H5PV2Mo10O40 as an efficient catalyst for the oxidation of thiols to the corresponding disulfides using hydrogen peroxide as the oxidant

Article abstract of DOI:10.2298/JSC100904086S

The H₅PV₂Mo₁₀O₄₀ catalyzed the oxidation of thiols to their homodisulfides using hydrogen peroxide as the oxidant under mild conditions is reported. This system provides an efficient, convenient and practical me

Full text of DOI:10.2298/JSC100904086S

J. Serb. Chem. Soc. 76 (7) 955–963 (2011)
JSCS–4174
UDC 547.367+547.279.3+54–328+
546.215:544.478
Original scientific paper
H5PV2Mo10O40 as an efficient catalyst for the oxidation
of thiols to the corresponding disulfides using
hydrogen peroxide as the oxidant
1
1
2
*
ABDOLLAH FALLAH SHOJAEI , MOHAMAD ALI REZVANI and MAJID HERAVI
1Department of Chemistry, Faculty of Science, University of Guilan, Rasht, 419961-3769 and
2Department of Chemistry, School of Science, Alzzahra University, Vanak, Tehran, Iran
(Received 4 September 2010, revised 10 January 2011)
Abstract: The H5PV2Mo10O40 catalyzed the oxidation of thiols to their homo-
disulfides using hydrogen peroxide as the oxidant under mild conditions is
reported. This system provides an efficient, convenient and practical method
for the syntheses of symmetrical disulfides. In this work, comparisons of Keg-
gin and Dawson type polyoxometalates are addressed in term of relative sta-
bility, hardness and acidity.
Keywords: heteropolyacids; thiols; disulfides; hydrogen peroxide; Keggin type;
polyoxometalate; Dawson.
INTRODUCTION
1–3
Disulfides play an important role in synthetic organic chemistry as well as
in biology, notably to control cellular redox potentials in biological systems in
4
which thiols are oxidized to prevent oxidative damage. The disulfide functio-
nality is used as a protecting group under oxidative conditions for thiols, which
5
can be regenerated by S–S bond cleavage. Disulfides have also found industrial
applications as vulcanizing agents and as important synthetic intermediates in
6,7
organic synthesis. Thiols can also be over-oxidized to sulfoxides and sulfones;
therefore, controlled and selective studies were performed in the present investi-
8
gation regarding the oxidation of thiols. Various reagents and oxidants have
9–16
been employed for the oxidation of thiols to homodisulfides.
Some of these
methods suffer from obvious disadvantages, such as long reaction times, limited
availability of the oxidant, toxicity of reagents and difficulty in the isolation of
the products. Consequently, the introduction of readily available, safe and stable
reagents for the oxidation of thiols to disulfide is still a necessity. In continuation
of on-going research on the synthetic application of heteropolyacids (HPAs) in
*Corresponding author. E-mail: shja47@gmail.com
doi: 10.2298/JSC100904086S
955
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956
SHOJAEI, REZVANI and HERAVI
17–23
organic synthesis,
the applicability of various mixed HPAs for the efficient
oxidation of thiols to their homodisulfides using hydrogen peroxide as the
oxidizing reagent and under mild conditions is reported herein.
The catalytic function of heteropolyacids (HPAs) and related polyoxome-
talate compounds has attracted great attention, particularly over the last two de-
16
cades. In this context, heteropolyacids (HPAs) are promising catalysts. A com-
mon and important class of these acids and those used in the majority of catalytic
applications is the Keggin class of compounds of general formula H XM O
n
12 40
24
(X = P, Si, As, Ge, B; M = Mo, W). These solid acids are usually insoluble in
non-polar solvents but highly soluble in polar ones. They can be used in bulk or
supported forms in both homogeneous and heterogeneous systems. Furthermore,
these HPAs have several advantages, including high flexibility in modification of
the acid strength, ease of handling, environmental compatibility, non-toxicity,
25
and experimental simplicity. Keggin type polyoxoanions have been widely stu-
died as homogeneous and heterogeneous catalyst for the oxidation of organic
26
compounds.
Another catalytically important subclass of Keggin compounds are the mixed
vanadium(V)-substituted HPAs of the general formula H PV M
O
12n 40
(M =
3+n
n
= Mo and W; n = 1–6). These compounds exhibit high activity in acid–base type
catalytic reactions; hence they are used in many catalytic areas as homogeneous
and heterogeneous catalysts. The most well-known of these HPAs is
H PV Mo O .
5
2
10 40
Previously reported procedures for the synthesis of large ring disulfides from
thiols using dichlorodioxomolybdenum(VI) as a catalyst produced poor to mo-
13
derate yields. However, a very efficient and simple method for the oxidative
coupling of thiols into the disulfides using hydrogen peroxide as an oxidizing
reagent catalyzed by mixed vanadium(V)-substituted HPAs under mild condi-
tions is now reported.
EXPERIMENTAL
All reagents and solvents used in this work are available commercially and were used as
received, unless otherwise indicated. Previously reported methods were used to purify the
thiols.24 Preparation of the H5PV2Mo10O40 catalyst and other mixed heteropolyacids and salts
were based on a literature procedure with the following modifications.25 The acids of
[NaP5W30O110]
14-, [P2W18O62]6- and [P2Mo18O62]6- were prepared according to published
methods and were identified by infrared spectroscopy.26–28 All chemicals were purchased
1
from Merck and used without purification. The H-NMR spectra were recorded on a FT-NMR
Bruker 100 MHz Aspect 3000 with tetramethylsilane as an internal standard and CDCl3 as the
solvent. The IR spectra were recorded on a Buck 500 Scientific Spectrometer in KBr pellets.
25
Preparation of H5PV2Mo10O4
Sodium metavanadate (12.2 g, 100 mmol) was dissolved by boiling in 50 mL of water
and then mixed with Na2HPO4 (3.55 g, 25 mmol in 50 mL of water). After cooling the solu-
tion, concentrated sulfuric acid (5 mL, 17 M, 85 mmol) was added, whereby the solution
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H5PV2Mo10O40 AS CATALYST FOR OXIDATION OF THIOLS
957
developed a red color. Na2MoO4·2H2O (60.5 g, 250 mmol) dissolved in 100 mL of water was
added to the red solution under vigorous stirring, followed by the slow addition of concen-
trated sulfuric acid (42 mL, 17 M, 714 mmol). The hot solution was allowed to cool to room
temperature. The 10-molybdo-2-vanadophosphoric acid was then extracted with 500 mL of
diethyl ether. Air was passed through the heteropoly etherate (bottom layer) to free it of ether.
The solid residue was dissolved in water, concentrated to first crystal formation, as already
described, and then allowed to crystallize further. The large red crystals that formed were
filtered, washed with water and air-dried.
Preparation of H4PVMo11O40
Na2HPO4 (3.55 g, 25 mmol) was dissolved in 50 mL of water and mixed with sodium
metavanadate (3.05 g, 25 mmol) that had been dissolved by boiling in 50 mL of water. The
mixture was cooled and acidified to a red color with concentrated sulfuric acid (2.5 mL, 17 M,
42.5 mmol). To this mixture was added a solution of Na2MoO4⋅2H2O (66.5 g, 274.8 mmol)
dissolved in 100 mL of water. Finally, 42.5 mL of concentrated sulfuric acid was added slow-
ly with vigorous stirring of the solution. With this addition, the dark red color changed to a
lighter red. After cooling the aqueous solution, the heteropoly acid was extracted with 200 mL
of diethyl ether. In this extraction, the heteropoly etherate was present as a middle layer; the
bottom layer (water) was yellow and probably contained vanadyl species. After separation, a
stream of air was passed through the heteropoly etherate layer to free it of ether. The orange
solid that remained was dissolved in 50 mL of water, concentrated to the first appearance of
crystals in a vacuum desiccator over concentrated sulfuric acid, and then allowed to crystallize
further. The orange crystals that formed were filtered, washed with water and air-dried.
General procedure for the oxidation of thiols to disulfides
H5PV2Mo10O40 (0.5 g, 0.2 mmol) was dissolved in a mixture of 25 mL of ethanol and 5 mL
of H2O. The substrate thiol (4 mmol) and 1.5 mL (13 mmol) of 30 % H2O2 were added to the
solution. The reaction mixture was stirred at room temperature until thin layer chromato-
graphy, TLC, indicated the reaction was complete. After completion of the reaction, the solid
product was filtered off and recrystallized, while in the case of liquid disulfides, after com-
pletion of the reaction, the excess H2O2 was destroyed by 2 mL of 3×10-3 M sodium sulfite.
The mixture was treated with dichloromethane (2×25 mL). The organic layer was dried by an-
hydrous MgSO4 and then concentrated to obtain the required product. General reaction for the
oxidation of thiols catalyzed by H5PV2Mo10O40 is shown in Scheme 1.
H5PV2Mo10O40
2RSH
RSSR
H2O2, Solvent
Scheme 1. General reaction for the oxidation of thiols catalyzed by H5PV2Mo10O40.
Recycling of the catalyst
In order to ascertain whether the catalyst had succumbed to poisoning and lost its
catalytic activity during the reaction, the reusability of the catalyst was investigated. For this
purpose, after completion of the model reaction (Scheme 1), dichloromethane was added to
the solid reaction mixture. All the products were soluble in dichloromethane but the catalyst
was not. Thus, the catalyst was separated by simple filtration, washed with dichloromethane
and dried at 90 °C for 1 h, and then reused in further reaction with the same substrate.
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958
SHOJAEI, REZVANI and HERAVI
RESULTS AND DISCUSSION
Effect of the solvent
4-Chlorothiophenol was taken as a model compound and the reaction con-
ditions were optimized by varying the solvent, Table I. In EtOH + H O, the
2
reaction was completed within 2 h. The other solvents provided moderate yields
with longer reaction times, except CH CN and CH NO , in which the yields
3
3
2
were very low even after 5 h (14 %, entry 12 and 18 %, entry 11, respectively).
Notably, in the absence of catalyst, very low yields were obtained (25 %, entry 2
and 21 %, entry 7).
TABLE I. Results of the oxidation of 4-chlorothiophenol using H2O2 as the oxidant and
H5PV2Mo10O40 as the catalyst in different solvents
Entry Amount of H5PV2Mo10O40, g
Solvent
Time, h Temperature, °C Yield, %a
1
2
0.5
None
1.0
0.5
0.5
0.5
None
0.5
0.5
0.5
EtOH + H2O
2
5
2
2
2
3
5
4
2
4
5
5
2
2
25
50
25
25
25
25
50
25
25
50
50
50
50
50
97
25
96
97
97
90
21
73
68
45
18
14
74
63
3
4b
5c
6
MeOH + H2O
7
8
9
10
11
12
13
EtOH
MeOH
H2O
CH3NO2
CH3CN
CH2Cl2
DMF
0.5
0.5
0.5
0.5
14
a
b
Isolated yield based on the weight of the pure product obtained; the reaction was performed with the first
c
recycled catalyst; the reaction was performed with the second recycled catalyst
Effect of the catalyst structure
The effect of the structure of the catalyst on the oxidation of 4-chlorothio-
phenol, as a model compound, in EtOH + H O is presented in Table II. The
2
amount of each catalyst was constant throughout the series. In the Keggin-type
polyoxometalates series, H PV Mo O showed the highest catalytic activity.
5
2
10 40
In general, the heteropoly salt type catalysts were less efficient than the hetero-
polyacids. The Keggin-type polyoxometalates led to more effective reactions in
comparison with the Wells–Dawson type polyoxometalates. However,
H P Mo O was more effective than H P W O in the oxidation of thiols,
6 2
18 62
6 2 18 62
possibly due to the difference in tungsten and molybdenum reduction potentials.
However, the results indicated that the highest yield of disulfide was obtained
with H PV Mo O as catalyst (Table II). This behavior was found to be quite
5
2
10 40
general. The high activity of H PV Mo O in comparison of other HPAs (Tab-
5
2
10 40
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H5PV2Mo10O40 AS CATALYST FOR OXIDATION OF THIOLS
959
+
5+
le II) confirmed that in addition to H , V probably play a catalytic role in the
reaction.
TABLE II. Oxidation of 4-chlorothiophenol by H2O2 in the presence of a variety of hetero-
polyacids
Entry
Catalyst
Time, h
Temperature, °C
Yield, %a
1
2
3
4
5
6
7
8
H5PV2Mo10O40
H4PVMo11O40
H6PV3Mo9O40
H3PMo12O40
H3PW12O40
H4SiW12O40
2
2
2
4
4
4
5
5
5
5
4
4
4
25
25
25
45
52
50
70
80
90
120
90
90
90
97
95
96
92
90
89
75
68
63
56
79
72
28
H14[NaP5W30O110
Na3PMo12O40
Na3PW12O40
(NH4)3H[PMo11VO40]
H6P2Mo18O62
H6P2W18O62
None
]
9
10
11
12
13
a
Isolated product
Effect of the substituent
The effects of various substituents on a range of aromatic thiols on the oxi-
dation yield in EtOH + H O were examined using H PV Mo O as the cata-
2
5
2
10 40
lyst. The results are given in Table III. Halogens were chosen as electron-with-
drawing groups (entries 3–5), while methyl, phenolic hydroxyl and methylthio
groups (entries 1, 6 and 7, respectively) were chosen as electron-donating substi-
tuents. One heteroaromatic thiol, i.e., pyridine-2-thiol, was successfully oxidized
in good yield (entry 9) as well as benzylthiol (entry 8) as a benzylic aliphatic re-
presentative. The yields were generally very good (>75 %) to excellent (>90 %)
with no obvious relationship between the aromatic substituent and yield (com-
pare entries 4 with 5 and 2 with 10). A highlight of the method is the ease by
which the product may be isolated via simple filtration followed by removal of
the solvent.
TABLE III. Oxidation of thiols with different substituents by H2O2 with H5PV2Mo10O40 as
catalyst
Time Yielda,b
M.p., °C
Entry
Thiol
Disulfide
Found Lit.12,14,29
h
2
%
1
97 43–44 44–45
H3C
S
S
CH3
CH3
SH
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960
SHOJAEI, REZVANI and HERAVI
TABLE III. Continued
Time Yielda,b
M.p., °C
Entry
Thiol
Disulfide
Found Lit.12,14,29
h
%
SH
2
S
S
2
96 60–61
61
3
4
2
2
95 90–92 91–93
97 72–73 70–71
SH
Br
Cl
Br
Cl
S
S
S
Br
Cl
SH
S
S
5
6
3
3
79 49–51
–
–
SH
F
S
F
F
OH
SH
OH
HO
89
Li-
quid29
S
S
S
S
7
8
3
3
82 40–43 40–43
78 69–71 69–70
CH3S
SCH3
CH3S
SH
CH2SH
SH
CH2S
SCH2
9
3
4
77 55–56 55–57
S
S
N
N
N
SH
S
S
10
78
144– 142–145
–146
a
b
Isolated yield based on the weight of the pure product obtained; The products were identified by comparison of their physical
and spectroscopic properties with the respective authentic compound
Reusability of the catalyst
After recovery of the catalyst, it was repeatedly used for further oxidation
reactions. Even after five consequent employments, the catalytic activity of
H PV Mo O was almost the same as that of fresh catalyst. The results are
5
2
10 40
summarized in Table IV. The IR spectra of the resulting solids indicated that the
catalyst can be recovered without structural degradation.
Structures of the obtained disulfides
1
The structures and purity of the obtained disulfides were verified by H- and
C-NMR spectroscopy and the obtained data are given in Table V, as well as by
13
their melting points that are given in Table III.
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H5PV2Mo10O40 AS CATALYST FOR OXIDATION OF THIOLS
961
TABLE IV. Reuse of the catalyst for oxidation of chlorothiophenol (see Table III, entry 4, for
the reaction conditions)
Entry
Isolated yield, %
1
2
3
4
5
96
94
95
96
92
TABLE V. 1H-NMR and 13C-NMR spectral data of the products
1H-NMR
13 C-NMR,
δ / ppm
Entry
Product
δ / ppm
1
2.42 (6H, s, –CH3),
7.07–7.14 (4H, m, Ar), (C4, C4′), 130 (C3,
7.29–7.33 (4H, m, Ar)
135.8 (C1, C1′), 134
H3C
S
S
CH3
C5, C3′, C5′), 127
(C2, C6, C2′, C6′),
21 (C7, C7′)
2
3
4
5
6
S
S
7.11–7.15 (2H, m, Ar), 136 (C1, C1′), 129
7.29–7.33 (4H, m, Ar), (C3, C5, C3′, C5′),
7.44–7.48 (4H, m, Ar) 126.4 (C4, C4′), 126
(C2, C6, C2′, C6′)
7.11–7.16 (4H, m, Ar), 135.8 (C1, C1′), 134
7.34–7.39 (4H, m, Ar) (C4, C4′), 130 (C3,
C5, C3′, C5′), 127
Br
S
S
S
S
Br
(C2, C6, C2′, C6′)
6.57–6.61 (4H, m, Ar), 135.3 (C4, C4′), 130
7.21–7.25 (4H, m, Ar)
Cl
Cl
(C2, C6, C2′, C6′),
127 (C3, C5, C3′,
C5′), 120 (C1, C1′)
6.37–6.44 (4H, m, Ar), 154 (C4, C4′), 132
7.33–7.42 (4H, m, Ar) (C1, C1′), 129 (C2,
C6, C2′, C6′), 115
F
S
S
F
(C3, C5, C3′, C5′)
155.8 (C2, C2′), 131
OH
HO
4.50 (2H, s, –OH),
6.72–6.76 (2H, d, Ar), (C4, C4′), 127 (C6,
6.83–6.88 (2H, t, Ar), C6′), 126 (C1, C1′),
7.17–7.22 (2H, d, Ar)
2.40 (6H, s, –SCH3),
6.67–6.73 (4H, m, Ar), (C1, C1′), 130 (C3,
7.19–7.23 (4H, m, Ar)
S
S
120 (C5, C5′), 113
(C3, C3′)
136 (C4, C4′), 134
7
CH3S
SCH3
S
S
C5, C3′, C5′), 129
(C2, C6, C2′, C6′),
16.4 (C7, C7′)
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962
SHOJAEI, REZVANI and HERAVI
TABLE V. Continued
1H-NMR
δ / ppm
13 C-NMR,
δ / ppm
Entry
Product
8
3.53 (4H, s, –SCH2),
137 (C1, C1′), 129
7.19–7.22 (2H, m, Ar), (C2, C2′), 128 (C3,
CH2S
SCH2
7.31–7.35 (4H, m, Ar) C5, C3′, C5′), 127.4
7.40–7.45 (4H, m, Ar)
(C4, C4′), 42.6
(C7, C7′)
9
6.89–6.92 (4H, m, Py), 158 (C2, C2′), 147
7.39–7.41 (2H, m, Py), (C6, C6′), 135 (C4,
8.21–8.25 (2H, m, Py) C4′), 122(C5, C5′),
118(C3, C3′)
7.40 (2H, m, Ar),
7.45 (2H, m, Ar),
7.62 (2H, m, Ar), 7.70 C3′), 134 (C1, C1′),
(2H, m, Ar), 7.74 (2H, 129.58 (C4, C4′), 128
m, Ar), 7.79 (2H, m,
S
S
S
N
N
S
10
138 (C10, C10′), 137
(C8, C8′), 133 (C3,
(C5, C5′), 127 (C9,
Ar), 7.96 (2H, m, Ar) C9′), 126 (C7, C7′),
125(C2, C2′),
122(C6, C6′)
CONCLUSIONS
The H PV Mo O heteropolyacid was the most successful heteropolyacid
5
2
10 40
in the oxidation of the thiols. Although it is difficult to offer an explanation for
the different activity between these HPAs, certainly there is a complex rela-
tionship between the activity and structure of the polyanion. By changing the
constituent elements of the polyanion (both hetero and addenda atoms), the acid
strength of HPAs as well as their catalytic activity are able to vary over a wide
18
range.
For the first time, using an inexpensive and easily prepared
H V PMo O solid catalyst, the oxidation of thiols with electron-withdrawing
5
2
10 40
and electron-donating groups to the corresponding disulfides was studied. The
results showed that the catalyst type is important as well as solvent and tempera-
ture, but that the reaction withstands a range of substituents. Furthermore, the
reusability of the catalysts was confirmed.
И З В О Д
H5PV2Mo10O40 КАО УСПЕШАН КАТАЛИЗАТОР ЗА ОКСИДАЦИЈУ ТИОЛА ДО
ОДГОВАРАЈУЋИХ ДИСУЛФИДА ПОМОЋУ ВОДОНИК-ПЕРОКСИДА КАО
ОКСИДАЦИОНОГ СРЕДСТВА
ABDOLLAH FALLAH SHOJAEI1, MOHAMAD ALI REZVANI1 и MAJID HERAVI2
1Department of Chemistry, Faculty of Science, University of Guilan, Rasht 419961-3769 и
2Department of Chemistry, School of Science, Alzzahra University, Vanak, Tehran, Iran
H5PV2Mo10O40 катализује оксидацију тиола до одговарајућих дисулфида помоћу водо-
ник-пероксида као оксидационог средства, под благим реакционим условима. Примењени
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H5PV2Mo10O40 AS CATALYST FOR OXIDATION OF THIOLS
963
реакциони услови омогућавају ефикасну и погодну синтезу симетричних дисулфида. У овом
раду извршено је поређење са Keggin-овим и Dawson-овим полиоксометалатима у погледу
релативне стабилности, јачине и киселости.
(Примљено 4. септембра 2010, ревидирано 10. јануара 2011)
REFERENCES
1. D. C. Jocelyn, Biochemistry of Thiol Group, Academic Press, New York, 1992
2. M. C. Carreno, Chem. Rev. 95 (1995) 1717
3. N. Iranpoor, B. Zeynizadeh, Synthesis 1 (1999) 49
4. J. C. Bardwell, J. Mol. Microbiol. 14 (1994) 199
5. A. Ogawa, Y. Nishiyama, N. Kambe, S. Murai, N. Sonoda, Tetrahedron Lett. 28 (1987)
3271
6. K. Ramadas, N. Srinivasan, Synth. Commun. 25 (1995) 227
7. V. Srivastav, R. Gupta, R. R. Gupta, Indian J. Chem. 39B (2000) 223
8. T. L. Ho, C. M. Wong, J. Org. Chem. 39 (1974) 562
9. C. C. Silveira, S. R. Mendes, Tetrahedron Lett. 48 (2007) 7469
10. J. Drabowicz, M. Mikolajczyk, Synthesis 1 (1980) 32
11. X. Wu, R. D. Rieke, L. Zhu, Synth. Commun. 26(1996)191.
12. A. McKillop, D. Kuyuncu, Tetrahedron Lett. 31 (1990) 5007
13. R. Sanz, R. Aguado, M. R. Pedrosa, F. J. Arnaiz, Synthesis 7 (2002) 856
14. M. M. Khodaei, I. Mohammadpoor-Baltork, K. Nikoofar, Bull. Korean Chem. Soc. 24
(2003) 885
15. V. Kesavan, D. Bonnet-Delpon, J.-P. Begue, Synthesis 2 (2000) 223
16. C. A. S. Regino, D. E. Richardson, Inorg. Chim. Acta 360 (2007) 3971
17. M. M. Heravi, K. Bakhtiari, F. F. Bamoharram, Catal. Commun. 7 (2006) 373
18. F. F. Bamoharram, M. M. Heravi, M. Roshani, A. Gharib, M. Jahangir, J. Mol. Catal. A:
Chem. 252 (2006) 90
19. M. M. Heravi, R. Motamedi, N. Seifi, F. F. Bamoharram, J. Mol. Catal. A: Chem. 249
(2006) 1
20. F. F. Bamoharram, M. M. Heravi, M. Roshani, M. Jahangir, A. Gharib, J. Appl. Catal. A:
Gen. 302 (2006) 42
21. F. F. Bamoharram, M. M. Heravi, J. Mol. Catal. A: Chem. 255 (2006) 193
22. M. M. Heravi, S. Taheri, K. Bakhtiari, Catal. Commun., in press
23. M. M. Heravi, F. K. Behbahani, J. Mol. Catal. A: Chem. 253 (2006) 16
24. I. V. Kozhevnikov, Chem. Rev. 98 (1998) 171
25. Y. Izumi, K. Urabe, M. Onaka, Zeolites Clay and Heteropolyacid in Organic Reactions,
Kodansha, Tokyo, 1992, p. 99
26. I. V. Kozhevnikov, Chem. Rev. 98 (1998) 171
27. I. V. Kozhevnikov, Catalysis for Fine Chemical Synthesis, Catalysis by Polyoxometa-
lates, Wiley, New York, 2002, p. 2
28. X. López, PhD Thesis, Universitat Rovira i Virgili, Tarragona, Spain, 2003
29. H. Golchoubian, F. Hosseinpoor, Catal. Commun. 8 (2007) 697.
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