Chiral Selenide-Catalyzed Enantioselective Construction of Saturated Trifluoromethylthiolated Azaheterocycles

Article abstract of DOI:10.1021/acs.orglett.7b01392

An indane-based, bifunctional, chiral selenide catalyst has been developed. The new catalyst is efficient for the enantioselective synthesis of saturated azaheterocycles possessing a trifluoromethylthio group. The desired products were obtained in good yields with high diastereo- and enantioselectivities.

Full text of DOI:10.1021/acs.orglett.7b01392

Letter
pubs.acs.org/OrgLett
Chiral Selenide-Catalyzed Enantioselective Construction of Saturated
Trifluoromethylthiolated Azaheterocycles
Jie Luo, Yannan Liu, and Xiaodan Zhao*
Institute of Organic Chemistry & MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen
University, Guangzhou 510275, P. R. China
S
* Supporting Information
ABSTRACT: An indane-based, bifunctional, chiral selenide catalyst
has been developed. The new catalyst is efficient for the
enantioselective synthesis of saturated azaheterocycles possessing a
trifluoromethylthio group. The desired products were obtained in
good yields with high diastereo- and enantioselectivities.
he synthesis of saturated azaheterocycles such as
pyrrolidines and piperidines has been a long-standing
selenium catalyst for bromoaminocyclization of trisubstituted
olefins.9 Despite these advances, studies in this field are still in
their infancy. Thus, developing new chiral selenium catalysts and
using them for valuable organic transformations is highly
desirable. Herein, we report a bifunctional selenide catalyst
that is efficient for the enantioselective CF3S aminocyclization of
internal alkenes to afford a series of saturated azaheterocycles
(Scheme 1b).12
T
endeavor of the synthetic community1 because these structural
motifs are present in many drugs and bioactive natural products.2
Among the developed methods for their preparation, the
electrophilic cyclization of alkenes represents an efficient strategy
to construct these valuable molecular frameworks.3 Normally, a
nitrogen-containing ring is formed with the concomitant
introduction of an additional functional group (−F, −Cl, −Br,
−I, −SR) in the cyclization step. Because of the significant
importance of fluorinated moieties,4 many efforts have been
devoted to generating fluorinated azaheterocycles in a similar
way.5,6 However, successful examples of enantioselective syn-
thesis are very rare.7 In particular, the enantioselective cyclization
of internal alkenes with incorporation of a fluorinated moiety to
produce saturated azaheterocycles has not been developed
(Scheme 1a).
Owing to the importance of the CF3S group in bioactive
molecules, methods for the preparation of achiral CF3S
compounds have been rapidly developed.13 In contrast,
enantioselective strategies for generating CF3S stereocenters
are scarce.14 Recently, we have brought chalcogenide catalysis
into the field of trifluoromethylthiolation.11 With the partic-
ipation of selenide/sulfide-captured CF3S+, the developed
system offered a chance for asymmetric trifluoromethylthiola-
tion. Accordingly, an indane-based chiral sulfide catalyst for the
enantioselective CF3S lactonization of alkenoic acids was
developed.11c It was efficient for 4-arylalkenoic acid substrates,
but when 4-alkylalkenoic acids were used under similar
conditions, the desired products were generated with moderate
enantioselectivities. These facts suggested that indane could act
as a privileged catalyst scaffold, and 4-alkyl-substituted substrates
were challenging. With these points in mind, we initiated a series
of CF3S aminocyclization reactions with the (E)-ethyl-
substituted, Ns-protected olefinic sulfonamide 1a as a model
substrate (Table 1).15 Sulfide C1 with an NHBoc group was first
examined in the cyclization of 1a. Not surprisingly, product 2a
was generated with only 18% ee at 0 °C using (PhSO2)2NSCF3
(Table 1, entry 1). To test the effect of a hydrogen bond donor,
the amine group on the catalyst was protected by different groups
such as Ts, Bz, and Tf (Table 1, entries 2−4). It was found that
catalyst C4, with its strong hydrogen bond donor group,
delivered a 39% ee for the formation of 2a.
Scheme 1. Synthesis of Saturated Azaheterocycles
Lewis basic selenium catalysis has emerged as a powerful tool
in organic synthesis.3h,8−11 Its utilization in asymmetric trans-
formations began several years ago. In 2011, Denmark
demonstrated BINAM-derived selenophosphoramide catalysts
for the asymmetric sulfenylation of olefins.8c−h In 2013, Yeung
reported a new, sugar alcohol-derived monofunctional cyclic
Received: May 9, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01392
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a
was placed at the ortho position to the methoxy group (C13), the
reaction almost shut down (Table 1, entry 16). Catalyst C14
bearing both o-methyl and methoxy groups afforded 2a in
excellent yield with 70% ee (Table 1, entry 17). In comparison to
the results from other catalysts, this improvement depended on
the appropriate electron effect and steric hindrance of catalyst
that affects the interaction between selenium and CF3S+ cation
and the chiral environment of the reaction, respectively.
Furthermore, the enantioselectivity could be improved when
the reactions were performed at −78 °C, but the yields were low
(Table 1, entries 18−20).
Table 1. Condition Evaluation with Sulfide Catalysts
In our previous studies,11a selenium had a special ability to
promote trifluoromethylthiolation compared with the other
Lewis base atoms such as N, P, and S. This drove us to do further
research to exploit the catalytic properties of selenium.
Subsequently, chiral selenide C15 was tested in the same
reaction (Scheme 2). Product 2a was formed in a higher yield
a
Scheme 2. Selenide-Catalyzed CF3S-aminocyclization
a
Conditions: As described in Table 1.
a
Conditions: 1a (0.05 mmol), (PhSO2)2NSCF3 (1.5 equiv), catalyst
with a higher ee compared to the use of catalyst C4 when the
reaction was performed at 0 °C. The reaction temperature was
further studied. The lower the reaction temperature, the higher
the enantioselectivity of the product. To our surprise, even at
−78 °C, the selenide catalyst still resulted in the formation of 2a
with 62% ee.
(20 mol %), TfOH (0.5 equiv), CH2Cl2 in 0.025 M concentration for
b
12 h. Unless noted, diastereoselectivity is >99:1. Refers to NMR yield
c
using trifluoromethylbenzene as the internal standard. Determined by
HPLC analysis.
The reactivity of 1a was heavily affected by the reaction
temperature (Table 1, entries 5−7). When the reaction was
carried out at −45 °C, the product was formed in only 7% yield
with 44% ee (Table 1, entry 6). The reaction did not occur at −78
°C (Table 1, entry 7). To improve the reactivity and
enantioselectivity, the steric hindrance and the effect of electron
density within the catalyst were studied at −45 °C (Table 1,
entries 8−17). The electron-rich and sterically hindered, ortho-
disubstituted phenyl sulfides were tested in the cyclization of 1a.
As expected, 2,6-dimethoxy-substituted phenyl sulfide C5 not
only enhanced reactivity but also raised the enantioselectivity to
66% (Table 1, entry 8). More electron-rich groups on the phenyl
ring did not help increase enantioselectivity and reactivity (Table
1, entry 9). In sharp contrast, replacement of the two methoxy
groups with larger methyl groups led to a dramatic drop in yield
and a decrease in enantioselectivity, which revealed that
substituents at the ortho position heavily affected the reactivity
(Table 1, entry 10).
From the above results, it was concluded that selenide catalysis
was promising for the cyclization. In analogy to the studies of
sulfide catalysis, steric hindrance and the effect of electron
density in the selenium catalyst were studied at −78 °C to
improve the enantioselectivity (Scheme 3). As expected, 2,6-
dimethoxy-substituted phenyl selenide C17 raised the enantio-
selectivity to 77%. In contrast, replacement of the two methoxy
groups with larger ethoxy groups (C18) led to a dramatic drop in
yield and a decrease in enantioselectivity. There was nearly no
reaction using difluoro-substituted catalyst C16 and dimethyl-
substituted catalyst C19. When less sterically hindered catalysts
were utilized, the product was formed in high yield, but with
reduced ee. To our surprise, catalyst C23 bearing an alkylseleno
group did not afford any product. Next, we focused on ortho-
monosubstituted catalysts. A methoxy group was better at the
ortho position than a methyl group. Other catalysts with one
methoxy group (C26-C28) were tested for the reactions.
Gratifyingly, catalyst C28 bearing both o-methyl and methoxy
groups afforded 2a with 81% ee. This discovery is similar to the
observation in sulfide catalysis.
We turned our attention to less sterically hindered catalysts.
When catalysts C8−C11 were utilized, the enantioselectivities
did not increase (Table 1, entries 11−14). Other catalysts with
one methoxy group (C12−C14) were tested for the reactions.
Catalyst C12 with a 3-tert-butyl substituent delivered a slightly
higher ee compared to catalyst C4, and the reactivity remained
(Table 1, entry 15). Interestingly, when another tert-butyl group
To further enhance the enantioselectivity, the reactions were
optimized with different acids since acids affect the activation of
electrophilic reagents and may change the chiral environment of
transition states in Lewis base-catalyzed transformations.8c−h,11
We were glad to discover that the reaction proceeded smoothly
B
DOI: 10.1021/acs.orglett.7b01392
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Letter
a
a
Scheme 3. Screening of Selenide Catalysts
Table 2. Substrate Scope
a
Conditions: As described in Table 1. The reaction was performed at
b
−78 °C. Refers to NMR yield using trifluoromethylbenzene as the
internal standard. Determined by HPLC analysis.
c
to afford product 2a with 86% ee when BF3·OEt2 was added to
the reaction instead of protic acid. After a screening of mixed
solvents and studies of different concentrations of 1a, the desired
product was formed in 89% ee in CH2Cl2/ClCH2CH2Cl at 0.025
M concentration in the presence of 1 equiv of BF3·OEt2 (see the
Supporting Information for details).
With these optimal conditions in hand, we began to evaluate
substrate scope (Table 2). When the ethyl group on the double
bond was replaced by other alkyl groups, the corresponding
products were still formed in high yields with excellent
enantioselectivities (1b−e, 87−93% ees). To establish the
robustness of the catalyst, a series of aryl-substituted olefinic
sulfonamides were tested under similar conditions. All of them
afforded the corresponding products in excellent yield with no
less than 90% ee except sterically hindered 2-methylphenyl
alkene 1g. When an electron-withdrawing group such as −Cl and
−Br was put on the phenyl ring of the substrates (1j and 1k), the
cyclization slowly proceeded to give the corresponding products
at −78 °C. Moreover, when the reactions were carried out at −60
°C, they worked very efficiently to generate the desired products
in high yield with excellent ee (2j, 94% ee; 2k, 95% ee).
Substrates with more electron-withdrawing groups, i.e., −CF3,
did not afford any products even at −60 °C. Pleasingly, alkene 1l
bearing a p-methoxy group on the phenyl ring still underwent
aminocyclization to afford product 2l in 85% yield with 93% ee.
In the literature, the selenium-catalyzed electrophilic cyclization
of the substrates bearinga p-methoxy group on the phenyl ring
was not efficient, and even racemization occurred in the
bromoaminocyclization.9a,11c The conditions were also suitable
for an alkene bearing a 2-thienyl group which afforded the
desired product 2n in 83% yield with 90% ee. Similarly, studies of
cyclizations of alkenes bearing a heterocycle are rare because
their high reactivities easily lead to side reactions. This method
proved to be efficient for the cyclization of (E)-alkenes. In
contrast, when the (Z)-alkene 1o was utilized, no desired product
was formed, which was attributed to inherent reactivity of the
alkene16 or steric hindrance in the alkene.
a
Conditions: 1 (0.1 mmol), (PhSO2)2NSCF3 (1.5 equiv), BF3·OEt2
(1.0 equiv), CH2Cl2/ClCH2CH2Cl = 1:1 (v/v), in 0.025 M
concentration, −78 °C, 12 h. Isolated yield. All the products were
obtained with >99:1 diastereoselectivities. Determined by HPLC
analysis. 1 mmol scale with 10 mol % of C28, 36 h. BF3·OEt2 (2.0
equiv), 24 h. NfOH (0.75 equiv) instead of BF3·OEt2, 24 h. Reaction
was carried out at −60 °C for 24 h. BF3·OEt2 (5.0 equiv), 24 h.
b
c
d
e
f
g
h
to evaluate what contribution would be made by the Thorpe−
Ingold effect, substrate 1q with two methyl groups on the alkyl
chain was synthesized. As expected, the cyclization of 1q
proceeded more efficiently to give the desired product 2q in 93%
yield with 86% ee. In order to test the selectivity of endo/exo
cyclization, alkyl-substituted olefinic sulfonamides 1r and 1s
were examined. Their cyclizations gave rise to two constitutional
isomers of the exo/endo products. For example, the reaction of 1r
produced the ca. 2:1 exo/endo products under the standard
conditions. Interestingly, when the amount of BF3·OEt2 was
increased to 5.0 equiv, the molar ratio of the exo/endo products
increased to 5:1, and the exo product 2r was obtained as the
major product in 77% yield with 97% ee. When a phenyl group
replaced the alkyl group on the double bond, the endo cyclization
took place as the main pathway to form the six-membered
piperidine 2t in 61% yield with 89% ee. It is noteworthy that
trisubstituted alkene 1u could still efficiently undergo CF3S-
aminocyclization to give the exo product 2u with good
enantioselectivity. The absolute configuration of 2 was
determined through the X-ray crystallographic studies of 2f
and 2r.
To extend this method to more challenging substrates,
terminal alkene 1p was examined. Product 2p was formed in
acceptable yield with good enantioselectivity (75% ee). In order
In summary, we have developed an indane-based, bifunctional
chiral selenide catalyst that is efficient for the enantioselective
C
DOI: 10.1021/acs.orglett.7b01392
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Sodeoka, M. J. Am. Chem. Soc. 2015, 137, 4865. (d) Chen, C.; Chen, P.;
CF3S aminocyclization of alkenes with construction of saturated
pyrrolidines and piperidines. The method provides a new
pathway for the synthesis of chiral azaheterocycles and is
complementary for chiral selenium catalysis.
Liu, G. J. Am. Chem. Soc. 2015, 137, 15648. (e) Yuan, W.; Szabo,
́
K. J.
Angew. Chem., Int. Ed. 2015, 54, 8533.
(7) (a) Kawai, H.; Kusuda, A.; Nakamura, S.; Shiro, M.; Shibata, N.
Angew. Chem., Int. Ed. 2009, 48, 6324. (b) Rauniyar, V.; Lackner, A. D.;
Hamilton, G. L.; Toste, F. D. Science 2011, 334, 1681. (c) Lozano, O.;
Blessley, G.; Campo, T. M.; Thompson, A. L.; Giuffredi, G. T.; Bettati,
M.; Walker, M.; Borman, B.; Gouverneur, V. Angew. Chem., Int. Ed.
2011, 50, 8105. (d) Xu, T.; Qiu, S.; Liu, G. Chin. J. Chem. 2011, 29, 2785.
(e) Kong, W.; Feige, P.; de Haro, T.; Nevado, C. Angew. Chem., Int. Ed.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b01392.
2013, 52, 2469. (f) Shunatona, H. P.; Fruh, N.; Wang, Y.-M.; Rauniyar,
̈
Experimental details, characterization data, NMR spectra
of new compounds, HPLC traces, and X-ray data (PDF)
X-ray crystallgraphic data for 2f (CIF)
V.; Toste, F. D. Angew. Chem., Int. Ed. 2013, 52, 7724. (g) Wolstenhulme,
J. R.; Rosenqvist, J.; Lozano, O.; Ilupeju, J.; Wurz, N.; Engle, K. M.;
Pidgeon, G. W.; Moore, P. R.; Sandford, G.; Gouverneur, V. Angew.
Chem., Int. Ed. 2013, 52, 9796. (h) Suzuki, S.; Kamo, T.; Fukushi, K.;
Hiramatsu, T.; Tokunaga, E.; Dohi, T.; Kita, Y.; Shibata, N. Chem. Sci.
2014, 5, 2754. (i) Hiramatsu, K.; Honjo, T.; Rauniyar, V.; Toste, F. D.
ACS Catal. 2016, 6, 151. (j) Lin, J.-S.; Dong, X.-Y.; Li, T.-T.; Jiang, N.-
C.; Tan, B.; Liu, X.-Y. J. Am. Chem. Soc. 2016, 138, 9357.
X-ray crystallgraphic data for 2r (CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zhaoxd3@mail.sysu.edu.cn.
ORCID
(8) (a) Denmark, S. E.; Collins, W. R. Org. Lett. 2007, 9, 3801.
(b) Denmark, S. E.; Kalyani, D.; Collins, W. R. J. Am. Chem. Soc. 2010,
132, 15752. (c) Denmark, S. E.; Kornfilt, D. J. P.; Vogler, T. J. Am. Chem.
Soc. 2011, 133, 15308. (d) Denmark, S. E.; Chi, H. M. J. Am. Chem. Soc.
2014, 136, 3655. (e) Denmark, S. E.; Chi, H. M. J. Am. Chem. Soc. 2014,
136, 8915. (f) Denmark, S. E.; Rossi, S.; Webster, M. P.; Wang, H. J. Am.
Chem. Soc. 2014, 136, 13016. (g) Lewis Base Catalysis in Organic
Synthesis; Vedejs, E., Denmark, S. E., Eds.; Wiley-VCH: Weinheim,
2016. (h) Denmark, S. E.; Kornfilt, D. J. P. J. Org. Chem. 2017, 82, 3192.
(i) Denmark, S. E.; Chi, H. M. J. Org. Chem. 2017, 82, 3826.
(9) (a) Chen, F.; Tan, C. K.; Yeung, Y.-Y. J. Am. Chem. Soc. 2013, 135,
1232. (b) Ke, Z.; Tan, C. K.; Chen, F.; Yeung, Y.-Y. J. Am. Chem. Soc.
2014, 136, 5627.
(10) (a) Balkrishna, S. J.; Prasad, C. D.; Panini, P.; Detty, M. R.;
Chopra, D.; Kumar, S. J. Org. Chem. 2012, 77, 9541. (b) Verma, A.; Jana,
S.; Prasad, C. D.; Yadav, A.; Kumar, S. Chem. Commun. 2016, 52, 4179.
(11) (a) Luo, J.; Zhu, Z.; Liu, Y.; Zhao, X. Org. Lett. 2015, 17, 3620.
(b) Wu, J.-J.; Xu, J.; Zhao, X. Chem. - Eur. J. 2016, 22, 15265. (c) Liu, X.;
An, R.; Zhang, X.; Luo, J.; Zhao, X. Angew. Chem., Int. Ed. 2016, 55, 5846.
(d) Luo, J.; Liu, X.; Zhao, X. Synlett 2017, 28, 397.
Xiaodan Zhao: 0000-0002-2135-5121
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Sun Yat-Sen University, the “One Thousand Youth
Talents” Program of China, and the Natural Science Foundation
of Guangdong Province (Grant No. 2014A030312018) for
financial support. We are grateful to our co-worker, Dr. Jinji Wu,
for the single-crystal structure analysis of 2f and 2r.
REFERENCES
■
(1) For selected references, see: (a) Wang, Y.-M.; Lackner, A. D.;
Toste, F. D. Acc. Chem. Res. 2014, 47, 889. (b) Mai, D. N.; Wolfe, J. P. J.
Am. Chem. Soc. 2010, 132, 12157. (c) Julian, L. D.; Hartwig, J. F. J. Am.
Chem. Soc. 2010, 132, 13813. (d) Bovino, M. T.; Chemler, S. R. Angew.
Chem., Int. Ed. 2012, 51, 3923. (e) Ingalls, E. L.; Sibbald, P. A.;
Kaminsky, W.; Michael, F. E. J. Am. Chem. Soc. 2013, 135, 8854. (f) Zhu,
H.; Chen, P.; Liu, G. Org. Lett. 2015, 17, 1485.
(12) Braga, A. L.; Ludtke, D. S.; Vargas, F.; Braga, R. C. Synlett 2006,
̈
1453.
(13) For selected examples, see: (a) Ferry, A.; Billard, T.; Langlois, B.
R.; Bacque, E. Angew. Chem., Int. Ed. 2009, 48, 8551. (b) Chen, C.; Chu,
́
L.; Qing, F.-L. J. Am. Chem. Soc. 2012, 134, 12454. (c) Yang, Y.-D.;
Azuma, A.; Tokunaga, E.; Yamasaki, M.; Shiro, M.; Shibata, N. J. Am.
Chem. Soc. 2013, 135, 8782. (d) Danoun, G.; Bayarmagnai, B.;
Gruenberg, M. F.; Goossen, L. J. Chem. Sci. 2014, 5, 1312. (e) Yin, G.;
Kalvet, I.; Englert, U.; Schoenebeck, F. J. Am. Chem. Soc. 2015, 137,
4164. (f) Mukherjee, S.; Maji, B.; Tlahuext-Aca, A.; Glorius, F. J. Am.
Chem. Soc. 2016, 138, 16200. (g) 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.
(2) For selected references, see: (a) O’Hagan, D. Nat. Prod. Rep. 2000,
17, 435. (b) He, R.; Kurome, T.; Giberson, K. M.; Johnson, K. M.;
Kozikowski, A. P. J. Med. Chem. 2005, 48, 7970. (c) Dragutan, I.;
Dragutan, V.; Demonceau, A. RSC Adv. 2012, 2, 719.
(3) For selected reviews, see: (a) Denmark, S. E.; Kuester, W. E.; Burk,
M. T. Angew. Chem., Int. Ed. 2012, 51, 10938. (b) Tan, C. K.; Yeung, Y.-
Y. Chem. Commun. 2013, 49, 7985. (c) Chemler, S. R.; Bovino, M. T.
ACS Catal. 2013, 3, 1076. (d) Cheng, Y. A.; Yu, W. Z.; Yeung, Y.-Y. Org.
Biomol. Chem. 2014, 12, 2333. For selected examples, see: (e) Zhou, L.;
Chen, J.; Tan, C. K.; Yeung, Y.-Y. J. Am. Chem. Soc. 2011, 133, 9164.
(f) Wang, Y.-M.; Wu, J.; Hoong, C.; Rauniyar, V.; Toste, F. D. J. Am.
Chem. Soc. 2012, 134, 12928. (g) Liu, G.-Q.; Li, Y.-M. J. Org. Chem.
2014, 79, 10094. (h) Denmark, S. E.; Hartmann, E.; Kornfilt, D. J. P.;
Wang, H. Nat. Chem. 2014, 6, 1056. (i) Mizar, P.; Niebuhr, R.;
Hutchings, M.; Farooq, U.; Wirth, T. Chem. - Eur. J. 2016, 22, 1614.
(4) For reviews about different fluorinated moieties, see: (a) Ma, J.-A.;
Cahard, D. Chem. Rev. 2008, 108, PR1. (b) Liang, T.; Neumann, C. N.;
Ritter, T. Angew. Chem., Int. Ed. 2013, 52, 8214. (c) Toulgoat, F.; Alazet,
S.; Billard, T. Eur. J. Org. Chem. 2014, 2014, 2415. (d) Xu, X.-H.;
Matsuzaki, K.; Shibata, N. Chem. Rev. 2015, 115, 731. (e) Yang, X.; Wu,
T.; Phipps, R. J.; Toste, F. D. Chem. Rev. 2015, 115, 826.
(14) (a) Bootwicha, T.; Liu, X.; Pluta, R.; Atodiresei, I.; Rueping, M.
Angew. Chem., Int. Ed. 2013, 52, 12856. (b) Wang, X.; Yang, T.; Cheng,
X.; Shen, Q. Angew. Chem., Int. Ed. 2013, 52, 12860. (c) Deng, Q.-H.;
Rettenmeier, C.; Wadepohl, H.; Gade, L.-H. Chem. - Eur. J. 2014, 20, 93.
(d) Zhu, X.-L.; Xu, J.-H.; Cheng, D.-J.; Zhao, L.-J.; Liu, X.-Y.; Tan, B.
Org. Lett. 2014, 16, 2192. (e) Zhao, B.-L.; Du, D.-M. Org. Lett. 2017, 19,
1036.
(15) All of the alkenes used in this paper are exclusively the E-
configuration unless otherwise noted.
(16) Ashtekar, K. D.; Marzijarani, N. S.; Jaganathan, A.; Holmes, D.;
Jackson, J. E.; Borhan, B. J. Am. Chem. Soc. 2014, 136, 13355.
(5) Serguchev, Y. A.; Ponomarenko, M. V.; Ignat’ev, N. V. J. Fluorine
Chem. 2016, 185, 1.
(6) For selected examples, see: (a) Wu, T.; Yin, G.; Liu, G. J. Am. Chem.
Soc. 2009, 131, 16354. (b) Zhang, Z.; Tang, X.; Thomoson, C. S.;
Dolbier, W. R. Org. Lett. 2015, 17, 3528. (c) Kawamura, S.; Egami, H.;
D
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