Selective oxidation of (hetero)sulfides with molecular oxygen under clean conditions

Article abstract of DOI:10.1039/c9gc03713f

The development of eco-friendly and switchable catalytic systems for the conversion of a sole raw-material into distinct high-value products is a particularly attractive concept and a daunting synthetic challenge. In the present work, the first example of efficient and selective oxidation of sulfides to sulfones and sulfoxides using molecular oxygen under clean conditions was established.

Full text of DOI:10.1039/c9gc03713f

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Selective oxidation of (hetero)sulfides with
molecular oxygen under clean conditions†
Cite this: DOI: 10.1039/c9gc03713f
b
Kai-Jian Liu,a Ji-Hui Deng,a Jie Yang,a Shao-Feng Gong,a Ying-Wu Lin,
a
Jun-Yi He,c Zhong Cao c and Wei-Min He
*
Received 29th October 2019,
Accepted 26th November 2019
The development of eco-friendly and switchable catalytic systems for the conversion of a sole raw-
material into distinct high-value products is a particularly attractive concept and a daunting synthetic
challenge. In the present work, the first example of efficient and selective oxidation of sulfides to sulfones
and sulfoxides using molecular oxygen under clean conditions was established.
DOI: 10.1039/c9gc03713f
rsc.li/greenchem
pounds, sulfones and sulfoxides are important structural
motifs existing in numerous natural products, pharmaceuti-
Introduction
Molecular oxygen is a perfect oxidant, given its natural abun- cals, pesticides and organic functional materials.5 They are
dance, low cost and eco-friendly features.1 From the viewpoint also serving as significant synthetic intermediates and versa-
of green chemistry, the direct oxidation of organic compounds tile building blocks in synthetic chemistry. The development
with molecular oxygen is one of the most ideal methods for of efficient and switchable catalytic systems for the conversion
preparing oxygenated chemicals.2 However, selective and of a sole raw-material into distinct high-value products has
efficient utilization of molecular oxygen is challenging due to become a particularly attractive concept and a daunting syn-
the triplet-ground-state of molecular oxygen, which imposes thetic challenge.6 Although there are plenty of methodologies
large kinetic barriers to oxygen reduction and leads to poorly for the production of sulfones and sulfoxides,7 to the best of
selective transformation, considering the electron disparity our knowledge, there are only two examples of selective syn-
between the four-electron molecular oxygen reduction and thesis of sulfones and sulfoxides through solvent-dependent
two-electron substrate oxidation.3 Most of the catalytic oxi- oxidation of sulfides. In 2012, L.-N. He and co-workers pio-
dation reactions with molecular oxygen have to rely on metal neered a catalyst-free protocol for the selective oxidation of sul-
catalysts and/or eco-unfriendly radical initiators, which usually fides with an inorganic oxidant oxone in ethanol and water,
generate undesired metal salts and by-products. In addition, respectively (Scheme 1a).8 Very recently, Jiang’s group devel-
the traditional oxidation reactions generally require a large oped a solvent-dependent selective synthesis of sulfones and
excess amount of volatile organic solvent as a reaction sulfoxides through visible-light-induced UO2(OAc)2·2H2O-cata-
medium, which not only leads to a potentially flammable and lyzed oxidation of sulfides using molecular oxygen with water
possibly explosive mixture in the presence of highly energetic and phosphoric acid as additives in pure acetonitrile and a
oxidants but also causes environmental and resource pro- solvent
mixture
of
acetonitrile/o-xylene,
respectively
blems. Thus, an oxidation reaction that could operate under (Scheme 1b).9 Despite these significant achievements men-
metal- and additive-free conditions would be highly desirable.
Owing to their diverse oxidation states, sulfur-containing
compounds play a vital role in fine chemicals, and medicinal
and materials chemistry.4 Among the sulfur-containing com-
tioned above, from the economic and environmental view-
aDepartment of Chemistry, Hunan University of Science and Engineering,
Yongzhou 425100, China. E-mail: weiminhe2016@yeah.net
bSchool of Chemistry and Chemical Engineering, University of South China,
Hengyang, 421001, China
cHunan Provincial Key Laboratory of Materials Protection for Electric Power and
Transportation, Changsha University of Science and Technology, Changsha, 410114,
China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9gc03713f
Scheme 1 Solvent-dependent selective oxidation of sulfides.
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Table 1 Optimization of conditionsa
point, innovative metal- and additive-free oxidation protocols
should be pursued, especially for the pharmaceutical industry
in which the removal of the unexpected metal residues could
be expensive.
Ethers could react with ground-state molecular oxygen to
produce peroxides.10 We envisioned that the in situ generated
peroxides could be used as oxidants for the oxidation of sul-
fides. Considering the very low rate of conversion of ethers to
peroxides at room temperature, we chose the heating con-
ditions to improve the reaction rate. Bis(2-alkoxyethyl) ethers,
including bis(2-methoxyethyl)ether (BME), bis(2-ethoxyethyl)
ether (BEE) and bis(2-butoxyethyl)ether (BBE), are cheap,
abundant and readily available high-boiling-point ether sol-
vents. As our research program is guided by the twelve prin-
ciples of green chemistry,11 herein we report a clean and prac-
tical method for the selective synthesis of sulfones and sulfox-
ides through solvent-controlled catalytic (Principle 9, catalyst)
oxidation of sulfides (Principle 2, atom economy) with mole-
cular oxygen (Principle 7, use of renewable feedstocks) as the
sole oxidant under clean conditions (Principle 1, prevention)
(Scheme 1c). In the present transformation, 3 equiv. of B (or 6
equiv. of Pegdme) (Principle 5, safer solvents and auxiliaries)
is applied and it has dual roles: catalyst and reaction medium.
Yieldb
Entry
Solvent (equiv.)
Temp.
Conv.
2a (%)
3a (%)
1
2
3
4
5
6
7
8
BME (3)
BEE (3)
BBE (3)
1,2-Diethoxyethane(3)
Pegdme (3)
Degme (3)
1,4-Dioxane (3)
THF (3)
BBE (4)
BBE (2)
Pegdme (4)
Pegdme (5)
Pegdme (6)
Pegdme (7)
BBE (3)
BBE (3)
BBE (3)
100 °C
100 °C
100 °C
100 °C
100 °C
100 °C
100 °C
100 °C
100 °C
100 °C
100 °C
100 °C
100 °C
100 °C
110 °C
90 °C
80 °C
70 °C
110 °C
90 °C
80 °C
70 °C
60 °C
100 °C
90 °C
100 °C
90 °C
93%
97%
100%
99%
99%
44%
38%
0%
78
83
98
55
31
32
12
N.D.
98
87
42
35
21
20
93
88
35
Trace
50
11
15
9
Trace
65
11
98
11
15
14
2
44
68
12
26
N.D.
2
9
100%
96%
97%
97%
98%
96%
99%
97%
41%
Trace
99%
96%
64%
44%
Trace
75%
63%
100%
96%
100%
100%
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24c
25c
26d
27d
28e
29f
9
55
62
77
76
6
9
6
Trace
49
85
49
35
Trace
10
52
2
BBE (3)
Pegdme (6)
Pegdme (6)
Pegdme (6)
Pegdme (6)
Pegdme (6)
BBE (3)
Pegdme (6)
BBE (3)
Pegdme (6)
BBE (3)
Results and discussion
Our preliminary research began with methyl 4-tolyl sulfide
(0.6 mmol) (1a) as a model substrate (Table 1). When the oxi-
dation reaction was performed with molecular oxygen as the
sole oxidant in the presence of 3 equiv. of bis(2-methoxyethyl)
ether (BME) as a solvent at 100 °C, a 78% yield of methyl
4-tolyl sulfone (2a) and a 15% yield of methyl 4-tolyl sulfoxide
(3a) based on 93% conversion of 1a were observed (Table 1,
entry 1). Subsequently, a series of ether solvents were investi-
gated to improve the reaction efficiency and tune the chemical
selectivity towards different oxidative products. It was found
that dialkyl-endether solvents (entries 1–5) can provide better
85
3
2
100 °C
100 °C
97
98
BBE (3)
a Conditions: methyl 4-tolyl sulfide (1a) (0.6 mmol), solvent, and O2
balloon (2.5 L; the amount of molecular oxygen is about 0.11 mol).
b The yield was estimated by GC-MS by the area normalization method.
c In air. d In the dark. e 1.5 L of O2 balloon was used. f 4 L of O2 balloon
was used. N.D.: No product was detected. Pegdme: poly(ethylene
glycol) dimethyl ether (average Mn, 250).
reaction outcomes than monoalkyl-end-capped ether (entry 6) (entries 15–18). The exploration on the temperature also indi-
and cyclic ethers (entries 7–8). The oxidation reaction pro- cated that 90 °C was an appropriate temperature to obtain 3a
ceeded to full conversion of 1a and provided a 98% yield of 2a (entry 20). Performing the reaction with air as an oxidant
and a 2% yield of 3a when 3 equiv. of BBE was employed as resulted in lower reaction efficiency (entries 24 and 25).
the solvent (entry 3). Using 3 equiv. of Pegdme as the reaction Moreover, when the transformation was conducted under dark
medium, a considerable decrease in the yield of sulfone 2a conditions, the conversion of 1a showed no change (entries 26
appeared, but notably, the selectivity towards sulfoxide 3a sig- and 27). Performing the reaction with 1.5 L or 4 L of molecular
nificantly increased (entry 5). Changing the solvent to a low oxygen as the oxidant had virtually no effect on the product
boiling point THF solvent led to no conversion of 1a, which yield (entries 28 and 29).
might be caused by the low boiling-point of THF (entry 8).
With the optimal reaction conditions established (Table 1,
Varying the loading of BBE did not provide the improved yield entries 3 and 20), we next proceeded to investigate the general-
of 2a (entries 3 and 9–10), whereas a 77% yield of 3a was ity of the present oxidation reaction. As shown in Scheme 2, all
observed when 6 equiv. of Pegdme was employed (entries 5 sulfide substrates were allowed, and the oxidation yields were
and 11–14). These results of the screening of reaction tempera- mostly >80%. Methyl phenyl sulfides bearing electron-donat-
ture suggested that the present oxidation reaction could be a ing or electron-withdrawing groups at the para-, meta- or ortho-
thermodynamically controlled process (entries 3 and 15–18; 5 positions of the aromatic ring (1a–1l) underwent the trans-
and 19–23). No improvement in the transformation of 1a into formation efficiently to deliver the targeted products. Neither
2a was observed by elevating or decreasing the temperature electronic nor steric factors of methyl phenyl sulfides had a
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Fig. 1 Reaction scope of complex compounds. Condition A for sul-
fones: sulfides 1 (0.6 mmol), BBE (1.8 mmol), and O2 balloon (2.5 L).
Condition B for sulfoxides: sulfides 1 (0.6 mmol), Pegdme (3.6 mmol),
and O2 balloon (2.5 L). a130 °C; b120 °C; c110 °C; and d150 °C. Isolated
yields were reported.
Scheme 2 Reaction scope. Condition
A for sulfones: sulfides 1
(0.6 mmol), BBE (1.8 mmol), O2 balloon (2.5 L), and 100 °C. Condition B
for sulfoxides: sulfides 1 (0.6 mmol), Pegdme (3.6 mmol), O2 balloon
(2.5 L), and 90 °C. a110 °C; b120 °C; c150 °C; d100 °C; and e130 °C.
Isolated yields were reported.
To further verify the synthetic practicability of the eco-
friendly protocol, some large-scale oxidation reactions of sul-
fides were carried out under the optimal reaction conditions
(Scheme 3). To our delight, the desired oxidative products were
formed in good to excellent yields and can be easily isolated
through recrystallization. The experimental results suggest
that this present oxidation reaction is easy to scale up.
considerable influence on the oxidation outcome except for
the strongly electron-withdrawing nitro group (1j). In the
known oxidation reactions of organic sulfides, it was difficult
to prepare dioxidation products due to the strong electron-
withdrawing effect of the sulfone (sulfoxide) groups on the
mono-oxidative product which are less reactive than their
sulfide precursors. Delightfully, the double-oxidation of 1,4-bis
(methylthio)benzene (1m) proceeded smoothly under standard
conditions to produce the targeted products 2m and 3m in
92% and 83% yields, respectively. We were pleased that the oxi-
dation of the methylthiosubstituted fused ring (1n) and
various aromatic heterocycles (1o–1r) proceeded in good to
excellent yields, thus further extending the reaction scope of
the present oxidation reaction. Other phenyl sulfides bearing
diverse substituted groups of the sulfur atom, such as ethyl
(1s) and cyclopropyl (1t), could be successfully converted into
the desired oxidation products in good to excellent yields.
Moreover, a series of symmetrical and unsymmetrical diaryl
sulfides (1u–1af) constituted suitable substrates and provided
the target oxidative products in good yields. This developed
protocol method could also be applied for dibenzo[b,d]thio-
phene (1ag) yielding the oxidative products in good yield.
Based on the metal- and additive-free, neutral reaction con-
ditions as well as excellent functional-group compatibility, a
series of sulfone- and sulfoxide-containing pesticides, pharma-
ceuticals and their derivatives were prepared through the
present oxidation (Fig. 1). Fenofibrate, an antilipemic agent,
allowed the incorporation of –SO2Me and –S(O)Me motifs
resulting in 91% (2ah) and 82% (3ah) yields, respectively.
Several sulfone-containing artificial pharmaceuticals, includ-
ing Etoricoxib (2ai), Mesotrione (2aj), Zolimidine (2ak), and
Rofecoxib (2al), were obtained in good yields. Sulfoxide-con-
taining synthetic drug Sulindac (3am), a non-steroidal anti-
inflammatory agent, can be easily prepared through the
present selective oxidation reaction.
The environmental friendliness of the developed method was
evaluated by calculating the atom economy (AE), molar
efficiency (Mol. E%),12 E-factor and EcoScale.13 For comparison
with the work by L.-N. He,8 an oxidation of methyl 4-tolyl sulfide
was selected as a template reaction (Table 2). For the preparation
of sulfone 2a, the present method presented a 6-fold increase in
AE, a 56.7-fold increase in Mol. E%, a 9.4-fold decrease in the
E-factor and basically no change in the EcoScale. For the prepa-
ration of sulfoxide 3a, the current protocol has a 3.2-fold
increase in AE, a 9.6-fold increase in Mol. E%, a 2.4-fold
decrease in the E-factor and a 16% increase in the EcoScale.
The effect of oxidation time on the conversion of methyl
4-tolyl sulfide (1a) was investigated under the optimal reaction
conditions and the results are shown in Fig. 2. With 3 equiv. of
BBE as the solvent, both methyl 4-tolyl sulfone (2a, 10%) and
methyl 4-tolyl sulfoxide (3a, 76%) were generated along with
the consumption of 1a (86% conversion), suggesting that the
conversion of 1a to 3a is the dominant reaction in the initial
reaction stage of 6 h. After the reaction continued for 12 h, a
52% yield of 3a and a 48% yield of 2a based on the full conver-
sion of sulfide 1a were detected. As the oxidation reaction
exceeded 20 h, 1a and 3a were almost completely converted
into 2a. These results suggest that the oxidation of 1a is more
liable to proceed through the sulfoxide pathway. Moreover, in
Scheme 3 Large-scale synthesis. Isolated yields were reported.
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Table 2 The comparison of Green Metrics and EcoScale
Method
Solvent
AE
Mol. E%
E-factor
EcoScale
1a
2a
3a
Ref. 8a
This work
Ref. 8a
H2o (277.7 mmol)
BBE (3 mmol)
EtOH (86 mmol)
Pegdme (6 mmol)
15%
91%
28.4%
92%
0.3%
17%
1.1%
10.6%
43.1
4.6
31.7
13
86.5
88.5
72
1 mmol
This work
83.5
Fig. 2 Time course for the conversion of methyl 4-tolyl sulfide.
the oxidation reaction with Pegdme as the solvent, the conver-
sion of 1a to 3a is the dominant reaction at all stages.
To further elucidate the reaction mechanism of the present
oxidation, several experiments were conducted as shown in
Scheme 4. First, the oxidation reaction was entirely inhibited
by adding two equiv. of a radical scavenger (TEMPO or BHT)
into the reaction mixture, which revealed that this oxidation
proceeds via a free-radical pathway. When 1,4-diazabicyclo
[2.2.2]octane (DABCO), a well-known singlet oxygen scavenger,
Scheme 4 Control experiments.
was added to the reaction mixture under the standard con- as the sole oxidant under a nitrogen atmosphere, a trace
ditions, no product 2a was observed, indicating that singlet amount of 2a and a moderate yield of 3a were observed
oxygen might be involved in the transformation (Scheme 4a). (Schemes 4c and d). These experimental results implied that
The reaction of 9,10-dimethylanthracene (4a) and 3 equiv. of molecular oxygen played a crucial role in the oxidation of sulf-
BBE in the presence of oxygen balloon at 100 °C delivered the oxides to sulfones. To confirm the source of the oxygen atom
endoperoxide product (4b) via a [4 + 2] cycloaddition reaction, in the oxidative product, the oxidation of sulfide 1a was
1
in which O2 was probably involved14 (Scheme 4b). The results carried out in the presence of 18O2, and 18O-labeled 2a was
of the electron paramagnetic resonance (EPR) experiments detected by GC-MS. This observation manifested that the
suggested that both singlet oxygen and the peroxyl radical oxygen atom of sulfone originates from molecular oxygen
were generated during the oxidation reaction (see the ESI†). (Scheme 4e). To clarify the role of the ether solvent in the
Performing the oxidation of 1a with 2 equiv. of TBHP or H2O2 transformation, the oxidation reaction of 1a was carried out
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Acknowledgements
We are grateful for the financial support from the Hunan
Provincial Natural Science Foundation of China (No.
2019JJ40090 and 2019JJ20008).
Scheme 5 Possible mechanism.
Notes and references
1 (a) X. Zhang, K. P. Rakesh, L. Ravindar and H.-L. Qin, Green
Chem., 2018, 20, 4790; (b) Q. Mei, H. Liu, Y. Yang, H. Liu,
S. Li, P. Zhang and B. Han, ACS Sustainable Chem. Eng.,
2018, 6, 2362; (c) F.-L. Zeng, X.-L. Chen, S.-Q. He, K. Sun,
Y. Liu, R. Fu, L.-B. Qu, Y.-F. Zhao and B. Yu, Org. Chem.
Front., 2019, 6, 1476; (d) Q. Huang, K. Dong, W. Bai, D. Yi,
J.-X. Ji and W. Wei, Org. Lett., 2019, 21, 3332; (e) L.-Y. Xie,
J.-L. Hu, Y.-X. Song, G.-K. Jia, Y.-W. Lin, J.-Y. He, Z. Cao and
W.-M. He, ACS Sustainable Chem. Eng., 2019, 7, 19993;
(f) L.-Y. Xie, Y.-S. Lu, H.-R. Ding, S.-F. Gong, J.-X. Tna,
J.-Y. He, Z. Cao and W.-M. He, Chin. J. Catal., 2019, DOI:
S1872-2067(19)63526-6.
2 (a) Y. Feng, N. Tian, Y. Li, C. Jia, X. Li, L. Wang and X. Cui,
Org. Lett., 2017, 19, 1658; (b) B. Liu, F. Jin, T. Wang,
X. Yuan and W. Han, Angew. Chem., Int. Ed., 2017, 56,
12712; (c) D.-F. Jiang, J.-Y. Hu, W.-J. Hao, S.-L. Wang,
S.-J. Tu and B. Jiang, Org. Chem. Front., 2018, 5, 189;
(d) H. Zhang, J. Shen, G. Cheng, Y. Feng and X. Cui, Org.
Lett., 2018, 20, 664; (e) K.-J. Liu, S. Jiang, L.-H. Lu,
L.-L. Tang, S.-S. Tang, H.-S. Tang, Z. Tang, W.-M. He and
X. Xu, Green Chem., 2018, 20, 3038; (f) B. Xiong, X. Zeng,
S. Geng, S. Chen, Y. He and Z. Feng, Green Chem., 2018, 20,
4521; (g) W. Wei, H. Cui, H. Yue and D. Yang, Green Chem.,
2018, 20, 3197; (h) L. Zhao, P. Li, H. Zhang and L. Wang,
Org. Chem. Front., 2019, 6, 87; (i) P. Hu, M. Tan, L. Cheng,
H. Zhao, R. Feng, W.-J. Gu and W. Han, Nat. Commun.,
2019, 10, 2425; ( j) J. Zhang, W. Xie, S. Ye and J. Wu, Org.
Chem. Front., 2019, 6, 2254; (k) B. Liu, L. Cheng, P. Hu,
F. Xu, D. Li, W.-J. Gu and W. Han, Chem. Commun., 2019,
55, 4817; (l) K.-J. Liu, Z.-H. Duan, X.-L. Zeng, M. Sun,
Z. Tang, S. Jiang, Z. Cao and W.-M. He, ACS Sustainable
Chem. Eng., 2019, 7, 10293; (m) Z. Cao, Q. Zhu, Y.-W. Lin
and W.-M. He, Chin. Chem. Lett., 2019, 30, 2132.
with various amounts of BBE. No oxidation product was
detected by GC-MS analysis in the absence of BBE. When the
reaction of 1a was conducted with 0.1 to 0.3 equiv. of BBE as
the reaction medium, a total yield of 31–60% of the oxidative
products (2a and 3a) was observed. In addition, the GC-MS
analysis of the reaction mixture resulting from the oxidation
revealed no change in BBE. These experimental results demon-
strated that the BBE not only acted as a reaction medium but
also served as a catalyst in the transformation (Scheme 4f).
On the basis of these experimental results obtained above
and literature precedent,9,14c two plausible mechanistic path-
ways for the oxidation of sulfides are presented in Scheme 5.
For the singlet oxygen oxidation pathway, hot molecular
oxygen was converted into singlet oxygen (1O2) in the presence
1
of BBE as a catalyst. Subsequently, the active O2 abstracts one
electron from the lone pair of the sulfur atom in sulfide 1a to
generate the intermediate IM1, which interacts with another
sulfide to form sulfoxide 3a. At a high temperature, sulfoxide
3a was further oxidized to the sulfone product 2a. For the free-
radical pathway, with the help of peroxide free radicals, sulfide
1a would undergo single electron oxidation (SET) to produce
the sulfide free-radical cation IM2, which reacts with ground
state molecular oxygen to generate the persulfoxide free-
radical IM3. Next, the intermediate IM3 reacts with another
molecule of 1a and forms sulfoxide 3a, which undergoes
further oxidation to deliver sulfone 2a.
Conclusions
In conclusion, a solvent-controlled selective oxidation of sul-
fides with molecular oxygen under metal- and additive-free
conditions was established. With 3 equiv. of BBE as the reac-
tion medium and catalyst, sulfone compounds were obtained
in good to excellent yields with high selectivity, while sulfoxide
compounds were efficiently produced with 6 equiv. of Pegdme.
Importantly, the desired products could be easily collected
through recrystallization in large-scale preparation. The clean-
ness, simplicity and low-cost of the developed oxidation will
make it very attractive in fine chemicals, agrochemicals and
pharmaceuticals.
3 (a) S. D. McCann and S. S. Stahl, Acc. Chem. Res., 2015, 48,
1756; (b) W. T. Borden, R. Hoffmann, T. Stuyver and
B. Chen, J. Am. Chem. Soc., 2017, 139, 9010.
4 (a) C. Jacob, Nat. Prod. Rep., 2006, 23, 851; (b) N. Wang,
P. Saidhareddy and X. Jiang, Nat. Prod. Rep., 2019, DOI:
10.1039/C8NP00093J; (c) J. Zhao and X. Jiang, Chin. Chem.
Lett., 2018, 29, 1079; (d) W. Wei, L. Wang, H. Yue,
Y.-Y. Jiang and D. Yang, Org. Biomol. Chem., 2018, 16, 7001;
(e) Z. Gan, Q. Yan, G. Li, Q. Li, X. Dou, G.-Y. Li and
D. Yang, Adv. Synth. Catal., 2019, 361, 4558; (f) L.-Y. Xie,
Y.-L. Chen, L. Qin, Y. Wen, J.-W. Xie, J.-X. Tan, Y. Huang,
Z. Cao and W.-M. He, Org. Chem. Front., 2019, 6, 3950;
(g) Y. Gao, Y. Liu and J.-P. Wan, J. Org. Chem., 2019, 84,
2243; (h) X. Wang, M. Yang, W. Xie, X. Fan and J. Wu,
Conflicts of interest
There are no conflicts to declare.
This journal is © The Royal Society of Chemistry 2019
Green Chem.

View Article Online
Paper
Green Chemistry
Chem. Commun., 2019, 55, 6010; (i) G. Li, Z. Gan, K. Kong,
and Z.-L. Wang, Chin. J. Catal., 2019, 40, 1494; (i) S. Ye,
X. Li, W. Xie and J. Wu, Eur. J. Org. Chem., 2019, DOI:
10.1002/ejoc.201900396; ( j) R. G. Almeida, R. L. de
Carvalho, M. P. Nunes, R. S. Gomes, L. F. Pedrosa, C. A. de
Simone, E. Gopi, V. Geertsen, E. Gravel, E. Doris and
E. N. da Silva Júnior, Catal. Sci. Technol., 2019, 9, 2742;
(k) X. Gong, G. Li, Z. Gan, Q. Yan, X. Dou and D. Yang,
Asian J. Org. Chem., 2019, 8, 1472; (l) S. Hou, N. Chen,
P. Zhang and S. Dai, Green Chem., 2019, 21, 1455.
8 (a) B. Yu, A.-H. Liu, L.-N. He, B. Li, Z.-F. Diao and Y.-N. Li,
Green Chem., 2012, 14, 957; (b) B. Li, A.-H. Liu, L.-N. He,
Z.-Z. Yang, J. Gao and K.-H. Chen, Green Chem., 2012, 14,
130.
X. Dou and D. Yang, Adv. Synth. Catal., 2019, 361, 1808;
( j) Q. Zhu, C. Liu, L. Zhou, L. Wu, K. Bian, J. Zeng,
J. Wang, Z. Feng, Y. Yin and Z. Cao, Biosens. Bioelectron.,
2019, 140, 175; (k) L. Peng, Z. Hu, Z. Tang, Y. Jiao and
X. Xu, Chin. Chem. Lett., 2019, 30, 1481; (l) K. Sun,
X.-L. Chen, Y.-L. Zhang, K. Li, X.-Q. Huang, Y.-Y. Peng,
L.-B. Qu and B. Yu, Chem. Commun., 2019, 55, 12615;
(m) G.-H. Li, D.-Q. Dong, Q. Deng, S.-Q. Yan and
Z.-L. Wang, Synthesis, 2019, 51, 3313; (n) Q. Yu, Y. Zhang
and J.-P. Wan, Green Chem., 2019, 21, 3436.
5 (a) A.-F. Wang, W.-J. Hao, Y.-L. Zhu, G. Li, P. Zhou, S.-J. Tu
and B. Jiang, ACS Omega, 2018, 3, 1482; (b) K. Sun, Z. Shi,
Z. Liu, B. Luan, J. Zhu and Y. Xue, Org. Lett., 2018, 20,
6687; (c) Y. Zong, Y. Lang, M. Yang, X. Li, X. Fan and J. Wu,
9 Y. Li, S. A.-e.-A. Rizvi, D. Hu, D. Sun, A. Gao, Y. Zhou, J. Li
and X. Jiang, Angew. Chem., Int. Ed., 2019, 58, 13499.
Org. Lett., 2019, 21, 1935; (d) J. Zhang, X. Li, W. Xie, S. Ye 10 (a) R. S. Drago and R. Riley, J. Am. Chem. Soc., 1990, 112,
and J. Wu, Org. Lett., 2019, 21, 4950; (e) H. Yue, P. Bo,
b. L. Wang, X. Lv, D. Yang and W. Wei, Chin. J. Org. Chem.,
215; (b) X. Fan, N. Li, T. Shen, X.-M. Cui, H. Lv, H.-B. Zhu
and Y.-H. Guan, Tetrahedron, 2014, 70, 256.
2019, 39, 463; (f) X.-Y. Qin, L. He, J. Li, W.-J. Hao, S.-J. Tu 11 (a) L.-H. Lu, Z. Wang, W. Xia, P. Cheng, B. Zhang, Z. Cao
and B. Jiang, Chem. Commun., 2019, 55, 3227; (g) L. Wang,
Y. Zhang, M. Zhang, P. Bao, X. Lv, H.-G. Liu, X. Zhao,
J.-S. Li, Z. Luo and W. Wei, Tetrahedron Lett., 2019, 60,
1845; (h) S. Liu, K. Chen, W.-J. Hao, X.-C. Tu, S.-J. Tu and
B. Jiang, J. Org. Chem., 2019, 84, 1964; (i) P. Bao, L. Wang,
Q. Liu, D. Yang, H. Wang, X. Zhao, H. Yue and W. Wei,
Tetrahedron Lett., 2019, 60, 214.
6 (a) K. Ni, L.-G. Meng, H. Ruan and L. Wang, Chem.
Commun., 2019, 55, 8438; (b) W. Wei, H. Cui, D. Yang,
H. Yue, C. He, Y. Zhang and H. Wang, Green Chem., 2017,
19, 5608.
and W.-M. He, Chin. Chem. Lett., 2019, 30, 1237;
(b) W.-H. Bao, Z. Wang, X. Tang, Y.-F. Zhang, J.-X. Tan,
Q. Zhu, Z. Cao, Y.-W. Lin and W.-M. He, Chin. Chem. Lett.,
2019, DOI: 10.1016/j.cclet.2019.06.052; (c) S. Peng,
Y.-X. Song, J.-Y. He, S.-S. Tang, J.-X. Tan, Z. Cao, Y.-W. Lin
and W.-M. He, Chin. Chem. Lett., 2019, 30, 2287;
(d) S. Peng, D. Hu, J.-L. Hu, Y.-W. Lin, S.-S. Tang,
H.-S. Tang, J.-Y. He, Z. Cao and W.-M. He, Adv. Synth.
Catal., 2019, DOI: 10.1002/adsc.201901163; (e) Q.-W. Gui,
X. He, W. Wang, H. Zhou, Y. Dong, N. Wang, J.-X. Tang,
Z. Cao and W.-M. He, Green Chem., 2019, DOI: 10.1039/
C9GC02657F.
7 (a) Y. Li, M. Wang and X. Jiang, ACS Catal., 2017, 7, 7587;
(b) C. Li, K. Suzuki, N. Mizuno and K. Yamaguchi, Chem. 12 F. I. McGonagle, H. F. Sneddon, C. Jamieson and
Commun., 2018, 54, 7127; (c) K.-S. Du and J.-M. Huang, A. J. B. Watson, ACS Sustainable Chem. Eng., 2014, 2, 523.
Green Chem., 2018, 20, 1405; (d) Q. Liu, L. Wang, H. Yue, 13 K. Van Aken, L. Strekowski and L. Patiny, Beilstein J. Org.
J.-S. Li, Z. Luo and W. Wei, Green Chem., 2019, 21, 1609; Chem., 2006, 2, 3.
(e) Y. Gao, H. Xu, S. Zhang, Y. Zhang, C. Tang and W. Fan, 14 (a) Y.-F. Liang, X. Wang, Y. Yuan, Y. Liang, X. Li and
Org. Biomol. Chem., 2019, 17, 7144; (f) Y.-C. Wu, S.-S. Jiang,
S.-Z. Luo, R.-J. Song and J.-H. Li, Chem. Commun., 2019, 55,
8995; (g) M. Wang, J. Zhao and X. Jiang, ChemSusChem,
2019, 12, 3064; (h) D.-Q. Dong, L.-X. Li, G.-H. Li, Q. Deng
N. Jiao, ACS Catal., 2015, 5, 6148; (b) M. C. Carreño,
M. González-López and A. Urbano, Angew. Chem., Int. Ed.,
2006, 45, 2737; (c) G. Li, Q. Yan, X. Gong, X. Dou and
D. Yang, ACS Sustainable Chem. Eng., 2019, 7, 14009.
Green Chem.
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