VO2F(dmpz)2: A new catalyst for selective oxidation of organic sulfides to sulfoxides with H2O2

Article abstract of DOI:10.1016/j.tetlet.2012.09.067

A newly developed and structurally characterized vanadium complex [VO ₂F(dmpz)₂] (dmpz = 3, 5 dimethyl pyrazole) is reported as recyclable catalyst for the selective oxidation of organic sulfides with H ₂O₂ in C

Full text of DOI:10.1016/j.tetlet.2012.09.067

Tetrahedron Letters 53 (2012) 6512–6515
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Tetrahedron Letters
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VO2F(dmpz)2: a new catalyst for selective oxidation of organic sulfides
to sulfoxides with H2O2
Sahid Hussain a, Dhrubajyoti Talukdar b, Saitanya K. Bharadwaj c, Mihir K. Chaudhuri b,c,
⇑
a Department of Chemistry, Indian Institute of Technology Patna, Patna 800 013, India
b Tezpur University, Tezpur 784028, India
c Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 June 2012
Revised 15 September 2012
Accepted 18 September 2012
Available online 2 October 2012
A newly developed and structurally characterized vanadium complex [VO2F(dmpz)2] (dmpz = 3, 5
dimethyl pyrazole) is reported as recyclable catalyst for the selective oxidation of organic sulfides with
H2O2 in CH3CN at sub-ambient temperature. [VO2F(dmpz)2]–H2O2 system chemoselectively oxidizes
alkyl as well as aryl sulfides in the presence of oxidation prone functional groups such as C@C, –CN,
and –OH. Refractory sulfides (dibenzothiophenes) are also oxidized to sulfoxides.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Selective oxidation
Refractory sulfide
Vanadium complex
Hydrogen peroxide
Chemoselectivity
Sulfoxides and sulfones are of immense interest because of their
extensive applications as reagents in organic chemistry as well as
synthetic intermediates for the construction of various chemically
and biologically active molecules.1 For this reason the oxidation
of sulfides to sulfoxides or sulfones has been the subject of exten-
sive studies. There are many reagents available for the oxidation
of sulfides such as halogen compounds,2 nitrates,3 transition metal
oxides,4 oxygen, and hydrogen peroxide.5 Incidentally, most of
these reagents are not satisfactory for medium to large-scale
synthesis for one or the other reasons like low content of effective
oxygen, over oxidation, the formation of environmentally unfavor-
able by-products, and cost effectiveness. Generally, it is important
to stop the oxidation at the sulfoxide stage by controlling the
electrophilic character of the oxidant, but this requirement is often
hard to meet and failure results in over oxidation to sulfones. The
reported methods rarely offer the ideal combination of simplicity
of method, selective reactions, and high yields of products and often
suffer from a lack of generality and economic viability, and hence
the search for newer methods for the selective oxidation of sulfides
to sulfoxides has continued. Over the years, hydrogen peroxide has
attracted much attention because it is an ideal waste-avoiding
oxidant, cheap, and the most attractive from the environmental
viewpoint. Although it is a powerful oxidant, the reactions of
hydrogen peroxide are generally rather slow, and the challenge is
to overcome this kinetic barrier in more cost-effective and ‘green
chemical’ ways. Therefore, the development of catalysts for tuning
the oxidation potentiality of H2O2 will be quite valuable. Inciden-
tally, vanadium(V) compounds are known for activating H2O2
through the formation of mono, di, tri, and tetra peroxovanadates
and they are useful in oxidizing organics.6 Dioxovanadium(V)
complexes are studied as biomimetic synthetic models,7 since they
can be converted into oxoperoxovanadate which resembles the
active site of vanadium haloperoxidase enzymes. Taking a lead
from our earlier experience in vanadium chemistry, we elected to
develop an effective penta-coordinated hetero-ligand vanadium(V)
complex as an efficacious catalyst for the titled selective oxidation.
A penta coordinated complex is expected to facilitate in situ
coordination of H2O2 thereby activating the oxidant. This Letter
involves the preparation of VO2F(dmpz)2, complete characteriza-
tion and its catalytic efficacy for selective oxidation of organic
sulfides at sub ambient temperature (Scheme 1).
Strategically, the preparation and characterization of the cata-
lyst was the first task. To this end, we have prepared the catalyst
by reacting V2O5 with NH4HF2 followed by the addition of dmpz
O
VO2F(dmpz)2, H2O2
S
R1
R2
S
CH3CN, 0-5 0
C
R1
R2
⇑
Corresponding author. Tel.: +91 3712 267003; fax: +91 3712 267006.
E-mail address: mkc@tezu.ernet.in (M.K. Chaudhuri).
Scheme 1. Oxidation of sulfide.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.09.067

S. Hussain et al. / Tetrahedron Letters 53 (2012) 6512–6515
6513
tion condition, we carried out the oxidation of methyl phenyl sul-
fide in acetonitrile at room temperature (Table 1). It was found that
methyl phenyl sulfide was oxidized to a 3:1 mixture of methyl
phenyl sulfoxide and sulfone in the presence of 5 mol% of the cat-
alyst and 2 equiv of H2O2. We also performed the oxidation in dif-
ferent solvents (Table 1) maintaining the same conditions.
Unfortunately, over-oxidation could not be averted. The over-
oxidation could not be overcome even by lowering the amount of
H2O2 to 1.1 equiv. The attention was then turned on to the temper-
ature of the reaction. Sulfoxide as the sole product was found when
the reaction was carried out at ice-bath temperature. To ascertain
the efficacy of the catalyst several reactions were carried out with
or without catalyst. The reactions took place in each case with the
best performance being in CH3CN with 1 mol % of catalyst. Accord-
ingly, all the reactions discussed herein were conducted with this
combination.17 To demonstrate the scope, a series of structurally
diverse sulfides were subjected to oxidation under the optimized
reaction conditions and the results are presented in Table 2. The
reactions went on well affording the products in high yields. It is
notable that sulfides were chemoselectively oxidized in the pres-
ence of some oxidation prone functional groups such as C@C,
–CN, and –OH (Table 2, entries 6–10). However, the oxidation of
sulfides containing electron-withdrawing groups (Table 2, entries
15 and 16) was sluggish producing the corresponding sulfoxides
in very low yields. The presence of electron-withdrawing (–NO2
in the present case) groups reduces the electron density on the
sulfur atom thereby decreasing its nucleophilicity. This probably
explains the low yields. Oxidation of refractory sulfur such as
dibenzothiophene (DBT) and substituted DBT is quite challenging
with the standard oxidation procedures.18 Considering the press-
ing need for reducing the sulfur content of transportation fuel to
abate environmental pollution, it was thought worthwhile to
explore the efficacy of the newly developed catalyst for this
purpose. Significantly, upon treatment with VO2F(dmpz)2–H2O2
system DBT and substituted DBT were oxidized to the correspond-
ing sulfoxides (Table 2, entries 11–13). It may be mentioned that
with the increase in alkyl chain length of the sulfides, the rate of
reaction becomes slower Table 2, entries 4, 5, and 7). This may
be due to the orientation of hydrophobic alkyl chain around the
sulfur atom. Dimethyl sulfide (DMS) can be selectively oxidized
to dimethyl sulfoxide (DMSO) with this protocol (entry 14).
Oxidation of DMS to DMSO has direct relevance in the commercial
preparation of ranitidine hydrochloride, where DMSO is one of the
essential ingredients and methyl sulfide is an unavoidable pollut-
ing effluent, which can be recycled through methylation followed
by selective oxidation.19 In catalysis, recyclability is one of the
important attributes. To this end, we have conducted a series of
reactions with methyl phenyl sulfide by using the aqueous phase
containing VO2F(dmpz)2, obtained after extraction of the reaction
mixture with ethyl acetate. This was charged with fresh substrate
and 1.1 equiv of H2O2. The catalyst could be reused for at least five
reaction cycles with consistent activity. Importantly, the reaction
Figure 1. The ORTEP view of VO2F(dmpz)2, the catalyst with selected bond
distances (Å) and bond angles (°): V–F, 1.607 (6); V–O1, 1.70 (3); V–O2, 1.75 (3); V–
N1, 2.07 (3); V–N3, 2.15 (3); F–V–O1, 126 (2); F–V–O2, 112 (2); F–V–N1, 92 (1); F–
V–N3, 93 (1); O1–V–O2, 122.1(7); O1–V–N1, 92 (1); O1–V–N3, 86 (1); O2–V–N1, 87
(1); O2–V–N3, 90 (1); N1–V–N3, 174.7 (5).
to afford VO2F(dmpz)2.8 The role of NH4HF2 was not only to per-
form fluoridation but also to provide mild acidity (pH ꢀ4) of the
reaction medium. This has facilitated the coordination of 3, 5
dimethyl pyrazole (dmpz) through its non-protonated N-donor
atom. A higher acidity is not conducive to the synthesis. The cata-
lyst is found to be a highly crystalline lemon yellow solid, stable in
air, soluble in nearly all-polar solvents, and decomposes at 156 °C.
Its solution electrical conductance value of 26-mho cm2 molÀ1 in
acetonitrile attests its neutral character.9 Magnetic susceptibility
measurement shows that the compound is diamagnetic with gram
susceptibility being À0.369 Â 10À6cgs.10 The IR spectrum of
VO2F(dmpz)2 shows characteristic absorption bands due to coordi-
nated dmpz,11 fluoride,12 and oxo ligands.13 A strong band at
446 cmÀ1 is due to
support of the dmpz coordination. The strong bands appearing at
953 and 922 are assigned to (V@O) stretching. Splitting of this
m(V–N) stretching. This is very important in
m
band is a clear indication of the occurrence of a cis-dioxovanadyl
center.13 The Raman spectrum shows complimentary signals at
547 cmÀ1 due to V–F, and at 947 and 930 cmÀ1 assigned to mV@O
m
originating from the cis-VO2 core. The UV–Vis spectrum showed
one intense broad band at 245 nm which is ascribed to ligand to
metal charge transfer.14
The X-ray structural analysis of the compound15 confirms that it
is
a penta coordinated mononuclear vanadium(V) species
[VO2F(dmpz)2] with space group Cc. The ORTEP diagram of the cat-
alyst shows trigonal bipyramidal geometry (Fig. 1) with dmpz li-
gands occupying the apical positions and the oxo and fluoride
groups in the equatorial site. The V@O bonds and O@V@O angle
are slightly greater (mean V@O bond = 1.723 Å and O@V@O angle
=122.1°) than generally encountered V@O bond 1.649 Å and
O@V@O angle 108.2–110.7°. The aforementioned results establish
the identity of the catalyst as claimed.
Our interest in oxidation6d,6g,16 inspired us to use this complex
as a catalyst for sulfide oxidation. The catalyst was screened for
oxidation of sulfides with aqueous 30% H2O2. To optimize the reac-
Table 1
Optimization of reaction condition for selective oxidation of methyl phenyl sulfide oxidation
Entry
Catalyst mol %
Time (h)
Temp (°C)
H2O2 equiv.
Solvent
Sulfoxide (%)
Sulfone (%)
1
2
3
4
5
6
7
8
9
5
5
5
5
5
3
1
1
0
5
2.5
2.5
5
27
27
27
2
2
2
1.1
1.1
1.1
1.1
0
CH3CN
C2H5OH
H2O
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
C2H5OH
75
70
55
65
95
95
95
0
20
23
20
20
<1
<1
<1
—
3
27
3.5
4.0
5
12
12
5.5
0–5
0–5
0–5
0–5
0–5
0–5
1.1
1.1
35
92
—
3
10

6514
S. Hussain et al. / Tetrahedron Letters 53 (2012) 6512–6515
Table 2
O
S
O
O
VO2F(dmpz)2 catalyzed oxidation of organic sulfides with H2O2 in CH3CN
H2O2
V
F
dmpz
R1
R2
dmpz
O
VO2F(dmpz)2, H2O2
S
CH3CN, 0-5 oC
S
R1
R2
R1
R2
H2O
R2
S
Entry
1
Substrate
Time (h)
Sulfoxidea
_
R1
O
O
O
..
S
S
O
R2
O
5
95,90b,86c,97d
CH3
O
V
F
V
F
dmpz
dmpz
R1
dmpz
dmpz
S
S
2
3
5
93
87
Scheme 2. Plausible mechanism of oxidation of sulfide.
C6H13
5.5
S
S
C17H35
C11H23
4
5
6.5
6
95
87
thereby rendering the VO2F(dmpz)2–H2O2 system efficacious, as
observed in the present study.
H7C3
In conclusion, a new penta-coordinated VO2F(dmpz)2 catalyst
has been developed and fully characterized. It efficiently catalyzes
the oxidation of alkyl as well as aryl sulfides by H2O2 also in the
presence of oxidation prone functional groups such as C@C, –CN,
and –OH. The selective oxidation of DMS to DMSO is industrially
important in the context of ranitidine hydrochloride. Refractory
sulfides are also capable of being oxidized quite effectively which
are especially relevant in desulfurization of transportation fuel.
The recyclability of the catalyst offers a potentially competitive
practical process.
S
S
6
7
3
97
86
6.5
C11H23
S
8
4.5
85
MeO
OH
S
9
4.5
2
82
85
S
CN
10
88e
75e
Acknowledgments
11
12
5
8
S
We thank Dr. Gopal Das of IIT Guwahati for his help in solving
X-ray structure. D.T and S.K.B are grateful to CSIR, New Delhi for
research fellowships.
S
13
14
15
12
0.5
5
80e
99
S
S
References and notes
CH3
S
H3C
O2N
O2N
1. (a) Fernandez, I.; Khiar, N. Chem. Rev. 2003, 103, 3651–3706; (b) Kowalski, P.;
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35
S
16
6
30
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a
b
c
Isolated yield.
Reaction in ethanol.
Yield after fifth cycle.
Yield at 5 g scale.
Reaction at room temperature.
d
e
can be performed on a relatively larger scale (5 g) to give good
yields (Table 2, entry 1) showing its prospects for scaled-up
applications.
We believe that the reaction proceeds via metal-oxygen shift
mechanism in the present reaction as depicted in Scheme 2 and
suggested by Chu and Trout.20 The ease of the formation of sulfox-
ide is likely to happen through the nucleophilic attack by the sul-
fide to the electrophilic O–O bond of peroxometal species thus
facilitating the regeneration of the catalyst.
It is relevant to mention that a commercially available vana-
dium complex, VO(acac)2 (acac = acetyl acetonate), might as well
be considered as a catalyst for such oxidations using H2O2 as the
oxidant. However, the efficacy and selectivity are comparatively
low.21 This is indeed expected because the metal has +IV oxidation
state (d1 system) in VO(acac)2 whereas +V (d0 system) in our com-
plex. Hence VO2F(dmpz)2 is more effective in activating H2O2
6. and references cited therein (a) Conte, V.; Floris, B. Inorg. Chim. Acta 2010, 363,
1935–1946; (b) Conte, V.; Fabbianesi, F.; Floris, B.; Galloni, P.; Sordi, D.; Arends,
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1277; (c) Wu, P.; Celik, C.; Santoni, G.; Dallery, J.; Rehder, D. Eur. J. Inorg. Chem.
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V.; Lesnugin, A. Z.; Senyavin, V. M.; Aslanov, L. A.; Rybakav, V. B.; Tarakanova, A.
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Chettri, S. K.; Chaudhuri, M. K.; Islam, N. S.; Barman, S. R. J. Mol. Catal. 1993, 78,
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7. (a) Wu, P.; Santoni, G.; Froeba, M.; Rehder, D. Chem. Biodiver. 2008, 5, 1913–
1926; (b) Wikete, C.; Wu, P.; Zampella, G.; De Gioia, L.; Licini, G.; Rehder, D.
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Compounds; Academic Press: New York, 1981.

S. Hussain et al. / Tetrahedron Letters 53 (2012) 6512–6515
6515
8. Synthesis of Dioxofluoro(bis-dimethylpyrazole) vanadium(V), VO2F(dmpz)2: An
aqueous suspension (15–20 mL) of 0.5 g (2.75 mmol) V2O5 was treated with
0.55 g (9.64 mmol) NH4HF2 followed by heating on a steam bath to get a clear
solution. An ethanolic solution (15–20 mL) of 1.33 g (13.73 mmol) of 3,5-
dimethyl pyrazole (dmpz) was then added to it and the solution was allowed to
concentrate (ca.10–12 mL) by heating on a steam bath. The concentrated
solution was kept in a freezer until shiny lemon yellow crystals of VO2F(dmpz)2
were obtained. The compound was separated by decantation and dried in
vacuo over conc. H2SO4. The yield was 1.3 g (81%). Spectral data: IR(KBr): 3257,
17. Typical procedure for the oxidation of methyl phenyl sulfide: methyl phenyl
sulfide (0.248 g, 2 mmol) in acetonitrile (2 ml) solvent was reacted with
VO2F(dmpz)2 (0.0059 g, 0.02 mmol) and H2O2 (30% aqueous solution, 25 lL,
2.2 mmol) under stirring at ice bath temperature for 5 h and monitored by TLC.
On completion of the reaction, acetonitrile was removed under reduced
pressure and 1 ml of water was added. The product was extracted with ethyl
acetate, dried over anhydrous MgSO4, and evaporated to dryness, while the
aqueous layer was retained for recovery of the catalyst. In order to remove any
traces of VO2F(dmpz)2, the product was transferred to silica gel (60–120 mesh)
column and eluted with ethyl acetate: hexane (1:7). The aqueous layer is
concentrated and reused. The spectral data of some representative compounds
are given below.
1577, 1285, 953, 922 cmÀ1 1HNMR (400 MHz, CDCl3): d 2.13 (s, 3H), 2.47 (s,
;
3H), 5.85 (s, 1H), 11.62 (br s, 1H, N-H); 13CNMR (100 MHz, CDCl3): d 11.74 (2C),
105.04, 145.76 (2C); 19FNMR (400 MHz, CDCl3): d -146.32 (br, 1F). Laser raman:
947, 930, 547 cmÀ1. Anal. Calculated for C10H14N4O2VF (294.21): C, 40.83; H,
5.48; N, 19.04; O, 10.88; V, 17.32; F, 6.46. Found: C, 40.62; H, 5.18; N, 19.31; V,
17.47; F, 6.21.
Methyl phenyl sulfoxide (entry 1): IR(neat): 1032 cmÀ1
CDCl3): d 2.74 (s, 3H), 7.51–7.53(m, 3H), 7.64–7.66 (m, 2H) ppm; MS: m/z 157
(M+); Butyl dodecyl sulfoxide (entry 5): IR (KBr): 1025 cmÀ1
1H NMR(400 MHz,
;
1H NMR (400 MHz,
;
9. Geary, W. J. Coord. Chem. Rev. 1971, 7, 81–122.
10. Eranshawan, A. Introduction to Magnetochemistry; Academic Press, Inc.: New
York, 1968.
CDCl3): d 0.89(t, J = 6.8 Hz, 3H), 0.98(t, J = 6.8 Hz, 3H), 1.22–1.56(m, 20H), 1.72–
1.8(m, 4H), 2.57–2.74(m, 4H); Allyl dodecyl sulfoxide (entry 7): IR (KBr):
1035 cmÀ1 1H NMR(400 MHz, CDCl3): d 0.9(t, J = 6.4 Hz, 3H), 1.31–1.46(m,
;
11. Dodge, R. P. D.; Templeton, H.; Zalkin, A. J. Chem. Phys. 1961, 35, 55–67.
12. Clark, H. C.; Emeleus, H. J. J. Chem. Soc. 1957, 2119.
16H), 1.78–1.88(m, 2H), 2.93(t, J = 8.4 Hz,2H), 3.69(d, J = 8.4 Hz, 2H), 5.42(d,
J = 16.4 Hz, 1H), 5.49(d, J = 10.8 Hz, 1H), 5.85–6.0(m, 1H); Allyl 4-methoxyphenyl
13. Griffith, W. P.; Wickins, T. D. J. Chem. Soc. A 1968, 400–404.
14. Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; 1984.
15. Crystal data: C10H14N4O2VF, MW = 294.21, monoclinic, space group Cc lemon
yellow crystal (0.29 mm £ 0.21 mm x 0.20 mm), a = 11.2074(4), b = 11.9492(5),
sulfoxide (entry 8): IR (neat): 1044 cmÀ1 1H NMR (400 MHz, CDCl3): d 3.45–
;
3.57(m, 2H), 3.78(s, 3H), 5.18(d, J = 16.8 Hz, 1H), 5.31(d, J = 10.8 Hz, 1H), 5.58–
5.68(m, 1H), 7.47–7.51(m, 3H), 7.54–7.57(m, 2H).
18. Noyori, R.; Aoki, M.; Sato, K. Chem. Commun. 2003, 1977–1986.
19. Green Chemistry Education: Facilitating Transition to Green Technologies
c = 9.7556(4) Å, b = 94.3860(10)°, V = 1302.64(9) Å3, Z = 4,
l ,
= 0.77 mmÀ1
T = 298 K., Mo
measured, 1976 reflections collected, No. of observed reflections
(IP3
(I)) = 163, Ra = 0.0449, Rwb = 0.0526, Goodness of fit = 1.324.
K
a
(k = 0. 71073 Å), h–2h scans, 2h = 50°, 2334 reflections
and
Processes
in
India
(Webpage:
acs.confex.com/acs/green09/
recordingredirect.cgi/id/452) browsed on 25th June 2012.
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Barbarini, A.; Maggi, R.; Muratori, M.; Sartori, G.; Sartorio, R. Tetrahedron:
Asymmetry 2004, 15, 2467–2473; (c) Bryliakov, K. P.; Karpyshev, N. N.;
Fominsky, S. A.; Tolstikov, A. G.; Talsi, E. P. J. Mol. Catal. A: Chem. 2001, 171, 73–
80.
r
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2009, 3319–3322; (b) Bharadwaj, S. K.; Sharma, S. N.; Hussain, S.; Chaudhuri,
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