Synthesis of novel chiral ionic liquids based on (-)-menthyl isonicotinate

Article abstract of DOI:10.1080/00397911.2011.562336

(Chemical Equation Presented) Thirteen novel chiral ionic liquids (CILs) containing the (-)-menthyl group can be easily prepared from isonicotinic acid with yields of 35-90%. The properties and characterization of these compounds were discussed and the alternation of optical rotation was analyzed. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397911.2011.562336

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Synthesis of Novel Chiral Ionic Liquids
Based on (-)-Menthyl Isonicotinate
Chengping Miao a b , Xiaohua Tu a b , Ziwei Xiang a b & Jianyi Wu a b
a College of Biological and Chemical Engineering, Jiaxing University ,
Zhejiang , China
b Key Laboratory of Clean Chemical Process of Jiaxing , Zhejiang ,
China
Accepted author version posted online: 02 Jan 2012.Published
online: 29 May 2012.
To cite this article: Chengping Miao , Xiaohua Tu , Ziwei Xiang & Jianyi Wu (2012) Synthesis of
Novel Chiral Ionic Liquids Based on (-)-Menthyl Isonicotinate, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 42:17, 2555-2563,
DOI: 10.1080/00397911.2011.562336
To link to this article: http://dx.doi.org/10.1080/00397911.2011.562336
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Synthetic Communications1, 42: 2555–2563, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.562336
SYNTHESIS OF NOVEL CHIRAL IONIC LIQUIDS
BASED ON (ꢀ)-MENTHYL ISONICOTINATE
Chengping Miao,1,2 Xiaohua Tu,1,2 Ziwei Xiang,1,2 and
Jianyi Wu1,2
1College of Biological and Chemical Engineering, Jiaxing University,
Zhejiang, China
2Key Laboratory of Clean Chemical Process of Jiaxing, Zhejiang, China
GRAPHICAL ABSTRACT
Abstract Thirteen novel chiral ionic liquids (CILs) containing the (ꢀ)-menthyl group
can be easily prepared from isonicotinic acid with yields of 35–90%. The properties and
characterization of these compounds were discussed and the alternation of optical rotation
was analyzed.
Keywords Chiral ionic liquids; isonicotinic acid; (ꢀ)-menthol
INTRODUCTION
Asymmetric catalytic synthesis is one of the most important synthesis methods
for chiral drugs in pharmaceutical industry. Asymmetric organocatalysts, with the
advantages of high selectivity, mild reaction condition, no contamination of heavy
metal and etc,[1,2] have been extensively used. With further investigations in the field
Received November 16, 2010.
Address correspondence to Jianyi Wu, College of Biological and Chemical Engineering, Jiaxing
University, Zhejiang 314001, China. E-mail: jianyi_wu@163.com
2555

2556
C. MIAO ET AL.
Figure 1. Some CILs based on (ꢀ)-menthol.
of asymmetric organocatalysts, the issues of recovery and utilization of them have
drawn much attention. Chiral ionic liquids (CILs), as a new special category of asym-
metric organocatalysts in recent decades, have emerged as promising candidates.
CILs contain asymmetric atoms in molecules and could provide a simple way
to probe the potential application of optically active solvents or catalysts.[3,4]
Recently, investigation in the fields of synthesis and application of CILs has seen
Scheme 1. General strategy for synthesis of CILs.

SYNTHESIS OF NOVEL CHIRAL IONIC LIQUIDS
2557
tremendously increased in popularity. The imidazolium- and pyridinium-based CILs
have been synthesized and reported, in which the chiral pool was mostly selected
from amino acids, a-pinene, ephedrine, etc.[5–9] However, only a few of them can
be applied in asymmetric synthesis.[10] Therefore, it is worthy to synthesize new CILs
from the chiral pool. The introduction of an ester function group, besides being an
easy task,[11] could improve some of the properties of these CILs, especially those
related to the stabilization of polymers.[12] Also, such synthetic method can maintain
the molecular configuration of the chiral pool. In this frame, we presented a simple
way to synthesize new chiral ionic liquids using (ꢀ)-menthol as the chiral pool.
Although many other (ꢀ)-menthol-based CILs have been reported, the (-)-
menthyl group is connected to nitrogen atom (Fig. 1) in most cases.[11,13,14] Herein,
we introduced chiral center into pyridine ring through esterification of isonicotinic
acid, and all 13 new CILs (Scheme 1) have been prepared using this method.
RESULTS AND DISCUSSION
All pyridinium salts were prepared by a four-step reaction, as presented in
Scheme 1. Seen from the first and second steps, the (1S,2S,5R)-2-isopropyl-4-methyl-
cyclohexyl isonicotinate with a yield of 98% was synthesized based on isonicotinic
acid and (ꢀ)-menthol by esterification. Then the corresponding esters were reacted
with RBr under solvent-free conditions to form the ILs with Brꢀ anion (50–85%
yields). In this step, a much lower temperature (0–5 ꢁC) had to be used because of
the violent reaction. The last step was the anion exchange reaction, which was
accomplished in aqueous solution of KPF6 or NaBF4. The CILs containing (ꢀ)-
menthol were obtained as white or light yellow solids (70–95% yields), except the
ꢀ
one with C2H5OSO3 anion, which was a viscous yellow liquid. The structures and
some main characteristics of these new CILs are shown in Table 1. The structures
1
of all compounds were confirmed by infrared(IR), H NMR, and 13C NMR.
Although the chiral center and the configuration of chiral alcohol were main-
tained after the synthesis, they presented various optical activities as a result of the alter-
nation of the anions andꢀcations, as shown in Table 1. After the bromide anion was
ꢀ
exchanged with the BF4 or PF6 anions, the absolute values of optical rotation
25
25
ꢀ
(j½aꢂD j) of CILs with BF4 were slightly increased but j½aꢂD j of CILs with PF6ꢀ, was
25
decreased, which shows a strict relation between j½aꢂD2j5of these CILs and the type of
the anion. Herein, it is noteworthy to mention that j½aꢂD j of CILs with BF4ꢀ is slightly
25
larger than that with PF6ꢀ, andj½aꢂD j of CILs with Brꢀ change by no regular law. This
25
phenomenon could also be seen in the literature.[6,15,16] Meanwhile, another obvious dif-
ference was discovered: j½aꢂD j of CILs generally decrease with the increase of voluꢀme of
anion, which was also found in the references,[13,15,17,18] except the one with PF6
.
The melting points of CILs decrease with increasing the length of carbon chain,
which is connected to the nitrogen atom, as shown in Table 1. However, the solubility
of these CILs in water changes with the length of the carbon chain. For example, CILs
that were synthesized from isonicotinic acid, (ꢀ)-menthol, and n-octyl bromide could
not dissolve in water, but the other CILs with Brꢀ anion did. It is also shown that the
melting points of CILs with BF4 are less than those with Brꢀ.
ꢀ
The thermal stability of the novel CILs was evaluated by thermogravimetry=
differential thermal analysis (TG=DTA). All CILs have high thermal stability with

2558
C. MIAO ET AL.
Table 1. Structures and properties of the CILs synthesized
[a]
Yields
[%]
Mp
[ꢁC]
Tg
[ꢁC]
25
½aꢂD
Number
Name
Structure
Property
Light yellow
solid
1
[MePy]Br
81
ꢀ55.0
184–185.7 215.0
Light yellow
solid
2
[MePy]BF4
90
ꢀ62.9 151.7–154.3 241.1
3
4
[MePy]PF6
White solid
93
96
ꢀ40.0 157.8–158.9 269.6
[MePy]- C2H5SO4
Yellow liquid
ꢀ53.8
—
260.7
Light yellow
solid
5
6
[MbPy]Br
62
80
ꢀ60.0 145.6–145.9 220.2
ꢀ58.0 116.3–117.4 255.6
[MbPy]BF4
White solid
White solid
7
[MbPy]PF6
86
ꢀ56.0 151.7–152.3 261.8
Light yellow
solid
8
9
[MhPy]Br
59
98
ꢀ56.0 135.4–136.4 231.5
Light yellow
solid
[MhPy]BF4
ꢀ68.0 131.3–133.0 258.2
(Continued )

SYNTHESIS OF NOVEL CHIRAL IONIC LIQUIDS
2559
Table 1. Continued
[a]
Yields
[%]
Mp
[ꢁC]
Tg
[ꢁC]
25
½aꢂD
Number
10
Name
Structure
Property
[MhPy]PF6
White solid
72
ꢀ52.0 153.6–156.8 259.3
Light yellow
solid
11
12
13
[MoPy]Br
[MoPy]BF4
[MoPy]PF6
52
93
91
ꢀ66.0 130.0–135.7 232.4
Yellow solid
ꢀ53.3 118.1–120.5 257.9
Light Yellow
solid
ꢀ46.5 175.2–177.5 256.4
the decompositꢀion temperatures between 210 and 260 ꢁC (as shown in Table 1). The
CILs with BF4 or PF6ꢀ anion have much greater thermal stability than those with
Brꢀ anion.
EXPERIMENTAL
General Experimental Procedure for the Synthesis of the CILs
Isonicotinic acid (6.16 g, 0.05 mol) and 10 mL thionyl chloride were added to a
50-mL flask successively, heated, stirred for 5 h, and then evaporated under atmos-
pheric pressure to remove excess thionyl chloride. A solution of 0.05 mol (ꢀ)-menthol
in 20 mL dichloromethane was added to the residue, stirred, heated at reflux tem-
perature for 8 h, cooled to room temperature, and washed with 50 ml ꢃ 2 saturated
solution of Na2CO3. The separated organic layer was washed with diluted water,
and the aqueous layer was extracted with dichloromethane (50 mL ꢃ 2). All organic
layers were combined, dried over anhydrous Na2SO4, and evaporated under atmos-
pheric pressure to give yellow liquid. The residue was heated at 75 ꢁC under stirring in
the presence of 1.2 equiv of halohydrocarbon for 12 h, giving a light yellow solid. For
the ion exchange, the residue was stirred in water at room temperature for 1 h with 1.0
equiv of sodium tetrafluoroborate or potassium hexafluorophosphate. After water
evaporation, the residue was diluted in CH2Cl2 (10 mL), filtered under vacuum,
and gave the corresponding chiral ionic liquids, which were further dried under
vacuum at 65 ꢁC for several hours.

2560
C. MIAO ET AL.
Characterization Data of Compounds 1–13
4-(ꢀ)-Formylmentholethylpyridinium bromide, [MePy]Br (1). Yield 81%;
[a]2D5 ꢀ55.0 (c 0.01, CH2Cl2); IR (film): 3116.37, 1724.98, 1625.75, 1578.71, 1460.15,
1
1400.28, 1279.96, 1124.20; H NMR (D2O): d 8.88 (d, J ¼ 5.1 Hz, 2H), 8.33 (d,
J ¼ 5.1 Hz, 2H), 4.80–4.86 (m, 1H), 4.54 (td, 11.1 Hz and 5.4 Hz, 2H), 1.92 (m,
1 H), 1.90 (m, 1 H), 1.62 (m, 6H), 1.35 (m, 1 H), 0.99 (m, 1H), 0.74 (m, 7 H), 0.57
(d, 5.4 Hz, 3H); 13C NMR (D2O): d 160.90, 146.59, 145.00, 127.59, 76.76, 57.81,
46.95, 40.53, 33.91, 31.47, 26.46, 23.31, 21.87, 20.70, 17.49, 16.24; mass (Mþ) 290.2.
4-(ꢀ)-Formylmentholethylpyridinium tetrafluoroborate, [MePy]BF4(2).
Yield ¼ 90%; [a]2D5 ꢀ62.9 (c 0.01, CH2Cl2); IR (film): 3120.34, 1714.31, 1620.71,
1
1571.51, 1472.13, 1401.28, 1271.64, 1122.76; H NMR (CD3Cl3): d 9.42 (d, J ¼
5.4 Hz, 2H), 8.47 (d, J ¼ 5.1 Hz, 2H), 5.04–4.94 (m, 3H), 2.07 (m, 1H), 1.85 (m,
1H), 1.74 (m, 6H), 1.55 (m, 1H), 1.11 (m, 1H), 0.94 (m, 7H), 0.76 (d, J ¼ 5.1 Hz,
3H); mass (Mþ) 290.2.
4-(ꢀ)-Formylmentholethylpyridinium hexafluorophosphate, [MePy]
PF6 (3). Yield ¼ 93%; [a]2D5 ꢀ40.0 (c 0.01, CH2Cl2); IR (film): 3126.01, 1720.14,
1
1619.21, 1580.41, 1469.01, 1398.86, 1269.94, 1120.16; H NMR (CD3Cl3): d 9.35
(d, J ¼ 5.2 Hz, 2H), 8.66 (d, J ¼ 5.4 Hz, 2H), 5.15–4.94 (m, 3H), 1.97 (m, 1 H), 1.91
(m, 1H), 1.76 (m, 6H), 1.60 (m, 1H), 1.13 (m, 1H), 1.01 (m, 7H), 0.79 (d, J ¼ 5.4 Hz,
Hz, 3H); mass (Mþ) 290.2.
4-(ꢀ)-Formylmenthylethylpyridinium ethylsulfate, [MePy]C2H5SO4 (4).
Yield ¼ 96%, [a]2D5 ꢀ53.8 (c 0.01, CH2Cl2); IR (film): 3125.23, 1729.55, 1623.15,
1580.26, 1459.38, 1400.08, 1292.25, 1126.57; 1H NMR (CD3Cl3): d 9.41(d,
J ¼ 8.0 Hz, 2H), 8.32 (d, J ¼ 8.0 Hz, 2H), 4.78 (td, J ¼ 8.0 Hz and 4.0 Hz), 4.02 (m,
1 Hz), 3.89 (td, J ¼ 8.0 Hz and 4.0 Hz), 1.88 (m, 1H), 1.58 (m, 1H), 1.56 (m, 4H),
1.40 (m, 2H), 1.22 (m, 1H), 1.08 (t, J ¼ 8.0 Hz, 3H), 0.99 (m, 2H), 0.75 (m, 7H),
0.56 (d, J ¼ 4 Hz, 3H); 13C NMR(CD3Cl3): 161.00, 146.62, 144.51, 127.66, 77.04,
63.52, 57.96, 46.75, 40.38, 33.79, 31.30, 26.26, 23.28, 21.73, 20.51, 16.81, 16.19,
16.11; mass (Mþ) 290.2.
4-(ꢀ)-Formylmentholbutylpyridinium bromide, [MbPy]Br (5). Yield ¼
62%; [a]2D5 ꢀ60.0 (c 0.01, CH2Cl2); IR (film): 3115.86, 1733.08, 1644.62, 1578.43,
1
1456.49, 1400.15, 1291.93, 1123.64; H NMR (CDCl3): d 9.90 (d, J ¼ 5.1 Hz, 2H),
8.49(d, J ¼ 4.8 Hz), 5.21 (td, J ¼ 10.2 Hz and 3.6 Hz, 2H), 5.02 (m, 1H), 2.07 (m,
3H), 1.81 (m, 4H), 1.58 (m, 2H), 1.43 (m, 2H), 1.14 (m, 2H), 0.96 (m, 9H), 0.78
(d, J ¼ 5.4 Hz, 3H); 13C NMR (CDCl3): 160.84, 146.60, 144.95, 127.39, 76.70,
62.18, 46.92, 40.50, 34.02, 33.87, 31.46, 26.40, 23.23, 21.88, 20.70, 19.34, 16.19,
13.52; mass (Mþ) 318.2.
4-(ꢀ)-Formylmentholbutylpyridinium tetrafluoroborate, [MbPy]BF4 (6).
Yield ¼ 80%; [a]2D5 ꢀ58.0 (c 0.01, CH2Cl2); IR (film): 3125.81, 1718.01, 1624.02,
1569.41, 1478.49, 1402.05, 1290.13, 1122.41; 1H NMR (CD3Cl3): d 9.81 (d,
J ¼ 5.4 Hz, 2H), 8.71(d, J ¼ 5.1 Hz, 2H), 5.32 (m, 2H), 5.10 (m, 1H), 2.01 (m, 3H),
1.79 (m, 4H), 1.60 (m, 2H), 1.41 (m, 2H), 1.12 (m, 2H), 0.91 (m, 9H), 0.73 (d,
J ¼ 5.4 Hz, 3H); mass (Mþ) 318.2.

SYNTHESIS OF NOVEL CHIRAL IONIC LIQUIDS
2561
4-(ꢀ)-Formylmentholbutylpyridinium hexafluorophosphate, [MbPy]
PF6 (7). Yield ¼ 86%; [a]25 ꢀ56.0 (c 0.01, CH2Cl2); IR (film): 3120.89, 1718.62,
D
1
1621.14, 1576.01, 1480.61, 1410.21, 1287.31, 1119.82; H NMR (CD3Cl3): d 9.92
(d, J ¼ 5.1 Hz, 2H), 8.84(d, J ¼ 5.4 Hz, 2H), 5.41 (m, 2H), 5.12 (m, 1H), 2.09 (m,
3H), 1.82 (m, 4H), 1.64 (m, 2H), 1.46 (m, 2H), 1.16 (m, 2H), 0.96 (m, 9H), 0.81
(d, J ¼ 5.1 Hz, 3H); mass (Mþ) 318.2.
4-(ꢀ)-Formylmentholhexylpyridinium bromide, [MhPy]Br (8). Yield ¼
59%; [a]25 ꢀ56.0 (c 0.01, CH2Cl2); IR (film): 3119.63, 1730.86, 1644.67, 1576.68,
D
1
1457.68, 1400.51, 1291.69, 1127.03; H NMR (CDCl3): d 10.06 (d, J ¼ 8 Hz, 2H),
8.51 (d, J ¼ 8 Hz, 2H), 5.18 (m, 2H), 5.02 (m, 1H), 2.13 (m, 3H), 1.76 (m, 3H),
1.57 (m, 2H), 1.41 (m, 2H), 1.35 (m, 4H), 1.30 (m, 2H), 1.16 (m, 7H), 0.95 (t,
J ¼ 4 Hz, 3H), 0.75 (d, 5.1 Hz, 3H); 13C NMR (CDCl3): 160.92, 146.83, 144.87,
127.43, 76.68, 62.29, 46.93, 40.52, 33.90, 32.13, 31.456, 31.09, 26.42, 25.67, 23.29,
22.32, 21.86, 20.69, 16.21, 13.90; mass (Mþ) 346.3.
4-(ꢀ)-Formylmentholhexylpyridinium tetrafluoroborate, [MhPy]BF4 (9).
Yield ¼ 98%; [a]25 ꢀ68.0 (c 0.01, CH2Cl2); IR (film): 3120.13, 1720.16, 1631.73,
D
1
1570.61, 1471.61, 1401.15, 1291.80, 1125.13; H NMR (CDCl3): d 10.12 (d, J ¼
5.2 Hz, 2H), 9.01 (d, J ¼ 5.4 Hz, 2H), 5.41 (m, 2H), 4.98 (m, 1H), 2.10 (m, 3H),
1.80 (m, 3H), 1.56 (m, 2H), 1.39 (m, 2H), 1.32 (m, 4H), 1.29 (m, 2H), 1.13–0.90
(m, 10H), 0.71 (d, J ¼ 5.1 Hz, 3H); mass (Mþ) 346.3.
4-(ꢀ)-Formylmentholhexylpyridinium hexafluorophosphate, [MhPy]
PF6 (10). Yield ¼ 72%; [a]2D5 ꢀ52.0 (c 0.01, CH2Cl2); IR (film): 3119.92, 1721.46,
1
1636.52, 1571.39, 1472.02, 1412.12, 1290.76, 1121.20; H NMR (CDCl3): d 9.97
(d, J ¼ 5.4 Hz, 2H), 8.97 (d, J ¼ 5.4 Hz, 2H), 5.39 (m, 2H), 4.91 (m, 1H), 2.02 (m,
3H), 1.78 (m, 3H), 1.51 (m, 2H), 1.32 (m, 2H), 1.29 (m, 4H), 1.23 (m, 2H),
1.10–0.87 (m, 10H), 0.73 (d, J ¼ 5.4 Hz, 3H); mass (Mþ) 346.3.
4-(ꢀ)-Menthyloctylpyridinium bromide, [MoPy]Br (11). Yield ¼ 52%;
[a]25 ꢀ66.0 (c 0.01, CH2Cl2); IR (film): 3113.68, 1727.91, 1642.73, 1576.53,
D
1460.06, 1400.32, 1293.45, 1125.73; 1H NMR (CDCl3): d9.98 (d, J ¼ 8 Hz, 2H),
8.51 (d, J ¼ 8 Hz, 2H), 5.17 (m, 2H), 5.02 (m, 2H), 2.10 (m, 1H), 1.76 (m, 3H),
1.59 (m, 3H), 1.40 (m, 2H), 1.35 (m, 4H), 1.27 (m, 6H), 1.24 (m, 2H), 1.15 (m,
7H), 0.95 (t, J ¼ 4 Hz, 3H), 0.73 (d, J ¼ 5.1 Hz, 3H); 13C NMR (CDCl3): 160.93,
146.76, 144.88, 127.46, 76.78, 62.40, 46.93, 40.52, 33.90, 32.16, 31.62, 31.46, 28.99,
28.95, 26.42, 26.03, 23.29, 22.51, 21.85, 20.69, 16.21, 13.78; mass (Mþ) 374.3.
4-(ꢀ)-Menthyloctylpyridinium tetrafluoroborate, [MoPy]BF4 (12).
Yield ¼ 93%; [a]25 ꢀ53.3 (c 0.01, CH2Cl2); IR (film): 3121.59, 1719.09, 1640.13,
D
1
1571.41, 1469.12, 1401.59, 1291.13, 1124.93; H NMR (CDCl3): d 10.02 (d, J ¼
7.4 Hz, 2H), 8.68 (d, J ¼ 7.1 Hz, 2H), 5.69 (m, 2H), 5.10 (m, 2H), 2.15 (m, 1H), 1.
78 (m, 3H), 1.71 (m, 3H), 1.43 (m, 2H), 1.37 (m, 4H), 1.30 (m, 6H), 1.26 (m, 2H),
1.16–0.98 (m, 10H), 0.79 (d, J ¼ 5.4 Hz, 3H); mass (Mþ) 374.3.
4-(ꢀ)-Menthyloctylpyridinium hexafluorophosphate, [MoPy]PF6 (13).
Yield ¼ 91%; [a]25 ꢀ46.5 (c 0.01, CH2Cl2); IR (film): 3119.61, 1720.18, 1639.21,
D
1569.39, 1470.23, 1400.97, 1290.01, 1121.39; 1H NMR (CDCl3):d 9.98 (d, J ¼ 7.4 Hz,
2H), 8.61 (d, J ¼ 7.2 Hz, 2H), 5.63 (m, 2H), 5.01 (m, 2H), 2.11 (m, 1H), 1.72 (m, 3H),

2562
C. MIAO ET AL.
1.68 (m, 3H), 1.41 (m, 2H), 1.35 (m, 4H), 1.28 (m, 6H), 1.24 (m, 2H), 1.12–0.95
(m, 10H), 0.76 (d, J ¼ 5.4 Hz, 3H); mass (Mþ) 374.3.
CONCLUSIONS
In conclusion, 13 chiral ionic liquids based on (ꢀ)-menthol were synthesized
by a straightforward four-step strategy under very mild conditions. The study of
applications of these chiral ionic liquids as chiral catalysts in some organic reactions
is in progress.
ACKNOWLEDGMENT
This work was supported by the Natural Science Foundation of Zhejiang
Province (Y406470).
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