Solvent-free Friedel-Crafts acylation of aromatic compounds with carboxylic acids in the presence of trifluoroacetic anhydride and aluminum dodecatungstophosphate

Article abstract of DOI:10.1016/S0040-4039(03)01199-7

The stable and non-hygroscopic aluminum dodecatungstophosphate (AlPW₁₂O₄₀), was found to be an effective catalyst (3 mol%) for the solvent-free Friedel-Crafts acylation of aromatic compounds with carboxylic acids in the presence of trifluoroacetic anhydride under mild reaction conditions.

Full text of DOI:10.1016/S0040-4039(03)01199-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 5343–5345
Solvent-free Friedel–Crafts acylation of aromatic compounds
with carboxylic acids in the presence of trifluoroacetic anhydride
and aluminum dodecatungstophosphate
Habib Firouzabadi,* Nasser Iranpoor* and Farhad Nowrouzi
Department of Chemistry, Shiraz University, Shiraz 71454, Iran
Received 15 March 2003; revised 27 April 2003; accepted 9 May 2003
This article is dedicated to Professor Michael P. Cava on the occasion of his 75th birthday.
Abstract—The stable and non-hygroscopic aluminum dodecatungstophosphate (AlPW12O40), was found to be an effective catalyst
(3 mol%) for the solvent-free Friedel–Crafts acylation of aromatic compounds with carboxylic acids in the presence of
trifluoroacetic anhydride under mild reaction conditions. © 2003 Elsevier Science Ltd. All rights reserved.
Heteropolyacids (HPAs) show very high catalytic activ-
ities for acid-catalyzed reactions and have been used for
the hydration of propylene1 and isobutylene2 and the
polymerization of tetrahydrofuran.3 Both the strong
Brønsted acidity4 and the softness of the heteropoly
anion are responsible for such high catalytic activities.5
HPAs are usually insoluble in non-polar but readily
soluble in polar solvents. The poor solubility of HPAs
in reaction products improves product selectivity and
also allows easy separation of the reactant and the
product.3a
In continuation of our work on the catalytic properties
of heteropolyacids9 herein, we report the use of alu-
minum dodecatungstophosphate (AlPW12O40) as a sta-
ble and effective heterogeneous catalyst (3 mol%) for
the efficient acylation of aromatic compounds using
carboxylic acids in the presence of trifluoroacetic anhy-
dride (TFAA) under mild reaction conditions in the
absence of solvent (Scheme 1).
The catalyst, AlW12O40 was prepared by a known pro-
cedure involving addition of aluminum nitrate10 to an
aqueous solution of tungstophosphoric acid which on
complete evaporation of water gave the desired com-
pound as a white powder in quantitative yield with
satisfactory analytical results. We have also used alu-
minum carbonate for the preparation of AlPW12O40
without any problem.
Friedel–Crafts acylation of aromatic compounds is the
most important and practical route for the synthesis of
aromatic ketones that are used in manufacturing fine
and speciality chemicals as well as pharmaceuticals.6
For the synthesis of aromatic ketones by Friedel–Crafts
acylation reactions the acylating agents are mostly acid
anhydrides or toxic acid chlorides. In the classical
procedures the most familiar Lewis acid; AlCl3 is used
as a promoter which requires more than 1 equiv. of the
Lewis acid due to its complexation with the aryl ketone
produced in the reaction mixture. In addition, AlCl3 is
unstable in aqueous media and cannot be recovered
from the reaction mixture.6c In order to minimize such
problems new catalytic reactions have been reported in
the literature.7 A literature survey shows that carboxylic
acids are scarcely used for Friedel–Crafts acylations.8
The results of the acylation reactions are presented in
Table 1. Acylation of anisole was conducted by acetic
or benzoic acid in the presence of trifluoroacetic anhy-
dride catalyzed by AlPW12O40 (3 mol%) in excellent
yields (Table 1, entries 1 and 2). 2-Methyl and 4-methyl
anisoles were also acylated with these acids under simi-
lar reaction conditions to give the desired ketones in
excellent yields (Table 1, entries 3–6).
* Corresponding authors. Fax: +098-711-228-6008/0926; e-mail:
firouzabadi@chem.susc.ac.ir; iranpoor@chem.susc.ac.ir
Scheme 1.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01199-7

5344
H. Firouzabadi et al. / Tetrahedron Letters 44 (2003) 5343–5345
Table 1. Acylation of aromatic compounds with carboxylic acids catalyzed by AlPW12O40 in the presence of trifluoroacetic
anhydride
catalyst for the efficient preparation of aromatic
ketones via Friedel–Crafts acylation reactions of
aliphatic and aromatic carboxylic acids in the presence
of trifluoroacetic anhydride.
1,2-Diethoxybenzene under similar conditions, gave the
desired acetyl and benzoyl ketones as the sole products
in this reaction in 93 and 91% yields, respectively
(Table 1, entries 7 and 8). We have also studied the
catalytic activity of AlPW12O40 for the preparation of
aryl ketones from polycyclic aromatic compounds. We
found that acylation and benzoylation of 2-methoxy-
naphthalene was successful with excellent yields at
room temperature and with high regioselectivity
whereas, similar reactions of anthracene needed higher
temperature (50°C) and proceeded in good yields
(Table 1, entries 9–12).
In a typical procedure anisole (2 mmol, 0.216 g), benz-
oic acid (2 mmol, 0.244 g), trifluoroacetic anhydride
(2.8 mmol, 0.39 mL) and AlPW12O40 (0.03 mmol, 0.174
g) were stirred using a magnetic stirrer at room temper-
ature for 2.5 h (Table 1, entry 3). After completion of
the reaction (monitored by TLC and GC), Et2O (10
mL) was added to the reaction mixture and the solid
material was separated by filtration. The filter cake was
washed with Et2O (2×5 mL) and the filtrates were
combined and washed with saturated sodium bicarbon-
ate solution (20 mL). The resulting ethereal solution
was separated and dried over anhydrous Na2SO4.
Evaporation of the solvent resulted in a highly pure
product.
In all the reactions we have studied, cleavage of the
OꢀR bond was not observed, which is usually detected
using other methods.8a
Furan is sensitive to acidic media and polymerizes
easily under Friedel–Crafts conditions. We observed
that the reaction of furan proceeded with acetic or
benzoic acid in excellent yields using a catalytic amount
(3 mol%) of AlPW12O40 in the presence of TFAA
(Table 1, entries 13 and 14). Formation of polymeric
materials was not observed in this reaction.
Acknowledgements
Valuable help for the correction and typing of the
manuscript conducted by Sara Sobhani and partial
support of this work by the Shiraz University Research
Council are highly appreciated.
In conclusion, we have demonstrated that AlPW12O40 is
a cheap, easily available, non-hygroscopic and water
stable compound which can be used as an effective

H. Firouzabadi et al. / Tetrahedron Letters 44 (2003) 5343–5345
5345
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