1
560
M. Kirihara et al.
LETTER
Oxidation of several sulfides with 30% hydrogen perox- pothesized that the TaC or NbC surface was oxidized by
ide catalyzed by NbC (f = 5 mm) in ethanol at 60 °C effi- hydrogen peroxide resulting in the peroxides which were
ciently afforded the corresponding sulfones 3 in all cases capable of oxidizing the sulfur atom of the sulfides. The
(
Table 11). The olefin moiety remained unchanged under details of the reaction mechanism are presently unclear
the reaction conditions (entry 2). It is notable that even the and are currently under investigation.
less reactive diaromatic sulfides gave the sulfones 3 under
the reaction conditions (entries 8 and 9).
Next, the oxidation of functionalities other than sulfide
with hydrogen peroxide in the presence of catalytic
amounts of TaC or NbC were examined. Interestingly, al-
cohols, ketones, and amines were completely inactive un-
der the reaction conditions for 24 hours. Moreover,
because the reaction conditions are neutral, an acetal com-
ponent was also inactive.
Table 11 Oxidation of Sulfides 1 with 30% Hydrogen Peroxide
Catalyzed by NbC in Ethanol
NbCa (0.04 equiv)
3
0% H2O2 (4.5 equiv)
O
O
S
R1
R2
S
R1
R2
EtOH, 60 °C
1
3
In conclusion, it has been shown that TaC and NbC act as
recyclable catalysts for the oxidation of sulfides with hy-
drogen peroxide. For TaC, the corresponding sulfoxides
were obtained in high yield. The sulfones were efficiently
produced using NbC as a catalyst. The reactions are high-
ly chemoselective, and the reaction conditions are mild
and neutral. The oxidation of sulfides is very important in
organic synthesis in the preparation of sulfoxides and sul-
Entry R1
R2
Time (h) Yield (%)b
1
2
3
4
5
6
7
8
9
Ph
Ph
Me
2.75
3.25
5.0
98
97
98
90
96
99
93
94
96
CH CH=CH
2
2
4-MeOC H4
Me
Me
Bn
Me
Bn
Ph
6
6
4-AcC H4
22.0
fones. This TaC- or NbC-catalyzed hydrogen peroxide
3,4
oxidation of sulfides is an excellent method for preparing
sulfoxides and sulfones.
Ph
4.0
Bn
Bn
Ph
1.75
1.75
6.5
Typical Experimental Procedure for the Oxidation of Sulfides
with Hydrogen Peroxide Catalyzed by TaC
To a stirred solution of 1 (1 mmol), MeOH (2 mL) and 30% H O
2
2
4-MeOC H4
4-MeOC H
4
6.5
6
6
(
0.5 mL, 5 mmol) was added 3.8 mg (0.02 mmol) of TaC (f = 2 mm,
a
purchased from Kojundo Chemical Laboratory Co., Ltd), and the
reaction mixture was stirred at 45 °C. The reaction was monitored
by TLC. After 1 disappeared from the TLC, the solvent was decant-
ed. MeOH (5 mL) was added to the residue, and the solvent was de-
f = 5 mm.
Isolated yields.
b
NbC was easily recovered from the reaction mixture using canted again. Saturated aq Na S O (6 mL) was added to the mixed
2
2
3
solvent, and the mixture was extracted with EtOAc (3 × 30 mL).
decantation and could be used as the catalyst without los-
ing its activity for at least four times (Table 12).
The combined organic phase was washed with H O and brine suc-
2
cessively. The extract was dried over anhyd MgSO , and the solvent
4
The treatment of 1a with an equimolar amount of TaC or was evaporated. Chromatography on silica gel afforded pure 2.
NbC in aqueous methanol or ethanol in the absence of hy-
drogen peroxide did not afford either 2a or 3a, and 1a was
quantitatively recovered. These results indicated that the
carbides themselves are not the oxidizing agents. It is hy-
Typical Experimental Procedure for the Oxidation of Sulfides
with Hydrogen Peroxide Catalyzed by NbC
Following a procedure similar to the TaC-catalyzed reaction, 1 (1
mmol), EtOH (2 mL), 30% H O (0.45 mL, 4.5 mmol) and 4.2 mg
2
2
(
0.04 mmol) of NbC (f = 5 mm, purchased from Aldrich) were re-
Table 12 NbC Repeatedly Used as the Catalyst for the Oxidation of
acted at 60 °C.
1a with 30% Hydrogen Peroxide
NbCa (0.04 equiv)
O
O
30% H2O2 (4.5 equiv)
S
References and Notes
S
Ph
Me
Ph
Me
EtOH, 60 °C
(
1) CRC Handbook of Chemistry and Physics: A Ready-
Reference Book of Chemical and Physical Data, 90th ed.;
David, L. R., Ed.; CRC Press: Boca Raton / FL, 2009.
2) (a) Kirihara, M.; Yamamoto, J.; Noguchi, T.; Hirai, H.
Tetrahedron Lett. 2009, 50, 1180. (b) Kirihara, M.;
Yamamoto, J.; Noguchi, T.; Itou, A.; Naito, S.; Hirai, Y.
Tetrahedron 2009, 65, 10477.
1
a
3a
Runb
Time (h)
2.25
Yield (%)c
(
1
2
3
92
89
96
97
2.08
(
3) Reviews: (a) Carreno, M. C. Chem. Rev. 1995, 95, 1717.
2.16
(
b) Frenanez, I.; Khiar, N. Chem. Rev. 2003, 103, 3651.
4
2.25
(4) Oxidation of sulfides with hydrogen peroxide catalyzed by
transition-metal compounds: (a) Kaczorowska, K.;
a
f = 5 mm.
Kolarska, Z.; Mitka, K.; Kowalski, P. Tetrahedron 2005, 61,
b
Catalyst was removed by decantation after each run and reused in
8315. (b) Mba, M.; Prins, L. J.; Licini, G. Org. Lett. 2007, 9,
the subsequent run.
Isolated yields.
21. (c) Jeyakumar, K.; Chand, K. Tetrahedron Lett. 2006,
c
Synlett 2010, No. 10, 1557–1561 © Thieme Stuttgart · New York