Oxidation of alcohols and pyridines by a water-soluble polyoxometalate with hydrogen peroxide

Article abstract of DOI:10.1080/00397911.2011.628134

A water-soluble catalyst based on a silicotungstate polyoxometalate, K ₈[β-SiW₁₁O₃₉] · 14H₂O, was developed for the oxidation of pyridines and alcohols with hydrogen peroxide. The reactions were carried out in water, and good yields of the corresponding heterocyclic N-oxides and ketones were obtained under relatively mild conditions. The catalyst could be easily recovered by extraction with ethyl acetate and reused several times. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications to view the free supplemental file.

Full text of DOI:10.1080/00397911.2011.628134

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Oxidation of Alcohols and Pyridines by
a Water-Soluble Polyoxometalate with
Hydrogen Peroxide
Zhenxin Zhang a , Qianqian Zhu a & Yong Ding a
a Key Laboratory of Nonferrous Metal Chemistry and Resource
Utilization of Gansu Province, College of Chemistry and Chemical
Engineering, Lanzhou University, Lanzhou, China
Accepted author version posted online: 29 Feb 2012.Published
online: 21 Feb 2013.
To cite this article: Zhenxin Zhang , Qianqian Zhu & Yong Ding (2013) Oxidation of Alcohols and
Pyridines by a Water-Soluble Polyoxometalate with Hydrogen Peroxide, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 43:9, 1211-1218, DOI:
10.1080/00397911.2011.628134
To link to this article: http://dx.doi.org/10.1080/00397911.2011.628134
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Synthetic Communications1, 43: 1211–1218, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.628134
OXIDATION OF ALCOHOLS AND PYRIDINES
BY A WATER-SOLUBLE POLYOXOMETALATE
WITH HYDROGEN PEROXIDE
Zhenxin Zhang, Qianqian Zhu, and Yong Ding
Key Laboratory of Nonferrous Metal Chemistry and Resource
Utilization of Gansu Province, College of Chemistry and
Chemical Engineering, Lanzhou University, Lanzhou, China
GRAPHICAL ABSTRACT
Abstract
A water-soluble catalyst based on a silicotungstate polyoxometalate,
K8[b-SiW11O39] ꢀ 14H2O, was developed for the oxidation of pyridines and alcohols with
hydrogen peroxide. The reactions were carried out in water, and good yields of the correspond-
ing heterocyclic N-oxides and ketones were obtained under relatively mild conditions. The cata-
lyst could be easily recovered by extraction with ethyl acetate and reused several times.
Supplemental materials are available for this article. Go to the publisher’s online edition
of Synthetic Communications1 to view the free supplemental file.
Keywords Alcohols; oxidation; polyoxometalate; pyridines
Received August 16, 2011.
Address correspondence to Yong Ding, Key Laboratory of Nonferrous Metal Chemistry and
Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou
University, Lanzhou 730000, China. E-mail: yongding278@163.com
1211

1212
Z. ZHANG, Q. ZHU, AND Y. DING
INTRODUCTION
Selective oxidation of alcohols to generate corresponding carbonyl compounds
is quite important, because carbonyl compounds are widely applied to the pro-
duction of various fine and specialty chemicals including drugs, vitamins, and fra-
grances[1] The procedure of pyridine oxidation to corresponding heterocyclic
N-oxides is of great importance because of the wide usefulness of the oxygenated
products, which are utilized as auxiliary agents, protecting groups, catalysts, oxi-
dants, ligands in metal complexes and surrogates for heterocyclic boronic acids[2]
Catalytic reaction using hydrogen peroxide as an oxidant offers advantages, because
hydrogen peroxide can oxidize organic compounds with relatively high atom
efficiency and theoretically generates only water as a by-product[3]
Polyoxometalates that are molecular metal–oxygen clusters have attracted
much attention and are applied to many applications, especially alcohol and pyridine
oxidation. Although oxidation of these organic compounds catalyzed by polyoxome-
talates is quite effective,the separation of the catalysts is difficult because of the good
solubility of polyoxometalates in normal solvents. Therefore, efficient separation and
reuse of polyoxometalate catalysts become the single most important issue that
should be emphasized for industrial application[4] It is very essential to develop a
catalytic system that allows polyoxometalate to be easily separated from the reaction
solution. Recently, a catalytic system based on water-soluble polyoxometalates was
developed to solve this problem by using liquid–liquid biphase separation. Such a
system has the advantages that the catalyst is recycled easily from the reaction sol-
ution by extraction, and the reactions performed in water are considered to be safer,
cheaper, and more environmentally friendly. Therefore, some research is concerned
with oxidation of alcohols and pyridines catalyzed by polyoxometalates with
hydrogen peroxide in water. For example, Na12[WZnZn2(H2O)2(ZnW9O34)2],[5]
Na6[SiW11ZnH2O40] ꢀ 2H2O,[6] K6[BW11O39],[7] and K8[c-SiW10O36][8] for alcohol
oxidation and K6[PW9V3O40][9] for pyridine oxidation show good catalytic activity
of alcohol oxidation and pyridine oxidation in water.
In the present work, a lacunary silicotungstate polyoxometalate, K8[b-
SiW11O39] ꢀ 14H2O, was synthesized, and this polyoxometalate was utilized as an
efficient catalyst for oxidation of alcohols and pyridines with hydrogen peroxide
at 90 ꢁC in water. The reaction was performed in a biphase reaction system, and
the water-soluble catalyst could be separated from the reaction solution by
extraction and reused without addition of fresh catalyst.
RESULTS AND DISCUSSION
Several kinds of catalytic materials were applied to the catalytic oxidation
using cyclohexanol as a model substrate with hydrogen peroxide. The results
(Table 1) showed that the catalyst of K8[b-SiW11O39] ꢀ 14H2O behaved as the most
effective catalyst for the oxidation of alcohol. After the essential reaction time,
99% of yield and selectivity were achieved (entry 1). Another kind of lacunary
polyoxometalate, K8[c-SiW10O36], also could obtain good result (yield ¼ 95%), but
K8[c-SiW10O36] was not easy to prepare (entry 2)[10] When saturated polyoxometa-
late, K4[SiW12O40], was applied to catalyze the reaction, only low catalytic activity
was observed (entry 3). Simple salts such as Na2WO4 ꢀ 2H2O, Na2SiO3 ꢀ 9H2O, and

OXIDATION CATALYZED BY POLYOXOMETALATE
1213
Table 1. Cyclohexanol oxidation catalyzed by different catalysts with hydrogen peroxidea
Entry
Catalyst
Yield (%)
Selectivity (%)
1
K8[b-SiW11O39] ꢀ 14H2O
99
95
5
99
99
99
99
99
99
—
2
K8[c-SiW10O36]
K4[SiW12O40]
3
4
Na2WO4 ꢀ 2H2O
13
7
5
Na2SiO3 ꢀ 9H2O
6b
7
Na2SiO3 ꢀ 9H2OþNa2WO4 ꢀ 2H2O
7
—
0
aReaction conditions: catalyst 9.3 mmol, water 1 mL, cyclohexanol 1 mmol, hydrogen peroxide 5 mmol,
reaction temperature 90 ꢁC, reaction time 7 h.
bNa2SiO3 ꢀ 9H2O 9.3 mmol, Na2WO4 ꢀ 2H2O 9.3 mmol.
the mixture of Na2SiO3 ꢀ 9H2O and Na2WO4 ꢀ 2H2O could not achieve satisfactory
yields (entries 4–6). When no catalyst was used, there was no catalytic activity
observed (entry 7). Therefore, K8[b-SiW11O39] ꢀ 14H2O seemed to be the best catalyst
for the oxidation of alcohols.
The lacunary silicotungstate, K8[b-SiW11O39] ꢀ 14H2O, could be applicable to
oxidation of various alcohols (such as benzylic alcohols and secondary alcohols)
with 30% hydrogen peroxide at 90 ꢁC, and the results are listed in Table 2. For some
cyclic alcohols, such as cyclohexanol and cyclopentanol, the substrates could be
transferred to corresponding ketones with good yields and selectivities (entries 1
and 2). Hydrophobic alcohols, such as 2-octanol and DL-menthol, could not be cat-
alyzed to carbonyl compounds under the current reaction conditions, because of the
poor solubility of the alcohols in water (entries 3 and 6). Therefore, adding a small
amount of phase-transfer catalyst, methyltrioctylammonium chloride, could obvi-
ously promote the yields of the ketones (entries 4 and 7). Benzyl alcohol was mainly
converted to benzoic acid (selectivity ¼ 85%) after 7 h at 90 ꢁC, when the molar ratio
of hydrogen peroxide to benzyl alcohol was 5:1 (entry 9).
The stability of the catalyst of K8[b-SiW11O39] ꢀ 14H2O was evaluated by cata-
lyst recycling and recovery experiment. As shown in Table 3, the water-soluble cata-
lyst could maintain relatively high catalytic activity after 5 times, and 80% of yield
was obtained by using the catalyst in the fifth cycle. However, compared with the
fresh catalyst, the used catalyst was not as active because of the structural changes
of the catalyst. It was reported that polyoxometalates anion could form a mixture
of peroxo species with hydrogen peroxide when the catalytic oxidation was com-
pleted. Typically for silicotungstate, a kind of peroxo species, [c-SiW10O32(O2)
(OOH)]3ꢂ, was demonstrated, which formed after the selective oxidation[10] Our
experimental data (Fig. 1) also gave evidence of the structural changes of K8[b-
SiW11O39] ꢀ 14H2O after reaction and demonstrated that the used catalyst was a
mixture of peroxo species. According to the Fourier transform—infrared (FT-IR)
spectra (Fig. 1), the peak at 838 cmꢂ1 could be attributed to peroxo species, which
were generated by mixing the polyoxometalate with hydrogen peroxide[11]
The catalyst of K8[b-SiW11O39] ꢀ 14H2O could also be applied to the oxidation
of pyridines to corresponding N-oxide compounds with hydrogen peroxide at 90 ꢁC
in water. For pyridine, the reaction was almost complete after 2 h and 95% of yield

1214
Z. ZHANG, Q. ZHU, AND Y. DING
Table 2. Alcohol oxidation catalyzed by K8[b-SiW11O39] ꢀ 14H2O with hydrogen peroxidea
Entry
Substrate
Product
Yield (mol%)
Selectivity (mol%)
1
2
99
97
99
99
3
12
99
99
99
4b
5
91
7
99
99
99
6
7b
97
8
99
85
99
85
9c
aReaction conditions: K8[b-SiW11O39] ꢀ 14H2O 9.3 mmol, water 1 mL, substrate 1 mmol, hydrogen
peroxide 5 mmol, reaction temperature 90 ꢁC, reaction time 7 h.
b20 mmol of methyltrioctylammonium chloride was added.
cBy-product was benzaldehyde.
was obtained (entries 1 and 2). Methyl-substituted pyridines were easier to oxidize
and good yields were achieved (entries 3–5), because the methyl group was an
electron-supplying group, which promoted the reactivity of the substrates. For
2-chloropyridine, the reactivity was quite poor (entry 6) because of two main rea-
sons: (1) chloro group was an electron-withdrawing group and (2) the solubility of
2-chloropyridine was poor in water. Phase-transfer catalyst was added to improve
Table 3. Recycling of the catalyst for the oxidation of cyclohexanola
Entry
Cycle time
Yield (%)
Selectivity (%)
1
2
3
4
5
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
99
98
88
81
80
99
99
99
99
99
aReaction conditions: K8[b-SiW11O39] ꢀ 14H2O 9.3 mmol, water 1 mL, cyclohexanol
1 mmol, hydrogen peroxide 5 mmol, reaction temperature 90 ꢁC, reaction time 7 h.

OXIDATION CATALYZED BY POLYOXOMETALATE
1215
Figure 1. FT-IR spectra of (a) the fresh catalyst of K8[b-SiW11O39] ꢀ 14H2O and (b) the used catalyst of
K8[b-SiW11O39] ꢀ 14H2O.
the catalytic performance of oxidation of 2-chloropyridine, but the yield of the
product was not promoted (entry 7).
It has been reported that polyoxometalate anions could form a mixture of per-
oxo species in hydrogen peroxide.[4(a),7,10(a),12] The generated peroxo species actually
catalyzed the oxidation of organic compounds. According to previous research and
Scheme 1. Proposed catalytic cycle for the oxidation of substrates by K8[b-SiW11O39] ꢀ 14H2O with hydro-
gen peroxide as oxygen source.

1216
Z. ZHANG, Q. ZHU, AND Y. DING
Table 4. Pyridine oxidation catalyzed by K8[b-SiW11O39] ꢀ 14H2O with hydrogen peroxidea
Entry
Substrate
Product
Time (h)
Yield (%)
Selectivity (%)
1
2
1
2
84
95
99
99
3
1
95
99
4
5
1
1
99
85
99
99
6
5.5
5.5
17
37
99
99
7b
aReaction conditions: K8[b-SiW11O39] ꢀ 14H2O 9.3 mmol, water 1 mL, substrate 1 mmol, hydrogen per-
oxide 5 mmol, reaction temperature: 90 ꢁC, isolated yield.
b20 mmol of methyltrioctylammonium chloride was added.
FT-IR spectra, the a mechanism is proposed in Scheme 1. First, the fresh catalyst of
K8[b-SiW11O39] ꢀ 14H2O reacted with hydrogen peroxide to generate a mixture of
peroxo species, which reacted with the substrate to yield the product. Because the
hydrogen peroxide was in excess, the peroxo species could be regenerated by reacting
with hydrogen peroxide after the reaction and used for the next cycle.
CONCLUSION
In summary, a water-soluble silicotungstate, K8[b-SiW11O39] ꢀ 14H2O, was per-
formed as an efficient and recyclable catalyst for oxidation of alcohols and pyridines
with hydrogen peroxide in water. Various alcohols and pyridines were converted to
the corresponding products with good yields and selectivities. The catalyst of K8[b-
SiW11O39] ꢀ 14H2O could be easily recovered from the reaction solution and reused
five times.
EXPERIMENTAL
K8[b-SiW11O39] ꢀ 14H2O was prepared using the previous method.[10] IR spec-
trum (KBr, cmꢂ1) was 989, 950, 880, 864, 805, 732, 611, 533, and 514. The catalytic
reaction was performed in a 15-mL flask equipped with an oil bath, a magnetic stir-
ring bar, and a reflux condenser. The reactants were added into the flask and the

OXIDATION CATALYZED BY POLYOXOMETALATE
1217
amounts of the reactants were as follows: 30 mg of catalyst, 1 mL of water, 1 mmol of
substrate, and 5 mmol of hydrogen peroxide. The flask was heated at 90 ꢁC. After
enough reaction time, the solution was extracted by 2 mL of ethyl acetate. The
organic layer was analyzed by gas chromatography. Assignments of products were
1
made by comparison with authentic samples and analyzed by H NMR and 13C
NMR. The aquatic layer was left for the next cycle without addition of fresh catalyst.
Please see the Supporting Information, available online, for complete details.
ACKNOWLEDGMENT
This work was financially supported by the National Natural Science
Foundation of China (Grant No. 20803032).
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