Stereoregular polycarbonate synthesis: Alternating copolymerization of CO2 with aliphatic terminal epoxides catalyzed by multichiral cobalt(III) complexes

Article abstract of DOI:10.1002/pola.24945

Completely stereoregular polycarbonate synthesis was achieved with the use of unsymmetric multichiral cobalt-based complexes bearing a derived chiral BINOL and an appended 1,5,7-triabicyclo[4.4.0] dec-5-ene as catalyst for the copolymerization of CO₂

Full text of DOI:10.1002/pola.24945

ARTICLE
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Stereoregular Polycarbonate Synthesis: Alternating Copolymerization
of CO2 with Aliphatic Terminal Epoxides Catalyzed by Multichiral
Cobalt(III) Complexes
Wei-Min Ren, Ye Liu, Guang-Peng Wu, Jie Liu, Xiao-Bing Lu
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China
Correspondence to: X.-B. Lu (E-mail: lxb-1999@163.com)
Received 24 April 2011; accepted 11 August 2011; published online 9 September 2011
DOI: 10.1002/pola.24945
ABSTRACT: Completely stereoregular polycarbonate synthesis
ages, and a Krel of 24.4 for the enchainment of (R)-epoxide
over (S)-epoxide. The isotactic PPC exhibits an enhanced
glass transition temperature of 47 ꢀC, which is 10–12 ꢀC
was achieved with the use of unsymmetric multichiral cobalt-
based complexes bearing
a derived chiral BINOL and an
appended 1,5,7-triabicyclo[4.4.0] dec-5-ene as catalyst for the
copolymerization of CO2 and aliphatic terminal epoxides at
mild conditions. The (S,S,S)-Co(III) complex 1c with sterically
hindered substituent group is more stereoregular catalyst for
the copolymerization of CO2 and racemic propylene oxide to
higher than that of the corresponding irregular polycarbonate.
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Chem 49: 4894–4901, 2011
KEYWORDS: carbon dioxide; catalysis; cobalt; copolymerization;
epoxide; isotactic; polymerization catalysis; regioselective ring
opening; ring-opening polymerization; stereochemistry control
afford
a perfectly regioregular poly(propylene carbonate)
(PPC), with >99% head-to-tail linkages, >99% carbonate link-
INTRODUCTION The accurate control of polymer stereo-
chemistry remains an important goal in polymerization catal-
ysis.1 For CO2/terminal epoxides [such as propylene oxide
(PO)] copolymerization,2 there also exists interesting regio-
chemistry concerning epoxide ring opening and stereochem-
to-tail linkage was observed in the resulting polymers.5–8
However, further improvement in copolymer regiochemistry
by decreasing reaction temperature is very limited. The
epoxide ring opening at the methine CAO bond may cause a
change in stereochemistry with inversion, so the absolutely
ring opening at the methylene CAO bond is a prerequisite
for achieving completely stereoregular polycarbonates.3
istry regarding carbonate unit sequence in
a polymer
(Scheme 1).3 Regioselective ring opening of the epoxide sig-
nificantly affects the resulting polymer stereochemistry,
because ring opening occurring at the methylene CAO bond
normally retains the stereochemistry at the methine carbon.
On the contrary, the ring opening at the methine CAO bond
may cause a change in stereochemistry at the methine car-
bon with inversion (Scheme 2). The formation of head-to-tail
carbonate linkage involves successive epoxide ring opening
at either the methylene CAO bond or the methine CAO
bond, while head-to-head or tail-to-tail carbonate linkages
produce in alternating ring opening at methylene CAO bond
of one epoxide and methine CAO bond of the following
epoxide. Aliphatic terminal epoxide ring opening is seemed
to be typically favored at the least hindered CAO bond, but
indeed cleavage is normally observed at both CAO bonds,
giving regioirregular polymers with ꢁ60% head-to-tail link-
age.4 Recently, some systems based on SalenCo(III) com-
plexes were reported as regioregular catalysts for CO2/termi-
nal epoxides copolymerization at ambient temperature, and
in the presence of a nucleophilic cocatalyst up to 95% head-
Recently, we succeeded in tuning Cr(III) complexes into
regioregular catalyst for the copolymerization of CO2 and ali-
phatic epoxides by designing sterically hindered coordination
environment around the central metal ion.9 Based on the
understanding on stereochemistry control mechanism of
coordination catalysis polymerization,10 we developed
unsymmetrical Salen ligand bearing derived chiral
a
a
BINOL.11 The cobalt complex of this ligand, in conjunction
with a nucleophilic cocatalyst, exhibited good activity and
stereoselectivity for asymmetric copolymerization of CO2 and
racemic PO. However, it is difficult to achieve high turnover
frequency at a high [epoxides]/[catalyst] ratio, which may
prevent the produced polymer from being high molecular
weight. More recently, Nozaki and coworkers reported a
novel stereogradient poly(propylene carbonate) (PPC) by
regioselective and enantioselective copolymerization of race-
mic PO with CO2.12 The stereogradient polymer is thermally
more robust and exhibits a decomposition temperature of
280 ꢀC, higher than that of the typical stereorandom PPC
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Regiochemistry and stereochemistry of polycarbonates from the alternating copolymerization of CO2 and terminal
epoxides.
ꢀ
(ꢁ240 C). The cobalt complexes anchoring organic bases or
quaternary ammonium salts can efficiently operate the alter-
nating copolymerization of epoxides with CO2 to afford the
corresponding polycarbonate at high [epoxides]/[catalyst] ra-
tio.13,14 Stimulated by the success such achieved, in this arti-
cle, we report novel multichiral catalyst systems (Fig. 1, 1a–
c, 2), which contain a derived chiral BINOL and a 1,5,7-tria-
bicyclo[4.4.0] dec-5-ene (designated as TBD, a sterically hin-
dered organic base) unit anchored on the ligand framework
for asymmetric, regioselective and stereoselective alternating
copolymerization of CO2 with racemic aliphatic terminal
epoxides. In these complexes, the derived BINOL can tune
the chiral environment around the central metal ion; mean-
while, the appended TBD may play an important role in
maintaining stability and high activity of the catalyst at low
temperatures and/or low catalyst loading.
has high molecular weight and narrow molecular weight dis-
tributions less than 1.1. Notably, the resulting polymers have
a high head-to-tail linkage of 99%. Such a microstructure is
consistent with a highly regioselective ring opening of PO
during the copolymerization with CO2. Interestingly, the
steric hindrance of 20-alkyloxy group in the chiral BINOL has
an important influence on kinetic resolution of the racemic
epoxide during the copolymerization with CO2. Among these
catalysts, complex 1c with isopropoxy as substituent group
proved to be more efficient in consuming (R)-epoxide over
(S)-epoxide with a Krel of 12.9. On the contrary, complex 1a
with methoxy as substituent group shows less efficient in ki-
netic solution of racemic PO, indicating that highly steric
RESULTS AND DISCUSSION
The unsymmetrical chiral Schiff-base ligands in the com-
plexes 1a–c were synthesized by the reaction of the corre-
sponding functionalized salicylaldehydes with the condensa-
tion product of (1S,2S)-diaminocyclohexane mono(hydrogen
chloride)
and
(S)-3-formyl-2-hydroxy-20-alkyloxy-1,10-
binaphthyl (Scheme 3). For a comparison purpose, complexes
2–4 were prepared by a similar procedure of complexes 1a–
c. As these cobalt complexes are easily dissolved in neat
epoxide surveyed, the catalyzed coupling of CO2 and PO does
not require any organic cosolvent.
Our initial studies show that complexes 1a–c could effi-
ciently catalyze CO2/racemic PO copolymerization to selec-
tively afford PPCs with more than 99% carbonate linkages at
a [PO]/[catalyst] ratio of 5000 at 20 ꢀC and under 1.0 MPa
CO2 pressure (Table 1, entries 1–3). The isolated polymer
SCHEME 2 The stereochemistry involved in the (S)-epoxide
ring opening in the central cobalt ion of SalenCo(III) complexes
during the polymerization with CO2.
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FIGURE 1 General structure of SalenCo(III)X complexes contain derived chiral BINOL and TBD.
hindrance helps to promote the competition coordination of
(R)-epoxide over (S)-epoxide to the central metal ion.
dec-5-ene (MTBD),6(b) the higher Krel observed in the present
catalysts probably ascribes to the enhanced chiral induction
in the presence of derived (S)-BINOL in complexes 1a–c. To
clarify it, we also synthesized cobalt-based complex 2 with a
In comparison with our previously reported binary catalyst
system of SalcyCo(III)X and 7-methyl-1,5,7-triazabicyclo[4.4.0]-
SCHEME 3 General synthetic procedure for unsymmetrical multichiral ligands.
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TABLE 1 The Alternating Copolymerization of Aliphatic
regioselective ring opening of epoxide during the polymer-
ization. Indeed, in previously reported binary catalyst sys-
tem of SalcyCo(III) complex in conjunction with an ionic or-
a
Terminal Epoxides with CO2
t
ganic ammonium salt, substitution of H for the Bu group
on the three position of Salcy ligand resulted in significant
decreases in polymer enantioselectivity and head-to-tail
linkages.6(b)
Temp. Time TOFb Mn
HTd
c
Generally, the reaction temperature has a significant influ-
ence on kinetic resolution of the racemic epoxide during
the copolymerization with CO2. A decrease in reaction tem-
perature from 20 to 0 ꢀC results in a Krel from 12.9 dra-
matically increasing to 19.0 (entry 7). Furthermore, when
the reaction was performed at ꢂ20 ꢀC, a Krel of 24.3 was
achieved, the highest record in this asymmetric copolymer-
ization (entry 8). The resultant polycarbonates have
more than 99% head-to-tail linkages [Fig. 2(B)]. It is worth
noting here that a decrease in CO2 pressure even at
0.1 MPa did not result in any change in polymer
(hꢂ1
)
(Kg/mol) PDIc (%) Krel
e
Entry R Catal. (ꢀC)
(h)
1
Me 1a
Me 1b
Me 1c
Me 2
20
20
20
20
20
20
20
ꢂ20
20
0
12
12
12
12
4
145
140
130
132
370
328
51
86.9
84.6
84.3
81.2
77.5
67.8
58.2
20.4
21.5
30.3
15.6
98.7
1.09 99
1.10 99
1.10 99
1.10 99
1.08 92
1.09 96
10.1
2
12.0
12.9
1.4
3
4
5
Me 3
3.5
6
Me 4
4
5.7
7
Me 1c
Me 1c
Me 1c
Et 1c
Bu 1c
Me 1c
24
24
6
1.09 >99 19.0
1.15 >99 24.3
8
16
9f
10g
11h
12i
a
64
1.08 99
13.1
24
24
6
20
1.16 >99 9.8
1.17 >99 6.7
0
7
20
236
1.09 100
–
The reaction was performed in neat racemic epoxides in 50-mL auto-
clave at 1.0 MPa CO2 pressure. The molar ratio of catalyst/epoxide is 1/
5000, excepting for entry 8 with 1/1000 and entry 11 with 1/500.
b
Turnover frequency of epoxide to polymer product.
c
Determined by gel permeation chromatography in THF, calibrated
with polystyrene standards.
d
HT
¼
head-to-tail linkage, determined by using 13C NMR
spectroscopy.
e
Krel ¼ ln[1 ꢂ c(1 þ ee)]/ln[1 ꢂ c(1 ꢂ ee)], where c is the conversion,
and ee is based on the enantiomeric excess of cyclic products given af-
ter complete degradation of polycarbonates.
f
The reaction was performed at 0.1 MPa CO2 pressure.
g
Racemic 1,2-butene oxide was used.
h
Racemic 1,2-hexene oxide was used.
(R)-Propylene oxide was used. Note: Polymer selectivity of more than
i
99% was observed in all cases, based on 1H NMR spectroscopy. All
resulted polymer contains more than 99% carbonate linkages as deter-
mined by 1H NMR spectroscopy.
(S,S,R) configuration originated from (1S,2S)-diaminocyclohex-
ane backbone and derived (R)-BINOL for asymmetric copoly-
merization of CO2 with racemic PO. Although complex 2
exhibited high activity as similar to complexes 1a–c and the
resulting polycarbonate also has a high head-to-tail linkage up
to 99%, the Krel is very low (entry 4), indicative of a poor chi-
ral induction for this reaction. This result demonstrates that
the S configuration of derived BINOL in complexes 1a–c plays
an important role in chiral induction during the asymmetric
copolymerization of CO2 with racemic PO.
For a comparison purpose, we also tested catalysts 3 and 4
for the kinetic resolution of PO in the copolymerization
with CO2. Although these complexes exhibited higher activ-
ity than catalysts 1a–c, the lower Krel and head-to-tail link-
age content were observed in these systems (entries 5 and
6). Obviously, the appended TBD on the three position of
one aromatic ring has a negative effect on the Krel and
FIGURE 2 Carbonyl region of the 13C NMR spectra of poly(pro-
pylene carbonate)s with: (A) 70% HT linkages, (B) more than
99% HT linkages with the system of catalyst 1c and racemic
propylene oxide obtained at 0 ꢀC, and (C)100% HT linkages
with the system of catalyst 1c and (R)-propylene oxide
obtained at 20 ꢀC (Table 1, entry 12).
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CH2Cl2 (150 mL) was stirred at ambient temperature for 20
min. At 15 ꢀC, compound A16 (3.0 g, 0.010 mol) in CH2Cl2
(100 mL) was added and stirred at ambient temperature for
12 h. The mixture was then partitioned between water (200
mL). The aqueous layer was extracted with CH2Cl2 (2 ꢃ 50
mL), and the combined organic layer was washed with water
(1 ꢃ 100 mL), 10% CuSO4 (2 ꢃ 200 mL), water (1 ꢃ 200
mL), saturated NaHSO3 (2 ꢃ 200 mL), and brine (1 ꢃ 200
mL), dried over Na2SO4, filtered, and concentrated under
reduced pressure. The residue was slurried in iPr2O (200
mL) and filtered. The filtrate was concentrated in vacuo and
purified by column chromatography on silica gel using petrol
ether/ethyl acetate (5/1) as the mobile phase to give the
afforded 4-(3-iodopropyl)-2-(1-adamanyl)-methoxybenzene
(B) as a white solid (3.2 g, 78%).
FIGURE 3 DSC thermograms of (A) stereoregular and (B) irreg-
1H NMR (400 MHz, CDCl3): d 7.02 (s, 1H), 6.98 (d, J ¼ 8.4
Hz, 1H), 6.77 (d, J ¼ 8.4 Hz, 1H), 3.83 (s, 3H), 3.18 (t, J ¼
7.4 Hz, 2H), 2.66 (t, J ¼ 7.4 Hz, 2H), 2.11 (s, 6H), 2.08 (s,
3H), 2.06–2.08 (m, 2H), 1.77 (s, 6H). HRMS (m/z) Calcd. for
[C20H28IO]þ: 411.1185, found: 411.1195.
ular poly(propylene carbonate)s.
microstructure, excepting for an obvious decrease in reac-
tion rate (entry 9).
The (S,S,S)-cobalt-based complex 1c is also a highly efficient
catalyst system for the copolymerization of CO2 and various
terminal epoxides with a long aliphatic chain to provide cor-
responding optically active polycarbonates under solvent-
free conditions (entries 10 and 11). As anticipated, the 13C
NMR spectra in the carbonyl region show that these polycar-
bonates all have a head-to-tail content of >99% (see the
Supporting Information Fig. S3).
Synthesis of 7-(3-(3-(1-Adamanyl)-4-methoxyphenyl)
propyl)-1,5,7-triabicyclo[4.4.0] Dec-5-ene (C)
A mixture of TBD (1.0 g, 7.4 mmol) and NaH (0.55 g, 0.022
mol) in THF (30 mL) was stirred for 2 h at ambient temper-
ature, and then was added slowly a solution of B (2.0 g, 4.9
mmol) in THF (10 mL). The mixture was stirred for another
24 h and then filtered. The filtrate was evaporated, and the
residue was purified by column chromatography on silica gel
using CH2Cl2/CH3OH (10:1) as the mobile phase to give the
7-(3-(3-(1-adamanyl)-4-methoxyphenyl)propyl)-1,5,7-triabi-
cyclo[4.4.0] dec-5-ene (C) as a while solid (1.5 g, 73%).
1H NMR (400 MHz, CDCl3): d 7.28 (s, 1H), 7.13 (d, J ¼ 8.4
Hz, 1H), 6.67 (d, J ¼ 8.4 Hz, 1H), 3.80 (s, 3H), 3.74 (t, J ¼
7.4 Hz, 2H), 3.51–3.53 (m, 2H), 3.25–3.32 (m, 6H), 2.64 (t, J
¼ 7.4 Hz, 2H), 2.08 (s, 6H), 2.05 (s, 3H), 1.94–2.03 (m, 6H),
1.73 (s, 6H). HRMS (m/z) Calcd. for [C27H40N3O]þ: 422.3171,
found: 422.3199.
The isotactic polymer is highly desired to show novel proper-
ties.15 As to the copolymerization of CO2 with aliphatic termi-
nal epoxides, it is very difficult to obtain completely isotactic
polycarbonates even though using the optically active epoxi-
des.3,9(a) The reason is that ring opening of a terminal epoxide
occurring at the methine CAO bond causes a change in stereo-
chemistry with inversion (Scheme 2). In this article, we were
gratified to discover that the (S,S,S)-cobalt-based complex 1c
exhibited a perfectly regioselective ring opening of racemic PO
at 20 ꢀC. As a result, this catalyst was also applied to the
copolymerization of CO2 and (R)-PO at the same reaction con-
dition (entry 12). The resulting polycarbonates have an unprec-
edented head-to-tail linkages of 100% [Fig. 2(C)], consistent
with a completely isotactic microstructure. Furthermore, chiral
GC analysis of cyclic carbonate originating from the depolymer-
ization of the resulting polycarbonate demonstrated that no
inversion in stereochemistry was observed in the methine car-
bon, indicating ring opening completely occurring at the meth-
ylene CAO bond of (R)-PO during the copolymerization with
CO2. The isotactic PPC resulted from the copolymerization of
CO2 and (R)-PO exhibits an enhanced glass transition tempera-
Synthesis of 7-(3-(3-(1-Adamanyl)-4-hydroxyphenyl)
propyl)-1,5,7-triabicyclo[4.4.0] Dec-5-ene (D)
Compound C (1.4 g, 3.5 mmol) was dissolved in CH2Cl2 (50
mL), cooled to ꢂ78 ꢀC, and BBr3 (1.8 mL, 0.018 mol) was
added. The mixture was allowed to stir at ꢂ78 ꢀC for 30
min and at room temperature for 12 h. The mixture was
cooled to 0 ꢀC, quenched with saturated NaHCO3 (50 mL),
and extracted with CH2Cl2. The organic layer was dried
(Na2SO4) and filtered. The filtrate was evaporated, and the
residue was purified by column chromatography on silica gel
using CH2Cl2/CH3OH (10:1) as the mobile phase to give the
7-(3-(3-(1-adamanyl)-4-hydroxyphenyl)propyl)-1,5,7-triabicy-
clo[4.4.0] dec-5-ene (D) as a while solid (1.5 g, 73%).
ꢀ
ꢀ
ture of 47 C [Fig. 3(A)], which is 10–12 C higher than that of
the corresponding irregular polycarbonate [Fig. 3(B)].
EXPERIMENTAL
Synthesis of 4-(3-Iodopropyl)-2-(1-adamanyl)-
methoxybenzene (B)
A mixture of triphenylphosphine (2.9 g, 0.011 mol), iodine
(2.8 g, 0.011 mol), and imidazole (0.2 g, 0.03 mol) in dry
1H NMR (400 MHz, CDCl3): d 7.00 (d, J ¼ 8.0 Hz, 1H), 6.94
(s, 1H), 6.82 (d, J ¼ 8.0 Hz, 1H), 3.56 (t, J ¼ 7.2 Hz, 2H),
3.38–3.40 (m, 2H), 3.22–3.26 (m, 6H), 2.64 (t, J ¼ 7.2 Hz,
2H), 2.14 (s, 6H), 2.09 (s, 3H), 1.96–2.02 (m, 6H), 1.78 (s,
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6H). HRMS (m/z) Calcd. for [C26H38N3O]þ: 408.3015, found:
Synthesis of Chiral Ligand (S,S,S)-L1b
408.3008.
It was synthesized as a similar procedure of (S,S,S)-L1a, only
with the substitution of (S)-3-formyl-2-hydroxy-20-butoxy-
1,10-binaphthyl for (S)-3-formyl-2-hydroxy-20-methoxy-1,10-
binaphthyl. Yield: 46%. [a]2D0 þ208 (c 1.0, CHCl3).
Synthesis of 7-(3-(5-(1-Adamanyl)-3-formyl-4-
hydroxyphenyl)propyl)-1,5,7-triabicyclo[4.4.0]
Dec-5-ene (E)
1H NMR (400 MHz, CDCl3): d 13.80 (s, 1H), 13.03 (s, 1H),
8.56 (s, 1H), 8.25 (s, 1H), 7.93 (d, J ¼ 8.4 Hz, 1H), 7.83 (d, J
¼ 8.4 Hz, 1H), 7.79 (d, J ¼ 8.4 Hz, 1H), 7.73 (d, J ¼ 8.4 Hz,
1H), 7.42 (t, J ¼ 8.4 Hz, 1H), 7.14–7.35 (m, 6H), 7.05 (d, J ¼
8.4 Hz, 1H), 6.91 (s, 1H), 3.95–4.03 (m, 2H), 3.62–3.65 (m,
2H), 3.40–3.48 (m, 3H), 3.19–3.23 (m, 3H), 3.02–3.10 (m,
4H), 2.77 (t, J ¼ 7.6 Hz, 2H), 2.18 (s, 6H), 2.11 (s, 3H), 1.50–
2.01 (m, 20H), 1.33–1.45 (m, 2H), 0.88–0.95 (m, 2H), 0.59 (t,
J ¼ 7.6 Hz, 2H). 13C NMR (100 MHz, CDCl3): d 166.0, 165.7,
162.3, 158.2, 154.2, 150.4, 138.1, 136.3, 134.1, 130.5, 130.3,
129.8, 129.7, 129.6, 128.7, 128.2, 127.3, 126.9, 126.8, 126.7,
124.2, 124.1, 124.0, 123.9, 120.2, 119.9, 74.1, 72.1, 68.7,
50.4, 48.3, 47.6, 45.9, 43.1, 39.8, 38.1, 35.8, 34.3, 33.0, 30.6,
26.9, 26.8, 24.1, 24.0, 20.8, 20.0, 17.9, 13.9. HRMS (m/z)
Calcd. for [C58H70N5O3]þ: 884.5479, found: 884.5498.
Compound D (1.1 g, 3.0 mmol) was dissolved in anhydrous
THF (50 mL). Paraformaldehyde (0.45 g, 15.0 mmol), trie-
thylamine (0.47 mL, 3.4 mmol), and magnesium chloride
(0.32 g, 3.4 mmol) were added under a nitrogen atmosphere.
After the reaction mixture was refluxed for 3 h, the solvent
was evaporated. CH2Cl2 (100 mL) and 10% aq. HCl (10 mL)
were added as the residue. The organic extracts were
washed by saturated NaHCO3 (100 mL), dried (Na2SO4), and
filtered. The filtrate was evaporated, and the residue was
purified by column chromatography on silica gel using
CH2Cl2/CH3OH (10:1) as the mobile phase to give the 7-(3-
(5-(1-adamanyl)-3-formyl-4-hydroxyphenyl)propyl)-1,5,7-tria-
bicyclo[4.4.0] dec-5-ene (E) as a yellow solid (0.7 g, 95%).
1H NMR (400 MHz, CDCl3): d 11.69 (s, 1H), 9.85 (s, 1H), 7.41
(s, 1H), 7.33 (s, 1H), 3.70–3.76 (m, 4H), 3.29–3.43 (m, 6H),
2.81 (t, J ¼ 7.2 Hz, 2H), 2.12 (s, 6H), 2.08 (s, 3H), 1.65–1.94
(m, 6H), 1.75 (s, 6H). 13C NMR (100 MHz, CDCl3): d 196.8,
158.5, 150.7, 145.2, 136.1, 128.7, 123.6, 121.7, 50.9, 48.6,
47.3, 46.1, 43.1, 38.1, 34.2, 29.8, 27.1, 26.8, 21.8, 21.5. HRMS
(m/z) Calcd. for [C27H38N3O2]þ: 435.2964, found: 435.2975.
Synthesis of Chiral Ligand (S,S,S)-L1c
It was synthesized as a similar procedure of (S,S,S)-L1a, only
with the substitution of (S)-3-formyl-2-hydroxy-20-isopro-
poxy-1,10-binaphthyl for (S)-3-formyl-2-hydroxy-20-methoxy-
1,10-binaphthyl. [a]D20 þ211 (c 1.0, CHCl3).
1H NMR (400 MHz, CDCl3): 13.80 (s, 1H), 12.98 (s, 1H), 8.57
(s, 1H), 8.18 (s, 1H), 7.89 (d, J ¼ 8.0 Hz, 1H), 7.81–7.84 (m,
2H), 7.74–7.76 (m, 2H), 7.37 (d, J ¼ 8.0 Hz, 1H), 7.31 (t, J ¼
8.0 Hz, 1H), 7.17–7.23 (m, 3H), 7.03–7.06 (m, 2H), 6.84 (s,
1H), 4.25–4.31 (m, 1H), 3.52–3.55 (m, 1H), 3.44–3.50 (m,
4H), 3.19–3.23 (m, 3H), 3.03–3.10 (m, 4H), 2.62 (t, J ¼ 7.6
Hz, 2H), 2.13 (s, 6H), 2.09 (s, 3H), 1.48–2.01 (m, 20H), 0.87
(d, J ¼ 6.0 Hz, 3H), 0.80 (d, J ¼ 6.0 Hz, 3H). 13C NMR (100
MHz, CDCl3): d 165.4, 165.3, 158.8, 154.5, 154.2, 150.1,
137.4, 135.3, 134.0, 133.1, 130.1, 129.8, 129.3, 129.0, 128.6,
128.1, 127.7, 127.2, 126.3, 125.4, 125.1, 123.9, 123.1, 121.6,
120.6, 118.9, 118.3, 117.5, 73.1, 73.0, 71.9, 50.4, 48.2, 47.2,
46.3, 40.4, 38.6, 37.2, 37.0, 33.1, 32.7, 31.8, 29.1, 28.8, 24.3,
24.2, 22.6, 22.4, 21.0, 20.7. HRMS (m/z) Calcd. for
[C57H68N5O3]þ: 870.5322, found: 870.5348.
Synthesis of Chiral Ligand (S,S,S)-L1a
A flask was charged with (1S,2S)-diaminocyclohexane mono
(hydrogen chloride) (0.15 g, 1.0 mmol), activated 5-Å molecu-
lar sieve (1.00 g), and anhydrous methanol (10 mL).
Compound (S)-3-formyl-2-hydroxy-20-methoxy-1,10-binaphthyl
(0.37 g, 1.0 mmol) was added in one portion, and the reaction
mixture was stirred at room temperature for 2 h. A solution
of E (0.36 g, 1.0 mmol) in anhydrous CH2Cl2 (10 mL) was
then added to the reaction system, followed by the slow addi-
tion of triethylamine (0.27 mL, 2.0 mmol). After stirring at
room temperature for an additional 8 h, the reaction mixture
was filtered through a short pad of dry silica gel. Then, all sol-
vents and the excessive triethylamine were removed in vacuo.
The residue was purified by column chromatography on silica
gel using petrol ether/ethyl acetate (5:1, 1% Et3N) as the mo-
bile phase to give the afforded compound (S,S,S)-L1a as a
bright yellow solid (0.37 g, 42%). [a]2D0 þ179 (c 1.0, CHCl3).
Synthesis of Chiral Ligand (S,S,R)-L2
It was synthesized as a similar procedure of (S,S,S)-L1c, only
with the substitution of (R)-3-formyl-2-hydroxy-20-isopro-
poxy-1,10-binaphthyl for (S)-3-formyl-2-hydroxy-20-isopro-
poxy-1,10-binaphthyl. Yield: 48%. [a]D20 þ53 (c 1.0, CHCl3).
1H NMR (400 MHz, CDCl3): 13.79 (s, 1H), 13.12 (s, 1H), 8.45
(s, 1H), 8.23 (s, 1H), 8.01 (d, J ¼ 8.0 Hz, 1H), 7.89 (d, J ¼ 8.0
Hz, 1H), 7.73–7.76 (m, 2H), 7.47 (t, J ¼ 8.0 Hz, 1H), 7.20–
7.31 (m, 5H), 7.01–7.05 (m, 2H), 6.99 (d, J ¼ 8.0 Hz, 1H),
3.78 (s, 3H), 3.60–3.64 (m, 2H), 3.41–3.48 (m, 3H), 3.20–
3.25 (m, 3H), 3.01–3.09 (m, 4H), 2.78 (t, J ¼ 7.6 Hz, 2H),
2.17 (s, 6H), 2.10 (s, 3H), 1.46–2.04 (m, 20H). 13C NMR (100
MHz, CDCl3): d 165.8, 165.6, 162.1, 158.0, 154.2, 150.2,
138.0, 136.2, 134.0, 130.4, 130.1, 129.8, 129.7, 129.5, 128.6,
128.2, 127.2, 126.8, 126.7, 124.1, 124.0, 123.8, 120.1, 119.8,
74.0, 72.0, 58.2, 50.1, 48.2, 47.3, 45.4, 43.0, 39.7, 38.3, 35.7,
34.3, 33.0, 30.4, 26.9, 26.8, 24.1, 24.0, 20.1. HRMS (m/z)
Calcd. for [C55H64N5O3]þ: 842.5009, found: 842.5038.
1H NMR (400 MHz, CDCl3): 13.80 (s, 1H), 13.12 (s, 1H), 8.47
(s, 1H), 8.24 (s, 1H), 8.03 (d, J ¼ 8.0 Hz, 1H), 7.90 (d, J ¼ 8.0
Hz, 1H), 7.74–7.78 (m, 2H), 7.49 (t, J ¼ 8.0 Hz, 1H), 7.21–
7.33 (m, 6H), 7.06 (d, J ¼ 8.0 Hz, 1H), 6.98 (d, J ¼ 8.0 Hz,
1H), 4.56–4.60 (m, 1H), 3.62–3.65 (m, 2H), 3.41–3.49 (m,
3H), 3.20–3.24 (m, 3H), 3.02–3.10 (m, 4H), 2.79 (t, J ¼ 7.6
Hz, 2H), 2.15 (s, 6H), 2.08 (s, 3H), 1.48–2.01 (m, 20H), 0.89
(d, J ¼ 6.0 Hz, 3H), 0.81 (d, J ¼ 6.0 Hz, 3H). 13C NMR (100
MHz, CDCl3): d 165.8, 165.5, 162.2, 158.0, 154.1, 150.3,
138.1, 136.1, 134.2, 130.6, 130.4, 129.9, 129.8, 129.7, 129.5,
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128.4, 128.0, 127.1, 126.9, 126.7, 124.1, 124.0, 123.9, 120.0,
119.5, 74.0, 72.0, 71.8, 50.0, 48.2, 47.4, 45.4, 41.3, 39.9, 38.2,
38.0, 35.6, 33.1, 30.2, 30.1, 24.6, 24.0, 22.3, 20.5, 20.0. HRMS
(m/z) Calcd. for [C57H68N5O3]þ: 870.5322, found: 870.5367.
1H NMR (400 MHz, DMSO-d6): d 8.47 (s, 1H), 8.45 (s, 1H),
8.02 (d, J ¼ 8.4 Hz, 1H), 7.89 (d, J ¼ 8.4 Hz, 1H), 7.81–7.88
(m, 2H), 7.63 (d, J ¼ 8.4 Hz, 1H), 7.35 (s, 1H), 7.07–7.23 (m,
6H), 6.45 (s, 1H), 4.69–4.72 (m, 1H), 3.59–3.64 (m, 2H),
3.41–3.43 (m, 2H), 3.30–3.35 (m, 8H), 2.95–3.13 (m, 4H),
2.13 (s, 6H), 2.06 (s, 3H), 1.49–2.00 (m, 18H), 1.14 (d, J ¼
6.0 Hz, 3H), 0.86 (d, J ¼ 6.0 Hz, 3H). 13C NMR (100 MHz,
DMSO-d6): d 167.0, 165.1, 161.7, 158.6, 153.8, 150.4, 141.9,
138.2, 137.4, 135.9, 134.3, 139.1, 128.9, 128.8, 128.6, 127.7,
127.6, 125.4, 125.3, 125.2, 123.6, 123.1, 122.8, 122.3, 121.7,
121.4, 118.3, 115.9, 72.2, 69.7, 69.3, 59.0, 55.8, 48.7, 47.0,
46.6, 45.7, 38.2, 36.7, 36.3, 30.6, 29.5, 28.1, 27.9, 24.1, 24.0,
22.7, 22.1, 20.3, 20.1, 18.4, 13.9. HRMS (m/z) Calcd. for
[C57H66N6O5Co]þ: 989.4376, found: 989.4389.
Synthesis of Complex (S,S,S)-1a
Cobalt(II)acetate (0.054 g, 0.30 mmol) and the ligand (S,S,S)-
L1a (0.20 g, 0.25 mmol) were dissolved in methanol (10
mL). After stirring for 12 h at room temperature, LiCl (0.022
g, 0.50 mmol) was added, and the mixture was further
stirred for 12 h while oxygen was bubbled through the reac-
tion mixture. The solvent was removed in vacuo, and the res-
idue was extracted with CH2Cl2. The organic layer was
washed with saturated NaHCO3 and brine and then dried
over anhydrous Na2SO4. The mixture was filtered, and the fil-
trate was concentrated under vacuum. After removal of the
solvent, the residue was dissolved in CH2Cl2 (10 mL) in a
40-mL Schlenk vial wrapped in aluminum foil was added
AgNO3 (0.051 g, 0.30 mmol). The mixture was stirred for 24
h and then filtered for removing the Ag byproduct. The sol-
vent was removed in vacuo. The resulted solid was further
treated with the mixture of CH2Cl2 and hexane to afford
(S,S,S)-1a as a dark green solid (0.18 g, 91%).
1H NMR (400 MHz, DMSO-d6): d 8.46 (s, 1H), 8.43 (s, 1H),
8.00 (d, J ¼ 8.0 Hz, 1H), 7.86 (d, J ¼ 8.0 Hz, 1H), 7.82–7.88
(m, 2H), 7.63 (d, J ¼ 8.0 Hz, 1H), 7.37 (s, 1H), 7.01–7.22 (m,
6H), 6.48 (s, 1H), 3.81 (s, 3H), 3.56–3.62 (m, 2H), 3.40–3.43
(m, 2H), 3.28–3.33 (m, 8H), 2.96–3.13 (m, 4H), 2.14 (s, 6H),
2.07 (s, 3H), 1.51–2.00 (m, 18H). 13C NMR (100 MHz, DMSO-
d6): d 166.9, 165.0, 161.6, 158.6, 153.7, 150.3, 141.9, 138.0,
137.2, 135.9, 134.4, 129.0, 128.9, 128.8, 128.7, 127.7, 127.6,
125.4, 125.2, 125.1, 123.6, 123.2, 122.8, 122.5, 121.5, 121.3,
118.6, 115.1, 70.1, 69.9, 58.2, 49.1, 47.8, 47.5, 46.3, 40.1,
39.0, 37.2, 36.9, 33.2, 30.0, 29.9, 28.4, 27.6, 24.2, 24.1, 20.2,
20.1. HRMS (m/z) Calcd. for [C55H62N6O5Co]þ: 961.4063,
found: 961.4096.
Synthesis of Complex (S,S,R)-2
It was synthesized as a similar procedure of (S,S,S)-1a, only
with the substitution of (S,S,R)-L2 for (S,S,S)-L1a. Yield: 86%.
1H NMR (400 MHz, DMSO-d6): d 8.56 (s, 1H), 8.51 (s, 1H),
8.04 (d, J ¼ 8.4 Hz, 1H), 7.90 (d, J ¼ 8.4 Hz, 1H), 7.82–7.88
(m, 2H), 7.64 (d, J ¼ 8.4 Hz, 1H), 7.35 (s, 1H), 7.07–7.25 (m,
6H), 6.56 (s, 1H), 4.71–4.78 (m, 1H), 3.59–3.65 (m, 2H),
3.42–3.44 (m, 2H), 3.31–3.35 (m, 8H), 2.98–3.14 (m, 4H),
2.15 (s, 6H), 2.07 (s, 3H), 1.49–2.01 (m, 18H), 1.14 (d, J ¼
6.0 Hz, 3H), 0.88 (d, J ¼ 6.0 Hz, 3H). 13C NMR (100 MHz,
DMSO-d6): d 167.4, 165.5, 161.9, 158.6, 153.7, 150.3, 142.0,
138.2, 137.4, 135.7, 134.5, 139.1, 1290, 128.9, 128.7, 127.7,
127.6, 125.4, 125.3, 123.6, 123.3, 122.8, 122.5, 121.8, 121.6,
118.5, 115.9, 72.3, 69.8, 69.2, 59.3, 55.6, 48.5, 47.0, 46.2,
45.2, 38.4, 36.7, 36.3, 30.5, 29.2, 28.0, 27.6, 24.1, 24.0, 22.6,
22.0, 20.5, 20.0, 18.8, 15.0. HRMS (m/z) Calcd. for
[C57H66N6O5Co]þ: 989.4376, found: 989.4342.
Polymerization Procedure
Typically, to a stirred mixture of complex (S,S,S)-1c (10.0 mg,
0.010 mmol, 1 equiv) was dissolved in PO (3.5 ml, 50 mmol,
5000 equiv) to form red-brown solution in a nitrogen atmos-
phere. The mixture solution was charged into a predried 50-
mL autoclave equipped with a magnetic stirrer underꢀa CO2
atmosphere. The autoclave was put into a bath of 20 C and
then pressurized to appropriate pressure with CO2. After the
allotted reaction time, the unreacted PO was isolated at am-
bient temperature by vacuum (10 mmHg) transfer into a
cooled (–20 ꢀC) receiving flask. A small amount of the resi-
Synthesis of Complex (S,S,S)-1b
It was synthesized as a similar procedure of (S,S,S)-1a, only
with the substitution of (S,S,S)-L1b for (S,S,S)-L1a. Yield: 88%.
1H NMR (400 MHz, DMSO-d6): d 8.48 (s, 1H), 8.44 (s, 1H),
8.00 (d, J ¼ 8.8 Hz, 1H), 7.90 (d, J ¼ 8.8 Hz, 1H), 7.80–7.86
(m, 2H), 7.63 (d, J ¼ 8.8 Hz, 1H), 7.37 (s, 1H), 7.07–7.25 (m,
6H), 6.39 (s, 1H), 4.00–4.05 (m, 1H), 3.90–9.94 (m, 1H),
3.60–3.65 (m, 2H), 3.19–3.30 (m, 8H), 2.91–3.10 (m, 4H),
2.15 (s, 6H), 2.12 (s, 3H), 1.54–2.05 (m, 18H), 1.37–1.48 (m,
2H), 0.85–0.93 (m, 2H), 0.56 (t, J ¼ 7.6 Hz, 2H). 13C NMR
(100 MHz, DMSO-d6): d 166.0, 164.2, 161.5, 158.6, 153.7,
154.9, 141.8, 138.3, 137.2, 135.8, 135.7, 134.3, 129.1, 128.9,
128.8, 128.5, 127.5, 127.4, 125.4, 125.3, 123.5, 123.0, 122.8,
122.1, 121.6, 121.5, 118.3, 115.8, 69.9, 69.2, 67.7, 49.4, 47.6,
47.2, 46.1, 40.3, 39.1, 36.9, 36.8, 30.8, 29.7, 29.6, 28.5, 28.0,
24.2, 24.1, 20.6, 20.4, 17.9, 13.3. HRMS (m/z) Calcd. for
[C58H68N6O5Co]þ: 1003.4532, found: 1003.4516.
1
due was removed for H NMR analysis to quantitatively give
the selectivity of PPC to cyclic propylene carbonate, as well
as carbonate linkages and also used for GPC analysis. The
crude polymer was dissolved in 10 mL CHCl3/MeOH (5/1,
v/v) mixture and precipitated from methanol or diethyl
ether. This process was repeated three to five times to com-
pletely remove the catalyst, and white polymer was obtained
by vacuum dry.
CONCLUSIONS
Several novel multichiral cobalt-based complexes containing
a derived chiral BINOL and an appended TBD were devel-
oped for asymmetric, regioselective and stereoselective alter-
nating copolymerization of CO2 with racemic aliphatic
Synthesis of Complex (S,S,S)-1c
It was synthesized as a similar procedure of (S,S,S)-1a, only
with the substitution of (S,S,S)-L1c for (S,S,S)-L1a. Yield: 91%.
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ARTICLE
Coates, G. W. J Am Chem Soc 2005, 127, 10869–10878; (c)
Cohen, C. T.; Coates, G. W. J Polym Sci Part A: Polym Chem
2006, 44, 5182–5191.
epoxides. The (S,S,S)-Co(III) complex 1c with sterically hin-
dered substituent group is more stereoregular catalyst for
this reaction to afford a perfectly regioregular PPC, with
more than 99% head-to-tail linkages and a Krel of 24.4 for
the enchainment of (R)-epoxide over (S)-epoxide, which all
are the highest records in this asymmetric copolymerization.
Furthermore, completely regioregular and isotactic polycar-
bonates with an enhanced glass transition temperature were
first synthesized by the use of optically active epoxide. Fur-
ther exploring the properties of these polymers will be the
subject of future investigations.
6 (a) Lu, X.-B.; Wang, Y. Angew Chem Int Ed 2004, 43,
3574–3577; (b) Lu, X.-B.; Shi, L.; Wang, Y.-M.; Zhang, R.; Zhang,
Y.-J.; Peng, X.-J.; Zhang, Z.-C.; Li, B. J Am Chem Soc 2006,
128, 1664–1674.
7 Paddock, R. L.; Nguyen, S. T. Macromolecules 2005, 38,
6251–6253.
8 (a) Niu, Y. S.; Li, H.; Chen, X. S.; Zhang, W.; Zhuang, X.; Jing,
X. Macromol Chem Phys 2009, 210, 1224–1229; (b) Liu, B. Y.;
Gao, Y.; Zhao, X.; Yan, W.; Wang, X. H. J Polym Sci Part A:
Polym Chem 2010, 48, 359–365.
This work was supported by National Natural Science Founda-
tion of China (NSFC) program (Grant 20634040) and National
Basic Research Program of China (973 Program:
2009CB825300). X.-B. Lu gratefully acknowledges the Outstand-
ing Young Scientist Foundation of NSFC (Grant 20625414).
9 (a) Li, B.; Wu, G.-P.; Ren, W.-M.; Wang, Y.-M.; Rao, D.-Y.; Lu,
X.-B. J Polym Sci Part A: Polym Chem 2008, 46, 6102–6113; (b)
Li, B.; Zhang, R.; Lu, X.-B. Macromolecules 2007, 40,
2303–2307; (c) Rao, D.-Y.; Li, B.; Zhang, R.; Wang, H.; Lu, X.-B.
Inorg Chem 2009, 48, 2830–2836; (d) Darensbourg, D. J.;
Ulusov, M.; Karroonnirum, O.; Poland, R. R.; Reibenspies, J. H.;
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