Synthesis and characterization of novel chiral imidazolium and pyridinium ionic liquids derived from tartaric acid and 2-oxazolidinone

Article abstract of DOI:10.1080/00397911.2019.1591455

Novel chiral imidazolium and pyridinium ionic liquids based on tartaric acid and 2-oxazolidinone were designed. Symmetrical dicationic ionic liquids based on tartaric acid have been synthesized and characterized. These chiral ionic liquids were designed b

Full text of DOI:10.1080/00397911.2019.1591455

Synthetic Communications
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Synthesis and characterization of novel chiral
imidazolium and pyridinium ionic liquids derived
from tartaric acid and 2-oxazolidinone
Sagar P. Nehate, Himanshu M. Godbole, Girij P. Singh, Jessy E. Mathew &
Gautham G. Shenoy
To cite this article: Sagar P. Nehate, Himanshu M. Godbole, Girij P. Singh, Jessy E. Mathew
& Gautham G. Shenoy (2019): Synthesis and characterization of novel chiral imidazolium and
pyridinium ionic liquids derived from tartaric acid and 2-oxazolidinone, Synthetic Communications,
DOI: 10.1080/00397911.2019.1591455
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https://doi.org/10.1080/00397911.2019.1591455
Synthesis and characterization of novel chiral imidazolium
and pyridinium ionic liquids derived from tartaric acid and
2-oxazolidinone
Sagar P. Nehatea, Himanshu M. Godbolea, Girij P. Singha, Jessy E. Mathewb, and
Gautham G. Shenoyb
aChemical Research and Development, Lupin Limited, Pune, MH, India;bDepartment of Chemistry,
Manipal Academy of Higher Education, Manipal, KA, India
ABSTRACT
ARTICLE HISTORY
Received 20 December 2018
Novel chiral imidazolium and pyridinium ionic liquids based on tar-
taric acid and 2-oxazolidinone were designed. Symmetrical dicationic
ionic liquids based on tartaric acid have been synthesized and char-
acterized. These chiral ionic liquids were designed by employing
very short and simple methods. Incorporation of alkyl halide over
tartaric acid and 2-oxazolidinone is an important step. N-methyl
imidazole and pyridine were used for preparation of quaternary salts.
These ionic liquids have been evaluated for the asymmetric sulfide
oxidation. Chiral ionic liquids based on tartaric acid showed superior
chiral inducing property as compare to 2-oxazolidinone based chiral
ionic liquids.
KEY WORDS
Chiral ionic liquids; tartaric
acid; imidazolium;
pyridinium; 2-oxazolidinone
GRAPHICAL ABSTRACT
Introduction
Ionic liquids (ILs) have been described as molten salts that are entirely ionic in nature
having a melting point below 100 ꢀC.[1] Ionic liquids that are liquid at room tempera-
ture are referred as room temperature ionic liquids (RTILs).[2] RTILs are thermally
stable up to 300 ꢀC. Generally ionic liquids have a large organic cation and an organic
or inorganic anion. Ionic liquids have a unique physicochemical property as compared
to the conventional organic solvents. The cations and anions in the ionic liquids made a
CONTACT Sagar P. Nehate
sagarnehate@lupin.com
Lupin Limited, 46 and 47A, Nande Village, Mulshi Taluka,
Pune 412115, India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data for this article is available online at on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

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S. P. NEHATE ET AL.
Figure 1. Starting materials.
large impact on their physical properties. Properties of ionic liquids can be altered by
modifying the structure of the cations or anions of ionic liquids in order to influence
the outcomes of reactions. Hydrogen bonding, charge distribution on the anions, polar-
ity, and dispersive interactions also influence the physical properties of ionic liquids.
Miscibility of ionic liquids with water depends on the nature of the anion. At room
ꢁ
temperature, ionic liquids having [PF6]ꢁ and [Tf2N]
anions are insoluble in water
while ionic liquids with nitrate, acetate, halides, etc are fully miscible with water. Chiral
ionic liquids are useful for the stereoselective polymerization, as a chiral stationary
phase in chromatography and chiral shift reagent in NMR spectroscopy.[3] Chiral ionic
liquids showed chiral introducing property in many reactions.[4] Various types of asym-
metric reactions have been reported using chiral ionic liquids as a catalyst or reaction
media like aldol reaction,[5] Baylis–Hillmann reaction,[6] Diels Alder reaction,[7] Michael
addition,[8] hydrogenation,[9] and asymmetric alkylation.[10] Chiral ionic liquids can be
recycled many times without affecting the reactivity.[5] Ionic liquids can be useful as
electrolytes for batteries, nuclear fuel reprocessing, solar thermal energy, and waste recy-
[11]
cling.
In view of the emerging importance of the ionic liquids as a chiral catalyst or
reaction media in organic synthesis, we designed and synthesized a novel imidazolium
as well as pyridinium ionic liquid from D-tartaric acid and 2-oxazolidinone (Fig. 1).
Derivatives of tartaric acid and oxazolidine were reported as chiral axillaries.[12] Kagan
and group reported the metal-catalyzed asymmetric sulfide oxidation using diethyl tartrate
in 1984.[13] Similar oxidation method was reported by Modena in 1984.[14] Both asym-
metric oxidation methods reported by Kagan and Modena were based on the Sharpless
asymmetric epoxidation.[15] Therefore, we designed ionic liquids structurally similar to
diethyl tartrate. We have synthesized symmetrical di-cationic ionic liquids from D-tartaric
acid. Recently, we have published the synthesis of novel chiral ionic liquids derived from
2-oxazolidinone.[16] In continuation with this, herein, we report the synthesis of novel
chiral ionic liquids through N-acylation of 2-oxazolidinone.
Results and discussion
Symmetrical di-cationic imidazolium and pyridinium ionic liquids 4 and 5 were
prepared from D-tartaric acid 1. D-tartaric acid reacted with an excess amount of
2-chloroethanol in presence of catalytic para-toluene sulfonic acid to afford Bis (chlor-
oethyl)-D-tartrate 3. Compound 3 is a low melting, off-white color solid. After quaterni-
zation reaction of 3 with N-methyl imidazole and pyridine gives imidazolium 4 and
pyridinium 5 ionic liquid as pale-yellow color oil. Higher temperature (90–100 ꢀC) is

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3
Scheme 1. Preparation of symmetrical di-cationic ionic liquids.
Scheme 2. Preparation of imidazolium ionic liquids from 2-oxazolidinone.
required for completion of quaternization reaction (Scheme 1). Presence of hydroxyl
and ester functional groups is the major difference between these ionic liquids and ear-
lier reported ionic liquids from tartaric acid.[17]
Furthermore, we synthesized some imidazolium and pyridinium ionic liquids sup-
ported on 2-oxazolidinone 2. In this case, 2 reacted with 2-chloro acetyl chloride in
tetrahydrofuran and sodium hydride as a base to afford 6a–c. Then 7a–c were prepared
by quaternization reaction of 6a–c with N-methyl imidazole at 50–60 ꢀC (Scheme 2).
Intermediates 7a–c are the solid compounds, therefore, the melting point of these
ionic liquids was recorded. Intermediate ionic liquids 7a–c have melting point more
than 100 ꢀC. Therefore, these solid intermediate ionic liquids were subjected to anion
exchange reaction with potassium hexafluorophosphate. The new imidazolium ionic
liquids bearing hexafluorophosphate anion 8a–c have melting points below 100 ꢀC.
Similarly, 6a–c was reacted with pyridine to afford intermediate ionic salts which
were directly subjected to anion exchange to form ionic pyridinium liquids bearing hex-
afluorophosphate anion 9a–c (Scheme 3).
Similar to imidazolium ionic liquids, these pyridinium ionic liquids derived from
2-oxazolidinone are also low melting solids except 9b (m.p. = 110–114 ꢀC) and hydro-
phobic. Ionic liquids having hexafluorophosphate anion are soluble in acetone, dichloro-
methane, and dimethyl sulfoxide while insoluble in water and alcohols.

4
S. P. NEHATE ET AL.
N
Scheme 3. Preparation of pyridinium ionic liquids from 2-oxazolidinone.
Scheme 4. Oxidation of sulfide to sulfoxide using ionic liquids.
Chiral inducing property of these ionic liquids was evaluated for sulfide oxidation of
sulfide 10. In the Kagan asymmetric sulfide oxidation method diethyl tartrate was used
as a chiral auxiliary. We tried titanium metal-catalyzed asymmetric sulfide oxidation of
sulfide 10 to sulfoxide 11 by replacing diethyl tartrate with ionic liquids (Scheme 4).
Chiral ionic liquids 4 and 5 are, therefore, liquids that were used as solvents for the
reaction. Ionic liquids 4 and 5 were also evaluated for sulfide oxidation with co-solvent.
Almost same enantiomeric excess was observed using ionic liquids 4 or 5 with and
without co-solvent. Other ionic liquids 8a–c and 9a–c are solid compounds; therefore,
co-solvent is necessary along with these ionic liquids to allow reaction mixture to stir
smoothly. Solvents like dichloromethane, toluene, and acetonitrile were evaluated as co-
solvents along with ionic liquids. Based on the solubility of ionic liquids dichlorome-
thane was used as co-solvent along with the TBHP as an oxidant for the reaction.
Initially, we carried out the oxidation reaction without a metal catalyst to evaluate the
catalytic property of the chiral ionic liquid. But racemic sulfoxide was obtained without
metal catalyst. The influence of the mole ratio of ionic liquids with respect to 10 was
evaluated for sulfide oxidation keeping other parameters constant. Ionic liquids with 10
moles% and 20 moles% have been explored for this reaction. There is a significant
change in the enantiomeric of sulfoxide using 10 moles% and 20 moles% of ionic liquid.
The results are summarized in Table 1. Form the table it is clear that enantiomeric
excess of sulfoxide obtained using tartaric acid based ionic liquids is higher (up to 81%

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5
Table 1. Summary of sulfide oxidation reaction.
R¼H
R¼CH3
Ionic liquids
Co- solvent
Yield (%)
eea (%)
[a]22
Yield (%)
eea (%)
[a]22
D
D
4
5
4
5
8a
8b
8c
9a
9b
9c
–
–
73
75
76
73
69
71
68
70
72
75
77
75
76
74
18
15
12
20
18
18
ꢁ101.9
–98.5
–101.2
–98.1
–23.9
–19.8
–16.8
–25.7
–23.5
–23.8
76
79
73
75
71
75
66
68
73
73
81
78
79
78
21
16
12
24
21
20
–115.6
–111.2
–113.4
–112.7
–31.5
–22.2
–18.1
–33.8
–29.7
–27.6
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
a
DCM: Dichloromethane; enantiomeric excess measured by chiral HPLC.
ee) than that obtained from 2-oxazolidinone. It has been also observed that enantio-
meric excess is slightly higher when R=CH3.
Experimental
Procedure for the synthesis of ionic liquid 4
Bis (chloroethyl) tartrate 3 (10 g, 36 mmol) and N-methyl imidazole (12 g, 146 mmol)
were added to the round bottom flask at room temperature. The reaction mixture was
slowly heated to 90–100 ꢀC and stirred for 24–48 h. The progress of the reaction was
monitored on thin layer chromatography. The reaction mixture was cool to room tem-
perature. Water (30 ml) and dichloromethane (60 ml) were added to the reaction mix-
ture. The reaction mixture was stirred at room temperature for 15 min. The aqueous
layer was separated and washed with dichloromethane (20 ml ꢂ 2). Then, the aqueous
layer was concentrated on the laboratory rotavapor under reduced pressure at 50–55 ꢀC.
Concentrated mass was purified on column chromatography using 30% methanol in
ethyl acetate. Pure compound was dried under reduced pressure at 50–55 ꢀC for 48 h on
laboratory rotavapor to get 4.
1,10-((((2S,3S)-2,3-dihydroxysuccinyl)bis(oxy))bis(ethane-2,1-diyl))bis(3-methyl-1H-
imidazol-3-ium). bis chloride (4): Pale yellow colour oil, (Yield: 14.3 g, 93%); [a]22
=
D
ꢁ31.0ꢀ (c ¼ 1.0, Methanol); Density q ¼ 1.21 g/cm3; H NMR (500 MHz, DMSO-d6): d,
3.87 (s, 6H), 4.40–4.54 (m, 10H), 5.90–5.92 (d, 2H), 7.73–7.74 (t, 2H), 7.85–7.86 (t, 2H),
9.36 (s, 1H); 13C NMR (125 MHz, DMSO-d6): 171.0, 137.3, 123.7, 122.6, 72.3, 62.8, 48.2,
35.9 IR (neat), ꢀ/cm-1: 3382, 1755, 1634; Anal. Calcd. for C16H24Cl2N4O6: C, 43.75; H,
5.51; N, 12.75. Found: C, 43.7; H, 5.50; N, 12.77.
1
Procedure for the synthesis of ionic liquid 7a
Acetonitrile (20 ml) and 6a (10 g, 48.6 mmol) were added in a round bottom flask at
room temperature. N-methyl imidazole (4.4 g, 53.4 mmol) was added to the above reac-
tion mixture and raised the temperature of reaction mixture to 50–55 ꢀC. Then tem-
perature of reaction was to maintained 50–55 ꢀC for 4–5 h. Progress of the reaction
monitored on thin layer chromatography. Then after completion of reaction ethyl acet-
ate (80 ml) was added slowly to the reaction mixture and cooled to room temperature.

6
S. P. NEHATE ET AL.
Reaction mixture was stirred at room temperature for 1 h. Filter the solid and washed
with ethyl acetate (20 ml). Solid was dried under reduced pressure to get the 7a.
(S)-3-(2-(4-isopropyl-2-oxooxazolidin-3-yl)-2-oxoethyl)-1-methyl-1H-imidazol-3-ium
1
chloride(7a); (Yield: 13g, 93%); [a]22 = þ39.2ꢀ (c ¼ 1.0, Methanol); mp: 127–129 ꢀC; H
D
NMR (500 MHz, MeOD): d, 0.92–0.96 (d, 6H), 2.30–2.35 (m, 1H), 4.01 (s, 3H), 4.44–4.50
(m, 3H), 5.71–5.79 (q, 2H), 7.62–7.66 (d, 2H), 8.98(s, 1H); 13C NMR (125 MHz, CDCl3):
167.1, 155.9, 139.5, 125.4, 124.4, 66.3, 60.2, 53.6, 36.6, 30.0 17.7, 15.1; IR (neat), ꢀ/cm-1:
3099, 1779, 1713; Anal. Calcd. for C12H18ClN3O3: C, 50.09; H, 6.31; N, 14.60. Found: C,
50.10; H, 6.34; N, 14.58.
Procedure for preparation of 8a
In a round bottom flask, 7a (5 g, 17.4 mmol) and water (50 ml) were added at
room temperature. Potassium hexafluorophosphate (3.2 g, 17.4 mmol) added to above
reaction mixture and stirred at room temperature for 1 h. Filter the solid and
washed with water (20 ml ꢂ 2). Solid was dried under reduced pressure at 40 ꢀC to
get 8a.
(S)-3-(2-(4-isopropyl-2-oxooxazolidin-3-yl)-2-oxoethyl)-1-methyl-1H-imidazol-3-ium
hexafluorophosphate (8a); (Yield: 6.5 g, 95%); [a]22 ¼ þ58.5ꢀ (c¼ 1.0, Acetone);
D
1
mp: 68–70ꢀC; H NMR (500 MHz, DMSO-d6): d, 0.86–0.88 (dd, 6H), 2.16–2.19 (m,
1H), 3.92 (s, 3H), 4.37–4.45 (m, 3H), 5.60–5.78 (dd, 2H), 7.63–7.71 (d, 2H), 8.99
(s, 1H); 13C NMR (125MHz, DMSO-d6): 166.0, 154.1, 137.8, 123.9, 123.1, 64.9,
58.2, 52.2, 35.9, 28.5, 17.2, 14.8; IR (neat), ꢀ/cm-1: 3170, 1774, 1709, 1391; Anal.
calcd for C12H18F6N3O3P: C, 36.28; H, 4.57; N, 10.58. Found: C, 36.25; H, 4.61;
N, 10.60.
General procedure for sulfide oxidation
In a round bottom flask, dichloromethane (10 ml) and sulfide 10a (2 g, 16 mmol) were
added at room temperature. Added 4 (0.7 g, 1.6 mmol), titanium isopropoxide (0.22 g,
0.8 mmol) and water (15 ml, 0.8 mmol) to above reaction mixture. Then the reaction
mixture was stirred at room temperature for 1 h. Tertiary butyl hydroperoxide (4.2 ml,
32 mmol) was slowly added to the reaction mixture at room temperature. Then the
reaction mixture was stirred at room temperature for 16 h. water was added (20 ml) to
the reaction mixture and stirred for 0.5 h. Filter the reaction mixture to remove the
solid precipitate. The organic layer separated from the filtrate and concentrated on
laboratory rotavapor to get the oily residue. The residue purified on column chromatog-
raphy using ethyl acetate-cyclohexane to get the pure methyl phenyl sulfoxide. HPLC:
The enantiomeric ratio was measured by HPLC (DAICEL OB-H, heptane/isopropanol
70:30, flow rate: 0.5 mL/min, 254 nm, Ret Time for R-isomer = 13.5 min and Ret Time
for S-isomer = 21.8 min).
Methyl phenyl sulfoxide (11a): (Yield: 1.65 g, 73%); [a]22 ¼ ꢁ101.9ꢀ (c ¼ 1.1,
D
1
Acetone); ee = 77%; H NMR (500 MHz, CDCl3): d, 2.74 (s, 3H), 7.50–7.67 (m, 5H);
13C NMR (125 MHz, CDCl3): 145.6, 131.0, 129.3, 123.5, 43.9; IR (ꢀ/cm-1) 2920, 1478,
1090, 1040; Anal. Calcd. for C7H8OS: C, 59.97; H, 5.75; Found: C, 60.1; H, 5.78.

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Conclusion
D- Tartaric acid and 2-oxazolidinone based imidazolium and pyridinium chiral ionic
1
liquids have been synthesized. All these functionalized ionic liquids characterized by H
NMR, 13C NMR, and FTIR. Physical properties like melting point, density and specific
optical rotation of these ionic liquids have also been recorded. Ionic liquids based on
tartaric acid are liquids while the ionic liquids based on N-acylation of 2-oxazolidinone
with hexafluorophosphate anion are low melting solids. Tartaric acid based ionic liquids
produces more optically active sulfoxide.
Full experimental details, 1H and 13C NMR spectra can be found via the
“Supplementary Content” section of this article’s webpage.
Acknowledgments
The authors are thankful to the M/S Lupin Limited and the authorities of Manipal Academy of
Higher Education for the research program. We also acknowledge the valuable guidance, support,
and suggestions from Dr. P. R. Upadhaya, and Dr. Vijaya Desai.
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