Chemo- and regioselective reduction of nitroarenes, carbonyls and azo dyes over nickel-incorporated hexagonal mesoporous aluminophosphate molecular sieves

Article abstract of DOI:10.1016/j.tetlet.2003.12.091

Nickel-incorporated hexagonal mesoporous aluminophosphate (NiHMA) molecular sieves were found to be highly efficient heterogeneous catalysts for the chemo- and regioselective reduction of nitroarenes and carbonyl compounds as well as the reductive cleavage of azo functions, including bulkier substrates, by the hydrogen transfer method.

Full text of DOI:10.1016/j.tetlet.2003.12.091

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 2003–2007
Chemo- and regioselective reduction of nitroarenes, carbonyls and
azo dyes over nickel-incorporated hexagonal mesoporous
aluminophosphate molecular sieves
Parasuraman Selvam,a,* Susanta K. Mohapatra,a Sachin U. Sonavaneb
and Radha V. Jayaramb
aDepartment of Chemistry, Indian Institute of Technology––Bombay, Powai, Mumbai 400076, India
bApplied Chemistry Division, University of Mumbai, Institute of Chemical Technology, Matunga, Mumbai 400019, India
Received 21 September 2003; revised 6 December 2003; accepted 15 December 2003
Abstract—Nickel-incorporated hexagonal mesoporous aluminophosphate (NiHMA) molecular sieves were found to be highly
efficient heterogeneous catalysts for the chemo- and regioselective reduction of nitroarenes and carbonyl compounds as well as the
reductive cleavage of azo functions, including bulkier substrates, by the hydrogen transfer method.
Ó 2004 Elsevier Ltd. All rights reserved.
The reduction of nitroarenes and carbonyl compounds
to the corresponding amines and alcohols, respectively,
is an important step in the industrial synthesis of dyes
and of biologically active compounds.1 In comparison
to the commonly used reduction processes, which
involve hazardous molecular hydrogen,2 catalytic
transfer hydrogenation (CTH) employing hydrogen
donors, for example, propan-2-ol (Meerwein–Pondorff–
Verley (MVP) reduction), is safer, highly selective and
eco-friendly.3 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 ions in solution.4 However,
these catalysts are not reusable due to various problems
such as deactivation by metal precipitation or ligand
degradation, separation difficulties, etc. On the other
hand, the use of heterogeneous catalysts offers several
advantages over homogeneous systems with respect to
easy recovery and recycling of the catalysts as well as
minimization of undesired toxic wastes. In this regard,
Raney-Ni has been extensively used for heterogeneous
CTH reactions.5;6 Although, CTH reactions are very
facile using this catalyst, they are not selective towards
functional groups such as –X (X ¼ a halogen) and
–NO2, and almost all labile functional groups undergo
reduction under these reaction conditions. Furthermore,
the Raney-Ni catalyst is flammable and presents hazards
during handling. Hence, attention has been focused
on the design of nickel-based oxide and nickel oxide
supported catalysts.7 However, these catalyst systems
also have several drawbacks such as long reaction
times, competing nucleophilic attack, the formation of
by-products, low yields and the lack of true heteroge-
neity, etc. In this communication, we report a very
efficient, highly selective and rapid method for the
reduction of nitroarenes and carbonyl compounds and
the reductive cleavage of azo compounds using a nickel-
containing hexagonal mesoporous aluminophosphate
(NiHMA) molecular sieve catalyst.
Hexagonal mesoporous aluminophosphates (HMA)8
are novel molecular sieves, analogous to mesoporous
silicates,9 having a high surface area, and large pore size
and volume. Various transition metal ion incorporated
HMA catalysts have been reported.10 They can be used
to carry out organic transformations more efficiently
than the corresponding microporous analogues or sup-
ported metal oxide systems. Since the CTH process
requires Lewis acidic sites, mesoporous NiHMA, which
possesses such characteristics,11 may be suited for this
purpose. Hence, in this investigation, we used NiHMA
for CTH reactions. The catalyst was prepared by a very
simple and highly reproducible procedure12 and
Keywords: Catalytic transfer hydrogenation; Nitro compounds; Car-
bonyl compounds; Mesoporous molecular sieves; Nickel.
* Corresponding author. Tel.: +91-22-2576-7155; fax: +91-22-2572-
3480/2576-7152; e-mail: selvam@iitb.ac.in
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.12.091

2004
P. Selvam et al. / Tetrahedron Letters 45 (2004) 2003–2007
characterized using several easily available analytical
and spectroscopic techniques.13 The CTH reactions were
carried out as per the standard procedure.14;15
CHO
CH2OH
NiHMA
KOH,
OCH3
(CH ) CHOH
3
OCH3
OCH3
2
OCH3
Tables 1 and 2 summarize the results of CTH of several
aromatic nitro and carbonyl functions attached to an
aromatic ring using the NiHMA catalyst, wherein the
compounds were reduced with excellent yields to the
corresponding alcohols and amines, respectively. It can
be seen from Tables 1 and 2 that the activity is signifi-
cantly influenced by the nature/position of the substit-
uents on the aromatic ring. 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/
donating groups such as chloride, amine and methoxy,
did not have a significant influence on the reaction (see
also Scheme 1).
Scheme 1.
The catalyst was highly chemoselective for the reduction
of nitro and carbonyl compounds, and other functional
groups, viz., –F, –Cl, –Br, –OH, –CN, –NH2 and
–OCH3, etc. were not affected.
The catalyst also showed promise for regioselective
(Table 3, entries 2 and 3) and chemoselective (Table 3;
entries 5–8) reductions. Since nitro groups attached to
aromatic rings can withdraw electrons more strongly in
comparison to carbonyl groups, they can be adsorbed
more easily onto the catalyst surface. This may explain
the chemoselective reduction of a nitro group ahead of a
carbonyl group. All these compounds were reduced in
excellent yields affording a single product, which
reduced the difficulties of separation of the products
from unreacted substrates. The yields and the reaction
times (1.5–4 h) for these transformations over NiHMA
were encouraging from the organic synthesis point of
view in terms of the practical utility of the catalyst
compared to other reported catalysts (reaction time
30–50 h).16 NiHMA also required less time for these
conversions to achieve optimum yields, compared to
mesoporous CoHMA molecular sieves. This may be due
to the fact that the Lewis acid characteristics of nickel
are higher than those of cobalt.
Table 1. Reduction of nitroarenes over NiHMA
NiHMA
NO
NH
2
2
R
R
KOH, (CH ) CHOH
3 2
refluxed at 356K
Entry
R
Time (h)
Yield (%)
First run
Sixth run
1
H
1.5
2.0
2.5
2.0
2.0
2.5
1.5
2.0
1.5
2.0
2.0
1.5
97
86
90
93
91
93
82
90
89
93
89
92
95
84
89
93
92
91
79
90
88
91
90
90
2
2-Cl
3-Cl
4-Cl
4-F
4-Br
3
4
5
6
7
2-CH3
3-CH3
4-CH3
4-OCH3
2-NH2
3-NH2
8
Furthermore, this reduction was also successfully car-
ried out using heterocyclic compounds (Table 4, entries
1–3) as well as bulkier compounds (Table 4, entries 4–6)
with high yields being obtained. Table 5 presents the
results of the rapid (1–2 h) reductive cleavage of azo
functions over the NiHMA catalyst in good yields
(ꢀ90%). It is interesting to note that the cleavage of azo
compounds over NiHMA is rapid (1–2 h) compared to
other catalysts used for this purpose.17 Unlike palla-
dium-based catalysts, many functional groups such as
–Cl, –Br and –OCH3 were tolerated over NiHMA.18 The
Pd-based catalysts also gave poor yields due to the
formation of intermediate hydrazo compounds, which
were not observed when using NiHMA. Furthermore,
the catalyst showed true heterogeneity for all the above
substrates over six cycles. In contrast, the use of the
NiO/ZrO2 supported system18 exhibited very good
activity for the reduction of nitrobenzene, however, it
showed a loss in activity upon recycling and the yield
decreased drastically. Isopropanol was found to be the
best hydrogen donor compared to the others.19
9
10
11
12
Table 2. Reduction of aromatic carbonyls over NiHMA
O
C
OH
R
C
H
R
NiHMA
R1
R1
KOH, (CH ) CHOH
3 2
refluxed at 356K
Entry
R
R1
Time (h)
Yield (%)
First run Sixth run
1
2
3
4
5
6
7
8
9
H
H
2.0
1.5
1.5
90
81
89
90
67
82
85
72
84
92
79
89
91
65
82
84
70
82
H
2-Cl
4-Cl
H
H
4-OCH3
4-N(CH3)2
4-CN
2.0
4.0
3.5
3.5
3.0
3.0
H
The reaction mechanism for the reactions under study
may relate to the MPV reduction of ketones that were
reported using aluminium isopropoxide.20 Accordingly,
adsorption of propan-2-ol onto the catalyst is followed
by the formation of an alkoxide, which is also known to
H
CH3
CH3
CH3
H
3-NH2
4-Cl

P. Selvam et al. / Tetrahedron Letters 45 (2004) 2003–2007
Table 3. Regio- and chemoselective reduction of aromatic compounds over NiHMA
2005
Entry
Substrate
Time (h)
Product
Yield (%)
Sixth run
First run
85
NO
2
NO
2
1
2.5
84
NO
NH
2
2
NH
NO
2
2
2
3
4
3.0
3.0
4.0
88
87
89
87
NO
NO
NO
2
2
2
Cl
Cl
NO
NH
2
2
87
89
NO
2
CH3
CH3
NO
NH
2
2
NO
NO
2
H N
2
H2N
2
NH2
NO2
CHO
CHO
CHO
CHO
5
6
3.5
3.0
84
88
82
88
NH2
NO2
CHO
CHO
7
8
2.5
2.0
89
92
87
91
NO2
NH2
Cl
Cl
NH
2
NO
2
COCH3
COCH3
be the activated H-donor in MPV reductions.21 Fur-
thermore, the coordinative interaction of the substrates
with the Lewis acidic centre allows the formation of a
six-membered transition state in which hydride transfer
takes place.22–24 Thus the role of NiHMA is to hold both
the H-donor and the H-acceptor through the Lewis acid
sites in close proximity so that direct hydrogen transfer
can take place smoothly.25 The surface alkoxide species
thus formed may lead to hydride ion transfer to the
substrate promoted by the presence of KOH.20;26 The
formation of aniline from nitrobenzene goes through
three steps via the formation of nitrosobenzene and
N-phenylhydroxylamine as the intermediates.27 In a
similar way, the reductive cleavage of azobenzene also
proceeds by hydride transfer from the H-donor to the
–N@N– of the adsorbed azobenzene at the Lewis acid
site, followed by a proton transfer, which in turn pro-
ceeds through the intermediate hydrazobenzene to form
aniline.
In conclusion, we have shown that NiHMA is a versatile
heterogeneous catalyst for the reduction of nitroarenes
and carbonyl compounds as well as for the reductive
cleavage of azo functions. The CTH process can be
accomplished using this novel catalyst without affecting
the reduction of any other reducible functional group.

2006
P. Selvam et al. / Tetrahedron Letters 45 (2004) 2003–2007
Table 4. Reduction of heterocyclic and bulky aromatic compounds over NiHMA
Entry
Substrate
Time (h)
Product
Yield (%)
Sixth run
First run
76
1
2
3.0
3.5
78
80
CHO
N
CH OH
N
S
2
82
80
S
CHO
CH OH
2
3
3.5
4.0
79
72
O
O
CH OH
2
CHO
NO
2
NH
2
4a
73
R
R
R
R
N
H C
3
N
H
O
H C
3
CH
CH
3
3
H
OH
OH
H
C
C
5
2.5
2.5
87
85
87
83
O
C
H
C
6
Br
Br
a R ¼ COOCH3.
Table 5. Reductive cleavage of azo compounds over NiHMA
NH
References and notes
NH
2
2
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N
N
+
KOH, (CH ) CHOH
3 2
refluxed at 356K
1
R
R
1
R
R
Entry
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R1
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First run Sixth run
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1
2
3
4
5
6
7
H
H
1.0
1.5
1.5
2.0
1.5
94
90
94
91
90
88
89
90
43
36
53
35
4-Cl
3-Br
41-Cl
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41-CH3
88
89
3-CH3
4-CH3
4-OCH3
H
93
41-OCH3 1.5
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4-NH2
2.0
45a
37b
54c
37d
8
H
3-CH3
2.0
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The authors thank Sophisticated Analytical Instrumen-
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P. Selvam et al. / Tetrahedron Letters 45 (2004) 2003–2007
2007
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TPD) confirmed the presence of Lewis acid sites in the
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12. The hydrothermal synthesis of NiHMA was carried out
with a final (molar) gel composition of Al2O3:P2O5:0.08
NiO:cetyltrimethylammonim chloride (CTAC):2.5 tetra-
methylammonium hydroxide (TMAOH):70H2O at 373 K
for 72 h. Nickel nitrate hexahydrate (>99%, Sarabhai) was
used as the metal source. Removal of the template
molecules was carried out by calcination of the sam-
ple at 823 K for 1 h in a flow of N2, followed by 2 h in air.
13. X-ray diffraction patterns (Rigaku-miniflex, Cu-Ka) of
both synthesized and calcined NiHMA samples showed a
typical hexagonal mesoporous structure.9;10 Further, N2
18. The reduction of nitrobenzene was carried out over NiO/
ZrO2 (7.85% Ni) under identical reaction conditions,
which gave a >95% yield in the first run, and 83% yield
for the sixth run.
19. The effect of various hydrogen donors such as primary and
secondary alcohols on the CTH of nitrobenzene was
performed over NiHMA. The former gave lower yields
(methanol/11%; ethanol/23%; propan-1-ol/72%; butan-1-
ol/55%; pentan-1-ol/46%; octan-1-ol/30%) while the latter
(propan-2-ol/97%; butan-2-ol/87%) gave higher yields of
aniline. In addition, the dehydrogenation product is a
ketone, which can easily be removed from the reaction
system. In the case of tertiary alcohols, for example,
2-methylpropan-1-ol, the reaction did not proceed as there
is no a-hydrogen and hence they cannot act as hydrogen
donors. Therefore, in this study, we used propan-2-ol as
the hydrogen donor.
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adsorption measurements (BET surface area, 720 m2 gꢁ1
;
pore volume, 0.32 cm3gꢁ1; and pore size, 26 A) support the
mesoporous nature of the sample. ICP-AES analysis
shows 3.5 wt% Ni loading in the catalyst.
ꢀ
14. In a typical CTH reaction, KOH pellets (20 mmol)
were dissolved in propan-2-ol (20 mL) to which the
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 (Eshika) fitted
with an OV-101 column. For recycling purposes, the
catalyst after filtration was washed several times with
acetone followed by thorough washing with water to
remove any alkali; it was then activated at 373 K and
reused.
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