Catalytic Conversion of Sulfides to Sulfoxides by the [PZnMo 2W9O39]5- Polyoxometalate

Article abstract of DOI:10.1246/cl.2003.1066

Keggin type polyoxometalates can be used as catalysts for the activation of 30% aqueous hydrogen peroxide for the selective transformation of various organic substrates. In this paper, results are presented for the oxidation of sulfides to sulfoxides in high to excellent yields using 30% aqueous H ₂O₂ as oxidant and [PZnMo₂W₉O ₃₉]^5- as catalyst in acetonitrile.

Full text of DOI:10.1246/cl.2003.1066

1066
Chemistry Letters Vol.32, No.11 (2003)
Catalytic Conversion of Sulfides to Sulfoxides by The [PZnMo2W9O39]5À Polyoxometalate1
Bahram Yadollahiꢀ
Institute for Advanced Studies in Basic Sciences (IASBS), Gava Zang, Zanjan 45138-79368, Iran
(Received August 7, 2003; CL-030734)
Keggin type polyoxometalates can be used as catalysts for
the activation of 30% aqueous hydrogen peroxide for the selec-
tive transformation of various organic substrates. In this paper,
results are presented for the oxidation of sulfides to sulfoxides
in high to excellent yields using 30% aqueous H2O2 as oxidant
and [PZnMo2W9O39]5ꢁ as catalyst in acetonitrile.
O
S
30% H2O2, Catalyst
CH3CN, rt
RSR'
R
R'
Scheme 1.
silicate (TS-1).13 However, many of these procedures require ei-
ther chlorohydrocarbon solvents that affect human health and the
environment, or anhydrous H2O2 in ethanol. We will describe
below our results on the use of the zinc-substituted polyoxome-
talate, [PZnMo2W9O39]5ꢁ, as catalyst for the oxidation of sul-
fides to sulfoxides using aqueous hydrogen peroxide (30%)as
oxidant in acetonitrile (Scheme 1).
In this paper, we wish to report our studies in the oxidation
of aromatic and aliphatic sulfides using H2O2 (30%)as oxidant
in the presence of catalytic amount of tetrabutylammonium salts
of transition-metal-substituted polyoxometalates ([P(Mnþ)-
Mo2W9O39]7ꢁ, M = Mn2þ, Co2þ, Zn2þ, Fe3þ). These catalysts
are readily available and easily prepared from ꢀ-K7PMo2W9O39
by a slight modification of the previously reported method.9 The
formation of Keggin structure and composition of the com-
pounds were confirmed by IR, NMR and elemental analysis.
Thermal gravimetric analysis was performed on all of the cata-
lysts (between 40 and 240 ꢂC). The results indicated that the hy-
dration numbers were between 3 to 6.
From initial studies carried out with methyl phenyl sulfide,
we found the best reaction condition. Initially using the methyl
phenyl sulfide, 30% H2O2 and catalytic amount of various tran-
sition metal substituted polyoxometalate salts in acetonitrile, we
searched the optimum conditions of reaction.14 The reactions
were conducted at ambient temperature with magnetic stirring.
The oxidation was efficiently achieved under this condition
(Table 1, Entry 1), and the presence of the catalyst was crucial
to obtain full conversion of the sulfide. After studied the various
amount of the catalyst, we found the 0.05 equivalent from Zn(II)
substituted polyoxometalate catalyst was the best. Lower cata-
lytic activity was observed with other, substituted catalysts
(about 20–30%). We chose the CH3CN as solvent and lower cat-
alytic activities were observed with other solvents such as
CH2Cl2, CHCl3, CH3OH, n-C6H6 and C2H6O. The optimum
amount of H2O2 30% in this system was found 1.2 mmol and
when the higher amount of oxidant was employed, a significant
increase in the sulfone was observed.
For methyl phenyl sulfide, separate experiments showed that
at the time sited in Table 1, there is little reaction without cata-
lyst (below 7%). From the results as summarized in Table 1, one
must divide substrates into classes: aromatic and aliphatic sul-
fides. Catalytic reactions and substrate/oxidant ratios of 1/1.2
yield sulfoxide as major product with selectivities of 80–92%,
and a trace amount of sulfone was observed for some of sulfides
(below 5% for Table 1, Entries 2, 13, 14).
Partial oxidation reactions are extensively used in industrial
processes to functionalize hydrocarbon substrates, increasing
their reactivity so that they can be utilized as intermediates in
chemical industry. Preparation of sulfoxide derivatives by effi-
cient methods is of continuous interest in organic chemistry.2
The selective oxidation of sulfides to sulfoxides has been a chal-
lenge for many years, partly because of the importance of sulf-
oxides as intermediates in organic synthesis.3
During the last years, very useful procedures involving an
oxidant and a catalytic activator have been developed to promote
this transformation.2–4 Conventional oxidants include, among
others, NaIO4,5a KMnO4,5b KHSO5,5c NaClO,5d NaBO3,5e
CF3CO3H,5f dimethyldioxirane,5g t-C4H9O2H5h and singlet oxy-
gen6 or molecular oxygen.7 Because of the low content of effec-
tive oxygen, the formation of environmentally unfavorable co-
products and high cost, most of these reagents are not satisfacto-
ry. Aqueous hydrogen peroxide (H2O2)as terminal oxidant due
to the effective oxygen content, low cost, safety in storage and
operation, and environmentally friendly character, is an ideal ox-
idant, however, a catalyst is necessary to activate this oxidant.8
In continuation of our research on the application of polyox-
ometalates in oxidation of organic substrates,9 we describe our
efforts in developing catalytic procedures for oxidation of organ-
ic sulfides to sulfoxides using aqueous hydrogen peroxide as ox-
idant and transition metal-substituted polyoxometalates as cata-
lysts.
The polyoxometalates comprise of a class of d0 metal com-
plexes of the group VB and VIB elements (excluding Cr)and ex-
hibit both diverse and tunable domains of sizes, shapes, charge
densities, acidities, and reversible redox potentials.10 The cata-
lytic function of the polyoxometalates has attracted much atten-
tion and they are used in solution as well as in the solid state.
These compounds have awoken interest for catalytic oxidations,
as they are inherently stable to oxidation. An especially interest-
ing subclass of the polyoxometalates is Keggin type compounds
where one or perhaps more of the metal atom are substituted by
lower valent transition metals. These so called transition-metal-
substituted polyoxometalates are uniquely synthetically attrac-
tive as well as interesting as oxidation catalysts because one
may visualize these compounds as having reactive low valent
transition metal centers complexed by inorganic oxometalate li-
gands which also have high capacity for electrons.10e,10f
8
Various tungsten (W)catalyst systems were used in addi-
tion to CH3ReO3,11 Na2MoO4 + (n-C4H9)3PO,12 and titanium
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.11 (2003)1067
Table 1. Oxidation of sulfides to sulfoxides with 30% H2O2
catalyzed by [PZnMo2W9O39]5ꢁ via Scheme 1
References and Notes
1
This paper dedicated to: Prof. Mohammad Hosein Habibi and
Prof. Shahram Tangestaninejad.
Entry
Sulfide
Time/min
Yield of
Solfoxidea,b/%
2
M. Hudlicky, in ‘‘Oxidation in Organic Chemistry,’’ ACS
Monograph Ser. 186, American Chemical Society, Washing-
ton DC (1990), pp 252–261 and references cited therein.
J. Drabowski, P. Kielbasinski, and M. Mikolajczyk, in
‘‘Synthesis of Sulfoxides,’’ Wiley, New York (1994).
a)G. J. Hollingworth, in ‘‘Comprehensive Organic Function-
al Group Transformations,’’ ed. by A. R. Katritzky, O. Meth-
Cohn, C. W. Rees, and G. Pattenden, Elsevier, Oxford (1995),
Vol. 2, pp 144–156. b)In ‘‘The Synthesis of Sulphones,
Sulphoxides and Cyclic Sulphides,’’ ed. by S. Patai and Z.
Rappoport, Wiley, New York (1994). c) J. March, in
‘‘Advanced Organic Chemistry,’’ 4th ed., Wiley, New York
(1992).
S
1
2
30
20
92
80
3
4
S
CHO
S
S
S
3
4
30
180
150
240
210
30
90
90
87
90
90
92
87
88
85
88
5
S
S
5
a)R. S. Varma, R. K. Saini, and H. M. Meshram, Tetrahedron
Lett., 38, 6525 (1997). b) G. W. Gokel, H. M. Gerdes, and D.
M. Dishong, J. Org. Chem., 45, 3634 (1980). c) B. M. Trost
and D. P. Curran, Tetrahedron Lett., 22, 1287 (1981). d) J. M.
Khurana, A. K. Panda, A. Ray, and A. Gogia, Org. Prep.
Proced., 28, 234 (1996). e) A. Mckillop and J A. Tarbin, Tet-
rahedron Lett., 24, 1505 (1983). f) C. G. Venier, T. G.
Squires, Y.-Y. Chen, G. P. Hussmann, J. C. Shei, and B. F.
Smith, J. Org. Chem., 47, 3773 (1982). g) W. Adam and L.
Hadjiarapoglou, Tetrahedron Lett., 33, 469 (1992). h) G.
W. Berton, J. D. Fields, and P. J. Kropp, Tetrahedron Lett.,
36, 3825 (1995).
6
7
S
8
S
9
30
S
10
11
12
60
S
6
7
8
a)S. M. Bonesi and A. Albini, J. Org. Chem., 65, 4532
(2000). b) D. Madhavan and K. Pitchumani, Tetrahedron,
57, 8391 (2001).
a)M. M. Dell’Anna, P. Mastrorilli, and C. F. Nobile, J. Mol.
Catal., 108, 57 (1996). b) V. Khanna, G. C. Maikap, and J.
Iqbal, Tetrahedron Lett., 37, 3367 (1996).
a)Y. Ishii, H. Tanaka, and Y. Nishiyama, Chem. Lett., 1994,
1. b)K. Sato, M. Hyodo, M. Aoki, X.-Q. Zheng, and R.
Noyori, Tetrahedron, 57, 2469 (2001)and references cited
therein.
60
S
S
60
13
14
30
30
85
87
S
aAll of the products were characterized by comparison of their
physical and spectral data with those of authentic samples.
bYields refer to isolated products.
9
S. Tangestaninejad and B. Yadollahi, Chem. Lett., 1998, 512.
10 a)M. T. Pope, ‘‘Heteropoly and Isopoly Oxometalates,’’
Springer-Verlag, Berlin (1983). b) M. T. Pope and A. Muller,
Angew. Chem., Int. Ed. Engl., 30, 34 (1991). c) C. L. Hill and
C. M. Prosser-McCartha, Coord. Chem. Rev., 143, 407
(1995). d) C. L. Hill, Chem. Rev., 98, 1 (1998). e) I. V.
Kozhevnikov, Chem. Rev., 98, 171 (1998). f) N. Mizuno
and M. Misono, J. Mol. Catal., 64, 1 (1990).
11 S. Yamazaki, Bull. Chem. Soc. Jpn., 69, 2955 (1996).
12 O. Bortolini, F. Di Furia, G. Modena, and R. Seraglia, J. Org.
Chem., 50, 2688 (1985).
13 a)J. M. Fraile, J. I. Garcia, B. Lazaro, and J. A. Mayoral,
Chem. Commun., 1998, 1807. b)D. J. Robinson, L. Davies,
N. McGuire, D. F. Lee, P. McMorn, D. J. Willock, G. W.
Watson, P. C. Bulman Page, D. Bethell, and G. Hutchings,
Phys. Chem. Chem. Phys., 2, 1523 (2000).
14 Typical experimental procedure: To a stirred solution of sul-
fide (1 mmol)in acetonitrile (3 mL)in the presence of
[PZnMo2W9O39]5ꢁ (0.05 mmol), was added at room temper-
ature 1.2 mmol of 30% H2O2. The progress of reaction moni-
tored by TLC. After the time reported in the table, the sulfox-
ide were purified by flash chromatography over silica gel and
fully characterized.
In the case of phenyl methyl sulfide, full oxidation of the
starting material was obtained (yield of sulfoxide 92%). The ox-
idation of benzyl phenyl sulfide results in the formation of ben-
zyl phenyl sulfoxide in 85% yields (Table 1, Entry 11). Allyl
methyl thioether (Table 1, Entry 8)was selected for study a
molecule having two functional groups for oxidation, namely
carbon–carbon double bond and the sulfur atom. However, no
oxidation products of the carbon–carbon double bond were ob-
served with [PZnMo2W9O39]5ꢁ as catalyst. The oxidation of di-
buthyl sulfide occurs to give the sulfoxide in 87% yield. We not-
ed that, as expected, the susceptibility of sulfides to H2O2 is
highly dependent on the substitution.
In conclusion this catalytic system efficiently promotes the
oxidation of aliphatic and aromatic sulfides with 30% hydrogen
peroxide and [PZnMo2W9O39]5ꢁ as catalyst in acetonitrile with
high to excellent yields. This system is a mild, simple, clean, and
cheap method for oxidation of sulfides to sulfoxides.
I am grateful to Institute for Advanced Studies in Basic Sci-
ences (IASBS)for financial support of this work.
Published on the web (Advance View)October 20, 2003; DOI 10.1246/cl.2003.1066