Synthesis and Characterization of 1,2-Dimethyl Imidazolium Ionic Liquids and Their Catalytic Activities

Article abstract of DOI:10.1080/00397911.2015.1067324

The synthesis of substituted imidazolium-type ionic liquids via a simple method is described. Our synthesized ionic liquids are more useful in the catalytic behavior of the Mannich reaction.

Full text of DOI:10.1080/00397911.2015.1067324

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Synthesis and Characterization of 1,2-Dimethyl
Imidazolium Type of Ionic Liquids and Its Catalytic
Activities
Ganapathi Pandurangana & Ganesan Kilivelua
a PG & Research Department of Chemistry, Presidency College (Autonomous), Chennai, India
Accepted author version posted online: 06 Jul 2015.
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SYNTHESIS AND CHARACTERIZATION OF 1,2-DIMETHYL IMIDAZOLIUM
TYPE OF IONIC LIQUIDS AND ITS CATALYTIC ACTIVITIES
Ganapathi Pandurangan1, Ganesan Kilivelu1
1PG & Research Department of Chemistry, Presidency College (Autonomous), Chennai,
India
Address correspondence to Ganesan Kilivelu, PG & Research, Department of Chemistry,
Presidency College, Chennai 600 005, India.
E-mail: kiliveluganesan@yahoo.co.in
Abstract
The synthesis of substituted imidazolium type ionic liquids via a simple method is
described. Our synthesized ionic liquids are more useful in the catalytic behavior of
Mannich reaction.
KEYWORDS: Ionic liquids, metathesis, Mannich reaction, recyclable
INTRODUCTION
Salts in their liquid state at room temperature are called room temperature ionic liquids
(RTIL) which has a variety of applications such as energy storage devices [1], solvent for
nanoparticle stabilization [2] and electrode position [3]. Combining a larger organic
segment with charge diffuse anion [4] is the possible method to prepare IL at room
temperature. IL’s are safer solvents due to non – toxic ions and insignificant vapor
1

pressure. Significant properties of IL’s are heat capacity and viscosity which are easily
controllable by selection of cation and anion segments [5]. The equal molar ratio of H2SO4
and IL are used as effective chlorinating agent [6], compared with powerful,
environmental damaging solvent and more toxic chlorinating reagents like COCl2, PCl3,
and SOCl2 etc. [7] IL’s are acted as product controller to improve the stereo selectivity for
Diels – Alder reaction [8-9]. Some of IL’s acted as both solvent and catalyst for Friedel –
Craft reaction with effective response [10-11]. Essawy and coworkers reported that Michael
Adducts of α-phenyl chalcone with various ketones are showed variety of medicinal
activities12. IL’s are more suitable solvent for oxidation reaction due to its stability even
under oxidizing condition and its inertness [13-19]. Most of the IL’s has dual function [20]
some of imidazolium salts are acted as mild antioxidative and reducing species [21]. The
solubility property of IL’s with hydrophobic (or) hydrophilic solvent can be tuned by
altering the cations and anions [22-24]. Ionic liquids acted as electrolytes with number of
merits when compared to conventional materials due to its high ionic conductivity and
non-volatile nature [25-27]. Room temperature ionic liquids are considered as
environmental friendly solvent for industrial actinide extraction [28]. Convenient synthesis
of bulky groups containing imidazolium / pyridinium type of ionic liquids has been
reported29. Herein, we wish to report the synthesis and catalytic activity of dimethyl
imidazolium type of ionic liquids.
RESULTS AND DISCUSSION
1, 2 – Dimethylimidazole (1.0 equi; 1.04x10-2 ) is treated with benzylbromide /
4 - nitrobenzylbromide (1.05 equi; 1.092x10-2) in the presence of dry CH3CN
2

under refluxing condition for 2 to 2.5 hours to give (1a-b) in 95 - 97 % yield. After
N – alkylation, anion exchange reaction is carried out with various inorganic salts
in the presence of minimum amount of deionized water, which is used as a solvent,
for about 1 hour to give anion exchanged products.
The anion exchanged product undergoes Soxhlet extraction in the presence of
dry THF for about 1 hour to remove the metal bromide to give pure form of
imidazolium salts (2a-f) in 90 - 93 % yield. Reaction between 1, 2 -
dimethylimidazole with 4 - nitrobenzylbromide is much faster than benzyl bromide
due to nitro benzyl C - Br bond which is weaker than simple benzyl bromide. The
same reaction is repeated with DMSO and observed that the reaction is much faster
than CH3CN, but work up is not easier due to water soluble nature of both solvent
and imidazolium bromide, therefore CH3CN is the more suitable solvent for N-
alkylation.
CATALYTIC ACTIVITIES
One pot multi component organic reactions are gaining more importance because of
quicker diversity and efficiency30. Mannich product (substituted oxazine) has more
features in the area of biologically important natural residues [31, 32]. Some of the
substituted oxazine and its derivatives are reported in literature [33-35]. Multicomponent
preparation of some of the naphthol heterocyclic substituted compounds in the presence
of ionic liquids needs more reaction time to complete with insignificant yield [36]. Deepak
and coworkers reported that one pot three components condensation reaction between
3

aldehyde, ketone and aromatic amine in the presence of mixed oxides MgO/ZrO2 in the
ratio of 1:3 needs 16 hours to complete the reaction with poor yield [37]. But our
synthesized ionic liquids for Mannich reaction afforded appreciable observation (Table –
1 & 2). We have tried with different concentration of ionic liquids such as 7.299x10-5,
1.497x10-4 , 2.245x10-4 and 2.994x10-4 concentrations. Among these concentrations, we
have observed that 2.245x10-4 concentration is the optimum concentration to complete
the reaction with less reaction time with higher conversion. In Mannich reaction, we have
used two types of aromatic amines (m/p-nitro aniline). From that we observed that m-
nitro aniline is two times faster than p-nitro aniline with higher yields (Table-1). m-
Nitroaniline shows better nucleophilic activity than para isomer. Mannich products are
thoroughly characterized by spectral and analytical data which are closely matching with
literature [38]
.
So our synthesized ionic liquids are potential candidates to accelerate the Mannich
reaction as quicker with higher yield. Our synthesized ionic liquids are recycled upon four
cycles and used for Mannich reaction with same reaction condition. Even after fourth
recycle, the product obtained was same as we observed in the fresh use shown in the
Table- 3.
CONCLUSION
We have used simple synthetic methodology to prepare imidazolium type of ionic liquids.
Our synthesized ionic liquids are more useful for catalytic behavior in Mannich reaction
to reduce the reaction time and improve the percentage of yield. We have examined the
4

catalytic activity of synthesized ionic liquids. Among the eight ionic liquids, 1a & 1b
showed better catalytic activity than the others due to better Lewis character. We have
tried the catalytic activities with different concentrations (7.299x10-5mol/L, 1.497x10-
4mol/L, 2.245x10-4mol/L and 2.994x10-4mol/L concentration) of our synthesized ionic
liquids. From the result, we have concluded that 2.245X10-4 concentration is the optimum
concentration to complete the Mannich reaction with higher yield and lesser reaction
time. The same reaction is repeated with our recycle IL’s showed good catalytic activity
with higher yield.
EXPERIMENTAL
Procedure For N-Alkylation
1, 2- dimethylimidazole (1.040x10-2mol/L; 1.0 equi.) is treated with benzylbromide
(1.092x10-2 mol/L; 1.05 equi.) in the presence of 30 mL of dry CH3CN under refluxing
condition for about 2 hours afforded the N-alkylated quaternary ammonium bromide 1a in
95% yield after the purification.
1,2-Dimethyl(3-Methylene Benzene)-Imidazolium Bromide 1a
2.75g; 95% Semisolid; 1H NMR (D2O); δ: 2.45(s, 3H); 3.66(s, 3H); 5.22(s, 2H); 7.20(d,
J=1.8 HZ, 2H) 7.24 (s, 2H) 7.33(t, J=7.2 HZ, 3H); 13C NMR δ: 9.19, 34.75, 51.42, 121.10,
122.37, 127.76, 129.26, 133.73, 144.58; MS:267; Elemental analysis:- Molecular
Formula (C12H15N2Br); Calculated: C: 53.93; H: 5.61; N: 10.48; Found C: 53.86; H:
5.52; N: 10.40.
5

Procedure For Anion Exchange Reaction
N-Alkylated quaternary ammonium bromide (1.0 equi.) is treated with NaBF4 (1.05 equi.)
in the presence of 10 mL deionized water at room temperature with stirring for about 1
hour afforded the anion exchanged ionic liquid. After the anion exchanged reaction, we
have used Soxhlet extraction for separation using dry THF for about 1 hour followed by
concentration will give ionic liquid 2a in 93% yield.
1,2-Dimethyl(3-Methylene Benzene)-Imidazolium Tetrafluoroborate 2a
0.67g; 93%; Semisolid; 1H NMR (D2O); δ: 2.43(s, 3H); 3.68(s, 3H); 5.26(s, 2H); 7.18(d,
J=1.9 HZ, 2H) 7.21 (s, 2H) 7.4 (t, J=7.4 HZ, 3H); 13C NMR δ: 9.17, 34.78, 51.44, 121.08,
122.35, 127.74, 129.27, 133.71, 144.55; MS:274; Elemental analysis:- Molecular
Formula (C12H15BF4N2); Calculated: C: 52.55; H: 5.47; N: 10.22; Found C: 52.48; H:
5.32; N: 10.14.
ACKNOWLEDGMENT
Author thanks to Dr. R. Sundaram, Assistant Professor, Department of Chemistry,
Presidency College, Chennai - 5. for cooperation and moral support.
SUPPORTING INFORMATION
Experimental procedure, 1H and C13 NMR spectrum of new compounds for this article
can be accessed on the publisher’s website.
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Table 1 One pot synthesis of m-nitrooxazine derivatives
S. No IL Time Yield (%)
Absence of catalyst 8 hours 40
1
2
3
4
5
6
7
8
9
1.5 min. 90
1.5 min. 85
1.5 min. 90
1.5 min. 88
1.5 min. 87
1.5 min. 80
1.5 min. 87
1.5 min. 84
1a
2a
2b
2c
1b
2d
2e
2f
Reagents and conditions: α – Naphthol (200mg;1.387 x10-3 mol/L); paraformaldehyde
(85mg; 2.843x10-3 mol/L);
m -nitroaniline (0.148g; 1.387x10-3 mol/L); CH3CN (10 mL); IL (2.245x10-4 mol/L); rt.
11

Table 2 One pot synthesis of p-nitrooxazine derivatives
S.
No
1
IL
Time
Yield
(%)
30
Absence of
10 hr.
catalyst
1a
2
3
4
5
6
7
8
9
2 min.
2 min.
2 min.
2 min.
2 min.
2 min.
2 min.
2 min.
82
78
81
81
77
72
75
75
2a
2b
2c
1b
2d
2e
2f
Reagents & conditions:- α – Naphthol (200mg;1.387 x10-3 mol/L); paraformaldehyde
(85mg; 2.843x10-3 mol/L );
p -nitroaniline (0.148g; 1.387x10-3 mol/L); CH3CN (10 mL ); IL (2.245x10-4 mol/L) ; rt.
12

Table 3 One pot synthesis of oxazine by using 4th cycle recycled IL’s
S. No
IL
1a
2a
2b
2c
1b
2d
2e
2f
Yield (%)
72 - 85
70 - 80
74 - 82
70 - 75
70 - 76
66 - 70
68 -73
1
2
3
4
5
6
7
8
65 - 70
13

Scheme 1. Synthesis of substituted imidazolium type of ionic liguids
14

Scheme 2. One pot preparation of oxazine using imidazolium type of ionic liquid
15