Organocatalysis in polysiloxane gels: A magnetic-stir-bar encapsulated catalyst system prepared by thiol-ene photo-click immobilization

Article abstract of DOI:10.1039/c4ra16351f

This manuscript presents a facile thiol-ene photo-click chemistry method to prepare magnetic stir bar-encapsulated polysiloxane-based organocatalyst gels under benign conditions, and develops a Stir Bar-Encapsulated Catalysis (SBEC) technique. Through thiol-ene addition chemistry, we graft olefin-terminated organocatalysts (i.e. MacMillan catalyst, proline catalyst, and N-heterocyclic carbene catalyst) onto poly[3-mercaptopropylmethylsiloxane], which is further photo-crosslinked to coat the embedded magnetic stir bar. The prepared magnetic stir bar-encapsulated polysiloxane-based organocatalyst gels can be put into reaction flasks to perform stirring and catalysis functions at the same time. The most important benefit of SBEC technique is to infinitely simplify the catalyst/product separation procedure by using a simple stir-bar-retriever, even without any precipitation/filtration steps. The catalytic performances of three different organocatalyst gels applied in asymmetric Diels-Alder reaction, asymmetric aldol reaction and benzoin condensation reaction respectively are also examined herein.

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PAPER
Organocatalysis in Polysiloxane Gels: A Magnetic-Stir-Bar
Encapsulated Catalyst System Prepared by Thiol-Ene Photo-Click
Immobilization
Hong Yang,*a Ming Xu,a Ling-Xiang Guo,a Hao-Fan Ji,a Jun-Yu Wang,a Bao-Ping Lin,*a Xue-Qin
5
Zhang,a and Ying Suna
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
This manuscript presents a facile thiolꢀene photoꢀclick chemistry method to prepare magnetic stir barꢀ
encapsulated polysiloxaneꢀbased organocatalyst gels under benign conditions, meanwhile develops a Stir
10 BarꢀEncapsulated Catalysis (SBEC) technique. Through thiolꢀene addition chemistry, we graft olefinꢀ
terminated organocatalysts (i.e. MacMillan catalyst, Proline catalyst, and Nꢀheterocyclic carbene catalyst)
on poly[3ꢀmercaptopropylmethylsiloxane], which is further photoꢀcrosslinked to coat the embedded
magnetic stir bar. The prepared magnetic stir barꢀencapsulated polysiloxaneꢀbased organocatalyst gels
can be put into reaction flasks to perform stirring and catalysis functions at the same time. The most
15 important benefit of SBEC technique is to infinitely simplify the catalyst/product separation procedure to
using a simple stirꢀbarꢀretriever, even without any precipitation/filtration steps. The catalytic
performances of three different organocatalyst gels applied in asymmetric DielsꢀAlder reaction,
asymmetric aldol reaction and benzoin condensation reaction respectively are also examined herein.
variable and unpredictable so that the catalyst grafting ratio is
beyond control; 2) nobel metal, platinum is very expensive and
meanwhile is difficult to be removed from the products, which
might complicate the following catalytic applications; 3) the
50 preparation procedure usually requires long reaction time, plenty
of solvents and a high reaction temperature.
Introduction
20 Heterogeneous catalysts, prepared by immobilizing catalysts on
surfaces of inorganic materials or functionalized organic
polymers,1ꢀ3 offer several engineering advantages such as easy
separation, high stability and facile catalyst recycling, thus play
an important role in “Green Chemistry” processes.4ꢀ6 Among
25 various support media, crossꢀlinked polysiloxane gels are very
attractive carrier materials and have risen up global interests over
the last decade for their wealth of advantageous properties such
as good chemical and thermal stabilities, superhydrophobicity,
highly flexible SiꢀOꢀSi bonds, as well as excellent permeability
30 which allows organic molecules to go through siloxane matrixes
with very fast diffusion velocities.7
To support catalysts on polysiloxane gels, conventional
noncovalent immobilization methods8 are to occlude catalysts
such as Grubbs’ catalysts,9,10 BINAPꢀRu,11,12 SalenꢀMn13 and
35 DuPHOSꢀRh,14 into polydimethylsiloxane (PDMS) films or slabs.
Although this strategy is very convenient and efficient, catalyst
leaching even in aqueous solution is an inevitable and serious
problem.14 In order to overcome this defect, covalent
immobilization method provides another solution by chemically
40 linking the catalysts onto the polysiloxane matrixes. Previously
Fig. 1 Preparation methods and molecular structures of polysiloxaneꢀgelꢀ
based organocatalysts herein this manuscript.
reported protocols always relied on
a platinumꢀcatalyzed
55
Herein, we present a facile thiolꢀene photoꢀclick chemistry
method18ꢀ20 to prepare polysiloxaneꢀgelꢀbased organocatalysts
under benign conditions. As shown in Figure 1, we use instead of
PMHS, poly[3ꢀmercaptopropylmethylsiloxane] (PMMS)21,22
which bears one mercapto group in every monomer unit, and
hydrosilylation reaction of polymethylhydrosiloxane (PMHS) and
olefinꢀterminated catalysts or ligands to build polysiloxaneꢀbased
catalysts.15ꢀ17 However, this approach also has several obvious
45 disadvantages: 1) Hydrosilylation reaction efficiency is quite
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graft olefinꢀterminated organocatalysts (MacMillan catalyst C1, 15 technique,23 we use organocatalystꢀimmobilized PMMS gel
Proline catalyst C2, and Nꢀheterocyclic carbene (NHC) catalyst
C3) onto PMMS chain. Meanwhile, by mixing the above systems
instead of PDMS, to coat magnetic stir bar, and develop a Stir
BarꢀEncapsulated Catalysis (SBEC) technique. As shown in
Figure 2, a plastic vial containing a magnetic stir bar and the
mixture of PMMS, organocatalyst C1, photoinitiator DMPA and
with
a photoꢀinitiator (2,2ꢀdimethoxyꢀ2ꢀphenylacetophenone,
5
DMPA) and a variety of olefinꢀfunctional crosslinkers, a series of
organocatalystꢀimmobilized polysiloxane gels can be synthesized 20 crosslinker L3, was UV illuminated for 20 minutes (Fig. 2AꢀC) to
by UVꢀinitiated thiolꢀene click chemistry. Compared with
traditional hydrosilylation procedure, this thiolꢀene photoꢀclick
protocol, as a greener and cleaner approach, has an almost 100%
form a crossꢀlinked gel (Fig. 2D). After breaking up the plastic
vial, the prepared magnetic stir barꢀencapsulated polysiloxaneꢀ
based organocatalyst gel (Fig. 2E) could be put into a reaction
flask to perform stirring and catalysis functions at the same time
10 reaction conversion; uses cheap photoꢀinitiators as catalysts,
which are much easier to be removed; and requires very mild 25 (Fig. 2F). The intrinsic motivation and the most important benefit
reaction conditions such as minuteꢀscale reaction time, solventꢀ
less environmentꢀfriendly process and ambient temperature, etc.
Furthermore, inspired by Stir BarꢀSorptive Extraction (SBSE)
of this approach are to infinitely simplify the catalyst/product
separation procedure to using a simple stirꢀbarꢀretriever (Fig. 2G),
even without any precipitation/filtration steps.
30
Fig. 2 Preparation protocol of magnetic stir barꢀencapsulated polysiloxaneꢀbased organocatalyst gels: (A) A magnetic stir bar and plastic pipetteꢀhead
vials. (B) The plastic vial was filled with a magnetic stir bar and the mixture of PMMS, organocatalyst C1, DMPA and crosslinker L3. The oily mixture
was UVꢀilluminated (C) and became a crosslinked gel (D), which was cut out off the vial (E). (F) The obtained organocatalyst gel was performing both
stirring and catalysis functions (S1.avi). (G) A stirꢀbarꢀretriever was used to separate the catalyst from products (S2.avi).
chromatography (HPLC) with chiral columns. A UV lamp (20
55 mW·cmꢀ2, λ = 365 nm; LPꢀ40A; LUYOR Corporation) was used
to irradiate the samples to perform the photoꢀcrosslinking
reactions.
Syntheses of organocatalyst monomers C1, C2, C3. All the
synthetic procedures and 1H NMR spectra are listed in the
Experimental Section
35 Materials and Instrumentation
Poly[3ꢀmercaptopropylmethylsiloxane] (PMMS, SMSꢀ992, M.W.
4000~7000, 95 cst) was purchased from Gelest Inc. Poly(ethylene
glycol) diacrylate (average Mn ~ 700) was purchased from 60 supporting information.
Aldrich Inc. 2,2ꢀDimethoxyꢀ2ꢀphenylacetophenone (DMPA), (s)ꢀ
40 phenylalanine methylester hydrochloride, allylamine, Transꢀ4ꢀ
Typical preparation procedure of stir bar-encapsulated
PMMS-g-organocatalyst gels. In a 10 mL glass vial, PMMS
(400 mg, 3.0 mmol based on ꢀSH, 1.0 equiv.), poly(ethylene
glycol) diacrylate L3 (158 mg, 0.225 mmol, 0.075 equiv.),
hydroxyꢀLꢀproline,
undecꢀ10ꢀenoyl
chloride,
4,5ꢀ
diphenylimidazole and 11ꢀbromoꢀ1ꢀundecene were purchased
from Aladdin (Shanghai) Inc. Dichloromethane, toluene and 65 DMPA (15 mg), and a solution of catalyst C1 (0.622 g, 2.55
DMF were distilled from CaH2 under argon. THF was distilled
45 from sodiumꢀbenzophenone ketyl under argon. Other chemical
reagents were used without further purification. All nonꢀaqueous
reactions were conducted in ovenꢀdried glassware, under a dry
mmol, 0.85 equiv.) in 0.2 mL CH2Cl2 were mixed well by
centrifuge. A plastic pipette was cut off the tip, the remaining
pipette head was charged with a magnetic stir bar and the above
mixed solution. The pipette vial was then UV illuminated at r.t.
nitrogen atmosphere. All flash chromatography was performed 70 for 20 minutes. After carefully cutting off the plastic vial by a
using MachereyꢀNagel MN Kieselgel 60 (0.063ꢀ1.2 mm).
All H NMR spectra were obtained using a Bruker HW500
MHz spectrometer (AVANCE AVꢀ500) and recorded in CDCl3
(internal reference 7.26 ppm). The enantiomeric excess (ee)
scissor, the magnetic stir barꢀencapsulated polysiloxaneꢀbased
organocatalyst gel PMMS-g-C1L3 was prepared. The
crosslinked gel was immersed and swelled in CH2Cl2 several
times to wash out the unreacted small molecules, and then stored
1
50
values were analyzed by Waters 1525 Highꢀperformance liquid 75 in a 20 mL black glass vial with a screw cap for future uses.
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Typical synthetic procedure of asymmetric Diels-Alder
reaction. In a 50 mL roundꢀbottom flask, freshly distilled
monomers including MacMillan catalyst C1, Proline catalyst C2,
and NHC catalyst C3 are shown in Scheme 1. The esterꢀamide
cinnamic aldehyde (0.66 g, 5.0 mmol), CH3CNꢀH2O mixture 60 exchange of (S)ꢀphenylalanine methyl ester hydrochloride with
(95:5, 10 mL), stir barꢀencapsulated polysiloxaneꢀbased
organocatalyst gel PMMS-g-C1L3 (estimated as 50 mol%, if all
the grafted catalysts could be reached) and CF3COOH (0.29 g,
2.5 mmol) were added. To the above solution freshly distilled
allyl amine, followed by condensation reaction with acetone gave
the imidazolinone catalyst C1.24 Proline catalyst C2 was prepared
by a selective Oꢀacylation of transꢀ4ꢀhydroxyꢀLꢀproline 3 in
trifluoroacetic acid.25 Starting from 4,5ꢀdiphenylimidazole 4,
5
cyclopentadiene (1.65 g, 25.0 mmol) were then added. The 65 NHC catalyst C3 was synthesized in two steps by alkylation with
reaction mixture was stirred at 0 ℃ for 24 hrs. The solution was
11ꢀbromoꢀ1ꢀundecene and further quaterisation treatment with
iodomethane.26 The detailed experimental procedures and 1H
NMR spectra are listed in the supporting information.
10 extracted by ethylacetate (3 X 50 mL). The catalyst gel PMMS-
g-C1L3 was removed by a stir bar retriever and immerseꢀwashed
by CH2Cl2 several times, stored for future uses. The resulting
organic layer was washed by brine (2 X 40 mL), dried over
MgSO4 and was further concentrated under vacuum to provide a
15 yellow oil. The crude product was further converted into the
corresponding alcohol by reduction with an excess NaBH4 in
CH3OH at 24°C for 1 hr. The endo/exo ratios were determined by
crude NMR, and enantiomeric excess (ee) values were analyzed
by chiral HPLC with Daciel Chiralcel OJꢀH column (eluent:
20 Hexane/isopropanol 7/3; 0.8 mL/min, λ = 225 nm).
Typical synthetic procedure of asymmetric aldol reaction.
In a 50 mL roundꢀbottom flask, 4ꢀnitrobenzaldehyde (0.50 g, 3.29
mmol), cyclohexanone (2.23 g, 23.0 mmol), H2O (10 mL) and
stir barꢀencapsulated polysiloxaneꢀbased organocatalyst gel
25 PMMS-g-C2L3 (estimated as 77 mol%, if all the grafted
catalysts could be used) were added. The reaction mixture was
stirred at 50 ℃ for 48 hrs. The solution was extracted by
ethylacetate (3 X 50 mL). The catalyst gel PMMS-g-C2L3 was
removed by a stir bar retriever and immerseꢀwashed by CH2Cl2
30 several times, stored for future uses. The resulting organic layer
was washed by brine (2 X 40 mL), dried over MgSO4 and was
further concentrated under vacuum to provide a yellow oil, which
was purified by flash column chromatography (10:1 petroleum
ether ꢀ ethylacetate) to give the desired product as a yellow solid.
35 The anti/syn ratios and enantiomeric excess (ee) values were
analyzed by chiral HPLC with Daciel Chiralpak ADꢀH column
(eluent: isohexane/isopropanol 9/1; 1.0 mL/min, λ = 254 nm).
Typical synthetic procedure of benzoin condensation
reaction. In a 50 mL roundꢀbottom flask, benzaldehyde (0.78 g,
40 7.4 mmol), DMSO (10 mL), DBU (0.168 g, 1.1 mmol) and stir
barꢀencapsulated polysiloxaneꢀbased organocatalyst gel PMMS-
g-C3L3 (estimated as 34 mol%, if all the grafted catalysts could
be used) were added. Under nitrogen atmosphere, the reaction
mixture was stirred at 25 ℃ for 48 hrs. The solution was
45 extracted by ethylacetate (3 X 50 mL). The catalyst gel PMMS-
g-C2L3 was removed by a stir bar retriever, regenerated by a
solution of 4.0 M HCl in 1,4ꢀdioxane, immerseꢀwashed by
CH2Cl2 several times, and stored for future uses. The resulting
organic layer was washed by brine (2 X 40 mL), dried over
50 MgSO4 and was further concentrated under vacuum to provide a
crude oil, which was purified by flash column chromatography
(10:1 petroleum ether ꢀ ethylacetate) to give the desired benzoin
70
Scheme 1 Syntheses of olefinꢀterminated MacMillan catalyst, Proline
catalyst, and NHC catalyst.
For preparing magnetic stir barꢀencapsulated polysiloxaneꢀ
based organocatalyst gels, efficient crosslinkage based on the
design of crosslinker and crosslinking ratio plays a crucial role in
75 building a stable polymeric network. Based on our previous
experiments, commercial PMMS are short oligomers with an
estimated degree of polymerization (D.P.) around 30.27,28 Thus, in
order to form a stable crossꢀlinked PMMS gel, the molar
percentage of the crosslinking sites should be at least higher than
80 7ꢀ8 mol% and herein we set 15 mol% as a constant crosslinking
ratio for all the experiments. Three crosslinkers, triallyl cyanurate
(L1, TAC), 1,6ꢀhexanediol diacrylate (L2), poly(ethylene glycol)
diacrylate (L3, average Mn ~ 700) were tested in the experiments
respectively. In comparison, although all three crosslinkers could
85 be successfully used to synthesize polysiloxane gels, the gels
containing a much longer and flexible crosslinker, poly(ethylene
glycol) diacrylate are more elastic and stable than other brittle
gels prepared by triallyl cyanurate or 1,6ꢀhexanediol diacrylate
crosslinkers.
90
As shown in Figure 2, before UV illumination, we first
dissolved an organocatalyst into a small amount of methylene
chloride which was then mixed with PMMS, photoinitiator and
crosslinker to form an oily liquid. The mixture was then poured
into a vial containing a magnetic stir bar. Herein, we chose a soft
95 plastic container (PE pipette head, Fig. 2A) in stead of glass vials,
because compared with scissorꢀcut plastic pieces, shattered glass
would easily damage the prepared gels in the last step. After UV
illumination, the oily liquid became a crosslinked gel (Fig. 2D),
which was cut out of the plastic vial and immersed in dry
100 methylene chloride several times to wash out unreacted small
molecules. The desired magnetic stir barꢀencapsulated
1
product (490 mg, Yield: 63%) as a white solid. H NMR (500
MHz, CDCl3): δ 7.91 (m, 2H), 7.51 (m, 1H), 7.39 (m, 2H), 7.30
55 (m, 5H), 5.95 (s, 1H).
Results and Discussion
The synthetic protocols of olefinꢀterminated organocatalyst
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polysiloxaneꢀbased organocatalyst gel was prepared. However,
to protonate the supported organocatalyst. Traditionally, HBF4
our prototype manufacturing system has two technique problems: 40 has been proven to be a very efficient acid in this reaction
1) magnetic stir bars are randomly embedded in PMMS gels and
we can not precisely arrange the locations and postures of stir
bars placed in the gels. Thus, the prepared organocatalyst gels
will have physically vulnerable points where the stir bars touch
system,24 however in our case, this choice provided negative
results (Entry 1), possibly due to the strong lewis acidity and Fꢀ
ion of HBF4 which might be able to destroy CꢀSꢀC and SiꢀO
bonds. The alternative use of trifluoroacetic acid in
5
on the walls of plastic container, and this imperfectness results in 45 acetonitrile/water (95/5) solvent provided an optimal 88% yield
a moderate stirring effect (see the stirring movie, SI1.avi). 2) The
organocatalyst gels are partially crosslinked and would be better
10 to be stored in organic solvents to maintain elasticity. For
with moderate endo (96%) and exo (78%) ee values (Entry 3).
The recovered PMMS-g-C1L3 gel was recycled five times to test
the catalytic performances. As can be seen from the reported data
(Entry 4ꢀ8), the conversion efficiency and catalyst
example, we have tried to remove all the solvent from the gels via
vacuum, which unfortunately caused the spontaneous fission of 50 stereoselectivity were maintained at around 70% reaction yield
gels.
Although some flaws exist in our prototype products at present,
15 future industrial manufacturing can be expected to realize
technique improvements. Herein, we prepared three magneticꢀ
and 77% ee, although slightly lower than the first trial’s result.
Nonetheless, PMMS-g-C1L3 gel can be conveniently employed
to catalyze asymmetric DielsꢀAlder cycloadditions.
Polymerꢀsupported Lꢀproline represents another very
stirꢀbarꢀencapsulated organocatalyst gels, PMMS-g-C1L3, 55 important class of organocatalysts for C−C bond constructions
PMMS-g-C2L3 and PMMS-g-C3L3, which were used in
catalyzing asymmetric DielsꢀAlder reaction, asymmetric aldol
20 reaction and benzoin condensation reaction respectively.
The imidazolidinone compound developed by MacMillan,29
such as asymmetric aldol reaction.25,35ꢀ46 Following literature
protocols, we tested the catalytic performance of polysiloxane gel
PMMS-g-C2L3 applied in a classical enantioselective aldol
reaction of 4ꢀnitrobenzaldehyde and cyclohexanone. As
might be the most famous organocatalyst which has been widely 60 illustrated in Table 2, solvent plays
a crucial role in
used in a variety of organocatalytic processes and has been
enantioselective property. The reaction carried out in
methanol/H2O (1/1, v/v) system provided moderate yields and
low ee (34ꢀ38%), while using pure water solution resulted in high
conversion (> 80%) and high ee values (96ꢀ99%). Unlike
unsurprisingly
immobilized
on
different
polymeric
25 supports.24,25,30ꢀ34 Polysiloxane gel catalyst, PMMS-g-C1L3
bearing MacMillan imidazolidinone was applied to promote a
classical asymmetric DielsꢀAlder reaction of cyclopentadiene and 65 traditional homogeneous reactions which would have
a
cinnamic aldehyde.
significant decrease in both stereoꢀ and enantioselectivity along
with raising reaction temperature,47ꢀ50 our PMMS-g-C2L3
catalyst gel slightly favors higher temperature possibly due to the
hydrophobicity of polysiloxanes expelling water from catalytic
70 centers to stabilize the transition state of forming enamine species
by excluding competitive hydrogen bonding with water. This
phenomena is consistent with Monteiro’s observation.46
Table 1 Enantioselective DielsꢀAlder reaction catalyzed by
30 catalyst PMMS-g-C1L3
Table 2 Enantioselective aldol reaction catalyzed by catalyst PMMS-g-
C2L3
Entrya Recycle Acid
number
Solvent
Yield exo/endo exo ee
(%)b
(%)c (endo ee)
(%)d
1
2
3
4
5
6
7
8
0
0
0
1
2
3
4
5
HBF4 CH3CN/H2O
(95/5)
0
ꢀꢀ
ꢀꢀ
TFA
MeOH/H2O
(95/5)
52
88
73
66
74
72
64
51/49
51/49
55/45
53/47
51/49
51/49
54/46
66 (83)
78 (96)
77 (77)
78 (79)
74 (77)
73 (72)
70 (79)
Entrya Recycle
number
Solvent
Temperature Yield anti/syn anti ee
TFA CH3CN/H2O
(95/5)
TFA CH3CN/H2O
(95/5)
TFA CH3CN/H2O
(95/5)
TFA CH3CN/H2O
(95/5)
TFA CH3CN/H2O
(95/5)
TFA CH3CN/H2O
(95/5)
(%)c
68
(%)d
(%)e
38
(℃)
1
0
MeOH/H2O
(1/1)
MeOH/H2O
(1/1)
25
89/11
2
0
50
65
92/8
34
3
4b
5
6
7
8
9
10
0
0
0
1
2
3
4
5
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
25
50
50
50
50
50
50
50
80
82
85
87
76
75
69
73
88/12
88/12
90/10
86/14
90/10
81/19
85/15
56/44
90
96
99
91
72
60
24
22
a Reactions were carried out using cinnamic aldehyde (1 equiv.) and
cyclopentadiene (5 equiv.) at 0 ℃ for 24 hrs. b Isolated yield. c
Determined by crude NMR. d Determined by chiral HPLC.
75 a Reactions were carried out using 4ꢀnitrobenzaldehyde (1 equiv.) and
cyclohexanone (7 equiv.) for 48 hrs. b Catalyst: PMMS-g-C2L2. c Iolated
yield. d Determined by chiral HPLC. e Determined by chiral HPLC.
As shown in Table 1, roughly 50 % : 50 % mixture of endo
35 and exo cycloadducts (determined by H NMR analysis of crude
1
The recovered PMMS-g-C2L3 gel was reused five times to
test the recyclability of catalyzing the asymmetric aldol reaction.
80 As shown in Table 2, the first two runs provided satisfying
products) were isolated in all the experimental trials. To convert
the grafted imidazolidinone C1 to the catalytically active
intermediate, an equimolar amount of a Bronsted acid is required
4
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reaction yields and high ee values (entry 5ꢀ6), however the
which might poison noble metals.
enantioselectivity decreased dramatically starting from the third
recycle (entry 7ꢀ10). Besides the lack of exploration in optimal
reaction conditions, one possible reason might be that since the
recovered PMMS-g-C2L3 gel was always kept in solvents to
avoid gel fission, some leftover chemicals might “poison” or
racemize the grafted LꢀProline catalyst.
45
Incorporating magnetic stir bars into crosslinked PMMS gels
can provide the corresponding organocatalyst gels an ability to
perform stirring and catalysis functions at the same time (SBEC
technique). The most important benefit of this technique is to
infinitely simplify the catalyst/product separation procedure to
5
50 using
a
simple stirꢀbarꢀretriever, even without any
Incorporating imidazolium salts into the polymer backbones or
side chains has been proven to be an efficient way to develop
precipitation/filtration steps. Although our organocatalyst gels are
prototype products bearing several technique problems and the
catalytic performances are modest, we hope this “proofꢀofꢀidea”
work would open interesting perspectives and bring some useful
10 recyclable polymeric NHC catalysts.26,51ꢀ58 Inspired from
Cowley’s work,26 we designed and synthesized an imidazolium
monomer C3, and grafted it onto crosslinked PMMS gels. With 55 information for heterogeneous catalysis community.
PMMS-g-C3L3 catalyst in hand, we examined its ability of
catalyzing a typical benzoin condensation reaction.
Acknowledgement
15 Table 3 Benzoin condensation reaction catalyzed by catalyst PMMS-g-
C3L3
This research was supported by National Natural Science
Foundation of China (Grant No. 21374016). The authors would
like to gratefully thank Dr. Lin Chen for his help with chiral
60 HPLC experiment measurements.
Notes and references
Entrya
Recycle Solvent Yieldb of benzoin
Yieldb of benzil
product (%)
a School of Chemistry and Chemical Engineering, Jiangsu Province Hi-
Tech Key Laboratory for Bio-medical Research, Southeast University,
Nanjing 211189, China. Fax: 86 25 52091096; Tel: 86 25 52091096; E-
65 mail: yangh@seu.edu.cn (H.Y.), lbp@seu.edu.cn (B.L.).
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
‡ Footnotes should appear here. These might include comments relevant
70 to but not central to the matter under discussion, limited experimental and
spectral data, and crystallographic data.
number
product (%)
1
2
3
4
5
6
7
8
0
0
0
1
2
3
4
5
H2O
DMF
trace
58
63
54
47
32
35
32
0
7
5
9
7
6
4
5
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
a Reactions were carried out using dry DMSO and DBU (15 mol%) at r.t.
under N2 for 48 hrs. b Isolated yield.
1
2
3
4
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