New heteropolyacids as catalysts for the selective oxidation of sulfides to sulfoxides with hydrogen peroxide

Article abstract of DOI:10.1055/s-2004-837195

Pyridinium salts of Keggin-type molybdovanadophosphates proved to be highly active catalysts for the hydrogen peroxide oxidation of sulfides to the corresponding sulfoxide and sulfone derivatives. High conversion and high selectivity for sulfoxide were observed.

Full text of DOI:10.1055/s-2004-837195

LETTER
75
New Heteropolyacids as Catalysts for the Selective Oxidation of Sulfides to
Sulfoxides with Hydrogen Peroxide
ulfoxides P. Romanelli,a,b Patricia G. Vázquez, Pietro Tundo*
a
c,d
H
G
eteropolyacids as
u
Catalysts fo
s
r
the Sele
t
ctive
O
a
xidation of
vS ulfides to So
a
b
c
d
Centro de Investigación y Desarrollo en Ciencias Aplicadas ‘Dr. Jorge J. Ronco’ (CINDECA) Facultad de Ciencias Exactas, Universidad
Nacional de La Plata – CONICET, Calle 47 Nº 257 (B1900AJK) La Plata, Argentina
Laboratorio de Estudio de Compuestos Orgánicos (LADECOR) Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Calles
47 y 115 (B1900AJL) La Plata, Argentina
Dipartamento di Scienze Ambientali, Universitá Ca’Foscari, Venice, Italy
E-mail: tundop@unive.it
Consorzio Interuniversitario Nazionale La Chimica per l’Ambiente, Venice, Italy
Received 16 September 2004
formed by a central tetrahedron XO , surrounded by 12
4
Abstract: Pyridinium salts of Keggin-type molybdovanadophos-
phates proved to be highly active catalysts for the hydrogen perox-
ide oxidation of sulfides to the corresponding sulfoxide and sulfone
octahedra MO . They operate either as multi-electron ox-
6
idants or as strong acids, with an acid strength higher than
17
derivatives. High conversion and high selectivity for sulfoxide were that of the classical acids. Currently, there is a consider-
observed.
able interest in exploiting both the structure of catalyst
precursor and the multicenter actives sites to facilitate
catalysis by HPA having Keggin structure.
Key words: sulfides, sulfoxides, oxidation, hydrogen peroxide,
Keggin-type molybdovanadophosphate pyridinium salts
The catalytic activity of some pyridinium salts of Keggin-
type molybdovanadophosphates in the selective oxidation
of sulfides to sulfoxides with hydrogen peroxide is here
The selective oxidation of sulfides to sulfoxides is an im-
portant reaction both in industrial processes and in basic
research; it has been a challenge for many years, owing to
reported. The pyridinium salts of the acids H PMo VO ,
4
11
40
1
H PMo V O , H PMo V O , obtained by vanadium
the importance of sulfoxides in organic synthesis. Since
5
10
2
40
9
6
6
40
substitution in the structure of H PMo O , were synthe-
the first report on synthesis of sulfoxides published by
Maercker at 1865, a number of stoichiometric and catalyt-
3
12 40
sized and characterized by FT-IR; the change in their acid
properties were determined by titration with n-butyl-
amine.1
2
ic methods has been developed. Various oxidizing re-
8,19
agents used for this purpose include, among others: nitric
acid, dinitrogen tetroxide, chromic acid, manganese diox-
ide, ozone, peracids, selenium dioxide, sodium periodate,
Catalytic oxidation of methyl phenyl sulfide (Scheme 1)
was selected as a test reaction: the sulfide (0.7 mmol),
3
hypervalent iodine reagents, sodium perborate, halogens,
3
5% aqueous H O (2.0 or 20.0 molar equiv) and 0.01 mo-
2 2
4
tetrabutylammonium peroxydisulfate, binuclear manga-
lar equivalents of catalyst, were reacted in acetonitrile.
Table 1 shows the results obtained for different catalysts
under different conditions. High selectivity for the sulfox-
ide was always obtained.
5
nese complexes–periodic acid, N-hydroxyphalimide–
6
molecular oxygen, camphorsulfonic acid–t-butyl hydro-
7
8
peroxide, Fe(NO ) –FeBr –molecular oxygen, FeBr –
3
3
3
3
9
nitric acid and (FeBr ) (DMSO) –nitric acid.
3
2
3
During the last years, useful procedures involving cataly-
sis have been reported; e.g. H WO , H PW O
O
O
O
3
5% H2O2
+
12 40
S
S
+
S
2
4
3
Ph
Me
1
0
Ph
Me
HPA catalyst, MeCN, r.t. Ph
Me
[
(C H ) N]Br, rhenium(V) oxophosphine complexes,
8 17 4
1
1
12
methyltrioxorhenium, Sc(OTf)3, (salen) Mn(III) and
Ti(IV) complexes,
Fe(III)Cl–imidazole. These catalysts have been specifi-
cally developed for hydrogen peroxide, since this reagent
has low cost, safe in storage and operation, and is environ-
mentally friendly.
Scheme 1
1
3,14
15
tellurium dioxide and TPP-
1
6
When PMo VO H Py is used as a catalyst, the reaction
11
40
3
gave rise to highest selectivity. After 30 minutes of reac-
tion at room temperature, 98% of methyl phenyl sulfide
conversion was obtained, with a H O /substrate ratio of
2
2
In this context, heteropolyacid compounds (HPA) with
the Keggin structure are polynuclear complexes principal-
ly constituted by molybdenum, tungsten or vanadium as
polyatoms (M) and phosphorus, silicon or germanium as
central atoms or heteroatoms (X). The Keggin structure is
2
.0, and 98% of selectivity to methyl phenyl sulfoxide. As
a comparison in the same conditions, after 15 minutes of
reaction and with PMo O Py as a catalyst, methyl phe-
nyl sulfide was 98% oxidized with 90% of selectivity to
methyl phenyl sulfoxide (10% sulfone). PMo V O Py
1
2
40
3
10
2
40
5
and PMo V O Py catalysts, after 2 hours of reaction
10
6
40
9
and a large excess of H O (H O /substrate ratio = 20),
SYNLETT 2005, No. 1, pp 0075–0078
0
5.
0
1.
2
0
0
5
2
2
2
2
Advanced online publication: 07.12.2004
DOI: 10.1055/s-2004-837195; Art ID: D28004ST
lead to only methyl phenyl sulfone (100%). This result
may be explained with the help of previous theoretical
©
Georg Thieme Verlag Stuttgart · New York

7
6
G. P. Romanelli et al.
LETTER
a
Table 1 Oxidation with 35% Aqueous H O of Methyl Phenyl Sulfide at Room Temperature in Acetonitrile Catalyzed by Heteropolyacids
2
2
Catalyst
Time (h)
H O (mol equiv)
Conversion (%)
Selectivity (%)
Sulfoxide
Selectivity (%)
Sulfone
2
2
No Catalyst
No Catalyst
PMo O H
16
16
0.25
2
2
20
2
0
0
–
–
–
–
20
100
97
80
8
0
12
40
3
3
3
3
PMo O H
2
95
3
12
40
PMo O H
2
20
20
2
100
100
98
20
92
10
19
100
1
12
40
PMo O H
16
0.25
0.25
2
12
40
PMo O Py
90
81
–
12
40
3
3
3
PMo O Py
2
99
12
40
PMo O Py
20
2
100
82
12
40
PMo VO H Py
0.25
0.5
2
99
98
60
–
11
40
3
PMo VO H Py
2
98
2
11
40
3
PMo VO H Py
20
20
2
100
100
2
40
100
–
11
40
3
PMo VO H Py
16
0.25
2
11
40
3
PMo VO HPy
100
95
73
–
11
40
3
3
3
3
PMo VO HPy
2
93
5
11
40
PMo VO HPy
2
20
20
2
99
27
100
–
11
40
PMo VO HPy
16
0.25
2
100
3
11
40
PMo VO Py
100
99.5
76
5
11
40
4
PMo VO P
40 y4
2
72
0.5
24
95
6
11
PMo VO Py
2
20
20
2
96
11
40
4
4
PMo VO Py
16
0.25
2
100
75
11
40
PMo V O Py
94
71
–
10
2
40
5
5
5
9
9
9
PMo V O Py
2
100
100
78
29
100
8
10
2
40
PMo V O Py
2
20
2
10
2
40
PMo V O Py
0.25
2
92
71
–
10
6
40
PMo V O Py
2
100
100
29
100
10
6
40
PMo V O Py
2
20
10
6
40
a
See experimentals for reaction conditions.
1
9
and computational studies: the vanadium atom, present oxidation protocol (Scheme 2 and Table 2), by using
as the V-OH group in classical Keggin-type catalysts, PMo VO H Py as the catalyst. All the reactions were
1
1
40
3
lowers its average oxidation state when present as the complete within a very short time and the sulfoxides were
(
–)
(+)
VO PyH
species. For PMo VO HPy
and obtained in excellent yields, as practically unique oxida-
1
1
40
3
PMo VO Py catalysts, the reaction requires more time tion products. Importantly, benzaldehyde is not affected
1
1
40
4
for completion as compared with PMo VO H Py under these reaction conditions (Table 2, note).
1
1
40
3
(
Table 1), due to their partial insolubility in the reaction
medium.
2 2
35% H O
RSR′
RSOR′
PMo11VO40H3Py catalyst, MeCN, r.t.
In order to explore the general applicability of the method
for the selective oxidation of sulfides to sulfoxides, vari-
ous functionalized sulfides were reacted according to this
Scheme 2
Synlett 2005, No. 1, 75–78 © Thieme Stuttgart · New York

LETTER
Heteropolyacids as Catalysts for the Selective Oxidation of Sulfides to Sulfoxides
77
a,b
Table 2 Oxidation with 35% Aqueous H O of Sulfides to Sulfoxides in Acetonitrile at Room Temperature Catalyzed by PMo VO H Py
2
2
11
40
3
Entry
Substrate
Product
Time (h)
0.5
Yield (%)
98
1
S
O
H3C
H3C
CH3
S
H3C
CH3
2
3
4
0.5
0.5
0.5
97
99
96c
S
CH3
S
O
S
H3C
H3C
CH3
H3C
CH3
O
S
CH3
S
O
S
CH3
CH3
5
2
95
S
O
S
6
7
0.5
1
94
92
O
S
S
S
O
S
8
0.5
0.5
91
89
S
O
S
CH3
CH3
H3CO
H3CO
9
S
O
S
a
See experimentals for reaction conditions.
Double bonds and alcohols are not oxidized if the reaction is carried out at r.t.: styrene, 1-undecene, phenyl allyl ether, benzylic alcohol, 1-
b
decanol and 2-decanol were independently reacted under the same conditions of Table 2: after 20 h, 20 h, 20 h, 20 h, 3 h ,and 3 h, respectively,
no trace of oxidation products was detected.
c
Benzaldehyde remains unaffected either if present in this reaction mixture or alone in an independent experiment: 0.7 mmol of PhCHO were
stirred for 5 h under the same conditions: no PhCOOH was observed. After 20 h, but operating at 70 °C, 10% PhCOOH was obtained.
In conclusion, we have found a new, convenient and se- Catalyst Preparation
The heteropoly compounds were prepared by a hydrothermal
synthesis method.
lective procedure for oxidizing sulfides to sulfoxides, with
1
8
3
5% aqueous H O and a catalytic amount of pyridinium
2 2
salt of heteropolyacids. The yields were excellent. Re- PMo11VO40
H : a stoichiometric mixture of 0.98 g of phosphoric
4
acid, 0.91 g of vanadium pentoxide and 14.4 g of molybdenum tri-
oxide was suspended in distilled water. The mixture was stirred for
a given time at 80–90 °C. After cooling to r.t. and removal of insol-
uble molybdates and vanadates, the heteropolyacid solution was
evaporated and dried: orange crystals of PMo VO H were ob-
agents and catalysts are cheap and easily available. The
oxidation is carried out at room temperature and requires
a short reaction time. Further studies on the oxidations of
other organic substrates and other polyoxometallates are
currently in progress.
1
1
40
4
tained.
Mixed proton-pyridinium and pyridinium counter cations (contain-
ing x pyridinium cations per heteropolyanion, 1<x<9) were pre-
pared from H PMo O , H PMo VO , H PMo V O and
3
12 40
4
11
40
5
10
2
40
H PMo V O . For H PMo VO acid, the salts are designated as
9
6
6
40
4
11
40
PMo VO H4-xPyx and PMo VO Py , respectively. Molybdo-
1
1
40
11
40
4
Synlett 2005, No. 1, 75–78 © Thieme Stuttgart · New York

7
8
G. P. Romanelli et al.
LETTER
vanadophosphate pyridinium salts were prepared by addition of the References
corresponding molar equivalent of pyridine to the aqueous solution
of heteropolyacid. The pyridine was added slowly to the solution of
heteropolyacid and the mixture was stirred for a few minutes at
(
(
1) Trost, B. M. Chem. Rev. 1978, 78, 363.
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8
0 °C. The resulting precipitates were filtered and dried at r.t.
(
(
(
(
(
38, 6525; and references cited therein.
General Procedure for the Oxidation of Sulfides to Sulfoxides
Table 1: The oxidation of methyl phenyl sulfide was typically car-
ried out by stirring a solution of 0.7 mmol of the substrate and 1%
molar equiv of the catalyst in 5 mL of MeCN, at 20 °C, with 35%
aq H O . During the reaction, about 20 mL of the reaction mixture
4) Tajbakhsh, M.; Hosseinzadeh, R.; Shakoori, A. Tetrahedron
Lett. 2004, 45, 1889; and references cited therein.
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7
055.
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9, 9059.
2
2
was taken and diluted in a mixture of H O–CH Cl (2 mL); the
2
2
2
3
CH Cl layer was shaken with anhyd Na SO and filtered. CG/MS
2
2
2
4
7) Bonadies, F.; De Angelis, F.; Locati, L.; Scettri, A.
analyses were performed on HP 5971 mass detector coupled to a HP
gas chromatograph fitted with a 30 m × 0.25 mm DB5 capillary
column. The percentages of each compound in the reaction mixture
were directly estimated from the corresponding chromatographic
peak areas.
Tetrahedron Lett. 1996, 37, 7129.
(
(
8) Martin, S.; Rossi, L. Tetrahedron Lett. 2001, 41, 7147.
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(
(
10) Xu, W.; Li, Y.; Zhang, Q.; Zhu, H. Synthesis 2004, 227.
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J. Tetrahedron Lett. 1998, 39, 5655.
Table 2: To a stirred solution of the sulfide (0.7 mmol) and
PMo VO H Py (0.01 mmol), in MeCN (5 mL), 2.0 mmol of 35%
11
40
3
aq H O were added at r.t. The solvent was evaporated and the sub-
2
2
(
(
(
12) Matteucci, M.; Bhalay, G.; Bradley, M. Org. Lett. 2003, 5,
strate was extracted with CH Cl and dried with anhyd Na SO ; fil-
2
2
2
4
2
35.
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992, 33, 7111.
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tration and evaporation afforded the corresponding pure crude
sulfoxides. The solid sulfoxides were purified by re-crystallization
to afford the pure products.
1
All the compounds were identified via comparison with authentic
samples and mass spectra analysis and their purity was established
by GLC.
(15) Kim, K. S.; Hwang, H.; Cheong, C.; Hahn, C. Tetrahedron
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(
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Acknowledgment
205.
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The authors thank to the Consorzio Interuniversitario Nazionale ‘La
Chimica per l’Ambiente’ (INCA), Consejo Nacional de Investiga-
ciones Científicas y Técnicas (CONICET) and University of La
Plata for the financial support.
Synlett 2005, No. 1, 75–78 © Thieme Stuttgart · New York