Tetrabutylammoniumhexatungstate (VI), [(n-C4H9) 4N]2[W6O19]: As a green and recylable isopolyanion catalyst in oxidation of benzaldehydes

Article abstract of DOI:10.1080/15533174.2012.680122

Oxidation of benzaldehydes to corresponding carboxylic acids has been studied in the presence of tetrabutylammoniumhexatungstate (VI), [(n-C ₄H₉)₄N]₂[W₆O ₁₉], as a green and recyclable catalyst. This isopolyanion was found to be suitable catalyst for the oxidation of benzaldehydes both in thermal conditions and microwave irradiation. The effects of various parameters such as the solvent, catalyst amount, reaction time, and temperature were examined. The catalyst can be easily recovered and recycled. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15533174.2012.680122

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Tetrabutylammoniumhexatungstate (VI), [(n-
C H ) N] [W O ] : As a Green and Recylable
4
9 4
2
6 19
Isopolyanion Catalyst in Oxidation of Benzaldehydes
a
b
a
Fatemeh F. Bamoharram , Majid M. Heravi , Raheleh Omidinia & Niloofar Tavakoli-
a
Hoseini
a
Department of Chemistry, Mashhad Branch , Islamic Azad University , Mashhad , I. R. Iran
b
Department of Chemistry, School of Sciences , Alzahra University , Tehran , I. R. Iran
Accepted author version posted online: 09 Oct 2012.Published online: 28 Dec 2012.
To cite this article: Fatemeh F. Bamoharram , Majid M. Heravi , Raheleh Omidinia & Niloofar Tavakoli-Hoseini (2013)
Tetrabutylammoniumhexatungstate (VI), [(n-C H ) N] [W O ] : As a Green and Recylable Isopolyanion Catalyst in Oxidation
4
9 4
2
6 19
of Benzaldehydes, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 43:2, 125-130, DOI:
0.1080/15533174.2012.680122
1
To link to this article: http://dx.doi.org/10.1080/15533174.2012.680122
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 43:125–130, 2013
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2012.680122
Tetrabutylammoniumhexatungstate (VI),
[
(n-C H ) N] [W O ] : As a Green and Recylable
4
9 4
2
6
19
Isopolyanion Catalyst in Oxidation of Benzaldehydes
1
2
1
Fatemeh F. Bamoharram, Majid M. Heravi, Raheleh Omidinia,
1
and Niloofar Tavakoli-Hoseini
Department of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad, I. R. Iran
Department of Chemistry, School of Sciences, Alzahra University, Tehran, I. R. Iran
1
2
basic oxygen surfaces, which means they are attractive units for
Oxidation of benzaldehydes to corresponding carboxylic acids use as building blocks in different reactions.[8]
has been studied in the presence of tetrabutylammoniumhex-
atungstate (VI), [(n-C N] [W 19], as a green and recyclable
However, until now only a few IPAs were known and only
a few examples have been reported for synthesis and catalytic
applications of them.
4
H
9
)
4
2
6
O
catalyst. This isopolyanion was found to be suitable catalyst for
the oxidation of benzaldehydes both in thermal conditions and
microwave irradiation. The effects of various parameters such as
[9]
Much attention has been devoted to the synthesis and applica-
the solvent, catalyst amount, reaction time, and temperature were tion of HPAs during the last couple of years[ and the application
9]
examined. The catalyst can be easily recovered and recycled.
of IPAs has been largely overlooked and very limited.
A literature survey shows only a few catalytic examples of
Keywords benzaldehyde, catalyst, isopolyanion, oxidation, tetabuty- IPAs have thus far been described.
lammoniumhexatungstate (VI)
In recent years we realized that HPAs can be used as excellent
catalysts, especially for tungsten structures.[
10–20]
Our interest in exploring the catalytic behavior of POMs
as green and eco-friendly catalysts prompted us to investigate
INTRODUCTION
One of the emphases in the field of catalysis is to replace the capability of tetrabutylammoniumhexatungstate (VI), [(n-
the existing acidic liquid catalysts by nontoxic, noncorrosive, C H ) N] [W O ] (1) as a IPA.
4
9 4
2
6
19
easy-to-handle, recyclable, highly selective, and environmen-
To the best of our knowledge catalytic activity of 1 is un-
tally friendly ones, so the search for suitable green and recy- known for oxidative transformation of benzaldehydes to car-
clable catalysts is still a significant challenge. In this context, boxylic acids.
the polyoxometalates (POMs) are definitely very attractive and
challenging when looking for the green, recyclable, and eco- ganic synthesis and since carbonyl compounds are widely used
The oxidation of aldehydes plays an important role in or-
nomic catalysts.[
1–3]
[21]
as intermediates both in manufacturing and laboratory,
the
Owing to structural and electronic diversity, POMs with dif- development of new catalytic protocols continues to receive at-
ferent metal centers and varying oxidation states undergo an ex- tention despite the availability of different methods to a achieve
ceptionally wide variety of reactions, which have applications such objectives.[
22,23]
[
4–7]
in many fields of chemistry.
In the oxidation of aldehydes, the use of a number of HPAs
The structural types in POMs can be split into three classes is very common, but these methods are associated with the use
including: heteropoly anions (HPAs), isopolyanions (IPAs), and of strong acidic/basic conditions.
Mo-blue and Mo-brown reduced POMs.
IPAs are composed of metal-oxide framework, but without because it is not acidic or basic and is clean and safe.
the internal heteroatom/hetero anion. As a result, they have
In this context, the application of 1 as a salt is appreciated
As a part of our ongoing research toward catalytic oxida-
interesting physical properties such as high charges and strongly tion of benzaldehydes to corresponding carboxylic acids, in
the presence of IPAS, we decided to extend our research with
IPAs.
Herein, for the first time, we report that the pairing of
W6O19], as an IPA, with tetrabutylammonium (TBA), is an
excellent catalyst in oxidation of benzaldehyes to carboxylic
acids in both thermal and microwave conditions.
Received 25 June 2011; accepted 26 December 2011.
[
Address correspondence to Fatemeh F. Bamoharram, Department
of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad, I.
R. Iran. E-mail: fbamoharram@mshdiau.ac.ir
125

126
F. F. BAMOHARRAM ET AL.
Interestingly, our findings showed that this salt reacts similar of the oxidation process with the same substrate. The results of
to Preyssler acid[ but catalyzes the reaction in shorter times. the first experiment and subsequent experiments were almost
This catalyst can be easily recovered and reused for several consistent in yields.
times that is very important in industry. The major aim of the
24]
work described herein is the development of applications for RESULTS AND DISCUSSION
IPAs.
Highly selective oxidation of aromatic benzaldehydes to car-
boxylic acids has been carried out for the first time by an
inexpensive, recyclable, green, and easily prepared 1 as cat-
alyst. A literature survey shows that, although oxidation of
aldehydes to carboxylic acids has been studied under aque-
EXPERIMENTAL
Chemicals and Instruments
All of the chemicals were commercially available chemicals.
IPA (1) was prepared according to the literature.
A MILESTONE APC-55E microwave (USA) was used for
all experiments. HNMR spectra were recorded on an FT-NMR
Brucker 100 MHZ Aspect 3000 (Germany). IR spectra were
recorded on a Buck 500 scientific spectrometer (KBr Pellets).
Mass spectra were obtained with a massens POEKTRO METER
CH- 7A VARIN MAT BREMEN spectrometer (USA).
ous/organic biphasic conditions with similar or lower yields in
our work,[
26,27]
the role of tetrabutylammoniumhexatungstate
[
25]
(
VI), [(n-C4H9)4N]2[W6O19], has been largely overlooked, so
1
the novelty of this work lies in selection of this IPA as
a study topic, and to the best of our knowledge no sys-
tematic study for oxidation of aldehydes to carboxylic acids
has been reported with these green and reusable IPA so
far.
The reactions were optimized according to the oxidation of
4-chloro benzaldehyde through the investigation of the catalytic
amount, temperature and reaction time.
First we checked for the optimum ratio of benzaldehyde to
H2O2 in different solvents. When we screened the reaction with
several solvents such as toluene, chloroform, acetone, ethyl ac-
etate, acetonitrile, and a mixture of acetonitrile and water, the
latest is found to be the most suitable solvent for the reaction.
This mixture was provided the highest yield of acid in the short-
est time.
Catalyst Characterization
The catalyst is characterized by IR and elemental analysis.
Anal. Calcd. for C32H72N2W6O19: C, 20.31; H, 3.84; N, 1.48; W,
5
8.30. Found: C, 20.46; H, 3.83; N, 1.52; W, 58.39. IR spectrum
–
1
(KBr pellet 1000–2000 cm , cation bands); 975(vs), 888(vw),
8
5
73 (vw), 812 (vs), 752 (vw), 736 (vw), 716 (vw), 664 (vw),
88 (m), 445 (vs), 402 (vw), and 368 (m).
General Procedure
First, all reaction mixtures were refluxed in a 10 mL two-
One of the important factors affecting the behavior of POMs
necked round-bottom flask equipped with magnetic stirrer, re- is the energy gap between the highest occupied molecular or-
flux condenser, and thermometer. In all cases, catalyst (0.03 g) bital (HOMO) and the lowest unoccupied molecular orbital
was added to mixed solvent (acetonitrile in water) and benzalde- (LUMO). The POMs are easily reducible chemical species, and
–
3
hyde (10 mol). The reaction mixture was stirred and refluxed thus the energy of the LUMO must be sufficiently low to accept
for mentioned times and temperatures. At equal intervals, 30% the incoming electron in electron-transfer reaction. The solvent
hydrogen peroxide (0.03 mol) was added to the final mixture, molecules can stabilize POMs and place these molecular orbitals
a 5% aqueous solution of NaHCO3 was added and the mix- at the appropriate level.
ture was filtered. The carboxylic acids were precipitated by
When, we checked for the optimum ratio of aldehyde to
adding HCl to the filtrate. The solid product was collected and H2O2, in CH3CN and H2O as solvent, our data showed that
washed with water and recrystallized in ethanol. The products 0.217 mol of H2O2 is the best in 6 h at reflux conditions.
were characterized by comparison of their spectroscopic (IR,
Then, we endeavored to optimize the amount of the cata-
HNMR, Mass) data, and melting points with those of authentic lyst. The observations are presented in Figure 1. Decrease of
yields with increase of catalyst amount can be attributed to
Second, under microwave irradiation, benzaldehyde (10–3 overoxidations and side reactions by the excess amount of the
mol), catalyst (0.03 g), and hydrogen peroxide (0.03 mol) were catalytic sites. Generally reactions catalyzed by HPAs may be
mixed thoroughly in a small beaker. The reaction mixture was represented by the conventional mechanisms of Bronsted acid
placed in a microwave oven and irradiated for 4–5 min at 350 W. catalysis. The mechanism may include the protonation of the
1
samples.
After cooling to room temperature and filtering of the mixture, substrate followed by the conversion of the ionic intermediate
to yield the reaction product.[
28,29]
Misono et al. advanced two
the procedure was the same as previous.
types of catalysis for heterogeneous acid catalysis by HPAs as
surface type and bulk type.[
30,31]
Unfortunately, unlike the HPAs
Reusability of the Catalyst
We investigated the reusability of the catalyst. For this pur- that have been widely used as acid and oxidation catalysts for
pose we first carried out the oxidation of benzaldehydes in the organic syntheses, the role of IPAs is largely overlooked and
presence of the catalyst. After completing the reaction, the cat- there is not sufficient information. With respect to the absence
alyst was removed and washed and subjected to a second run of protons in 1, we believe that the large anion of 1 provides

OXIDATION OF BENZALDEHYDES
127
after 6 h. Addition of fresh oxidant again after 6 h takes reaction
to completion.
Under optimum conditions for all of the benzaldehydes effect
of temperature were studied. The results are shown in Figure 4.
The results show that carboxylic acid yield increased as the
reaction temperature was raised. Increase in temperature can
accelerate the oxidation reaction.
Tetrabutylammoniumhexatungstate (VI), [(n-C4H9)4N]2
[
W6O19] has also been considered as a quaternary ammonium
+
salt having an organic cation part, [(n-Bu)4N ], and a poly-
2–
oxometalate anion part, W6O19 and therefore can act as a
phase transfer catalyst (PTC) in an aqueous/organic biphasic
system. Hence, we believe that the catalyst acts as a PTC in our
method.
Although we did not investigate the reaction mechanism,
a plausible mechanism may proceed as depicted in Scheme
1. The reaction occurs via initial formation of the perhydrate
FIG. 1. Relation of catalyst amount with percent yield of carboxylic acids.
intermediate [I] as the result of a nucleophilic attack of H2O2
at the carbonyl group of aldehyde. The elimination of water
from the tetrahedral intermediate [I] via a Baeyer-Villiger type
fragmentation yields the carboxylic acids as final products. The
catalyst facilitates the formation of the intermediate [I], and
also the elimination of water from this intermediate. Tetrabutyl
many sites on the oval-shaped molecule that are likely to render
the catalyst effective.
For further studies, we have examined the effect of catalyst
amount for the other benzaldehydes. The results are shown in
Figure 2. Interestingly, in all cases the observed yields were
higher with 0.03 g. In optimum conditions, effect of the time
was studied in the time range of 2–8 h and the results are shown
in Figure 3.
+
ammonium ion [(n-Bu)4N ] induces polarization of carbonyl
group in aldehydes. Furthermore, both tetrabutyl ammonium ion
+
2–
[
(n-Bu)4N ] and polyoxometalate anion, W6O19 , can probably
facilitate the elimination of water from the intermediates [I] and
The yield of benzoic acids increases with the reaction time.
At reaction time of 6 h, the yield increases and it has not increase
thereafter. This has been found to be due to the nonavailability
of enough oxidant in the reaction mixture after 6 h. This is
confirmed by the determination of unreacted hydrogen peroxide
by titration method that shows little oxidant left in the mixture
therefore can promote the reactions.
FIG. 2. Percent yield of carboxylic acids with different amounts of catalyst (color figure available online).

128
F. F. BAMOHARRAM ET AL.
FIG. 3. Relation of time with percent yield of carboxylic acids (color figure available online).
The blank oxidation of benzaldehydes was carried out with
In order to have a proper comparison, all reactions were
H2O2 alone under similar reaction conditions without using performed under optimum conditions (270 s and 350 W). The
catalyst. The oxidation was found to be very slow and gave very results are shown in Figure 5.
poor yield.
The results show that all of the benzaldehydes could be easily
oxidized in a short time under solvent-free conditions.
Comparison between yields of products in thermal conditions
and microwave irradiation shows that benzaldehydes show dif-
ferent behaviors. In thermal conditions yield of carboxylic acids
increased in the following order: 4-nitro benzaldehyde > 4-
chloro benzaldehyde > 4-bromo benzaldehyde > 3- chloro ben-
zaldehyde > 4-methyl benzaldehyde > 3-nitro benzaldehyde.
While under microwave irradiation yield increased in this order:
Microwave Irradiation
For the first time, we describe herein our results on this en-
vironmentally benign microwave approach for the synthesis of
aromatic carboxylic acids in the presence of 1 as catalyst. We
show that 1 in solvent-free conditions renders effective the alde-
hyde oxidation to carboxylic acid in the presence of hydrogen
peroxide under microwave irradiation, in 4–5 min.
FIG. 4. Relation of temperature with percent yield of carboxylic acids (color figure available online).

OXIDATION OF BENZALDEHYDES
129
FIG. 5. Oxidation of benzaldehydes under microwave irradiation (color figure available online).
4-nitro benzaldehyde > 4-chloro benzaldehyde > 4-bromo ben- of preparation, excellent yields of products, and simple workup
zaldehyde > 3-nitro benzaldehyde > 3- chloro benzaldehyde > make this an attractive and facile synthetic tool for the oxidation
4-methyl benzaldehyde.
of various aromatic aldehydes to acids.
It is proposed that this behavior is due to loss factor of the
substrates. Microwave heating of substances to a great extent
depends on its dissipation factor, which is the ratio of dielectric
loss, or loss factor due to the dielectric constant of the mate-
rial. The dielectric constant is a measure of the ability of the
material to retard microwave energy as it passes through; loss
factor is a measure of the ability of the material to dissipate
the energy. In other words, loss factor represents the amount
of input microwave energy that is lost in the material by being
dissipated as heat. Therefore, materials with different loss factor
show different behaviors.
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