Tetrabutylammonium phosphomolybdate on fluorapatite: An efficient solid catalyst for solvent-free selective oxidation of sulfides

Article abstract of DOI:10.1016/j.tetlet.2004.11.002

A new synthetic method of sulfoxides and sulfones using solvent-free oxidations of sulfides with urea-hydrogen peroxide complex (urea-H ₂O₂) and tetrabutylammonium phosphomolybdate catalyst on fluorapatite ((Bu₄N)₃[PMo₁₂O ₄₀]/FAp). In the solid-phase system the oxidations of aromatic and alkyl sulfides proceeded at 4-25°C and the corresponding sulfoxides or sulfones were selectively obtained in good yields by controlling the amount of urea-H₂O₂.

Full text of DOI:10.1016/j.tetlet.2004.11.002

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 9513–9515
Tetrabutylammonium phosphomolybdate on fluorapatite:
an efficient solid catalyst for solvent-free
selective oxidation of sulfides
Yoh Sasaki,a Kyohei Ushimaru,a Katsuma Iteya,a Hirokazu Nakayama,b
Shunro Yamaguchic and Junko Ichiharac,*
aFaculty of Science and Engineering, Kinki University, Kowakae, Higashiosaka, Osaka 577-8502, Japan
bKobe Pharmaceutical University, Motoyama-kitamachi, Higashinada, Kobe, Hyogo 658-8558, Japan
cThe Institute of Scientific and Industrial Research, Osaka University, Mihogaoka, Ibaraki, Osaka 567-0047, Japan
Received 8 October 2004; revised 2 November 2004; accepted 2 November 2004
Available online 13 November 2004
Abstract—A new synthetic method of sulfoxides and sulfones using solvent-free oxidations of sulfides with urea–hydrogen peroxide
complex (urea–H2O2) and tetrabutylammonium phosphomolybdate catalyst on fluorapatite ((Bu4N)3[PMo12O40]/FAp). In the
solid-phase system the oxidations of aromatic and alkyl sulfides proceeded at 4–25°C and the corresponding sulfoxides or sulfones
were selectively obtained in good yields by controlling the amount of urea–H2O2.
Ó 2004 Elsevier Ltd. All rights reserved.
A solid-phase-assisted reaction system without organic
solvent has been increasingly attracted as an environ-
mentally benign organic reaction system.1,2 Recently,
we have developed a new solvent-free catalytic reaction
system using apatite as a harmless solid disperse-phase.
A tungstic acid dispersed on fluorapatite solid phase
(H2WO4/FAp) catalyzed epoxidation of alkenes and
allylic alcohols with solid urea–H2O2 without solvent
under ordinary temperature and pressure.3–5 We have
also found the effectiveness of Keggin-type of ammo-
nium phosphomolybdate ((NH4)3[PMo12O40]) in the
solvent-free epoxidations with urea–H2O2 by dispersing
on fluorapatite solid phase.4 In the solid-phase system
the phosphomolybdate was superior to the phospho-
tungstate, in contrast to the conventional liq-
uid-biphase-system.6,7 In the related studies, we now
report that the combination of solid urea–H2O2 and tetra-
butylammonium phosphomolybdate on fluorapatite
((Bu4N)3[PMo12O40]/FAp) promotes the selective oxida-
tion of sulfides to the sulfoxides and the sulfones.
Several inorganic and organic salts of Keggin-type of
phosphomolybdates on fluorapatite (M3[PMo12O40]/
FAp) were examined in the solvent-free oxidation of p-
tolyl methyl sulfide with urea–H2O2. The solvent-free
reaction was carried out under solid phase conditions
as follows. A solid catalyst phase of M3[PMo12O40]/
FAp (0.005mmol/0.25g, 1.0mol%) was not previously
prepared by impregnating in solution but prepared by
simple mixing in the powders.5,8 The solid catalyst phase
was permeated by the liquid sulfide (0.5mmol). After the
resulting solid mixture was cooled at 4°C for 30min,
solid urea–H2O2 (1.25mmol) cooled at 4°C was added
and sufficiently mixed. Then, the solid mixture was left
without stirring at 4°C. The reaction proceeded in pow-
dery state, which was followed by gas chromatography
using the internal standard.
In the oxidation of p-tolyl methyl sulfide, the phospho-
molybdates modified by organic cations such as tetra-
butylammonium or cetylpyridinium cation were much
more effective than the salts of inorganic cations such
as ammonium or sodium. For example, in the use of
(Bu4N)3[PMo12O40]/FAp the conversion of the sulfide
to the sulfone was over 99% at 4°C after 6h. Under
the mild conditions, the sulfide was not oxidized with
urea–H2O2 without additives, and FAp itself did not
catalyze the reaction.
Keywords: Oxidation; Sulfides; Phosphomolybdate; Fluorapatite;
Urea–H2O2; Solvent-free reaction; Solid NMR; Sulfoxides; Sulfones.
*
Corresponding author. Tel.: +81 6 6879 8467; fax: +81 6 6879
8469; e-mail: ichihara@sanken.osaka-u.ac.jp
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.11.002

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Y. Sasaki et al. / Tetrahedron Letters 45 (2004) 9513–9515
Table 1. The oxidation of sulfides to sulfones using (Bu4N)3[PMo12O40]/FAp and urea–H2O2
Substrate
Reaction conditionsa Temp/°C, time/h
Product
Yield/% (selectivity/%)c
93 (93)
O
25, 24
S
Me
S
Me
O
O
S
25, 24
25, 48
99 (100)
93 (95)
Me
Cl
S Me
Me
Cl
Me
O
O
S
Me
S
Me
O
O
S
25, 72
4, 48b
62 (64)
87 (97)
S
O
O
CH2S Me
CH2-S Me
O
O
S
O
4, 48b
4, 31b
89 (100)
81 (99)
S
S
S
O
O
a (Bu4N)3[PMo12O40]/FAp/urea–H2O2/substrate = 0.025mmol/2.5g/6.25mmol/2.5mmol.
b 0.005mmol of (Bu4N)3[PMo12O40] was used.
c The selectivity was determined by GC or 1H NMR. The other component was the sulfoxide. See Ref. 9.
The solid-phase system using (Bu4N)3[PMo12O40]/FAp/
urea–H2O2 was effectively used for the solvent-free oxi-
dations of some aromatic and aliphatic sulfides. As
shown in Table 1, when 2.5 times amount of urea–
H2O2 to a substrate was used, the oxidation of the sul-
fide to the corresponding sulfone proceeded under the
mild conditions (at 4–25°C for 24–72h). The sulfones
were obtained in good to excellent yields by the simple
isolation procedure described in Ref. 9. The oxidation
of sulfides is a consecutive reaction via sulfoxides to sulf-
ones. The selective oxidation of the sulfides to the corre-
sponding sulfoxides was accomplished by using 1.05
times amount of urea–H2O2 to the substrate at 4°C
(Table 2). Aromatic and aliphatic sulfoxides were mostly
obtained with over 85% of the selectivity. The use of
smaller amount of the catalyst (0.2mol%) was more
effective for the oxidation of aliphatic sulfides.
to the high selectivities. In our system the peroxo-species
derived from (Bu4N)3[PMo12O40]/FAp had electrophili-
city to preferentially attack the sulfides leading to the
selective synthesis of the sulfoxides.
In order to elucidate the peroxo-species, we carried out
the analysis of 31P solid-state NMR on the solid-phase-
activation of (Bu4N)3[PMo12O40] catalyst. NMR spectra
did not change after the activation at 25°C for 24h (Fig.
1).10 In the NMR spectra on the solid-phase-activation
of (NH4)3[PMo12O40], the formation of the active
species with partial degradation of the framework was
observed.4 Therefore, the Keggin-cluster structure of
(Bu4N)3[PMo12O40] was not degraded through the acti-
vation and a novel peroxo-species having the parent
framework could be formed. In contrast to the solid-
phase reactions, the degraded peroxo-species of
{PO4[MoO(O2)2]4}3À (PMo4) is formed in the biphasic
reaction of CetylPy3[PMo12O40] with aq H2O2.6 Further
degraded species may be formed, which would cause the
poor catalytic activity of the phosphomolybdate.
O
0.2-1 mol% (Bu4N)3[PMo12O40] / FAp
R
S
R’
R
S
R’
urea-H2O2
In our system urea–H2O2 was quantitatively used for the
sulfide oxidation without loss; when the excess amount
of the substrate to urea–H2O2 was used, the sulfoxide
and sulfone were formed over 99% yield based on the
urea–H2O2 amount. In the system, probably, the
peroxo-species with the parent framework has not been
degraded during the repeated catalytic cycle, which can
lead to the high H2O2 efficiency and the high activity
obtained above.
R= Aryl, Alkyl
R’=Alkyl
In the oxidation of aryl methyl sulfides to the sulfoxides,
methyl substitution to para-position of phenyl ring did
not affect the reactivity and the selectivity, while p-
chloro substitution decreased the selectivity to afford
the sulfoxide in a lower yield. The electron-withdrawing
substituent such as Cl may be favorable for the second
nucleophilic oxidation of the sulfoxide. Except for the
p-chloro sulfide, the rate of electrophilic oxygen transfer
to the sulfides was over ten times faster than that of
nucleophilic oxygen transfer to the sulfoxides to lead
In conclusion, the simple solid-phase system, Keggin-
type of (Bu4N)3[PMo12O40] dispersed on FAp phase, is
effective for the selective oxidation of the sulfides with
urea–H2O2. In the system the reaction can be carried

Y. Sasaki et al. / Tetrahedron Letters 45 (2004) 9513–9515
9515
a
Table 2. The oxidation of sulfides to sulfoxides using (Bu4N)3[PMo12O40]/FAp and urea–H2O2
Substrate
Time/h
Product
Yield/% (selectivity/%)c
S
Me
72
85 (4/88/8)
S
Me
O
Me
Cl
S
Me
Me
72
89 (1/89/10)
49 (26/56/18)
93 (0/93/7)
77 (0/89/11)
77 (7/86/7)
Me
Cl
S
S
Me
Me
O
S
24
O
49b
48b
51b
CH2-S Me
O
CH2S Me
S
S
O
S
S
O
a (Bu4N)3[PMo12O40]/FAp/urea–H2O2/substrate = 0.025mmol/2.5g/2.63mmol/2.5mmol at 4°C.
b 0.005mmol of (Bu4N)3[PMo12O40] was used.
c The selectivity of sulfide/sulfoxide/sulfone was determined by GC and 1H NMR. See Ref. 9.
References and notes
1. Mizuno, N.; Misono, M. Chem. Rev. 1998, 98, 199–217;
Misono, M. Chem. Commun. 2001, 1141–1152.
2. Kozhevnikov, I. V. Chem. Rev. 1998, 98, 171–198.
3. Ichihara, J. Tetrahedron Lett. 2001, 42, 695–697.
4. Ichihara, J.; Yamaguchi, S.; Nomoto, T.; Nakayama, H.;
Iteya, K.; Naitoh, N.; Sasaki, Y. Tetrahedron Lett. 2002,
43, 8231–8234.
5. Ichihara, J.; Iteya, K.; Kambara, A.; Sasaki, Y. Catal.
Today 2003, 87, 163–169.
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4.
7. Yasuhara, Y.; Yamaguchi, S.; Ichihara, J.; Nomoto, T.;
Sasaki, Y. Phosphorus Res. Bull. 2000, 11, 43–46.
8. Fluorapatite powder Ca10(PO4)6F2 (particle size, 5–20lm;
specific surface area. 8–12m2/g) were purchased form
Taihei Chemical Industrial Co. Ltd and used.
9. The reactions were carried out under the conditions
described in Tables 1 and 2. The solid reaction mixture
was extracted with n-hexane–ethyl acetate. In most cases,
after evaporating the solvent, the mixture of the starting
materials and the described products were obtained
without loss of weight. The yields of the products were
calculated without further separation and purification of
1
the extracts by means of GC and/or H NMR.
10. In NMR spectra the behavior of the catalyst on CaF2
disperse phase was similar to that on FAp disperse phase.
Since the PO4 signal of FAp overlapped in the latter
case, the NMR spectra in the reaction of the former
(Bu4N)3[PMo12O40]/CaF2 with urea–H2O2 were shown.
11. Tungstate/aq H2O2: Sato, K.; Hyodo, M.; Aoki, M.;
Zheng, X.-Q.; Noyori, R. Tetrahedron 2001, 57, 2469–
2476.
3À
Figure 1. 31P solid-state NMR spectra on the activation of (Bu4N)3-
[PMo12O40]/CaF2 with urea–H2O2; immediately after mixing (0h) and
after 24h at 25°C using a-zirconium phosphate (a-ZrP) as an internal
standard.
12. Molybdate- or tungstate-exchanged hydrotalcite catalyst/
aq H2O2: Choudary, B. M.; Bharathi, B.; Reddy, C. V.;
Kantam, M. L. J. Chem. Soc., Perkin Trans. 1 2002, 2069–
2074.
13. Urea–H2O2 with heating: Varma, R. S.; Naicker, K. P.
Org. Lett. 1999, 1.
out under solvent-free, neutral, and mild reaction condi-
tions with easy isolation procedure.11–13 In addition to
the effectiveness and the convenience, it is notified that
the simple Keggin-type of phosphomolybdates excep-
tionally catalyzed the oxidation in this system.