Efficient acylation of toluene and anisole with aliphatic carboxylic acids catalysed by heteropoly salt Cs2.5H0.5PW12O40

Article abstract of DOI:10.1039/b207915c

Heteropoly salt Cs_2.5H_0.5PW₁₂O₄₀ is a highly efficient and reusable solid acid catalyst for the liquid-phase acylation of toluene or anisole with C₂ - C₁₂ aliphatic carboxylic acids.

Full text of DOI:10.1039/b207915c

Efficient acylation of toluene and anisole with aliphatic carboxylic acids
catalysed by heteropoly salt Cs2.5H0.5PW12O40
Jaspal Kaur and Ivan V. Kozhevnikov*
Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, Liverpool,
UK L69 7ZD. E-mail: kozhev@liverpool.ac.uk
Received (in Cambridge, UK) 13th August 2002, Accepted 9th September 2002
First published as an Advance Article on the web 26th September 2002
Heteropoly salt Cs2.5H0.5PW12O40 is a highly efficient and
reusable solid acid catalyst for the liquid-phase acylation of
toluene or anisole with C2 – C12 aliphatic carboxylic acids.
or OMe]. CsPW is compared with bulk and silica-supported
H3PW12O40 (PW) which is the strongest acid in the HPA
series.†
The acylation of toluene was carried out at a molar ratio
PhMe/RCOOH = 50 and 110 °C (reflux) in the presence of ca.
10 wt% CsPW in the reaction mixture. The reaction was clearly
heterogeneous, as CsPW is insoluble; it stopped when the
catalyst was filtered off the reacting mixture. With acetic,
propionic and butyric acids, the yield of acylated products was
very low, though increasing in this series, similar to that
observed for zeolites.3 This may be due to the preferential
adsorption of the lower acids on CsPW, blocking access for
toluene to the catalyst surface. The higher acids C6–C12 were
more reactive in acylation, yielding 31–51% aromatic ketones
(Table 1). All three possible isomers, ortho, meta and para,
were formed, the para isomers being the major products
(55–73%), as expected. The reaction selectivity is virtually
100%, no other products were found. The yield increases in the
series of acids from hexanoic to dodecanoic acid like in the
reaction with CeY zeolite3 and cation-exchanged montmor-
illonite.4 CsPW (SBET 112 m2 g21) is a much more efficient
catalyst than bulk PW (SBET 7 m2 g21) (cf. entries 3 and 5),
which may be explained by a greater number of H+ surface sites
in CsPW.7 The Cs salt is also more active than silica-supported
HPA, 40% PW/SiO2 (entry 6), which may be the result of the
higher hydrophobicity of CsPW,7 favouring the adsorption of
nonpolar reactants on the catalyst surface and making CsPW
more resistant towards deactivation by co-produced water
compared to the more hydrophilic PW. After the reaction,
CsPW can be easily separated by filtration and reused (entry 4).
Some catalyst deactivation was observed, though, which was
probably caused by coking. Similar behaviour has been
observed for the acylation of anisole and toluene over HPA9d
and zeolite.2
Friedel–Crafts aromatic acylation catalysed by strong acids is
the most important route for the synthesis of aromatic ketones
that are intermediates in manufacturing fine and speciality
chemicals as well as pharmaceuticals.1 Present practice uses
acyl chlorides or acid anhydrides as acylating agents and
requires a stoichiometric amount of soluble Lewis acids (e.g.
AlCl3) or mineral acids (e.g. HF) as the catalysts, which results
in substantial amounts of waste and corrosion problems.2
Recently, in view of the increasingly strict environmental
legislation, considerable effort has been put into developing
cleaner Friedel–Crafts chemistry using solid acid catalysts such
as zeolites, clays, Nafion-H, heteropoly acids, etc., zeolites
being the most studied catalysts.2 Likewise, the environmen-
tally benign aromatic acylation with carboxylic acids [eqn. (1)]
instead of the anhydrides and acyl chlorides, resulting in the
formation of water as the only by-product, has been attempted
in a few studies, mostly with zeolites as catalysts.3–5 The
acylation of anisole with acetic anhydride using a zeolite
catalyst has been commercialised by Rhodia.2
(1)
Keggin type heteropoly acids (HPAs) that are very strong
Brønsted acids of formula H82x[XW12O40], where X is the
heteroatom (e.g. P5+ or Si4+) and x is its oxidation state,6–8 are
promising solid acid catalysts for aromatic acylation.6–9 HPAs
have been widely used as catalysts for organic synthesis and
have found several industrial applications.6–8 The insoluble
acidic caesium salt Cs2.5H0.5PW12O40 (CsPW) introduced by
Misono et al.7 has attracted considerable interest as a solid acid
catalyst for various reactions, including Friedel–Crafts reac-
tions, because of its strong acidity, high surface area and
hydrophobicity.7,8
The most important advantage of the CsPW catalyst is that it
gives much higher productivity in aromatic ketones than the
zeolite and clay catalysts reported so far, which may be
attributed to the stronger acidity of CsPW.7 Thus, for the
acylation of toluene with dodecanoic acid, CsPW gives a 1.0%
yield of ketone based on toluene (Table 1, run 3) which is three
times that reported for CeY (0.31%) and for Al3+-montmor-
illonite (0.32%). For CeY, a 96% yield based on dodecanoic
acid at PhMe/acid = 313 has been obtained (150 °C, 48 h).3a
Here we report on a very efficient heterogeneous catalysis by
CsPW for the acylation of toluene and anisole with C2–C12
aliphatic carboxylic acids in the liquid phase [eqn. (1), RA = Me
Table 1 Acylation of toluene (100 mmol) with carboxylic acids (2.0 mmol) at 110 °C (reflux), 48 h
Product distribution (%)
Catalyst/g
Acid
Yield (%)a
para
ortho
meta
1
2
3
4
5
6
CsPW (1.0)
CsPW (1.0)
CsPW (1.0)
CsPW (1.0)b
PW (1.0)
Hexanoic
Octanoic
Dodecanoic
Dodecanoic
Dodecanoic
Dodecanoic
31
47
51
44
14
35
55
72
71
73
60
83
37
22
22
21
23
11
8
6
7
6
17
6
40% PW/SiO2 (2.5)
a The yield of aromatic ketones based on carboxylic acid; the yield based on toluene is 50 times lower. b A reuse of run 3. The catalyst was filtered off, washed
with CH2Cl2, dried at 150 °C/0.1–0.5 Torr/1.5 h and rerun.
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For Al3+-montmorillonite, a 60% yield at PhMe/acid = 187
(110 °C, 24 h) has been reported.4 It should be noted, however,
that CeY gives a higher selectivity towards para-acylation
(94%)3a than CsPW, which may be the result of shape
selectivity of the zeolite.
In conclusion, the heteropoly salt CsPW is a very efficient
catalyst for the environmentally friendly aromatic acylation
with aliphatic acids in heterogeneous liquid-phase systems.
The authors are grateful to Johnson Matthey for financial
support.
The acylation of more reactive anisole with C2–C12 acids was
carried out under the same conditions as that of toluene, except
a shorter reaction time (5 h). The acylated anisole formed as the
major product (para/ortho = 59+1–96+1 and no meta isomers)
together with esterification products—methyl esters of car-
boxylic acids and phenol (Table 2). No phenyl esters were
found. The selectivity to esters increases from acetic to
dodecanoic acid, reaching 40% for the latter. The acylation of
anisole, in contrast to that of toluene, is most efficient with C2–
C6 acids, giving a 62–65% yield of acylated products and only
2–6% of methyl esters. When the acylation of anisole by acetic,
propanoic or butyric acid was carried out in air instead of
nitrogen, 4-methoxy-4A-acyl derivatives of diphenyl ether, in a
molar ratio to acylated anisole of 1+3–1+5, were found
(identified by GC-MS). Only traces of such products formed in
the case of C6–C12 acids. Apparently, these products are formed
by the C–O oxidative coupling of phenol and anisole, followed
by acylation.
The acylation of anisole with HZSM-5 zeolite (Si/Al = 30)
as a catalyst has been reported5 to proceed differently. With C2–
C3 acids, at 120 °C, PhOMe/acid = 4 and 20% HZSM-5, the
phenyl esters are the main products; no methyl esters have been
found. At 150 °C and otherwise the same conditions, a 2+1–5+1
mixture of acylated anisole and phenyl ester forms at an
87–100% acid conversion. The conversion drops sharply for
acids higher than C3, down to 0.6% for C12, probably because of
restricted access into zeolite pores. Thus CsPW is a more active
as well as a more selective catalyst than HZSM-5 for anisole
acylation.
Notes and references
† Experimental. The caesium salt CsPW was prepared by titrating PW by
caesium carbonate in aqueous solution according to the literature method10
and calcined in nitrogen flow at 300 °C for 3 h. The salt CsPW had a BET
surface area of 112 m2 g21. 40% PW/SiO2 was prepared by impregnating
Aerosil 300 silica (SBET = 300 m2 g21) with an aqueous solution of PW.11
Bulk PW and 40% PW/SiO2 were calcined at 150 °C/0.1–0.5 torr/1.5 h and
had SBET values of 7 and 130 m2 g21, respectively. The acylation was
carried out under nitrogen in a 50 ml round-bottomed flask equipped with
a condenser, a drying tube and a stirrer. A mixture of a carboxylic acid (2.0
mmol) and toluene or anisole (100 mmol) and the precalcined CsPW (1 g)
was stirred at 110 °C for a period of 5 h for anisole and 48 h for toluene. The
reaction samples were taken out periodically, filtered and analysed by gas
chromatography (Varian 3380 with autosampler) using a 30 m 3 0.25 mm
BP1 capillary column and tetradecane as internal standard. The products
were identified by GC-MS.
1 G. A. Olah, in Friedel–Crafts and Related Reactions, Wiley-Inters-
cience, New York, vol. I–IV, 1963–1964; G. A. Olah, in Friedel–Crafts
and Related Reactions, Wiley-Interscience, New York, 1973.
2 P. Metivier, in Fine Chemicals through Heterogeneous Catalysis, eds.
R.A.Sheldon and H. van Bekkum, Wiley-VCH, Weinheim, 2001, p. 161
and references therein.
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Organic Reactions, Kodansha/VCH, Tokyo, 1992.
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(b) M. Misono, Chem. Commun., 2001, 1141.
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Catalysis by Polyoxometalates, Wiley & Sons, Chichester, UK, 2002;
(b) I. V. Kozhevnikov, Chem. Rev., 1998, 98, 171.
Table 2 Acylation of anisole (100 mmol) with carboxylic acids (2.0 mmol)
catalysed by CsPW (1.0 g) at 110 °C, 5 h
Product distribution (%)a
Acid
Conversion (%)
para
ortho
methyl ester
9 (a) C. Castro, J. Primo and A. Corma, J. Mol. Catal. A, 1998, 134, 215;
(b) C. Castro, A. Corma and J. Primo, J. Mol. Catal. A, 2002, 177, 273;
(c) B. M. Devassy, S. B. Halligudi, S. G. Hedge, A. B. Halgeri and F.
Lefebvre, Chem. Commun., 2002, 1074; (d) J. Kaur, K. Griffin, B.
Harrison and I. V. Kozhevnikov, J. Catal., 2002, 208, 448; (e) E. F.
Kozhevnikova, E. G. Derouane and I. V. Kozhevnikov, Chem.
Commun., 2002, 1178.
Acetic
Propionic
Butyric
Hexanoic
Octanoic
Dodecanoic
65
53
66
71
45
42
96
93
89
89
73
59
1
3
2
2
1
1
3
4
9
9
26
40
10 Y. Izumi, M. Ono, M. Kitagawa, M. Yoshida and K. Urabe,
Microporous Mater., 1995, 5, 255.
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H. van Bekkum, J. Mol. Catal. A, 1996, 114, 287.
a Based on carboxylic acid converted; para and ortho are the corresponding
acylated anisoles. Phenol in a molar ratio of 1+4–1+5 to acylated anisole is
also formed.
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