Photosensitizer-free synthesis of β-keto sulfones: Via visible-light-induced oxysulfonylation of alkenes with sulfonic acids

Article abstract of DOI:10.1039/d1ob00552a

A practical and environment-friendly methodology for the construction of β-keto sulfones through visible-light induced direct oxysulfonylation of alkenes with sulfonic acids at ambient temperature under open-air conditions was developed. Most importantly, the reaction proceeded smoothly without the addition of any photocatalyst or strong oxidant, ultimately minimizing the production of chemical waste.

Full text of DOI:10.1039/d1ob00552a

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Biomolecular Chemistry
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Photosensitizer-free synthesis of β-keto sulfones
via visible-light-induced oxysulfonylation of
alkenes with sulfonic acids†
Cite this: DOI: 10.1039/d1ob00552a
Zhen Peng, Yun-Yun Hong, Sha Peng, Xiang-Qun Xu, Shan-Shan Tang, Li-Hua Yang
and Long-Yong Xie
*
Received 21st March 2021,
Accepted 27th April 2021
A practical and environment-friendly methodology for the construction of β-keto sulfones through
visible-light induced direct oxysulfonylation of alkenes with sulfonic acids at ambient temperature under
open-air conditions was developed. Most importantly, the reaction proceeded smoothly without the
addition of any photocatalyst or strong oxidant, ultimately minimizing the production of chemical waste.
DOI: 10.1039/d1ob00552a
rsc.li/obc
pounds as side products are often unavoidable through an
in situ generated vinyl radical intermediate via an atom trans-
Introduction
Organo sulfones are an important class of extremely versatile fer radical addition (ATRA) process. On the other hand,
compounds in organic chemistry.1 Sulfonyl scaffolds are alkenes as versatile and readily available materials have been
associated with a broad range of biological activities, such as widely used in organic synthesis. In particular, oxidative
anticancer, antibacterial, antifungal, neuroprotective and anti- difunctionalization of alkenes with dioxygen has become
inflammatory activities (Fig. 1).2
an efficient strategy for the construction of oxygenated com-
Among various organo sulfones, β-keto sulfones are of par- pounds, such as dioxygenation,16 aminooxygenation,17 oxyalk-
ticular interest, as they not only show a broad spectrum of bio- ylation,18 and oxyphosphorylation.19 From a green and sustain-
logical properties,2a,3 but also act as significant intermediates able chemistry perspective, the direct oxidative difunctionaliza-
for the synthesis of various important organic compounds tion of alkenes with sulfonyl precursors under ambient air
such as olefins,4 allenes,5 disubstituted acetylenes,4 ketones,6 conditions is therefore an ideal strategy for constructing β-keto
chiral vinylsulfones,7 polyfunctionalized 4H-pyrans,8 and opti- sulfones. Over the past few years, the synthesis of β-keto sul-
cally active β-hydroxysulfones.9 As a consequence, extensive fones has been successfully achieved through the difunctiona-
efforts have been devoted to developing novel protocols to syn- lization of olefins mediated by transition metal catalysts (Ag,20
thesize β-keto sulfones, mainly including the alkylation of sul- Cu,21 Fe,22 etc.) and/or stoichiometric chemical oxidants.23
finate salts with α-halo-ketones,10 acylation of alkyl sulfones,11 The use of metals and strong oxidants limits their utilization
sulfonylation of alkyl ketones or enol acetates12 and oxidation in pharmaceutical chemistry. In recent years, several protocols
of β-keto sulfides13 or β-hydroxysulfones.14 However, most of through visible-light initiated direct oxysulfonylation of
them suffer from some drawbacks such as the use of pre-func- alkenes with various sulfonylation reagents leading to β-keto
tionalized materials, relatively complicated or harsh con- sulfones under homogeneous or heterogeneous photoredox
ditions, limited substrate scope, and undesirable byproducts.
In the past decades, direct aerobic difunctionalization of
terminal alkynes with sulfonyl precursors (e.g., RSO2Na,
RSO2NHNH2 and RSO2H) has emerged as a promising method
for the synthesis of β-keto sulfones.15 However, the alkyne
functionalization suffers from two major drawbacks. First, the
terminal alkynes easily undergo self-homocoupling to generate
undesirable dimerization products, and second, vinyl com-
College of Chemistry and Bioengineering, Hunan University of Science and
Engineering, Yongzhou 425100, China. E-mail: longyongxie@yeah.net
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1ob00552a
Fig. 1 Representative sulfonyl scaffold-based bioactive molecules.
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Table 1 Optimization of reaction conditionsa
Entry
Light source
Oxidant
Solvent
Yieldb
1
2
3
4
5
6
7
8
405–410 nm
405–410 nm
405–410 nm
405–410 nm
405–410 nm
405–410 nm
405–410 nm
405–410 nm
405–410 nm
415–420 nm
420–425 nm
440–445 nm
450–455 nm
455–460 nm
545–550 nm
440–445 nm
440–445 nm
Dark conditions
440–445 nm
440–445 nm
440–445 nm
440–445 nm
Sunlight
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
O2 balloon
TBHP
H2O2
K2S2O8
Air
MeCN
Acetone
DCE
THF
EtOAc
EtOH
12%
11%
6%
trace
N.R.
N.R.
34%
46%
17%
48%
52%
57%
53%
32%
N.R.
82%
73%
NR
Acetone/H2O (1 : 1)
CH3CN/H2O (1 : 1)
DCE/H2O (1 : 1)
CH3CN/H2O (1 : 1)
CH3CN/H2O (1 : 1)
CH3CN/H2O (1 : 1)
CH3CN/H2O (1 : 1)
CH3CN/H2O (1 : 1)
CH3CN/H2O (1 : 1)
CH3CN/H2O (2 : 1)
CH3CN/H2O (3 : 1)
CH3CN/H2O (2 : 1)
CH3CN/H2O (2 : 1)
CH3CN/H2O (2 : 1)
CH3CN/H2O (2 : 1)
CH3CN/H2O (2 : 1)
CH3CN/H2O (2 : 1)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Scheme 1 Visible-light induced direct synthesis of β-keto sulfones.
catalysis conditions have been reported (Scheme 1a–d).24 The
critical defects of these methods were the requirement of photo-
sensitizer and peroxide, or need for super-stoichiometric
amounts of acid or base. It should be emphasized that though
wei and co-workers have reported photocatalyst-free visible light
induced synthesis of β-keto sulfones via oxysulfonylation of
alkenes with arylazo sulfones as sulfonylation reagents
(Scheme 1e),25 while multi step reactions involving potentially
dangerous diazotization were required for the preparation of
arylazo sulfones. As a part of our ongoing interest in sulfonation
reactions,26 herein we disclose an efficient visible light induced
direct oxysulfonylation of alkenes to access β-keto sulfones with
readily available sulfonic acids by using ambient air as the term-
inal oxidant under photosensitizer-free conditions (Scheme 1f).
81%
54%
33%
61%
N.R.
a Conditions:
1-methoxy-4-vinylbenzene
(0.1mmol),
4-methyl-
benzenesulfinic acid (0.2 mmol), 6w LED, solvent (0.6 mL), rt, 24 h.
b Estimated by 1H NMR using diethyl phthalate as an internal refer-
ence. N.R.: no reaction.
was obtained in 82% yield. No benefit was obtained by chan-
ging ambient air to pure dioxygen gas and more by-product
4-methoxybenzaldehyde was observed via GC-MS (entry 19).
When ambient air was replaced with 2 equiv. of tert-butyl
hydroperoxide (TBHP) or H2O2, lower yield of 3ga was observed
(entries 20–22). Besides, when the reaction was conducted
under sun-light irradiation, no reaction occurred (entry 23).
With the optimal reaction conditions in hand (Table 1,
Results and discussion
In our initial investigation, a template reaction of 1-methoxy-4-
vinylbenzene (1g) and 4-methylbenzenesulfinic acid (2a) was entry 16), we first explored the generality of the reaction by
performed to screen the reaction conditions (Table 1). employing various aryl-alkenes with p-toluenesulfinic acid
Treatment of 1g with 2a in MeCN under 6w LED irradiation (2a). As revealed in Table 2, whether the phenyl ring is modi-
(405–410 nm) in air for 24 h afforded the desired product (3ga) fied by an electron-donating group, an electron-withdrawing
in 12% yield (Table 1, entry 1). Screening of common organic group or a sterically hindered group, the present reaction can
solvents revealed that aqueous acetonitrile was suitable for the be easily conducted and the corresponding β-keto sulfones
sulfonylation reaction (entries 2–9). Interestingly, the trans- (3aa–3oa) can be obtained in moderate to good yields. In
formation in the presence of water was more efficient (entries addition, a range of functional groups were tolerated, includ-
7–9). Then, the effect of the visible light source (405–550 nm, ing methyl (3ba–3da), tertiary butyl (3fa), methoxy (3ga),
entries 10–15) was investigated. Compared to other light phenoxy (3ha), halide (3ja–3la), trifluoromethyl (3ma) and
sources, the reaction was performed under the irradiation of a nitro (3na) groups. Unexpectedly, N-heterocyclic alkenes were
440–445 nm wavelength LED resulting in a higher yield of 3ga. highly selective to give the protonated products 3pa and 3qa,
When the transformation was conducted under the irradiation without the formation of β-keto sulfone. With (E)-1,2-diphenyl-
of a 545–550 nm wavelength LED (entry 15) or dark conditions ethylene as the substrate, a trace of the sulfonyl product was
(entry 18), no reaction occurred. The ratio of acetonitrile to detected (3ra). Besides, altering the terminal olefins with cyclic
water greatly affected the reaction (entry 12 vs. entries 16 and olefins could also give the desired product in good yield (3sa).
17), and when the ratio of acetonitrile to water was 2 : 1, 3ga Unfortunately, the desired products 3ta and 3ua were not
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Table 2 Reaction scopea
Table 3 Reaction scopea
a Conditions: 1 (0.3 mmol), 2 (0.6 mmol), CH3CN/H2O (v/v 2 : 1)
(1.8 mL), 6 W LED lamp (440–445 nm), air, r.t.
a Conditions: 1 (0.3 mmol), 2 (0.6 mmol), CH3CN/H2O (v/v 2 : 1)
(1.8 mL), 6 W LED lamp (440–445 nm), air, r.t.
Scheme 2 Gram-scale synthesis of 3ga.
obtained when aliphatic alkenes were subjected to standard
conditions. To our delight, styrene modified by naturally
occurring citronellol (3va) was also a suitable substrate.
Next, the reaction scope of this process with respect to sulfinic
acids was evaluated (Table 3). Aryl sulfinic acid bearing either
electron-donating (Me and OMe) or electron-withdrawing groups
(F, Cl, Br, CF3 and CN) at the para-, meta-, and ortho-positions of
the aromatic ring could afford the desired products in good
yields (3ab–3al). 3-Chloro-4-fluoro benzene sulfonic acid also dis-
played good reactivity to afford the corresponding product (3am)
in excellent yield. Notably, thiophene-2-sulfinic acid was used
under the prevailing conditions, and the desired product was
obtained in a good yield (3an). Besides, aliphatic sulfonic acids,
such as methanesulfinic acid and propane-1-sulfinic acid, could
also generate the desired products in moderate yields (3ao–3ap);
however, with phenylmethanesulfinic acid as a sulfonyl substrate, Scheme 3 Control experiments.
no desired product was detected (3aq).
The practicability of this sulfonylation reaction was
explored by employing 1g (10 mmol, 1.34 g) with 2a under the reaction conditions, a trace amount of 3aa was obtained
standard reaction conditions (Scheme 2). As anticipated, the (Scheme 3a), suggesting that a free-radical pathway might be
desired 3ga was formed in 78% isolated yield, which suggested involved in the catalytic cycle. When 2 equiv. of 1,1-diphenyl-
that the methodology is a practical process for the preparation ethylene were added to the reaction mixture, the sulfonyl
of β-keto sulfones.
radical adduct 4b was observed by GC-MS. (Scheme 3b). Under
To gain some insights into the possible mechanism, several the standard reaction conditions, 9,10-dimethylanthracene 5a
controlled experiments were carried out (Scheme 3). When the was converted into the endoperoxide product 5b in 76% yield
reaction was performed in the presence of 2 equiv. of a radical through a [4 + 2] cycloaddition reaction with 1O2 being
scavenger (TEMPO or BHT) as an additive under standard involved (Scheme 3c).27 Taken together, these results
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2020JJ5203), Hunan Provincial Education Department General
Project (no. 19C0799) and the construct program of applied
characteristic discipline in Hunan University of Science and
Engineering.
Scheme 4 Possible mechanism.
Notes and references
1 (a) P. De Vries, C. M. Villalón and P. R. Saxena,
Eur. J. Pharmacol., 1999, 375, 61–74; (b) R. Ettari, E. Nizi,
M. E. Di Francesco, M.-A. Dude, G. Pradel, R. Vičík,
T. Schirmeister, N. Micale, S. Grasso and M. Zappalà,
J. Med. Chem., 2008, 51, 988–996; (c) S. Kotha and
A. S. Chavan, J. Org. Chem., 2010, 75, 4319–4322;
(d) Z.-Y. Mo, T. R. Swaroop, W. Tong, Y.-Z. Zhang,
H.-T. Tang, Y.-M. Pan, H.-B. Sun and Z.-F. Chen, Green
Chem., 2018, 20, 4428–4432; (e) K. G. Petrov, Y.-M. Zhang,
M. Carter, G. S. Cockerill, S. Dickerson, C. A. Gauthier,
Y. Guo, R. A. Mook, D. W. Rusnak, A. L. Walker, E. R. Wood
and K. E. Lackey, Bioorg. Med. Chem. Lett., 2006, 16, 4686–
4691.
2 (a) C. Curti, M. Laget, A. O. Carle, A. Gellis and P. Vanelle,
Eur. J. Med. Chem., 2007, 42, 880–884; (b) A. Guerrini,
A. Tesei, C. Ferroni, G. Paganelli, A. Zamagni, S. Carloni,
M. Di Donato, G. Castoria, C. Leonetti, M. Porru, M. De
Cesare, N. Zaffaroni, G. L. Beretta, A. Del Rio and G. Varchi,
J. Med. Chem., 2014, 57, 7263–7279; (c) E. E. Parent,
C. S. Dence, C. Jenks, T. L. Sharp, M. J. Welch and
J. A. Katzenellenbogen, J. Med. Chem., 2007, 50, 1028–1040;
(d) S. Y. Woo, J. H. Kim, M. K. Moon, S.-H. Han, S. K. Yeon,
J. W. Choi, B. K. Jang, H. J. Song, Y. G. Kang, J. W. Kim,
J. Lee, D. J. Kim, O. Hwang and K. D. Park, J. Med. Chem.,
2014, 57, 1473–1487; (e) J. Xiang, M. Ipek, V. Suri,
W. Massefski, N. Pan, Y. Ge, M. Tam, Y. Xing, J. F. Tobin,
X. Xu and S. Tam, Bioorg. Med. Chem. Lett., 2005, 15, 2865–
2869.
suggested that the singlet oxygen might be a reactive inter-
mediate in the present reaction. In addition, when
β-hydroxysulfone 6a was used under the standard conditions,
the desired product 3aa could not be detected (Scheme 3d),
indicating that β-hydroxysulfone might not be the possible
intermediate in the current process. Besides, we have not
detected H2O2 with Starch-KI test paper.
Although various mechanistic details of this reaction are
still unclear, a proposed mechanism for the reaction is
depicted in Scheme 4 according to the abovementioned obser-
vations and previous relevant literature reports.27b,28 Initially,
under visible-light irradiation in our reaction system, oxygen is
converted to singlet oxygen, and then the active 1O2 quickly
reacted with sulfonic acid 2 through a hydrogen-atom-transfer
(HAT) process to form a hydroperoxy radical (HO2•) and a sulfo-
nyl radical. Subsequently, the addition of the sulfonyl radical to
alkene 1 generated a carbon-centered radical IM1, which is cap-
tured by dioxygen to generate the peroxy radical IM2. Finally, the
•
peroxy radical IM2 interacted with OOH to form the monoalkyl
tetroxide intermediate IM3 (we tried to capture intermediate IM3,
but failed),21b,24d,29 which would be decomposed into product 3
with the release of dioxygen and water.
Conclusions
In summary, we have established a visible-light induced direct
oxysulfonylation of alkenes with readily available sulfonic acids
using ambient air as the sole oxidant at ambient temperature in
aqueous acetonitrile solution. This radical reaction provides a
simple and efficient process to construct β-keto sulfones in good
to excellent yields. The environmentally beneficial oxidant,
metal-, base- and photosensitizer-free conditions, high scalability
and operational simplicity showed that the developed reaction is
an eco-friendly and useful synthetic method.
3 G. Li, X. Kong, Q. Liang, L. Lin, K. Yu, B. Xu and Q. Chen,
Eur. J. Org. Chem., 2020, 2020, 6135–6145.
4 M. Ihara, S. Suzuki, T. Taniguchi, Y. Tokunaga and
K. Fukumoto, Tetrahedron, 1995, 51, 9873–9890.
5 J. E. Baldwin, R. M. Adlington, N. P. Crouch, R. L. Hill and
T. G. Laffey, Tetrahedron Lett., 1995, 36, 7925–7928.
6 M. J. Kurth and M. J. O’Brien, J. Org. Chem., 1985, 50,
3846–3848.
7 S. Sengupta, D. Sen Sarma and S. Mondal, Tetrahedron:
Asymmetry, 1998, 9, 2311–2316.
8 J.-L. Marco, I. Fernandez, N. Khiar, P. Fernandez and
A. Romero, J. Org. Chem., 1995, 60, 6678–6679.
Conflicts of interest
9 R. E. Swenson, T. J. Sowin and H. Q. Zhang, J. Org. Chem.,
2002, 67, 9182–9185.
There are no conflicts to declare.
10 N. Suryakiran, P. Prabhakar, K. Rajesh, V. Suresh and
Y. Venkateswarlu, J. Mol. Catal. A: Chem., 2007, 270, 201–
204.
Acknowledgements
We are grateful for financial support from the Hunan 11 A. R. Katritzky, A. A. A. Abdel-Fattah and M. Wang, J. Org.
Provincial Natural Science Foundation of China (no.
Chem., 2003, 68, 1443–1446.
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2021

View Article Online
Organic & Biomolecular Chemistry
Paper
12 V. K. Yadav, V. P. Srivastava and L. D. S. Yadav, Tetrahedron 22 W. Wei, J. Wen, D. Yang, M. Wu, J. You and H. Wang, Org.
Lett., 2016, 57, 2236–2238. Biomol. Chem., 2014, 12, 7678–7681.
13 B. M. Trost and D. P. Curran, Tetrahedron Lett., 1981, 22, 23 X. Wan, K. Sun and G. Zhang, Sci. China: Chem., 2017, 60,
1287–1290. 353–357.
14 T. Zweifel, M. Nielsen, J. Overgaard, C. B. Jacobsen and 24 (a) J. Wen, X. Yang, Z. Sun, J. Yang, P. Han, Q. Liu,
K. A. Jørgensen, Eur. J. Org. Chem., 2011, 2011, 47–52.
15 (a) S. Cai, D. Chen, Y. Xu, W. Weng, L. Li, R. Zhang and
M. Huang, Org. Biomol. Chem., 2016, 14, 4205–4209;
(b) S. Handa, J. C. Fennewald and B. H. Lipshutz, Angew.
Chem., Int. Ed., 2014, 53, 3432–3435; (c) N. Kumar and
A. Kumar, ACS Sustainable Chem. Eng., 2019, 7, 9182–9188;
(d) Q. Lu, J. Zhang, G. Zhao, Y. Qi, H. Wang and A. Lei,
H. Dong, M. Gu, L. Huang and H. Wang, Green Chem.,
2020, 22, 230–237; (b) J. Wu, Y. Zhang, X. Gong, Y. Meng
and C. Zhu, Org. Biomol. Chem., 2019, 17, 3507–3513;
(c) D. Yang, B. Huang, W. Wei, J. Li, G. Lin, Y. Liu, J. Ding,
P. Sun and H. Wang, Green Chem., 2016, 18, 5630–5634;
(d) X. Yang, J. Yang, K. Yan, H. Qin, W. Dong, J. Wen and
H. Wang, Eur. J. Org. Chem., 2020, 3456–3461.
J. Am. Chem. Soc., 2013, 135, 11481–11484; (e) B. Ni, 25 Q. Liu, F. Liu, H. Yue, X. Zhao, J. Li and W. Wei, Adv. Synth.
B. Zhang, J. Han, B. Peng, Y. Shan and T. Niu, Org. Lett., Catal., 2019, 361, 5277–5282.
2020, 22, 670–674; (f) A. K. Singh, R. Chawla and 26 (a) L.-Y. Xie, T.-G. Fang, J.-X. Tan, B. Zhang, Z. Cao,
L. D. S. Yadav, Tetrahedron Lett., 2014, 55, 2845–2848;
(g) B. Sreedhar and V. S. Rawat, Synlett, 2012, 2012, 413–
417; (h) N. R. Yalavarthi, N. Gundoju, R. Bokam and
M. G. Ponnapalli, J. Chem. Sci., 2019, 131, 8; (i) I. Yavari
and S. Shaabanzadeh, Org. Lett., 2020, 22, 464–467.
16 B. C. Giglio, V. A. Schmidt and E. J. Alexanian, J. Am. Chem.
Soc., 2011, 133, 13320–13322.
17 H. Wang, Y. Wang, D. Liang, L. Liu, J. Zhang and Q. Zhu,
Angew. Chem., Int. Ed., 2011, 50, 5678–5681.
18 Z.-Q. Wang, W.-W. Zhang, L.-B. Gong, R.-Y. Tang,
X.-H. Yang, Y. Liu and J.-H. Li, Angew. Chem., Int. Ed., 2011,
50, 8968–8973.
L.-H. Yang and W.-M. He, Green Chem., 2019, 21, 3858–
3863; (b) L.-Y. Xie, Y.-J. Li, J. Qu, Y. Duan, J. Hu, K.-J. Liu,
Z. Cao and W.-M. He, Green Chem., 2017, 19, 5642–5646;
(c) L.-Y. Xie, S. Peng, F. Liu, G.-R. Chen, W. Xia, X. Yu,
W.-F. Li, Z. Cao and W.-M. He, Org. Chem. Front., 2018, 5,
2604–2609; (d) L.-Y. Xie, S. Peng, J.-X. Tan, R.-X. Sun, X. Yu,
N.-N. Dai, Z.-L. Tang, X. Xu and W.-M. He, ACS Sustainable
Chem. Eng., 2018, 6, 16976–16981.
27 (a) M. J. Cabrera-Afonso, S. R. Lucena, Á. Juarranz,
A. Urbano and M. C. Carreño, Org. Lett., 2018, 20, 6094–
6098; (b) G. Li, Q. Yan, X. Gong, X. Dou and D. Yang, ACS
Sustainable Chem. Eng., 2019, 7, 14009–14015.
19 W. Wei and J.-X. Ji, Angew. Chem., Int. Ed., 2011, 50, 9097– 28 (a) W.-T. Xu, B. Huang, J.-J. Dai, J. Xu and H.-J. Xu, Org.
9099.
Lett., 2016, 18, 3114–3117; (b) M. Zhang, H. Lan, N. Li,
Q. Zhong, H. Zhu, C. Liu and H. Zhao, J. Org. Chem., 2020,
85, 8279–8286.
20 A. K. Singh, R. Chawla and L. D. S. Yadav, Tetrahedron Lett.,
2014, 55, 4742–4746.
21 (a) W. Min, G. Guo, C. Yang and C. Huo, Tetrahedron Lett., 29 (a) J. A. Howard and K. U. Ingold, J. Am. Chem. Soc., 1968,
2020, 61, 152029; (b) W. Wei, C. Liu, D. Yang, J. Wen,
J. You, Y. Suo and H. Wang, Chem. Commun., 2013, 49,
10239–10241.
90, 1058–1059; (b) G. A. Russell, J. Am. Chem. Soc., 1957, 79,
3871–3877; (c) Y. Su, X. Sun, G. Wu and N. Jiao, Angew.
Chem., Int. Ed., 2013, 52, 9808–9812.
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