Chemoselective transfer hydrogenation of nitroarenes, aldehydes and ketones with propan-2-ol catalysed by Ni-stabilized zirconia

Article abstract of DOI:10.1039/a701518f

Nickel-stabilized zirconia (Zr_0.8Ni_0.2O₂), a reusable, solid catalyst, efficiently catalyses the chemoselective reduction of nitroarenes, aldehydes and ketones using propan-2-ol and KOH, in the liquid phase, exhibiting the reactivity order NO₂ ? C=O > C-X ? C=C.

Full text of DOI:10.1039/a701518f

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Chemoselective transfer hydrogenation of nitroarenes, aldehydes and ketones
with propan-2-ol catalysed by Ni-stabilized zirconia
T. T. Upadhya, S. P. Katdare, D. P. Sabde, Veda Ramaswamy and A. Sudalai*
National Chemical Laboratory, Pune-411 008, India
Nickel-stabilized zirconia (Zr0.8Ni0.2O2), a reusable, solid
catalyst, efficiently catalyses the chemoselective reduction of
nitroarenes, aldehydes and ketones using propan-2-ol and
KOH, in the liquid phase, exhibiting the reactivity order NO2
>> CNO > C–X >> CNC.
80:20) with tetramethylammonium hydroxide (25% aqueous
solution) as a precipitating agent under vigorous stirring until a
pH of 8.0 was achieved. The resultant hydroxide precipitate was
washed with deionised water, acetone and dried at 110 °C for 24
h and was subsequently heated to 500 °C under a programmed
heating rate of 2 °C min21 and then calcined for 8 h. It was then
characterized by powder X-ray diffraction, FTIR, surface area
determination by the BET method, X-ray fluorescence (XRF),
temperature programmed reduction (TPR) and cyclic voltam-
metry (CV) techniques.
The XRD profiles of nickel-stabilized zirconia shows the
formation of a cubic (fluorite) phase The FTIR spectrum in the
framework region shows a broad band at ca. 480 cm21
characteristic of the cubic phase of ZrO2.9 Scanning electron
microscopy indicates the presence of uniform particles with
sizes ranging between 0.5 and 1.0 mm. The surface area of the
sample as determined by the BET method† was 96 m2 g21. The
chemical composition determined by XRF10 was 79.97 mol%
Zr and 20.03 mol% Ni. TPR11 shows a single peak at ca. 417 °C
which can be attributed to a single step reduction of Ni2+ to Ni0.
The cyclic voltammogram of nickel-stabilized zirconia shows a
reversible system (Epa + 0.32 V, Epc = 0.40 V vs. SCE with
E1/2 = 0.04 V). This electrochemical behaviour is attributed to
the Ni2+–Ni0 redox cycle indicating that Ni in the solid matrix
of zirconia undergoes a facile two-electron transfer process.12
The selective and rapid reduction of nitro compounds is an
interesting area of research, particularly when other potentially
reducible moieties are present in the molecule.13 The results of
the reduction of nitrobenzene to aniline over various catalysts
are presented in Table 1 which shows that nickel-stabilized
zirconia exhibits significantly higher activity and selectivity
than pure ZrO2, Ni(O2CMe)2 or Raney Ni. In the absence of
either catalyst or KOH, no reaction took place. In a typical
reaction procedure, a mixture of nitrobenzene (1 g, 0.0081 mol),
KOH pellets (0.045 g, 0.0008 mol) and Zr0.8Ni0.2O2 (10% m/m)
The chemoselective reduction of organic compounds is synthet-
ically important, both in the laboratory and in industry. In
comparison with catalytic reduction using molecular hydro-
gen,1 transfer reduction using hydrogen donors, viz. propan-
2-ol, has real and potential advantages because neither hydro-
gen containment nor a pressure vessel is required. Moreover,
the transfer hydrogenation method could offer enhanced
selectivity in the reduction process. A wide variety of
homogeneous and heterogeneous catalytic systems in combina-
tion with different hydrogen donors have been employed to
selectively reduce most major functional groups attached to
aliphatic and aromatic structures.2 Chemoselective reduction of
the functional groups can be achieved by classical Meerwein–
Ponndorf–Verley reductions. However, the use of aluminium
isopropoxide in stoichiometric amounts and under drastic
reaction conditions often leads to undesired side reactions.3
Recently, transfer hydrogenation of ketones using homoge-
neous complexes of Cu, Fe, Rh, Ru, Ir, Pd and Ni with propan-
2-ol has been reported.4 However, it has been observed that
controlling the reduction rates is difficult with these active
catalysts.5 On the other hand, the use of heterogeneous catalysts
offers several advantages over the homogeneous systems such
as ease of recovery, recycling and enhanced stability. Raney Ni
catalyst has also been extensively used in combination with
hydrazine hydrate or propan-2-ol as a hydrogen source for
reductions. However, Raney Ni is pyrophoric and does not
show any selectivity towards functional groups such as C–X,
CNO and NO2, reducing all of them simultaneously. Further it
has been observed that the use of Raney Ni for the reduction of
aromatic ketones leads to hydrogenolysis of alcohols quite
easily at the benzylic position.6
Table 1 Transfer hydrogenation of nitrobenzene to aniline with propan-2-ol
over different catalystsa
Considerable efforts have been devoted in use of zirconium
oxide, which is known to be a bifunctional catalyst, either as a
single oxide or in a mixed metal oxide, for application in
organic synthesis as is evident from the literature.7 Here, we
report the potential application of nickel-stabilized zirconia as
an active, heterogeneous catalyst for chemoselective reduction
of nitro and carbonyl functions under transfer hydrogenation
conditions in the liquid phase using propan-2-ol (Scheme 1).
Zirconia samples stabilized by Cr, Mn, Fe, Co or Ni were
prepared by the procedure reported from our laboratory.8 In
particular, Zr0.8Ni0.2O2 was prepared by treating an aqueous
solution of zirconyl nitrate and nickel nitrate (molar ratio
Yieldb
Unreacted
Surface area/ Aniline
nitrobenzene
Entry
Catalyst
m2 g21
(mass%) (mass%)
1
2
3
4
5
6
7
8
9
No catalyst/No KOH
ZrO2
Ni(O2CMe)2
Raney Ni
Zr0.8Cr0.2O2
Zr0.8Mn0.2O2
Zr0.8Fe0.2O2
Zr0.8Co0.2O2
Zr0.8Cu0.2O2
Zr0.8Ni0.2O2
—
56
—
0
100
14.5
10.0
21.2
28.1
39.0
38.4
50.0
70.8
96.0
85.5
90.0
78.8
71.9
61.0
61.6
50.0
29.2
4.0
—
118
105
102
98
94
96
O
O
R
R
i
10
O2N
H2N
a Reaction conditions: nitrobenzene (8 mmol), KOH pellets (8 mmol) and
catalyst (10% m/m) in propan-2-ol (15 ml), refluxing for 3 h. b Determined
by capillary gas chromatography.
Scheme 1 Reagents and conditions: i, Zr0.8Ni0.2O2 (cat.), PriOH, KOH,
reflux, 3 h
Chem. Commun., 1997
1119

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Table 2 Transfer hydrogenation of nitroarenes and carbonyl compounds with propan-2-ol catalysed by Ni-stabilized zirconia (Zr0.8Ni0.2O2)a
Yieldc
(mass%)
Entry
Substrate
t/h
Productb
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Nitrobenzene
3
3
3
2
6
5
6
5
4
4
5
5
5
4
4
6
3
3
3
Aniline
4-Chloroaniline
4-Aminoacetophenone
o-Anisidine
Benzyl alcohol
4-Methoxybenzyl alcohol
4-Chlorobenzyl alcohol
4-Hydroxybenzyl alcohol
2-Furfuryl alcohol
1-Phenethyl alcohol
Benzhydrol
4-Methoxybenzhydrol
4-Bromobenzhydrol
3,4-Dichlorobenzhydrol
Octan-2-ol
96
85
88
86
87
86
90
88
4-Chloronitrobenzene
4-Nitroacetophenone
2-Nitroanisole
Benzaldehyde
4-Methoxybenzaldehyde
4-Chlorobenzaldehyde
4-Hydroxybenzaldehyde
Furan-2-carbaldehyde
Acetophenone
81
90
91d
70
90
95
68 (25e)
58
90
80
Benzophenone
4-Methoxybenzophenone
4-Bromobenzophenone
3,4-Dichlorobenzophenone
Octan-2-one
2-Methylcyclohexanone
4-Nitrobenzophenone
2-Nitrobenzaldehyde
Cinnamaldehyde
2-Methylcyclohexanol
4-Aminobenzophenone
2-Aminobenzyl alcohol
Cinnamyl alcohol
72
a Reaction conditions: substrate (8 mmol), KOH pellets (8 mmol) and Zr0.8Ni0.2O2 catalyst (10% m/m) in propan-2-ol (15 ml); reflux for 3–6 h. b Products
were characterized by IR, 1H, 13C NMR and MS. c Isolated yield after chromatographic purification, the remainder is essentially unreacted starting material.
d Diphenylmethane is obtained with Raney Ni.6 e Yield with Raney Ni.6
S. P. K. thanks CSIR (New Delhi) for the award of Senior
Me
Me
H
Research Fellowship and T. T. U. and D. P. S. are grateful to the
Director, NCL for providing facilities. Special thanks are given
to Dr S. G. Hegde and Dr A. V. Ramaswamy for fruitful
discussions.
R1
R2
R1
R2
Me
Me
O
OH
+
O
H
O
A
H
B
Scheme 2
Footnotes
* E-mail: otech@ems.ncl.res.in
† Determination of specific surface area by BET (Brunner, Emmett, Teller),
using N2 adsorption on a Omnisorp 100CX apparatus.
in propan-2-ol (15 ml) was refluxed for 3–5 h. The progress of
the reaction was monitored by TLC, the catalyst was then
filtered off and the product purified by flash chromatography to
afford aniline (96% yield).
References
As summarized in Table 2, in refluxing propan-2-ol (82 °C),
the present catalytic system efficiently reduces both nitroarenes
and carbonyl compounds in excellent yields. Both aromatic and
aliphatic aldehydes and ketones are reduced to the correspond-
ing alcohols. It is remarkable that both C–Br and C–Cl bonds
are not reduced and are stable under the reaction conditions
(Table 2: entries 2, 7, 13 and 14). Another notable feature of this
catalytic system is that both 4-nitroacetophenone and 4-ni-
trobenzophenone undergo chemoselective reduction to 4-ami-
noacetophenone and 4-aminobenzophenone, respectively, in
high yields without reduction of the carbonyl function. Further,
cinnamaldehyde is reduced chemoselectively to cinnamyl
alcohol without reducing CNC. The present catalytic system did
not reduce functional groups such as CO2H, CN, CO2R, etc.,
under the present experimental conditions. The catalyst was
easily recovered by filtration, and could be reused three times
for the reduction of nitrobenzene without affecting the activity
or selectivity of the process. The XRD pattern of the used
catalyst shows no structural deterioration.
Mechanically, stabilization of zirconia by nickel does not
alter its bifunctional behaviour.7 Propan-2-ol is adsorbed on the
basic site (B) and the ketone on the adjacent acidic site (A); the
Ni2+ on the surface promotes dehydrogenation of propan-2-ol
by KOH, and is reduced to the active Ni0 species as evident
from CV measurements and finally the hydrogen is transferred
as hydride as shown in Scheme 2. It appears that the active Ni0
species, formed instantaneously during the reaction, is responsi-
ble for the catalytic reduction.
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In conclusion, incorporation of Ni into the bulk structure of
ZrO2, not only stabilizes ZrO2 in its cubic (fluorite) form but
also moderates the reactivity of Ni in the stabilized zirconia
enabling chemoselective functional group reduction.
Received in Cambridge, UK, 4th March 1997; Com.
7/01518F
1120
Chem. Commun., 1997