Enantioselective Resolution Copolymerization of Racemic 2,3-Disubstituted cis-Epoxides with CO2 Mediated by Binuclear Cobalt(III) Catalyst?

Article abstract of DOI:10.1002/cjoc.202100252

Enantioselective resolution copolymerization of racemic internal epoxides with carbon dioxide (CO₂) is a challenging issue because of their poor reactivity and complicated regio/stereoselectivity. Herein, we describe the first enantioselective

Full text of DOI:10.1002/cjoc.202100252

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Title: Enantioselective Resolution Copolymerization of Racemic 2,3-Disubstituted
cis-Epoxides with CO2 Mediated by Binuclear Cobalt(III) Catalyst
Authors: Yan-Lan Liu, Guang-Hui He, Ye Liu, and Xiao-Bing Lu*
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Cite this paper: Chin. J. Chem. 2021, 39, XXX—XXX. DOI: 10.1002/cjoc.202100XXX
Enantioselective Resolution Copolymerization of Racemic
2,3-Disubstituted cis-Epoxides with CO2 Mediated by Binuclear
Cobalt(III) Catalyst
Yan-Lan Liu, Guang-Hui He, Ye Liu, and Xiao-Bing Lu*
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China
Keywords
Asymmetric copolymerization | Internal cis-epoxides | Carbon dioxide | Hammett constant
Main observation and conclusion
Enantioselective resolution copolymerization of racemic internal epoxides with carbon dioxide (CO2) is a chal-
lenged issue because of their poor reactivity and complicated regio/stereoselectivity. Herein, we describe the first
enantioselective resolution copolymerization of racemic aromatic 2,3-disubstituted cis-epoxides and CO2 using
enantiopure dinuclear cobalt(III) complexes as catalyst under mild conditions, affording the corresponding poly-
carbonates with completely alternating structure and good enantioselectivity in the range of 70% to 97% ee. The
isotactic polycarbonate from β-Methylstyrene oxide is a typical semicrystalline material, possessing a melting point
of 241 oC; while other isotactic-enriched copolymers from 2,3-disubstituted cis-epoxides bearing a substitute group
on the aromatic ring are amorphous, having glass transition temperatues between 86 to 124 oC. Interestingly, the
copolymer selectivity is correlated well with the hammett substituent constant, and the highest polymer selectivity
of 98% was found in the system using the epoxides bearing an electron-donating group on the benzene ring.
Comprehensive Graphic Content
O
O
O
O
R
O
R
R
S
S
R,R,R,R
n
(
)-catalyst
R
S
+
R
+
+
CO2
O
R
R
R
R
>
99% carbonate linkages;
ee
to 97%
for
Up
copolymer
O
F
N
O
N
o
-F-OMSy
Co
O
O
O
O
O
tBu
2
3
X
R
=
F
Cl
R
X
R
tBu
O
N
O
m
m
m
-Me-OMSy
-Cl-OMSy
-F-OMSy
Co
O
O
O
O
N
Et
p-Et-OMSy
F
Cl
p-Cl-OMSy p-F-OMSy
OMS
y
Monomer: Substituent effect
Catalyst: Bimetallic
complexes
*E-mail: xblu@dlut.edu.cn
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Supporting Information
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© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
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through the copyediting, typesetting, pagination and proofreading process which may lead to
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Liu, He, Liu, & Lu
Concise Report
Background and Originality Content
O
F
The development of efficient transformation from carbon di-
oxide (CO2) to economically competitive products is of great in-
terest and has been a long-standing goal for chemists1,2. One of
the most promising strategies is to synthesize biodegradable pol-
ycarbonates via the alternating copolymerization of CO2 and
epoxides bearing diverse structures3. In recent decades, numerous
o
-F-OMSy
O
O
O
O
2
3
=
F
Cl
catalyst systems have been developed for this transformation3-11
.
R
m
m
m
-Me-OMSy
-Cl-OMSy
-F-OMSy
In addition, the stereochemistry inherent to the epoxide mono-
mer provides the possibility to synthesize stereoregular polycar-
bonates with enhanced thermal property10,12-14. In 2004, Lu and
coworkers reported the enantioselective resolution copolymeriza-
tion of racemic terminal epoxides and CO2 using binary catalyst
systems consisting a chiral salenCoX complex 1 and one quater-
nary ammonium salt. Excellent activity and moderate enantiose-
lectivity were observed in the copolymerization reaction of race-
mic propylene oxide (PO) and CO2. The resulting polycarbonates
possessed perfectly alternating structure and more than 95%
head-to-tail linkages15. Also, the desymmetrization copolymeriza-
tion of meso-epoxides and CO2 has been individually investigated
by several groups, since Nozaki et al. first described this method-
ology in 199912,16-20. The most significant development in this area
was reported in 2013 by Lu et al., who demonstrated a catalyst
system comprising an enantiopure, biphenol-linked, dinuclear
salenCo(III)DNP catalyst 2, and a nucleophilic cocatalyst, PPN-DNP
O
O
O
O
Et
F
Cl
p-Cl-OMSy
p-Et-OMSy
OMSy
p-F-OMSy
Figure 1 Structure of employed salenCo(III)DNP complexes and aromatic
cis-internal epoxides.
Results and Discussion
We initiated our studies by using the alternating copolymeri-
zation of racemic β-Methylstyrene oxide (OMSy) and CO2 as a
model reaction, with the use of binary salenCo(III)DNP/PPN-DNP
catalyst systems (Figure 1). The classical mononuclear salen com-
plex (R,R)-1 exhibited no activity in this reaction within 48 hours
(Table 1, entry 1). On the contrary, when activated by PPN-DNP,
the biphenol-linked binuclear catalysts 2a-2d were found to be
highly active and enantioselective in this copolymerization. For
example, binuclear complex (R,R,R,R)-2b, bearing isopropyl sub-
stitute on the phenolate ortho-position, predominantly consumed
(2S,3R)-isomer to produce (R,R)-configured polycarbonate and
retained (2R,3S)-isomer. This resolution copolymerization exhib-
ited a moderate kinetic resolution coefficient of 4.7 and produced
enantioenriched polycarbonate with 86% enantiomeric excess (ee)
through the chiral analysis of the resultant diol after the copoly-
mer hydrolysis (entry 3). As we described in our previous reports,
the intramolecular bimetallic cooperation mechanism was recog-
nized to be responsible for the superior enantioselectivity and
activity of the bimetallic complexes in the asymmetric copolymer-
ization of epoxides and CO2.25 Interestingly, the steric hindrance of
the phenolate ortho-position substituents of the ligand strongly
influenced the catalytic activity, the krel and the polymer enanti-
oselectivity. For example, a very low activity was observed with
the catalyst system (R,R,R,R)-2a bearing tert-butyl groups on the
phenolate ortho-position (entry 2), while less-hindered
(R,R,R,R)-2c bearing methyl groups showed a relatively high activ-
ity(entry 4). Replacement with a further less-hindered (R,R,R,R)-2d
without any substituent on the phenolate ortho-position in-
creased the krel to a certain extent (entry 5), affording enantioen-
riched polycarbonate with up to 91% ee in the main chain. Besides,
when the reaction was conducted at 0 oC, the reaction activity
apparently decreased, and only 18.9% conversion could be
achieved after a prolonged reaction time of 48h. Nevertheless, the
copolymer selectivity, krel and product enantioselectivity all in-
creased to a certain extent. The highest enantioselectivity of 94%
ee for polycarbonate was obtained (entry 6). The increase of the
reaction temperature was favorable for the copolymerization
activity, but negative effect on the enantioselectivity and copoly-
(PPN
=
bis(triphenylphosphine)iminium,
DNP
=
2,4-dinitrophenoxide). This system provided the efficient and
highly enantioselective copolymerization of CO2/meso-epoxides to
afford isotactic polycarbonates with enantioselectivities of up to
99% ee 21-23. Instead, unlike the wide research on the enantiose-
lective copolymerization using terminal and meso-epoxides as
substrates, the racemic internal epoxide was less investigated
mainly due to their poor activity and regio/stereoselectivity. Re-
cently, we reported an unprecedent enantioconvergent copoly-
merization of racemic cis-internal epoxides and anhydrides using
binuclear aluminum complexes in conjunction with PPN-Cl as cat-
alyst systems, where the selectivity factor (s-factor) for the
enantioselective formation of copolymer significantly exceeds the
kinetic resolution coefficient (krel) based on the unreacted epoxide
at various conversions24.
Herein, we reported the first enantioselective resolution
copolymerization of aromatic cis-internal epoxides with CO2
mediated by the catalyst systems based on enantiopure binuclear
salenCo(III) complex 2 (Figure 1). The system exhibited highly
catalytic
enantioselectivity
activity,
good
for
copolymer
enantioselective
selectivity
and
resolution
copolymerization of CO2 and racemic aromatic 2,3-disubstituted
cis-epoxides.
N
O
N
O
Co
X
tBu
N
O
N
O
R
Co
X
R
tBu
tBu
tBu
O
N
O
N
X
Co
tBu
tBu
2a: R tBu
=
2b: R iPr
=
O2N
1
2
=
2c: R Me
=
O
NO2
=
H
X
2d: R
1
mer selectivity (entry 7). The H and 13C NMR spectra of isotactic
poly(OMSy-alt-CO2) were showed in Figure 2. No detectable poly-
ether peaks was observed in the 1H or 13C NMR spectra, indicating
the perfectly alternating structure of the resultant polycarbonate.
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Chin. J. Chem. 2021, 39, XXX－XXX

Enantioselective Resolution Copolymerization of Racemic 2,3-Disubstituted cis-Epoxides with CO2
Chin. J. Chem.
a
Table 1 Enantioselective copolymerization of OMSy and CO2
O
O
O
O
O
R
R
S
Chiral Catalyst
R
S
R
S
n
R
+
+
CO2
+
O
Conv.b
Sel.c
(%)
Mn
eeo
eep
d
e
g
Temp.
(oC)
Time
(h)
Entry
Cat.
Ðd
krel
f
(%)
<5
(kg/mol)
--
(%)
--
(%)
1
2
3
4
5
6
7
(R,R)-1
25
25
25
25
25
0
48
24
24
24
12
48
3
--
--
--
--
--
--
(R,R,R,R)-2a
(R,R,R,R)-2b
(R,R,R,R)-2c
(R,R,R,R)-2d
(R,R,R,R)-2d
(R,R,R,R)-2d
<5
--
--
--
--
43.1
55.6
29.3
18.9
29.2
85
87
90
97
85
10.1
12.8
9.4
1.25
1.26
1.15
1.14
1.52
40(2R,3S)
59(2R,3S)
30(2R,3S)
20(2R,3S)
28(2R,3S)
4.7
4.9
8.3
15.9
6.8
86(R,R)
81(R,R)
91(R,R)
94(R,R)
90(R,R)
5.2
50
4.4
a Conditions: The reaction was performed in neat epoxide under 2 MPa CO2 a 20 mL autoclave, catalyst/PPN-DNP/epoxide = 1/2/500 (molar ratio). b Cal-
c
culated using 1H NMR spectroscopy using epoxide as the limiting reagent.
Calculated using 1H NMR spectroscopy, selectivity
=
ar-
d
e
ea(polymer)/[area(polymer) + area(cyclic carbonate)]. Determined by using gel permeation chromatography in THF, calibrated using polystyrene.
f
Measured by chiral HPLC, the absolute configuration was confirmed from literature results. krel = ln[(1-c)(1-eeo)]/ln[(1-c)(1+eeo)], where c is the conver-
sion and eeo is the enantiomeric excess of the recovered epoxide expressed as %/100. g Measured by hydrolyzing the polymer and analyzing the resulting
diol by chiral HPLC, and the (R,R)-diol was the major enantiomer when (R,R,R,R)-catalyst was used.
Vauious racemic 2,3-disubstituted cis-epoxides bearing
a
substituent on the benzene ring were also investigated for the
resolution copolymerization with CO2 using complex (R,R,R,R)-2d
catalyst system. To our delight, epoxides bearing diverse
electron-donating groups (EDG) or electron-withdrawing groups
(EWG) on the benzene ring were found to be reactive and
enantioselective in the copolymerization reaction with CO2,
affording (R,R)-configuration-enriched polycarbonates with
70%~97% ee. For example, 41.2% conversion and 98% copolymer
selectivity could be obtained within 24 hours at room temperature
in the reaction of p-Et-omsy bearing a ethyl group on the
para-position of the benzene ring. A krel of 13.2 was determined in
this case, affording the polycarbonate with 87% ee in the main
chain (Table 2, entry 1). The reaction of CO2 and epoxides bearing
a EWG such as fluorine or chlorine on the para-position of
benzene ring also exhibited good enantioselectivity, with krels of
∗
O
a
O
∗
b
O
c
c
a
b
o
4.6 and 7.6 at 25 C, respectively (entries 2 and 4). Lowering the
reaction temperature dramatically increased the krels, 15.4 and
19.4 for p-F-OMSy and p-Cl-OMSy, affording the polycarbonates
with 90% and 94% ee, respectively (entries 3 and 5). For epoxides
with substituent at the meta-position, the resulting
polycarbonates had moderate to good enantioselectivity. For
instance, the reaction of CO2 and m-Cl-OMSy exhibited a krel of 3.5
at 0 oC, producing enantioenriched polycarbonate with 70% ee
(entry 8). For m-F-OMSy case, good enantioselectivity (krel = 8.2)
could be obtained at room temperature, and the produced
polycarbonate possessed 92% ee (entry 9). Similarly, lowering the
reaction temperature is beneficial for the enantioselective
resolution copolymerization of m-F-OMSy and CO2. An increased
krel of 26.6 was observed when the reaction was performed at 0 oC
(entry 10). However, extremely low reactivity was observed in the
∗
d
O
a
O
∗
b
O
c
c
a
d
b
copolymerization reaction of CO2 with epoxides bearing
a
substituent on the ortho-position of the benzene ring, possibly
duing to the steric hindrance of the ortho-position near the
reaction center (entry 11). It is wothing noticing that the polymer
selectivity was found to be highly relevant to the electronic effect
of the substituent on the benzene ring. The excellent copolymer
Figure 2 1H and 13C NMR spectra of poly(OMSy-alt-CO2) in CDCl3. Top: 1H
NMR spectrum. Bottom: 13C NMR spectrum
Chin. J. Chem. 2021, 39, XXX－XXX
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Liu, He, Liu, & Lu
Concise Report
a
Table 2 Substrate scope of enantioselective copolymerization of aromatic cis-internal epoxides with CO2
O
O
O
O
R
O
R
S
R
S
2d
n
R
R,R,R,R
S
(
)-
R
+
+
+
CO2
O
R
R
R
R
Conv.b
(%)
Sel.c
Mn
eeo
(%)
eep
Tg
d
e
g
h
Temp.
(oC)
25
25
0
Entry
Monomer
Ðd
krel
f
(%)
98
52
97
63
95
98
<1
87
38
95
--
(kg/mol)
9.4
(%)
(oC)
99
--
1
2
p-Et-OMSy
p-F-OMSy
p-F-OMSy
p-Cl-OMSy
p-Cl-OMSy
m-Me-OMSy
m-Cl-OMSy
m-Cl-OMSy
m-F-OMSy
m-F-OMSy
o-F-OMSy
41.2
40.3
39.8
45.4
48.6
44.5
25.9
33.9
37.2
49.8
--
1.24
1.33
1.51
1.34
1.67
1.33
--
54(2R,3S)
36(2R,3S)
53(2R,3S)
53(2R,3S)
75(2R,3S)
46(2R,3S)
16(2R,3S)
24(2R,3S)
41(2R,3S)
82(2R,3S)
--
13.2
4.6
15.4
7.6
19.4
5.7
3.1
3.5
8.2
26.6
--
87(R,R)
93(R,R)
90(R,R)
97(R,R)
94(R,R)
90(R,R)
--
4.0
3
10.1
6.7
99
--
4
25
0
5
8.2
124
98
--
6
25
25
0
10.8
--
7
8
7.1
1.41
1.23
1.51
--
70(R,R)
92(R,R)
87(R,R)
--
86
--
9
25
0
7.2
10
12.7
99
--
11
25
--
a Conditions: The reaction was performed in neat epoxide with the molar ratio of catalyst/PPN-DNP/epoxides = 1/2/500 in a 20 mL autoclave under 2 MPa
CO2 for 24 hours at 25oC and 3 days at 0oC. b Calculated using 1H NMR spectroscopy using epoxide as the limiting reagent. c Calculated using 1H NMR spec-
d
troscopy, selectivity = area(polymer)/[area(polymer) + area(cyclic carbonate)]. Determined by using gel permeation chromatography in THF, calibrated
e
using polystyrene. Measured by chiral GC or HPLC, the absolute configuration of OMSy was confirmed from literature results and the other epoxides
f
were assigned as analogy. krel = ln[(1-c)(1-eeo)]/ln[(1-c)(1 + eeo)], where c is the conversion and eeo is the enantiomeric excess of the recovered epoxide
expressed as %/100. g Measured by hydrolyzing the polymer and analyzing the resulting diol by chiral HPLC, and the (R,R)-diol was the major enantiomer
when (R,R,R,R)-catalyst was used. h Determined by differential scanning calorimetry (DSC).
selectivity of 98% was observed in the copolymerization
reaction of CO2 and epoxides bearing an EDG substitute such as
ethyl or methyl at room temperature (entries 1 and 6). On the
1.2
contrary, epoxides bearing a EWG exhibited inferior polymer
m-Me
p-Et
selectivity at the same reaction condition. The Hammett
substituent constant has been widely applied to evaluate the
electronic effect of substituents in the para- or meta- position of
the benzene. Also, it is beneficial for the mechanistic
understanding of some organic reactions.26 A good correlation
between the polymer selelctivity and the Hammett constant of the
substituent was observed in the reaction (Figure 3). As expected,
lowering the reaction temperature effectively increased the
copolymer selectivity for all tested epoxides.
p-H
1.0
0.8
0.6
0.4
0.2
0.0
p-Cl
p-F
m-F
m-Cl
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
σ
Figure 3 Correlation between the polymer selectivity and Hammett sub-
stituent constant when the reaction was conducted at room temperature
using (R,R,R,R)-2d as catalyst
Moreover, the thermal properties of the resultant
polycarbonates were inverstigated by differential scanning
calorimetry (DSC). The isotactic poly(OMSy-alt-CO2) with 94% ee
was discovered to be
a
typical semi-crystalline material,
o
possessing a melting point of 241 C, while only a glass transition
o
temperature (Tg) of 120 C was observed in the atactic copolymer
(Figure 4). The crystallizability of the stereoregular polycarbonate
was further identified by powder X-ray diffraction (XRD), in which
diffraction peaks at 9.6o, 13.7o, 16.2o, and 19.2o were observed, no
diffraction signal was found in the atactic copolymer. However,
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Chin. J. Chem. 2021, 39, XXX－XXX

Enantioselective Resolution Copolymerization of Racemic 2,3-Disubstituted cis-Epoxides with CO2
Chin. J. Chem.
1
other isotactic-enriched polycarbonates from 2,3-disubstituted
cis-epoxides bearing a substitute group on the aromatic ring were
amorphous materials with Tgs ranging from 86 to 124 oC, probably
resulting from the hindrance effect of the substituent, which was
thought to significantly affect the orderly assembly of polymer
chain.
from the autoclave for H NMR analysis to quantitatively give the
selectivity of polycarbonate to cyclic carbonate, as well as car-
bonate linkages. The remainder mixture was then dissolved in
CH2Cl2 and precipitated into an excess of MeOH. This process was
1
repeated until no residual monomer was detected in the H NMR
spectrum of the product. After several rounds of precipitation, the
material was collected and dried under vacuum.
Supporting Information
1.0
exo
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2021xxxxx.
0.5
B
0.0
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In summary, we have demonstrated the enantioselective res-
olution copolymerization of racemic aromatic 2,3-disubstituted
cis-epoxides with CO2 mediated by the biphenol-linked binuclear
cobalt(III) catalyst systems at mild conditions, affording enantio-
merically enriched polycarbonates with completely alternating
structure and relatively low polydispersity index. The highly iso-
tactic polycarbonates from racemic aromatic cis-internal epoxides
have been prepared in optically active form, for the first time. The
copolymer selectivity was found to be correlated well with the
Hammett substituent constant, in which the epoxides bearing an
electron-donating group on the benzene ring exhibited better
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tably, the isotactic-enriched poly(OMSy-alt-CO2) was found to be a
typical semicrystalline material with a melting endothermic peak
at 241 °C, while isotactic-enriched polycarbonates from
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In a predried 20 mL autoclave equipped with a magnetic stir-
rer, salenCo(III)DNP catalyst (0.0016 mmol, 1 equiv.), PPNDNP
(PPN = 2,4-dinitrophenoxide, 0.0032 mmol, 2 equiv.) and aromatic
cis-internal epoxides (0.8 mmol, 500 equiv.) in a argon atmos-
phere. After CO2 was introduced, the reaction mixture was stirred
at a desired temperature for an appropriate time. Then CO2 was
released, a small amount of the resultant mixture was removed
Chin. J. Chem. 2021, 39, XXX－XXX
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
www.cjc.wiley-vch.de

Liu, He, Liu, & Lu
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Manuscript received: XXXX, 2021
Manuscript revised: XXXX, 2021
Manuscript accepted: XXXX, 2021
Accepted manuscript online: XXXX, 2021
Version of record online: XXXX, 2021
Liu, Y.; Ren, W. M.; Wang, M.; Liu, C.; Lu, X. B. Crystalline Stereocom-
www.cjc.wiley-vch.de
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
Chin. J. Chem. 2021, 39, XXX－XXX

Enantioselective Resolution Copolymerization of Racemic 2,3-Disubstituted cis-Epoxides with CO2
Chin. J. Chem.
Entry for the Table of Contents
Title
Author, Corresponding Author,* Author
Chin. J. Chem. 2021, 39, XXX—XXX. DOI: 10.1002/cjoc.202100XXX
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Chin. J. Chem. 2021, 39, XXX－XXX
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
www.cjc.wiley-vch.de