Cycloaddition of carbon dioxide and epoxides using pentaerythritol and halides as dual catalyst system

Article abstract of DOI:10.1002/cssc.201301273

The combination of pentaerythritol with nucleophilic halide salts such as nBu₄NI is used as a dual catalyst system for the cycloaddition of carbon dioxide (CO₂) with a broad range of organic epoxides yielding the respective cyclic ca

Full text of DOI:10.1002/cssc.201301273

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DOI: 10.1002/cssc.201301273
Cycloaddition of Carbon Dioxide and Epoxides using
Pentaerythritol and Halides as Dual Catalyst System
Michael E. Wilhelm, Michael H. Anthofer, Mirza Cokoja,* Iulius I. E. Markovits,
Wolfgang A. Herrmann, and Fritz E. Kꢀhn[a]
The combination of pentaerythritol with nucleophilic halide
salts such as nBu4NI is used as a dual catalyst system for the
cycloaddition of carbon dioxide (CO2) with a broad range of or-
ganic epoxides yielding the respective cyclic carbonates. Due
to synergistic effects of the organocatalysts, excellent yields
and selectivities could be achieved under mild reaction condi-
tions. Moreover, the nontoxic, cost-efficient, and readily avail-
able system is easily recyclable without significant loss of reac-
tivity, representing an exceptional sustainable approach for the
fixation of CO2.
sisting of a hydroxy-containing electrophile (phenols,[8] glycer-
ol,[9] chitosan,[10] lignin,[11] cellulose[12]) and a halide. Meanwhile,
some of the most active two-component organocatalysts by
the teams of Shi,[13] Kleij,[8] Han,[9] and Zhang[14] exhibit similar
activities as metal catalysts under comparable reaction condi-
tions.
Following our previous studies on the cycloaddition of CO2
and epoxides with halide-based catalysts and electrophilic co-
catalysts (NbCl5,[15] imidazolium-based ionic liquids[16]), we fo-
cused on investigating the reactivity of pentaerythritol (PETT),
which is a cheap and non-toxic compound, as cycloaddition
catalyst. The catalytic experiments were performed in a Fisher-
Porter bottle charged with equimolar amounts of PETT and
halide, flushed with argon, filled with PO, and set under a con-
stant pressure of 4 bar CO2 for 1 min before the reaction mix-
ture was heated to the desired temperature. The selectivity
and yield of PC were determined by gas chromatography. The
results are given in Table 1.
In the last years, catalysis research has increasingly been devot-
ed to finding ways to develop high-performance catalysts for
the valorization of renewable carbon sources.[1] Carbon dioxide
is in a special spotlight as a renewable C1 feedstock, because
it is ubiquitous and readily available, either from decomposi-
tion of organic matter, or artificially by human activities. Of all
possible transformations of CO2 to C1 or higher chemicals with
molecular catalysts in solution, one of the most promising re-
actions in terms of market need, catalyst performance, and re-
usability is the conversion of CO2 and epoxides (particularly
propylene oxide; PO) to cyclic carbonates (e.g., propylene car-
bonate; PC) and polycarbonates, respectively.[2] The synthesis
of carbonates and polycarbonates from CO2 and epoxides is
a well-known and established procedure using various high-
performance metal catalysts,[3] with some of them already
being applied in industry.[4] With regards to cheap, sustainable,
and green processes, avoiding metal waste by employing
(heavy) metal-free catalysts would, however, be much more fa-
vorable. In essence, the organocatalysts for the synthesis of
cyclic carbonates act as nucleophiles, opening the epoxide
ring prior to addition of CO2 and subsequent cyclization.[5] Sev-
eral types of metal-free catalysts have so far been presented,
such as halides containing weakly interacting cations (e.g., am-
monium, imidazolium),[6] and nitrogen donor bases.[7] However,
these catalysts require significantly higher temperatures and
pressures than metal catalysts. For this reason, more efficient
two-component tandem catalyst systems have emerged, con-
The results show that the activity of the binary system origi-
nates from a synergistic effect between the catalysts, as no or
very little conversion was obtained when using only PETT or
nBu4NI (Table 1, entry 1, 2). Computational studies by Kleij
et al.[8] have shown that the mechanism of the reaction
Table 1. Screening of nucleophilic catalysts in combination with PETT as
co-catalyst for the formation of PC.[a]
Entry
Catalyst
Yield[b] [%]
[5 mol%]
1
2
3
4
PETT
nBu4NI
PETT/nBu4NI
PETT/nBu4NBr
0
10
96
97
5
PETT/
79
6
PETT/
86
[a] M. E. Wilhelm,+ M. H. Anthofer,+ Dr. M. Cokoja, I. I. E. Markovits,
Prof. Dr. W. A. Herrmann, Prof. Dr. F. E. Kꢀhn
7
8
PETT/
87
6
Chair of Inorganic Chemistry/Molecular Catalysis
Catalysis Research Center, Technische Universitꢁt Mꢀnchen
Ernst-Otto-Fischer Str. 1, 85747 Garching bei Mꢀnchen (Germany)
Fax: (+49)89-289-13473
PETT/KI
[a] 10.0 mmol PO, 0.5 mmol catalysts, p(CO2)=4 bar, 708C, 22 h. [b] Yields
based on GC analysis, selectivity ꢀ99% for propylene carbonate.
E-mail: mirza.cokoja@ch.tum.de
[+] These authors have equally contributed to this work.
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changes through the addition of hydrogen-bond donors, when
compared to the only use of nBu4NI. Accordingly, OH-contain-
ing electrophiles interact with epoxides via hydrogen bonds,
thus polarizing the CÀO ring bond and facilitating the nucleo-
philic attack by the halide and the ring opening, respective-
ly.[8,17] Further, the intermediates and transition states are stabi-
lized through hydrogen bonds, while the relative energy of the
rate-limiting step decreases significantly in comparison to the
use of nBu4NI only.[8] As a result of this synergistic effect, it is
possible to carry out the reaction with our catalytic system
based on PETT and nBu4NI under relatively mild reaction condi-
tions. It is important to note that the catalytic system PETT/KI
is active for the cycloaddition of CO2 and PO, however at sig-
nificantly higher temperatures and pressures (1308C, 20 bar
CO2).[18] Under the reaction conditions applied this mixture
leads to only 6% PO conversion (entry 8). This can be attribut-
ed to the fact that weaker-interacting cations, that is, the
degree of electrostatic interaction between the ions, have
a substantial effect on the nucleophilicity of the halide.[19] That
the reactivity of the catalyst system depends on the structure
of the cation of the respective nucleophile is further shown by
the use of imidazolium-based ionic liquids, which result in
lower yields of PC compared to PETT (entries 5–7). This can be
explained by the bulkiness of the tetrabutylammonium cation,
which weakens the electrostatic interaction between the ions
and therefore leads to a higher nucleophilicity of the anion.[6b]
The different reactivity presumably also results from the hydro-
gen-bonding ability of the imidazolium cation (entry 5), which
can interact with the bromide anion[19] thus competing with
the hydrogen bonding of PETT to PO, thereby lowering the
product yield. This presumption is further supported by the
fact that the C2- and tetramethylated imidazolium-based ionic
liquids, which exhibit weaker hydrogen bond donors, give
a slightly higher yield (entries 6 and 7). When only imidazolium
halides are employed as catalysts, alkylation at the C2-position
leads to a decrease of epoxide conversion due to a lack of H–
epoxide contacts.[16]
Figure 1. Time dependence of the yield of propylene carbonate. Reaction
conditions: PO (10.0 mmol), catalysts PETT/nBu4NX (0.5 mmol),
p(CO2)=4 bar, T=708C.
Figure 2. Effect of the reaction temperature on the yield of PC. Reaction
conditions: PO (10.0 mmol), catalysts PETT/nBu4NI (0.5 mmol), p(CO2)=4 bar,
t=16 h.
To investigate the influence of the nucleophile on the activi-
ty of the dual catalyst PETT/halide, kinetic studies of the cata-
lytic conversion of CO2 and PO to PC were undertaken using
nBu4NI and nBu4NBr as co-catalysts. The resulting kinetic
curves are shown in Figure 1. Accordingly, under these reaction
conditions iodide exhibits a higher catalytic activity since all
product yields are higher after the same reaction time than
with nBu4NBr. This behavior is likely caused by the fact that
the bromide anion is a stronger hydrogen-bond acceptor than
iodide.[21] Hence, it is stabilized by interaction with PETT, which
may also lead to a decrease of the activation of PO through
competitive hydrogen bonding. As a result the duration of the
reaction can be reduced to 16 h, as compared to 22 h when
using nBu4NI as co-catalyst. For this reason all further investiga-
tions were carried out with iodide as nucleophile.
The catalytic activity of the binary system also strongly de-
pends on the concentration of the catalysts. The nearly quanti-
tative conversion of PO with a catalyst loading of 5 mol%
(Table 2, entry 5) decreases to 54% yield when the concentra-
tion of the catalysts is 1 mol% (Table 2, entry 1).
For a potential application in a green and economic process
the recyclability of a catalyst plays an important role. To exam-
ine the reusability and the stability of the binary catalytic
system PETT/nBu4NI, the reaction of CO2 with PO was carried
out under the previously described reaction conditions (T=
708C, t=16 h, p(CO2)=4 bar). After the precipitation of the
catalysts and extraction of propylene carbonate with diethyl
ether, the catalysts were dried in vacuo (see the Experimental
Section for details) and used for the next run under the same
reaction conditions. The results (Figure 3) show that the activi-
ty of the catalytic system remains the same for at least eight
catalytic cycles. The slightly irregular yields of PC are within
the standard deviation. Notably, leaching effects were not ob-
served. While known catalysts such as pyrogallol[8] give lower
Additionally, the influence of the catalyst loading and the re-
action temperature on the PC yield was investigated. A reduc-
tion of the temperature from 708C to 508C results in a strong
decreasing yield of PC from 96% to 76%, while the difference
in yield at 608C is marginal (Figure 2).
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Table 2. Influence of catalyst loadings on the formation of propylene car-
bonate (PC) from PO and CO2.[a]
Table 3. Cycloaddition of CO2 with different epoxides catalyzed by the
binary system PETT/nBu4NI.[a]
Entry
PETT [mol%]
nBu4NI [mol%]
Yield[b] [%]
Entry
Substrate
Conversion [%]
1
2
3
4
5
1.0
2.0
3.0
4.0
5.0
1.0
2.0
3.0
4.0
5.0
54
67
74
93
96
1
2
3
4
96[b]
99[c]
100[c]
92[c]
[a] 10.0 mmol PO, p(CO2)=4 bar, T=708C, t=16 h. [b] Yields based on
GC analysis, selectivity ꢀ99% for propylene carbonate.
5
6
82[c]
30[c]
[a] 10.0 mmol epoxide, 0.5 mmol catalysts, p(CO2)=4 bar, T=708C, t=
16 h. [b] Conversion based on GC analysis, selectivity ꢀ99% for propyl-
ene carbonate. [c] Conversion determined by integration of the epoxide
1
and cyclic carbonate peaks in the H NMR spectrum.
The dual catalyst system is easily recyclable and can be reused
for at least eight times without loss of reactivity. Further, no or-
ganic solvents or metals are needed for the cycloaddition reac-
tion, rendering this catalytic system sustainable and economi-
cal. Our future research work will focus on the design of tail-
ored hydrogen-bond donor catalysts for the cycloaddition of
CO2 and epoxides without nucleophilic co-catalysts at lower re-
action temperatures and pressures in order to improve the sus-
tainability merits of the reaction.
Figure 3. Influence of the catalyst recycling on the yield of PC; Conditions:
PO (10.0 mmol), catalysts PETT/nBu4NI (0.5 mmol), p(CO2)=4 bar, 708C, 16 h.
yields in the second catalytic run, PETT is easily recyclable with-
out significant loss of activity. Hence, PETT/nBu4NI represents
a very feasible and reusable catalytic system for the cycloaddi-
tion of CO2 and PO.
Experimental Section
All organic epoxides and commercially available catalysts were pur-
chased from Sigma Aldrich. Carbon dioxide (99.995%) was ob-
tained from Westfalen AG. All chemicals were used as received
without any further purification. The imidazolium-based ionic liq-
uids were synthesized as reported in literature[16] and dried under
vacuum for 6 h at 708C. The yields of propylene carbonate were
analyzed by gas chromatography (GC) on a Hewlett–Packard In-
strument HP 5890 Series II with a Hewlett–Packard integration unit
The dual catalyst system PETT/nBu4NI was applied to the cy-
cloaddition of CO2 with various epoxides (Table 3). The results
show that all examined substrates are converted to the corre-
sponding cyclic carbonates with
a
selectivity of ꢀ99%.
Because of the electron-withdrawing effect of their substitu-
ents, which facilitates nucleophilic attack at the epoxide ring
carbon atoms, epichlorohydrin (entry 2) as well as glycidol
(entry 3) gave nearly quantitative conversions under the inves-
tigated reaction conditions. Also cyclohexene oxide (entry 6),
which is known to be a more difficult substrate for the cyclo-
addition reaction with CO2, is converted to the corresponding
carbonate in 30% yield.
1
3396 Series II, a FID and a Supelco column Alphadex 120. H NMR
spectra were recorded at a 400 MHz Bruker Avance DPX-400 spec-
trometer in CDCl3 or [D6] DMSO and referred to the residual signal
of the deuterated solvents.
Catalysis experiments: A Fisher-Porter bottle was charged with
PETT and nBu4NI (0.5 mmol each) and a magnetic stirring bar. The
epoxide (10.0 mmol) was added under Ar atmosphere and the
bottle was pressurized (p(CO2)=4 bar) for 1 min. The reaction mix-
ture was heated to the desired temperature and stirred for the re-
quired time period. After the respective reaction time the bottle
was cooled to 08C and the excess of CO2 was vented. The reaction
mixture was collected with 5 mL chloroform and a sample for the
GC analysis of the yield and selectivity was taken.
In conclusion, the binary system PETT/nBu4NI is able to cata-
lyze the formation of cyclic carbonates via cycloaddition of or-
ganic epoxides with CO2. The combination of the cost-efficient,
commercially available, and nontoxic components represents
one of few metal-free systems that catalyze the conversion of
different organic epoxides at rather mild reaction conditions
(T=60–708C, p(CO2)=4 bar, 4–5 mol% catalyst concentration).
Catalyst recycling studies: After the sample for the GC analysis was
taken, the solvent and unreacted substrate were removed under
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Keywords: carbon dioxide fixation · cooperative effects ·
cycloaddition · green chemistry · organocatalysis
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Received: November 27, 2013
Published online on && &&, 0000
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Doubleteam: The combination of pen-
taerytrithol with tetrabutylammonium
iodide leads to an efficient catalytic
system for the cycloaddition of carbon
dioxide (CO2) with various epoxides to
cyclic carbonates. The nontoxic, metal-
free, and cost-efficient dual catalysts, as
well as the easy recyclability result in an
exceptional sustainable organocatalytic
approach for the fixation of carbon di-
oxide.
M. E. Wilhelm, M. H. Anthofer, M. Cokoja,*
I. I. E. Markovits, W. A. Herrmann,
F. E. Kꢀhn
&& – &&
Cycloaddition of Carbon Dioxide and
Epoxides using Pentaerythritol and
Halides as Dual Catalyst System
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 4
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