyrins or III, the catalyst and
NC
NC
[
(
a] Reaction conditions: Method A: substrate (1 mmol), 2 (2 mol%), PhSiH
5 mmol), toluene (0.4 mL), 60 °C, 3 h; Method B: substrate (1 mmol), 2 (10
mol%), PhSiH (5 mmol), toluene (0.4 mL), 20 °C, 15 h. [b] Isolated yield.
3
I
2
phenylsilane were added, and the reaction was maintained
along 15 hours at 20 °C. After easy work-up and
chromatographic purification, the desired porphyrinic amino
compounds II and IV have been obtained with isolated yields of
3
Moreover, the present catalytic system also showed remarkable
chemoselectivity in the reduction of a variety of substrates
bearing other easily reducible functional groups such as ketone
86 and 94%, respectively. This is a quite remarkable result that
opens the way for the large scale preparation on amino-
porphyrins, very important for the preparation of new
functionalized materials,[18] without the use of highly toxic
Sn/HCl approaches.[18,19,20]
(
Table 3, entry 1), amide (Table 3, entry 2), hydroxyl (Table 3,
entry 3), methyl (Table 3, entry 4), methylthio (Table 3, entry 5),
ester (Table 3, entry 6), and methoxy (Table 3, entry 7). In all
cases high yields (up to 81%) have been obtained. Exceptions
are the reduction of nitrobenzenes substituted by amide and
methylthio groups, where moderate yields were obtained
NO2
NH2
(
entries 2 and 5, Table 3). Furthermore, the reduction of
nitrobenzene substituted with an aldehyde or alkene gave, in
both cases, a mixture of compounds attributed to the reducing of
both functionalities. In addition, nitrocyclohexane and 1-
nitrohexane were reduced to corresponding amines in good
yield (entries 8 and 9, Table 3).
NH
N
N
NH
N
N
(i)
O2N
NO2
2
H N
NH2
HN
HN
NO2
NH2
II
using 2[a]
3
Table 3. Reduction of nitrobenzenes to amines with PhSiH
I
Entry
1
Nitroarene
Amine
Yield (%)[b]
96
NO2
NH2
NO2
NO2
NH2
NH2
N
N
N
N
(ii)
N
N
N
O
O
Cu
Cu
N
2
59
O
O
NH2
NH2
III
IV
NO2
NO2
NH2
NH2
3
4
97
94
Scheme 3. Reduction of 5,10,15,20-tetra-(nitrophenyl)porphyrin, TNPP (I) and
Cu(II) -nitroporphyrin (III). Reaction conditions: (i) catalyst 2 (10 mol%),
HO
Me
HO
Me
PhSiH
3
(20 mmol), toluene (0.4 mL), 20 °C, 15 h; (ii) catalyst 2 (10 mol%),
PhSiH
3
(5 mmol), toluene (0.4 mL), 20 °C, 15 h.
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In order to get an insight into the reaction mechanism, we
performed the stoichiometric reaction of complex 2 with one
Method A: In a closed vessel, under nitrogen atmosphere, a mixture of
catalyst (2 mol%), nitroarene (1 mmol), phenylsilane (5 mmol), and
toluene (0.4 mL) was heated to 60 ºC for 3 h. All volatiles were removed
under vacuo and the remaining residue was purified by column
chromatograpy (with a mixture ethyl acetate:hexane and MeOH). The
equivalent of 1-chloro-4-nitrobenzene in C
mixture was heated at 60 °C and monitored by H NMR.
Complex 2 did not react with the substrate after being heated for
6
D
6
. The reaction
1
NMR spectra produced was compared with the reported data for
identification of products.[21]
6
h. We also explored the stoichiometric reaction of 2 with
PhSiH . The reaction was performed in a J Young valve NMR
3
tube. Phenylsilane was added to a solution of 2 in deuterated
benzene, and the reaction was heated to 60 °C and monitored
by H NMR and Si NMR during 24 h. Complex 2 did not react
with phenylsilane. This result contrast with those reported by
Mandal, which reported that a Ni complex bearing an abnormal
Method B: In a closed vessel, under nitrogen atmosphere, a mixture of
catalyst (10 mol%), nitroarene (1 mmol), phenylsilane (5 mmol), and
toluene (0.4 mL) was stirred for 15 h at 20 ºC. All volatiles were removed
under vacuo and the remaining residue was purified by column
chromatograpy (with a mixture ethyl acetate:hexane and MeOH). The
NMR spectra produced was compared with the reported data for
identification of products.[21]
1
29
3
NHC reacted with an stoichiometric amount of PhSiH forming a
Ni-silyl complex.17 Similar result was observed by Shimada when
reacted a (salicylaldiminato)Ni(II) complex with an stoichiometric
Catalytic reduction of 5,10,15,20-tetra-(amino-phenyl)porphyrin (I)
1
6i
amount of Ph
intermediary silyl complex based on 29Si NMR experiments. In
012, we reported the formation of the silyl Ni complex (Cp*-
2
SiH
2
.
They proposed the formation of an
Under nitrogen atmosphere, a 5 mL screw-capped vial was charged with
compound I (1.0 mmol), PhSiH (20.0 mmol), catalyst 2 (10 mol%), and
3
toluene (0.4 mL). After being stirred for 15 h at room temperature, all
volatiles were evaporated, and the remaining residue was purified by
2
NHCMe)Ni-SiH
alkoxide (Cp*-NHC )Ni-OBu with PhSiH
Ph formed by stoichiometric reaction of the Ni
2
Me
t
5a
3
.
The silyl complex
2
9
displayed a resonance at δ -18.7 ppm in its Si NMR spectrum,
and the silyl hydrogen resonances at δ 4.67 and δ 4.44 ppm in
column chromatography (with a mixture of MeOH/ACN/Acetone/Et
yield 5,10,15,20-tetra-(amino-phenyl)porphyrin.[19]
3
N) to
1
its H NMR spectrum. Therefore, if complex 2 were reacted with
forming the expected silyl complex (Cp*-NHCnBu)Ni-
Catalytic reduction of Cu(II) -nitroporphyrin (III)
PhSiH
3
SiH
2
Ph, a resonance close to δ -18 ppm would be observed in
the 9Si NMR spectrum. Nevertheless, we only observed a
2
Under nitrogen atmosphere, a 5 mL screw-capped vial was charged with
compound II (1.0 mmol), PhSiH (5.0 mmol), catalyst 2 (10 mol%),, and
toluene (0.4 mL). After being stirred for 15 h at room temperature, all
volatiles were evaporated, and the remaining residue was purified by
column chromatography (with a mixture of MeOH/ACN/Acetone/Et N) to
3
yield 5,10,15,20-tetra-(amino-phenyl)porphyrin.[20]
3
3
resonance at δ -59.5 ppm, which corresponds to free PhSiH .
We believe that complex 2 reacts with phenylsilane under
catalytic conditions in which a large excess of silane is present.
We have attempted to explore the nature of the Ni species that
remained after catalysis. We isolated a brown residue, which
corresponds to a complex mixture as showed by its 1H NMR.
The 29Si NMR of the residue did not displayed any resonance
characteristic of a Ni-silyl species. The kinetic profile of complex
Acknowledgements
2
indicates the rapid formation of the active species, since 29%
We gratefully acknowledge financial support from FCT of
Portugal (UID/Multi/04551/2013), grant fellowship
yield of the corresponding product is formed after 15 min of
reaction (Figure 1).
SFRH/BD/52372/2013 (RL) and IF consolidation contract
IF/00346/2013 (BR). We thank the Analytical Services at ITQB
for elemental analyses, and the UniMS-Mass Spectrometry Unit,
ITQB/IBET. The NMR spectrometers are part of the national
NMR facility supported by FCT (RECI/BBB-BQB/0230/2012).
Conclusions
A new catalytic system capable to perform the selective
reduction of functionalised nitroarenes at 20 °C was developed.
The half-sandwich Ni-NHC complex 2 catalysed the reduction of
nitroarenes to anilines with phenylsilane without adding any
base or additives. Under the optimised conditions, the reduction
of nitroarenes was chemoselective, displaying tolerance to a
wide variety of fuctional groups including halogens, cyano,
hydroxyl, ketone, amide, methoxy, and ester groups. Notable,
nitroporphyrins were selectively reduced to the corresponding
aminoporphyrins. The described catalytic system represents a
synthetically useful method for the reduction of nitroarenes.
Keywords: nickel• N-heterocyclic carbenes • hydrosilylation •
reduction nitroarenes • porphyrins
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ChemCatChem
10.1002/cctc.201700218
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Well-defined Ni(II)-NHC complexes
were used as catalysts for the
Rita Lopes, Mariette M. Pereira, Beatriz
Royo*
reduction of nitroarenes to amines
with silanes at 20 °C. Excellent yields
and chemoselectivities were obtained
for a wide range of nitroarenes,
including nitroporphyrines.
Page No. – Page No.
Selective reduction of nitroarenes
with silanes catalysed by nickel N-
heterocyclic carbenes
NO2
2
, PhSiH3
NH2
0
20 C, 15 h
R
R
catalyst 2
Ph
Ni
I
N
N
This article is protected by copyright. All rights reserved.