Effective preparation of hexahydroquinolines under ambient and solvent-free conditions

Article abstract of DOI:10.1016/j.molliq.2015.01.013

The mild acidic ionic liquids such as N-methyl-2-pyrrolidonium dihydrogen phosphate [NMP][H₂PO₄], N-methyl-2-pyrrolidonium hydrogen sulfate [NMP][HSO₄], Triethylamine hydrogen sulfate [Et₃N-H][HSO₄],

Full text of DOI:10.1016/j.molliq.2015.01.013

Journal of Molecular Liquids 204 (2015) 15–20
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Effective preparation of hexahydroquinolines under ambient and
solvent-free conditions
a,
a
a
a
b
Mahdi Ghorbani ⁎, Hamid Reza Shaterian , Soheila Noura , Fereshte Khammar , Kazem Behbodi ,
c
d
Bahareh Reisi , Mohsen Oftadeh
Department of Chemistry, Faculty of Sciences, University of Sistan and Baluchestan, PO Box 98135-674, Zahedan, Iran
Department of Chemistry, Shahreza Branch, Islamic Azad University, 311-86145 Shahreza, Isfahan, Iran
Department of Chemistry, Yasouj University, Yasouj 75918-74831, Iran
a
b
c
d
Chemistry Department, Payame Noor University, 19395-4697 Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 October 2014
Received in revised form 5 January 2015
Accepted 8 January 2015
Available online 10 January 2015
The mild acidic ionic liquids such as N-methyl-2-pyrrolidonium dihydrogen phosphate [NMP][H PO ], N-methyl-
2
4
4 3 4
2-pyrrolidonium hydrogen sulfate [NMP][HSO ], Triethylamine hydrogen sulfate [Et N-H][HSO ], L-prolinium
sulfate [L-Pro ][SO ] and L-prolinium nitrate [L-Pro][NO ] were used as efficient catalysts for the one-pot four-
2
4
3
component condensations of aromatic aldehydes, dimedone, ethyl acetoacetate, and ammonium acetate under
solvent-free conditions. Some derivatives of ethyl 4-(aryl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquino-
line-3-carboxylate were obtained under solvent-free conditions in high yields. Simple reaction, mild condition
and easy work-up procedures are advantages of the present work. Also, the ionic liquids can be recycled several
times without significant loss of their activity.
Keywords:
Ionic liquid
Catalysis
Dimedone
Aldehyde
Solvent free
© 2015 Published by Elsevier B.V.
1
. Introduction
Multicomponent reactions (MCRs) in ionic liquids (ILs) are impor-
hexahydroquinoline-3-carboxylate derivatives during the reaction of
aromatic aldehydes, dimedone, ethyl acetoacetate, and ammonium ace-
tate under solvent-free conditions for the first time (Scheme 1).
tant in organic synthesis [1]. Ionic liquids act as solvent, co-catalyst, or
catalyst [2]. IL-MCR causes the rapid synthesis of highly functionalized
heterocyclic molecules with high potential applications in medicinal
chemistry [3,4]. Yielding the one-pot multi-component symmetrical
2. Experimental
All reagents were purchased from Merck and Aldrich and used with-
1
,4-dihydropyridines (1,4-DHPs) during Hantzsch reaction was first
out further purification. The mild acidic ionic liquids such as N-methyl-
established in 1881 [5]. 1,4-DHPs form a class of heterocyclic com-
pounds which present interesting pharmacological and biological prop-
erties [6]. In order to research on ionic liquids and their application on
IL-MCRs [7], recently several alternate and more efficient methods in-
cluding the use of microwaves [8], autoclave [9], ionic liquids [10], io-
dine [11], metal triflates [12], ceric ammonium nitrate (CAN) [13], L-
proline [14], PTSA-SDS [15] and BINOL-phosphoric acid derivatives
2-pyrrolidonium hydrogen phosphate [NMP][H
pyrrolidonium hydrogen phosphate [NMP][HSO
hydrogen sulfate [Et N-H][HSO ] [19], [L-Pro ][SO
[NO ] [20] as catalysts were prepared according to the reported proce-
dure. The NMR spectra were recorded on a Bruker Avance DPX 300 or
400 MHz instrument. IR spectra were recorded on a JASCO FT-IR 460
plus spectrophotometer. Melting points were determined in open cap-
illaries with a BUCHI 510 melting point apparatus. TLC was performed
on silica-gel Poly Gram SIL G/UV 254 plates. All yields refer to isolated
products after purification.
2
PO
] [18], Triethylamine
] [20] and [L-Pro]
4
] [17], N-methyl-2-
4
3
4
2
4
3
[
16] have been developed for the synthesis of polyhydroquinoline de-
rivatives. Herein, we report the new application of some ILs such as N-
methyl-2-pyrrolidonium dihydrogen phosphate [NMP][H PO ], N-
methyl-2-pyrrolidonium hydrogen sulfate [NMP][HSO ], Triethylamine
hydrogen sulfate [Et N-H][HSO ], L-prolinium sulfate [L-Pro ][SO ], and
L-prolinium nitrate [L-Pro][NO ] as catalysts (Fig. 1) for the preparation
of some functionalized ethyl 4-(aryl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-
2
4
4
3
4
2
4
2.1. General procedure for the synthesis of ethyl 4-(aryl)-2,7,7-trimethyl-5-
oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate derivatives under
solvent-free conditions
3
The mixture of the aldehydes (10 mmol), dimedone (10 mmol),
ethyl acetoacetate (10 mmol) and ammonium acetate (15 mmol) and
⁎
Corresponding author.
E-mail address: ghorbanii.mahdi@gmail.com (M. Ghorbani).
http://dx.doi.org/10.1016/j.molliq.2015.01.013
167-7322/© 2015 Published by Elsevier B.V.
0

16
M. Ghorbani et al. / Journal of Molecular Liquids 204 (2015) 15–20
2 4 4 3 4 2 4 3
Fig. 1. The molecular structure of [NMP][H PO ], [NMP][HSO ], [Et N-H][HSO ], [L-Pro ][SO ] and [L-Pro][NO ].
Scheme 1. The preparation of ethyl 4-(aryl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate derivatives.
ILs containing [NMP][H
H][HSO ] (10 mol%), [L-Pro
10 mol%) as mild acidic ionic liquid catalysts was stirred at room tem-
perature for the specific time. At the end of the reaction, it was cooled to
room temperature. Then, 5 mL of water was added to the mixture. The
ionic liquid was dissolved in water, and filtered for separation of the
crude product. The separated product was washed twice with water
2
PO
4
] (10 mol%), [NMP][HSO
4
] (10 mol%), [Et
3
N-
3
]
All of the spectral data have been presented in the supported informa-
tion and the selected spectra for three known products are given below:
Ethyl 4-(4-methoxyphenyl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-
hexahydroquinoline-3-carboxylate (Table 2, entry 6): m.p = 256–258
°C; IR (KBr): νmax = 3276, 2956, 1703, 1648, 1606, 1496, 1381, 1215,
1031, 765 cm . H NMR (300 MHz, DMSO-d ): δ (ppm) 0.85 (s, 3H),
6
1.00 (s, 3H), 1.14 (t, J = 7.0 Hz, 3H), 1.96 (d, J = 16.0 Hz, 1H), 2.15 (d,
J = 16.1 Hz, 1H), 2.27 (s, 3H), 2.37–2.49 (m, 2H), 3.66 (s, 3H), 3.97 (q,
4
2
][SO ] (10 mol%) and [L-Pro][NO
4
(
−
1 1
(
2 × 5 mL). The solid product was purified by recrystallization proce-
dure in ethanol. All of the desired products were characterized by com-
parison of their physical and spectral data (melting points, IR, H NMR)
J = 7.0 Hz, 2H), 4.79 (s, 1H), 6.73 (d, J = 8.3 Hz, 2H), 7.05 (d, J =
8.3 Hz, 2H), 8.99 (s, 1H); C NMR (75 MHz, DMSO-d ): δ (ppm) 14.6,
6
1
13
with those of known compounds [1,22,24,26]. For recycling the cata-
lysts, after washing the solid products with water completely, the
water containing the ionic liquid (IL is soluble in water) was evaporated
under reduced pressure and the ionic liquid was recovered and reused.
18.7, 26.9, 29.6, 32.6, 35.4, 50.6, 55.3, 59.4, 104.4, 110.7, 113.5, 128.8,
140.5, 145.1, 149.7, 157.7, 167.4, 194.7.
Ethyl 4-(4-hydroxyphenyl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexa-
hydroquinoline-3-carboxylate (Table 2, entry 8): m.p = 229–231 °C;
Table 1
3 4 4 2 4 2 4 3
Optimal amounts of A: [Et N-H][HSO ], B: [NMP][HSO ], C: [NMP][H PO ], D: [L-Pro ][SO ] and E: [L-Pro][NO ] as catalysts in four-component synthesis of ethyl 4-(aryl)-2,7,7-trimethyl-5-
oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate from the reaction of aldehydes, dimedone, ethyl acetoacetate, and ammonium acetate under solvent-free and ambient conditions.
Entry
Catalyst (mol%)
Temp (°C)
Time (min)
A
Yield (%)a
A
B
C
D
E
B
C
D
E
1
2
3
4
5
–
25
25
25
25
25
120
9
8
7
5
120
10
9
6
5
120
11
9
7
6
120
9
8
6
4
120
12
10
9
Trace
90
92
95
96
Trace
89
91
96
97
Trace
87
92
94
95
Trace
88
91
95
96
Trace
85
89
92
94
5
10
15
20
7
a
Yields refer to isolated pure product based on the reaction of aldehydes (10 mmol), dimedone (10 mmol), ethyl acetoacetate (10 mmol) and ammonium acetate (15 mmol).

M. Ghorbani et al. / Journal of Molecular Liquids 204 (2015) 15–20
17
Table 2
The preparation of ethyl 4-(aryl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate derivatives in the presence of A: [Et
D: [L-Pro ][SO ] and E: [L-Pro][NO ].
3 4 4 2 4
N-H][HSO ], B: [NMP][HSO ], C: [NMP][H PO ],
2
4
3
Entry
Aldehyde
Product
Time (min)
Yield (%)a
M.P/M.P [ref](°C)
A
B
C
D
E
A
B
C
D
E
1
8
8
9
8
9
9
8
8
10
9
92
91
92
91
89
202–204/203–205
22,28]
[
2
92
92
93
91
90
236–238/233–235
22,26]
[
3
4
8
7
7
8
8
9
8
8
9
9
93
93
92
91
93
94
92
92
90
91
206–208/207–209
22]
[
242–244/242–244
22,23,26]
[
5
6
8
5
9
6
10
9
6
10
94
95
93
93
95
95
93
94
91
92
171–173/172–174
22]
[
7
8
256–258/257–259
22]
[
7
8
9
8
9
8
8
9
9
8
8
9
10
13
96
96
93
94
94
92
95
95
94
95
95
93
93
93
92
263–265/262–264
22]
[
229–231/228–230
22]
[
10
11
11
12
11
12
261–263/262–263
14]
[
10
12
13
14
92
91
93
94
90
190–192/188–190
22,26]
[
(continued on next page)

18
M. Ghorbani et al. / Journal of Molecular Liquids 204 (2015) 15–20
Table 2 (continued)
Entry Aldehyde
Product
Time (min)
Yield (%)a
M.P/M.P [ref](°C)
A
B
C
D
E
A
B
C
D
E
11
12
12
15
13
16
92
90
92
95
91
185–187/184–186
[
22]
1
2
7
7
8
9
8
7
9
92
93
93
92
91
93
92
93
91
92
201–203/202–204
22,26]
[
1
1
3
4
7
9
8
8
242–244/241–243
22]
[
8
10
8
11
93
93
92
91
91
254–256/255–256
1]
[
1
1
5
6
n-Heptanal
n-Octanal
–
–
120
120
120
120
120
120
120
120
120
120
–
–
–
–
–
–
–
–
–
–
–
–
a
Yields refer to the pure isolated products. The structures of all known products were confirmed by comparison of melting point and spectral data (FT-IR, 1H NMR) with literature data
for known compounds.
IR (KBr): νmax = 3416, 3281, 3076, 2959, 1687, 1614, 1484 cm− , H
NMR (300 MHz, DMSO-d ): δ (ppm) 0.86 (s, 3H), 1.00 (s, 3H), 1.14 (t,
J = 7.0 Hz, 3H), 1.96 (d, J = 16.0 Hz, 1H), 2.15 (d, J = 16.1 Hz, 1H),
1 1
catalysts for the preparation of the entitled products based on this
model were used. According to Table 1 and its general optimization pro-
6
cedure, 10 mol% of [NMP][HSO
[L-Pro ][SO ] and 10 mol% of [L-Pro][NO
ture were chosen, respectively (Table 1).
4
], 10 mol% of [Et
3 4
N-H][HSO ], 10 mol% of
2
6
1
3
1
.26 (s, 3H), 2.36–2.49 (m, 2H), 3.96 (q, J = 7.0 Hz, 2H), 4.74 (s, 1H),
2
4
3
] as catalysts at room tempera-
.56 (d, J = 8.1 Hz, 2H), 6.93 (d, J = 8.1 Hz, 2H), 8.94 (s, 1H), 9.01 (s,
H); 13C NMR (75 MHz, DMSO-d
): δ (ppm) 14.6, 18.7, 26.9, 29.6,
Next, the one-pot four-component condensations of aromatic alde-
hydes, dimedone, ethyl acetoacetate and ammonium acetate under op-
timized conditions for the preparation of ethyl 4-(aryl)-2,7,7-trimethyl-
5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate derivatives were
carried out (Table 2). The obtained data show that the wide ranges of
substituted and structurally diverse aldehydes could synthesize the cor-
responding products in high to excellent yields using the mild acidic
ionic liquid as catalyst. Aliphatic aldehyde such as n-heptanal and n-
octanal was intact in this reaction conditions and all starting materials
were observed without any reaction even after 120 min (Table 2, entries
15 and 16).
6
2.6, 35.3, 50.8, 59.4, 104.6, 110.8, 114.9, 128.8, 138.9, 144.8, 149.6,
55.7, 167.5, 194.7.
Ethyl 4-(4-bromophenyl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexa-
hydroquinoline-3-carboxylate (Table 2, entry 14): m.p = 253–255 °C;
IR (KBr): νmax = 3280, 3217, 3065, 2951, 1695, 1637, 1608, 1489,
1
1
381, 1263, 1210, 1092 cm−1
;
H NMR (300 MHz, DMSO-d
6
): δ
(
ppm) 0.83 (s, 3H), 0.99 (s, 3H), 1.10 (t, J = 6.9 Hz, 3H), 1.96 (d, J =
1
2
7
6.0 Hz, 1H), 2.16 (d, J = 16.1 Hz, 1H), 2.29 (s, 3H), 2.38–2.49 (m,
H), 3.97 (q, J = 7.0 Hz, 2H), 4.84 (s, 1H), 7.11 (d, J = 7.2 Hz, 2H),
.37 (d, J = 7.2 Hz, 2H), 9.09 (s, 1H); 13C NMR (75 MHz, DMSO-d
): δ
6
(
1
ppm) 14.6, 18.8, 26.9, 29.5, 32.6, 36.2, 50.6, 59.5, 103.5, 110.1, 119.1,
30.2, 131.0, 145.8, 147.4, 150.0, 167.1, 194.7.
The suggested mechanism for the formation of the products is
shown in Scheme 2.
According to the suggested mechanism first dimedone is converted to
3
. Results and discussions
4
its enol form by using [NMP][HSO ] and readily undergoes Knoevenagel
condensation with aldehyde to generate an alkylidenedimedone (1). On
At first, to find optimal conditions, the solvent-free reaction of benz-
the other hand, the activated β-ketoester (by the catalyst) with liberation
aldehydes (10 mmol), dimedone (10 mmol), ethyl acetoacetate
10 mmol) and ammonium acetate (15 mmol) in the presence of
NMP][H PO ] (10 mol%) as catalyst was selected as a model. The reac-
tion was carried out with different amounts of the catalyst (5, 10, 15,
0 mol%) at room temperature (Table 1). As it was shown from
Table 1, 10 mol% of [NMP][H PO ] as catalyst at room temperature
afforded ethyl 2,7,7-trimethyl-5-oxo-4-phenyl-1,4,5,6,7,8-hexahydro-
quinoline-3-carboxylate in 9 min with 92% of yield. Then other ionic liq-
3 4 4 2 4 3
uids [Et N-H][HSO ], [NMP][HSO ], [L-Pro ][SO ] and [L-Pro][NO ] as
4
of ammonia from NH OAc gives enamine (2). Afterward, the
(
[
alkylidenedimedone (1) and enamine (2) react together by Michael addi-
tion to afford intermediate (3). The intermediate (3) is converted to inter-
mediate (4) by tautomerization, and the intermediate (4) affords
2
4
2
2
intermediate (5) by intramolecular nucleophilic attack of the NH group
to the activated carbonyl group. The mechanism is confirmed by the liter-
atures [1,11,21,27,28]. The recycling of the ILs under solvent-free condi-
tions based on the model reaction has also been investigated. After
completion of the reaction, water was added and the precipitated mixture
2
4

M. Ghorbani et al. / Journal of Molecular Liquids 204 (2015) 15–20
19
Scheme 2. The suggested mechanism for the formation of ethyl 4-(aryl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate derivatives.
was filtered off for separation of crude products completely. The water
containing ionic liquid (IL is soluble in water) was evaporated under re-
duced pressure and ionic liquid was recovered and reused (Fig. 2). The re-
covered catalysts were reused four runs without any loss of their
activities.
3
and [L-Pro][NO ] as catalysts are the most efficient catalysts with respect
to the reaction time, low amount of the catalyst and low temperature,
and exhibit broad applicability in terms of the obtained yields.
4. Conclusion
In order to show the accessibility of the present work in comparison
with the reported results in the literature, some of the results for the
preparation of ethyl 4-(aryl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexa-
hydroquinoline-3-carboxylate were summarized in Table 3. The results
In this research, five mild acidic ionic liquids [NMP][H
2 4
PO ], [NMP]
[HSO ], [Et N-H][HSO ], [L-Pro ][SO ] and [L-Pro][NO ] were used for
4
3
4
2
4
3
the preparation of ethyl 4-(aryl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-
hexahydroquinoline-3-carboxylate derivatives under ambient and
2 4 4 3 4 2 4
show that [NMP][H PO ], [NMP][HSO ], [Et N-H][HSO ], [L-Pro ][SO ]

20
M. Ghorbani et al. / Journal of Molecular Liquids 204 (2015) 15–20
products make it a useful protocol for the synthesis of these classes of
compounds.
Acknowledgments
We are thankful to the University of Sistan and Baluchestan and
Payame Noor University for the partial support of this research.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.molliq.2015.01.013.
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3 4 4 2 4 2 4
Fig. 2. The recycling of [Et N-H][HSO ], [NMP][HSO ], [NMP][H PO ], [L-Pro ][SO ] and [L-
Pro][NO ] as mild acidic ionic liquids under solvent-free conditions.
3
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[Et
[L-Pro
[L-Pro][NO
HClO –SiO
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[
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4
2
10
30
SSA (silica sulfuric
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94 [23]
8
9
Yb (OTf
TiO NPS
Hf (NPf2)
Sc (OTf)
3
)
5 mol%, in EtOH, r.t
10 mol%, reflux in EtOH
240
30
240
40
94 [12a]
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(
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a
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