Friedel-crafts acylation of arenes with carboxylic acids using polystyrene-supported aluminum triflate

Article abstract of DOI:10.2298/JSC100215010P

Cross-linked polystyrene-supported aluminum triflate (Ps-Al(OTf) ₃) has been shown to be a mild, efficient, and chemoselective heterogeneous Lewis acid catalyst for the acylation of aromatic compounds. The catalyst can be easily prepared from cheap starting materials, is stable (as a bench top catalyst) and is reusable.

Full text of DOI:10.2298/JSC100215010P

J. Serb. Chem. Soc. 76 (2) 155–163 (2011)
UDC 66.095.11+547.857.8:678.746:547–316
JSCS–4108
Original scientific paper
Friedel–Crafts acylation of arenes with carboxylic acids
using polystyrene-supported aluminum triflate
1
2
*
KAVEH PARVANAK BOROUJENI and KAMRAN PARVANAK
1Department of Chemistry, Shahrekord University, Shahrekord 115 and
2Islamic Azad University, Shahr-e-Rey Branch, Iran
(Received 15 February, revised 1 September 2010)
Abstract: Cross-linked polystyrene-supported aluminum triflate (Ps–Al(OTf)3)
has been shown to be a mild, efficient, and chemoselective heterogeneous
Lewis acid catalyst for the acylation of aromatic compounds. The catalyst can
be easily prepared from cheap starting materials, is stable (as a bench top ca-
talyst) and is reusable.
Keywords: acylation; ketones; aluminum triflate; polystyrene.
INTRODUCTION
Friedel–Crafts acylation of arenes is of great importance in both laboratory
1,2
work and industry processes to synthesize aromatic ketones. Usually, this re-
3–12
13–25
action is performed using acid chlorides or anhydrides
or carboxylic acids
as acylating agents in the presence of protic acids or Lewis acids. Acylation of
arenes using carboxylic acids is preferable to the acylation via acid chlorides or
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 the corresponding acid chlorides or an-
hydrides. However, only a few studies on the use of carboxylic acids as acylating
agents have been reported and many of these procedures have serious drawbacks,
such as a need for the use of stoichiometric or excess amounts of catalyst in the
reactions, tedious work-up, environmental pollution, long reaction times, high
reaction temperatures, highly corrosive conditions, the formation of various by-
products and the use of moisture-sensitive, non-recyclable or difficult to handle
catalysts. In view of this, a reliable method for this useful reaction involving
heterogeneous catalysts is demanded.
*Corresponding author. E-mail: parvanak-ka@sci.sku.ac.ir
doi: 10.2298/JSC100215010P
155
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156
BOROUJENI and PARVANAK
Triflic acid (trifluoromethanesulfonic acid), known to be a strong acid, is
26–28
suitable for use as a catalyst for synthetic applications.
However, the re-
covery of the triflic acid from the reaction mixture results in the formation of
large amounts of waste, which is environmentally unacceptable. In continuation
of our ongoing program to develop environmentally benign methods using hete-
rogeneous Lewis acid catalysts, it was found that Ps–Al(OTf) is a good solid
3
29
catalyst for highly chemoselective dithioacetalization of carbonyl compounds.
Along this line, it is herein reported that Ps–Al(OTf) is also an effective and
3
highly chemoselective catalyst for the acylation of aromatic compounds with
carboxylic acids under mild reaction conditions (Scheme 1).
Scheme 1. Acylation of arenes with carboxylic acids using Ps–Al(OTf)3.
EXPERIMENTAL
The employed chemicals were either self-prepared or were purchased from Merck and
Fluka. Polystyrene (8 % divinylbenzene, prepared via suspension polymerization, using poly-
(vinylpyrrolidone) 90 K as the suspension agent, grain size range: 0.25–0.6 mm) was obtained
from the Iran Polymer and Petrochemical Institute. Capacity of the catalyst was determined by
gravimetric method and atomic absorption technique using a Philips PU9400X atomic absorp-
tion spectrophotometer. Reaction monitoring and purity determination of the products were
accomplished by GLC or TLC on silica-gel polygram SILG/UV254 plates. Gas chromatogra-
phy was performed on a Shimadzu GC 14-A. The IR spectra were recorded on a Shimadzu
model 8300 FT-IR spectrophotometer. The NMR spectra were recorded on a Bruker Advance
DPX-300 spectrometer.
Preparation of Ps–Al(OTf)3
This catalyst was prepared as reported in the literature.29 The determined loading of
Al(OTf)3 was 0.41 mmol g-1.
Typical experimental procedure
To a solution of arene (5 mmol) and carboxylic acid (3.5 mmol) was added 0.35 mmol of
Ps–Al(OTf)3 and the reaction mixture was stirred at 80 °C. After completion of the reaction
(monitored by TLC and GC), the catalyst was filtered off and washed with CH2Cl2. The fil-
trate was washed with 10 % NaHCO3 solution (2×10 mL) and water (2×10 mL) and the orga-
nic layer was dried over anhydrous Na2SO4. The unreacted anisole was removed by vacuum
distillation and the aryl ketone product was isolated in high purity in yields from 87 to 97 %.
Whenever required, the products were purified by column chromatography (silica gel) using
petroleum ether–ethyl acetate as the eluent, whereby pure ketone was obtained.
RESULTS AND DISCUSSION
Ps–Al(OTf) was prepared by the exchange reaction between cross-linked
3
polystyrene supported AlCl (Ps–AlCl ) and triflic acid in Freon-113 under re-
3
3
flux conditions. Using catalytic amounts of this catalyst in the Friedel–Crafts
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FRIEDEL–CRAFTS ACYLATION
157
acylation of arenes with carboxylic acids as acylating agents under solvent-free
conditions, high to excellent yields of product were obtained (Table I). The
optimum molar ratio of Ps–Al(OTf) to carboxylic acid was 0.1:1. From the
3
results, it is clear that Ps–Al(OTf) is capable of catalyzing not only the acylation
3
of activated arenes, but also of deactivated arenes (Table I, entries 1–23). The
methodology showed the excellent positional selectivity as the para substituted
product was formed almost exclusively. Naphthalene, 2-methoxynaphthalene and
anthracene underwent acylation with high regioselectivity in 87–93 % yields
(Table I, entries 24–30). It was satisfying 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 I, entries 31–36). These reactions were
regioselective, affording only the 2-acyl product in high to excellent yields. The
acylation of indole with benzoic acid in the presence of Ps–Al(OTf) was also
3
studied and the corresponding indolyl aryl ketone was obtained with high
regioselectivity in 90 % yield (Table I, entry 37). No N-substituted products were
observed under these reaction conditions. Acylation of highly deactivated arenes,
such as nitrobenzene and 1,2-dichlorobenzene failed.
TABLE I. Acylation of arenes with carboxylic acids catalyzed by Ps–Al(OTf)3 (all reactions
performed at 80 °C in the absence of solvent, unless otherwise indicated. The molar ratio of
Ps–Al(OTf)3:carboxylic acid is 0.1:1)
Time Yield, %
h
Entry
1
Arene
Carboxylic acid
PhCO2H
Product
Ref.
(o:m:p)a
3.3
90
–
20,30
2
PhCO2H
3
92
(6:3:91)
90
(3:0:97)
95
25
25
3
p-NO2C6H4CO2H
CH3(CH2)2CO2H
CH3(CH2)5CO2H
CH3(CH2)6CO2H
CH3(CH2)10CO2H
PhCO2H
3.1
2.9
2.9
3
4
13
(4:3:93)
5
94
(5:3:92)
88
(4:0:96)
85
(4:0:96)
13
6
13
7
3.1
3
13
8
94
25
(4:0:96)
95
(5:3:92)
9
p-MeOC6H4CO2H
MeCO2H
2.9
2.9
15,25
31–33
10
94
(6:4:90)
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BOROUJENI and PARVANAK
TABLE I. Continued
Time Yield, %
h
Entry
11
Arene
Carboxylic acid
Product
Ref.
34
(o:m:p)a
PhCH2CO2H
PhCH=CHCO2H
CH3(CH2)2CO2H
CH3(CH2)5CO2H
CH3(CH2)6CO2H
CH3(CH2)10CO2H
CH3(CH2)5CO2H
2.8
94
(6:4:90)
90
(3:0:97)
97
12
13
14
15
16
17
3.1
2.7
2.8
3
25,35
16,18
16,18
16,18
16,18
21
(5:4:91)
95
(5:3:92)
93
(3:0:97)
3
92
(3:0:97)
90
3
(95:5)b
18
19
20
PhCO2H
MeCO2H
PhCO2H
2.9
2.8
3.1
93
–
25
18,36
30
94
–
88
(3:4:97)c
21
22
23
24
PhCO2H
PhCO2H
MeCO2H
MeCO2H
3
91
(7:3:90)
87
(4:3:93)
89
7,30
30
3.5
3.3
3.3
30
(5:3:92)
88
18
(3:97)c,d
25
26
PhCO2H
3.1
3
93c
–
25
25
p-NO2C6H4CO2H
90c
–
27
MeCO2H
3.1
90c
–
18,37
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FRIEDEL–CRAFTS ACYLATION
159
TABLE I. Continued
Time Yield, %
Entry
28
Arene
Carboxylic acid
Product
Ref.
h
(o:m:p)a
PhCH2CO2H
3
90c
–
18,38
29
30
PhCO2H
3.4
3.3
87c
–
25
MeCO2H
89c
–
18,37
31
32
33
34
35
PhCO2H
MeCO2H
3.5
3.3
3.6
3.6
3.4
89e
–
90e
20,39
5,40
5
p-NO2C6H4CO2H
MeCO2H
89e
–
92e
5,41
5,42
PhCO2H
87e
–
36
37
MeCO2H
PhCO2H
3.3
4
89e
–
18,42
90c
–
43–45
a
1
Isolated yields. Isomer distribution based on 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 authentic samples;
b
c
isomer distribution of the 3,4-dimethylphenyl isomer to the 2,3-dimethylphenyl isomer; the reaction was
d
e
performed in 1,2-dichloroethane; α:β ratio; the reaction was performed at 60 °C.
When Ps–Al(OTf) was used as the catalyst in the acylation reactions, no
3
band corresponding to –CO stretching in the IR spectrum of Ps–Al(OTf) was
3
observed after the reactions in either the presence or absence of substrate, in-
dicating that polystyrene itself did not undergo acylation under the employed
experimental conditions. Probably acylation reactions are not favored with Ps–
29,46
–Al(OTf) as a π complex is formed between the polystyrene and Al(OTf) .
3
3
By-product formation was not observed in the studied reactions. Steric crowding
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160
BOROUJENI and PARVANAK
of the supported catalyst influences the positional selectivity (isomer distribution)
observed in the acylation of aromatic compounds.
To determine whether the reaction occurs in the solid matrix of Ps–Al(OTf)
3
or whether Al(OTf) simply released into the reaction medium is responsible for
3
the acylation reaction, Ps–Al(OTf) was added to toluene and the mixture was
3
stirred at 80 °C for 2 h. Then, the catalyst was filtered off and the filtrate was
analyzed for its aluminum content, which showed a negligible release of Al(OTf) .
3
The filtrate was found to be inactive for acylation reaction. These observations
indicate that Ps–Al(OTf) is stable under the employed reaction conditions and
3
there was no leaching of acid moieties during the studied reactions.
One notable achievement of this solid acid catalyst was intramolecular Frie-
del–Crafts acylation. For example, 4-phenylbutanoic acid cyclized in nitroben-
zene at 80 °C in the presence of Ps–Al(OTf) to afford the desired 1-tetralone in
3
94 % yield (Scheme 2).
Scheme 2. Intramolecular Friedel–Crafts acylation using Ps–Al(OTf)3.
Ps–Al(OTf) recovered after a reaction can be washed with dichloromethane
3
and used again at least five times without any noticeable loss of catalytic activity
(Scheme 3). The capacity of the catalyst after five uses was 0.40 mmol Al(OTf)
per gram of polymeric catalyst.
3
Scheme 3. Acylation of toluene using recovered Ps–Al(OTf)3 (recovered catalyst was
successfully reused).
A comparison of the efficiency of Ps–Al(OTf) catalyst with some of those
3
reported in the literature is given in Table II. As can be seen, in addition to hav-
ing the general advantages attributed to solid supported catalysts, Ps–Al(OTf)
also has good efficiency compared to other recently reported catalysts.
3
Representative spectral data of some of the obtained compounds are given
below.
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FRIEDEL–CRAFTS ACYLATION
161
TABLE II. Comparison of the catalytic activity of Ps–Al(OTf)3 against other reported cata-
lysts for the acylation of anisole and toluene with carboxylic acids
Arene Carboxylic acid
Anisole Benzoic acid
Catalyst
Temperature, °C Time, h Yield, % (o:p)
Ps–Al(OTf)3
AlPW12O40
80
120
25
3
10
2.5
5
94(4:96)
9218
AlPW12O40/TFAAa
P2O5/Al2O3
9618
85
6525
Anisole Heptanoic acid
Anisole Octanoic acid
Ps–Al(OTf)3
HZSM-5 zeolite
Eu(NTf2)3
Ps–Al(OTf)3
Cs2.5H0.5PW12O40
Beta zeolite
Ps–Al(OTf)3
FePW12O40
Ps–Al(OTf)3
CeNaY zeolite
80
2.8
48
6
3
5
6
3
1
3
95(5:92)
150
250
80
110
155
80
160
80
150
514
87(3:97)21
93(3:97)
45(1:73)16
42.6(1:38)22
92(3:97)
97(2:98)24
88(4:96)
75(3:94)13
Anisole Dodecanoic acid
Toluene Octanoic acid
48
aTrifluoroacetic anhydride
–1
1
4-Methylbenzophenone. IR (KBr, cm ): 1660 (C=O). H-NMR (300 MHz,
CDCl , δ / ppm): 2.51 (3H, s), 7.51 (2H, d, J = 8.31 Hz), 7.87–8.15 (7H, m).
3
–1
1-(4-Methoxyphenyl)-3-phenyl-2-propen-1-one. IR (KBr, cm ): 1650 (C=O).
1
H-NMR (300 MHz, CDCl , δ / ppm): 3.91 (3H, s), 6.99 (2H, d, J = 8.71 Hz),
3
7.47 (3H, m), 7.59 (1H, d, J = 15.15 Hz), 7.69 (2H, m), 7.86 (1H, d, J = 15.15
Hz), 8.11 (2H, d, J = 8.71 Hz).
–1
1
1-(3,4-Dimethylphenyl)-1-heptanone. IR (KBr, cm ): 1685 (C=O). H-NMR
(300 MHz, CDCl , δ / ppm): 0.82–0.91 (3H, m), 1.23–1.41 (6H, m), 1.63–1.74
3
(2H, m), 2.26–2.33 (6H, m), 2.80 (0.1H, t, J = 7.2Hz, 2,3-dimethyl isomer), 2.90
(1.9H, t, J = 7.2 Hz, 3,4-dimethyl isomer), 7.11 (0.05H, t, J = 7.2 Hz, 2,3-di-
methyl isomer), 7.13–7.20 (1H, m), 7.25 (0.05H, d, J = 7.2 Hz, 2,3-dimethyl
isomer), 7.65 (0.95H, d, J = 7.2 Hz, 3,4-dimethyl isomer), 7.70 (0.95H, d, J = 7.2
Hz, 3,4-dimethyl isomer).
–1
1
2,4,6-Trimethylbenzophenone. IR (KBr, cm ): 1669 (C=O). H-NMR (300
MHz, CDCl , δ / ppm): 2.2 (6H, s, 2- and 6-Me), 2.4 (3H, s, 4-Me), 6.91 (2H, s,
3
aromatic protons of mesityl group), 7.45 (2H, m), 7.55 (1H, m), 7.81 (2H, m).
–1
1
1-Benzoyl-2-methoxynaphthalene. IR (KBr, cm ): 1665 (C=O). H-NMR
(300 MHz, CDCl , δ / ppm): 3.88 (3H, s), 7.31–7.52 (7H, m), 7.55–8.12 (4H, m).
3
CONCLUSIONS
In conclusion, a convenient and chemoselective method of the acylation of
aromatic compounds has been devised. The significant advantages of this me-
thodology are mild reaction conditions, high to excellent yields, solvent-free
conditions and easy preparation and handling of the catalyst. In addition, the use
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162
BOROUJENI and PARVANAK
of Ps–Al(OTf) resulted in a reduction in the unwanted and hazardous waste that
3
is produced during conventional homogeneous processes.
Acknowledgements. We gratefully acknowledge the partial support of this study by the
Shahrekord University and the Islamic Azad University, Shahr-e-Ray branch Research Coun-
cil, Iran.
И З В О Д
ФРИДЕЛ–КРАФТСОВО АЦИЛОВАЊЕ АРЕНА КАРБОКСИЛНИМ КИСЕЛИНАМА
ПОМОЋУ АЛУМИНИЈУМ-ТРИФЛАТА НА ПОЛИСТИРЕНУ
KAVEH PARVANAK BOROUJENI1 и KAMRAN PARVANAK2
1Department of Chemistry, Shahrekord University, Shahrekord 115 и 2Islamic Azad University,
Shahr-e-Rey Branch, Iran
Показано је да је Луисова киселина алуминијум-трифлат на унакрсно повезаном поли-
стирену благ, ефикасан и хемoселективан катализатор за ациловање ароматичних једињења.
Катализатор се лако припрема из јефтиних полазних једињења, стабилан je и може се више
пута користити.
(Примљено 15. фебруара, ревидирано 1. септембра 2010)
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