Green and efficient cycloaddition of CO2 toward epoxides over thiamine derivatives/GO aerogels under mild and solvent-free conditions

Article abstract of DOI:10.1007/s11426-016-0376-3

Thiamine derivatives that are cheap, readily available, non-toxic and green are used as heterogeneous catalyst for the generation of cyclic carbonates through cycloaddition of CO₂ to epoxides without the need of co-catalyst and solvent. The interaction between thiamine hydrochloride (VB₁-Cl) and substrates (CO₂ and propylene oxide) was proven by ultraviolet-visible spectroscopy and ¹H nuclear magnetic resonance analysis, and it is deduced that the synergistic action among multi-functional groups (hydroxyl, halide anion and amine) is a favorable factor for cycloaddition reaction. A series of VB₁/GO aerogels were facilely prepared through the addition of aqueous VB₁ derivatives to a suspension of GO in ethanol at room temperature. It was found that the aerogel generated through the interaction of VB₁-Cl with GO shows catalytic activity and stability higher than those of VB₁-Cl. It is because the electrostatic interaction between GO and VB₁-Cl enhances the nucleophilicity and leaving ability of anion. The effects of reaction temperature, catalyst loading, CO₂ pressure and reaction time on CO₂ cycloaddition to propylene oxide were thoroughly studied.

Full text of DOI:10.1007/s11426-016-0376-3

SCIENCE CHINA
Chemistry
doi: 10.1007/s11426-016-0376-3
•
ARTICLES •
·
SPECIAL ISSUE · Green Chemistry
Green and efficient cycloaddition of CO toward epoxides over
2
thiamine derivatives/GO aerogels under mild and solvent-free
conditions
1†
1,2†
1
1
1,2
1*
Ziqi Yi , Donghui Lan , Ying Wang , Lang Chen , Chaktong Au & Shuangfeng Yin
1
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University,
Changsha 410082, China
Department of Chemistry, College of Chemistry and Chemical Engineering, Hunan Institute of Engineering, Xiangtan 411104, China
2
Received September 11, 2016; accepted October 29, 2016; published online November 30, 2016
Thiamine derivatives that are cheap, readily available, non-toxic and green are used as heterogeneous catalyst for the generation of
cyclic carbonates through cycloaddition of CO to epoxides without the need of co-catalyst and solvent. The interaction between
thiamine hydrochloride (VB -Cl) and substrates (CO
nuclear magnetic resonance analysis, and it is deduced that the synergistic action among multi-functional groups (hydroxyl, halide
anion and amine) is a favorable factor for cycloaddition reaction. A series of VB /GO aerogels were facilely prepared through the
addition of aqueous VB derivatives to a suspension of GO in ethanol at room temperature. It was found that the aerogel generated
through the interaction of VB -Cl with GO shows catalytic activity and stability higher than those of VB -Cl. It is because the
electrostatic interaction between GO and VB -Cl enhances the nucleophilicity and leaving ability of anion. The effects of reaction
temperature, catalyst loading, CO pressure and reaction time on CO cycloaddition to propylene oxide were thoroughly studied.
2
1
1
2
and propylene oxide) was proven by ultraviolet-visible spectroscopy and H
1
1
1
1
1
2
2
1
thiamine B , graphene oxide, aerogels, synergetic effect, carbon dioxide, cycloaddition reaction
Citation:
2
Yi Z, Lan D, Wang Y, Chen L, Au C, Yin S. Green and efficient cycloaddition of CO toward epoxides over thiamine derivatives/GO aerogels under mild
and solvent-free conditions. Sci China Chem, 2017, 60: 1–7, doi: 10.1007/s11426-016-0376-3
1
Introduction
additives [4,5].
A large number of homogeneous and heterogeneous cata-
lysts were developed for the generation of cyclic carbonates,
such as metal salts [6], organic bases [7], ionic liquids (ILs)
Carbon dioxide (CO ) is a greenhouse gas blamed for global
2
warming [1]. Nonetheless, being abundant, inexpensive, safe
and renewable, it has the potential of replacing harmful reac-
tants such as CO and phosgene for the production of valu-
able chemical materials [2,3]. From the viewpoints of “green
[8,9], metal complexes [10,11], metal oxides [12], supported
catalysts [13], metal-organic frameworks [14], and carbon
materials [15]. With the metal-based ones holding a promi-
nent position [16], there is a compilation of elements that are
considered “endangered” [17]. For the sake of sustainability,
it is desirable to design efficient catalysts for the cycloaddi-
tion reactions based on metal-free resources that are cheap,
non-toxic and readily available. Furthermore, the method has
to be tunable, easy-to-handle, and solvent-free.
chemistry” and “atom economy”, it is attractive to utilize CO
2
for the generation of cyclic carbonates that are used as polar
solvents, high-permittivity electrolyte components and fuel
†
*
These authors contributed equally to this work.
Corresponding author (email: sf_yin@hnu.edu.cn)
It is well known that thiamine (VB ) is an essential nutrient
1
©
Science China Press and Springer-Verlag Berlin Heidelberg 2017
chem.scichina.com link.springer.com

2
Yi et al. Sci China Chem
for animals. It contains a pyrimidine ring (with a primary
amine) and a thiazolium salt (with a hydroxyl and quaternary
ammonium salt) that are linked together by a methylene
bridge. The VB derivatives are cheap, readily available, and
1
non-toxic. They were used as green catalysts for a number
of reactions but not for cycloaddition [18–20]. In previous
studies, it was found that the synergetic effects of acid sites
and halide anions for ring opening of epoxide and the role of
1 1
Figure 1 Structure of VB derivative VB -X (X=Cl, Br, I).
change. First, 5 mmol of VB -Cl was added to a flask with
1
basic sites for CO
the cycloaddition reactions [21–24]. It is hence envisaged
that in a case such as thiamine hydrochloride (VB -Cl) with
multifunctional groups (hydroxyl, quaternary ammonium
salt, pyrimidine ring and primary amine), there should be
2
adsorption and activation are essential for
30 mL ethanol, and the mixture was stirred for 10 min. Then
12 mmol of NaI was added and the mixture was stirred for 24
h. The as-obtained white powder was collected by centrifu-
gation and washed 7 times with ethanol/glycerin (9:1, v/v),
followed by washing with ethanol to remove residual glyc-
1
efficient catalytic activity for cycloaddition of CO
epoxides. However, VB -Cl is stable only up to 100 °C at
a pH of 3.5 [25]. It is hence meaningful to find a method
to improve the thermal stability and activity of VB -Cl.
As reported by Song et al. [26], the thermal stability and
mechanical properties of polycaprolactone could be en-
hanced through composition with graphene oxide (GO). As
reported, GO has plenty donors for hydrogen bonding (e.g.,
hydroxyl and carboxyl) and becomes negatively charged
2
toward
erin. Finally, the purified powder named herein as VB -I was
1
1
obtained after drying at 60 °C in a vacuum oven. The VB -Br
1
was prepared likewise, employing LiBr instead of NaI.
GO was prepared by following the procedure published
elsewhere [22]. The typical procedure for the preparation of
1
VB -Cl/GO aerogel is as follows. First, the purified GO (0.2
1
g) was dispersed in 60 mL of ethanol/H O (2:1, v/v) and sub-
2
ject to ultrasonication for 2 h. After the addition of 4 mL
of aqueous VB -Cl (1.6 mmol), the mixture was stirred for
1
upon hydration [27,28]. In this regard, a VB
1
/GO composite
24 h. With drying under vacuum using a rotary evapora-
tor at 50 °C, the VB -Cl/GO was ready for use. The VB -
material could be stabilized by effects such as electrostatic
interaction, hydrogen bonding and esterification reaction.
The consequence is improvement of thermal stability and
mechanical properties as well as promoted catalytic activity
as a result of enhancement in nucleophilicity of anion.
1
1
Br/GO and VB -I/GO was prepared similarly, using VB -Br
1
1
and VB -I instead of VB -Cl, respectively.
1
1
2
.3 Catalyst characterization
In the present work, VB
pared by adding aqueous VB
in ethanol at room temperature. The process is simple and
efficient. For the first time, VB derivatives and VB deriva-
tive/GO aerogels are tested as heterogeneous catalyst for the
synthesis of cyclic carbonates through CO cycloaddition to
epoxides under mild conditions without the use of co-cata-
lyst and solvent. In addition, VB -Cl interaction with propy-
lene oxide (PO) and CO was investigated by ultraviolet-vis-
1
derivative/GO aerogels were pre-
1
derivatives to a GO suspension
The catalysts were characterized by UV-Vis diffuse re-
1
flectance spectroscopy (UV-Vis DRS), H NMR, scanning
1
1
electron microscopy (SEM) and thermogravimetry (TG).
The UV-Vis DRS spectra of samples were obtained over a
UV-Vis spectrophotometer (Cary 100, Agilent, USA) using
2
1
BaSO as reference. The H NMR spectra were recorded
4
1
over a Bruker AV 400/500 M NMR spectrometer (Germany)
using DSS (sodium 2,2-dimethyl-2-silapentane-5-sulfonate)
as internal reference and (methyl sulfoxide)-d (DMSO-d )
2
1
1
ible (UV-Vis) and H nuclear magnetic resonance ( H NMR)
methods. A possible mechanism is proposed for the cycload-
6
6
as solvent. The morphology of the samples was observed by
SEM (HITACHI S-4800 microscope, Japan). The TG curve
was obtained by heating the sample from 50 to 550 °C at
dition reaction over VB -Cl.
1
a constant rate of 10 °C/min under a N
ZSCH-STA-449C equipment (Germany).
2
flow on a NET-
2
Experimental
2.1 Materials
2
.4 Procedure for cycloaddition reactions
All chemicals for the synthesis of materials and testing of
catalytic activity were purchased from Sinopharm Chemical
Reagent Co., Ltd. (China), and directly used without purifi-
cation.
The cycloaddition reaction was conducted in a stainless steel
autoclave of 30 mL equipped with a magnetic bar. In a typ-
ical run, catalyst, biphenyl, and epoxide were added into the
flask. Then, the reactor was pressurized with an appropriate
amount of CO
bath. After a designated period, the reactor was cooled to
0 °C, followed by venting of the remaining CO . The cyclic
2
and heated to a desired temperature in an oil
2
.2 Catalysts preparation
The synthesis of VB -I (Figure 1) was by means of ion ex-
1
2

Yi et al. Sci China Chem
3
carbonate yield and selectivity were qualified by gas chro-
matography (GC) analysis (7820A GC, Agilent, USA).
The used catalysts can be easily recovered by centrifuga-
tion. After being washed with PO for five times and dried at
60 °C for 12 h, the recovered catalyst was used again for the
next run.
3
Results and discussion
3.1 Catalyst characterization
Enriched with oxygen-containing functional groups such
as hydroxyl, carboxyl and epoxy, GO is a carbon material
that can be dispersed uniformly in hydrophilic solvent [29].
Shown in Figure 2(a) are the photographic images of a
GO dispersion (2 mg/mL) after ultrasonication for 1 h in
ethanol and followed by static settling for 1 h. One can see
well dispersion of GO in ethanol. When 1 mL of aqueous
1
Figure 3 Generation of VB -X/GO (X=Cl, Br, I) (color online).
VB -Cl solution (5 mg/mL) was added to 1 mL of the GO (2
1
mg/mL) dispersion, a hydrogel as illustrated by having the
tube inverted was quickly formed (Figure 2(b, c)). A gray
precipitate was immediately observed upon the mixing of
aqueous VB -Cl solution and aqueous GO dispersion. The
1
formation of the gray precipitate is attributed to the elec-
trostatic interaction between GO (negatively charged) and
1
VB -Cl (with thiazole ring that is positively charged). The
generation of the VB
1
-Cl/GO aerogel could be a combined
result of electrostatic interaction, hydrogen bonding and
esterification reaction (Figure 3).
1
Figure 4 SEM images of GO and VB -Cl/GO.
Displayed in Figure 4 are the SEM images of GO and VB -
1
Cl/GO. The SEM images of GO show a sheet structure with
wrinkles plausibly induced upon drying as a result of volume
contraction. The cross section of VB -Cl/GO shows a close-
1
packed layered structure. The amplified SEM image of VB
Cl/GO aerogel also shows a sheet structure with wrinkles.
1
-
The TG curves of GO, VB
played in Figure 5. One can see that VB
58 °C, and pyrolysis starts above 158 °C and major pyrolysis
1
-Cl and VB
1
-Cl/GO are dis-
1
-Cl is stable up to
1
1 1
Figure 5 TG curve of GO, VB -Cl and VB -Cl/GO (color online).
occurs between 219 and 226 °C. However, the pyrolysis of
VB
1
-Cl/GO begins above 221 °C. The slight weight loss of
-Cl/GO below 221 °C is ascribed to the loss of adsorbed
VB
1
species such as H
2
O and CO
2
. It is apparent that the thermal
stability of VB -Cl is improved by forming the aerogel. It is
1
an indication that there is interaction between GO and VB
Cl.
1
-
Figure 2 Photographs of (a) GO dispersion in EtOH (2 mg/mL GO); (b)
VB -Cl/GO aerogel; (c) VB -Cl/GO aerogel inverted (color online).
1
1

4
Yi et al. Sci China Chem
3.2 Catalytic performance
the 4-h period (Table 1, entry 7). Since following the normal
procedure the yield of PC at 120 °C for 4 h was 77.6%, there
is obvious decline of PC yield (55.4%) as a consequence of
3.2.1 Catalytic performance of catalysts
The cycloaddition of CO
model reaction to test the activity of VB
are summarized in Table 1. One can see that over VB
the propylene carbonate (PC) yield is 24.7% at 100 °C
for 6 h, and PC yield is up to 83.3% when the reaction is
prolonged to 14 h (Table 1, entries 1 and 2). A higher tem-
perature means more effective collision between the reactant
2
towards PO was chosen as the
-Cl, and the results
-Cl,
the pretreatment. The results indicate that VB
act as a pre-catalyst, and the decline of activity of VB
after pretreatment is due to the decomposition of VB
result of PO presence.
1
-Cl does not
-Cl
-Cl as a
1
1
1
1
Previously, we reported the synergetic effect of GO and
Bu NBr on cycloaddition [15]. In this regard, the presence of
GO should enhance the catalytic activity of VB -Cl (Table 1,
entries 5, 8 and 9). In the case of VB -Cl/GO, the yield of PC
is up to 85.3%, higher than that of VB -Cl and GO (Table 1,
entries 5, 8 and 10). It may be ascribed to the electrostatic
interaction between GO and VB -Cl that enhances the nu-
cleophilicity and the leaving ability of anion. As shown in
Figure 5, the thermal stability of VB -Cl was improved by
4
1
molecules. The catalytic activity of VB
1
-Cl is sensitive to
1
reaction temperature: the PC yield increases rapidly with the
rise of temperature and is up to 99.4% at 120 °C (Table 1,
entries 2–4). However, the increase of reaction temperature
1
1
results in the decomposition of VB
1
-Cl. The VB -Cl changed
1
from a white powder to a red and sticky substance when the
reaction temperature was increased from 100 to 120 °C. It
1
forming aerogel with GO, similar to that reported by Song et
al. [26] and He et al. [31]. As shown in Figure S1 (Sup-
porting Information online), there is obvious decline of cat-
was observed that VB
then to red when the experiment was conducted at 100 °C
in the absence of CO . The above results are ascribed to the
fact that PO is a weak base and there is partial decomposition
of VB -Cl under basic conditions [25]. Also, the stability
of VB -Cl is enhanced in the presence of CO , and there is
interaction between PO and CO under high pressure [30].
In order to eliminate the possibility of VB -Cl being a pre-
catalyst, the experiment was conducted with VB -Cl and PO
pretreated at 120 °C for 2 h without CO , and then in 2 MPa
of CO for 4 h; and 51.9% yield of PC was obtained over
1
-Cl changed from white to yellow, and
2
alytic performance for VB
VB -Cl/GO in a test of five runs. The above results clearly
demonstrate that VB -Cl/GO is superior to VB -Cl not only
in catalytic performance but also in stability.
1
-Cl while only slight decline for
1
1
1
1
1
2
2
As reported, the nucleophilicity and leaving ability of an-
ions have a remarkable influence on the activity of a catalyst
in the cycloaddition reaction [32]. The catalytic efficiency of
1
1
2
VB
1
derivatives with different anions can be arranged in the
2
order of VB
1
-I>VB
1
-Br>VB
1
-Cl (Table 2, entries 1–3). The
a)
Table 1 The catalytic activity of VB
1
-Cl for cycloaddition of CO
2
toward PO
Entry
T (°C)
100
100
110
120
120
120
120
120
120
120
t (h)
6
Yield (%)
24.7
83.3
38.1
99.4
77.6
55.4
51.9
1.3
Selectivity (%)
99.9
1
2
3
4
5
14
6
99.9
99.9
6
99.9
4
99.9
b)
6
4
99.3
c)
7
4
99.8
d)
8
4
99.9
e)
9
4
84.6
85.3
99.8
f)
1
0
4
99.8
a) Reaction conditions: 28.6 mmol PO with 2.2 mol% VB
with 2.2 mol% VB
of GO and 2.2 mol% VB
1
-Cl, initial CO
2
pressure 2 MPa; b) recovered VB
2
1
-Cl after the second cycle; c) 28.6 mmol PO
1
-Cl was stirred at 120 °C for 2 h, then pressurized with CO and stirred at 120 °C for 4 h; d) using 80 mg of GO as catalyst; e) using 80 mg
1
-Cl as catalyst; f) using VB
1
-Cl/GO (containing 80 mg of GO and 2.2 mol% VB
1
-Cl) as catalyst.
a)
Table 2 The catalytic activity of VB
1
derivatives for cycloaddition of CO
2
toward PO
Entry
Catalysts VB derivatives (mol%)
1
GO (mg)
Yield (%)
24.7
Selectivity (%)
99.9
1
2
3
VB
1
-Cl
2.2
2.2
2.2
1.1
1.1
1.1
2.2
–
–
VB
1
-Br
50.6
99.5
VB
1
1
-I
-I
–
86.8
99.3
b)
4
VB
–
41.5
99.4
b)
5
VB
VB
VB
1
-I/GO
1
-I/GO
1
-I/GO
80
160
80
69.3
99.5
b)
6
93.2
99.5
b)
7
92.3
99.5
2
a) Reaction conditions: 28.6 mmol PO, 100 °C, initial CO pressure 2 MPa, 6 h; b) reaction temperature 90 °C.

Yi et al. Sci China Chem
5
excellent performance of VB
cleophilicity and leaving ability of I [32]. To further improve
the activity of VB -I, VB -I/GO aerogel was prepared and
tested. The PC yield over VB -I/GO is significantly higher
than that of VB -I. In addition, the catalytic activity of VB
I/GO increases with the increased amount of GO or VB -I,
suggesting that both GO and VB -I are essential ingredients
for excellent performance of VB -I/GO. In addition, VB
1
-I is attributed to the strong nu-
sults are displayed in Table 4. Obviously, the VB
1
-I/GO cat-
–
alyst is applicable to a variety of terminal epoxides, showing
high yield and selectivity. In the case of cyclohexene oxide,
the catalytic activity is relatively low plausibly due to steric
hindrance and electronic effect [36,37].
1
1
1
1
1
-
1
1
3
.3 Mechanism investigation
1
1
-
It was reported that the synergetic effect of hydroxyl groups
and halide anions is important for ring opening of epoxides
I/GO is efficient for the cycloaddition reaction even under
mild conditions (Figure S2).
[
37–42]. The hydrogen bonding between PO and VB
1
-Cl was
studied by H NMR. Figure 6 shows the H NMR spectra of
PO, VB -Cl and PO mixed with VB -Cl. One can see a clear
upfield shift of the VB -Cl H signal, and a new signal at 5.3
ppm assignable to the OH proton of VB -Cl. In addition, there
is an upfield shift of the PO H (CH ) signal (from 1.2 to 1.1
ppm). The above results suggest hydrogen bonding between
PO and VB -Cl as previously reported [37–42].
On the other hand, the basic sites is effective for CO
sorption and activation [43,44]. The process of CO activa-
tion and fixation was studied using a homogeneous system of
VB -Cl in H O monitored by UV-Vis technique. Shown in
Figure 7 are the UV-Vis DRS spectra of aqueous VB -Cl so-
lution before and after bubbling with CO . Before CO bub-
bling, the aqueous VB -Cl solution shows two typical signals:
the strong one at 232 nm from pyrimidine ring and the weak
one at 263 nm from thiazole ring. After CO bubbling, there
1
1
3
.2.2 Comparison of different heterogeneous catalysts
The performance of various heterogeneous catalysts for cy-
cloaddition of PO and CO is depicted in Table 3. One can
see that the immobilized ILs catalysts (F-PIL-Br, [BisAm-
OH-i-PS]I , CS-[BuPh P]Br, GO-DMEDA-I) show excellent
1
1
1
2
1
3
2
3
catalytic activity at 120 °C, and GO-DMEDA-I performs the
best. However, there is obvious decline of catalytic activ-
ity with these immobilized ILs catalysts when the cycload-
dition reactions were conducted at 90 °C. Compared to the
1
2
ad-
2
–1
VB
are inferior in catalytic activity at 90 °C. Furthermore, the
preparation of VB -I/GO is environment-benign and not as
complicated and time-consuming. Taking into account the
1
-I/GO catalyst (TOF: 14.1 h ), the immobilized catalysts
1
2
1
1
2
2
cost and performance, it can be concluded that VB
superior among the listed heterogeneous catalysts.
1
-I/GO is
1
2
3
.2.3 Catalytic activity towards different epoxides
To demonstrate the applicability of the process, we conducted
the cycloaddition of CO toward other epoxides, and the re-
is a clear shift of the pyrimidine ring peak (from 232 to 237
nm). The red shift may be attributed to the adsorption and ac-
2
tivation of CO at the pyrimidine ring. Prasetyanto et al. [45]
2
2
Table 3 Catalytic performance of various heterogeneous catalysts for cycloaddition of CO to PO
a)
–1
Catalyst
T (°C)
120
130
90
t (h)
9
Cat. (mol%)
Yield (%)
94.0
TOF (h )
10.4 [33]
53 [34]
F-PIL-Br
1
[
BisAm-OH-i-PS]I
2
2
2.5
2.5
4
0.75
0.75
1.5
99.3
[
BisAm-OH-i-PS]I
21.0
11.2 [34]
16.0 [35]
10.5 [35]
45.8 [22]
13.2 [22]
7.2 [9]
CS-[BuPh
CS-[BuPh
3
P]Br
P]Br
120
90
96.3
3
4
1.5
63.0
GO-DMEDA-I
GO-DMEDA-I
120
90
3
0.65
0.65
2
89.4
6
51.6
+
–
SiO
2
-(CH
2
)
3
(EtOH)
3
N I
90
6
86.0
b)
VB
1
-I/GO
90
6
1.1
93.2
14.1
a) TOF=mole of synthesized propylene carbonate over per mole ILs in per hour; b) this work.
a)
Table 4 Catalytic activity of VB
1
-I/GO for CO
2
cycloaddition toward various epoxides
Entry
Substrate
Yield (%)
95.4
Selectivity (%)
1
2
3
4
5
6
Ethylene oxide
Propylene oxide
Epichlorohydrin
1,2-Epoxybutane
Styrene oxide
99.2
99.5
97.9
98.6
98.9
93.8
93.2
96.5
76.1
59.2
Cyclohexene oxide
6.3
1 2
a) Reaction conditions: 0.3 g VB -I/GO, 28.6 mmol epoxide, temperature 90 °C, initial CO pressure 2 MPa, time 6 h.

6
Yi et al. Sci China Chem
proposed that the combination of secondary and tertiary
amine at a certain position resulted in a CO activation rate
higher than that of the single type amine species. It is sug-
gested that with a primary amine in pyrimidine ring, VB -Cl
activates CO through synergetic interaction between the
primary amine and the tertiary amine.
is adsorbed and activated under the synergetic influence of
primary amine and tertiary amine. Second, the nucleophilic
Cl attacks the less hindered carbon atom of the activated ring
2
–
1
to generate the oxygen anion that is stabilized through hydro-
2
gen bonding with the hydroxyl group of VB
1
-Cl. Then, the
oxygen anion reacts with an activated CO molecule to give
2
Based on the above results and those of previous reports
21,22], a possible mechanism is suggested for the catalytic
an alkylcarbonate anion. Finally, PC is formed through an in-
tramolecular cyclic step, together with the release of catalyst.
[
reaction as depicted in Figure 8. First, there is hydrogen
bonding of PO with the hydroxyl group of VB
1
-Cl, while CO
2
4
Conclusions
Cheap, readily available, and non-toxic thiamine derivatives
are used as heterogeneous catalyst for the synthesis of cyclic
carbonates through CO
confirmed by UV-Vis and H NMR studies, there is interac-
tion between VB -Cl and substrates (CO and PO). A pos-
2
cycloaddition toward epoxides. As
1
1
2
sible mechanism for the cycloaddition reaction referring to
the synergistic action of multi-functional groups (hydroxyl,
halide anion and amines) is suggested. To achieve better cat-
alytic activity and stability of thiamine derivatives, GO was
employed for the generation of VB
aqueous VB derivatives to a GO ethanol suspension at room
temperature. Among the prepared catalysts, VB -I/GO per-
1
/GO aerogels by adding
1
1
forms the best under mild conditions without the need of us-
ing any solvent or co-catalyst.
1
Figure 6 H NMR spectra of (a) PO, (b) VB
1
-Cl, (c) PO and VB
1
-Cl.
Conflict of interest
The authors declare that they have no conflict of
interest.
Supporting information The supporting information is available online
at http://chem.scichina.com and http://link.springer.com/journal/11426.
The supporting materials are published as submitted, without typesetting
or editing. The responsibility for scientific accuracy and content remains
entirely with the authors.
1
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Figure 7 UV-Vis DRS spectra of aqueous VB
before and (b) after bubbling CO for 0.5 h.
1
-Cl solution (8 mg/L) (a)
2
4
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