Efficient α-chlorination of aryl ketones using aluminum chloride/urea-hydrogen peroxide in ionic liquid

Article abstract of DOI:10.1080/00397910500449641

Effective α-chlorination reactions of aryl ketones into the corresponding α-chloroketones have been accomplished with aluminum chloride hexahydrate and urea-hydrogen peroxide in [bmim]BF₄ ionic liquid. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910500449641

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Synthetic Communications: An International
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Efficient α‐Chlorination of Aryl Ketones Using
Aluminum Chloride/Urea–Hydrogen Peroxide
in Ionic Liquid
Jong Chan Lee a & Hyun Jung Park a
a Department of Chemistry, Chung‐Ang University, Seoul, South Korea
Published online: 16 Aug 2006.
To cite this article: Jong Chan Lee & Hyun Jung Park (2006) Efficient α‐Chlorination of Aryl Ketones
Using Aluminum Chloride/Urea–Hydrogen Peroxide in Ionic Liquid, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 36:6, 777-780, DOI:
10.1080/00397910500449641
To link to this article: http://dx.doi.org/10.1080/00397910500449641
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Synthetic Communicationsw, 36: 777–780, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500449641
Efficient a-Chlorination of Aryl Ketones
Using Aluminum Chloride/Urea–Hydrogen
Peroxide in Ionic Liquid
Jong Chan Lee and Hyun Jung Park
Department of Chemistry, Chung-Ang University, Seoul, South Korea
Abstract: Effective a-chlorination reactions of aryl ketones into the corresponding
a-chloroketones have been accomplished with aluminum chloride hexahydrate and
urea–hydrogen peroxide in [bmim]BF4 ionic liquid.
Keywords: Chlorination, ionic liquid, ketones, peroxide
a-Chloroketones has received considerable attention because of their versatile
utilities in various organic syntheses.[1,2] In particular, a-chloroarylketones
are important intermediates in variety of important organic transform-
ations.[3–5] Synthesis of a-chloroarylketones has usually been achieved by
indirect routes, which include the reaction of silyl enol ethers with transition
metal chlorides[6] and reaction of aromatic compounds with chloroacetyl
chloride.[4] Direct chlorination at the alpha carbon of ketones also has been
achieved using various chlorinating agents such as iron(III) chloride,[3]
N-chlorosuccinimide,[7] and benzyltrimethylammonium dichloroiodate.[8]
However, all of these methods often provided undesired dichlorinated
by-products and are invariably conducted in toxic volatile organic solvents.[9]
In recent years, there has been a good deal of interest in the application of
ionic liquids to organic synthesis as environmentally benign reaction media.
Their high thermal stability, nonflammability, and negligible vapor pressure
make them a safe alternative to the conventional organic solvents.[10,11]
Received in Japan July 28, 2005
Address correspondence to Jong Chan Lee, Department of Chemistry,
Chung-Ang University, Seoul 156-756, South Korea. Fax: þ82-2-825-4736; E-mail:
jclee@cau.ac.kr
777

778
J. C. Lee and H. J. Park
Although numerous hydrogen peroxide–mediated oxidation methods
have been reported in organic synthesis, the utilization of the more stable
urea hydrogen peroxide addition compound (UHP) has been rather
scarce.[12] UHP has been known for a long time, and its nontoxicity, high
stability, and ease of manipulation make it a safe carrier of unstable
hydrogen peroxide. It is only recently that UHP has begun to receive some
interest in organic synthesis.[13,14] In conjunction with our efforts in
widening application of UHP to environmentally benign organic synthesis,
we now report a novel method for the synthesis of a-chloroarylketones. Our
new protocol involves reaction of aryl ketones with aluminum chloride hexa-
hydrate in the presence of UHP in 1-butyl-3-methylimidazolium tetrafluorobo-
rate, [bmim]BF4, ionic liquid medium. Treatment of ketones with aluminum
chloride hexahydrate (1.1 equiv) and UHP (2.0 equiv) in 1 mL of 1-butyl-3-
methylimidazolium tetrafluoroborate ([bmim]BF4) at 608C for 4–20 h
smoothly afforded the corresponding a-chloroketones (Scheme 1). At the
present reaction conditions, both aryl methyl ketones and aryl methylene
ketones were successfully converted into the corresponding a-chloroketones
in high yields. The results are summarized in Table 1. As shown in Table 1,
the reactions of aryl methylene ketones (entries 5–10) with high enol
content generally gave superior yields than the cases of aryl methyl ketones.
Presumably, the chlorination reactions occurred by the reaction of the enolic
form of aryl ketones with the chloronium ion of preformed hypochlorous
acid (ClOH) by interaction of aluminum chloride with UHP.
In conclusion, we have developed a new and efficient method for the
preparation of a-chloroarylketones from the reaction of aryl ketones with
aluminum chloride and UHP in environmentally friendly [bmim]BF4 ionic liquid.
EXPERIMENTAL
The 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4) was
obtained from Fluka. Merck silica gel 60 (230–400 mesh) was used for
flash column chromatography.
General Procedure for Chlorination Reaction of Ketones
Aluminum chloride hexahydrate (0.266 g, 1.1 mmol) and UHP (0.188 g,
2.0 mmol) to a stirred solution of ketone (1.0 mmol) in [bmim]BF4 (1 mL)
Scheme 1.

a-Chlorination of Aryl Ketones
779
Table 1. Alpha chlorination of aryl ketones
Time
(h)
Yield
(%)a
Entry
Substrate
Product
1
2
3
4
5
6
7
8
9
6
10
4
77
72
77
72
90
88
94
80
97
18
20
4
6
9
4
10
6
89
aIsolated yield.
were added and stirring was continued at 608C for the time indicated in
Table 1. After completion of the reaction, the product was extracted with
dichloromethane (2 ꢀ 25 mL), washed with water (40 mL), and dried over
MgSO4. The solvent was removed in vacuo, and the crude mixture was
purified by silica-gel chromatography using dichloromethane to give the
pure a-chloroketone.
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