Highly active, binary catalyst systems for the alternating copolymerization of CO2 and epoxides under mild conditions

Article abstract of DOI:10.1002/anie.200453998

Excellent activity and selectivity in the copolymerization of CO ₂ with epoxides at extremely mild temperature and pressure are observed in the presence of binary nucleophile-electrophile catalyst systems based on chiral [(salcy)Co^IIIX] complexes and quaternary ammonium salts (see scheme). Completely alternating copolymers are obtained with > 95% head-to-tail linkages.

Full text of DOI:10.1002/anie.200453998

Communications
[
6–12]
heterogeneous catalysts.
Unfortunately, these catalysts
do not readily polymerize CO and aliphatic epoxides such as
2
[
13]
propylene oxide (PO). Recently, Coates and co-workers
reported the first highly active zinc b-diiminate catalysts
bearing electron-withdrawing groups for the synthesis of
poly(propylene carbonate) (PPC) with carbonate linkages up
to 99%, but along with the formation of cyclic propylene
carbonate (PC; 7–25%). A simple decrease in temperature
and increase in CO pressure suppressed the formation of PC
2
and increased the selectivity for PPC. A similar effect was
reported by Darensbourg and co-workers, who used chiral
salen chromium chloride, alone or in conjunction with N-
[
14]
methylimidazole as a catalyst, for this reaction. Herein, we
report the use of a highly active binary catalyst system, which
iii
consists of a chiral cobalt complex [(salcy)Co X] and a
quaternary ammonium salt (nBu NY) for completely alter-
4
nating copolymerization of CO and aliphatic epoxides under
2
extremely mild conditions.
Pursuant to our own efforts toward the development of
Copolymerization
highly efficient catalysts for coupling CO with epoxides
2
[
15]
under mild conditions,
we became interested in the
Highly Active, Binary Catalyst Systems for the
possibility of developing catalyst systems for the direct
synthesis of optically active cyclic carbonates from racemic
epoxides, which are inexpensive or easily accessible from
inexpensive commercial starting materials. Based on the fact
Alternating Copolymerization of CO and
2
Epoxides under Mild Conditions**
iii
that chiral [(salcy)Co ] complexes (Jacobsen catalyst) were
Xiao-Bing Lu* and Yi Wang
shown to be highly efficient and enantioselective catalysts for
[
16]
the hydrolytic kinetic resolution of terminal epoxides, we
recently designed a bifunctional nucleophile–electrophile
Chemical fixation of CO is of great interest with respect to
2
iii
the development of truly environmentally benign processes,
catalyst system consisting of chiral [(salcy)Co ] complexes
as there are many possibilities for CO to be used as a safe and
and quaternary ammonium salts for the direct synthesis of
2
[
1]
[17]
cheap C feedstock in organic synthesis. One of the most
optically active cyclic carbonates from racemic epoxides. In
1
iii
promising reactions in this area is the alternating copolymer-
this system, the chiral [(salcy)Co ] complex was proposed to
[
2]
ization of CO and epoxides to polycarbonates, which was
serve as an electrophile for selectively complexing one
enantiomer of the racemic epoxides, and the anion of the
quaternary ammonium salt serves as a nucleophile. The
2
[
3]
first reported by Inoue et al. These polycarbonates not only
exhibit interesting material properties, but also have the
additional environmental advantages resulting from their
biodegradability. In recent decades, numerous catalyst
systems have been developed for this transformation.
regioselective attack of a nucleophile or activated CO at the
2
[
4]
coordinated epoxide on the less substituted carbon atom leads
to enantioselective ring opening of the epoxide with subse-
quent formation of the chiral cyclic carbonates through
intermolecular cyclic elimination (Scheme 1). We were
delighted to find that the solvent-free reaction of racemic
PO (0.2 mol) with CO2 in the presence of [(salcy)-
[
5]
Although the advances have been significant, high catalyst
loading, elevated CO pressure, and long reaction time are
2
usually prerequisites for obtaining appreciable amounts of
polymer, which are often not perfectly alternating and exhibit
broad molecular-weight distributions. Recently, several effi-
iii
Co (O CCCl )] (1b; 0.05 mol%) as a chiral electrophile in
2
3
cient catalyst systems for the copolymerization of CO and
conjunction with nBu NBr (0.05 mol%) as nucleophile pro-
2
4
alicyclic epoxides such as cyclohexene oxide have been
reported to offer significant advantages over traditional
ceeded within 3 h at room temperature and 2.0 MPa pressure
to afford a mixture of unconverted PO and PC with moderate
enantioselectivity (Table 1, entry 2), along with a small
quantity of PPC. To our surprise, the resulting PPC shows
[*] Dr. X.-B. Lu, Y. Wang
1
State Key Laboratory of Fine Chemicals
Dalian University of Technology
Dalian 116012 (P. R. China)
Fax: (+86)411-8363-3080
> 99% carbonate linkage in the H NMR spectrum. Further
investigation showed that simple changes in the axial group X
iii
of chiral [(salcy)Co X] and the anion Yof nBu NY drastically
4
affect the PPC/PC selectivity (Table 1, entries 1–8). It seems
that the use of a cobalt complex with an electron-withdrawing
axial group X and a quaternary ammonium salt, whose anion
exhibits poor leaving ability, increases the selectivity for PPC.
E-mail: lxb-1999@163.com
[**] Gratitude is expressed to the NationalScience Foundation of China
(
NSFC) program (Grant 20204002) and the Science Foundation of
Liaoning Province (Grant 20031074) for financialsupport.
For example, in the presence of nBu NBr cocatalyst, a change
in the axial group X of chiral [(salcy)Co X] from acetate to
dinitrophenol resulted in an increase in the selectivity of PPC
Supporting information for this article (experimental procedures
and characterization of polymers) is available on the WWW under
http://www.angewandte.org or from the author.
4
iii
3
574
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200453998
Angew. Chem. Int. Ed. 2004, 43, 3574 –3577

Angewandte
Chemie
Scheme 1. A bifunctional nucleophile–electrophile catalyst system for the direct synthesis of enantiomerically pure cyclic carbonates or linear poly-
carbonates from racemic epoxides and CO2.
from 3 to 78%. On the other hand, with complex 1d as
pressure does not lead to an observable decrease in selectivity
À
catalyst, a change in the anion of cocatalyst nBu NY from Br
for PPC formation. In contrast, in the presence of [(bdi)-
4
À
À
[13]
to Cl or CH COO resulted in an increase in the selectivity
ZnOAc] catalyst (bdi = b-diiminate),
an increase in the
3
for PPC from 78 to 99%. The results may be explained in
terms of greater advantages for producing linear polycarbon-
pressure of CO is beneficial for suppressing PC formation
2
and effectively increases the selectivity for polymer while
moderately decreasing the catalyst activity for PPC forma-
tion. Surprisingly, a change in the reaction temperature from
25 to 408C, only significantly affects the activity of the
catalyst, rather than the selectivity for PPC formation
(Table 1, entry 16).
ates by successive insertion of CO /epoxide towards the M–O
2
bond as compared with the formation of cyclic carbonates by
intermolecular cyclic elimination (Scheme 1) in the catalyst
systems that consist of a strong electrophile in conjunction
with a nucleophile with poor leaving ability. Notably, these
binary catalyst systems based on chiral [{(1R,2R)-(tBu) sal-
cy}CoX] complexes all preferentially consume (S)-PO over
All the isolated polymers have narrow molecular-weight
distributions, consistent with controlled polymerization; how-
2
(R)-PO with Krel = 2.8–3.5. Notably, these cobalt complexes
ever, the M values are not close to the expected values, which
n
or quaternary ammonium salts by themselves show no (or
only very low) catalytic activity under the conditions
employed. Interestingly, the binary catalyst systems are
inactive for the homopolymerization of PO over 1 day.
indicates the existence of chain transfer during the reaction.
1
The H NMR spectra of all PPCs produced show > 99%
1
3
carbonate linkages, and the C NMR spectra show that these
polymers have an unprecedented head-to-tail content of
> 95% (Figure 1). Such a microstructure is consistent with a
highly stereoselective ring opening of PO, and is in contrast to
A significant drawback in the use of CO as a reagent in
2
organic synthesis is the potential danger associated with
operating at high temperatures and pressures. Prior to the
present work, Coates et al. reported the first discrete cobalt
complexes alone as catalysts for the copolymerization of CO2
[
19]
polymers prepared in the presence of zinc glutarate (~ 60%
head-to-tail linkages) and [(bdi)ZnOAc] (~ 54% head-to-tail
linkages). Also, the catalyst system was found to be applicable
to other aliphatic epoxides, providing the corresponding
polycarbonates with > 99% carbonate linkages (Table 1,
entries 17 and 18).
and PO, but these complexes are inactive at low CO
pressures. We were gratified to discover that the binary
catalyst system can operate very efficiently at low temper-
2
[
18]
iii
atures and with CO at atmospheric pressure. The highest
In summary, chiral [(salcy)Co X] in conjunction with a
2
activity occurs at 1.0–2.0 MPa and is maintained at 50% of
the optimum TOF at only 0.2 MPa. An increase in the
pressure beyond the optimal range results in a dramatic
decrease in activity. Interestingly, the variation of CO2
quaternary ammonium salt is a highly efficient catalyst for the
copolymerization of CO with epoxides under extremely mild
2
temperatures and pressures to afford the corresponding
completely alternating copolymers with > 95% head-to-tail
Angew. Chem. Int. Ed. 2004, 43, 3574 –3577
www.angewandte.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3575

Communications
[
a]
Table 1: CO /Epoxides copolymerization results.
2
[
b]
À1
[c]
[d]
À1
[e]
Entry
1
Y
Epoxide
t [h]
P [MPa]
TOF [h
]
Selectivity
Carbonate
linkages [%]
Mn [gmol
]
PDI(M /M )
Krel
w
n
[
c]
[%PPC]
[
[
f]
f]
[f]
[f]
1
2
3
4
5
6
7
8
9
a
b
d
d
d
d
c
Br
Br
Br
ClPO
I
PO
PO
PO
3
3
3
2.0
3
3
2.0
2.0
24
2.0
0.2
0.4
1.0
4.0
6.0
4.0
6
2.0
2.0
2.0
228
264
289
3
6
78
>99
>99
>99
>99
>99
>99
>99
>99
>99
–
–
–
–
3.9
4.7
3.2
3.4
2.8
3.1
2.9
3.0
23500
30400
22100
18200
21000
27500
1.29
1.36
1.34
1.23
1.48
1.43
3
257
99
PO
PO
2.0
2.0
272
167
69
99
OAc
ClPO
ClPO
–
ClPO
ClPO
ClPO
ClPO
ClPO
ClPO
ClPO
Cl1,2-BuO
Cl1,2-HO
3
3
185
248
96
99
e
[
f]
[f]
[f]
d
–
d
d
d
d
d
d
d
d
PO
2.0
2
<1
95
0
–
–
–
–
–
[f]
[f]
[f]
[f]
10
11
12
13
14
15
16
17
18
24
3
3
3
3
–
–
130
190
243
219
173
371
98
99
99
99
99
99
>99
>99
>99
>99
>99
>99
>99
>99
18200
28300
29900
28700
25500
24000
11600
7300
1.29
1.31
1.41
1.42
1.33
1.37
1.26
1.11
3.2
3.5
3.3
3.3
3.4
2.8
3
[
[
g]
g]
3
[
h]
[f]
2.0
2.0
61
48
99
98
–
[i]
[f]
8
–
[
[
a] The reaction was carried out with neat epoxide (14 mL, 200 mmol; catalyst/co-catalyst/epoxide=1:1:2000) at 258C, unless otherwise noted.
b] Turnover frequency of epoxide to products (polycarbonate and cyclic carbonate). [c] Determined by using H NMR spectroscopy. [d] Determined by
1
means of gelpermeation chromatography in THF at 35 8C, calibrated with polystyrene standards. [e] Krel =ln[1-c(1+ee)]/ln[1-c(1-ee)]; c=conversion,
ee=enantiomeric excess of unconverted epoxide. [f] Not applicable. [g] 408C. [h] 1,2-Butene oxide. [i] 1,2-hexene oxide.
merization. Furthermore, they offer unprecedented opportu-
nities for controlling the regio-and stereochemistry of
copolymerization. Further studies are focused on elucidating
the catalytic mechanism and developing new bifunctional
nucleophile–electrophile catalyst systems that exhibit higher
regio-and stereoselectivity for epoxide ring opening in CO 2/
epoxide copolymerization.
Received: February 11, 2004 [Z53998]
Keywords: carbon dioxide fixation · copolymerization ·
.
epoxides · homogeneous catalysis · polycarbonates
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ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 3574 –3577

Angewandte
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[
[
1
2
Angew. Chem. Int. Ed. 2004, 43, 3574 –3577
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ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3577