Indian Journal of Chemistry
Vol. 39A, October 2000, pp. 1041-1043
Reduction of nitroarenes with isopropanol and potassium hydroxide
over metal oxide catalysts
T M Jyothi* & B S Rao
Catalysis Division, National Chemical Laboratory, Pune 411 008, India
and
S Sugunan
Department of Applied Chemistry, Cochin University of Science and Technology, Cochin 682 022, Indi a
Received 20 October /999; revised 20 December 1999
Redu ction of aromatic nitrocompounds have been carried out in presence of isopropanol and KOH over some metal
oxide catalysts, viz., Sn0 2, La 20 r Sn0 2 and La 20 3. Reduction of p-nitrobenzophenone has resulted in the chemoselective
formation of p-aminobenzophenone. It is proposed th at reducti on of nitroarenes proceed via a mechanism similar to
Meerwin -Pondroff-Yerly (M-P- V) reduction of ketones.
A wide variety of homogeneous as well as
heterogeneous catalyst systems in combination with
different hydrogen donors have been employed for
selective functional group reductions 1'2. Reduction of
nitrobenzene
can
afford
nitrosobenzene,
phenylhydroxylamine, p-aminophenol, azobenzene,
azoxybenzene, hydrazobenzene or aniline depending
on the reaction conditions. Complete reduction of
nitro compounds to amines is very important as they
are very important intermediates for the manufacture
of polyurethanes, rubber chemicals, agricultural
products, dru gs and photographic chemicals 3.4.
Utilization of heterogeneous catalysts, like metal
oxides often provides eco-friendly alternative to
conventional homogenous catalysts. In recent years
different catalyst systems like Ni stabilized zirconia,
montmorillonite and Fe20rMgO have been reported
for transfer hydrogenation reactions 5-7 . Here, we
present the results of transfer hydrogen reduction of
some aromatic nitrocompounds using isopropanol and
KOH over Sn02, La20 3 and SnOr La20 3.
Materials and Methods
Different catalysts viz. Sn02, La 20 3 and Sn02La20 3 were prepared from the corresponding
hydroxides . Mixed hydroxides of tin and lanthanum
were prepared by mixing required quantities of
Sn(IV) chloride solution and lanthanum nitrate
solution followed by the addition of l: l aq. ammonia,
at a final pH of 8-9 . The precipitate was washed
several times with distilled water till free of chloride
ions and dried at 383 K for 12 hours, and sieved to
mesh size < I 00 micron after calcination in air at
773 K for 8 hours. Tin chloride and lanthanum nitrate
were obtained from Ranbaxy Laboratories Ltd., and
Indian Rare Earths Ltd., respectively.
The different oxide phases were detected by X-ray
diffraction (Rigaku D/MAX-VC with Ni filler Cu Ka
radiation, '). _ = 1.5404 A) and Omnisorb 1()() ex
(supplied by COULTER corporation USA) unit was
used for the measurement of nitrogen adsorption to
determine surface area.
All the reactions were carried out batch wise in a
I 00 ml round bottom flask fitted with reflux
condenser with continuous stirring. ln a typical run
123 mg of catalyst was dispersed in a solution
containing 1.23 g (10 mmoles) of nitrobenzene,
0.56 g KOH (10 mmoles) pellets and 10 ml
isopropanol. The mixture was vigorously stirred and
heated under reflux for 3-5 hours in an oil bath . After
the completion of the reaction (TLC), catalyst was
filtered off, washed with isopropanol (5 mJ) and
excess distilled water was added to get an emulsion.
Then it was extracted with dichloromethane and
excess solvent removed by rotary evaporation. The
products were analyzed in a gas chromatograph
(Shimadzu GC 15A), fitted with an SE 30 column and
FID detector. The identity of the products was further
established by IR, 1H NMR and GC-MS (Schimadzu,
QP2000A).
1042
INDIAN J CHEM, SEC. A, OCTOBER 2000
Table !-Reduction of nitrobenzene with isopropanol and
KOH over different catalyis
Catalyst
Nitrobenzene yield %
SnOrLa 20 3
1-00
Sn0 2
72
La 20 3
59
Reaction conditions Substrate 10 mmol, KOH 10 mmol , Catalyst
I 0 wt % of substrate, propan-2-ol I 0 ml, reflux 3 h.
Table 2-Reduction of different aromatic nitrocompounds with
isopropanol and KOH over SnOr La 20 3 catalyst
Substrate
Time
(h)
Product
Yield
(%)
3
Aniline
100
p-Chloronitrobenzene
3
p-Anisidine
89
p-Nitroanisole
3
p-Anisidine
82
Nitrobenzene
o-Nitrotoluene
4
o-Toludine
81
m-Dinitrobenzene
5
m-Nitroaniline
75
Phenylenediamine
12
p-Nitrobenzophenone
3
p-Aminobenzophenone
89
Reaction conditions: Substrate 10 mnol, KOH 10 mmol, Catalyst
10 wt% of substrate, propan-2-ol 10 ml, reflux, blsolated by
column chromatography.
Results and Discussion
The important physico-chemical characteristics of
the present catalyst systems have been already
8
reported elsewhere . In the reduction of nitrobenzene,
aniline was obtained as the only product under
optimized conditions. The hydrogen transfer
reduction of nitrobenzene to aniline over metal oxide
catalysts is depicted in Table I. Conversion to aniline
is maximum in the case of mixed oxide catalyst.
Hence, reduction of vari ous nitro compounds were
performed over SnOrLa20 3 catalyst under optimized
conditions (Table 2). Remarkably, p-nitrobenzophenone is chemoselectively reduced to p-aminobenzophenone. Moreover, C-CI , C-CH 3 and C-OCH 3
bonds were not affected by reduction. It is interesting
to note that m-dinitrobenzene is regioselectively
reduced to m-nitroaniline . The catalyst after filtration
was washed several times with dichloromethane
followed by thorough washing with distilled water to
remove alkali, dried at 383 K and finally calcined in
air and the regenerated catalyst showed the same
act ivity . But recycling of the catalyst after each
reaction resulted in the decrease of nitrobenzene
conversion.
An attempt to carry out the reaction taking lesser
amount of potass ium hydroxide resulted in a
Scheme I
Q
NH,
Scheme 2
diminution in nitrobenzene conversion. Moreover,
under these conditions an appreciable amount of
azoxybenzene was formed as a result of the partial
reduction of nitro group . It is worth noting that
reaction was not proceeded either in the absence of
catalyst or potassium hydroxide. Conversion of
nitrobenzene decreases linearly with decrease in
KOH concentration. An increase in the KOH
concentration enhances the aniline selectivity where
as azoxybenzene is formed selectively at lower
KOH concentration . KOH/substrate molar ratio
concentration was varied from 0 .2 to 1.0. The effect
of KOH concentration on product selectivity is shown
in Table 3. Aniline is the end hydrogenation product
of this reaction where as azoxybenzene is resulted
from the partial reduction of N0 2 group. In the
reduction of nitro group the oxygen is progressively
being replaced by hydrogen. The reaction is highly
exothermic, the heat of reaction being estimated to be
about 130 kcal/mol (545 kJ/mol) 9 . The reaction
proceeds via the intermediates as proposed by Haber
from his work on the electrochemical reduction of
nitrobenzene 10"11 . ·The mechanism of formation of
different products is shown in Scheme I. The final
product aniline can be formed either from the partialy
hydrogenated product N-phenylhydroxylamine or
from azobenzene or hydrazobenzene by reductive
cleavage of these molecules. However, it is very
interesting to note that these compounds when
separately tested, failed to undergo reductive cleavage
to aniline under identical conditions over these
catalysts. Hence formation of aniline via azobenzene
JYOTHI et al.: REDUCTION OF NITROARENES
Table 3-Effect of KOH concentration on product selevctivity
and nitrobenzene conversion
KOHl
substrate
molar ratio
0.2
Nitrobenzene
conversion
%
39
0.5
69
1.0
100
Selectivity %
Azoxybenzene
Aniline
79 .7
36.1
20.3
63 .9
100.0
Reaction conditions: Substrate I 0 mmol, catalyst I 0 wt % of
substrate, propan-2-ol 10 ml, reflux .
is excluded. Formation of azoxybenzene, which might
result from the intermediates nitrosobenzene and Nphenylhydroxylamine,
further
confirms
the
participation of this intermediates in the formation of
aniline.
Moreover,
N-phenylhydroxylamine
is
completely converted to aniline over these catalysts.
This reaction is similar to the classical MeerwinPondroff-Verly (M-P-V) reduction of ketones which
takes place in the presence of aluminium
12
isopropoxide. Posner et al. have used dehydrated
alumina in the selective reduction of aldehydes using
isopropyl alcohol. It is proposed that propan-2-ol is
adsorbed on the basic site and ketone on the adjacent
acidic site and finally hydrogen is transferred as
hydride ion to the substrate. As pointed out earlier,
reaction does not proceed in the absence of basic
conditions or in the absence of catalyst. So it can be
concluded that the catalyst promotes the transfer of
hydrogen from isopropyl alcohol by potassium
hydroxide. Nitro group attached to the benzene ring
can pull electron more strongly from benzene ring
compared to keto group and hence can be relatively
easily adsorbed on the catalyst surface. This may be
1043
the reason for chemoselective reduction of nitro
group in the case of p-nitrobenzophenone. Moreover,
it seems that the C-T character of the p-aminobenzoyl
structure formed prevents further hydrogenation of
the keto group. We believe that nitro group attached
to the acid site of the catalyst is reduced to
nitrosobenzene by hydrogen transfer from isopropyl
alcohol adsorbed on the adjacent basic site as in the
case of M-P-V reduction of ketones (Scheme 2).
Nitrosobenzene can undergo complete reduction to
form aniline. Both the basicity of the catalyst and its
redox nature must be the deciding factors affecting
transfer hydrogen reduction of aromatic substrates.
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