, 2005, 15(3), 113–116
O
Cr
O
No reaction
4
OH
O
wet SiO2, CH2Cl2,
room temperature
SiO2
O
O
SiO2
Scheme 8
without wet SiO2
CH2Cl2, room temperature
0.75 h
To the best of our knowledge, this is the first example of a
silica chromium based oxidant with covalent linkages (silica
vulcanised with CrO2 linkages). Thus, we hope that silica
chromate would be superior to all of the previously reported
chromium-based oxidants1–6 because (i) toxic chromium cations
could be collected from the reaction mixture easily, (ii) work-up
is very simple and pure products obtained solely by filtration
and removing of solvent, (iii) no need in chelating agents such
as pyridine in pyridinium chlorochromate (PCC).2 Moreover, in
older procedures, the isolation of oxidation products from the
reaction mixture and chelating agents such as pyridine in PCC
is difficult.2,3
90%
Scheme 4
CH2OH
No reaction
wet SiO2, CH2Cl2,
room temperature
Scheme 5
Trimethylsilyl ethers 3 were subjected to the deprotection
reaction in the presence of silica chromate and wet SiO2 in
dichloromethane at room temperature with good to excellent
yields (Scheme 6 and Table 2).§
In conclusion, the cheapness and availability of the reagents,
easy and clean work-up and high yields make this method attrac-
tive for chemists. We believe that the present methodology
could be an important addition to the existing methodologies.
O
SiO2
O
Cr
O
O
SiO2
OTMS
O
OH
R2
R1
R2
R1
R2
R1
This work was supported by the research affair, Bu-Ali Sina
University, Hamadan, Iran.
wet SiO2, CH2Cl2,
room temperature
3
2
1
Scheme 6
References
Silica chromate is a very mild and chemoselective oxidant
because in the case of aliphatic alcohols (Table 2, entries 8, 9,
11–13) and benzyl alcohols containing electron-withdrawing
groups, deprotection did occur. Three competitive reactions
(Scheme 7) were carried out.
As shown in Scheme 7 for aliphatic alcohols and electron-
deficient aromatic alcohols, only deprotection occurred. Depro-
tection did not occur when using wet SiO2 (50/50, w/w) in the
absence of silica chromate (Scheme 8).
1
2
3
A. R. Bressette, and L. C. Glover, IV, Synlett., 2004, 738.
R. A. Fernandes, Synlett., 2003, 741.
P. Salehi, H. Firouzabadi, A. Farrokhi and M. Gholizadeh, Synthesis,
2001, 2273.
(a) B. F. Mirjalili, M. A. Zolfigol, A. Bamoniri and A. Zarei, Bull.
Korean Chem. Soc., 2003, 24, 400; (b) F. Shirini, M. A. Zolfigol and
T. Yamato and N. Shinoda, J. Chem. Res. (S), 2002, 400.
and K. Amani, Synthesis, 2003, 408 and references cited therein.
K. Sakuratani and H. Togo, Synthesis, 2003, 21.
4
5
O
Cr
O
CH2OTMS
6
7
CH2OTMS
SiO2
O
O
SiO2
wet SiO2, CH2Cl2,
room temperature
8
9
A. Tashiro, A. Mitsuishi, R. Irie and T. Katsuki, Synlett., 2003, 1868.
4
10 S. S. Kim and D. W. Kim, Synlett., 2003, 1391.
11 A. Minatti, Synlett., 2003, 140.
H
O
CH2OH
12 S. S. Kim and K. Nehru, Synlett., 2002, 616.
13 L. S. V. D. Vondervoort, S. Bouttemy, J. M. Padron, J. L. Bras, J. Muzart
and P. L. Alsters, Synlett., 2002, 243.
14 L. Blackburn, C. Pei and R. J. K. Taylor, Synlett., 2002, 215.
15 C. Li, Y. Xu, M. Lu, Z. Zhao, L. Liu, Z. Zhao, Y. Cui, P. Zheng, X. Ji
and G. Gao, Synlett., 2002, 2041.
5
100%
100%
16 H. B. Friedrich, F. Khan, N. Singh and M. V. Staden, Synlett., 2001,
869.
O
Cr
O
CH2OTMS
NO2
CH2OH
NO2
17 (a) B. F. Mirjalili, M. A. Zolfigol, A. Bamoniri and Z. J. Zaghaghi,
J. Chem. Res. (S), 2003, 273; (b) B. F. Mirjalili, M. A. Zolfigol,
A. Bamoniri, Z. Zaghaghi and A. Hazar, Acta Chem. Slov., 2003, 50,
563; (c) F. Shirini, M. A. Zolfigol and M. Khaleghi, Phosphorous
Sulfur Silicon Relat. Elem., 2003, 178, 2107.
SiO2
O
O
SiO2
4
5
wet SiO2, CH2Cl2,
room temperature
100%
100%
18 T. T. Tidwell, Synthesis, 1990, 875.
19 P. M. Price, J. H. Clark and D. J. Macquarrie, J. Chem. Soc., Dalton
Trans., 2000, 101.
O
SiO2
O
Cr
O
O
SiO2
5
T. Madrakian, E. Ghaemi, A. Afkhami, S. Azizian and S. Afshar, Green
4
wet SiO2, CH2Cl2,
room temperature
S
CH2OH
100%
S
CH2
100%
OTMS
Scheme 7
§
General procedure for the deprotection of trimethylsilyl ethers. A sus-
pension of trimethylsilyl ether (2 mmol), silica chromate (0.6 g) and wet
SiO2 (0.4 g, 50% w/w) in CH2Cl2 (10 ml) was stirred at room temperature
for 15–50 min (the progress of the reaction was monitored by TLC) and
then filtered. The residue was washed with CH2Cl2 (20 ml). Anhydrous
Na2SO4 (3 g) was added to the filtrate and filtered off after 20 min.
Dichloromethane was removed. The pure carbonyl compound or alcohol
was obtained.
Mendeleev Commun. 2005 115