Friedel-Crafts acylation of arenes with carboxylic acids using silica gel supported AlCl3

Article abstract of DOI:10.3906/kim-0912-359

Aromatic compounds react smoothly with carboxylic acids in the presence of silica gel supported aluminium trichloride to afford the corresponding ketones with high regioselectivity in high to excellent yields. The catalyst is stable (as a bench top catalyst) and can be easily recovered and reused without appreciable change in its efficiency. tuebitak.

Full text of DOI:10.3906/kim-0912-359

Turk J Chem
34 (2010) , 621 – 630.
¨
˙
c
ꢀ TUBITAK
doi:10.3906/kim-0912-359
Friedel-Crafts acylation of arenes with carboxylic acids
using silica gel supported AlCl3
Kaveh PARVANAK BOROUJENI
Department of Chemistry, Shahrekord University, Shahrekord 115, IRAN
e-mail: parvanak-ka@sci.sku.ac.ir
Received 12.12.2009
Aromatic compounds react smoothly with carboxylic acids in the presence of silica gel supported alu-
minium trichloride to afford the corresponding ketones with high regioselectivity in high to excellent yields.
The catalyst is stable (as a bench top catalyst) and can be easily recovered and reused without appreciable
change in its efficiency.
Key Words: Acylation, aluminium trichloride, ketones, silica gel
Introduction
In recent years, the use of heterogeneous catalysts has received considerable attention in different areas of organic
synthesis. The high selectivity, nontoxicity, easy separation from the reaction media, recyclability, moisture
and air tolerance, and easier handling make the use of heterogeneous catalysts an attractive alternative to
conventional homogeneous catalysts.1
Friedel-Crafts acylation reactions are of great importance in both laboratory work and industrial processes
to synthesize aromatic ketones.2,3 Usually, these reactions are performed using acid chlorides or acid anhydrides
in the presence of protic acids or Lewis acids.4−12 The major disadvantages associated with the reported methods
for the acylation of arenes are the use of toxic acid chlorides or anhydrides as acylating agents, a need for the
use of stoichiometric or excessive amounts of catalyst in reactions, tedious work-up, environmental pollution,
and the use of moisture sensitive, unrecyclable, hazardous, or difficult-to-handle catalysts. These disadvantages
could be avoided by performing the acylation of arenes with carboxylic acids as acylating agents over recoverable
solid acid catalysts. The Friedel-Crafts acylation of arenes using carboxylic acids is preferable to acylation via
acid chlorides and anhydrides because the former reaction produces only water as a by-product, which meets
recent requirements for environmentally benign chemical processes. Furthermore, carboxylic acids are stable
and more available compounds, and their handling is much easier than that of corresponding acid chlorides and
621

Friedel-Crafts acylation of arenes with carboxylic acids..., K. PARVANAK BOROUJENI
anhydrides. However, the aromatic acylation using carboxylic acids, especially in the presence of heterogeneous
Lewis acid catalysts, is scarcely reported.13−25
In a continuation of our ongoing program to develop environmentally benign methods using heterogeneous
Lewis acid catalysts, we found that silica gel supported aluminium trichloride (SiO2 -AlCl3) was a good catalyst
for the Friedel-Crafts sulfonylation of aromatic compounds.26 Along this line, we now wish to report that
SiO2 -AlCl3 is also an effective and highly chemoselective catalyst for the acylation of aromatic compounds
with carboxylic acids under mild reaction conditions (Scheme 1).
O
SiO2-AlCl3
ArH
R = Aryl or Alkyl
Ar
R
°
RCO2H / 80 C
Scheme 1. Acylation of arenes with carboxylic acids using SiO2 -AlCl3 .
Experimental
General: Chemicals were either prepared in our laboratory or purchased from Merck and Fluka. Capacity of
the catalyst was determined by the gravimetric method and atomic absorption technique using a Philips atomic
absorption instrument. Reaction monitoring and purity determination of the products were accomplished by
GLC or TLC on silica gel polygram SILG/UV254 plates. Gas chromatography was performed in an apparatus
from Shimadzu with a flame ionization detector and SE-30 coated (5% on Chromosorb W-HP) packed 3-ft
column. The temperature was programmed to rise from 80 to 280 ◦ C at 10 ◦ C/min (injection temperature:
300 ◦ C). IR spectra were run on a Shimadzu model 8300 FTIR spectrophotometer. NMR spectra were recorded
on a Bruker Avance DPX-300 spectrometer.
Preparation of SiO2 -AlCl3
Anhydrous AlCl3 (5.1 g) was added to silica gel (Merck, grade 60, 230-400, washed with 1 M HCl and dried
under vacuum at 80 ◦ C for 72 h, 10.2 g) in carbon tetrachloride (30 mL). The mixture was stirred using a
magnetic stirrer under reflux conditions for 2 days in an N2 atmosphere and filtered and washed with 50 mL of
dry CCl4 , and then dried under vacuum at 60 ◦ C for 3 h. The determined loading of AlCl3 was 1.3 mmol/g.27
Typical experimental procedure
To a solution of o-xylene (5 mmol) and heptanoic acid (4 mmol), 0.4 mmol of SiO2 -AlCl3 was added, and the
reaction mixture was stirred magnetically at 80 ◦ C. After completion of the reaction (monitored by TLC and
GC), the catalyst was filtered off and washed with CH2 Cl2 . The filtrate was washed with an aqueous solution
of sodium bicarbonate and water, and the organic layer was dried (Na2 SO4) and concentrated on a rotary
evaporator under reduced pressure to give the corresponding ketone: 1 H-NMR (300 MHz; CDCl3)δ 0.82-0.91
(m, 3H), 1.23-1.41 (m, 6H), 1.63-1.74 (m, 2H), 2.26-2.33 (m, 6H), 2.80 (t, J = 7.2 Hz, 0.1H, 2,3-dimethyl isomer),
2.90 (t, J = 7.2 Hz, 1.9H, 3,4-dimethyl isomer), 7.11 (t, J = 7.2 Hz, 0.05H, 2,3-dimethyl isomer), 7.13-7.20 (m,
1H), 7.25 (d, J = 7.2 Hz, 0.05H, 2,3-dimethyl isomer), 7.65 (d, J = 7.2 Hz, 0.95H, 3,4-dimethyl isomer), 7.70
622

Friedel-Crafts acylation of arenes with carboxylic acids..., K. PARVANAK BOROUJENI
(d, J= 7.2 Hz, 0.95H, 3,4-dimethyl isomer); IR (KBr) ν : 1685 cm−1 (C=O). Whenever required, the products
were purified by column chromatography (silica gel) using petroleum ether-ethyl acetate as an eluent so as to
afford the pure ketone.
Results and discussion
SiO2 -AlCl3 was prepared by reacting silica gel with aluminium trichloride in refluxing carbon tetrachloride.
Using catalytic amounts of this catalyst in the Friedel-Crafts acylation of arenes with carboxylic acids as
acylating agents in solvent-free conditions, high to excellent yields of product were obtained (Table 1). The
optimum molar ratio of SiO2 -AlCl3 to carboxylic acid was 0.1:1. From the results, it is clear that SiO2 -AlCl3
is capable of catalyzing not only the acylation of activated arenes but also that of deactivated arenes (Table
1, entries 1-23). The methodology showed the excellent positional selectivity as the para-substituted product
was formed exclusively. Naphthalene, 2-methoxynaphthalene, and anthracene underwent acylation with high
regioselectivity in 88%-91% yields (Table 1, entries 24-30). It was pleasing to observe that even heterocyclic
compounds such as furan, thiophene, and pyrrole were smoothly converted into the corresponding ketones, a
conversion which is otherwise problematic in the presence of strong acid catalysts (Table 1, entries 31-36). These
reactions were regioselective, producing only the 2-acyl product in high to excellent yields. The acylation of
indoles with carboxylic acids in the presence of SiO2 -AlCl3 was also studied, and the corresponding indolyl
aryl ketones were obtained with high regioselectivity in high to excellent yields. For example, indole and 5-
bromoindole were converted to their corresponding ketones with benzoic acid in 89% and 90% yields, respectively
(Table 1, entries 37-38). No N -substituted products were observed under these reaction conditions. Acylation
of highly deactivated arenes such as nitrobenzene and 1,2-dichlorobenzene failed. Side product formation was
not observed in the reactions we studied. SiO2 -AlCl3 was stable under the reaction conditions, and during the
reactions no leaching of acid moieties was observed due chemical bonding to the support.
One notable achievement of this solid acid catalyst is intramolecular Friedel-Crafts acylation. For
example, 4-phenylbutanoic acid cyclized in nitrobenzene at 80 ◦ C in the presence of SiO2 -AlCl3 to give the
desired 1-tetralone in 95% yield (Scheme 2).
SiO2-AlCl3 (0.12 mmol)
nitrobenzene 80 3 h
OH
O
/
°
C /
95%
O
Scheme 2. Intramolecular Friedel-Crafts acylation using SiO2 -AlCl3 .
Following these results, we further investigated the potential of SiO2 -AlCl3 for the selective acylation of
different types of substituted benzenes. The results showed that SiO2 -AlCl3 is able to discriminate between
aromatic compounds containing electron donating and electron withdrawing groups, a transformation that
is difficult to accomplish via conventional methods. For example, toluene was selectively converted to the
corresponding ketone in the presence of chlorobenzene (Scheme 3). Furthermore, it was also observed that the
catalyst is efficient for the selective acylation of anisole over toluene. Although the reason for the observed
chemoselectivities is not exactly clear, it is possibly due to the mild catalytic activity of the solid catalyst.
623

Friedel-Crafts acylation of arenes with carboxylic acids..., K. PARVANAK BOROUJENI
Table 1. Acylation of arenes with carboxylic acids catalyzed by SiO2 -AlCl3 .a
Time Yield (%)
Entry Arene
Carboxylic acid
Product
(h)
2
(o:m:p)b
1
2
9020,28
PhCO2H
COPh
91(5:4:91)25
91(5:0:95)29
COPh
1.8
1.7
PhCO2H
Me
Me
Me
COC6H4Me-p
3
״
p-MeC6H4COOH
95(4:0:96)13
94(4:0:96)13
CO(CH2)2CH3
CO(CH2)5CH3
4
5
״
״
1.7
1.7
CH3(CH2)2CO2H
CH3(CH2)5CO2H
Me
Me
CO(CH2)6CH3
6
״
״
1.8
1.9
1.6
1.5
1.5
1.6
1.9
1.5
1.6
1.6
90(3:0:97)13
CH3(CH2)6CO2H
CH3(CH2)10CO2H
PhCO2H
Me
CO(CH2)10CH3
7
89(3:0:97)13
Me
93(6:3:91)25
COPh
OMe
8
MeO
MeO
MeO
MeO
MeO
MeO
MeO
MeO
COC6H4OMe-p
COMe
9
״
״
״
״
״
״
״
95(6:3:91)15,25
96(6:4:90)30,31,32
95(6:3:91)33
p-MeOC6H4CO2H
MeCO2H
10
11
12
13
14
15
COCH2Ph
PhCH2CO2H
90(4:0:96)25,34
92(5:4:91)16,18
91(5:0:95)16,18
90(3:0:97)16,18
COCH=CHPh
CO(CH2)2CH3
CO(CH2)5CH3
CO(CH2)6CH3
PhCH=CHCO2H
CH3(CH2)2CO2H
CH3(CH2)5CO2H
CH3(CH2)6CO2H
624

Friedel-Crafts acylation of arenes with carboxylic acids..., K. PARVANAK BOROUJENI
Table 1. Continued
Time Yield (%)
Entry Arene
Carboxylic acid
CH3(CH2)10CO2H
CH3(CH2)5CO2H
Product
(h)
1.7
(o:m:p)b
90(3:0:97)16,18
CO(CH2)10CH3
16
17
״
MeO
Me
Me
90(95:5)c,21
9125
Me
Me
1.7
1.8
CO(CH2)5CH3
COPh
Me
Me
Me
18
19
PhCO2H
MeCO2H
Me
Me
Me
Me
Me
COMe
Me
״
1.7
9218,35
COPh
20
21
1.8
1.6
88(3:4:93)d,28
90(7:0:93)28,29
PhCO2H
PhCO2H
COPh
NMe2
Cl
Me2N
COPh
22
23
2
89(5:0:95)28
87(4:3:93)28
PhCO2H
MeCO2H
Cl
Cl
COMe
״
1.9
COPh
24
25
26
27
2.5
2.4
2.5
2.2
91(7:93)d,e,28
PhCO2H
COPh
OCH3
OMe
90d,25
PhCO2H
COC6H4NO2-p
OCH3
״
״
89d,25
p-NO2C6H4CO2H
MeCO2H
COMe
OCH3
91d,18,36
625

Friedel-Crafts acylation of arenes with carboxylic acids..., K. PARVANAK BOROUJENI
Table 1. Continued
Time Yield (%)
Entry Arene
Carboxylic acid
Product
(h)
(o:m:p)b
COCH2Ph
OCH3
28
29
״
2.3
91d,18,37
PhCH2CO2H
COPh
2.7
88d,25
PhCO2H
COMe
30
31
32
33
34
״
״
״
2.6
2.3
2.2
2.5
2.5
90d,18,36
90f,20,38
90f,5,39
93f,5
MeCO2H
PhCO2H
COPh
O
S
O
O
S
S
MeCO2H
COMe
p-NO2C6H4CO2H
MeCO2H
COC6H4NO2-p
COMe
95f,5,40
COPh
35
36
37
2.3
2.3
3
88f,5,41
PhCO2H
N
H
N
H
COCH2Ph
COPh
״
89f,5,18
PhCH2CO2H
PhCO2H
N
H
90d,42,43,44
N
H
N
H
626

Friedel-Crafts acylation of arenes with carboxylic acids..., K. PARVANAK BOROUJENI
Table 1. Continued
Time Yield (%)
Entry Arene
Carboxylic acid
Product
(h)
(o:m:p)b
COPh
Br
Br
38
3.1
89d,42,43,44
PhCO2H
N
H
N
H
a All reactions carried out at 80 ◦ C in the absence of solvent, unless otherwise indicated. The molar ratio of
SiO2 -AlCl3 to carboxylic acid is 0.1:1.
b Isolated yields. Isomer distribution based on 1 H-NMR spectroscopy and GC. All products are known com-
pounds 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.
eα:β ratio.
f The reaction was performed at 60 ◦ C.
(2 mmol)
(2 mmol)
H3C
Cl
H3C
Cl
COMe
COMe
SiO2-AlCl3 (0.2 mmol)
90%
0%
°
MeCO2H (2 mmol) / 1.7 h / 80 C
H3CO
H3C
COPh
COPh
H3CO
H3C
(2 mmol)
(2 mmol)
91%
4%
SiO2-AlCl3 (0.2 mmol)
PhCO H (2 mmol) / 1.7 h / 80 C
°
2
Scheme 3. Selective acylation of different types of substituted benzenes using SiO2 -AlCl3 .
SiO2-AlCl3 (0.2 mmol)
Me
(3 mmol)
Me
COPh
PhCO2H (2 mmol)
80 C / 1.8 h
°
Usea
1
2
3
4
5
Yield (%)
91
91
90
90
89
aRecovered catalyst was used successively (Use 2,3,4,…)
Scheme 4. Acylation of toluene using recovered SiO2 -AlCl3 .
SiO2 -AlCl3 can be used several times with negligible loss of catalytic activity and there is no need for
regeneration (Scheme 4). The capacity of the catalyst after 5 uses was 1.3 mmol AlCl3 per gram.
627

Friedel-Crafts acylation of arenes with carboxylic acids..., K. PARVANAK BOROUJENI
A comparison of the efficiency of SiO2 -AlCl3 catalyst with some of those reported in the literature is
given in Table 2. As it is seen, in addition to having the general advantages attributed to the solid supported
catalysts, SiO2 -AlCl3 has a good efficiency compared to other recently reported catalysts.
Table 2. Comparison of the catalytic activity of SiO2 -AlCl3 against other reported catalysts for the acylation of anisole
and toluene with carboxylic acids.
Arene
Carboxylic acid
Catalyst
SiO2-AlCl3
Temp. (◦C) Time (h) Yield (%) (o : p)
Anisole
Benzoic acid
80
120
25
1.6
10
2.5
5
93(6:91)
9218
9618
”
”
AlPW12O40
”
”
AlPW12O40/TFAAa
P2O5/Al2O3
SiO2-AlCl3
”
”
85
6525
Anisole
Heptanoic acid
80
1.7
48
6
91(5:95)
514
”
”
HZSM-5 zeolite
Eu(NTf2)3
150
250
80
”
”
87(3:97)21
90(3:97)
45(1:73)16
42.6(1:38)22
90(3:97)
97(2:98)24
90(3:97)
75(3:94)13
Anisole
Octanoic acid
SiO2-AlCl3
1.6
5
”
”
”
”
Cs2.5H0.5PW12O40
Beta zeolite
110
155
80
6
Anisole Dodecanoic acid
SiO2-AlCl3
1.7
1
”
Toluene
”
”
FePW12O40
160
80
Octanoic acid
”
SiO2-AlCl3
1.8
48
CeNaY zeolite
150
aTrifluoroacetic anhydride.
Conclusion
A convenient and chemoselective method of acylation of aromatic compounds has been devised. The significant
advantages of this methodology are mild reaction conditions, high to excellent yields, short reaction times,
solvent-free conditions, low cost, and easy preparation and handling of the catalyst. In addition, the use of
SiO2 -AlCl3 resulted in a reduction in the unwanted and hazardous waste that is produced during conventional
homogeneous processes. Finally, the reaction work-up is effected through a simple filtration process, and the
catalyst recovered after a reaction can be washed with solvent and used again at least 5 times with no impaired
yield.
Acknowledgements
The author thanks the Research Council of Shahrekord University for partial support of this work.
628

Friedel-Crafts acylation of arenes with carboxylic acids..., K. PARVANAK BOROUJENI
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630

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