A recyclable AgI/OAc- catalytic system for the efficient synthesis of α-alkylidene cyclic carbonates: Carbon dioxide conversion at atmospheric pressure

Article abstract of DOI:10.1039/c7gc00276a

The cyclization of carbon dioxide and propargylic alcohols, especially challenging substrates, were efficiently catalyzed by a green and recyclable AgI/OAc^- system under atmospheric pressure, which is shown to be the most recyclable system with 20 recycle rounds and has the lowest loading among all the reported recyclable systems that work under atmospheric pressure.

Full text of DOI:10.1039/c7gc00276a

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A Recyclable AgI/OAc- Catalytic System for the Efficient Synthesis
of α-Alkylidene Cyclic Carbonates: Carbon Dioxide Conversion at
Atmospheric Pressure
Received 00th January 20xx,
Accepted 00th January 20xx
Ye Yuan,a,b Yu Xie,a Cheng Zeng,b Dandan Song,b Somboon Chaemchuen,a Cheng Chen,*,a Francis
Verpoort*,a,b,c,d
DOI: 10.1039/x0xx00000x
www.rsc.org/
The cyclizations of carbon dioxide and propargylic alcohols adduct,28 N-heterocyclic olefin/CO2 adduct,29 azole-anion-
especially challenging substrates were efficiently catalyzed by a based aprotic ionic liquids30 and the transition-metallic Cu,31-34
green and recyclable AgI/OAc- system under atmospheric pressure, Ag,22, 35-44 Co,45 Pd,46 Ru,47, 48 W,49 Zn50 systems as the efficient
which exhibited as the most recyclable system with 20 recycle catalysts for this carboxylative cyclization have been
rounds and the lowest loading system among all the recyclable developed. T. Yamada et al. reported a AgOAc/DBU system
systems that worked under atmospheric pressure.
which efficiently catalyzed this cyclization and obtained the
carbonates as pure Z-isomers.42 Furthermore, they achieved
the enantioselective incorporation of CO2 into bispropargylic
alcohols by employing a AgOAc/chiral Schiff base ligand
system.41 He et al. reported a AgOAc/(n-C7H15)4NBr system
with a very high turnover number of up to 6024.22 Wang et al.
reported a AgOAc/[P66614][DEIm] system in which the ionic
liquids worked as both adsorbents and activators of CO2.38
Based on the principles of green and sustainable chemistry,
a catalyst exhibiting excellent activity as well as everlasting
reusability is more favourable for practical application. Up to
now, however, most of the catalysts reported for this
cyclization do not possess the recyclability. Although several
As one of the crucial subjects of sustainable development,
carbon dioxide capture and utilization (CCU) is attracting more
and more interests in view of energy, environmental and social
issues.1-4 In terms of synthetic chemistry, using the ubiquitous,
cheap, renewable and nontoxic carbon dioxide (CO2) as a
chemical feedstock for the synthesis of high-value compounds
is extraordinarily attractive.5-7 However, the inherent
thermodynamic and kinetic stability of CO2 impedes its
efficient incorporation into the organic framework.7 Thus,
effective protocols and catalysts are still urgently desired for
the potential valuable CO2 transformation reactions.
35, 37-40, 44
heterogeneous catalysts have been developed,32,
Gratefully, remarkable progress has been achieved in this
area during the past few decades.8, 9 Incorporation of CO2 and
epoxides,10, 11 alkynes,12-16 alkenes,17, 18 amines,19-21 etc. has
successfully yielded the desired product with the catalysis of
the respective catalyst. Among them, the carboxylative
cyclization of CO2 and propargylic alcohols has evoked great
interests since the resultant products, α-alkylidene cyclic
carbonates, are a series of versatile heterocyclic compounds
harsh reaction conditions including elevated CO2 pressure,
high catalyst loading and poor recyclability were still inevitably
involved in these systems. Therefore, the design of robustly
active and extraordinarily reusable catalysts still remains a
great challenge for academic researches and industrial
applications. Gratifyingly, ionic liquids (ILs), a class of emerging
green materials, offer us a novel methodology that combines
the seemingly incompatible merits of homogeneous catalysis
(excellent activity) and heterogeneous catalysis (outstanding
reusability) together based on their unique structures and
properties,51-54 which could offer high solubility for
homogeneous catalysis to dissolve the starting materials and
provide high-polar immiscibility that facilitates the
heterogeneous extraction of products with low-polar solvents.
Herein, based on the aforementioned strategy, we designed
an ionic liquid based AgI/OAc- system which exhibited
outstanding activities for the carboxylative cyclization of
propargylic alcohols and CO2. Numerous substrates, even
sterically hindered substrates, could be converted into the
target α-alkylidene cyclic carbonates rapidly and smoothly
with potential bioactivities for pharmaceutical chemistry and
23
wide applications in organic synthesis.22,
To date, many
elegant strategies employing the metal-free phosphine,24, 25
bicyclic guanidine,26 alkoxide-functionalized imidazolium
betaines/CO2 adduct,27 N-heterocyclic carbene (NHC)/CO2
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Table 1. Screening of the reaction conditions.a
Table 2. Screening of the substrates.a
O
OH
[Ag]/IL system
+
O
CO2
O
(1 bar)
N
N
N
N
N
N
Yieldb
Time
OAc-
Substrate
Product
-
OAc-
BF4
(Selectivity)
IL2
IL1
IL3
93%
N
N
N
N
I-
1a
1b
1c
2a
2b
2c
3 h
(CF3SO2)2N-
IL4
(>99%)
IL5
Entry
Catalytic system
Yieldb
96%
4 h
(>99%)
Ag salt
AgI
IL
1
/
0
2
/
IL1
IL1
IL1
IL1
IL1
0
91%
3
AgI
91%
75%
48%
43%
44%
60%
48%
58%
86%
41%
0
3 h
(98%)
4
AgBr
AgCl
Ag2O
5
6
7
Ag2WO4
Ag2CO3
AgOAc
AgNO3
AgPF6
AgI
IL1
IL1
IL1
IL1
IL1
IL2
IL3
IL4
IL5
IL1
99%
1d
1e
1f
2d
2e
2f
6 h
8
(>99%)
9
10
11
12
13
14
15c
16d
88%
8 h
AgI
(99%)
AgI
0
AgI
0
AgI
56%
99%
a Reaction conditions: [Ag] (0.05 mmol), IL (1 mL. details for the mass and amount
of ILs are supplied in Table S2 of the supporting information), 2-methylbut-3-yn-
2-ol (5 mmol), CO2 (1 bar) at 45°C for 2.5 h. b Yields were determined by 1H NMR
spectroscopy using 1,1,2,2-tetrachloroethane as the internal standard. c IL5 (1.5 g,
6.301 mmol), 80°C. d 25°C
6 h
(>99%)
97%
1g
1h
2g
2h
6 h
(98%)
under atmospheric CO2 pressure with a competitively low
catalyst loading. Notably, this system performed excellent
constancy and recyclability, which is considered to be the most
recyclable catalytic system that could be continuously reused
for at least 20 times without loss of its high catalytic activity.
Moreover, it is the lowest catalyst loading system ever
reported among all the recyclable systems that worked under
atmospheric CO2.
44%
12 h
(97%)
a Reaction conditions: AgI (0.05 mmol), IL1 (1 mL, 1.102 g, 6.474 mmol), substrate
(5 mmol), CO2 (1 bar) at 45°C. b Yields were determined by 1H NMR spectroscopy
using 1,1,2,2-tetrachloroethane as the internal standard.
The carboxylative cyclization of 2-methylbut-3-yn-2-ol and
CO2 was selected as the model catalytic reaction for exploring
the optimal reaction conditions (Table 1). Initially, AgI or IL1
was individually employed as the catalyst, while they were
demonstrated to be ineffective for the cyclization (entries 1, 2).
Interestingly, the combinations of the silver salts and ILs
provided good to excellent yields of the desired products
(entries 3-12). Among these active catalytic systems, AgI
together with IL1 exhibited the best catalytic performance
(entry 3). Furthermore, iodide was replaced with different
halogens and the corresponding systems showed activities
following the order of I>Br>Cl (entries 3-5). Various anions,
such as O2-, WO42-, CO32-, OAc-, NO3 , PF6 were also introduced
into the systems in forms of different silver salts and moderate
to good yields were successively achieved (entries 6-11). On
the other hand, the IL components were systematically tuned
with different cations and anions. Replacing the ethyl group of
the cation with an n-butyl group led to a significantly reduced
-
-
2 | J. Name., 2012, 00, 1-3
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-
yield of 41% (entry 12). When another anion such as BF4 ,
(CF3SO2)2N-, or I- was employed instead of OAc-, no activity was
observed for the catalytic cyclization (entries 13-15). Further
investigation implied that slight heat was favorable for the
catalytic performance of the AgI/IL1 system (entry 16).
Consequently, the optimal reaction conditions were identified
as the AgI/IL1 system working at 45°C under atmospheric CO2
pressure. More details of the experimental section are
available in supporting information.
Having established the optimal catalytic system and reaction
conditions, the scope of the substrates was subsequently
explored (Table 2). To our delight, numerous propargylic
alcohols with various alkyl or aryl substituents (1a-1h) could be
converted into the desired α-alkylidene cyclic carbonates (2a
-
Figure 1. Reusability of AgI/IL1 system. Conditions: AgI (0.05 mmol), IL1 (1 mL), 1a
(5 mmol), CO2 (1 bar) at 45°C for 4h
2h) in excellent yields with the catalysis of the AgI/IL1 system.
Notably, this conversion proceeded rapidly and smoothly
under atmospheric CO2 pressure. It is worth noting that the
reactivity of the substrates was strongly related to the steric
effect of the substituted R1 and R2 group, indicated by the
requirements of various reaction times for different
substituted substrates. Despite this, the AgI/IL1 catalytic
system showed robust activities to the challenging substrates
such as 1e with a five-membered ring and 1g with a sterically
hindered isopropyl group, which were seldom reported to be
active and elevated CO2 pressure or extensive reaction time
were required for the catalysts. In contrast, excellent yields of
2e and 2g could be obtained rapidly and efficiently under
atmospheric CO2 pressure catalyzed by the AgI/IL1 system. The
O
OH
O
2
1
R1
R2
O
R1
AgI
+
R2
n OAc-
O
C
O
AcO
O
O
I
O
O
Ag
O
I
HOAc
O
Ag
R1
R1
R2
R2
HOAc
III
I
O
O
O
I
HOAc
Ag
R1
R2
isolated yields of 2a-2h are applied in supporting information.
Having identified the robust activity and wide generality
exhibited by the AgI/IL1 system with a competitively low
catalyst loading, the constancy and recyclability of this
catalytic system were subsequently explored, which was the
pivotal goal of this work. To our delight, this system behaved
exceptionally in the reusability evaluation employing 1a as the
model substrate. With only 1 mol% of the Ag loading which is
even lower than almost all the homogeneous catalysts, this
catalytic system could rapidly promote the transformation in
excellent yield and be reused under atmospheric CO2 for more
than 20 times without significant loss of the high catalytic
activity (as shown in Figure 1). To the best of our knowledge,
this AgI/IL1 system is considered to be the most recyclable
catalytic system ever reported with 20 recycle rounds.
Meanwhile, it is the lowest Ag loading system among all the
recyclable systems that worked under atmospheric CO2.
II
Scheme 1. Proposed catalytic mechanism of the AgI/IL1 system
of OAc- in high concentration. Consequently, a plausible
mechanism for the carboxylative cyclization of CO2 and
propargylic alcohols catalyzed by the AgI/IL1 system was
proposed in Scheme 1. Initially, the high concentrated OAc-
activates the hydroxyl group from the substrate and enhances
the nucleophilicity of the hydroxyl oxygen to the silver atom.
Meanwhile, CO2 is induced by the OAc- and receives a partial
negative charge,55 followed by the favorable approaching to
the positive silver center and the generation of intermediate
I 50
carbon atom on the hydroxyl oxygen atom occurs to give
intermediate II 50
Moreover, silver activates the triple bond to
.
Afterwards, the electrophilic attack of the carboxylic
.
According to the previous work, the activation of the
hydroxyl group of the substrate was considered as the crucial
procedure for the catalysis of this carboxylative cyclization,
bring the connection of the adjacent oxygen and the carbon in
C≡C, resulting in the formation of the intermediate five-
membered ring III 36
Finally, product 2 is afforded with the
.
which could be indicated by the variation of peak shape and its
chemical shift in 1H NMR spectra.22,
regeneration of the catalyst, following the general transition
metal catalytic mechanism.36, 50
36-38, 50
Further
investigations for this AgI/IL1 system revealed that the
activation of the hydroxyl proton was accomplished by the IL
component of the system (more details see supporting
information). On the other hand, the interaction between OAc-
and CO2 were thoroughly investigated by J. A. Steckel,55 which
is considered to be the possible CO2 activation mechanism
existing in this AgI/IL1 system containing an abundant amount
Conclusions
In conclusion, we developed a green and recyclable ionic liquid
based AgI/OAc- catalytic system (AgI/IL1 system) for the
carboxylative cyclization of propargylic alcohols and CO2.
Numerous substrates especially those with sterically hindered
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20 T. Ishida, S. Kikuchi, T. Tsubo and T. Yamada, Org. Lett., 2013,
15, 848-851.
substituents underwent this catalytic process rapidly and
smoothly under atmospheric CO2 pressure with the catalysis of
this system, generating excellent yields of the desired α-
alkylidene cyclic carbonates. Notably, this ionic liquid based
system exhibits unprecedentedly robust constancy and
recyclability. A record-high number of recycle rounds (20 times)
was achieved by this catalytic system with only 1 mol% of Ag
loading, which is considered as the most recycle rounds and
the lowest catalyst loading among all the recyclable systems
that worked under atmospheric CO2.
21 T. Ishida, S. Kikuchi and T. Yamada, Org. Lett., 2013, 15
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National Science Foundation of China (No. 21502062, No.
21502146), the Fundamental Research Funds for the Central
Universities (WUT: 2017IVA091).
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O
R1
R2
OH
+
CO2
O
O
R1
R2
AgI/OAc-
Atmospheric CO2
Sterically hindered substrates
Up to 20 times recycle
Low to 1 mol% of AgI