Phosphorus-based bifunctional organocatalysts for the addition of carbon dioxide and epoxides

Article abstract of DOI:10.1002/cssc.201402477

Bifunctional phosphonium salts were synthesized and employed as organocatalysts for the atom efficient synthesis of cyclic carbonates from CO₂ and epoxides for the first time. These catalysts were obtained in high yields by a modular, straightforward one-step synthesis. The hydrogen-bond donating alcohol function in the side chain leads to a synergistic effect accelerating the catalytic reaction. The desired cyclic carbonates are obtained in high yields and selectivity under solvent-free reaction conditions without the use of any co-catalyst. Under optimized reaction conditions various epoxides were converted to the corresponding cyclic carbonates in excellent yields. The products were obtained analytically pure after simple filtration over a silica gel pad. This protocol is even applicable for a multigram reaction scale. Moreover, the catalysts could be easily recovered and reused up to five times.

Full text of DOI:10.1002/cssc.201402477

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DOI: 10.1002/cssc.201402477
Phosphorus-based Bifunctional Organocatalysts for the
Addition of Carbon Dioxide and Epoxides
Thomas Werner* and Hendrik Bꢀttner[a]
Bifunctional phosphonium salts were synthesized and em-
ployed as organocatalysts for the atom efficient synthesis of
cyclic carbonates from CO2 and epoxides for the first time.
These catalysts were obtained in high yields by a modular,
straightforward one-step synthesis. The hydrogen-bond donat-
ing alcohol function in the side chain leads to a synergistic
effect accelerating the catalytic reaction. The desired cyclic car-
bonates are obtained in high yields and selectivity under sol-
vent-free reaction conditions without the use of any co-cata-
lyst. Under optimized reaction conditions various epoxides
were converted to the corresponding cyclic carbonates in ex-
cellent yields. The products were obtained analytically pure
after simple filtration over a silica gel pad. This protocol is
even applicable for a multigram reaction scale. Moreover, the
catalysts could be easily recovered and reused up to five
times.
efficiently below 1008C, and such systems often require co-cat-
alysts, solvents, and/or multistep catalyst preparation.
There have been several reports on synergistic effects in cat-
alytic systems.[13] Especially in the presence of hydrogen-bond
donors such as alcohols,[9a,13e,14] carboxylic acids,[15] silanols,[13d]
chitosan,[16] cellulose,[17] b-cyclodextrin,[18] amino acids,[19] and
amino alcohols[20] the addition of CO2 and epoxides to the cor-
responding cyclic carbonate was significantly accelerated.
We envisioned that bifunctional phosphonium salts bearing
hydrogen-donating alcohol functions should display a similar
effect. To the best of our knowledge the application of bifunc-
tional catalysts based on phosphorus derivatives has not been
described so far. Thus, we established a straightforward syn-
thesis of tunable bifunctional phosphonium salts 3 (Scheme 1).
The simple change of either the phosphine 1 or the halo deriv-
ative 2 leads to a series of air-stable and structurally diverse
catalysts 3.[21]
In the past two decades transformations employing carbon di-
oxide as a readily available, inexpensive, nontoxic, and abun-
dant carbon source have been studied extensively.[1] However,
carbon dioxide is the end product of thermal combustion and
its thermodynamic stability as well as its kinetic inertia are
challenging for CO2 utilization.[2] Hence, the development of
suitable catalysts creating value-added products from carbon
dioxide is of particular interest. The atom-efficient addition of
CO2 and epoxides produces cyclic carbonates and provides ef-
ficient carbon dioxide fixation as well as the production of val-
uable outputs.[3] Cyclic carbonates serve as polar green sol-
vents and display some outstanding properties such as a high
boiling point, being odorless, and low toxicity.[4] In addition,
carbonates are applied as electrolytes in lithium-ion batteries,[5]
as monomers in polymerization reactions,[6] and as intermedi-
ates for fine chemicals.[7]
Scheme 1. Synthesis of bifunctional phosphonium catalysts 3.
These catalysts can activate the epoxide in dual mode via
hydrogen-bond donation of the alcohol moiety and electro-
static interactions of the phosphonium salt center. This should
enable an easier opening of the epoxide by the counter anion
of the salts 3.[21] Thus, it allows to perform the reaction under
mild conditions. In this context the bifunctional organocata-
lysts 3 were employed in the model reaction of butylene oxide
(4a) and CO2 producing 5a to deduce structure–activity rela-
tionships as well as the most active catalysts 3 (Table 1). While
trimethylphosphine-based bifunctional catalyst 3a–c were
present, only poor yields of cyclic carbonate 5a were observed
(entries 1–3). However, the yield increased from ClÀ <BrÀ <IÀ,
emphasizing the impact of the halide. Tri-n-butyl substituted
phosphonium salts 3d–f showed significantly higher yields in
the model reaction than the methyl-substituted analogues (en-
tries 4–6). Amongst these, again the iodide salt tri-n-butyl-(2-
hydroxyethyl) phosphonium iodide (3 f) gave the best result
with a yield of 95% of cyclic carbonate 5a. Additionally, we
were interested in effects concerning the distance between nu-
cleophilic and electrophilic groups. When the linkage between
electrophilic and nucleophilic center was extended to propyl
to form 3g, the catalytic activity decreased slightly (entry 7). To
emphasize the catalytic activity of the bifunctional catalyst we
employed simple monofunctional tetra-n-butyl phosphonium
salts 6a–c (entries 8–10). In the presence of the best mono-
There are numerous reports on catalysts for the coupling of
CO2 with epoxides. Especially organocatalysts based on pyridi-
nium,[8] imidazolium,[9] ammonium,[10] and phosphonium
salts[11] as well as carbenes[12] have been studied extensively as
both single catalysts and co-catalysts. However, under metal-
free conditions these catalysts usually require harsh reaction
conditions: temperatures>1008C and/or pressures>2.0 MPa.
There are very few examples of catalytic systems that operate
[a] Dr. T. Werner, H. Bꢀttner
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: thomas.werner@catalysis.de
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201402477.
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First, the effect of the reaction temperature was examined at
p(CO2)=1.0 MPa and 2 h (entries 1–4). The yield of 1,2-butyl-
ene carbonate (5a) increased significantly with increasing tem-
perature from 60–908C. Even at a low temperature of 608C
a yield of 97% of 5a was observed when prolonging the reac-
tion time to 24 h (entry 2). The coupling reaction was most
sensitive to the temperature and 908C was found to be opti-
mal for the reaction. Shortening the reaction time to 1 h gave
also a good yield of 85% but for full conversion 3 h was neces-
sary and 98% of 5a was obtained (entries 5–6). The effect of
the initial CO2 pressure was also investigated in the range of
0.3–2.0 MPa (entries 7–10). Even at a low pressure of 0.3 MPa
a good yield of 89% of 5a was observed. However, to prefera-
bly obtain quantitative conversion the reaction conditions
were set to 1.0 MPa, 908C and 3 h reaction to evaluate the
substrate scope (Table 3).
Table 1. Model reaction and screening results for the catalysts 3 and 6.
Entry
Catalyst
Yield 5a [%][a]
1
2
3
4
5
6
7
3a
3b
3c
3d
3e
3 f
0
2
21
57
86
95
78
3g
8
9
6a
6b
6c
7a
7b
7c
36
25
19
51
24
19
10
11
12
13
Table 3. Substrate scope for catalyst 3 f under optimized reaction condi-
tions.[a]
Entry
1
Substrate
Product
Yield [%][b]
97
4a
5a
[a] Yields were determined by GC and hexadecane as internal standard.
2
3
4
5
6
4b
4c
4d
4e
4 f
5b
5c
5d
5e
5 f
99
99
97
99
93
functional catalyst 6a only a poor yield of 36% was observed.
The utilization of the corresponding simple ammonium salt 7a
led to a somewhat improved yield while 7b and 7c gave re-
sults comparable to their phosphonium counterparts (en-
tries 11–13). In contrast to bifunctional organocatalyst 3d–f the
yield increased from IÀ <BrÀ <ClÀ and followed the order of
nucleophilicity. When bifunctional organocatalysts 3 were em-
ployed the order turned to ClÀ <BrÀ <IÀ, which presumably
can be explained by stronger hydrogen-bond interactions of
the alcohol moiety and the chloride compared to the bromide
and iodide.
7
8
4g
4h
5g
5h
97
98
Nevertheless, these results confirmed the acceleration of the
CO2 addition to epoxide 4a due to synergistic effects. Cata-
lyst 3 f was identified as the most active one and chosen for
optimizing reaction conditions of the model reaction produc-
ing 1,2-butylene carbonate (5a) from 4a and CO2 (Table 2).
9
4i
4j
5i
5j
99
10
17 (69)[c]
Table 2. Optimization of the reaction conditions employing catalyst 3 f.[a]
[a] Reaction conditions: epoxide 4 (2.00 g), 3 f (2 mol%), 908C, 3 h
p(CO2)=1.0 MPa. [b] Isolated yields. [c] 1208C, 6 h, p(CO2)=4.0 MPa.
Entry
T [8C]
p(CO2)[b] [MPa]
t [h]
Yield 5a[c] [%]
1
2
3
4
5
6
7
8
9
60
60
80
90
90
90
90
90
90
90
1.0
1.0
1.0
1.0
1.0
1.0
0.3
0.5
2.0
3.0
2
24
2
2
1
3
3
3
3
45
97[d]
85
95
85
98
89
96
96
97
Tri-n-butyl-(2-hydroxyethyl) phosphonium iodide (3 f) cata-
lyzed effectively the coupling reaction of terminal alkyl- and
phenyl-substituted epoxide 4a–d to the corresponding cyclic
carbonate 5a–d in quantitative yields (Table 3; entries 1–4).
The catalytic system was applicable for electron-withdrawing
substituted epoxide 4e and tolerated even the olefin side
chain in epoxide 4 f. The corresponding carbonates 5e–f were
produced in excellent yields (entries 5–6). Furthermore, several
glycidyl ether and ester derivatives 4g–i were converted with
CO2 (entries 7–9). Noteworthy, even the methyl methacrylate-
10
3
[a] Reaction conditions: 4a (2.00 g, 27.7 mmol), 2 mol% 3 f (208 mg,
0.556 mmol). [b] Initial pressure; after reaching the desired temperature
p(CO2)=const. [c] Yields were determined by GC-FID. [d] Isolated yield.
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substituted carbonate 4i was obtained in 99% yield although
it is sensitive to a polymerization side reaction.[22] Internal ep-
oxides are known to be challenging substrates in this reaction.
Nevertheless, cyclohexene oxide (4j) could be converted in
17% to the carbonate 5j under optimized reaction conditions.
Under more drastic conditions of 1208C, 4.0 MPa, and 6 h the
yield was increased up to 69%. In all reactions cyclic carbo-
nates were formed exclusively and no side products were ob-
served. It is worth mentioning that the catalyst 3 f could be
easily removed by filtration over silica gel to obtain analytically
pure carbonates 5.
In addition, we were interested in the catalytic activity of
the organocatalyst 3 f even in multigram scale. Hence, the
model reaction of 1,2-epoxybutane (4a) with carbon dioxide
was scaled up by a factor 20 and performed in a 100 mL stain-
less steel reactor and monitored by in situ ATR-FTIR spectrosco-
py (Figure 1).
Figure 2. Recycling of the catalyst 3 f in the coupling reaction of 4a and CO2
under optimized reaction conditions: 908C, 3 h, p(CO2)=1.0 MPa.
In conclusion, we present a straightforward route to tunable
phosphorus-based bifunctional organocatalysts 3. The pre-
pared salts are employed in a model reaction of CO2 with
1,2-butylene oxide (4a). We determine structure–activity rela-
tionships and identify the most active catalyst. The catalyst ac-
tivity is significantly influenced by the substituents at the
phosphorus center and the anion. A synergistic effect of the
catalyst shows a superior catalytic activity compared to simple
tetra-n-butyl phosphonium salts 6. In the presence of the most
active organocatalyst 3 f full conversion of several epoxides is
achieved under solvent-free reaction conditions at 908C and
1.0 MPa within 3 h. In general, the corresponding products are
obtained in excellent yields and can be easily isolated by filtra-
tion over silica. Notably, even at a low temperature of 608C an
almost quantitative yield of 97% is isolated. The protocol is
even applicable on a multigram scale and can be easily moni-
tored by using in situ ATR-FTIR spectroscopy. When examining
the reusability of the catalyst 3 f in five consecutive runs, ex-
cellent yields up to 97% of 1,2-butylene carbonate (5a) are ob-
tained in the first three runs.
Figure 1. Conversion of 1,2-epoxybutane (4a) with CO2 followed by in situ
ATR-FTIR spectroscopy. Reaction conditions: 1,2-epoxybutane (4a, 38.7 g,
0.537 mol), 1 mol% 3 f (2.81 g, 7.51 mmol) 908C, p(CO2)=1.0 MPa.
From offline ATR-FTIR spectroscopy we determined specific
bands of 1,2-epoxybutane (4a) and 1,2-butylene carbonate
(5a). According to these measurements, we were able to moni-
tor the reaction by reference to the intensity of the carbonyl
vibration band at 1790 cmÀ1 as well as the bending vibrations
at 1190 and 1060 cmÀ1. The rapid increases of the intensity cor-
relate with the results from optimizing reaction conditions
after 1 h at 908C and 1.0 MPa (Table 2, entry 4). In the same
time period the intensity of the corresponding substrate bands
at 904 and 833 cmÀ1 decreased. A maximum of the intensity of
1,2-butylene carbonate (5a) bands was observed after 255 min
and the reaction was stopped. After work up 84% of cyclic car-
bonate 5a was isolated. We were also interested in the recy-
cling of 3 f (Figure 2). The catalyst was separated from the pro-
duct 5a by Kugelrohr distillation and reused in five consecu-
tive runs. In the first three runs no loss of activity was observed
and 5a was obtained in excellent yields up to 97%. In the fol-
lowing two runs the yield decreased to 76 and 65%, respec-
tively. We assume partial decomposition of the catalyst 3 f due
to the thermal stress during Kugelrohr distillation. Tri-n-butyl-
phosphine and the corresponding phosphine oxide were iden-
tified by GC-MS as the major decomposition products.
Experimental Section
In a typical coupling reaction a 45 mL stainless steel reactor was
charged with catalyst 3 f (208 mg, 0.556 mmol) and 1,2-butylene
oxide (4a, 2.00 g, 27.7 mmol). The reactor was purged once with
CO2, pressurized to p(CO2)=1.0 MPa and heated to 908C for 3 h.
After reaching the desired reaction temperature the pressure was
kept constant. Subsequently, the reactor was cooled to ambient
temperature with an ice bath and CO2 was released slowly. The re-
action mixture was filtered over silica gel (CH2Cl2) and all volatiles
were removed under reduced pressure. The product 5a was ob-
tained as a colorless oil (3.15 g, 27.2 mmol, 97%).
Acknowledgements
We wish to thank the Federal Ministry of Research and Education
(BMBF) for financial support (Chemische Prozesse und stoffliche
Nutzung von CO2: Technologien fꢀr Nachhaltigkeit und Klima-
schutz, grant 01 RC 1004A) as well as Prof. Beller for support and
advice.
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Keywords: carbon dioxide fixation · coupling reactions ·
carbonates · organocatalysis · phosphorus
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Received: May 28, 2014
Revised: June 23, 2014
Published online on October 10, 2014
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