Polystyrene-supported amino acids as efficient catalyst for chemical fixation of carbon dioxide

Article abstract of DOI:10.1002/adsc.201000261

Four new polystyrene-supported amino acids have been synthesized and applied to the chemical fixation of carbon dioxide for the first time. Two series of experiments with polystyrene-supported threonine (PS-Thr) and polystyrene-supported tyrosine (PS-Tyr) as catalyst, respectively, were conducted to study the effect of the reaction conditions on the carboxylation of propylene oxide/carbon dioxide. There was no considerable decrease in the yield of propylene carbonate after the polystyrene-supported amino acids were used five times, indicating that these catalysts are very stable. It was demonstrated that these catalysts were very efficient in the carboxylation of various epoxides and aziridines with carbon dioxide under mild conditions without any solvents. The mechanism for this carboxylation is also discussed.

Full text of DOI:10.1002/adsc.201000261

FULL PAPERS
DOI: 10.1002/adsc.201000261
Polystyrene-Supported Amino Acids as Efficient Catalyst for
Chemical Fixation of Carbon Dioxide
Chaorong Qi,a Jinwu Ye,a Wei Zeng,a and Huanfeng Jianga,*
a
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, Peopleꢀs
Republic of China
Fax : (+86)-20-8711-2906; e-mail: jianghf@scut.edu.cn
Received: April 5, 2010; Revised: June 27, 2010; Published online: August 16, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000261.
Abstract: Four new polystyrene-supported amino ported amino acids were used five times, indicating
acids have been synthesized and applied to the that these catalysts are very stable. It was demon-
chemical fixation of carbon dioxide for the first time. strated that these catalysts were very efficient in the
Two series of experiments with polystyrene-support- carboxylation of various epoxides and aziridines with
ed threonine (PS-Thr) and polystyrene-supported ty- carbon dioxide under mild conditions without any
rosine (PS-Tyr) as catalyst, respectively, were con- solvents. The mechanism for this carboxylation is
ducted to study the effect of the reaction conditions also discussed.
on the carboxylation of propylene oxide/carbon diox-
ide. There was no considerable decrease in the yield Keywords: amino acids; aziridines; carbon dioxide;
of propylene carbonate after the polystyrene-sup- cycloaddition reation; epoxides; polystyrene
Introduction
The cycloaddition of CO2 with epoxides or aziri-
dines is known to be catalyzed by alkali metal salts,[5]
Due to human industrial activities, the fossil fuel organic bases,[6] metal oxides,[7] transition metal com-
source is limited and the production of carbon dioxide plexes,[8] quaternary ammonium and phosphonium
is increasing which causes a variety of environmental salts,[9] ionic liquids,[10] and dual catalysts.[11] Recently,
problems such as global warming and acid rain. many functional polymers[10d,12] have been used as cat-
Therefore, there is an urgent need to develop new alysts or catalyst supports for this transformation due
chemical processes using renewable resources and to to their attractiveness for technological applications
reduce carbon dioxide emissions. The transformation and manufacturing processes with simplified product
of carbon dioxide into industrially valuable chemicals recovery. The use of polymers instead of homogene-
is generally regarded as an excellent route in this re- ous catalysts in the insertion of CO2 will lead to envi-
spect because of its significant values in both environ- ronmentally more benign processes for the manufac-
mental preservation and resource utilization.[1] In this ture of chemicals using CO2 as a raw material. Di-
respect, a great deal of work, in many different fields, vinylbenzene (DVB) cross-linked polystyrene is one
has been undertaken to produce organic carbonates of the most useful polymers owing to its compatibility
using CO2 as the feedstock.[1a,2] In particular, one with a wide range of reaction conditions.[13] Further-
promising methodology in the chemical fixation of more, because of its easy preparation, recyclability,
CO2 is to efficiently convert CO2 to five-membered environmental stability, low cost, and non-solubility in
heterocycles such as oxazolidinones or cyclic carbo- any commonly used organic solvent and water, DVB
nates. The resulting cyclic carbonates are excellent cross-linked polystyrene has attracted much attention
aprotic solvents, pharmaceutical and fine chemical in- as an efficient heterogeneous catalysts support.[12a,14]
termediates, precursors for polycarbonate materials,
Although the addition of CO2 with epoxides or
intermediates in organic synthesis,[3] and oxazolidi- aziridines to produce five-membered heterocycles has
nones are also important heterocyclic compounds been studied extensively, there is continuing motiva-
showing a large scope of application as intermediates tion for developing efficient catalysts for chemical fix-
and chiral auxiliaries in organic synthesis.[4]
ation of CO2 which would ideally carboxylate both
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Chaorong Qi et al.
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epoxides and aziridines. Our studies showed that a- Results and Discussion
amino acids, which are commonly found in nature
and hence relatively inexpensive, are highly efficient Divinylbenzene cross-linked polystyrene was func-
catalysts for the coupling of epoxides and carbon di- tionalized with four different a-amino acids using a
oxide under mild conditions.[11a] However, there are modification of a procedure in the literature.[15] The
two existing problems that need to be solved. One is preparation of PS-Tyr is shown in Scheme 1. The
how to avoid the use of solvents under mild condi- polymer obtained was characterized by using IR,
tions, and the other is how to separate the catalyst SEM, TGA/DSC and elemental analysis.
from the product without decomposition of the cata-
The organic composition of the four polymers (C,
lyst or formation of by-products. Considering the ad- H, N and Cl) was determined by elemental analysis
vantages of using DVB cross-linked polystyrene as (EA) (see Table S1 in the Supporting Information).
catalyst support, we envisioned that functionalized The effect of immobilization was explicitly evident
polystyrene, with a-amino acid as a catalytically from the elemental analysis. The increase of the
active species being covalently grafted onto polystyr- amount of nitrogen, which may correspond to the
ene, could be utilized as an active and recyclable het- amino group of the amino acids, indicated that the
erogeneous catalyst for the chemical fixation of CO2. amino acid is present in the polystyrene resin. More-
Herein, we have developed four new polystyrene-sup- over, the amount of chlorine changed noticeably after
ported amino acids catalysts: polystyrene-supported the polymer had been functionalized. This is probably
threonine (PS-Thr), polystyrene-supported tyrosine due to the fact that the OH group of a-amino acids
(PS-Tyr), polystyrene-supported hydroproline (PS- reacts with the Cl of the DVB cross-linked chlorome-
À
HPro), and polystyrene-supported serine (PS-Ser) thylated polystyrene, forming C O bonds and elimi-
(Figure 1). During the course of this work, we discov- nating HCl during covalent anchoring to the polystyr-
ered that these catalysts have high chemical and ther- ene resin. The loading of the catalysts on each sup-
mal stability leading to high catalytic activity for the port (2.39–2.62 mmol/g) was estimated by the nitro-
carboxylation of both epoxide and aziridine sub- gen content of each polymer.
strates, and also could be easily separated from the
FT-IR spectroscopy provided clear evidence for
product and reused. To the best of our knowledge, amino acid incorporation and organo-functionaliza-
this is the first work to combine these two kinds of tion (Figure S1 in the Supporting Information). The
compounds for the chemical fixation of CO2. We be- IR peaks at 3400 cmÀ1 are probably due to the N H
À
lieve that these catalysts could have great potential in stretching mode of the functionalized amino acids.
industrial application, because they have some unusu-
al advantages, such as their easy preparation, low
cost, high activity and selectivity, stability, recyclabili-
ty.
Figure 1. Structures of the four cross-linked-polystyrene-sup-
ported amino aicds.
Scheme 1. Synthetic route to cross-linked-polystyrene-sup-
ported l-tyrosine.
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Polystyrene-Supported Amino Acids as Efficient Catalyst for Chemical Fixation
Table 1. Comparison of catalytic activities of polymer-sup-
And, in principle, they can also correspond to the
overtones of the bands in the range 1560–1640 cmÀ1.
Furthermore, evidence for the presence of amino
acids was obtained from the sharp, characteristic
peaks at 1600 cmÀ1 which corresponds to the C=O
group of amino acids. It should be noted that these
peaks are absent in non-functionalized polystyrene
resin. In addition, the bands at 1262 cmÀ1 and
ported amino acids with those of small molecular amino
acids.[a]
671 cmÀ1 which correspond to C Cl group become
À
Entry
Catalyst
Yield[b] [%]
weak in the polystyrene-supported amino acids owing
to the elimination of HCl in the process.
1
0
0
The morphology of PS-amino acids was observed
using scanning electron microscopy (SEM: see Fig-
ure S2 and Figure S3 in the Supporting Information).
The figure shows that the microscopic appearance of
the polymer changed obviously after it had been fixed
with amino acids. The surface of the functionalized
polymer became rough compared with the chlorome-
thylated polystyrene (PS-CH2Cl). Although the ag-
glomeration of the particles is irregular, the size of
the particles was on the nanometer scale both on the
surface and inside of the polystyrene-supported
amino acids.
2
PS-CH2Cl
l-tryrosine
l-threonine
l-serine
l-hydroproline
PS-Tyr
PS-Thr
PS-Ser
PS-HPro
trace
36
34
14
31
93
96
91
95
3[c]
4[c]
5[c]
6[c]
7
8
9
10
[a]
Reaction conditions: propylene oxide 20 mmol, CO2 pres-
sure 9 MPa, catalyst 0.6 mol%, time 24 h, temperature
1308C.
Determined by GC using tridecane as internal standard.
The catalyst was 0.8 mol% of PO, reaction time: 48 h.
[b]
[c]
For a polymer support, a certain thermal stability is
required because the polymer has to be subjected to
vigorous shaking, filtration, and drastic acid-base
treatment during the reaction. The thermal stability sumably attributed to the benefits from changes in
of our catalyst system was estimated using TAG and the physical properties[17] of the reaction mixture,
DSC (Table S2, Figure S4 and Figure S5 in the Sup- such as low viscosity and increased solubility for reac-
porting Information). It is observed that the weight tants. Besides, the special structure of the PS-amino
loss of the catalysts occurred at slightly under 2008C, acids (analyzed by SEM in the Supporting Informa-
which may correspond to the loss of physisorbed tion) would have a great influence on the catalysts ac-
water molecules in the polymer matrix.[16] The signifi- tivity. Consequently, the a-amino acids anchored on
cant weight loss that occurred in the region 370– the 2% DVB-cross-linked polystyrene can be consid-
4508C is attributed to the decomposition of the poly- ered as the active species for the reaction and the
mer.
CO2-expandable polymer improves the catalytic activ-
Structural characterization results show that amino ity. For all the experiments with different catalysts, no
acids are firmly held inside the polystyrene resin. by-product was detected.
Hence, the present polymers were applied to the
Two series of experiments with PS-Thr and PS-Tyr
chemical fixation of carbon dioxide. At first, studies as catalyst, respectively, were conducted to study the
were performed to investigate the effect of different carboxylation of PO under a variety of reaction con-
amino acids on the coupling of CO2 and propylene ditions, which included changes in catalyst loading
oxide (PO) under the standard conditions. The results (0.1–0.8 mol%), temperature (90–1408C), CO2 pres-
are summarized in Table 1. Without any catalyst, the sure (2–11 MPa) and reaction time (6–48 h).
coupling reaction of PO/CO2 did not occur at all
The catalytic activities of PS-Thr and PS-Tyr for the
(Table 1, entry 1), and PS-CH2Cl alone was found to reaction of PO/CO2 were first studied as shown in
be inactive for the reaction (Table 1, entry 2). Al- Figure 2. It is obvious that both the two catalysts were
though an a-amino acid itself can catalyze the cou- active for the carboxylation of PO with CO2. Even
pling of CO2 and PO under supercritical conditions, when the amount of PS-Thr was as low as 0.2 mol%,
their activities remain low even under conditions of the coupling reaction proceeded smoothly to give an
0.8 mol% catalyst over 48 h (Table 1, entries 3–6). In 80% yield of propylene carbonate (PC). It is notewor-
contrast, polystyrene-supported amino acids showed thy that decrease in catalyst amount reduced the reac-
superior activity without the need of any additives tion rate, but the reaction selectivity was not changed.
(Table 1, entries 7–10). It is evident that all of the Shown in Figure 3 are the activities of PS-Thr and PS-
four supported amino acids have a marked influence Tyr as a function of reaction time in the reaction of
on the reaction rate. The enhancement of catalytic CO2 and PO. This illustrates that the yield of PC in-
performance for the PS-supported amino acids is pre- creased smoothly with the reaction time, and nearly
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Figure 2. Dependence of yield of PC on the amount of PS-
Thr and PS-Tyr. Reaction conditions: 20 mmol PO, 9 MPa
CO2, 1308C, 24 h.
Figure 4. Dependence of yield of PC on the reaction temper-
ature. Reaction conditions: 20 mmol PO, 0.6 mol% PS-Thr/
PS-Tyr, 9 MPa CO2, 24 h.
Figure 3. Dependence of yield of PC on the reaction time.
Reaction conditions: 20 mmol PO, 0.6 mol% PS-Thr/PS-Tyr,
9 MPa CO2, 1308C.
Figure 5. Dependence of yield of PC on CO2 pressure. Reac-
tion conditions: 20 mmol PO, 0.6 mol% PS-Thr/PS-Tyr,
1308C, 24 h.
all the PO could be converted within 24 h in the two yield reached a maximum at about 9 MPa near the
series of experiments. critical pressure of CO2 and a further increase in pres-
The influence of temperature on the yield of PC sure to 11 MPa led to a depression in the yield. This
was investigated using PS-Thr and PS-Tyr as catalysts interesting phenomenon is almost certainly due to the
at 9 MPa CO2 with a reaction time of 24 h. As is phase behaviour in this set of experiments and is in
easily seen from Figure 4, the activity of our catalyst general agreement with a previous report.[18]
system is strongly dependent on reaction temperature.
Furthermore, the more important investigations
In the lower temperature region (90 to 1308C), the about the recyclability of PS-Thr and PS-Tyr were
total yield of PC increases sharply with increasing also performed using PO as the substrate at the opti-
temperature, while no significant change is observed mized reaction conditions. In each cycle, PS-Thr/PS-
from 130 to 1408C.
Tyr could be readily recovered by filtration and
On the basis of the above results, we continued to rinsed with acetone, respectively. After drying, the
examine the dependence of PC yield on pressure at catalyst was reused for the next run. The yields of PC
1308C with a reaction time of 24 h. As is easily seen for the five repeated runs are shown in Figure 6.
from Figure 5, pressure has a great influence on the There was no considerable decrease in the yield of
reaction rate with variations of the CO2 pressure from PC, indicating that the catalyst was very stable. The
2 to 11 MPa in the two series of experiments. The excellent stability was also supported by the results of
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Polystyrene-Supported Amino Acids as Efficient Catalyst for Chemical Fixation
Table 2. Synthesis of various cyclic carbonates under the op-
timized conditions.[a]
Entry
1
Product
Catalyst
Yield [%][b]
PS-Thr
PS-Tyr
98
93
PS-Thr
PS-Tyr
90
89
Figure 6. Reuse of PS-Thr. Reaction conditions: 20 mmol
PO, 0.6 mol% PS-Thr, 9 MPa CO2, 1308C, 24 h.
2
PS-Thr
PS-Tyr
87
88
TGA/DSC analyzed before. The decomposition of the
polymer occurred in the region 370–4508C, which is
much higher than the reaction temperature (1308C)
in our catalytic experiments.
3
4
5
6
Once having established that the polystyrene-sup-
ported amino acid was an excellent catalyst for the
coupling reaction of CO2 and PO, the capabilities of
PS-Thr and PS-Tyr as catalysts were further investi-
gated using a variety of challenging epoxide sub-
strates to synthesize the corresponding cyclic carbo-
nates. The results are summarized in Table 2. Both
PS-Thr and PS-Tyr were found to be applicable to a
variety of terminal epoxides with different substituted
groups, providing the corresponding cyclic carbonates
as the sole product in good to high yields. With PS-
Thr as a catalyst, epoxides 1a–1f gave the correspond-
ing cyclic carbonates in over 85% yields (Table 2, en-
tries 1–6). When styrene oxide (1g) was used as sub-
strate, the yield was reduced to 78%, which is proba-
bly due to the low reactivity of its b-carbon centre
(Table 2, entry 7). Besides the terminal epoxides, cy-
clohexene oxide (1h) was also used as substrate with
PS-Thr as catalyst under the same reaction conditions
(Table 2, entry 8). Although the selectivity was nearly
100%, the yield of the product was only 70%, which
may result from the high steric hindrance of cyclohex-
ene oxide.
PS-Thr
PS-Tyr
92
90
PS-Thr
PS-Tyr
99
99
PS-Thr
PS-Tyr
87
94
PS-Thr
PS-Tyr
78
91
7
Encouraged by the successful results of CO2-fixa-
tion using epoxides as substrate, we next targeted the
aziridine substrates, which are the nitrogen analogues
of epoxides. In contrast with the analogous epoxide/
CO2 coupling, the carboxylation of aziridines with
CO2 could proceed under milder conditions with PS-
Thr as catalyst. As shown in Table 3, the use of PS-
Thr gave rise to a high yield and regioselectivity for
oxazolidinones synthesis under organic solvent-free
conditions. It is worth mentioning that 5-aryl-2-oxazo-
8
PS-Thr
70
[a]
Reaction conditions: epoxide (20 mmol), catalyst
(0.6 mol%), CO2 pressure (9 MPa), 1308C, 24 h.
Yields were determined by GC using tridecane as inter-
nal standard.
[b]
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Table 3. Oxazolidinones synthesis under solvent-free condi-
Table 3. (Continued)
tions.[a]
Entry Substrate
Conversion[b]/ Molar
yield[c] [%]
ratio of
4:5[d]
10
11
>99/95
99:1
99:1
100:0
Entry Substrate
Conversion[b]/ Molar
yield[c] [%]
ratio of
4:5[d]
>99/96
>99/90
1
>99/61
85:15
95:5
12
2
>99/80
>99/98
>99/93
>99/96
53/50
[a]
Reaction conditions: substrates (2 mmol), PS-Thr (5.0 mg,
0.012 mmol), 24 h, CO2 8 MPa, 1008C.
Determined by GC.
[b]
[c]
[d]
[e]
The total yield of 4 and 5.
Determined by H NMR.
The reaction time is 48 h.
1
3
98:2
lidinones were preferentially formed with high regio-
selectivity with a variety of aziridines under identical
reaction conditions. Especially, 2-phenylaziridine (3a)
and 1-methyl-2-phenylaziridine (3b) afforded relative-
ly low yields of the desired products because self-olig-
omers were formed during the reaction (Table 3, en-
tries 1 and 2). Aziridines (3c–3e and 3g–3i) bearing
alkyl groups or cyclohexyl at the nitrogen atom af-
forded the corresponding oxazolidinones in good to
high yields (Table 3, entries 3–5 and 7–9), and 5-sub-
stituted oxazolidinones were the major isomer. Un-
fortunately, substrate 3f showed low activity which
may result from the high steric hindrance of the nitro-
gen atom, however, it gave 3-tert-butyl-5-phenyloxa-
zolidin-2-one (4f) as the sole product (Table 3,
entry 6). It can be suggested from the above results
that the product regioselectivity mainly depends on
the substituent group at the nitrogen atom; increasing
steric hindrance of N-substituted group R leads to an
enhancement of the regioselectivity of the product. In
this study, the regioselectivity can be also enhanced
from 85:15 to 100:0 with variation of the alkyl sub-
stituent at the nitrogen atom. Table 3 also shows that
aziridines with either an electron-donating group or
an electron-withdrawing group on the C-1-aryl group
could react with CO2 and give oxazolidinones in high
yield and regioselectivity (Table 3, entries 10–12).
As for the catalytic mechanism of the carboxylation
of both epoxides and aziridines, we speculated a uni-
fying mechanism containing hydrogen binding based
4
90:10
95:5
5
6[e]
100:0
97:3
7
>99/96
>99/94
>99/97
8
99:1
9
100:0
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Polystyrene-Supported Amino Acids as Efficient Catalyst for Chemical Fixation
Scheme 2. Plausible reaction mechanism.
on the special structure of polystyrene-supported lectivity, stability, low cost and simple work-up proce-
amino acids and the experimental results. The mecha- dure.
nism shown in Scheme 2 involves the activation of the
ring of the epoxide or aziridine by an amino acid
through hydrogen bonding, followed by the nucleo-
philic ring-opening of the substrate by another amino
Experimental Section
acid, and subsequent insertion of CO2 to form the
final product via an intramolecular cyclization and re-
Carbon dioxide with a purity of 99.99% was commercially
available. Dichloromethane, triethylamine and N-methyl-
generate the catalyst. Likewise, the nucleophilic prrolidone (NMP) were distilled from calcium hydride.
Compounds 3a–3k were produced according to the reported
literatures.[20] All the other chemicals were purchased from
Aldrich Chemicals and used as received. 1H NMR
(400 MHz) and 13C NMR (100 MHz) spectra were recorded
attack would not occur by the carboxylate ion of the
same amino acid which activated the substrate ring
through hydrogen bonding according to the Baldwinꢀs
rule for cyclization.[19]
using
a Bruker Avance 400 MHz NMR spectrometer
(100 MHz for carbon) and respectively referenced to 7.27
and 77.0 ppm for chloroform-d solvent with TMS as internal
standard. Mass spectra were recorded on a Shimadzu
GCMS-QP5050A at an ionization voltage of 70 eV
equipped with a DB-WAX capillary column (internal diam-
Conclusions
In summary, we have developed an efficient, simpler
and environmentally friendly process for the carboxy- eter=0.25 mm, length=30 m). IR spectra were obtained as
potassium bromide pellets or as liquid films between two
potassium bromide pellets with a Brucker Vector 22 spec-
trometer. Elemental analysis was performed on a Vario EL
elemental analyzer. Analytical thin-layer chromatography
(TLC) was performed using commercially prepared 100–400
mesh silica gel plates (GF254), and visualization was effect-
ed at 254 nm. GC analyses were performed on a Shimadzu
GC-2014, equipped with a capillary column (RTX-5, 30 mꢂ
0.25 mm) using a flame ionization detector. TGA-DSC was
performed on a TAINC-Q600SDT system at a heating rate
lation of various epoxides and aziridines with high
conversion and selectivity by using polystyrene-sup-
ported amino acids as solid catalyst without any sol-
vents or additives. This catalyst system is easy to pre-
pare and can be easily separated from the products
and reused. We believe that this route to synthesize
the five-membered heterocycles will have great po-
tential in industrial applications, because the polystyr-
ene-supported amino acids have some unusual advan-
tages, such as their easy preparation, high activity, se- of 108CminÀ1. SEM was performed on QUANTA400F.
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Chaorong Qi et al.
FULL PAPERS
nal standard. The crude product was purified by distillation
and cyclic carbonates were obtained and identified by IR,
GC-MS and H NMR.
General Procedure for the Synthesis of Polystyrene-
Supported Amino Acids (PS-Tyr)
1
To a suspension of l-Tyr (5 g, 27.6 mmol) in 100 mL NMP,
chlorotrimethylsilane (2.98 g, 27.6 mmol) was added at room
temperature. A clear solution was obtained after 2 h of stir-
ring at room temperature. To this homogeneous solution
2 equiv. of NaH (1.325 g, 55.2 mmol) were added portion-
wise at 08C for 15 min. 2% DVB cross-linked chloromethy-
lated polystyrene (ca. 5 mmol Cl/g) (2.5 g) was added at
once and whole mixture was stirred for 3 days at room tem-
perature. Then the mixture was heated to 1208C with stir-
ring for 1 day. After cooling, the polymer was treated with
1N HCl aqueous solution to complete removal of trimethyl-
silyl groups, followed by neutralization with aqueous ammo-
nia. The polymer obtained on a glass filter was washed with
MeOH (100 mL), THF (100 mL), THF-H2O (100 mL) and
CHCl3 (100 mL), respectively. After drying under vacuum at
508C for 24 h, white beads of polystyrene-supported tyro-
sine were obtained; yield: 4.2 g.
General Procedure for the Carboxylation of
Aziridines with CO2
Aziridine (2 mmol) and PS-Thr (5 mg, 0.012 mmol) were
added to a 15-mL stainless autoclave reactor with a magnet-
ic stirrer. CO2 was introduced into the autoclave and then
the pressure was generally adjusted to 8 MPa at 1108C. The
mixture was stirred continuously at 1108C for 24 h, and the
pressure was kept constant during the reaction. After the re-
action, the reactor was cooled in ice-water and extra CO2
was vented slowly. An aliquot of sample was taken from the
resultant mixture and dissolved in CH2Cl2 for GC analysis.
The residue was purified by flash silica gel chromatography
using petroleum ether:ethyl acetate=10:1 to 1:1 as eluant to
afford the product. The products were further identified by
1H NMR, 13C NMR and MS.
General Procedure for the Synthesis of Aziridines
(1) Bromodimethylsulfonium bromide SA: Dimethyl sulfide
(6.2 g, 0.1 mol) and bromine (16.0 g, 0.1 mol) were separate-
ly dissolved in dry dichloromethane (20 mL and 20 mL, re-
spectively). The bromine solution was then added dropwise
over 30 min to the ice-cooled solution of dimethyl sulfide.
During the addition, light orange crystals of bromodimethyl-
sulfonium bromide began to separate. After the addition of
bromide was completed, the crystals of SA were collected
by filtration and then washed with dry ether and dried
under vacuum; yield: 70%.
Acknowledgements
We thank the National Natural Science Foundation of China
(Nos. 20625205, 20772034 and 20932002), the National Basic
Research Program of China (2010CB732206), Doctoral Fund
of Ministry of Education of China (20090172110014) and
Guangdong
Natural
Science
Foundation
(No.
8451064101000236) for financial support.
(2) Styrenesulphonium bromide SB: The olefin (70 mmol)
was added dropwise to 70 mL CH3CN solution of compound
SA (31.2 g, 70 mmol) in an ice-water bath. During the addi-
tion, a white solid began to separate. The solution was
stirred for 10 min after the addition of olefin was completed.
The crystals of SB were collected by filtration and dried
under vacuum; yield: 28%.
(3) Synthesis of aziridines: To a stirred solution of styre-
nesulphonium bromide SB (10 mmol) in 20 mL of water at
room temperature, a solution of amine (20–50 mmol) in
water was added dropwise and the resulting mixture was
stirred overnight. The mixture was poured into 20 mL of sa-
turated brine, extracted with diethyl ether (3ꢂ20 mL), dried
with MgSO4 and the solvent evaporated under reduced pres-
sure. The pure product was obtained by distillation under
reduced pressure; yield: 80–100%.
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General Procedure for the Carboxylation of Epoxides
with CO2
Epoxide (20 mmol) and PS-Thr (50 mg, 0.12 mmol) were
added to a 15-mL stainless autoclave reactor with a magnet-
ic stirrer. CO2 was introduced into the autoclave and then
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1932
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