Immobilized bifunctional phosphonium salts as recyclable organocatalysts in the cycloaddition of CO2 and epoxides

Article abstract of DOI:10.1039/c7gc01782k

Several bifunctional phosphonium salt catalysts were prepared and immobilized on silica and polystyrene supports. The immobilized systems were compared with their homogeneous analogs in cyclic carbonate synthesis. Interestingly, in some cases, higher acti

Full text of DOI:10.1039/c7gc01782k

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Immobilized bifunctional phosphonium salts as
recyclable organocatalysts in the cycloaddition of
Cite this: DOI: 10.1039/c7gc01782k
CO2 and epoxides†
a
J. Steinbauer,a L. Longwitz,a M. Frank, b J. Epping,c U. Kragld and T. Werner
*
Several bifunctional phosphonium salt catalysts were prepared and immobilized on silica and polystyrene
supports. The immobilized systems were compared with their homogeneous analogs in cyclic carbonate
synthesis. Interestingly, in some cases, higher activities were observed for the immobilized catalysts. The
most active system was a phenol-functionalized phosphonium salt supported on a silica surface. The
covalent linkage of the phosphonium unit to the silica was verified by solid-state NMR and FT-IR. SEM and
EDX measurements revealed a homogeneous distribution of the phosphonium salt on the particle
surface. This catalyst facilitated the addition of CO2 to epoxides under mild conditions. The evaluation of
the substrate scope and the catalyst recycling were combined in one set of experiments. In 15 consecu-
tive runs, the synthesis of 12 cyclic carbonates in yields of up to 98% was achieved. The investigation of
the catalyst after the recycling experiments revealed the loss of the original anion (bromide) as well as a
decrease in the number of phosphonium units, which explained the observed deactivation of the catalyst
during the recycling experiments.
Received 16th June 2017,
Accepted 28th July 2017
DOI: 10.1039/c7gc01782k
rsc.li/greenchem
CO2, from localized point sources is available and used for
industrial purposes.7 The available quantities from these
Introduction
The greenhouse gas CO2 is generally considered the biggest localized sources exceed those needed for large-scale synthesis
waste product, and more than 35 Gt of this gas was produced processes. In this context, the interest in the utilization of CO2
in 2014 by human activities.1 The increasing CO2 concen- as an energy carrier as well as a carbon synthon in organic
tration is closely related to global warming, which is related to chemistry has tremendously increased in the past decade.3e,4a,d
climate change.2 Current industrial and academic research is Certain CO2-based transformations are kinetically hindered3e or
evaluating concepts to address this challenge, such as carbon have thermodynamic constraints,8 which render their large-scale
capture and storage and carbon capture and utilization.3 The application infeasible. However, since the 1950s, cyclic carbonates
chemical utilization of CO2 as a feedstock in current processes such as ethylene carbonate (2b) have been produced commercially
has only negligible impact on the reduction of global CO2 from epoxides and CO2 in an exothermic reaction (Scheme 1).9
emissions.1,4 However, the use of CO2 as a C1 building block
The global annual production directly utilizing about 40 ×
in industrial processes can contribute to the saving of limited 103 t CO2 is approx. 80 × 103 t.1 However, the cyclic carbonate
fossil resources.5 Even though CO2 can be captured directly market is constantly growing mainly due to the properties10 of
from ambient air, this process is relatively energy intensive.6 these compounds, which can be used in a broad range of
Commonly, concentrated, and sometimes even pressurized applications, for example, as green solvents,11 electrolytes,12 or
as additives in medicines and cosmetics.10 The use of cyclic
carbonates as monomers in polymer chemistry13 and as inter-
mediates in organic chemistry14 has also been explored over
aLeibniz-Institute for Catalysis e.V. at the University of Rostock,
Albert-Einstein-Straße 29a, 18059 Rostock, Germany.
E-mail: Thomas.Werner@catalysis.de
bMedical Biology and Electron Microscopy Centre, University Medicine Rostock,
Strempelstraße 14, 18057 Rostock, Germany
cBerlin University of Technology, Institute of Chemistry, Straße des 17. Juni 135,
10623 Berlin, Germany
dInstitute of Chemistry, Technische Chemie, University of Rostock,
Albert-Einstein-Str. 3a, 18059 Rostock, Germany
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c7gc01782k
Scheme 1 The atom economic reaction of epoxide 1a with CO2 to
yield the valuable cyclic carbonate 2a.
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the past few years. Due to these reasons, the development of
new catalysts for the atom economic synthesis of cyclic carbon-
ates from epoxides and CO2 is a highly active research area.15
This reaction can be catalyzed by abundant metal catalysts15b
or even organocatalysts,15e,16 and appropriately fits the 12 prin-
cipals of Green-17 and CO2-chemistry.18 The great potential of
organocatalysis in this CO2-fixation reaction has been high-
lighted in various recent reviews.15e,16a,b,19a–c In general,
organocatalysts offer several advantages; for example, they are
often robust and bench-stable and do not necessarily require
inert reaction conditions.20 Another significant upside is the
carbon-based scaffold, which is associated with a big modifi-
cation potential, especially regarding the catalyst immobiliza-
tion.16a,21 Lately, highly active OH-functionalized organocata-
lysts have been reported for the synthesis of cyclic carbon-
ates.21b,22a,b–i The superior activity of these catalysts is attribu-
ted to the epoxide activation and the stabilization of inter-
mediates by hydrogen bonding. While the development of
bifunctional N-based catalyst systems has been extensively
studied, P-based systems are comparatively less investiga-
ted.16a,b,19a We are generally interested in the design and appli-
cation of bifunctional phosphonium salts as recyclable cata-
lysts.22a,23 One method for obtaining reusable catalyst systems is
the immobilization on an organic or inorganic support.21a,24a,b
Previously, a few monofunctional P-based systems, which were
immobilized on various supports operating at reaction temp-
eratures ≥100 °C, have been reported.25 Most recently, pioneer-
ing work has been carried out by Dai et al. and Liu et al., who
established heterogeneous bifunctional phosphonium salt
catalysts.26 However, for efficient application of these catalysts,
high reaction temperatures ≥100 °C were necessary. We esta-
blished bifunctional phosphonium salts bearing OH-moieties
as efficient catalysts for the synthesis of cyclic carbonates
under mild reaction conditions (T ≤ 90 °C). Herein, we report
the immobilization of these bifunctional structural motifs on
polystyrene and silica, as well as the comparison with their
homogeneous analogs.22a,c,g
Scheme 2 Synthesis of various polystyrene-supported phosphonium
salts 6a–d and their homogeneous analogs 5a–d.
able phosphane groups.28 The synthesized catalysts 6a–d were
also analyzed by IR spectroscopy. The successful immobiliz-
ation of the alcohol functionality could be verified by a broad
band at around 3300 cm−1 for the O–H-vibration. The catalytic
activities of the synthesized phosphonium salts were tested in
the reaction of butylene oxide (1b) as the model substrate with
CO2 at a temperature of 90 °C, a CO2 pressure of 1.0 MPa,
2 mol% catalyst loading, and a reaction time of 2 h.
The unloaded support 4b was able to yield 11% of cyclic
carbonate (Table 1, entry 1).‡ All of the employed catalysts
showed excellent selectivity of >99% toward the cyclic carbon-
ate 2b. Note that the best result was obtained when using the
homogeneous iodide salt 5a, which yielded 78% of the
product (2b, entry 2). The corresponding heterogeneous cata-
lyst 6a had a lower activity and only converted 56% of butylene
oxide (1b, entry 3). The introduction of an additional hydroxyl
group in 5c compared to 5b facilitated the conversion of 1b
with CO2 (entries 4 and 6). In the presence of 5b, the desired
product was obtained in 1% yield, while the use of 5c led to
11% yield. The same trend was observed for the supported
analogs 6b and 6c, even though significantly higher yields of
34% to 42% were obtained, respectively (entries 5 and 7).
Immobilized catalysts often suffer from activity loss compared
to their homogeneous counterparts, which is why we were sur-
prised to observe that the immobilized bromide salts 6b and
6c had higher activities than the salts 5b and 5c (entries 4–7).
The heterogeneous catalysts offer multiple functional groups
in close proximity, which might enhance the activity. Tassaing
et al. reported that the introduction of a CF3-group adjacent to
the hydroxyl group is beneficial for the catalytic activity.27b
Thus, we prepared bifunctional phosphonium salt 5d, which
led to an improved yield of 30% compared to that for 5b (entry
8 vs. 4). Interestingly, the supported catalyst 6d gave the
desired product 2b in only 10% yield (entry 9).
Results and discussion
We first prepared four bifunctional phosphonium salt catalysts
5a–d by the alkylation of triphenylphosphane (4a) with
different halohydrins (3a–d) and their polystyrene-supported
analogs 6a–d (Scheme 2). The iodide 5a and bromide 5b
represent the most basic bifunctional structures, which we
have reported previously. Catalyst 5c is functionalized with an
additional OH-moiety, which might benefit the activation of
the epoxide by additional hydrogen bonding. The derivate 5d
bears a CF3-group, which possibly improves the hydrogen
bond donor capabilities of the corresponding alcohols by
increasing the acidity of the hydroxyl group.27 The polystyrene-
supported derivatives 6a–d were easily accessible by alkylation
of polystyrene-supported triphenylphosphane 4b in the pres-
ence of the respective halohydrins (3a–d). Solid-state 31P NMR
‡When investigating the support 4b, a significant amount of phosphonium salt
was detected in the solid-state 31P NMR. The elemental analysis showed a
chlorine content of 3.8%. We attributed this to the presence of phosphonium
measurements revealed a quantitative conversion of all avail- chlorides, which were probably formed during the polymer synthesis.
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Table 1 Reaction of butylene oxide (1b) with CO2 under standard reac-
tionalized silica support 9a and to the polystyrene support 9b
to yield catalysts 12 and 13. These two catalysts can be
considered as the direct heterogeneous analogs to catalysts 10
and 11. This enabled the comparison between the different
supports (immobilized catalysts 12 and 13) and to their homo-
geneous counterparts (12 vs. 10 and 13 vs. 11). The catalysts
were tested in our model reaction under the same conditions
as those shown in Table 1. The supports 9a and 9b showed no
activity on their own (Table 2, entries 1 and 2). All four cata-
lysts gave excellent yields of cyclic carbonate 2b (entries 3–6).
The exchange of the propyl group to benzyl group in the homo-
geneous catalysts 10 and 11 had almost no effect on their
activities (entries 3 and 4). In contrast, the silica-supported
derivative 12 showed a significantly higher activity compared
to the polystyrene-supported counterpart 13 (entries 5 and 6).
In the presence of the silica-supported catalyst 12, the desired
product 2b was obtained in 96% yield (entry 5). This gain in
activity compared to homogeneous catalyst 10 and polystyrene-
supported catalyst 9 can be attributed to the Si-OH functional-
ities on the support surface, which act as additional hydrogen
bond donors.25a
Subsequently, we investigated the performance of catalyst
12 under various reaction conditions (entries 6–11). Reducing
the catalyst loading from 2 to 1 mol% led to 77% yield of 2b
(entry 7). Furthermore, the reaction was carried out at different
pressures. The reduction from 1.0 MPa CO2 pressure to
0.5 MPa had negligible effect on the yield (entries 7 and 8).
However, at 0.25 MPa CO2 the yield decreased to 56% (entry 9).
Furthermore, the reaction could be performed under atmos-
pheric pressure (entry 10). Therefore, we chose tert-butyl glyci-
dyl ether (1c) as the substrate, because of its lower volatility
compared to that of 1b. The catalyst loading was set to
2 mol%, and after 2 h, a yield of 32% on 2c was obtained.
Extending the reaction to 24 h, we observed full conversion of
the substrate. However, only 61% yield was achieved (entry 10).
This is probably due to the loss of epoxide 1c out of the open
reaction system at 90 °C under atmospheric CO2 pressure. It
was possible to obtain 2b in a yield of 18% and even 93% at a
lower temperature of 45 °C after 2 h and 24 h, respectively
(entry 11). The immobilized catalysts 12 and 13 were analyzed
by solid-state NMR, IR, and elemental analysis.28 Catalyst 12
showed a broad signal at 20 ppm in the solid-state 31P NMR
spectrum, which was in the range of a typical phosphonium
salt. The 13C solid-state NMR further confirmed the incorpor-
ation of phosphane 7 (Fig. 1). When comparing the spectrum
of support 9a (Fig. 1a) with the phosphonium salt catalyst 12
tion conditions with several bifunctional catalystsa
Entry
1
Catalyst
Yield 2b a/%
4b
11
2
3
5a
6a
78
56
4
5
5b
6b
1
34
6
7
5c
6c
11
42
8
9
5d
6d
30
10
a Reaction conditions: 1.0 equiv. 1b (1.00 g, 13.9 mmol), 2 mol% cata-
lyst, 90 °C, 2 h, p(CO2) = 1.0 MPa, solvent-free. The catalyst loadings of
the polystyrene-supported catalysts 6a–d were calculated based on the
halogen content determined by elemental analysis. Yields and selectiv-
ities were determined by 1H NMR using mesitylene as internal stan-
dard. All selectivities were >99%.
In our recent work on phosphonium-based catalysts, we (b), the signal at around 8 ppm vanished (blue). It is very likely
also discovered that the introduction of phenol groups can that this signal corresponded to the carbon bound to bromine
enhance the catalytic activity.22g Bifunctional phosphonium (–CH2Br), which was not present in the loaded catalyst.
salt 10 proved to be highly active in the synthesis of bio-based
cyclic carbonates. For this reason, we envisioned to immobilize heterogeneous catalyst 12 (b) were in great agreement overall.
10 on halogen-functionalized support. The bromide- Furthermore, the signal at 160 ppm in the spectrum of the
The spectra of homogeneous catalyst 10 (Fig. 1c) and
a
functionalized silica support 9a and the polystyrene support supported catalyst (b) suggested the successful immobilization
9b are commercially available and suitable for the synthesis of of the phenol moiety, which is crucial for the catalytic activity
immobilized derivatives of 10 (Scheme 3). The easily prepared (red). The SEM images of the silica-supported 9a and the
phosphane 7 29 was successfully bound to bromopropyl-func- immobilized catalyst 12 are displayed in Fig. 2. The EDX
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Scheme 3 Immobilization of phosphane 7 on two different supports to give catalysts 9 and 12 and their homogeneous counterparts 11 and 10.
Table 2 Catalyst screening and parameter optimization for immobi-
lized catalyst 12 a
Entry
Catalyst
mol%
T/°C
p/MPa
Yielda/%
1
2
3
4
5
6
7
8
9a
9b
10
11
12
13
12
12
12
12
12
2
2
2
2
2
2
1
1
1
2
2
90
90
90
90
90
90
90
90
90
90
45
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.25
0.1c
1.0
0
<1
89
91
96
80
77
74
9
56
10b
11
32 (61)d
18 (93)d
a Reaction conditions: 1.0 equiv. 1b (1.00 g, 13.9 mmol), 1–2 mol%
catalyst, 45–90 °C, 2 h, p(CO2) = 0.1–1.0 MPa, solvent-free. The catalyst
loadings of catalysts 12 and 13 were calculated based on the phos-
phorus content determined by elemental analysis. Yields and selectiv-
ities were determined by 1H NMR using mesitylene as internal stan-
dard. All selectivities were >99%. b tert-Butyl glycidyl ether (1c) was
used as substrate. c p(CO2) = 1 atm. d 24 h.
mapping indicated that there was no significant amount of
phosphorus on the silica support 9a (Ic). In contrast, after
immobilization of phosphane 7 on 9a, the EDX mapping
demonstrated highly dispersed phosphorus on the surface
without any accumulation (IIc). We decided to combine the
substrate screening and catalyst recycling in one set of experi-
Fig. 1 (a) 13C NMR spectra of homogeneous catalyst 10 in CDCl3.
(b) Solid-state 13C NMR spectra of catalyst 12 and (c) silica-based
support 9a.
ments under standard reaction conditions (2 mol% 12, 90 °C, carbonate (2e) was only obtained in moderate yield of 60% in
6 h, p(CO2) = 1.0 MPa). In addition, we chose a reaction time the recycling experiments, we used 2 mol% of fresh catalyst 12
of 6 h to allow the conversion of more challenging substrates. to convert styrene oxide (1e) and obtained an increased iso-
After each reaction CH2Cl2/Et2O was added and the dissolved lated yield of 80% (entry 4). To probe the possible deactivation
product was separated from the catalyst, the catalyst was dried, of the catalyst, we decided to use our model substrate 1b once
and subsequently, a different epoxide was converted under the more and could still isolate 89% of butylene carbonate (2b) in
standard reaction conditions. We started with our model sub- cycle 6 (entry 6). Subsequently, we converted three epoxides
strate 1b, which gave the desired product in 89% isolated yield containing different functional groups in cycles 7 to 9 (entries
(Table 3, entry 1).
7–9). Substrate 1g is an interesting bio-based epoxide, which
The conversion of tert-butyl glycidyl ether (1c), n-butyl-func- was converted in 76% yield (entry 7). The unsaturated carbon-
tionalized epoxide 1d, styrene oxide (1e), and n-hexyl-substi- ates 2h and 2i were obtained in excellent yields of 96% and
tuted epoxide 1f led to the respective carbonates in yields of 83%, respectively (entries 8 and 9). In cycle 10, the challenging
up to 98% in the recycling runs 2–5 (entries 2–5). Since styrene disubstituted epoxide 1j was converted and only 23% of the
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Fig. 2 SEM images of the silica support 9a (Ia) and the supported catalyst 12 (IIa) on 20 μm scale. EDX mapping with color-coded intensity range of
silicon (Ib) and phosphorus (Ic) for silica support 11. EDX mapping with color-coded intensity range of silicon (IIb) and phosphorus (IIc) for the sup-
ported catalyst 12.
carbonate 2j was isolated (entry 10). When the experiment was original weight. During the recycling experiments, 31P NMR
repeated with a fresh catalyst, only a marginal increase in yield spectra of the extracted products did not show any signs of
(28%) was observed (entry 10).
leaching, but after 15 cycles, the phosphorus content deter-
For this reason, the low yield can rather be explained by the mined by elemental analysis was lowered from 1.60% to 0.77%
substrate and not a severe deactivation of the catalyst. (Fig. 4). The amount of bromine in the catalyst dropped below
Propylene oxide (1k) was chosen as the next substrate and 72% the limit of detection by elemental analysis, which explains
of the cyclic carbonate 2k was isolated in cycle 11, which is the decrease in conversion and yield from cycle 1 to 15 for
only slightly lower than the yield obtained in the control butylene carbonate (2b) (Table 3, entries 1 and 15). When the
experiment with fresh catalyst 12 (entry 11). Afterward, we used catalyst was investigated with SEM, considerable irregula-
used butylene oxide (1b) as the substrate in another control rities were observed on the surface of the particles (Fig. 3, IIa).
experiment for the third time, but only a yield of 66% could be The bromine content detected with EDX spectroscopy
achieved (entry 12). In cycle 13, epichlorohydrin (1l) was decreased drastically (Fig. 4); however, the catalytic centers
employed and a very good yield of 83% was obtained, showing were still spread evenly across the particles (Fig. 3, IIb). It is
that the catalyst could still achieve high yields even after being possible that small particles with high loading of active sides
recycled 12 times (entry 13). The iso-propyl-substituted deri- got detached from the catalyst surface and washed out during
vate gave carbonate 2m in a yield of 45% in reaction cycle 14 the recycling experiment. Due to their insolubility in common
(entry 14). Since the activity of the catalyst was already lowered, NMR solvents, it would explain why we did not observe any
fresh catalyst was used with 1m to give 92% of the desired leaching during our experiments with 31P NMR. Additionally,
product.
when examining the used catalyst after 15 cycles, we could
Finally, after 14 recycling runs, the model substrate 1b was detect a considerable amount of chlorine, indicating a halogen
once again selected and a yield of 33% of butylene carbonate exchange (ESI Fig. 41†).28 We were not able to detect any chlor-
(2b) was isolated (entry 15). Even though the conversion and ine when analyzing the catalyst after run 8 with EDX. This
isolated yield in the first two runs with 1b as the substrate likely occurred during run 13 of the recycling experiments,
stayed the same (entries 1 and 6), a strong decline in activity when epichlorohydrin (1l) was used as the substrate (ESI
was observed in later runs (entries 12 and 15). The conversion Scheme 2†). The solid-state 31P NMR spectrum of the catalyst
of two challenging internal epoxides, cyclohexene oxide (1n) stayed the same even after the recycling experiments, showing
and epoxidized methyl oleate (1o, EMO), was also investigated one broad signal of a phosphonium salt (ESI Fig. 33†). The
(entries 16 and 17). While the yield was only around 30% for amount of halide counter ions in the used catalyst were not
both substrates, the catalyst showed activity under mild con- sufficient compared to the number of phosphonium cations,
ditions for internal epoxides.
which leads us to the conclusion that the deactivation of the
Subsequently, the recycled catalyst was analyzed by elemen- catalyst was at least partly induced by an ion exchange. The
tal analysis, solid-state NMR experiments, SEM, and EDX. The trace amount of water present in the extraction solvent could
mass of the recovered catalyst after cycle 15 was 63% of the also lead to the formation of a phosphonium hydroxide. Even
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Table 3 Substrate scope for the reaction of epoxides 1 with CO2 to
form cyclic carbonates 2 while recycling catalyst 12 a
Entry
1
Product
Cycle
1
Yieldb/%
89
2b
2c
2d
2e
2f
2
3
4
5
6
7
2
3
4
5
6
7
95
98
60 (80)
85
Fig. 3 SEM images of the fresh catalyst 12 (Ia) and the supported cata-
lyst 12 after 15 reaction cycles (IIa) on 20 μm scale. EDX mapping with
color-coded intensity range of bromine (Ib) for the fresh catalyst 12.
EDX mapping with color-coded intensity range of bromine (IIb) for the
supported catalyst 12 after 15 reaction cycles.
2b
2g
89
76
8
2h
8
96
9
2i
9
83
10
11
12
2j
10
11
12
23 (28)
72 (86)
66
2k
2b
13
14
2l
13
14
83
2m
46 (92)
Fig. 4 Section of bromine, silicon, and phosphorus in the EDX spectra
between 1.25 and 2.25 keV of fresh catalyst 12 (blue) after 8 cycles
(black) and 15 cycles (red).
15
16
2b
2n
15
33
—
(33) (cis : trans > 99 : 1)c
though onium hydroxides could also facilitate the reaction,30
the activity under mild reaction conditions was inferior to that
of halides.
17
2o
—
(26) (cis : trans 43 : 57)c
The solid-state 13C NMR spectrum showed an overall loss of
aromatic carbon signal and the addition of a broad signal
around 70 ppm (ESI Fig. 32†).28 Presumably, this signal corre-
sponded to butylene carbonate, which was still adsorbed on
the catalyst surface even after thoroughly washing the catalyst.
We assumed that the mechanism of this transformation was
very similar to other bifunctional phosphonium salt-catalyzed
reactions.22c,28 The catalytic cycle includes the activation of the
epoxide through hydrogen bonding by a hydroxyl moiety, and
a Reaction conditions: 1.0 equiv. 1b (1.00 g, 13.9 mmol), 2 mol% cata-
lyst, 90 °C, 6 h, p(CO2) = 1.0 MPa, solvent-free. The catalyst loading of
catalyst 12 was calculated based on the phosphorus content deter-
mined by elemental analysis. Isolated yields are given. b Yield when
fresh catalyst was used is given in parenthesis. c t = 24 h; yield was
determined by 1H NMR using mesitylene as the internal standard.
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subsequently, the ring opening by the nucleophilic attack of Et2O. Subsequently, all volatiles were removed in vacuo to yield
the halide anion. It is followed by the insertion of CO2 and an silica-supported catalyst 12 or PS-supported catalyst 13.
intramolecular ring closing, which leads to the product and
General procedure for the catalyst screening and parameter
optimization (GP4)
simultaneously regenerates the catalyst.
A stainless-steel autoclave was charged with the catalyst
(0.01–0.02 equiv.) and 1.00 equiv. of butylene oxide (1b, 1.00 g,
13.9 mmol). The mixture was immediately purged with CO2.
Conclusions
Subsequently, the reactor was heated to 45–90 °C for 2–24 h,
while p(CO2, 45–90 °C) was kept constant at 0.1–1 MPa.
Afterward, the reactor was cooled with an ice bath below 20 °C
and CO2 was released slowly. The yields for 2b were deter-
mined by 1H NMR using mesitylene as an internal standard
directly from the reaction mixture.
We presented several bifunctional phosphonium salts and
could synthesize comparable immobilized analogs. The
heterogeneous catalysts were characterized by solid-state NMR,
EA, IR, and SEM/EDX mapping. The catalysts showed great
activity in the cycloaddition of epoxides and CO2, and the
silica-supported phenol-functionalized phosphonium bromide
12 could convert 96% of butylene oxide (1b) at 90 °C, a CO2
pressure of 1.0 MPa, and a catalyst loading of 2 mol% in 2 h to
the corresponding cyclic carbonate. The reaction could also be
performed at atmospheric CO2 pressure or a reaction tempera-
ture of 45 °C. The catalyst was also used in a recycling experi-
ment with different epoxides to determine the substrate scope.
In total, 12 different carbonates were synthesized in moderate
to excellent yields over 15 reaction cycles. Furthermore, two
challenging internal epoxides were successfully synthesized
with a fresh catalyst. The used catalyst was analyzed by solid-
state NMR, IR, SEM, EDX mapping, and elemental analysis. A
gradual leaching of phosphorus and bromide and an anion
exchange led to the catalyst deactivation.
General procedure for the substrate scope and recycling
experiments (GP5)
A
stainless-steel autoclave was charged with catalyst 12
(0.02 equiv.) and 1.00 equiv. of the epoxide (1, 13.9 mmol),
and was immediately purged with CO2. Subsequently, the
reactor was heated to 90 °C for 6 h, while p(CO2, 90 °C) was
kept constant at 1 MPa. Afterward, the reactor was cooled with
an ice bath below 20 °C and CO2 was released slowly. The
reaction mixture was removed by extraction with CH2Cl2
(3 × 30 mL) and once with Et2O (10 mL). All volatiles were
removed in vacuo to yield cyclic carbonate 2. The catalyst was
dried in air overnight and reused.
Synthesis of the catalysts
(2,3-Dihydroxypropyl)triphenylphosphonium bromide (5c).
According to GP1, a mixture of Ph3P (4a, 1.00 equiv., 1.01 g,
3.85 mmol) and 3-bromopropane-1,2-diol (3c, 2.20 equiv.,
1.34 g, 8.61 mmol) was stirred at 110 °C for 24 h.
Subsequently, the product was precipitated twice from di-
chloromethane with diethyl ether. Afterward, the purification
proceeded according to GP1 and the product 5c was obtained
as a colorless solid with a yield of 63% (1.02 g, 2.43 mmol).
1H NMR (300 MHz, CDCl3): δ = 3.41–3.55 (m, 1H), 3.69–3.89
(m, 2H), 3.98–4.11 (m, 2H), 4.67–4.77 (m, 2H), 7.62–7.83 (m,
15H) ppm.
Experimental
General procedure for the preparation of the homogeneous
phosphonium salt catalysts (GP1)
A
mixture of triphenylphosphane (4a) or phosphane 7
(1.00 equiv.) and halohydrin 3a–d, 1-bromopropane (8a,
5.00 equiv.), or benzyl bromide (8b, 5.00 equiv.) was stirred for
24 h at 60–110 °C under an argon atmosphere. The crude
product was washed 3× with EtOAc and 3× with Et2O.
Subsequently, all volatiles were removed in vacuo to yield 5a–d.
Triphenyl(3,3,3-trifluoro-2-hydroxypropyl)phosphonium
bromide (5d). In accordance with GP1, a mixture of triphenyl-
phosphine (4a, 1.00 equiv., 1.04 g, 3.97 mmol) and 3-bromo-
1,1,1-trifluoropropan-2-ol (3d, 2.20 equiv., 1.28 g, 6.63 mmol)
was reacted and product 5d was obtained as a colorless solid
with a yield of 82% (1.47 g, 3.16 mmol).
General procedure for the alkylation of PS-supported PPh3
(GP2)
A mixture of PS-supported PPh3 4b (1.00 equiv.) and the halo-
hydrin 3a–d was mixed in a shaking device for 96 h at
60–110 °C under an argon atmosphere. The crude product was
washed 6× with Et2O. Subsequently, all volatiles were removed
in vacuo to yield PS-supported catalyst 6a–d.
1H NMR (300 MHz, CDCl3): δ = 3.22–3.32 (m, 1H),
3.81–3.949 (m, 1H), 4.51–4.67 (m, 1H), 6.86–6.88 (d, J = 7.1 Hz,
1H), 7.66–7.84 (m, 15H) ppm. 13C{1H} NMR (300 MHz, CDCl3):
δ = 25.9 (d, J = 58.7, CH2), 64.1 (d, J = 36.7 Hz, CH), 117.2 (d,
J = 88.2, CF3), 130.5 (CH), 130.6 (CH), 133.8 (CH), 133.9 (CH),
General procedure for preparation of phosphonium salts on
Br-functionalized PS and silica (GP3)
A mixture of PS-supported benzyl bromide (9b, 1.00 equiv.) or 135.4 (CH), 135.4 (CH) ppm.
4-bromopropyl-functionalized silica gel (9a, 1.00 equiv.) and
Polystyrene-supported catalyst (6a). According to GP2, a
phosphane 7 (2.00 equiv.) was mixed in a shaking device for mixture of 4b (1.01 g, 1.4–2.0 mmol g−1 P-loading) and
96 h at 110 °C in toluene under an argon atmosphere. The 2-iodoethanol (3a, 5 g, 29.1 mmol) was reacted at 60 °C and
crude product was washed 4× with hot CH2Cl2 and once with product 6a was isolated (1.081 g).
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13C NMR (solid state) δ = 2–15 (CH2, br.), 16–59 (CH2, br.),
Benzyl(2-hydroxyphenyl)diphenylphosphonium bromide (11).
102–137 (Ar, br.), 137–156 (Ar, br.), 213–233 (br.), 233–255 (br.) According to GP1, a mixture of 2-(diphenylphosphanyl)phenol
ppm. 31P NMR (solid state) δ = −49–23 (br.), 5–33 (br.), 72–106 (7, 1.00 equiv., 0.50 g, 1.80 mmol) and benzyl bromide (8b, 5
(br.) ppm. IR (ATR): λ−1 = 533 (m), 695 (br.), 721 (s), 747 (s), equiv., 1.54 g, 8.98 mmol) was stirred at 110 °C for 24 h.
820 (s), 1008 (s), 1069 (s), 1112 (m), 1180 (m), 1406 (s), 1436 Subsequently, the product was precipitated from DCM and
(s), 1450 (s), 1490 (s), 1598 (m), 2920 (br.), 3024 (m), 3328 (br.) washed 3× with ethyl acetate and 3× with diethyl ether and
cm−1. Elemental analysis: found I 13.31%, (1.05 mmol g−1 dried in vacuo. Product 11 was obtained as a colorless solid
I-loading).
with a yield of 79% (640 mg, 1.42 mmol).
Polystyrene-supported catalyst (6b). According to GP2, a
1H NMR (300 MHz, CDCl3): δ = 4.58 (s, 1H), 4.63 (s, 1H),
mixture of 4b (1.07 g, 1.4–2.0 mmol g−1 P-loading) and 2-bro- 6.68–6.76 (m, 1H), 6.85–6.91 (m, 1H), 7.00–7.04 (m, 2H),
moethanol (3b, 7.93 g, 63.5 mmol) was reacted at 110 °C and 7.13–7.33 (m, 7H), 7.48–7.59 (m, 5H), 7.66–7.73 (m, 2H),
product 6b was isolated (1.06 g).
8.12–8.17 (m, 2H), 11.32 (s, 1H) ppm; 13C{1H} NMR (300 MHz,
13C NMR (solid state) δ = 2–16 (CH2, br.), 16–72 (CH2, br.), CDCl3): δ = 30.5 (CH2), 116.9 (C), 118.1 (C), 118.9 (CH), 119.0
104–137 (Ar, br.), 137–156 (Ar, br.), 211–236 (br.), 236–256 (br.) (CH), 120.5 (CH), 120.6 (CH), 127.5 (C), 127.6 (C), 128.5 (CH),
ppm. 31P NMR (solid state) δ = −108–86 (br.), −51–23 (br.), 128.6 (CH), 128.9 (CH), 129.0 (CH), 129.6 (CH), 129.8 (CH),
7–27 (br.), 27–34 (br.), 34–56 (br.), 71–111 (br.) ppm. IR (ATR): 130.9 (CH), 131.0 (CH), 133.7 (CH), 133.8 (CH), 134.3 (CH),
λ−1 = 534 (m), 695 (m), 747 (s), 820 (s), 1008 (s), 1068 (s), 1112 134.5 (CH), 134.6 (CH), 134.6 (CH), 137.5 (CH), 162.3 (C) ppm.
(m), 1406 (s), 1437 (s), 1491 (s), 1598 (m), 2920 (br.), 3023 (s),
Silica-supported catalyst (12). According to GP3, a mixture of
3283 (br.) cm−1
.
Elemental analysis: found Br 14.14%, 9a (1.00 g, 1.50 mmol g−1 Br-loading) and 7 (832 mg,
(1.77 mmol g−1 Br-loading).
2.98 mmol) was reacted and product 12 was isolated (1.11 g).
13C NMR (solid state) δ = −4–6 (CH2, br.), 6–64 (CH2, br.),
Polystyrene-supported catalyst (6c). According to GP2, 4b
(630 mg, 1.4–2.0 mmol g−1 P-loading) and 3-bromopropane- 91–108 (Ar, br.), 108–122 (Ar, br.), 122–145 (br.), 153–163 (br.),
1,2-diol (3c, 5 g, 32.3 mmol) were mixed in a shaking device at 207–236 (br.) ppm. 31P NMR (solid state) δ = −53–19 (br.), 1–33
110 °C under argon for 96 h. The obtained solid was washed (br.), 33–43, 68–100 (br.) ppm. IR (ATR): λ−1 = 445 (br.), 796 (s),
3× with DCM, 3× with ethyl acetate, and 8× with diethyl ether. 1062 (br.), 1446 (s) cm−1. Elemental analysis: found P 1.60%,
Then, the obtained solid was dried in vacuo, and subsequently, (0.517 mmol g−1 P-loading).
6c was isolated (640 mg).
Polystyrene-supported catalyst (13). According to GP3, a
13C NMR (solid state) δ = 4–15 (CH2, br.), 15–50 (CH2, br.), mixture of 9b (1.00 g, 2.96 mmol g−1 Br-loading) and 7 (1.65 g,
103–137 (Ar, br.), 137–158 (Ar, br.); 215–235 (br.); 235–258 (br.) 5.93 mmol) was reacted and product 13 was isolated (1.55 g).
ppm. 31P NMR (solid state) δ = −115–89 (br.), −56–20 (br.),
13C NMR (solid state) δ = 4–58 (CH2, br.), 108–138 (Ar, br.),
4–49 (br.), 76–111 (br.) ppm. IR (ATR): λ−1 = 534 (m), 695 (m); 138–155 (Ar, br.), 155–168 (br.), 217–256 (br.) ppm. 31P NMR
747 (s); 820 (s), 1069 (s), 1111 (m), 1406 (s), 1437 (s), 1491 (s), (solid state) δ = −60–23 (br.), 1–52 (br.), 69–110 (br.) ppm.
1598 (m), 2920 (br.), 3023 (s), 3297 (br.) cm−1. Elemental ana- IR (ATR): λ−1 = 485 (m), 526 (s), 687 (m), 747 (s), 839 (m),
lysis: found Br 16.03%, (2.01 mmol g−1 Br-loading).
997 (s), 1018 (s), 1075 (s), 1107 (m), 1133 (s), 1164 (s), 1217 (s),
Polystyrene-supported catalyst (6d). According to GP2, a 1301 (m), 1351 (s), 1438 (m), 1489 (s), 1509 (s), 1590 (m),
mixture of 4b (630 mg, 1.4–2.0 mmol g−1 P-loading) and 1690 (s), 1989 (s), 2052 (s), 2912 (br.) cm−1. Elemental analysis:
3-bromo-1,1,1-trifluoropropan-2-ol (3d, 4 g, 20.7 mmol) was found Br 10.69%, (1.34 mmol g−1 Br-loading).
reacted at 110 °C and 6d was isolated (570 mg).
13C NMR (solid state) δ = 3–15 (CH2, br.), 15–77 (CH2, br.),
Synthesis of the cyclic carbonates
4-Ethyl-1,3-dioxolan-2-one (2b).22c According to GP5, butyl-
104–137 (Ar, br.), 137–162 (Ar, br.), 209–236 (br.), 236–257 (br.)
ppm. 31P NMR (solid state) δ = −111–90 (br.), −52–24 (br.),
3–34, 34–44 (br.), 75–106 (br.), 139–169 (br.) ppm. IR (ATR): ene oxide (1b, 1.00 g, 13.9 mmol) was allowed to react with
λ−1 = 534 (m), 694 (br.), 720 (s), 748 (s), 821 (s), 1008 (s), CO2 in the presence of 0.02 equiv. catalyst (12, 536 mg,
1070 (s), 1116 (m), 1181 (m), 1405 (s), 1436 (s), 1451 (s), 1491 (s), 0.278 mmol). After the extraction, 2b (1.43 g, 12.3 mmol, 89%)
1599 (s), 2920 (br.), 3024 (s), 3300 (br.) cm−1. Elemental analysis: was isolated as a colorless liquid. This procedure was repeated
found Br 10.69%, (1.34 mmol g−1 Br-loading).
in cycle 6 to afford 1b in 89% yield (1.43 g, 12.3 mmol), cycle
(2-Hydroxyphenyl)diphenyl(propyl)phosphonium bromide (10). 12 to afford 2b in 66% yield (1.06 g, 9.13 mmol), and cycle 15
According to GP1, a mixture of 2-(diphenylphosphanyl)phenol to afford 2b in 30% yield (491 mg, 4.22 mmol).
(7, 1.00 equiv., 430 mg, 1.55 mmol) and 1-bromopropane (8b,
1H NMR (300 MHz, CDCl3): δ = 1.04 (t, J = 7.4 Hz, 2.1 Hz,
5 equiv., 1.10 g, 8.94 mmol) was stirred at 110 °C for 24 h. 3H), 1.68–1.91 (m, 2H), 4.09 (dt, J = 7.1 Hz, 1.5 Hz, 1H), 4.53
Subsequently, the product was washed 6× with diethyl ether (t, J = 7.9 Hz, 1H), 4.63–4.72 (m, 1H) ppm.
and dried in vacuo. Product 10 was obtained as a colorless
solid with a yield of 94% (581 mg, 1.46 mmol).
4-(tert-Butoxymethyl)-1,3-dioxolan-2-one (2c).22c According
to GP5, tert-butyl glycidyl ether (1c, 1.81 g, 13.9 mmol) was
1H NMR (300 MHz, CDCl3): δ = 1.06–1.12 (m, 3H), 1.61 (m, allowed to react with CO2 in the presence of 0.02 equiv. catalyst
2H), 3.06–3.16 (m, 2H), 6.80–6.94 (m, 2H), 7.49–7.64 (m, 9H), (12, 536 mg, 0.278 mmol). After the extraction, 2c (2.30 g,
7.70–7.77 (m, 2H), 8.02–8.07 (m, 1H), 11.01 (s, 1H) ppm.
13.2 mmol, 95%) was isolated as a colorless liquid.
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1H NMR (300 MHz, CDCl3): δ = 1.20 (s, 9H), 3.57 (dd, J = J = 8.4 Hz, 6.2 Hz, 1H), 4.50 (t, J = 8.3 Hz, 1H), 4.77–4.87 (m,
10.2 Hz, 3.6 Hz, 1H), 3.58 (dd, J = 10.2 Hz, 3.6 Hz, 1H), 4.39 1H), 5.19–5.34 (m, 2H), 5.79–5.95 ppm (m, 1H) ppm.
(dd, J = 8.3 Hz, 5.3 Hz, 1H), 4.47 (t, J = 8.2 Hz, 1H), 4.73–4.80
(m, 1H) ppm.
4-(Chloromethyl)-4-methyl-1,3-dioxolan-2-one (2j).31 According
to GP5, 2-(chloromethyl)-1,2-epoxypropane (1j, 1.48 g,
4-Butyl-1,3-dioxolan-2-one (2d).22c According to GP5, hexene 13.9 mmol) was allowed to react with CO2 in the presence of
oxide (1d, 1.39 g, 13.9 mmol) was allowed to react with CO2 in 0.02 equiv. catalyst (12, 536 mg, 0.278 mmol). After
the presence of 0.02 equiv. catalyst (12, 536 mg, 0.278 mmol). extraction, 2j (480 mg, 3.19 mmol, 23%) was isolated as a
After the extraction, 2d (1.97 g, 13.7 mmol, 98%) was isolated colorless liquid.
as a colorless liquid.
1H NMR (300 MHz, CDCl3): δ = 1.64 (s, 3H), 3.66 (dd, J =
1H NMR (300 MHz, CDCl3): δ = 0.82 (t, J = 7.4 Hz, 3H), 12 Hz, 41 Hz, 2H), 4.17 (d, J = 8.7 Hz, 1H), 4.52 (d, J = 8.7 Hz,
1.28–1.49 (m, 4H), 1.58–1.88 (m, 2H), 4.05 (dd, J = 8.3 Hz, 1H) ppm.
7.1 Hz, 1H), 4.51 (t, J = 8.1 Hz, 1H), 4.64–4.75 (m, 1H) ppm.
4-Methyl-1,3-dioxolan-2-one (2k).22c According to GP5, pro-
4-Phenyl-1,3-dioxolan-2-one (2e).22c According to GP5, styrene pylene oxide (1k, 0.81 g, 13.9 mmol) was allowed to react with
oxide (1e, 1.67 g, 13.9 mmol) was allowed to react with CO2 in CO2 in the presence of 0.02 equiv. catalyst (12, 536 mg,
the presence of 0.02 equiv. catalyst (12, 536 mg, 0.278 mmol). 0.278 mmol). After extraction, 2k (1.02 g, 9.99 mmol, 72%) was
After extraction and further purification via column chromato- isolated as a colorless liquid.
graphy (SiO2, cHex : EtOAc = 20 : 1 to 5 : 1), carbonate 2e (1.37 g,
1H NMR (300 MHz, CDCl3): δ = 1.49 (d, J = 6.2 Hz, 3H), 4.03
(dd, J = 8.3 Hz, 7.3 Hz, 1H), 4.55 (t, J = 7.9 Hz, 1H), 4.80–4.89
8.34 mmol, 60%) was isolated as a colorless liquid.
1H NMR (300 MHz, CDCl3): δ = 4.36 (dd, J = 8.5 Hz, 7.7 Hz, (m, 1H) ppm.
1H), 4.81 (dd, J = 8.6 Hz, 8.3 Hz, 1H), 5.68 (t, J = 8.2 Hz, 1H),
7.34–7.56 (m, 5H) ppm.
4-(Chloromethyl)-1,3-dioxolan-2-one (2l).22c According to
GP5, epichlorohydrin (1l, 1.28 g, 13.9 mmol) was allowed to
4-Hexyl-1,3-dioxolan-2-one (2f).22c According to GP5, octene react with CO2 in the presence of 0.02 equiv. catalyst (12,
oxide (1f, 2.17 g, 13.9 mmol) was allowed to react with CO2 in 536 mg, 0.278 mmol). After the extraction, 2l (1.72 g,
the presence of 0.02 equiv. catalyst (12, 536 mg, 0.278 mmol). 12.6 mmol, 91%) was isolated as a colorless liquid.
After extraction and further purification via column chromato-
graphy (SiO2, cHex : EtOAc = 20 : 1 to 5 : 1), carbonate 2f (2.37 g, (dd, J = 8.9 Hz, 5.7 Hz, 1H), 4.60 (t, J = 8.2 Hz, 1H), 4.92–5.33
1H NMR (300 MHz, CDCl3): δ = 3.69–3.84 (m, 2H), 4.42
11.8 mmol, 85%) was isolated as a colorless liquid.
(m, 1H) ppm.
1H NMR (300 MHz, CDCl3): δ = 0.90 (t, J = 7.0 Hz, 3H),
4-(Iso-propoxymethyl)-1,3-dioxolan-2-one (2m).22c According
1.22–1.53 (m, 8H), 1.58–1.90 (m, 2H), 4.07 (dd, J = 8.3 Hz, to GP5, iso-propyl glycidyl ether (1m, 1.61 g, 13.9 mmol) was
7.1 Hz, 1H), 4.53 (t, J = 7.9 Hz, 1H), 4.65–4.76 (m, 1H) ppm.
allowed to react with CO2 in the presence of 0.02 equiv. catalyst
Methyl 9-(2-oxo-1,3-dioxolan-4-yl)nonanoate (2g).22c According (12, 536 mg, 0.278 mmol). After the extraction, 2m (1.00 g,
to GP5, epoxidized methyl 9-(oxiran-2-yl)nonanoate (1g, 2.99 g, 6.24 mmol, 45%) was isolated as a colorless liquid.
13.9 mmol) was allowed to react with CO2 in the presence of
1H NMR (300 MHz, CDCl3): δ = 1.17 (d, J = 6.1 Hz, 6H),
0.02 equiv. catalyst (12, 536 mg, 0.278 mmol). After extraction 3.57–3.70 (m, 3H), 4.39 (dd, J = 8.1, 6.0 Hz, 1H), 4.49 (t, J =
and further purification via column chromatography (SiO2, 8.4 Hz, 1H), 4.73–4.83 (m, 1H) ppm.
cHex : EtOAc = 20 : 1 to 5 : 1), carbonate 2g (2.72 g, 10.5 mmol,
76%) was isolated as a colorless liquid.
1H NMR (300 MHz, CDCl3): δ = 1.22–1.91 (m, 14H), 2.31
(t, J = 7.6 Hz, 2H), 3.67 (s, 3H), 4.08 (dd, J = 8.4 Hz, 7.2 Hz, 1H),
Acknowledgements
This research project is part of the Leibniz ScienceCampus
Phosphorus Research Rostock and is co-funded by the funding
line strategic networks of the Leibniz Association. We thank
Ursula Bentrup, Christine Rautenberg and Andrea Bellmann
for their support.
4.53 (dd, J = 8.2 Hz, 7.8 Hz, 1H), 4.64–4.74 (m, 1H) ppm.
4-(But-3-en-1-yl)-1,3-dioxolan-2-one (2h).22c According to
GP5, 1,2-epoxy-5-hexene (1h, 1.36 g, 13.9 mmol) was allowed to
react with CO2 in the presence of 0.02 equiv. catalyst (12,
536 mg, 0.278 mmol). After the extraction, 2h (1.90 g,
13.4 mmol, 96%) was isolated as a colorless liquid.
1H NMR (300 MHz, CDCl3): δ = 1.70–1.87 (m, 1H), 1.87–2.04
(m, 1H), 2.11–2.36 (m, 2H), 4.09 (dd, J = 8.5 Hz, 7.3 Hz, 1H),
4.54 (dd, J = 8.4 Hz, 7.9 Hz, 1H), 4.68–4.79 (m, 1H), 5.02–5.16
(m, 2H), 5.71–5.88 (m, 1H) ppm.
Notes and references
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