Silica gel supported AlCl3 catalyzed Friedel-Crafts acylation of aromatic compounds

Article abstract of DOI:10.1016/j.cclet.2010.07.024

Silica gel supported aluminium trichloride (SiO₂-AlCl₃) has been shown to be a mild, efficient, and chemoselective heterogeneous Lewis acid catalyst for the acylation of aromatic compounds with acid chlorides. The catalyst can be reused up to five times after simple washing with ether and is stable (as a bench top catalyst).

Full text of DOI:10.1016/j.cclet.2010.07.024

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 1395–1398
www.elsevier.com/locate/cclet
Silica gel supported AlCl3 catalyzed Friedel-Crafts acylation of
aromatic compounds
Kaveh Parvanak Boroujeni
Department of Chemistry, Shahrekord University, Shahrekord (115), Iran
Received 10 March 2010
Abstract
Silica gel supported aluminium trichloride (SiO2-AlCl3) has been shown to be a mild, efficient, and chemoselective
heterogeneous Lewis acid catalyst for the acylation of aromatic compounds with acid chlorides. The catalyst can be reused up
to five times after simple washing with ether and is stable (as a bench top catalyst).
# 2010 Kaveh Parvanak Boroujeni. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Acylation; Ketones; Aluminium trichloride; Silica gel
Friedel-Crafts acylation is a fundamental and important process in organic synthesis as well as in
industrial chemistry [1,2]. Protic acids and Lewis acids have been used as catalysts in conventional Friedel-Crafts
acylation reactions [3–19]. The major disadvantages associated with the reported methods for the acylation
of arenes are a need for the use of stoichiometric or excess amounts of catalyst in reactions (due to
consumption of the catalyst by complexation to starting materials and/or products), the use of unrecyclable
catalysts which eventually result in generation of a large amount of environmentally hazardous waste
materials, less selectivity of para/ortho isomers, tedious work-up, and the use of moisture-sensitive, expensive,
hazardous or difficult to handle catalysts. Furthermore, in most of these methods applicable substrate range is very
limited.
Over the last years, SiO2-AlCl3 has been reported to catalyze several organic reactions [20–22]. In a
previous study, we also reported the use of SiO2-AlCl3 as a remarkably efficient heterogeneous catalyst for the
Friedel-Crafts sulfonylation of arenes [23]. Along this line, we now wish to report that SiO2-AlCl3 is also highly
chemoselective catalyst for the acylation of aromatic compounds with acid chlorides in solvent-free conditions
(Scheme 1).
1. Experimental
Chemicals were either prepared in our laboratory or were purchased from Merck and Fluka. Capacity of the catalyst
was determined by gravimetric method and atomic absorption technique using a Philips atomic absorption instrument.
E-mail address: parvanak-ka@sci.sku.ac.ir.
1001-8417/$ – see front matter # 2010 Kaveh Parvanak Boroujeni. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.07.024

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K.P. Boroujeni / Chinese Chemical Letters 21 (2010) 1395–1398
Reaction monitoring and purity determination of the products were accomplished by GLC or TLC on silica gel
polygram SILG/UV254 plates. Gas chromatography was recorded on Shimadzu GC 14-A. IR spectra were run on a
Shimadzu model 8300 FT-IR spectrophotometer. NMR spectra were recorded on a Bruker Advance DPX-300
spectrometer.
[(Scheme_1)TD$FIG]
Scheme 1.
Anhydrous AlCl3 (5.1 g) was added to silica gel (Merck grade 60, 230–400, washed with 1 mol/L HCl and dried
under vacuum at 80 8C for 72 h, 10.2 g) in CCl4 (30 mL). The mixture was stirred using a magnetic stirrer under reflux
condition for two days under N2 atmosphere and filtered and washed with 50 mL of dry CCl4, and then dried under
vacuum at 60 8C for 3 h. The loading of AlCl3 determined was 1.3 mmol/g [23].
To a solution of anisole (5 mmol) and benzoyl chloride (4 mmol) was added 0.4 mmol of SiO2-AlCl3 and
the reaction mixture was stirred magnetically at room temperature. After completion of the reaction (monitored by
TLC), the catalyst was filtered off and washed with CH2Cl2. The filtrate was washed with aqueous solution of
sodium bicarbonate and water and the organic layer was dried (Na2SO4) and concentrated on a rotary
evaporator under reduced pressure to give the corresponding ketone. Whenever required, the products were purified
by column chromatography (silica gel) using petroleum ether-ethyl acetate as eluent so as to afford the pure
ketone.
2. Results and discussion
SiO2-AlCl3 was prepared by reaction of silica gel with aluminium trichloride in refluxing carbon tetrachloride. By
using this catalyst various substituted benzenes were converted into their corresponding ketones with benzoyl chloride
and acetyl chloride as acylating agents in high to excellent yields at room temperature in solvent-free conditions (Table
1). The optimum molar ratio of SiO2-AlCl3 to acid chloride was 0.1:1. The methodology showed the excellent
positional selectivity as the para substituted product was formed almost exclusively. Naphthalene, 2-methoxy-
naphthalene, and anthracene underwent acylation with high regioselectivity in 89–91% yields (see the supporting
information). Acylation of furan, thiophene, and pyrrole is not a straightforward reaction and usually in the presence of
acid catalysts undergo polymerization reactions. We found that SiO2-AlCl3 is a suitable catalyst for such important
acylation reactions. These reactions were regioselective and produced only the 2-acyl product in 89–92% yields. The
acylation of indoles with acid chlorides in the presence of SiO2-AlCl3 was also studied and the corresponding indolyl
aryl ketones were obtained with high regioselectivity. For example, indole was converted to its corresponding ketone
with acetyl chloride in 91% yield. No N-substituted products were observed under these reaction conditions. Acylation
of highly deactivated arenes such as nitrobenzene and 1,2-dichlorobenzene failed. SiO2-AlCl3 was stable under the
reaction conditions and during reactions no leaching of acid moieties was observed due to being chemically bonded to
the support.
It was observed that, 4-phenylbutanoic acid chloride cyclized in nitrobenzene at 40 8C in the presence of SiO2-
AlCl3 to give the desired 1-tetralone in 94% yield (Scheme 2).
The feasibility of recycling the catalyst was also examined. After each reaction the catalyst was washed with ether
and reused. As it is shown in Scheme 3 the efficiency of the recycled catalyst did not change appreciably after five
cycles.
As shown in Table 2, SiO2-AlCl3 has a good efficiency compared to other recently reported catalysts.
In conclusion we have introduced a highly efficient solid catalyst for the acylation of arenes with acid chlorides to
afford ketones in high to excellent yields. The mild reaction conditions, short reaction times, easy work-up, high to
excellent yields, chemoselectivity, reuse of the catalyst for at least five times without significant change in its
catalytic activity, low cost, and easy preparation and handling of the catalyst are the obvious advantages of the present
method.

K.P. Boroujeni / Chinese Chemical Letters 21 (2010) 1395–1398
1397
Table 1
Acylation of arenes with acid chlorides catalyzed by SiO2-AlCl3.a.
Entry
1
Arene
Acid chloride
PhCOCl
Product
Time (h)
1
Yield (%) (o:m:p)b
91 [7]
[DT$INLE]
[TD$INLINE]
2
3
MeCOCl
PhCOCl
1.1
0.9
92 [7]
[DT$INLE]
[TD$INLINE]
92 (6:3:91) [19]
[TD$INLINE]
[DT$INLE]
4
5
MeCOCl
PhCOCl
0.9
0.8
92 (5:4:91) [7]
95 (5:3:92) [19]
[TD$INLINE]
[DT$INLE]
[TD$INLINE]
[DT$INLE]
[TD$INLINE]
[DT$INLE]
6
PhCOCl
0.9
92 (95:5)c [7]
[DT$INLE]
[TD$INLINE]
7
8
MeCOCl
PhCOCl
0.9
0.9
92 (95:5)c [7]
92 [19]
[TD$INLINE]
[DT$INLE]
9
PhCOCl
PhCOCl
PhCOCl
MeCOCl
1
89 (4:3:93)d [7]
91 (6:0:94) [7]
89 (6:4:90) [7]
90 (6:4:90) [7]
[TD$INLINE]
[DT$INLE]
10
11
1.1
1.2
1.3
[TD$INLINE]
[DT$INLE]
[TD$INLINE]
[DT$INLE]
12
[TD$INLINE]
[DT$INLE]
a
All reactions carried out at room temperature in the absence of solvent, unless otherwise indicated. The molar ratio of SiO2-AlCl3:acid chloride is
0.1:1.
b
Isolated yields. Isomer distribution based on 1H-NMR spectroscopy and GC. All products are known compounds and were identified by
comparison of their physical and spectral data with those of the authentic samples.
c
Isomer distribution of 3,4-dimethylphenyl isomer to 2,3-dimethylphenyl isomer.
d
The reaction was performed in 1,2-dichloroethane.
[(Scheme_2)TD$FIG]
Scheme 2.
[(Scheme_3)TD$FIG]
Scheme 3.

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K.P. Boroujeni / Chinese Chemical Letters 21 (2010) 1395–1398
Table 2
Comparison of the catalytic activity of SiO2-AlCl3 against other reported catalysts for the acylation of anisole with benzoyl chloride.
Catalyst
Solvent
Temp. (8C)
Time (h)
Yield (%) (o:p)
SiO2-AlCl3
Sc(OTf)3
Neat
r.t
0.8
18
6
95 (5:92)
79 [4]
CH3NO2
Neat
50
BiCl3
110
96 [6]
Zn powder
AlPW12O40
Al metal powder
InCl3-[C4mim]Cla
SmI3
Neat (MW)
Neat
120
10 (s)
2
71 [7]
60–70
82 (MW)
100
94 (20:80) [8]
81 [11]
Neat
10
18
4
Neat
94 (6:94) [12]
61 [13]
CH3CN
45
a
Chloroindate(III) ionic liquid.
Acknowledgment
The author thanks the Research Council of Shahrekord University for partial support of this work.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cclet.
2010.07.024.
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