Chemoselective Hydrazine-mediated Transfer Hydrogenation of Nitroarenes by Co3O4 Nanoparticles Immobilized on an Al/Si-mixed Oxide Support

Article abstract of DOI:10.1002/asia.201700025

Cobalt oxide nanoparticles (size 2 to 3.5 nm) were successfully impregnated on an alumina–silica (mixed oxide) support through an experimentally viable and easily reproducible protocol. The prepared material was well characterized by XRD, HR-TEM, BET surface area, EDX and XPS analyses. Porous alumina–silica having a high surface area served as a protective heterogeneous support on which the well-dispersed Co₃O₄ nanoparticles served as an active catalytic species for the hydrazine-mediated transfer hydrogenation of nitroarenes. About 2 mol % of the active catalyst in ethanol at 60 °C was adequate for a successful conversion. Moreover, transfer hydrogenation of nitroarenes by the catalyst was found to take place chemoselectively in the presence of other labile functional groups such as halide, alkene, nitrile, carbonyl, and ester. This inexpensive catalyst was also able to catalyze the reaction on a gram scale reaction and found to be robust and recyclable up to eight runs.

Full text of DOI:10.1002/asia.201700025

DOI: 10.1002/asia.201700025
Full Paper
Transfer Hydrogenation
Chemoselective Hydrazine-mediated Transfer Hydrogenation of
Nitroarenes by Co O Nanoparticles Immobilized on an Al/Si-
3
4
mixed Oxide Support
[
a]
[a]
[b]
[a]
P. Linga Reddy, Mohit Tripathi, R. Arundhathi, and Diwan S. Rawat*
Abstract: Cobalt oxide nanoparticles (size 2 to 3.5 nm) were
successfully impregnated on an alumina–silica (mixed oxide)
support through an experimentally viable and easily repro-
ducible protocol. The prepared material was well character-
ized by XRD, HR-TEM, BET surface area, EDX and XPS analy-
ses. Porous alumina–silica having a high surface area served
as a protective heterogeneous support on which the well-
dispersed Co O nanoparticles served as an active catalytic
of nitroarenes. About 2 mol% of the active catalyst in etha-
nol at 608C was adequate for a successful conversion. More-
over, transfer hydrogenation of nitroarenes by the catalyst
was found to take place chemoselectively in the presence of
other labile functional groups such as halide, alkene, nitrile,
carbonyl, and ester. This inexpensive catalyst was also able
to catalyze the reaction on a gram scale reaction and found
to be robust and recyclable up to eight runs.
3
4
species for the hydrazine-mediated transfer hydrogenation
Introduction
troarenes mediated by various hydrogen donors such as
formic acid derivatives, alcoholic solvents (EtOH, iPrOH, glycer-
ol, etc.), siloxanes and hydrazine are such viable options. Al-
though hydrazine is an unstable toxic liquid, its hydrate form
(i.e., hydrazine-hydrate (N H ·H O)), with its inherent advantag-
Transfer hydrogenation (TH) is a practical and more convenient
alternative for reduction of various unsaturated molecules in-
cluding alkenes, alkynes, carbonyls, nitriles and nitro com-
pounds since it circumvents the use of hazardous pressurized
hydrogen gas and pyrophoric catalysts. The applicability of this
protocol is significant for the reduction of nitro compounds to
the corresponding amines as the latter are vital building blocks
for the production of agrochemicals, dyes, pigments, herbi-
cides, pharmaceuticals, polymers and various specialty chemi-
cals. The global market of amines is estimated to grow from
2
4
2
es such as stability, convenience of use, cost effectiveness and
ease of availability, has attracted the attention of chemists as
a practical hydrogen source for TH processes. However, hydra-
zine itself does not produce the desired anilines in good yields
through transfer hydrogenation and requires sealed-tube con-
[2]
ditions. However, the use of transition metal catalysts, as first
[
3]
exemplified in 1929 by Busch (using Pd/CaCO ) and subse-
3
[1]
[4]
USD 13.35 Billion in 2015 to USD 19.90 Billion by 2020 and
hence, improvements to the reductive routes to their prepara-
tion, evading occupational hazards are extremely significant.
Currently, the sole route to industrial production of anilines
involves the reduction of nitroarenes, and although hydroge-
nation processes catalyzed by expensive metal-based catalysts
quently by Pietra (using Pd/C) for the hydrazine-mediated TH
of nitroaromatics, carved a path for milder catalytic TH pro-
[5]
cesses as hydrazine spontaneously decomposes under these
conditions to generate hydrogen while producing nitrogen
(N ) gas and water as the only by-products. This assures
2
a cleaner reduction, while at the same time minimizes the oc-
(Pd, Pt, Ru, Rh, etc.)/Raney nickel are still employed, safer and
cupational risks associated with the use of H gas. Apart from
2
greener alternatives are slowly paving the way for such trans-
formations. Of these, the catalytic transfer hydrogenation of ni-
the precious transition metals, various metal oxides/hydroxides
have been found to catalyze this reduction. Notable examples
[6]
[7]
include iron(III)oxide,
iron oxide–hydroxide,
nickel–iron
[8]
[9]
[
a] P. L. Reddy, M. Tripathi, Prof. D. S. Rawat
Department of Chemistry
University of Delhi
mixed oxide and molybdenum dioxide. Nanotechnology
has also generated a lot of interest in this area, and various
supported nanocatalysts having metal/metal oxide nanoparti-
cles as the active species have been lately developed for hy-
drazine-mediated nitroarene reduction, including Rh–Fe O
Delhi-110007 (India)
Fax: (+91)11-27667501
E-mail: dsrawat@chemistry.du.ac.in
3
4
[
10]
[11]
[12]
[b] Dr. R. Arundhathi
nanocrystals,
Pd/C nanospheres,
Fe O nanocrystals,
3
[15]
4
Corporate Research&Development Centre
Bharat Petroleum Corporation Limited
Greater Noida, Uttar Pradesh-201306 (India)
[13]
[14]
0
Fe O /alumina,
Ru@CNTs,
Ni /SiO
2,
PtPd/AuPd/AuPt-
3
4
[16]
[17]
[18]
mixed metal oxide,
g-Fe O /C,
Ni/Al O -Pt/TiO
and
Even though the above
2
3
2
3
2
[19]
oxygen-implanted MoS2 catalyst.
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/asia.201700025.
methods are efficient, many of these present one or more of
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the following limitations such as use of precious metals and
expensive catalyst preparations, generation of toxic waste, low
chemoselectivity, requirement of microwave irradiation, and
leaching of active catalyst species. In lieu of these limitations,
the development of new, cost-effective and sustainable cata-
lysts for hydrazine-mediated TH of nitroarenes is highly desira-
ble.
In view of the importance of metal oxides as catalysts for TH
[
7–9,12,13,17]
processes
been found to act as active catalysts for a variety of organic
and the fact that cobalt and its oxides have
[
20]
transformations,
we thought to devise a supported Co-
based nanocatalyst for the hydrazine-mediated TH of nitroar-
enes. Moreover, some recent reports on the use of supported
3 4 2 3 2
Scheme 1. Schematic illustration for the preparation of Co O @Al O /SiO
catalyst.
CoO nanocatalysts for nitroarene reduction, albeit by hydroge-
x
[
21]
nation at elevated temperatures and pressures, or on the
[
22]
[23]
use of CO/H O, and HCOOH-Et N as the H source, further
2
3
2
spurred our interest to develop a catalytic system employing
cobalt-oxide nanoparticles (NPs) as an active catalyst for the
said purpose. However, it was envisaged that the cobalt oxide
NPs (active species) must be dispersed on a heterogeneous
support that not only stabilizes it but also protects it from
leaching under reaction conditions to generate a robust TH
NO , H O) efficiently. Finally the optimum calcination condi-
2 2
tions were found to be as follows: calcination at 3008C for the
first 1 h and at 8008C for the next 3 h, under continuous N
2
À1
flow (30 mLmin ), to yield Co O @Al O /SiO as a black
3
4
2
3
2
powder.
BET-surface area analysis of the calcined material
(Co O @Al O /SiO ) revealed that the TSA (total surface area) of
[
24]
catalyst. Based on literature precedents, and our own previ-
3
4
2
3
2
[
25]
2
À1
ous experience, alumina–silica-mixed oxide was used as the
preferred support for cobalt oxide nanospecies since the
mixed oxide supports have been found to not only prevent
the active nanoparticles from aggregation but also to protect
them from leaching and impart thermal stability. Furthermore,
the alumina–silica-mixed oxide support could also improve the
catalytic activity of the active species by furnishing desired
metal/metal oxide interfaces, deemed important for the cata-
lytic activity, or by providing surrounding acid/base and redox
the material was 172.3 m g , MiPV (micropore volume) was
À1
2
À1
0.006951 ccg , MiPA (micropore Area) was 6.94 m g , ESA
2
À1
(external surface area) was 165.4 m g , and TPV (total pore
À1
volume) was 0.169 ccg . HR-TEM images show highly dis-
persed cobalt oxide nanoparticles on Al O /SiO (Figure 1). The
2
3
2
majority of the nanoparticles are spherical in shape and evenly
distributed on the mixed oxide support with a size ranging be-
tween 2 and 3.5 nm (Figure 1E). EDX analysis revealed that the
loading of cobalt on the heterogeneous support was up to
3.16% (Figure 1F). Thus, it can be concluded that the prepared
Co O @Al O /SiO material consisted of well-dispersed active
[
26]
sites. With these points of view in mind, and in continuation
of our endeavor to prepare novel nanocatalysts for important
3
4
2
3
2
[
27]
organic conversions,
herein we report the preparation of
cobalt oxide nanoparticles (2 to 3.5 nm) on highly porous
Al O /SiO support.
highly dispersed cobalt-oxide nanoparticles (Co O4 NPs) on
3
2
3
2
a
mixed oxide (alumina–silica) support, termed as
Figure 2 shows the powder X-Ray Diffraction (PXRD) spec-
trum for Co O @Al O /SiO . The XRD spectrum contains alumi-
Co O @Al O /SiO , and its applicability as highly efficient cata-
3
4
2
3
2
3
4
2
3
2
lyst for chemoselective hydrazine-mediated TH of nitroarenes
to anilines.
na–silica peaks overlapping the cobalt-oxide signals. However,
on closer look, the peaks (2q) at 19.2, 31.3, 36.8, 44.8, 59.2, and
o
6
5.2 in Figure 2 match the JCPDS card no: 073-1701 file, indi-
[28]
cating Co O as the cobalt species.
3
4
Results and Discussion
To further affirm the elemental oxidation states of cobalt in
the prepared material, XPS analyses were performed. As shown
in Figure 3A, the binding energy peaks at 780.8 and 796.5 eV
correspond to the Co 2p3/2 and Co 2p1/2 core levels, respective-
ly, clearly showing that cobalt is present in its oxide form
(Co O ). The presence of diminished satellite peaks at 786.5
A schematic representation of the protocol followed for the
preparation of Co O @Al O /SiO catalyst is provided in
3
4
2
3
2
Scheme 1. Cobalt nitrate and alumina–silica were stirred in de-
ionized water at 608C for 12 h. The mixture was then filtered
and oven-dried (1108C, 12 h) to yield a pale pink powder of
3
4
II
Co @Al O /SiO . It was then subjected to four different calcina-
and 803.0 eV corresponding to the Co 2p3/2 level of CoO, fur-
2
3
2
À1
tion atmospheres (air/50% steam: 30 mLmin ; air:
ther confirm the presence of cobalt as a mixed Co(II,III)oxide
À1
À1
À1
[22,23a,b]
3
0 mLmin ; air: 50 mLmin ; N : 30 mLmin ) and eight dif-
(Co O ).
The XPS spectrum showed an intense peak for
2
3
4
ferent calcination temperatures (range: 450–8508C), making
the total number of samples 32. Both the post-calcination ni-
trogen content and the cobalt dispersion were measured. The
results demonstrated that in order to maximize the cobalt dis-
persion, it is necessary to use optimized calcination tempera-
tures and remove the precursor decomposition products (NO,
O 1s at ꢀ531.5 eV due to the high abundance of oxygen pres-
ent in the alumina–silica support. However, on closer observa-
tion, the O 1s core level comprises two distinct peaks at 530.3
and 532.4 eV, clearly indicating the presence of Co O oxide
3
4
[29]
ions (Figure 3B). Thus, it can be concluded that cobalt is
present as Co O , well dispersed over alumina–silica.
3
4
&
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3 4 2 3 2
Figure 1. HR-TEM images (A–C) with SAED pattern (D), particle size distribution (E) and EDX spectrum (F) of the prepared Co O @Al O /SiO catalyst.
Figure 3. XPS high-resolution narrow scan of A) Co 2p and B) O 1s core
levels of Co @Al /SiO
3
O
4
2
O
3
2
.
as using lesser equivalents of hydrazine gave lower yields
entry 7, Table 1). Also, reactions with Al O /SiO , commercial
3 4 2 3 2
Figure 2. Powder XRD spectrum of Co O @Al O /SiO .
(
2
3
2
Co O NPs and CoCl ·6H O instead of Co O @Al O /SiO cata-
3
4
2
2
3
4
2
3
2
With the well-characterized Co O @Al O /SiO material in
lyst were carried out, which either gave no reaction or gave
poor yields (entries 8–10, Table 1), validating that the adsorbed
cobalt oxide nanospecies is required as a catalyst for this trans-
formation.
3
4
2
3
2
hand, we employed it as a catalyst for the model TH reaction
of p-nitrotoluene to p-toluidine, using hydrazine hydrate as the
reductant. Initially, we used water as a solvent for this reaction,
but it gave moderate yields even after 4 h (entry 1, Table 1).
Better yields were obtained with toluene and THF (entries 5
and 6, Table 1). However, ethanol emerged as the best solvent
for this reaction (entry 3, Table 1) and was preferentially used
for subsequent reactions. The reaction without hydrazine (con-
trol experiment) led to no reaction. It was found that four
equivalents of hydrazine are required for a successful reduction
Next, we checked the generality of the optimized reaction
conditions by applying them on various nitroarene substrates
as shown in Scheme 2. Nitrophenols (o- and p-), nitroanilines
(o-, m- and p-), and p-nitroanisole were reduced to the corre-
sponding anilines (2c–d, 2 f–h and 2i) with excellent yields.
1,3-Dinitrobenzene was also reduced to 1,3-phenylenediamine
(2g) by using an excess of hydrazine hydrate. O-, m- and p-
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Furthermore, we examined the chemoselective aspects of
the present protocol, and the results are summarized in
Scheme 3. p-Bromonitrobenzene, having a more labile -Br sub-
stituent, was successfully reduced to p-bromoaniline (3a), and
no de-bromination was observed. While amide-substituted ni-
Table 1. Optimization for hydrazine-mediated catalytic transfer hydroge-
nation of p-nitrotoluene to p-toluidine.
[
a]
Entry
Catalyst
Solvent
T
t
Yield
[
b]
[
8C]
[h]
4
4
2
4
4
4
2
[%]
54
45
98
12
58
75
1
2
3
4
5
6
7
Co
3
Co
3
Co
3
Co
3
Co
3
Co
3
Co
3
O
O
O
O
O
O
O
4
@Al
4
@Al
4
@Al
4
@Al
4
@Al
4
@Al
4
@Al
2
2
2
2
2
2
2
O
O
O
O
O
O
O
3
/SiO
3
/SiO
3
/SiO
3
/SiO
3
/SiO
3
/SiO
3
/SiO
2
2
2
2
2
2
2
H
2
O
60
60
60
60
60
60
60
MeOH
EtOH
Ethylene glycol
Toluene
THF
[c]
[d]
[f]
EtOH
nr, 22
[
e]
5
2, 89
8
9
Al
Co
CoCl
2
O
3
/SiO
NPs
·6H
2
EtOH
EtOH
EtOH
60
80
80
4
2
6
nr
30
18
3
O
4
10
2
2
O
[a] Reaction conditions: p-nitrotoluene (1.0 mmol), hydrazine hydrate
(4.0 mmol), catalyst (2 mol%) and solvent (3 mL) were stirred at 608C.
[b] Isolated yield. [c]–[f] Reaction performed with 0 mmol, 2 mmol,
3
mmol hydrazine hydrate.
Scheme 3. Chemoselective transfer hydrogenation of nitroarenes to corre-
sponding anilines catalyzed by Co
arene (1.0 mmol), hydrazine hydrate (4.0 mmol), catalyst (2 mol%) and EtOH
3 mL) were stirred at 608C for an appropriate time. [a] 10 g scale. [b] Hydra-
3 4 2 3 2
O @Al O /SiO . Reaction conditions: nitro-
(
zone was obtained. Values in parantheses denote the reaction time, isolated
1
yield (%)/selectivity (%) by H NMR).
troarenes gave good yields (3b–d), there was a slight drop in
catalytic activity with a -COOH substituent present ortho to the
nitro group (3e,f), possibly due to complexation with the
active cobalt species. Nevertheless, the reactions went to com-
pletion after 5 h, and this protocol was also applied for the re-
duction of industrially important 4,5-dimethoxy-2-nitrobenzoic
acid on a gram scale (10 g, 44 mmol). No special experimental
set-up was required for the scale-up reaction and it proceeded
well to give a high yield of 3 f (87%).
Although hydrazine is known to reduce alkenes, with the
present protocol a high level of chemoselectivity was obtained
Scheme 2. Scope of Co
nitroarenes. Reaction conditions: nitroarene (1.0 mmol), hydrazine hydrate
4.0 mmol), catalyst (2 mol%) and EtOH (3 mL) were stirred at 608C for 2 h.
Values in parentheses indicate isolated yield. [a] With m-nitroaniline. [b] With
m-dinitrobenzene (required 8 mmol of hydrazine hydrate). [c] Required 5 h.
3 4 2 3 2
O @Al O /SiO -catalyzed hydrazine-mediated TH of
(
Scheme 3, 3g, h), with no appreciable harm to the olefinic
bond. Selective reduction of the nitro group in conjunction
with an aromatic nitrile (3i) and with a combination of aliphat-
ic nitriles and alkene functionalities (3j) was also successfully
performed. When p-nitrobenzaldehyde was employed, it gave
the corresponding hydrazone on reaction with hydrazine
(Scheme 3, 3k). p-Nitroacetophenone was successfully transfer
hydrogenated to the corresponding aniline (3l). Lastly, selec-
tive reduction of the nitro group in the presence of an ester
group was also performed employing ethyl 4-nitrobenzoate as
a TH substrate to afford 3m. From these experiments, the che-
moselectivity of the developed TH protocol towards nitroar-
enes was established.
(
chloroanilines (2j–l) were also obtained in good yields from
their nitro precursors. The substituent -CH OH was also well
2
tolerated, and 2e was obtained in 94% yield. Nitroarene sub-
stituted with a carbocyclic ring was also reduced, and 2n was
obtained in good yield. This protocol was also applied success-
fully to a medicinally relevant scaffold, affording 2o in good
yield.
&
&
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th
To confirm the heterogeneous nature of the Co O @Al O /
8 run) showed no appreciable change in the oxidation state
of the catalyst (see Figure S1, Supporting Information), proving
the robustness and stability of the catalyst under the em-
ployed reaction conditions.
3
4
2
3
SiO catalyst, a hot-filtration test was also performed wherein
2
the TH reaction of p-nitrotoluene was stopped midway
(40 min) and the catalyst was filtered off. Continuation of the
reaction with the filtrate for further 4 h showed no additional
formation of p-toluidine, proving that no active catalyst species
leached into the reaction mixture and confirming the hetero-
geneity of the catalyst. Figure 4 compares the kinetics of the
model reaction (blue curve) and the reaction with the hetero-
geneity test (red curve), clearly showing that no reaction takes
place after hot-filtration.
On comparison of the catalytic activity of Co O @Al O /SiO
2
3
4
2
3
with other reported nanocatalysts for the synthesis of 2a via
catalytic hydrazine-mediated TH of nitrobenzene (entries 1–7
vs. 8, Table 2), it can be observed that the present protocol is
Table 2. Comparison of Co
3 4 2 3 2
O @Al O /SiO catalyst with reported nanoca-
talysts for transfer hydrogenation of nitrobenzene to 2a.
Entry Catalyst [mol%]
H
2
Solvent T
[8C]
t [h] Yield
Ref.
source
[%]
[
a,c]
1
2
3
4
5
6
7
8
Rh-Fe
3
O
4
(1.0)
N
2
H
4
EtOH
80
1
99
[10]
[11]
[12]
[13]
[14]
[15]
[17]
·
2
H O
[
b]
a]
b]
a]
nano-Pd/C (0.05)
N
2
H
4
EtOH/ 80 2.5 100
·
H
2
O
2
H O
[
[
[
nano-Fe
3
O
4
(0.25)
(1.0)
N
2
H
4
MeOH 150 0.03 99
·
H
2
O
MW
MeOH 150 0.03 99
MW
Fe @Al
3
O
4
2
O
3
N
2
H
4
·
H
2
O
Ru@CNT (0.4)
N
2
H
4
H
2
O
RT
6
95
·
H
2
O
0
[b]
a]
Ni /SiO
2
(0.12)
N
2
H
4
EtOH
EtOH
EtOH
100 2
93.8
·
H
2
O
[
g-Fe
2
O
3
/C (2.5)
N
2
H
4
85
60
3
2
100
·
H
2
O
[
b]
Figure 4. Kinetics of the formation of p-toluidine through Co
3
O
4
@Al
2
O
3
/SiO
2
-
Co O @Al O /SiO
N H
4
98
This
3
4
2
3
2
2
catalyzed hydrazine-mediated TH of p-nitrotoluene (the blue curve repre-
sents the kinetics of the model reaction, the red curve corresponds to the
hot-filtration test, and the green curve shows the kinetics of the reaction
with 7-times reused catalyst).
(2.0)
·H O
work
2
[
a] GC yields. [b] Isolated yields. [c] Also reduces C=C.
milder, requiring a temperature of 608C to complete. More-
over, no microwave irradiation is required. Furthermore, the
Co O @Al O /SiO catalyst was found to be recyclable and
Recyclability of the catalyst was also studied. After comple-
tion of the reaction, the catalyst was separated from the reac-
tion mixture by centrifugation. The recovered catalyst was
washed thrice with ethanol to remove any organic impurities
and was then dried at 658C in vacuo. It was then subjected to
subsequent runs and was recycled up to seven times, without
showing any major changes in the reaction kinetics (green
curve, Figure 4) or any appreciable loss in its catalytic activity
3
4
2
3
2
offers several advantages including facile synthesis, simplicity
in handling, easy purification, and excellent yields in a relatively
shorter reaction time. In addition, the catalytic activity per sur-
face area for the Co O @Al O /SiO catalyst was calculated to
3
4
2
3
2
À1
À2
be 20.82 h
m
and for the commercial Co O4 NPs was
3
À1
À2
0.568 h
m
for a 30% formation of p-toluidine, which vali-
(see Figure 5). XPS analysis of the recovered material (after the
dates the high catalytic efficiency of the present protocol (see
the Supporting Information for calculations). The TON (turn-
over number) and TOF (turnover frequency) of the present cat-
À1
alyst were calculated to be 49 and 24.5 h , respectively, for
the reduction of p-nitrotoluene. As per the green chemistry
principles, the present Co O @Al O /SiO -catalyzed hydrazine-
3
4
2
3
2
mediated TH process of nitroarenes fulfills 10 of the 12 princi-
ples (i.e., waste prevention, safer synthesis, safer products by
design, safer solvent/auxillaries, energy efficiency, reduction of
derivatives, catalyst, degradability, pollution prevention and ac-
cident prevention. The use of hydrazine hydrate presents in-
herent associated advantages as a cleaner substitute for pres-
surized hydrogen gas and the fact that any excess of hydrazine
hydrate spontaneously decomposes to non-hazardous nitro-
gen, hydrogen (unused) and water in the presence of the
Co O @Al O /SiO catalyst during the course of reaction. More-
Figure 5. Recyclability of the catalyst for the reduction of p-nitrotoluene to
p-toluidine up to eight runs.
3
4
2
3
2
over, the feasibility of the present TH protocol towards large-
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II
scale operations makes it a useful and safer alternative against
conventional hydrogenation processes.
obtain a pale pink slurry of Co @Al
2
O
3
/SiO
. Subsequently, the
2
II
cobalt-impregnated support (Co @Al O /SiO ) was filtered, washed
thoroughly with deionized water and dried at 1108C for 12 h to
obtain 113 g of Co @Al O /SiO as a pale pink powder. Next, 50 g
of Co @Al O /SiO were taken in a crucible and calcined in the
2 3 2
2
3
2
II
2
3
2
II
Conclusions
presence of N at 3008C for 1 h and then at 8008C for 3 h. The cal-
2
To conclude, the present protocol is the first report on a recy-
clable cobalt oxide-based nanocatalyst employed for the che-
moselective hydrazine-mediated transfer hydrogenation of var-
ious nitroarenes to anilines. Successful impregnation of the
Co O nanoparticles (2–3.5 nm size) on the protective porous
cined catalyst was then cooled to room temperature to obtain
4
2 g of Co O @Al O /SiO as a black powder.
3 4 2 3 2
General procedure for transfer hydrogenation of nitroarenes
3
4
Al O /SiO support not only contributed to the ruggedness of
An oven-dried 10 mL round-bottom flask was charged with nitroar-
2
3
2
ene (1 mmol), hydrazine hydrate (4 mmol), Co O @Al O /SiO cata-
the catalyst but also rendered a high catalytic activity to the
system. The catalyst preparation is easy and reproducible, and
the prepared catalyst was found to chemoselectively reduce
various nitroarene substrates in the presence of other labile
functional groups such as halide, alkene, nitrile, carbonyl, and
ester under mild conditions and short reaction times. The het-
erogeneous nature and stability of the catalyst was confirmed,
and the catalyst was found to be recyclable. These advantages,
coupled with milder reaction conditions and the use of ethanol
as a solvent, make the present method sustainable and envi-
ronmentally benign. Further investigations on the applicability
of the catalyst for other related transformations are currently
underway.
3
4
2
3
2
lyst (2 mol%) and 3 mL of EtOH. It was then sealed and allowed to
stir at 608C for an appropriate time. On completion of the reaction,
as monitored by TLC, EtOAc (5 mL) was added to the reaction mix-
ture at room temperature. The mixture was then centrifuged to
remove the catalyst. The catalyst was further washed with 5–10 mL
of EtOAc to remove any organic matter, dried and used for the
next run. The collected organic phase was evaporated in vacuo to
yield a crude product which was purified by column chromatogra-
phy on silica gel (using 20–50% EtOAc/hexane as eluent) to yield
the desired aniline product. For the gram-scale reaction, a similar
procedure was used with a larger reaction vessel (250 mL for 10 g
reaction batch). The characterization data for all aniline products is
available in the Supporting Information.
Acknowledgements
Experimental Section
General
DSR thanks DST-SERB, DST-PURSE and University of Delhi for
the financial support. PLR, MT and RA thank UGC for the award
of SRF, DST for INSPIRE-SRF and BPCL for their encouragement,
respectively. We would also like to acknowledge USIC-CIF, Uni-
versity of Delhi for analytical support.
High purity chemicals and solvents (analytical grade) were pur-
chased from Sigma–Aldrich, Alfa-Aesar, and Merck India Pvt. Ltd.,
and were used as received. Alumina/silica (Al O /SiO ) was pur-
chased from Sasol, Germany. Column chromatography for purifica-
tion of organic compounds was performed on silica gel (100–200
mesh) and thin layer chromatography was performed on Merck
pre-coated silica gel 60-F254 plates. H and C spectra were record-
ed on a Jeol Spectrospin spectrometer at 400 MHz and 100 MHz,
respectively. Chemical shifts are reported in ppm relative to the re-
sidual CDCl3 and DMSO; coupling constants (J) are reported in
2
3
2
1
13
Keywords: chemoselective
· cobalt oxide · hydrazine ·
supported nanocatalysts · transfer hydrogenation
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FULL PAPER
Transfer Hydrogenation
Providing great support: Co O nano-
3
4
particles were successfully impregnated
on porous alumina–silica. The support-
ed catalyst exhibited a high catalytic ac-
tivity and chemoselectivity for the hy-
drazine-mediated transfer hydrogena-
tion of nitroarenes.
P. Linga Reddy, Mohit Tripathi,
R. Arundhathi, Diwan S. Rawat*
&& – &&
Chemoselective Hydrazine-mediated
Transfer Hydrogenation of Nitroarenes
by Co O Nanoparticles Immobilized
3
4
on an Al/Si-mixed Oxide Support
&
&
Chem. Asian J. 2017, 00, 0 – 0
www.chemasianj.org
8
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!