Chiral oligomers of spiro-salencobalt(III)X for catalytic asymmetric cycloaddition of epoxides with CO2

Article abstract of DOI:10.1016/j.catcom.2016.04.006

Several new chiral oligomers of spiro-salenCo(III)X (spiro = 1.1′-spirobiindane-7.7′-diol) complexes have been designed, synthesized, and characterized by nuclear magnetic resonance (NMR), infrared (IR), and elemental analyses, in which, the chiral spiro moieties are first introduced into a scaffold of chiral salenCo catalysts. They were used to catalyze the asymmetric cycloaddition of epoxides with carbon dioxide. Under very mild reaction conditions, a kinetic resolution of racemic epoxides with CO₂ was smoothly initiated by these chiral oligomer catalysts with good enantioselectivities, which can be attributed to the match effect between chiral backbones of salen and spiro. High stability and easy recyclability are their major advantages.

Full text of DOI:10.1016/j.catcom.2016.04.006

Chiral oligomers of spiro-salencobalt(III)X for catalytic asymmetric cycload-
dition of epoxides with CO2
Zhouhe Zhu, Yuqian Zhang, Kai Wang, Xiying Fu, Fengjuan Chen,
Huanwang Jing
PII:
DOI:
Reference:
S1566-7367(16)30126-1
doi: 10.1016/j.catcom.2016.04.006
CATCOM 4629
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
29 January 2016
9 April 2016
11 April 2016
Please cite this article as: Zhouhe Zhu, Yuqian Zhang, Kai Wang, Xiying Fu, Fengjuan
Chen, Huanwang Jing, Chiral oligomers of spiro-salencobalt(III)X for catalytic asym-
metric cycloaddition of epoxides with CO2, Catalysis Communications (2016), doi:
10.1016/j.catcom.2016.04.006
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ACCEPTED MANUSCRIPT
Chiral oligomers of spiro-salencobalt(III)X for catalytic asymmetric
cycloaddition of epoxides with CO2
Zhouhe Zhua, Yuqian Zhanga, Kai Wanga, Xiying Fua, Fengjuan Chena, Huanwang
Jinga,b*
aState Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous
Metals Chemistry and Resources Utilization, Gansu; College of Chemistry and
Chemical Engineering, Lanzhou University, 222 South Tianshui Road, Lanzhou,
Gansu 730000, People’s Republic of China
bState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, People’s Republic of China
Corresponding author:
Huanwang Jing
Tel: +86 931 891 2585
Fax: +86 931 891 2582
E-mail: hwjing@lzu.edu.cn
Highlights
1 Several new chiral oligomers of spiro-salenCo(III)X catalysts were synthesized.
2 The spiro backbones were first applied to produce chiral cyclic carbonates.
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3 These chiral catalysts could be easily recovered and reused.
Abstract
Several
new
chiral
oligomers
of
spiro-salenCo(III)X
(spiro
=
1,1’-spirobiindane-7,7’-diol) complexes have been designed, synthesized, and
characterized by nuclear magnetic resonance (NMR), infrared (IR), and elemental
analyses, in which, the chiral spiro moieties are first introduced into a scaffold of
chiral salenCo catalysts. They were used to catalyze the asymmetric cycloaddition of
epoxides with carbon dioxide. Under very mild reaction conditions, a kinetic
resolution of racemic epoxides with CO2 was smoothly initiated by these chiral
oligomer catalysts with good enantioselectivities, which can be attributed to the match
effect between chiral backbones of salen and spiro. High stability and easy
recyclability are their major advantages.
Keywords: Asymmetric cycloaddition, Chiral oligomer catalyst, Carbon dioxide,
Epoxide, Kinetic resolution
1. Introduction
Carbon dioxide (CO2) fixation has attracted much research attention because of
its relative abundance, low cost, nontoxicity, and renewability.13 In the past
decades, >20 reactions with CO2 as a starting material have been developed. Among
them, the synthesis of cyclic carbonates via cycloaddition of epoxides and CO2 is one
of the most promising strategies,4 because of its 100% atom economy. The cyclic
carbonates are widely used as aprotic polar solvents, monomers, organic synthetic
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intermediates, and fine chemical intermediates. Chiral cyclic carbonates might be
applied to produce pharmaceuticals and useful chiral organic intermediates.57
Optically pure cyclic carbonates were traditionally fabricated by chiral diols and
phosgene in the presence of zinc8–10 or palladium complexes.11 Some chiral
salenCo(III) complexes were shown to be a highly efficient catalyst for the
asymmetric ring-opening and kinetic resolution of aliphatic terminal epoxides.1214
Efficient bifunctional chiral catalysts were also reported by our group.15,16 Recently,
Lu and coworkers have reported a new type of chiral salenCo(III) complexes as
efficient catalysts for the preparation of chiral cyclic carbonates.17 In order to
investigate the effect of more chiral centers in one molecule, chiral ionic liquids were
used in these reactions as cocatalysts,18 the polymeric binol–salenCo(III) catalysts
with multiple chiral centers exhibited more activity, and enantioselectivity and easy
recyclability were documented by our group.19 It is well known that the spirocyclic
framework as chiral backbones is widely used in the area of asymmetric catalysis.20,21
In this study, we report several new oligomeric spiro-based salenCo(III)X catalysts
(Fig. 1) and their application to the kinetic resolution of epoxides.
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Figure 1. Configurations of chiral catalysts.
2. Experimental
2.1. General
Racemic epoxides were distilled from CaH2 before use. Other commercially
available chemicals and reagents were used without purification unless specified.
1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Varian
Mercury Plus 300-MHz spectrometer using CDCl3, DMSO-d6, or acetone-d6 as
the solvent and tetramethylsilane (TMS) as an internal standard. Mass
spectrometry data were obtained with a HP_5988 spectrometer and ZAB-HS
spectrometer. Gel permeation chromatography (GPC) was performed using a
Waters 1515 chromatograph at 35°C with tetrahydrofuran (THF) as eluent at a
flow rate of 1.0 ml/min calibrated by polystyrene standards. Infrared (IR) spectra
were recorded on a Fourier transform infrared (FTIR) spectrometer and reported
in cm−1 (%T). Gas chromatographic (GC) analyses were carried out on a Varian
CP-3800 gas chromatograph equipped with a flame ionization detector (FID)
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using Supelco-DEX series (225) chiral columns. High-performance liquid
chromatography (HPLC) analyses (Daicel Chiralcel OD chiral column) were
conducted on a Water 600 controller with 2996 Photodiode Array Detector.
2.2. Synthesis of Catalysts
Following the synthetic routes (Scheme 1) reported in the literature,22,23 compounds
2–7 were successfully produced and purified, and their fully characterized data are
listed in supporting information.
Scheme 1. Synthetic routes for rac-1,1’-spirobiindane-7,7’-diol: (a) acetone, NaOH,
EtOH–H2O, r.t., 4 h; (b) Raney Ni, acetone, r.t., 1 atm. H2, 24 h; (c) NaBr, H2O2,
HOAc, r.t.; (d) CH3SO3H, 50°C, 12 h; (e) n-BuLi, THF, −78°C, 1 h; EtOH (quench);
(f) BBr3, CH2Cl2, −78°C to r.t. overnight.
The synthetic routes for the chiral compounds 7–10 are shown in Scheme 2, and their
data of characterization are listed in supporting information.
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Scheme 2. (a) MgCl2, paraformaldehyde, Et3N, THF, 12 h, reflux; (b)
(1R,2R)-(+)-1,2-diaminocyclohexane L-tartrate, K2CO3, water/ethanol, THF, 4 h; (c)
Co(OAc)2·4H2O, CH3OH, Ar, reflux, 4 h; (d) CCl3COOH, CH2Cl2, O2, room
temperature, 12 h.
2.3 General procedure for coupling of epoxides with CO2
Catalyst of (S,R,R,S)-ZSS-2b (631 mg, 0.1 mmol), cocatalyst of tetrabutylammonium
fluoride (TBAF, 0.0522 g, 0.2 mmol), and racemic propylene oxide (PO, 7 mL, 100
mmol) were introduced into a 100-mL stainless steel autoclave to form a brown
solution. The reactor was purged thrice with carbon dioxide, and charged it to 0.8
MPa. Then, the asymmetric catalytic reaction took place at room temperature. After
some time, the pressure in the reactor decreased to an expected value, and it was
vented to terminate the reaction. After removing the unreacted epoxide, chiral cyclic
carbonate (R = Me, Et, CH2Cl), weighed to calculate the yield of cyclic carbonate,
was distilled under vacuum as a colorless liquid or it (R = Ph, PhOCH2) was obtained
by column chromatography through a short silica gel column (ethyl acetate:petroleum
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ether = 5:1).
Results and Discussion
The synthetic routes of the new oligomeric chiral catalysts are shown in Schemes 1
and 2. Compounds 24 were fabricated via routine strategies in high yields. For
synthesizing the key racemic spiro compound 5, cyclization of symmetric ketone 4
was first performed in the presence of polyphosphoric acid at 105°C for 6 h, which led
to only 2030% yield.21 Alternatively, when the catalyst polyphosphoric acid was
replaced by MsOH, this cyclization generated 5 in 62% yield under mild reaction
conditions (50°C, 6 h).22 Therefore, four chiral oligomer catalysts with various
absolute configurations of spiro and diaminocyclohexane were fabricated and named
as (S,S,S,S)-ZSS-1, (R,R,R,R)-ZSS-4, (S,R,R,S)-ZSS-2, and (R,S,S,R)-ZSS-3.
Asymmetric cycloaddition of racemic PO and CO2 in the presence of the
aforementioned chiral catalysts was then carefully investigated. Data in Table 1 reveal
the enantionselective effects between the chirality of spiro and diaminocyclohexane.
Catalysts 1a and 2a with OAc as counterion were used in this reaction with TBAF as
cocatalyst (Table 1, entries 1, 6). Catalyst 2a is superior to 1a and exhibits a higher
enantioselectivity (57.1% vs. 48.5% ee of chiral propylene carbonate, CPC). It is
evident that the enantioselectivity is mainly affected by the chirality of spiro skeleton.
It can also be observed that catalysts 2a (S,R,R,S) and 3b (R,S,S,R) possessing mixed
chirality exhibit higher ee values (Table 1, entries 6 and 13, 57.1% (S) and 60.3% (R)
of CPC). Catalysts 1a (S,S,S,S) and 4b (R,R,R,R) having the same absolute
configurations exhibit lower ee values (Table 1, entries 14, 48.5% (R) and 50.2% (S)
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of CPC). The cocatalyst effect was also screened by using catalysts 1a and 2a (Table 1,
entries 1–5, entries 6–10). The obtained reactivity order is PTAT > TBAI > TBAB >
TBAC > TBAF; however, the enantioselectivity order shows an opposite trend:
TBAF > TBAC > TBAB > TBAI > PTAT. The results also reveal that the anion of the
cocatalyst in the binary catalyst system has a significant effect on the activity of
catalysts and enantioselectivity of CPC: the cocatalyst TBAI with a large anion (I)
shows more activity and less enantioselectivity of CPC (Table 1, entries 4 and 9). By
contrast, the cocatalyst TBAF with a small anion (F) shows relatively less activity and
more enantioselectivity of CPC (Table 1, entries 1 and 6). The cocatalyst PTAT with
the largest anion shows the highest reaction activity and the lowest enantioselectivity
in CPC formation (Table 1, entries 5 and 10). The counterion effect was also studied
with catalysts 2a, 2b, and 2c. The order of enantioselectivity for counterions is:


CCl3CO2 > CF3CO2 > OAc (Table 1, entries 6, 11, and 12). Therefore, the size of
the counterion X group also affects the enantioselectivity. The catalyst 2b with a bulky
X group (X = CCl3CO2) shows a better performance with a higher ee value in this
reaction. Furthermore, these reactions were also investigated at a lower temperature
(0°C) and enantioselectivities of CPC are improved (Table 1, entries 1518).
Table 1. Results of asymmetric cycloaddition of CO2 and POa.
c
Entry
Catalyst Cocatalys Time (h)
Conv. (%)
eeb (%)
krel
t
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1
2
TBAF
TBAC
TBAB
TBAI
PTAT
TBAF
TBAC
TBAB
TBAI
PTAT
TBAF
TBAF
TBAF
TBAF
TBAF
TBAF
TBAF
TBAF
4
3
2
2
2
4
3
2
2
2
4
4
4
4
8
8
8
8
42.1
44.1
43.4
47.1
50.2
40.3
42.7
43.1
47.7
49.1
40.2
41.1
40.4
44.7
41.1
40.1
40.3
40.7
48.5(R)
45.9(R)
22.3(R)
16.3(R)
9.9(R)
4.0
3.8
1.8
1.6
1.3
5.3
4.9
2.3
1.7
1.5
6.1
5.6
6.0
4.4
5.4
8.0
7.6
5.5
1a
1a
1a
1a
1a
2a
2a
2a
2a
2a
2b
2c
3b
4b
1b
2b
3b
4b
3
4
5
6
57.1(S)
54.1(S)
30.1(S)
18.5(S)
11.2(S)
60.8(S)
58.4(S)
62.7(R)
50.2(S)
57.2(R)
66.7(S)
66.8(R)
58.1(S)
7
8
9
10
11
12
13
14
15d
16d
17d
18d
a Reaction conditions: cat. 0.1 mmol, cocat. 0.2 mmol, PO 100 mmol, CO2 0.8 MPa, T
= 25°C.
b The ee value of CPC was determined by Varian CP-3800 GC on a Supelco-DEX
series (225) chiral column.
c krel was calculated as (ln[1 − c (1 + ee)])/(ln[1 − c (1 − ee)]), where c = conversion
and ee = enantiomeric excess.
d T = 0°C.
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A higher ee value of CPC was obtained by using (S,R,R,S)-ZSS-2b as chiral
catalyst (60.8% vs. 50% ee CPC) than chiral salenCo(III) catalyst.14 It can be
inferred from these results that the configuration of spiro fragment has an effect
on the optical selectivity of the epoxide. The oligomer catalysts (S,R,R,S)-ZSS-2
and (R,S,S,R)-ZSS-3 with the mixed configuration can increase the
enantioselectivity of CPC. On the contrary, (S,S,S,S,)-ZSS-1 and (R,R,R,R)-ZSS-4
with the same configuration reduces the enantioselectivity of CPC. In order to
extend the scope of substrate, various epoxides were used in this reaction in the
presence of good catalyst 2b and cocatalyst TBAF (Table 2). It can be also seen
that the terminal epoxides are converted to chiral cyclic carbonates with
considerable ee values under optimal condition. Otherwise, the krel values
decrease significantly for the styrene oxide or glycidyl phenyl ether (Table 2,
entries 4 and 5). The observation suggested that the substrates with small
substituted groups could acquire products with a higher ee value (Table 2, entries
1–3), but those with large substituted groups acquired products with much lower
ee values (Table 2, entries 4 and 5), because of their steric hindrance, which led to
insufficient transfer of chirality.19
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Table 2. Asymmetric cycloaddition of CO2 with various epoxides in the presence of
2b/TBAFa.
Entry
R=
CH3
Time (h)
Conv. (%)
40.2
ee (%)
60.7(S)
54.2(S)
57.8(S)
3.3(S)
krel
6.1
5.1
5.2
1.1
1.2
1
2
4
8
ClCH2
CH2CH3
Ph
44.5
3
12
16
8
37.7
4b
5b
39.2
PhOCH2
46.7
5.6(S)
a Reaction conditions: cat. 0.1 mmol, cocat. 0.2 mmol, epoxide 100 mmol, CO2 0.8
MPa, T 25°C.
b The ee value of cyclic carbonates was determined by HPLC (Daicel Chiralcel OD,
n-hexane/2-propanol (9:1 v/v), 1.0 mL/min, 254 nm).
Recycling of the catalyst 2b was further investigated in this cycloaddition reaction,
and the results are presented in Table 3. Addition of suitable ethanol to the mixture
facilitated the immediate precipitation of these oligomeric chiral catalysts and no
obvious loss of activity and enantioselectivity even after five cycles of catalysis.
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Table 3. Recycle test results of catalysts in the coupling reaction of CO2 with POa.
Entry
Cycle number
Catalyst
2b
Cocatalyst
TBAF
TBAF
TBAF
TBAF
TBAF
TBAF
Conv. (%)
39.1
eeb (%)
67.9(S)
67.2(S)
66.5(S)
65.8(S)
65.7(S)
65.8(S)
krel
8.0
7.7
7.4
7.1
7.1
7.1
1
2
3
4
5
6
fresh
1
2
3
4
5
38.7
2b
38.4
2b
37.9
2b
37.5
2b
37.7
2b
a Reaction conditions: cat. 0.1 mmol, cocat. 0.2 mmol, PO 100 mmol, CO2 0.8 MPa, t
6 h, T 0°C.
b
The ee value of CPC was determined by Varian CP-3800 GC on a Supelco-DEX
series (225) chiral column.
Conclusion
In conclusion, the chiral spiro structural units were used for the first time to modify
salenCo(III)X complexes, which performed well in the asymmetric cycloaddition
reaction of PO and carbon dioxide with good enantioselectivity. The effects of
cocatalyst and counterions were also carefully studied and discussed. The scope of
terminal epoxides was screened with the catalyst 2b and cocatalyst TBAF producing
the corresponding cyclic carbonates. Furthermore, the catalyst 2b could be recycled
five times without obvious loss of activity and enantioselectivity, thereby exhibiting
good stability and recyclability.
Acknowledgments
This study was financially supported by the National Natural Science Foundation of
China (NSFC 20973086, 21173106, and 21401091) and the Foundation of State Key
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Laboratory of Coal Conversion (Grant No. J16-17-913).
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Graphical Abstract
Chiral oligomers of spiro-salencobalt(III)X for catalytic asymmetric
cycloaddition of epoxides with CO2
Zhouhe Zhu, Yuqian Zhang, Kai Wang, Xiying Fu, Fengjuan Chen, Huanwang Jing*
15