Solvent-free catalytic Friedel - Crafts acylation of aromatic compounds with carboxylic acids by using a novel heterogeneous catalyst system: p-Toluenesulfonic acid/graphite

Article abstract of DOI:10.1002/hlca.200590162

TsOH/graphite was found to be an effective catalyst system for the Friedel-Crafts acylation of aromatic compounds with carboxylic acids. Both aliphatic and aromatic carboxylic acids reacted smoothly under TsOH/graphite catalysis to afford the corresponding aromatic ketones in high yields. The graphite was easily recovered by simple extraction and could be reused without decrease of activity in the presence of fresh TsOH.

Full text of DOI:10.1002/hlca.200590162

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Helvetica Chimica Acta Vol.88 (2005)
Solvent-Free Catalytic Friedel Crafts Acylation of Aromatic Compounds
with Carboxylic Acids by Using a Novel Heterogeneous Catalyst System:
p-Toluenesulfonic Acid/Graphite
by Mona Hosseini Sarvari and Hashem Sharghi*
Chemistry Department, College of Sciences, Shiraz University, Shiraz 71454, I.R. Iran
(phone: ‡ 98-711-2284822; fax: ‡ 98-711-2280926; e-mail: shashem@chem.susc.ac.ir)
TsOH/graphite was found to be an effective catalyst system for the Friedel Crafts acylation of aromatic
compounds with carboxylic acids.Both aliphatic and aromatic carboxylic acids reacted smoothly under TsOH/
graphite catalysis to afford the corresponding aromatic ketones in high yields.The graphite was easily recovered
by simple extraction and could be reused without decrease of activity in the presence of fresh TsOH.
Introduction.
Recently, many studies on catalytic Friedel Crafts acylation
reactions with acid chlorides or anhydrides as acylating agents have been reported [1
5].However, only few studies on Friedel Crafts acylations with carboxylic acids as
acylating agents have been published [6].
Although numerous methods to achieve Friedel Crafts acylation are known, newer
methods continue to attract attention for their experimental simplicity and effective-
ness.We were interested in the development of a method for Friedel Crafts acylation
that a) circumvents chromatographic purification, b) avoids the use of acid anhydrides
or toxic acid chlorides, c) is easy to perform, d) is economical for application to large-
scale preparations, and e) allows to use solvent-free conditions.
Results and Discussion. During the course of our studies aimed at developing
solvent-free procedures [7], we reasoned that an immobilized system, via the
application of solid-phase reagents, could be a more-efficient and cleaner variant of
the classical Friedel Crafts reaction.So, we examined several reactions, focusing on
high efficiency as well as generation of activated acylating species under catalytic
conditions (Table 1).Finally we found that a mixture of graphite and p-toluenesulfonic
acid (TsOH) accelerated the catalytic Friedel Crafts acylation of anisole systemati-
cally.The effect of the amounts of graphite and TsOH in the acylation of anisole is
summarized in Table 1.While no acylated products were obtained in the presence of
TsOH and graphite alone (Entries 5 and 6), a 67% yield of 4-methoxybenzophenone
was obtained when combining 0.3 g of graphite and 0.1 g of TsOH (Entry 7).The yield
was improved when 0.3 g of graphite and 0.03 g of TsOH were used (Entry 10).In the
presence of a large amount of graphite (Entries 11 and 12), the yield decreased due to
adsorption of an appreciable amount of product and starting material on the graphite.
The effect of solvents in this reaction was detrimental since only a trace amount of the
product was detected in CH2Cl2 or CHCl3 solutions (Entries 13 and 14).
¹ 2005 Verlag Helvetica Chimica Acta AG, Z¸rich

Helvetica Chimica Acta Vol.88 (2005)
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Table 1.Friedel Crafts Acylation of Anisole (1 mmol) under Various Reaction Conditions
Entry
Conditions
Acylating agent
Time [h]
Yielda [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
H2SO4, 808
polyphosphoric acid, 808
(CF3CO)2, Al2O3, microwave
[Eu(NTf2)3], 2508
TsOH (0.1 g), 908
graphite (0.3 g), 908
graphite (0.3 g), TsOH (0.1 g, 0.6 mmol), 908
graphite (0.3 g), TsOH (0.05 g, 0.3 mmol), 908
graphite (0.3 g), TsOH (0.04 g, 0.25 mmol), 908
graphite (0.3 g), TsOH (0.03 g, 0.19 mmol), 908
graphite (0.5 g), TsOH (0.03g, 0.19 mmol), 908
graphite (1 g), TsOH (0.03g, 0.19 mmol), 908
graphite (0.3 g), TsOH (0.03g, 0.19 mmol), CH2Cl2
graphite (0.3 g), TsOH (0.03g, 0.19 mmol), CHCl3
PhCO2H
PhCO2H
PhCO2H
Me(CH2)5CO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
3
3
0.17
6
3
3
1
2
2.
3
trace
10
80
87
0
0
67
77
5
86
60
54
trace
trace
80
6
6
6
6
a) Yield of isolated material.
Several examples of the Friedel Crafts acylation of aromatic compounds with
various types of carboxylic acids in the presence of a catalytic amount of TsOH and
graphite are shown in Table 2 ( ! 1 14).In a typical experimental procedure, anisole
(1 mmol) and benzoic acid (1 mmol) were added to TsOH (0.03 g) and graphite (0.3 g)
at 908 (oil bath).After 3 h, the product was isolated by simple extraction of the solid
mass with CHCl3.In all cases (except for Entries 6 9), the reactions proceeded
smoothly giving the corresponding aromatic ketones in high yields (see Table 2) and
purities.No chromatographic separation was necessary, except in a few cases ( Entries 2
and 4) where the product contained some starting material, the conversion being
incomplete.Acylation occurred exclusively at the position para to the MeO or Me
group of the aromatic compounds studied.The acetylation of benzene, anthracene, and
unactivated benzenes with acetic acid, themselves volatile and less reactive, seemed
more difficult to perform (Entries 6 9).Even after vigorous stirring for 8 h, the
reaction did not occur since no acylated products were obtained, the reactants were not
partially consumed, and no side reaction took place.In addition, it was very exciting to
find that unactivated carboxylic acids such as 4-nitrobenzoic acid reacted smoothly in
the presence of TsOH and graphite to afford the corresponding aromatic ketones in
high yields (Entry 12).Furthermore, the reaction was reasonably fast even with the
higher acids and cyclohexanecarboxylic acid (Entries 14 and 15).
The use of graphite for the acylation of activated aromatic compounds has been
reported previously [8], but our new method using a mixture of graphite and TsOH
offers significant improvements over the earlier method, i.e., a considerable decrease of

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Helvetica Chimica Acta Vol.88 (2005)
Table 2.Friedel Crafts Acylation of Aromatic Compounds (1 mmol) with Carboxylic Acids (1 mmol) in the
Presence of a Catalytic Amount of TsOH and Graphite at 908
Entry ArH
RCOOH
MeCO2H
Producta)
Time Yield M.p. or b.p.
[h]
[%]b) [8/Torr]
1
1
3
96
129/637
([9]: 154/760)
2
3
4
5
MeCO2H
MeCO2H
MeCO2H
MeCO2H
2
3
4
5
6
87
48 ([9]: 51)
6
85
81
89/637
([9]: 113/760)
5
186/637
([9]: 213/740)
0.5
97c) 46/637
([9]: 67/760)
6
7
MeCO2H
MeCO2H
MeCO2H
MeCO2H
No reaction
No reaction
No reaction;
No reaction
6
8
8
8
8
9
8
10
2.5
90
95
165
11
7
1.5
125 ([9]:
127.5 128)

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Table 2 (cont.)
Entry ArH
RCOOH
Producta)
Time Yield M.p. or b.p.
[h]
[%]b) [8/Torr]
12
13
14
15
16
17
18
8
4
90
85
90
92
86
84
92
125
220
92.5
110
9
3.5
10
1
Me(CH2)8COOH
11
12
3
3
55 ([9]: 56)
BrCH2(CH2)9COOH 13
4
190
120
14
3.5
a) Products were characterized by their melting points, IR, and NMR spectra; for 13 and 14, see Exper. Part.
b) Yields refer to pure isolated products. c) The reaction was carried out at room temperature.

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Helvetica Chimica Acta Vol.88 (2005)
the reaction time (3 h vs. to 8 h), a minimization of the amount of graphite (1 g vs.
0.3 g), remarkable increase of the yield, solvent-free conditions, and above all, the use
of both electron-donating and specially electron-withdrawing substituted carboxylic
acids as starting materials instead of the expensive and polluting acid halides that
require special cares in handling and workup.
Another interesting behavior of graphite lies in the fact that it can be reused after
simple washing with AcOEt and H2O, thus rendering the process more economic.The
yields of 4-methoxyacetophenone (1) in the 2nd, 3rd, 4th, and 5th use of the graphite
were almost the same as that in the 1st use (Table 3).It should be noted that TsOH was
not adsorbed on the graphite during the reaction and, after extraction with AcOEt,
TsOH was not found on the graphite; thus, it is necessary to add TsOH again for the
2nd use of recovered graphite.No attempt was made to probe the mechanism of the
reaction and further understanding of the catalysis by graphite.
Table 3. Reuse of Graphite in the Acetylation of Anisole with Acetic Acid
Number of use
Yield of 1 [%]
Recovery of graphite [%]
1
2
3
95
90
87
94
87
85
In conclusion, catalytic Friedel Crafts acylation was successfully carried out in the
presence of a new heterogeneous catalyst system, TsOH/graphite.The presented
method provides a very convenient and efficient procedure for the acylation of
aromatic ethers.The notable advantages are the direct use of a wide variety of
carboxylic acids, fast reaction, operational simplicity, generality, excellent regioselec-
tivity, no side reactions, solvent-free conditions, and more or less quantitative yields.
Thus, a higher turnover is expected, and this report may be the answer to the long-term
Friedel Crafts acylation problems.The application of graphite to other synthetic
reactions is now under investigation.
The authors are thankful to the Shiraz University Research Council for the partial support of this work.
Experimental Part
General.Chemicals were purchased from Fluka, Merck, B. D. H., and Aldrich Chemical Companies.TLC
Monitoring: silica gel Polygrams SIL G/UV 254 plates.GC Monitoring: Shimadzu gas chromatograph GC-14A,
flame ionization detector 3-m glass column packed with DC-200 stationary phase; N2 as the carrier gas.IR
Spectra: Perkin-Elmer 781 or Nicolet Impact-400-D FT-IR spectrophotometers; in cmÀ1.NMR Spectra: Bruker-
DPX-250-MHz instrument; d in ppm rel.to SiMe .MS: in m/z (rel.%).
4
Acylation of Aromatic Compounds, General Procedure: To a mixture of graphite (CAS No.7782-42-5; fine
powder extra pure, from Merck; 0.3 g, 25 mmol) and anh. TsOH (0.03 g, 0.17 mmol) at 908 in an oil bath the
aromatic compound (1 mmol) and carboxylic acid (1 mmol) were added.The mixture was kept at 90 8 while
stirring with a mechanical stirrer for the time required to complete the reaction (see Table 2; TLC or GC
monitoring).The solid mass (graphite) was then extracted with AcOEt (20 ml) and the AcOEt extract washed
with aq.NaHCO 3 soln., dried (Na2SO4) and evaporated: practically pure product.The structure of the products
was confirmed by 1H-NMR, IR, m.p. or b.p., and comparison with authentic samples obtained commercially or
prepared by reported methods.Data for new compounds (see Table 2, Entries 17 and 18) are given below.

Helvetica Chimica Acta Vol.88 (2005)
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11-Bromo-1-(4-methoxyphenyl)undecan-1-one (13): IR (KBr): 1676. 1H-NMR (CDCl3): 1.19 1.30 (m,
6 CH2); 1.65 (m, CH2CH2Br); 1.99 (m, COCH2CH2); 2.87 (t, COCH2); 3.75 (s, MeO); 4.07 (t, CH2Br); 6.87 (d, 2
arom.H); 7.89 ( d, 2 arom.H). 13C-NMR (CDCl3): 28.17; 33.20; 40.97; 55.15; 112.61; 129.80; 163.10; 195.40. MS:
‡
354 (100, M ), 356.12 (97.2), 355.12 (20.2). Anal. calc. for C18H27BrO2; C 60.85, H 7.66; found: C 60.60, H 7.40.
1-(4-Methoxyphenyl)-2-phenylethan-1-one (14).IR (KBr): 1679. 1H-NMR (CDCl3): 3.87 (s, MeO); 4.35 (s,
COCH2); 6.94 (d, 2 arom. H); 7.26 7.39 (m, 5 arom.H); 80. 3 ( d, 2 arom.H). 13C-NMR (CDCl3): 50.48; 56.17;
‡
113.44; 114.66; 120.51; 126.97; 128.72; 130.06; 162.12; 191.10. MS: 226 (100, M ).Anal.calc.for C 15H14O2: C
79.62, H 6.24; found: C 79.41, H 6.08.
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Received December 20, 2004