Visible-Light-Induced Metal-Free Synthesis of 2-Phosphorylated Thioflavones in Water

Article abstract of DOI:10.1002/cssc.201902817

The introduction of phosphorus functional groups into the skeleton of thioflavones is an attractive task and of great significance. Herein, a metal-free visible-light-induced radical cascade cyclization was developed for the preparation of 2-phosphorylate

Full text of DOI:10.1002/cssc.201902817

Accepted Article
Title: Visible-Light-Induced Metal-Free Synthesis of 2‑Phosphorylated
Thioflavones in Water
Authors: Xiao-Ceng Liu, Xiao-Lan Chen, Yan Liu, Kai Sun, Yu-Yu
Peng, Ling-Bo Qu, and Bing Yu
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10.1002/cssc.201902817
ChemSusChem
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Visible-Light-Induced Metal-Free Synthesis of 2-Phosphorylated
Thioflavones in Water
Xiao-Ceng Liu,[a] Xiao-Lan Chen,*[a] Yan Liu,[a][b] Kai Sun,*[a] Yu-Yu Peng,[c] Ling-Bo Qu,[a] and Bing
Yu*[a]
Dedicated to the 100th anniversary of Nankai University
Abstract: Using water as a reaction medium, a large number of 2-
phosphorylated thioflavones were prepared via the reaction of
methylthiolated alkynones and phosphine oxides catalyzed by
4CzIPN under visible-light irradiation. These reactions can be carried
out at ambient temperature, feature simple operation, wide reaction
scope, and recyclability of aqueous media.
reactions in water is an attractive strategy but is less developed.
Therefore, the development of photocatalytic systems using water
as a green solvent is still highly desirable for green organic
synthesis.[7]
Phosphorus-containing compounds have received increasing
attention in the past few decades due to their wide range of
applications in medicinal chemistry, materials science, and
organic synthesis.[8] In recent years, P-centered radicals added to
unsaturated double and triple bonds have been recognized as
powerful and efficient approach for the construction of P-
containing compounds.[9] The synthesis of structurally diverse
phosphorus-containing compounds via P-centered radicals
initiated reaction has been witnessed.[10] However, the generation
of P-center radicals in these transformations generally requires
the use of transition-metal salts and high reaction temperatures,
limiting their practical applications (Scheme 1a).[11]
Thioflavones are meaningful functional motifs that occur in
numerous nature products, drugs and functional materials.[12] The
intrduction of phosphorus functional groups to the skeleton of
thioflavones is an attractive task and of great significance. With
our continuing interest in green chemistry and phosphorus
chemistry,[13] we herein report a metal-free visible-light-induced
radical cascade cyclization in water for the preparation of 2-
phosphorylated thioflavones from methylthiolated alkynones and
phosphine oxides (Scheme 1b). By using the organophotocatalyst
4CzIPN, a range of 2-phosphorylated thioflavones were prepared
via the aqueous reaction with satisfactory yields at room
temperature. To the best of our knowledge, this is the first
example of the photocatalytic construction of C-P bonds by using
4CzIPN as a photocatalyst in water, in which the waste and
toxicity issues of organic solvents could be avoided, and aqueous
media could be recycled.
The increasing concern about minimizing the environmental
impact of chemical synthesis has become an area of growing
significance and has prompted the development of
environmentally friendly and green procedures for organic
transformations. One of the major impacts of classical organic
synthesis on the environment is due to the utilization of toxic and
volatile organic solvents. Additionally, the application of organic
solvents in heat- or gas-releasing reactions may present potential
safety issues, such as combustion and explosion. Since the
definition of “green chemistry” was proposed by Anastas and
Warner in 1991, organic transformation in green solvents, which
were considered as well-accepted and effective solutions for
these problems, has attracted widespread attention.[1] As a
natural liquid with abundant reserves, water is uniquely
advantageous as a reaction medium with obvious merits, such as
non-toxicity, low cost, and easy acquisition.[2] In addition, the use
of water instead of traditional organic solvents can effectively
avoid toxicity and potential risks in chemical processes, as water
is perfectly bio-compatible and is a non-flammable liquid with high
heat capacity.[3] From economic, environmental points of view,
organic reactions carried out in water are attractive and highly
desired, which is in harmony with the philosophy of green
chemistry.[4]
Visible-light represents a highly economical and eco-friendly
energy source. Recently, visible-light-induced reactions have
emerged as versatile tools for chemical transformations.[5]
Although significant advances have been made in the past
decade, visible-light catalysis is generally carried out in traditional
organic solvents.[6] Obviously, running photocatalytic organic
[a]
X. C. Liu, Prof. Dr. X.-L. Chen, Dr. Y. Liu, Dr. K. Sun, Prof. L.-B. Qu
and Prof. Dr. B. Yu
College of Chemistry, Zhengzhou University, Zhengzhou 450001,
China. E-mail: chenxl@zzu.edu.cn, bingyu@zzu.edu.cn
Dr. Y. Liu
College of Biological and Pharmaceutical Engineering, Xinyang
Agriculture & Forestry University, Xinyang 464000, China
Y. Y. Peng
[b]
[c]
Hunan Provincial Key Laboratory of Materials Protection for Electric
Power and Transportation, Changsha University of Science and
Technology, Changsha, 410114, China
Scheme 1. Radical reaction for the construction of organophosphorus
compounds
Supporting information for this article is given via a link at the end of
the document.
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For the optimization of reaction conditions, methylthiolated
phenylpropynone (1a) and diphenylphosphine oxide (2a) were
utilized as model substrates, and the results were summarized in
Table 1. When the model reaction was firstly performed in water
by employing eosin Y as photocatalyst in the presence of
TBHP/NaHCO3 under the irradiation of white LEDs for 24 h, it is
pleased to see that the desired product 3a was obtained in 24%
yield (entry 1). Inspired by this intriguing result, other frequently
used photocatalysts, including eosin B, Na2Eosin Y, Na2Eosin B,
rhodamine B, rose bengal, 4CzIPN, and fluorescein, were
evaluated (entries 2-8). Among the above photocatalysts
examined, 4CzIPN demonstrated the highest catalytic activity,
affording the desired product 3a in moderate yield (45%, entry 7).
To further improve the reaction efficiency, various inorganic and
organic bases were tested (entries 9-12). However, no better
results were obtained. Surprisingly, this visible-light-induced.
transformation also proceeded smoothly under the base-free
conditions, delivering the product of 3a with 47% yield (entry 13).
Furthermore, various oxidants, such as DTBP, BPO, K2S2O8, LPO
and O2 were investigated (entries 14-18). To our delight, by
employing LPO as oxidant, a satisfactory yield of 3a was obtained
(entry 17). It is especially worth mentioning here that 53% yield of
3a can be obtained when O2 was employed as green oxidant in
this photooxidation processes (entry 18). Due to the higher
efficiency, LPO was employed as oxidant for further investigation.
Subsequent study on the investigation for the dosages of oxidant
indicated that 2 equiv of LPO was the best choice for this
transformation (entries 19-21). When the reaction was performed
under air, a slight decrease in yield was observed (entry 22).
Notably, no desired product was observed in the absence of either
4CzIPN or visible light (entries 23-24). Therefore, the optimized
reaction conditions are illustrated as follows: 1a (0.2 mmol), 2a
(0.4 mmol), 4CzIPN (5 mol%) and LPO (2 equiv) were mixed in
H2O with the irradiation of white LEDs under N2 for 24 h.
Table 1. Optimization of the reaction conditions
Table 2. Substrate scope for 2-phosphorylated thioflavones
Entry
1
PC
eosin Y
Oxidant
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
DTBP
BPO
Base [equiv]
Yield [%]
24
NaHCO3 (2)
2
eosin B
NaHCO3 (2)
23
3
Na2Eosin Y
Na2Eosin B
rhodamine B
rose bengal
4CzIPN
fluorescein
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
---
NaHCO3 (2)
31
4
NaHCO3 (2)
30
5
NaHCO3 (2)
13
6
NaHCO3 (2)
19
7
NaHCO3 (2)
45
8
NaHCO3 (2)
trace
trace
11
9
Cs2CO3 (2)
10
11
12
13
14
15
16
17
18c
19d
20e
21
22f
23
24g
K2CO3 (2)
DABCO (2)
17
Et3N (2)
---
18
47
---
22
---
27
K2S2O8
LPO
---
58
78 (75)b
---
O2
---
53
LPO
---
67
LPO
---
77
---
---
N.D.
66
LPO
---
LPO
---
N.D.
N.D.
4CzIPN
LPO
---
a Reaction conditions: 1 (0.2mmol), 2 (0.4 mmol), 4CzIPN (5 mol%), LPO (2
equiv), H2O (2.5 mL), white LEDs, under N2 for 24 h, RT. Isolated yields were
given.
a Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), PC (5 mol%), oxidant
(2 equiv), H2O (2.5 mL) for 24 h under N2. PC = photocatalyst, 4CzIPN =
1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene, TBHP
=
tert-butyl
hydroperoxide, DTBP = di-tert-butyl peroxide, BPO = benzoyl peroxide, LPO
= dilauroyl peroxide, N.D. = not detected. Yield were determined by 31P NMR
using trioctylphosphine oxide as an internal standard. Isolated yield.
Under O2 balloon. d LPO (1 equiv). e LPO (3 equiv). f Under Air. g In dark
With optimized reaction conditions identified, a systematic
study of substrate scope was carried out to evaluate the
applicability of this visible-light-induced transformation by
b
c
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10.1002/cssc.201902817
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COMMUNICATION
examining different methylthiolated alkynones 1 and phosphine
oxides 2, and the results are illustrated in Table 2. A variety of
methylthiolated alkynones with diverse functionalities at the
phenyl ring (R1) worked well in the optimal condition reacting with
diphenylphosphine oxide (2a), affording the corresponding
products 3a–m in moderate to excellent yields (up to 89%). In
these cases, substrates bearing electron-donating groups (–Me,
–OMe, –Et, and –tBu) generally showed better reactivities than
those attached with electron-withdrawing substituents (–F, –Cl, –
Br, –CN, and –CF3). Furthermore, the trifluoromethyl group,
widespread in drugs and agrochemicals, was also well
accommodated, leading to the corresponding CF3-containing
compound 3i in moderate yields (59%). In addition, the reactivities
of various phosphine oxides were also investigated. We were
delighted to find that phosphine oxides with electron-donating and
electron-withdrawing substituents were compatible under the
optimized reaction conditions (3n-3w). The electron-withdrawing
and electron-donating groups have no significant effect on the
efficiency of this visible-light-induced transformation. In addition,
reactant 2 containing naphthalene rings (R2) was also suitable for
this transformation, resulting in product 3w in satisfactory yield.
Additionally, products were not detected when dibutyl phosphine
oxide, diethyl H-phosphonate, dimethyl H-phosphonate as well as
diphenylphosphine sulfide were employed as phosphorus
reaction partners, indicating that dialkyl phosphine oxides, dialkyl
H-phosphinates and diarylphosphine sulfides were not suitable
for this transformation.
radical pathway should be involved in this visible-light-promoted
reaction. Moreover, a BHT-P(O)Ph2 adduct 4 was detected in the
reaction mixture by high-resolution mass spectrometry (HRMS),
indicating that phosphoryl radical was generated in the reaction
process (Scheme 3b). Additionally, quantum yield for the model
reaction was measured.[14] The low value (0.24) determined
indicated that a radical chains process should not account for the
full reaction productivity in this photochemical transformation (for
details see the Supporting Information).
Scheme 3. Control experiments
Based on the experimental results as well as the previous
literature,[15]
a
plausible reaction mechanism for this
transformation is thereby proposed as shown in Scheme 4.
Initially, the ground state of 4CzIPN was converted into the
excited-state species, i.e., 4CzIPN*, under the irradiation of
visible-light. Diphenylphosphine oxide 2a is usually in equilibrium
between tri-coordinated and tetra-coordinated forms. The tri-
coordinated isomer 2a’ could react with the excited 4CzIPN* via
a single-electron transfer (SET) process, leading to the formation
of phosphoryl radical 5 and radical anion 4CzIPN•−. Importantly,
the high redox potential of the photoexcited state of 4CzIPN
(E1/2(P*/P-) = +1.35 V vs SCE) make this process credible and
reasonable.[16] Next, the addition of phosphoryl radical 5 to the
triple bond of 1a, followed by an intramolecular cyclization,
generated the final product 7, releasing methyl radical. Moreover,
LPO can oxidize the radical anion 4CzIPN•− regenerating the
ground state of 4CzIPN to complete the photoredox cycle.
Moreover, to evaluate the practicability of this photocatalytic
phosphorylation reaction, a gram-scale synthesis of 3a was
conducted in a specially designed flow reactor under optimal
conditions (Scheme 2). It is pleasing to see that the desired
product 3a was obtained in good yield (67%) when the reaction
was performed on the 5-mmol scale, implying a promising
industrial application in preparative synthesis.
Scheme 2. Gram scale synthesis of 3a
To obtain more potential mechanistic insights into this
transformation, several control experiments were performed. As
can be seen in Scheme 3, the purposeful addition of a radical
scavenger, (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO)
or 2,6-di-tert-butyl-4-methylphenol (BHT), to the model reaction
severely inhibited the reaction. These results suggested that a
Scheme 4. Proposed mechanism
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10.1002/cssc.201902817
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Additionally, recycling study of aqueous media was carried out
(see the Supporting Information). The experimental results show
that water could be recycled and applied as solvent for this
reaction at least five times without significant loss of reactivities
(Figure 1).
diverse phosphorus-containing compounds is currently ongoing
in our laboratory.
Experimental Section
80
75
74
72
71
70
70
A mixture of methylthiolated phenylpropynones (0.2 mmol, 1 equiv),
phosphine oxides (0.4 mmol, 2 equiv), LPO (2 equiv), 4CzIPN (5 mol%)
were sequentially added in a 10-mL reaction vial. Then, H2O (2.5 mL) was
added into this reaction system. The reaction vial was sealed under N2 and
stirred under the irradiation of white LEDs at room temperature until TLC
(petroleum ether/ethyl acetate= 2/1) revealed that conversion of the
starting material was completed. Next, the solvent was extracted with ethyl
acetate (3 × 5 mL). The combined organic layers were dried over
anhydrous Na2SO4. After filtration, the solvent was evaporated in vacuo.
The crude product was purified by silica gel chromatography (petroleum
ether/ethyl acetate = 2/1) to give the desired products.
60
40
20
0
1
2
3
4
5
6
Run
Figure 1. Recyclability of water.
Acknowledgements
We acknowledge the financial support from National Natural
Science Foundation of China (21501010, 21971224), China
Postdoctoral Science Foundation (2019M652561), Key Research
Projects of Universities in Henan Province (20A150006), Henan
Province Young Talents Elevation Program (2019HYTP023) and
Hunan Provincial Key Laboratory of Materials Protection for
Electric Power and Transportation (2019CL03).
Keywords: Photocatalysis • Metal-free • Aqueous reaction•
Phosphorylation • Thioflavones
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Scheme 5. Preparation of phosphorylated N-heterocycles
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Encouraged by these exciting results, we attempted to extend
this green and sustainable strategy to different radical partners for
the preparation of significant phosphorus-containing compounds.
Gratifyingly, the conditions were suitable for the synthesis of
phosphorylated N-heterocycles, including phosphorylated
oxindole (8), phosphorylated quinoline-2,4(1H,3H)-dione (10) as
well as phosphorylated benzothiazole (12) in water, as shown in
Scheme 5, implying the broad applicability of our approach.
In conclusion, a metal-free photocatalytic system for C-P bond
formation at room temperature was successfully developed under
visible-light irradiation in water. With this green and sustainable
strategy, a wide range of 2-phosphorylated thioflavones were
prepared with satisfactory yields via the reaction of
methylthiolated alkynones and phosphine oxides. Moreover,
some phosphorylated N-heterocycles, including phosphorylated
oxindole, phosphorylated quinoline-2,4(1H,3H)-dione as well as
phosphorylated benzothiazole could be smoothly synthesized in
good isolated yields by using this green and sustainable strategy.
This is the first example of the photocatalytic construction of C-P
bond using 4CzIPN as a photocatalyst in water, in which the
waste and toxicity issues of organic solvents could be avoided,
and aqueous media could be recycled. Further application of this
photocatalytic strategy for the construction of more structurally
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10.1002/cssc.201902817
ChemSusChem
COMMUNICATION
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COMMUNICATION
Xiao-Ceng Liu,a Xiao-Lan Chen,*a Yan
Liu,a,b Kai Sun,*a Yu-Yu Peng,c Ling-Bo
Qu,a and Bing Yu*a
Page No. – Page No.
Using water as a reaction medium, a large number of 2-phosphorylated thioflavones
were prepared via the reaction of methylthiolated alkynones and phosphine oxides
catalyzed by 4CzIPN under visible-light irradiation. These reactions can be carried
out at ambient temperature, feature simple operation, wide reaction scope, and
recyclability of aqueous media.
Visible-Light-Induced Metal-Free
Synthesis of 2-Phosphorylated
Thioflavones in Water
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