Microstructure Analysis of Poly(cyclopentene carbonate)s at the Diad Level

Article abstract of DOI:10.1021/acs.macromol.5b01759

The spectroscopic assignment of poly(cyclopentene carbonate)s at the diad level was performed by using two kinds of model compounds: isotactic and syndiotactic dimers of cyclopentene carbonate unit. By comparing the signals in the carbonyl region, we concluded that the signals at 153.85 and 153.78 ppm in the ¹³C NMR spectrum of poly(cyclopentene carbonate) were attributed to m-diad and r-diad, respectively. The signals at 82.61 and 82.53 ppm in the ¹³C NMR spectrum were assigned to m-diad and r-diad peak of methine resonance, respectively. It was found that the carbonate carbon signals were sensitive toward the stereocenters on adjacent epoxide ring-opening units. The syndiotactic and isotactic diads matched well with the microstructures of the stereoregular poly(cyclopentene carbonate)s that were prepared by using chiral dinuclear Co(III) complex catalysts.

Full text of DOI:10.1021/acs.macromol.5b01759

Article
pubs.acs.org/Macromolecules
Microstructure Analysis of Poly(cyclopentene carbonate)s at the
Diad Level
Ye Liu, Rong-Rong Li, and Xiao-Bing Lu*
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China
S
* Supporting Information
ABSTRACT: The spectroscopic assignment of poly(cyclopentene
carbonate)s at the diad level was performed by using two kinds of
model compounds: isotactic and syndiotactic dimers of cyclopentene
carbonate unit. By comparing the signals in the carbonyl region, we
concluded that the signals at 153.85 and 153.78 ppm in the 13C NMR
spectrum of poly(cyclopentene carbonate) were attributed to m-diad
and r-diad, respectively. The signals at 82.61 and 82.53 ppm in the 13C
NMR spectrum were assigned to m-diad and r-diad peak of methine
resonance, respectively. It was found that the carbonate carbon signals
were sensitive toward the stereocenters on adjacent epoxide ring-
opening units. The syndiotactic and isotactic diads matched well with
the microstructures of the stereoregular poly(cyclopentene carbonate)s
that were prepared by using chiral dinuclear Co(III) complex catalysts.
oligoether carbonates as the model compounds, R-
INTRODUCTION
■
(PO)nOCO2(PO)nR, where R = Me, Et, or H, n = 1, 2, 3, 4,
10, 30, and PO = propylene oxide ring-opened unit. The
13C{1H} NMR investigations showed that the carbonate carbon
signals had both regio- and stereosensitivity at the diad and the
tetrad levels to its adjacent propylene oxide (PO) ring-opened
units.12 In 2012, Rieger and co-workers have studied the
microstructures of the different poly(propylene carbonate)s by
a combination of chiral GC and high-resolution NMR
spectroscopy.13 In a recent study, we synthesized three kinds
of model compounds of poly(styrene carbonate)s with tail-to-
tail, head-to-tail, and head-to-head linkages for the micro-
structural assignment by NMR spectroscopy.14 It was found
that the syndiotactic and isotactic diads matched well with the
microstructures of the stereoregular poly(styrene carbonate)s
in our previous studies.15 Compared with the study on the
microstructures analysis of CO2-based polycarbonates from
terminal epoxides, CO2 copolymers from meso-epoxide have
historically received far less attention, except for cyclohexene
oxide.16,17 In 2001, Nozaki and co-workers completed the
spectroscopic assignment of poly(cyclohexene carbonate)
(PCHC) based on the spectroscopic analysis of model
oligomers, the signal at 153.7 ppm includes isotactic diads,
and the ones at 153.3−153.1 ppm include syndiotactic diads
(Scheme 2).16 Using Bernoullian statistical methods, PCHC
tetrad and triad sequences were assigned in the 13C NMR
spectra by Coates and co-workers.17 Recently, we reported a
chiral catalyst system based on enantiopure dinuclear Co(III)
There is a great correlation between the main-chain stereo-
chemistry of a polymer and its physical properties.1 For
example, the isotactic polypropylene is a typical semicrystalline
material, possessing a melting temperature (Tm) of ∼170 °C,
while the amorphous polypropylene is a viscous polymer at
ambient temperature with a Tg of ∼0 °C. For the biodegradable
polymer, e.g. polylactides, the isotactic polymers show a slower
degradation rate compared to the amorphous analogue due to
their higher crystallinity.2 As a consequence, the accurate
control of polymer microstructure is one of the most important
goals in the area of coordination polymerization catalysis.3
Indeed, the relative stereochemistry (the spatial arrangement of
atoms or groups in a polymeric unit) of adjacent locations in
the polymeric chain also preserves the traces of the reaction
pathway.4 Therefore, the assignment of the stereochemistry
information on a polymer chain is beneficial for understanding
the polymerization mechanism and further designing the more
active and enantioselective catalyst systems.
The alternating copolymerization of epoxides and CO2 to
generate biodegradable CO2-based polycarbonates was first
reported in 1969 by Inoue and co-workers.5 Since this
discovery, many hetereo- and homogeneous catalyst systems
have been developed for this transformation.6,7 Especially, the
well-defined binary or bifunctional catalysts based on metal−
salen complexes have proven to be very efficient in approaching
the high activity and excellent regio- and/or stereoselectivity for
the coupling reactions.8−11 Accompanied by the discovery of
these catalysts, the study on the regiochemistry of epoxide ring-
opening and stereochemistry of the carbonate unit sequence in
the resulting copolymer chains was also reported.12−14 In 2004,
Chisholm and co-workers have synthesized a series of
Received: August 7, 2015
Revised: September 11, 2015
© XXXX American Chemical Society
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Scheme 1. (a) Microstructure Study of Poly(propylene carbonate)s by Chisholm and Co-workers;12 (b) Microstructure Study of
Poly(propylene carbonate)s by Rieger and Co-workers;13 (c) Microstructure Study of Poly(styrene carbonate)s by Lu and Co-
workers14
Scheme 2. Copolymerization of CO2 with Cyclohexene Oxide and the Synthesis of a Model Compound for Studying the
Microstructure of Poly(cyclohexene carbonate)s by Nozaki and Co-workers16
complexes with a rigid bridging biphenol linker, which exhibit
excellent activity and enantioselectivity for the alternating
copolymerization of CO2 with various meso-epoxides under
mild reaction conditions.18−20 Based on the analysis of 13C
NMR spectroscopy, there are significant differences in both the
carbonyl, methine, or methylene region between the atactic and
highly isotactic copolymers from various meso-epoxides.
Usually, the carbon resonance was found to be splitted to
multiplets in the 13C NMR spectroscopy for their atactic
structures. In contrast with the clear discrimination of atactic
PCHC at carbonyl region for isotactic and syndiotactic
structure, the 13C NMR spectra of atactic poly(cyclopentene
carbonate)s (PCPC) at the carbonyl region were found to be
very complicated (Figure 1).18 Herein, the assignment of the
stereochemistry information on poly(cyclopentene carbonate)s
is performed by synthesizing the model compounds and the
analysis of their 13C NMR spectra.
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Figure 1. Difference in the carbonyl region of 13C NMR spectra of atactic PCHC and PCPC.
Scheme 3. Stereochemistry in CO2-Based Polycarbonates from the Copolymerization with meso-Epoxides
complicated.18 In order to assign the carbonate signals of
PCPC microstructure, we first synthesized two model
compounds (isotactic and syndiotactic dimers) to simulate
carbonate linkages in this CO2 copolymer (Scheme 4). Unlike
the kinetic resolution of racemic terminal epoxides by means of
catalytic hydrolysis mediated by chiral cobalt(III) complexes
can produce optically active 1,2-diol and unreacted enantiopure
epoxides with 99% enantiomeric excess (ee),21 the desymmet-
rization ring-opening of meso-epoxides using water as a
nucleophile suffered from relative low enantioselectivity.22 In
order to obtain optically active (R,R)- or (S,S)-cyclopentane-
1,2-diol with 99% ee, the chiral dinuclear Co(III) complexes
based on biphenol linking bridge were employed to catalyze the
RESULTS AND DISCUSSION
■
For CO2 copolymers from meso-epoxides, there are two
possible linkages for neighboring carbonate units, dependent
on the stereoselectivity of the desymmetrization ring-opening
of meso-epoxides. The isotactic structure is produced by
successive epoxide ring-opening at the one of the two chiral
centers with the same configuration, while the formation of
syndiotactic structure involves alternating ring-opening at (R)-
C−O bond then open at (S)-C−O bond for the following
epoxide, or in a reverse order (Scheme 3).
In contrast with the clear discrimination of PCHC at
carbonyl region for isotactic and syndiotactic structure, the 13C
NMR spectra of PCPCs at carbonyl region are very
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Scheme 4. Synthetic Strategy for Model Dimers (iso-I and syndio-I) for Simulating Isotactic and Syndiotactic Poly(cyclopentene
carbonate)s
Key: (a) BnOC(=NH)CCl3, TfOH (0.10 eq), cyclohexane, CH2Cl2, room temperature; (b) TBSOTf (1.1 eq), Et3N (2.0 eq), CH2Cl2, 0 °C to room
temperature; (c) NaH, THF, room temperature, then Im2CO; (d) (S,S)-2, NaH, reflux; (e) BF3·OEt2, CH3CN, 0 °C; (f) H2, Pd/C, EtOH, room
temperature. (g) ClCO2Me, Py, CH2Cl2, room temperature
asymmetric copolymerization CO2 with cyclopentene oxide,
and the hydrolysis of the produced isotactic CO2 polymer can
afford the optically active (R,R)- or (S,S)-cyclopentane-1,2-diol
with 99% ee.18
benzyl group by hydrogenolysis with palladium on carbon.
Because the dimers diols of iso-6 and syndio-6 having hydroxyl
groups do not exist in the main chain of the copolymer, iso-6
and syndio-6 further reacted with methyl chloroformate to
afford the model dimers, iso-I and syndio-I with methox-
ycarbonyl groups, which resemble the main chain structure of
PCPCs.
Starting with (S,S)- or (R,R)-trans-cyclopentane-1,2-diol, we
prepared the syndiotactic and isotactic dimers in moderate
yields (for details of experimental procedures, see the
Experimental Section). First, the strategy of protecting group
was employed. For example, benzyl (Bn) was introduced as one
hydroxyl-protecting group to afford the product (S,S)- or
(R,R)-2, and tert-butyldimethylsilyl (TBS) was introduced as
another hydroxyl-protecting group to afford the product (S,S)-
or (R,R)-3 (Scheme 4). Then, 1,1′-carbonyldiimidazole
(Im2CO) was used as a carbonyl equivalent for the reaction
with Bn-protected-2 or TBS-protected-3 with the same or
different configurations to afford iso- or syndio-4. Then iso- and
syndio-4 were converted into the dimer diol by the removal of
the silyl group with BF3·OEt2 and the following removal of
The 13C NMR spectra of dimer oligomers iso-I and syndio-I
as well as atactic and isotactic PCPCs are shown in Figure 2.
The chemical shift at carbonyl region of iso-I (153.85 ppm) is
almost identical with that isotactic PCPC (153.84 ppm), and
the chemical shift of syndio-I appeared at 153.78 ppm, 0.07 ppm
lower shift than that of iso-I (153.85 ppm). Compared with the
carbonyl region, the splits of the chemical shift in the methine
region are more clear. The methine resonance of iso-I and
syndio-I 13C NMR spectra appears at δ = 82.61 (82.49) and
82.53 (82.39) ppm, respectively. These results clearly suggest
that the copolymer should show a m-diad peak at around
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Figure 2. Carbonyl (left) and methine (right) region of 13C NMR spectra of (a) iso-I, (b) syndio-I, (c) atactic PCPC with ee = 0,23 (d) isotactic
PCPC with 81% ee,23 and (e) isotactic PCPC with >99% ee.23
isotactic (S,S)- or (R,R)-poly(cyclopentene carbonate)s were accord-
ing to a published method.18 The 1H NMR spectrum was recorded on
a Varian INOVA-400 MHz type (1H, 400 MHz), and the 13C NMR
spectrum was recorded on a Bruker 500 MHz type (13C, 125 MHz)
spectrometer. Their peak frequencies were referenced versus an
153.85 ppm and a r-diad peak at 153.78 ppm for carbonyl
resonance as well as 82.61 and 82.53 ppm for m-diad and r-diad
peak of methine resonance, respectively. In comparison with
the relatively high chemical shift differences (0.7 ppm) between
m-diad and r-diad for PCHCs, there is a minor difference for
PCPCs, suggesting the more complicated NMR spectra of
PCPC.
1
internal standard (TMS) shift at 0 ppm for H NMR and against the
solvent chloroform-d at 77.0 ppm for 13C NMR. A Micromass Q-Tof
(Micromass, Wythenshawe, UK) mass spectrometer equipped with an
orthogonal electrospray source (Z-spray) used for the cobalt
complexes in positive ion mode (capillary = 2000 V, sample cone =
20 V).
CONCLUSION
■
In conclusion, two kinds of model compounds for the
microstructural assignment of isotactic and syndiotactic
PCPCs, iso-I and syndio-I, were synthesized by the use of
enantiopure cyclopentane-1,2-diols as starting materials, in
which diols resulted from the hydrolysis of isotactic (S,S)- or
(R,R)-PCPCs produced from the asymmetric copolymerization
of CO2 with meso-cyclopentene oxide mediated by chiral
catalyst system based on dinuclear Co(III) complex. 13C NMR
investigations of these compounds revealed that the carbonate
carbon signals showed the stereosensitivity at the diad level
toward the adjacent epoxide ring-opened unit. It could be
concluded that the signal of PCPC at 153.85 ppm was
attributed to m-diad and that at 153.78 ppm to r-diad for
carbonyl resonance, while the signals at 82.61 and 82.53 ppm
were assigned to m-diad and r-diad peak of methine resonance,
respectively. The syndiotactic and isotatic diads matched well
with the microstructures of the stereoregular PCPCs in our
previous study.
(R,R)-Cyclopentane-1,2-diol [(R,R)-1]. The synthesis and hydrol-
ysis of (R,R)-poly(cyclopentene carbonate)s were according to a
published method (Scheme 4).18 1H NMR (400 MHz, CDCl3): δ 4.01
(m, 2H), 2.06−2.01 (m, 2H), 1.78−1.71 (m, 2H), 1.52−1.33 (m, 2H).
13C NMR (100 MHz, CDCl3): δ 79.28, 31.27, 19.68. HRMS (m/z):
Calcd for [C5H10NaO2]+: 125.0578. Found: 125.0579. [α]20
−21.3° (c = 1.1, CHCl3).
=
D
(S,S)-Cyclopentane-1,2-diol [(S,S)-1]. The synthesis and hydrol-
ysis of (S,S)-poly(cyclopentene carbonate)s were according to a
published method.18 1H NMR (CDCl3): δ 4.01 (m, 2H), 2.06−2.01
(m, 2H), 1.78−1.71 (m, 2H), 1.52−1.33 (m, 2H). 13C NMR
(CDCl3): δ 79.28, 31.27, 19.68. HRMS (m/z): Calcd for
[C5H10NaO2]+: 125.0578. Found: 125.0579. [α]20 = +21.8° (c =
D
1.1, CHCl3).
Synthesis of Bn-Protected-2 and TBS-Protected-3. (1S,2S)-2-
Benzyloxy-1-cyclopentanol [(S,S)-2]. To a solution of (S,S)-cyclo-
pentane-1,2-diol (1.92 g, 18.8 mmol) and benzyl trichloroacetimidate
(3.1 mL, 16.9 mmol) in a mixture of dichloromethane (30 mL) and
cyclopentane (15 mL) in a dry 100 mL Schlenk tube was added
trifluoromethanesulfonic acid (200 μL, 2.0 mmol). The mixture was
stirred at room temperature for 20 h and then poured into diethyl
ether (80 mL). The resulting organic layer was washed with saturated
aqueous NaHCO3 (30 mL × 2) and with brine (30 mL), dried over
anhydrous MgSO4, filtered, and concentrated by rotary evaporation.
Addition of hexane (20 mL) to the residue afforded insoluble
trichloroacetamide, which was removed by filtration. The filtrate was
concentrated in vacuo, and the residue was purified by column
chromatography on silica gel using hexane/ethyl acetate (4:1) as the
EXPERIMENTAL SECTION
■
All manipulations involving air- and/or water-sensitive compounds
were carried out in a glovebox or with the standard Schlenk techniques
under dry nitrogen. CO2 (99.995%) was purchased from Dalian
Institute of Special Gases and used as received. Cyclopentene oxide
was purchased from Acros and distilled over calcium hydride. All of
the solvents were distilled under an argon atmosphere after drying
over an appropriate drying reagent. The synthesis and hydrolysis of
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mobile phase to give (S,S)-2 (1.87 g, 58% yield) as a colorless oil. H
NMR (400 MHz, CDCl3): δ 7.40−7.24 (m, 5H), 4.60−4.50 (m, 2H),
4.22−4.15 (m, 1H), 3.80−3.72 (m, 1H), 2.06−1.95 (m, 2H), 1.78−
1.50 (m, 4H). 13C NMR (100 MHz, CDCl3): δ 138.64, 128.56,
dissolved in dichloromethane (4.0 mL), pyridine (1.0 mL), and
DMAP (2.0 mg, 0.02 mmol). To this mixture was slowly added methyl
chloroformate (0.3 mL, 3.90 mmol) in dichloromethane (2.0 mL) at 0
°C, and then the resulting mixture was stirred at 0 °C for 5 h. Workup
and purification by column chromatography on silica gel using
hexane/ethyl acetate (7:1) as the mobile phase to give iso-I (0.20 g,
127.85, 127.76, 86.67, 71.60, 71.55, 32.19, 29.46, 20.60. [α]20
+22.9° (c = 1.0, CHCl3).
=
D
1
60% yield) as a colorless oil. IR (CHCl3): 1750 cm−1. H NMR (400
(1S,2S)-2-t-Butyldimethylsiloxy-1-cyclopentanol [(S,S)-3]. To an
ice-cold solution of (S,S)-cyclopentane-1,2-diol (2.04 g, 20 mmol) and
triethylamine (5.60 mL, 40 mmol) in dichloromethane (20 mL)
placed in a dry 50 mL Schlenk tube was added slowly a solution of tert-
butyldimethylsilyl triflate (4.6 mL, 20 mmol) in dichloromethane (5.0
mL). The reaction mixture was stirred at 0 °C for 2 h and then poured
into diethyl ether (100 mL). The solution was washed with saturated
aqueous NaHCO3 (30 mL × 2) and then with brine (30 mL), dried
over anhydrous Na2SO4, filtered, and concentrated in vacuo.
Purification of the residue by column chromatography on silica gel
using hexane/ethyl acetate (7:1) as the mobile phase to give (S,S)-3
(1.90 g, 44% yield) as a colorless oil. 1H NMR (400 MHz, CDCl3): δ
3.98−3.90 (m, 2H), 2.06−1.46 (m, 6H), 0.87 (s, 9H), 0.07 (s, 3H),
0.05 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 79.86, 79.60, 32.15,
MHz, CDCl3): δ 5.05−5.00 (m, 4H), 3.78 (s, 6H), 2.23−2.09 (m,
4H), 1.86−1.70 (m, 8H). 13C NMR (125 MHz, CDCl3): δ 155.20,
153.85, 82.61, 82.49, 55.08, 30.32, 30.28, 21.49. [α]20 = −42.0° (c =
D
0.2, CHCl3).
Synthesis of syndio-I (Scheme 4). (1S,2S)-1-Benzyloxy-2-
{[(1R,2R)-2-(tert-butyldimethylsiloxy)-1-cyclopentyloxy]carbonyl-
oxy}cyclopentane (syndio-4). In a dry 50 mL Schlenk tube, sodium
hydride (0.15 g, 6.0 mmol) was placed and washed with dry hexane
(1.0 mL × 2). Then, a solution of (R,R)-3 (1.29 g, 6.0 mmol) in THF
(4.0 mL) was added. The resulting mixture was stirred at room
temperature for 30 min and added to a solution of 1,1′-carbon-
yldiimidazole (0.97 g, 6.0 mmol) in THF (2.0 mL) placed in a 100 mL
Schlenk tube. The resulting mixture was stirred at room temperature
for 3 h. To this reaction mixture was added a mixture of sodium
hydride (0.15 g, 6.0 mmol) and (S,S)-2 (1.14 g, 6.0 mmol) in THF
(4.0 mL) generated in a manner as described above. The whole
mixture was heated to reflux for 12 h before concentration under
reduced pressure. The residue was dissolved by diethyl ether (30 mL)
and washed with 1 M aqueous HCl (15 mL × 2), and the aqueous
layer was extracted with diethyl ether (30 mL × 2). The combined
organic layer was washed with brine (50 mL), dried over anhydrous
Na2SO4, filtered, and concentrated under reduced pressure.
Purification of the residue by column chromatography on silica gel
using hexane/ethyl acetate (20:1) as the mobile phase to give syndio-4
(1.20 g, 46% yield) as a colorless viscous oil. IR (CHCl3): 1735 cm−1.
1H NMR (400 MHz, CDCl3): δ 7.36−7.22 (m, 5H), 5.05−4.99 (m,
31.15, 26.02, 19.94, −4.44, −4.54. [α]20 = +26.2° (c = 1.0, CHCl3).
D
Synthesis of iso-I (Scheme 4). (1S,2S)-1-Benzyloxy-2-{[(1S,2S)-
2-(tert-butyldimethylsiloxy)-1-cyclopentyloxy]carbonyloxy}-
cyclopentane (iso-4). In a dry 20 mL Schlenk tube was placed sodium
hydride (6.8 mg, 2.82 mmol). Then, a solution of (S,S)-3 (0.61 g, 2.82
mmol) in THF (7.0 mL) was added. The resulting mixture was stirred
at room temperature for 1 h and added to a solution of 1,1′-
carbonyldiimidazole (0.45 g, 2.82 mmol) in THF (5.0 mL) placed in a
100 mL Schlenk tube. The resulting mixture was stirred at room
temperature for 3.5 h. To this reaction mixture was added a mixture of
sodium hydride (6.8 mg, 2.82 mmol) and (S,S)-2 (0.54 g, 2.82 mmol)
in THF (7.0 mL) generated in a manner as described above. The
whole mixture was heated to reflux for 18 h. Workup and purification
by column chromatography on silica gel using hexane/ethyl acetate
(20:1) as the mobile phase to give iso-4 (0.50 g, 41% yield) as a
colorless oil. IR (CHCl3): 1735 cm−1. 1H NMR (400 MHz, CDCl3): δ
7.36−7.22 (m, 5H), 5.05−4.99 (m, 1H), 4.81−4.76 (m, 1H), 4.65−
4.51 (m, 2H), 4.20−4.12 (m, 1H), 4.00−3.92, (m, 1H), 2.15−1.50 (m,
12H), 0.86 (s, 9H), 0.07 (s, 3H), 0.06 (s, 3H). 13C NMR (100 MHz,
CDCl3): δ 154.30, 138.50, 128.52, 127.80, 127.72, 85.38, 83.90, 82.90,
77.05, 71.40, 32.80, 30.60, 29.40, 25.85, 21.82, 20.90, 18.20, −4.65,
1H), 4.81−4.76 (m, 1H), 4.65−4.51 (m, 2H), 4.20−4.12 (m, 1H),
4.00−3.92, (m, 1H), 2.15−1.50 (m, 12H), 0.86 (s, 9H), 0.07 (s, 3H),
0.06 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 154.34, 138.55, 128.52,
127.79, 127.72, 85.38, 83.92, 82.90, 77.06, 71.44, 32.80, 30.60, 29.38,
25.86, 21.82, 20.88, 18.18, −4.62, −4.68. [α]20 = −11.5° (c = 0.1,
D
CHCl3).
(1R,2R)-2-{[(1S,2S)-2-Benzyloxy-1-cyclopentyloxy]carbonyloxy}-1-
cyclopentanol (syndio-5). To an ice-cold solution of syndio-4 (0.55 g,
1.27 mmol) in acetonitrile (8.0 mL) in a dry 20 mL Schlenk tube was
added slowly a solution of BF3·OEt2 (0.3 mL, 2.54 mmol) in
acetonitrile (3.0 mL). The mixture was stirred at 0 °C for 3.5 h, and
the solvent was removed under reduced pressure. The residue
dissolved in diethyl ether (30 mL) was washed with saturated aqueous
NaHCO3 (30 mL × 2); the aqueous layer was extracted with ethyl
acetate (50 mL). The combined organic layer was washed with brine
(50 mL), dried over anhydrous Na2SO4, filtered, and concentrated
under reduced pressure. Purification of the residue by column
chromatography on silica gel using hexane/ethyl acetate (2:1) as the
mobile phase to give syndio-5 (0.37 g, 91% yield) as a colorless viscous
−4.70. [α]20 = +24.6° (c = 1.0, CHCl3).
D
(1S,2S)-2-{[(1S,2S)-2-Benzyloxy-1-cyclopentyloxy]carbonyloxy}-1-
cyclopentanol (iso-5). To an ice-cold solution of iso-4 (0.25 g, 0.57
mmol) dissolved in acetonitrile (7.0 mL) and placed in a dry 100 mL
Schlenk tube was added slowly a solution of BF3·OEt2 (1.26 mL, 1.14
mmol) in acetonitrile (7.0 mL), and then the resulting mixture was
stirred at 0 °C for 1.5 h. Workup and purification by column
chromatography on silica gel using hexane/ethyl acetate (1:1) as the
mobile phase to give iso-5 (0.14 g, 76% yield) as a colorless oil. IR
(CHCl3): 1740 cm−1. 1H NMR (400 MHz, CDCl3): δ 7.40−7.22 (m,
5H), 5.08−5.00 (m, 1H), 4.80−4.68 (m, 1H), 4.64−4.52 (m, 2H),
4.23−4.15 (m, 1H), 4.00−3.93, (m, 1H), 2.60 (s, 1H), 2.20−1.95 (m,
4H), 1.85−1.55 (m, 8H). 13C NMR (CDCl3): δ 154.33, 138.32,
128.48, 127.74, 127.68, 86.67, 83.79, 83.28, 78.11, 71.39, 32.53, 30.48,
1
oil. IR (CHCl3): 1740 cm−1. H NMR (400 MHz, CDCl3): δ 7.40−
7.22 (m, 5H), 5.08−5.00 (m, 1H), 4.80−4.68 (m, 1H), 4.64−4.52 (m,
2H), 4.23−4.15 (m, 1H), 4.00−3.93 (m, 1H), 2.60 (s, 1H), 2.20−1.95
(m, 4H), 1.85−1.55 (m, 8H). 13C NMR (100 MHz, CDCl3): δ 155.35,
138.43, 128.55, 127.79, 127.70, 86.72, 83.85, 83.46, 78.08, 71.48,
30.29, 30.07, 21.72, 21.49. [α]20 = −10.6° (c = 1.4, CHCl3).
D
(1S,2S,1′S,2′S)-2,2′-Carbonyldioxydicyclohexanol (iso-6). In a 20
mL Schlenk tube were placed iso-5 (0.12 g, 0.37 mmol), Pd/C (10 wt
%, 90 mg), and ethanol (5.0 mL). The mixture was stirred under a
hydrogen atmosphere (1 atm) at room temperature for 21 h, filtered,
and concentrated in vacuo. Purification of the residue by column
chromatography on silica gel using hexane/ethyl acetate (1:2) as the
mobile phase to give iso-6 (0.083 g, 96% yield) as a colorless oil. IR
(CHCl3): 1740 cm−1. 1H NMR (400 MHz, CDCl3): δ 4.80−4.70 (m,
2H), 4.20 (s, 2H), 2.95 (s, 2H), 2.25−1.95 (m, 4H), 1.85−1.55 (m,
8H). 13C NMR (100 MHz, CDCl3): δ 156.41, 87.03, 77.98, 32.57,
32.59, 30.55, 30.40, 30.14, 21.78, 21.52. [α]20 = +18.5° (c = 1.0,
D
CHCl3).
(1R,2R)-2-{[(1S,2S)-2-Hydroxy-1-cyclopentyloxy]carbonyloxy}-1-
cyclopentanol (syndio-6). In a 20 mL Schlenk tube were placed
syndio-5 (0.20 mg, 0.62 mmol), Pd/C (10 wt %, 150 mg) and ethanol
(4.0 mL). The mixture was stirred under a hydrogen atmosphere (1
atm) at room temperature for 43 h, filtered, and concentrated in vacuo.
Purification of the residue by column chromatography on silica gel
using hexane/ethyl acetate (1:1) as the mobile phase to give syndio-6
30.12, 21.82. [α]20 = −34.8° (c = 0.7, CHCl3).
D
1
(0.14 mg, 95% yield) as a colorless oil. IR (CHCl3): 1740 cm−1. H
(1S,2S)-1-(Methoxycarbonyloxy)-2-[{(1S,2S)-2-(methoxy-
carbonyloxy)-1-cyclopentyloxy}carbonyloxy}cyclopentane (iso-I). In
a dry 20 mL Schlenk tube were placed iso-6 (0.22 g, 0.10 mmol)
NMR (400 MHz, CDCl3): δ 4.80−4.70 (m, 2H), 4.20 (s, 2H), 2.95 (s,
2H), 2.25−1.95 (m, 4H), 1.85−1.55 (m, 8H). 13C NMR (100 MHz,
F
DOI: 10.1021/acs.macromol.5b01759
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
CDCl3): δ 156.38, 87.00, 77.96, 32.56, 30.10, 21.80. [α]20 = 0° (c =
Zhang, X.-H.; Du, B.-Y.; Sun, X.-K.; Fan, Z.-Q.; Qi, G.-R.
Macromolecules 2013, 46, 3693−3697.
D
1.3, CHCl3).
(1R,2R)-1-(Methoxycarbonyloxy)-2-[{(1S,2S)-2-(methoxy-
carbonyloxy)-1-cyclopentyloxy}carbonyloxy}cyclopentane (syndio-
I). In a dry 20 mL Schlenk tube were placed syndio-6 (0.08 mg, 0.35
mmol) dissolved in dichloromethane (2.0 mL), pyridine (1.0 mL), and
DMAP (7.6 mg, 0.04 mmol). To the mixturewas slowly added methyl
chloroformate (1.1 mL, 14.00 mmol) in dichloromethane (2.0 mL) at
0 °C. The resulting mixture was stirred at 0 °C for 4 h, diluted with
diethyl ether (20 mL), washed with 1 M aqueous HCl (10 mL × 2)
and brine (10 mL), dried over anhydrous Na2SO4, filtered, and
concentrated under reduced pressure. Purification of the residue by
column chromatography on silica gel using hexane/ethyl acetate (2:1)
as the mobile phase to give syndio-I (0.09 mg, 74% yield) as a colorless
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1
oil. IR (CHCl3): 1750 cm−1. H NMR (400 MHz, CDCl3): δ 5.05−
5.00 (m, 4H), 3.78 (s, 6H), 2.23−2.09 (m, 4H), 1.86−1.70 (m, 8H).
13C NMR (125 MHz, CDCl3): δ 155.13, 153.78, 82.53, 82.39, 55.05,
30.23, 30.18, 21.49. [α]20 = −0.1° (c = 0.8, CHCl3).
D
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.5b01759.
NMR spectra of compounds 1−6, iso-I, and syndio-I
(PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail xblu@dlut.edu.cn (X.-B.L.).
■
Author Contributions
Y.L. and R.-R.L. contributed equally to this work.
Notes
(14) Wu, G.-P.; Zu, Y.-P.; Xu, P.-X.; Ren, W.-M.; Lu, X.-B. Chem. -
Asian J. 2013, 8, 1854−1862.
The authors declare no competing financial interest.
(15) Wu, G.-P.; Wei, S.-H.; Ren, W.-M.; Lu, X.-B.; Li, B.; Zu, Y.-P.;
Darensbourg, D. J. Energy Environ. Sci. 2011, 4, 5084−5092.
(16) Nakano, K.; Nozaki, K.; Hiyama, T. Macromolecules 2001, 34,
6325−6332.
ACKNOWLEDGMENTS
■
This work is supported by the National Natural Science
Foundation of China (NSFC, Grant 21134002) and Program
for Changjiang Scholars and Innovative Research Team in
University (IRT13008). X.-B. Lu gratefully acknowledges the
Chang Jiang Scholars Program (T2011056) from the Ministry
of Education of China.
(17) Cohen, C. T.; Thomas, C. M.; Peretti, K. L.; Lobkovsky, E. B.;
Coates, G. W. Dalton Trans. 2006, 237−249.
(18) Liu, Y.; Ren, W.-M.; Liu, J.; Lu, X.-B. Angew. Chem., Int. Ed.
2013, 52, 11594−11598.
(19) Liu, Y.; Wang, M.; Ren, W.-M.; He, K.-K.; Xu, Y.-C.; Liu, J.; Lu,
X.-B. Macromolecules 2014, 47, 1269−1276.
(20) Liu, Y.; Ren, W.-M.; He, K.-K.; Lu, X.-B. Nat. Commun. 2014, 5,
5687.
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DOI: 10.1021/acs.macromol.5b01759
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