Ionic liquids containing plant derived benzoate as anions, exhibiting supramolecular polymeric aggregation: Impact of the aggregation on organic catalysis in aqueous medium

Article abstract of DOI:10.1016/j.molliq.2021.116329

Naturally occurring benzoic acid (BA) derived anions have been used to prepare halogen free hydrophilic ionic liquids (ILs 1–4). The supramolecular polymeric aggregation behaviour of these ILs has been studied through high-resolution electron spray ioniza

Full text of DOI:10.1016/j.molliq.2021.116329

Journal of Molecular Liquids 336 (2021) 116329
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Ionic liquids containing plant derived benzoate as anions, exhibiting
supramolecular polymeric aggregation: Impact of the aggregation on
organic catalysis in aqueous medium
Muhammad Naveed Javed a, Imran Ali Hashmi a, Shoaib Muhammad a, Ahmed Bari b,
Syed Ghulam Musharraf c, Syed Junaid Mahmood d, Saima Javed a, Firdous Imran Ali a, , Faisal Rafique e,
⇑
Muhammad Amjad Ilyas e, Waqas Ahmed Waseem a
a Department of Chemistry, University of Karachi, Main University Road, Karachi - 75270, Sindh, Pakistan
b Department of Pharmaceutical Chemistry, College of Pharmacy, King Saud University, P.O. Box 2457, Riyadh 11451, Saudi Arabia
c H.E.J. Research Institute of Chemistry, International Center for Chemical and Biological Sciences (ICCBS), Main University Road, Karachi - 75270, Sindh, Pakistan
d PCSIR Labs Complex, Karachi 75280, Pakistan
e Department of Applied Physics, University of Karachi, Main University Road, Karachi - 75270, Sindh, Pakistan
a r t i c l e i n f o
a b s t r a c t
Article history:
Naturally occurring benzoic acid (BA) derived anions have been used to prepare halogen free hydrophilic
ionic liquids (ILs 1–4). The supramolecular polymeric aggregation behaviour of these ILs has been studied
through high-resolution electron spray ionization mass spectrometry (HR-ESI-MS). Furthermore, these
ILs were applied as catalyst/phase transfer catalyst in the synthesis of tetrahydrobenzo[b]pyran and its
derivatives. These reactions were conducted in aqueous medium under ultrasound assistance. The results
suggested that 1-ethyl-3-methylimidazolium 2-hydroxybenzoate (IL-1) was a highly efficient catalyst yield-
ing 86.9% product in 30 min. In addition, the impact of aggregation behaviour of ILs on their catalytic
activity was also investigated.
Received 29 September 2020
Revised 26 March 2021
Accepted 24 April 2021
Available online 28 April 2021
Keywords:
Ionic liquids
Supramolecular polymeric aggregation
Tetrahydrobenzo[b]pyran
Benzoates
Ó 2021 Elsevier B.V. All rights reserved.
Catalytic activity
1. Introduction
literature [7–9]. Since, on one hand above mentioned properties of
ILs are responsible to reduce risk towards air pollution but halide
The reaction medium and catalyst both play a significant role in
many organic reactions [1]. Selection of suitable reaction media is
one of the crucial conditions among the other important variables
for organic transformations [2]. Many researchers are involved in
modifying the reaction medium to enhance efficiency of organic
reactions in terms of time, yield and conditions besides curtailing
toxicity levels. The need of environmentally safe and clean proce-
dures for organic synthesis is demanded widely to overcome the
diverse effects on the ecosystem [3]. Recently, we employed a vari-
ety of ionic liquids (ILs) as effective catalysts in C-C bond forming
reactions and as metal extracting agents from water due to their
intrinsic properties associated with these unique materials [4–6].
ILs or molten salts exhibit negligible vapor pressure and bulk
non-flammability [7]. These features make them green substitu-
tions of volatile organic solvents and applications of ILs as reaction
media or as catalyst in the organic reactions is well documented in
containing ILs pose a serious threat of water contamination via
accidental spill or waste disposal [10–13]. Studies reveal that imi-
dazolium is the less toxic cation among other nitrogen containing
cation like pyridinium, ammonium etc. The studied data further
disclose that toxicity of ILs is related to size of alkyl chain
appended to cation or anion, longer the alkyl chain greater the tox-
icity [14]. Hydrophobic ILs with bigger alkyl moiety can release
these longer alkyl chains that can penetrate through cell mem-
brane and act as neurotoxin during metabolism. On the other hand,
water-soluble ILs containing halogens may release HCl or HF in
water due to hydrolysis [11–12]. Boething et. al. [15–16] have pro-
vided guidelines regarding biodegradability of organic compounds.
In the light of environmental concerns and solubility of ILs, we
have prepared halogen free hydrophilic ILs using imidazolium
cation with smaller side chains i.e. methyl and ethyl and plant
based benzoic acid (BA) anions. These include two phenolic acids
and two aromatic amino acids; a) the pant hormone salicylic acid
or ortho-Hydroxy benzoic acid (SA, oHBA or 2-HBA) naturally
found in Filipendula ulmaria and many food items i.e. olive oil, nuts,
⇑
Corresponding author.
E-mail address: firdous@uok.edu.pk (F. Imran Ali).
https://doi.org/10.1016/j.molliq.2021.116329
0167-7322/Ó 2021 Elsevier B.V. All rights reserved.

M.N. Javed, I. Ali Hashmi, S. Muhammad et al.
Journal of Molecular Liquids 336 (2021) 116329
tea, meat, fish, eggs, coffee etc.; b) p-hydroxybenzoic acid (pHBA or
4-HBA) naturally found in Cocos nucifera, Spongiochloris spongiosa
etc.; c) Anthranilic acid or ortho-amino benzoic acid (AA, 2-ABA,
oABA) naturally occur in all living species ranging from bacteria
to homo sapiens and d) p-amino benzoic acid (pABA or 4-ABA), a
significant component of folic acid and naturally occur in a wide
range of food items including eggs, meat, grains etc [17–20].
Another reason for picking substituted benzoates, as anions are
evidence of cluster formation by these anion in gaseous state [5–
6,21–22].
flask containing 0.3 mol methylimidazole. After complete addition
of EtI, the ice bath was removed and reaction mixture was stirred
for one hour at room temperature. Reaction was monitored
through TLC using pre-coated silica plates. On completion of reac-
tion, the solvent was evaporated on a rotary vacuum. White hygro-
scopic solid obtain in 97% yield. M.p. 79 °C. The product was
identified through comparison of spectral and physical data with
published literature [23], Moisture content = 2.11%.
2.3. General Ion-Exchange metathesis method (GIEMM) for ILs 1–4:
ILs display supramolecular polymeric aggregation behavior due
to non-covalent interactions especially electrostatic forces and
hydrogen bonding [21–22]. Since, the formation of aggregates in
the ILs with organic anions is rare. We have previously observed
that unlike halogens, phenols do not exhibit cluster formation.
But benzoates have significant potential of aggregate formation
and is detectable through ESI-MS. Molecular self assemblies result-
ing into aggregation, influence the effectiveness of catalyst in
chemical reactions [21]. But, investigation on relationship between
aggregation behaviour and catalytic activity of IL in organic reac-
tions is infrequent. Supramolecular self-assembled cluster forma-
tion ability beside unique physio-chemical properties of ILs i.e.
thermal stability, non-volatility, non-flammability, ease of han-
dling and recyclability makes them potential candidates for catal-
ysis [7–9].
In continuation of our previous work [4,6], we here report the
relationship between the supramolecular aggregation behavior
and catalytic activity of four newly synthesized hydrophilic ILs
(1–4). The synthesis of tetrahydrobenzo[b]pyran by condensation
of aromatic aldehyde, dimedone and malononitrile catalyzed by
ILs 1–4 under ultrasonic assistance in aqueous media carried out.
Synthesis of tetrahydrobenzo[b]pyran and its analogues involve
Knoevenagel condensation and Michael addition reactions, two
important base catalysed C-C bond forming reactions. The results
indicated that, aggregate formation potential, thermal stability
and catalytic activity of ILs depends on the position and nature
of substituent on benzoate anion.
In a two neck round bottomed flask, a mixture of sodium bicar-
bonate (20 mmol) and respective benzoic acid (20 mmol) in water
was taken followed by addition of 1-ethyl-3-methylimidazolium
iodide (20 mmol). The reaction was stirred for 30 min at room tem-
perature and then water was evaporated on a rotary evaporator.
After removal of water, cold absolute ethanol was added to reac-
tion mixture. Insoluble inorganic salt was removed by filtration/
decantation and ethanolic solution was subjected to evaporation
to obtain pure desired product.
2.4. Synthesis of 1-ethyl-3-methylimidazolium 2-hydroxybenzoate
(IL-1):
The IL-1 was prepared as described in section 2.3. A white
coloured highly hygroscopic product was obtained. The product
was immediately stored in a desiccator to protect from moisture.
Yield: 95.2%, Moisture content = 10.28%.
1H NMR (DMSO d6, 700 MHz) d ppm: 16.02 (s, 1H), 9.18 (s, 1H),
7.79 (s, 1H), 7.71 (s, 1H), 7.68 (d, J = 7.35, 1H), 7.15 (t, J = 7.63, 1H),
6.63 (d, J = 8.33, 1H), 6.60 (d, J = 7.21 Hz, 1H), 4.19 (q, J = 7.21, 2H),
3.85 (s, 3H), 1.41 (t, J = 7.21, 3H).
13C NMR (DMSO d6, 700 MHz) d ppm: 172.38, 163.09, 136.74,
131.94, 130.47, 124.01, 122.42, 120.73, 116.58, 44.59, 36.19, 15.59.
HR-ESI-MS
+ ve: m/z 111.0915 [calcd. m/z 111.0922 for
C6H11N+2]; m/z 359.2079 [calcd. m/z 359.2083 for C19H27N4O+3].
HR-ESI-MS -ve: m/z 137.0242 [calcd. m/z 137.0238 for C7H5O-3];
m/z 275.0560 [calcd. m/z 275.0555 for (C14H10O6 + H)-]; m/z
297.0380 [calcd. m/z 297.0375 for (C14H10O6
+
Na)-]; m/z
2. Methods and materials
385.1404 [calcd. m/z 385.1399 for C20H21N2O-6].
2.1. General
FTIR: 3421.72 (phenolic OH), 2976.16 (C-H stretch), 1639.49
(C = O), 1577.77 (C = N imidazole ring), 1489.05 (C = C imidazole
ring), 1386.82 (C-O), 1163.08 (C-C).
For all chemical reactions, oven dried apparatus was used. All
commercially available chemicals were used without any addi-
tional purification. A rotary evaporator was used for solvent evap-
oration maintaining water bath temperature 45 °C. Pre-coated
aluminium plates, Kieselgel 60, F254, 0.25 mm, e. Merck, Darm-
stadt, Germany was used for thin layer chromatography (TLC).
NMR (1H & 13C) were obtained from AVANCE AV-700 and AVANCE
AV-175 BRUKER. An AB SCIEX QSTA R XL LCMSMS Q-Tof was used
for high resolution electron spray ionization mass spectrometry
(HR-ESI-MS). FSF-020S 220 V/50 Hz was used for ultrasound. Ther-
mogravimetric analysis (TGA) were obtained from a Mettler Toledo
2.5. Synthesis of 1-ethyl-3-methylimidazolium 4-hydroxybenzoate
(IL-2):
The IL-2 was prepared as described in section 2.3. The pure IL is
white coloured hygroscopic solid product was obtained. The pro-
duct was stored in a desiccator to protect from moisture. Yield:
97.6%, Moisture content = 19.46%
1H NMR (DMSO d6, 700 MHz) d ppm: 9.25 (s, 1H), 7.78 (s, 1H),
7.74 (d, J = 8.12, 2H), 7.69 (s, 1H), 6.69 (d, J = 8.19 Hz, 2H), 4.17 (q,
J = 7.21, 2H), 3.83 (s, 3H), 1.37 (t, J = 7.28, 3H).
(TGA/SDTA 851). TGA of ILs (10 mg 0.1
lg) was conducted at
isothermal mode (100 °C to 500 °C at heating rate of 5 °C/min,
N2 flow 60 mL/min, platinum pan). pH experiments were recorded
using JENCO VisionPlus pH6175 instrument. Moisture content
recoreded on Mettler Toledo Karl Fischer titrator Compact V30S
serial # B824972927 (version 5.2.0). Stuart Digital Melting Appara-
tus SMP 10 was used for melting point (M.p.) analysis.
13C NMR (DMSO d6, 700 MHz) d ppm: 171.46, 159.67, 136.82,
131.23, 123.97, 122.38, 114.42, 44.58, 36.20, 15.61.
HR-ESI-MS
+ ve: m/z 111.0925 [calcd. m/z 111.0922 for
C6H11N+2]; m/z 359.2085 [calcd. m/z 359.2083 for C19H27N4O+3].
HR-ESI-MS -ve: m/z 137.0244 [calcd. m/z 137.0238 for C7H5O-3];
m/z 275.0548 [calcd. m/z 275.0555 for (C14H10O6 + H)-]; m/z
385.1391 [calcd. m/z 385.1399 for C20H21N2O-6]; m/z 633.2552
[calcd. m/z 633.2560 for C33H37N4O-9].
2.2. Synthesis of 1-ethyl-3-methylimidazolium iodide [EMIM]I
FTIR: 3431.36 (phenolic OH), 2970.00 (C-H stretch), 1639.49
(C = O), 1462.04 (C = C imidazole ring), 1381.03 (C-O), 1134.14
(C-C).
Then 0.3 mol ethyl iodide (EtI) in a suitable amount of tetrahy-
drofuran (THF) was added dropwise in an ice cooled round bottom
2

M.N. Javed, I. Ali Hashmi, S. Muhammad et al.
Journal of Molecular Liquids 336 (2021) 116329
2.6. Synthesis of 1-ethyl-3-methylimidazolium 2-aminobenzoate
(IL-3):
3. Results & discussion
3.1. Synthesis and characterization of ILs (1–4)
The IL-3 was prepared as described in section 2.3. The pure IL is
light reddish brown coloured hygroscopic solid substance was
obtained. The product was stored in a desiccator to protect from
moisture. Yield is 93.4%, Moisture content = 8.62%
1H NMR (DMSO d6, 700 MHz) d ppm: 9.26 (s, 1H), 7.79 (s, 1H),
7.74 (s, 1H), 7.71 (s, 1H), 6.942 (s, 1H), 6.72 (s, 2H), 6.51 (s, 1H),
6.36 (d, J = 5.39 Hz, 1H), 4.20 (s, 2H), 3.85 (s, 3H), 1.41 (d, broad,
3H).
The most commonly investigated type of cations in ILs are N,N’-
dialkylimidazolium and 1-alkyl-3-methylimidazolium. Quater-
nization of methylimidazole using equimolar amount of ethyl
iodide in THF yielded 1-ethyl-3-methylimidazolium iodide
[EMIM][I] as a white hygroscopic solid in excellent yield under
mild condition [23]. The 1H NMR spectrum of [EMIM][I] exhibited
a downfield signal at d 9.2 ppm, suggesting the formation of imida-
zolium moiety. In addition, 13C NMR spectra showing signal at d
136 ppm confirms synthesis of imidazolium ion. As our strategy,
to prepare halogen free, environmentally benign anion with a dif-
fused or delocalized negative charge, Group-I metal salts of substi-
tuted benzoates were prepared by reacting corresponding
naturally occurring benzoic acids (o-HBA, p-HBA, o-ABA and p-
ABA) with sodium bicarbonate in situ during ion exchange
metathesis (Scheme 1). Sodium bicarbonate (instead of sodium
hydroxide) was used to avoid N-heterocyclic carbene (NHC) forma-
tion. Sodium salts were used to provide strong preference to coun-
ter ion i.e. iodide for optimum ion exchange.
Post synthetic purification of ILs, especially for hydrophilic ILs,
is very challenging due to their inherent properties. For removal
of alkali metal and halide salt, we have used cold absolute ethanol
(vide experimental). For the removal of water, neither chemical dry-
ing agents nor molecular sieves or sorbents were used, as these are
the main sources of particulate contaminations of ILs. Synthesis of
ILs (1–4) at room temperature via ion exchange of metathesis in
water fulfilled principles of green and sustainable chemistry. For
characterization of synthesized ILs, 1H NMR, 13C NMR and HR-
ESI-MS techniques were employed. IL-1 displayed a downfield sin-
13C NMR (DMSO d6, 700 MHz) d ppm: 173.04,150.59, 136.87,
132.21, 130.10, 124.02, 122.42, 121.16, 115.52, 114.12, 44.57,
36.16, 15.61.
HR-ESI-MS
+ ve: m/z 111.0929 [calcd. m/z 111.0922 for
C6H11N+2]; m/z 358.2251 [calcd. m/z 358.2242for C19H28N5O+2].
HR-ESI-MS -ve: m/z 136.0400 [calcd. m/z 136.0398 for
C7H6NO-2]; m/z 273.0883 [calcd. m/z 273.0875for (C14H12N2-
O4 + H)-]; m/z 295.0703 [calcd. m/z 295.0694 for (C14H12N2-
O4
+
Na)-]; m/z 454.1000 [calcd. m/z 454.0991 for
(C21H18N3O6
+
Na2)-]; m/z 613.1294 [calcd. m/z 613.1287
for (C28H24N4O8
+
Na3)-]; m/z 772.1585 [calcd. m/z
772.1583 for (C35H30N5O10 + Na4)-].
FTIR: 3441.01 (H2O masked NH2), 1622.13 (Conjugated C = O),
1523.76 (C = N), 1379.10 (C = C), 1136.07 (C-O).
2.7. Synthesis of 1-ethyl-3-methylimidazolium 4-aminobenzoate
(IL-4):
The IL-4 was prepared as described in section 2.3. The pure IL is
light greenish brown coloured hygroscopic solid was obtained. The
product was stored immediately in a desiccator to protect from
moisture, Yield: 98.2%, Moisture content = 15.61%
glet at
d 16.17, indicating intermolecular/intramolecular H-
bonding. In addition, 13C NMR spectra of ILs (1–4) displayed car-
boxylate carbon around d 171–173 ppm (vide Supplementary Infor-
mation Figure-S1). FTIR spectra of ILs 1–4 displayed a broad signal
in the range of 3440–3460 cmÀ1, indicative of H-bonded OH
(Figure S14-S17). In addition, a signal with medium intensity
around 1620–1640 affirms presence of conjugated carbonyl of
benzoate.
1H NMR (DMSO d6, 700 MHz) d ppm: 9.26 (s, 1H), 7.79 (s,
1H), 7.70 (s, 1H), 7.59 (d, J = 8.05, 2H), 6.42 (d, J = 8.33 Hz,
2H), 5.13 (s, 2H), 4.19 (q, J = 7.28, 2H), 3.85 (s, 3H), 1.40 (t,
J = 7.28, 3H).
13C NMR (DMSO d6, 700 MHz) d ppm: 171.67, 149.96, 136.87,
130.94, 124.01, 122.41, 112.63, 44.57, 36.17, 15.62.
HR-ESI-MS
+ ve: m/z 111.0913 [calcd. m/z 111.0922 for
C6H11N+2]; m/z 358.2235 [calcd. m/z 358.2242for C19H28N5O+2].
HR-ESI-MS -ve: m/z 136.0391 [calcd. m/z 136.0398 for
C7H6NO-2]; m/z 273.0870 [calcd. m/z 273.0875for (C14H12N2-
O4 + H)-]; m/z 295.0689 [calcd. m/z 295.0694 for (C14H12N2-
O4
C
3.2. Thermal properties of ILs:
The high onset decomposition temperature (Tonset) defines the
thermal stability of ILs [24]. The thermal stability of ILs mostly
were depended on the structure of ions, the cations and anions. Lit-
erature shows that, there is a close relationship between thermal
stability and anion type because by varying the anions, the decom-
position temperature of ILs was changed from 200 to 400 °C [24–
25]. The thermal stabilities of newly synthesized ILs (1–4) were
investigated via thermogravimetric analyser. Thermogravimetric
analysis (TGA) was performed at temperature ranging from
100 °C to 500 °C at heating rate of 5 °C/min under N2 gas flow.
The thermogram (Figure-1) shows start of weight lose at 100 0C
due to water content, indicated by Karl Fischer titration (section).
Literature revealed that, imidazolium cations were more stable as
compared to tetraalkyl ammonium cations [26–27]. Herein, we
evaluated the impact of ortho/para hydroxy and ortho/para amino
substituted benzoate-based anions on thermal stability of ILs 1–4.
It was evident from data that the thermal stability increases in the
order of IL-3 > IL-4 > IL-2 > IL-1 showing Tonset at 305 °C, 290 °C,
245 °C and 238 °C, respectively. These results were suggestive of
comparatively higher thermal stability of amino benzoates. This
was because the size, and molar mass of amino benzoates were
also increased. Hydrogen bonding can be another contributing
+
Na)-]; m/z 383.1725 [calcd. m/z 383.1719 for
20H23N4O-4].
FTIR: 3462.22 (H2O masked NH2), 1639.49 (Conjugated C = O),
1386.82 (C = C), 1136.07 (C-O).
2.8. General method (GM) for tetrahydrobenzo[b]pyrans:
In a round bottomed flask an aromatic aldehyde (9 mmol),
malononitrile (9 mmol) and dimedone (9 mmol) were mixed in
10–20 mL of water and IL-1 (4 mol %) was added. The reaction mix-
ture was then subjected to irradiation under ultrasound (Table 2).
The solid insoluble product was started to appear within minutes.
Completion of reaction was confirmed through TLC. After comple-
tion of reaction as indicated by TLC, the solid product was filtered,
washed with deionized water and dried. Recrystallization of prod-
ucts was carried out with 95% ethanol. Comparisons of pure pro-
duct with publish literature and TLC with authentic sample
confirmed the formation.
3

M.N. Javed, I. Ali Hashmi, S. Muhammad et al.
Journal of Molecular Liquids 336 (2021) 116329
Table 1
Catalyst evaluations for tetrahydrobenzo[b]pyrans synthesisa.
Catalyst (mol%)
Conditions
Time (min)
Yield (%)b
1
2
3
4
5
6
6a
6b
6c
6d
7
None
None
None
None
None
2 mol %
IL-1
IL-2
IL-3
IL-4
IL-1
H2O, 50 °C
180
360
360
20
150
30
Traces [35]
[36]
[37]
[36]
[37]
H2O, Reflux
Ethanol, Reflux
))) c, H2O
))), Ethanol
))), H2O, 45 °C
84.67
84.55
78.96
82.56
))), H2O, 45 °C
30
7a
7b
7c
7d
4 mol %
6 mol %
8 mol %
10 mol %
86.13
86.41
86.74
87.10
a
Reaction Conditions: Benzaldehyde (9 mmol); Dimedone (9 mmol); Malononitrile (9 mmol); ILs (mol %); H2O; 45 °C. bIsolated yield. c))) = Ultrasound irradiation.
Table 2
Preparation of tetrahydrobenzo[b]pyrans analogues using IL-1 (4 mol %)a.
Sr No.
R1
R2
Time (min)
Yield (%)[b]
M.p. °C (lit.)
1
2
3
4
5
6
7
8
H
OH
H
OCH3
OH
OCH3
H
H
H
Cl
Br
H
H
OH
H
OCH3
OH
Cl
Br
30
25
30
15
20
20
30
30
15
20
25
35
86.93
84.23
85.97
85.76
86.90
86.44
86.13
83.68
83.12
87.19
87.25
84.14
231 (233–234) [38]
235 (230–232) [39]
226 (220–222) [40]
187 (188–190) [41]
228 (230–232) [42]
240 (238–239) [43]
215 (214–215) [40]
204 (202–203) [40]
191 (191–193) [39]
232 (230–231) [38]
290 (289–291) [41]
208–210 (212–213) [44]
9
F
H
H
10
11
12
H
N(CH3)2
a
Reaction Conditions: Benzaldehyde (9 mmol); Dimedone (9 mmol); Malononitrile (9 mmol); IL-01 (4 mol %); H2O; 45 °C. bIsolated yield.
Scheme 1. Synthesis of ILs (1–4).
factor. After decomposition, the residual amounts of 3-ethyl-1-
methylimidazolium aminobenzoates were also higher than those
of 1-ethyl-3-methylimidazolium hydroxybenzoates, as shown in
Fig. 1.
that ILs (1–4) exhibited peaks around m/z 111 corresponding to
C+ (C6H11N+2). Furthermore, it also showed singly positive charge
aggregation around m/z 359 which were attributed to C2A+. How-
ever, HR-ESI-MS (negative) of IL-1 exhibited
a peak at m/z
137.0242, which represents to A– (C7H5O–3), along with singly neg-
ative charged aggregates, which corresponds to CA–2. In addition to
this, anion-anion cluster formation with additional proton
[A2 + H]– or sodium [A2 + Na]–, observed at m/z 275.0560 and m/
z 297.0380 respectively (as shown in Fig. 2). Similar behaviour
was witnessed in ILs 2–4 (Figs. 3-5).
3.3. Aggregation behavior of ILs:
ILs exhibit supramolecular polymeric aggregation behaviour
[28]. Therefore, ILs 1–4 were subjected to HR-ESI-MS to investigate
cluster formation. Previously, through ESI-MS, we observed that ILs
containing phenolate anions [4] did not show self-assembly to
form clusters/aggregates while benzoates containing ILs did [6].
The results are in agreement with the previous results i.e. anion-
anion aggregation reduces catalytic activity while cation–anion
aggregates support it (Table-4). Figures (S9-S12) in SI displayed
The overall HR-ESI-MS results were suggestive of two different
types of supramolecular polymeric aggregates i.e. usual cations-
anions aggregates and anion-anion aggregates. Presence of remain-
ing sodium ion (inorganic salt) in ILs was also obvious. Thus, addi-
tional purification (vide experimental) was carried out and impurity
4

M.N. Javed, I. Ali Hashmi, S. Muhammad et al.
Journal of Molecular Liquids 336 (2021) 116329
Fig. 1. Thermal behavior of benzoates based ILs.
Fig. 2. HR-ESI-MS of IL-1 in negative mode exhibited singly charged negative aggregates.
free ILs were obtained and verified through 1H NMR spectra (SI).
This behavior of ILs was correlated with catalytic activity in the
synthesis of tetrahydrobenzo[b]pyran derivatives.
the high cost of ILs, water was selected as solvent and ILs were
investigated only for their catalytic activity. For model reaction,
benzaldehyde, dimedone and malononitrile were reacted to pre-
pare tetrahydrobenzo[b]pyran, using different ILs (Scheme 2).
Literature shows that, ultrasound can break the hydrogen bond,
depending on counter anion and accelerates ILs catalysed reaction.
Fusion of ultrasound with ILs has proved to be ideal combination in
the organic synthesis especially multicomponent reactions [33–
34]. The results of reactions under ultrasonic conditions are
depicted in Table 1. The results presented in table-1 clearly indi-
cate IL-1 as the best catalyst for these reactions at 45 °C under
ultrasonic irradiation.
3.4. Catalytic activity of ILs towards the synthesis of tetrahydrobenzo
[b]Pyrans:
Application of ILs in the chemical reactions has been under-
taken in different ways including as a reaction medium, as a phase
transfer catalyst, as a co-catalyst / catalyst activator and many
more [29–30]. ILs with modified anions exhibiting potential for
H-bonding can serve as conjugate bases and phase transfer catalyst
in organic synthesis. Furthermore, the use of IL catalyst in combi-
nation of solvent is reported to be more effective [31–32]. Due to
The ILs 1 – 4 contains different acidic and basic functional
groups in anions. Moreover, the catalytic reactions are carried
5

M.N. Javed, I. Ali Hashmi, S. Muhammad et al.
Journal of Molecular Liquids 336 (2021) 116329
Fig. 3. HR-ESI-MS of IL-2 in negative mode exhibited series of singly charged negative aggregates.
Fig. 4. HR-ESI-MS of IL-3 in negative mode exhibited series of singly charged anion aggregates.
out in water, therefore, the activity of the catalyst could be related
to the pH. Hence, we check the pH of the reaction medium before
(pH = 6.59) and after addition of IL-4 (pH = 6.44) but no significant
change in pH observed.
6

M.N. Javed, I. Ali Hashmi, S. Muhammad et al.
Journal of Molecular Liquids 336 (2021) 116329
Fig. 5. HR-ESI-MS of IL-4 in negative mode exhibited singly charged negative aggregates.
There are different approaches reported in literature for
tetrahydrobenzo[b]pyrans synthesis using different materials as
catalyst and reaction medium with or without solvent. All previ-
ously reported procedures exhibit harsh reaction conditions
(Table 3). However, in this study recycled and reusable greener cat-
alyst was used under ultrasonic irradiation with high selectivity,
mild reaction conditions, and short reaction time with simple
and effortless work-up procedures. Table 3 shows the assessment
and comparison of our study with previously reported work.
Scheme 2. Preparation of tetrahydrobenzo[b]pyran using IL under ultrasound.
The scope of methodology was examined using IL-1 (4%) with
12 different aromatic aldehydes resulting in excellent yields
(Table 2). Halogenated aldehydes at meta position (entry 10 and
11) furnish excellent yields suggesting presence of electron with-
drawing group at meta position may enhance the yield of product.
3.5. Recyclability of the Catalyst:
A greater advantage of ILs is their recyclability. Although, recy-
clability of water miscible ILs is challenging, especially for the
removal of inorganic by-products. After every run, the IL-1 was
recycled. Solid product was removed through filtration and filtrate
containing IL was evaporated and washed with cold ethanol and
repeatedly used further for four times and no considerable loss
of its efficiency and activity was observed (Fig. 6). Recycling of
4 mol % of water miscible IL is quite difficult. Therefore, we con-
ducted higher concentration experiments to investigate recyclabil-
ity of the catalyst.
Table 3
Evaluation of existing work with previously reported methodologies for the model reaction among malononitrile, dimedone and benzaldehyde.
Sr No.
Catalyst (mol/g)
Condition
Time (min)
Yield (%)[a]
Ref
1
2
3
4
5
6
7
MgFe2O4 nanoparticles
MNPs–PhSO3H (0.01 g)
[Py-SO3H]Cl (10%)
EtOH,))) [c], 65 °C
H2O/ EtOH (1:1), 100 °C
Solvent free, 95 °C
H2O, Reflux
Solvent free, 100 °C
H2O, rt
12
10
8
13
20
10
30
74
91
91
80
93
95
86.13
[45]
[46]
[47]
[48]
[44]
[52]
PPA–SiO2 (0.1 g)
Fe3O4@SiO2@TiO2 (0.01 g)
[ADPPY][OH] (10 mol%)
IL-1 (4 mol %)
[b]
H2O,))), 45 °C
-
aIsolated Yield. bOur Work. c))) = Ultrasound irradiation.
7

M.N. Javed, I. Ali Hashmi, S. Muhammad et al.
Journal of Molecular Liquids 336 (2021) 116329
the model reaction. On the other hand, ILs 3–4 (amino benzoates)
offered moderate yield in tetrahydrobenzo[b]pyrans synthesis. The
results also revealed that, there is a close relationship between
thermal stability and catalytic activity of ILs as shown in Table 4.
Moreover, IL-1 that showed intermolecular/intramolecular hydro-
gen bonding (Supplementary Information Figure S1) exhibited com-
paratively lower thermal stability and enhanced catalytic activity.
This was attributed to strong ion packing between cation and
anion. The overall results suggested that, anion-anion interactions
occur in hydroxyl/amino benzoates influence catalytic activity.
Furthermore, it was also realized that greater the anion-anion
interaction, weaker the cation–anion packing and reduced catalytic
activity of ILs.
3.8. Proposed Mechanism:
In 2009, Chakraborti et. al. [49] reported a dual role (elec-
trophilic and nucleophilic) of imidazolium based ILs as catalyst in
the organic reactions [49]. On the other hand, Gauchot et. al. [50]
showed that anion of the ILs involves in the aldol condensation
by activating aldehydes through hydrogen bonding. While Tao et.
al. [51] demonstrated that deprotonation of C-2 proton of imida-
zolium moiety induces electrophilic activation to anhydride in
the presence of DMF or DMSO [51]. In the light of these observa-
tions, results obtained from present study (especially that of IL-1)
we have proposed a mechanism demonstrated in Scheme 3.
Here, the role of water cannot be excluded, as water is known to
solvate ionic species. Cammarata et. al. carried out experiments on
water-IL interaction and suggested the possibility of carboxylate
anions binding with water in case of deliberate addition of water.
Further, they experimentally proved that ILs, containing 1-butyl-
3-methyl imidazolium as cation, capture water from the atmo-
sphere with different types of anion [53].
Fig. 6. Recycling of IL-1 in the model reaction of tetrahydrobenzo[b]pyran.
3.6. Conductivity of ILs:
To further ascertain the formation of aggregates in IL 1–4 elec-
trical conductivities of these ILs along with parent cation [EMIM][I]
were recorded at different concentrations of ILs in water. Critical
aggregation concentration (CAC) and degree of ionization (a) were
determined using method reported by Wang et. al. [54] . The
results are presented in Fig. 7 and Table S13.
The concentration at point of interaction of two linear frag-
ments corresponds to CAC while
a is the ratio of slope of linear
lines above and below CAC. The higher CAC value (1100 mmol.
dmÀ3) in IL-3 indicates that negative ion is loosely bonded to
cation. However, lowest
a value (0.241) in IL-3 supports presence
of anion-anion [A2 + H]– aggregation. These finding fully support
the HR-ESI-MS observation showing highest relative intensity of
[A2 + H]– Fig. 3.
4. Conclusion
In short, we have synthesized four new ILs 1–4. Thermally
stable and water-soluble hydroxy/amino benzoates anions paired
with methyl alkyl imidazolium to furnish environmentally benign
ILs as catalyst. In the present studies IL-1 was found to be excellent
catalyst. IL-1 showed stronger supramolecular aggregation
behaviour (possibly due to H-bonding) as exhibited in HR-ESI-MS
3.7. Relationship of thermal stability, Conductivity, and aggregation
behavior with catalytic activity of ILs:
The effect of anions on catalytic behavior of ILs was investi-
gated. ILs 1–2 (hydroxy benzoates) furnished excellent result in
Fig. 7. Conductivity of ILs in aqueous medium.
8

M.N. Javed, I. Ali Hashmi, S. Muhammad et al.
Journal of Molecular Liquids 336 (2021) 116329
Table 4
Correlation of thermal stability, Conductivity at CAC, and relative intensity of ILs (1–4) with catalytic activity in tetrahydrobenzo[b]pyrans reactions.
Relative Intensity (105)
Conductivity at CAC
(mS/cm)
CAC
a
Tonset
Catalytic activity
Time (min)
–
ILs
[A2 + H]–
[CA2
]
[C2A3]–
(mM)
(°C)
Yield (%)
IL-1
IL-2
IL-4
IL-3
0.5
0.8
0.8
1
0.72
0.5
0.39
0
0
0.21
0
33
38
21
30
850
760
750
1100
0.374
0.364
0.391
0.241
238
245
290
305
30
30
30
30
84.67
84.55
82.56
78.96
0
Scheme 3. Proposed mechanism for the synthesis of tetrahydrobenzo[b]pyran.
(Supplementary information; Figure S9) and 1H NMR (Supplementary
information; Figure S1), and in consistence with previously
reported results [5–6]. The less intense [A2 + H]+ signal, exhibiting
anion-anion interaction is suggestive of intermolecular H-bonding.
9

M.N. Javed, I. Ali Hashmi, S. Muhammad et al.
Journal of Molecular Liquids 336 (2021) 116329
[18] A.R. Swain, S.P. Dutton, A.S. Truswell, Salicylates in foods, Journal of American
Dietetic association 85 (1985) 950–960.
[19] K. Shahriar, J.M. Robin, Monocyclic Phenolic Acids, Hydroxy- and
Polyhydroxybenzoic Acids: Occurrence and Recent Bioactivity Studies,
Molecules 15 (2010) 7985–8005.
On the basis of these observations, we can conclude that inter-
molecular H-bonding lowers thermal stability, reduces anion-
anion interaction and enhance catalytic activity.
[20] A. Kluczyk, T. Popek, T. Kiyota, P. de Macedo, P. Stefanowicz, C. Lazar, Y.
Konishi, Yasuo, Drug Evolution: p-Aminobenzoic Acid as a Building Block,
Current Medicinal Chemistry 9 (2002) 1871–1892.
Declaration of Competing Interest
[21] T. Xin, L. Xiaomin, Y. Xiaokian, Z. Yaqin, J. Kun, Theoretical Study of Ionic Liquid
Clusters Catalytic Effect on the Fixation of CO2, Industrial & Engineering
Chemistry Research 58 (2019) 34–43.
[22] N. Thomas, Z. Dimitri, S. Anne, V. Alaxendar, L. Ralf, Cationic clustering
influences the phase behaviour of ionic liquids, Scientific Reports 8 (2018)
14753.
[23] G.A. Anthony, A.C. Penelop, P.D. Martin, R.S. Kenneth, W. Thomas, Evidence for
Hydrogen Bonding in Solutions of I -Ethyl-3- methylimidazolium Halides, and
its Implications for Room-temperature Halogenoaluminate(llI), Ionic Liquids,
Journal of Chemical Society, Dalton Transaction (1994) 3405–3413.
[24] Y. Cao, M. Tiancheng, Comprehensive Investigation on the Thermal Stability of
66 Ionic Liquids by Thermogravimetric Analysis, Industrial and Engineering
Chemical Research 53 (2014) 8651–8664.
[25] Y. Cao, M. Tiancheng, Stability of Ionic Liquids, in: S. Zhang (Ed.), Encyclopedia
of Ionic Liquids, Springer, Singapore, 2020, https://doi.org/10.1007/978-981-
10-6739-6_103-1.
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgment
The authors thank Research Centre, College of Pharmacy and
Deanship of Scientific Research, King Saud University for support-
ing this study. The authors also thank Prof. Yanan Gao (Dalian
Institute of Chemical Physics, Chinese Academy of Sciences, China)
for their valuable comments and suggestions.
[26] L.N. Helen, L. Karen, H. Liesel, B.M. Alan, Thermal properties of imidazolium
ionic liquids, Thermochimica Acta 357–358 (2000) 97–102.
[27] Z. Ling, W. Ligang, Z. Shangru, Z. Dingwei, S. Jian, A. Qingda, Hydrogen bond
promoted thermal stability enhancement of acetate based ionic liquid, Chinese
Journal of Chemical Engineering 28 (2020) 1293–1301.
[28] A.D.N. Brenno, S. John, The impressive chemistry, applications and features of
ionic liquids: properties, catalysis & catalysts and trends, Journal of Brazilian
Chemical Society 23 (2012) 987–1007.
[29] P. Wasserschied, T. Welton (Eds.), Ionic Liquids in Synthesis, VCH Wiley,
Weinheim, 2002.
[30] D. Zhao, M. Wu, Y. Kou, E. Min, Ionic Liquids: Applications in Catalysis 74
(2002) 157–159.
[31] R.A. Sheldon, Green solvents for sustainable organic synthesis: state of the art,
Green Chemistry 7 (2005) 267–278.
[32] N. Isambert, M. del, M.S. Duque, J.-C. Plaquevent, Y. Genisson, J. Rodriguez, T.
Constantieux, Multicomponent reactions and ionic liquids: a perfect synergy
for eco-compatible heterocyclic synthesis, Chemical Society Reviews 40 (2011)
1347–1357.
[33] K. Li, T. Kobayashi, A FT-IR spectroscopic study of ultrasound effect on aqueous
imidazole based ionic liquids having different counter ions, Ultrasonics
Sonochemistry 28 (2016) 39–46.
[34] G. Kaur, A. Sharma, B. Banerjee, Ultrasound and Ionic Liquid: An ideal
Combination for Organic Transformations, Chemistry Select 3 (2018) 5283–
5295.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molliq.2021.116329.
References
[1] P. Liu, J.-W. Hao, L.-P. Mo, Z.-H. Zhang, Recent advances in the application of
deep eutectic solvents as sustainable media as well as catalysts in organic
reactions, RSC Advances 5 (2015) 48675–48704.
[2] P.E. Savage, Organic chemical reactions in supercritical water, Chemical
reviews 99 (1999) 603–622.
´
´
ˇ
[3] M.C. Bubalo, K. Radoševic, I.R. Redovnikovic, J. Halambek, V.G. Srcek, A brief
overview of the potential environmental hazards of ionic liquids,
Ecotoxicology and environmental safety 99 (2014) 1–12.
[4] M.N. Javed, S. Muhammad, I.A. Hashmi, A. Bari, S.G. Musharraf, F.I. Ali, Newly
designed pyridine and piperidine based Ionic Liquids: Aggregation behavior in
ESI-MS and catalytic activity in CC bond formation reactions, Journal of
Molecular Liquids 272 (2018) 84–91.
[5] S. Muhammad, F.I. Ali, A. Aslam, V.U. Ahmad, A.N. Khan, I.A. Hashmi, Synthesis
of Thiosalicylate based Hydrophobic Ionic Liquids and their Applications in
Metal Extraction from aqueous solutions, Journal of the Chemical Society of
Pakistan 37 (2015).
[6] S. Muhammad, M.N. Javed, F.I. Ali, A. Bari, I.A. Hashmi, Supramolecular
polymeric aggregation behavior and its impact on catalytic properties of
imidazolium based hydrophilic ionic liquids, Journal of Molecular Liquids 300
(2020) 112372.
[7] T. Welton, Room-Temperature Ionic liquids, solvents for synthesis and
catalysis, Chemical Review 99 (1999) 2071–2083.
[8] R.L. Vekariya, A review of ionic liquids: Applications towards catalytic organic
transformations, Journal of Molecular Liquids 227 (2017) 44–60.
[9] Z.S. Qureshi, K.M. Deshmukh, B.M. Bhanage, Applications of ionic liquids in
organic synthesis and catalysis, Clean Technologies and Environmental Policy
16 (2014) 1487–1513.
[10] A. Oskarsson, M.C. Wright, Ionic Liquids: New Emerging Pollutants, Similarities
with Perfluorinated Alkyl Substances, PFASs, Environmental Science and
Technology 53 (2019) 10539–10541.
[11] R.P. Swatloski, J.D. Holbrey, R.D. Rogers, Ionic Liquids are not always green:
hydrolysis of 1-butyl-3-methyl-imidazolium hexafluorophosphate, Green
Chemistry 2 (2003) 361–363.
[12] M.G. Freire, C.M.S.S. Neves, I.M. Marrucho, J.A.P. Coutinho, A.M. Fernandes,
Hydrolysis of tetrafluoroborate and hexafluorophosphate counter ions in
imidazolium-based ionic liquids, Journal of Physical Chemistry A 114 (2010)
3744–3749.
[35] R. Baharfar, S. Asghari, N. Shariati, Green synthesis of 2-amino-3-cyclo-4H-
chromenes in water using nano-silica bonded 5-N-propyl-octahydro-pyrtmido
[1, 2-4] azepinium chloride as as effective and reusable nano catalyst, Journal
of the Chilean Chemica; Society, 60 (2015) 2900-2904.
[36] M. Esmaeilpour, J. Javidi, F. Dehghani, F.N. Dodeji, A green one-pot three-
component synthesis of tetrahydrobenzo [b] pyran and 3, 4-dihydropyrano [c]
chromene derivatives using
a Fe 3 O 4@ SiO 2–imid–PMA n magnetic
nanocatalyst under ultrasonic irradiation or reflux conditions, RSC Advances
34 (2015) 26625–26633.
[37] N.M. Abd El-Rahman, R.M. Borik, Eco-friendly solvent-free synthesis of
tetrahydrobenzo [b] pyran derivatives under microwave irradiation, World
Applied Sciences Journal 31 (2014) 1–6.
[38] D. Tahmassebi, J.A. Bryson, S.I. Binz, 1, 4-Diazabicyclo [2.2. 2] octane as an
efficient catalyst for a clean, one-pot synthesis of tetrahydrobenzo [b] pyran
derivatives via multicomponent reaction in aqueous media, Synthetic
Communications, 41 (2011) 2701-2711.
[39] D. Fang, D.; H.B. Zhang, Z. –L. Liu, Synthesis of 4H-benzopyrans catalyzed by
acyclic acidic ionic liquids in aqueous media, Journal of Heterocyclic
Chemistry, 47 (2010) 63-67
[40] W.-B. Sun, P. Zhang, J. Fan, S.-H. Chen, Z.-H. Zhang, Lithium bromide as a mild,
efficient, and recyclable catalyst for the one-pot synthesis of tetrahydro-4 H-
chromene derivatives in aqueous media, Synthetic Communications 40 (2010)
587–594.
[41] M.R. Islami, E. Mosaddegh, Sulfur,; Silicon, Ce (SO4) 2Á 4H2O as a Recyclable
Catalyst for an Efficient, Simple, and Clean Synthesis of 4H-Benzo [b] pyrans,
Phosphorous, Sulfur and Silicon and the Related Elements 184 (2009) 3134–
3138.
[42] R. Balaskar, S. Gavade, M. Mane, P. Pabrekar, M. Shingare, D. Mane,
Triethylammonium Acetate [TEAA]: An Efficient Catalyst for One Pot
Synthesis of Tetrahydro-4H-chromene Derivatives, Letters in Organic
Chemistry 8 (2011) 282.
[43] A. Patra, T. Mahapatra, T Synthesis of tetrahydrobenzo [b] pyran derivatives
catalysed by AliquatÒ 336 in water under microwave irradiation, Journal of
Chemical Research 34 (2010) 689–693.
[44] A. Khazaei, F. Gholami, V. Khakyzadeh, A.R. Moosavi-Zare, J. Afsar, Magnetic
core–shell titanium dioxide nanoparticles as an efficient catalyst for domino
Knoevenagel–Michael-cyclocondensation reaction of malononitrile, various
aldehydes and dimedone, RSC Advances 5 (2015) 14305–14310.
[13] S. Stolte, J. Arning, U. Bottin-Weber, M. Matzke, F. Stock, K. Thiele, M.
Uredingen, U. Welz-Biermann, B. Jastorff, J. Ranke, Anion effects on
cytotoxicity of ionic liquids, Green Chemistry 8 (2006) 621–629.
[14] C. Chul-Woong, P.T.P. Thi, J. You-Chul, K. Vijayaraghavan, C. Woo-Seok, Y.
Yeong-Sang, Toxicity of imidazolium salt with anion bromide to
phytoplankton Selenastrum capricornutum: Effect of alkyl-chain length,
Chemosphere 69 (2007) 1003–1007.
a
[15] R.S. Boethling, Cationic Surfactants, in Surfactant Science Series, Marcel
Dekker, New York 53 (1994) 95–135.
[16] P.H. Howard, R.S. Boethling, W. Stiteler, W. Meylan, J. Beauman, Development
of a predictive model for biodegradability based on BIODEG, the evaluated
biodegradation data base, Science Total Environment 109–110 (1991) 635–
641.
[17] A. Wood, G. Baxter, F. Thies, J. Kyle, G. Duthie,
A systematic review of
salicylates in foods: estimated daily intake of a Scottish population, Molecular
Nutrition and Food Research 55 (Suppl 1) (2011) S7–S14.
10

M.N. Javed, I. Ali Hashmi, S. Muhammad et al.
Journal of Molecular Liquids 336 (2021) 116329
[45] B. Eshtehardian, M. Rouhani, Z. Mirjafary, Green protocol for synthesis of MgFe
2 O 4 nanoparticles and study of their activity as an efficient catalyst for the
synthesis of chromene and pyran derivatives under ultrasound irradiation,
Journal of the Iranian Chemical Society 17 (2019) 1–13.
[49] A.K. Chakraborti, S.R. Roy, On catalysis by ionic liquids, Journal of the American
Chemical Society 131 (2009) 6902–6903.
[50] V. Gauchot, A.R. Schmitzer, Asymmetric aldol reaction catalyzed by the anion
of an ionic liquid, The Journal of Organic Chemistry 77 (2012) 4917–4923.
[51] Y. Tao, R. Dong, I.V. Pavlidis, B. Chen, T. Tan, Using imidazolium-based ionic
liquids as dual solvent-catalysts for sustainable synthesis of vitamin esters:
inspiration from bio-and organo-catalysis, Green Chemistry 18 (2016) 1240–
1248.
[46] H.F. Niya, N. Hazeri, M.R. Kahkhaie, M.T. Maghsoodlou, Preparation and
characterization of MNPs–PhSO
3 H as a heterogeneous catalyst for the
synthesis of benzo [b] pyran and pyrano [3, 2-c] chromenes, Research on
Chemical Intermediates 46 (2019) 1–20.
[47] M.A. Zolfigol, A. Khazaei, A.R. Moosavi-Zare, J. Afsar, J.; V. Khakyzadeh, O.
Khaledian, Knoevenagel-Michael-cyclocondensation Tandem Reaction of
Malononitrile, Various Aldehydes and Dimedone Catalyzed by Sulfonic Acid
Functionalized Pyridinium Chloride as a New Ionic Liquid and Catalyst, Journal
of the Chinese Chemical Society, 62 (2015) 398-403
[48] A. Davoodnia, S. Allameh, S. Fazli, N. Tavakoli-Hoseini, One-pot synthesis of 2-
amino-3-cyano-4-arylsubstituted tetrahydrobenzo [b] pyrans catalysed by
silica gel-supported polyphosphoric acid (PPA-SiO 2) as an efficient and
reusable catalyst, Chemical Papers 65 (2011) 714–720.
[52] P.P. Salvi, A.M. Mandhare, A.S. Sartape, D.K. Pawar, S.H. Han, S.S. Kolekar, An
efficient protocol for synthesis of tetrahydrobenzo[b]pyrans using amino
functionalized ionic liquids, Comptes Rendus Chimie 14 (2011) 878–882.
[53] L. Cammarata, S.G. Kazarian, P.A. Salter, T. Welton, Molecular states of water in
room temprature ionic liquids, Phys. Chem. Chem. Phys. 3 (2001) 5192–5200.
[54] Huiyong Wang, Jianji Wang, Shibiao Zhang, Xiaopeng Xuan, Structural Effects
of Anions and Cations on the Aggregation Behavior of Ionic Liquids inAqueous
Solutions, The Journal of Physical Chemistry B 112 (51) (2008) 16682–16689,
https://doi.org/10.1021/jp8069089.
11