Synthesis of β-Difluoroalkylated Acrylonitriles in the Presence of Copper Powder

Article abstract of DOI:10.1021/acs.orglett.6b02627

A highly regio- and stereoselective copper-mediated cyanodifluoroalkylation of alkynes with ethyl difluoroiodoacetate and trimethylsilyl cyanide (TMSCN) is described. The three-component coupling reaction provides straightforward access to a variety of us

Full text of DOI:10.1021/acs.orglett.6b02627

Letter
pubs.acs.org/OrgLett
Synthesis of β‑Difluoroalkylated Acrylonitriles in the Presence of
Copper Powder
†
†
†
‡
,†,§
Yu-Tao He, Lian-Hua Li, Qiang Wang, Wangsuo Wu, and Yong-Min Liang*
†
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China
‡
Radiochemistry Laboratory, School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, P. R. China
§
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou, 730000, P.
R. China
*
S Supporting Information
ABSTRACT: A highly regio- and stereoselective copper-
mediated cyanodifluoroalkylation of alkynes with ethyl difluor-
oiodoacetate and trimethylsilyl cyanide (TMSCN) is described.
The three-component coupling reaction provides straightforward
access to a variety of useful difluoroalkyl-substituted acrylonitriles.
The introduction of the nitrile unit is of great importance in drug
discovery for the modification of this fragment. Preliminary mechanistic investigations indicate that a vinyl iodide intermediate
and a difluoroalkyl radical might be involved in this transformation.
rganofluorine compounds usually exhibit specific proper-
Scheme 1. Transition-Metal-Catalyzed or -Mediated
O
ties as compared with their parent molecules in organic
Difluoroalkylation of Alkynes
1
synthesis. Consequently, considerable efforts have been
devoted for the introduction of fluorinated moieties into target
2
,3
molecules. In contrast to the progress achieved in the
trifluoromethylation field, strategies to access CF R-containing
2
compounds are less abundant for the lack of good
4
difluoromethylation reagents, even if they exhibit the ability
5
to be modificated into other fluorinated moieties. The
traditional methods to synthesize CF -containing molecules
2
are mainly from the following three aspects: (1) Transition-
metal-mediated cross-coupling reactions of difluoroalkyl species
6
7
8
with halogenated arenes, alkenes, or arylboronic acids; (2)
Difluoroalkylation of the C−H bonds of unsaturated
9
,10
compounds;
(3) Difluoroalkylation of alkenes or alkynes
11
based on an intermolecular difunctionalization strategy.
Recently, Zhang, Pannecoucke, and related groups
12
13
14,15
have made significant advancements in this area. Nevertheless,
using these methods, there are merely three types of transition-
metal-catalyzed difluoroalkylation reactions for compounds
comprising a less reactive CC bond. One straightforward
approach is the halodifluoroalkylation of alkynes reported by
1
6
17
the Hu and Besset group in 2014, respectively (Scheme 1a).
For the three-component aryldifluoroalkylation reactions,
1
8
19
20
Nevado, Chaładaj, and our group independently reported
the palladium-catalyzed difunctionalization of alkynes with
ethyl difluoroiodoacetate and arylboronic acids (Scheme 1b). In
It is well-known that nitriles are an important class of organic
22
compounds. The preparations of such compounds have
received significant attention. In 2014, our group reported a
copper-catalyzed, intermolecular cyanotrifluoromethylation of
2
016, a palladium catalyzed stereoselective difluoroalkylation
and carbonylation of alkynes was also reported by our group
Scheme 1b). Due to few reports and great potential in this
23
alkenes. On this basis, the formation of two C−C bonds
2
1
across the alkynes can be achieved concurrently in a single pot
(
area, the development of concise approaches to introduce the
difluoroalkyl group synchronously with other functional groups
into alkynes remains a challenging task.
Received: September 1, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02627
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
operation. In contrast to alkyl nitriles, α,β-unsaturated nitriles
are widely applied as feedstocks in the Diels−Alder reaction,
which is a powerful synthetic tool in the design of bioactive
Scheme 2. Substrate Scope for the Cyanodifluoroalkylation
of Alkynes
,
a b
24
molecules and natural products. Consistent with the previous
work of our group, we herein report the highly regio- and
stereoselective copper-mediated cyanodifluoroalkylation of
alkynes with ethyl difluoroiodoacetate and trimethylsilyl
cyanide (TMSCN) (Scheme 1c). This work provides rapid
assemblies of valuable difluoroalkyl-substituted acrylonitriles.
We began the optimization of reaction conditions by utilizing
phenylacetylene 1a, ethyl difluoroiodoacetate, and TMSCN as
model substrates. The iododifluoroalkylation product 4a was
isolated in 70% yield with the CuBr catalyst at 80 °C under
argon in 1,4-dioxane (Table 1, entry 1), which is the same as
Table 1. Optimization of the Reaction Conditions for
a
Cyanodifluoroalkylation of Alkynes
b
entry
additive
base (equiv)
solvent
yield (%)
1
2
3
4
5
6
7
8
CuBr (10%)
Cu (10%)
Cs CO (1.0)
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
DMF
3a/4a (0/70)
3a/4a (0/71)
3a/4a (0/65)
3a/4a (0/81)
3a/4a (31/30)
3a/4a (0/78)
3a/4a (29/30)
3a/4a (25/32)
3a/4a (69/9)
3a/4a (74/6)
0
2
3
Cs CO (1.0)
2
3
Cu (10%)








Cu (2.0 equiv)
Cu (2.0 equiv)
Cu (2.0 equiv)
Cu (2.0 equiv)
Cu (2.0 equiv)
Cu (2.0 equiv)
Cu (2.0 equiv)
Cu (2.0 equiv)
Cu (1.0 equiv)

toluene
NMP
DMAc
9
DMSO
c
10
11
12
13
DMSO
Cs CO (1.0)
DMSO
2
3

DMSO
3a/4a (67/7)
0
a
d

DMSO
Reaction conditions: 1 (0.2 mmol), 2a (0.3 mmol), TMSCN (0.4
mmol), Cu powder (2.0 equiv), DMSO (1.0 mL), 12 h, 90 °C, under
a
Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), TMSCN (0.4
mmol), catalyst, 12 h, 80 °C, under argon. Isolated yield. The
reaction was conducted at 90 °C. Without copper catalyst.
b
b
c
argon. Isolated yields.
d
stituted acrylonitrile 3c was confirmed by X-ray diffraction.
Phenylacetylene bearing electron-donating substituents gave
higher yields than those bearing electron-withdrawing ones on
the aryl ring. The methoxy naphthyl substrate 1o participated
in this cyanodifluoroalkylation reaction and provided the
corresponding product in a satisfactory yield. A significant
drop in the reactivity was found when phenylacetylene with a
keto-moiety on the para-position of phenyl ring 1p was
explored under the optimal conditions. It should be mentioned
that the linear alkyne 1q showed a competent reaction partner
for this transformation, leading to the desired product in 40%
yield. Sterically hindered substrates such as 2,4,5-trimethyl- and
2-chlorophenylacetylene failed to undergo this cyanodifluor-
oalkylation reaction as expected.
To gain more insights into the cyanodifluoroalkylation
process, some necessary trapping experiments were performed.
First, two reactions were conducted to investigate whether the
iododifluoroalkylation product was the reaction intermediate.
As a result, only 16% of 4a was isolated in the absence of
TMSCN (Scheme 3a). The iododifluoroalkylation compound
4a used as starting material under the optimized conditions
resulted in an even lower conversion; a 9% yield of the
corresponding cyanodifluoroalkylation product 3a was thus
obtained (Scheme 3b). These experiments suggest that part of
Hu’s results (Scheme 1a). A similar yield of 4a was also
obtained when copper powder was used instead of CuBr (Table
1
, entry 2). Stoichiometric amounts of copper powder were
used in the following investigation. A short survey on solvents
indicated that the property of the solvent was crucial for this
transformation; the desired product difluoroalkyl-substituted
acrylonitrile 3a was successfully obtained in a polar solvent. A
6
9% yield of product 3a was isolated in dimethyl sulfoxide
(
DMSO) (Table 1, entry 9). A higher temperature of 90 °C
could increase the yield to 74% (Table 1, entry 10). No
significant progress in the yield was obtained in subsequent
attempts. Control reactions demonstrated that no reaction
occurred in the absence of copper powder. As a consequence,
the optimized reaction conditions were confirmed by using
copper powder (2.0 equiv), TMSCN (2.0 equiv), and ethyl
difluoroiodoacetate (1.5 equiv) in DMSO at 90 °C under
argon.
Subsequently, the scope of a series of alkynes 1 with ethyl
difluoroiodoacetate and TMSCN was investigated under the
optimized conditions. As described in Scheme 2, the reaction
proceeded smoothly in most cases to give the corresponding
products in moderate to good yields with excellent stereo-
selectivity. The NMR-based structure of difluoroalkyl-sub-
B
DOI: 10.1021/acs.orglett.6b02627
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Trapping Experiments
Scheme 4. A Possible Radical Pathway
stereodefined manner, and the introduction of a nitrile unit is
of great importance in drug discovery for the modification of
these fragments. We expect that this transformation may
provide new insight into the difluoroalkylation reaction of
alkynes and find applications to synthesize complex hetero-
cycles. Further explorations to uncover the reaction mechanism
are currently underway in our laboratory.
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.6b02627.
Detailed experimental procedures, spectral data for all
new compounds, crystallographic data (PDF)
Crystallographic data for 3c (CIF)
this transformation proceeded via a vinyl iodide intermediate.
No β-difluoroalkylated acrylonitrile was obtained when ethyl
bromodifluoroacetate was used instead of ethyl difluoro-
iodoacetate as the source of CF2 (Scheme 3c), and a
hydrodifluoroalkylation product 5a (49%) could be obtained
without TMSCN (Scheme 3d). Second, when the reaction was
carried out in the presence of TEMPO, a radical scavenger, the
desired transformation was shut down and a trace amount of
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: liangym@lzu.edu.cn.
Notes
the TEMPO−CF COOEt adduct was detected by GC−MS
The authors declare no competing financial interest.
2
(
Scheme 3e). Additionally, when a stoichiometric amount of
1
,1-diphenylethylene 6, in the presence or absence of
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation (NSF
1272101, 21472074, and 21472073), the supported by the
Fundamental Research Funds for the Central Universities
lzujbky-2014-243 and lzujbky-2014-194). We also acknowl-
edge support from the “111” Project, J1103307, Program for
Changjiang Scholars, and Innovative Research Team in
University (IRT1138).
■
phenylacetylene 1a, was added as a radical trapper to this
control experiment (Scheme 3f), no desired coupling product
2
3
a could be isolated. However, the CF COOEt-containing
2
compounds 7 and 8 with a 1:1 ratio were mainly obtained in
both cases, in which the total yield of 7 and 8 could be
increased to 34% without alkyne 1a. On the basis of these
preliminary results, a mechanism involving a difluoroalkyl
radical pathway might be involved in the present reactions. As
described in Scheme 4, the difluoroalkyl radical might come
from the redox reaction of ethyl difluoroiodoacetate with
copper powder. The difluoromethyl radical species was added
to the C−C triple bond of alkyne, and the radical intermediate
A was formed. The radical intermediate A reacted with copper
powder and TMSCN to generate the copper(II) cyanide
complex B in trans-stereochemistry. Subsequent reductive
elimination of the copper(II) cyanide complex B produced
the desired cyanodifluoroalkylation product 3 with the
simultaneous release of Cu(0).
(
REFERENCES
■
(
1) (a) Filler, R.; Kobayashi, Y.; Yagupolskii, L. M. Organofluorine
Compounds in Medicinal Chemistry and Biomedical Applications;
Elsevier: Amsterdam, 1993. (b) Purser, S.; Moore, P. R.; Swallow,
S.; Gouverneur, V. Chem. Soc. Rev. 2008, 37, 320. (c) Furuya, T.;
Kuttruff, C.; Ritter, T. Curr. Opin. Drug Discovery Dev. 2008, 11, 80.
2) (a) Prakash, G. K. S.; Hu, J. Acc. Chem. Res. 2007, 40, 921.
b) Xue, F.; Li, H.; Delker, S. L.; Fang, J.; Martasek, P.; Roman, L. J.;
Poulos, T. L.; Silverman, R. B. J. Am. Chem. Soc. 2010, 132, 14229.
c) Tomashenko, O. A.; Grushin, V. V. Chem. Rev. 2011, 111, 4475.
d) Kong, W.; Casimiro, M.; merino, E.; Nevado, C. J. Am. Chem. Soc.
013, 135, 14480. (e) Liang, T.; Neumann, C. N.; Ritter, T. Angew.
Chem., Int. Ed. 2013, 52, 8214.
3) (a) Zhao, G.; Sun, H.-l.; Qian, Z.-s.; Yin, W.-X. J. Fluorine Chem.
(
(
́
(
(
2
In summary, we have developed an unprecedented copper-
mediated cyanodifluoroalkylation of alkynes with trimethylsilyl
cyanide and ethyl difluoroiodoacetate. This three-component
coupling reaction provides a general method for the expeditious
synthesis of various β-difluoroalkylated acrylonitriles in a
(
2001, 111, 217. (b) Jiang, H.; Huang, C.; Guo, J.; Zeng, C.; Zhang, Y.;
Yu, S. Chem. - Eur. J. 2012, 18, 15158. (c) Iqbal, N.; Jung, J.; Park, S.;
C
DOI: 10.1021/acs.orglett.6b02627
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Cho, E. J. Angew. Chem., Int. Ed. 2014, 53, 539. (d) Liu, J.; Zhuang, S.;
Gui, Q.; Chen, X.; Yang, Z.; Tan, Z. Eur. J. Org. Chem. 2014, 2014,
(13) (a) Belhomme, M.; Poisson, T.; Pannecoucke, X. Org. Lett.
2013, 15, 3428. (b) Belhomme, M. C.; Poisson, T.; Pannecoucke, X. J.
Org. Chem. 2014, 79, 7205. (c) Caillot, G.; Dufour, J.; Belhomme, M.
C.; Poisson, T.; Grimaud, L.; Pannecoucke, X.; Gillaizeau, I. Chem.
Commun. 2014, 50, 5887. (d) Besset, T.; Poisson, T.; Pannecoucke, X.
Chem. - Eur. J. 2014, 20, 16830. (e) Belhomme, M. C.; Bayle, A.;
Poisson, T.; Pannecoucke, X. Eur. J. Org. Chem. 2015, 2015, 1719.
(14) (a) Wang, J.-Y.; Su, Y.-M.; Yin, F.; Bao, Y.; Zhang, X.; Xu, Y.-M.;
Wang, X.-S. Chem. Commun. 2014, 50, 4108. (b) Wang, J.-Y.; Zhang,
X.; Bao, Y.; Xu, Y.-M.; Cheng, X.-F.; Wang, X.-S. Org. Biomol. Chem.
2014, 12, 5582. (c) Tang, X.-J.; Thomoson, C. S.; Dolbier, W. R. Org.
Lett. 2014, 16, 4594. (d) Zhang, Z.; Tang, X.; Thomoson, C. S.;
Dolbier, W. R. Org. Lett. 2015, 17, 3528. (e) Tang, X.-J.; Dolbier, W.
R. Angew. Chem., Int. Ed. 2015, 54, 4246. (f) Zhang, Z.; Tang, X.-J.;
Dolbier, W. R. Org. Lett. 2016, 18, 1048.
3196. (e) Yu, C.; Park, S.; Cho, E. J.; Iqbal, N. Chem. Commun. 2014,
50, 12884. (f) Xu, P.; Hu, K.; Gu, Z.; Cheng, Y.; Zhu, C. Chem.
Commun. 2015, 51, 7222.
4) (a) Yang, Z. Y.; Burton, D. J. J. Org. Chem. 1991, 56, 5125.
b) Yang, Z. Y.; Burton, D. J. J. Org. Chem. 1992, 57, 4676. (c) Sato,
(
(
K.; Nakazato, S.; Enko, H.; Tsujita, H.; Fujita, K.; Yamamoto, T.;
Omote, M.; Ando, A.; Kumadaki, I. J. Fluorine Chem. 2003, 121, 105.
(
d) Sato, K.; Ishida, Y.; Murata, E.; Oida, Y.; Mori, Y.; Okawa, M.;
Iwase, K.; Tarui, A.; Omote, M.; Kumadaki, I.; Ando, A. Tetrahedron
007, 63, 12735. (e) Ponomarenko, M. V.; Serguchev, Y. A.;
Ponomarenko, B. V.; Roschenthaler, G.-V.; Fokin, A. A. J. Fluorine
Chem. 2006, 127, 842. (f) Ohtsuka, Y.; Yamakawa, T. Tetrahedron
011, 67, 2323.
5) (a) Nesmeyanov, A. N.; Pecherskaya, K. A.; Uretskaya, G. Y. Izv.
Akad. Nauk SSSR, Ser. Khim. 1948, 240. (b) Kim, Y. S. J. Korean Chem.
963, 7, 189. (c) Battais, A.; Boutevin, B.; Pietrasanta, Y. J. Fluorine
Chem. 1979, 14, 467. (d) Ghattas, W.; Hess, C. R.; Iacazio, G.; Hardre,
R.; Klinman, J. P.; Reglier, M. J. Org. Chem. 2006, 71, 8618.
e) Fujikawa, K.; Fujioka, Y.; Kobayashi, A.; Amii, H. Org. Lett. 2011,
3, 5560.
6) For selected examples: (a) Sato, K.; Omote, M.; Ando, A.;
2
̈
2
(
(15) (a) Murakami, S.; Ishii, H.; Tajima, T.; Fuchigami, T.
Tetrahedron 2006, 62, 3761. (b) Nguyen, J. D.; Tucker, J. W.;
Konieczynska, M. D.; Stephenson, C. R. J. J. Am. Chem. Soc. 2011, 133,
1
4
160. (c) Wallentin, C.-J.; Nguyen, J. D.; Finkbeiner, P.; Stephenson,
́
C. R. J. J. Am. Chem. Soc. 2012, 134, 8875. (d) Jung, J.; Kim, E.; You,
Y.; Cho, E. J. Adv. Synth. Catal. 2014, 356, 2741. (e) He, Z.; Tan, P.;
Ni, C.; Hu, J. Org. Lett. 2015, 17, 1838.
́
(
1
(
16) Xu, T.; Cheung, C. W.; Hu, X. Angew. Chem., Int. Ed. 2014, 53,
910.
17) Belhomme, M.-C.; Dru, D.; Xiong, H.-Y.; Cahard, D.; Besset, T.;
Poisson, T.; Pannecoucke, X. Synthesis 2014, 46, 1859.
18) Li, Z.; García-Domínguez, A.; Nevado, C. J. Am. Chem. Soc.
015, 137, 11610.
19) Domanski, S.; Chaładaj, W. ACS Catal. 2016, 6, 3452.
20) He, Y.-T.; Wang, Q.; Li, L.-H.; Liu, X.-Y.; Xu, P.-F.; Liang, Y.-M.
Org. Lett. 2015, 17, 5188.
21) Wang, Q.; He, Y.-T.; Zhao, J.-H.; Qiu, Y.-F.; Zheng, L.; Hu, J.-
Y.; Yang, Y.-C.; Liu, X.-Y.; Liang, Y.-M. Org. Lett. 2016, 18, 2664.
22) (a) Larock, R. C. Comprehensive Organic Transformations; Wiley-
(
4
(
Kumadaki, I. J. Fluorine Chem. 2004, 125, 509. (b) Guo, Y.; Shreeve, J.
n. M. Chem. Commun. 2007, 3583. (c) Guo, C.; Wang, R.-W.; Qing,
F.-L. J. Fluorine Chem. 2012, 143, 135. (d) Jiang, H.; Lu, W.; Yang, K.;
Ma, G.; Xu, M.; Li, J.; Yao, J.; Wan, W.; Deng, H.; Wu, S.; Zhu, S.;
Hao, J. Chem. - Eur. J. 2014, 20, 10084. (e) Ge, S.; Chaładaj, W.;
Hartwig, J. F. J. Am. Chem. Soc. 2014, 136, 4149. (f) Ge, S.; Arlow, S.
I.; Mormino, M. G.; Hartwig, J. F. J. Am. Chem. Soc. 2014, 136, 14401.
(
2
(
(
́
(
7) For selected examples: (a) Yokomatsu, T.; Suemune, K.; Murano,
(
T.; Shibuya, S. J. Org. Chem. 1996, 61, 7207. (b) Han, X.; Yue, Z.;
Zhang, X.; He, Q.; Yang, C. J. Org. Chem. 2013, 78, 4850.
(
(
8) For selected examples: (a) Qi, Q.; Shen, Q.; Lu, L. J. Am. Chem.
VCH: New York, 1989; p 819. (b) Fleming, F. F.; Zhang, Z.
Tetrahedron 2005, 61, 747.
Soc. 2012, 134, 6548. (b) Ma, G.; Wan, W.; Hu, Q.; Jiang, H.; Wang,
J.; Zhu, S.; Hao, J. Chem. Commun. 2014, 50, 7527.
(
23) He, Y.-T.; Li, L.-H.; Yang, Y.-F.; Zhou, Z.-Z.; Hua, H.-L.; Liu,
X.-Y.; Liang, Y.-M. Org. Lett. 2014, 16, 270.
24) Nicolaou, K. C.; Snyder, S. A.; Montagnon, T.;
Vassilikogiannakis, G. Angew. Chem., Int. Ed. 2002, 41, 1668.
(
9) (a) Surapanich, N.; Kuhakarn, C.; Pohmakotr, M.; Reutrakul, V.
Eur. J. Org. Chem. 2012, 2012, 5943. (b) Lin, Q.; Chu, L.; Qing, F.
Chin. J. Chem. 2013, 31, 885. (c) Su, Y.-M.; Hou, Y.; Yin, F.; Xu, Y.-M.;
Li, Y.; Zheng, X.; Wang, X. Org. Lett. 2014, 16, 2958. (d) Wang, L.;
Wei, X.-J.; Lei, W.-L.; Chen, H.; Wu, L.-Z.; Liu, Q. Chem. Commun.
(
2
014, 50, 15916. (e) Wang, L.; Wei, X.-J.; Jia, W.-L.; Zhong, J.-J.; Wu,
L.-Z.; Liu, Q. Org. Lett. 2014, 16, 5842.
10) (a) Li, Y.; Zhu, J.; Xie, H.; Li, S.; Peng, D.; Li, Z.; Wu, Y.; Gong,
(
Y. Chem. Commun. 2012, 48, 3136. (b) Li, Y.; Zhang, L.; Zhang, L.;
Wu, Y.; Gong, Y. Eur. J. Org. Chem. 2013, 2013, 8039. (c) Zhou, Q.;
Ruffoni, A.; Gianatassio, R.; Fujiwara, Y.; Sella, E.; Shabat, D.; Baran, P.
S. Angew. Chem., Int. Ed. 2013, 52, 3949. (d) Zhou, Q.; Gui, J.; Pan, C.-
M.; Albone, E.; Cheng, X.; Suh, E. M.; Grasso, L.; Ishihara, Y.; Baran,
P. S. J. Am. Chem. Soc. 2013, 135, 12994. (e) Shi, S.-L.; Buchwald, S. L.
Angew. Chem., Int. Ed. 2015, 54, 1646.
(
11) (a) Qiu, Z.-M.; Burton, D. J. Tetrahedron Lett. 1993, 34, 3239.
b) Li, Y.; Li, H.; Hu, J. Tetrahedron 2009, 65, 478. (c) Ma, G.; Wan,
W.; Li, J.; Hu, Q.; Jiang, H.; Zhu, S.; Wang, J.; Hao, J. Chem. Commun.
014, 50, 9749. (d) Tang, X.-J.; Zhang, Z.; Dolbier, W. R. Chem. - Eur.
(
2
J. 2015, 21, 18961. (e) Lin, Q.-Y.; Xu, X.-H.; Zhang, K.; Qing, F.-L.
Angew. Chem., Int. Ed. 2016, 55, 1479. (f) Arai, Y.; Tomita, R.; Ando,
G.; Koike, T.; Akita, M. Chem. - Eur. J. 2016, 22, 1262. (g) Yi, N.;
Zhang, H.; Xu, C.; Deng, W.; Wang, R.; Peng, D.; Zeng, Z.; Xiang, J.
Org. Lett. 2016, 18, 1780.
(
12) (a) Feng, Z.; Chen, F.; Zhang, X. Org. Lett. 2012, 14, 1938.
(
b) Xiao, Y.-L.; Zhang, B.; Feng, Z.; Zhang, X. Org. Lett. 2014, 16,
4
1
822. (c) Feng, Z.; Xiao, Y.-L.; Zhang, X. Org. Chem. Front. 2014, 1,
13. (d) Feng, Z.; Min, Q.-Q.; Xiao, Y.-L.; Zhang, B.; Zhang, X. Angew.
Chem., Int. Ed. 2014, 53, 1669. (e) Xiao, Y.-L.; Guo, W.-H.; He, G.-Z.;
Pan, Q.; Zhang, X. Angew. Chem., Int. Ed. 2014, 53, 9909. (f) Min, Q.-
Q.; Yin, Z.; Feng, Z.; Guo, W.-H.; Zhang, X. J. Am. Chem. Soc. 2014,
136, 1230. (g) Feng, Z.; Min, Q.-Q.; Zhao, H.-Y.; Gu, J.-W.; Zhang, X.
Angew. Chem., Int. Ed. 2015, 54, 1270.
D
DOI: 10.1021/acs.orglett.6b02627
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