A novel and effective Ni complex catalyst system for the coupling reactions of carbon dioxide and epoxides

Article abstract of DOI:10.1039/b305617a

The coupling of carbon dioxide and mono-substituted terminal epoxides or cyclohexene oxide to form cyclic carbonates under a Ni complex catalyst system without using additional organic solvents was achieved in excellent selectivity and TOF.

Full text of DOI:10.1039/b305617a

A novel and effective Ni complex catalyst system for the coupling
reactions of carbon dioxide and epoxides
Fuwei Li, Chungu Xia,* Liwen Xu, Wei Sun and Gexin Chen
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 73000, P. R. China. E-mail: cgxia@ns.lzb.ac.cn; Fax: (+86)
931-827-7088
Received (in Cambridge, UK) 21st May 2003, Accepted 26th June 2003
First published as an Advance Article on the web 4th July 2003
The coupling of carbon dioxide and mono-substituted
terminal epoxides or cyclohexene oxide to form cyclic
carbonates under a Ni complex catalyst system without
using additional organic solvents was achieved in excellent
selectivity and TOF.
synthesis of cyclic carbonates from the cycloaddition between
carbon dioxide and terminal epoxides or cyclehexene oxide
under mild conditions. In this catalyst system, there is no co-
solvent and the seperation of product from catalyst is extremely
simple, therefore, it is an environmentally benign and green
catalyst process.
The search for environmentally benign and economic processes
has been the impetus for much of the research involving
epoxides and carbon dioxide coupling. During the last two
decades, the chemical fixation of carbon dioxide has received
much attention. This is in part due to the economic and
environmental benefits of carbon dioxide 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 cycloaddition
between epoxides and carbon dioxide to afford the five-
membered cyclic carbonates, which are excellent aprotic polar
solvents and are used extensively as intermediates in the
production of pharmaceuticals and fine chemicals.2 The
cycloaddition between epoxides and carbon dioxide is usually
carried out in conventional noxious organic solvents by various
catalyst systems.3 However, these catalyst systems all suffer
from either low catalyst stability/reactivity, the need for a co-
solvent, or the requirement for high pressure and/or high
temperatures. Recently, Nguyen and Shi reported that Cr(III),
Cu(II), Co(II) and Zn(II) salen-type metal complexes can
efficiently catalyze the reaction of epoxide and carbon dioxide
to synthesize cyclic carbonates in high TOF.4 But these catalyst
systems are all expensive and catalyst reactivities are not high
enough.
During our investigation of the cycloaddition between
propylene oxide 1a and carbon dioxide catalyzed by a Ni(II
)
complex in the presence of quaternary salts and the reducing
agent Zn powder (eqn. 1), we found that the propylene
carbonate 2a could be obtained in 99% yield and 3544 h21 TOF
without using any co-solvents (Table 1, entry 4). To the best of
our knowledge, this is the best result in this reaction. When the
reducing agent Zn powder was not used or there was only n-
Bu4NBr in the catalyst system, the reaction activity was poor
(Table 1, entries 1 and 2); the yield of the product was very low
in the absence of the quaternary salt (Table 1, entry 3); however,
increasing the quaternary salt from 1.0 equiv to 3.0 equiv of Ni
complex catalyst, respectively, the yields and TOF increase
markedly (Table 1, entries 5–7). We believe this may be due to
the quaternary salt taking part in the catalysis cycle and only the
Ni(0) complex generated in situ could not efficiently catalyze
the reaction. These results ensured it was the cooperative effect
between the Ni(0) complex generated in situ and the quaternary
salt which led to the very efficient reaction. As shown in Table
1 (entries 4, 8–9), the catalyst system is quite sensitive to
reaction temperature. When the temperature was decreased
from 120 °C to 90 °C, the yields and TOF decreased. Hence,
120 °C is the optimum reaction temperature in this catalyst
system. Besides these conditions, we also investigated the effect
of the different quaternary salts on the reaction: among n-
Bu4NBr, n-Et4NBr and n-Bu4NCl, n-Bu4NBr is the best
quaternary salt under the same conditions (Table 1, entries 5, 10,
11). The best reaction conditions are as follows: propylene
oxide (10.38 g, 0.179 mol), catalyst (0.028 mol%);
Ni(PPh3)2Cl2/PPh3/n-Bu4NBr/Zn = 1 : 2 : 4 : 20; time: 1 h; CO2
initial pressure: 2.5 MPa; reaction temperature 120 °C.10
De Pasquale disclosed a number of unsaturated nickel(0)
complexes which exhibited excellent reactivity.5 However, this
catalyst system needs noxious benzene as a co-solvent and the
reaction time is long; furthermore, the catalysts are very
sensitive to air and are expensive. This result stimulated us to
explore solvent-free, air-insensitive, cheap, shorter reaction
time and simple separation catalyst systems. Herein, we report
a very highly active, green Ni(II) catalyst system for the
Table 1 Effect of reaction parameters on the coupling of carbon dioxide and propylene oxidea
Selectivity
(%)
Entry
Catalyst
Quaternary saltb
T/°C
Yieldc (%)
TOFd/h21
1
2
3
4
5
6
7
8
9
Ni(PPh3)2Cl2/PPh3
—
n-Bu4NBr
n-Bu4NBr
—
120
120
120
120
120
120
120
110
90
12
5
2
98
98
97
430
179
72
3544
358
2220
3043
2971
1002
358
Ni(PPh3)2Cl2/PPh3/Zn
Ni(PPh3)2Cl2/PPh3/Zn
Ni(PPh3)2Cl2/PPh3/Zn
Ni(PPh3)2Cl2/PPh3/Zn
Ni(PPh3)2Cl2/PPh3/Zn
Ni(PPh3)2Cl2/PPh3/Zn
Ni(PPh3)2Cl2/PPh3/Zn
Ni(PPh3)2Cl2/PPh3/Zn
Ni(PPh3)2Cl2/PPh3/Zn
n-Bu4NBr
n-Bu4NBre
n-Bu4NBrf
n-Bu4NBrg
n-Bu4NBr
n-Bu4NBr
n-Et4NBr
n-Bu4NCl
99
10
62
85
83
28
10
66
100
100
100
100
100
100
100
100
10
11
120
120
2363
a Reaction conditions: propylene oxide (12.5 ml, 10.38 g, 0.179 mol), catalyst (32.7 mg, 0.028 mol%), Ni(PPh3)2Cl2/PPh3/n-Bu4NBr/Zn = 1 : 2 : 4 : 20, time:
1 h, CO2 initial pressure: 2.5 MPa. b Used 4.0 equiv of Ni catalyst. c Isolated yields. d Moles of propylene carbonate produced per mole of catalyst per hour.
e Used 1.0 equiv of Ni complex catalyst. f Used 2.0 equiv of Ni complex catalyst. g Used 3.0 equiv of Ni complex catalyst.
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This journal is © The Royal Society of Chemistry 2003

ring of the epoxides is low; it is the cooperative effect between
the Ni(0) complex and quaternary salts which leads to the very
efficient reaction.
(1)
In conclusion, Ni(PPh3)2Cl2/PPh3/Zn with n-Bu4NBr10 is a
highly efficient catalyst system in the cycloaddition of carbon
dioxide to epoxides under mild reaction conditions. It is an air
stable, easily synthesized, cheap, extremely robust and envir-
onmentally benign catalyst system, which is free of co-solvent
and can tolerate multiple substrates. These characteristics make
it an ideal catalyst system in terms of potential industrial
application in chemical carbon dioxide fixation. In addition, a
reaction mechanism has been proposed which plausibly ex-
plains the reaction result.
Under optimized reaction conditions, we next examined the
cycloaddition reaction of the other mono-substituted terminal
epoxides 1b–1e and cyclohexene oxide 1f with carbon dioxide.
The results are summarized in Table 2. From Table 2, we can
see that this catalyst system is very effective and almost all of
the mono-substituted terminal epoxides can be completely
transformed to the corresponding five-membered cyclic carbon-
ates 2b–2e in very high yields, selectivity and TOF. Cyclohex-
ene oxide was used to synthesis the corresponding cyclic
carbonate in 68% yield and 1519 h21 TOF, this is due to the
effect of high steric hindrace of cyclohexene epoxide. To the
best of our knowledge, this is the first report of the synthesis of
cyclic cyclohexylcarbonate 2f directely from cyclohexene
oxide and carbon dioxide in such a high selectivity. Judged by
GC and NMR, the purities of the produced cyclic carbonates are
> 99%.
From the results above mentioned, we deduced a plausible
mechanism for this reaction. The Ni(II) complex was reduced to
a Ni(0) complex by the reducing agent Zn powder,6,7 and the
Ni(0) complex Ni(PPh3)3 could efficiently activate the inert
carbon dioxide to afford a three-membered Ni(0) active
intermediate.5 In another catalytic cycle, the quaternary salts
open the ring of the epoxide by means of nucleophilic attack,
which leads to an oxyanion species,8 which attacks the three-
membered Ni(0) intermediate to afford another oxyanion
species which eventually yields the cyclic carbonate product.
This mechanism can explain why there is almost no reaction in
the absence of quaternary salts (Table 1, entry 3), while the
reaction is very efficient in the presence of quaternary salts
(Table 1, entry 4). Based on the above result, we think the Ni(0)
complex can activate carbon dioxide5,9 but its ability to open the
We thank the National Science Foundation of China for
financial support (29933050).
Notes and references
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Complexes, VCH Publishers, New York, 1988; (e) Carbon Dioxide
Fixation and Reduction in Biological and Model Systems, ed. C.-I.
Brauden and G. Schneider, Oxford University Press: Oxford, UK,
1994.
2 K. Biggadike, R. M. Angell, C. M. Burgess, R. M. Farrekk, A. P.
Hancick, A. J. Harker, A. J. Irving, W. R. Irving, C. Ioannou, P. A.
Procopiou, R. E. Shaw, Y. E. Solanke, O. M. P. Singh, M. A. Snowden,
R. Stubbs, S. Walton and H. E. Weston, J. Med. Chem., 2000, 43,
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H. Kisch, R. Millini and I. J. Wang, Chem. Ber., 1986, 119, 1090; (c) M.
Ratzenhofer and H. Kish, Angew. Chem., Int. Ed. Engl., 1980, 19, 317;
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Kang, Angew. Chem., Int. Ed., 2000, 39, 4096; (e) T. Aida and S.
Inooue, J. Am. Chem. Soc., 1983, 105, 1304; (f) K. Yamaguchi, K.
Ebitani, T. Yoshida, H. Ishiguro and K. Kaneda, J. Am. Chem. Soc.,
1999, 121, 4526; (g) H. Kawanami, A. Sasaki, K. Matsui and Y.
Ikushima, Chem. Commun., 2003, 896; (h) J. J. Peng and Y. Q. Deng,
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Table 2 Cycloaddition between CO2 and various epoxides catalyzed by the
Ni(II) complex in the presence of Zn powder and n-Bu4NBra
Entry
1
Epoxide
Product
Yieldb (%) TOFc/h21
98
94
96
94
3234
3122
2000
2066
2
3
4
9 (a) L. M. Venanzi, J. Chem. Soc., 1958, 719; (b) F. A. Cotton, O. D. Faut
and D. M. Goodgame, J. Am. Chem. Soc., 1961, 93, 344.
10 Representative procedure for the reaction of epoxide with carbon
dioxide: All cycloaddition reactions were performed in a 70 ml stainless
steel autoclave equipped with a magnetic stirrer. For each typical
reaction, Ni(PPh3)2Cl2 (32.7 mg, 0.05 mmol), PPh3 (26.2 mg, 0.1
mmol), n-Bu4NBr (64 mg, 0.2 mmol), Zn powder (65 mg, 1 mmol) and
12.5 ml propylene oxide 1a (10.4 g, 0.18 mol) were successively
charged into the reactor without using any additional solvent. The
reactor vessel was placed under a constant pressure of carbon dioxide
for 2 min to allow the system to equilibrate and then heated to 120 °C
for 1 h. After cooling to ambient temperature, the result mixture was
transferred to a 50 ml round bottom flask. Firstly, Unreacted propylene
oxide was removed in vacuo, then the product propylene carbonate 2a
was obtained as a colorless liquid. All the cyclic carbonates were
identified by GC/MS (HP6890/5973) and 400 MHz NMR.
5d
68
1519
a Reaction conditions: all the catalyst components are 1/3 equiv of the
optimum reaction conditions, epoxides 4.2 ml, the others are the same as in
Table 1, the product selectivities are all > 98%. b Isolated yields. c Moles of
carbonate produced per mole of catalyst per hour. d 4.0 equiv PPh3 and 8.0
equiv n-Bu4NBr of Ni complex catalyst were used.
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