Friedel-Crafts acylation of aromatic compounds with carboxylic acids in the presence of P2O5/SiO2 under heterogeneous conditions

Article abstract of DOI:10.1016/j.tetlet.2008.09.062

A convenient and efficient procedure for the Friedel-Crafts acylation of aromatic compounds with carboxylic acids in the presence of P₂O₅/SiO₂ is described. Both aromatic and aliphatic carboxylic acids reacted easily to afford the corresponding aromatic ketones. The use of non-toxic and inexpensive materials, simple and clean work-up, short reaction times and good yields of the products are the advantages of this method.

Full text of DOI:10.1016/j.tetlet.2008.09.062

Tetrahedron Letters 49 (2008) 6715–6719
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Friedel–Crafts acylation of aromatic compounds with carboxylic acids
in the presence of P2O5/SiO2 under heterogeneous conditions
Amin Zarei a, , Abdol R. Hajipour b,c, Leila Khazdooz c
*
a Department of Science, Islamic Azad University, Fasa Branch, PO Box No. 364, Fasa, 7461713591 Fars, Iran
b Department of Pharmacology, University of Wisconsin, Medical School, 1300 University Avenue, Madison, 53706-1532 WI, USA
c Pharmaceutical Research Laboratory, College of Chemistry, Isfahan University of Technology, Isfahan 84156, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 June 2008
Revised 31 August 2008
Accepted 12 September 2008
Available online 17 September 2008
A convenient and efficient procedure for the Friedel–Crafts acylation of aromatic compounds with car-
boxylic acids in the presence of P2O5/SiO2 is described. Both aromatic and aliphatic carboxylic acids
reacted easily to afford the corresponding aromatic ketones. The use of non-toxic and inexpensive mate-
rials, simple and clean work-up, short reaction times and good yields of the products are the advantages
of this method.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Friedel–Crafts acylation
Carboxylic acids
P2O5/SiO2
Friedel–Crafts acylation is one of the most important methods
to prepare aromatic ketones, which are used in manufacturing fine
and speciality chemicals as well as pharmaceuticals.1 Typical
procedures include the use of acid chlorides or acid anhydrides
as acylating agents and an excess amount of aluminium trichloride
as a Lewis acid, which entails environmental pollution and a
tedious work-up. In order to minimize these problems, several
catalytic Friedel–Crafts acylations have been developed.2 A litera-
ture survey indicates that the use of carboxylic acids as acylating
agents is a superior method with respect to procedures utilizing
acyl chlorides and anhydrides. Zeolites,3 heteropolyacids and their
salts,4 clay,5 alumina/TFAA,6 MeSO3H/graphite7 and Lewis acids8
are reported to catalyze Friedel–Crafts acylations using carboxylic
acids as acylating agents. Recently, the use of catalysts and
reagents on solid supports has been developed because such
reagents not only simplify purification processes but also help to
prevent release of reaction residues into the environment.9
Although there are many reports using phosphorus pentoxide
as a reagent in organic reactions,10 P2O5 is difficult to handle due
to its moisture sensitivity at room temperature. P2O5 on silica gel
(P2O5/SiO2) is easy to prepare and to handle, and can be removed
from the reaction mixture by simple filtration.11 Herein, we report
an efficient and convenient procedure for the conversion of carbox-
ylic acids to the corresponding aryl ketones in the presence of
P2O5/SiO2. These reactions are easily carried out under hetero-
geneous and reflux conditions (Scheme 1).
O
C
P2O5 / SiO2
R-COOH +
ArH
R
Ar
Reflux, 1-5 h
R= Aryl, Alkyl, Alkenyl
Scheme 1.
The acylation reactions were carried out by heating a stirring
mixture of the carboxylic acid, aromatic compound and P2O5/
SiO2 at reflux.12,13 However, for reagents such as anisole, 1,3-di-
methoxybenzene, 2-methoxynaphthalene, naphthalene, anthra-
cene, 2-methylthiophene, thioanisole and biphenyl, the acylation
reactions were carried out in 1,2-dichloroethane under reflux con-
ditions.14 The products were isolated by simple filtration of the
reaction mixture and then by usual work-up. These reaction condi-
tions were successfully applied for the preparation of different aryl
ketones from electron-rich and electron-poor aromatic com-
pounds. The results of this study are presented in Table 1. The reac-
tions are remarkably clean, convenient and no chromatographic
separation was necessary to obtain spectroscopically pure com-
pounds, except in a few cases (Table 1, entries 26, 29 and 30). Using
this reagent, acylation occurs at the para position with good
selectivity. However, in cases where the para positions are blocked
(Table 1, entries 2 and 16), the acyl group is introduced ortho to the
substituent. This procedure also succeeds for the acylation of thio-
phene and 2-methylthiophene (Table 1, entries 8, 9 and 20), as well
as polycyclic aromatic hydrocarbons (Table 1, entries 7, 11, 26 and
27), producing the corresponding acylated products in good yields.
These reactions are rapid even with higher carboxylic acids.
* Corresponding author. Tel.: +98 9171302528; fax: +98 7313333411.
E-mail address: aj_zarei@yahoo.com (A. Zarei).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.062

6716
A. Zarei et al. / Tetrahedron Letters 49 (2008) 6715–6719
Table 1
Direct acylation of aromatic compounds with carboxylic acids in the presence of P2O5/SiO2 under reflux conditions
Entry
Carboxylic acid
Aromatic hydrocarbon
Product
Time (h)
Yielda (%)
COOH
O
C
1
Toluene
5
68
O2N
NO2
O
C
2
3
p-Xylene
3
3
70
52
O2N
O2N
O
C
Cumene
O
C
4
Mesitylene
Anisole
2
5
66
67
O2N
O2N
O2N
O
C
5be
OMe
OMe
O
C
6c
1,3-Dimethoxybenzene
2-Methoxynaphthalene
3
2
78
86
MeO
NO2
7d
C O
OMe
O
C
S
S
8b
2-Methylthiophene
Thiophene
3
3
58
52
O2N
O2N
O
C
9
O
C
10b
Thioanisole
5
2
62
88
O2N
SMe
NO2
11d,f
Naphthalene
C
O
O
C
12d
13
Biphenyl
3
3
85
37
NO2
O
C
Bromobenzene
O2N
O2N
Br
O
C
NO2
14
Nitrobenzene
3
0

A. Zarei et al. / Tetrahedron Letters 49 (2008) 6715–6719
6717
Table 1 (continued)
Entry
Carboxylic acid
Aromatic hydrocarbon
Product
O2N
Time (h)
Yielda (%)
CH2COOH
O
CH2C
15
16
Toluene
3
80
76
NO2
O
CH2C
p-Xylene
3
O2N
O
CH2C
17
Cumene
Anisole
3
5
45
88
O2N
O2N
O
CH2C
18b
OMe
OMe
O
CH2C
19c
1,3-Dimethoxybenzene
3
90
O2N
MeO
O
CH2C
S
20b
21c
22
2-Methylthiophene
Thioanisole
3
5
3
55
60
82
O2N
O2N
O
CH2C
SMe
COOH
O
C
m-Xylene
O
C
23
24
Mesitylene
2
5
80
28
O
C
Chlorobenzene
Cl
O
C
25c
1,3-Dimethoxybenzene
3
3
84
57
OMe
MeO
26d
Anthracene
C
O
O
27d
28b
2-Methoxynaphthalene
Anisole
3
3
76
86
C
OMe
CH2CH2COOH
O
CH2CH2C
OMe
29
Toluene
3
16
O
(continued on next page)

6718
A. Zarei et al. / Tetrahedron Letters 49 (2008) 6715–6719
Table 1 (continued)
Entry
Carboxylic acid
Aromatic hydrocarbon
Product
Time (h)
Yielda (%)
30
31b
32
33
m-Xylene
Anisole
3
23
O
O
C
3
2
1
3
88
72
70
70
CH CH
OMe
CH CHCOOH
O
ClꢀCH2COOH
m-Xylene
Mesitylene
Anisole
C CH2Cl
O
C CH2Cl
O
34b,g
MeO
C CH2Cl
a
The yield, based on the starting acid, refers to the isolated pure products which were characterized from their spectral data and were compared with authentic
samples.15,16
b
c
d
e
f
The molar ratio of aromatic compound/carboxylic acid is 5/1.5 and the reaction is carried out in 1,2-dichloroethane under reflux conditions.
The molar ratio of aromatic compound/carboxylic acid is 3/1.5 and the reaction is carried out in 1,2-dichloroethane under reflux conditions.
The molar ratio of aromatic compound/ carboxylic acid is 1/1.5 and the reaction is carried out in 1,2-dichloroethane under reflux conditions.
15% of the ortho-isomer was detected by 1H NMR spectroscopy.
18% of the b-isomer was detected by 1H NMR spectroscopy.
g
12% of the ortho-isomer was detected by 1H NMR spectroscopy.
However, for deactivated aromatic rings such as bromobenzene
and chlorobenzene (Table 1, entries 13 and 24), the corresponding
4-acylated products were obtained in low yields. With nitrobenz-
ene, no product was obtained (Table 1, entry 14). The reaction be-
tween 3-phenylpropionic acid and anisole in the presence of P2O5/
SiO2 produced the corresponding 4-acylated product in high yield
(Table 1, entry 28); however, the reaction between 3-phenylpropi-
onic acid and toluene or m-xylene produced 2,3-dihydro-30H-
[1,20]biindenyliden-10-one as a major product of a subsequent aldol
condensation (Table 1, entries 29 and 30).
Acknowledgements
We gratefully acknowledge the funding support received for
this project from the Islamic Azad University of Fasa and the
Isfahan University of Technology (IUT).
References and notes
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Maier, A. H.; Schmider, J. Synthesis 2000, 1427; (d) Bartoli, G.; Bosco, M.;
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6331; (e) Repichet, S.; Roux, C. L.; Roques, N.; Dubac, J. Tetrahedron Lett. 2003,
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8. (a) Suzuki, K.; Kitagawa, H.; Mukaiyama, T. Bull. Chem. Soc. Jpn. 1993, 66, 3729;
(b) Kobayashi, S.; Moriwaki, M.; Hachiya, I. Tetrahedron Lett. 1996, 37, 4183; (c)
Kawamura, M.; Cui, D. M.; Hayashi, T.; Shimada, S. Tetrahedron Lett. 2003, 44,
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1994; p 2.
10. (a) Eaton, C. L. J. Org. Chem. 1973, 38, 4071; (b) Basavaiah, D.; Bakthadoss, M.;
Reddy, G. J. Synthesis 2001, 919; (c) Kato, Y.; Okada, S.; Tomimoto, K.; Mase, T.
Tetrahedron Lett. 2001, 42, 4849.
11. (a) Hajipour, A. R.; Zarei, A.; Khazdooz, L.; Zahmatkesh, S.; Ruoho, A. E.
Phosphorus, Sulfur, Silicon 2006, 181, 387; (b) Mirjalilli, F.; Zolfigol, M. A.;
According to our observations, the present method occurs by
in situ generation of a carboxylic anhydride derived from a mixed
carboxylic-phosphoric anhydride.17,18
To evaluate the role of the SiO2, we studied the acylation of ani-
sole with 4-nitrobenzoic acid, 4-nitrophenylacetic acid, 3-phenyl-
propionic, cinnamic acid and 2-chloroacetic acid in the absence
of SiO2. These reactions were carried out in refluxing 1,2-dichloro-
ethane in the presence of P2O5 alone. We found that the yields
using P2O5/SiO2 were greater (average, 20%) than those with P2O5
alone under the same conditions. The effect of SiO2 may be due
to good dispersion of P2O5 on the surface of silica gel leading to sig-
nificant improvements in its reactivity. SiO2 as a support may also
minimize cross contamination between the product and H3PO4
generated during the course of the reaction.9,17
In conclusion, P2O5/SiO2 is an inexpensive, easily available, non-
corrosive and environmentally benign compound. In this work, we
have reported a convenient and efficient procedure for the prepa-
ration of aryl ketones in good yields and short reaction times. The
notable advantages of this methodology are direct use of a wide
variety of carboxylic acids, operational simplicity, generality, good
regioselectivity, availability of reactants and easy work-up.

A. Zarei et al. / Tetrahedron Letters 49 (2008) 6715–6719
6719
Bamoniri, A. H.; Khazdooz, L. Bull. Korean Chem. Soc. 2003, 24, 1009; (c)
Hajipour, A. R.; Koshki, B.; Ruoho, A. E. Tetrahedron Lett. 2005, 46, 5503; (d)
Eshghi, H.; Hassankhani, A. Synth. Commun. 2006, 36, 2211; (e) Tamaddon, F.;
Khoobi, M.; Keshavarz, E. Tetrahedron Lett. 2007, 48, 3643.
Musiu, C.; Scintu, F.; Putzolu, M.; Colla, P. L. Eur. J. Med. Chem. 1997, 32, 143; (n)
Shirude, S. T.; Patel, P.; Giridhar, R.; Yadav, M. R. Indian J. Chem. 2006, 45B, 1080.
16. Spectral data of new compounds: Table 1, entry 7: Yellow solid; mp 172–174 °C;
1H NMR (300 MHz, CDCl3) d = 8.25 (2H, d, J = 8.82 Hz), 7.98 (3H, m), 7.74 (1H, d,
J = 7.35Hz), 7.52 (1H, d, J = 9.1 Hz), 7.4 (2H, m), 7.32 (1H, d, J = 9.1 Hz), 3.8 (3H,
s). 13C NMR (75 MHz, CDCl3) d = 196.6, 155.5, 151.2, 143.3, 133.2, 132.3, 131,
129.6, 129.2, 128.7, 125.2, 124.6, 124.2, 122, 113.5, 57.2. EIMS m/z (%): 307 (M+,
56), 290 (16), 276 (13), 260 (12), 185 (100), 142 (25), 127 (21), 120 (27), 114
(26), 106 (15), 92 (14), 76 (14), 43 (34). IR (KBr) cmꢀ1: 3040, 2920 1675, 1600,
1530, 1340, 1235, 1080, 890, 840, 800. Anal. Calcd for C18H13NO4: C, 70.03; H,
4.23; N, 4.56. Found: C, 70.16; H, 4.21; N, 4.51. Table 1, entry 8: Yellow solid;
mp 124–126 °C; 1H NMR (300 MHz, CDCl3) d = 8.16 (2H, d, J = 9.2 Hz), 7.97 (2H,
d, J = 9.2 Hz), 7.44 (1H, d, J = 4.1 Hz), 6.87 (1H, d, J = 4.1 Hz), 2.6 (3H, s). 13C NMR
(75 MHz, CDCl3) d = 189, 149.1, 147, 143.2, 142.5, 136.5, 130, 127.2, 123.9,
16.3. EIMS m/z (%): 247 (M+, 2), 246 (3), 232 (2), 125 (100), 106 (5), 97 (6), 78
(2), 53 (15), 45 (4). IR (KBr) cmꢀ1: 3060, 2875, 1630, 1590, 1520, 1450, 1350,
1300, 1050, 870, 840, 700. Anal. Calcd for C12H9NSO3: C, 58.3; H, 3.64; N, 5.66;
S, 12.95. Found: C, 58.22; H, 3.55; N, 5.76; S, 12.86. Table 1, entry 10: Bright
yellow solid; mp 140–142 °C; 1H NMR (300 MHz, CDCl3) d = 8.36 (2H, d,
J = 9 Hz), 7.92 (2H, d, J = 9 Hz), 7.75 (2H, d, J = 8.4 Hz), 7.34(2H, d, J = 8.4 Hz), 2.6
(3H, s). 13C NMR (75 MHz, CDCl3) d = 196, 147.1, 143.8, 140.5, 135, 130.8, 130.7,
125.1, 123.7, 18.5. EIMS m/z (%): 273 (M+, 47), 243 (24), 196 (10), 151 (100),
123 (13), 120 (45), 108 (18), 76 (16), 45 (20). IR (KBr) cmꢀ1: 3065, 2885, 1640,
1585, 1515, 1360, 1290, 1090, 935, 850. Anal. Calcd for C14H11NSO3: C, 61.53;
H, 4.03; N, 5.13; S, 11.71. Found: C, 61.62; H, 4.13; N, 5.04; S, 11.81. Table 1,
entry 16: Yellow solid; mp 86–88 °C; 1H NMR (300 MHz, CDCl3) d = 8.2 (2H, d,
J = 8.5 Hz), 7.55 (1H, s), 7.4 (2H, d, J = 8.5Hz), 7.22 (1H, d, J = 8.05 Hz), 7.15 (1H,
d, J = 8.05 Hz), 4.37 (2H, s), 2.42 (3H, s), 2.39 (3H, s). 13C NMR (75 MHz, CDCl3)
d = 195.1, 145.3, 137, 134.5, 134, 133.2, 132.7, 131.1, 129.7, 128.4, 124.2, 48.1,
24.9, 21.6. IR (KBr) cmꢀ1: 3065, 2890, 1680, 1600, 1510, 1340, 1170, 985, 960,
820, 720. Anal. Calcd for C16H15NO3: C, 71.37; H, 5.57; N, 5.20. Found: C, 71.30;
H, 5.50; N, 5.28. Table 1, entry 17: Yellow solid; mp 122–124 °C; 1H NMR
(500 MHz, CDCl3) d = 8.2 (2H, d, J = 8.3 Hz), 8 (2H, d, J = 7.96 Hz), 7.55 (2H, d,
J = 8.28 Hz), 7.44 (2H, d, J = 7.93 Hz), 4.6 (2H, s), 2.99 (1H, septet, J = 6.8 Hz),
1.24 (6H, d, J = 6.8 Hz). 13C NMR (75 MHz, CDCl3) d = 195.8, 145.2, 143.1, 134.3,
131.3, 130.7, 130.6, 127, 123.6, 45.8, 34.5, 24.1. EIMS m/z (%): 283 (M+, 2), 147
(100), 119 (9), 104 (8), 91 (15), 77 (7), 41 (5). IR (KBr) cmꢀ1: 3060, 2925, 1670,
1600, 1520, 1350, 1230, 990, 840, 725. Anal. Calcd for C17H17NO3: C, 72.08; H,
6.05; N, 4.94%. Found: C, 72.15; H, 6.11; N, 5.01. Table 1, entry 19: Yellow solid;
mp 119–121 °C; 1H NMR (300 MHz, CDCl3) d = 8.18 (2H, d, J = 8.7 Hz), 7.85 (1H,
d, J = 8.65 Hz), 7.38 (2H, d, J = 8.7 Hz), 6.55 (1H, dd, J1 = 8.65 Hz, J2 = 2.48 Hz),
6.48 (1H, d, J = 2.48 Hz), 4.4 (2H, s), 3.92 (3H, s), 3.88 (3H, s). 13C NMR (75 MHz,
CDCl3) d = 194.5, 163, 160.1, 146.2, 142.3, 134.1, 131.4, 124.2, 117.5, 106.1,
98.7, 56.3, 56.1, 49.4. IR (KBr) cmꢀ1: 3045, 2920, 1660, 1600, 1515, 1355, 1310,
1270, 1140, 990, 830, 735. Anal. Calcd for C16H15NO5: C, 63.78; H, 4.98; N, 4.65.
Found: C, 63.71; H, 5.08; N, 4.72.
12. P2O5/SiO2 was prepared by mixing phosphorus pentoxide (3 g) and 3 g of dried
Silica Gel 60 (0.063–0.200 mm, previously heated at 120 °C for 24 h) in a round
bottomed flask with a glass spatula. After 10 min, a fine and homogenous
powder was obtained. This reagent was heated in an oven at 120 °C for 1 h and
then stored in a sealed flask for later use. General procedure for the acylation of
aromatic compounds using carboxylic acids and P2O5/SiO2: To a mixture of a
carboxylic acid (1.5 mmol) and an aromatic compound (5 mL), P2O5/SiO2
(0.6 g) was added and the reaction mixture was stirred under reflux conditions
for the appropriate reaction times (Table 1). After completion of the reaction
(monitored by TLC), the mixture was diluted with EtOAc and filtered. The
organic layer was washed with 10% NaHCO3 solution and then dried over
anhydrous Na2SO4. The solvent was evaporated under reduced pressure to give
the corresponding pure aryl ketone.
13. Typical procedure for the acylation of toluene using 4-nitrophenylacetic acid and
P2O5/SiO2: To a mixture of 4-nitrophenylacetic acid (1.5 mmol, 0.27 g) and
toluene (5 mL), P2O5/SiO2 (0.6 g) was added and the reaction mixture was
stirred under reflux conditions for 3 h. After cooling, the mixture was diluted
with EtOAc (15 mL) and after vigorous stirring was filtered. The residue was
extracted with EtOAc (2 ꢁ 10 mL) and the collected organic layer was washed
with 10% NaHCO3 solution and then dried over anhydrous Na2SO4. The solvent
was evaporated under reduced pressure to give 1-(p-tolyl)-2-(4-
nitrophenyl)ethanone in 80% yield (0.15 g, Table 1, entry 15).
14. Typical procedure for the acylation of 1,3-dimethoxybenzene using 4-nitrobenzoic
acid and P2O5/SiO2 in 1,2-dichloroethane under reflux conditions: To a mixture of
4-nitrobenzoic acid (1.5 mmol, 0.25 g), 1,3-dimethoxybenzene (3 mmol,
0.4 mL) and 1,2-dichloroethane (5 mL), P2O5/SiO2 (0.6 g) was added and the
reaction mixture was stirred under reflux conditions for 3 h. After cooling, the
mixture was diluted with CH2Cl2 (15 mL) and after vigorous stirring was
filtered. The residue was extracted with CH2Cl2 (2 ꢁ 10 mL) and the collected
organic layer was washed with 10% NaHCO3 solution and then dried over
anhydrous Na2SO4. The solvent was evaporated under reduced pressure. The
crude product was washed with cold n-hexane to give (2,4-
dimethoxyphenyl)(4-nitrophenyl)methanone in 78% yield (0.165 g, Table 1,
entry 6).
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