Efficient method for oxidation of ketones to esters with 4-aminoperoxybenzoic acid supported on silica gel

Article abstract of DOI:10.1080/00397910801997835

4-Aminoperoxybenzoic acid supported on silica gel was found to be a versatile and efficient oxidant for the oxidation of ketones to esters. Copyright Taylor & Francis Group, LLC.

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Efficient Method for Oxidation
of Ketones to Esters with 4-
Aminoperoxybenzoic Acid
Supported On Silica Gel
Dadkhoda Ghazanfari a , Mohammed M. Hashemi b &
Mehdi Shahidi-Zandi a
a Department of Chemistry, Kerman Branch, Islamic
Azad University, Kerman, Iran
b Department of Chemistry, Sharif University of
Technology, Tehran, Iran
Version of record first published: 12 Jun 2008.
To cite this article: Dadkhoda Ghazanfari, Mohammed M. Hashemi & Mehdi
Shahidi-Zandi (2008): Efficient Method for Oxidation of Ketones to Esters with 4-
Aminoperoxybenzoic Acid Supported On Silica Gel, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 38:12,
2037-2042
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Synthetic Communications1, 38: 2037–2042, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910801997835
Efficient Method for Oxidation of Ketones to
Esters with 4-Aminoperoxybenzoic Acid
Supported on Silica Gel
Dadkhoda Ghazanfari,1 Mohammed M. Hashemi,2 and
Mehdi Shahidi-Zandi1
1Department of Chemistry, Kerman Branch, Islamic Azad University,
Kerman, Iran
2Department of Chemistry, Sharif University of Technology, Tehran, Iran
Abstract: 4-Aminoperoxybenzoic acid supported on silica gel was found to be a
versatile and efficient oxidant for the oxidation of ketones to esters.
Keywords: 4-Aminoperoxybenzoic acid; Esters; Ketones; Oxidation; Silica gel
INTRODUCTION
Traditionally, solids, especially chromatographic adsorbents, have been
used in reactions to provide a surface on which a reagent spread out.
Empirically, such procedures were often found to provide high yields
under mild conditions. The ease of setup and workup was often matched
only by the cleanliness of the reactions.[1,2] However, little thought
appeared to go into the choice of the solid, except to make sure that it
offered ample active surface. Progress in surface science, the improved
understanding of heterogenous catalysis, and the availability of numer-
ous new solids provide opportunities for better design of catalysts as well
as supported reagents, which can lead to enhanced performance in these
processes.[3] Microporous solids such as silica gel offer a wide range of
active sites for supporting reagents and most can often be regenerated
if deactivated during a reaction. Silica gel as an inorganic support has
Received in the U.K. August 22, 2007.
Address correspondence to Dadkhoda Ghazanfari, Department of Chemistry,
Kerman Branch, Azad University, P. O. Box 194 Kerman, Iran. E-mail:
dadkhodaghbk@yahoo.com
2037

2038
D. Ghazanfari et al.
been used in various chemical reactions.[1–5] However, there are few
examples of chemical reactions for supporting organic molecules on silica
gel as oxidizing agents.[6–9]
The Baeyer–Villiger oxidation of ketones has been widely investi-
gated, and numerous procedures are known.[10] There has been few
reports concerning the conversion of ketones to esters. The representative
examples include the oxidation with peroxyacids, especially trifluroper-
oxyacetic acid,[11–13] with sodium percarbonate,[14] photo catalytic
oxidation of ketones in the gas phase over TiO2 thin films,[15] and so
forth. A silica gel–supported peroxyacid is described in the present report.
RESULTS AND DISCUSSION
In course of our systematic study of the development of supported reagents
and catalysts for the oxidation of organic compounds,[16–24] we have nearly
presented preparation and use of 4-aminoperoxybenzoic acid supported
on silica gel as a convenient procedure for the oxidation of benzylic meth-
ylene compounds to corresponding ketones,[21] and for selective oxidation of
aryl ketones to a-diketones.[23] Now we report that 4-aminoperoxybenzoic
acid supported on silica gel is an efficient and convenient oxidant for
the oxidation of ketones to esters in high yields (Scheme 1). Silica gel–
supported 4-aminoperoxybenzoic acid can be prepared in the reaction
of 4-aminobenzoic acid with activated silica gel, which is then oxidized
to 4-aminoperoxybenzoic acid using hydrogen peroxide as an oxidant.
The oxidation of ketones is performed in chloroform as solvent, and
workup is a simple extraction and separation of the heterogeneous sup-
ported oxidant by filtration. Removal of the solvent followed by purifi-
cation of products by column chromatography gives esters in high
yields, although low-yield by-products were observed from oxidation of
unsymmetrical ketones; for example, oxidation of acetophenone yielded
7% of methyl benzoate as side product from column chromatography.
It is noteworthy to mention that in the absence of hydrochloric
acid, the oxidation of acetophenone with the same amount of this
heterogeneous supported oxidant can be completed with higher reaction
Scheme 1.

Efficient Method for Oxidation of Ketones
2039
time (22 h) in 88% conversion of the ketone. The reaction can also be
carried out with unsupported peroxybenzoic acid, but a long reaction
time is needed and the yields are low. For example, when peroxybenzoic
acid was used for oxidation of acetophenone in CHCl3, only 58% of
phenyl acetate was obtained even after stirring for 24 h. Representative
reaction times and yields are given in Table 1.
Table 2 shows reusability of this heterogeneous supported oxidant
system after oxidating acetophenone three times. In each experiment,
the supported 4-aminobenzoic acid was recovered quantitatively.
EXPERIMENTAL
Chemicals were purchased from Merck, Aldrich, and Riedel de Haen
AG and were used without further purification. IR and NMR spectra
Table 1. Oxidation of ketones with 4-aminoperoxybenzoic acid supported on
silica gel
Entry
1
Substrate
Time (h)
11
Product (s)
Yield (%)a
C6H5COCH3
CH3CO2C6H5
[C6H5CO2CH3]
91
[7]
91
[7]
93
[3]
90
[6]
96
[0]
100
78
[14]
82
2
3
4
5
o-BrC6H4COCH3
p-BrC6H4COCH3
o-ClC6H4COCH3
p-ClC6H4COCH3
12
12
12
12
o-BrC6H4OCOCH3
[o-BrC6H4CO2CH3]
p-BrC6H4OCOCH3
[p-BrC6H4CO2CH3]
o-ClC6H4OCOCH3
[o-ClC6H4CO2CH3]
p-ClC6H4OCOCH3
[p-ClC6H4CO2CH3]
C6H5CO2C6H5
6
7
C6H5COC6H5
CH3COCH2CH3
8
14
CH3CO2CH2CH3
[CH3CH2CO2CH3]
CH3CH2CO2CH2CH3
8
9
CH3CH2COCH2CH3
14
12
93
10
11
14
12
88
96
aIsolated yields.

2040
D. Ghazanfari et al.
Table 2. Reusability of 4-aminoperoxybenzoic acid supported on
silica gel for oxidation of acetophenone
Experiment
1
Products
Yield (%)
CH3CO2C6H5
[C6H5CO2CH3]
CH3CO2C6H5
[C6H5CO2CH3]
CH3CO2C6H5
[C6H5CO2CH3]
91
[7]
84
[7]
83
[7]
2
3
were recorded on FT-IR Unicam Mattson 1000 and Bruker AC-80
(80 MHz). All products are known compounds, and they were character-
ized by comparison of their physical and spectral data with those of
authentic samples.[25–27] All yields refer to pure isolated products.
Activation of Silica Gel
Chromatographic-grade silica gel (30 g, 70–230 mesh) was heated in an
electric furnace at 700 ꢀC for 3 h. Then it was cooled to room temperature
to obtain 27.9 g of activated silica gel.
Supporting of 4-Aminobenzoic Acid on Activated Silica Gel
To a stirred solution of 6.9 g of 4-aminobenzoic acid (50 mmol) in
200 cm3 of ethylacetate, 27.9 g of activated silica gel were added, and
the mixture was refluxed under magnetic stirring for 16 h. Then it was
cooled to room temperature and filtered, and the solid matter was
continuously extracted with ethyl acetate in a Sextet apparatus for 18 h.
Then it was dried at 80 ꢀC for 3 h to yield 33.7 g of silica gel–supported
4-aminobenzoic acid. 4-aminobenzoic acid (5.8 g, 42 mmol) was sup-
ported on 27.9 g of activated silica gel.
Oxidation of Silica Gel–Supported 4-Aminobenzoic Acid
to the Corresponding Peroxyacid
Silica gel–supported 4-aminobenzoic acid (33.7 g) containing 5.8 g of
4-aminobenzioc acid was added to a stirred solution of a commercial
30% solution of H2O2 at room temperature, and the reaction mixture was
stirred for 8 h. Then it was filtered, washed with distilled water (5 ꢁ 20 cm3),
and dried over anhydrous CaCl2 at room temperature in a vacuum (40 torr)
to obtain 34.3 g of silica gel–supported 4-aminoperoxybenzoic acid.

Efficient Method for Oxidation of Ketones
2041
Oxidation of Ketones to Esters, General Procedure
To a stirred solution of ketone (20 mmol) in 100 cm3 of CHCl3, one
drop of hydrochloric acid (0.1 N) was added. Then silica–gel supported
4-aminoperoxybenzoic acid (33 g, containing 40 mmol of 4-aminoperoxy-
benzoic acid) was added, and the mixture was stirred at room tempera-
ture for the indicated time. Progress of the reaction was monitored by
TLC (chloroform–methanol ¼ 4:1). Then the reaction mixture was
filtered to recover the solid material. The solid material was washed
thoroughly with 2 ꢁ 10 cm3 of CHCl3. The solvent was evaporated, and
the product was purified by column chromatography using silica gel.
The pure products were obtained by yields indicated in Table 1.
Reusability of Silica Gel–Supported 4-Aminoperoxybenzoic Acid
After using silica gel–supported 4-aminoperoxybenzoic acid as oxidant
for the oxidation of ketones, it can be reconverted to the corresponding
peroxyacid by treatment with hydrogen peroxide. Then it was used for
the oxidation of acetophenone to phenyl acetate. The second use of this
oxidant for the oxidation of acetophenone to phenyl acetate showed
about 7% decrease in the reactivity. However, the third use showed no
further decrease in reaction yields (see Table 2).
CONCLUSION
In summary, mild reaction conditions, high yields, ease of workup, high
selectivity, and reusability of supported reagent are the most significant
aspects of this method.
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