The addition of carbon dioxide to epoxides was reported
as early as 1956 by Lichtenwalter and Cooper.8 As the result
of extensive catalyst optimization, it was found that a
combination of a Lewis acid catalyst and a nucleophilic
cocatalyst (binary system) is most effective for this purpose.9
This bifunctional mode of catalysis can be rationalized by
the reaction mechanism shown in Scheme 1.10
DIANANE-salen-chlorid I (Figure 1) in combination with
nBu4NCl, which provides a selectivity factor of 5.3.13
Scheme 1. Proposed Mechanism for PO/CO2 Coupling
Catalyzed by a Binary Systema
Figure 1. CrIII-DIANANE-salen-chlorid I and CoIII-DACH-salen-
tosylate II.
Lu et al. reported that the addition of CO2 to PO can also
be conducted at atmospheric pressure, with the binary system
CoIII-DACH-salen-tosylate II (Figure 1)/nBu4NCl.14 We
found that the enantioselectivity (at atmospheric pressure)
can be increased by carrying out the reaction at lower
temperature. At -50 °C, the ee of propylene carbonate (PC)
was found to be 87%, with s ) 15.0 (Figure 2). At that point
a LA: Lewis acid. LB: Lewis base.
Although the addition of CO2 to epoxides has been known
for quite some time, enantioselective versions are compara-
tively little studied. This is due to the fact that the
enantioselectivity suffers from the harsh reaction conditions,
such as elevated CO2 pressures and high temperatures.10a One
of the first and still best catalyst systems was reported by
Lu and co-workers. It operates under mild reaction conditions
(15 bar, 0 °C), providing a conversion of 40% with 70% ee
for the rac-PO/CO2 coupling.11 This corresponds to a
selectivity factor s of 9.0.12
In this paper, we report the addition of carbon dioxide to
propylene oxide at low temperatures and at atmospheric
pressure, in the presence of a catalytic amount of a chiral
Lewis acid complex and a nucleophilic cocatalyst. As it
turned out, the best selectivity factor obtained was 18.7 -
unprecedented for this type of reaction.
Figure 2. Enantiomeric purity of PC versus temperature at
atmospheric pressure.
We first carried out experiments at 13 bar CO2 pressure.
The best catalyst system under these conditions was the CrIII-
in time, these values already constituted the best enantiose-
lectivity and selectivity factor reported for the asymmetric
addition of CO2 to PO. To further increase the activity of
the catalyst system, and to develop a more efficient kinetic
resolution, we optimized the individual parts of the binary
catalyst system, starting with the Lewis acidic metal center
M (Figure 3).
Changing the metal to AlIII (3) or CrIII (4), or using CoII-
salen (2) is detrimental for the catalyst activity at atmospheric
CO2 pressure (Table 1, entries 1-3). We therefore main-
tained the CoIII ion and investigated the influence of the
counterion X, and that of the cocatalyst.
(6) Arakawa, H.; Aresta, M.; Armor, J. N.; Brateau, M. A.; Beckmann,
E. J.; Bell, A. T.; Bercaw, J. E.; Creutz, C.; Dinjus, E.; Dixon, D. A.; Dome,
K.; DuBois, D. L.; Eckert, J.; Fujita, E.; Gibson, D. H.; Goddard, W. A.;
Goodman, D. W.; Keller, J.; Kubas, G. J.; Kung, H. H.; Lyons, J. E.; Manzer,
L. E.; Marks, T. J.; Morokuma, K.; Nicholas, K. M.; Periana, R.; Que, L.;
Rostrup-Nielson, J.; Sachtler, W. M. H.; Schmidt, L. D.; Sen, A.; Somorjai,
G. A.; Stair, P. C.; Stults, B. R.; Tumas, W. Chem. ReV. 2001, 101, 953.
(7) Noyori, R. Chem. Commun. 2005, 1807.
(8) Lichtenwalter, M.; Cooper, J. F. U.S. Patent 2,773,070, Dec. 4, 1956.
(9) (a) Darensbourg, D. J.; Billodeaux, D. R. Inorg. Chem. 2005, 44,
1433. (b) Darensbourg, D. J.; Phelps, A. L. Inorg. Chem. 2005, 44, 4622.
(c) Peretti, K. L.; Ajiro, H.; Cohen, C. T.; Lobkovsky, E. B.; Coates, G.
W. J. Am. Chem. Soc. 2005, 127, 11566. (d) Cohen, C. T.; Chu, T.; Coates,
G. W. J. Am. Chem. Soc. 2005, 127, 10869. (e) Darensbourg, D. J.;
Mackiewicz, R. M.; Phelps, A. L.; Billodeaux, D. R. Acc. Chem. Res. 2004,
37, 836.
(10) (a) Shen, Y.-M.; Duan, W.-L.; Shi, M. J. Org. Chem. 2003, 68,
1559. (b) Huang, J.-W.; Shi, M. J. Org. Chem. 2003, 68, 1559. (c) Shen,
Y.-M.; Duan, W.-L.; Shi, M. Eur. J. Org. Chem. 2004, 69, 3080. (d)
Darensbourg, D. J.; Mackiewicz, R. M. J. Am. Chem. Soc. 2005, 127, 14026.
(11) Lu, X.-B.; Liang, B.; Zhang, Y.-J.; Tian, Y.-Z.; Wang, Y.-M.; Bai,
C.-X.; Wang, H.; Zhang, R. J. Am. Chem. Soc. 2004, 126, 3732.
(12) The selectivity factor, s, is determined by using the conversion,
C, and ee of the propylene carbonate product in the following equation:
s ) ln[1 - C(1 + ee)]/ln[1 - C(1 - ee)].
(13) For the synthesis and catalytic applications of DIANANE salen
complexes see: (a) Berkessel, A.; Schro¨der, M.; Sklorz, C. A.; Tabanella,
S.; Vogl, N.; Lex, J.; Neudo¨rfl, J. M. J. Org. Chem. 2004, 69, 3050. (b)
Berkessel, A.; Ertu¨rk, E. AdV. Synth. Catal. 2006, in press. (c) Berkessel,
A.; Vogl, N. Eur. J. Org. Chem. 2006, in press.
(14) Lu, X.-B.; Shi, L.; Wang Y.-M.; Zhang, R.; Zhang, Y.-J.; Peng,
X.-J.; Zhang, Z.-C.; Li, B. J. Am. Chem. Soc. 2006, 128, 1664.
4402
Org. Lett., Vol. 8, No. 20, 2006