Studies on –SO3H functionalized Br?nsted acidic imidazolium ionic liquids (ILs) for one-pot, two-step synthesis of 2-styrylquinolines

Article abstract of DOI:10.1080/00397911.2016.1193754

Four task-specific –SO₃H functionalized imidazolium ionic liquids (ILs) were investigated for Br?nsted acidities by Hammett functions. After knowing their thermal stabilities, the catalytic activity was observed for the preparation of 2-styrylq

Full text of DOI:10.1080/00397911.2016.1193754

Synthetic Communications
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Studies on –SO3H Functionalized Brønsted Acidic
Imidazolium Ionic Liquids (ILs) for One-Pot Two
Step Synthesis of 2-styrylquinolines
Parishmita Sarma, Susmita Saikia & Ruli Borah
To cite this article: Parishmita Sarma, Susmita Saikia & Ruli Borah (2016): Studies on –SO3H
Functionalized Brønsted Acidic Imidazolium Ionic Liquids (ILs) for One-Pot Two Step Synthesis
of 2-styrylquinolines, Synthetic Communications, DOI: 10.1080/00397911.2016.1193754
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Date: 23 June 2016, At: 21:57

Studies on –SO3H Functionalized Brønsted Acidic Imidazolium Ionic Liquids (ILs)
for One-Pot Two Step Synthesis of 2-Styrylquinolines
Parishmita Sarma1, Susmita Saikia1, Ruli Borah1
1Department of Chemical Sciences, Tezpur University, Assam, India
E-mail :ruli@tezu.ernet.in
Abstract
Four task-specific –SO3H functionalized imidazolium ionic liquids were investigated for
the Brønsted acidities by Hammett functions. After knowing their thermal stabilities, the
catalytic activity was observed for the preparation of 2-styrylquinolines by following a
consecutive Friedländer and Knoevenagel reactions in solvent-free thermal energy. The
acidity order ([Dsim][OOCCF3]>[Dsim][OTs]>[Dsim][OOCCl3]> [Msim][OOCCF3]) of
three ILs was consistent with their activity order observed in the acid-catalyzed synthesis
of 2-styrylquinolines under solvent-free condition at 90° C except [Dsim][OTs]. The best
catalytic activity showed by 25 mol% of [Dsim][OOCCF3] IL. The less acidic ILs
required 50 mol% to give good yields of 2-styrylquinolines under the optimized
conditions.
Graphical Abstract
KEYWORDS: Acidic ionic liquids; Hammett plot; one-pot; Friedländer and
Knoevenagel condensation; recyclable IL; 2-styryl quinoline
1

INTRODUCTION
Designing of one pot multistep synthetic route is a sustainable structure
development method in organic synthesis [1-2]. A large numbers of individual
reactions are brought about in a single vessel through various steps. It makes
provision for creating complex library of small organic molecules without isolating
the intermediates involve in various steps with less chemical waste and greater
economic benefit[3-4]. Styrylquinolines derivatives have shown extensive
potential biological activities including anti-inflammatory, antitumor, antifungal,
and anti-allergic, acting as HIV-1 integrase, lipoxygenase inhibitors and
leukotriene d4-antagonists [5-15].Two such derivatives, FZ41 and KHD161
(Fig.1), provide a novel and promising strategy for AIDS therapy[13-15]. Some of
them are effective for the treatment of Alzheimer’s disease [16]. Other applications
include the design of molecular logic devices [17] and supramolecular system [18].
Several known methods are available for the direct introduction of alkyl
substituent into the quinoline nucleus using organometallic compounds, active
methylene compounds or Wittig reagents[19-22]. However, most of the 2-
styrylquinolines are obtained from the acid or base catalyzed condensation of 2-
methyl quinoline derivatives with excess amount of appropriate aldehydes in acetic
anhydride at high temperature (100-140° C) which produce only moderate yields
of products during longer reaction time (20h)[23]. Few modified procedures
utilized microwave energy to generate the title compound in good to moderate
2

yield from 2-methyl quinoline using ZnCl2, Ac2O/SiO2 as catalyst in neat
conditions at high temperature [24-29].They obtained the key intermediate
quinalidine from Friedländer, Doebner–von Miller,Combes, and Pfitzinger
reactions[30-32]. Another two step method also describes the preparation of 2-
styryl-1, 2, 3, 4-tetrahydroquinolines via a vinylogous Povarov reaction followed
by aromatization which isn’t yet commonly applicable in terms of the substitution
on the styryl side chain [13, 33-34]. All these multistep protocols provide low
yield, time consuming and produce side products.
Dabri et al first employed 1-methyl-imidazolium trifluoroacetate IL as acidic
medium for the one pot synthesis of 2-styrylquinolines via Friedländer annulation
of 2-aminoarylketone and methylketone followed by Knoevenagel reaction with
aromatic aldehyde at 80° C in 2h (Scheme 1)[35]. Recently Kumar et al also
exploited the synthesis of title compounds using this approach in presence of 10
mol% of In(OTf)3 as catalyst at 100° C within 2-4 h[36]. Both methods required
longer reaction time to give good to excellent results. Although the both one pot
methods have several advantages such as simple work up procedure, high yields of
required product, no side product, use of recyclable acidic medium/catalyst, but the
major limitation is the use of stoichiometric amount of less acidic IL and longer
reaction time (2-4 h). As results there is a scope to use strong acidic ILs as reusable
homogeneous catalytic system for the preparation of 2-styrylquinolines.
3

The -SO3H functionalized ionic liquids represent a class of strong task specific
Br nsted acidic ILs and they have many applications as recyclable acid catalyst in
organic reactions under mild conditions, which is rather difficult to obtain with other
Lewis or Br nsted acid catalysts [37]. The acidity of such ILs can be varied with the
ø
ø
incorporation of one or more –SO3H groups into the cations. In this study, four acidic
sulfoimidazolium ILs 3-methyl-1-sulfoimidazolium trifluoroacetate
([Msim][OOCCF3]) 5 , 1, 3-disulfoimidazoliumtosylate ([Dsim][OTs])
6
and 1,3-
, [
(Fig. 2).
disulfo-imidazolium carboxylate ILs ([Dsim][X]) where X = [CCl3COO]-
7
CF3COO]-
8
were investigated as catalyst/medium for the preparation of 4
RESULTS AND DISCUSSION
At first we synthesized the three known ILs ([Msim][OOCCF3]) 5 ,
([Dsim][OOCCCl3]) 7 and ([Dsim][OOCCF3]) 8 (Fig. 2) from the reaction of 1-sulfo-3-
methylimidazolium chloride ([Msim]Cl) with CF3COOH and 1,3-disulfo-imidazolium
chloride ([Dsim]Cl) with CCl3COOH and CF3COOH respectively[38-39]. In next phase,
we prepared ([Dsim][OTs])
6 IL from the reaction of [Dsim]Cl with p-toluene
sulfonic acid at room temperature (Scheme 2).
The FT-IR spectra of IL 6 indicated three absorption bands around 1208, 1048 and 582
cm-1 which are assigned for the stretching and bending vibrations of -SO3H group (Fig.
3). It has another strong peak at 823 cm-1 corresponding to N-S stretching vibration. The
1H NMR spectra of 6 has two proton singlet at 14.1 ppm for the –SO3H acidic protons.
The elemental analyses further confirmed the structure of the new IL ([Dsim][OTs]) 6.
4

From the previous study, we knew the thermal stability of ILs 7 and 8 up to 260° C [39].
Therefore, we performed the thermal analysis of two ILs (5, 6) (Fig. 4). The TGA profile
of IL 5 showed slight weight loss at 67.7° C which can be assigned for the elimination of
absorbed water from IL. It has higher thermal stability up to 364° C than 6, 7 and 8. In
contrast, the TGA curve of ([Dsim][OTs]) expressed 40 % decomposition at 57.5° C. The
reason may be the weaker strength of ionic bond between imidazolium cation and
tosylate anion in ([Dsim][OTs]) with increasing temperature and thus, the –OTs group
can easily eliminate as strong leaving group in the form of p-toluene sulfonic acid
through abstraction of acidic proton from electrophilic -N–SO3H group of imidazolium
cation. This was further confirmed from the 1HNMR spectra of [Dsim][OTs] IL after
heating at 60° C for one hour This spectra expressed this IL as a mixture of p-TSA and 1-
sulfonic acid imidazole displaying an acidic proton at δ 14.2 ppm and two distinct
doublets for four aromatic proton of p-TSA at 7.10 ppm and 7.47 ppm respectively.
Hammett function (Ho) is an effective way to measure the acidity of various acidic ionic
liquids using UV-Visible spectrophotometer according to equation (1) in presence of 4-
nitroaniline as weak base [40-41]. In the equation- (1), the term pK(I)aq represents pKa
value of the weak base indicator in aqueous solution and [I] is the absorbance of
indicator without protonation.
Ho pK I aq+log I / IH
(1)
The acidities of ILs were expressed in terms of Hammett function in table-1 by the extent
of protonation of the indicator with the decreasing absorbance [I] of basic indicator. The
5

protonated form [HI]+ of the indicator didn’t show any absorbance due to smaller molar
absorptivity. The procedure started with the mixing of basic indicator (0.04mmol /dm3,
pKa＝0.99) and ILs (5 mmol/dm3) solution in ethanol with equal concentration which
gave maximum absorbance at 378 nm.
The observed acidities of ILs was arranged in the decreasing trend by increasing the
values of Hammett function (table-1) according to the Uv-vis plot as
follows:([Dsim][OOCCF3])>([Dsim][OTs]) > ([Dsim][OOCCl3])>([Msim][OOCCF3])
(Fig. 5).
Table-2 includes the standardization results of 2-styryl quinoline 4a starting from 2-
aminobenzophenone (1mmol), acetoacetic ester (1mmol) and p-tolualdehyde (1mmol)
using the acidic ILs (5/6/7/8) as catalysts/medium at various temperature. As reaction
medium, they produced 22-39 % of 2-methylquinoline 3a at room temperature during 12
h stirring (table-2, entry-1). By increasing the temperature to 100° C, the reaction was
completely converted to 3a on TLC observation using 50 mol % of 5/7/8 ILs within 10-
40 min (table-2, entry-2) except 6 which decomposed at 57.7o C in the TGA analysis. In
next step, the crude intermediate 3a gave 84-96 % of 4a after treatment with 1 mmol of
p-toluadehyde at 100° C (table-2, entry-3). The three ILs formed similar yields of 4a at
90° C (table-2, entry- 4). Among the three ILs, the best catalytic activity obtained from
25 mol % of 8 at 90° C (table-2, entries 5-6). We optimized 50 mol % of weak acidic ILs
5 and 7 as task specific reaction medium for the preparation of 4a at 90° C (table-2, entry
6

4). The model reaction yielded 62-70 % of 4a with 25 mol % of 8 in methanol and water
solvents during 4 h reflux in 2nd step (table-2, entry 7).
The standard reaction conditions were expanded with various 2-aminoaryl ketones and β-
keto ester and substituted aromatic aldehydes. The results were summarized in table-3.
The product formation was observed to be satisfactory within 1-2 h min time in the 2nd
step regardless of the nature of substituted aromatic aldehydes. The condensation reaction
didn’t occur with aliphatic aldehyde using IL 8 as catalyst (table-3, entry 11).
Single crystal X-ray analysis was performed on the 2-styrylquinoline derivative 4j(table-
3, entry-10) which confirmed the presence of basic 2-styrylquinoline moiety (Fig. 6)
along with (E)-conformation of two hydrogen atoms on the styrene double bond [42].
The reusability of strong acidic ionic liquid catalyst 8 was performed up to 4th cycle for
the preparation of 4a from the mixture of 2-amino benzophenone (1 mmol),
ethylacetoacetate (1 mmol) and p- tolualdehyde (1mmol) at 90° C and it was expressed
by the bar diagram (Fig. 7).
EXPERIMENTAL SECTION
General Remarks
The 1H and 13C NMR spectroscopic data of all products were recorded on JEOL (400
MHz), Bruker (400 MHz) and (300 MHz) spectrometer using Me4Si as internal standard.
The NMR samples were run in CDCl3 and DMSO-d6. The coupling constants were (J)
7

expressed in Hertz (Hz). FT-IR spectra of all compounds were recorded on a Nicolet
Impact-410 spectrometer. Single crystal data of compound 4j was recorded on Bruker
SMART APEX II CCD diffractometer. Elemental analyses of the products were
performed on Perkin Elmer 20 analyzer. Melting points were determined on Buchi M-
560. All products were identified by FT-IR, 1H NMR, 13CNMR, and melting point data
and also by comparison with literature reported data[35-37].The1H NMR and 13C NMR
spectra of new ionic liquid and 2-styrylquinoline derivatives were included in
supplimentary file.
Experimental Procedures
General Procedure For The Synthesis Of Ionic Liquids
The known three ionic liquids ([Msim][OOCCF3]) 5 , ([Dsim][OOCCCl3]) 7 and
([Dsim][OOCCF3]) 8 were prepared according to the previously reported procedures[38].
The novel IL 1, 3- disulfo-imidazolium tosylate([Dsim][OTs]) 6 was synthesized by
following the reaction Scheme 2. In a 50 mL two necked round bottomed flask
containing 5 mmol (1.32g) of [Dsim]Cl ionic liquid, equal amount of p-toluene sulfonic
acid (5 mmol, 0.86 g) was added over a period of 10 mins and then stirred at room
temperature for 1 hour. To remove the HCl gas produced during the period, an alkali trap
was connected through a vacuum system. After completion of one hour stirring, the
viscous reaction mixture was washed with dry CH2Cl2 (25 mL) and dried under reduced
pressure to give [Dsim][OTs] as colourless oil with 97 % (1.94 g) yield.
Typical Procedure For The Synthesis Of 2-Styrylquinoline Derivatives 4
8

A mixture of 2-aminoaryl ketone (1.0 mmol), β-keto ester (1.0 mmol) and 25 mol% of
acidic ILs was heated in a 50 mL round bottom at 90° C for the specified time. After
completion of the reaction as monitored by thin layer chromatography (using 1:10 ethyl
acetate and petroleum ether as developing solvent) to the corresponding 2-methyl
quinoline derivatives, 1 mmol of aromatic aldehyde was added to the reaction mixture.
The reaction was again continued for the respective reaction time as represented in table-
3 at 90° C for the formation of 2-styrylquinolines. The crude product was extracted from
the ionic liquid phases using dichloromethane (2 x 3 mL) as solvent which was isolated
from the dichloromethane solution after distillation under reduced pressure. The catalyst
ionic liquid was retained as viscous layer in the reaction vessel and it was again utilized
for next cycle of reaction. Further purification of the solid product was completed by
recrystallization from ethanol to get analytically pure product.
Spectral Data Of New Ionic Liquid And 2-Styrylquinoline Derivatives
1, 3- disulfo-imidazolium tosylate [Dsim][OTs] 6(new): FT-IR(KBr): 3464, 1638, 1208,
1048, 823, 688, 582, 478, 418 cm-1 ; 1H NMR (400 MHz, DMSO-d6): δ = 2.11(s, 3H),
7.04(s, 2H), 7.46-7.50 (m, 4H), 8.91(s, 1H), 14.13(s, 2H); 13C NMR (100 MHz, DMSO-
d6): 21.2, 119.8, 125.9, 129, 134.6, 139.5, 139.7,143.8,144.1.
Ethyl-2-[(E)-2-(naphthalene-3-yl)vinyl]- 4-phenyl-quinoline-3-carboxylate 4c (Table 3,
Entry 3, new ): Yellow solid, Mp. 158.6-159.2° C, FT-IR (KBr) cm-1:3451, 3242, 3048
,2927, 2377, 2271, 1624, 1582, 1504, 1434, 1362, 1301, 1265, 1168, 1098, 980, 865, 821,
742;1HNMR (400MHz, CDCl3) δ ppm :0.92(t, J = 6.9Hz, 3H), 4.05(q, J = 6.9Hz, 2H),
9

7.28-7.43(m,9H), 7.52(d, J = 8.2Hz, 1H), 7.64-7.85(m, 5H),7.97(s, 1H), 8.18(d, J = 8.6
Hz, 1H), 8.26(d, J = 15.6Hz, 1H);13CNMR(75MHz, CDCl3) δ ppm : 168.2, 151.0, 148.1,
146.8, 143.4, 136.6, 135.7, 134.6, 134.1, 133.5, 130.5, 129.5, 129.4, 129.1, 128.6, 128.4,
128.3, 128.0, 127.7, 127.3, 127.0, 126.6,126.3,125.7,124.5,123.8, 123.5,61.6,13.7;CHN
analysis (%) : C30H23NO2, Cal. C 83.89, H 5.40, N 3.26; Found C 83.86, H 5.44, N 3.22.
CONCLUSION
In summary, this study has identified the catalytic activity of IL 8 among four different
sulfoimidazolium BAILs for the one pot synthesis of 2-styrylquinolines after comparing
their acidities and thermal stabilities. This is the first report of –SO3H functionalized
ionic liquid catalyzed synthesis of 2-styrylquinoline with excellent yields at 90 °C within
1 h. The weak acidic ILs 5 and 7 promoted the reaction as reaction medium at the same
temperature. The new IL 6 decomposed at 57.5° C and didn’t catalyze the reaction under
the optimized condition.
SUPPLEMENTARY MATERIAL
Supplemental data for this article can be accessed on the publisher’s website. This file
contains the spectra of 2-styryl quinolines 4 and new ionic liquid 6.
ACKNOWLEDGEMENT
The authors are thankful to Sophisticated Analytical Instrumentation Centre, T.U. for
analyses of various samples for this work and also to CSIR, New Delhi, India for
granting a research project no. 02(0067)/12/EMR-II to R. B.
10

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14

Table 1. Calculation of the Hammett Function for various ILs in ethanol
[a]
Entry IL
Amax [I]% [IH]% H0
1
2
Blank
1.394 100.0 0
-
[Msim][OOCCF3] 0.815 58.5 41.5
1.14
5
3
[Dsim][OOCCl3] 0.798 57.2 42.8
1.12
7
4
5
[Dsim][OTs] 6
0.743 53.3 46.7
1.05
0.79
[Dsim][OOCCF3] 0.541 38.8 61.2
8
[a] Indicator: 4-nitroaniline
15

Table 2. Optimization of the amount of ionic liquids and temperature for the synthesis of
3a and 4
Entry
ILs Tem Time
(mol p. (°
% Yield of
products a,b
%)
C)
3a(min)
Step I
12 h
4a(h)
Step II
—
3a
4a
1
5/6/ 25
22/35/2 -
7/39
7/8
(100
)
2
3
5/7/ 100
40/25/10
—
97/97/9 -
8
(50)
5/7/ 100
40/25/10 2 /1.5/1 -
84/87/9
6
8
(50)
4
5
6
5/7/ 90
40/30/10 2/1.5/1
-
83/85/9
5
8
(50)
5/7/ 90
8
3 h
—
70/80/9 –
/2.5h/15
7
(25)
8(25 90/8 15/35
1
–
94/83
)
0
/85min
16

7
8(25 70/1 1 h
00
4
82/77
70/62
)
a Reactions were carried from the 1:1 ratio of 2-amino benzophenone and
ethylacetoacetate using ionic liquid (5 /6 /7 /8) followed by addition of 1 mmol of p-
tolualdehyde ;bIsolated yields.
17

Table 3. Evaluation of the catalytic activity of ILs 5, 7 and 8 for the synthesis of 2-
styrylquinoline derivatives 4
Entr 1
y
2
R2CHO
Time (method)a
3 (min)
Product 4
Mp. 4
Yieldb (%) [reported]35
4 (hour)
5/7/8
83/85/94
4a
°C
1
2
3
4
5
6
7
8
9
4-
40(I)/30(II)/15(I 2 /1.5/ 1
150.7-152.8
1
a
1
a
1
a
1
b
1
b
1
b
1
a
1
a
1
a
2a
2a
2a
2b
2b
2a
2a
2a
2a
CH3C6H4- II)3a
4-
2 /1.5/ 1
2 /1.5/ 1
81/84/904b 171.4-172.5
80/84/884c 158.6-159.2
80/83/954d 146.3-146.9°
78/81/934e 182.7-184.3
3a
3a
NO2C6H4-
2-
naphthyl-
4-
48(I)/40(II)/20(I 2 /1.5/ 1
MeOC6H4- II)3b
4-
2 /1.5/ 1
3b
HOC6H4-
4-
50(I)/45(II)/25(I 2 /1.5/ 1
75/80/924f 145.7-147.2
[147-148]
MeOC6H4- II)3c
Ph-
2 /1.5/ 1
2 /1.5/ 1
2 /1.5/ 1
81/84/904g 169.6-172.1
3a
3a
3a
CH=CH-
4-ClC6H4-
75/79/94
4h
150.5-154
[150-153]
4-
80/83/934i 233-234°
[232-234]
HOC6H4-
18

10
11
4-
2 /1.5/ 1
2 h
83/85/954j 126.7-129
1
a
1
a
2a
2a
3a
MeOC6H4-
C4H9-
3a (III)
No
—
reaction
a Method : (I) using 50 % of IL 5 at 90° C in neat (II) using 50 mol% of IL 7 at 90° C in
neat (III) using 25 mol% of IL 8 at 90° C in neat ;b Isolated yields
19

Scheme 1. Preparation of 2-styrylquinoline derivatives
4
20

Scheme 2. Synthesis of [Dsim][OTs] ionic liquid
21

Figure 1. Structure of FZ41 and KHD161 as 2-stryrylquinoline derivatives
22

Figure 2. Structure of three sulfoimidazolium BAILs
23

Figure 3. FT-IR spectra of IL 6
24

Figure 4. TGA and DTGA analysis of the ionic liquids 5 and 6
25

Figure 5. Absorption spectra of basic indicator
26

Figure 6. Single crystal structure of 4j. The displacement ellipsoids are drawn at the 50%
probability level and H atoms are represented by circles of arbitrary radii
27

Figure 7. Reusability of IL 8 as catalyst for the preparation of 4a
28