Graphitic Carbon Nitride Polymer as a Recyclable Photoredox Catalyst for Decarboxylative Alkynylation of Carboxylic Acids

Article abstract of DOI:10.1002/adsc.202000777

Visible-light-induced heterogeneous photocatalysis for decarboxylative alkynylation has been performed. The using of cheap, metal-free and recyclable graphitic carbon nitride (g-C₃N₄) as the photoredox catalyst in the process enables

Full text of DOI:10.1002/adsc.202000777

Title: Graphitic Carbon Nitride Polymer as a Recyclable Photoredox
Catalyst for Decarboxylative Alkynylation of Carboxylic Acids
Authors: Jia-Qi Guo, Ya-Ting Wang, Yu-Hang Li, Kai-Lin Lu, Shi-Hui
Liu, Wei Wang, and Yongqiang Zhang
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.202000777
Link to VoR: https://doi.org/10.1002/adsc.202000777

Advanced Synthesis & Catalysis
10.1002/adsc.202000777
Very Important Publication
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Graphitic Carbon Nitride Polymer as a Recyclable Photoredox
Catalyst for Decarboxylative Alkynylation of Carboxylic Acids
a
b
b
a
a
ac
Jiaqi Guo, Yating Wang, Yuhang Li, Kailin Lu, Shihui Liu, Wei Wang,*
a
Yongqiang Zhang*
a
State Key Laboratory of Bioreactor Engineering, Shanghai Key Laboratory of New Drug Design, and School of
Pharmacy, East China University of Science and Technology, Shanghai 200237, P. R. China
E-mail: yongqiangzhang@ecust.edu.cn
Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of
Hierarchical Nanomaterials, School of Materials Science and Engineering, East China University of Science and
Technology, Shanghai 200237, P. R. China
b
c
Department of Pharmacology and Toxicology and BIO5 Institute, University of Arizona, Tucson, AZ 85721-0207,
USA
E-mail: wwang@pharmacy.arizona.edu
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract:
Visible-light-induced
heterogeneous The photocatalytic reactions could also be performed in a
photocatalysis for decarboxylative alkynylation has been continuous flow fashion and scaled up to gram-scale.
performed. The using of cheap, metal-free and recyclable Furthermore, the preparative power of this strategy is
graphitic carbon nitride (g-C N ) as the photoredox catalyst in further highlighted by its application in the 1, 2-amido
3 4
the process enables the facile transformation of a variety of alkynylation of unactivated alkenes.
carboxylic acids into structurally diverse alkyne-containing
molecular architectures under mild and environmentally- Keywords: graphitic carbon nitride; heterogeneous
benign conditions. Notably, the heterogeneous nature of the photoredox catalysis; decarboxylative alkynylation;
reaction system allows for the recovery and reuse of the
catalyst in multiple runs without loss of reactivity.
sustainable synthesis
CO as a by-product. Despite advances, an important
2
Introduction
challenge that has not been fully addressed in this field
is how to activate carboxylic acids for further
transformations under environmentally-benign and
sustainable conditions. Recently, visible-light-induced
photoredox catalysis has been identified as an ideal
approach to generate active carbon radical species
In the last couple of years, visible-light-induced
photoredox catalysis has emerged as a powerful tool
for the development of new [ and valuable
1]
transformations in organic synthesis. However, the
[3]
primary contributions are based on the homogeneous
photoredox catalysts, such as ruthenium- or iridium-
based transition metal complexes or various organic
dyes. The photoredox catalysis using heterogeneous
photocatalysts, which exhibits the inherent advantage
of easy catalyst separation and recyclability and is of
great interest from the industrial point of view, remains
less explored.[
from carboxylic acids. This strategy has also been
extended to decarboxylative alkynylation reaction. For
example, in 2015, Chen group reported a visible-light-
induced photocatalytic decarboxylative alkynylation
reaction of α-keto acids with ethynylbenziodoxolones
(EBXs) using Ru(bpy)
3
(PF
6
2
) as the photocatalyst, in
which an extra hypervalent iodine (III) reagent was
employed as the co-oxidant to activate the carboxyl
2]
[4]
Carboxylic acids are among the most suitable
starting materials for chemical synthesis owing to their
ready availability, structural diversity, as well as the
non-toxic and easy to handle properties. One of the
most fascinating transformations of carboxylic acids is
the decarboxylative alkynylation reaction, which
enables the synthesis of various functional alkyne
molecules along with the simply releasing of non-toxic
group. These transformations were also achieved
under the direct irradiation of sunlight without the use
[5]
of any photocatalysts by Wang group. Shortly
afterward, the using of iridium complexes as the
alternative photocatalysts in the process, enables the
extension of substrate scope to various aliphatic acids
[
6]
while avoiding the use of the external co-oxidant.
Recently, a more cost-effective procedure has been
1
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Advanced Synthesis & Catalysis
10.1002/adsc.202000777
developed using cheap and easily handled 9,10-
dicyanoanthracene as the photocatalyst.[ While the
above mentioned strategy of photocatalytic
decarboxylative alkynylation is quite remarkable, the
homogeneous nature of the photocatalysts used in the
process makes their separation from the product as
well as their recycling difficult, which has
significantly restricted the industrial application of
these methods.
1a and phenyl ethynylbenziodoxolone (EBX) 2a as the
7]
model substrates, g-C
3
N
4
(1.5 mg/mL) as the
photocatalyst, K CO
2
3
(2 equiv) as the base, and CHCl
3
(0.1 M) as the solvent, the desired alkynylation
product 3a could be isolated in moderate yield (entry
1, 69% yield, Table S1). Lowering the catalyst loading
to 0.75 mg/mL led to comparable yield (entry 2, 67%
yield, Table S1), while low yield was achieved using
0.25 mg/mL of g-C
3
N
4
in the reaction (entry 3, 43%
was used for
yield, Table S1). 0.75 mg/mL of g-C
3
N
4
Scheme 1. The proposed g-C
3
N
4
mediated photocatalytic
further screening study. Screening of the blue LEDs
with precise wavelength identified 427 nm blue LEDs
as the most suitable light source (entries 1-3, 69% - 87%
yields, Table 1).
decarboxylative alkynylation reactions.
Table 1. The optimization and validation of the reaction
a
conditions.
blue
LEDs
entry
base (x equiv) solvent
yield b
Graphitic carbon nitride (g-C
3
N ), a metal-free
4
1
2
3
4
5
6
7
8
9
456 nm
427 nm
390 nm
427 nm
427 nm
427 nm
427 nm
427 nm
427 nm
427 nm
427 nm
427 nm
-
427 nm
427 nm
427 nm
427 nm
427 nm
427 nm
427 nm
K
K
K
2
2
2
CO
CO
CO
3
3
3
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
CHCl
CHCl
CHCl
CHCl
CHCl
CHCl
CHCl
CHCl
CHCl
CHCl
3
3
3
3
3
3
3
3
3
3
69%
87%
72%
82%
86%
83%
76%
80%
93%
95%
97%
polymer that is easily accessible by thermal
polymerisation of cheap precursors like urea and
cyanamide, is able to absorb light in the visible area
Cs
Na
HPO
CsF (2.0)
NaOAc (2.0)
2
CO
3
(
band gap: 2.7 eV; λEx ≈ 460 nm) and shows a high
photo- and thermal-stability. These properties render it
a promising sensitizer for visible-light-induced
heterogeneous photoredox catalysis. We envisioned
that the uncovering of new reactivity of g-C in
2
CO
3
K
2
4
(2.0)
[8]
3
N
4
K
K
K
K
K
CO
CO
CO
CO
CO
(1.0)
(0.5)
(0.5)
(0.5)
(0.5)
2
2
2
2
2
3
3
3
3
3
photocatalytic decarboxylation would enable the
development a visible-light-induced heterogeneous
photocatalytic decarboxylative alkynylation reaction.
1
1
1
1
1
1
1
1
1
1
2
0
1
2
3
4
5
6
7
8
9
0
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
2
2
2
2
2
2
2
2
2
2
Cl
2
2
2
2
2
2
2
2
2
2
c
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
90%
d, e
The proposed mechanism of this reaction is outlined in
n.d
Scheme 1.[ The excitation of g-C
9]
N with visible-
-
n.d d
3
4
f
light could result in the formation of oxidative valence
band holes and reductive conduction band electrons.
The hole oxidation is expected to promote the
decarboxylation process, enabling the generation of
various carbon-centered radicals. The addition of these
reactive species to ethynylbenziodoxolone (EBX)
reagents following by a radical elimination would
deliver the alkynylation products accompanied with
the generation of benziodoxolonyl radical (BIC, II).
Finally, BIC is reduced by the reductive conduction
band electrons to produce 2-iodobenzoic acid anion
K
K
K
K
K
K
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
(0.5)
(0.5)
(0.5)
(0.5)
(0.5)
(0.5)
45%
d, g
n.d
n.d
n.d
d, h
d, i
j
9%
k
83%
a)
Standard reaction conditions: a mixture of 1a (0.2 mmol),
a (0.3 mmol), g-C (1.5 mg), and base (0.4 - 0.1 mmol)
in solvent (2 mL) was irradiated by blue LEDs with specific
wavelengths at 25 C under N for 16 h, unless otherwise
2
noted. Isolated yields were reported. The reaction time
was shortened to 10 h. Not detected. In darkness. In the
absence of g-C
employed in the reaction. 3.0 equiv of BHT was employed
in the reaction. 3.0 equiv of K S O was employed in the
2 2 8
2
3 4
N
o
b)
c)
(
III) as a by-product and complete the photocatalytic
cycle.
d)
e)
f)
g)
h)
3
N
4
.
Under air. 3.0 equiv of TEMPO was
i)
j)
k)
Results and Discussion
reaction. 3.0 equiv of ammonia oxalate was employed in
the reaction.
Considering the fact that g-C
3
N possesses significant
4
[
8]
absorption in blue light region, we started our
investigations using multi-wavelength blue LEDs (15
Several commonly used bases in photocatalytic
W) as the light source. g-C
3
N
4
is prepared by
decarboxylation, such as Cs
CsF and NaOAc, were then examined, and K
preferred (entries 4-8, 76% - 86% yields, Table 1).
Interestingly, the decrease of the amount of K CO
2
CO
3
, Na
2
CO
3
, K
2
HPO
3
4
,
o
thermolysis of urea at 550 C according to a previously
reported protocol (see supporting information for
details).[ Gratifyingly, using Boc-protected proline
2
CO
was
10]
2
3
2
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Advanced Synthesis & Catalysis
10.1002/adsc.202000777
resulted in improved yields, and the yield could be
– 3k, 64% - 88% yields), while the electron-rich aryl
groups significantly reduced the yields (3m – 3n, 39%
- 52% yields). Nevertheless, the phenyl-EBX reagent
with a cyan substituent on the phenyl motif gave the
corresponding alkynylation product in low yield (3l,
40% yield), which might attribute to its inferior photo-
stability. Furthermore, the use of tert-butyldiphenysilyl
raised to 95% using only 0.5 equiv of K
2
CO in the
3
reaction (entries 9-10, 93% - 95% yields, Table 1).
Screening of the reaction solvents revealed that the
more
economic
and
relatively
non-toxic
dichloromethane was optimal for the reaction (Table
S2). The shortening of the reaction time resulted in
inferior reaction efficiency (entry 12, 90% yield, Table
(
(
TBDPS) in place of aryl also resulted in high yield
3o, 77%). It is of note that the TBDPS group in the
1
). Control experiments indicate that visible-light,
photocatalyst and base are essential for this reaction
entries 13-15, Table 1). The complete inhibition of the
product 3o could be easily removed to give the
terminal alkyne for further transformations such as the
Sonogashira cross-coupling and alkyne-azide click
reaction. However, the reaction of octyl-substituted
EBX reagent was complicated and only trace product
was detected (3p, Scheme 2).
(
reactivity was observed by the presence of molecular
oxygen or radical scavenger (TEMPO and BHT)
(
entries 16-18, Table 1), again suggesting the radical
nature of the reaction. The control experiments using
The scope of carboxylic acids in the decarboxylative
alkynylation reaction was examined next. As revealed
in Scheme 3, we were delighted to find that a broad
range of α-amino acid readily participate as coupling
partners in this protocol, generating synthetically
versatile alkynylamines products in moderate to
excellent yields (5a – 5p, 38% - 94% yields). Fmoc-
and Cbz-amino protecting groups were well tolerated
ammonium oxalate as the hole scavenger or K
S
2 2
8
O as
the electron scavenger gave the product 3a in lower
yields (9% - 83% yields, entries 19-20, Table 1), which
indicates that both hole and electron play essential
roles in the process.[
11]
Accordingly, the optimal
reaction conditions are established as shown in entry
1 (427 nm blue LEDs, g-C 0.75 mg/mL, K CO
.5 equiv, CH Cl , N , rt, 16 h).
1
0
N
3 4
2
3
2
2
2
(
5a – 5b, 88% - 92% yields). The introduction of
methyl substituent on the nitrogen atom of amino acid
resulted in a remarkable increase of reaction efficiency
Scheme 2. Scope of alkynylating agents. a
(
5e – 5f, 89% - 94% yields). Notably, the acyclic
amino acids equipped with various functional groups,
including alkyl, aryl, thioether, ether, amide and
guanidine groups, were easily converted to the
phenylethynylated products 5f – 5p in 38% - 89%
yields, further suggesting the excellent functional
group compatibility of this method.
In addition to α-amino acids, various acyclic and
cyclic α-oxy acids were also proved effective reactants
(
7a – 7c, 87% - 96% yields). In particular, the
hydrocarbon-substituted carboxylic acids, which lack
the radical stabilizing α-heteroatoms were compatible
with this protocol (7d – 7J, 59% - 85%). Among them,
the secondary and tertiary carboxylic acids are highly
reactive and gave the products 7e – 7j in 70% - 82%
yields. However, the primary carboxylic acids
displayed inferior reaction efficiency (7d, 59% yield).
It should be noted that radical-sensitive cyclopropyl
and cyclobutyl were well tolerated (7g – 7h, 74% -
7
6% yields). As expected, the reaction also works well
in the case of α-keto acids (7l – 7m, 61% - 70% yields).
However, different from the reported homogeneous
photocatalytic decarboxylative functionalization of
oxalic acid monoesters, which typically results in the
a)
See general procedure A for the experimental details
unless otherwise noted, isolated yields were reported.
formation of alkylation products via the release of two
[12]
CO molecules, only the esterification product was
2
observed in this heterogeneous process (7m, 61%
yield).
With the optimized conditions established, we
studied the reactivity of various EBX reagents in the
transformation (Scheme 2). Various aryl-substituted
EBX reagents worked well in the alkynylation process,
giving products 3b – 3n in 39% - 88% yields. Avariety
of functional groups, such as borate, cyan, bromine,
and chlorine, were well tolerated, providing the
handles for further synthetic elaboration. In general,
high yields were observed for the EBX reagents with
electron-deficient and electron-neutral aryl groups (3b
Furthermore, the preparative power of this protocol
is further highlighted by the applicability in late-stage
modification of diverse drug molecules and nature
products with the carboxylic acid moiety. For example,
three structurally diversified and highly functionalized
drug molecules, including isoxepac, indomethacin and
fenofibric acid, were readily coupled with phenyl EBX
reagent (9a – 9c, 65% - 97% yields). Gratifyingly, the
3
naturally occurring and Csp -enriched carboxylic
acids also readily underwent the decarboxylative
3
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Advanced Synthesis & Catalysis
10.1002/adsc.202000777
alkynylation, providing the products 9d – 9f in 57% -
Troc-protected amino-oxy-acid (Troc = 2, 2, 2-
trichloroethoxycarbonyl) as the amidyl radical
precursor and phenyl-EBX as the alkynylation reagent,
various acyclic unactivated alkenes were found to be
competent reactants in this three-component reaction
(12a – 12d, 48% - 80% yields). The reaction of
cyclohexene also proceeded smoothly, providing the
products in moderate yield and excellent
diastereoselectivity (12e, 57% yield, dr > 20: 1).
Moreover, the product 12f, resulting from a typical 5-
exo cyclization, was obtained in 43% yield. This result,
as well as the fact that the cyclization products were
not observed for the substrates with a free hydroxyl
group (11a and 11c), further suggest that the reaction
performs through a radical-based mechanism.
9
0% yields, while the configuration remains
unaffected. It should be noted that the reversed alkynyl
C-nucleoside 9e could be readily accessed in excellent
yield and diastereoselectivity (90% yield, dr > 20: 1),
which clearly demonstrates the potential utility of this
new coupling protocol for late-stage and orthogonal
nucleoside functionalization (an area of significant
interest among pharmaceutical agencies developing
antiviral agents).[
13]
This heterogeneous photocatalytic decarboxylative
alkynylation strategy was also extended to the 1, 2-
amidoalkynylation of unactivated alkenes (Scheme 4),
where the amidyl radical was in-situ generated via the
[14]
decarboxylation-inducedN-Obondcleavage. Using
Scheme 3. Scope of carboxylic acids. a
a)
b)
See general procedure A for the experimental details unless otherwise noted, isolated yields were reported.
1
Diastereomeric ratio (d.r.) determined using H NMR spectroscopy: dr > 20 : 1.
4
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Advanced Synthesis & Catalysis
10.1002/adsc.202000777
Scheme 4. The application in radical 1, 2-amidoalkynylation of unactivated alkenes. a
a)
b)
See general procedure B for the experimental details unless otherwise noted, isolated yields were reported.
1
Diastereomeric ratio (d.r.) determined using H NMR spectroscopy: dr > 20: 1.
To further demonstrate the practicability of this
heterogeneous protocol, a recycling experiment was
performed. As shown in Figure 1, g-C
3
N was recycled
4
Scheme 5. Simplified scheme of the flow reactor. a
eight times without losing its catalytic activity,
highlighting its high potential for sustainable
photocatalysis. The sustainability and photo-stability
of g-C
3
N was also supported by the XRD pattern
4
analysis (Fig. S2). Prompted by this result, we decided
to perform the reaction in continuous flow. Using a
column, glass beads, silica gel and g-C
3
N
4
, we built a
simple and recyclable continuous-flow photoreactor
9]
(
Scheme 5).[ After slight optimization of the flow
reaction conditions, the model reaction shown in Table
1
could be successfully conducted in continuous flow
even on a gram-scale (1.3 g), giving the desired
product 3a in 74% yield.
Figure 1. Evaluation of the catalyst recycling.
a)
l = length of the filled column, din = internal diameter of
r
the column, F = flow rate, t = residence time.
Conclusion
In summary, we have demonstrated for the first time
that the cheap and metal-free polymer g-C can act
3
N
4
as an effective photocatalyst for the visible-light
promoted heterogeneous photocatalytic decarboxyla
tive alkynylation reactions. The newly established
method is mild and cost-effective and features
excellent functional group compatibility, which allows
the transformation of a variety of simple α-amino acids,
α-oxy acids, α-keto acids and hydrocarbon-substituted
carboxylic acids, as well as the complex bio-active
carboxylic acids, into structurally diverse alkyne-
containing molecular architectures in moderate to
excellent yields. Notably, the heterogeneous nature of
5
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Advanced Synthesis & Catalysis
10.1002/adsc.202000777
the reaction system enables the recovery and reuse of
the catalyst in multiple runs without loss of reactivity.
The reaction can also be performed in a continuous
flow fashion, even on a gram-scale, which further
illustrates the practicability of this protocol.
Furthermore, this strategy could also be extended to
the 1, 2-amidoalkynylation of unactivated alkenes
where the amidyl radical is in-situ generated via
decarboxylation-induced N-O bond cleavage.
Dixneuf, J.-F. Soulé, Chem. Rev., 2018, 118, 7532-7585;
d) K. L. Skubi, T. R. Blum, T. P. Yoon, Chem. Rev.,
2016, 116, 10035-10074; e) N. A. Romero, D. A.
Nicewicz, Chem. Rev., 2016, 116, 10075-10166; f) T.-Y.
Shang, L.-H. Lu, Z. Cao, Y. Liu, W.-M. He, B. Yu, Chem.
Commun., 2019, 55, 5408-5419; g) Y.-Y. Chen, L.-Q. Lu,
D.-G. Yu, C.-J. Zhu, W.-J. Xiao, Sci. China. Chem.,
2
019, 62, 24-57.
[
[
2] a) J. Z. Bloh, R. Marschall, Eur. J. Org. Chem., 2017,
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017, 2085-2094; b) J. Chen, J. Cen, X. Xu, X. Li, Catal.
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Sustainable Chem. Eng., 2020, 8, 2682-2687.
Experimental Section
Genaral procedure A: carboxylic acids (0.2 mmol,
1
.0 equiv), EBX reagents (0.3 mmol, 1.5 equiv),
K
2
CO
3
(0.1 mmol, 0.5 equiv) and g-C
3
N (1.5 mg)
4
3] a) J. Xuan, Z.-G. Zhang, W.-J. Xiao, Angew. Chem. Int.
Ed., 2015, 54, 15632-15641; b) Z.-W. Zuo, D. W. C.
MacMillan, J. Am. Chem. Soc., 2014, 136, 5257-5260;
c) S.-Q. He, X.-L. Chen, F.-L. Zeng, P.-P. Lu, Y.-Y.
Peng, L.-B. Qu, B. Yu, Chin. Chem. Lett., 2019, DOI:
were weighed into a dried glass vial equipped with a
stir bar and dissolved in 2 mL of DCM. The vial was
then sealed with a Teflon-lined septum screw cap. The
reaction mixture was degassed by bubbling N inside
2
the glass vial via syringe for five minutes. The tightly
sealed vial was then irradiated with 427 nm blue LEDs
1
0.1016/j.cclet.2019.12.031; d) L. Xiong, H. Hu, C.-W.
Wei, B. Yu, Eur. J. Org. Chem., 2020. DOI:
0.1002/ejoc.201901581.
(
Kessil Lighting: intensity 50; the distance between the
1
tube and the light source was about 10 cm) and
simultaneously cooled by a fan to keep the reaction
temperature at 25 ℃. After 16 hours, the mixture was
filtered and purified by flash chromatography on silica
gel using the indicated solvent system to afford the
desired product.
[
[
[
4] H. Huang, G. Zhang, Y. Chen, Angew. Chem. Int. Ed.,
015, 54, 7872-7876.
2
5] H. Tan, H. Li, W. Ji, L. Wang, Angew. Chem. Int. Ed.,
015, 54, 8374-8377.
2
6] a) Q.-Q. Zhou, W. Guo, W. Ding, X. Wu, X. Chen, L.-
Q. Lu, W.-J. Xiao, Angew. Chem. Int. Ed., 2015, 54,
Genaral procedure B: 2-methyl-2-((((2,2,2-
1
1196-11199; b) F. L. Vaillant, T. Courant, J. Waser,
trichloroethoxy) carbonyl) amino) oxy) propanoic acid
Angew. Chem. Int. Ed., 2015, 54, 11200-11204.
(
0.1 mmol, 1.0 equiv), EBX reagents (0.15 mmol, 1.5
equiv), Na
2
HPO
4
(0.1 mmol, 1.0 equiv) and g-C
3
N
4
[
[
7] C. Yang, J.-D. Yang, Y.-H. Li, X. Li, J.-P. Cheng, J. Org.
Chem., 2016, 81, 12357-12363.
(
5.0 mg) were weighed into a dried glass vial equipped
with a stir bar and dissolved in 2 mL of DCM. The vial
was then sealed with a Teflon-lined septum screw cap.
8] a) A. Savateev, M. Antonietti, ACS Catal., 2018, 8,
9
790-9808; b) A. Savateev, I. Ghosh, B. König, M.
Antonietti, Angew. Chem. Int. Ed., 2018, 57, 15936-
5947.
The reaction mixture was degassed by bubbling N
2
inside the glass vial via syringe for five minutes.
Afterwards, alkenes (0.3 mmol, 3.0 equiv) was added
1
by syringe under N
vial was then irradiated with 427 nm blue LEDs
Kessil Lighting: intensity 50; the distance between the
2
atmosphere. The tightly sealed
[
[
[
[
9] a) Y. Cai, Y. Tang, L. Fan, Q. Lefebvre, H. Hou, M.
Rueping, ACS Catal., 2018, 8, 9471-9476; b) M. Baar,
S. Blechert, Chem. Eur. J., 2015, 21, 526-530.
(
tube and the light source was about 10 cm) and
simultaneously cooled by a fan to keep the reaction
temperature at 25 ℃. After 12 hours, the mixture was
filtered and purified
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