An efficient and green method for regio- and chemo-selective Friedel-Crafts acylations using a deep eutectic solvent ([CholineCl][ZnCl2]3)

Article abstract of DOI:10.1039/c6ra03551e

[CholineCl][ZnCl₂]₃, a deep eutectic solvent between choline chloride and ZnCl₂, has been used as a dual function catalyst and green solvent for the Friedel-Crafts acylation of aromatic compounds instead of using the moisture-sensitive Lewis acids and volatile organic solvents. The reactions are performed with high yields under microwave irradiation with short reaction times for the synthesis of ketones. Interestingly, indole derivatives are regioselectively acylated in the 3-position under mild conditions with high yields without NH protection. Three new ketone products are synthesized. [CholineCl][ZnCl₂]₃ is easily synthesized from choline chloride and zinc chloride at a low cost, with easy purification and environmentally benign compounds. [CholineCl][ZnCl₂]₃ can be reused up to five times without loss of catalytic activity, making it ideal in industrial processes.

Full text of DOI:10.1039/c6ra03551e

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An efficient and green method for regio- and
chemo-selective Friedel–Crafts acylations using
a deep eutectic solvent ([CholineCl][ZnCl2]3)†
Cite this: RSC Adv., 2016, 6, 37031
a
a
b
a
*
Phuong Hoang Tran, Hai Truong Nguyen, Poul Erik Hansen and Thach Ngoc Le
[CholineCl][ZnCl2]3, a deep eutectic solvent between choline chloride and ZnCl2, has been used as a dual
function catalyst and green solvent for the Friedel–Crafts acylation of aromatic compounds instead of using
the moisture-sensitive Lewis acids and volatile organic solvents. The reactions are performed with high
yields under microwave irradiation with short reaction times for the synthesis of ketones. Interestingly,
indole derivatives are regioselectively acylated in the 3-position under mild conditions with high yields
without NH protection. Three new ketone products are synthesized. [CholineCl][ZnCl2]3 is easily
Received 7th February 2016
Accepted 1st April 2016
synthesized from choline chloride and zinc chloride at
a low cost, with easy purification and
DOI: 10.1039/c6ra03551e
environmentally benign compounds. [CholineCl][ZnCl2]3 can be reused up to five times without loss of
www.rsc.org/advances
catalytic activity, making it ideal in industrial processes.
catalytic systems containing the catalyst and ionic liquids are
dried under vacuum for a period of from one to three hours
before being used in the next cycle.23 Various homogeneous and
Introduction
The Friedel–Cras acylation is an important tool for organic
syntheses of aromatic ketones, which are useful precursors in
the synthesis of pharmaceuticals, agrochemicals, dyes and
fragrances.1–6 The traditional Lewis acids catalyzing Friedel–
Cras acylations are always used in more than stoichiometric
amounts, and cannot be recovered and reused aer aqueous
workup.2,7 Thus, traditional Lewis acids are not useful in
industrial processes due to environmental problems. Conse-
quently, there is considerable interest in the development of
green catalysts and efficient methods for regio- and chemo-
selectivity in the Friedel–Cras acylation.8–15 Over the past
decade there has been an explosion in the development of green
catalysts for Friedel–Cras acylation, and a large number of
papers have been published.7,16–20 Among these catalysts, ionic
liquids have attracted increasing interest as solvents because of
their unique chemical and physical properties, such as low or
non-volatility, thermal stability and large liquid range.21–23
Consequently, ionic liquids gain a special attraction as green
solvents to replace volatile organic solvents.24
heterogeneous catalysts dissolved in ionic liquids gave the best
conversion.24 However, high cost, environmental toxicity and
high purity requirement limit the use of ionic liquids in organic
synthesis.23 Recently, the rst integrated ionic liquids have been
easily prepared in high purity,25–27 such as chloroaluminate
ionic liquid, which was reported as an efficient catalyst for
Friedel–Cras acylation, but its poor stability to moisture
generated undesired products necessitating the use of an inert
atmosphere.28–31 In addition, the recovery and reuse of the rst
integrated ionic liquids led to decrease of reaction yields due to
the loss of metal chloride into the product stream as benzo-
phenone–metal chloride adduct.29 In addition, gradual
decomposition of the catalyst is also an environmental
problem.32
Recently, Abbott and co-workers have promoted and devel-
oped a new class of ionic liquids called deep eutectic solvents
(DES) which are oen composed of choline chloride and one or
two other components.33 Generally, DES are easily formed
through hydrogen bond interaction, resulting in a lower
The Friedel–Cras acylation using ionic liquids as green
solvents aims to increase the yield and to recycle the catalytic
system without signicant loss of the catalytic activity.23 The
melting point than those of the individual components.34,35
A
slightly different type of DES is formed between choline chlo-
ride and zinc chloride, which can be used as stable Lewis acids
and green solvents for organic syntheses and electrochemical
applications.36 The advantages of DES are easy synthesis with
high purity, non-toxicity, biodegradability and lower price than
traditional ionic liquids.35,37–39
In this paper, we report a green and efficient method with
high regio- and chemoselective Friedel–Cras acylation using
acid anhydrides and [CholineCl][ZnCl2]3 as catalyst under
aDepartment of Organic Chemistry, Faculty of Chemistry, University of Sciences,
Vietnam National University, Ho Chi Minh City 70000, Vietnam. E-mail: lenthach@
yahoo.com; thphuong@hcmus.edu.vn
bDepartment of Science, Systems and Models, Roskilde University, DK-4000 Roskilde,
Denmark
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra03551e
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microwave irradiation. A deep eutectic solvent was used as results are summarized in Table 2. Interestingly, all acid anhy-
catalyst for many organic transformations.40–48 In particular, drides, such as acetic anhydride, propionic anhydride, butyric
DES was used as Lewis acid catalyst in Friedel–Cras alkylation anhydride, iso-butyric anhydride and benzoic anhydride, gave
including alkenylation/alkylation of indole with 1,3-dicarbonyl ketone products with major p-isomer and no demethylation
compounds,49 alkylation of indoles,50 alkylation of electron-rich products were observed. Surprisingly, pivalic anhydride was not
arenes with aldehyde51 and alkylation of thiophenic reactive under the same reaction conditions (Table 2, entries 9–
compounds.52 However its use as a catalyst for Friedel–Cras 11). Anisole is acylated to afford the corresponding ketones in
acylation reactions remains unreported. This is the rst appli- excellent yields at 120 ꢀC for 5 min under microwave irradiation.
cation, to our knowledge, of [CholineCl][ZnCl2]3 as a catalyst for Among the tested acid anhydrides, propionic and benzoic
Friedel–Cras acylation reactions. The [CholineCl][ZnCl2]3 used anhydride provide the highest yields. The above mentioned
in this work had a melting point of 45 ꢀC.36 Choline chloride and
zinc chloride are both inexpensive and the processes of using
deep eutectic solvents like [CholineCl][ZnCl2]3 can be easily
Table 2 Acylation scope with respect to acid anhydridea
applied in industry.
Results and discussion
[CholineCl][ZnCl2]3 was easily prepared by heating and stirring
Entry –R
Temperature (ꢀC) Conversionb (%) Selectivityc (%)
a mixture of choline chloride (20 mmol) and zinc chloride (60
mmol) at 100 C until a clear, colorless liquid was obtained.36
ꢀ
1
CH3
100
120
100
120
100
120
90
95
87
97
54
86
79
93
0
5/0/95
5/0/95
3/0/97
8/0/92
3/0/97
2/0/98
2/0/98
3/0/97
—
First, our investigation focused on nding the optimal
mixture of choline chloride and zinc chloride. The Friedel–
Cras acylations of anisole and indole with propionic anhy-
2
3
4
5
C2H5
C3H7
ꢀ
dride were tested under microwave (MW) irradiation at 120 C
6
for 5 min (see Table 1). The best conversions were obtained
under microwave irradiation with high regio-selectivity when
[CholineCl][ZnCl2]3 was used as the catalyst (Table 1, entries 4
and 8). It could be explained by the stronger Lewis acidity with
more zinc chloride used. [CholineCl][ZnCl2]3 was used in a less
than stoichiometric amount (35 mol%) and was easily recovered
and reused without signicant loss of activity (see below).
Anisole was chosen as a model substrate, and [CholineCl]-
[ZnCl2]3 catalyst was used to screen for the optimal condition
under microwave irradiation at 100–140 ꢀC for 5 min. The
7
8
i-C3H7 100
120
9
t-C4H9 100
120
10
11
12
13
0
—
140
0
—
C6H5
100
120
71
97
8/0/92
0/0/100
a
Anisole (1 mmol), acylating reagent (1 mmol), [CholineCl][ZnCl2]3
b
c
(0.35 mmol). Conversion was reported by GC. The ratio of ortho/
meta/para isomers was determined by GC.
Table 1 Optimization of the ratio between choline chloride and zinc chloride
Entry
Substrate
Anisolea
Catalyst
Conversionc (%)
Selectivityc,d (%)
1
2
3
4
5
6
7
8
ZnCl2
48
60
48
99
63
66
69
99
5/0/95
6/0/94
2/0/98
2/0/98
4/0/96
4/0/96
7/0/93
1/2/97
[CholineCl][ZnCl2]
[CholineCl][ZnCl2]2
[CholineCl][ZnCl2]3
ZnCl2
[CholineCl][ZnCl2]
[CholineCl][ZnCl2]2
[CholineCl][ZnCl2]3
Indoleb
a
Anisole (1 mmol), propionic anhydride (1 mmol), MW (120 ꢀC, 5 min). b Indole (1 mmol), propionic anhydride (1 mmol), MW (120 ꢀC, 10 min).
Conversion and selectivity were determined by GC. Selectivity: anisole (ortho/meta/para isomers), indole (1/2/3 position).
c
d
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Table 3 Friedel–Crafts acylation of various aromatic compounds and five-membered heterocyclesa
Conditions
Entry
1
Substrate
R
(ꢀC, min)
Product
Yieldb (%)
92
Selectivityc (%)
98d
C6H5
120, 5
2
3
4
5
C6H5
C2H5
C6H5
C6H5
120, 5
120, 5
120, 10
120, 10
94
80
90
78
100
100
95e
100
6
7
8
C6H5
C6H5
C2H5
130, 5
80
71
70
100
100
100
140, 20
120, 15
9
C6H5
C2H5
C6H5
140, 10
140, 10
140, 25
80
64
78
89f
10
11
93g
100
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Table 3 (Contd.)
Conditions
(ꢀC, min)
Entry
12
Substrate
R
Product
Yieldb (%)
80
Selectivityc (%)
100
C6H5
140, 20
140, 20
13
C6H5
91
93h
14
15
16
17
18
19
CH3
120, 10
120, 10
120, 10
120, 10
120, 10
120, 10
81
92
83
81
79
80
7/0/93i
1/2/97
3/2/95
3/0/97
5/0/95
9/0/91
C2H5
C3H7
i-C3H7
t-C4H9
C6H5
20
C2H5
120, 10
70
14/0/86
21
22
23
24
25
C2H5
C2H5
C2H5
C2H5
C2H5
100, 20
100, 20
120, 10
120, 10
120, 10
85
82
88
85
92
100
100
100
8/0/92
5/0/95
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Table 3 (Contd.)
Conditions
(ꢀC, min)
Entry
26
Substrate
R
Product
Yieldb (%)
Selectivityc (%)
C2H5
120, 10
90
100
27
C6H5
120, 10
120, 10
72
92
10/0/90
28
C2H5
98j
Arene (1 mmol), acylating reagent (1 mmol), [CholineCl][ZnCl2]3 (0.35 mmol). b Yields are for the isolated, pure isomer. c Selectivity is determined
a
d
e
f
by GC.
ortho/para
¼
2/98.
2,6-Dimethoxybenzophenone/2,4-dimethoxybenzophenone
¼
5/95.
2,6-Dimethylbenzophenone/2,4-
g
h
i
dimethylbenzophenone ¼ 11/89. 2,6-Dimethylpropiophenone/2,4-dimethylpropiophenone ¼ 7/93. ortho/para ¼ 7/93. For indoles and
pyrrole the selectivity is given as 1-/2-/3- isomers. j 1-(Benzofuran-2-yl)propan-1-one/1-(benzofuran-3-yl)propan-1-one ¼ 2/98.
conditions were applied to the Friedel–Cras acylation of more sterically hindered than the others, was also reactive in
a variety of aromatic compounds as seen in Table 3.
this method, giving a product in 79% yield (entry 18).
The aromatic compounds with electron-donating (methoxy)
Table 3 shows a variety of reactions in which the reactivity of
substituents are reactive under optimized conditions, affording indoles bearing electron-poor (halogens) or electron-rich
the benzoylated products in good to excellent yields (entries 1, substituents at position 5 was investigated. The halogen-
2, 4, 5). No demethylation was observed in this method, with the containing indoles selectively afforded 3-propionylation prod-
exception of 1,2,4-trimethoxybenzene (less than 10%). The ucts in good yields in spite of weakly deactivating substituents
Friedel–Cras propionylation of veratrole gave a lower yield (entries 21–23). 4-Bromoindole was propionylated in 70% yield
than benzoylation under similar conditions. Although alkyl- with 86% selectivity at position 3 due to the steric effect of the
benzenes were acylated in good yields (64–80%), higher bromo substituent in the benzene ring. 5-Methylindole was
temperatures and longer reaction times were required than for propionylated in 85% yield (entry 24). 5-Methoxyindole, with
methoxybenzene derivatives. Thioanisole was reactive under electron-donating substituent (methoxy) making it more reac-
optimized conditions in excellent yield.
tive, provided 92% yield (entry 25). Furthermore, a negligible
Indoles are important compounds used in many pharma- quantity of N-acylated products (1–5%) were generated and no
ceuticals. Especially, the Friedel–Cras acylation of indoles at 1,3-diacylation or polymerization occurred in our method.
position
3 has attracted much attention in the past Pyrrole and benzofuran also afforded 3-acylated products in
decade.13,53–60 So far the use of DES as catalyst for this reaction excellent yields (entries 26–28).
has not, to our knowledge, been reported. In this paper, we
The recovery and reuse of [CholineCl][ZnCl2]3 is necessary
report the Friedel–Cras acylation of indoles at position 3 for economic and environmental reasons. Aer extraction,
without N-protection.
[CholineCl][ZnCl2]3 is dried under vacuum at 80 ꢀC for one
Minor modication of the optimized conditions were made hour. Then the recycled [CholineCl][ZnCl2]3 is used in further
when the Friedel–Cras acylation of indole with six types of acid Friedel–Cras acylations (Scheme 1). Interestingly, the catalyst
anhydrides was investigated at 120 ꢀC for 10 min under was stable aer ve consecutive cycles without signicant loss
microwave irradiation. In most cases, the major product was the of the activity. Hence, this result is useful for future industrial
3-substituted one (>90%). The highest yield was obtained with applications.
propionic anhydride. Interestingly, pivalic anhydride, which is
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¨
performed on a Buchi B-545. GC-MS analyses were performed
on an Agilent GC System 7890 equipped with a mass selective
detector (Agilent 5973N) and a capillary DB-5MS column (30 m
ꢁ 250 mm ꢁ 0.25 mm). The 1H and 13C NMR spectra were
recorded on Bruker Avance 500 and Varian Mercury 300
instruments using DMSO-d6 or CDCl3 as solvent and solvent
peaks or TMS as internal standards. HRMS (ESI) data were
recorded on a Bruker micrOTOF-QII MS at 80 eV.
General procedure for Friedel–Cras acylation
A mixture of [CholineCl][ZnCl2]3 (0.192 g, 0.35 mmol), anisole
(0.108 g, 1 mmol) and benzoic anhydride (0.226 g, 1 mmol) was
heated under microwave irradiation at 120 ꢀC for 5 min in
a CEM Discover apparatus. Aer being cooled, the mixture was
extracted with diethyl ether (3 ꢁ 15 mL). The organic layer was
decanted, washed with H2O (10 mL), aqueous NaHCO3 (2 ꢁ 20
mL), and brine (10 mL), and dried over Na2SO4. The solvent was
removed on a rotary evaporator. The crude product was puried
by ash chromatography (n-hexane, then 10% ethyl acetate in n-
hexane) to give 4-methoxybenzophenone (0.195 g, 92% yield).
The purity and identity of the product were conrmed by GC-MS
spectra which were compared with the spectra in the NIST
Scheme 1 Recycling of [CholineCl][ZnCl2]3.
1
library, and by H and 13C NMR spectroscopy.
Experimental
Chemicals, supplies and instruments
Recycling of [CholineCl][ZnCl2]3
(2-Hydroxyethyl)trimethylammonium (choline chloride, purity
$ 99.0%) was obtained from HiMedia Laboratories Pvt. Ltd
(India). Zinc chloride (purity $ 98%) was obtained from Sigma-
Aldrich. Anisole (analytical standard, GC, purity $ 99.9%),
indole (purity $ 99%), propionic anhydride (purity $ 96%),
acetic anhydride (purity $ 99%), butyric anhydride (purity $
97%), isobutyric anhydride (purity $ 99%), t-butyric anhydride
(purity $ 99%), benzoic anhydride (purity $ 95%), 1,2-dime-
thoxybenzene (purity $ 99%), 1,3-dimethoxybenzene (purity >
98%), 1,4-dimethoxybenzene (purity > 99%), 1,2,4-trimethox-
ybenzene (purity $ 97%), mesitylene (purity $ 99%), m-xylene
(purity $ 98%), p-xylene (purity $ 99%), cumene (purity $
98%), thioanisole (purity $ 99%), 4-bromoindole (purity $
96%), 5-bromoindole (purity $ 99%), 5-chloroindole (purity $
98%), 5-uoroindole (purity $ 98%), 5-methylindole (purity $
98%), 5-methoxyindole (purity $ 99%), pyrrole (purity $ 98%)
and benzofuran (purity $ 99%) were obtained from Sigma-
Adrich. Silica gel 230–400 mesh, for ash chromatography
was obtained from HiMedia Laboratories Pvt. Ltd (India). TLC
plates (silica gel 60 F254) were obtained from Merck. Ethyl
acetate (purity $ 99.5%), n-hexane and chloroform (purity $
99%) were obtained from Xilong Chemical Co., Ltd (China).
Chloroform-d, 99.8 atom% D, stabilized with Ag, was obtained
from Armar (Switzerland).
This procedure was also carried out in a monomode microwave
oven, on indole and anisole. In order to recover the catalytic
[CholineCl][ZnCl2]3, aer completion of the reaction, diethyl
ether was applied to wash the reaction mixture as many times as
necessary to completely remove both substrates and products.
Then, the mixture containing [CholineCl][ZnCl2]3 was dried in
a vacuum at 80 ꢀC for 60 min. This recycled system was used for
four consecutive runs and it is worth noting that the isolated
yield of the product decreased slightly aer each run. The
process for recycling [CholineCl][ZnCl2]3 is simple and efficient
so it could be applied on a large scale.
Conclusions
We have developed a novel catalyst taking advantage of green
and efficient catalytic activity under microwave irradiation. The
use of [CholineCl][ZnCl2]3 allows regioselective acylation of
aromatic compounds and ve-membered heterocycles under
mild conditions. A variety of electron-rich compounds such as
alkylbenzenes, anisole derivatives, and ve-membered hetero-
cycles are reactive using the present method. This catalyst
possesses several advantages such as low toxicity, low cost, easy
handling, and easy recycling. The procedure is simple, good to
excellent yields are obtained, and further potential applications
can be foreseen.
All starting materials, reagents and solvents were used
without further purication.
Microwave irradiation was performed on a CEM Discover
BenchMate apparatus which offers microwave synthesis with
safe pressure regulation using a 10 mL pressurized glass tube
with Teon-coated septum and vertically-focused IR tempera-
ture sensor controlling reaction temperature. Melting point was
Acknowledgements
This research is funded by Vietnam National University-Ho Chi
Minh City (VNU – HCM) under grant number C2016-18-21. We
thank Duy-Khiem Nguyen Chau (University of Minnesota –
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