Activation of (salen)CoI complex by phosphorane for carbon dioxide transformation at ambient temperature and pressure

Article abstract of DOI:10.1039/c7gc01458a

We report the activation of (salen)CoI complex 3g by a phosphorane to form a bifunctional catalyst for the reaction of carbon dioxide with terminal epoxides or aziridines at ambient temperature and 1 bar carbon dioxide pressure. Only 1.0 mol% of both (salen)CoI 3g and phosphorane 4d are required for cyclic carbonate synthesis, and the catalyst loading could even be lowered down to 0.1 mol%. Under these conditions, no polycarbonate formation is detected by NMR analysis. It is proposed that the high efficiency originates from the activation of (salen)CoI by a phosphorane to form a phosphorane-salen Co(iii) complex with enhanced Lewis acidity for the electrophilic activation while generating an iodide anion as a Lewis base co-catalyst to facilitate the ring-opening of epoxides. Further investigation revealed that the phosphorane-(salen)CoI complex could also successfully catalyze the coupling of CO₂ with aziridines under ambient conditions at a catalyst loading of 2.5 mol%.

Full text of DOI:10.1039/c7gc01458a

View Article Online
View Journal
Green
Chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: F. Zhou, S. Xie, X.
Gao, R. Zhang, C. Wang, G. Yin and J. Zhou, Green Chem., 2017, DOI: 10.1039/C7GC01458A.
This is an Accepted Manuscript, which has been through the
Volume 18 Number
7 7 April 2016 Pages 1821–2242
Royal Society of Chemistry peer review process and has been
accepted for publication.
Green
Chemistry
Accepted Manuscripts are published online shortly after
Cutting-edge research for a greener sustainable future
www.rsc.org/greenchem
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1463-9262
CRITICAL REVIEW
G. Chatel et al.
Heterogeneous catalytic oxidation for lignin valorization into valuable
chemicals: what results? What limitations? What trends?
rsc.li/green-chem

Please do not adjust margins
Page 1 of 8
Green Chemistry
View Article Online
DOI: 10.1039/C7GC01458A
Journal Name
ARTICLE
Activation of (salen)CoI complex by phosphorane for carbon
dioxide transformation at ambient temperature and pressure
Received 00th January 20xx,
Accepted 00th January 20xx
Feng Zhou,*a,b Shi-Liang Xie,a Xiao-Tong Gao,a Rong Zhang,*c Cui-Hong Wang,a Guang-Qiang Yin,a
and Jian Zhou*a,d
DOI: 10.1039/x0xx00000x
We report the activation of (salen)CoI complex 3g by a phosphorane to form a bifunctional catalyst for the reaction of
carbon dioxide with terminal epoxides or aziridines at ambient temperature and 1 bar carbon dioxide pressure. Only 1.0
mol% of both (salen)CoI 3g and phosphorane 4d are required for cyclic carbonate synthesis, and the catalyst loading could
even be lowered down to 0.1 mol%. Under this condition, no polycarbonate formation is detected by NMR analysis. It is
proposed that the high efficiency originates from the activation of (salen)CoI by a phosphorane to form a phosphorane-
salen Co(III) complex with enhanced Lewis acidity for the electrophilic activation, whilst generating an iodide anion as a
Lewis base co-catalyst to facilitate the ring-opening of epoxides. Further investigation revealed that the phosphorane-
(salen)CoI complex could also catalyze the coupling of CO2 with aziridines successfully under ambient conditions at a 2.5
mol% of catalyst loading.
www.rsc.org/
During the past decade, a variety of elegant catalyst systems
capable of mediating cyclic carbonates synthesis at ambient
conditions (25 C, 1 atm of CO2) have been developed by Lu,
Introduction
The development of efficient catalytic protocols that use carbon
dioxide (CO2) as an inexpensive, nontoxic and renewable C1
feedstock for the synthesis of value-added products is of current
research interest, since it provides an attractive complement to
carbon capture and storage.1 Among the known more than 20
reactions using CO2 as starting material, the coupling reaction
of epoxides and CO2 has attracted considerable attention,2 as it
can be controlled to give either cyclic carbonates3 or
polycarbonates4; both products have a range of applications.5
In particular, the cyclic carbonate production has been
commercialized for over 50 years, because cyclic carbonates
can be used as electrolytes for lithium ion batteries, polar
aprotic solvents, synthons for the synthesis of value-added
chemicals and monomers for polymer synthesis. However,
although the synthesis of cyclic carbonates from epoxides with
o
North, Williams and other groups.7,8,9 Most of them are based
on the binary catalyst system, consisted of a metal Lewis acid
together with a suitable nucleophilic co-catalyst (often a halide),
as shown in Scheme 1. This is because the cooperation of the
Lewis acid and the co-catalyst can make the ring-opening
procedure energetically less demanding and the following CO2
insertion easier. A generally accepted mechanism for the metal-
catalyzed coupling of CO2 and epoxides is depicted in Scheme
1A. The epoxides are first activated by the Lewis acid through
M-O coordination, followed by the nucleophilic attack of the
co-catalyst to facilitate the ring-opening process. The in situ
formed metal alkoxide intermediate I then reacts with CO2 to
afford the desired cyclic carbonates.
CO2 is
a highly exothermic reaction, all the current
commercialized processes are net CO2 emitter rather than
consumer, because these commercial processes require high
reaction temperatures and pressures, along with the use of
highly purified CO2.6 Therefore, it is important and urgent to
develop efficient catalysts for the synthesis of cyclic carbonates
from epoxides and CO2 under mild conditions.
a.Shanghai Key Laboratory of Green Chemistry and Chemical Process, School of
Chemistry and Molecular Engineering, East China Normal University, 3663N
Zhongshan Road, Shanghai 200062, P. R. China. E-mail:
fzhou@chem.ecnu.edu.cn; rzhang@dlut.edu.cn; Jzhou@chem.ecnu.edu.cn.
b.Shanghai Engineering Research Center of Molecular Therapeutics and New Drug
Development, School of Chemistry and Chemical Engineering, East China Normal
University, Shanghai, 200062, P. R. China.
c. State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian
116012, P R China.
d.State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032, P R China.
Scheme 1. Typical binary catalyst system toward CO2 coupling with terminal epoxides
under ambient conditions.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 2 of 8
ARTICLE
Taking advantage of such a cooperation, notable protocols of undesired polycarbonates.3c,
Journal Name
4c
In addition, it is easy to
View Article Online
DOI: 10.1039/C7GC01458A
are capable of performing the reaction at ambient conditions prepare and modify the phosphorane-salen metal complex by
with only 2.0-2.5 mol% of both metal complex and the varying the backbone of salen ligands and the substituent of the
nucleophilic co-catalyst.7 However, further lowering down the phosphorane. Herein, we wish to exhibit the power of such
catalyst loading was not successful to date in terms of epoxides bifunctional catalysts by a highly efficient coupling reactions of
conversion and substrate scope. Therefore, the development of CO2 with terminal epoxides or aziridines catalyzed by
new catalyst systems is very much in demand.
phosphorane-(salen)CoI complex.
With the above hypothesis, we first examined the
acceleration effect of different phosphoranes
4 in the
Results and discussion
Recently, we found that Jacobsen’s privileged catalyst,
(salen)AlCl 3a 10
, can be effectively activated by phosphorane
(salen)AlCl complex 3a catalyzed coupling of epoxide 1a and
CO2, as shown in Table 1. All reactions were carried out at 25
oC under solvent-free conditions with CO2 held within a
balloon to maintain a 1 atmosphere pressure, in the presence of
4a to produce an enhanced Lewis acid for catalytic
enantioselective ketone cyanosilylation.11 Both electrical
conductivity experiments and density functional theory
calculations supported that the binding of 4a to 3a led to the
formation of a cationic aluminum complex via the ionization of
chloride, a kind of Lewis base activation of Lewis acid12 (A,
Scheme 2). Notably, the thus formed phosphorane-(salen)AlCl
complex showed much higher catalytic activity, because
(salen)AlCl itself was unable to catalyze the ketone
cyanosilylation.
3a and phosphorane
4 (2.5 mol%, each). As expected, no
reaction took place when (salen)AlCl 3a was used alone, in
accordance to North’s result13 (entry 1). It was found that the
substituent of phosphorane
4 obviously influenced the reaction
outcome (entries 2-5). While phosphorane 4a-b bearing an ester
or a phenyl group had almost no acceleration effect, the
addition of 4c with a smaller methyl group slightly promoted
this reaction (entry 4). Therefore, we tried phosphorane 4d, and
found that 27% conversion of 1a could be detected after a
reaction time of 24 h (entry 5).
Table 1. The influence of phosphoranes.
Entry
Phosphorane
Conv. (%)a
Yield (%)a
1
2
3
4
5
no
---
Trace
Trace
9
---
---
---
8
4a: R = CO2Et
4b: R = Ph
4c: R = Me
4d: R = H
27
27
a Determined by 1H NMR using mesitylene as internal standard.
To improve the reactivity, we next tried the merger of
different metal-salen complexes with phosphorane 4d as the co-
catalyst. Since iodide was a better nucleophile and leaving
group than chloride, we prepared (salen)AlI 3b and found it
was indeed more active, as the conversion of 1a was raised to
82% (entry 1 vs 2, Table 2). On the other hand, the screening of
Scheme 2. Phosphorane-(salen)MX catalysts
Based on this mechanistic insight, along with the fact that
metal salen complexes are the most widely used catalysts for
the reactions of epoxides and CO2,3c we speculate that
phosphorane-(salen)MX complexes might be
conceptually new bifunctional catalysts for this valuable
transformation, for following reasons. First, while the in situ
a type of
salen complexes 3c-e with different metals revealed that
(salen)CoCl 3e was the most active one (entry 5 vs 1, 3 and 4).
Next, we tried varying axial halide ligands of cobalt-salen
complexes to improve the activity. As expected, the activity of
formed halide is largely
a
spectator in the ketone
cyanosilylation, it may act as an indispensable nucleophilic co-
catalyst to facilitate the ring-opening of epoxides (B, Scheme 2).
Second, the enhanced Lewis acidity of the metal center
resulting from the activation of the corresponding salen
complex by a phosphorane is promising to inhibit the formation
cobalt complexes 3e-g was significantly influenced by the axial
ligands, and the conversion of epoxide 1a obviously increased
with the halide ligands changing from chlorine to iodine
(entries 5-7). When (salen)CoI 3g was used, product 2a could
be obtained in 96% isolated yield (entry 7). This observation
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 8
Green Chemistry
Please do not adjust margins
Journal Name
ARTICLE
supported our hypothesis that the binding of phosphorane 4d to (entry 4). The use of 2.0 equivs of 4d relative tVoiew3AgrtidcliedOnnlionet
(salen)CoX complexes could polarize the Co-X bond to improve the reactivity (entry 5), which oDbOvIi:a1t0e.d10t3h9e/Cp7oGsCs0ib14il5i8tyA
generate in situ the halide co-catalyst, and (salen)CoI 3g gave that 4d acted as a nucleophilic co-catalyst to facilitate the ring-
the best result simply because it gave iodide, the best co- opening of epoxide. Furthermore, the use of 3.0 equivs of 4d
catalyst and leaving group. It was not surprising that relative to 3g almost terminated the reaction (entry 6), possibly
(salen)CoOTs 3h in combination with 4d failed to catalyze the due to the formation of the six-coordinated 1/2 complex, which
reaction (entry 8), because p-toluenesulfonate anion was known has no catalytic activities as phosphorane had occupied all
to be an inefficient nucleophilic co-catalyst.14 These results empty coordinating sites of cobalt center (for details, see
clearly showed that both the central metal and the axial ligand section 3-3 of SI).
of the salen-metal complexes greatly influenced the catalytic
Table 3 Control experiment.
properties. With axial ligand (X-) of high nucleophility and
leaving ability, the corresponding metal salen complex had a
higher catalytic activity. Considering the substituents on the
salen ligands might have a great influence on the Lewis acidity
of the central metal,3c we also tried some other salen ligands to
further improve the activity, but in vain. For details, see
Entry
X
Y
Conv. (%)a
Yield (%)a
supporting information, SI.
1
2
3
4
5
6
1.0
---
---
---
---
88
70
84
13
---
---
88
68
84
12
Table 2 The influence of metal-salen complexes.
1.0
1.0
0.5
2.0
3.0
1.0
1.0
1.0
1.0
a Determined by 1H NMR using mesitylene as internal standard.
Conv.
(%)a
Yield
(%)a
Entry
(salen)MX complex
Y
Table 4 Substrate scope of epoxides.
1
2
3a: M = Al, X= Cl
3b: M = Al, X= I
3c: M = Mn, X= Cl
3d: M = Cr, X= Cl
3e: M = Co, X= Cl
3f: M = Co, X= Br
3g: M = Co, X= I
3h: M = Co, X= OTs
3g: M = Co, X= I
3g: M = Co, X= I
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
1.0
1.0
27
82
27
80
7
3
8
4
40
17
44
77
96
---
88
95c
5
50
Time Conv.
Yield
(%)b
Entry
1
2
(h)
36
24
36
24
24
24
36
24
24
36
36
36
(%)a
97
6
80
7
100
Trace
88
1
2
1a: R = Ph
2a
2b
2c
2d
2e
2f
95
91
80
90
87
92
91
90
93
95
88
90
8
1b: R = 4-MeC6H4
1c: R = 4-ClC6H4
1d: R = Me
full
85
9
3
10b
97
4
98
5
1e: R = Et
93
a
Determined by 1H NMR using mesitylene as internal standard. b On a 5.0 mmol
scale, 36 hours. c isolated yield.
6
1f: R = n-Bu
92
7
1g: R = PhCH2
1h: R = ClCH2
1i: R = i-PrOCH2
1j: R = BnOCH2
1k: R = allylOCH2
1l: R = propargylOCH2
(S)-1a (>99% ee)
(S)-1d (>99% ee)
2g
2h
2i
98
Gratifyingly, if lowering the loading of 3g and 4d from 2.5
mol% to 1.0 mol%, the reaction could still furnish the carbonate
2a in 88% yield (entry 9). If prolonging the reaction time to 36
h, an isolated 95% yield was obtained for product 2a (entry10).
8
97
9
97
10
11
12
13
14
2j
full
full
95
The efficiency of 3g/4d is comparable to North’s dinuclear
2k
2l
(salenAl)2O complex (shown in Scheme 1), one of the most
efficient catalytic systems for the coupling of CO2 and epoxide.
The combination of (salenAl)2O with (n-Bu)4NBr (1.0 mol%,
each) could afford 86% conversion in 24 h.13, 15
(S)-2a, 36 h, 86% yield, 93% ee
(S)-2d, 24 h, 84% yield, >99% ee
To further confirm our hypothesis, control experiments
were conducted as shown in Table 3. Either 1 mol% (salen)CoI
a Determined by 1H NMR. b Isolated yield.
3g or 1 mol% phosphorane 4d alone was unable to catalyze this
In the following, the scope with respect to different
o
reaction at 25 C (entries 1-2). However, the merger of both epoxides was examined by running the reaction on a 5.0 mmol
could achieve a conversion of 88% after 24 h (entry 3). The use scale at ambient conditions (25 oC, 1 atm of CO2), in the
of 0.5 equiv of 4d relative to 3g resulted a lower conversion presence of (salen)CoI 3g and phosphorane 4d (1.0 mol%,
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 4 of 8
ARTICLE
Journal Name
each), as shown in Table 4. To our delight, no matter aryl could be easily activated by phosphorane as well, the reason
View Article Online
DOI: 10.1039/C7GC01458A
epoxides 1a-c or aliphatic substituted epoxides 1d-l, all worked why the resulting complex was impotent to catalyze the
well under this mild condition, to afford the desired cyclic coupling was simply because the in situ formed sulfonate was a
carbonates 2a
in high to excellent isolated yield (entries 1-12). weak nucleophile.14
-
l
In addition, different functional groups such as chloromethyl,
ether, allyl and propargyl groups on the side chain of the
epoxides could be tolerated. We also tried the synthesis of
optically active carbonates from the enantiopure epoxides. For
example, while 2-phenyloxirane (S)-1a (99% ee) afforded
chiral product (S)-2a in slightly diminished 93% ee (entry 13),
the aliphatic (S)-1d furnished (S)-2d in 99% ee (entry 14).
It should be noted that 1H NMR analysis of the crude
mixture of each reaction did not show the presence of
polycarbonates (see SI). This exhibited the enhanced Lewis
acidity of the cobalt center resulting from the activation of
(salen)CoI by phosphorane was indeed effective to suppress
this side copolymerization. This is impressive, because the
synthesis of thermodynamically more stable cyclic carbonates
was usually favored when the reactions were performed at
enhanced temperatures, or in the presence of an excess of
nucleophilic cocatalyst.16
100
80
60
40
20
0
10
8
6
4
2
0
0
1
2
3
4
0
1
2
3
4
Figure 1. Conductivity changes of (salen)CoX in response to the addition of
phosphonane 4d at 25 oC. Inset: Conductivity changes of 4d.
To further confirm the generation of a halide after the
activation of (salen)CoX complex by a phosphorane and to
investigate whether the in situ formed halide involve in the
ring-opening step, we tried to detect the intermediate resulting
from the attack of a halide to epoxide 1a by GC-MS analysis.
This was inspired by the successful trapping of the
halogenoalcohol to confirm the role of halogen anion of the
ammonium salt by the groups of Chisholm20 and Ren & Lu21
respectively. Fortunately, when (salen)CoBr 3f in combination
with phosphorane 4d (0.1 mmol, each) was stirred in 0.5 mL of
styrene oxide, we could detect the signal at m/z 200, 202 (1/1),
To demonstrate the scalability of this protocol, a 50 mmol
scale synthesis using only 0.1 mol% of 3g and 0.1 mol% of 4d
was tried. Although the reaction proceeded slowly, the catalyst
kept active during a reaction time of 15 days, giving product 2e
in 79% isolated yield.17
Now that phosphorane-(salen)CoI complex had been
established as an efficient bifunctional catalyst for the coupling
corresponding to the bromoalcohol
7 (For details, see section 3-
o
2
of SI). Unfortunately, the reaction proceeded very slowly and
of CO2 with terminal epoxides at 25 C, under 1 atm of CO2,
was accompanied by severe hydrolysis of epoxide, so the
reaction system was in a mess, difficult to clearly detect the
we next tried to gain a better understanding of the reaction
mechanism. In the following, electrical conductivity
experiments were conducted to probe the ionization of Co-X
bond resulting from the binding of phosphorane to (salen)CoI
complex. According to the theory of Lewis base catalysis
defined by Denmark,18 the activation of a metal complex by a
Lewis base may stem from a redistribution of electron-density
in the thus formed Lewis adduct, which polarizes adjacent
bonds to generate a hypervalent species with enhanced Lewis
acidity.19 Given a strong polarization, ionization would happen
to produce cationic species with greatly enhanced
electrophilicity. The electrical conductivity was measured by
using a two-probe method, as we previously reported.11a
presence of bromoalcohol
7 by NMR analysis. This result
suggested the in situ generated halide facilitated the ring-
opening of epoxide by nucleophilic attack at the less substituted
methylene carbon of oxide, as shown in Scheme 2B.
To get more information about the phosphorane-(salen)CoI
complex, mass spectroscopy studies were conducted. Initially,
the MALDI-TOF MS analysis of the 1:1 mixture of 3g and 4d
Although the conductivity of (salen)CoCl 3e, (salen)CoBr 3f
,
detected a characteristic signal [3g
/
4d - I]+ at m/z 907.43,
(salen)CoI 3g, (salen)CoOTs 3h and phosphonane 4d in
CH2Cl2 (2 × 10-3 M) were all weak, the addition of 4d to the
consistent with the 1/1 phosphorane-(salen)CoI complex 3g
/4d
,
which was further confirmed by ESI-HRMS. Further study of
the 1:3 mixture of 3g and 4d via ESI-MS revealed that the
binding of a second molecule of 4d was also possible, but the
resulting six-coordinated 1/2 complex was less stable as the
CID study shown in Figure 2 and it probably had no catalytic
activity as revealed in Table 3. (For details, see section 3-3 of
SI)
CH2Cl2 solution of complexes 3e, 3f, 3g and 3h all resulted in a
dramatic increase in the conductivity. When 1.0 equiv of 4d
was added, the conductivity was increased by around 133, 378,
43.5 and 13.8 times, respectively. These results strongly
supported that ionization process took place to form a halide or
a sulfonate after the activation of (salen)CoX complexes by
phosphorane 4d. Although it seemed that (salen)CoOTs 3h
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 8
Green Chemistry
Please do not adjust margins
Journal Name
ARTICLE
As shown in the selected part of NMR spectra, when 1.0
View Article Online
equiv of (salen)CoI 3g was added to a soDluOtiI:o1n0.o10f349d/C7inGCC0D14C58l3A,
1H NMR analysis showed that the quaternary H(1)-cis signal
slightly shifted from 3.68 (JPH = 24.5 Hz) to 3.63 ppm (JPH
=
23.0 Hz), while the H(1)-trans signal at 4.10 ppm (JPH = 19.5
Hz) disappeared. On the other hand, 13C NMR analysis
revealed that the characteristic signal of doublet C(1)-cis carbon
shifted downfield from 54.87 (JPC = 99.4 Hz) to 63.43 ppm (JPC
= 99.6 Hz), and the C(1)-trans carbon signal at 56.56 ppm (JPC
= 109.6 Hz) disappeared. These observations implied that the
binding of the cis isomeric form of phosphorane 4d to
(salen)CoI 3g was favored, and the trans isomeric form of 4d
readily balanced to the corresponding cis form for coordination.
According to literature reports, the C-coordination of a
phosphorane to a metal complex would result in the upfield-
shift of the signal of the C1 carbon, along with the downfield-
shift of the signal of phosphorous.22c The current studies
detected an obvious downfield-shift from 54.87 to 63.43 of the
signal of C1 carbon, together with the upfield shift from 15.22
to 14.38 of that of phosphorous. Therefore, we believed that 4d
coordinated to (salen)CoI 3g in a cis O-coordination, as shown
in eq 3. For details of NMR analysis, see section 3-4 of SI.
Based on the aforementioned studies, we proposed a
mechanism for the bifunctional phosphorane-(salen)CoI
Figure 2. ESI-HRMS of 1/1 complex (left); Plots of [3g/(4d)2-I]+ abundance versus CID
voltage (right).
We further conducted the NMR analysis to investigate the
coordination fashion of the phosphonane 4d to (salen)CoI 3g 22
.
It was known that phosphorane might form various C-
coordinated (I) and O-coordinated (II) metal complexes (eq 1,
Figure 3).22c Previously, by NMR studies, we safely arrived at a
conclusion that phosphorane 4c bound to (salen)AlCl 3a in an
O-coordination fashion.11a However, in the case of
phosphonane 4d, the analysis was more difficult, because
according to the study of Kayser and Hatt,22a 4d existed in a
stable enolate ion, as a mixture of isometric forms of 4d-cis and
4d-trans (eq 2). Fortunately, NMR studies provided persuasive
proofs to support the formation of an O-coordinated complex
derived from phosphorane 4d and (salen)CoI 3g (eq 3).
complex 3g
/4d catalyzed coupling reaction of epoxide and CO2,
as shown in Figure
4
. The coordination of 1.0 equiv of
phosphorane 4d to (salen)CoI 3g readily afforded the desired
bifunctional cobalt complex. With electrophilic activation of
epoxide by cobalt (III), the in situ generated nucleophilic iodide
attacked the less hindered methylene carbon of the epoxide,
followed by the insertion of CO2 into the Co-O bond to form
the metal carbonate. Finally, ring closure afford the desired
cyclic carbonate along with the regeneration of the catalyst.
However, the reaction of enantiopure epoxide (S)-1a afforded
(S)-2a in only 93% ee, which indicated that the reaction did not
proceed with 100% retention of the stereochemistry. In light of
this, there might be other mechanism involved23 and the
nucleophilic attack at the methine carbon was also possible.
Figure 4. Proposed reaction mechanism.
Encouraged by the successful results of the phosphorane-
(salen)CoI 3g/4d complex catalyzed cycloaddition of epoxide
and CO2, we further examined the potential of phosphorane-
(salen)CoI complexes for the coupling of aziridines with CO2
under ambient conditions. It was found that under the catalysis
Figure 3. 1H, 13C and 31P NMR study.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 6 of 8
ARTICLE
Journal Name
of (salen)CoI 3g and phosphorane 4b (2.5 mol%, each), a could also catalyze the coupling of CO2 with aziVriiedwinAretiscleuOnndliener
DOI: 10.1039/C7GC01458A
variety of aryl aziridines
Table 5.24 Both N-alkyl aziridines 5a
aziridine 5d afforded the corresponding 1,3-oxazolidin-2-ones phosphorane-(salen)MX complexes can be readily tuned by
6a in moderate to high yield (entries 1-4). In addition, no varying the axial halide ligand, the metal center and the
matter electron-withdrawing or electron-donating substituent on substituent of phosphorane and salen ligands. This offers the
the phenyl ring of the aziridines, the desired products 5e were promise for further improving catalyst properties. The current
all obtained in high yield (entries 5-7). 2-Naphthyl substituted protocol using phosphorane-(salen)CoI catalyst 3g 4d has a
5
worked well with CO2, as shown in ambient conditions successfully.
-
c
and unprotected
Importantly, the catalytic properties of such bifunctional
-
d
-
g
/
aziridine 5h was also viable under this condition, furnishing 6h limitation that 1,2-disubstituted epoxides work inefficiently.28
in 40% yield (entry 8). When enantiopure (R)-5a was used, (R)- Further optimization of the catalyst structures to improve the
6a could be obtained in >99% ee with 82% yield (entry 9).
catalytic efficiency along with the application of the catalytic
It is worth mentioning that as compared with the coupling system in the kinetic resolution of epoxides for the synthesis of
of epoxides with CO2, the corresponding coupling of aziridines chiral cyclic carbonates are now in progress in our laboratory29.
is much less well studied.25 A challenge is how to avoid the
Acknowledgements
The financial support from 973 Program (2015CB856600) and
NSFC (21502053, 21472049) is appreciated.
unselective synthesis of 4- or 5-substituted oxazolidinones.26
While several elegant protocols for the selective synthesis of 5-
substituted oxazolidinones have been developed by Nguyen, He
and Lu groups et al.,27 only very limited way could be
Notes and references
27d,
h
performed at ambient conditions.8a,
To the best of our
knowledge, metal-catalyzed coupling of aziridines and CO2
usually required high pressure of CO2.21, 27a, b
1
(a) D. B. Dell’Amico, F. Calderazzo, L. Labella, F. Marchetti and
G. Pampaloni, Chem. Rev., 2003, 103, 3857; (b) T. Sakakura, J.-
C. Choi and H. Yasuda, Chem. Rev., 2007, 107, 2365; (c) D. M.
D’Alessandro, B. Smit and J. R. Long, Angew. Chem. Int. Ed.,
2010, 49, 6058; (d) M. Cokoja, C. Bruckmeier, B. Rieger, W. A.
Table 5 Substrate scope of aziridines.
Herrmann and F. E. Kühn, Angew. Chem. Int. Ed., 2011, 50
8510; (e) C. Maeda, Y. Miyazaki and T. Ema, Catal. Sci.
Technol., 2014, , 1482; (f) G. Fiorani, W. Guo and A. W. Kleij,
,
4
Green Chem., 2015, 17, 1375; (g) W. Leitner, Coord. Chem. Rev.,
1996, 153, 257; (h) I. Omae, Catal. Today, 2006, 115, 33; (i) M.
Aresta and A. Dibenedetto, Dalton Trans., 2007, 2975; (j) D. J.
Darensbourg, Inorg. Chem., 2010, 49, 10765; (k) H. Zhou and X.
Lu, Sci. China Chem., 2017, 60, 904; (l) A. W. Kleij, M. North,
and A. Urakawa, ChemSusChem, 2017, 10, 1036.
Conv.
(%)a
Yield
(%)b
Entry
5
6
1
2
3
4
5
6
7
8c
9
5a: R = Ph, R’ = Me
6a
6b
6c
6d
6e
6f
full
84
84
65
54
38
83
90
76
40
2
3
(a) Q. Liu, L. Wu, R. Jackstell and M. Beller, Nat. Commun.,
5b: R = Ph, R’ = Et
2015,
6, 5933; (b) D. J. Darensbourg and M. W. Holtcamp,
Coord. Chem. Rev., 1996, 153, 155; (c) S. N. Riduan and Y.
Zhang, Dalton Trans., 2010, 39, 3347.
5c: R = Ph, R’ = Bn
58
5d: R = Ph, R’ = H
full
full
full
full
48
(a) C. Martín, G. Fiorani and A. W. Kleij, ACS Catal., 2015,
1353; (b) M. Yoshida and M. Ihara, Chem.-Eur. J., 2004, 10
5
,
,
5e: R = 4-MeC6H4, R’ = Me
5f: R = 4-ClC6H4, R’ = Me
5g: R = MeOCH2C6H4, R’ = Me
5h: R = 2-Naphthyl, R’ = Me
(R)-5a (>99% ee)
2886; (c) A. Decortes, A. M. Castilla and A. W. Kleij, Angew.
Chem. Int. Ed 2010, 49, 9822; (d) M. North, R. Pasquale and C.
.,
Young, Green Chem., 2010, 12, 1514; (e) J. W. Comerford, I. D.
V. Ingram, M. North and X. Wu, Green Chem., 2015, 17, 1966;
(f) X.-D. Lang and L.-N. He, Chem. Rec., 2016, 16, 1337.
6g
6h
4
(a) E. J. Beckman, Science, 1999, 283, 946; (b) Y. Zhu, C.
Romain and C. K. Williams, Nature, 2016, 540, 354; (c) G. W.
Coates and D. R. Moore, Angew. Chem. Int. Ed., 2004, 43, 6618;
(d) D. J. Darensbourg, Chem. Rev., 2007, 107, 2388; (e) W.
Kuran, Prog. Polym. Sci., 1998, 23, 919; (f) S. Klaus, M. W.
Lehenmeier, C. E. Anderson and B. Rieger, Coord. Chem. Rev.,
2011, 255, 1460; (g) X.-B. Lu, W.-M. Ren and G.-P. Wu, Acc.
Chem. Res., 2012, 45, 1721; (h) D. J. Darensbourg and A. D.
(R)-6a, 82%, >99% ee
a Determined by 1H NMR. b Isolated yield. c DMF (1.0 M) was used as solvent.
Conclusions
In summary, we have demonstrated that a bifunctional
phosphorane-(salen)CoI complex as a conceptually new type of
efficient catalysts for the coupling reaction of CO2 with
Yeung, Polym. Chem., 2014,
5, 3949; (i) S. Paul, Y. Zhu, C.
Romain, R. Brooks, P. K. Saini and C. K. Williams, Chem.
Commun., 2015, 51, 6459; (j) Y. Qin, X. Sheng, S. Liu, G. Ren,
X. Wang and F. Wang, J. CO2 Util., 2015, 11, 3; (k) G. A.
Luinstra, Polym. Rev., 2008, 48, 192; (l) M. R. Kember, A.
Buchard and C. K. Williams, Chem. Commun., 2011, 47, 141;
(m) M. I. Childers, J. M. Longo, N. J. V. Zee, A. M. LaPointe
and G. W. Coates, Chem. Rev., 2014, 114, 8129.
Applications of cyclic carbonates: (a) B. Schäffner, F. Schäffner,
S. P. Verevkin and A. Börner, Chem. Rev., 2010, 110, 4554; (b)
T. Sakakura and K. Kohno, Chem. Commun., 2009, 1312; (c) J.
H. Clements, Ind. Eng. Chem. Res., 2003, 42, 663; Applications
o
terminal epoxides under ambient conditions (25 C, 1 atm of
CO2), with a catalyst loading of 1.0 mol%. The key to the
efficiency of the protocol is the activation of (salen)CoI
complex by a phosphorane co-catalyst, which not only
enhances the Lewis acidity of the Co(III) center for
electrophilic activation, but also simultaneously generates an
iodide to facilitate the ring-opening of the epoxide. Further
investigation revealed the phosphorane-(salen)CoI complex
5
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 8
Green Chemistry
Please do not adjust margins
Journal Name
ARTICLE
of polycarbonates: (d) Y. Qin and X. Wang, Biotechnol. J., 2010,
, 1164.
M. Mikkelsen, M. Jørgensen and F. C. Krebs, Energy Environ.
Sci., 2010, , 43.
For metal catalysts: (a) X.-B. Lu, Y.-J. Zhang, B. Liang, X. Li 18 (a) S. E. Denmark and G. L. Beutner, Angew. Chem. Int. Ed.,
Chem. Sci., 2012, 3, 2094.
17 When the same reaction was performedDaOtI:1100.a10tm39/oCf7GCCO0214w5i8tAh
View Article Online
5
6
7
0.1 mol% of 3g
/4d, 58% conversion could be obtained in only 48
3
h, which further demonstrated the scalability of the reaction.
and H. Wang, J. Mol. Catal. A-Chem., 2004, 210, 31; (b) M.
2008, 47, 1560; (b) G. L. Beutner and S. E. Denmark, Angew.
North and R. Pasquale, Angew. Chem. Int. Ed., 2009, 48, 2946; (c)
Chem. Int. Ed., 2013, 52, 9086.
M. North and C. Young, Catal. Sci. Technol., 2011,
North, S. C. Z. Quek, N. E. Pridmore, A. C. Whitwood and X.
Wu, ACS Catal., 2015, , 3398; (e) J. A. Castro-Osma, C. 20 N. D. Harrold, Y. Li and M. H. Chisholm, Macromolecules, 2013,
Alonso-Moreno, A. Lara-Sánchez, J. Martínez, M. North and A. 46, 692.
Otero, Catal. Sci. Technol., 2014, , 1674; (f) J. A. Castro-Osma, 21 W.-M. Ren, Y. Liu and X.-B. Lu, J. Org. Chem., 2014, 79, 9771.
M. North and X. Wu, Chem.-Eur. J., 2016, 22, 2100; (g) J. A. 22 a) M. M. Kayser and K. L. Hatt, Can. J. Chem., 1991, 69, 1929;
1
, 93; (d) M. 19 V. Gutmann, The Donor-Acceptor Approach to Molecular
Interactions; Plenum Press, New York, 1978, chap. 1.
5
4
Castro-Osma, K. J. Lamb and M. North, ACS Catal., 2016,
6
,
(b) P. A. Byrne, K. Karaghiosoff and H. Mayr, J. Am. Chem. Soc.,
2016, 138, 11272; (c) L. R. Falvello, S. Fernández, R. Navarro, I.
Pascual and E. P. Urriolabeitia, J. Chem. Soc. Dalton Trans.,
1997, 763.
5012-; (h) A. Buchard, M. R. Kember, K. G. Sandeman and C. K.
Williams, Chem. Commun., 2011, 47, 212; (i) Y. Yang, Y.
Hayashi, Y. Fujii, T. Nagano, Y. Kita, T. Ohshima, J. Okuda and
K. Mashima, Catal. Sci. Technol., 2012,
S. Shimada, Chem. Commun., 2009, 1136; (k) H. V. Babu and K.
Muralidharan, Dalton Trans., 2013, 42, 1238; (l) B. Barkakaty, K. 24 Mechanistic study for the coordination of phosphorane 4b to
2
, 509; (j) S.-F. Yin and 23 J. A. Castro-Osma, M. North, W. K. Offermans, W. Leitner
and T. E. Müller, ChemSusChem, 2016, , 791.
9
Morino, A. Sudo and T. Endo, Green Chem., 2010, 12, 42; (m) I.
Shibata, I. Mitani, A. Imakuni and A. Baba, Tetrahedron Lett.,
2011, 52, 721; (n) A. Monassier, V. D’Elia, M. Cokoja, H. Dong,
3g was also performed, however, apart from electrical
conductivity study, we tried in vain to get more information
for their interaction, for detail see section 3-1 of SI.
̈
J. D. A. Pelletier, J.-M. Basset and F. E. Kuhn, ChemCatChem, 25 (a) P. Tascedda and E. Duñach, Chem. Commun., 2000, 449; (b)
2013, , 1321. H. Kawanami and Y. Ikushima, Tetrahedron Lett., 2002, 43
For Organic catalysts: (a) H. Zhou, G.-X. Wang, W.-Z. Zhang
and X.-B. Lu, ACS Catal., 2015, , 6773; (b) L. Wang, G. Y.
Zhang, K. Kodama and T. Hirose, Green Chem., 2016, 18, 1229;
(c) N. Aoyagi, Y. Furusho and T. Endo, Tetrahedron Lett., 2013
54 7031. Du, Y. Wu, A.-H. Liu and L.-N. He, J. Org. Chem., 2008, 73,
5
,
8
9
3841; (c) A. Sudo, Y. Morioka, E. Koizumi, F. Sanda and T.
Endo, Tetrahedron Lett., 2003, 44, 7889; (d) M. T. Hancock and
A. R. Pinhas, Tetrahedron Lett., 2003, 44, 5457; (e) Y.-M. Shen,
W.-L. Duan and M. Shi, Eur. J. Org. Chem., 2004, 3080; (f) Y.
5
,
,
For other examples at mospheric pressure or room temperature:
4709; (g) C. Phung, R. M. Ulrich, M. Ibrahim, N. T. G. Tighe, D.
(a) X.-B. Lu, B. Liang, Y.-J. Zhang, Y.-Z. Tian, Y. M. Wang, C.-
L. Lieberman and A. R. Pinhas, Green Chem., 2011, 13, 3224; (h)
X. Bai, H. Wang and R. Zhang, J. Am. Chem. Soc., 2004, 126
3732; (b) X.-B. Lu and Y. Wang, Angew. Chem. Int. Ed., 2004,
43, 3574; (c) A. Decortes, M. M. Belmonte, J. Benet-Buchholz
,
T. Y. Ma and S. Z. Qiao, ACS Catal., 2014,
D. B. Shinde, R. Banerjee and B. M. Bhanage, Catal. Sci.
Technol., 2016, , 6152.
4, 3847; (i) V. Saptal,
6
and A. W. Kleij, Chem. Commun., 2010, 46, 4580; (d) A. 26 S. Stanković, M. D’hooghe, S. Catak, H. Eum, M. Waroquier, V.
Decortes and A. W. Kleij, ChemCatChem, 2011,
3
, 831; (e) A.
V. Speybroeck, N. D. Kimpe and H.-J. Ha, Chem. Soc. Rev.,
Berkessel and M. Brandenburg, Org. Lett., 2006,
8, 4401; (f) R.
2012, 41, 643.
Ma, L.-N. He and Y.-B. Zhou, Green Chem., 2016, 18, 226;
27 (a) A. W. Miller and S. T. Nguyen, Org. Lett., 2004,
D. Adhikari, A. W. Miller, M.-H. Baik and S. T. Nguyen, Chem.
Sci., 2015, , 1293; (c) Z.-Z. Yang, L.-N. He, S.-Y. Peng and A.-
6, 2301; (b)
10 (a) M. S. Sigman and E. N. Jacobsen, J. Am. Chem. Soc., 1998,
120, 5315; (b) T. P. Yoon and E. N. Jacobsen, Science, 2003, 299
1691.
11 (a) X.-P. Zeng, Z.-Y. Cao, X. Wang, L. Chen, F. Zhou, F. Zhu,
C.-H. Wang and J. Zhou, J. Am. Chem. Soc., 2016, 138, 416; (b)
X.-P. Zeng and J. Zhou, J. Am. Chem. Soc., 2016, 138, 8730; (c)
X. Ye, X.-P. Zeng and J. Zhou, Acta Chim. Sinica, 2016, 74, 984.
12 For review of Lewis base catalysis, see: (a) S. E. Denmark and G.
,
6
H. Liu, Green Chem., 2010, 12, 1850; (d) Z.-Z. Yang, Y.-N. Li,
Y.-Y. Wei and L.-N. He, Green Chem., 2011, 13, 2351; (e) A.-H.
Liu, R. Ma, C. Song, Z.-Z. Yang, A. Yu, Y. Cai, L.-N. He, Y. N.
Zhao, B. Yu and Q.-W. Song, Angew. Chem. Int. Ed., 2012, 51
,
11306; (f) X.-Z. Lin, Z.-Z. Yang, L.-N. He and Z.-Y. Yuan,
Green Chem., 2015, 17, 795; (g) H. Zhou, Y.-M. Wang, W.-Z.
Zhang, J.-P. Qu and X.-B. Lu, Green Chem., 2011, 13, 644; (h) F.
Fontana, C. C. Chen and V. K. Aggarwal, Org. Lett., 2011, 13,
34547; And ref 8a.
L. Beutner, Angew. Chem
Beutner and S. E. Denmark, Angew. Chem. Int. Ed., 2013, 52
9086; For selected recent examples: (c) S. E. Denmark and A.
. Int. Ed., 2008, 47, 1560; (b) G. L.
,
Jaunet, J. Am. Chem. Soc., 2013, 135, 6419; (d) S. E. Denmark 28 We found that phosphorane-(salen)CoI catalyst 3g/4d catalyzed
o
and H. M. Chi, J. Am. Chem. Soc., 2014, 136, 8915; (e) S. E.
Denmark, E. Hartmann, D. J. P. Kornfilt and H. Wang, Nat.
Chem., 2014, 6, 1056.
the coupling of epoxide 1m with CO2 inefficiently at 25 C,
with 10 atm of CO2, but no better results were obtained at
elevated temperature, possibly due to the instability of salen
Co(III) complexes at elevated temperature, as pointed out by Lu
and co-workers (ref 16c).
13 W. Clegg, R. W. Harrington, M. North and R. Pasquale, Chem.-
Eur. J., 2010, 16, 6828.
14 The nucleophility and leaving ability of the anion has great
influence on the reaction. For example: (a) X.-B. Lu, X.-J. Feng,
R. He, Appl. Catal. A-Gen., 2002, 234, 25; (b) X.-B. Lu, Y.-J.
Zhang, K. Jin, L.-M. Luo and H. Wang, J. Catal., 2004, 227, 537.
15 A comparative study at the early stage of reaction was also
performed: 3g/4d (1.0 mol%), 3 h, 37% conversion, 37% yield;
(salenAl)2O/Bu4NBr (1.0 mol%), 3 h, 38% conversion (ref 13).
16 For review: (a) X.-B. Lu and D. J. Darensbourg, Chem. Soc. Rev.,
2012, 41, 1462; For example: (b) C. T. Cohen, T. Chu and G. W.
Coates, J. Am. Chem. Soc., 2005, 127, 10869; (c) X.-B. Lu, L.
Shi, Y.-M Wang, R. Zhang, Y.-J. Zhang, X.-J. Peng, Z.-C. Zhang
and B. Li, J. Am. Chem. Soc., 2006, 128, 1664; (d) W.-M. Ren,
G.-P. Wu, F. Lin, J.-Y. Jiang, C. Liu, Y. Luo and X.-cB. Lu,
29 A preliminary attempt revealed that using CH2Cl2 as solvent,
chiral cyclic carbonate 2a could be obtained in 23% ee with 10%
conversion over 15 hours, for the 1 mol% of 3g/4d catalyzed
coupling of 1a with CO2, which demonstrated the feasibility of
the corresponding kinetic resolution reaction.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Green Chemistry
Page 8 of 8
View Article Online
DOI: 10.1039/C7GC01458A
The activation of (salen)CoI complex by phosphorane in situ formed a bifunctional catalyst for the
reaction of carbon dioxide with terminal epoxides or aziridines at ambient temperature and 1 bar
carbon dioxide pressure.