Regio- and chemoselective catalytic transfer hydrogenation of aromatic nitro and carbonyl as well as reductive cleavage of azo compounds over novel mesoporous NiMCM-41 molecular sieves

Article abstract of DOI:10.1021/ol026936p

(equation presented) Regio-and chemoselective reduction of nitroarenes and carbonyl compounds and reductive cleavage of azo compounds, including bulkier molecules, was achieved by the catalytic transfer hydrogenation method (CTH) using a novel nickel-containing mesoporous silicate (NiMCM-41) molecular sieve catalyst. In addition, the catalyst was also found to behave as a truly heterogeneous catalyst as the yield was practically unaffected.

Full text of DOI:10.1021/ol026936p

ORGANIC
LETTERS
2002
Vol. 4, No. 24
4297-4300
Regio- and Chemoselective Catalytic
Transfer Hydrogenation of Aromatic
Nitro and Carbonyl as Well as Reductive
Cleavage of Azo Compounds over Novel
Mesoporous NiMCM-41 Molecular Sieves
Susanta K. Mohapatra,† Sachin U. Sonavane,‡ Radha V. Jayaram,‡ and
,†
Parasuraman Selvam*
Department of Chemistry, Indian Institute of Technology-Bombay,
Powai, Mumbai 400 076, India, and Applied Chemistry DiVision, UniVersity of
Mumbai Institute of Chemical Technology, Matunga, Mumbai 400 019, India
selVam@iitb.ac.in
Received September 19, 2002
ABSTRACT
Regio- and chemoselective reduction of nitroarenes and carbonyl compounds and reductive cleavage of azo compounds, including bulkier
molecules, was achieved by the catalytic transfer hydrogenation method (CTH) using a novel nickel-containing mesoporous silicate (NiMCM-
41) molecular sieve catalyst. In addition, the catalyst was also found to behave as a truly heterogeneous catalyst as the yield was practically
unaffected.
The reduction of nitroarenes and carbonyl compounds to the
corresponding amines and alcohols, respectively, is an
important step in the industrial synthesis of dyes, biologically
active compounds, pharmaceuticals, rubber chemicals, and
photographic and agricultural chemicals.1 A variety of
methods have been developed for this purpose.2 In compari-
son to the commonly used reduction processes, which involve
hazardous molecular hydrogen or Fe/ HCl3 or Sn/HCl,4
catalytic transfer hydrogenation (CTH) employing hydrogen
donors, e.g., propan-2-ol, is safer, highly selective, and eco-
friendly.5 Furthermore, unlike conventional hydrogenation
methods, CTH reactions do not require any elaborate
experimental setup or high-pressure reactors. A wide variety
of homogeneous metal complexes have been reported for
the CTH process and most of them involve metal-catalyzed
hydrogenations, complex hydrides, or metal ion in solution.6
However, it has been observed that controlling the reduction
rates is difficult with these active catalysts.7 On the other
hand, the use of heterogeneous catalysts offers several
advantages over homogeneous systems with respect to easy
recovery and recycling of catalysts as well as minimization
of undesired toxic wastes. However, these processes require
* Corresponding author. Also at New Industry Creation Hatchery Center,
Tohoku University, Aoba-yama 04, Sendai 980 8579, Japan. E-mail:
selvam@aki.che.tohoku.ac.jp.
† Indian Institute of Technology-Bombay.
‡ University of Mumbai Institute of Chemical Technology.
(1) Kabalka, G. W.; Verma, R. S. In ComprehensiVe Organic Synthesis;
Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford, UK, 1991; Vol. 8, pp
363.
(4) Doxsee, K. M.; Fiegel, M.; Stewart, K. D.; Canary, J. W.; Knobler
C. B.; Cram, D. J. J. Am. Chem. Soc. 1987, 109, 3089.
(5) Smith, G. V.; Notheisz, F. Heterogeneous Catalysis in Organic
Chemistry; Academic: New York, 1999; p 71.
(6) For a monograph, see: Hudlicky, M. Reduction in Organic Chemistry,
2nd ed.; American Chemical Society: Washington, DC, 1996.
(7) Pasto, D. J. J. Am. Chem. Soc. 1979, 101, 6852.
(2) Larock, R. C. ComprehensiVe Organic Transformations: A Guide
to Functional Group Preparation, 2nd ed; Wiley-VCH: New York, 1999;
p 823.
(3) Merlic, C. A.; Motamed, S.; Quinn, B. J. Org. Chem. 1995, 60, 3365.
10.1021/ol026936p CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/30/2002

moisture-sensitive reagents or catalysts3,8,9 such as using
Raney Ni, Pd/C, PtO2, etc. Among these, Raney Ni catalyst
has frequently been used with hydrazine hydrate or isopropyl
alcohol for CTH reations.8,9 Further it has been observed that
the use of Raney Ni for the reduction of ketones leads to
hydrogenolysis.8 Although CTH reactions are very facile over
these catalysts, they are, however, not selective toward
functional groups such as -CO, -CX, and -NO2, and
almost all labile functional groups undergo reduction under
reaction conditions. Furthermore, the Raney Ni catalyst is
flammable and presents considerable hazards during hand-
ling. Hence, attention has been focused on the design of
nickel-based oxide and nickel oxide supported catalysts.10
However, these catalyst systems too have several drawbacks
such as typically longer reaction times, nucleophilic attack,
byproducts, low yield, etc. Furthermore, the activity of most
of these catalysts decreases with subsequent recycling. In
this letter, we report here, for the first time, a very efficient,
highly selective, and rapid method for the reduction of
nitroarenes, carbonyl functions, and reductive cleavage of
azo compounds using newly developed nickel-incorporated
mesoporous silicate (NiMCM-41) molecular sieve catalyst
(Scheme 1). To the best of our knowledge, this study also
mesoporous NiMCM-41, which possesses such characteris-
tics,16 may be very well suited for this purpose. Hence, in
this investigation, we used NiMCM-41 as the catalyst for
the CTH process. The CTH reactions were carried out as
per standard procedures17 using propan-2-ol as the hydrogen
donor.18
Tables 1 and 2 summarize the results of CTH of several
aromatic nitro and carbonyl functions attached to the aromatic
Table 1. Reduction of Nitroarenes
yield (%)
entry
R
time (h)
1st run
6th run
1
2
3
4
5
6
7
8
9
H
2-Cl
3-Cl
4-Cl
4-F
4-Br
2-CH3
3-CH3
4-CH3
4-OCH3
2-NH2
3-NH2
4.0
5.0
4.5
4.0
4.0
4.5
4.5
4.0
4.5
4.0
4.0
3.5
93
85
89
92
88
91
80
88
80
90
85
88
92
84
89
90
88
90
80
87
81
87
84
89
Scheme 1. Reduction of Aromatic Compounds over
10
11
12
NiMCM-41 by the CTH Method
ring over the NiMCM-41 catalyst, wherein the compounds
were reduced with excellent yields. However, the activity
Table 2. Reduction of Aromatic Carbonyls
forms first of its kind on the reduction of azo groups to the
corresponding amines by catalytic hydrogenation using
molecular sieves.
Mesoporous silicate materials are novel molecular sieves,11,12
having high surface area and large pore size and volume.
The transition metal ion incorporated mesoporous catalysts
can be used to carry out certain important organic transfor-
mations13 more efficiently than the corresponding mi-
croporous analogues or supported metal oxide systems. The
NiMCM-41 catalyst was hydrothermally synthesized14 and
characterized using several analytical and spectroscopic
techniques.15 Since the CTH process requires acidic sites,
yield (%)
entry
R
R1
time (h)
1st run
6th run
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
CH3
CH3
CH3
H
2.5
3.5
3.5
4.0
4.0
5.5
3.0
3.0
3.5
94
77
87
85
86
68
92
68
84
94
77
87
85
86
65
90
65
84
2-Cl
4-Cl
4-OH
4-OCH3
4-N(CH3)2
H
3-NH2
4-Cl
(8) (a) Andrews, M. J.; Pillai, C. N. Indian J. Chem. B 1978, 16, 465.
(b) Ayyanger, N. R.; Lugade, A. G.; Nikrad, P. V.; Sharma, V. K. Synthesis
1981, 640.
(9) Gilchrist, T. L. In ComprehensiVe Organic Synthesis; Trost, B. M.,
Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 8, p 388.
(10) (a) Upadhya, T. T.; Ramaswamy, V.; Sabade, D. P.; Katdure, D.
P.; Sudalai, A. Chem. Commun. 1997, 1119. (b) Sonavane, S. U.; Jayaram,
R. V. Synth. Commun. In press.
(11) Kresge, C. T.; Leonowicz, M. E.; Roth, W. T.; Vartuli, J. C.; Beck,
J. S. Nature 1992, 359, 710.
was significantly influenced by the nature/position of the
substituents on the aromatic ring. For example, in the case
of nitroarenes, only amines were obtained in the product.
The presence of a methyl group ortho to the nitro group
decreased the yield to a larger extent than at the para position
due to steric effects. On the other hand, electron withdrawing/
(12) Selvam, P.; Bhatia, S. K.; Sonwane C. G. Ind. Eng. Chem. Res.
2001, 40, 3237.
4298
Org. Lett., Vol. 4, No. 24, 2002

donating groups such as chloride, amine, and methoxy do
not have a significant influence on the reaction. Furthermore,
several functional groups, viz., -F, -Cl, -Br, -OH, -CN,
-NH2, -CH3, and -OCH3, are tolerated. The catalyst also
shows promise for regioselective (Table 3; entries 1-4) and
catalyst surface. This may be the reason for the chemose-
lective reduction of a nitro group ahead of a carbonyl group.
Likewise, the catalyst also shows promise for regioselective
reduction of dinitro compounds.
Furthermore, this reduction was also successfully carried
out for certain heterocyclic (Table 4; entries 1-4) compounds
Table 3. Regio- and Chemoselective Reduction of Aromatic
Compounds over NiMCM-41
Table 4. Reduction of Heterocyclic and Bulky Aromatic
Compounds over NiMCM-41
as well as for bulkier molecules (Table 4; entries 4-8) with
high yields being obtained. Table 5 presents the results of
an elegant and rapid (1-2 h) reductive cleavage of azo
(14) The hydrothermal synthesis of NiMCM-41 is carried out by the
final molar gel composition: 1 SiO2:0.27 CTAB:0.26 NaOH:0.26 TMAOH:
60 H2O:0.02 NiO at 373 K for 24 h. Nickel nitrate hexahydrate was used
as the metal source. Removal of the template molecules was carried out by
calcination of the as-synthesized sample at 823 K for 2 h in a flow of N2,
followed by 6 h in air. For further details, see: Sakthivel, A.; Badamali, S.
K.; Selvam, P. Micropor. Mesopor. Mater. 2000, 39, 457.
(15) The powder X-ray diffraction (XRD) patterns of both as-synthesized
(ao ) 44.5 Å) and calcined (ao ) 40.8 Å) NiMCM-41 samples are typical
of hexagonal mesoporous MCM-41 structure.11,12 Further, N2 sorption
measurements (BET surface area ) 920 m2 g-1, pore volume ) 0.52 cm3
g-1, and pore size ) 30 Å) support the mesoporous nature of the sample.
Inductively coupled plasma-atomic emission (ICP-AES) analysis shows 4.3
wt % Ni loading in the catalyst.
chemoselective (Table 3; entries 5-9) reductions with high
yields. Since the nitro groups attached to aromatic rings can
withdraw electrons more strongly from benzene compared
with carbonyl groups, they can easily be adsorbed on the
(13) (a) Mahalingam, R. J.; Badamali, S. K.; Selvam, P. Chem. Lett.
1999, 1141. (b) Badamali, S. K.; Sakthivel, A.; Selvam, P. Catal. Lett. 2000,
65, 153. (c) Sakthivel, A.; Dapurkar, S. E.; Selvam, P. Catal. Lett. 2001,
77, 155. (d) Sakthivel, A.; Selvam, P. J. Catal. 2002, 211, 134. (e)
Mohapatra, S. K.; Sahoo, B.; Keune, W.; Selvam, P. Chem. Commun. 2002,
1466.
(16) Thermogravimetric (TG) analysis of calcined NiMCM-41 shows a
20% weight loss indicating its acidic nature. This is well supported by the
temperature-programmed desorption of ammonia studies. Differential
thermal analysis (DTA) shows corresponding endothermic transition.
Org. Lett., Vol. 4, No. 24, 2002
4299

that it can very well be reused without affecting either the
activity (see Tables 1-5) or the catalyst characteristics.20
On the other hand, the use of a NiO/ZrO2 supported system21
exhibited very good activity for the reduction process of
nitrobenzene; it, however showed a continuous loss in
activity upon recycling and the yield was decreased drasti-
cally. Although the method described here is safer, rapid,
and highly selective compared to the reduction processes with
molecular hydrogen, the use of KOH, however, makes the
workup procedure a bit difficult.
Table 5. Reductive Cleavage of Azo Compounds
yield (%)
entry
R
R1
time (h)
1st run
6th run
1
2
3
4
5
6
7
H
4-Cl
3-Br
3-CH3
4-CH3
4-OCH3
H
H
2.0
2.0
3.0
2.0
2.5
3.5
3.0
91
88
85
90
91
90
41a
39b
52a
35b
89
88
85
91
90
86
38a
32b
50a
33b
41-Cl
Acknowledgment. The authors thank RSIC, IIT-Bombay
for TG, DTA, and ICP-AES data.
31-Br
31-CH3
41-CH3
41-OCH3
4-NH2
Supporting Information Available: Powder X-ray dif-
fraction (XRD) patterns of NiMCM-41 catalyst before and
after CTH reaction. This material is available free of charge
via the Internet at http://pubs.acs.org.
8
H
3-CH3
4.0
OL026936P
a Aniline. b Substituted Anilines.
(18) The effect of various hydrogen donors such as primary and
secondary alcohols on the CTH of nitrobenzene was performed over
NiMCM-41. The former gave lower yields (ethanol/15%; propan-1-ol/66%;
butan-1-ol/59) while the latter (propan-2-ol/93%; butan-2-ol/81%) gave
higher yields of aniline. In addition, the dehydrogenation product is ketone,
which can easily be removed from the reaction system. In the case of tertiary
alcohols, e.g., 2-methylpropan-1-ol, the reaction did not proceed as there
is no R-hydrogen and hence they cannot act as hydrogen donors. Therefore,
in this study, we used propan-2-ol as the hydrogen donor.
functions over NiMCM-41 catalyst. All the compounds
reduced were obtained in good yields. Many functional
groups such as -Cl, -Br, -CH3, and -OCH3 are tolerated.
Furthermore, the catalyst is more effective than several other
catalytic systems such as hydrazine/Raney Ni,9 cyclohexane/
Pd on asbestos,19 and nickel-based oxide catalysts.10 Fur-
thermore, all these systems require longer reaction times,
typically 22-48 h under refluxing conditions. The NiMCM-
41 catalyst was also tested for reusability and it was found
(19) Ho, T. L.; Olah, G. A. Synthesis 1988, 91.
(20) The powder XRD (see Supporting Information: the diffraction
patterns show all the prominent reflections characteristic of hexagonal
mesoporous MCM-41 structure, and it can also be deduced from the pattern
that the structure remains intact even after the recycling experiments, as no
significant change in the pattern is observed) as well as N2 adsorption data
of NiMCM-41 catalyst before and after recycling (6th run) experiments
indicate that the structure remains intact. However, a slight broadening of
the reflections can be noticed, which may possibly be due to the finer particle
size of the catalyst generated upon cycling. The unit cell parameter (ao )
41.1 Å) of the catalyst is nearly the same even after the 6th run. Furthermore,
the mesoporous nature of this catalyst is also confirmed by N2 sorption
measurements (BET surface area ) 885 m2 g-1, pore volume ) 0.47 cm3
g-1, and pore size ) 30 Å).
(17) In a typical CTH reaction, KOH pellets (20 mmol) were dissolved
in propan-2-ol (20 mL) to which substrate (20 mmol) was added along
with 100 mg of catalyst. It was then refluxed at 356 K for a few hours
depending upon the nature of the substrate. The products were analyzed
using a gas chromatograph fitted with an OV-101 column. For recycling
purposes, the catalyst was recovered, after the first reaction, by simple
filtration and washed three times with acetone followed by water, then it
was dried at 373 K. This catalyst was reused for the subsequent cycles.
(21) The reduction of nitrobenzene was carried out over NiO/ZrO2 (7.85%
Ni) under identical reaction conditions, which gives >95% yield in the 1st
run, and 83% yield for the 6th run.
4300
Org. Lett., Vol. 4, No. 24, 2002