A highly efficient and reusable mesoporous supported Co(II) catalyst for chemoselective deprotection of aryl acetates

Article abstract of DOI:10.1016/j.tetlet.2009.10.012

Chemoselective deprotection of aryl acetates is successfully carried out in excellent yield using a mesoporous silica-supported (Salen) Co(II) catalyst. The catalyst shows high thermal stability and also can be recovered and reused at least 10 times without any significant loss of its catalytic activity. The present process is environmentally benign and economical.

Full text of DOI:10.1016/j.tetlet.2009.10.012

Tetrahedron Letters 50 (2009) 7256–7258
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A highly efficient and reusable mesoporous supported Co(II) catalyst
for chemoselective deprotection of aryl acetates
Fatemeh Rajabi *
Department of Science, Payame Noor University, PO Box 878 Qazvin, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Chemoselective deprotection of aryl acetates is successfully carried out in excellent yield using a meso-
porous silica-supported (Salen) Co(II) catalyst. The catalyst shows high thermal stability and also can be
recovered and reused at least 10 times without any significant loss of its catalytic activity. The present
process is environmentally benign and economical.
Received 19 July 2009
Revised 23 September 2009
Accepted 2 October 2009
Available online 8 October 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Phenol derivatives are a class of organic compounds which has
attracted attention regarding their various biological and pharma-
ceutical activities. Thus, selective protection and subsequent
During our recent research program on the development of
organic transformations employing efficient and recyclable
solid-based catalysts, an immobilized cobalt(II) Schiff base on
mesoporous silica showed good stability and activity in aerobic
1
deprotection of phenol groups is important in multi-step organic
syntheses. Generally, protection of phenolic compounds as their
acetates is preferred due to their ease of formation and cleavage.
Removal of an acetyl group can be achieved either using either
acidic or basic conditions. Commonly employed deprotection
methods include acid or base hydrolysis. Several acids and bases
have been reported to be effective in promoting selective
1
6
oxidation and protection reactions. In this Letter, a convenient
and efficient procedure for the chemoselective deprotection of aro-
matic acetates in the presence of aliphatic acetates using a catalytic
amount of supported cobalt(II) catalyst is developed (Scheme 1).
Previously, we reported the chemoselective acetylation of alco-
hols and phenols catalyzed by the supported Co(II) catalyst in excel-
2
3
4
8
17
deprotection of acetates such as NaOMe, micelles, Zn–MeOH,
lent yield. However, to the best of our knowledge, there is no report
5
6
7
cyclodextrin, enzymes, metallo-enzymes, metal complexes,
on the use of this catalyst for the selective deprotection of aryl ace-
tates. In this regard, deprotection of phenyl acetate in the presence
of solid-supported cobalt(II) in MeOH was investigated under vari-
ous conditions. The optimized conditions required phenyl acetate
(1 mmol), heterogeneous cobalt(II) catalyst (0.01 mmol), and MeOH
(5 mL) at room temperature (Scheme 1, Table 1, entry 2).
To prove that the immobilized Co(II) catalyst was required for
the deprotection of phenyl acetate, we examined the reaction in
the absence of the supported catalyst. However, no appreciable
deprotection was observed under similar reaction conditions even
after 12 h (Table 1, entry 1).
9
10
11
12
and antibodies, Montmorillonite K-10,
2
I ,
NaBO
3
,
and
1
3
HCOONH
4
–SiO
2
,
These methods, while useful, suffer from limita-
tions including harsh reaction conditions, the use of unrecoverable
and expensive catalysts, long reaction times, large amounts of re-
agents, tedious work-up procedures, and difficulty in separation
of the catalyst, poor atom efficiency, and limited practical applica-
tion on a large scale. Therefore, the development of new and recov-
erable heterogeneous catalysts for selective deprotection of
aromatic acetates which work efficiently under economically and
environmentally appropriate conditions are highly desirable.
Recently, solid-supported catalysts have gained increased con-
sideration for organic transformations due to their ease of separa-
tion from the reaction mixture and the possibility of reuse as well
To test the catalytic activity and heterogeneity of the supported
cobalt(II) catalyst, the reaction was conducted with phenyl acetate
in the presence of supported cobalt(II) catalyst for 20 min which
14
as for economical and environmental reasons. Among solid-sup-
ported catalysts, silica-based catalysts have attracted significant
attention due to their interesting properties including thermal sta-
bility and their commercial availability for liquid phase reactions.
It is believed that mesoporous silicas with tunable pore sizes are
able to facilitate reactions by localizing reactants in their pores
and providing a high local concentration of active centers (typically
N
O
Co
(
1 mol% )
O
N
1
5
acidic or basic).
ArOAc
ArOH
MeOH, rt
*
Tel.: +98 281 3336366; fax: +98 281 3344081.
E-mail address: f_rajabi@pnu.ac.ir
Scheme 1. Deprotection of aromatic acetates using a supported cobalt(II) catalyst.
0
040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.10.012

F. Rajabi / Tetrahedron Letters 50 (2009) 7256–7258
7257
Table 1
Table 2
Deprotection of aromatic acetates with a supported cobalt(II) catalyst in MeOH at
room temperature
Recycling of the supported cobalt(II) catalyst for the deprotection of phenyl acetate
according to Scheme 1
Entry Substrate
Product
Time Yielda
Cycle
1
2
3
4
5
6
7
8
9
10
(
h)
(%)
Time (h)
Yield (%)
1.5
95
1.5
97
1.5
95
2
93
2
90
2
91
2.5
90
2.5
90
3
89
3.5
85
7b
a
1
2
C
6
H
5
OAc
C
6
H
5
OH
12
a
OAc
OH
Isolated yield.
1.5 98
gave a conversion of 55%. The catalyst was then removed from the
reaction mixture. The filtered, ‘catalyst-free’ reaction liquor was re-
turned to the reaction conditions. It was observed that no further
conversion of substrate occurred. The catalyst showed excellent
selectivity for the deprotection of aryl acetates to the correspond-
3
4
5
6
7
4-Cl(C
6
H
4
)OAc
)OAc
4-Cl(C
6
H
4
)OH
)OH
2.5 88
2.5 90
4-N0 (C
2
6
H
4
4-N0 (C
2
6
H
4
4-Br(C
2-Br(C
2-Cl(C
6
H
4
)OAc
)OAc
4-Br(C
2-Br(C
2-Cl(C H )OH OH
6 4
6
H
4
)OH
)OH
3
92
6
H
4
6
H
4
3.5 94
3.5 95
ing phenols. Alkyl acetates (benzyl acetate, 4-CH
3 6 4
(C H )-
H
6 4
)OAc
CH OAc, and butyl acetate) were unaffected under similar
2
experimental conditions even after 12 h. Furthermore, the che-
moselectivity of the reaction was demonstrated. Thus, in a compet-
itive reaction, when a mixture of benzyl acetate and phenyl acetate
was treated with the mesoporous silica-supported Co(II) catalyst in
methanol, benzyl acetate remained intact while phenyl acetate
was easily deprotected to phenol.
In order to explore the generality of this procedure, a variety of
aromatic acetates were subjected to the optimized deprotection
conditions. Aromatic acetates containing electron-donating or -
withdrawing groups were deprotected to the corresponding phe-
nols in excellent yields.
The reusability of supported catalysts is an important benefit and
makes them useful for commercial applications. Therefore, the reus-
ability of the supported cobalt(II) catalyst was investigated. After the
first use of the catalyst for the deprotection of phenyl acetate, it was
removed from the reaction mixture via filtration and successfully
used in 10 subsequent runs without any significant loss in catalytic
activity under similar experimental conditions. However, the reac-
tion time increased from 1.5 h to 3.5 h over the 10 runs (Table 2).
No pretreatment step was required, although the recovered catalyst
was washed with ca. 5 mL of MeOH to remove traces of the previous
reaction mixture and dried before the next cycle.
OAc
OH
8
2
2
91
94
OAc
OH
9
0
1
1
1
1.5 93
AcO
AcO
O
O
O
O
HO
HO
O
O
O
O
1.5 90
OAc
OH
In conclusion, a novel and efficient protocol for selective cleav-
age of aromatic acetates using a supported cobalt(II) Salen complex
in methanol at room temperature is developed. The advantages of
the present method are the simplicity of operation, the high yield
of deprotected products, and the recyclability of the catalyst. The
use of the supported cobalt(II) Salen complex catalyst provides a
useful alternative to the selective deprotection of aryl acetates
with the notable advantages that the work-up procedure is re-
duced to a simple filtration. The catalyst is very robust and shows
high thermal stability (up to 480 °C). The use of a small amount of
supported catalyst (1 mol %) makes this procedure an environmen-
tally friendly chemical process. Further experiments are ongoing in
our laboratory in order to investigate the catalytic activity of the
mesoporous silica-supported Co(II) catalyst in various organic
transformations.
1
2
3
12
0
OAc
OH
1
1
95
H3C
H3C
O
O
1
4
2.5 93
AcO
HO
General procedure: aryl acetate (1 mmol) and supported cobal-
t(II) catalyst (0.01 mmol, 0.033 g) were stirred in methanol (5 mL)
at room temperature. After completion of the reaction (TLC), the
cobalt catalyst was removed by filtration and then rinsed twice
with methanol (15 mL) and reused. The filtrate was concentrated
under vacuum to give the phenol in almost pure form which could
be purified by flash chromatography through a short pad of silica
gel using an appropriate solvent, if necessary.
OAc
OAc
1
1
5
6
2
91
AcO
AcO
HO
HO
CHO
CHO
2.5 90
Acknowledgement
a
Payame Noor University is thanked for financial support of this
work.
Isolated yield.
b
The reaction was conducted in the absence of the supported cobalt(II) catalyst.

7
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F. Rajabi / Tetrahedron Letters 50 (2009) 7256–7258
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Supplementary data
Supplementary data (copies of thermogravimetric analysis, sur-
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