Intramolecularly two-centered cooperation catalysis for the synthesis of cyclic carbonates from CO2 and epoxides

Article abstract of DOI:10.1016/j.tetlet.2008.09.035

A catalyst system containing an electrophilic center and a sterically hindered nucleophilic center in one molecule was applied to the cycloaddition reaction of CO₂ and epoxides. This intramolecularly two-centered cooperation catalyst showed activity even at a high [epoxide]/[catalyst] ratio up to 50 000 under mild conditions such as solvent-free, ambient temperature, and low CO₂ pressure. The reaction of CO₂ with (S)-propylene oxide at 80 °C in the presence of the bifunctional catalyst gives (S)-propylene carbonate in 96% ee with retention of stereochemistry.

Full text of DOI:10.1016/j.tetlet.2008.09.035

Tetrahedron Letters 49 (2008) 6589–6592
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Intramolecularly two-centered cooperation catalysis for
the synthesis of cyclic carbonates from CO2 and epoxides
a
a
a,b,
*
Xiang Zhang a, Yin-Bao Jia a, Xiao-Bing Lu a, , Bo Li , Hui Wang , Li-Cheng Sun
*
a State Key Laboratory of Fine Chemicals, Dalian University of Technology (DUT), 158 Zhongshan Road, 116012 Dalian, China
b KTH Chemistry, Organic Chemistry, Royal Institute of Technology, 10044 Stockholm, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 July 2008
Revised 5 September 2008
Accepted 9 September 2008
Available online 11 September 2008
A catalyst system containing an electrophilic center and a sterically hindered nucleophilic center in one
molecule was applied to the cycloaddition reaction of CO2 and epoxides. This intramolecularly two-cen-
tered cooperation catalyst showed activity even at a high [epoxide]/[catalyst] ratio up to 50000 under
mild conditions such as solvent-free, ambient temperature, and low CO2 pressure. The reaction of CO2
with (S)-propylene oxide at 80 °C in the presence of the bifunctional catalyst gives (S)-propylene carbon-
ate in 96% ee with retention of stereochemistry.
Ó 2008 Elsevier Ltd. All rights reserved.
Catalytic transformation of CO2 into useful organic compounds
has received much attention because of its potential use as an
abundant carbon source and its indirect role as an environmental
pollutant.1 The coupling of CO2 with epoxides affording cyclic car-
bonates is one of the most extensively studied reactions with
respect to the chemical fixation of CO2.2 These carbonates can be
used as aprotic polar solvents and as precursors for polymeric
materials.3 In recent decades, numerous catalyst systems including
amines or in conjunction with phenols,4 quaternary ammonium or
phosphonium salts,5 ionic liquids,6 alkali metal or zinc salts,7
organotin halides,8 carbonyl metal complexes,9 metal complexes
of porphyrin, phthalocyanine and Salen10–12 have been developed
for this transformation. Prominent among these are binary catalyst
systems, being the most efficient for the CO2/epoxides coupling
reported an elegant design of catalyst system containing a Lewis
acidic metal center and a quaternary ammonium salt unit in a
molecule, which showed high activity for polycarbonate synthesis
even at high temperature and high [epoxide]/[catalyst] ratio.14
Also, a similar bifunctional cobalt–Salen complex was designed
and used as catalyst for cyclic carbonates synthesis.15
In 2001, Nguyen group reported binary salenCr(III)Cl/ DMAP
catalyst systems for epoxides/CO2 coupling to cyclic carbonates,
in which a cooperative mechanism involving two different metal
species was proposed based on the requirement of both Lewis
base-activation of CO2 and Lewis acid-activation of the epoxide.12a
Unfortunately, the binary systems did not efficiently operate at
ambient temperature and low CO2 pressure. In the present Letter,
we report a catalytic system based on pyrrolidine SalenCr(III)X
complexes containing an electrophilic center (Lewis acid metal
ion) and a nucleophilic center (sterically hindered strong organic
base) in one molecule (Fig. 1a–c). The selective anchoring of steri-
cally hindered strong organic base such as 1,5,7-triazabicyclo-
[4.4.0] dec-5-ene (TBD) is based on the fact that CO2 can be
activated by bicyclic amidines through the formation of the zwit-
terionic compound, which can add to the epoxide via nucleophilic
attack.4a,16 Suppressed by steric repulsion, the organic base unit in
this complex cannot coordinate to the central metal ion, so we can
expect a high activity for the CO2/epoxides coupling by intramole-
cular cooperation catalysis of bifunctional electrophile–nucleo-
phile units even at high [epoxide]/[catalyst] ratio and under low
CO2 pressure.
reaction even at ambient temperature and low CO2 pressure.13
A
general pattern in the reaction of CO2 with epoxides has emerged,
wherein catalysis involves activation of both epoxides and CO2.
Our recent research found that the synergistic actions of an elec-
trophile in conjunction with a nucleophile were more effective in
catalyzing this reaction.12,13 Since the reaction rate is dependent
on the catalyst concentration, these binary systems usually show
substantial loss in activity at a high [epoxide]/[catalyst] ratio. We
believe that the nucleophilic attack of a nucleophile or an activated
CO2 at the activating epoxide, which could be activated by its coor-
dination to an electrophile, is similar to a bimolecular process
(Scheme 1). Therefore, the rate is directly proportional to the con-
centration of the nucleophile or the activated CO2 in connection
with the nucleophile and the activated epoxide with regard to
the concentration of the electrophile. Recently, Lee and co-workers
Since pyrrolidine SalenCr(III)X complexes 1a–e and 2a are eas-
ily dissolved in neat epoxides surveyed, the catalyzed coupling of
CO2 and epoxides does not require any organic co-solvent. Our ini-
tial studies showed that 1a, a Salen chromium complex with an an-
chored sterically hindered TBD on its pyrrolidine ring, could
effectively catalyze the cycloaddition of CO2 to propylene oxide
* Corresponding authors. Tel.: +86 411 88993864; fax: +86 411 83633080
(X.-B.L.).
E-mail address: lxb-1999@163.com (X.-B. Lu).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.035

6590
X. Zhang et al. / Tetrahedron Letters 49 (2008) 6589–6592
E
O
E
C
O
O
O
O
E
O
O
O
R
H
Ha
Hb
Ha
Hb
Ha
Hb
O
O
R
H
R
H
R
Ha
H
Hb
Nu
Nu
Nu
Path A
E
E
O
Nu
O
E
O
Ha
Hb
O
R
Ha
O
O
R
R
Hb
Ha
O
O
H
Hb
R
H
Hb
H
O
O
Ha
E
= Electrophile
Nu
O
O
Nu
Nu = Nucleophile
Path B
Scheme 1. The possible synergistic mechanisms of the cycloaddition of CO2 to epoxides catalyzed by binary electrophile–nucleophile systems.
Table 1
The catalyzed coupling of CO2 and propylene oxide (PO) at various conditionsa
n
N
N
N
PO/catal. (mol/mol) T (°C) P (MPa) t (h) TOFb (hÀ1
)
N
Entry Catal.
1
2
3
4
5
1a
2a
MTBD
1a
1a
1a
1a
2a/MTBD
5000
5000
5000
5000
20,000
50,000
20,000
5000
1000
500
50,000
5000
5000
5000
5000
5000
5000
5000
25
25
25
80
25
25
80
25
25
25
25
80
25
80
25
80
25
25
0.5
0.5
0.5
2.0
0.5
0.5
2.0
0.5
0.5
0.5
0.5
2.0
0.5
2.0
0.5
2.0
0.5
0.5
24
24
24
1
24
24
1
24
24
24
24
1
24
1
24
1
24
24
48
<1
<1
1936
46
49
1788
<1
6
10
42
1819
42
2120
<1
27
N
O
N
O
Nucleophilic center
tBu
1a:
Cr
tBu
X
n= 6, X = Cl;
1b: n= 6, X = BF4;
1c:
tBu
tBu
6
7
Electrophilic center
n= 1, X = Cl
8c
9c
10c
11
12
13
14
15
16
17
18d
2a/MTBD
2a/MTBD
N
N
1b
1b
1c
1c
1d
1d
1e
1a
N
N
6
N
N
N
<1
<1
N
N
O
N
O
N
O
Cr
Cr
Cl
tBu
O
tBu
tBu
tBu
a
The reaction was performed in neat propylene oxide.
The rate is expressed in terms of turnover frequency (TOF, moles of cyclic
Cl
1d
b
tBu
tBu
tBu
tBu
carbonate product per mole of catalyst per hour).
c
1e
Compound 2a/MTBD = 1:1 (molar ratio).
In the presence of 2 equiv N-methylimidazole.
d
N
[epoxide]/[catalyst] ratio between 5000 and 50000 did not lead to
an obvious change in activity (entries 1, 4–7). In contrast, with the
use of binary catalyst system consisting of complex 2a and MTBD,
the reaction rate was significantly dependent on the catalyst con-
centration (entries 8–10), and a complete loss in catalyst activity
was observed at 25 °C with [PO]/[catalyst] ratio of 5000. A long
induction period of about 3 h was observed from the plot of con-
version versus time at [PO]/[catalyst] ratio of 5000, at 40 °C and
under 2.0 MPa pressure, while with complex 1a as catalyst, no
induction period was observed under the same conditions
(Fig. 2). These results demonstrate that intramolecular two-cen-
tered cooperation catalysis of bifunctional units in complex 1a, in
which the central metal ion serves as an electrophilic center and
the anchored TBD as a nucleophilic center, is the sole factor for
maintaining activity of the catalyst at a high [epoxide]/[catalyst]
ratio. Recently, Heldebrant et al. have reported that they were
unable to spectroscopically observe adduct formation between 1,8-
dizazbicyclo[5,4,0]undec-7-ene (DBU, a sterically hindered strong
organic base) and CO2 under moderate CO2 pressure and at ambi-
ent temperature, while single crystals of the bicarbonate salt of
N
O
N
N
Cr
tBu
O
tBu
N
N
Cl
tBu
tBu
MTBD
2a
Figure 1. The structure of various pyrrolidine SalenCr(III)X complexes.
(PO) at a high [epoxide]/[catalyst] ratio of 5000 at ambient tem-
perature (Table 1, entry 1), while 2a or 7-methyl-1,5,7-triazabicy-
clo[4.4.0] dec-5-ene (MTBD) alone did not show any activity under
the same conditions (entries 2 and 3). No by-product such as poly-
carbonates or polyester was observed.
The reaction temperature had a strong influence on the rate. An
increase in the temperature from 25 to 80 °C resulted in a dramatic
increase of TOF from 48 hÀ1 to 1936 hÀ1. As expected, altering the

X. Zhang et al. / Tetrahedron Letters 49 (2008) 6589–6592
6591
Table 2
Reaction time (hour)
The coupling reaction of CO2 with various epoxides catalyzed by the complex 1aa
0
1
2
3
4
5
6
7
16
14
12
10
8
Entry
1
Substrate
Product
Time (h)
Yield (%)
TOF (hÀ1
)
O
O
1
66.7
3338
(A)
O
O
O
O
O
O
2
3
4
1
8
1
38.7
97.4
23.0
1936
609
6
(B)
CH3
4
CH3
O
O
O
2
O
O
O
0
O
O
O
O
1150
0
2
4
6
8
10 12 14 16 18 20
Reaction time (hour)
C2H5
CH2Cl
CH3
C2H5
Figure 2. Plots of conversion of the epoxide versus time, with the use of (A) 1a
alone, and (B) binary 2a/MTBD as catalyst for the cycloaddition of CO2 to PO at a
condition of [PO]/[catalyst] ratio of 5000, 40 °C and 2.0 MPa pressure.
O
5
1
1
30.0
30.0
1500
1498
CH2Cl
DBU were isolated in the presence of adventitious water.17 Darens-
bourg and co-workers further concluded from some control exper-
iments that bicarbonate generated by adventitious water was not
likely the cocatalyst in the catalyst systems which consisted of
SalenCr(III) and 4-(dimethylamino)pyridine.18 Indeed, although
every effort has been made to keep the coupling reaction anhy-
drous, we were concerned that trace quantities of water might
be present, thereby leading to the formation of the bicarbonate salt
of the anchored TBD under CO2 atmosphere, which might be
shown to have a deleterious effect on the reaction rate.
O
6
CH3
a
The reaction was performed in neat epoxide at 80 °C and 2.0 CO2 MPa, epoxide/
1a = 5000 (molar ratio).
sion (Table 2, entry 3). Notably, the reaction of CO2 with (S)-propyl-
ene oxide gave (S)-propylene carbonate in 96% ee with retention of
stereochemistry at 80 °C. However, complex 1a was inactive for
the coupling reaction of CO2 and cyclohexene oxide, perhaps due
to the steric hindrance of the epoxide, significantly affecting the
nucleophilic ring-opening of the coordinated cyclohexene oxide at-
tacked by sterically hindered nucleophilic center. An attempt for
direct synthesis of optically active cyclic carbonates from the race-
mic epoxides by a catalytic kinetic resolution process13 was also
performed in the cycloaddition of CO2 and PO in the presence of
chiral (R,R)-1a as catalyst, but the enantioselectivity of the resulted
propylene carbonate was less than 40% ee and the corresponding
Krel was in the range of 1.5–2.5.
In summary, a catalyst containing both an electrophilic center
and a sterically hindered nucleophilic center in one molecule was
synthesized and applied to the coupling of CO2 and epoxides. The
bifunctional catalyst system is active even at a high [epoxide]/[cat-
alyst] ratio up to 50000, while the conventional binary electro-
phile–nucleophile catalyst system completely loses its activity
under the same conditions. The reaction was carried out with per-
fect atom economy under solvent-free and very mild conditions
such as ambient temperature and low CO2 pressure. The design
of two-centered catalyst in the present study is in connection with
a green chemistry approach to catalysis: solvent-free and very low
catalyst loading.19 Further exploration of intramolecularly cooper-
ative catalysis for other applications is now underway in our
laboratory.
A change in the axial X group of pyrrolidine SalenCr(III)X from
ClÀ to BF4 (a non-nucleophilic anion, 1b) does not cause an obvi-
À
ous decrease in activity (entries 11 and 12). It is worth noting that
the properties of the spacer linked through the nitrogen atoms in
the backbone of the pyrrolidine Salen ligand and nucleophilic
TBD play an important role in maintaining intramolecularly two-
centered cooperation catalysis of the bifunctional chromium com-
plexes. A flexible spacer is usually a prerequisite for obtaining
excellent catalytic activity at high [epoxide]/[catalyst] ratio. The
complex 1d with the p-xylylene spacer hardly showed any activity
at a [epoxide]/[catalyst] ratio of 5000 (entries 15 and 16). These
results further indicate that the enhanced activity in the cyclo-
addition of CO2 to PO be completely ascribed to the synergistic
actions between the electrophilic metal ion and the nucleophilic
TBD in the complexes 1a–c.
Surprisingly, substituted imidazole (a coordination base, 1e) for
sterically hindered TBD anchored on the pyrrolidine ring of Sale-
nCr(III)X resulted in nearly complete loss in activity at ambient
temperature (entry 17). Interestingly, in the presence of 2 equiv
N-methylimidazole, the complex 1a also did not show any activity
for this reacton under the same conditons (entry 18). It could be
ascribed to the coordination of N-methylimidazole to the central
metal ion, thereby retarding the activation of the epoxide. These
results indicate that the steric repulsion originating from TBD
effectively inhibits its coordination to Lewis acidic metal ion,
thereby remains as intramolecular cooperative catalysis of nucleo-
philic–electrophilic centers during the reaction.
The complex 1a containing nucleophilic and electrophilic cen-
ters was found to be applicable to a variety of terminal epoxides,
providing the corresponding cyclic carbonates in high rate and
100% selectivity (Table 2). Among the epoxides surveyed, ethylene
oxide was found to be the most reactive, one and a TOF up to
3338 hÀ1 was observed in the coupling reaction of CO2 at 80 °C.
A prolonged time of 8 h nearly resulted in the quantitative conver-
Acknowledgments
We are grateful to Fok Ying Tung Education Foundation (Grant
No. 104023) and the program for New Century Excellent Talents in
University (NCET-05-0277) for the support of this research. X.-B. Lu
gratefully acknowledges the Outstanding Young Scientist Founda-
tion of NSFC (Grant No. 20625414).

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X. Zhang et al. / Tetrahedron Letters 49 (2008) 6589–6592
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