DABCO functionalized dicationic ionic liquid (DDIL): A novel green benchmark in multicomponent synthesis of heterocyclic scaffolds under sustainable reaction conditions

Article abstract of DOI:10.1016/j.molliq.2016.10.039

A novel DABCO functionalized dicationic ionic liquid (DDIL) has been synthesized using diazabicyclo[2,2,2]octane (DABCO), 1,3-dichloro-2-propanol and NaBF₄ in acetonitrile. The IL was fully characterized by IR, NMR and mass spectroscopic techniques. The presence of BF₄^? anion in IL was confirmed by ¹⁹F NMR and also supported by mass analysis. The TGA analysis showed that the IL is thermally stable up to 180?°C temperature. We demonstrated that the presence of the tertiary nitrogen sites and hydroxyl group in the DDIL network enhances the overall activity of DDIL. These make them compatible for base catalyzed one pot multicomponent synthesis of ortho-amino carbonitriles and 3-methyl-4-arylmethylene-isoxazol-5(4H)-ones under grinding without solvent. In addition the activity of DDIL was also studied for synthesis of tetrahydrobenzo[b]pyrans under ultrasound irradiation in water. Furthermore the DDIL was easily recoverable and recyclable many times with modest decrease in activity.

Full text of DOI:10.1016/j.molliq.2016.10.039

DABCO functionalized dicationic ionic liquid (DDIL): A novel green bench-
mark in multicomponent synthesis of heterocyclic scaffolds under sustainable
reaction conditions
Trushant Lohar, Arjun Kumbhar, Madhuri Barge, Rajshri Salunkhe
PII:
DOI:
Reference:
S0167-7322(16)31739-1
doi:10.1016/j.molliq.2016.10.039
MOLLIQ 6443
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
29 June 2016
8 October 2016
8 October 2016
Please cite this article as: Trushant Lohar, Arjun Kumbhar, Madhuri Barge, Rajshri
Salunkhe, DABCO functionalized dicationic ionic liquid (DDIL): A novel green bench-
mark in multicomponent synthesis of heterocyclic scaffolds under sustainable reaction
conditions, Journal of Molecular Liquids (2016), doi:10.1016/j.molliq.2016.10.039
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ACCEPTED MANUSCRIPT
DABCO Functionalized Dicationic Ionic Liquid (DDIL): A Novel Green Benchmark in
Multicomponent Synthesis of Heterocyclic Scaffolds under Sustainable Reaction
Conditions
a
a
a
a*
Trushant Lohar , Arjun Kumbhar , Madhuri Barge , Rajshri Salunkhe
Department of Chemistry, Shivaji University, Kolhapur, 416004, M.S., India,
Email: rsschem1@gmail.com
*
Corresponding author. Tel.: +91 231 260 9240; fax: +91 231 269 2333.
ABSTRACT: A novel DABCO functionalized Dicationic Ionic Liquid (DDIL) has been
synthesized using diazabicyclo[2,2,2]octane (DABCO), 1,3-dichloro-2-propanol and NaBF in
4
acetonitrile. The IL was fully characterized by IR, NMR and mass techniques. The presence of
1
9
BF4- anion in IL was confirmed by F NMR and also supported by mass analysis. The TGA
analysis showed that the IL is thermally stable up to 180°C temperature. We demonstrated that
the presence of the tertiary nitrogen sites and hydroxyl group in the DDIL network enhances the
overall activity of DDIL. These makes them compatible for base catalyzed one pot
multicomponent synthesis of Ortho-amino carbonitriles and 3-methyl-4-arylmethylene-isoxazol-
5
(4H)-ones under grinding without solvent. In addition the activity of DDIL was also studied for
synthesis of tetrahydrobenzo[b]pyrans under ultrasound irradiation in water. Furthermore the
DDIL was easily recoverable and recyclable many times with modest decrease in activity.
KEYWORDS DABCO Functionalized Ionic Liquids, Grinding, Ultrasound, Multicomponent
Reactions
INTRODUCTION
In the annals of heterocyclic chemistry, the academic and industrial research groups have
been increasingly focused on the development of multicomponent reactions (MCRs). MCRs can
lead to green and robust synthetic methodologies to afford rapid synthesis of small drug-like
1
-2
molecules with several degrees of structural diversity. This technique simultaneously engage
three or more components in one pot, resulting in formation of complex frameworks that
3
incorporate the elements of all the starting materials with good synthetic efficiency.
1

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4
Nowadays, solvent-free reactions have become paradigms of synthetic chemistry. These
reactions utilizes alternative energy inputs such as mechanical (grinding), ultrasound and
microwaves. Among these, mechanical grinding is simple and efficient method of synthesis
which combines economic aspects with environmental concerns. The reactions initiated by
5
grinding involves transfer of very small amount of energy through friction. In the mechanical
grinding, solid-state reactions occur more efficiently and more selectively than in the solution
6
phase reactions. This methodology facilitates the organic reactions with high yields, requires
stoichiometric amount of reactants, avoid the use of volatile organic solvents, short reaction time
,
and better energy balance with straightforward work-up. The above mentioned benefits offered
by mechanical grinding have also been widely used in the field of ionic liquid (ILs) catalyzed
7
MCRs.
ILs have been recognized as potential new green alternatives to conventional organic
8
solvents for a wide range of synthetic, catalytic, and electrochemical applications. The ILs are
characterized by their unique properties, including non-volatility, low inflammability, tunable
9
hydrophobicity, environmental friendly nature, easy recoverability and recyclability.
Moreover, it is well known that physical and chemical properties of an IL can be changed
by varying the structure of constituent cations and anions. This modification of ILs can
1
0
dramatically influence the outcome of various reactions. During the past few years a number of
1
1
dicationic and polycationic ILs, with a large variety of tunable properties, have been explored.
1
2
The dicationic ILs contain two head groups, linked by a rigid or flexible spacer. This kind of
13
ILs demonstrate unique features than monocationic ILs and other traditional solvents. Besides
the changes in the length of the spacer, and the incorporation of functional groups such as thiol,
ether, hydroxy and amino groups in the cations allow the physical properties of the dicationic ILs
1
4
to be tailored for specific applications.
The Poly(ethylene glycol)-linked dicationic neutral IL
1
5
16
(
PEG-DILs) and poly(ethylene glycol)-linked dicationic acidic ILs (PEG-DAILs) have been
explored as a powerful catalysts for various transformations.
The synthesis of ILs is complicated and often suffers from halogen impurities but
hydroxide based ILs now offers the simplest synthetic tool for the preparation of large number of
1
7
halogen free ILs. Recently we have demonstrated the application of this methodology for the
1
8
synthesis of multicationic ILs and its applications for MCR under MW. There are several
2

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19-20
reports for monocationic DABCO based ionic liquids in organic synthesis
dicationic DABCO functionalized ionic liquids till date.
but no report on
Heterocyclic motifs such as ortho-aminocarbonitriles, 3-methyl-4-arylmethylene–
isoxazol-5(4H)-ones and tetrahydrobenzo[b]pyrans have a wide range of applications. Ortho-
2
1
aminocarbonitriles, is probably the most well-known important skeleton in organic synthesis
2
2-23
and widely used in synthesis of various heterocyclic compounds.
Similarly, pyran
frameworks are also most abundant in variety of important natural compounds including
2
4
alkaloids, antibiotics, pheromones and iridoids. Specially, tetrahydrobenzo[b]pyrans themselves
have a broad spectrum of biological properties such as spasmolytic, anticancer, diuretic,
2
5-26
anticoagulant, and antiancaphylactia activities.
Isoxazole scaffolds bearing both oxygen and
nitrogen atoms are important heterocyclic compounds largely employed in the area of
2
7
pharmaceuticals and therapeutics such as antiviral, analgesic, antitumor, antioxidant etc .
In continuation of our interest in the development of greener methodologies for organic
2
8-29
30
synthesis,
and the use of DABCO as a ligand for Pd catalyst, herein we report the synthesis
of heterocyclic scaffolds under catalytic umbrella of DDIL under sustainable reaction conditions.
RESULTS AND DISCUSSION
Our study started with synthesis of DDIL, bearing a one mono quaternary ammonium
unit and a one free tertiary amine site on the same DABCO unit (Scheme 1). This free tertiary
amine can acts as a basic catalytic site for synthesis of above multifunctional heterocyclic
scaffolds. First the DABCO was quaternized with 1,3-dichloro-2-propanol in acetonitrile to
produce the corresponding chloride salt in 96 % yield at room temperature. With the chloride
-
salts in hand, introduction of desired anion (BF ) was readily accomplished by anion-exchange
4
reaction. The reaction of chloride salt with NaBF in acetonitrile furnished the desired dense
4
DDIL in excellent yield (94 %) at room temperature. The DDIL was easily miscible with water,
methanol, and acetonitrile while immiscible in diethyl ether.
N
N
N
N
Cl
Cl
N
OH
NaBF / MeCN
4
2
N
N
2 BF4
2
Cl
MeCN
RT, 24 h
N
OH
OH
RT, 24 h
N
N
DABCO
DDIL
3

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Scheme 1 Synthesis of DDIL
1
13
19
The synthesized DDIL was characterized by IR, H, C, F NMR and MS (ESI)
1
9
spectroscopy (See SI). In F NMR (Fig. 1) (400 MHz, CDCl ) signal at δ -150 (s, BF )
3
4
-
confirmed the formation of DDIL containing BF . This was also supported by MS of DDIL
4
which shows fragment at 82 m/z. The DDIL is stable up to 180 °C temperature in air as analyzed
by TGA analysis (Fig. 2).
1
9
Fig. 1 F NMR spectrum of DDIL
Fig. 2 TGA analysis of DDIL
4

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Our next task was to demonstrate the catalytic activity of synthesized DDIL in the multi-
component synthesis of ortho-amino carbonitriles by the reaction of aryl aldehydes,
malononitrile and cyclohexanone. Very few efforts have been paid for the construction of ortho-
3
1
32
33
34
amino carbonitriles including [BPy] BF4, morpholine, primary 1,2-diamine,
Et N,
3
35
36
complex substrate, and Glucose-containing imidazolium salt
under various reaction
conditions. Though, these methods showed varying degrees of success these have limitations
such as prolonged reaction time, low yields, and use of toxic solvents. Thus, the development of
an alternate mild and cleaner procedure, which surpasses these limitations, is very much relevant
for the synthesis of ortho-amino carbonitriles.
Cl
N
CHO
O
N
N
N
DIL (20 mol %)
Grinding, RT
+
2
+
NH2
Cl
1
2
3
4
N
Scheme 2 Synthesis of ortho-amino carbonitrile by DDIL
To begin with, the reaction of malononitrile, p-chloro benzaldehyde and cyclohexanone
molar ratio: 2:1:1) was chosen as the model reaction for the synthesis of ortho-amino
(
carbonitriles (Scheme 2). To prove the efficiency of DDIL under different conditions, a
systematic study considering different factors affecting the yield of the ortho-amino carbonitriles
was performed, and the results are summarized in Table 1.
Table 1 Optimization of the reaction conditions for preparation of ortho-amino
a
carbonitriles
b
Entry
Catalyst
Reaction conditions
Time
Yield
(%)
(
20 mol %)
1
2
-
-
H O/reflux
6 h
6 h
Trace
Trace
2
Ethanol/ reflux
5

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3
4
5
6
7
8
9
-
CH CN/ reflux
6 h
6 h
4 h
2 h
3 h
Trace
Trace
83
3
o
-
DMF/100 C
DDIL
DDIL
DDIL
DDIL
Ethanol/ reflux
o
Water /Ultrasound (70 C)
86
o
Solvent free/ 80 C
86
Grinding /RT
Grinding /RT
5 min
7 min
90
DDIL
83
(10 mol %)
Grinding /RT
7 min
1
0
DDIL
80
(5 mol %)
1
1
1
1
1
2
3
4
DABCO
Grinding /RT
Grinding /RT
Grinding /RT
Grinding /RT
10 min
12 min
12 min
15 min
65
60
56
51
Morpholine
Piperidine
N-methyl
Imidazole
1
1
5
6
NaOH
KOH
Grinding /RT
Grinding /RT
15 min
15 min
59
65
a
Reaction conditions: 4-chloro benzaldehyde (1 mmol), malononitrile (2 mmol), cyclohexanone (1mmol)
b
Isolated yields
Preliminary optimization experiments indicated that no desired product was obtained in
the absence of any catalyst in various solvents even under reflux conditions (Table 1, entries, 1-
). In all above cases only the Knoevenagel condensation product of malononitrile and aldehyde
4
was formed quantitatively. Interestingly, when the same reaction was carried out in the presence
of catalytic amount of DDIL (20 mol%) in ethanol under reflux, the desired ortho-amino
carbonitrile was obtained in 83 % in 4 h (Table 2, entry 5). Similar results were obtained for
6

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°
DDIL under ultrasound irradiation in water at 70 C (Table 1, entry 6) and solvent-free
0
conditions at 80 C, (Table 1, entry 7). On the other hand, to our surprise, when the model
reaction was carried out using DDIL under mechanical grinding at room temperature, incredibly
decrease in reaction time (5 min) was observed with excellent yield (90 %) of the desired product
(Table 1, entry 8). All above results indicated that the catalyst plays a crucial role in ensuring
very efficient reaction rate with excellent yield of the desired product. When same reaction was
carried out with various organic bases like DABCO, morpholine, piperidine, N-methyl imidazole
and inorganic bases like NaOH and KOH, dramatic decreased product yield was noticed (Table
1
, entries 11-16). Next the effect of amount of DDIL on yield of the product was studied. The
results showed that lowering the amount of DDIL decreases the overall yield the desired product
Table 1, entries 8-10).
According to the above results, the promising activity of DDIL under grinding can be
(
explained on the basis of dual character of DDIL. In grinding heat is evolved due to friction and
is absorbed by DDIL, which is driving force for the reaction. Another reason is that, DDIL
contains two basic nitrogen sites along with hydroxyl substituent in IL framework which is
primary condition for the synthesis of ortho-amino carbonitriles.
In order to attempt generality of reaction, a various ortho-amino carbonitrile derivatives
were synthesized by the reaction of differently substituted aromatic aldehydes with
cyclohexanone and malononitrile in the presence of DDIL (20 mol%) under grinding conditions.
The results summarized in Table 2 showed that all the products were obtained in good to
excellent yields (80-90 %) in 10-15 min.
7

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The plausible mechanism for the synthesis of ortho-amino carbonitriles in the presence of
DDIL is represented in Fig.3. It was presumed that, the electron lone pair present on the N atom
of the IL removes the active hydrogen of malononitrile to form corresponding carbanions.
8

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Simultaneously, the hydroxyl group of the IL increases the electrophilicity of the aldehyde and
cyclohexanone carbon atoms and assists the formation of Knovenagel condensation product of
aldehyde 5 second Knovenagel condensation type product 6 of cyclohexanone. The resulting
Knovenagel condensation products undergo [4+2] Diels-Alder reaction gave the final desired
carbonitrile product 4.
N
N
2
BF4
N
O
CN
N
H
O
H
OH
NC
NC
NC
NC
H
NC
2
1
-
H O
2
5
CN
CN
Diels-Alder Reaction
4+2]Cycloaddition
CN
C
CN
[
NH
NH2
CN
CN
6
4
H
N
N
N
C
2
BF4
CN
N
-H O
2
O
N
HO
H
O
NC
NC
NC
NC
H
NC
NC
2
3
Fig.3 Mechanistic picture for formation of ortho-amino carbonitrile catalyzed by DDIL
9

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Next, we extended the applicability of DDIL (20 mol%)/Grinding/RT for the synthesis of
various 3-methyl-4-arylmethylene–isoxazol-5(4H)-ones by reacting various benzaldehydes,
hydroxylamine hydrochloride and ethyl acetoacetate (Scheme 3).
H
O
O
O
O
DDIL (20 mol %)
H
+
O
R
R
NH OH.HCl +
OEt Grinding, RT
2
N
1
7
8
9
Scheme 3 Synthesis of 3-methyl-4-arylmethylene isoxazol-5(4H)-ones by DDIL
To have a better understanding of the catalytic system, the effectiveness of DDIL was compared
3
7-42
to those of the catalysts reported previously.
The results for benzaldehyde are listed in Table
3
. Thus from the results it is observed that our present system is far more efficient as that of
reported methods.
Table 3 Comparison of catalytic activity of DDIL with various catalysts for the synthesis of 3-
a
methyl-4-arylmethyleneisoxazol-5(4H)-one
b
Entry
1
Catalyst
DDIL
Reaction conditions
Grinding/RT
Time
Yield
(%)
[Ref.]
(min)
8
88
Present
work
2
3
4
5
6
-
Grinding/RT
Ethanol/Reflux
Ethanol/Reflux
48
180
18
60
60
70
90
90
-
Pyridine
DABCO
Boric acid
[37]
[38]
[39]
[40]
Water/RT
Water/RT
100
180
Potassium hydrogen
phthalate
Water/RT
7
Pthalimide N-oxyl
salts
60
90
[41]
1
0

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8
Sodium acetate
95 % ethanol/Visible
light
15
61
[42]
a
Reaction conditions: benzaldehyde (1mmol), ethyl acetoacetate (1mmol), hydroxyl amine hydrochloride (1 mmol),
catalysts (20 mol %) under grinding at RT.
b
Isolated yields.
The synthesis of a variety of substituted isoxazol-5(4H)-one could be successfully
accomplished by reacting a range of aryl aldehydes (Table 4). It can be seen that, various
aldehydes including heterocycles could reacted efficiently to give the desired products in good to
excellent yields. Unfortunately the aldehydes containing NO and Cl groups were highly
2
inefficient.
1
1

ACCEPTED MANUSCRIPT
We then extended these above protocol for one pot three component synthesis of
bioactive
tetrahydrobenzo[b]pyrans
under
ultrasound
irradiation
(Scheme
4).
4
3-44
Tetrahydrobenzo[b]pyrans has been synthesized by different methods in good yields.
The
1
2

ACCEPTED MANUSCRIPT
diverse application of the pyran derivatives in different fields have been encouraged us to
develop highly efficient procedures for the preparation of these kinds of compounds.
Cl
O
O
O
CN
CN
H
+
20 mol % DDIL
+
CN
H O, ))))), RT
Cl
O
2
O
NH2
1
2
10
11
Scheme 4 Synthesis of tetrahydrobenzo[b]pyrans by DDIL
Initially, equimolar mixture of p-chloro benzaldehyde, malononitrile and dimedone was
irradiated by ultrasound in presence of 20 mol % DDIL at room temperature. The yield of the
reaction was 89 % for 1 h (Table 5, entry 1).
a
Table 5 Optimization of reaction conditions for synthesis of tetrahydrobenzo[b]pyrans
b
Entry
Catalyst
Time (h)
Yield (%)
1
2
3
4
DDIL
DABCO
DDIL
1.0
2.5
89
68
c
2.0
84
d
DDIL
15 min
80
a
Reaction conditions: p-chlorobenzaldehyde (1 mmol), malononitile (1 mmol), dimedone (1 mmol), catalyst (20
mol %), water (5 mL) at room temperature.
b
Isolated yields.
c
Reaction without ultrasound irradiation,
d
Reaction under grinding condition.
When the same reaction was carried out without ultrasound in the presence of DDIL in
water at room temperature, the yield was decreased to 84 % with increased reaction time to 2 h.
This indicates that ultrasound waves play a crucial role for enhancing the reaction yield.
Remarkably higher rates of reaction in the aqueous solution of DDIL under ultrasound
1
3

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irradiation can be rationalized on the basis of synergistic effect (Table 5, Entry 1). The driving
energy for rate enhancement is provided by cavitation, the formation and collapse of bubbles,
which liberates considerable energy in short time. The chemical effects of cavitation are mainly
dependent on the contents of the collapsing bubble and hence on the nature of solvent. As the
DDIL is miscible in water and has negligible vapor pressure, it provides an interesting approach
to control the chemical contents of the collapsing bubble through the reduction of solvent vapor
pressure. This leads to greater compressional heating of the reactants thereby enhancing the rates
4
5
of the reactions. Though under grinding the reaction time decreased to 15 min the yield of
reaction was 80 % (Table 5, entries 3 and 4). When the same reaction was carried out in presence
of DABCO under ultrasound irradiations no quantitative conversion was observed (Table 5,
entry 2).
With the satisfactory results of optimization in hand, a series of tetrahydrobenzo[b]pyrans
were synthesized in good to excellent yields by reacting different aldehydes and results
aresummarized in Table 6.
1
4

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Recycling of the catalyst is important for the large scale operations and industrial point of
view. The recyclability study of DDIL was studied for model reaction of synthesis of
tetrahydrobenzobenzo[b]pyran. After completion of the model reaction the product was
separated by filtration and the aqueous solution containing DDIL was washed with diethyl ether
and reused in the next four cycles with negligible loss in final product yield (Fig. 4). The
1
5

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1
13
recycled IL after fifth cycle was separated from aqueous phase and characterized by H and C
NMR spectroscopy which was same as that of the original catalyst.
x
1
00
8
9 %
8
6 %
8
1 %
7
6 %
8
0
0
7
2 %
6
Yield
%
4
0
2
0
Y
0
1
2
3
Cycle
4
5
Fig. 4 Recyclability of DDIL for synthesis of tetrahydrobenzobenzo[b]pyran
CONCLUSION
In summary, novel DDIL was proved to be an effective basic catalyst as well as a green reaction
medium for the synthesis of ortho-amino carbonitriles, 3-methyl-4-arylmethylene-isoxazol-
5
(4H)-ones and tetrahydrobenzo[b]pyrans under sustainable reaction conditions. This
environmentally friendly methodology with excellent green chemistry credentials, such as atom
economy, use of non-toxic, thermally stable ionic liquid and shorter reaction time without any
by-product may find a wide range of promising industrial applications.
ACKNOWLEDGMENT
One of the authors Mr. Trushant Lohar is very much thankful to the Department of Science and
Technology (DST), New Delhi for providing DST-INSPIRE fellowship (IF 140918). We also
acknowledge Department of Chemistry, Shivaji University, Kolhapur for providing spectral data
analysis.
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ACCEPTED MANUSCRIPT
DABCO functionalized Dicationic Ionic Liquid: a novel green benchmark in
multicomponent synthesis of heterocyclic scaffolds under sustainable reaction
conditions
Graphical abstract
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ACCEPTED MANUSCRIPT
HIGHLIGHTS
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Preparation and Characterization of Novel DABCO functionalized dicationic ionic liquid
Applicability of this ionic liquid for the synthesis of bioactive heterocyclic scaffolds
Green reaction conditions
Recyclability of catalyst
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