Asymmetric Catalysis with CO2: Direct Synthesis of Optically Active Propylene Carbonate from Racemic Epoxides

Article abstract of DOI:10.1021/ja049734s

This communication describes a convenient route to optically active propylene carbonate by a catalytic kinetic resolution process resulting from the coupling reaction of CO₂ and racemic epoxides using simple chiral SalenCo(III)/quaternary ammonium halide catalyst systems. Copyright

Full text of DOI:10.1021/ja049734s

Published on Web 03/05/2004
Asymmetric Catalysis with CO2: Direct Synthesis of Optically Active
Propylene Carbonate from Racemic Epoxides
Xiao-Bing Lu,*,† Bin Liang,† Yin-Ju Zhang,† Yu-Zeng Tian,‡ Yi-Ming Wang,† Chen-Xi Bai,†
Hui Wang,† and Rong Zhang†
State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology, Dalian 116012, P. R. China, and
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China
Received January 15, 2004; E-mail: lxb-1999@163.com
Scheme 1
Chemical fixation of CO2 is of great interest in connection with
the development of a truly environmentally benign process, since
there are many possibilities for CO2 to be used as a safe and cheap
C1 building block in organic synthesis.1 One of the most promising
methodologies in this area is the synthesis of five-membered cyclic
carbonates via the coupling of CO2 and epoxides.2 These carbonates
are valuable as precursors for polymeric materials such as poly-
carbonates, aprotic polar solvents, pharmaceutical/fine chemical
intermediates, and in many biomedical applications.3 In recent
decades, numerous catalyst systems have been developed for this
transformation.4 The preparation of optically pure cyclic carbonates
is based on the cyclization of chiral diols with triphosgene5 or the
insertion of CO2 into chiral epoxides catalyzed by zinc(II)6a or
palladium(0) complexes.6b Also, optically active cyclic carbonates
process resulting from the coupling reaction of CO2 and racemic
could be obtained by enzyme-mediated enantioselective hydrolysis
epoxides using simple chiral SalenCo(III)/quaternary ammonium
of racemic cyclic carbonates7 or asymmetric hydrogenation of
halide catalyst systems.
5-methylene-1,3-dioxolan-2-ones catalyzed by chiral ruthenium
Indeed, the exploration of highly efficient catalysts for the
coupling reaction of CO2 and epoxides under low temperature and
complexes.8
Pursuant to our own efforts toward the development of highly
low CO2 pressure still remains a challenging problem.4c,d,12 For
efficient catalysts for coupling CO2 with epoxides under extremely
example, a recently reported catalyst system is Cr(III)Salen/DMAP,
mild conditions,9 we became interested in the possibility of
which operates at 25 °C and 100 psig CO2 pressure, but the
developing catalyst systems for direct synthesis of optically active
formation rate of propylene carbonate is less than 3 h-1 4k
We
.
cyclic carbonates from the racemic epoxides, which are inexpensive
or easily accessible from inexpensive commercial starting materials.
The key to accomplishing this goal is the rational design of
improved catalysts based on mechanistic understanding. A general
pattern in the coupling of CO2 with epoxides has emerged wherein
catalysis involves activation of both epoxides and CO2.4i Our recent
research found that the synergistic actions of an electrophile for
epoxide activation in conjunction with a nucleophile for CO2
activation were more effective in catalyzing this reaction.9,10 If an
electrophile can selectively complex one enantiomer of racemic
epoxides, the attack of nucleophile or activated CO2 to the
coordinated epoxide on the less substituted carbon regioselectively
will lead to enantioselective ring-opening of epoxides and then
further form chiral cyclic carbonates via intramolecular cyclic
elimination (Scheme 1). Recently, we noted that chiral SalenCo-
(III) complexes (Jacobsen catalyst) had been demonstrated to be a
highly efficient and enantioselective catalyst for the hydrolytic
kinetic resolution of terminal epoxides.11 In this system, one
SalenCo(III) molecule was proposed to serve as Lewis acid for
epoxide activation and another as counterion for the nucleophile.
The result stimulated us to design a bifunctional nucleophile-
electrophile catalyst system based on chiral SalenCo(III) complexes
for directly synthesizing optically pure cyclic carbonates from
racemic epoxides. Herein, we describe a convenient route to
optically active propylene carbonate by a catalytic kinetic resolution
delightfully found that the solvent-free reaction of racemic propy-
lene oxide (0.5 mol) with 0.55-0.60 equiv of CO2 in the presence
of 0.1 mol % of SalenCo(III)(O2CCCl3) complex 1b using as chiral
electrophile in conjunction with 0.1 mol % of n-Bu4NBr as
nucleophile proceeded within 2 h at room temperature to afford a
mixture of unreacted epoxide and propylene carbonate with
moderate enantioselectivity (Table 1, run 2), which could be
separated conveniently by simple fractional distillation. The residual
chiral SalenCo complex was easily recovered by suspension in
methanol and collection by vacuum filtration and could be recycled
with no observable loss in activity or enantioselectivity (run 3).
The formation rate of propylene carbonate is up to turnover
frequency (TOF) of 245 h-1. To the best of our knowledge, the
catalytic activity should be the highest for the coupling of CO2
with terminal epoxides under ambient temperature among the
reported catalysts, though organometallic Pd(0) complexes have
been demonstrated to be effective in coupling CO2 with the much
higher reactivity of vinyl epoxides under mild conditions.6b,12a If
only 1b was used as catalyst (0.1 mol %), no reaction occurred
(run 7). The coexistence of a quaternary ammonium salt such as
n-Bu4NBr is essential to promote this reaction.
It is interesting that the anion of quaternary ammonium salts in
the binary catalyst systems has a great effect on enantiomeric purity
and reaction rate (runs 1, 9, and 10). It seems that the use of n-Bu4-
NCl was more beneficial for improving enantiomeric purity of the
products, but had a pronounced negative effect on the rate. The
ratio of n-Bu4NCl to complex 1e had a significant effect on the
† Dalian University of Technology.
‡ Chinese Academy of Sciences.
9
3732
J. AM. CHEM. SOC. 2004, 126, 3732-3733
10.1021/ja049734s CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Racemic Propylene Oxide/CO2 Coupling Resultsa
was carried out with perfect atom economy under solvent-free and
extremely mild conditions. Further optimization of such bifunctional
nucleophile-electrophile catalyst systems and exploration into the
mechanism are now underway in our laboratory.
Acknowledgment. Gratitude is expressed to the National
Science Foundation of China (NSFC) program (Grant 20204002)
and the Science Foundation of Liaoning Province (Grant 20031074)
for financial support.
Supporting Information Available: Experimental procedures and
detailed results (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
cocat
equiv
time
[h]
conv
[%]
TOFb
PC ee
[%]
1
c
run
cat
cocat
[h-
]
Krel
1
1a
n-Bu4NBr
1
1
1
1
1
1
0
1
1
1
1
2
10
2
2
2
1
1
2.5
2.0
2.0
2.2
4.0
52.4
49.0
48.2
51.1
48.2
50.8
0
210
245
241
232
120
203
0
<1
160
91
126
187
240
316
73
42.9
50.5
51.1
46.1
39.8
51.6
-
3.9
4.8
4.8
4.3
3.3
5.2
-
-
3.0
5.7
6.4
6.0
5.8
2.8
7.2
9.0
1.1
2
1b n-Bu4NBr
1b n-Bu4NBr
1c
1d n-Bu4NBr
1e n-Bu4NBr
1b none
no
1a
1a
1e
1e
1e
1e
1e
1e
2
References
3d
4
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n-Bu4NI
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43.9
40.0
48.4
2.9
37.3
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56.7
57.2
55.9
35.2
63.5
70.2
2.9
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n-Bu4NCl
n-Bu4NCl
n-Bu4NCl
n-Bu4NI
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15.0
11.0
14.0
27
44
3
2
DMAP
-
-
a
Reaction was carried out in 35 mL (500 mmol) of neat propylene
oxide (PO), cat. (0.5 mmol, 0.001 equiv), CO2 (275-300 mmol, 0.55-
b
0.60 equiv) at 25 °C unless otherwise noted. The rate is expressed in
terms of turnover frequency [TOF, mol of product (mol of catalyst‚h)-1].
c
Krel ) ln[1 - c(1 + ee)]/ln[1 - c(1 - ee)], where c is the conversion
and ee is the enantiomeric excess of the resulting propylene carbonate (PC).
d
e
f
g
h
The chiral cobalt complex was recycled. 45 °C. 15 °C. 0 °C.
Reference 4k.
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11-13). Axial X-group of chiral SalenCo(III)X complexes also
affects enantiomeric purity of the resulted propylene carbonate and
propylene oxide. The chiral SalenCo(III)X complexes with a
sterically bulk axial X-group, such as p-toluene-sulfonate, are
essential for attaining high enantioselectivity in this reaction (runs
1-6). Runs 14-16 show the strong influence of reaction temper-
ature. The decrease of reaction temperature from 45 to 0 °C resulted
in TOF from 316 h-1 rapidly decreasing to 27 h-1 and Krel from
2.8 increasing to 9.0, respectively.
Unfortunately, chiral chromium analogues in conjunction with
n-Bu4NI could also catalyze this reaction with good reactivity, but
with only 2.9% enantioselectivity (run 17). Very recently, the
Darensbourg group reported that chiral SalenCr(III)Cl 2 alone or
accompanying N-methyl imidazole was an effective catalyst for
the coupling of propylene oxide and CO2 to afford poly(propylene
carbonate) as the dominant product at ambient temperature.13
However, with n-Bu4NI as cocatalyst, cyclic carbonate is the sole
product, even though the convertion of propylene oxide approachs
100%.
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In conclusion, a convenient route to optically active cyclic
carbonates by a catalytic kinetic resolution process resulting from
the coupling reaction of CO2 and racemic epoxides has been
discovered by using simple and highly efficient chiral SalenCo-
(III)/quaternary ammonium halide catalyst systems. The reaction
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