Catalytic and kinetic study of the liquid-phase hydrogenation of various organic substrates over a polymer-anchored Pd(II) catalyst

Article abstract of DOI:10.1007/s11243-011-9522-y

A polymer-anchored Pd(II) complex has been prepared and characterized using scanning electron microscopy, elemental analysis, atomic absorption spectroscopy and FTIR. The catalyst shows excellent catalytic activity in the liquid-phase hydrogenation of substituted nitrobenzenes at normal pressure of hydrogen gas at 25 °C in DMF medium. We have also studied the liquid-phase hydrogenation of other organic substrates such as alkenes, alkynes, aromatic aldehydes, etc. The influences of various parameters such as amount of catalyst, concentration of substrate, temperature and solvent have been studied. The catalyst can be used five times without much loss in activity.

Full text of DOI:10.1007/s11243-011-9522-y

Transition Met Chem (2011) 36:699–706
DOI 10.1007/s11243-011-9522-y
Catalytic and kinetic study of the liquid-phase hydrogenation
of various organic substrates over a polymer-anchored
Pd(II) catalyst
•
Sk. Manirul Islam Sanchita Mondal
•
•
Paromita Mondal Anupam Singha Roy
•
•
Noor Salam Sumantra Paul Manir Mobarak
•
Received: 19 May 2011 / Accepted: 22 July 2011 / Published online: 11 August 2011
Ó Springer Science+Business Media B.V. 2011
Abstract A polymer-anchored Pd(II) complex has been
prepared and characterized using scanning electron
microscopy, elemental analysis, atomic absorption spec-
troscopy and FTIR. The catalyst shows excellent catalytic
activity in the liquid-phase hydrogenation of substituted
nitrobenzenes at normal pressure of hydrogen gas at 25 °C in
DMF medium. We have also studied the liquid-phase
hydrogenation of other organic substrates such as alkenes,
alkynes, aromatic aldehydes, etc. The influences of various
parameters such as amount of catalyst, concentration of
substrate, temperature and solvent have been studied. The
catalyst can be used five times without much loss in activity.
been used for the homogeneous catalysis of these reactions,
but the inherent disadvantages associated with homogeneous
catalysis can be minimized by using supported metal com-
plexes. Polymer-anchored heterogeneous catalysts often
offer advantages for carrying out organic transformations
[7–10]. Ease of work-up, higher yields, product selectivity
and re-usability of the catalysts make them more attractive
than their homogeneous counterparts. Among supported
noble metal catalysts, palladium is one of the most frequently
used for hydrogenation reactions at laboratory as well as
industrial scale. However, heterogeneous catalysts normally
suffer from limited mass transfer, low specificity and
selectivity and leaching of the catalytic species from the
surface of the support [11]. Cross-linked poly(styrene–
divinylbenzene) resin is one of the most widely used mac-
romolecular supports for functionalization and attachment to
various metals. Polymer supported catalysts for hydrogena-
tion of organic substrates, notably involve complexes of
Rh(I) [12] and Ru(II) [13]. These have been studied as
mimics of their homogeneous counterparts which include,
among others, Wilkinson’s hydrogenation catalyst. Later
examples of supported Pd(II) catalysts include polystyrene
functionalized with phosphine [14–16], 2,20-bipyridyl [17]
and anthranilic acid [18]. The catalytic activity of these
systems is greatly influenced by factors such as the nature
of ligand bound to the polymeric backbone, type of olefinic
substrate, percentage cross-linking and polarity of solvent,
etc.
Introduction
The catalytic hydrogenation of substituted nitrobenzenes
(NB) is commonly used to manufacture substituted anilines
(AN), which are important intermediates for polyurethanes,
dyes, pharmaceuticals, explosives and agricultural products
[1–4]. Aromatic chloroamines are important intermediates
in the synthesis of dyes, drugs, herbicides and pesticides [5].
At present, these organic amines are generally produced
through selective hydrogenation of the corresponding aro-
matic chloronitro compounds over transition metal catalysts
such as noble metals and nickel. In this process, the hyd-
rodechlorination of the aromatic chloroamines often occurs
due to the electron-donating effect of the amino groups in the
aromatic ring [6]. Various metals and their complexes have
In the present work, we have developed a simple synthetic
route for anchoring 1,10-Phenanthroline-5-amine on a
moderately crosslinked chloromethylated poly(styrene–
divinylbenzene) matrix followed by complexation with
Pd(II). This material was evaluated for its performance as a
heterogeneous catalyst in hydrogenation of substituted
nitrobenzenes and other substrates. The effect of various
Sk. M. Islam (&) ꢀ S. Mondal ꢀ P. Mondal ꢀ
A. S. Roy ꢀ N. Salam ꢀ S. Paul ꢀ M. Mobarak
Department of Chemistry, University of Kalyani, Kalyani,
Nadia, West Bengal 741235, India
e-mail: manir65@rediffmail.com
123

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Transition Met Chem (2011) 36:699–706
reaction parameters on hydrogenation as well as the kinetics
of the reaction have been studied at room temperature under
atmospheric pressure of H2 gas.
ionization detector and identified by Trace DSQ II GC–MS
equipped with a 60 m TR-50MS capillary column.
Synthesis of the catalyst
Experimental
The preparation for the polymer-anchored Pd(II) catalyst is
shown in Scheme 1.
Analytical grade reagents and freshly distilled solvents,
along with pure and dry hydrogen gas, were used throughout
the experiments. Liquid substrates were distilled and dried
with appropriate molecular sieve, and solid substrates were
recrystallized before use. Distillation and purification of the
solvents and substrate were done by standard procedures
[19]. Chloromethylated polystyrene and 1,10-phenanthro-
line-5-amine were supplied by Aldrich. Palladium acetate
and other organic reagents were purchased from Merck and
used without further purification.
Synthesis of polymer-bound 1,10-phenanthroline
(PS-phen)
Chloromethylated polystyrene (1.2 g) was swelled for 36 h
in dioxane and then added to a solution of 5-NH2–phen
(0.983 g) in dioxane (100 mL). After refluxing and stirring
for 26 h, Et3N (0.71 g) was added. With further refluxing
for an additional 32 h, the solution became a yellow–brown
color. The mixture was cooled to room temperature and
then filtered. The polymer was washed with ethanol until
the filtrate was colorless. The brown–yellow polymer was
washed with hot dioxane three times and then immersed in
dioxane overnight. The mixture was boiled for 5 min and
then filtered. After drying under vacuum, the polymer so
obtained was yellow–brown.
The FTIR spectra of the samples were recorded from
400 to 4,000 cm-1 on a Perkin-Elmer FTIR 783 spectro-
photometer using KBr pellets. Surface morphology of the
samples was measured using a scanning electron micro-
scope (SEM) (ZEISS EVO40, England) equipped with
EDX facility. Palladium content in the catalyst was deter-
mined using a Varian AA240 atomic absorption spectro-
photometer (AAS). The reaction products were quantified
(GC data) using a Varian 3400 gas chromatograph equip-
ped with a 30 m CP-SIL8CB capillary column and a flame
(PS) found (%); C (70.8), H (5.9), Cl (23.2)
(5-NH2–phen) found (%); C (70.2), H (5.3), N (24.6)
(PS-phen) found (%); C (85.1), H (5.4), N (9.4)
Scheme 1 Synthesis of the
polymer-anchored Pd(II)
catalyst
P
P
N
N
Et3N / Dioxane
+
reflux 3 days
N
N
NH2
CH2Cl
CH2NH
(PS)
(5-NH2-Phen)
(PS-phen)
= polystyrene framework
P
Pd(OAc)
2 /AcOH
reflux 24 h
P
N
N
OAc
OAc
Pd
CH2NH
(PS-Pdphen)
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Transition Met Chem (2011) 36:699–706
701
Synthesis of the palladium complex (PS-Pdphen)
The typical method for synthesis of the Pd-complex (PS-
Pdphen) is as follows: a mixture of PS-phen (3 g) and
Pd(OAc)2 (0.50 g) in acetic acid (40 mL) was refluxed for
24 h with stirring, whereupon the color of solution changed
from yellow to deep brown. The deep brown catalyst was
filtered off, washed thoroughly with methanol until the
washings were colorless and dried in room temperature
under vacuum.
(PS-Pdphen) found (%); C (49.1), H (3.7), N (9.05)
Hydrogenation procedure
In a typical procedure, a solution of the desired substrate
(5 mmol) in DMF (10 mL) containing the catalyst (5 mg,
9.02 9 10-4 mmol) was subjected to hydrogenation under
hydrogen (1.0 atm) in a magnetically stirred glass reactor.
The detailed experimental setup and hydrogenation pro-
cedure have been described earlier [20]. The reaction
mixture was sampled at regular time intervals and analyzed
with a Varian 3400 gas chromatograph equipped with a
30 m CP-SIL8CB capillary column and Flame Ionization
Detector.
Fig. 1 FE SEM image of PS-phen (a) and PS-Pdphen (b)
Results and discussion
Characterization of the polymer-anchored Pd(II)
complex
5-NH2 phen, and near 3,371 cm-1 for the polymer-bound
ligand and its complex. The stretching vibrations of the C=N
and C=C double bonds were observed at: 1,613 and
1,596 cm-1 for PS-phen, and 1,622 and 1,601 cm-1 for PS-
Pdphen. N-coordination of the phenanthroline moiety to the
palladium is suggested from the bond at 567 cm-1 [21]
assigned to the Pd–N stretching vibration. Two new peaks at
1,629 cm-1 (vC=O, free) and 1,313 cm-1 (vC–O coordi-
nated) [22] were observed in the spectrum of the complex.
From the microanalytical and spectroscopic data, a probable
structure of the polymer-anchored palladium(II) complex
may be proposed according to Scheme 1.
Due to the insolubility of the polymer-anchored Pd(II)
complex in all common organic solvents, its characteriza-
tion was limited to its physicochemical properties, chemi-
cal analysis, SEM, EDAX, IR and UV–Vis spectra. The
metal contents of the catalyst determined by atomic
absorption spectroscopy suggested 1.92% (w/w) Pd in the
immobilized metal complex.
The morphology of the PS-Pdphen sample was studied
using SEM. The micrographs of the PS-phen ligand and PS-
Pdphen catalyst are presented in Fig. 1a and b, respectively.
The morphological change is quite evident from these
images and suggests the loading of palladium on the surface
of the polymer matrix. Energy dispersive spectroscopy
analysis of X-rays (EDAX) data for the free ligand and the
complex polymers are given in Fig. 2a and b, respectively.
The EDX data are also consistent with the attachment of
palladium on the surface of the polymer matrix.
Catalytic reduction experiments
The performance of the supported palladium catalyst was
evaluated in the hydrogenation of various substrates at
constant temperature (25 °C) and pressure of hydrogen
(1 atm). PS-PdPhen catalyzed the hydrogenation of various
alkenes, alkynes, aldehydes and substituted nitrobenzenes
with high yield. For a proper understanding of the efficacy
of the present polymer supported catalyst compared to its
homogeneous counterpart, direct comparison of catalytic
The mode of attachment of the palladium onto the support
was confirmed by comparison of the FTIR spectra of the
metal catalyst in various steps of its synthesis. The IR spectra
showed a stretching vibration for N–H at 3,463 cm-1 for
123

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Transition Met Chem (2011) 36:699–706
Fig. 2 EDX of PS-phen
(a) and PS-Pdphen (b)
activities under optimized reaction conditions was essen-
tial. In the absence of relevant data on hydrogenation of
nitrobenzene using homogeneous Pdphen catalysts, an
attempt was made to generate in situ a simple 1,10-phe-
nanthroline complex of Pd(II) by mixing Phen with
Pd(OAc)2 in 1:1 molar ratio in acetic acid. After a reaction
period of 6 h at 25 °C, an aliquot of the solution containing
an equivalent amount of Pd (5 mg) as in PS-PdPhen was
taken up for subsequent hydrogenation of p-hydroxynitro-
benzene. The overall yield for this species was only
slightly lower than that obtained with the supported cata-
lyst (Fig. 3). The effects of temperature, substrate con-
centration, catalyst amount, reaction time, solvent and
electronic effects on the catalytic hydrogenation of various
substituted nitrobenzenes and other substrates with the PS-
PdPhen catalyst were investigated (Tables 1, 2). Addi-
tionally, comparative evaluation was made with a com-
mercially available hydrogenation catalyst, namely Pd on
activated carbon. These results are shown in Table 3. The
polymer-anchored palladium complex appears to be supe-
rior to Pd/C in the rate of reduction in substrates and yields
of products (Table 3). We also compared the activity of
100
80
60
40
20
0
Ps-Pdphen
Pdphen
0
1
2
3
4
5
6
Time (h)
Fig. 3 Hydrogenation of p-hydroxynitrobenzene using PS-Pdphen
and PS-phen catalyst. Substrate = 5 mmol, catalyst = 9.02 9 10-4
mmol, P = 1 atm, T = 25 °C, DMF = 10 mL and time = 6 h
H2
this catalyst in the hydrogenation reaction with other
reported catalysts (Table 4).
The hydrogenation reactions of nitro compounds,
alkenes, alkynes, aromatic aldehydes and Schiff bases were
123

Transition Met Chem (2011) 36:699–706
703
Table 1 Hydrogenation of
Expt.
various substrates and yields of
no
Substrates
Reaction
time (h)
% of
yield
TOF
(min-1
Products
)
products at 1 atm of H2 and
25 °C using polymer-anchored
1.
Nitrobenzene
6
8
98
86.5
89
88
96
92
94
96
96
93
87
85
15.08
10.1
Aniline
palladium catalyst
2.
o-Chloronitrobenzene
o-Nitrotoluene
o-Chloroaniline
o-Toluidine
3.
8
10.28
8.13
4.
m-Chloronitrobenzene
p-nitroanisole
10
6
m-Chloroaniline
p-Aminoanisole
p-Bromoaniline
p-Toluidine
Reaction condition:
substrate = 5 mmol;
catalyst = 5 mg
(9.02 9 10-4 mmol); solvent
(DMF) = 10 mL
5.
14.78
14.16
14.47
12.67
14.32
8.58
6.
p-Bromonitrobenzene
p-Nitrotoluene
6
7.
6
8.
m-Dinitrobenzene
p-Hydroxynitrobenzene
p-Chloronitrobenzene
p-Nitroacetophenone
p-Nitrobenzoicacid
7
m-Phenylenediamine
p-Hydroxyanilene
p-Chloroaniline
Yield refers to GC & GC–MS
analysis
9.
6
10.
11.
12.
10
12
12
Turn over frequency
(TOF) = moles of substrate
converted per mole of Pd per
min
6.7
p-Aminoacetophenone
p-Aminobenzoicacid
6.55
from 25 to 50 °C at a fixed substrate concentration
(5 mmol) and catalyst amount (5 mg) at 1 atm of H2 in
DMF. The hydrogenations of the above substrates were not
influenced by temperature, giving near-constant yields. The
optimum temperature was therefore taken as 25 °C. We
also studied the effect of solvent on the hydrogenation of
nitrobenzene at a fixed substrate concentration (5 mmol),
catalyst amount (5 mg), reaction temperature (25 °C) and
1 atm of H2. The nature of the solvent proved to be an
important variable. DMF was found to be the best solvent;
the rate of hydrogenation was slow in DMSO and very
slow in co-ordinating solvents like trimethylamine. No
catalytic hydrogenation was observed in non-coordinating
solvents like CH2Cl2 and petroleum ether.
Table 2 Hydrogenation of various substrates and yields of products
at 1 atm of H2 and 25 °C using polymer-anchored palladium catalyst
Expt. Substrates
no
Reaction % of TOF
yield min-1
Products
time (h)
1.
2
Styrene
6
10
12
10
92
87
85
90
14.17
8.03
6.55
8.35
Ethylbenzene
Pentane
Pent-1-ene
Hex-1-ene
3.
4.
Hexane
Phenyl
Ethylbenzene
acetylene
5.
6.
7.
Cyclohexene
Maleic acid
6
8
83
78
90
12.78
9.00
8.31
Cyclohexane
Succinic acid
Benzylalcohol
Benzaldehyde 10
Reaction condition: substrate = 5 mmol; catalyst = 5 mg (9.02 9
10-4 mmol); solvent (DMF) = 10 mL
The yield of the product for the reduction of nitroben-
zene was found to increase with increase in catalyst amount
within the range 2.5–7.5 mg. Above 5 mg of catalyst, the
yield remained almost constant. The substrate concentra-
tion (5 mmol), temperature (25 °C) and H2 pressure of
1 atm in DMF solvent were kept constant during these
experiments. The results are shown in Fig. 4.
Yield refers to GC & GC–MS analysis
Turn over frequency (TOF) = moles of substrate converted per mole
of Pd per min
employed to investigate the influence of temperature on
the rate of hydrogenation using PS-Pdphen catalyst. The
hydrogenations were conducted by varying the temperature
Table 3 Comparison of catalytic activities of PS-Pdphen and Pd/C
Entry
Substrates
PS-Pdphen
Yield (%)
Pd/C
Products
TOF (min-1
)
Yield (%)
TOF (min-1
)
1.
2.
3.
4.
5.
6.
Nitrobenzene
m-dinitrobenzene
o-nitrotoluene
Phenylacetylene
Styrene
98
96
89
90
92
90
15.08
12.67
10.28
8.35
96
91
87
92
94
87
12.3
7.18
8.3
Aniline
m-Phelenediamine
o-Toluidine
8.38
10.48
5.63
Ethylbenzene
Ethylbenzene
Benzylalcohol
14.17
8.31
Benzaldehyde
Reaction condition: substrate = 5 mmol; catalyst = 5 mg; solvent (DMF) = 10 mL
Yield refers to GC & GC–MS analysis
Turn over frequency (TOF) = moles of substrate converted per mole of Pd per min
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Transition Met Chem (2011) 36:699–706
Table 4 Comparison of
different catalysts in the
hydrogenation reaction
Substrates
Catalyst
TOF
(min-1
Products
Aniline
Reference
)
Nitrobenzene
PS-Pdphen
FeII/EDTANa2
15.08
8.82
0.23
This study
[23]
Poly(S-DVB) supported
amino acid Pd(II) complex
[24]
o-Nitrotoluene
PS-Pdphen
FeII/EDTANa2
10.28
7.62
8.58
6.67
0.88
14.17
1.33
2.05
o-Toluidine
This study
[23]
p-Chloronitrobenzene
PS-Pdphen
FeII/EDTANa2
Chloroaniline
This study
[23]
Al-MCM-41supported Pd catalyst
PS-Pdphen
[25]
Styrene
Ethylbenzene
This study
[26]
NH2-MCM-41-[RuHCl(CO)(PPh3)2]
NH2-SBA-15-[RuHCl(CO)(PPh3)2]
[26]
100
80
60
40
20
0
100
80
60
40
20
0
nitrobenzene
phenylhydroxylamine
anilene
5 mg
2.5 mg
7.5 mg
1
2
3
4
5
6
Time (h)
0
1
2
3
4
5
6
Time (h)
Fig. 4 Effect of catalyst amount on the hydrogenation of nitroben-
zene using PS-Pdphen catalyst. Substrate = 5 mmol, PH = 1 atm,
T = 25 °C, DMF = 10 mL and time = 6 h
2
Fig. 6 Hydrogenation of nitrobenzene using PS-Pdphen catalyst.
Substrate = 5 mmol, catalyst = 9.02 9 10-4 mmol, PH = 1 atm,
2
T = 25 °C, DMF = 10 mL and time = 6 h
The yield of the product for the reduction in p-bromo-
nitrobenzene was independent of substrate concentration in
the range 5–20 mmol at a fixed catalyst amount (5 mg) and
other conditions as before (Fig. 5).
95
p-bromonitrobenzene
94
93
92
91
90
Electronic effects on the hydrogenation of substituted
nitrobenzenes
The yields of products for the hydrogenation of substituted
aromatic nitro compounds were lower than those for
hydrogenation of nitrobenzene (Table 1). The reduction of
nitrobenzene proceeds via the intermediate formation of
phenyl hydroxylamine, but coupling products such as azo-
or azoxy-compounds were not formed at any stage during
their reduction (Fig. 6). High yield was obtained for p-
nitroanisole, while relatively low yields were obtained for
p-, o-nitrotoluenes and p-bromonitrobenzene. Figure 7
shows, in the case of dinitrobenzene reduction, the presence
0
5
10
15
20
25
Substrate conc. (mmol)
Fig. 5 Effect of substrate concentration on the hydrogenation of p-
bromonitrobenzene using PS-Pdphen catalyst. Catalyst = 9.02 9
10-4 mmol, PH = 1 atm, T = 25 °C, DMF = 10 mL and time =
6 h
2
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Transition Met Chem (2011) 36:699–706
705
which are part of a delocalized system are reduced more
quickly than localized double bonds, since styrene was
reduced more quickly than hex-1-ene. Phenylacetylene was
successively reduced first to styrene and than to ethylben-
zene. In general, increase in chain length of alk-1-enes
caused decrease in reduction rate, probably due to steric
factors which would prevent approach of the substrate to
the metal centre in the polymer matrix.
m-dinitrobenzene
m-nitroanilene
m-phenylenediamine
100
80
60
40
20
0
Catalyst recycling and heterogeneity tests
One of the main advantages of a heterogeneous catalyst is
enhanced lifespan. To investigate the reusability of this
polymer-anchored complex, the catalyst was separated by
filtration after the first run, washed, dried under vacuum
and then subjected to a second run under the same condi-
tions. This was repeated with further addition of substrate,
and the nature and yield of the final products were com-
parable to the original results. As shown in Table 5, the
catalyst can be reused up to five times, without any
appreciable loss of its catalytic activity.
0
1
2
3
4
5
6
7
8
Time (h)
Fig. 7 Hydrogenation of m-dinitrobenzene using PS-Pdphen catalyst.
Substrate = 5 mmol, catalyst = 9.02 9 10-4 mmol, PH = 1 atm,
T = 25 °C, DMF = 10 mL and time = 7 h
2
of phenylenediamine was detected only after 73–77%
formation of mono-nitroaniline. As seen from Table 1,
hydrogenation of p-nitroacetophenone and p-nitrobenzoic-
acid gave the corresponding amino derivatives without
affecting the other reducible groups. Even the hydrogena-
tion of halonitrobenzenes showed high selectivity for
haloanilines without any dehalogenation of halonitrobenz-
enes. o-Chloronitrobenzene and o-nitrotoluene were reduced
at slower rates than those of non-substituted and
p-substituted compounds (Table 1).
To determine whether the catalyst was actually func-
tioning in a heterogeneous manner, a hot-filtration test was
performed in the hydrogenation of nitrobenzene. The solid
catalyst was filtered out after the reaction had proceeded
for 4 h, at which point the determined conversion was 59%,
and the liquid phase of the reaction mixture was collected
by filtration at the reaction temperature. Atomic absorption
spectrometric analysis of the liquid phase confirmed the
absence of Pd. The obtained filtrate was then stirred under
the reaction conditions. After 2 h, the conversion was still
59%. These results indicate that the catalytic reaction was
caused by the solid catalyst. Pd was also not detected in the
liquid phase of the reaction mixture after the completion
of the reaction. It was noteworthy that the DMF remains
completely colorless on addition of Pd catalyst. These
Reduction in alkenes, alkynes and aromatic aldehydes
The catalyst proved to be highly effective for the hydro-
genation of linear and cyclic alkenes, alkynes and aromatic
aldehydes under atmospheric pressure of H2. The yields of
various products are presented in Table 2. From the rates of
reduction in various substrates, it appears that double bonds
Table 5 Recycling of the catalyst for hydrogenation of various organic substrates
Expt. no Substrates
Products
1st cycle
3rd cycle
5th cycle
Yield (%) TOF (min-1
)
Yield (%) TOF (min-1
)
Yield (%) TOF (min-1
)
1.
Nitrobenzene
Aniline
98
92
15.08
14.16
6.55
96
91
82
90
90
14.78
14.01
6.31
95
90
81
89
89
14.63
13.86
6.23
2.
3a.
p-Bromonitrobenzene p-Bromoaniline
p-Nitrobenzoicacid
Styrene
p-Aminobenzoicacid 85
4.
5b.
Ethylbenzene
Benzylalcohol
92
90
14.17
8.31
13.85
8.31
13.70
8.22
Benzaldehyde
Reaction condition: substrate = 5 mmol; catalyst = 5 mg (9.02 9 10-4 mmol); solvent (DMF) = 10 mL; PH = 1 atm, time = 6 h,
2
T = 25 °C
a
Time = 12 h
b
Time = 10 h
Yield refers to GC & GC–MS analysis
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Conclusion
By immobilizing palladium on the surface of a polymer-
anchored phenanthroline ligand, an efficient and highly
recyclable heterogeneous catalyst was prepared. This cat-
alyst shows high activity in liquid-phase hydrogenation
reactions of substituted nitrobenzenes at normal pressure.
The catalyst exhibits excellent performance in the reduc-
tion in alkenes, alkynes, aromatic aldehydes and acids. It
can be easily recovered by filtration and good yields are
obtained even when the catalyst is reused at least five
times.
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Acknowledgments We thank the Indian Association for the Culti-
vation of Science, Kolkata for providing the instrumental support.
MI acknowledges Department of Science and Technology (DST),
Council of Scientific and Industrial Research (CSIR) and University
Grant Commission (UGC), New Delhi, India for funding.
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