An Efficient and Versatile Lanthanum Heteroscorpionate Catalyst for Carbon Dioxide Fixation into Cyclic Carbonates

Article abstract of DOI:10.1002/cssc.201700898

A new lanthanum heteroscorpionate complex has shown exceptional catalytic activity for the synthesis of cyclic carbonates from epoxides and carbon dioxide. This catalyst system also promotes the reaction of bio-based epoxides to give an important class of bis(cyclic carbonates) that can be further used for the production of bio-derived non-isocyanate polyurethanes. The catalytic process requires low catalyst loading and mild reaction conditions for the synthesis of a wide range of cyclic carbonates.

Full text of DOI:10.1002/cssc.201700898

Accepted Article
Title: An Efficient and Versatile Catalyst for Carbon Dioxide Fixation
into Cyclic Carbonates
Authors: Antonio Otero, Javier Martinez, Juan Fernandez-Baeza, Luis
F Sanchez-Barba, Jose A Castro-Osma, and Agustin Lara-
Sanchez
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To be cited as: ChemSusChem 10.1002/cssc.201700898
Link to VoR: http://dx.doi.org/10.1002/cssc.201700898
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10.1002/cssc.201700898
ChemSusChem
COMMUNICATION
An Efficient and Versatile Catalyst for Carbon Dioxide Fixation
into Cyclic Carbonates
Javier Martínez,[a] Juan Fernández-Baeza,[a] Luis F. Sánchez-Barba,[b] José A. Castro-Osma,*,[c]
Agustín Lara-Sánchez*,[a] and Antonio Otero*,[a]
Dedication ((optional))
Abstract: A new heteroscorpionate lanthanum complex has shown
exceptional catalytic activity for the synthesis of cyclic carbonates
from epoxides and carbon dioxide. The new catalyst system also
promotes the reaction of bio-based epoxides to give an important
class of bis(cyclic carbonates) that can be further used for the
production of bio-derived non-isocyanate polyurethanes. The catalytic
process requires low catalyst loading and mild reaction conditions for
the synthesis of a wide range of cyclic carbonates.
applications and can be used as electrolytes in lithium–ion
batteries, as polar aprotic solvents and as chemical intermediates
in organic synthesis.[5] Numerous catalyst systems have been
developed for the synthesis of cyclic carbonates from epoxides
and carbon dioxide and these contain a combination of a Lewis
acid and a nucleophile as a cocatalyst.[3]
The use of carbon dioxide (CO2) as a starting material for the
production of organic molecules and/or materials has recently
received a great deal of attention from the scientific community.[1]
This approach is of great interest not only because carbon dioxide
is a cheap, abundant, non–toxic and versatile C1-building block
but also because it is one of the most significant greenhouse
gases.[1] Therefore, the efficient transformation of CO2 into fuels,
chemicals and polymers, as a complement to carbon capture and
storage strategies, would contribute towards a transition from a
fossil fuel-based industry to a sustainable and circular one.[2] The
main challenge involves overcoming the thermodynamic stability
of CO2.[1e] Hence, catalysis is crucial for the use of CO2 as a viable
feedstock since the kinetic barrier of the reaction would be
reduced.[1i] Moreover, the use of high free energy reactants
facilitates the conversion of CO2 to the corresponding products
from a thermodynamic point of view.[1,2] One of the most important
processes that uses CO2 is the production of cyclic carbonates[3]
or polycarbonates[4] from epoxides and carbon dioxide as these
processes have 100% atom–economy and are highly exothermic
(Scheme 1). However, current commercial processes are net CO2
emitter due to the high reaction temperatures and pressures
required.[1] Cyclic carbonates have important commercial
Scheme 1. Synthesis of cyclic– and polycarbonates from epoxides and CO2.
During the last decades, rare-earth metal complexes and their use
as catalysts in organic synthesis and polymerization processes
have received significant attention.[6] The hard Lewis acidity, easy
tuning of the steric and electronic environments and the capacity
to bind and dissociate rapidly make rare-earth complexes
promising initiators for a range of catalytic reactions.[6] Despite the
fact that many efficient catalyst systems for the reaction between
epoxides and carbon dioxide have been developed, very little
attention has been paid to lanthanide complexes. In fact, only
three lanthanide complexes have been studied as catalysts for
this reaction.[7] Our research group has wide experience in the
development of new heteroscorpionate rare-earth metal-based
complexes as highly efficient homogeneous catalysts for a range
of catalytic processes, including ring-opening polymerization and
hydroelementation reactions.[8] Thus, bearing in mind both the
high catalytic activity and versatility of lanthanide complexes, we
focused our efforts on developing the most active rare-earth
catalyst for the synthesis of cyclic carbonates from epoxides and
carbon dioxide. We demonstrate that the catalyst system displays
broad substrate scope and functional group tolerance. The
remarkable versatility of this system allowed us to synthesize a
range of bio-based cyclic carbonates that can be prepared from
waste biomass, including limonene carbonate and limonene bis-
carbonate, in the highest yield reported to date.
[a]
Dr. Javier Martínez, Dr. Juan Fernández-Baeza, Dr. Agustín Lara-
Sánchez and Prof. Antonio Otero
Universidad de Castilla-La Mancha, Departamento de Química
Inorgánica, Orgánica y Bioquímica-Centro de Innovación en Química
Avanzada (ORFEO-CINQA), Facultad de Ciencias y Tecnologías
Químicas, 13071-Ciudad Real, Spain.
E-mail: Antonio.Otero@uclm.es, Agustin.Lara@uclm.es
Dr. Luis F. Sánchez-Barba
Universidad Rey Juan Carlos, Departamento de Biología y Geología,
Física y Química Inorgánica, Móstoles, 28933 Madrid, Spain.
Dr. José A. Castro-Osma
[b]
[c]
Universidad de Castilla-La Mancha, Departamento de Química
Inorgánica, Orgánica y Bioquímica-Centro de Innovación en Química
Avanzada (ORFEO-CINQA), Facultad de Farmacia, 02071-Albacete,
Spain. E-mail: JoseAntonio.Castro@uclm.es.
Supporting information including copies of the 1H and 13C NMR
spectra for complex 1, bio-derived epoxides and cyclic carbonates 2a-
n, 3a-h and 4a-g for this article is given via a link at the end of the
document.
Scheme 2. Synthesis of complex 1.
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10.1002/cssc.201700898
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COMMUNICATION
The new lanthanum complex [La{N(SiHMe2)2}2{3-bpzcp}] (1) was
synthesized in a straightforward manner by an amine elimination
route, as previously reported for Y, Lu, Nd and Sm complexes,[8e]
involving the reaction of the heteroscorpionate precursor
order to carry out the synthesis of cyclic carbonates at relatively
mild reaction conditions.
Figure 1. Synthesis of cyclic carbonates 2a–n using complex 1.a
bpzcpH[9] [bpzcp
=
2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-
diphenylethylcyclopentadienyl] and [La{N(SiHMe2)2}3(thf)2][10] in
toluene (Scheme 2).
We first optimized the reaction conditions for the reaction of
styrene oxide and CO2 using a combination of complex 1 and
tetrabutylammonium bromide (TBAB) as the catalyst system
(Table S1 in Supporting Information). The use of a complex 1 and
cocatalyst loading of 0.2 mol% led to 100% conversion of styrene
oxide into styrene carbonate 2a after one hour (Table S1, entry 1).
The effect of reducing the catalyst loading whilst keeping the
1:TBAB molar ratio at 1:1 was subsequently investigated. A
catalyst loading of 0.05 mol% led to quantitative conversion at 90
oC and 10 bar CO2 pressure (Table S1, entry 3). It was therefore
o
decided to decrease the reaction temperature from 90 to 70 C
(Table S1, entries 4–6). Under these reaction conditions, 100%
conversion was obtained on using 0.1 mol% of the catalyst
system. In an effort to reduce further the catalyst loading to 0.05
mol%, a reaction time of 4 hours was required to achieve 100%
conversion (Table S1, entry 8).
The effect of the halide counterion was investigated (Table S1,
entries 9–10), but both chloride and iodide counterions led to
lower catalytic activity when compared to the bromide anion. In
an attempt to increase the catalytic activity, the amount of TBAB
was increased to 0.25 mol% whilst using 0.05 mol% of complex 1
(Table S1, entries 11–12). However, conversions did not improve
and it was decided that 0.05 mol% was the optimal loading for
both complex 1 and TBAB for reactions at 70 oC. Control
experiments revealed that low conversions were obtained when
complex 1 or TBAB were used as the catalyst in the absence of
the other catalyst component (Table S1, entries 13–14), which
demonstrates that both catalyst components are essential to
achieve excellent conversions. All reactions selectively gave
cyclic carbonate 2a as the only reaction product.
aCyclic carbonates were obtained from their corresponding epoxides using a
combination of complex 1 and TBAB as the catalysts system in a molar ratio
1:TBAB = 1:1. Reactions were carried out at 70 oC and 10 bar carbon dioxide
pressure in the absence of solvent. Selectivity towards the cyclic carbonate was
determined by 1H NMR of the crude reaction mixture and was >99% in all cases.
b4 h. c16 h.
We then investigated the use of reduced amounts of catalyst
system in an effort to increase the productivity of complex 1 for
the synthesis of styrene carbonate (Table S2). We were pleased
Having defined the optimal reaction conditions for the synthesis
o
of styrene carbonate 2a from styrene oxide and CO2 as
to find that complex 1 displayed catalytic activity at 70 C when
o
0.05 mol% of lanthanum complex 1 and TBAB at 70 C and 10
0.003 mol% of catalyst loading was used. Under these reaction
conditions 9% conversion was achieved in 2 hours resulting in a
TOF value of 1500 h−1 per lanthanum center. The influence of the
reaction temperature on the catalytic activity was studied and the
bar carbon dioxide pressure, we proceeded to investigate the
reaction of a range of terminal epoxides and CO2 for the formation
of the corresponding cyclic carbonates 2a–n (Figure 1). In general,
good to excellent conversions with a selectivity >99% to the cyclic
carbonates were achieved in 4–16 hours and the products were
isolated in high yields. It is worth noting that the catalyst system
1:TBAB is not only compatible with aryl epoxides but also with a
range of alkyl epoxides and functionalities such as alcohol, ether,
alkene, alkyne and ester. These results show that the catalyst
system 1:TBAB is highly tolerant to various functional groups and
confirm its catalytic potential for the production of cyclic
carbonates.
The substrate scope of the complex 1:TBAB catalyst system was
studied further by investigating internal epoxides (Figure 2).
These substrates are more challenging for cyclic carbonate
formation than terminal epoxides, although excellent
achievements have recently been reported in this area.[7a,11] We
initially optimized the reaction conditions using cyclohexene oxide
as a model substrate (See Table S4 in Supporting Information).
o
activities were measured at 70–150 C (Table S2, entries 1–8).
The results showed that there is a maximum of catalytic activity at
o
110 C, giving an initial TOF of 8500 h−1 per metal center using
0.003 mol% of complex 1. To study the effect of the reaction
temperature on the initial TOF value, reactions were carried out
at 0.5, 1, 1.5, 2, 4, 6 and 10 h (Table S2, entries 4, 9–14) showing
that the initial TOF remained unchanged after 2 h and slowly
decreases for longer reaction times. When the catalyst loading
and reaction time were fixed at 0.0003 mol% and 2 hour, 9%
conversion of styrene oxide into styrene carbonate was achieved
o
with a TOF of 15000 h−1 at 110 C (Table S2, entry 15). Under
these reaction conditions, the TON value reached 306667 after
48 hours, indicating that the catalyst is stable during the reaction.
Although this catalyst system displayed outstanding catalytic
o
o
activity at 110 C, we selected 70 C as optimal temperature in
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10.1002/cssc.201700898
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COMMUNICATION
Figure 2. Synthesis of cyclic carbonates 2a–n using complex 1.a
diamines.[14] Challenging highly substituted bio-based internal
epoxides derived from limonene were also investigated.
Figure 3. Synthesis of bio-based cyclic carbonates 4a–g using complex 1.a
aCyclic carbonates were obtained from their corresponding epoxides using a
combination of complex 1 and TBAB as the catalyst system in a molar ratio
1:TBAB = 1:4. Reactions were carried out at 70 oC and 10 bar carbon dioxide
pressure in the absence of solvent for 18 h unless otherwise stated. Selectivity
towards the cyclic carbonate was determined by 1H NMR of the crude reaction
mixture and was >99% in all cases. b66 h. ccis:trans = 60:40. d90 oC.
We were pleased to find that the catalyst system allowed the
synthesis of a range of disubstituted cyclic carbonates in good to
excellent yields and with selectivities above 99% – even for
cyclohexene oxide, which is very likely to form a polycarbonate.
This is due to the excess and nature of the bromide which favors
back-biting as previously reported.[4,11g] Cyclic carbonates 3a and
3d were isolated in good yield from their corresponding epoxides
using 0.2 mol% of complex 1. However, 0.5 mol% of complex 1
was required to obtain reasonable yields of cyclic carbonates 3b–
c,e–h. It was observed by NMR that cyclic carbonates 3a,b,d–h
were obtained with retention of epoxide stereochemistry. The cis
cyclic carbonate 3f, however, was obtained in lower yield as the
synthesis of cyclic carbonates from cis-1,2-dimethyloxirane is
more challenging than from the trans-isomer.[11j,12] Due to the solid
nature of trans-stilbene oxide, the synthesis of trans-1,2-
diphenylethylene carbonate 3g was carried out at 90 oC and this
was obtained in 98% isolated yield. Cyclooctadiene oxide was
also used as a substrate and gave carbonate 3c in 35% isolated
yield with a 60:40 mixture of cis- and trans-isomers of 3c. In this
case, the reaction time was increased to 66 h. To our knowledge,
there is only one example of the synthesis of 3c by the reaction
of carbon dioxide and cyclooctadiene oxide,[11a] thus showing the
potential of complex 1 as an efficient catalyst to transform bulky
epoxides into their corresponding cyclic carbonates.
In view of the high activity and broad substrate scope of complex
1:TBAB as a catalyst system for the synthesis of cyclic carbonates
from epoxides and carbon dioxide, we focused our attention on
the synthesis of a range of bio-derived cyclic carbonates, which
to date have received very little attention (Figure 3).[13] Almost
quantitative conversion to furan-based cyclic carbonates 4a and
4b was achieved after 4 h at 70 oC using 0.05 mol% of the catalyst
system. Bio-based diacid-derived (succinic acid, fumaric acid and
glutaric acid) cyclic carbonates 4c–e were isolated in excellent
yields with high selectivity (>99%) after 16 h. These cyclic
carbonates are particularly interesting due to the possibility of
using them as building blocks for the synthesis of non-isocyanate
polyurethanes by reaction of bis-cyclic carbonates with
aCyclic carbonates were obtained from their corresponding epoxides using a
combination of complex 1 and TBAB as the catalyst system in a molar ratio
1:TBAB = 1:1. Reactions were carried out at 70 oC and 10 bar carbon dioxide
pressure in the absence of a solvent. Selectivity towards the cyclic carbonate
was determined by 1H NMR of the crude reaction mixture and was >99% in all
cases. b4 h. c16 h. dCyclic carbonates 4f–g were obtained using a combination
of complex 1 and TBAC as catalyst system in a molar ratio 1:TBAC = 1:4.
Reactions were carried out at 100 oC and 10 bar carbon dioxide pressure in the
absence of solvent.
The preparation of cyclic carbonates 4f–g is noteworthy as there
are very few reports on the metal-catalyzed synthesis of
trisubstituted cyclic carbonates from epoxides and carbon
dioxide.[15] Due to the challenging nature of these substrates, the
synthesis of cyclic carbonates 4f–g from their corresponding
o
terpene epoxides proceeded slowly at 70 C and reaction times
of 66 h were required to obtain reasonable conversions using a
combination of complex 1 and tetrabutylammonium chloride
(TBAC) as the catalyst system (Figure 3). It is worth noting that
zero conversion was observed when TBAB was used as the
nucleophile source since the nucleophilic attack occurs at the
most substituted carbon atom of limonene oxide.[16] Higher
reaction temperatures improved the activity of the catalyst system
and quantitative conversions of trans-limonene oxide and
o
limonene bis-epoxide were achieved after 16 h at 100 C. Cyclic
carbonate 4f was synthesized from commercially available
limonene oxide in a 60:40 trans/cis mixture, which resulted in the
formation of 4f in a 92:8 trans:cis ratio in 43% isolated yield. The
use of limonene bis-epoxide as a substrate provided limonene
bis-carbonate 4g in a 66:34 trans:cis ratio in 69% isolated yield
along with cyclic carbonate 4g* in 14% isolated yield with a 31:69
trans:cis ratio. To our knowledge, this lanthanum complex is the
most active catalyst for the synthesis of limonene bis-carbonate
4g.
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10.1002/cssc.201700898
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COMMUNICATION
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In conclusion, we report a novel lanthanum heteroscorpionate
complex that is highly active for the versatile synthesis of cyclic
carbonates from a broad range of epoxides and CO2. The activity
displayed by complex 1 is higher than that of any other reported
rare-earth catalyst. The catalyst system is remarkably versatile
and it allows the preparation of a wide range of (bio-based) cyclic
carbonates from terminal and highly substituted internal epoxides.
These bio-based cyclic carbonates are attracting significant
interest as potential building blocks for the chemical and polymer
industries. Taking into account the high catalytic activity of the
reported lanthanum heteroscorpionate complex, we are currently
working towards understanding the mechanism of catalysis and
further developing and optimizing rare-earth metal complexes for
the chemical fixation of CO2 into organic molecules. Moreover, we
are working on the production of novel bio-based non-isocyanate
polyurethanes from bis-cyclic carbonates and polyamines.
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Experimental Section
All epoxide substrates and reagents commercially available (Alfa, Aldrich,
Fluka) were used as received unless specified otherwise. [LaCl3(thf)x],
Li[N(SiHMe2)2], [La{N(SiHMe2)2}3(thf)x], bpzcpH and epoxides to
synthesize cyclic carbonates 4a-4e and 4g were prepared according to
literature procedures.[9,10,14] Carbon dioxide was purchased from Air
Liquide and used without further purification. Reactions for the synthesis
of bpzcpH and complex 1 were performed using standard Schlenk-tube
techniques under an atmosphere of dry nitrogen. Solvents were distilled
from appropriate drying agents and degassed before use. Microanalyses
were carried out with a Perkin-Elmer 2400 CHN analyzer. 1H, and 13C NMR
spectra were recorded on a Varian Inova FT-500, (1H NMR 500 MHz and
13C NMR 125 MHz), spectrometer and referenced to the residual
deuterated solvent. The NOESY-1D spectra were recorded with the
following acquisition parameters: irradiation time 2 s and number of scans
256, using standard VARIAN-FT software. Two-dimensional NMR spectra
were acquired using standard VARIAN-FT software and processed using
an IPC-Sun computer.
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Acknowledgements
The authors gratefully acknowledge financial support from the
Ministerio de Economía y Competitividad (MINECO), Spain
(Grant Nos. CTQ2014-51912-REDC and CTQ2014-52899-R). Dr.
José A. Castro-Osma acknowledges financial support from the
Plan Propio de la Universidad de Castilla-La Mancha..
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Keywords: Carbon Dioxide • Catalysis • Cyclic Carbonates •
Rare-earth complexes • Epoxide
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10.1002/cssc.201700898
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(Bio-derived) cyclic
Javier Martínez,[a] Juan Fernández-
carbonates were synthesized by a
mononuclear lanthanum complex via
reaction of epoxides and CO2 in
quantitative yield under mild reaction
conditions.
Baeza,[a] Luis F. Sánchez-Barba,[b] José
A. Castro-Osma,*,[c] Agustín Lara-
Sánchez*,[a] and Antonio Otero*,[a]
Page No. – Page No.
An Efficient and Versatile Catalyst for
Carbon Dioxide Fixation into Cyclic
Carbonates
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