ring required a longer reaction time to complete the reaction
(entries 11 and 14).
The present catalytic system was also applied to the oxida-
tion of the sulfide (I) to produce well-known antiulcer drug,
omeprazole (II) in 86.6% yield and 99.7% purity (Scheme 1).22,23
solution was stirred for 2 minutes, 30% aqueous hydrogen
peroxide (2.5 mmol) was dropwise added into the solution
under stirring. The progress of the reaction was monitored
by TLC. After completion of the reaction, a small amount of
sodium thiosulfate was introduced into the reaction solution in
order to eliminate the unreactive H2O2. Then 10 ml of ethyl
acetate (or methanol) was added to the reaction mixtures to
self-precipitate the catalyst, and the catalyst was separated by
filtration. The solvent was removed under reduced pressure
and the crude products were analyzed by gas chromatography
or 1H NMR with dimethyl maleate or bromobenzene as
internal standard. The further purification of sulfoxides was
performed by flash chromatography (ethyl acetate/hexane or
ethyl acetate/methanol) on silica gel.
Scheme
1
Reaction conditions: I/H2O2/NEt3/[(n-C4H9)4N]4(a-
Mo8O26) = 400:420:40:1, 30% aqueous H2O2, isopropanol, 0 ◦C, 55 h.
Experimental
Conclusions
Preparation of [(n-C4H9)4N]4(a-Mo8O26)24
In summary, the oxidation of various sulfides to sulfoxides
catalyzed by tetra-(tetraalkylammonium)octamolybdates was
achieved by using 30% aqueous hydrogen peroxide as oxidant
in high ratio of substrate to catalyst (up to 10000:1) and
short reaction time with excellent selectivity (95–100%). It is
Dihydrate sodium molybdate (12.220 g, 0.05 mol) was dissolved
into 30 ml of water and the solution was acidified with aqueous
HCl (6 mol/l) to pH 4.5. After stirring for 1 h, (n-C4H9)4NCl
(8.0 g, 0.0282 mol) was added to the solution and stirred for 4 h.
The precipitate was filtered and thoroughly washed successively
with water, absolute ethanol, acetone, and diethyl ether. The
white solid was dried in vacuum at room temperature to give
[(n-C4H9)4N]4(a-Mo8O26) (10.200 g) in 75.8% yield. Anal. Calcd.
for [(n-C4H9)N]4(a-Mo8O26): C, 35.69%; H, 6.75%; N, 2.60%.
noteworthy that actively functional groups, including hydroxyl
21
=
group and C C bonds that were easy to be oxidized, were
tolerated in the oxidation. In addition, the preparation of
the catalysts was simple and convenient, and they could be
easily recovered with simple self-precipitation and filtration after
reaction and showed high activity and selectivity even after
being used four times. Moreover, the present catalytic system
is cheap, safe, reproductive, easy to handle and environmentally
benign. In addition to mild reaction conditions, the catalytic
procedure provides a green alternative because it involves
recyclable pollution-free polyoxomolybdate catalysts, halogen-
free solvent and a green oxidant.
Found: C, 35.47%; H, 6.78%; N, 2.60%. H-NMR (d6-DMSO,
1
300 MHz), d (ppm) 0.94 (t, J = 7.5 Hz, 3H ¥ 4), 1.26–1.38 (m,
2H ¥ 4), 1.52–1.62 (m, 2H ¥ 4), 3.17 (t, J = 8.6 Hz, 2H ¥ 4). IR
spectrum (KBr, cm-1): 2961, 2938, 2872, 1633, 1481, 1460, 1379,
1345, 1149, 1104, 1053, 1025, 934, 921, 910, 872, 848, 807, 730,
712, 662, 555, 521, 409, 370, 311.
Preparation of [(n-C4H9)(p-C5H5N)]4(b-Mo8O26)25
Acknowledgements
A mixture of 12% dilute hydrogen chloride (9.5 ml, 0.038 mol)
and an aqueous solution of dihydrate sodium molybdate (6.11 g,
0.025 mol) was dropwise added into 50 ml of aqueous solution of
n-butylpyridinium chloride (8.233 g, 0.047 mol) under stirring
at 70 ◦C. After continuously stirring for 25 minutes, the resulted
white precipitate was filtered, washed successively with water
and diethyl ether, and dried in vacuum at room temperature to
give [(n-C4H9)(p-C5H5N)]4(b-Mo8O26) (2.559 g) in 47.4% yield.
Anal. Calcd. for [(n-C4H9)(p-C5H5N)]4(b-Mo8O26): C, 25.02%;
H, 3.27%; N, 3.24%. Found: C, 24.85%; H, 3.31%; N, 3.36%.
1H-NMR (d6-DMSO, 300 MHz), d (ppm) 0.91 (t, J = 7.6 Hz,
3H), 1.26–1.33 (m, 2H), 1.85–1.95 (m, 2H), 4.62 (t, J = 7.5 Hz,
2H), 8.16 (t, J = 7.3 Hz, 2H), 8.61 (t, J = 7.9 Hz, 1H), 9.11 (d,
J = 5.6 Hz, 2H). IR spectrum (KBr, cm-1): 3127, 3079, 3061,
2956, 2931, 2868, 1631, 1579, 1496, 1484, 1462, 1435, 1376, 1321,
1215, 1174, 941, 935, 912, 897, 837, 766, 713, 683, 664, 644, 573,
554, 520, 473, 449, 408, 366, 290.
This work has been funded by the National Natural Science
Foundation of China (No. 20025205 and 20572108) and the
Chinese Academy of Sciences.
Notes and references
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General procedure for the oxidation of sulfides
At room temperature, 0.005 mmol of catalyst and 2.5 mmol of
sulfide were added into a 15 cm-height and 2.5 cm-diameter
round tube and 4 ml of solvent was added. After the reaction
1404 | Green Chem., 2009, 11, 1401–1405
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