Enantioselective Resolution Copolymerization of Racemic Epoxides and Anhydrides: Efficient Approach for Stereoregular Polyesters and Chiral Epoxides

Article abstract of DOI:10.1021/jacs.9b02722

Herein we report an efficient strategy for preparing isotactic polyesters and chiral epoxides via enantioselective resolution copolymerization of racemic terminal epoxides with anhydrides, mediated by enantiopure bimetallic complexes in conjunction with a nucleophilic cocatalyst. The chirality of both the axial linker and the diamine backbones of the ligand are responsible for the chiral induction of this kinetic resolution copolymerization process. The catalyst systems exhibit exceptional levels of enantioselectivity with a kinetic resolution coefficient exceeding 300 for various racemic epoxides, affording highly isotactic copolymers (selectivity factors of more than 300) with a completely alternating structure and low polydispersity index. Most of the produced isotactic polyesters are typical semicrystalline materials with melting temperatures in the range from 77 to 160 °C.

Full text of DOI:10.1021/jacs.9b02722

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Article
Enantioselective Resolution Copolymerization of
Racemic Epoxides and Anhydrides: Efficient Approach
for Stereoregular Polyesters and Chiral Epoxides
Jie Li, Bai-Hao Ren, Zhao-Qian Wan, Shi-Yu Chen, Ye Liu, Wei-Min Ren, and Xiao-Bing Lu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02722 • Publication Date (Web): 11 May 2019
Downloaded from http://pubs.acs.org on May 12, 2019
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Enantioselective Resolution Copolymerization of Racemic Epoxides and Anhydrides:
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Efficient Approach for Stereoregular Polyesters and Chiral Epoxides
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Jie Li, Bai-Hao Ren, Zhao-Qian Wan, Shi-Yu Chen, Ye Liu, Wei-Min Ren, and Xiao-Bing Lu*
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China.
ABSTRACT:
Herein we report an efficient strategy for preparing isotactic polyesters and chiral
epoxides via enantioselective resolution copolymerization of racemic terminal epoxides
with anhydrides, mediated by enantiopure bimetallic complexes in conjunction with a
nucleophilic cocatalyst. Chirality of both the axial linker and the diamine backbones of
the ligand are responsible for the chiral induction of this kinetic resolution
copolymerization process. The catalyst systems exhibit exceptional levels of
enantioselectivity with a kinetic resolution coefficient exceeding 300 for various racemic
epoxides, affording highly isotactic copolymers (selectivity factors of more than 300)
with a completely alternating structure and low polydispersity index. Most of the
produced isotactic polyesters are typical semi-crystalline materials with melting
temperatures in the range from 77 to 160 °C.
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INTRODUCTION
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Kinetic resolution of racemic substrates by chiral catalysts or reagents that mediate the
selective reaction of one enantiomer is a powerful strategy for the preparation of
enantiopure compounds.1 A representative example is the hydrolytic kinetic resolution of
racemic terminal epoxides catalyzed by chiral salen-Co(III) complexes,2 a general and
effective method for producing highly enantioenriched epoxides, which has found
widespread applications in both academic and industrial fields. When such a powerful
strategy is applied to the enantioselective resolution copolymerization of racemates with
other substrates, optically active polymers with main-chain chirality can be produced,
owing to the configurational retention of one enantiomer of the racemates incorporated
into the copolymer.3 This process affords two desirable products, an enantiopure
stereoregular copolymer and the unreacted enantiomer. Employing this strategy, several
groups independently reported the kinetic resolution copolymerization of racemic
propylene oxide and CO2 using binary or bifunctional catalyst systems based on chiral
salen-Co(III)X.4-6 Although the resultant copolymers have a completely alternating
structure and more than 95% head-to-tail linkages, the kinetic resolution coefficients (Krel)
are less than 10. The highest Krel of 24.3 was obtained for the multichiral (S,S,S)-Co(III)
catalyst containing a (1S,2S)-1,2-diaminocyclohexane backbone and (S)-configured
2′-isopropyloxy-1,10-binaphthyl, for which the resulting poly(propylene carbonate)s
were highly regioregular with >99% head-to-tail linkages for the selective
copolymerization of CO2 with the (R)-epoxide over the (S)-enantiomer.7 A great
significant improvement in the kinetic resolution of epoxides by homopolymerization
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was achieved by using a chiral bimetallic cobalt complex with a binaphthol linker. The
catalyst system was highly active for the polymerization of aliphatic terminal epoxides
and exhibited selectivity factors (s-factors) in the range from 60 to 370, affording the
corresponding polyethers with >99% isotactic [mm] content.8
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The alternating copolymerization of epoxides and anhydrides is a promising method to
prepare various polyesters with easily adjustable properties, due to the wide availability
of both monomers.9 For terminal epoxide/anhydride copolymerization, highly
regioselective ring-opening of the epoxide is a prerequisite for obtaining stereoregular
polyesters, since only ring-opening at the methylene C-O bond retains the
stereochemistry at the methine carbon of the epoxide incorporated into the copolymer. In
2013, a highly regioregular ring-opening step was observed with a concomitant >99%
retention of the configuration at the methine carbon center of the epoxide incorporated
into the polyester during binuclear salen-Cr(III)Cl-complex-mediated copolymerization
of (S)-phenyl glycidyl ether and maleic anhydride.10 Subsequently, Coates et al.
described the synthesis of highly isotactic poly(propylene succinate) (96-97%
head-to-tail linkages) via the regioselective ring-opening alternating copolymerization of
enantiopure propylene oxide and succinic anhydride catalyzed by the chiral
(Cl-salcy)Co(III)NO3 complex.11
Herein, we report the first example of enantioselective resolution copolymerization of
racemic epoxides and anhydrides using enantiopure bimetallic complexes 1-4, which
exhibit exceptional levels of enantioselectivity with a Krel of more than 300, affording
highly isotactic polyesters (s-factors of >300) with a completely alternating structure and
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low polydispersity index.
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RESULTS AND DISCUSSION
Coates et al.8,12 discovered that binaphthol-linked bimetallic cobalt complexes were
highly active and enantioselective catalysts for kinetic resolution polymerization of
racemic epoxides, in which the axial chirality of the binaphthol linker determined the
enantioselectivity of monomer enchainment in epoxide homopolymerization.
Interestingly, our recent study on asymmetric copolymerization of CO2 and
meso-epoxides catalyzed by enantiopure biphenol-linked binuclear cobalt complexes
demonstrated that both the axial chirality of the biphenol linker and the chiral diamine
backbones of the salen ligands were responsible for copolymer enantioselectivity.13
Therefore, we initially investigated the enantiopure binaphthol-linked bimetallic cobalt
complexes 1a and 1b for copolymerizing phenyl glycidyl ether (PGE) and phthalic
anhydride (PA) in the presence of PPNDNP (PPN = bis(triphenylphosphine)iminium),
DNP = 2,4-dinitrophenoxide) as the cocatalyst. Unfortunately, only polyether, rather than
polyester, was rapidly formed within minutes at ambient temperature (Table 1, Entries 1
and 2). Changing either the phenolate ortho-substitutes on the ligand or the nucleophilic
cocatalyst did not improve the product selectivity. Surprisingly, we delightfully
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discovered that the enantiopure binaphthol-linked bimetallic aluminum complexes 2a-2c
in conjunction with PPNCl were highly active in catalyzing PGE/PA copolymerization,
affording polyesters with a completely alternating structure and low polydispersity index.
The catalyst systems based on the enantiopure binaphthol-linked aluminum complexes
with (R,R,R,R,R)-configuration preferentially consume (S)-PGE over (R)-PGE with Krel =
~33-152 (Table 1, Entries 3-5). When the reaction temperature was decreased to 0 °C, the
(R,R,R,R,R)-2a/PPNCl catalyst system afforded the polyester with up to 99%
enantioselectivity, corresponding to an s-factor of more than 300 (calculated from the
resulting (S)-diol by hydrolyzing the resultant copolymer), implying a preference for
(S)-PGE and the retention of its configuration in the copolymerization (Entry 6). When
(S,S,S,S,S)-2a was used, the copolymerization selectively consumed (R)-PGE instead of
(S)-PGE, giving an isotactic polyester with an (R)-configuration (Entry 7).
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Moreover, the enantioselective resolution copolymerization of racemic PGE and PA
was monitored at various times by determining the enantioselectivities of the unreacted
epoxide and copolymer at different conversions. As shown in Figure 1, the enantiopure
excess (ee) value of the unreacted epoxide increases with the conversion, whereas the ee
values of the resulting copolymers do not change, and remain at 99% during the
enantioselective resolution copolymerization. This result indicates an exceptional level of
kinetic resolution of racemic epoxides and a highly enantioselective copolymerization
process.
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Table 1. Outcomes of PGE/PA copolymerization mediated by enantiopure bimetallic complexes.a
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d
e
f
g
Temp.
[°C]
25
Time
[h]
<0.1
<0.1
1.7
2
Conv.b
TOFc
[h−1]
Mn
ee(epo)
Krel
ee(poly)
Entry
Catalyst
Đd
s-factorh
[%]
[kDa]
[%]
[%]
1
2
(R,R,R,R,R)-1a
(R,R,R,R,R)-1b
(R,R,R,R,R)-2a
(R,R,R,R,R)-2b
(R,R,R,R,R)-2c
(R,R,R,R,R)-2a
(S,S,S,S,S)-2a
(R,R,S,R,R)-2a
(R,R,R,R,R)-3
(R,R,R,R,R)-4a
(R,R,R,R,R)-4b
polyether
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25
polyether
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3
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40
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40
45
288
135
165
67
43.3
20.3
34.3
43.1
46.2
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39.3
39.8
40.3
1.13
1.13
1.23
1.16
1.21
1.15
1.17
1.23
1.19
92(R)
34(R)
45(R)
67(R)
75(S)
61(S)
15(R)
58(R)
63(R)
152
33
96(S)
89(S)
91(S)
99(S)
99(R)
70(R)
11(S)
85(S)
73(S)
163
24
4
25
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25
2
35
33
6
0
6
>300
>300
13
>300
>300
10
7
0
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72
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25
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90
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25
4.5
3
93
2
1
10
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25
133
114
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aConditions: The reactions were performed under the following conditions:
PGE/PA/catalyst/PPNX/toluene molar ratio = 1000/500/1/2/500. bCalculated using 1H NMR
c
spectroscopy, based on the epoxide. Turnover frequency (TOF) = mol of product (polyester)/mol of
catalyst per hour. dDetermined using gel permeation chromatography in THF, calibrated with
polystyrene. Measured by analyzing the resulting mixture via HPLC. fCalculated using Krel = ln[(1 −
e
g
c)(1 − ee(epo))]/ln[(1 – c)(1 + ee(epo))], where c is the conversion of epoxides. Measured by
hydrolyzing the polymer and analyzing the resulting diol via HPLC. hCalculated using s-factor = ln[1
− c(1 + ee(poly))]/ln[1 − c(1 − ee(poly))], where c is the conversion of epoxides.
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90
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40
ee(poly)
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ee(epo)
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0
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% conversation of PGE
Figure 1. The ee values of unreacted PGE and resulting polyesters at various conversions of PGE for
enantioselective resolution copolymerization of racemic PGE and PA mediated by
(R,R,R,R,R)-2a/PPNCl at 0 °C.
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Although the accurate assignment of the microstructure of PGE/PA copolymer has not
been reported, significant differences in the carbonyl and methylene regions of the atactic
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and isotactic copolymers can be easily identified via C NMR spectroscopy (Figure 2).
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Two single peaks at 166.75 and 166.60 ppm for the carbonyl carbons, and a single peak
at 63.60 ppm for the methylene carbon, were detected for the isotactic PGE/PA
copolymer, whereas the atactic polyester showed multiple peaks for the carbonyl carbons
and double peaks for the methylene region.
A
B
Figure 2. 13C NMR spectra of carbonyl (left) and methylene (right) regions of PGE/PA copolymer:
(A) atactic polymer and (B) isotactic polymer.
On the contrary, when activated with PPNCl, (R,R,S,R,R)-2a bearing (R,R)-diamine
backbones and the binaphthol linker with (S)-axial chirality predominantly afforded the
polyester with a preference for the (R)-configuration enchainment and a low s-factor of
10 in this copolymerization. The catalytic activity was less than one third that of the
(R,R,R,R,R)-2a/PPNCl catalyst system. These results demonstrate that both the axial
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chirality of the binaphthol linker and the stereochemistry of the diamine backbones are
responsible for the enantioselectivity of the monomer enchainment in the kinetic
resolution copolymerization of racemic PGE and PA. In addition, the steric hindrance of
the phenolate ortho substituents was found to have significant influence on the catalytic
activity and product enantioselectivity. In comparison with (R,R,R,R,R)-2a,
(R,R,R,R,R)-2b with methyl groups on the phenolate ortho position showed a relatively
lower activity and a moderate s-factor of 24 in catalyzing this copolymerization at the
same conditions (Table 1, Entries 3 and 4).
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Although the hydrogenated binaphthol-linked Co(III) complex 3 was found to be
active in catalyzing PGE/PA copolymerization, affording a copolymer with a perfect
alternating structure, the Krel and the s-factor were very low (Entry 9). Similar activity
but improved chiral induction was observed for the catalyst systems based on the
corresponding aluminum complexes 4a and 4b. However, the enantioselectivity was
significantly lower than that of the enantiopure binaphthol-linked aluminum complex 2a.
With the highly enantioselective catalyst (R,R,R,R,R)-2a in hand, we explored
enantioselective resolution copolymerization of PA with a variety of racemic terminal
epoxides. When activated with PPNCl, (R,R,R,R,R)-2a mediated copolymerization of PA
with aliphatic epoxides, such as propylene oxide (PO), 1,2-epoxybutane (BO), and
1,2-hexylene oxide (HO), exhibited excellent activities, Krels, and s-factors (Table 2,
Entries 2-6). A reduction in the reaction temperature from 25 to 0 °C resulted in an
obvious improvement in both the Krel and s-factor for the copolymerization of PA with
PO or BO. Glycidyl ethers, such as methyl glycidyl ether (MEGE), ethyl glycidyl ether
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(ETGE), isopropyl glycidyl ether (IPGE), and tert-butyl glycidyl ether (TBGE), are
highly reactive monomers that copolymerize with PA to give the corresponding
polyesters with both high s-factors and Krels of >300 at ambient temperature (Entries
7-10). An s-factor of more than 300 was also observed for the enantioselective resolution
copolymerization of PA and furfuryl glycidyl ether (FurGE), which is made from
biorenewable feedstocks (Entry 11). It is worth noting that 1-naphthol glycidyl ether
(1-NGE) and 2-naphthol glycidyl ether (2-NGE) bearing a bulky substitute group showed
high reactivities and excellent stereoselectivities in copolymerization with PA at 25 °C,
affording the corresponding isotactic polyesters with s-factors of >300 (Entries 12 and
13). 1,2-Epoxy-4-phenylbutane (BBO), possessing a structure similar to that of PGE, was
found to be highly reactive in copolymerization with PA at room temperature, affording
the copolymer with 99% enantioselectivity for the (R)-configuration (Entry 14). To our
surprise, the catalytic system also exhibited good activity and moderate enantioselectivity
for the copolymerization of PA with racemic epoxides bearing an electron-withdrawing
group, e.g., styrene oxide (SO) and epichlorohydrin (ECH), with s-factors of 24 and 7,
respectively (Entries 15 and 16).
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Moreover, with racemic PGE as a model epoxide monomer, other cyclic anhydrides,
including naphthyl anhydride (NA), 4,5-dimethylphthalic anhydride (4,5-DMPA), maleic
anhydride (MA), and succinic anhydride (SA), were employed for enantioselective
resolution copolymerization mediated by (R,R,R,R,R)-2a/PPNCl. The catalyst system is
highly efficient in the enantioselective resolution copolymerization of both NA and
4,5-DMPA, affording the corresponding polyesters with 95% and 93% ee, respectively
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(Entries 17 and 18). However, poor chiral induction was observed for the
copolymerization of racemic PGE with MA or SA (Entries 19 and 20). Surprisingly,
replacing (R,R,R,R,R)-2a with the hydrogenated binaphthol-linked Co(III) complex 3
resulted in excellent enantioselectivity (93% ee) for PGE/MA copolymerization at −10
°C, with an s-factor of 56 (Entry 21). Notably, (R,R,R,R,R)-3 was discovered to be an
enantioselective catalyst for the resolution copolymerization of racemic PO and MA,
exhibiting a high s-factor of >300 at a high catalyst loading (Entry 22). Additionally,
high s-factors of more than 300 were observed for the copolymerization of MA with
racemic 1-NGE or 2-NGE mediated by the (R,R,R,R,R)-3/PPNDNP catalyst system
(Entries 23 and 24). In addition, (R,R,R,R,R)-3/PPNDNP was highly efficient in
enantioselective resolution copolymerization of SA and racemic 1-NGE or 2-NGE,
affording the corresponding polyesters with high isotacticity (Entries 25 and 26).
Notably, most of these resulting isotactic polyesters were semicrystallic materials with
melting temperatures of 77-160 °C. For example, a fairly sharp endothermic peak (ΔHm
= 30.33 J/g) was observed at 101 °C for the isotactic PO/PA copolymer with 95% ee,
whereas the DSC thermogram of the atactic polyester featured a peak corresponding to a
glass transition temperature (Tg) of 63 °C (Figure 3, left). Figure 3 (right) shows the
wide-angle X-ray diffraction (WAXD) profiles of atactic and isotactic (R)-configured
PO/PA copolymers. No diffraction peaks were observed for the atactic copolymer, in
agreement with its amorphous nature. In contrast, sharp diffraction peaks at 2θ = 11.9°,
15.8°, 16.2°, 18.6°, 20.2°, 22.0°, and 27.0° were observed for the isotactic (R)-copolymer,
demonstrating that this polymer is a typical semicrystalline material.
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Table 2. Substrate scope of enantioselective resolution copolymerization of racemic epoxides with
anhydrides.a
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d
e
g
Temp. Time Conv.b TOFc Mn
ee(epo)
ee(poly)
Tg/Tm
[°C]
f
Entry Reactants
Ɖd
Krel
s-factorh
[°C]
0
[h]
6
[%]
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43j
47 j
40 j
48 j
43j
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43 j
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33
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45
[h−1
67
]
[kDa]
43.1
4.5
[%]
[%]
1
PGE/PA
1.16
1.08
1.19
1.12
1.22
1.09
1.21
1.19
1.16
1.17
1.21
1.25
1.28
1.14
1.24
1.18
1.17
1.11
67(R)
72(S)
82(S)
63(S)
84(S)
72(S)
89(R)
86(R)
96(R)
89(R)
95(R)
75(R)
88(R)
91(S)
--m
>300 99(S)
>300
84
--/120
--/101
--/104
--/127
--/139
55/--
2i
PO/PA
0
1
86
93
65
67
56
93
95(R)
91(R)
94(R)
90(R)
95(R)
3
PO/PA
25
0
3
157
53
9.7
53
4i
BO/PA
1.5
4
5.1
62
5
BO/PA
25
25
25
25
25
25
25
25
25
25
0
120
108
157
153
163
157
98
10.1
52.2
11.5
13.5
34.5
43.4
52.9
57.8
55.3
46.7
26.8
18.3
61.4
59.2
35.4
30.6
9.2
49
6
HO/PA
4
83
7
MEGE/PA
ETGE/PA
IPGE/PA
TBGE/PA
FurGE/PA
1-NGE/PA
2-NGE/PA
BBO/PA
ECH/PA
SO/PA
3
>300 99(S)
>300 99(S)
>300 99(S)
>300 99(S)
>300 99(S)
>300 99(S)
>300 99(S)
>300 99(R)
>300
>300
>300
>300
>300
>300
>300
>300
7
48/153
--/101
--/77
8
3
9
3
10
11
12
13
14
15
16
17
18
19
20
21k
22l
23k
24k
25k
26k
3
--/86
5
--/88
10
10
3
43
73/--
47
--/113
72/--
160
72
6
--
63(R)
82(R)
95(S)
93(S)
45(S)
25(S)
93(S)
57/105
--/160
--/127
78/--
25
25
0
3
163
59
86(S)
85(R)
77(R)
50
128
77
5
24
PGE/NA
PGE/4,5-DMPA
PGE/MA
PGE/SA
8
104
63
5
90
42/--
50
50
−10
0
8
51
1.27 40(R)
1.25 27(R)
4
12/--
10
2.8
4
39
3
2
PGE/MA
PO/MA
150
5
1.22
1.23
1.25
1.24
1.23
1.29
67(R)
75(S)
64(R)
49(R)
64(R)
81(R)
52
56
--/130
33/--
1.8
>300 99(R)
>300
>300
>300
30
1-NGE/MA
2-NGE/MA
1-NGE/SA
2-NGE/SA
0
5
78
10.4
11.3
13.6
12.8
>300 >99(S)
>300 >99(S)
74/--
25
25
25
5
66
--/93
5
84
31
88(S)
40/--
5
90
>300 99(S)
>300
57/95
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aConditions:
The
reactions
were
performed
under
the
following
conditions:
5
epoxide/anhydride/complex-2a/PPNCl/toluene molar ratio = 1000/500/1/2/500, except for entries 2, 4,
6
7
8
and 21-26. bCalculated using 1H NMR spectroscopy, based on the epoxide, except for entries 2-6 and
c
22, which were based on anhydride conversion. Turnover frequency (TOF) = mol of product
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d
(polyester)/mol of catalyst per hour. Determined using gel permeation chromatography in THF,
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e
calibrated with polystyrene. Measured by analyzing the resulting mixture via HPLC or derivatizing
the resulting epoxide using 2-mercaptobenzothiazole, and then determining the ee via HPLC.
fCalculated using Krel = ln[(1 − c)(1 − ee(epo))]/ln[(1 – c)(1 + ee(epo))], where c is the conversion of
g
epoxides. Measured by hydrolyzing the polymer and analyzing the resulting diol via HPLC or
derivatizing the resulting diol using acetic anhydride, and then determining the ee via GC. hCalculated
using s-factor = ln[1 − c(1 + ee(poly))]/ln[1 − c(1 − ee(poly))], where c is the conversion of epoxides. iThe
reactions were performed under the following conditions: epoxide/PA/complex-2a/PPNCl/toluene
j
1
molar ratio = 200/100/1/2/100. Calculated using H NMR spectroscopy, based on anhydrides as the
k
limiting reagents and the conv.(epoxides) = conv.(anhydrides)/2. The reactions were performed under the
following conditions: epoxide/anhydride/complex-3/PPNDNP/toluene molar ratio = 400/200/1/2/200.
lThe reactions were performed under the following conditions: PO/MA/complex-3/PPNDNP/toluene
molar ratio = 50/25/1/2/50. mNot detected.
exo
B
B
A
A
50
100
150
0
10
20
30
40
50
Temperature (oC)
2θ/deg
Figure 3. DSC thermograms (left) and WAXD profiles (right) of poly(PO-alt-PA)s: (A) atactic copolymer,
(B) isotactic-enriched polymer (95% ee).
Since (R,R,R,R,R)-2a/PPNCl is an advantageous chiral catalyst system for
enantioselective resolution copolymerization of racemic terminal epoxides with
anhydrides, PO/PA copolymerization mediated by this catalyst system was chosen as a
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model reaction for calculating the Gibbs free energies of activation (Δ‡G) of various
ring-opening paths of the two-configuration epoxides. To simplify calculations, the
polymer chain was replaced with 2-(methoxycarbonyl)benzoate to study the nucleophilic
ring-opening process of the coordinated PO molecule. In this model, a PO molecule
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coordinated
to
one
Al(III)
ion
is
subsequently
ring-opened
by
2-(methoxycarbonyl)benzoate bound to the other Al(III) ion (Supporting information,
Figure S87). The Gibbs free energies of activation (Δ‡G) of the four transition states (red,
blue, green, and orange lines), corresponding to the ring-opening at the methylene and
methine C-O bond of (R)- or (S)-PO coordinated to one aluminum center, are different
because of the significant difference in the effect of the chiral environment provided by
the (R,R,R,R,R)-2a (Figure 4). For binaphthol-linked dinuclear Al(III) complex
(R,R,R,R,R)-2a, the Gibbs free energy of the transition state produced via an attack at the
(R)-C-O of the methylene carbon (red line) of (R)-PO is 21.6 kcal/mol, which is much
lower than that of the other three possible epoxide ring-opening processes during chain
propagation. As a result, (R)-PO in the racemates is selectively coordinated to the
(R,R,R,R,R)-2a complex, and regioselectively ring-opened at the methylene C-O bond to
yield the (R)-polyester, which is in agreement with the experimental findings.
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(R)-PO, attacked at methylene carbon
50.0
(S)-PO, attacked at methylene carbon
(R)-PO, attacked at methine carbon
(S)-PO, attacked at methine carbon
Transition State
45.0
40.0
35.0
(TS)
P
O
O
O
Al
Cl
Al
Cl
9
30.0
25.0
20.0
15.0
10.0
27.2
26.8
25.6
O
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
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P
P
O
O
O
O
Al
Cl
21.6
Al
Cl
O
Al
Al

Cl
Cl
12.4
10.3
7.8
10.8
7.6
5.1
P
O
O
O
Al
4.0
5.0
0
Al
Cl
Cl
0.0
0.0
Reaction Coordinate
Figure 4. Gibbs free energies of activation (Δ‡G) of the four transition states corresponding to
ring-opening at methylene and methine C-O bond of (R)- or (S)-PO coordinated to one aluminum
center of (R,R,R,R,R)-2a. Energies are given in kcal/mol.
CONCLUSIONS
In summary, herein we report the first highly enantioselective resolution
copolymerization of racemic epoxides with anhydrides, mediated by enantiopure
bimetallic complexes in conjunction with a nucleophilic cocatalyst. The catalyst systems
based on the enantiopure binaphthol-linked bimetallic Al(III) complex (R,R,R,R,R)-2a or
hydrogenated binaphthol-linked Co(III) complex (R,R,R,R,R)-3 exhibit unprecedented
levels of enantioselectivity, affording the unreacted epoxides in high enantiopurity, and
the highly isotactic copolymers with a perfectly alternating structure and narrow
molecular weight distribution. Various racemic terminal epoxides and anhydrides were
tested, and most of these copolymerization systems exhibited both high Krel and s-factors
of more than 300.
The extraordinary level of enantioselection in the enantioselective resolution
copolymerization outlined above renders this approach extremely attractive for the
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practical synthesis of a wide range of chiral polyesters, which are mostly semicrystalline
materials with melting temperatures between 77 and 160 °C. Further investigation should
focus on understanding the mechanism of this enantioselective copolymerization, and
developing novel stereoselective resolution copolymerizations of racemic epoxides with
other nucleophilic reagents.
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ASSOCIATED CONTENT
Synthetic procedures of all new compounds, supplementary characterization data
1
13
including H NMR and C NMR spectra, DSC and XRD data, as well as chiral GC or
HPLC analysis of various compounds.
AUTHOR INFORMATION
Corresponding Author
*E-mail: xblu@dlut.edu.cn
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work is supported by the National Natural Science Foundation of China (NSFC,
Grant 21690073), and Program for Changjiang Scholars and Innovative Research Team
in University (IRT-17R14).
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SYNOPSIS TOC:
O
R
O
O
O
O
+
(R,R,R,R,R)-
Catalyst
R
O
m
O
R
R
+
O
M
M
Krel > 300, s-factor > 300
R
+
O
Bimetallic synergistic effect
(S,S,S,S,S)
O
O
O
-Catalyst
O
O
+
O
O
R
n
O
R
R
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Efficient approach for stereoregular polyesters with main-chain chirality is realized via enantiopure bimetallic
complexes mediated enantioselective resolution copolymerization of racemic terminal epoxides with
anhydrides, in an extraordinary level of enantioselection through multi-chiral synergistic induction.
157x78mm (300 x 300 DPI)
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