Catalytic production of cyclic carbonates mediated by lanthanide phenolates under mild conditions

Article abstract of DOI:10.1039/c4cc02065k

Readily available lanthanide complexes stabilized by a bridged poly(phenolate) ligand have been used for the first time as efficient catalysts for the insertion of CO₂ into epoxides to generate cyclic carbonates with high activity, high selectivity, and a wide substrate scope under mild conditions. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02065k

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COMMUNICATION
Catalytic production of cyclic carbonates
mediated by lanthanide phenolates under
mild conditions†
Cite this: Chem. Commun., 2014,
0, 10952
5
Received 19th March 2014,
Accepted 27th March 2014
a
a
a
a
a
ab
Jie Qin, Peng Wang, Qingyan Li, Yong Zhang, Dan Yuan and Yingming Yao*
DOI: 10.1039/c4cc02065k
www.rsc.org/chemcomm
Readily available lanthanide complexes stabilized by a bridged
poly(phenolate) ligand have been used for the first time as efficient
2
catalysts for the insertion of CO into epoxides to generate cyclic
carbonates with high activity, high selectivity, and a wide substrate
scope under mild conditions.
2
Scheme 1 Synthesis of cyclic carbonates from epoxides and CO .
2 1
Carbon dioxide (CO ) is an abundant, non-toxic, and renewable C
building block in synthetic chemistry, and chemical fixation of CO
2
Recently, important progress has been achieved in this area. North
has received considerable attention as it is consistent with the et al. reported bimetallic aluminium–Salen complexes which cata-
1
requirement of green chemistry and sustainability. Among all lyzed the cycloaddition of CO2 with monosubstituted epoxides
methods to transform CO , the coupling with epoxides (Scheme 1) at atmospheric pressure with 2.5 mol% catalyst loading to give
2
5
b,c
has attracted much interest due to its 100% atom efficiency. More- 34–98% yields.
over, the resulting cyclic carbonates have found wide applications as stabilized by an amino bridged triphenolate ligand, which catalyzed
excellent aprotic polar solvents, fine chemical intermediates (e.g., this transformation for both mono- and disubstituted epoxides with
to prepare diols and methanol)
produce polycarbonates which are useful as engineering thermo- yields.
Kleij et al. reported an aluminum complex
2
a
2
b–d
and valuable raw materials to 0.05–0.1% catalyst loading at 10 bar CO
2
pressure to give 40–99%
5g
2e
plastics in sheets, food packings and adhesives. To date, a variety
Lanthanide metals are oxyphilic, and many of their complexes
of promising catalysts, especially organometallic complexes, have possess unusually high Lewis acidity, properties which are expected
3
been studied extensively to catalyze this transformation. Among to benefit the activation of epoxides and the subsequent coupling
4
3c,4c,5g
them, transition metal (e.g., Zn, Cr, Co, and Fe) and main-group with CO
2
.
However, lanthanide-based complexes have seldom
5
metal (e.g., Al) complexes have been widely reported. However, been applied in this area. To the best of our knowledge, only two
most catalysts suffer from some limitations such as requiring harsh examples have been reported involving simple lanthanide salts
3
b,c,4c,e
6
6a
reaction conditions and displaying narrow substrate scopes.
SmOCl and Sc(OTf)
2
, which required either supercritical CO
2
or
6
b
Therefore, it is still a challenge to develop catalytic systems which 20 bar CO
2
at 120 1C. Since ligands largely influence the catalytic
can conduct this transformation under mild conditions (e.g., ideally properties of lanthanide complexes, the introduction of a suitable
atmospheric pressure) with high efficiency (at low catalyst loadings) ligand is expected to be an efficient method to improve their
for a broad range of substrates (especially for internal epoxides). catalytic performance. A bridged poly(phenolate) ligand with multi-
ple coordination sites will stabilize the electrophilic lanthanide
centre, and may also give rise to complexes with enhanced catalytic
activities. Hence, we developed four new lanthanide complexes
bearing a bridged tetra(phenolate) ligand, which were found to be
a
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials Science, Dushu Lake Campus,
Soochow University, Suzhou 215123, People’s Republic of China.
2
efficient in catalysing the coupling reaction of CO to epoxides with
high activity, high selectivity, and a wide substrate scope under
relatively mild conditions.
E-mail: yaoym@suda.edu.cn; Fax: +86 512 65880305; Tel: +86 512 65882806
b
The Institute of Low Carbon Economy, Suzhou 215123, People’s Republic of China
†
Electronic supplementary information (ESI) available: An initial TOF study, the
1
experimental section, copies of H NMR spectra of cyclic carbonate products,
The ligand precursor ethylenediamine-bridged tetra(phenol)
1
variable-temperature H NMR spectra of complex 2, and crystallographic data for
7
(
LH
4
) was easily prepared according to a literature method. Reac-
complexes 1–4, and solid state structures of complexes 1–4. CCDC 977124–
tions of LH
4
with Ln(C
5
H
5
)
3
(THF) (Ln = Yb, Y, Sm, Nd) in THF for
977127. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4cc02065k
4 hours at 25 1C afforded the corresponding lanthanide complexes
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solvent-free conditions. From the results listed in Table 1, it can be
seen that cyclic carbonates formed in good to excellent yields and
4
99% selectivities in the presence of 10 bar CO after 1 h reaction
2
at 85 1C (Table 1, entries 1–4). Moreover, lanthanide complexes 3
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and 4 with larger ionic radii showed higher activities (entries 1–4).
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This can be attributed to the open coordination sphere of the
larger lanthanide metal, which facilitates the coordination of the
propylene oxide. Reactions at different temperatures revealed that
the best result was obtained at 85 1C (entries 4–7). CO at higher
2
pressure led to similar results (entries 4 and 8), whereas lowering
the pressure to 7 bar resulted in only 6% drop in yield (entry 9).
Further attempts to conduct the reaction at the ideal atmospheric
pressure, however, failed, as propylene oxide with a boiling point of
3
4 1C mainly evaporated into the balloon at 85 1C and did not
participate in the coupling reaction. Different co-catalysts have also
been tested with NBu I being the best performer (entries 9–12). The
co-existence of the lanthanide complex and NBu I proved to be
essential for this transformation, as neither complex 4 nor NBu
alone could catalyze this reaction to give satisfactory yields (entries
3 and 14). After screening different loadings of the catalyst as well
Scheme 2 Synthesis of complexes 1–4.
4
4
1–4 [Ln = Yb(1), Y(2), Sm(3), Nd(4)] in good yields (75–80%)
4
I
(Scheme 2). Solid state structures of complexes 1–4 (ESI†) reveal
that complexes 1 and 2 are isostructural, in which the central metal
ions are six-coordinated with five donors from the ligand as well as
one oxygen atom from THF with one pendent phenol moiety.
However, for complexes 3 and 4 with larger ionic radii, the oxygen
atom of phenol is bound to the lanthanide centres.
1
as the co-catalyst, 0.2 mol% of 4 and 0.8 mol% of NBu I proved
4
to be the best combination. Lowering the catalyst loading to
0.01 mol% resulted in a moderate yield of 40% and a good TOF
of 4000 (entry 19).
1
The H NMR spectrum of diamagnetic yttrium complex 2
To study the scope of the cycloaddition, a series of mono-
substituted terminal epoxides bearing an alkyl, aryl, halogen,
alkenyl, hydroxyl, ether, morpholine or ester substituent were tested.
Since they all have relatively high boiling points, the reactions were
conducted at atmospheric pressure (balloon) under solvent-free
conditions. To our delight, all monosubstituted epoxides were
commendably converted to the corresponding cyclic carbonates in
moderate to excellent yields (60 to 97%) (Table 2, entries 1–9).
Notably, substrates containing substituents which might coordinate
to the lanthanide centre and deactivate the catalyst (such as 1c, 1f,
and 1j) also yielded the corresponding cyclic carbonates in 60–95%
recorded at 25 1C shows only broad signals. At temperature
lower than À10 1C, splitting of the broad signals occurred as
1
indicated by variable-temperature H NMR spectra, suggesting
a certain degree of fluctionality of this system (ESI†).
The cycloaddition of CO to propylene oxide was first examined
2
4
using 0.2 mol% of 1–4 and 0.4 mol% of NBu I as a co-catalyst under
Table 1 Catalytic synthesis of cyclic carbonate via insertion of CO
propylene oxide (1a) catalysed by complexes 1–4
2
to
a
5g
b,c
d
À1
yields. These results suggest that complex 4 not only had broad
functional group toleration but also showed good catalytic perfor-
mance even under atmospheric pressure. Furthermore, bulky/
Entry Cat. T (1C)
PCO2 (bar) Co-cat. Conversion (%) TOF (h )
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
1
2
3
4
4
4
4
4
4
4
4
4
4
—
4
4
4
4
4
85
85
85
85
70
55
30
85
85
85
85
85
85
85
85
85
85
85
85
10
10
10
10
10
10
10
20
7
7
7
7
7
7
7
7
7
7
7
NBu
NBu
NBu
NBu
NBu
NBu
NBu
NBu
NBu
NBu
NBu
4
4
4
4
4
4
4
4
4
4
4
I
I
I
I
I
I
I
I
I
64
66
93
92
88
70
31
93
87
320
330
465
460
440
350
155
465
435
415
320
415
0
internal epoxides, which were generally considered as sub-
3c,4c–e,5c,g
strates of low reactivity,
were also tested, and all of them
were transformed into the corresponding cyclic carbonates in
66–97% yields at elevated pressure (10 bar CO ) after prolonged
2
reaction time (entries 10–13). The background reactions with-
out complex 4 ceteris paribus have also been studied for selected
substrates including both terminal (1d) and non-terminal (1n)
epoxides. The yields dropped by around 60%, which strongly
suggests the imperative role of lanthanide complexes to attain
good yields (entries 3 and 13). Moreover, for substrate 1l
comprised of 78% trans and 22% cis isomers, the corres-
ponding carbonate 2l with trans and cis configurations formed
in approximately the same ratio, revealing that the configu-
ration was retained during the transformation (entry 11).
0
1
2
3
4
5
6
7
8
9
Br 83
Cl 64
Br 83
0
I
I
I
I
I
I
NOc
—
4
NBu
NBu
NBu
NBu
NBu
NBu
4
4
4
4
4
4
7
76
92
90
65
40
—
e
f
380
460
450
1970
4000
g
h
i
a
b
Reactions conditions: 0.2 mol% catalyst, 0.4 mol% co-catalyst, 1 h.
1
c
Determined by H NMR spectroscopy. Selectivity for the cyclic Similarly, cis-carbonates 2m and 2n were obtained exclusively
d
e
carbonate product were all 499%. mol 1a/mol 4/h. 0.2 mol% of
from cis-configured internal epoxides 1m and 1n (entries 12
f
g
h
NBu
4
I. 0.8 mol% of NBu
4
I. 1.2 mol% of NBu
4
I. 0.033 mol%
i
and 13). This finding is consistent with previous reports that
catalyst and 0.132 mol% of NBu
4
I were used. 0.01 mol% catalyst
4
d,5f
and 0.04 mol% of NBu I were used.
4
the substrate configuration is retained,
whereas a recent
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Table 2 Synthesis of various cyclic carbonates catalysed by complex 4 decreased concentration of the substrate. In addition, reasonable
a
and NBu
4
I
À1
À1
TOF values were obtained for 1c (180 h ) and 1e (175 h ). Thus,
4f,5f
b,c
our system achieves good initial TOFs at atmospheric pressure,
Entry
Substrate
Product
Yield (%)
À1
while a much higher initial TOF value of 36 000 h has been
5
g
reported by Kleij et al. under 10 bar CO2.
Overall, complex 4 is among the most active catalysts that
catalyze the formation of cyclic carbonates from CO and a broad
1
2
95
96
2
range of epoxides with a low catalyst loading under relatively low
pressure compared with other catalytic systems. Both terminal and
3
non-terminal substrates bearing different types of functional groups
were transformed into cyclic carbonates in good yields. It is also
noteworthy that all substrates were used as received without prior
removal of either oxygen or moisture, implying that lanthanide
complex 4 is relatively robust. These results strongly suggest that
organolanthanide complexes have great potential as catalysts for
d
3
93 (35 )
4
5
6
97
60
84
chemical fixation of CO
designing efficient organometallic catalysts in this field. Further
studies of these lanthanide complexes on the transformation of CO
2
, which provides some new insights for
2
are in process in our laboratory.
Financial support from the National Natural Science Foun-
dation of China (Grants 21174095, 21132002, and 21372172),
PAPD, and the Qing Lan Project is gratefully acknowledged. We
thank Professor Pierre H. Dixneuf and Professor Evgeny Kirillov
for helpful suggestions.
7
85
8
9
95
95
Notes and references
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8
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