Catalytic asymmetric cycloaddition of CO2 to epoxides via chiral bifunctional ionic liquids

Article abstract of DOI:10.1016/j.molcata.2015.10.008

A series of new chiral ionic liquid catalysts composed of the N,N'-bis(salicyclidene) cyclohexene diaminatocobalt and an imidazolium salt were designed, prepared and applied for the chiral cyclic carbonate synthesis from racemic epoxides and carbon dioxide. All reactions exhibit good enantioselectivity for the chiral cyclic carbonate without polycarbonate and other by-products. The order of The order of catalytic activity toward the axial anions is OAc^- > CF₃CO₂^- > CCl₃CO₂^- > OTs^- and the order of enantioselectivity is OTs^- > OAc^- > CCl₃CO₂^- > CF₃CO₂^-.

Full text of DOI:10.1016/j.molcata.2015.10.008

Accepted Manuscript
Title: Catalytic Asymmetric Cycloaddition of CO2 to
Epoxides via Chiral Bifunctional Ionic Liquids
Author: Shuhui Duan Xinyao Jing Dandan Li Huanwang Jing
PII:
S1381-1169(15)30111-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.10.008
MOLCAA 9651
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
26-7-2015
6-10-2015
7-10-2015
Please cite this article as: Shuhui Duan, Xinyao Jing, Dandan Li, Huanwang
Jing, Catalytic Asymmetric Cycloaddition of CO2 to Epoxides via Chiral
Bifunctional Ionic Liquids, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2015.10.008
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Catalytic Asymmetric Cycloaddition of CO2 to Epoxides via Chiral
Bifunctional Ionic Liquids
Shuhui Duan, Xinyao Jing, Dandan Li and Huanwang Jing*
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical
Engineering.
Lanzhou University, 222 South Tianshui Road, Lanzhou, Gansu 730000, PR China
Corresponding author Tel: +86 931 891 2585; fax: +86 931 891 2582.
E-mail address: hwjing@lzu.edu.cn
Graphic Abstract
Catalytic Asymmetric Cycloaddition of CO2 to Epoxides via Chiral Bifunctional Ionic Liquid
Shuhui Duan, Xinyao Jing, Dandan Li, and Huanwang Jing
O
N
O
N
O
IL Cat.
Co
Y
O
O
O
CO2
O
X
R
R
X
N
N
N
R
N
R
R
Catalyst
1

ABSTRACT
series
A
of
new
chiral
ionic
liquid
catalysts
composed
of
the
N,N’-bis(salicyclidene)cyclohexene diaminatocobalt and an imidazolium salt were designed,
prepared and applied for the chiral cyclic carbonate synthesis from racemic epoxides and
carbon dioxide. All reactions exhibit good enantioselectivity for the chiral cyclic carbonate
without polycarbonate and other by-products. The order of The order of catalytic activity
toward the axial anions is OAc¯> CF3CO2¯ > CCl3CO2¯ > OTs¯ and the order of
enantioselectivity is OTs¯> OAc¯ >CCl3CO2¯>CF3CO2¯.
Keywords: Asymmetric cycloaddition, Chiral ionic liquid, Carbon dioxide, Epoxide, Kinetic
resolution
2

1. Introduction
Transformation of carbon dioxide into useful organic compounds, as an environmentally
benign process, has attracted much attention during the last two decades [1-7]. The
preparation of cyclic carbonates via coupling reaction of CO2 and epoxides is one of means
for CO2 fixation [8-15]. Chiral cyclic carbonates are important building blocks of chiral
intermediates and precursors of pharmaceuticals and fine chemicals [16]. A variety of
methodologies such as the cyclization of chiral diol with triphogene [17], enzyme mediated
enatioselective hydrolysis of racemic cyclic carbonates [18] or the insertion of CO2 into chiral
epoxides catalysed by zinc(II) [19] or palladium(0) complexes [20] have been applied to their
fabrication. As far as we know, the kinetic resolution of racemic epoxides with CO2 in the
presence of a chiral SalenCo(Ⅲ) complex leading to chiral cyclic carbonates is rare reported
(Scheme 1) [21-26] in spite of the kinetic resolution of racemic epoxides with water obtaining
only enantiopure epoxides [27,28].
A plenty of catalyst systems have been developed for the synthesis of cyclic carbonates
including inorganic and organic salts, metal oxides, organometallic compounds, ionic liquids,
transition-metal and main group complexes etc. [29-35]. In which, the ionic liquids (ILs),
well-known as green solvents and catalysts in organic synthesis [36-41], are few used in the
asymmetric reactions, such as coupling reaction of CO2 and epoxides to generate chiral cyclic
carbonates till now [42].
In this regard, modifying chiral ligand with imidazolium cation in metal complex catalysts has
been recognized as a useful methodology to improve their catalytic efficiency [39]. According
to our own efforts toward the development of highly efficient catalysts for the coupling of
3

CO2 and epoxides including bifunctional catalysts [43] that have been used on the asymmetric
coupling reaction of CO2 and epoxides, we devised a new series of chiral SalenCo(III)Y
catalysts supported on the alkyl imidazole ionic liquids (Figure 1).
2. Experimental
2.1 General
Propylene oxide was distilled from CaH2. 1,2-Diaminocyclohexane, L-(+)-tartaric acid (2R,3R)
and other epoxides were purchased from Aldrich. Imidazole, 1-bromobutane, 1-bromooctane,
1-bromododecane were analytical reagents and used without further purification.
NMR spectra of compounds and ILs were determined with a Varian AM-400 or Am-300
spectrometer using TMS as an internal standard. IR spectra of catalysts were recorded on a
Nicolet NEXUS 670 FT-IR spectrometer. Elemental analyses of catalysts were carried out on
Carioel elemental analyzer. GC analyses of chiral cyclic carbonates were carried out on a
Varian CP-3800 gas chromatograph equipped with FID detectors utilizing a Supelco-DEX
series (225) chiral column to determine the enantiomeric excess value. HPLC analyses of
chiral cyclic carbonates were carried out on a Waters 600 controller with a 2996 photodiode
array detector using a chiral OD column. The mass spectra/ HRMS (ESI) were obtained by
utilizing an Applied Biosystems Mariner Biospectrometry Workstation/ Bruker APEX II mass
spectrometer. Melting Points were carried out on Digital Display Microscopic Melting Point
Locator X-4 purchased from Beijing Tech Instrument Co., Ltd.
2.2 General procedure for coupling of epoxide and CO2
A solution of freshly prepared SalenCo(Ⅲ)Y (0.1 mmol) and epoxide (100 mmol) was
4

introduced into a 100 mL stainless-steel autoclave, which was purged three times and charged
with CO2 to 1.2 MPa. The reaction mixture was stirred at room temperature. When the
pressure of reactor was fall down to a presetting value, it was then vented carefully. After
removing the excess epoxide, the residue was weighed to measure the conversion of epoxide,
chiral cyclic carbonate (R = Me, Et, CH2Cl) was distilled under vacuum as a colourless liquid
or it (R = Ph, PhOCH2) was obtained by column chromatography through a short silica-gel
column to yield white solid product (ethyl acetate/ petroleum ether = 5:1).
2.3 Synthesis of catalysts
2.3.1 Synthesis of N-alkylimidazoles (Scheme 3):
N-Butylimidazole and N-octylimidazole were synthesized following the procedure of
literature [44]: To
a
round-bottomed flask, imidazole (6.0 g, 88 mmol),
1-bromobutane/1-bromooctane (90 mmol), acetonitrile (50 mL) and potassium hydroxide (9.9
g, 177 mmol), were added in sequence. The reaction mixture was refluxed for 4 h and then
cooled down to room temperature. After evaporating the solvent, the residue was purified by a
flash column chromatography utilizing ethyl acetate/methanol (v/v = 25:1) as eluent. The pure
product of 1-butylimidazole/1-octylimidazole was isolated as a pale yellow oil (~83%).
N-Butylimidazole: 1H NMR (CDCl3): δ (ppm) = 0.85 (t, J = 7.3 Hz, 3H, CH3), 1.20-1.27 (m,
2H, CH2), 1.62-1.72 (m, 2H, CH2), 3.85 (t, J = 7.3 Hz, 2H, CH2), 6.83 (s, 1H, ImH), 6.94 (s,
1H, ImH), 7.37(s, 1H, ImH). 13C NMR (CDCl3): δ (ppm) = 13.3, 19.5, 32.8, 46.5, 118.7,
128.9, 136.8.
N-Octylimidazole: 1H NMR: (CDCl3): δ (ppm) = 0.87 (t, J = 6.5 Hz, 3H, CH3), 1.28 (br, 10H,
5

CH2), 1.77 (br, 2H, CH2), 3.92 (t, J = 7.2 Hz, 2H, CH2), 6.90 (s, 1 H, ImH), 7.04 (s, 1H, ImH),
7.45 (s, 1H, ImH). 13C NMR (CDCl3): δ (ppm) = 13.9, 22.5, 26.4, 28.9, 29.0, 31.0, 31.6, 46.9,
118.7, 129.2, 136.9.
N-Dodecylimidazole was obtained following the procedure of literature [45]. To a solution of
imidazole (0.7 g, 10 mmol) in NaOH (50%) solution (1.0 g, 11 mmol), a solution of
1-bromododecane (2.5 g, 10 mmol) in THF (10 mL) was added dropwise. The obtained
mixture was refluxed for three days. After cooling, THF was removed by a rotary evaporator.
The residue was extracted with dichloromethane three times. The combined organic layer was
washed with water and then dried over anhydrous Na2SO4. The filtrate was concentrated and
purified by a column chromatography to produce clear yellow oil (2.0 g, 81%).
1H NMR (CDCl3): δ (ppm) = 0.83 (t, J = 5.1 Hz, 3H, CH3), 1.10-1.21 (m, 18H, CH2), 1.69 (br,
J = 7.0 Hz, 2H, CH2), 3.84 (t, J = 6.2 Hz, 2H, CH2), 6.89 (s, 1H, ImH), 7.04 (s, 1H, ImH),
7.44 (s, 1H, ImH). 13C NMR (CDCl3): δ (ppm) = 13.7, 22.3, 26.1, 28.7, 28.9, 29.0, 29.2, 20.7,
31.5, 46.5, 118.3, 128.2, 136.6
6

2.3.2 Synthesis of ligands
Synthesis of N-alkylimidazolium salicylaldehydes, Aa-Ca and Ab-Cb (Scheme 4): A solution
of 3-tert-Butyl-5-bromomethyl-2-hydroxybenzaldehyde [46, 47] in toluene (5.0 mL) was
added dropwise to a solution of N-alkylimidazole (10.0 mmol) in toluene (20.0 mL) over 10
min at room temperature. The mixture was then heated to reflux for 5 h. After cooling to room
temperature, the precipitate product was filtered, washed with toluene (35 mL) and dried
under reduced pressure.
1
Aa: yield 97%. H NMR (CDCl3): δ (ppm) = 0.95 (t, J = 6.9 Hz, 3H, CH3), 1.39 (m, 11H,
CH2+tBu), 1.91 (t, J = 7.2 Hz, 2H, CH2), 4.31 (t, J = 7.2 Hz, 2H, Im-CH2), 5.65 (s, 2H,
ph-CH2-Im), 7.42 (s, 1H, ImH), 7.58 (s, 1H, ArH), 7.64 (s, 1H, ArH), 7.95 (s, 1H, ImH), 9.96
13
(s, 1H, ImH), 10.51 (s, 1H, CHO), 11.91 (s, 1H, OH); C NMR (CDCl3): δ (ppm) = 13.3,
19.4, 29.0, 31.9, 35.0, 50.0, 52.6, 120.5, 122.0, 123.8, 133.2, 134.4, 139.7, 161.7, 197.2. MS
(ESI-MS): m/z [M-Br]+ calcd. for C19H27N2O2Br: 315.2; found: 315.3.
Ba: yield 98%. 1H NMR (CDCl3): δ (ppm) = 0.86 (t, J = 6.2 Hz, 3H, CH3), 1.24-1.40 (m, 19H,
CH2+tBu), 1.93-1.98 (m, 2H, CH2), 4.29 (t, J = 7.7 Hz, 2H, ImCH2), 5.66 (s, 2H, ph-CH2-Im),
7.28 (s, 1H, ImH), 7.44 (s, 1H, ArH), 7.61 (s, 1H, ArH), 7.94 (s, 1H, ImH), 9.96 (s, 1H, ImH),
10.74 (s, 1H, CHO), 11.94 (s, 1H, OH); 13C NMR (CDCl3): δ (ppm) = 14.0, 22.5, 26.2, 28.85,
28.98, 29.1, 30.1, 31.6, 35.0, 50.3, 52.7, 120.6, 121.6, 123.7, 133.2, 134.3, 137.2, 139.8, 161.8,
197.2. MS (ESI): m/z [M-Br]+ calcd. for C23H35N2O2Br: 371.3; found: 371.4.
Ca: yield 98%. 1H NMR (CDCl3): δ (ppm) = 0.88 (t, J = 6.2 Hz, 3H, CH3), 1.24-1.40 (m, 27H,
CH2+tBu), 1.92 (br, 2H, CH2), 4.28 (t, J = 7.5 Hz, 2H, CH2), 5.67 (s, 2H, CH2), 7.28 (s, 1H,
ImH), 7.43 (s, 1H, ArH), 7.61 (s, 1H, ArH), 7.94 (s, 1H, ImH), 9.96 (s, 1H, ImH), 10.79 (s,
7

1H, CHO), 11.94 (s, 1H, OH). 13C NMR (CDCl3): δ (ppm) = 14.1, 22.6, 26.2, 28.9, 29.1,
29.25, 29.31, 29.4, 29.5, 30.1, 31.8, 35.0, 50.3, 52.6, 120.6, 121.9, 123.9, 133.3, 134.4, 137.0,
139.8, 161.8, 197.2. MS (ESI): m/z [M-Br]+ calcd. for C27H43N2O2Br: 427.3; found 427.4.
Ab: yield 97%. 1H NMR (CDCl3): δ (ppm) = 0.94 (t, J = 6.8 Hz, 3H, CH3), 1.23-1.39 (m, 11H,
CH2+tBu), 1.90-2.00 (m, 2H, CH2), 4.38 (t, J = 8.4 Hz, 2H, CH2), 5.65 (s, 2H, CH2), 7.42 (s,
1H, ImH), 7.57 (s, 1H, ArH), 7.64 (s, 1H, ArH), 7.94 (s, 1H, ImH), 9.88 (s, 1H, ImH), 10.86
13
(s, 1H, CHO), 11.90 (s, 1H, OH); C NMR (CDCl3): δ (ppm) = 13.2, 19.3, 28.9, 31.8, 34.8,
49.7, 52.3, 120.4, 122.0, 124.1, 133.1, 134.3, 137.0, 139.5, 161.5, 197.1. MS (ESI): m/z
[M-Cl]+ calcd. for C19H27N2O2Cl: 315.2; found 315.3.
Bb: yield 98%. 1H NMR (CDCl3): δ (ppm) = 0.86 (t, J = 3.3 Hz, 3H, CH3), 1.25-1.36 (m, 19H,
CH2+tBu), 1.91-1.98 (m, 2H, CH2), 4.28 (t, J = 7.2 Hz, 2H, CH2), 5.65 (s, 2H, CH2), 7.23 (s,
1H, ImH), 7.40 (s, 1H, ArH), 7.60 (s, 1H, ArH), 7.90 (s, 1H, ImH), 9.94 (s, 1H, ImH), 10.96
13
(s, 1H, CHO), 11.93 (s, 1H, OH); C NMR (CDCl3): δ (ppm) = 13.2, 19.3, 28.9, 29.5, 31.8,
34.5, 34.8, 49.7, 52.3, 120.4, 122.0, 124.1, 133.1, 134.3, 137.0, 139.5, 161.5, 197.1. MS (ESI):
m/z [M-Cl]+ calcd. for C23H35N2O2Cl: 371.3; found 371.4.
Cb: yield 97%. 1H NMR (CDCl3): δ (ppm) = 0.84 (t, J = 5.9 Hz 3H, CH3), 1.21-1.35 (m, 27H,
CH2+tBu), 1.88 (br, 2H, CH2), 4.24 (t, J = 7.5 Hz, 2H, CH2), 5.63 (s, 2H, CH2), 7.29 (s, 1H,
ImH), 7.46 (s, 1H, ArH), 7.58 (s, 1H, ArH), 7.89 (s, 1H, ImH), 9.92 (s, 1H, ImH), 10.97 (s,
1H, CHO), 11.89 (s, 1H, OH); 13C NMR (CDCl3): δ (ppm) = 14.0, 22.6, 26.2, 28.9, 29.0, 29.2,
29.3, 29.4, 29.5, 30.1, 31.8, 34.9, 50.1, 52.4, 120.5, 121.7, 133.2, 134.4, 139.7, 161.6, 197.2.
MS (ESI): m/z [M-Cl]+ calcd. for C27H43N2O2Cl: 427.3; found 427.4.
General procedure for the preparation of Salen ligands (1Aa-1Ca, 1Ab-1Cb (Scheme 3): (1R,
8

2R)-1,2-diaminocyclohexane (0.5 mmol) and N-alkylimidazolium salicylaldehyde (Aa-Ca,
Ab-Cb, 1.0 mmol) were dissolved in ethanol (10.0 mL) in a two-necked round-bottomed
flask equipped with a condenser and gas-protector. The mixed solution was refluxed for 5 h
under argon. The solvent was evaporated near to dry, and diluted by hexane (15 mL) to
precipitate the crude product that was filtered, washed with hexane and dried in vacuum [48].
1Aa: m.p. 74 - 75 °C, []58918 = -199º(c = 1.0, acetone). 1H NMR (CDCl3): δ (ppm) = 0.81 (t,
J = 7.8 Hz, 6H, CH3), 1.18-1.28 (m, 26H, CH2+tBu), 1.63-1.82 (m, 8H, CH2), 1.86-1.94 (m,
2H, CH), 4.14-4.28 (m, 4H, CH2), 5.27-5.46 (m, 4H, CH2), 7.17 (s, 2H, ArH), 7.25 (s, 2H,
ImH), 7.30 (s, 2H, ImH), 7.52 (s, 2H, ArH), 8.67 (s, 2H, CH=N), 10.29 (s, 2H, ImH), 14.15 (s,
13
2H, OH); C NMR (CDCl3): δ (ppm) = 13.3, 19.3, 19.6, 22.4, 24.0, 29.1, 29.6, 31.9, 32.9,
34.9, 46.7, 49.8, 52.9, 71.6, 118.3, 118.8, 121.6, 122.4, 128.8, 129.9, 130.6, 136.3, 138.8,
161.4, 165.0. MS (ESI): m/z [M-2Br]2+/2 calcd. for C44H64Br2N6O2: 354.3; found 354.5; IR
(KBr) (cm-1): 3363.0, 2958.5, 2918.5, 2850.0, 1740.9, 1629.1, 1561.3, 1462.2, 1379.5, 1270.5,
1210.6, 1158.0, 1089.3, 1047.1, 879.6, 723.4.
1Ba: m.p. 83 - 84 °C, []58918 = -206º (c = 1.0, acetone). 1H NMR (CDCl3): δ (ppm) = 0.86 (t,
J = 6.9 Hz, 6H, CH3), 1.25-1.48 (m, 42H, CH2+tBu), 1.77-1.97 (m, 8H, CH2), 1.99-2.02 (m,
2H, CH), 4.28 (t, J = 7.8 Hz, 4H, CH2), 5.34-5.54 (m, 4H, CH2), 6.91 (s, 2H, ArH), 7.18 (d, J
= 1.8 Hz, 2H, ImH), 7.33 (d, 2H, J = 1.8 Hz, ImH), 7.51 (s, 2H, ArH), 8.26 (s, 2H, CH=N),
13
10.48 (s, 2H, ImH), 14.23 (s, 2H, OH); C NMR (CDCl3): δ (ppm) = 14.0, 22.5, 22.6, 24.1,
26.2, 28.9, 29.0, 29.3, 30.2, 31.6, 31.9, 35.0, 37.0, 50.2, 52.9, 71.7, 118.4, 121.1, 121.6, 130.0,
138.9, 165.1; MS (ESI): m/z: [M-2Br]2+/2 calcd. for C52H80Br2N6O4: 410.3; found 410.4; IR
(KBr) (cm-1): 3383.9, 3130.7, 3062.9, 2926.1, 2857.1, 2193.7, 1731.9, 1664.4, 1629.4, 1559.6,
9

1444.2, 1377.9, 1318.1, 1211.7, 1158.6, 1096.2, 918.7, 732.2, 643.2, 511.8.
1Ca: m.p. 75 - 76 °C, []58918 = -203.5º(c = 1.0, acetone). 1H NMR (CDCl3): δ (ppm) = 0.88 (t,
6H, J = 6.6 Hz, CH3), 1.18-1.37 (m, 58H, CH2+tBu), 1.76-1.91 (m, 8H, CH2), 2.02-2.06 (m,
2H, CH), 4.28 (t, J = 7.4 Hz, 4H, CH2), 5.35-5.67 (m, 4H, CH2), 7.06 (s, 2H, ArH), 7.18 (s, J
= 2.1 Hz, 2H, ImH), 7.33 (d, J = 2.1 Hz, 2H, ImH), 7.56 (s, 2H, ArH), 8.29 (s, 2H, CH=N),
13
10.52 (s, 2H, ImH), 14.23 (s, 2H, OH); C NMR (CDCl3): δ (ppm) = 14.1, 22.6, 26.2, 28.9,
29.2, 29.3, 29.4, 29.5, 29.6, 30.2, 31.8, 34.9, 50.1, 52.9, 71.7, 118.3, 121.1, 121.6, 122.2,
123.0, 130.7, 136.4, 136.9, 161.5, 165.1; MS (ESI): m/z [M-2Br]2+/2 calcd. for
C60H96Br2N6O4: 466.3; found 466.6; IR (KBr) (cm-1): 3388.8, 3124.0, 3058.4, 2925.1, 2854.6,
2191.6, 1731.8, 1629.1, 1596.3, 1559.2, 1444.1, 1359.3, 1273.9, 1212.0, 1160.2, 1096.4,
1026.2, 921.2, 861.2, 777.2, 732.4, 643.1.
1Ab: m.p. 86 - 87 °C, []58918= -223º (c = 1.0, acetone). 1H NMR (CDCl3): δ (ppm) = 0.96 (t,
J = 4.8 Hz, 6H, CH3), 1.28-1.36 (m, 26H, CH2+tBu), 1.71-1.88 (m, 8H, CH2), 1.97-1.99 (m,
2H, CH), 4.28 (t, J = 7.2 Hz, 4H, CH2), 5.33-5.54 (m, 4H, CH2), 6.94 (s, 2H, ArH), 7.04 (s,
1H, 2mH), 7.47 (s, 2H, ImH), 7.55 (s, 2H, ArH), 8.28 (s, 2H, CH=N), 10.74 (s, 2H, ImH),
14.24 (s, 2H, OH); 13C NMR (CDCl3): δ (ppm) = 13.4, 19.5, 24.1, 24.6, 29.2, 29.6, 32.0, 32.5,
34.9, 49.9, 53.3, 60.5, 72.6, 87.3, 121.8, 129.8, 147.7, 161.5, 165.3; HRMS (ESI): m/z
[M-2Cl]2+/2 calcd. for C44H64Cl2N6O4: 354.2540; found 354.2536; IR (KBr) (cm-1): 3417.0,
3131.0, 2956.2, 2930.2, 2862.5, 2609.7, 2193.6, 1729.5, 1663.6, 1629.7, 1560.5, 1443.0,
1362.1, 1272.5, 1211.3, 1159.7, 1096.8, 1028.5, 995.1, 924.1, 804.3, 731.1, 642.4.
1Bb: m.p. 81 - 82 °C, []58918 = -211º (c = 1.0, acetone). 1H NMR (CDCl3): δ (ppm) = 0.87 (t,
3H, J = 2.7 Hz, CH3), 1.25-1.49 (m, 19H, CH2+tBu), 1.77-1.90 (m, 6H, CH2), 1.97-2.00 (m,
10

1H, CH), 4.27 (br, 2H, CH2), 5.33-5.49 (m, 2H, CH2), 6.97 (s, 1H, ArH), 7.05 (s, 1H, ImH),
7.12 (s, 1H, ImH), 7.44 (s, 1H, ArH), 8.25 (s, 1H, CH=N), 10.88 (s, 1H, ImH), 14.22 (s, 1H,
13
OH); C NMR (CDCl3): δ (ppm) = 13.9, 22.4, 23.1, 24.0, 26.1, 28.8, 28.9, 29.1, 29.6, 30.1,
31.5, 32.4, 34.8, 50.0, 52.8, 118.3, 121.1, 121.7, 129.8, 130.5, 138.8, 165.0, 175.1; HRMS
(ESI): m/z [M-2Cl]2+/2 calcd. for C52H80Cl2N6O4: 410.3166; found 410.3170; IR (KBr) (cm-1):
3421.3, 3133.1, 3060.6, 2954.2, 2927.9, 2857.7, 2398.0, 1727.1, 1660.7, 1629.9, 1560.0,
1443.1, 1363.3, 1272.7, 1211.5, 1159.3, 1029.3, 863.1, 776.1, 730.5, 643.3, 535.7.
1Cb: m.p. 54 - 55 °C, []58918 = -229º (c = 1.0, acetone). 1H NMR (CDCl3): δ (ppm) = 0.87 (t,
J = 6.9 Hz, 3H, CH3), 1.17-1.35 (m, 27H, CH2+tBu), 1.82-1.88 (m, 6H, CH2), 1.96-2.06 (m,
1H, CH), 4.27 (t, J = 7.5 Hz, 2H, CH2), 5.33-5.53 (m, 2H, CH2), 6.86 (s, 1H, ArH), 7.03 (s,
1H, ImH), 7.29 (s, 1H, ImH), 7.44 (s, 1H, ArH), 8.27 (s, 1H, CH=N), 10.94 (s, 1H, ImH),
14.21 (s, 1H, OH); 13C NMR (CDCl3): δ (ppm) = 14.0, 22.6, 24.0, 26.2, 28.9, 29.2, 29.5, 29.6,
30.2, 31.8, 32.9, 34.8, 46.9, 50.1, 52.9, 118.3, 121.0, 122.2, 124.9, 130.7, 138.1, 138.9, 161.4,
168.6, 186.5; HRMS (ESI): m/z [M-2Cl]2+/2 calcd. for C60H96Br2N6O4: 466.3792; found
466.3785; IR (KBr) (cm-1): 3389.1, 3130.5, 3061.3, 2925.6, 2854.9, 2190.7, 1919.3, 1629.5,
1597.5, 1560.7, 1463.2, 1444.2, 1379.1, 1361.2, 1273.2, 1211.0, 1161.0, 1096.8, 1027.0,
905.5, 863.8, 777.1, 731.8, 643.6, 622.6. (Im = imidazolium)
2.3.3 Synthesis of IL catalysts
General procedure for the synthesis of the imidazole ionic liquid bifunctional metal complex
catalysts (Scheme 2): In a round-bottomed flask, Co(OAc)2·4H2O (1.0 mmol) dissolved in
methanol (1.0 mL) was added into an ethanol solution (5.0 mL) of N-imidozolium Salen
11

ligand (1Aa-1Ca, 1Ab-1Cb, 1.0 mmol) under argon. Then, the obtained solution was heated
and refluxed for 2 h. After evaporating the solvent, the crude SalenCo(II) complex was gained.
The desired catalysts of SalenCo(III)Y were prepared by oxidizing the SalenCo(II) complex
dissolved in dichloromethane in the presence of relevant acid (0.1 mmol) under oxygen.
3Aap: m.p. 101 - 102 °C, []58918 = -229º (c = 1.0, EtOH). MS (ESI): m/z [M-OAc+K]+ calcd.
for C46H65Br2CoN6O4: 966.2; found 966.3; Anal. calcd. for C46H65Br2CoN6O4: C, 56.10; N,
8.53; H, 6.65. Found C, 56.28; N, 8.64; H, 6.54.
3Bap: m.p. 119 -120 °C, []58918 = -286º (c = 1.0, EtOH). MS (ESI): m/z [M-OAc-Br+2Na]+
calcd. for C54H81Br2CoN6O4: 1002.5; Found 1002.6; Anal. calcd. for C54H81Br2CoN6O4·1/2
hexane·1/2CH2Cl2 (%): C, 58.40; N, 7.11; H, 7.59. Found C, 58.53; N, 6.81; H, 8.02.
3Cap: m.p. 115 - 116 °C, []58918 = -235º (c = 1.0, EtOH). MS (ESI): m/z [M-OAc-Br+2Na]+
calcd. for C62H97Br2CoN6O4: 1114.6; Found 1114.7; Anal. calcd. for C62H97Br2CoN6O4·1/2
CH2Cl2: C, 59.88; N, 6.70; H, 8.04. Found C, 60.19; N, 6.92; H, 7.69.
3Abp: m.p. 156 - 157 °C, []58918= -274º (c = 1.0, EtOH). MS (ESI): m/z [M-OAc+NH4]+
calcd.
for
C46H65Cl2CoN6O4:
855.4;
Found
855.2;
Anal.
calcd
for
C46H65Cl2CoN6O4·1/3CH2Cl2: C, 60.21; N, 9.09; H, 7.16. Found C, 60.07; N, 8.85; H, 7.31.
18
3Bbp: m.p. 130 - 131 °C, []589 = -239º (c = 1.0, EtOH). MS (ESI): m/z [M]+ calcd. for
C54H81Cl2CoN6O4: 1008.1; found 1008.2; Anal. calcd. for C54H81Cl2CoN6O4·1/2CH2Cl2: C,
62.31; N, 8.00; H, 7.87. Found C, 62.16; N, 8.24; H, 7.96.
3Cbp: m.p. 101 - 102 °C, []58918 = -242º (c = 1.0, EtOH). MS (ESI): m/z [M-OAc+NH4]+
calcd. for C54H81Cl2CoN6O4: 1079.3; found 1079.4; Anal. calcd for C62H97Cl2CoN6O4 1/4
CH2Cl2(%): C, 65.50; N, 7.36; H, 8.61. Found C, 65.74; N, 7.66; H, 8.46.
12

3Cbq: m.p. 102 - 103 °C, []58918 = -235º (c = 1.0, EtOH). MS (ESI): m/z [M-CF3CO2+H]+
calcd. for C62H97Cl2CoN6O4:1061.6; found 1061.6; Anal. calcd for C62H94Cl2CoF3N6O4·3/2
CH2Cl2: C, 58.59; N, 6.46; H, 7.51. Found C, 58.20; N, 6.38; H, 7.12.
3Cbr: m.p. 97 - 98 °C, []58918 = -245º (c = 1.0, EtOH). MS (ESI): m/z [M-CCl3COO+NH4]+
calcd.
for
C62H94Cl5CoN6O4:
1079.3;
found
1079.5;
Anal.
calcd.
for
C62H94Cl5CoN6O4·hexane: C, 62.35; N, 6.42; H, 8.31. Found C, 62.48; N, 6.60; H, 8.28.
3Cbs: m.p. 101 - 102 °C, []58918 = -254º (c = 1.0, EtOH). MS (ESI): m/z [M-2Cl]2+/2 calcd.
for C67H101Cl2CoN6O5S: 580.8; found 580.8; Anal. calcd. for C67H101Cl2CoN6O5S·1/2CH2Cl2:
C, 63.59; N, 6.59; H, 8.06. Found C, 63.80; N, 6.50; H, 8.07.
3. Results and discussion
Understanding the mechanism of catalytic coupling reaction between epoxide and CO2
concerning two catalytic sites: Lewis acid and Lewis base, new catalysts should include metal
as Lewis acid and halogen anion as nucleophile (Lewis base). Following our own efforts to
develop highly efficient catalysts for the asymmetric cycloaddition of carbon dioxide to
epoxides, new bifunctional catalysts of imidazole ionic liquid combined a chiral metal-salen
complex with a quaternary ammonium salt in one molecule were devised and prepared
successfully.
The asymmetric cycloaddition of racemic propylene oxide (PO) and CO2 catalysed by these
chiral bifunctional IL was carefully investigated (Table 1). The results demonstrated that the
axial anions (Y¯) had both effects on the catalytic activity and enantioselectivity. The order of
catalytic activity toward the axial anions is OAc¯> CF3CO2¯ > CCl3CO2¯ > OTs¯ and the
13

order of enantioselectivity is OTs¯> OAc¯ >CCl3CO2¯>CF3CO2¯ (Table 1, entries 3-6 and
12-15). Taking one with another, the chiral IL bifunctional catalyst with OAc¯ as axial anion
gave better enantioselectivity and activity than other IL complexes in 47.9% yield and 23.2%
ee value at room temperature (Table 1, entry 7). Furthermore, when the reaction temperature
was decreased from 25 oC to 0 oC, the enantioselectivity of PC was increased from 37.1% up
to 57.2% ee for catalyst 3Abp (Table 1, entries 10 vs 19). Thus, the temperature was
indispensable factor in this kinetic resolution reaction.
To optimize and select the best catalyst system, screening of the coordinated counterions (X¯)
of the catalyst system was also proceeded (Table 2). The bifunctional ionic liquid catalysts
with various counterion X¯ and axial anion (OAc¯) influenced both the catalytic activity and
enantioselectivity of the reaction (Table 2, entries 1,4,7 and 2,5,8 and 3,6,9). The order of
activity for counterion is Br¯>Cl¯>F¯; and the order of enantioselectivity is Cl¯>Br¯>F¯.
This can be explained to the dissociation of a halide ion from the imidazolium moiety, which
attacks onto the terminal carbon atom of epoxide with more nucleophilic bromide ions based
on the understanding of the mechanism (Scheme 1) [25,49,50].
We must point out the more nucleophilicity of counterions of catalyst, the more catalytic
activity and enantioselectivity in a certain degree. However, what is the effect of imidazolium
cation? In order to answer this question, the investigations were carried out using catalysts
with various imidazolium moities (Table 2, Figure 1). The catalytic results revealed that the
14

alkyl chain length of imidazolium moities can also affect the activity of catalysts for
asymmetric coupling reaction of CO2 and epoxides in spite of their enantioselectivity. The
reaction rates are dramatically enhanced almost 4 times when increasing the alkyl chain
length from 4 (n-butyl) to 12 (n-dodecyl) (TOF 5.1 vs 19.5 h-1) (Table 2, entries 4-6). This is
attributed to the better CO2 solubility with longer alkyl chain length. Therefore, the best
catalyst 3Cbp was choosen in the next investigations considering all factors.
To further extend the scope of this reaction, various substituted terminal epoxides were
investigated using 3Cbp as catalyst under optimized conditions (Table 3). Epichlorohydrin
and 1,2-epoxydodecane were converted to corresponding chiral cyclic carbonates in moderate
yield and considerable enantioexcess values (Table 3, entries 2, 3). However, styrene oxide
and phenyl glycidyl ether were transformed into their cyclic carbonates in valuable yield and
lower enantioselectivies (3 and 2 % ee, respectively).
Considering the mechanism of coupling reaction of epoxide and CO2 reported in literatures
[51-53], a proposed mechanism is illustrated in Scheme 2. The epoxide is firstly activated by
a Lewis acidic center of SalenCo(Ⅲ)Y to form a complex 1 that is attacked by a nucleophile
X¯ to produce a requisite metal alkoxide intermediate 2. CO2 inserts into the intermediate 2
leading to an intermediate 3. After an intramolecular cyclization of 3 releasing a halogen
anion to stabilize the imidazolium of the ionic liquid, complex 4 is generated and releases a
chiral propylene carbonate when an epoxide approaches to it. The complex 1 is recycled
thereof and supplies the next catalytic cycle. According to this mechanism, it insists that both
Lewis acidic and Lewis basic center have the same importance in the coupling reaction.
Noteworthy, our chiral bifunctional IL catalysts can synchronously provide Lewis acid and
15

Lewis base that satisfy the asymmetric catalytic cycloaddition of CO2 to epoxide without
additional co-catalyst.
4. Conclusion
In summary, new bifunctional catalysts based on imidazolium ionic liquid were successfully
o
synthesized. The most melting points of these new ionic liquid are around 100 C. A kinetic
resolution of racemic epoxides with CO2 via these new chiral bifunctional ionic liquids
yielding chiral cyclic carbonates in reasonable to high yield under very mild reaction
conditions were carefully investigated. Furthermore, the effects of axial anion, counterions
and the chain length of alkyl group in the imidazolium cations were fully studied and
discussed, respectively. The counterion acts as nucleophile and gives deep infection to the
activity and selectivity of catalyst. The imidazolium moiety with longer alkyl chain length can
dramatically enhance the catalytic activity and the reaction rate except their enantioselectivity.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China
(NSFC 20973086, 21173106).
16

Notes and references
aState Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical
Engineering, Lanzhou University, 222 South Tianshui Road, Lanzhou, Gansu730000, P.R.
China. Fax: (+86)0931-8912582; Tel: (+86)0931-8912585; E-mail: hwjing@lzu.edu.cn
[1] D. J. Darensbourg, R. M. Mackiewicz, A. L. Phelps, D. R. Billodeaux, Acc. Chem. Res. 37
(2004) 836-844.
[2] M. Aresta, A. Dibenedetto and A. Angelini. Chem. Rev. 114 (2014) 1709-1742.
[3] T. Sakakura, J. C. Choi, H. Yasuda, Chem. Rev. 107 (2014) 2365-2387.
[4] X.-B. Lu and D. J. Darensbourg, Chem. Soc. Rev. 41 (2012) 1462–1484.
[5] D. M. d’Alessandro, B. Smit, J. R. Long, Angew. Chem. Int. Ed, 49 (2010) 6058- 6082.
[6] N. Kielland, C. J. Whiteoak and A. W. Kleij, Adv. Synth. Catal. 355 (2013) 2115-2138.
[7] D. S. Bai, S. H. Duan, L. Hai, and H. W. Jing, ChemCatChem, 4 (2012) 1752-1758.
[8] R. L. Paddock, S. T. Nguyen, J. Am. Chem. Soc. 123 (2001) 11498-11499.
[9] H. Jing, S. K. Edulji, J. M. Gibbs, C. L. Stern, H. Zhou, S. T. Nguyen, Inorg. Chem. 43
(2004) 4315-4327.
[10] D. J. Darensbourg, Chem. Rev. 107 (2007) 2388-2410.
[11] Z. Z. Yang, Y. N. Zhao, L. N. He, J. Gao, Z. S. Yin, Green Chem. 14 (2012) 519- 527.
[12] T. Chang, H. W. Jing, L. L. Jin, W. Y. Qiu, J. Mol. Catal. A-Chem, 264 (2007) 241-247.
[13] M. Cokoja, C. Bruckmeier, B. Rieger, W. A. Herrmann and F. E. Kühn, Angew. Chem.
Int. Ed. 50 (2011) 8510-8537.
[14] Y. Tsuji and T. Fujihara, Chem. Commun. (2012) 9956-9964.
[15] C. Maeda, Y. Miyazaki and T. Ema, Catal. Sci. Technol. 4 (2014) 1482-1497.
17

[16] J. H. Clements, Ind. Eng. Chem. Res. 42 (2014) 663-674.
[17] S. K. Kang, J. H. Jeon, K. S. Nam, C. H. Park, H. W. Lee, Synth. Commun. 24 (1994)
305-312.
[18] M. Shimojo, K. Matsumoto, M. Hatanaka, Tetrahedron, 56 (2000) 9281-9288.
[19] H. Hisch, R. Millini, I. J. Wang, Chem. Ber. 119 (1986) 1090-1094.
[20] B. M. Trost, S. R. Angle,. J. Am. Chem. Soc. 107 (1985) 6123-6124.
[21] B. Albrecht, B. Marc, Org. Lett. 8 (2006) 4401-4404.
[22] S. Chen, R. B. Kawthekar, G. J. Kim, Tetrahedron Lett. 48 (2007) 297-300.
[23] X. B. Lu, B. Liang, Y. J. Zhang, Y. Z. Tian, Y. M. Wang, C. X. Bai, H. Wang, R. Zhang, J.
Am. Chem. Soc. 126 (2004) 3732-3733.
[24] W. Yamada, Y. Kitaichi, H. Tanaka, T. Kojima, M. Sato, T. Ikeno, T. Yamada, Bull. Chem.
Soc. Jpn. 80 (2007) 1391-1401.
[25] D. Y. Jang, H. G. Jang, G. R. Kim, G-J. Kim, Catal. Today, 185 (2012) 306-312.
[26] L. Jin, Y. Huang, H. Jing, T. Chang, P. Yan, Tetrahedron. Asym. 19 (2008) 1947-1953.
[27] M. Tokunaga, J. F. Larrow, F. Kakiuchi, E. N. Jacobsen, Science 277 (1997) 936-938.
[28] A. Gansauer, A. Baechuk, F. Keller, M. Schmitt, S. Grimme, M. Gerenkmp, C.
Muck-Lichtenfeld, K. Daasbjerg, H. Svith, J. Am. Chem. Soc. 129 (2007) 1359-1371.
[29] F. Li, C. Xia, F. Xu, W. Sun, G. Chen, Chem. Commun. (2003) 2042-2043.
[30] P. Yan, H. W. Jing, Adv. Synth. Catal. 351 (2009) 1325-1332.
[31] H. Jing, S. T. Nguyen, J. Mol. Catal. A-Chem. 261 (2007) 12-15.
[32] D. Bai, X. Wang, Y. Song, B. Li, L. Zhang, P. Yan, H. Jing, Chin. J. Catal. 31 (2010)
176-180.
18

[33] Y. Song, D. Bai, B. Li, H. Jing, Chem. Euro. J. 17 (2011) 8731-8738.
[34] Y. Y. Song, C. Cheng, H. W. Jing, Chem. Euro. J. 20 (2014) 12894-12900.
[35] J. Liu, A. Wang, H. W. Jing, Chin. J. Catal. 35 (2014) 1669-1675.
[36] T. Welton, Chem. Rev. 99 (1999) 2071-2084.
[37] J. D. Holbrey, K. R. Seddon, J. Chem. Soc. Dalton Trans. (1999) 2133-2139.
[38] D. B. Zhao, M. Wu, Y. Kou, E. Z. Min, Catal. Today, 74 (2002) 157-159.
[39] J. M. Sun, S. I. Fujita, M. Arai, J. Organomet. Chem. 690 (2005) 3490-3497.
[40] W. S. Miao, T. H. Chan, Acc. Chem. Res. 39 (2006) 897-908.
[41] A. Taheri, B. Lai, C. Cheng, Y. Gu, Green Chem. 17 (2015), 812-816.
[42] S. Zhang, Y. Huang, H. Jing, W. Yao, P. Yan, Green Chem. 11 (2009) 935-938.
[43] T. Chang, L. L. Jin, H. W. Jing, ChemCatChem, 1 (2009) 379-383.
[44] J. Y. Cheng, Y. H. Chu, Tetrahedron Lett. 47 (2006) 1575-1579.
[45] M. Lee, Z. B. Niu, C. Slebodnick, H. W. Gibson, J. Phys. Chem. B, 114 (2010)
7312-7319.
[46] R. Tan, D. Yin, N. Yu, L. Tao, Z. Fu, D. Yin, J. Mol. Catal. A-Chem. 259 (2006) 125-132.
[47] F. Minutolo, D. Pini, A. Petri, P. Salvadori, Tetrahedron-Asymmetry, 7 (1996)
2293-2302.
[48] R. I. Kureshy, N. H. Khan, S. H. R. Abdi, I. Ahmad, S. Singh, R. V. Jasra, J. Catal. 221
(2004) 234-240.
[49] T. Chang, H. W. Jing, L. L. Jin, W. Qiu, J. Mol. Catal. A-Chem. 264 (2007) 241-247.
[50] H. S. Kim, J. J. Kim, H. Kim, H. G. Jang, J. Catal. 220 (2003) 44-46.
[51] X. Xu, C. Wang, H. Li, Y. Wang, W. Sun, Z. Shen, Polymer, 48 (2007) 3921-3924.
19

[52] K. B. Hansen, J. L. Leighton, E. N. Jacobsen, J. Am. Chem. Soc. 118 (1996)
10924-10925.
[53] W. N. Sit, S. M. Ng, K. Y. Kwong, C. P. Lau, J. Org. Chem. 70 (2005) 8583-8586.
20

Figure caption
Figure 1. Structures of chiral bifunctional ILs
21

Scheme 1 Kinetic resolution of epoxide by CO2
22

Y
X
A
N
Co
O
N
N
N
A
R
X
2
CO2
Y
Y
C
Y
X
A
X
A
X
A
X
N
Co
N
N
N
Co
O
o
N
N
N
N
N
N
N
O
N
A
A
A
O
X
O
+
C
O
R
R
O
1
O
X
O
O
3
R
R
Y
A = alkyl
R = Me, ClCH2, Ph, PhOCH2, CH3(CH2)9
X
N
Co
N
N
N
O
A
A
O
4
O
R
Scheme 2. Proposed mechanism for the coupling of CO2 and epoxide
23

Scheme 3 Synthesis of N-alkylimidazoles
24

Scheme 4. Synthesis of ligands and IL catalysts
25

Table caption
Table 1 The effect of axial anion on the synthesis of chiral PO a
EntryCatalyst Axial anion
Time(h) Yield(%)
ee% TON[b] TOF[c]
1
2
3
4
5
6
7
8
9
3Aap
3Aaq
3Bap
3Baq
3Bar
3Bas
3Cap
3Caq
3Car
OAc
10
34.0
42.5
39.1
28.1
34.1
29.1
47.9
34.0
40.9
37.3
24.0
48.9
38.1
39.8
41.2
25.3
7.2
19.8 340
12.6 425
22.6 391
15.4 281
19.4 341
33.1 291
23.2 479
13.7 340
14.6 409
37.1 373
32.1 240
38.8 489
37.7 381
38.6 398
39.9 412
35.4 253
38.6 72
34
CF3CO2
OAc
10
42.5
48.8
37.4
31
8
CF3CO2
CCl3CO2
OTs
7.5
11
4(d)
7.5
23
3.0
OAc
63.8
14.7
43.0
5.1
CF3CO2
CCl3CO2
OAc
9.5
3(d)
46
10 3Abp
11 3Bbp
12 3Cbp
13 3Cbq
14 3Cbr
15 3Cbs
16d 3Aap
17d 3Baq
18d 3Cap
19d 3Abp
OAc
5.2
OAc
25
19.5
15.8
16.5
8.5
CF3CO2
CCl3CO2
OTs
24
34
48
OAc
62
4.0
OAc
15
4.8
OAc
90
37.7
6.2
28.2 377
57.2 62
4.1
OAc
9(d)
0.2
26

20d 3Bbp
21d 3Cbp
OAc
OAc
6(d)
5(d)
3.8
43.5 38
0.2
0.8
10.1
50.5 101
a Reaction conditions: PO 100 mmol; PCO2 = 1.2 MPa; catalyst 0.1 mmol; T = 25 oC.
b Turnover number.
c Moles of PC produced by per mole of catalyst per hour.
27

Table 2 The effect of counterions on the synthesis of chiral PO a
Entry Catalyst
Counterions
Time(h)
10
Yield(%)
34.0
39.1
47.9
37.3
24.0
48.9
2.4
ee% TON[b] TOF[c]
1
2
3
4
5
6
7
8
9
3Aap
3Bap
3Cap
3Abp
3Bbp
3Cbp
3Acp
3Bcp
3Ccp
Br
Br
Br
Cl
Cl
Cl
F
19.8 340
22.6 391
23.2 479
37.1 373
32.1 240
38.8 489
18.9 24
34
8
48.8
63.8
5.1
7.5
3 (d)
46
5.2
25
19.5
0.2
6(d)
2 (d)
30.5
F
49.7
38.8
33.8 497
32.2 388
10.3
12.7
F
a Reaction conditions: PO 100 mmol; PCO2 = 1.2 MPa; catalyst 0.1 mmol; T = 25 oC.
b Turnover number.
c Moles of PC produced by per mole of catalyst per hour.
28

Table 3 The effect of counterions on the synthesis of chiral PO a
Substrate
Entry Catalyst
Time(h)
Yield(%)
ee% TON[b]
TOF[c]
(R =)
1
2
3
4
5
3Cbp
3Cbp
3Cbp
3Cbp
3Cbp
CH3
48.9
36.1
34.1
39.1
28.6
25
38.8 489
29.7 361
27.8 341
19.5
17.6
15.1
8.1
ClCH2
CH3(CH2)9
Ph
20.5
22.5
2(d)
1.3(d)
3
2
391
286
PhOCH2
9.1
a Reaction conditions: PO 100 mmol; PCO2 = 1.2 MPa; catalyst 0.1 mmol; T = 25 oC.
b Turnover number.
c Moles of PC produced by per mole of catalyst per hour.
29