Chemical CO2 fixation: CR(III) salen complexes as highly efficient catalysts for the coupling of CO2 and epoxides

Full text of DOI:10.1021/ja0164677

11498
J. Am. Chem. Soc. 2001, 123, 11498-11499
Table 1. Effect of Reaction Parameters on the Coupling of CO2
and PO Catalyzed by Complexes 1a-da
Chemical CO2 Fixation: Cr(III) Salen Complexes as
Highly Efficient Catalysts for the Coupling of CO2
and Epoxides
Robert L. Paddock and SonBinh T. Nguyen*
Department of Chemistry and Institute for
EnVironmental Catalysis, Northwestern UniVersity
2145 Sheridan Road, EVanston, Illinois 60208-3113
DMAP
equiv
CO2
(psig) (°C)
temp time
total
TONb (h-1
)
TOFd
entry catalyst
(h)
1
2
3
4
5c
6
7
8
9
la
lb
lc
1
1
1
1
1
1
0
0.5
1
2
4
1
1
100
100
100
100
100
150
150
150
150
150
150
100
100
100
75
75
75
75
75
75
75
75
75
75
75
25
50
100
2
2
2
2
2
2
2
2
2
2
2
14
7
1
323
338
253
507
386
0
162
169
127
254
193
0
ReceiVed June 21, 2001
Carbon dioxide is an attractive C1 building block in organic
synthesis as it is highly functional, abundant, inexpensive,
nontoxic, and nonflammable. As petroleum reserves are depleted,
the development of efficient catalytic processes employing CO2
as a feedstock has become increasingly important as evidenced
by the intense research in this area in recent years.1-3 However,
due to the inert nature of CO2, efficient catalytic processes for
chemical fixation remain elusive. Thus, in addition to the practical
merit, chemical CO2 fixation remains a significant synthetic
challenge.
1d
1d
none
1d
1d
1d
1d
1d
1d
1d
1d
0
0
302
340
458
30
151
170
229
15
10
11
12
13
14
39
3
179
916
26
916
1
One of the most promising methodologies in this area has been
the synthesis of cyclic carbonates via the metal-catalyzed coupling
of CO2 and epoxides (eq 1).4 Cyclic carbonates are valuable as
monomers, aprotic polar solvents, pharmaceutical/fine chemical
intermediates, and in many biomedical applications.5,6 In recent
decades numerous catalyst systems have been developed for this
transformation.4,7-11 While the advances have been significant,
all suffer from either low catalyst stability/reactivity, air sensitiv-
ity, the need for co-solvent, or the requirement for high pressures
and/or high temperatures. For example, a recently reported catalyst
system is a mixed-metal Mg/Al oxide which operates at a
reasonable CO2 pressure (5 atm) but requires a substantial amount
of solvent (85% v/v DMF) and takes 24 h at 100 °C to convert
just 0.28 g of propylene oxide in 88% yield and 92% selectivity,
even with a very high catalyst loading of 1.8 g catalyst/ g of
substrate.12
a Reaction Conditions: PO (4 mL, 3.32 g, 5.72 × 10-2 mol), CH2C12
(0.5 mL), catalyst (0.075 mol %). b Moles of propylene carbonate
produced per mole of catalyst. c Reaction carried out in neat PO (4
mL). d Moles of propylene carbonate produced per mole of catalyst
per hour.
Cr(III) porphyrin complex as a catalyst for this reaction.13,14
Changing the coordination environment around the Cr metal
center from a porphyrin to a salen15 ligand offers several
advantages: (1) salens are easily and efficiently synthesized in
stark contrast to porphyrins, which are typically obtained in yields
of less than 20%,16 and (2) the modular construction of salens
from diamines and salicylaldehydes enables easy tuning of catalyst
steric and electronic properties. We were further encouraged by
several recent reports of the catalytic nucleophilic ring opening
of epoxides using transition metal salen complexes17-19 and felt
that salen complexes could potentially be useful for the electro-
philic addition of CO2 to epoxides.
Our initial studies showed that la successfully catalyzes the
coupling of CO2 and propylene oxide (PO) in the presence of a
Lewis-basic co-catalyst such as (4-dimethylamino)pyridine (DMAP).
No reaction was observed in the absence of DMAP, which is
consistent with that observed for the Cr porphyrin-catalyzed
reaction.13 It is important to note that DMAP by itself did not
catalyze the reaction. Reaction 2 was run in 1:1 (v/v) mixtures
of PO and various co-solvents; however, running the reaction in
neat PO gave the best yield of propylene carbonate (PC). This
yield could be increased further by the addition of a small amount
of CH2Cl2, presumably to help solubilize the catalyst system
(Table 1, entries 4-5). Several (salen)Cr(lII) complexes (la-d)
with varying diamine backbones were investigated as catalysts
for reaction 2. Complex 1d exhibited the highest catalytic activity
of all the catalysts and is at least twice as active as the racemic
trans analogue lc (Table 1 entries 1-4). We believe this may be
due to the more accessible coordination site available in complex
1d. The trans-cyclohexyl salen catalyst la and the propylene salen
catalyst 1b are also slightly more active than 1c. Complex 1d
Herein, we report a new highly active (salen)Cr(III)-based
catalyst system for the synthesis of cyclic carbonates from the
coupling of CO2 and terminal epoxides under extremely mild
conditions. Kruper et al. have previously reported the use of a
(1) Leitner, W. Coord. Chem. ReV. 1996, 155, 257-284.
(2) Behr, A. Carbon Dioxide ActiVation by Metal Complexes; VCH
Publishers: New York, 1988.
(3) Green Chemistry: Frontiers in Benign Chemical Syntheses and
Processes; Anastas, P. T., Williamson, T. C., Eds.; Oxford University Press:
New York, 1998.
(4) Darensbourg, D. J.; Holtcamp, M. W. Coord. Chem. ReV. 1996, 153,
155-174.
(5) Biggadike, K.; Angell, R. M.; Burgess, C. M.; Farrell, R. M.; Hancock,
A. P.; Harker, A. J.; Irving, A. J.; Irving, W. R.; Ioannou, C.; Procopiou, P.
A.; Shaw, R. E.; Solanke, Y. E.; Singh, O. M. P.; Snowden, M. A.; Stubbs,
R.; Walton, S.; Weston, H. E. J. Med. Chem. 2000, 43, 19-21.
(6) Shaikh, A.-A. G.; Sivaram, S. Chem. ReV. 1996, 96, 951-976.
(7) Ratzenhofer, M.; Kisch, H. Angew. Chem., Int. Ed. Engl. 1980, 9, 317-
318.
(13) Kruper, W. J.; Dellar, D. D. J. Org. Chem. 1995, 60, 725-727.
(14) Kruper et al. (ref 13) did not report precise conditions for their
reactions. Instead, they gave a range of conditions (i.e., 0.013-0.07 mol %
catalyst and 4-10 equiv co-catalyst, 60-100 °C, 730-780 psig, 18-64 h,
typically >95% yield).
(15) For the remainder of this article, the term salen will be used to describe
the general class of bis(salicylaldimine) ligands and complexes.
(16) Shanmugathasan, S.; Edwards, C.; Boyle, R. W. Tetrahedron 2000,
56, 1025-1046.
(8) Kihara, N.; Hara, N.; Harando, N. T. J. Org. Chem. 1993, 58, 6198-
6202.
(9) Kawanami, H.; Ikushima, Y. Chem. Commun. 2000, 2089-2090.
(10) Aida, T.; lnoue, S. J. Am. Chem. Soc. 1983, 105, 1304-1309.
(11) Kim, H. S.; Kim, J..; Kimee,. J. Lee, B. G.; Jung, O. S.; Jang, H. G.;
Kang, S. O. Angew. Chem., Int. Ed. 2000, 39, 4096-4098.
(12) Yamaguchi, K.; Ebitani, K.; Yoshida, T.; Yoshida, T.; Yoshida, H.;
Kaneda, K. J. Am. Chem. Soc. 1999, 121, 4526-4527.
10.1021/ja0164677 CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/30/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 46, 2001 11499
Table 2. Coupling of CO2 and Various Epoxides Catalyzed by
Complex 1da
was then employed to further investigate and optimize the effects
of the various reaction parameters on propylene carbonate
production.
The ratio of DMAP to complex 1d had a significant effect on
the turnover frequency (TOF) (Table 1, entries 6-11). The TOF
increased as the number of equiv of DMAP increased, up to two
equiv. Increasing the DMAP concentration any further resulted
in a loss of activity to the point where the reaction was almost
completely shut down when 4 equiv of DMAP were used.
A significant drawback associated with using CO2 as a reagent
in organic synthesis is the potential dangers associated with
operating at high temperatures and pressures. Thus, we were
gratified to discover that our catalyst system can operate very
efficiently at low CO2 pressures and temperatures. The highest
catalytic activity occurs at 50 psig and is maintained at 80% of
this optimum TOF at only 25 psig. Increasing the pressure beyond
the optimal level resulted in a dramatic decrease in activity (see
Supporting Information), which to the best of our knowledge is
the only instance in chemical CO2 fixation where an increase in
CO2 pressure causes a decrease in TOF.
Increasing the reaction temperature had a pronounced positive
effect on the TOF (Table 1, entries 4 and 12-14). However, we
were pleased to discover that the coupling of PO with CO2 could
be carried out at a reasonable rate at ambient temperature. The
catalyst system exhibits remarkable longevity. The activity does
not deteriorate over a period of 50 h, attaining over 8000 turnovers
(see Supporting Information).
a Reaction conditions: ld (27.8 mg, 3.81 × l0-5 mol), DMAP (4.6
mg, 3.81 × 10-5 mol), epoxide (3.81 × 10-3 mol), CO2 (50 psig),
CH2Cl2 (0.5 mL). b Yields were determined by comparing the ratio of
product to substrate in the 1H NMR spectrum of an aliquot of the
reaction mixture. c Reaction run in neat epoxide.
Scheme 1. Proposed Mechanism for Reaction 2
The catalyst system was found to be applicable to a variety of
terminal epoxides,20 providing the corresponding cyclic carbonates
in near quantitative yield and 100% selectivity (Table 2). Both
aromatic 2f and aliphatic 2a,d,e epoxides are good substrates for
reaction 1. Epichlorohydrin 2b was found to be the most reactive
epoxide, while vinyl epoxide 2c exhibited the lowest activity of
the epoxides surveyed.
Jacobsen et al. have shown that the nucleophilic ring opening
of epoxides catalyzed by Cr(III) salen complexes occurs via
catalyst activation of both the electrophilic epoxide and the
incoming nucleophile.21 Previous reports on the synthesis of cyclic
carbonate from CO2 and epoxides also suggest the parallel
requirement of both Lewis-base-activation of the CO2 and Lewis-
acid-activation of the epoxide.4,12,22 We propose that the starting
(salen)CrCI complex in our system fulfills the role of activating
the epoxide.21 The DMAP co-catalyst is necessary for the
formation of the more electron-rich Cr(III) center in the (salen)-
Cr(III)‚DMAP complex 4 which is required to activate CO2.
Complex 4 can then attack the activated epoxide complex 3 at
the least sterically hindered carbon,23 leading to the formation of
the dimeric intermediate 5 which eventually yields the cyclic
carbonate product (Scheme 1). This mechanism can explain the
dramatic losses in activity that accompany an increase, beyond
an optimal concentration, in either DMAP molar equivalents or
CO2 pressure. Both of these situations would lead to an increased
formation of 4 at the expense of 3, thereby reducing the reaction
rate. Detailed mechanistic studies will be reported in due course.
In summary, Cr(III) salen complexes are highly efficient
catalysts in the coupling of carbon dioxide with epoxides at mild
temperatures and pressures. They represent an air stable, easily
synthesized, extremely robust catalyst system which requires no
solvent and tolerates multiple substrates. These characteristics
make them ideal catalysts for chemical CO2 fixation.
(17) Wu, M. H.; Jacobsen, E. N. J. Org. Chem. 1998, 63, 5252-5254.
(18) Tokunaga, M.; Larrow, J. F.; Kakiuchi, F.; Jacobsen, E. N. Science
1997, 277, 936-938.
Acknowledgment. This work was supported by the EMSI program
of the NSF and the DOE (NSF No. CHE-9810378) at the Northwestern
University Institute for Environmental Catalysis. S.T.N. is an Alfred P.
Sloan Research Fellow. Additional support from the DuPont and Union
Carbide Companies and the Packard Foundation are appreciated. We thank
Dr. W. Jack Kruper and Professor Kenneth R. Poeppelmeier for helpful
discussions.
(19) Martinez, L. E.; Leighton, J. L.; Carsten, D. H.; Jacobsen, E. N. J.
Am. Chem. Soc. 1995, 117, 5897-5898.
(20) Cyclic epoxides such as cyclohexene oxide yield the corresponding
polycarbonates.
(21) Hansen, K. B.; Leighton, J. L.; Jacobsen, E. N. J. Am. Chem. Soc.
1996, 118, 10924-10925.
(22) Yano, T.; Matsui, H.; Koike, T.; Ishiguro, H.; Fujihara, H.; Yoshihara.
M.; Maeshima, T. Chem. Commun. 1997, 1129-1130.
(23) The coupling of chirally pure (R)-(+)propylene oxide with CO2 using
1d results in pure (R)-(+)propylene carbonate, a complete retention of
stereochemistry that is consistent with a ring-opening attack at the least
hindered carbon.
Supporting Information Available: Experimental procedures are
available (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA0164677