Water as an efficient solvent for oxygenation transformations with 34% hydrogen peroxide catalyzed by some heteropolyoxometalates

Article abstract of DOI:10.1007/s00706-007-0660-z

A highly efficient, selective, fast, and cheap protocol is developed for oxidation of aromatic amines and alcohols utilizing 34% hydrogen peroxide in water catalyzed by some W- and Mo-based heteropolyoxometalates. Findings showed that dodecatungstophosphoric acid, H₃PW₁₂O₄₀, was the most efficient catalyst in the examined oxidation reactions. This methodology may prove to be a valuable alternative for eco-friendly green oxidation. Inherent simplicity, easy work up, and using regenerable catalysts were other key aspects of this oxidation protocol.

Full text of DOI:10.1007/s00706-007-0660-z

Monatshefte f u¨ r Chemie 138, 763–769 (2007)
DOI 10.1007/s00706-007-0660-z
Printed in The Netherlands
Water as an Efficient Solvent for Oxygenation Transformations
with 34% Hydrogen Peroxide Catalyzed by some Heteropolyoxometalates
Reza Tayebee1 and Mohammad H. Alizadeh2
;Ã
1
Department of Chemistry, Sabzevar University, Sabzevar, Iran
2
Department of Chemistry, Ferdowsi University, Mashhad, Iran
Received October 8, 2006; accepted (revised) January 31, 2007; published online May 23, 2007
#
Springer-Verlag 2007
Summary. A highly efficient, selective, fast, and cheap pro-
tocol is developed for oxidation of aromatic amines and alco-
hols utilizing 34% hydrogen peroxide in water catalyzed by
some W- and Mo-based heteropolyoxometalates. Findings
showed that dodecatungstophosphoric acid, H PW O , was
the most efficient catalyst in the examined oxidation reactions.
This methodology may prove to be a valuable alternative for
eco-friendly green oxidation. Inherent simplicity, easy work
up, and using regenerable catalysts were other key aspects of
this oxidation protocol.
Despite the vast array of homogeneously cata-
lyzed reactions that has been developed in the past
decades, industrial fine chemical manufacture still
heavily depends on classical organic chemistry based
on stoichiometric reagents. Development of highly
active homogeneous catalysts that meet industrial
demands is a challenging task. Considerable re-
search, most of quite recent vintage, on homoge-
neous catalysis by heteropolyoxometalates as early
transition metal oxygen anion clusters is a rapidly
growing area, in part as a consequence of the unusu-
al versatility of these catalysts and their compatibility
with environmentally friendly conditions [8–11].
Heteropolyoxometalates are an extraordinarily ver-
satile class of complexes with potential catalytic ap-
plications for a variety of attractive processes. These
include O - and H O -based oxidations and processes
that take place under environmentally benign condi-
tions. The selected examples reviewed show the broad
scope of potentially promising applications of hetero-
polyoxometalates as acid and redox catalysts in liquid
phase organic reactions including epoxidation, oxi-
dative dehydrogenation, and oxidative cleavage pro-
cesses [12–20]. Due to their unique physicochemical
properties, heteropolyoxometalates can be profitably
used in homogeneous, biphasic, or heterogeneous sys-
tems, providing a broad operational choice [21–23].
Selective oxidation of arylamines and alcohols to
their corresponding oxygenated products are of the
most fundamental functional transformations through-
3
12 40
Keywords. Oxidation; Heteropolyacid; Catalysis; Hydrogen
peroxide.
Introduction
Waste minimization is a very important aspect of an
environmentally benign protocol [1, 2]. Practically,
this means performing reactions with high conver-
sions, using green reagents, and a green reaction me-
dium followed by an efficient recycling program.
Although, several effective protocols based on reus-
ing of the reaction medium have been introduced,
the recycling of organic solvents is complex and
requires purification by distillation. Whereas the use
of water (without co-solvents) as reaction medium
makes recycling processes very easy and cheap. In
addition, the aqueous medium allows one-pot multi-
step processes to be performed [3–7].
2
2 2
à Corresponding author. E-mail: rtayebee@yahoo.com

7
64
R. Tayebee and M. H. Alizadeh
out chemistry and have been a challenge for many
years [24–30]. A variety of stoichiometric and cat-
alytic routes have been reported for preparation of
these crucial industrial precursors. The nature of the
oxidation products formed in these reactions de-
pends on the type of the oxidant used and the reac-
tion conditions. Various reagents, including metal
compounds, organic peroxides, and hydrogen perox-
ide, have been utilized to form oxygen containing
derivatives of many organic precursors [27–30, 32–
Results and Discussion
Oxygenation of Aniline and Cyclohexanol
in Water with H O Catalyzed by Different
Heteropolyoxometalates
2
2
Table 1 introduces an efficient new methodology for
catalytic oxygenation of aniline and cyclohexanol
with 34% H O in drinking water using simple het-
2
2
eropolyoxometalates.
Among the examined catalysts for the oxygenation
of aniline with hydrogen peroxide, K SiW Mo O ,
and H PW O showed the best results. They pro-
5
2, 57–59]. However, some of the homogeneous
4
9
2 39
and heterogeneous catalysts disclosed in the litera-
ture are often required in stoichiometric amounts.
Many of these methods are usually toxic and haz-
ardous and suffer from harsh reaction conditions,
over-oxidation, low yields of the desired products,
and cause environmental problems [53, 54]. There-
fore, designing a catalytic formulation using envi-
ronmentally benign oxidants by the mediation of
simple water soluble metal complexes in aqueous
reaction media are desirable in organic chemistry,
biochemistry, and atmospheric chemistry.
Herein, we wish to report a simple, clean, effi-
cient, and eco-friendly process for oxygenation of
anilines and alcohols with 34% hydrogen peroxide
catalyzed by some simple heteropolyoxometalates.
Indeed, in contrast to almost all presently em-
ployed processes, reactions were performed in wa-
ter effectively.
3
12 40
vided 67% of product with >97% selectivity after
0min. Previously, it has been demonstrated that
6
nitrosobenzene was the sole product of aniline oxida-
tion with aqueous hydrogen peroxide by the media-
tion of dodecatungstophosphoric acid [31].
The results of oxygenation of cyclohexanol with
H O in water demonstrated that H PW O and
K Na PW O were the best catalysts in this oxida-
2
2
3
12 40
5
2
11 39
tion system and they led to complete conversion of cy-
clohexanol to cyclohexanone after 2.5 h. H PMo O
and its ammonium salt [(CH ) N] PMo O show-
3
12 40
3
4
3
12 40
ed less catalytic activity and resulted in 30 and
3% yields, respectively, under the same reaction
1
conditions. Other mixed metal substituted Keggin-
type heteropolyanions revealed moderate reactiv-
ity and provided 67–70% of cyclohexanone under
Table 1. Oxidation of aniline and cyclohexanol in normal drinking water with 34% H O catalyzed by different
2
2
a
heteropolyoxometalates
Entry
Catalyst
H PW O
Substrate
Product
Conv.=%
Sel.=%
Time=min
1
2
3
4
5
6
7
8
9
Ph-NH (1)
2
Ph-NO (2)
C H O (4)
67
100
56
30
64
70
67
67
53
100
98
97
100
98
95
97
92
100
98
96
100
60
150
60
180
60
210
60
210
60
3
12 40
C H OH (3)
6
11
6
10
H PMo O
3
1
2
12 40
3
1
3
1
3
1
3
1
3
4
2
4
2
4
2
4
2
4
Na PW Mo O
3
9
3
40
K SiW Mo O
4
9
2
39
K Na PW O
11 39
5
2
10
11
12
100
54
13
150
60
180
(TBA) PMo O
3
12 40
a
To a solution of catalyst (0.015 mmol for aniline and 0.018 mmol for cyclohexanol) and 34% H O (8 mmol for aniline and
2
2
3
mmol for cyclohexanol) in water (4 cm for aniline and 5 cm for cyclohexanol) as solvent was added substrate (4 mmol for
3
5
aniline and 0.94 mmol for cyclohexanol). The reaction mixture was stirred at room temperature for the required time. Progress of
the reactions was followed by aliquots withdrawn directly and periodically from the reaction mixture and analyzed by gas
chromatography. Products were isolated as described in the experimental section

Water as an Efficient Solvent
765
According to the results in Table 2, 69% of nitro-
sobenzene was produced in water, whereas 27% of
conversion was observed in CH Cl under the same
2
2
reaction conditions within 60 min. It seems that in-
organic impurities in the water do not interfere into
the reaction progress. Ethanol was also capable of
aniline oxygenation with 71% yield at the same
time. Other organic solvents revealed less reactivity
than water in the oxygenation of aniline. Acetone,
dichloromethane, and chloroform produced 12–
2
7% of conversions to the corresponding products
t
after 60 min. Meanwhile, BuOH was the most effi-
cacious organic solvent and led to 50% yield at the
same time.
Fig. 1. Oxygenation of cyclohexanol in drinking water with
H O catalyzed by different heteropolyoxometalates
2
2
Table 2 also shows results of oxidation of cyclo-
hexanol with H O catalyzed by H PW O in dif-
ferent solvents. As in the oxygenation of aniline,
water was the best solvent for selective oxidation
of cyclohexanol. Complete conversion of cyclohex-
anol to cyclohexanone was obtained in water with
the reaction conditions reported here (Fig. 1 and
Table 1).
2
2
3
12 40
Effects of Different Solvents on the Oxygenation
of Aniline and Cyclohexanol with 34% H O
Catalyzed by H PW O
2
2
100% selectivity after 2.5 h. Fortunately, inorganic
3
12 40
impurities in water had no effect on the efficiency
of cyclohexanol oxidation. Other organic solvents
such as chloroform, ethanol, and acetone were not
suitable for the oxygenation system. These solvents
gave rise to 5, 15, and ꢀ10% of cyclohexanone,
respectively, within 2.5 h (Fig. 2).
To test the range of solvents that may promote oxi-
dation reactions, a number of usual solvents were
screened. Table 2 presents the results of catalytic ox-
ygenation of aniline and cyclohexanol with H O
2
2
mediated by H PW O in different solvents. Water
12 40
3
(
3
electrical conductivity, 550 S; total dissolved solids,
3
Different reactivity patterns observed for the
introduced catalysts in different solvents may in part
be explained considering solubility of them. For
50 mg=dm ; and pH, 8.3) turned out as the best
solvent for these oxygenation transformations. Find-
ings clearly approved that water was more effective
than dichloromethane.
example, K Na PW O
2
and (TBA) PMo O ,
3 12 40
5
11 39
which are slightly soluble in CH Cl , revealed little
2
2
Table 2. Effect of different solvents on the oxidation of aniline and cyclohexanol with 34% H O catalyzed by H PW O
12 40
2
2
3
Entry
Solvent
Water
Substrate
Product
Conv.=%
Sel.=%
Time=min
1
2
3
4
5
6
7
8
9
Ph-NH (1)
2
Ph-NO (2)
C H O (4)
69
100
98
15
12
<10
50
3
85
27
6
25
5
100
100
71
95
93
>95
95
>97
100
93
60
150
60
150
60
150
60
150
300
60
C H OH (3)
11
6
6
10
Ethanol
Acetone
t-Butanol
1
2
3
1
3
1
3
5
1
3
1
3
4
2
4
2
4
6
2
4
2
4
1
1
1
1
0
1
2
3
Dichloromethane
Chloroform
>95
92
>95
150
60
150
Reactions were carried out as described below Table 1

7
66
R. Tayebee and M. H. Alizadeh
This methodology provided a new environmen-
tally friendly route to the conversion of alcoholic
functions to carbonyl groups with excellent selectiv-
ities. Table 3 shows the results of oxygenation of
different alcohols with H O under the influence of
2
2
ꢁ
H PW O in water at 70 C. Findings showed that
3
12 40
3-NO -benzyl alcohol and 1-phenylethanol were the
2
most reactive substrates in the oxygenation system.
They produced 90 and 85% yields of the correspond-
ingketonesafter2.5 h, whereas 3-OMe-benzyl alcohol,
benzyl alcohol, and isopropanol resulted in 67–78%
conversions after the required times. n-Butanol and
n-heptanol, as linear primary alcohols, showed little
reactivity toward oxidation. They exhibited 30 and
Fig. 2. Effect of different solvents on the oxygenation of
cyclohexanol with H O catalyzed by H PW O
12 40
2
2
3
9% conversions after 2.5 and 4.5 h, respectively.
catalytic activity, whereas good yields were obtained
with these two catalysts in water.
As a fortuitous extension of the solvent study, the
oxidation of aniline and cyclohexanol with hydrogen
peroxide in water has cleanly and selectively pro-
vided nitrosobenzene and cyclohexanone.
Effect of Different Oxidants on the Oxygenation
of Cyclohexanol
A large number of oxidants have been extensively
investigated in context with homogeneous catalytic
oxidation processes. The primary characteristics that
define the attractiveness of an oxidant are the active
oxygen percent (the higher the better) and the selec-
tivity associated with its use, in addition to the prag-
matic issues of cost and effect on the environment.
Oxygenation of some Anilines and Alcohols
into the Corresponding Oxygenated Products
Treatment of some aromatic amines with hydrogen
peroxide in the presence of dodecatungstophospho-
ric acid as catalyst in water gave the corresponding
nitroso derivatives with good to excellent yields and
with 100% selectivities (Table 3). These results
showed that phenylenediamine was the most reac-
tive substrate toward oxidation, whereas electron
À
While the oxidants such as OCl , Cl , and ClO lead
2
2
in varying yields to chlorinated organic compounds,
including the highly toxic and carcinogenic dioxins,
and are rapidly becoming environmentally unaccept-
able, many other oxidants that produce only mini-
mally toxic inorganic waste as by-products, such as
sulfate from persulfate, are also becoming environ-
mentally and hence politically and economically
deficient 4-NO -aniline led to only 32% of the
2
nitroso product.
Table 3. Oxidation of some aromatic amines and alcohols with 34% H O in water catalyzed by H PW O
12 40
2
2
3
Entry
Class
Substrate
Ph-NH2
4-NO -Aniline
4-Me-Aniline
1,2-Phenylenediamine
Product
C H –NO
Conv.=%
Sel.=%
Time=min
1
2
3
4
AnilinesÃ
71
32
77
100
100
100
100
92
60
60
60
30
6
5
4-NO –C H –NO
2 6 4
2
4-Me–C H –NO
6
4
1-NO-2-(NH )C H
6 4
2
5
6
7
8
Alcohols
Isopropanol
C H O
3
67
73
78
30
9
100
100
100
75
60
100
100
180
120
150
150
270
150
150
6
4-OMe-Benzyl alcohol
Benzyl alcohol
n-Butanol
n-Heptanol
1-Ph-Ethanol
4-OMe–C H O
7 5
C H O
7
6
CH (CH ) CHO
3
2 2
CH (CH ) CHO
2 5
3
9
0
Ph-COCH3
3-NO –C H O
85
90
1
3-NO -Benzyl alcohol
2
2
7 5
Ã
t
Water: BuOH (3:1) mixture was used as solvent. Reactions were carried out as described below Table 1

Water as an Efficient Solvent
767
Table 4. Oxidation of cyclohexanol to cyclohexanone with
different oxidants catalyzed by H PW O
chromate, showed less reactivity in the oxygenation
system and their reactivities were independent into
the presence of catalyst. Potassium persulfate and
potassium dichromate led to ꢀ44 and <10% of cy-
clohexanone in this oxygenation system (Fig. 3 and
Table 4).
3
12 40
Entry
Oxidant
H O
H O
Yield=%
70
Sel.=%
Time=h
1
2
3
4
5
6
7
8
100
–
4
4
3.5
3.5
5
5
7
7
2
2
2
0Ã
2
KMnO4
KMnO4
K S O
72
30
43
44
<10
90
95
93
93
100
–
Ã
Ã
2
2
8
8
Conclusion
K S O
2
2
K Cr O
2
2
7
7
<5Ã
In conclusion, we have demonstrated an efficient,
eco-friendly, and effective one-pot protocol to oxi-
dize aromatic amines and alcohols directly into their
corresponding oxygenated products in water. These
reactions were facile, high-yielding, and easy to
work up and should provide a mild oxidative alter-
native for organic chemists. We have shown that
mentioned water-soluble heteropolyoxometalates
were effective, stable, and recyclable catalysts in
biphasic reaction media. The in situ prepared cata-
lysts were as active as the recycled ones. Simple
polyoxotungstophosphates used in this protocol can
be readily synthesized on a large scale from cheap
salts and are degraded at a very slow rate [60–62].
K Cr O
2
2
Reactions were carried out as described below Table 1
Ã
ꢁ
In the absence of catalyst at 70 C
unacceptable. The most attractive oxidants with re-
spect to the criteria articulated above are O and
2
H O . Hydrogen peroxide readily transfers one of
2
2
its oxygen atoms to a substrate to produce as a by-
product a ‘green’ substance, water, which is a great
advantage of this oxidant in comparison with all
other oxidants [55–56].
Table 4 describes oxygenation of cyclohexanol with
some important oxidants catalyzed by H PW O .
3
12 40
Findings demonstrated that hydrogen peroxide was
the best oxidant in this catalytic oxygenation system.
It gave rise to 70% of cyclohexanone in the presence
of a catalyst within 4 h, whereas no conversion oc-
curred in the absence of the catalyst with this oxidant
at the same time. Sodium permanganate was also
suitable for oxidation of cyclohexanole. It led to
Experimental
Materials and Instrumentation
Solvents, reagents, and other chemicals used in this study were
of the highest grade available and were purchased from SD
ꢁ
Fine Chemicals (India). The reagents were stored at 5 C
and purified just before use. Silica gel 60 (70–230 mesh, pur-
chased from E-Merck A.G., Darmstadt, West Germany) was
used for column chromatography. Purity of the substances and
progress of the reactions were monitored by gas chromatog-
raphy. GLC analyses were performed on a Shimadzu GC-
7
2 and 30% of cyclohexanone in the presence and
in the absence of the acid catalyst within 3.5 h. Other
oxidants, potassium persulfate and potassium di-
1
7A instrument equipped with a flame ionization detector
using 25 mÂ0.25mm CPB 5-20 and fused silica WCOT
5m 0.23mm capillary columns. The heteropolyoxometa-
lates H PW O , H PMo O , (TBA) PMo O , Na PW
2
3
12 40
3
12 40
3
12 40
3
9
Mo O , and K Na PW O were prepared and characterized
3
40
5
2
11 39
according to literature procedures [60–67].
Synthesis of Na PW O Á 7H O
3
12 40
2
3
Na WO Á 2H O (30 mmol, 10g) was slowly added to 20cm
2
4
2
ꢁ
distilled water and the mixture was warmed to 60 C with
3
complete stirring. Then, 1 cm 85% H PO (15 mmol) and
3
4
3
cm HCl (100mmol) were added and the resulting mixture
8
was stirred for 1 h. The white afforded precipitate was washed
with water and was recrystallized twice from hot water.
Synthesis of Na HPMo O Á 14H O
2
12 40
2
3
Fig. 3. Oxygenation of cyclohexanol to cyclohexanone with
different oxidants in presence or absence of H PW O
To 420 cm of a 2.85 M aqueous solution of Na MoO were
2 4
3
3
3
12 40
added successively 6.8cm 85% H PO and 284 cm 70%
3 4

7
68
R. Tayebee and M. H. Alizadeh
HClO. The disodium salt was precipitated from the yellow
lukewarm solution. After the mixture was cooled to room tem-
perature, the microcrystalline powder was filtered off and air-
General Procedure for Oxidation of Alcohols in Water
To a solution of 0.018mmol catalyst and 5 mmol 34% H O in
2
2
3
cm drinking water as solvent, was added 0.94 mmol alcohol
5
3
3
ꢁ
dried. Recrystallization in a mixture of 40cm EtOH=200 cm
H O gave 180 g of greenish microcrystals.
and the reaction mixture was allowed to stir at 65–70 C for
the required time. Progress of the reaction was followed by
aliquots withdrawn directly from the reaction mixture, which
were analyzed by gas chromatography using an internal stan-
dard. After completion of the reaction, the products were
resolved from water. The extract was dried over anhydrous
sodium sulfate, filtered, and was concentrated under reduced
pressure. Finally, the concentrated filtrate was treated with 2,4-
dinitrophenylhydrazine in 6% HCl to give 2,4-dinitrophenyl-
hydrazone of the corresponding carbonyl compound. After
five runs using the recycled catalyst, efficiency of the oxyge-
nation system was ꢀ10% decreased.
2
Synthesis of H PMo O Á 14H O
3
12 40
2
H PMo O Á 14H O was obtained from a solution of 250 g
3
12 40
2
3
3
Na HPMo O Á 14H O in 200 cm H O, acidified by 50cm
2
12 40
2
2
3
1
2M HC1, and extracted by 400 cm Et O. The heavy layer
2
3
300 cm ), to which 150 cm H O were added, gave 190 g of
3
(
yellow crystals after desiccation.
2
Synthesis of (TBA) PMO O
3
12 40
3
H PMo O Á 14H O (10 g) was dissolved in 20 cm water,
3
12 40
2
3
5
g [(n-C H )N] Br were dissolved in 50cm CH C1 , and
4 9 4 2 2
Recycling of the Catalyst
At the end of the reaction, to regenerate the catalyst and de-
compose the excess hydrogen peroxide, the aqueous phase
the mixture was poured into the molybdophosphate solution
with vigorous stirring. Three phases were formed: a yellow
solid one, a lower liquid one (CH C1 ), yellow colored, and
2
2
3
ꢁ
(
ꢀ7 cm ) of the reaction mixture was dried slowly at 50 C
an aqueous upper one, poorly colored. The solid was washed
with Et O, providing 8.5 g of product. A further yield of 2.5 g
ꢁ
under intense light for 2 h and then at 130 C for 3 h. The regen-
erated solid acid catalyst was washed with dichloromethane
and dried at 130 C for 1 h. Usual spectroscopic methods show-
2
was obtained by addition of Et O to the CH C1 phase. Recrys-
2
2
2
ꢁ
tallization was performed in CH C1 .
2
2
ed no changes in the purity of the recycled catalyst after several
apes without considerable loss of activity. Further experiments
are on the way to check this protocol in large scale synthesis.
Synthesis of Na PW MO O
2 39
7
9
Na HPW O Á 24H O (11 g) was dissolved in a mixture of
5
9
34
2
3
0cm 1 N aqueous sodium molybdate and 16 cm 1 N aque-
3
2
3
ous hydrochloric acid. Then about 12cm 1 N hydrochloric
acid were added drop wise until the pH lies between 6 and
References
6
tated by addition of solid sodium chloride.
.5. The sodium salt Na PW MO O Á 19H O was precipi-
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7
9
2
39
2
[2] Tundo P, Anastas PT (1999) Green Chemistry: Challeng-
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3 40
3
9
3
3
Dioxane (20 cm ), 30 cm 5.45 N aqueous hydrochloric acid,
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3
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[
[
[
[
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7
9
2
39
2
yellow salt Na PW MO O Á 25H O precipitates. This com-
3
9
3
40
2
pound, unstable in aqueous solution, is stable in hydro-organic
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4
2: 1131
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General Procedure for Oxidation of Anilines to
Nitrosoderivatives in Water
To a solution of 0.015mmol catalyst and 8 mmol 34% H O in
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2
2
3
4
cm water as solvent was added 4 mmol amine and the
reaction mixture was stirred at room temperature for the
required time. Progress of the reaction was followed by ali-
quots withdrawn directly and periodically from the reaction
mixture, which were analyzed by gas chromatography using a
CPB 10 capillary column. Eventually, the organic phase was
resolved using a decantor. The extract was dried over anhy-
drous sodium sulfate, filtered, and was concentrated under
reduced pressure. The products were purified by column chro-
matography on silica gel (1=5, ethyl acetate=hexane). The
boiling point and spectral data of each product were compared
with those of authentic samples and literature values. After
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Water as an Efficient Solvent
769
[
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