Efficient and recyclable rare earth-based catalysts for Friedel-Crafts acylations under microwave heating: Dendrimers show the way

Article abstract of DOI:10.1039/c3gc40720a

The catalytic system involving Sc(OTf)₃ and a dendritic terpyridine ligand is able to promote the Friedel-Crafts acylation of a wide range of aromatics under microwave irradiation. The expected products are obtained in high yields after short reaction times and the nano-sized catalyst can be recovered and successfully used in 12 consecutive runs.

Full text of DOI:10.1039/c3gc40720a

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Efficient and recyclable rare earth-based catalysts for
Friedel–Crafts acylations under microwave heating:
dendrimers show the way†
Cite this: DOI: 10.1039/c3gc40720a
Received 16th April 2013,
Accepted 20th May 2013
Arnaud Perrier,a,b Michel Keller,a,b Anne-Marie Caminade,*a,b Jean-Pierre Majoral*a,b
and Armelle Ouali*a,b
DOI: 10.1039/c3gc40720a
www.rsc.org/greenchem
The catalytic system involving Sc(OTf)3 and a dendritic terpyridine variety of reactions.13 To our knowledge, although terpyridines
ligand is able to promote the Friedel–Crafts acylation of a wide are known to coordinate RE,14 only one example reported the
range of aromatics under microwave irradiation. The expected use of a terpyridine–RE complex in a catalytic reaction.15 We
products are obtained in high yields after short reaction times and thus planned to prepare dendritic terpyridines and to use
the nano-sized catalyst can be recovered and successfully used in them as ligands for RE(OTf)3. The nanometric size of the
12 consecutive runs.
expected resulting complexes should enable their separation
from the reaction mixture and recovery by precipitation.
We focused on Friedel–Crafts (FC) arene acylation [eqn (1)],
one of the most efficient methods for functionalizing aromatic
compounds which affords access to targets or key intermedi-
ates in the pharmaceutical and agrochemical industries.16
Although these reactions are of great value in both academic
and industrial contexts, more than stoichiometric amounts of
Lewis acids such as AlCl3 have to be used because of the
coordination of the latter to the aromatic ketones produ-
ced.1a,16 Moreover, the reaction has to be quenched under
acidic conditions, creating large volumes of corrosive acidic
wastes.17 Other catalysts involving solid Lewis acids (zeolites,
clays, and metal oxides),18 methanesulfonic or trifluoroacetic
anhydrides,19 indium complexes,20 gallium21 or bismuth tri-
flates22 have been developed but except for the last two cases,
the use of stoichiometric amounts, of high quantities of
additional explosive LiClO4 or of harsh temperature conditions
is required. Surprisingly, despite the obvious advantages of RE
(OTf)3-based Lewis acids, their ability to promote FC acylations
has been scarcely studied (<25 publications).4a,c,5c,d,6,23 More-
over, to our knowledge, only a few recyclable systems based on
RE(OTf)3 for FC acylations have been reported so far and they
involve ionic liquids,5c,d a large excess of LiClO4 as the cocata-
lyst,4a,c stoichiometric amounts of RE(OTf)3 6c or high tempera-
ture (160 °C).6b,7a,f
Lewis acid (LA) catalysis provides access to unique reactivity
and selectivity under mild conditions. Although various types
of Lewis acids have been developed and used in industry, most
of the reported systems require strictly anhydrous conditions
and/or stoichiometric amounts.1 Therefore, the search for
more robust Lewis acids attracted much attention and in these
lines,2 rare earth triflates (RE(OTf)3) revealed appealing candi-
dates.3 Indeed, RE(OTf)3 are air- and moisture-stable and can
be used in catalytic amounts in various reactions.3 Noteworthy,
although the development of recyclable catalysts appears as
one of the most exciting challenges for chemists, the recycling
of RE(OTf)3 has only been scarcely studied. The few methodo-
logies published so far, generally displaying low recycling
capabilities, involve the extraction of RE(OTf)3 in water (with
removing of water between two runs under harsh conditions:
190 °C/vacuum),4 their encapsulation in ionic liquids5 or their
immobilization on solid supports.6,7 Besides, we and others
demonstrated that dendritic supports generally offer the possi-
bility to recover and reuse catalysts.8,9 Beyond this advantage,
dendritic ligands may strongly enhance the catalytic activity of
the metal (‘dendritic effect’).8,9g,10 We also recently reported
their surprising ability to dramatically decrease palladium
leaching in Suzuki couplings.9k Moreover, terpyridines consti-
tute attractive ligands of a wide range of metals due to their
strong π-acceptor character11 and their thermal stability12 and
the corresponding complexes are able to promote a wide
aLCC-CNRS, 205 Route de Narbonne, BP 44099, 31077 Toulouse Cedex 4, France
bUniversité de Toulouse, UPS, INPT, LCC, 31077 Toulouse, France.
E-mail: armelle.ouali@lcc-toulouse.fr, jean-pierre.majoral@lcc-toulouse.fr,
anne-marie.caminade@lcc-toulouse.fr
†Electronic supplementary information (ESI)
available. See DOI:
ð1Þ
10.1039/c3gc40720a
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performances being obtained with 2-G4 for which a significant
enhancement of the activity was observed (80% against 60%
without ligand). Noteworthy, in all cases, the amount of ligand
was calculated to reach a Sc/terpyridine moiety ratio of 1 : 1.
Moreover, other RE(OTf)3 tested were less efficient under the
same conditions and the best performances were obtained in
CH3CN.27 The observed positive dendritic effect might be due
to the hydrophobic interiors of phosphorus dendrimers which
are known to act as nanosponges for hydrophobic molecules.28
Dendritic ligand skeletons could therefore favor the concen-
tration of hydrophobic arenes to be acylated, thus creating a
high local concentration of substrates next to the catalytic
sites. The size of dendrimer interiors increasing from the first
generation to the fourth one, this could contribute to explain-
ing the increasing performances of the resulting catalyst
moving from 2-G1 to 2-G4.
The conditions were then applied to other substrates and
1,3-dimethoxybenzene, 1,2-dimethoxybenzene as well as 1,3,5-
trimethoxybenzene could be quantitatively converted into the
corresponding acylated compounds 6b, 6c and 6d (Fig. 2).
More challenging substrates such as thioanisole or dimethyl-
aniline could however not be functionalized under the same
conditions. Noteworthy, the substrate scope of RE(OTf)3
without any co-catalyst is known to be limited to strongly elec-
tron-donating arenes involving methoxy substituents.23i Inter-
estingly, although MWs permit acceleration of a wide range of
organic transformations,29 their application to RE(OTf)3-cata-
lyzed FC acylations has only been scarcely studied and is
limited to a very narrow scope of activated substrates.5d,30 In
the case of anisole, the yield of 6a could be improved from 80
to 99% within only 45 minutes under MW activation (30 W,
Fig. 2, c) instead of 16 h under classical heating (the perform-
ances were the same in the presence of naked Sc(OTf)3 after
45 min). Along the same lines, ketones 6b, 6c, and 6d were
quantitatively obtained within only 30 minutes instead of 16 h
(Fig. 2, c). Use of MWs thus enabled to greatly accelerate reac-
tions of activated substrates. Even more interestingly, it also
Fig. 1 Terpyridine-decorated dendrimers.
Herein we report a highly stable and recyclable LA based on
Sc(OTf)3 and dendritic terpyridines which is able to catalyti-
cally and efficiently promote the FC acylation of aromatics
with various electrophiles. Moreover, microwave (MW) acti-
vation allowed to improve the yields of aromatic ketones and
to widen the substrate scope. This corresponds to the first
application of dendritic terpyridines in the field of catalysis,24
to the first use of terpyridine–RE complexes for FC acylation
and to the second use of such complexes in catalysis. This is
also the first example of a recyclable dendritic catalyst based
on RE in catalysis25 and the first time that MW activation is
applied with dendritic catalysts.
New dendrimers 2-Gn (n = 1, 2, 3, 4) were prepared from
dendrimers 1-Gn bearing respectively 6, 12, 24 and 48 P(S)Cl2
end groups26a,b and 4′-(4-hydroxyphenyl)-2,2′:6′,2′′-terpyridine
326c in the presence of cesium carbonate (Fig. 1, ESI†). Mono-
meric terpyridine M functionalized with a 4-methoxyphenyl
substituent and mimicking the end groups of dendrimer
branches was also prepared according to the literature.26c
The FC acylation of anisole 4a with acetic anhydride 5a was
first performed in acetonitrile in the presence of Sc(OTf)3
(Scheme 1). In the absence of a ligand, the corresponding
ketone 6a was obtained in 60% yield. The latter decreased to
47% when using monomeric terpyridine M as the ligand (Sc/M
ratio = 1 : 1). Moreover, the activity conferred on Sc by dendritic
terpyridines was improved when the dendrimer generation
increased (55% for 2-G1, 59% for 2-G2, 66% for 2-G3), the best
Scheme 1 Acylation of anisole in the presence of terpyridine ligands. Con-
ditions: Sc(OTf )3 (0.035 mmol), M (10 mol%: 0.035 mmol), 2-G1 (0.8 mol%:
0.0029 mmol), 2-G2 (0.4 mol%: 0.00145 mmol); 2-G3 (0.2 mol%:
0.00073 mmol), 2-G4 (0.1 mol%: 0.00035 mmol), 4a (0.35 mmol), 5a
(0.70 mmol), CH3CN (2 mL), 16 h. GC yields determined by using trimethoxy-
benzene as the standard.
Fig. 2 Acylation of different aromatic substrates in the presence of Sc(OTf)3
and 2-G4 a under classicalb or MW heatingc. aConditions: Sc(OTf)3 (0.035 mmol),
2-G4 (0.00035 mmol), substrate (0.35 mmol), acetic anhydride (0.70 mmol),
CH3CN (2 mL). GC yields determined by using anisole or 1,3,5-trimethoxyben-
zene as the standard. bReflux, 16 h. c MWs 30 W (for the corresponding temp-
eratures: see the general procedure). dpara–ortho (4 : 1).
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allowed to extend the application scope of Sc(OTf)3. Indeed, cycles were carried out with little loss of activity. This work
the MW-assisted acylation of thioanisole could successfully be also aimed at extending the scope of Sc(OTf)3-2-G4 to hetero-
achieved and ketone 6e, an important pharmaceutical inter- arenes and other electrophiles (Table 1).
mediate,18,31 was obtained as one regioisomer in 80% yield
Heteroarenes such as furan, thiophene and pyrrole under-
within 2 h (Fig. 2). The substrate scope could also be extended went FC acylation in the presence of 5a, the corresponding
to dimethylaniline which gave rise to ketone 6f in 95% yield as ketones 6h, 6i and 6j being quantitatively isolated within only
a mixture of para- and ortho-substituted arenes (para–ortho 15 or 30 min in the first three runs (Table 1, runs 1 to 3, yields
(4 : 1)) under the same conditions. Noteworthy, in the litera- in bold characters). In the 4th run, ketone 6d was also
ture, only a few methods enable RE(OTf)3-catalyzed FC acyla- obtained in quantitative yield. The catalytic activity did not
tion of these last two substrates and they all involve high decrease during the first 4 uses (see yields obtained for single
amounts of catalyst (≥20 mol%) and/or a large excess of runs given in brackets) which is in accordance with the results
LiClO4 believed to enhance the reaction rates (4 to 16 equi- obtained when recycling the catalyst in the case of dimethyl-
valents compared to the aromatic substrate).4a,c,23f,i However, aniline (Scheme 2).
xylene was not functionalized under the same conditions
The first loss of activity was observed for the 5th run, 6b
(Fig. 2, 6g). Acylation of such unactivated benzene derivatives being obtained in 87% yield (bold characters) instead of the
is indeed known to require the replacement of triflate counter- 99% expected (yield in brackets, see also Fig. 2). Nevertheless,
anions in RE(OTf)3 by perfluorinated anions associated with from the 5th to the 12th run and despite the loss of activity of
high temperatures and long reaction times (180–250 °C, the catalyst (from 10 to 20% depending on the substrates), all
6–24 h).7a,23c,e,g
acylated arenes could be isolated in good to excellent yields.
The recycling ability of the dendritic catalyst based on 2-G4 Therefore, ketones 6a and 6c could be obtained in 85 and 80%
and Sc(OTf)3 was next tested under classical heating by using yields respectively from 5a and anisole or 1,2-dimethoxy-
1,3-dimethoxybenzene as the substrate and under MW heating benzene respectively (6th and 7th runs).
for dimethylaniline (Scheme 2). The recovery procedure was
Moreover, the scope of the catalyst Sc(OTf)3-2-G4 could be
optimized and consisted of the addition of diethyl ether32 at successfully extended to other electrophiles such as acetyl
the end of the reaction to enable the precipitation of the cata- chloride 5b (run 8) or benzoic anhydride 5c (run 9), the corres-
lyst, and a further filtration. In the first case, the catalyst could ponding arenes 6b and 6k being prepared in good yields (78
be successfully reused twice without significant loss of activity and 82 respectively in 1 h). Even after 9 runs, the performances
(from 99 to 95%) but the yield of 6b decreased to 75% in the of the catalyst in the case of more challenging substrates
4th run. In contrast, under MWs, the catalytic system was such as thioanisole and dimethylaniline were found to be com-
efficient for at least four consecutive runs without loss of petitive, 6e and 6f being obtained in 62 and 75% yields
activity. Noteworthy, recycling is not possible when using the respectively (11th and 12th runs).
monomeric terpyridine as the ligand. The superiority of MW
Lastly, by increasing the reaction times compared to those
activation in terms of recycling efficiency might be rationalized used for single runs, it was possible to quantitatively isolate 6f
by the shorter reaction times required (2 h instead of 16) and 6d after 6 h and 2 h respectively instead of 2 h and
which might limit its deactivation. For all these reasons and 30 minutes respectively (11th and 12th runs). Such recycl-
given the fact that MW-assisted acylations are much faster and ability makes the catalyst Sc(OTf)3-2-G4 one of the most
thus attractive in the context of green chemistry, we planned to efficient reported to date for FC acylation.4a,c,5c,d,6b
test the robustness of the Sc(OTf)3-2-G4 catalyst when changing
the substrate at each run. In total, 12 consecutive reaction
Conclusions
In summary, dendritic terpyridines have been prepared and
tested as ligands for Sc(OTf)3 in FC acylation and the fourth
generation dendritic ligand 2-G4 was found to enhance the
catalytic activity of the RE. The corresponding air- and moist-
ure-stable catalytic system is able to promote the acylation of
anisole derivatives by acetic anhydride in refluxing acetonitrile
and the use of MW irradiation enabled both to greatly acceler-
ate the reactions (from 16 h to 15–45 min) and to enlarge the
substrate scope to poorly reactive thioanisole and aniline
derivatives. The application field could next be extended to
heteroarenes and other acylating agents such as acyl chlorides
and less reactive anhydrides. The reported catalyst is very com-
petitive in terms of catalytic performances (high yields, very
short reaction times: 15 minutes to 2 h) and recycling abilities
(12 successful consecutive runs). Dendritic terpyridine ligands
Scheme 2 Recycling tests under classic or MW heating. Conditions: see
Scheme 1.
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Table 1 Recycling of the catalyst based on Sc(OTf)3 and dendritic terpyridine 2-G4 by changing the substrate at each runa
Substrate
RCOX
Product
Time
Yieldb
Run 1
Run 2
Run 3
5a
15 min
99 (99)
5a
5a
15 min
30 min
99 (99)
99 (99)
Run 4
5a
30 min
99 (99)
Run 5
Run 6
Run 7
5a
5a
5a
30 min
45 min
30 min
87 (99)
85 (99)
80 (99)
Run 8
Run 9
Run 10
5b
5c
5a
1 h
1 h
2 h
78 (88)
82 (90)
62 (80)
Run 11
Run 12
5a
5a
2 h
6 h
75c (95)
96c (99)
2 h
98 (99)
a Acylation of ArH and Het-H (0.35 mmol) with various electrophiles (0.70 mmol) in the presence of Sc(OTf)3 (0.035 mmol) and 2-G4
(0.00035 mmol), CH3CN (2 mL), MWs (30 W). b Isolated yields when recycling the catalyst Sc(OTf)3-2-G4 (in bold characters) and GC yields
obtained when using the catalyst for a single run in italics and brackets (GC yield determined by using 1,3-dimethoxybenzene or 1,3,5-
trimethoxybenzene as the standard). c para–ortho (4 : 1).
thus allowed us to propose practical answers to specific chal- catalytic Lewis acid can dramatically cut the environmental
18
lenges of RE catalysis and in particular those concerning the impact of AlCl3 currently still used on the industrial scale
crucial need to dispose of RE(OTf)3-based catalysts with high for FC acylations. The proposed catalytic system is also very
recycling capabilities (the few reported examples do generally innovative since we report here the first application of dendri-
not allow more than 4 successful reuses).4–6 The dendritic tic terpyridines to the field of catalysis,24 the first use of
system also displays unprecedented recyclability compared to terpyridine–RE complexes for FC acylation and the second use
any other system known to promote FC acylations.16–22 of such complexes in catalysis.25 This is also the first example
Additionally, the recovery technique (precipitation/filtration) of a recyclable dendritic catalyst based on RE in these reac-
being convenient to apply on a large scale, the reported tions and the first time that MW activation is applied with
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dendritic catalysts. Studies on the nature of catalytic species
formed in the FC reactions as well as the extension of the
application field of our RE(OTf)3-based catalytic system to FC
acylation of very challenging unactivated benzene derivatives
as well as to other reactions are now under way.
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Experimental
General procedure for recycling tests under microwaves
A CEM, Discover, SP apparatus was used for all reactions under
MW irradiation. A 10 mL reactor equipped with a magnetic
stirring bar was charged with Sc(OTf)3 (0.035 mmol, 15.0 mg),
2-G4 (0.00035 mmol, 18.1 mg) and CH3CN (2 mL). After
15 minutes stirring at room temperature, arene (0.35 mmol)
and an acylating agent (0.70 mmol) were introduced. The tube
was closed, stirred, and heated by MWs (30 W) for the required
time period. After cooling to room temperature, Et2O (8 mL)
was added and the resulting precipitate filtered and washed
twice with Et2O (5 mL). The filtrate was concentrated under
vacuum and the expected product isolated by column chrom-
atography if necessary. The precipitate was directly used for a
new catalytic run: CH3CN (2 mL), arene (0.35 mmol) and an
acylating agent (0.70 mmol) were introduced. The flask was
closed, stirred and heated by MWs for the required time
period.
Note: The maximum temperature reached in the reaction
mixtures mainly depends on the reaction time and to a lesser
extent on the substrate. The temperature is in the range of
130–140 °C for reactions of less than 45 min and in the range
of 150–160 °C for reaction times exceeding one hour (in all
cases, the pressure did not exceed 2 bars in the reactor). Note-
worthy, the performances of acylations at these temperatures
(130–160 °C) under classic heating were the same as those
obtained in refluxing CH3CN.
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Acknowledgements
We thank CNRS and ANR Globucat for funding.
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