OMS-2/H2O2/Dimethyl Carbonate: An Environmentally-Friendly Heterogeneous Catalytic System for the Oxidative Synthesis of Benzoxazoles at Room Temperature

Article abstract of DOI:10.1021/acs.oprd.7b00315

A manganese octahedral molecular sieve (OMS-2) was found to be an efficient and recyclable heterogeneous catalyst for the oxidative synthesis of benzoxazoles in the gram-scale from phenolic imines at room temperature. H₂O₂ and biobased reagent dimethyl carbonate (DMC) were successfully employed as the environmentally friendly oxidant and solvent, respectively, in an OMS-2-catalysted redox reaction for the first time. Benzoxazoles could also be obtained from N-substituted 2-aminophenols via Cu(OH)_x/OMS-2-catalyzed sequential oxidative transformation at elevated temperatures.

Full text of DOI:10.1021/acs.oprd.7b00315

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Full Paper
OMS-2/H2O2/Dimethyl Carbonate: an Environmentally-
Friendly Heterogeneous Catalytic System for Oxidative
Synthesis of Benzoxazoles at Room Temperature
Xu Meng, Yuanguang Wang, Baohua Chen, Gexin Chen, Zhenqiang Jing, and Peiqing Zhao
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.7b00315 • Publication Date (Web): 09 Nov 2017
Downloaded from http://pubs.acs.org on November 9, 2017
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OMS-2/H2O2/Dimethyl Carbonate: an
Environmentally-Friendly Heterogeneous Catalytic
System for Oxidative Synthesis of Benzoxazoles at
Room Temperature
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Xu Meng,a Yuanguang Wang,a Baohua Chenb Gexin Chen,a Zhenqiang Jing,c and Peiqing
Zhao*,a
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research Institute of
LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou,
730000, P. R. China
b State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Gansu Lanzhou,
730000, P. R. China
c
Suzhou Institute of Nano-Tech and Nano-Bionic (SINANO), Chinese Academy of Sciences,
Suzhou 215123, P. R. China
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ABSTRACT: Manganese octahedral molecular sieve (OMS-2) was found to be an efficient and
recyclable heterogeneous catalyst for the oxidative synthesis of benzoxazoles in gram-scale from
phenolic imines at room temperature. H2O2 and bio-based reagent dimethyl carbonate (DMC)
were successfully employed as the environmentally-friendly oxidant and solvent respectively in
OMS-2-catalysted redox reaction for the first time. Benzoxazoles could also be obtained from N-
substituted 2-aminophenols via Cu(OH)x/OMS-2-catalyzed sequential oxidative transformation
at elevated temperature.
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KEYWORDS: heterogeneous catalysis; catalytic oxidation; benzoxazole; green synthesis
INTRODUCTION
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Benzoxazole and its derivatives are frequently found as structure motifs in biologically active
natural products and used as building blocks in pharmaceuticals and organic materials,1
therefore, they are of great importance. Traditional methodologies of obtaining benzoxazoles are
through the condensation of 2-aminophenols with carboxylic-acid derivatives followed by
cyclization under either acidic or harsh reaction conditions.2 In recent years, several innovative
catalytic oxidative systems have emerged.3 For example, Bäckvall`s group developed an
alternative Ru-catalyzed biomimetic aerobic system towards the preparation of benzoxazoles
from amines and 2-aminophenols using air as the terminal oxidant.4 More recently, our group
reported a heterogeneous Cu(OH)x/OMS-2-catalyzed the oxidative synthesis of benzoxazoles
between catechols and amines via quick electron-transfer under a low-energy pathway.5
Nevertheless, one of the most general and facile pathways for the synthesis of benzoxazoles is
the oxidative cyclization of phenolic imines generated in situ from the condensation of aldehydes
and 2-aminophenols using various stoichiometric amount of strong oxidants.6 Catalytic oxidative
methods have been developed by the use of transition metals, TEMPO and NaCN.7 Due to the
ease in separation, recovery and recycling of catalysts, heterogeneous catalytic processes based
on the aerobic cyclization of phenolic imines have also been realized in recent years. Recyclable
Cu-based nanoparticles, Co-based nanocomposite, ZnO nanoparticles, active carbon, molecular
sieve and polymer-incarcerated Pt nanoclusters have been found to be efficient heterogeneous
catalysts associated with oxygen.8 However, in the aforementioned cases, base additives,
elevated reaction temperatures or toxic catalyst and reaction solvents were employed to achieve
satisfied catalytic performance. From the perspective of sustainable industrial chemistry, it is
desirable to develop an environmentally-safe catalytic system that consists of recyclable catalyst,
green oxidant, non-toxic solvent and mild reaction conditions.
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In recent years, manganese octahedral molecular sieves (OMS-2) composed of 2 × 2 edge-
and corner-shared MnO6 octahedral chains (Scheme 1) have gained importance because of its
superior properties, such as unique microporous tunnel structure, large surface areas, semi-
conductivity and oxygen reduction ability.9 More importantly, the co-existence of various
oxidation states of Mn in OMS-2 is critical for electron transfer in a redox process. As a result,
OMS-2 has been applied as heterogeneous redox catalyst in various oxidations, including CO
oxidation,10a,10b water oxidation,10c epoxidation,10d oxidation of alcohols/sulfides,10e,10f oxidative
C-C/N formation,11 aerobic cyanation/amidation.12 Although previously reported OMS-2-
catalyzed oxidations have demonstrated merits of sustainable chemistry, green oxidant H2O2 and
solvent are not successfully used to establish a cleaner synthetic system yet to date. H2O2 is
relatively easy to handle and also only produces H2O as the by-product in aerobic oxidations,
while the H2O in commercial H2O2 sometimes limits its scope of applications in organic
synthesis because water is not compatible with the entire system. On the other hand, it is well
known that the major waste generated in organic synthesis results from the use of solvents that
are usually produced from fossil resources. To avoid using toxic solvents, water or bio-based
solvents become alternatives. Among these, dimethyl carbonate (DMC), as a biomass-derived
reagent, has been considered a benign solvent because it is nontoxic and biodegradable and can
be produced from CO2.13
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In continuation of our research toward sustainable catalysis,14 herein, we describe an
environmentally-friendly protocol for aerobic oxidative cyclization of phenolic imines into
benzoxazoles using OMS-2 as the catalyst (Scheme 1). The absence of additives and the use of
bio-based DMC as reaction media under very mild reaction conditions by using H2O2 as the
terminal oxidant are advantages of this approach. More importantly, OMS-2 can be reused many
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times in gram-scale synthesis without the loss of catalytic activity, which makes this catalytic
system more practical and competitive with previous processes. This study is meant to be an
important step towards enriching the content of OMS-2-catalyzed oxidations due to achieving
the clean synthesis in larger scale.
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Scheme 1. Clean Synthesis of Benzoxazoles Under OMS-2/H2O2/DMC System
RESULTS AND DISCUSSION
Initially, a screening was performed at room temperature to optimize the catalyst, oxidant and
solvent through the cyclization of phenolic imine 1a (Table 1). Using ethanol as the solvent,
OMS-2-supported copper catalyst (20 mg, Cu: 1.48 wt.%) provided benzoxazole 2a in good
yields under aerobic conditions, although oxygen could accelerate the reaction (Table 1, entries 1
and 2). Interestingly, it was found that an additional transition metal was not necessary in the
catalysis and OMS-2 was able to catalyze the cyclization of 1a into 2a in around 85% yield with
some yellow pigment using air as the terminal oxidant under very mild conditions. Moreover, O2
did not show superior oxidizing ability compared with air under OMS-2-catalysis in EtOH when
the reaction was run for 12 h (Table 1, entries 3 and 4). To further better the reaction, several
OMS-2 materials, including OMS-2 prepared via different methods and ion-exchanged ones,
with different properties were tested, and the results were unfruitful (Table 1, entries 5-8).
Moreover, amorphous manganese oxide (AMO) and α-MnO2 did not lead to higher yields (Table
1, entries 9 and 10). After that, we paid our attention to using more environmentally-friendly
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reagent as the reaction media. Delightedly, EtOH was successfully replaced by biomass-derived
reagent DMC in the reaction, and 78% yield was obtained in the presence of air at room
temperature (Table 1, entry 11). More importantly, we found that H2O2 could be a highly
efficient oxidant for the aerobic cyclization in DMC under mild conditions, and the water in
commercial H2O2 did not make the reaction sluggish (Table 1, entry 12). Furthermore, the
reaction time could be decreased to 6 h with full conversion of raw, which led to a 95% yield of
benzoxazole 2a without yellow pigment (Table 1, entry 13). Due to the high conversion and
yield and the absence of pigment under OMS-2/H2O2/DMC system, the isolation and purification
of desired product become very simple. Without column chromatographic isolation, 95% yield of
pure 2a was readily obtained by filtration of solid catalyst followed by removal of DMC under
reduced pressure (For NMR spectrum of crude product, Fig. S1 and S2, in SI). However, the
reaction did not proceed at all when EtOH or water was used as the reaction solvent in the
presence of H2O2 (Table 1, entry 12). Similarly, the reaction did not occur without catalyst no
matter what oxidant was used (Table 1, entries 14 and 15). Finally, more experiments
demonstrated that the low catalyst loading (5 mol%) and N2 atmosphere was deleterious for the
reaction and decreased reaction time also slashed the yield of 2a (Table 1, entries 16 and 17).
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Table 1. Optimization of The Oxidative Cyclization of Phenolic Imine 1aa
Entry
Catalyst
Oxidant
Solvent
Time [h]
Yield [%]b
1
2
3
4
Cu(OH)x/OMS-2
Cu(OH)x/OMS-2
OMS-2
air
O2
air
O2
EtOH
EtOH
EtOH
EtOH
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6
12
6
75
79
87
OMS-2
77 (85[c])
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OMS-2-Urea
OMS-2- H2O2
Ce-OMS-2
Cr-OMS-2
AMO
α-MnO2
OMS-2
OMS-2
OMS-2
-
air
air
air
air
air
air
air
H2O2
H2O2
air
H2O2
H2O2
H2O2
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
DMC
DMC
DMC
DMC
DMC
DMC
DMC
12
12
12
12
12
12
12
12
6
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3
72
75
53
85
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65
78
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92 (0d)
95
0
Trace
70e (65f)
75
-
OMS-2
OMS-2
a Reaction conditions: 1a (0.2 mmol), catalyst (20 mg), solvent (1 mL), room temperature, H2O2
(50 uL) was used in entries 12, 13 and 15-17.
b Isolated yields.
c
For 12 h.
d Using EtOH or H2O replace DMC as the solvent.
e 10 mg of OMS-2 was used.
f Under N2.
After initially establishing a clean synthetic system by the combination of H2O2 and DMC,
more biomass-derived reagents were examined as reaction solvents in the synthesis of
benzoxazole 1a from 2a (Table 2). Firstly, polyethylene glycol (PEG) and glycerol were not
compatible with H2O2 in the reaction and phenolic imine 1a did not convert at all (Table 2,
entries 1-3). Other biomass-based reagents, like 2-Me-THF, ethyl lactate and γ-valerolactone,
could work with OMS-2/H2O2 to offer benzoxazole 2a with yields ranging from 42% to 76%
under mild conditions (Table 2, entries 4-6). After that, other alkyl or cyclic carbonates that have
been applied in C-H functionalization reactions13 were investigated in our system. The results
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showed that diethyl carbonate (DEC) acted as reaction media very well, while cyclic ethylene
carbonate also successfully worked with OMS-2/H2O2 to give 2a in slightly low yield of 82%
(Table 2, entries 7 and 8). As a result of the optimization process, we concluded that the best
reaction conditions for the cyclization of phenolic imines involve the use of OMS-2/H2O2/DMC
system.
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Table 2. Investigation of Bio-Derived Reagents as Reaction Mediaa
Entry
Solvent
Isolated yield [%]
1
2
3
4
5
6
7
PEG-200
PEG-400
glycerol
2-Me-THF
ethyl lactate
γ-valerolactone
DEC
N.R.
N.R.
N.R.
42
48
76
90
8
Ethylene carbonate
Reaction conditions: 1a (0.2 mmol), OMS-2 (20 mg, 10 mol%), H2O2
82
a
(50 uL), solvent (1 mL), room temperature, 6 h.
According the optimization, it was found that air was also a quite efficient green oxidant for
the cyclization of 1a in DMC (Table 1, entry 11). The reaction profiles for OMS-2-catalyzed
cyclization of 1a to 2a in DMC at room temperature with air or H2O2 as the oxidant were
carefully monitored (Figure 1). The results showed that H2O2 could accelerate the reaction
significantly and offered full conversion of 1a and 95% yield of 2a in 6 h (Figure 1, left). In air,
the reaction provided nearly 40% conversion and 35% yield under the standard conditions. So, in
our catalytic system, H2O2 was superior and favorable environmentally-friendly oxidant. As we
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known, previously reported oxidative cyclizations of phenolic imines were generally performed
in polar and aprotic solvents, such as CH2Cl2, toluene and EtOAc, or alcohol.6-8 When we used
CH2Cl2 as the solvent under the present conditions, the reaction proceeded faster in initial stage
and led to full conversion and excellent yield in 6 h (Figure 1, right). Comparatively, the reaction
in DMC proceeded slightly slow in initial stage, but eventually showed very similar kinetic
profile like it was run in CH2Cl2. So, it was proved that DMC was a suitable alternative reaction
media for the aerobic cyclization of phenolic imine.
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Figure 1. Reaction profiles for the OMS-2-catalyzed cyclization of 1a to 2a in DMC for 6 h at
room temperature with different oxidant (left). Reaction profiles for the OMS-2-catalyzed
cyclization of 1a to 2a using H2O2 as the oxidant for 6 h at room temperature in different
reaction medium (right).
To verify whether the observed catalysis was truly caused by OMS-2 or leached Mn species
from it, the hot filtration experiment of phenolic imine 1a in DMC under the standard conditions
was performed (For experimental details, see SI). After the reaction proceeded for 1 h, OMS-2
was removed by filtration from DMC, and product 2a was observed in around 32% NMR yield.
Meanwhile, the filtrate was kept stirring without adding fresh catalyst for further 5 h, and no
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more benzoxazole 2a was detected (Figure S6, in SI). ICP-AES was employed to analyze the
filtrate, which shows manganese species were hardly observed in filtrate. These facts indicated
manganese did not leach from OMS-2 and the observed catalysis was truly heterogeneous. Next,
the recyclability of OMS-2 was tested in gram-scale synthesis of benzoxazole 2a to show the
practicality of this catalytic system (Table 3). Firstly, we delightedly found that the reaction
could proceed smoothly in gram-scale by the use of 1 g of OMS-2 and 10 mmol of raw at room
temperature and the product was obtained in 1.97 g (96% yield). After the reaction, the catalyst
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o
was readily recycled by filtration, washing and drying at 120 C for overnight. The results of
BET, XRD and TEM analysis showed that OMS-2 was highly stable in structure and
morphology (Figures S3-S5, in SI). Under gram-scale conditions, the catalyst was employed in 7
successive runs without any loss of activity.
Table 3. Recycling of OMS-2 in Gram-Scale Reactiona
Cycle
1
2
3
4
5
6
7
Yield [%]
96
96
95
94
96
95
95
a
Reaction conditions: 1a (10 mmol, 1.97 g), OMS-2 (1 g), DMC (50 mL),
H2O2 (2.5 mL), r.t., 6 h, isolated yields.
With the optimized conditions in hand, the substrate scope of the oxidative cyclization was
investigated. As show in Table 4, OMS-2/H2O2/DMC system could tolerate a lot of substrates
with functional groups. Phenolic imines derived from the condensation of substituted 2-amino
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phenols and benzaldehydes were readily converted into corresponding 2-subustituted
benzoxazoles in excellent yields under the presence conditions, although some substrates, such
as 1h, 1r and 1t, had to react for prolonging time in order to realize full conversion. It is worth
mentioning that hydroxyl group (2r) was tolerated in the reaction very well and multi-halogen-
substituted benzoxazoles (2f and 2g) were also isolated in excellent yields. Moreover,
heterocyclic phenolic imines participated into the reactions successfully resulting into high yields
of benzoxazoles (2s and 2t). In addition, phenolic imine generated from alkyl aldehyde and 2-
amino phenol was oxidized to benzoxazole 2u in 96% yield with the help of base to ensure full
conversion. Unfortunately, sensitive group nitro-substituted phenolic imine could not react under
the standard conditions and it was recovered after the reaction.
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Table 4. Scope of The Reactions for The Synthesis of Benzoxazolesa
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a Reaction conditions: phenolic imine (0.2 mmol), OMS-2 (20 mg, 10 mol%),
DMC (1 mL), H2O2 (50 uL) r.t., 6-12 h, isolated yields.
b For 20 h.
c For 12 h.
d For 24 h.
e
Reaction conditions: phenolic imine (0.2 mmol), OMS-2 (20 mg, 10 mol%),
H2O2 (100 uL), K2CO3 (0.2 mmol), DMC (1 mL), r.t., 12 h, isolated yield.
To assess the versatility of the present catalytic system, we turned our attention to the
oxidative cyclization of thiophenolic imine 1aa. The corresponding product benzothiazole 2aa
was obtained in excellent yield without any changes in the standard conditions, which indicated
OMS-2 was not poisoned by sulphur (Scheme 2). Afterwards, we examined the catalytic system
in one-pot synthesis of benzaxazole from aldehyde and 2-aminophenol (Scheme 3). Under mild
reaction conditions, benzoxazole 2a was successfully synthesized directly from benzaldehyde
and 2-amino phenol via the in-situ generated phenolic imine with good yield. Then, a multigram-
scale synthesis of benzoxazole 2a was performed to demonstrate the reaction is also feasible for
process, and the reaction gave us a satisfactory yield of 80% (Scheme 4).
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Scheme 2. The oxidative synthesis of benzothiazole from thiophenolic imine. Reaction
conditions: 1aa (0.2 mmol), OMS-2 (20 mg, 10 mol%), H2O2 (50 uL), DMC (1 mL), r.t., 6 h,
isolated yield.
O
H2N
HO
O
OMS-2, DMC
+
r.t., H2O2, 12 h
N
85% yield
Scheme 3. The one-pot synthesis of benzoxazole 2aa from benzaldehyde and 2-amino phenol.
Reaction conditions: benzaldehyde (0.2 mmol), 2-amino phenol (0.24 mmol), OMS-2 (20 mg, 10
mol%), DMC (1 mL), H2O2 (50 uL), r.t., 12 h, isolated yield.
OH
O
N
OMS-2, DMC
N
r.t., H2O2, 12 h
1a, 250 mmol, 49.25 g
2a, 80% yield, 39 g
Scheme 4. The multigram-scale synthesis of benzoxazole via OMS-2/H2O2/DMC system.
Reaction conditions: phenolic imine 1a (250 mmol, 49.25 g), OMS-2 (25 g, 10 mol%), DMC
(1.25 L), H2O2 (70 mL), r.t., 12 h, isolated yield.
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Finally, more challenging staring substrate N-substituted 2-aminophenol 3a was used for the
synthesis of benzoxazole 2a via a sequential oxidative process (Scheme 5).15 At the first place, it
was found that the standard catalytic system could not give desired product with green oxidants
even at elevated reaction temperature, because OMS-2 could not catalyze the oxidative
dehydrogenation of C-N bond. Then, the introduction of a transition metal in the OMS-2 showed
an obvious fruitful effect in the conversion. In the presence of O2 or H2O2, OMS-2-supported
copper catalyst made the reaction proceed under heating conditions probably via a multi-electron
redox cycle we proposed previously.[5] The yield of benzoxazole 2a could reach to 70% yield
when using H2O2 as the oxidant in DMC through the efficient oxidative dehydrogenation of C-N
bond of 3a and following aerobic cyclization, which provides a potential of applying the
sustainable Cu-OMS-2/H2O2/DMC synthetic system in other oxidative dehydrogenation
reactions for the synthesis of valuable organic molecules.16
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OH
O
N
OMS-2-based material
oxidant, DMC, 18 h
N
H
3a
2a
OMS-2 (20 mg, 10 mol%), H2O2 or O2, r.t.- 60 oC
Cu(OH)x/OMS-2 (40 mg, Cu: 4 mol%), H2O2 or O2, r.t.
Cu(OH)x/OMS-2 (40 mg, Cu: 4 mol%), O2, 80 oC
Cu(OH)x/OMS-2 (40 mg, Cu: 4 mol%), H2O2, 80 oC
trace
trace
55% yield
70% yiled
Scheme 5. The sequential reaction for the synthesis of benzoxazole 2a from 3a (2 mmol) via
oxidative dehydrogenation and cyclization.
CONCLUSION
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In conclusion, we have developed an environmentally-safe reaction condition by the use of
OMS-2 as heterogeneous catalyst, H2O2 as oxidant and DMC as reaction medium for the
oxidative synthesis of 2-substituted benzoxazoles and 2-phenyl benzothiazole at room
temperature. Moreover, via introducing transition metal, Cu(OH)x/OMS-2 can catalyze
benzoxazole synthesis from N-substituted 2-aminophenol through oxidative dehydrogenation of
C-N bond followed by oxidative cyclization. The use of environmentally-friendly reagents, mild
reaction conditions, and catalyst reusability in gram-scale synthesis and easy isolation of
products make this scalable catalytic methodology practical and potential for industrial
application, which will prompt us to develop other sustainable chemical transformation
processes.
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EXPERIMENTAL SECTION
General. Unless otherwise noted, all reagents were purchased from commercial suppliers and
used without further purification. Metal salts were commercially available and were used
directly. H2O2 (35 wt.% in water) purchased from Sigma-Aldrich was used in this work and
DMC (CAS NO. 616-38-6, ReagentPlus®, 99%) was purchased from Sigma-Aldrich with a
mark of Greener Alternative Product. All reactions were performed in vials or flasks capped
under air. AMO (amorphous manganese oxide) and a-MnO2 were prepared according previous
report. Flash chromatography was carried out with Merck silica gel 60 (200-300 mesh) when
column chromatographic isolation is necessary. Analytical TLC was performed with Merck
silica gel 60 F254 plates, and the products were visualized by UV detection.
Preparation of OMS-2. 5.89 g of KMnO4 in 100 mL of deionized water was added to a solution
of 8.8 g of MnSO4·H2O in 30 mL of deionized water and 3 mL concentrated HNO3. The solution
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was refluxed at 100 oC for 24 h, and the product was filtered, washed, and dried at 120 oC for 8 h.
Finally, the OMS-2 was calcined in a muffle furnace at 350 oC for 2 h. Then, the black powder
OMS-2 was obtained.
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General procedure for OMS-2-catlayzed 2-substituted benzoxazoles synthesis. OMS-2 (20
mg, 10 mol%), phenolic imine (0.2 mmol), H2O2 (50 uL) and DMC (1 mL) were added to a vial
with a bar. The mixture was stirred at ambient temperature for 6 h. Then, the mixture was diluted
with ethyl acetate and filtered. The filtrate was removed under reduced pressure to get the crude
product or pure product for some cases, and the crude product was further purified by silica gel
chromatography (petroleum/ethyl acetate = 40/1 as eluent) to yield corresponding product if it is
necessary.
Procedure for Cu(OH)x/OMS-2-catlayzed 2-phenyl benzoxazole synthesis from N-
substituted 2-aminophenol 3a. Cu(OH)x/OMS-2 (40 mg, Cu: 4 mol%), 3a (0.2 mmol), H2O2
(50 uL) and DMC (1 mL) were added to a vial with a bar. The mixture was stirred at 80 oC for
18 h. Then, the mixture was diluted with ethyl acetate and filtered. The filtrate was removed
under reduced pressure to get the crude product, and the crude product was purified by silica gel
chromatography (petroleum/ethyl acetate = 40/1 as eluent) to yield pure 2a.
Synthesis of benzoxazole 2a in gram-scale. OMS-2 (1 g, 10 mol%), phenolic imine 1a (10
mmol, 1.97 g), H2O2 (2.5 mL) and DMC (50 mL) were added to a 250 mL flask with a bar. The
mixture was stirred at ambient temperature for 6 h. Then, the mixture was filtered to remove the
catalyst, and remove the solvent under reduced pressure to yield corresponding product 2a in
almost 96% yield directly.
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Synthesis of benzoxazole 2a in multigram-scale. OMS-2 (25 g, 10 mol%), phenolic imine 1a
(250 mmol, 49.25 g) and DMC (1.25 L) were added to a 2 L reaction flask with a bar under air
and the reaction flask was capped. H2O2 (70 mL) was added dropwise in the reaction mixture via
a constant pressure dropping funnel within 2 h. After that, the reaction was allowed to vigorously
stir at room temperature for 12 h. Then, the mixture was filtered to remove the catalyst, and
remove the solvent under reduced pressure to yield corresponding product 2a by flash
chromatography on silica gel in almost 80% yield.
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AUTHOR INFORMATION
Corresponding Author
zhaopq@licp.cas.cn
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
Funding Sources
The Natural Science Foundation of China (No. 21403256, 21372102, 21573261) and the Suzhou
Industrial Technology and Innovation Project (Grant No. SYG201531).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
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We gratefully acknowledge the Natural Science Foundation of China (No. 21403256, 21372102,
21573261) and the Suzhou Industrial Technology and Innovation Project (Grant No.
SYG201531).
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SUPPORTING INFORMATION
Experimental information, characterization of catalyst, hot filtration experiments and copies of
1H and 13C NMR for all products.
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