Room temperature ionic liquid choline chloride-oxalic acid: A versatile catalyst for acid-catalyzed transformation in organic reactions

Article abstract of DOI:10.1016/j.molliq.2013.11.036

An efficient and facile synthesis of hydrazones, bis(indolyl)methanes and bis(4-hydroxycoumarin)methanes is facilitated by room temperature ionic liquid choline chloride-oxalic acid (ChCl:Ox). ChCl:Ox (10 mol%) efficiently catalyzes condensation of aldehydes with phenylhydrazine giving corresponding hydrazones in high yield (92-96%) within short reaction time of 3-5 min. Electrophilic cyclization reaction of indole and 4-hydroxy coumarin with aromatic aldehyde was effectively promoted by 30 mol% of ChCl:Ox giving corresponding bis(indolyl)arylmethanes and bis (4-hydroxycoumarin)methanes in good yield at room temperature. Procedure is very simple, solvent free and completely eliminates use of toxic acid catalyst. ChCl:Ox is biodegradable, can be recycled and reused without the loss of efficiency with respect to yield.

Full text of DOI:10.1016/j.molliq.2013.11.036

Journal of Molecular Liquids 191 (2014) 137–141
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Short Communication
Room temperature ionic liquid choline chloride–oxalic acid: A versatile
catalyst for acid-catalyzed transformation in organic reactions
⁎
Urmiladevi Narad Yadav, Ganapati Subray Shankarling
Dyestuff Technology Department, Institute of Chemical Technology, Matunga, Mumbai 400 019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient and facile synthesis of hydrazones, bis(indolyl)methanes and bis(4-hydroxycoumarin)methanes is
facilitated by room temperature ionic liquid choline chloride–oxalic acid (ChCl:Ox). ChCl:Ox (10 mol%) efficiently
catalyzes condensation of aldehydes with phenylhydrazine giving corresponding hydrazones in high yield (92–
96%) within short reaction time of 3–5 min. Electrophilic cyclization reaction of indole and 4-hydroxy coumarin
with aromatic aldehyde was effectively promoted by 30 mol% of ChCl:Ox giving corresponding bis(indolyl)
arylmethanes and bis (4-hydroxycoumarin)methanes in good yield at room temperature. Procedure is very
simple, solvent free and completely eliminates use of toxic acid catalyst. ChCl:Ox is biodegradable, can be recycled
and reused without the loss of efficiency with respect to yield.
Received 16 November 2013
Received in revised form 27 November 2013
Accepted 29 November 2013
Available online 14 December 2013
Keywords:
Choline chloride
Oxalic acid
© 2013 Elsevier B.V. All rights reserved.
Phenylhydrazone
Bis(indolyl)methane
Acidic ionic liquid
Solvent free synthesis
1. Introduction
These acid catalysts are highly corrosive, unsafe to handle and pose
severe threat to the environment. Hence several modifications for
Heterocycles such as phenylhydrazone, bis(indolyl)lmethane and
bis(coumarin)methane derivatives have widespread application in the
fields of biological and pharmacological sciences. Hydrazones have
attracted the chemist as they are building blocks in the synthesis
of compounds for applications in DSSC [1], NLO [2], medicinal chemistry
[3–5] and synthesis of heterocycles such as oxadiazoles [6], bodipy [7],
formazons [8] and thiadiazolidines [9]. Bis(indolyl)lmethanes and
bis(coumarin)methanes show apoptosis and antiangiogenic activities
[10–12]. Strepindole, member of bis(indolyl)methane family, is a potent
genotoxic metabolite [13]. Bis(indolyl)methane is used as highly selective
colorimetric and ratiometric fluorescent molecular chemosensors for
Cu2+ cations [14] and F− anions [15]. Versatile applications of these
heterocycles have drawn attention of many scientists for their synthesis
and to explore their property.
preparation of hydrazones are implemented such as design of new
catalyst [21,22], zeolite [23], and use of different sources of energy
like microwave [24], ultrasound [25] and ionic liquid [26].
Similarly, numerous reports on synthesis of bis(indolyl)methane has
been reported using series of catalyst such as SO24−/ZrO2 [27], SbCl3 [28],
(NH4)H2PW12O40 [29], PEG-SO3H [30], 3-methyl-1-sulfonic acid
imidazolium chloride [31], camphor sulphonic acid or ZrOCl2∙8H2O
[32], In(OTf)3 [33], and [bmim][MeSO4] [34]. A solvent free synthesis
has been reported by Patil et al. which requires heating up to 80 °C
and the reaction time ranges from 40 to 90 min [35]. But these reactions
are encountered with drawbacks related to use of expensive catalyst
and their reuse, high reaction temperature, multi-step synthesis of
catalyst, stability and storage of catalyst, and use of organic solvents,
tedious product isolation step, and long reaction hours.
Synthesis of phenyl hydrazones involves reaction of carbonyl
compound with phenyl hydrazine in the presence of acid catalyst
such as PSSA [16] (poly styrene sulphonic acid), Mg(ClO4)2 [17],
glac∙CH3COOH [18], and Bronsted and Lewis acid catalyst [19,20].
In continuation of our research work [36–39] on the use of green
catalyst for organic reaction we have carried out synthesis of hydrazones,
bis(indolyl)methanes and bis(4-hydroxycoumarin)methanes using
room temperature ionic liquid (RTIL) of choline chloride and oxalic acid
(ChCl:Ox). Room temperature ionic liquids (RTILs) composed of choline
chloride are synthesized by heating of two solids which forms eutectic
mixture [40]. These eutectics are liquid at room temperature and are
commonly called as ‘deep eutectic solvents’ or ‘deep eutectic mixtures’
[41–43]. We have explore the catalytic activity of ChCl:Ox for efficient
synthesis and separation of aromatic and hetero-aromatic hydrazones,
bis(indolyl)arylmethanes and bis(4-hydroxycoumarin)methanes.
⁎
Corresponding author at: Department of Dyestuff Technology, Institute of Chemical
Technology, N. P. Marg, Matunga, Mumbai 400 019, India. Tel.: +91 2233612708;
fax: +91 22 33611020.
E-mail addresses: gsshankarling@gmail.com, gs.shankarling@ictmumbai.edu.in
(G.S. Shankarling).
0167-7322/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molliq.2013.11.036

138
U.N. Yadav, G.S. Shankarling / Journal of Molecular Liquids 191 (2014) 137–141
Table 2
2. Results and discussion
Synthesis of various heterocyclic hydrazone derivative catalyzed choline chloride and
oxalic acid ChCl:Ox.
Reaction of phenylhydrazine with aldehydes such as benzaldehyde, p-
nitrobenzaldehyde, and pyridine-3-carboxaldehyde yields the corre-
sponding hydrazones either under neat conditions or in solvents. Howev-
er, the conversion consumes 0.5–3 h in the absence of any catalyst at
room temperature giving hydrazones in the range of 80–90% at room
temperature. For the condensation of phenylhydrazine with less electro-
philic aldehydes/ketones such as N, N-dimethyl-2-hydroxybenzaldehyde,
4-methoxybenzaldehyde, indole-3-carboxaldehyde, 4-methyl acetophe-
none, acetophenone and 4-bromoacetophenone no significant phenyl-
hydrazone formation was observed when these substrates were treated
with phenylhydrazine in absence of catalyst. Thus, we carried out the syn-
thesis of phenylhydrazone derivatives with the above-mentioned sub-
strate using ChCl:Ox as greener catalyst.
In order to study the catalytic efficiency of ChCl:Ox we used 4-
methylacetophenone (1 mmol) as model substrate for reaction with
phenyl hydrazine (1 mmol). The reaction was carried out in different
sets of reaction conditions to find the most suitable parameters for
hydrazone formation (Table 1).
Optimization result indicated that 10 mol% of ChCl:Ox was efficient
to promote synthesis of hydrazone, further increase in mol% had no pro-
nounced enhancement in yield indicating that the acidity requirement
is well fulfilled by 10 mol% of catalyst. Conventional synthesis requires
glacial acetic acid in refluxing ethanol; the yield obtained after 5 h is
only 54%. Using biodegradable ChCl:Ox as catalyst affords 97% yield in
just 5 min. Thus, the current process is highly efficient and simple as
compared to other reported methodologies.
Product
Catalyst
Time (min)
Yield (%)
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
\p-OHC6H4, \H
\p-NO2C6H4, \H
\p-FC6H4, \H
1
94
99
96
98
92
90
91
98
96
95
99
94
94
92
30 s
30 s
30 s
1
4
2
1
4
5
3
\p-ClC6H4, \H
\p-OCH3C6H4, \H
\p-(N,N-diethyl)C6H4, \H
\m-(OC6H5)C6H4, \H
C6H5, \H
C6H5, \CH3
\p-CH3C6H4, \CH3
\p-NO2C6H4, \CH3
\p-BrC6H4, \CH3
C4H3O, \H
4
2
3
, \H
3o
4
92
To elaborate the scope of ChCl:Ox as efficient catalyst, aldehydes and
ketones with different electron donating and electron withdrawing sub-
stituents were investigated (Table 2). It was observed that reaction with
electron withdrawing substituent (Table 2, entries 3b, 3c, 3d & 3k) was
very fast which is due to high electrophilicity of carbonyl group induced
by the substituent. Electron donating groups had pronounced effect on re-
action rate. Electron donating group (Table 2, entry 3a, 3f, 3j) lowered the
electrophilicity of carbonyl group and hence took longer reaction times to
produce comparable yields to those obtained with simple and electron-
withdrawing counterparts. Reaction with heterocycles such as acid sensi-
tive furan-2-carboxaldehyde (3m), indole-3-carboxaldehyde (3n) and
isatin (3o) occurred in short time of 3–5 min with 92–96%yield. The
Reaction condition: aldehydes/ketone:phenyl hydrazine 1:1 (mole ratio), ChCl:Ox
(10 mol%), and room temperature.
catalytic behavior of ChCl:Ox is well depicted in Scheme 1. The presence
of ChCl:Ox catalyst results in enhanced polarization of carbonyl group
through hydrogen bonding thereby assisting the reaction towards com-
pletion. Result in Table 2 summarizes the operational protocol for
phenylhydrazone derivative synthesis thereby completely eliminating
the drawbacks of use of organic solvents and toxic acid catalyst.
Mechanistic pathway (Scheme 1) indicates the catalytic role of ChCl:
Ox. Electrophilicity of carbonyl group of aldehyde/ketone is increased
by hydrogen bonding with acidic hydrogen of oxalic acid. An alcoholic
hydroxyl group generated acts as base by donating a pair of electrons
(oxygen lone pair) to form a covalent bond with H+, thus acquiring a
positive charge. Acid catalyzed dehydration occurs by exit of stable
leaving group— +OH2. Finally protonation and deprotonation result in
the formation of final product.
Table 1
Comparisons of ChCl:Ox catalyst with various homogeneous or heterogeneous catalysts in
the condensation of 4-methyl acetophenone with phenylhydrazine.
Since, bis(indoly)arylmethanes and bis(4-hydroxycoumarin)meth-
anes are important intermediates for the synthesis of moiety with po-
tential anticancer activities [10–12], we next planned to extend the
application of ChCl:oxalic acid as a catalyst for their synthesis. 4-
Methylbenzaldehyde and indole were chosen as model substrate for
bis(indoly)methane synthesis. Reactions were carried out to optimize
the mol% of catalyst required for reaction completion (Table 3). It is ob-
served that in the presence of green catalyst like silica gel, amberlyst
high temperature is required for reaction. In the presence of high tem-
perature and acidic medium, polymerization of indole results in poor
or low yield. However, by use of 30 mol% ChCl:oxalic cid, the reaction
completes in 20 min at room temperature giving 98% conversion.
High mol% of catalyst is required for bis(indolyl)methane synthesis as
compared to hydrazones where 10 mol% of catalyst was sufficient for
reaction conversion.
Sr.
no.
Catalyst
Solvent
Reaction
temp (°C)
Time
5 h
15 min 85
4 min 93
Yield
(%)
Ref.
1.
2.
3.
4.
glc.CH3COOH
glc.CH3COOH
[bbim]Br, 1 equiv.
anhyd.Mg(ClO)4,
5 mol%
Conc.H2SO4,
2drops
ChCl:oxalic acid,
10 mol%
ChCl:oxalic acid,
10 mol%
ChCl:oxalic acid,
10 mol%
ChCl:oxalic acid,
5 mol%
ChCl:oxalic acid,
15 mol%
Ethanol
Glc.CH3COOH
80
25
28
54
[44]
[45]
[46]
[17]
–
Dichloromethane 20
2 h
95
5.
6.
7.
8.
9.
10
Methanol
70
RT
RT
RT
RT
RT
4 h
92
[47]
–
Ethanol
10 min 94
10 min 96
5 min 95
10 min 89
10 min 96
Solvent free
Solvent free
Solvent free
Solvent free
–
–
Different derivatives were synthesized in order to study the effect of
various substituents over the yields. Nature of substituents on aldehyde
and electron availability showed considerable effect in reaction time.
Rate of electrophilic addition of indole with electron rich aldehyde
(Table 4, 6a, 6b, 6c, & 6 k) was well facilitated giving high conversion
–
–
Reaction condition: 4-methylacetophenone:phenyl hydrazine 1:1 (mole ratio).

U.N. Yadav, G.S. Shankarling / Journal of Molecular Liquids 191 (2014) 137–141
139
Scheme 1. Tentative reaction pathway for hydrazone synthesis catalyzed by ChCl:Ox.
Table 3
Results of 3,3′-(p-tolylmethylene)bis(1H-indole)synthesis using different catalyst and
Table 4
Condensation of indole with various aromatic and heterocyclic aldehydes catalyzed by
ChCl:Ox.
reaction conditions.
Producta Ar
R
Time (in min) Yield (%)b M.P. (°C)
Sr.
no.
Catalyst
Solvent
Reaction temp
(°C)
Time
Yield
(%)
Ref.
6a
6b
6c
6d
\p-CH3C6H4
\p-OHC6H4
\p-(N, N-CH3)C6H4
\H
\H
\H
\H
15
18
14
25
94
92
96
89
96–98
120
212–214
202
1.
2.
3.
SBA-15/SO3H (70%)
KHSO4, 1 mmol
FeCl3∙6H2O, 5 mol%
70
24 h
2 h
4 h
62
54
85
[48]
[49]
[50]
Methanol RT
[omim]
PF6
RT
4.
–
[omim]
BF4
RT
4 h
91
[51]
6e
\H
20
91
188
5.
6.
Amberlyst 15
La(NO3)3∙6H2O,
5 mol%
–
80
20
2.5 h
2 h
96
98
[52]
[53]
–
6f
6g
6 h
\m-(OC6H5)C6H4,
\C5H4N
\H
\H
\H
20
24
18
94
88
89
82–84
136–138
148
7.
8.
Silica gel
Sodium
–
100
RT
1 h
2.5 h
98
96
[54]
[55]
Water
dodecylsulfate
ChCl:oxalic acid,
30 mol%
ChCl:oxalic acid,
30 mol%
ChCl:oxalic acid,
20 mol%
ChCl:oxalic acid,
40 mol%
9
Ethanol
RT
RT
RT
RT
RT
30 min 92
20 min 98
20 min 90
10 min 97
30 min NR
–
–
–
–
–
6i
\H
25
88
232
10.
11.
12.
13.
Solvent
free
Solvent
free
Solvent
free
Solvent
free
6j
6k
6l
\C6H5
\p-OCH3C6H4
\p-NO2C6H4, –H
\CH3 10
\CH3 18
\CH3 20
\CH3 20
95
96
93
89
242–244
210
242–244
238
6m
ChCl:urea
a
Reaction conditions: indole (2 mmol), aldehyde (1 mmol), ChCl:Ox (30 mol%), and
room temperature.
b
Reaction condition: 4-methylbenzaldehyde:indole (1:2) (mole ratio).
Isolated yields. All products are characterized by IR and proton NMR.

140
U.N. Yadav, G.S. Shankarling / Journal of Molecular Liquids 191 (2014) 137–141
Scheme 2. A tentative mechanistic pathway for ChCl:oxalic acid (1:1) catalyzed synthesis of bis(indolyl)methanes.
in short period. Electron withdrawing moiety like nitro group (6i),
benzoxazole (6l) and pyridine (6g) consumed longer reaction times to
produce comparable yields than those of their simple and electron-
rich counterparts. Different bicyclic aldehydes of naphthalene series
(6d & 6e) gave bis(indolyl)methane derivative in good yield, however
the time required was more. The reason for lower reaction rate may
be due to steric hindrance during the attack of second indole moiety
(Scheme 2). We found that a number of heteroaromatic aldehydes
(6h, 6i & 6m) afforded the corresponding bis(indolyl)methanes in
good yields.
Encouraged by excellent results obtained using ChCl:Ox, we then in-
vestigated the scope for synthesis of bis(4-hydroxycoumarin)methanes
using the same reaction condition. To our surprise ChCl:oxalic acid (1:1)
was found to catalyze the reaction between 4-hydroxy coumarin and al-
dehyde very efficiently (Table 5). Novel derivatives were obtained with-
in a time span of 20–25 min and are confirmed by FT-IR and 1H NMR.
A reasonable mechanistic interpretation to explain the formation of
the observed bis(indolyl)methanes might assume a reaction path as
shown in Scheme 2. ChCl:oxalic acid (1:1) facilitates the reaction by
making the carbonyl group electrophilic through hydrogen bonding.
The reaction between 1H of indole with aldehydes produces intermedi-
ate azafulvenium salts which further react with a second indole mole-
cule to form the corresponding bis(indol-3-yl)methanes.
From economical and environmental points of view reuse of catalyst is
extremely important hence a scale-up was set up. For hydrazone forma-
tion, scale-up batch for 50 g was carried using 4-methyl acetophenone
and phenyl hydrazine as model substrate while for recyclability study of
bis(indolyl)methane, indole, and 4-methylbenzaldehyde were taken as
substrate. After completion of reaction as indicated on TLC water was
added. Solid product was filtered out. ChCl:oxalic acid (1:1) was recov-
ered by vacuum evaporation of water at 80 °C and dried over sodium sul-
fate. t was reused for the next cycles (Fig. 1). Recyclability study indicated
that ChCl:oxalic acid (1:1) remains in high catalytic activity even after five
successive runs.
Table 5
Synthesis of bis(coumarin)methanes catalyzed by ChCl:Ox.
3. Conclusion
In summary, ChCl:Ox proves to be a potential catalyst for the facile
synthesis of phenylhydrazone derivatives, bis(indolyl)arylmethanes
and bis(4-hydroxycoumarin)methanes. Additional catalyst and envi-
ronmentally unfavorable volatile organic solvents have been completely
avoided by adopting this strategy. The most attractive part of the meth-
odology is its simple experimental procedure, high yield in short time
Product
9a
Producta
Time
(min)
Yield
(%)b
M.P. (°C)
observed
25
96
250
Yield (1) %
94
92
92
95
Yield (2) %
94
90
91
90
94
100
80
60
40
20
89
92
90
9b
20
92
N300
0
No. of cycles
a
Reaction conditions: 4-hydroxy coumarin (2 mmol), aldehyde (1 mmol), ChCl:Ox
(30 mol%), and room temperature.
Fig. 1. Recyclability study, yield (1) refers to percentage of 4-methyl acetophenone
phenylhydrazone product; yield (2) refers to percentage of 3,3′-(p-tolylmethylene)
bis(1H-indole) product.
b
Isolated yields. All products are characterized by IR and H-NMR.

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141
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Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.molliq.2013.11.036.
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