Copper-Catalyzed, Stereoselective Bis-trifluoromethylthiolation of Propiolic Acid Derivatives with AgSCF3

Article abstract of DOI:10.1021/acs.orglett.7b01366

A copper-catalyzed chemo- and stereoselective oxidative bis-trifluoromethylthiolation of propiolic acid derivatives was achieved by using carboxylic acid as the activating group and formic acid as a cosolvent. The reaction of propiolic acid derivatives and AgSCF₃ in the presence of (NH₄)₂S₂O₈ and catalytic Cu(OAc)₂ in MeCN/HCO₂H afforded bis-trifluoromethylthiolated acrylic acids in moderate to excellent yields with E selectivity. Further derivatization of the resultant products gave a series of polysubstituted SCF₃-containing alkenes.

Full text of DOI:10.1021/acs.orglett.7b01366

Letter
pubs.acs.org/OrgLett
Copper-Catalyzed, Stereoselective Bis-trifluoromethylthiolation of
Propiolic Acid Derivatives with AgSCF3
Shen Pan,† Huan Li,† Yangen Huang,† Xiu-Hua Xu,‡ and Feng-Ling Qing*,†,‡
†College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, 2999 North Renmin Lu, Shanghai 201620,
China
‡Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Science, 345 Lingling
Lu, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: A copper-catalyzed chemo- and stereoselective
oxidative bis-trifluoromethylthiolation of propiolic acid de-
rivatives was achieved by using carboxylic acid as the activating
group and formic acid as a cosolvent. The reaction of propiolic
acid derivatives and AgSCF3 in the presence of (NH4)2S2O8
and catalytic Cu(OAc)2 in MeCN/HCO2H afforded bis-
trifluoromethylthiolated acrylic acids in moderate to excellent
yields with E selectivity. Further derivatization of the resultant
products gave a series of polysubstituted SCF3-containing
alkenes.
he vicinal difunctionalization of alkenes and alkynes is a
type of fundamentally important transformation and thus
Herein, we describe an efficient and practical copper-catalyzed
stereoselective oxidative bis-trifluoromethylthiolation of inter-
nal alkynes with nucleophilic AgSCF3 using carboxylic acid as
the activating group to form the bis-trifluoromethylthiolated
products in high yields with E selectivity (Scheme 1b).
T
has been applied in different research areas.1 Recently, the
difunctionalization strategy has been widely applied in the
preparation of fluorinated compounds2 because of their
increasing importance in various fields.3 Although the
simultaneous introduction of a F,4 CF3,5 or SCF36 with another
functional group into alkenes and alkynes has made great
progress, the vicinal difluorination and bis-trifluoromethyl-
(thiol)ation are quite limited.
As an extension of our radical bis-trifluoromethylation of
alkenes,8 we became interested in the analogous bis-
trifluoromethylthiolation reactions. According to our experi-
ences and other works,10 the readily available and stable
AgSCF3 is an ideal SCF3 radical source. Thus, we initially
investigated the bis-trifluoromethylthiolation of styrene or
ethynylbenzene with AgSCF3 in the presence of catalytic
Cu(OAc)2 and persulfates. No bis-trifluoromethylthiolated
product was detected from the reaction of styrene. The bis-
trifluoromethylthiolation of ethynylbenzene (1a) took place to
give the bis-trifluoromethylthiolated product (3a) in 18% yield
along with the formation of trifluoromethylthiolated alkyne 2
(Scheme 2) after screening of additives, oxidants, and solvents
(see Table S1 in the Supporting Information (SI)), and it was
also found that the use of HCO2H as a cosolvent was crucial for
bis-trifluoromethylthiolation reaction.
In 2016, Jacobsen and Gilmour reported catalytic difluori-
nation of alkenes with a nucleophilic fluoride source by I(I)/
I(III) catalysis, respectively.7 The bis-trifluoromethylation of
alkenes was recently developed by our group using CF3SO2Na
in the presence of t-BuOOH/CuCl.8 In the case of bis-
trifluoromethylthiolation, Tlili and Billard very recently
disclosed a novel bis-trifluoromethylthiolation of electron-
deficient or heteroatom-containing terminal alkynes with
electrophilic TsNMeSCF3 to give the bis-trifluoromethylthio-
lated alkenes with Z selectivity (Scheme 1a).9 However, this
method suffers from the narrow substrate scope and low yields.
On the basis of this primary result, we next decided to
introduce an activating group to the alkyne to improve the yield
and chemoselectivity of bis-trifluoromethylthiolation reaction.
We anticipated that the activating group would activate the
alkyne to give higher yield and coordinate with the copper salt
to form the bis-trifluoromethylthiolated product selectively. A
series of activating groups, including ketone, ester, ether,
carboxylic acid, and amide, were then investigated (Table 1).
Scheme 1. Bis-trifluoromethylthiolation of Alkynes
Received: May 7, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01366
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
beneficial for yield and stereoselectivity (entries 10−15).
Among the ligands (L1−L6) tested, 4,4′-dimethyl-2,2′-
bipyridine (L4) afforded 3e in highest yield (96%) with
excellent stereoselectivity (E/Z > 99:1) (entry 13). Moreover,
when the reaction was carried out in the presence of other
oxidants such as PhI(OAc)2 and 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ), none of the desired product 3e was
detected. It was noteworthy that product 3e was confirmed as
the trans-addition product by the X-ray crystallographic analysis
(see the SI), which is in contrast to Tlili and Billard’s work on
the cis-bis-trifluoromethylthiolation of alkynes.9
Scheme 2. Strategy of Introducing an Activating Group
The scope of the bis-trifluoromethylthiolation of propiolic
acid derivatives is shown in Scheme 3. The aromatic propiolic
a
Table 1. Optimization of Reaction Conditions
Scheme 3. Bis-trifluoromethylthiolation of Propiolic Acid
a
Derivatives
b
b
entry
1
Cu salt
ligand
yield (%)
E/Z (%)
1
1a
1b
1c
1d
1e
1f
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
CuCl2
18
trace
trace
15
9:1
2
3
4
9:1
9:1
5
63
6
20
9:1
7
1e
1e
1e
1e
1e
1e
1e
1e
1e
54
9:1
8
CuI
49
9:1
9
CuCN
32
9:1
10
11
12
13
14
15
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
L1
L2
L3
L4
L5
L6
85
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
79
72
96
a
Reaction conditions: 1 (0.2 mmol), AgSCF3 (0.6 mmol), Cu(OAc)2
60
(0.04 mmol), (NH4)2S2O8 (0.6 mmol), L4 (0.04 mmol), MeCN/
HCO2H (3.0 mL/3.0 mL), under N2, 70 °C, 3 h, isolated yields.
74
a
Reaction conditions: 1 (0.1 mmol), AgSCF3 (0.3 mmol), Cu salt
(0.02 mmol), (NH4)2S2O8 (0.3 mmol), ligand (0.02 mmol), MeCN/
b
acids were transformed into the bis-trifluoromethylthiolated
acrylic acids in good to excellent yields. Substrates bearing
electron-donating (1g−i) and electron-withdrawing (1j−p)
groups as well as halides (1q−s) were all effective. The position
and number of substitutions (1t−y) did not interfere with this
transformation. In the cases of aliphatic propiolic acids (1z and
1aa), the desired products (3z and 3aa) were obtained in
moderate yields. Notably, this protocol could also be applied
for complex molecules such as estrone derivative (1ab) to give
the bis-trifluoromethylthiolated product (3ab) in good yield.
Compounds that contain a bis-SCF3-substituted acrylic acid
moiety are useful precursors for the preparation of SCF3-
containing compounds. As shown in Scheme 4, the
decarboxylation of compound 3y in CH3CN at 90 °C
proceeded smoothly to afford 4y in high yield. Moreover, 3y
was subjected to several standard transformations to give bis-
trifluoromethylthiolated α,β-unsaturated ester (5), amide (6),
aldehyde (8), and ketone (9) and as well as allylic alcohol (7).
The trifluoromethylthio group was proven to be a good leaving
HCO2H (1.5 mL/1.5 mL), under air, 70 °C, 3 h. Yields and
stereoselectivities determined by 19F NMR spectroscopy using
trifluoromethybenzene as an internal standard.
The ketone and ester derivatives (1b and 1c) were transformed
into other CF3S-containing products, but not the desired
products (entries 2 and 3). The bis-trifluoromethylthiolation of
alkynes 1d and 1f afforded the desired products in low yields
(entries 4 and 6). Interestingly, the acid derivative (1e) gave the
bis-trifluoromethylthiolated product 3e in 63% yield (entry 5).
The employment of carboxylic acid as a activating group has
been well studied in C−H bond functionalization reactions.11
Herein, the carboxylic acid was a powerful activating group for
difunctionalization reactions. To improve the yield of
compound 3e further, copper salts and ligands were examined.
Switching Cu(OAc)2 into other Cu salts including CuCl2, CuI,
and CuCN led to lower yields (entries 7−9). In general, the
addition of catalytic monodentate or bidentate ligand was
B
DOI: 10.1021/acs.orglett.7b01366
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
On the basis of the above results, a plausible reaction
mechanism was proposed (Scheme 6). First, oxidation of
Scheme 4. Transformation of Compound 3y
Scheme 6. Proposed Reaction Mechanism
a
Reaction conditions: (a) CH3CN, 90 °C, 2 h; (b) H2SO4, MeOH,
100 °C, 24 h; (c) SOCl2, rt, 4 h; then NH3/H2O, 0 °C to rt, 30 min;
(d) DIBAL-H, CH2Cl2, −78 °C, 3 h; (e) MnO2, CH2Cl2, reflux, 24 h;
(f) SOCl2, rt, 4 h; then AlMe3, CH2Cl2; (g) BnMgCl, THF, 0 °C, 2 h;
(h) NaOMe, THF, 0 °C, 2 h.
AgSCF3 by (NH4)2S2O8 generates AgIISCF3, which could be
further transformed to CF3S radical or CF3SSCF3.10 In fact, we
could detect the formation of CF3SSCF3 in the reaction mixture
by 19F NMR spectroscopy. On the other hand, substrate 1 is
transformed into copper complexes A9 and B.12a,d There is an
equilibrium between A and B, which is affected by the acidity of
the reaction mixture. In formic acid, complex A is primary and
reacts with CF3S radical to generate possible intermediates A-1.
Finally, the copper-assisted trifluoromethylthiolation with
CF3SSCF3 affords products 3. In less acidic solution, the
addition of CF3S radical to complex B produces the radical
intermediate B-1, which is further oxidized to cationic
intermediate B-2. Compound B-2 may undergo decarbox-
ylation or nucleophilic attack/decarboxylation to give the
byproducts 2 and 12. However, the exact mechanism of this
transformation remains unclear at the present stage.
group when bis-trifluoromethylthiolated alkene was subjected
to nucleophiles. For example, treatment of compound 4y with
BnMgCl gave α-CF3S-substituted naphthyl alkene (10). On the
other hand, the substitution of compound 5 by NaOMe
afforded β-CF3S-substituted naphthyl alkene (11). These
unique detrifluoromethylthiolation reactions probably proceed
through a nucleophilic addition followed by the elimination of
trifluoromethanethiolate.
To gain mechanistic insights into this reaction process, some
preliminary studies were conducted. Only a trace of the desired
product was detected when a radical scavenger, 1,4-
benzoquinone or 2,6-di-tert-butyl-4-methylphenol (BHT), was
added to the reaction mixture (Scheme 5a), which indicated
In conclusion, we have disclosed an unprecedented copper-
catalyzed, oxidative bis-trifluoromethylthiolation of propiolic
acid derivatives with AgSCF3. The use of HCO2H as the
cosolvent was crucial for this reaction. The resulting bis-
trifluoromethylthiolated acrylic acids and their derivatives are
previously unknown, difficult to obtain by other methods, and
potentially useful in drug discovery and material science.
Further investigation of the reaction mechanism and the
applications of bis-trifluoromethylthiolated products are
currently in progress in our laboratory.
Scheme 5. Preliminary Mechanistic Studies
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01366.
Table S1, experimental procedures, characterization data,
1
mechanistic study data, H, 19F, and 13C NMR spectra,
and X-ray crystal structure of 3-e (PDF)
this transformation probably proceeded through a radical
pathway. Moreover, the reaction in MeCN/AcOH gave 3e in
49% yield along with the decarboxylation products, trifluor-
omethylthiolated alkyne (2) and α-trifluoromethylthiolated
ketone (12), in low yields (Scheme 5b).12 Compound 3e was
formed in only 11% yield when the reaction was performed in
MeCN (Scheme 5c). These results revealed the importance of
HCO2H in this reaction.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: flq@mail.sioc.ac.cn.
ORCID
Feng-Ling Qing: 0000-0002-7082-756X
C
DOI: 10.1021/acs.orglett.7b01366
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(6) For selected examples, see: (a) Liu, J.; Chu, L.; Qing, F.-L. Org.
Lett. 2013, 15, 894. (b) Sheng, J.; Li, S.; Wu, J. Chem. Commun. 2014,
50, 578. (c) Zhu, L.; Wang, G.; Guo, Q.; Xu, Z.; Zhang, D.; Wang, R.
Org. Lett. 2014, 16, 5390. (d) Yang, T.; Lu, L.; Shen, Q. Chem.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Commun. 2015, 51, 5479. (e) Fuentes, N.; Kong, W.; Fernan
́
dez-
■
Sanchez, L.; Merino, E.; Nevado, C. J. Am. Chem. Soc. 2015, 137, 964.
́
The National Natural Science Foundation of China (21421002,
21332010) and Strategic Priority Research Program of the
Chinese Academy of Sciences (XDB20000000) are greatly
acknowledged for funding this work.
(f) Luo, J.; Zhu, Z.; Liu, Y.; Zhao, X. Org. Lett. 2015, 17, 3620. (g) Xu,
C.; Shen, Q. Org. Lett. 2015, 17, 4561. (h) Liu, X.; An, R.; Zhang, X.;
Luo, J.; Zhao, X. Angew. Chem., Int. Ed. 2016, 55, 5846. (i) Wu, W.;
Dai, W.; Ji, X.; Cao, S. Org. Lett. 2016, 18, 2918. (j) Jin, D.-P.; Gao, P.;
Chen, D.-Q.; Chen, S.; Wang, J.; Liu, X.-Y.; Liang, Y.-M. Org. Lett.
2016, 18, 3486.
REFERENCES
■
(7) (a) Banik, S. M.; Medley, J. W.; Jacobsen, E. N. J. Am. Chem. Soc.
(1) For recent reviews, see: (a) Chemler, S. R.; Bovino, M. T. ACS
Catal. 2013, 3, 1076. (b) Romero, R. M.; Woste, T. H.; Muniz, K.
2016, 138, 5000. (b) Molnar
́
, I. G.; Gilmour, R. J. Am. Chem. Soc.
, I. G.; Thiehoff, C.; Holland, M. C.;
̈
̃
2016, 138, 5004. (c) Molnar
́
Chem. - Asian J. 2014, 9, 972. (c) Song, R.-J.; Liu, Y.; Xie, Y.-X.; Li, J.-
H. Synthesis 2015, 47, 1195. (d) Huple, D. B.; Ghorpade, S.; Liu, R.-S.
Adv. Synth. Catal. 2016, 358, 1348. (e) Li, C.-C.; Yang, S.-D. Org.
Biomol. Chem. 2016, 14, 4365. (f) Du, X.; Huang, Z. ACS Catal. 2017,
7, 1227.
(2) For recent reviews, see: (a) Wolstenhulme, J. R.; Gouverneur, V.
Acc. Chem. Res. 2014, 47, 3560. (b) Egami, H.; Sodeoka, M. Angew.
Chem., Int. Ed. 2014, 53, 8294. (c) Merino, E.; Nevado, C. Chem. Soc.
Rev. 2014, 43, 6598. (d) Xu, X.-H.; Qing, F.-L. Curr. Org. Chem. 2015,
19, 1566. (e) Besset, T.; Poisson, T.; Pannecoucke, X. Eur. J. Org.
Chem. 2015, 2015, 2765. (f) Chen, P.; Liu, G. Eur. J. Org. Chem. 2015,
2015, 4295. (g) Koike, T.; Akita, M. Acc. Chem. Res. 2016, 49, 1937.
(h) Chatterjee, T.; Iqbal, N.; You, Y.; Cho, E. J. Acc. Chem. Res. 2016,
49, 2284. (i) Yin, G.; Mu, X.; Liu, G. Acc. Chem. Res. 2016, 49, 2413.
(j) Serguchev, Y. A.; Ponomarenko, M. V.; Ignat’ev, N. V. J. Fluorine
Chem. 2016, 185, 1.
Gilmour, R. ACS Catal. 2016, 6, 7167.
(8) Yang, B.; Xu, X.-H.; Qing, F.-L. Org. Lett. 2015, 17, 1906.
(9) Tlili, A.; Alazet, S.; Glenadel, Q.; Billard, T. Chem. - Eur. J. 2016,
22, 10230.
(10) (a) Zhang, K.; Liu, J.-B.; Qing, F.-L. Chem. Commun. 2014, 50,
14157. (b) Yin, F.; Wang, X.-S. Org. Lett. 2014, 16, 1128. (c) Guo, S.;
Zhang, X.; Tang, P. Angew. Chem., Int. Ed. 2015, 54, 4065. (d) Wu, H.;
Xiao, Z.; Wu, J.; Guo, Y.; Xiao, J.-C.; Liu, C.; Chen, Q.-Y. Angew.
Chem., Int. Ed. 2015, 54, 4070. (e) Fuentes, N.; Kong, W.; Fernan
́
dez-
Sanchez, L.; Merino, E.; Nevado, C. J. Am. Chem. Soc. 2015, 137, 964.
́
(f) Jin, D.-P.; Gao, P.; Chen, D.-Q.; Chen, S.; Wang, J.; Liu, X.-Y.;
Liang, Y.-M. Org. Lett. 2016, 18, 3486.
(11) For a review about the use of carboxylic acids as directing
groups for CH bond functionalization, see: (a) Pichette Drapeau,
M.; Gooßen, L. J. Chem. - Eur. J. 2016, 22, 18654. For a new
application of carboxylic acids as deciduous directing groups in Heck
reactions, see: (b) Tang, J.; Hackenberger, D.; Goossen, L. J. Angew.
Chem., Int. Ed. 2016, 55, 11296.
(12) For selected examples of decarboxylative functionalization of
propiolic acid derivatives, see: (a) He, Z.; Zhang, R.; Hu, M.; Li, L.; Ni,
C.; Hu, J. Chem. Sci. 2013, 4, 3478. (b) Priebbenow, D. L.; Becker, P.;
Bolm, C. Org. Lett. 2013, 15, 6155. (c) Xu, Y.; Zhao, J.; Tang, X.; Wu,
W.; Jiang, H. Adv. Synth. Catal. 2014, 356, 2029. (d) Zhang, P.; Zhang,
L.; Gao, Y.; Xu, J.; Fang, H.; Tang, G.; Zhao, Y. Chem. Commun. 2015,
51, 7839. (e) Zhang, L.; Hang, Z.; Liu, Z.-Q. Angew. Chem., Int. Ed.
2016, 55, 236. (f) Cai, S.; Xu, Y.; Chen, D.; Li, L.; Chen, Q.; Huang,
M.; Weng, W. Org. Lett. 2016, 18, 2990.
(3) For recent reviews, see: (a) Wang, J.; Sanchez-Rosello, M.; Acena,
́
́
̃
J. L.; del Pozo, C.; Sorochinsky, A. E.; Fustero, S.; Soloshonok, V. A.;
Liu, H. Chem. Rev. 2014, 114, 2432. (b) Boltalina, O. V.; Popov, A. A.;
Kuvychko, I. V.; Shustova, N. B.; Strauss, S. H. Chem. Rev. 2015, 115,
1051. (c) Tirotta, I.; Dichiarante, V.; Pigliacelli, C.; Cavallo, G.;
Terraneo, G.; Bombelli, F. B.; Metrangolo, P.; Resnati, G. Chem. Rev.
2015, 115, 1106. (d) Gillis, E. P.; Eastman, K. J.; Hill, M. D.; Donnelly,
D. J.; Meanwell, N. A. J. Med. Chem. 2015, 58, 8315. (e) Preshlock, S.;
Tredwell, M.; Gouverneur, V. Chem. Rev. 2016, 116, 719.
(4) For selected examples, see: (a) Barker, T. J.; Boger, D. L. J. Am.
Chem. Soc. 2012, 134, 13588. (b) Zhang, C.; Li, Z.; Zhu, L.; Yu, L.;
Wang, Z.; Li, C. J. Am. Chem. Soc. 2013, 135, 14082. (c) Emer, E.;
Pfeifer, L.; Brown, J. M.; Gouverneur, V. Angew. Chem., Int. Ed. 2014,
53, 4181. (d) Okoromoba, O. E.; Han, J.; Hammond, G. B.; Xu, B. J.
́
Am. Chem. Soc. 2014, 136, 14381. (e) Yuan, W.; Szabo, K. J. Angew.
Chem., Int. Ed. 2015, 54, 8533. (f) Yuan, Z.; Wang, H.-Y.; Mu, X.;
Chen, P.; Guo, Y.-L.; Liu, G. J. Am. Chem. Soc. 2015, 137, 2468.
(g) Ilchenko, N. O.; Cortes, M. A.; Szabo, K. J. ACS Catal. 2016, 6,
́ ́
447. (h) Woerly, E. M.; Banik, S. M.; Jacobsen, E. N. J. Am. Chem. Soc.
2016, 138, 13858.
(5) For selected examples, see: (a) Li, Y.; Studer, A. Angew. Chem.,
Int. Ed. 2012, 51, 8221. (b) Yasu, Y.; Koike, T.; Akita, M. Angew.
Chem., Int. Ed. 2012, 51, 9567. (c) Wu, X.; Chu, L.; Qing, F.-L. Angew.
Chem., Int. Ed. 2013, 52, 2198. (d) Jiang, X.-Y.; Qing, F.-L. Angew.
Chem., Int. Ed. 2013, 52, 14177. (e) Mizuta, S.; Verhoog, S.; Engle, K.
M.; Khotavivattana, T.; O’Duill, M.; Wheelhouse, K.; Rassias, G.;
́
Medebielle, M.; Gouverneur, V. J. Am. Chem. Soc. 2013, 135, 2505.
(f) Iqbal, N.; Jung, J.; Park, S.; Cho, E. J. Angew. Chem., Int. Ed. 2014,
53, 539. (g) Wang, F.; Qi, X.; Liang, Z.; Chen, P.; Liu, G. Angew.
Chem., Int. Ed. 2014, 53, 1881. (h) Wang, F.; Wang, D.; Mu, X.; Chen,
P.; Liu, G. J. Am. Chem. Soc. 2014, 136, 10202. (i) Tang, X.-J.; Dolbier,
W. R., Jr. Angew. Chem., Int. Ed. 2015, 54, 4246. (j) Bagal, D. B.;
Kachkovskyi, G.; Knorn, M.; Rawner, T.; Bhanage, B. M.; Reiser, O.
Angew. Chem., Int. Ed. 2015, 54, 6999. (k) Tomita, R.; Koike, T.; Akita,
M. Angew. Chem., Int. Ed. 2015, 54, 12923. (l) Kawamura, S.; Sodeoka,
M. Angew. Chem., Int. Ed. 2016, 55, 8740. (m) Ye, J.-H.; Song, L.;
Zhou, W.-J.; Ju, T.; Yin, Z.-B.; Yan, S.-S.; Zhang, Z.; Li, J.; Yu, D.-G.
Angew. Chem., Int. Ed. 2016, 55, 10022. (n) Wang, F.; Wang, D.; Wan,
X.; Wu, L.; Chen, P.; Liu, G. J. Am. Chem. Soc. 2016, 138, 15547.
D
DOI: 10.1021/acs.orglett.7b01366
Org. Lett. XXXX, XXX, XXX−XXX