[C8dabco]Br: a mild and convenient catalyst for intramolecular cyclization of 2-aminochalcones to the corresponding 2-aryl-2,3-dihydroquinolin-4(1H)-ones

Article abstract of DOI:10.1007/s10593-016-1840-8

[Figure not available: see fulltext.] A new and convenient synthesis of 2-aryl-2,3-dihydroquinolin-4(1H)-ones has been described using the intramolecular cyclization of 2-aminochalcones catalyzed by 1-octyl-4-aza-1-azoniabicyclo[2.2.2]octane bromide ([C

Full text of DOI:10.1007/s10593-016-1840-8

DOI 10.1007/s10593-016-1840-8
Chemistry of Heterocyclic Compounds 2016, 52(2), 99–103
[C8dabco]Br: a mild and convenient catalyst
for intramolecular cyclization of 2-aminochalcones
to the corresponding 2-aryl-2,3-dihydroquinolin-4(1H)-ones
Chamseddine Derabli1, Sara Mahdjoub1, Raouf Boulcina1*,
Boudjemaa Boumoud1, Hocine Merazig2, Abdelmadjid Debache1
1 Laboratory of Synthesis of Molecules with Biological Interest, University of Constantine 1,
Constantine 25000, Algeria; e-mail: boulcinaraouf@yahoo.fr
2 Chemistry Research Unit in Environmental and Molecular Structure,
University of Constantine 1, Constantine 25000, Algeria
e-mail: h_merazig@yahoo.fr
Published in Khimiya Geterotsiklicheskikh Soedinenii,
2016, 52(2), 99–103
Submitted October 11, 2015
Accepted after revision January 20, 2016
A new and convenient synthesis of 2-aryl-2,3-dihydroquinolin-4(1H)-ones has been described using the intramolecular cyclization of
2-aminochalcones catalyzed by 1-octyl-4-aza-1-azoniabicyclo[2.2.2]octane bromide ([C8dabco]Br). Recyclability of the catalyst, high
yields, simple isolation of the products, and high atom economy are the noteworthy aspects of the protocol.
Keywords: 2-aryl-2,3-dihydroquinolin-4(1H)-ones, [C8dabco]Br, ionic liquids, aza-Michael reaction, cyclization.
Scheme 1
2-Aryl-2,3-dihydroquinolin-4(1H)-ones are valuable
precursors1 for the synthesis of medicinally important
substances;2 these compounds also are common structural
intermediates in the biogenesis of naturally occurring
flavonoid-type derivatives.3 Derivatives of 2-arylquinolin-
4-ones have displayed interesting biological properties.4
Over the last years, the interest in 2-arylquinolin-4(1H)-
ones and their analogs has prompted extensive studies of
their properties, such as cytotoxicity against human tumor
cell lines and tubulin polymerization inhibition,5 anticancer
activity in the xenograft ovarian OVCAR-3 model, 13%
increase in lifespan of mice,6 hepatoprotective activity,7
and potential use as scintillator dyes in photooxidative
stability as well as antiplatelet agents.8
successfully used for such cyclization. Intramolecular
cyclization in the presence of basic catalyst such as
NaOH21 is well documented in literature. Other methods
like thermolysis,22 electrolysis,23 photolysis,24 as well as
use of Ni/Zn/K halides25 or ZnO-supported metal oxides,26
were also employed for the cyclization.
However, most of these procedures have limited
synthetic utility because of the use of corrosive acids,
strong alkalis, hazardous and expensive reagents. More-
over, many of them are of limited synthetic scope because
of poor yields, long reaction times, need for large amount
of catalyst, high temperature, specialized solvents,27 some
others employ microwave irradiation with solid support.28
Therefore, the development of new methods that lead to a
more convenient procedure and better yield is still
desirable.
The methodology most widely used to prepare 2-aryl-
2,3-dihydroquinolin-4(1H)-ones involves the cyclization of
the corresponding 2-aminochalcones, bearing substituents
in the aromatic rings, by an intramolecular aza-Michael
reaction (Scheme 1). This cyclization has been carried out
under a variety of conditions: using acids,9 such as
orthophosphoric10 or sulfuric,11 zeolites,12 silica gel,13
PEG-400,14 microwave irradiation,15 chiral Brønsted acids
and bases,16 and others.17
In the last decade, ionic liquids (ILs) have increasingly
found use in various organic transformations due to their
tunable chemical and physical properties and catalytic
20
Very recently, InCl3,18 CeCl3·7H2O,19 and TaBr5
immobilized on inorganic solid support have also been
0009-3122/16/52(2)-0099©2016 Springer Science+Business Media New York
99

Chemistry of Heterocyclic Compounds 2016, 52(2), 99–103
behavior.29 In particular, Lewis basic ILs [Cn+1dabco]X
derived from 1,4-diazabicyclo[2.2.2]octane (DABCO) have
proved their catalytic activity30 and attracted the attention
of scientific community due to their easy preparation from
commercially available and relatively inexpensive starting
materials. Besides, Carlo Pretti et al. have reported that
acute toxicity and biodegradability of DABCO-based ILs
are comparable with those of biodegradable functionalized
imidazolium-based ILs.31 The synthesis and general
structure of DABCO-based ILs used in this work are
shown in Scheme 2.
Table 1. Conditions of aza-Michael cyclization
of 2-aminochalcone 3a and yields of dihydroquinolinone 4a*
Amount
of catalyst,
mol %
Yield,
%
Entry
Catalyst
Solvent
Time, h
1
2
–
–
MeCN
24
24
24
24
24
24
24
24
4
Traces
10
ArB(OH)2
(NH4)2Ce(NO3)6
Bi(OTf)3
10
MeCN
3
10
DMF
15
4
10
MeCN
0
5
POCl3
10
DMF
10
6
Et3N
10
MeCN
Traces
Traces
0
7
Piperidine
PPh3
10
10% aq KOH
Scheme 2
8
10
PhMe
Me
Br
( )n
N
N
N
NaBF4
9
[C14dabco]Br
[C14dabco]BF4
[C10dabco]Br
[C10dabco]BF4
[C8dabco]Br
100
100
100
100
100
–
–
–
–
–
65
+
+
10
11
12
13
4
61
EtOAc
rt, 24 h
EtOAc
rt, 24 h
N
N
N
–
Br–
BF4
3
80
DABCO
( )n
( )n
Me
Me
3
67
n = 7, 9, 13
[Cn+1dabco]Br
[Cn+1dabco]BF4
2
90
88 (85)**
In this paper we would like to report an efficient process
for the synthesis of 2-aryl-2,3-dihydroquinolin-4(1H)-one
derivatives by employing Lewis basic DABCO-based ILs
as efficient and recyclable catalysts.
14
[C8dabco]BF4
100
–
2
83
* Load of 2-aminochalcone 3a – 1 mmol; reaction temperature: reflux (for
entries 1–8) or 150°C (for entries 9–14).
** Yields with recovered (twice recovered) catalyst.
We first investigated the possibility of synthesizing
2-phenyl-2,3-dihydroquinolin-4(1H)-one (4a) directly from
2-aminoacetophenone (1) and benzaldehyde (2a) in one
step, using different reaction conditions including the use
of acids or bases (Bi(OTf)3, SbCl3, and Et3N) as catalysts.
However, the desired product 4a was formed in very poor
yield (<10%); the efforts made so far to improve the yields
of compound 4a have failed and furnished the cor-
responding imine 5a as the major product with yields up to
90% (Scheme 3).
fair yields and the reactions gave complex mixtures
including starting material 3a (entries 2–8).
We have observed that there are few reports in the
literature on the use of ionic liquids in the synthesis of
dihydroquinolinones.32 Hence, we decided to investigate
catalytic performance of a series of DABCO-based ILs in
the preparation of the target compounds (Scheme 3).
For this purpose, IL [C14dabco]Br was used as the
catalyst, and we were pleased to see that the reaction under
solvent-free conditions proceeded efficiently, resulting
after 4 h in good yield of the desired product 4a (entry 9).
The effect of using other ILs on the reaction was also
investigated. The [C14dabco]BF4, too, showed a good
activity, but lower yield (entry 10). [C10dabco]BF4 gave
similar results (entry 12), whereas [C10dabco]Br was found
to be more active, affording the corresponding dihydro-
quinolinone 4a in very good yield (entry 11).
However, when [C8dabco]Br and [C8dabco]BF4 were
used, excellent yields of 90 and 83% respectively has been
achieved (entries 13, 14). Therefore, [C8dabco]Br was
identified as the most effective IL catalyst, and was thus
chosen as the model catalyst for further investigation. This
shows that all the DABCO-based ILs can very efficiently
catalyze the cyclization of 2-aminochalcones, when
100 mol % of the catalyst was used. Reduction of the
concentration of the catalyst from 100 to 50 or 20 mol %
resulted in decreasing yield of cyclized product.
Scheme 3
O
O
O
PhCHO
2a
Me
NH2
Me
Ph
+
N
H
Ph
N
1
4a
5a
(Major product)
2a
Catalyst
O
Ph
NH2
3a
This prompted us to explore an alternative strategy
based on the intramolecular cyclization of 2-aminochalcone
3a which was initially carried out under catalyst-free
conditions using MeCN as the solvent. However, also in
this case the reaction was very slow, and the yields of
dihydroquinolinone 4a were also very poor (Table 1, entry 1).
In order to improve the yield, the same reaction was
initially examined in the presence of a series of acidic or
basic catalysts traditionally used for such cyclization. In all
cases, the desired product 4a was obtained in moderate to
It is well known that the stability and reusability of the
catalyst system are the two key factors that determine its
potential of practical application in industry. To test the
catalyst recyclability, the reaction was carried out under
identical conditions in the presence of a catalytic amount of
[C8dabco]Br. The catalyst was recovered after the product
was separated out from the reaction mixture by distillation
100

Chemistry of Heterocyclic Compounds 2016, 52(2), 99–103
Table 2. Conditions of aza-Michael cyclization
of 2-aminochalcones 3a–m catalyzed by [C8dabco]Br,
yields and mp of obtained dihydroquinolinones 4a–m
under reduced pressure and then used for the next run
without further purification under the same conditions. The
results as listed in Table 1 indicate that no significant drop
in the yield of compound 4a was detected after three
successive cycles (entry 13).
Having successfully optimized the reaction conditions
by using [C8dabco]Br as catalyst, we investigated the scope
of this intramolecular cyclization by utilizing various
chalcones 3a–m as substrates. As shown in Table 2,
electron-rich and electron-poor 2-aminochalcones 3a–m,
including ortho-substituted ones, were converted into the
corresponding dihydroquinolinones 4a–m in good to
excellent yields. In general, electron-donating amino-
chalcones reacted better and gave the desired products 4a–f
in excellent yields. Functionalized chalcones bearing
electron-withdrawing substituents, such as chloro, bromo,
fluoro, or nitro, were readily transformed into the cor-
responding condensation products 4g–m albeit in moderate
to good yields.
The structures of the products were established from
their spectral properties (IR, 1H and 13C NMR) and also by
comparison with available literature data.32,33 In the case of
compounds 4f,i, suitable single crystals could be obtained,
and their structures were additionally confirmed by the
single crystal X-ray diffraction data (Figs. 1, 2).
A plausible reaction mechanism can be envisioned for
the ring closure of aminochalcones. Being quite strong
bases,34 the ionic liquid would deprotonate the amine group
in compounds 3 to give the intermediate species. It is
expected that enhanced nucleophilicity of amino group and
ability of ionic liquid to stabilize the formation of charged
species are probable reasons for the increased reaction rate.
It is expected also that this activates the nucleophilic attack
at β-carbon inducing the cyclization. Products 4 were
obtained by tautomerization of the corresponding inter-
mediates (Scheme 4).
Com-
pound
Reaction
time, h
Yield,
%
R
H
Mp, °C
2
2
2
2
3
90
92
76
83
80
148–150
(148–149)33a
150–152
4a
4b
4c
4d
4e
4-Me
2-OMe
4-OMe
4-NMe2
(148–149)32a
128–130
(126–128)33e
142–144
(144–146)33a
179–181
(183–184)33b
186–188
4-Ph
2-Cl
2
2
86
75
4f
4g
143–145
(146–147)33b
170–172
4-Cl
3,4-Cl2
4-Br
3
2
3
3
3
2
73
72
76
81
70
82
4h
4i
(170–172)33a
204–206
(205–208)33b
160–162
4j
(160)33d
4-F
134–136
4k
4l
(134)33c
3-NO2
2-Cl-5-NO2
160–162
(159–160)32b
168–170
4m
Scheme 4
Figure 1. Molecular structure of compound 4f. Thermal ellipsoids
are shown at the 50% probability level.
In conclusion, a very efficient method has been deve-
loped for the synthesis of substituted dihydroquinolinones
catalyzed by [C8dabco]Br under conventional heating. This
method has several advantages, such as high conversion
level, simplicity in operation, cost efficiency, and use of an
ionic liquid as catalyst and solvent, which significantly
contribute to the practice of green chemistry. The use of
Figure 2. Molecular structure of compound 4i. Termal ellipsoids
are shown at the 50% probability level.
101

Chemistry of Heterocyclic Compounds 2016, 52(2), 99–103
ionic liquid as a non-volatile medium, simple work-up,
δ, ppm (J, Hz): 8.54 (1H, d, J = 2.4, H Ar); 8.04 (1H, dd,
J = 8.8, J = 2.8, H Ar); 7.76 (1H, dd, J = 8.0, J = 1.6,
H Ar); 7.51 (1H, d, J = 8.8, H Ar); 7.31 (1H, td, J = 8.0,
J = 1.6, H Ar); 6.78–6.74 (2H, m, H Ar); 5.21 (1H, dd,
J = 12.0, J = 3.6, 2-CH); 4.64 (1H, s, NH); 2.90–2.60 (2H,
m, 3-CH2). 13C NMR spectrum, δ, ppm: 190.7; 150.1;
149.8; 146.1; 139.7; 138.3; 134.8; 130.0; 126.5; 123.0;
121.8; 118.3; 115.4; 53.4; 42.8. Found, m/z: 325.0378
[M+Na]+. C15H11ClN2NaO3. Calculated, m/z: 325.0356.
Found, %: C 59.65; H 3.66; N 9.05. C15H11ClN2O3.
Calculated, %: C 59.52; H 3.66; N 9.25.
X-ray structural study of compounds 4f,i. The
crystals of compounds 4f,i for X-ray study were obtained
by slow crystallization from EtOH. The data collection for
crystallographic analysis were performed on a Bruker
APEXII diffractometer with mirror MoKα radiation
(λ 0.71073 Å) at room temperature. The structure was
solved by direct methods and refined by full-matrix least-
squares methods with SHELXL-97 programs.35 Crystallo-
graphic data (excluding structure factors) for compounds
4f,i have been deposited at the Cambridge Crystallographic
Data Center (deposits CCDC 997180 and CCDC 997181,
respectively).
neutral reaction conditions, and high yields of the products
make our methodology a valid contribution to the existing
processes in the field of dihydroquinolinone synthesis.
Experimental
IR spectra were obtained on a Shimadzu FT IR 8201 PC
spectrophotometer in KBr, only significant absorption band
frequencies are listed. 1H and 13C NMR spectra were
recorded on a Bruker DPX-250 Avance spectrometer (250
and 63 MHz, respectively, compound 4f) and on a Bruker
Avance 400 instrument (400 and 100 MHz, respectively,
compound 4m), solvent – CDCl3, internal standard – TMS.
High-resolution mass spectra were recorded with a
MicroTof-Q 98 mass spectrometer using electrospray
ionization method. Elemental analyzes were performed on
a CHN ThermoScientific Flash 2000 apparatus. Melting
points were determined on an Electrothermal capillary fine
control apparatus and are uncorrected. Column chromato-
graphy was carried out using silica gel 60 (Merck, 230–
400 mesh ASTM). TLC was performed on Merck
aluminum plates coated with silica gel 60, layer thickness
0.2 mm, eluent hexane–EtOAc, 4:1. The chemicals were
used as obtained commercially.
Supplementary information file to this article containing
selected spectral and analytical data of the synthesized
compounds is available online at http://link.springer.com/
journal/10593.
Synthesis of 2-aryl-2,3-dihydroquinolin-4(1H)-ones
4a–m (General method). 2-Aminochalcone 3a–m (0.5 mmol)
was added to a stirred solution of ionic liquid [C8dabco]Br
(153 mg, 0.5 mmol). The reaction mixture was heated at
150°C for 2–3 h, and the reaction progress was monitored
by TLC. After completion of the reaction, the mixture was
cooled to room temperature, then poured into water. The
product was recovered by extracting several times with
Et2O and evaporation of the ether extracts. The residue was
percolated through a band of silica gel (60–120 mesh)
using hexane–EtOAc, 9:1, as an eluent to afford the
respective product 4a–m. The catalyst was recovered by
evaporation of the aqueous layer under reduced pressure
and then used for the next run without further purification
under the same conditions. Alternatively, the reaction
mixture was directly applied on a silica gel column and
eluted with hexane–EtOAc, 9:1, to afford the pure
products. Selected spectroscopic data and analytical data
are presented below.
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102

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