Phenanthroline-based microporous organic polymer as a platform for an immobilized palladium catalyst for organic transformations

Article abstract of DOI:10.1039/c9ra00460b

Porous organic polymers have attracted significant attention owing to their large specific surface area, excellent chemical and thermal stability, and controllable skeletons. phenanthroline-based microporous organic polymer (Phen-MOP) has been synthesized via a cost-effective method based on the Scholl reaction. The Phen-MOP polymer exhibits high surface area and good stability. Owing to the phenanthroline skeleton embedding into the microporous polymer framework, the Phen-MOP can serve as a platform to support a transition metal catalyst. After being post-modified with palladium acetate, the synthesized Phen-Pd-MOP framework can serve as a highly efficient heterogeneous catalyst for the Suzuki-Miyaura coupling reaction and the Heck coupling reaction. Moreover, the Phen-Pd-MOP catalyst could be reused at least 10-12 times without any significant loss of the catalytic activity.

Full text of DOI:10.1039/c9ra00460b

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Phenanthroline-based microporous organic
polymer as a platform for an immobilized palladium
Cite this: RSC Adv., 2019, 9, 8239
catalyst for organic transformations†
a
a
b
a
a
Chang-An Wang, * Kun Nie, Guo-Dong Song, Yan-Wei Li and Yin-Feng Han*
Porous organic polymers have attracted significant attention owing to their large specific surface area,
excellent chemical and thermal stability, and controllable skeletons. phenanthroline-based
microporous organic polymer (Phen-MOP) has been synthesized via a cost-effective method based
on the Scholl reaction. The Phen-MOP polymer exhibits high surface area and good stability. Owing to
the phenanthroline skeleton embedding into the microporous polymer framework, the Phen-MOP can
serve as a platform to support a transition metal catalyst. After being post-modified with palladium
acetate, the synthesized Phen-Pd-MOP framework can serve as a highly efficient heterogeneous
catalyst for the Suzuki–Miyaura coupling reaction and the Heck coupling reaction. Moreover, the
Phen-Pd-MOP catalyst could be reused at least 10–12 times without any significant loss of the
catalytic activity.
Received 18th January 2019
Accepted 27th February 2019
DOI: 10.1039/c9ra00460b
rsc.li/rsc-advances
a catalyst. Compared to the traditional polymerization reactions
Introduction
(expensive and rare transition/noble metal as the catalysts) used
1
Porous organic polymers (POPs) with their high surface area for the synthesis of CMPs, PAFs, or other POPs frameworks, the
and easily functionalization have diverse potential applications Scholl coupling reaction (AlCl as the catalyst) used for con-
3
2
3
in the elds of gas storage/separation, light-harvesting, and structing POPs frameworks is economical and abundant,
heterogeneous catalysis. During the past decade, various POPs, although the amount of catalyst required is equivalent. More
such as conjugated microporous polymers (CMPs), hyper- importantly, there is no need for the design and synthesis of
crosslinked polymers (HCPs), polymers of intrinsic micropo- organic building blocks with special functional groups (e.g. –Br,
4
5
6
7
8
rosity (PIMs), covalent organic framework (COFs), and porous –I, –NH
2
, –C^CH, –CHO, –CN, and –B(OH)
2
) used in the
9
aromatic framework (PAFs), have been successfully designed synthesis of POPs frameworks. It means that we can save a lot of
and synthesized based purely on structural and functional costs by avoiding tedious synthesis steps to prepare monomers.
organic building blocks. However, many expensive and rare The microporous polymers formed by the Scholl reaction with
transition/noble metal catalysts are used in constructing POPs the microporous and conjugated structure exhibit excellent
materials, and functional groups (e.g. –Br, –I, –NH
CHO, –CN, and –B(OH)
) are demanded in the organic separation, sensor, luminescence and semiconductor
building blocks for the synthesis of POPs frameworks, which materials.
2
, –C^CH, properties for potential applications in the area of gas storage,
–
2
hinders their practical application on a large scale. Accordingly,
Phenanthroline, which is a nitrogen donor-based chelating
it is essential to develop a cost-effective method to synthesize ligand, has been extensively employed in coordination chem-
1
2
functional POPs frameworks.
istry. In particular, owing to the robust redox stability, easy
10
11
13
In this context, recently, Tan's group and Zhu's group
functionalization, and favored coordinate with metal ions,
independently reported a cost-effective approach to synthesize metal–phenanthroline ligands have been routinely used as
14
POPs frameworks based on the Scholl reaction. This synthetic homogeneous catalysts for organic transformations. For
method forms a new aryl–aryl bond by eliminating two aryl- example, Yu's group reported a series of Pd-catalyzed C-3
bound hydrogen atoms in the presence of Lewis acid AlCl as selective C–H activations of pyridines with 1,10-phenanthro-
3
15
line as the ligand. However, these homogeneous catalysts are
quite expensive and it is difficult to separate the catalysts from
the reaction system, which limits their practical application.
Therefore, exploration of heterogeneous catalysts based on
phenanthroline and phenanthryl-derived ligands has attracted
a
College of Chemistry and Chemical Engineering, Taishan University, Tai'an,
Shandong 271000, P. R. China. E-mail: wangcha@tsu.edu.cn; han@tsu.edu.cn
b
Weifang University of Science and Technology, Shandong Peninsula Engineering
Research Center of Comprehensive Brine Utilization, Weifang 262700, P. R. China
16
signicant attention. In this context, Lin and co-workers re-
ported the synthesis of a series of robust and porous
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra00460b
1
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phenanthryl-based metal–organic frameworks (MOFs) as highly
active single site solid catalysts for tandem catalytic organic
17
transformation. In consideration of the rigid skeleton struc-
ture and strong coordinating ability of phenanthroline, herein,
we report a cost-effective strategy to synthesize phenanthroline-
based microporous organic polymers based on the Scholl
coupling reaction. The resulting POPs, denoted Phen-MOP,
exhibit high surface areas and high thermal and chemical
stability. Aer being post-modied with palladium ions, this
porous polymer framework (denoted as Phen-Pd-MOP, Scheme
1) could be applied as a highly efficient and recyclable hetero-
geneous catalyst for organic transformations, such as the
Suzuki–Miyaura coupling reaction and the Heck coupling
18
reaction.
Results and discussion
Synthesis and characterization of Phen-MOP and Phen-Pd-
MOP
In order to realize the industrial application of functional
POPs frameworks, the key factors for synthesizing POPs
should be considered as follows: (i) the synthetic method
should be easy to operate; (ii) the catalyst used must be low-
cost; (iii) the polymers can be prepared on a large scale.
1
0
Inspired by the pioneering work of Tan's group, in which
multifunctional microporous organic polymers were easily Fig. 1 (a) N2 sorption isotherms measured at 77 K. (b) Pore size
synthesized based on the Scholl coupling reaction, we have distribution (PSD) curve for Phen-MOP calculated by NLDFT method.
selected 4,7-diphenyl-1,10-phenanthroline 1 as the functional
building monomer and 1,3,5-triphenylbenzene 2 as the
Emmett–Teller (BET) surface area of Phen-MOP is estimated as
structural building monomer to synthesize a phenanthroline-
based microporous organic polymer (Phen-MOP) via this cost-
effective approach (Scheme 1). The obtained Phen-MOP
framework was precipitated from the solution as a brown
powder, and it was insoluble in all organic solvents tested
owing to the highly cross-linked structure. The porosity of
Phen-MOP was investigated by physisorption of nitrogen at 77
K. The representative curve of Phen-MOP exhibits a type I
nitrogen gas sorption isotherm (Fig. 1(a)), which suggests that
there are abundant micropores in the polymer. Based upon the
calculations of the non-local density functional theory
2
ꢀ1
3
ꢀ1
6
55 m g with a total pore volume of 0.32 cm g
The structure of Phen-MOP was initially characterized by
.
1
3
solid-state C cross-polarization magic-angle spinning (CP/
1
3
MAS) NMR analysis (Fig. 2). The C CP/MAS NMR analysis
conrmed the successful growth of a microporous network. As
shown in Fig. 2, the resonance peaks at d ¼ 150, 145, 123, and
112 ppm can be assigned to the 1,10-phenanthroline skeleton,
which suggests that the functional structure has been success-
fully embedded into the framework. The peaks at d ¼ 140 and
127 ppm can be assigned to the 1,3,5-triphenylbenzene skel-
eton. Additionally, elemental analysis of Phen-MOP identied
the nitrogen content (2.34%), and the phenanthroline loading
(
NLDFT), the pore size distribution (PSD) of Phen-MOP is
distributed around 0.5–1.5 nm (Fig. 1(b)). The Brunauer–
Scheme 1 Synthesis of phenanthroline– and Pd(II)–phenanthroline-based microporous organic polymers (Phen-MOP and Phen-Pd-MOP).
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framework is amorphous in nature (Fig. S3†), the same as
1
9
for previous POPs reported from our laboratory.
The Phen-MOP framework, with bidentate nitrogen-binding
sites, was simply post-treated with Pd(OAc)2 to yield the
palladium-incorporated porous organic catalyst Phen-Pd-MOP.
ꢁ
TGA showed that Phen-Pd-MOP was stable up to around 350 C
under a nitrogen atmosphere (Fig. 3(c)). Compared with the
Phen-MOP framework, the BET surface area of Phen-Pd-MOP
2
ꢀ1
decreased to 403 m g (Fig. S1 in the ESI†), the reason is that
the Pd(OAc)2 is handing in the nanopores volume. Next, we
selected X-ray photoelectron spectroscopy (XPS) measurements
and energy-dispersive X-ray spectroscopy (EDX) to survey the
coordination of palladium within the Phen-Pd-MOP framework.
As shown in Fig. 4 (red), the binding energy (BE) of Pd3d5/2 in
Phen-Pd-MOP is 337.9 eV, which indicated that the Pd species
in the Phen-Pd-MOP framework was present in a 2+ oxidation
state. In comparison with the BE of 338.2 eV for free Pd(OAc)
Fig. 4 (black)), the Pd(II) BE in Phen-Pd-MOP was shied
2
13
Fig. 2 Cross-polarization (CP) C MAS NMR spectrum of Phen-MOP.
(
negatively by 0.3 eV, which indicated the strong coordination of
Pd(OAc) with the bidentate nitrogen group of Phen-MOP. We
2
could be calculated based on the nitrogen content. All of the also found that the BE of Pd3d5/2 in Phen-Pd-MOP has the same
results conrmed the successful synthesis of the Phen-MOP value in Pd/Phen (ref. 20) (this structure was synthesized from
framework.
2
1,10-phenanthroline and Pd(OAc) ). The chemical state of N
Scanning electron microscopy (SEM) revealed that the elements in Phen-MOP and Phen-Pd-MOP has also been
polymer consists of plate-shaped monoliths of several investigated by XPS analysis (Fig. S2 in the ESI†). As shown in
micrometers in size (Fig. 3(a)). High-resolution TEM image Fig. 5, elemental mapping using energy-dispersive X-ray spec-
showed that abundant nanometer-scale cavities were troscopy (EDX) showed that a homogeneous distribution of
present in the Phen-MOP framework (Fig. 3(b)). Thermog- palladium accompanied by the well-dispersed element nitrogen
ravimetric analysis (TGA) showed that the decomposition of were found in the Phen-Pd-MOP framework. All of the results
ꢁ
the framework starts at 450 C under a nitrogen atmosphere conrm that the Pd(II) is successfully immobilized on the Phen-
(
Fig. 3(c)) and the modied weight (%) changed so much at MOP by coordination to phenanthroline functional groups
ꢁ
the low temperature of 100 C owing to these polymers rather than by physical adsorption of Pd(OAc)
2
on the surface.
adsorbing water when they are exposed to moist air for
a long time. The PXRD patterns indicate that the Phen-MOP
Heterogeneous catalytic application of Phen-Pd-MOP
Aer the palladium-incorporated porous polymers were
synthesized and characterized, we then investigated the cata-
lytic activity of the Phen-Pd-MOP framework as a robust and
powerful heterogeneous catalyst. The resultant Phen-Pd-MOP
was rstly tested on the Suzuki–Miyaura coupling reaction of
Fig. 3 (a) SEM image of Phen-MOP. (b) TEM image of Phen-MOP. (c)
TGA curves of Phen-MOP and Phen-Pd-MOP.
2
Fig. 4 XPS spectra of Phen-Pd-MOP (red) and free Pd(OAc) (black).
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Pd-MOP-catalyzed Suzuki–Miyaura reaction between phenyl-
boronic acid and various bromo- and iodo-benzene derivatives.
As shown in Table 1, all of the reactions were completed very
efficiently, giving excellent yields of the corresponding prod-
ucts. For the aryl bromide derivatives, regardless of the effects of
electron-donating or -withdrawing functions, high catalytic
activity was observed (Table 1, entries 1–7). Especially for aryl
iodides, the coupling reactions were rapidly completed within
Fig. 5 EDX elemental mapping of Phen-Pd-MOP.
0.5 h and gave high yields (entries 8 and 9). Next, in order to
arylboronic acids with various types of aryl halides in different determine that the reaction was indeed catalyzed by the Phen-
solvents. We chose the reaction of phenylboronic acid with Pd-MOP catalyst and not the dissolved homogeneous Pd species
bromobenzene as the model reaction to optimize the reaction leached from the supports, the following strategy was adopted.
conditions, and the results are listed in Table S1 (see the ESI†). When the phenylboronic acid conversion reached about 50%,
As shown in Table S1,† the reaction did not work when the the reaction mixture was quickly centrifuged to remove the
Phen-MOP framework was used as the catalyst or in the absence Phen-Pd-MOP catalyst and then the hot mother liquor was
of Phen-Pd-MOP, which conrmed that the Phen-Pd-MOP allowed to react for another 2 h under similar conditions. No
played the catalytic role (Table S1, entries 10 and 11†). signicant changes were observed in either the conversion or
Screening experiments with different solvents revealed that the the yield, indicating that the catalytic species was not dissolved
Suzuki–Miyaura reaction gave the highest yield when performed Pd(II) leached from the Phen-Pd-MOP framework.
ꢁ
in EtOH/H
2
O (1 : 1, v/v) at 80 C (Table S1, entry 8†). Under the
In order to verify the universality of the Phen-Pd-MOP cata-
optimized conditions, we then examined the scope of the Phen- lyst, we then chose the palladium-catalyzed Heck coupling
reaction to further investigate the catalytic activity of the Phen-
Pd-MOP framework. Firstly, we selected the Heck coupling
reaction of iodobenzene with methyl acrylate to optimize the
reaction conditions, and the results are listed in Table S3 (see
the ESI†). Screening experiments with different solvents and
Table 1 Phen-Pd-MOP-catalyzed Suzuki–Miyaura reaction of various
aryl halides with phenylboronic acid
a
temperatures showed that the best reaction conditions were:
DMF as the solvent, 0.6 mol% Phen-Pd-MOP as the catalyst, in
ꢁ
the presence of Et
3
N, and at 130 C (Table S3, entry 1†). With the
best reaction conditions established, various aryl iodides and
bromides were tested to examine the scope of the Heck coupling
reaction, and the results are shown in Table 2. For both
electron-donating aryl iodides (entries 1–4) and electron-
withdrawing aryl iodides (entries 5 and 6), as well as sterically
hindered aryl iodides (entries 7–9), the reaction proceeded
efficiently to give the corresponding products in excellent yields
b
c
Entry
Ar–X
Time (h)
Yield (%)
TON
d
1
0.5
8
99
95
165
158
2
3
(94–99%) within 2 h. However, for aryl bromides (entries 10–12),
2
6
97
94
162
157
the reaction proceeded more slowly to afford the products in
good to excellent yields (89–94%). With styrene as a substrate,
the reaction also afforded the corresponding products with high
yields, although a longer reaction time was required (entries 13
and 14). These results showed that the Phen-Pd-MOP frame-
work could work as a highly efficient heterogeneous catalyst for
the Suzuki–Miyaura reaction and the Heck coupling reaction.
The recyclability is a signicant indicator for evaluating
robust and powerful heterogeneous catalysts. Therefore, the
recyclability of the Phen-Pd-MOP catalyst was also investigated.
The catalytic recyclability of Phen-Pd-MOP was estimated by
examining both the Suzuki–Miyaura reaction of phenylboronic
acid with bromobenzene (see ESI, Table S2†) and the Heck
coupling reaction of iodobenzene with methyl acrylate (see ESI,
Table S4†). The recycling experiment was performed by recov-
ering the Phen-Pd-MOP by using a simple centrifugation
4
5
6
7
5
93
92
97
99
155
153
162
165
2
2
d
8
0.5
9
0.5
98
196
a
Reaction conditions: aryl halide (0.5 mmol), phenylboronic acid (0.75 method and the recovered catalyst was then washed with EtOH
mmol), K
2
3 2
COb (1.0 mmol), Phen-Pd-MOP (0.6 mol%), EtOH/H O (1.0
and EtOAc to remove the adherent products. Aer being dried
ꢁ
mL), 80 C. Isolated yield aer silica gel column chromatography.
ꢁ
c
d
under vacuum in 80 C, the heterogeneous catalyst could be
TON ¼ (moles of product)/(moles of Pd in the catalyst). Aryl halide
(0.75 mmol), phenylboronic acid (0.5 mmol).
reused directly without further purication. As shown in Fig. 6,
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Table 2 Phen-Pd-MOP catalyzed the Heck coupling reaction of
a
various aryl halides to olefin
b
c
Entry
Aryl halide
Olen
Time (h)
Yield (%)
TON
1
2
1
2
99
95
165
158
3
4
5
6
2
2
2
2
96
99
99
97
160
165
165
162
7
2
95
158
8
9
1
1
98
94
163
157
Fig. 6 Phen-Pd-MOP recycling experiments for the Suzuki–Miyaura
coupling reaction (a) and Heck coupling reaction (b).
1
1
1
1
0
1
2
3
5
92
89
94
80
153
148
157
133
5
Conclusions
5
In summary, we have reported a new and cost-effective strategy
to synthesize a phenanthroline-functionalized microporous
organic polymer (Phen-MOP) based on the Scholl coupling
reaction. The new polymer has outstanding stability and good
porosity with a high BET surface area. Owing to the phenan-
throline skeleton embedded in the microporous polymer
framework, Phen-MOP can serve as an ideal platform to support
transition metal catalysts for heterogeneous catalysis. Aer
being post-modied with palladium acetate, the synthesized
Phen-Pd-MOP framework can serve as a highly efficient
heterogeneous catalyst for a wide range of organic trans-
formations. Catalyzed by the Phen-Pd-MOP catalyst, the Suzuki–
Miyaura coupling reaction of aryl halides to phenylboronic acid
gave the corresponding products in excellent yields (92–99%)
and it could be reused at least 12 times without loss of catalytic
activity. Meanwhile, catalyzed by the Phen-Pd-MOP catalyst, the
Heck coupling reaction of aryl halides to olen also gave the
desired products in good to excellent yields (82–99%) and it
could be recycled and reused 10 times without an obvious
decrease in the catalytic efficiency. We believe that the low-cost
Phen-MOP framework prepared in this work may act as an ideal
platform to support transition metal catalysts for application on
a large scale.
24
1
4
15
82
137
a
General conditions: aryl halide (0.5 mmol), olen (0.75 mmol), Et
3
N
(
1
0.75 mmol, 1.5 eq.), DMF (1.0 mL) and Phen-Pd-MOP (0.6 mol%),
30 C. Isolated yields. TON ¼ (moles of product)/(moles of Pd in
ꢁ
b
c
the catalyst).
owing to the strong coordination ability of Phen-MOP and the
highly dispersed Pd(II) ions embedded into the Phen-MOP
framework, this Phen-Pd-MOP catalyst could be recycled and
reused at least 12 times for the Suzuki–Miyaura coupling reac-
tion or 10 times for the Heck coupling reaction without loss of
the catalytic activity. Meanwhile, we did not nd any Pd(II) ions
leaching from the Phen-Pd-MOP catalyst aer every cycle, as
determined by ICP. The BET surface area of the Phen-Pd-MOP
2
ꢀ1
th
framework decreased to 272 m g aer the 12 cycle of use
Fig. S1 in the ESI†), which could be due to the partial blocking
(
of the polymeric nanopores by substrates.
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General procedure for Heck coupling reaction catalyzed by
Phen-Pd-MOP
Experimental section
Materials
In a typical run of a catalytic activity test of Phen-Pd-MOP for the
Heck coupling reaction, aryl halides (0.5 mmol), methyl acrylate
4
,7-Diphenyl-1,10-phenanthroline and 1,3,5-triphenylbenzene
were received from J&K. Pd(OAc) and chloroform and anhy-
drous AlCl were purchased from Energy Chemical (Shanghai,
2
3
(64.5 mg, 0.75 mmol, 1.5 eq.), Et N (0.75 mmol, 1.5 eq.), and
3
Phen-Pd-MOP (3.0 mg, 0.6 mol%) were added to 1.0 mL of DMF.
China) and used as received. All reagents were obtained from
commercial suppliers and used without further purication. All
catalytic reactions were performed in a 15 mL pressure tube.
ꢁ
The reaction mixture was stirred at 130 C under a N
2
atmo-
sphere. Aer the reaction was completed (monitored by TLC),
the mixture was cooled to room temperature and centrifuged,
and the obtained Phen-Pd-MOP catalyst was washed with EtOAc
(
3 ꢂ 5 mL). The combined organic phase was washed with water
Synthesis of Phen-MOP
and dried over anhydrous Na SO . Aer the evaporation of the
2
4
Anhydrous AlCl
round-bottomed ask. Then, aer pumping to vacuum, the
system was inated with inert gas N three times. Next, 20 mL of
3
(665 mg, 5.0 mmol) was added to a 100 mL
solvent under vacuum, the residue was puried by ash column
chromatography with petroleum ether–EtOAc (6 : 1 to 20 : 1) as
the eluent.
2
dried CHCl
heated to 60
3
was injected through a syringe and the mixture was
ꢁ
C for 30 min. Then 4,7-diphenyl-1,10-
Recyclability of Phen-Pd-MOP catalyst
phenanthroline (166 mg, 0.5 mmol) and 1,3,5-triphe-
nylbenzene (102 mg, 0.33 mmol) dissolved in 20 mL of dried The Phen-Pd-MOP catalyst was recycled by centrifugation
CHCl were added into the system and the mixture was kept method. The recovered Phen-Pd-MOP catalyst was washed with
3
ꢁ
stirring at 60 C for 24 h. Aer the reaction had completed, the EtOH and EtOAc to remove the residual product and then
crude product was obtained by ltration and washed with 1 M simply dried before reuse. We chose the Suzuki–Miyaura
hydrochloric acid solution, methanol, and acetone to remove coupling reaction of bromobenzene with phenylboronic acid
unreacted monomers and catalyst residues. Further purica- and the Heck coupling reaction of iodobenzene with methyl
tion of the polymer was carried out by Soxhlet extraction with acrylate to investigate the recyclability of the Phen-Pd-MOP
ꢁ
methanol for 48 h. The polymer was dried at 80 C under catalyst, and the results are summarized in Tables S2 and S4.†
vacuum for 6 h to give a brown powder. Yield: 262 mg (98%).
Elemental analysis (%) found: C 48.73, N 2.34, H 4.09.
Conflicts of interest
There are no conicts to declare.
Synthesis of Phen-Pd-MOP catalyst
Palladium acetate (45 mg, 0.2 mmol) was dissolved in 10 mL of
acetone, and then Phen-MOP (100 mg, 0.08 mmol) was added. Acknowledgements
The mixture was kept stirring for 2 h at room temperature. Aer
We gratefully acknowledge the nancial support by the National
the reaction was complete, the resulting solid was ltered and
Science Foundation of China (No. 21502136 and 21571137), the
washed thoroughly with acetone (10 mL ꢂ 5), distilled water
ꢁ
Higher Educational Science and Technology Program of Shan-
dong Province (No. J15LC18), and the Doctoral Fund Project of
Weifang University of Science and Technology (No. 2017BS08).
(
10 mL ꢂ 5) and MeOH (10 mL ꢂ 5), then dried at 80 C under
vacuum for 12 h to yield Phen-Pd-MOP as a brown powder (110
mg). The Pd content in Phen-Pd-MOP was 10.60%, as deter-
mined by ICP.
Notes and references
General procedure for the Suzuki–Miyaura reaction catalyzed
by Phen-Pd-MOP
1
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515.
1
In a typical run of a catalytic activity test of Phen-Pd-MOP, aryl
halides (0.5 mmol), phenylboronic acid (91.5 mg, 0.75 mmol,
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1
.5 eq.), K CO (138 mg, 1.0 mmol, 2.0 eq.), and Phen-Pd-MOP
2 3
(
(
3.0 mg, 0.6 mol%) were added to 1.0 mL of 1.0/1.0 EtOH–H
v/v) mixture. The reaction mixture was stirred at 80 C under an
2
O
ꢁ
ambient atmosphere. Aer the reaction was completed (moni-
tored by TLC), the mixture was centrifuged and the solid was
washed with EtOH (1 ꢂ 5 mL) and EtOAc (3 ꢂ 5 mL). The
combined organic phase was washed with water to remove
K CO residue. Aer the evaporation of the solvent under
2
3
vacuum, the residue was puried by ash column chromatog-
raphy with petroleum ether or petroleum ether–EtOAc ¼ 10 : 1
as the eluent.
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