Highly Enantioselective Catalytic System for Asymmetric Copolymerization of Carbon Dioxide and Cyclohexene Oxide

Article abstract of DOI:10.1002/chem.201403088

A new ligand can be easily prepared, and its intramolecular dinuclear zinc complexes act as a high performance catalyst for the asymmetric alternating copolymerization of cyclohexene oxide and CO₂under very mild conditions (1 atm CO₂, room temperature), affording completely alternating polycarbonates with up to 93.8 % enantiomeric excess (ee) and 98 % yield. A high M_nvalue of 28 600 and a relatively narrow polydispersity (M_w/M_nratio) of 1.43 were also achieved.

Full text of DOI:10.1002/chem.201403088

DOI: 10.1002/chem.201403088
Communication
&
Copolymers
Highly Enantioselective Catalytic System for Asymmetric
Copolymerization of Carbon Dioxide and Cyclohexene Oxide
Yuan-Zhao Hua, Liu-Jie Lu, Pei-Jin Huang, Dong-Hui Wei, Ming-Sheng Tang, Min-
[a]
Can Wang,* and Jun-Biao Chang*
ity (73% ee) by using a catalytic amount of chiral catalyst
(Figure 1) at 408C and 30 atm CO . In the presence of addi-
Abstract: A new ligand can be easily prepared, and its in-
tramolecular dinuclear zinc complexes act as a high per-
formance catalyst for the asymmetric alternating copoly-
1
2
tional ethanol, the enantioselectivity increased up to 80%
[2c]
ee. Subsequently, Coates et al. developed chiral zinc oxazo-
merization of cyclohexene oxide and CO under very mild
2
line catalyst 2 (Figure 1), which demonstrated similar enantio-
conditions (1 atm CO , room temperature), affording com-
2
selectivity (72% ee) at 208C and 6.8 atm CO , for this copoly-
2
pletely alternating polycarbonates with up to 93.8% enan-
[2b]
merization.
Lu et al. reported chiral (R,R)-salen cobalt(III)
tiomeric excess (ee) and 98% yield. A high M value of
n
complex 3a (Figure 1) for the asymmetric copolymerization of
2
8600 and a relatively narrow polydispersity (M /M ratio)
w n
CHO with CO in the presence of quaternary ammonium salt
2
of 1.43 were also achieved.
[2g,h]
co-catalysts.
This method exhibited lower enantioselectivity
Optically active polymers are considered as candidates for new
and valuable materials because of the well-defined chemical
[1]
structures and special physical properties of these polymers.
Catalytic asymmetric polymerization is an efficient method to
prepare chiral polymers from achiral monomers because
a small amount of chiral catalyst repeatedly activates prochiral
molecules, controlling the stereochemistry to yield a large
amount of chiral polymer. Recently, asymmetric copolymeriza-
[
2]
tion of cyclohexene oxide (CHO) and CO2, affording chiral
polycarbonates, has attracted much attention for several rea-
sons: 1) CO is a relatively nontoxic, naturally abundant one-
2
carbon chemical feedstock, and is also the major greenhouse
gas; 2) meso-epoxides, such as CHO are an ideal substrate for
[
3]
desymmetrization using chiral catalysts; 3) the resulting chiral
polycarbonates not only have a relatively high glass-transition
[4]
temperature and tensile strength, but are also degradable,
giving optically active cyclohexane-1,2-diol.
Since the initial report by Inoue et al. on the copolymeriza-
[
5]
tion of CO with epoxides in 1969, this type of copolymeriza-
2
tion reaction has been studied extensively, and great progress
[
6]
has been made. However, very few examples of the asym-
Figure 1. The structures of chiral catalysts 1–6.
metric copolymerization of CO and CHO have been report-
2
[
2]
[2a]
ed. In 1999, Nozaki et al. described the first asymmetric co-
polymerization of CHO and CO with moderate enantioselectiv-
2
[2h]
(
up to 38% ee) at 258C and 6 atm CO2. A moderate to good
enantioselectivity (60–80% ee) was also observed in the co-
[
a] Y.-Z. Hua, L.-J. Lu, P.-J. Huang, D.-H. Wei, Prof. M.-S. Tang,
Prof. Dr. M.-C. Wang, Prof. Dr. J.-B. Chang
College of Chemistry and Molecular Engineering
Zhengzhou University
polymerization of CO and CHO catalyzed by optically active
2
dinuclear aluminum complexes 5, in conjunction with a bulky
[2j]
Lewis base as catalyst activator. General drawbacks of cur-
rently existing methods include: 1) Relatively low enantioselec-
tivity, which is in great contrast to the highly enantioselective
desymmetrization of meso-epoxides (>90% ee) with chiral cat-
7
5, Daxue Street, Zhengzhou City, 450052 (P.R. China)
Fax: (+86)371-67767895
E-mail: wangmincan@zzu.edu.cn
changjunbiao@zzu.edu.cn
[7]
alysts; 2) substantial pressures of CO , significantly increasing
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403088.
2
the overall energy requirement of the process.
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Communication
Recently, Lu et al. have made great progress in the highly
[
8]
enantioselective copolymerization of CO with CHO. They re-
2
ported a highly enantioselective catalyst system for this asym-
metric copolymerization by using the dissymmetric chiral (S,S)-
salen cobalt(III) complex 3b, bearing an adamantyl group on
[
8a]
the phenolate ortho position. It was found that the presence
of stoichiometric quantities of a chiral induction agent
Scheme 1. Synthesis of the chiral ligand 9.
(3 equivalents), such as (S)-propylene oxide or (S)-2-methylte-
trahydrofuran, could significantly improve the enantioselectivi-
ty. In the presence of (S)-2-methyltetrahydrofuran (3 equiva-
lents), at À258C and 8 atm CO , up to 96% ee was achieved in
The chiral ligand 9 was easily prepared from azetidin-2-yl-
(diphenyl)-methanol 7 and 2,6-bis(bromomethyl)-p-cresol 8, ac-
cording to a procedure by Trost et al. Initially, we conducted
2
the resultant polycarbonates, although this asymmetric alter-
[10]
nating copolymerization showed low conversions (45%) and
À1
lower M values (less than 10000 gmol ). A high enantioselec-
the copolymerization of CO and CHO under the conditions re-
n
2
[2d]
tivity of up to 98% ee was also observed by the same group in
the presence of dinuclear cobalt(III) complexes (S,S,S,S)-6 at
ported by Ding et al. The copolymerization reaction was car-
ried out at one atm of CO , in toluene at 608C, for 36 h in the
2
[
8b]
0
8C and 20 atm CO2.
Therefore, it is highly desirable that a new catalytic system,
presence of 5 mol% of the ligand 9, 10 mol% ZnEt , and
2
2 mol% EtOH. Gratifyingly, the chiral compound 9 showed sim-
using only a catalytic amount of chiral ligand and no additional
chiral induction agent, is developed for the copolymerization
ilar activity to that of the ligand by Trost et al., giving poly(cy-
clohexene carbonate, PCHC) 10 in a quantitative yield (>99%)
À1
À1
of CO and CHO, resulting in a high M value (>20000 gmol )
and with a relatively high M value of 17000 gmol and a rela-
2
n
n
and providing high catalytic performance, enantioselectivity,
and chemical yield under mild conditions. Meeting these goals
is challenging, and focuses mainly on catalyst design.
tively narrow polydispersity (M /Mn ratio) of 1.21 (Table 1,
w
entry 1). The enantioselectivity of the copolymerization reac-
tion was also determined by a procedure described by Nozaki
[2a]
Based on the mechanistic understanding that a dimeric zinc
complex was involved in the transition state of the epoxide
et al. The treatment of PCHC 10 with aqueous NaOH afford-
ed cyclohexane-1,2-diol 11 in 85% yield, followed by the trans-
formation of 11 into dibenzoate 12. The ee value of dibenzoate
12 was determined by HPLC analysis with a chiral column
(Sino-Chiral AS). To our delight, the enantioselectivity of the re-
sulting dibenzoate 12 was up to 86.7% ee with (S,S)-configura-
tion (Table 1, entry 1).
ring-opening event during CO /CHO copolymerization, Ding
2
[
2d,f]
et al. developed a chiral dinuclear metal catalyst 4,
which
was coordinated with Trost’s multidentate ligand. This catalyst
exhibited moderate activity and very low enantioselectivity (8–
1
8% ee) for copolymerization of CHO and CO , but most inter-
2
estingly, the catalyst was more active under only
one atm CO2 pressure. We believe that the main
reason for the low ee value could be the mismatch of
chiral microenvironment. The development of new
chiral ligands plays a key role for overcoming this
limitation because subtle changes in conformational,
steric, and/or electronic properties of the ligands can
often result in dramatic variation of the enantioselec-
tivity.
[a]
2
Table 1. Asymmetric alternating copolymerization of CO and CHO.
[
c]
Entry Solvent
T
t
n
Yield of 10 M of PDI Yield of 11 Yield of 12 ee of 12
[
b]
[c]
[b]
[b]
[d]
[
8C] [h] [%]
10
[%]
[%]
[%]
In recent years, we have explored the use of chiral
small-ring heterocycle ligands in catalytic asymmetric
1
2
3
4
5
6
7
8
9
PhCH
hexane 60 36
CCl 60 36
benzene 60 36
3
60 36 >99
17000 1.21 85
12900 1.12 79
13300 1.17 83
18700 1.33 97
9550 1.08 87
12500 1.17 96
11300 1.17 96
11200 1.18 87
13100 1.19 80
11200 1.21 89
28200 1.33 97
26000 1.46 90
28600 1.43 97
29800 1.15 91
83
97
93
80
77
79
83
90
97
98
93
99
96
99
86.7
63.7
74.3
48.8
85.6
68.6
75.3
80.2
71.0
69.6
89.4
90.9
93.8
93.3
66
80
73
40
87
87
83
[
9]
synthesis. We have found that among these chiral
nitrogen heterocycles containing a b-amino alcohol
moiety, the use of four-membered heterocycles as
chiral ligands affords the best enantioselectivity in
the catalytic asymmetric addition of diethylzinc to
benzaldehyde. This activity is owing to the relatively
rigid ligand skeleton and appropriate chiral microen-
vironment provided by the four-membered heterocy-
4
DCE
60 36
60 24
60 12
PhCH
PhCH
PhCH
3
3
3
60
6
PhCH3
60 48 >99
10
PhCH
PhCH
PhCH
PhCH
3
3
3
3
80 36
40 36
30 36
20 36
10 36
79
96
99
98
99
1
1
1
1
14
2
3
[
9a]
cle. Herein, we report the synthesis of new chiral
ligand 9 with a more rigid azetidine ring compared
with that of pyrrolidine (Scheme 1), and its applica-
tion in the asymmetric copolymerization of CHO and
PhCH3
[
a] All reactions performed by using 5 mol% ligand 9, 10 mol% ZnEt
EtOH, 1 atm CO pressure. [b] Yield of isolated product. [c] Estimated by gel-permea-
2
2
, and 2 mol%
CO with excellent enantioselectivity of up to 93.8%
tion chromatography using a polystyrene standard. [d] Determined by HPLC analysis
with a chiral column (Sino-Chiral AS). The product chromatograms were compared
against a known racemic mixture.
2
ee.
&
&
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Communication
To explore the effect of solvents on the efficiency and enan-
tioselectivity of the copolymerization reaction, we varied the
tion of solvent. The direct determination of the identity of this
solid by using electrospray mass spectrometry showed that
a series of peaks between m/z 857–865 were consistent with
solvent from toluene to hexane, CCl , benzene, or 1,2-dichlor-
4
+
ethane (DCE) (Table 1, entries 2–5), among which toluene was
found to be the best solvent with respect to both yield and
enantioselectivity. DCE afforded high ee values (85.6%), but
the yield of copolymerization was very low (40%).
the formula C H N O Zn , corresponding to the [M+H]
48
44
2
5
2
peak of 14, containing a benzoic acid anion, whereas a series
of peaks between m/z 979–987 were in accordance with the
+
formula C H N O Zn , corresponding to the [M+H] peak of
55
50
2
7
2
The effect of reaction time on the efficiency and enantiose-
lectivity of the copolymerization reaction was also examined
15, containing a benzoic acid anion and a benzoic acid mole-
cule. ESI-MS studies indicated that the zinc complex involved
an intramolecular dinuclear structure. Although the precise
structures of the dinuclear zinc complexes 14 are currently not
known, we favor the carboxylate-bridged dinuclear zinc struc-
ture, as depicted in 14, because some related carboxylate-
bridged dinuclear zinc complexes have already been deter-
(Table 1, entries 1 and 6–9). Reducing the reaction time from
36 to 24, 12, and 6 h led to a decrease in the reaction yield
and the M value. These results suggested that a chain-transfer
n
reaction existed. However, the enantioselectivity showed no
obvious regular change with the variation of reaction time.
Temperature proved to have a distinctive effect on the ee
value. An increase in temperature from 60 to 808C resulted in
a decrease in enantioselectivity from 86.7 to 69.6% ee (Table 1,
entries 10 and 1), whereas decreasing the reaction temperature
from 60 to 408C led to an enhancement in enantioselectivity
from 86.7 to 89.4% ee (Table 1, entries 11 and 1). Lowering the
temperature to 208C resulted in an ee value of up to 93.8%
[12]
[2d,13]
mined or proposed.
Compared with the pyrrolidine-based ligand of the same
type, azetidine-based ligand 9 showed a substantial improve-
ment in enantioselectivity. The only difference between these
ligands is the ring size of the pyrrolidine and azetidine. Ligand
9 possesses a more rigid-ring backbone and is more sterically
congested than five-membered heterocycle-based ligands.
Therefore, the intramolecular dinuclear zinc structure of the
catalytically active species containing an azetidine ring is more
rigid than that containing a pyrrolidine ring. This rigidity is re-
sponsible for the large improvement in enantioselectivity.
To obtain more information about the active species and the
origins of enantioselectivity, theoretical calculations were car-
(Table 1, entry 13). On decreasing the reaction temperature to
108C, the alternating copolymerization showed almost the
same enantioselectivity as that at 208C (93.3% ee and 93.8%
ee, respectively).
Because the resultant chiral poly(cyclohexene) carbonate
was easily hydrolyzed to trans-cyclohexane-1,2-diol by using
aqueous NaOH, this highly enantioselective copolymerization
ried out to study the intramolecular S 2 attack of the carbon-
N
[14]
reaction of CHO and CO offers a promising alternative strat-
ate ester at the back side of the cis-epoxide. This step is the
2
[
5,11]
egy for the synthesis of chiral cyclohexane-1,2-diol.
stereoselectivity-determining and rate-determining step ac-
[15]
[16]
To demonstrate that the zinc complex involved an intramo-
lecular dinuclear structure, which is critical for two proximal
zinc species to activate the substrates cooperatively during the
catalysis, the zinc complex was prepared by reacting 9 with di-
ethylzinc in toluene, followed by benzoic acid (Scheme 2). The
resulting solution was left to stand for several days in order to
obtain a single crystal of the dinuclear zinc complex 14.
Although numerous attempts to isolate 14 proved to be un-
successful, a pale yellow solid was afforded after slow evapora-
cording to the reports of Rieger et al. and Williams et al.
We optimized the transition states and calculated the free en-
ergies of the S 2 reaction step in toluene at the wB97XD/6-
N
[17]
31G(d) level by using the conductor-like polarizable continu-
[18]
um model (CPCM) method, which was successfully applied
[16]
by Williams et al. on the study of similar reaction mecha-
nisms. Figure 2 illustrates the optimized structures of the two
key transition states (TS), that is, TS-re (associated with the
Scheme 2. The possible structures of intramolecular dinuclear zinc com-
plexes.
Figure 2. The structures of intramolecular S
the wB97XD/6-31G(d) level.
N
2 transition states optimized at
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Communication
(
R,R)-product) and TS-si (associated to the (S,S)-product). All cal-
In conclusion, a novel ligand with the azetidine ring shows
excellent enantioselectivity for the catalytic asymmetric copoly-
[
19]
culations were carried out with Gaussian 09 programs.
The calculated results shows that TS-si, which formed the ex-
merization of CO and CHO catalyzed by dinuclear zinc(II). The
2
perimentally observed major (S,S)-product, was favored, by
salient features of this catalytic system include: 1) The alternat-
ing copolymerization required only a catalytic amount of the
chiral ligand without the presence of stoichiometric amounts
of additional chiral induction agents; 2) this catalytic system
needed only one atm pressure and room temperature; 3) ex-
cellent enantioselectivity, very high yields, and relatively high
molecular weights were obtained. The application of this
ligand for other asymmetric transformations, as well as the de-
termination of the precise structure of the active species for
À1
1.9 kcalmol , compared to TS-re (Figure 2). This energy value
is in agreement with an ee value of approximately 92.2% (see
the Supporting Information), which is very close to the value
obtained experimentally (93.8%, Table 1, entry 13).
Based on the model of the transition state for the azetidine-
based catalyst system, the prediction of the pyrrolidine-based
system was attempted. The energy difference between the
transition states for the (R,R)- and (S,S)-products in the azeti-
À1
dine system is higher, by 0.89 kcalmol , than the energy dif-
CHO/CO copolymerization, is currently underway in our labo-
2
ference in the pyrrolidine system (see the Supporting Informa-
tion). This finding suggests that the azetidine-based catalyst
can afford higher enantioselectivity than the pyrrolidine-based
ratory.
catalyst. This outcome is in accordance with the experimentally Experimental Section
observed result.
General procedure for the copolymerization of CO and CHO
2
The chemical structure and physical properties of the CHO/
CO copolymer formed at 108C (Table 1, entry 14) were investi-
In a flame-dried Schlenk tube, a solution of diethylzinc (0.20 mL,
2
À1
1
.0 molL in hexane, 0.20 mmol) was added to a solution of the
gated. Previous studies demonstrated that stereochemical in-
formation of copolymers can be conveniently collected by
chiral ligand 9 (61.2 mg, 0.1 mmol) in dry toluene (2.0 mL) under
13
[20] 13
nitrogen. The mixture was stirred at room temperature for 30 min.
C NMR spectroscopy.
C NMR signals of poly[cyclohexene
À1
Then a solution of dry ethanol (0.04 mL, 1.0 molL in toluene) was
oxide-alt-carbon dioxide], in the carbonyl region, were split
into two parts: d=153.7 and 153.3–153.1 ppm. The large peak
at d=153.7 ppm was assigned to isotactic diads, whereas the
peaks at d=153.3–153.1 ppm were attributed to syndiotactic
diads. In this case, the absence of any peak at d=153.3–
added to the mixture. The solution was stirred at ambient temper-
ature for an additional 15 min, and then cyclohexene oxide
(0.2 mL, 2.0 mmol) was introduced. Carbon dioxide was bubbled
into the solution whilst stirring at the necessary reaction tempera-
ture for 36 h. The mixture was then cooled/heated to room tem-
perature. The mixture was diluted with CH Cl (40 mL), and washed
153.1 ppm, as shown in Figure 3(a), indicated clearly that iso-
2
2
À1
with aqueous HCl (1 molL , 3ꢁ20 mL) and brine (2ꢁ20 mL). The
organic layer was dried over Na SO and concentrated to 5 mL by
tactic diads were absolutely predominant in this asymmetric
13
2
4
copolymerization of CO and CHO. The C NMR spectrum, in
2
vacuum evaporation. The copolymer was precipitated by adding
MeOH (40 mL), filtered, washed with MeOH, and dried in vacuo to
a constant weight. Analysis of the copolymer by gel-permeation
chromatography gave molecular weight and molecular-weight dis-
tribution.
the carbonyl region, of the copolymer was also consistent with
the excellent enantioselectivity (up to 93.3% ee). In addition,
1
the H NMR spectrum of the copolymer in Figure 3(b) shows
that the chemoselectivity was also outstanding. The complete-
ly alternating structure (>99% carbonate linkage) of the PCHC
was confirmed by the absolute predominance of a methine
proton peak at d=4.62 ppm (assigned to carbonates), and the
absence of a methine proton peak around d=3.45 ppm (at-
tributed to a polyether from homopolymerization of the
Acknowledgements
We are grateful to the National Natural Sciences Foundation of
China (NNSFC: 21272216, 20972140), and the Department of
Science and Technology of Henan Province for financial sup-
port.
[
21]
CHO). Differential scanning calorimetry analysis of the copo-
lymer showed that a high crystallization endothermic peak was
observed at 2408C, implying that the PCHC with over 90%
[
8]
enantioselectivity exhibited crystallization.
Keywords: asymmetric copolymerization · carbon dioxide ·
cyclohexene oxide · polycarbonates · zinc
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Received: April 14, 2014
2687; b) L. Zhao, B. Han, Z. Huang, M. Miller, H. Huang, D. S. Malashock,
Published online on && &&, 0000
Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
&
Copolymers
Y.-Z. Hua, L.-J. Lu, P.-J. Huang, D.-H. Wei,
M.-S. Tang, M.-C. Wang,* J.-B. Chang*
&& – &&
Highly Enantioselective Catalytic
System for Asymmetric
Copolymerization of Carbon Dioxide
and Cyclohexene Oxide
Highly enantioselective copolymeriza-
tion of carbon dioxide with cyclohexene
oxide has been achieved under very
rings have been used to catalyze this
transformation, affording completely al-
ternating polycarbonates with excellent
enantioselectivity (up to 93.8% ee),
mild conditions (1 atm CO , room tem-
2
perature; see scheme, ee=enantiomeric
excess). Intramolecular dinuclear zinc
complexes containing four-membered
a high M value of 28600, and a relative-
n
ly narrow polydispersity (M /M ratio) of
w
n
1.43.
&
&
Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!