Copper-Assisted Oxidative Trifluoromethylthiolation of 2,3-Allenoic Acids with AgSCF3

Article abstract of DOI:10.1021/acs.orglett.7b02249

The oxidative trifluoromethylthiolation of 2,3-allenoic acids with AgSCF₃ in the presence of (NH₄)₂S₂O₈ and catalytic copper salt was investigated. A series of 4-aryl-2,3-allenoic acids underwent radical trifluoromethylthiolation/intramolecular cyclization to afford β-trifluoromethylthiolated butenolides, which were conveniently transformed into trifluoromethylthiolated furan derivatives. In contrast, 2-monosubstituted 2,3-allenoic acids were converted into the corresponding 3,4-bis(trifluoromethylthio)but-2-enoic-acids under similar reaction conditions.

Full text of DOI:10.1021/acs.orglett.7b02249

Letter
pubs.acs.org/OrgLett
Copper-Assisted Oxidative Trifluoromethylthiolation of 2,3-Allenoic
Acids with AgSCF3
Shen Pan,† 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, University of Chinese Academy of Science,
Chinese Academy of Science, 345 Lingling Lu, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: The oxidative trifluoromethylthiolation of 2,3-
allenoic acids with AgSCF3 in the presence of (NH4)2S2O8 and
catalytic copper salt was investigated. A series of 4-aryl-2,3-
allenoic acids underwent radical trifluoromethylthiolation/
intramolecular cyclization to afford β-trifluoromethylthiolated
butenolides, which were conveniently transformed into
trifluoromethylthiolated furan derivatives. In contrast, 2-
monosubstituted 2,3-allenoic acids were converted into the
corresponding 3,4-bis(trifluoromethylthio)but-2-enoic-acids
under similar reaction conditions.
Scheme 1. Fluorination, Trifluoromethylation, and
Trifluoromethylthiolation of Allenes
he incorporation of a SCF3 moiety into organic molecules
Tcan improve the physical, chemical, and biological
properties of parent compounds.1 Consequently, the develop-
ment of efficient approaches to SCF3-containing compounds
has attracted extensive attention.2 Traditional synthetic
methods of these compounds mainly rely on halogen−fluorine
exchange of halomethyl sulfides or the trifluoromethylation of
thiols.3 Recently, the direct construction of C−SCF3 bonds has
emerged as an attractive research area in organofluorine
chemistry. Strategies for trifluoromethylthiolation through C−
X (Cl, Br, I) bond transformation,4 oxidative cross-coupling,5
7
C−H bond activation,6 and reduction of SO2CF3 have been
reported. On the other hand, the vicinal difunctionalization of
alkenes and alkynes has also been developed for the
introduction of a SCF3 group and another functional group.8
However, a strategy based on functionalization of allenes to
incorporate a SCF3 group has not been established.
In recent years, the reactions of allenes have been
investigated intensively due to their unique structures and
diverse reactivities.9 Among them, the fluorination and
trifluoromethylation of allenes have been disclosed.10,11 For
example, Gouverneur reported the electrophilic fluoro-
desilylation of allenylmethylsilanes for the synthesis of 2-
fluoro-1,3-dienes (Scheme 1a).10a Ma and co-workers described
the inter- and intramolecular oxyfluorination of allenes,
respectively (Scheme 1b).10b,c Liu developed a silver-catalyzed
intramolecular aminofluorination of the activated allenes for the
synthesis of fluorinated dihydropyrroles (Scheme 1c).10d In the
cases of trifluoromethylation of allenes, Ma’s group developed
an efficient copper-catalyzed cyclic oxytrifluoromethylation of
2,3-allenoic acids in 2013 (Scheme 1d).11a Recently, the
regioselective oxytrifluoromethylation, trifluoromethylazidation,
and trifluoromethylthiocyanation of allenes with Togni reagent
have also been reported (Scheme 1e11b and 1f11c). Herein, we
describe an efficient and practical oxidative cyclic oxytrifluoro-
methylthiolation and regioselective bis-trifluoromethyl-
thiolation of 2,3-allenoic acids with AgSCF3 (Scheme 1g).
The resulting β-trifluoromethythiolated butenolides and 3,4-
bis(trifluoromethylthio)but-2-enoic-acids are previously un-
known and potentially useful in different research areas.
Our study commenced with the trifluoromethylthiolation of
2-methy-4-phenylbuta-2,3-dienoic acid (1a) with AgSCF3 (2)
(Table 1). The reaction in the presence of (NH4)2S2O8 in
Received: July 21, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02249
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization of Reaction Conditions
Scheme 2. Cyclic Oxytrifluoromethylthiolation of 2,3-
a
Allenoic Acids
b
entry
Cu salt
solvent
yield (%)
1
2
3
4
5
6
7
8
9
10
−
−
−
−
−
−
−
CuI
MeCN
DMF
THF
22
trace
NR
15
MeCN/HCO2H (1:1)
MeCN/HOAc (1:1)
MeCN/HOAc (2:1)
MeCN/HOAc (4:1)
MeCN/HOAc (4:1)
MeCN/HOAc (4:1)
MeCN/HOAc (4:1)
38
60
65
67
Cu(OAc)2
CuCN
77
85
a
Reaction conditions: 1a (0.1 mmol), AgSCF3 (0.2 mmol), Cu salt
(0.02 mmol), (NH4)2S2O8 (0.2 mmol), solvent (2.0 mL), under N2,
b
70 °C, 3 h. Yields determined by 19F NMR spectroscopy using
trifluoromethylbenzene as an internal standard.
MeCN gave the cyclized trifluoromethylthiolated product 3a in
22% yield (entry 1). THF and DMF were not suitable for the
reaction (entries 2 and 3). Based on our experience of bis-
trifluoromethylthiolation of propiolic acid derivatives,8k
HCOOH and HOAc were tested as the co-solvents (entries
4 and 5). It was found that MeCN/HOAc was more effective
than MeCN/HCO2H. A careful survey of solvent ratios and
copper catalysts was then conducted (entries 6−10), and the
combination of MeCN/HOAc (4:1) and CuCN (0.2 equiv)
was optimal, giving the desired product 3a in 85% yield. Finally,
the temperature, oxidant, and ligand were also examined and no
better result was realized (for detailed data, see the Supporting
Information).
a
Reaction conditions: 1 (0.2 mmol), AgSCF3 (0.4 mmol), CuCN
(0.04 mmol), (NH4)2S2O8 (0.4 mmol), MeCN/HOAc (3.2 mL/0.8
mL), under N2, 70 °C, 3 h; isolated yields. The reaction was carried
out on a 1.0 mmol scale.
b
a
Table 2. Optimization of Reaction Conditions
With the optimized reaction conditions in hand, the substrate
scope of this cyclic oxytrifluoromethylthiolation was examined
(Scheme 2). A series of substrates bearing electron-donating
(3b and 3c) and electron-withdrawing (3d and 3e) groups as
well as halides (3f−3h) were all effective. The structure of the
product was confirmed by X-ray diffraction studies of 3f (see
the Supporting Information). The position and number of
substitutions (3i−3m) on the benzene ring did not interfere
with this transformation. Me, Et, and Ph were all competent
substituents for R2 (3n−3p), and this protocol could also be
applied for the substrate with n-butyl as R3 (3q). Notably, when
2-monosubstituted buta-2,3-dienoic acid 1r (R1 = R2 = H) was
subjected to this reaction, only a trace of the desired
trifluoromethylthiolated butenolide 3r was detected. Instead,
an unexpected bis-trifluoromethylthiolated product 4r was
obtained in 12% yield.
b
entry
Cu salt
solvent
yield (%)
1
2
3
4
5
6
7
8
CuCN
MeCN/HOAc (4:1)
16
24
19
12
28
38
51
45
Cu(OAc)2
CuCl2
MeCN/HOAc (4:1)
MeCN/HOAc (4:1)
CuI
MeCN/HOAc (4:1)
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
MeCN/HCO2H (4:1)
MeCN/HCO2H/H2O (4:1:1)
MeCN/HCO2H/H2O (4:1:2)
MeCN/HCO2H/H2O (4:1:3)
a
Reaction conditions: 1r (0.1 mmol), AgSCF3 (0.3 mmol), Cu salt
(0.02 mmol), (NH4)2S2O8 (0.3 mmol), solvent (2.8 mL), under N2,
b
70 °C, rt, 3 h. Yields determined by 19F NMR spectroscopy using
trifluoromethylbenzene as an internal standard.
In continuation of our research on bis-trifluoromethyl-
thiolation of propiolic acid derivatives,8k we then focused on
the bis-trifluoromethylthiolation of 2-monosubstituted buta-
2,3-dienoic acids. The optimization of reaction conditions of 1r
was undertaken to improve the yield of 4r (Table 2). Increasing
the amounts of AgSCF3 and (NH4)2S2O8 to 3.0 equiv led to a
slightly higher yield (entry 1). The screening of different copper
salts demonstrated that Cu(OAc)2 was more effective than
other Cu salts including CuCN, CuCl2, and CuI (entries 1−4).
It was found that the solvent was crucial for this reaction
(entries 5−8). The yield of 4r was improved by adding H2O as
the co-solvent, and the highest yield of 4r was achieved in
MeCN/HCO2H/H2O (4:1:2). Other attempts to promote this
transformation proved to be ineffective (see the Supporting
Information).
Next, the scope of the bis-trifluoromethylthiolation reaction
was evaluated under the modified conditions, and the results
are listed in Scheme 3. A series of 2-monosubstituted buta-2,3-
dienoic acids 1r−1w reacted smoothly to afford the bis-
trifluoromethylthiolated products 4r−4w in moderate yields. In
all cases, the cyclic oxytrifluoromethylthiolated products were
also formed in very low yields.
B
DOI: 10.1021/acs.orglett.7b02249
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
standard reaction conditions. However, the target product 3a
was not formed (Scheme 5b). On the other hand, the bis-
trifluoromethylthiolation reaction could not take place without
a copper catalyst (Scheme 5c), which revealed the important
role of the copper salt in this reaction. It was noteworthy that
buta-2,3-dienoate ester (8) was not a suitable substrate for the
bis-trifluoromethylthiolation reaction (Scheme 5d), proving
that the carboxylic acid of 2-monosubstituted buta-2,3-dienoic
acids was a key directing group for promoting bis-trifluoro-
methylthiolation reaction.
Scheme 3. Bis-trifluoromethylthiolation of 2-
Monosubstituted Buta-2,3-dienoic Acids
a
On the basis of the above results, a plausible reaction
mechanism was proposed (Scheme 6). First, oxidation of
Scheme 6. Proposed Reaction Mechanism
a
Reaction conditions: 1 (0.2 mmol), AgSCF3 (0.6 mmol), Cu(OAc)2
(0.04 mmol), (NH4)2S2O8 (0.6 mmol), MeCN/HCO2H/H2O (3.2
mL/0.8 mL/1.6 mL), under N2, 70 °C, 3 h; isolated yields.
Subsequently, the transformation of trifluoromethylthiolated
butenolide 3a was investigated. As shown in Scheme 4,
treatment of 3a with LDA/Ac2O or NaHMDS/ClP(O)(OPh)2
afforded trifluoromethylthiolated furan derivatives 5a or 6a in
good yields, respectively.
Scheme 4. Transformation of Compound 3a
AgSCF3 by (NH4)2S2O8 generates AgIISCF3, which could be
further transformed to the CF3S radical or CF3SSCF3.5g,h,12
The addition of the SCF3 radical to allenes 1 would generate
allyl radical species A and A′.11 Species A′ would isomerize to
the thermodynamically more favored A, because of the steric
hindrance between COOH and SCF3 groups.11c Then, the
reaction of intermediate A could proceed in two different ways
depending on the substrate. Intermediate A with aromatic
substitutions was easily oxidized by CuII or (NH4)2S2O8 to
yield the allyl cation B. Finally, the intramolecular attack of
intermediate B delivered the cyclized products 3 (path a).11a In
the case of substrates with R1 and R2 as H atoms, we speculated
that trapping species A by a Cu salt formed SCF3-allyl-Cu
complex C.8k,10b,11a Complex C was converted to the bis-
trifluoromethylthiolated products 4 by copper-assisted
trifluoromethylthiolation with CF3SSCF3 (path b).8k However,
the exact mechanism of this transformation remains unclear at
the present stage.
To gain mechanistic insights into this reaction process, some
preliminary studies were performed. Only a trace of the desired
product 3a was detected when a radical scavenger, 1,4-
benzoquinone or 2,6-di-tert-butyl-4-methylphenol (BHT), was
added to the standard reaction of 1a, which indicated this
transformation probably proceeded through a radical pathway
(Scheme 5a). Moreover, 3-methyl-5-phenylfuran-2(5H)-one
(7) was synthesized and reacted with AgSCF3 under the
Scheme 5. Mechanistic Investigation
In conclusion, we have disclosed a copper-assisted oxidetive
cyclic oxytrifluoromethylthiolation and bis-trifluoromethyl-
thiolation of 2,3-allenoics with AgSCF3. A series of previously
unknown β-trifluoromethylthiolated butenolides, trifluoro-
methylthiolated furans, and bis-trifluoromethylthiolated allyllic
acids were obtained in good yields. Further investigation of the
reaction mechanism and the applications of this protocol are
currently in progress.
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.7b02249.
C
DOI: 10.1021/acs.orglett.7b02249
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Optimization of reaction conditions; experimental
Letter
2014, 16, 2046. (d) Yin, W.; Wang, Z.; Huang, Y. Adv. Synth. Catal.
2014, 356, 2998. (e) Shao, X.; Xu, C.; Lu, L.; Shen, Q. J. Org. Chem.
2015, 80, 3012. (f) Xiong, H.-Y.; Besset, T.; Cahard, D.; Pannecoucke,
X. J. Org. Chem. 2015, 80, 4204. (g) Wang, Q.; Xie, F.; Li, X. J. Org.
Chem. 2015, 80, 8361. (h) Honeker, R.; Ernst, J. B.; Glorius, F. Chem. -
Eur. J. 2015, 21, 8047. (i) Zhang, P.; Li, M.; Xue, X.-S.; Xu, C.; Zhao,
Q.; Liu, Y.; Wang, H.; Guo, Y.; Lu, L.; Shen, Q. J. Org. Chem. 2016, 81,
7486. (j) Yoshida, M.; Kawai, K.; Tanaka, R.; Yoshino, T.; Matsunaga,
S. Chem. Commun. 2017, 53, 5974. (k) Liu, X.-G.; Li, Q.; Wang, H.
Adv. Synth. Catal. 2017, 359, 1942.
(7) For selected examples, see: (a) Yang, Y.-D.; Azuma, A.;
Tokunaga, E.; Yamasaki, M.; Shiro, M.; Shibata, N. J. Am. Chem.
Soc. 2013, 135, 8782. (b) Huang, Z.; Yang, Y.-D.; Tokunaga, E.;
Shibata, N. Org. Lett. 2015, 17, 1094. (c) Jiang, L.; Qian, J.; Yi, W.; Lu,
G.; Cai, C.; Zhang, W. Angew. Chem., Int. Ed. 2015, 54, 14965.
(d) Chachignon, H.; Maeno, M.; Kondo, H.; Shibata, N.; Cahard, D.
Org. Lett. 2016, 18, 2467. (e) Jiang, L.; Yi, W.; Liu, Q. Adv. Synth.
Catal. 2016, 358, 3700. (f) Lu, K.; Deng, Z.; Li, M.; Li, T.; Zhao, X.
Org. Biomol. Chem. 2017, 15, 1254.
procedures; characterization data; mechanistic study
1
data; copies of H, 19F, and 13C NMR spectra; X-ray
crystal structure of 3f (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: flq@mail.sioc.ac.cn.
ORCID
Feng-Ling Qing: 0000-0002-7082-756X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
National Natural Science Foundation of China (21332010,
21421002), the Strategic Priority Research Program of the
Chinese Academy of Sciences (XDB20000000), Youth
Innovation Promotion Association CAS (No. 2016234), and
Shanghai Pujiang Program (No. 15PJ1410300) are greatly
acknowledged for funding this work.
(8) For selected examples, see: (a) Sheng, J.; Li, S.; Wu, J. Chem.
Commun. 2014, 50, 578. (b) Zhu, L.; Wang, G.; Guo, Q.; Xu, Z.;
Zhang, D.; Wang, R. Org. Lett. 2014, 16, 5390. (c) Yang, T.; Lu, L.;
Shen, Q. Chem. Commun. 2015, 51, 5479. (d) Fuentes, N.; Kong, W.;
́ ́
Fernandez-Sanchez, L.; Merino, E.; Nevado, C. J. Am. Chem. Soc. 2015,
137, 964. (e) Luo, J.; Zhu, Z.; Liu, Y.; Zhao, X. Org. Lett. 2015, 17,
3620. (f) Xu, C.; Shen, Q. Org. Lett. 2015, 17, 4561. (g) Liu, X.; An,
R.; Zhang, X.; Luo, J.; Zhao, X. Angew. Chem., Int. Ed. 2016, 55, 5846.
(h) Pan, S.; Huang, Y.; Qing, F.-L. Chem. - Asian J. 2016, 11, 2854.
(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. (k) Pan, S.; Li, H.; Huang, Y.; Xu, X.-H.;
Qing, F.-L. Org. Lett. 2017, 19, 3247.
(9) For selected reviews, see: (a) Marshall, J. A. Chem. Rev. 2000,
100, 3163. (b) Lu, X.; Zhang, C.; Xu, Z. Acc. Chem. Res. 2001, 34, 535.
(c) Bates, R.; Satcharoen, V. Chem. Soc. Rev. 2002, 31, 12. (d) Sydnes,
L. Chem. Rev. 2003, 103, 1133. (e) Wei, L. L.; Xiong, H.; Hsung, R. P.
Acc. Chem. Res. 2003, 36, 773. (e) Ma, S. Acc. Chem. Res. 2003, 36, 701.
(g) Ma, S. Chem. Rev. 2005, 105, 2829. (h) Brasholz, M.; Reissig, H.-
U.; Zimmer, R. Acc. Chem. Res. 2009, 42, 45.
REFERENCES
■
(1) For recent reviews, see: (a) Cametti, M.; Crousse, B.;
Metrangolo, P.; Milani, R.; Resnati, G. Chem. Soc. Rev. 2012, 41, 31.
(b) Nakajima, T. J. Fluorine Chem. 2013, 149, 104. (c) 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.
(2) For recent reviews, see: (a) Chu, L.; Qing, F.-L. Acc. Chem. Res.
2014, 47, 1513. (b) Toulgoat, F.; Alazet, S.; Billard, T. Eur. J. Org.
Chem. 2014, 2014, 2415. (c) Xu, H.-H.; Matsuzaki, K.; Shibata, N.
Chem. Rev. 2015, 115, 731. (d) Shao, X.; Xu, C.; Lu, L.; Shen, Q. Acc.
Chem. Res. 2015, 48, 1227. (e) Zhang, K.; Xu, X.-H.; Qing, F.-L. Youji
Huaxue 2015, 35, 556. (f) Chachignon, H.; Cahard, D. Chin. J. Chem.
2016, 34, 445.
(3) For selected examples, see: (a) Umemoto, T.; Ishihara, S. J. Am.
Chem. Soc. 1993, 115, 2156. (b) Billard, T.; Large, S.; Langlois, B. R.
Tetrahedron Lett. 1997, 38, 65. (c) Kieltsch, I.; Eisenberger, P.; Togni,
A. Angew. Chem., Int. Ed. 2007, 46, 754. (d) Kremsner, J. M.; Rack, M.;
Pilger, C.; Kappe, C. O. Tetrahedron Lett. 2009, 50, 3665.
(10) (a) Pacheco, M. C.; Gouverneur, V. Org. Lett. 2005, 7, 1267.
(b) Zhou, C.; Li, J.; Lu, B.; Fu, C.; Ma, S. Org. Lett. 2008, 10, 581.
(c) Zhou, C.; Ma, Z.; Gu, Z.; Fu, C.; Ma, S. J. Org. Chem. 2008, 73,
772. (d) Xu, T.; Mu, X.; Peng, H.; Liu, G. Angew. Chem., Int. Ed. 2011,
50, 8176.
(4) For selected examples, see: (a) Teverovskiy, G.; Surry, D. S.;
Buchwald, S. L. Angew. Chem., Int. Ed. 2011, 50, 7312. (b) Zhang, C.-
P.; Vicic, D. A. J. Am. Chem. Soc. 2012, 134, 183. (c) Xu, J.; Mu, X.;
Chen, P.; Ye, J.; Liu, G. Org. Lett. 2014, 16, 3942. (d) Yin, G.; Kalvet,
I.; Englert, U.; Schoenebeck, F. J. Am. Chem. Soc. 2015, 137, 4164.
(e) Yin, G.; Kalvet, I.; Schoenebeck, F. Angew. Chem., Int. Ed. 2015, 54,
6809. (f) Weng, Z.; He, W.; Chen, C.; Lee, R.; Tan, D.; Lai, Z.; Kong,
D.; Yuan, Y.; Huang, K. Angew. Chem., Int. Ed. 2013, 52, 1548.
(g) Nguyen, T.; Chiu, W.; Wang, X.; Sattler, M. O.; Love, J. A. Org.
Lett. 2016, 18, 5492. (h) Yang, Y.; Xu, L.; Yu, S.; Liu, X.; Zhang, Y.;
Vicic, D. A. Chem. - Eur. J. 2016, 22, 858.
(11) (a) Yu, Q.; Ma, S. Chem. - Eur. J. 2013, 19, 13304. (b) Wang, Y.;
Jiang, M.; Liu, J.-T. Adv. Synth. Catal. 2014, 356, 2907. (c) Zhu, N.;
Wang, F.; Chen, P.; Ye, J.; Liu, G. Org. Lett. 2015, 17, 3580.
(12) (a) Yin, F.; Wang, X.-S. Org. Lett. 2014, 16, 1128. (b) Zhang, K.;
Liu, J.-B.; Qing, F.-L. Chem. Commun. 2014, 50, 14157.
(5) For selected examples, see: (a) Chen, C.; Xie, Y.; Chu, L.; Wang,
R.-W.; Zhang, X.; Qing, F.-L. Angew. Chem., Int. Ed. 2012, 51, 2492.
(b) Chen, C.; Chu, L.; Qing, F.-L. J. Am. Chem. Soc. 2012, 134, 12454.
(c) Zhang, C.-P.; Vicic, D. A. Chem. - Asian J. 2012, 7, 1756. (d) Zhai,
L.; Li, Y.; Yin, J.; Jin, K.; Zhang, R.; Fu, X.; Duan, C. Tetrahedron 2013,
69, 10262. (e) Chen, C.; Li, H.; Xu, X.-H.; Yang, B.; Qing, F.-L. Org.
Lett. 2014, 16, 3372. (f) Danoun, G.; Bayarmagnai, B.; Gruenberg, M.
F.; Goossen, L. J. Chem. Sci. 2014, 5, 1312. (g) Guo, S.; Zhang, X.;
Tang, P. Angew. Chem., Int. Ed. 2015, 54, 4065. (h) Wu, H.; Xiao, Z.;
Wu, J.; Guo, Y.; Xiao, J.-C.; Liu, C.; Chen, Q.-Y. Angew. Chem., Int. Ed.
2015, 54, 4070. (i) Mukherjee, S.; Maji, B.; Tlahuext-Aca, A.; Glorius,
F. J. Am. Chem. Soc. 2016, 138, 16200.
(6) For selected examples, see: (a) Tran, L. D.; Popov, I.; Daugulis,
O. J. Am. Chem. Soc. 2012, 134, 18237. (b) Xu, C.; Ma, B.; Shen, Q.
Angew. Chem., Int. Ed. 2014, 53, 9316. (c) Xu, C.; Shen, Q. Org. Lett.
D
DOI: 10.1021/acs.orglett.7b02249
Org. Lett. XXXX, XXX, XXX−XXX