AgSCF3-Mediated Trifluoromethylthiolation/Radical Cascade Cyclization of 1,6-Enynes

Article abstract of DOI:10.1021/acs.orglett.5b01657

A AgSCF₃-mediated radical cascade cyclization/trifluoromethylthiolation of 1,6-enynes triggered by a C-C triple bond is developed. This protocol also provides another opportunity to construct a valuable trifluoromethylthio-substituted polycyclic fluorene system through the formations of one C-SCF₃ bond and two C-C bonds in a single step.

Full text of DOI:10.1021/acs.orglett.5b01657

Letter
pubs.acs.org/OrgLett
AgSCF3‑Mediated Trifluoromethylthiolation/Radical Cascade
Cyclization of 1,6-Enynes
Yi-Feng Qiu,† Xin-Yu Zhu,† Ying-Xiu Li,† Yu-Tao He,† Fang Yang,‡ Jia Wang,† Hui-Liang Hua,†
Lan Zheng,† Li-Chen Wang,† Xue-Yuan Liu,† and Yong-Min Liang*,†
†State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, People’s Republic of China
‡College of Science, Northwest A&F University, 3 Taicheng Road, Yangling 712100, People’s Republic of China
S
* Supporting Information
ABSTRACT: A AgSCF3-mediated radical cascade cyclization/
trifluoromethylthiolation of 1,6-enynes triggered by a C−C
triple bond is developed. This protocol also provides another
opportunity to construct a valuable trifluoromethylthio-
substituted polycyclic fluorene system through the formations
of one C−SCF3 bond and two C−C bonds in a single step.
Scheme 1. AgSCF3 Participating in Oxidative/Aromatic
Cyclization Reaction and Our New Anticipation towards
Trifluoromethylthiolation/Cascade Cyclization of 1,6-Enynes
rganofluorine compounds have been identified to show
Owide application and evolution in the pharmaceutical
chemistry, pesticide industry, and material science.1 Among such
numerous fascinating fluorine-containing groups, the trifluor-
omethylthio group (SCF3) has occupied a significant field due to
its desirable bioactivities, such as high electronegativity, lip-
ophilicity, and metabolic stability, which led to a great promotion
of membrane permeability and absorption rate in bioavailability.2
Therefore, advancing aspiration and interest in new routes to
introduce the trifluoromethylthio group stimulates further
studies in this area.3 With the persistent efforts of many chemists,
adequate evolutions have been achieved on the development of a
series of trifluoromethylthiolation reagents.4 Very recently,
various electrophilic trifluoromethylthiolation reagents
5
+
(SCF3 ) have been exploited for an efficient trifluoromethylth-
iolation cyclization reaction.6 Nevertheless, the incompatibility
with a strong acidic system and the sensitivity for electronic effect
force reconsideration for some substrates with sensitive func-
tional groups.6 In subsequent research, AgSCF3, astraightforward
and easy-operation reagent,7 has emerged as a valuable tool in
trifluoromethylthiolation/oxidative aromatic cyclization reac-
tions.8 In 2014, the Wang group reported the first AgSCF3-
mediated aryltrifluoromethylthiolation cyclization of activated
alkenes in a radical process (Scheme 1a).8b Soon afterward, Yang
demonstrated a series of trifluoromethylthio-substituted chro-
effect, having extensive functional group compatibility, and
providing economies of time, labor, and cost.9b,10 Meanwhile,
1,6-enynes, as interesting and important starting materials with
multiple reaction sites, have become star molecules in synthetic
methodology.11 By taking into consideration our current interest
in the synthesis of fluorine-containing compounds,12 as well as
the continued anticipation of new approaches to polycyclic
skeletons,13 we designed a silver-mediated trifluoromethylth-
iolation/radical cascade cyclization of 1,6-enynes. This is the first
paper on the trifluoromethylthiolation−cyclization reaction
mones in a “SCF3 ” course (Scheme 1b).8d Very recently, a novel
+
trifluoromethylthiolation oxidative aromatic cyclization involving
a 1,4-aryl migration and desulfonylation was presented by
Nevado (Scheme 1c).8f Still, the development of direct oxidative
aromatic radical cascade cyclization with trifluoromethylthiola-
tion on a C−C triple bond is still fueled by the strong and growing
aspiration in organofluorine chemistry.
It is known that the status of cascade cyclization as a powerful
and ingenious strategy for the synthesis of polycyclic compounds
is irreplaceable.9 Herein, radical cyclization shows undeniable
benefits in most cases, including being insensitive to electronic
Received: June 7, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01657
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Letter
triggered by a C−C triple bond in a radical pathway, with a
polycyclic fluorene system constructed in a single step (Scheme
1d).
Our initial attempt was started by employing compound 1aa
(0.2 mmol) as the model substrate with AgSCF3 (1.5 equiv),
K2S2O8 (3.0 equiv), and HMPA (0.5 equiv) in MeCN at 80 °C
under an argon atmosphere for 12 h. To our delight, our
anticipated product 2aa was isolated in 42% yield (Table 1, entry
addition of ligand 2,2′:6′,2″-terpyridine L1 (10 mol %) proved to
be a good option, by which 87% of product 2aa was isolated
(entries 13−17). The role of ligand L1 was further investigated in
MeCN and DMF, respectively (entries 18−19). The coordina-
tion between ligand/HMPA and AgSCF3 may reduce the redox
potential of the high valent silver species, which hindered the
further oxidative decomposition of substrates and products.8b
Additional control experiments indicated that an argon
atmosphere and base were necessary for a high yield (entries
20−21).
a
Table 1. Optimization of the Reaction Conditions
A series of substituted 1,6-enynes were prepared to investigate
the scope of this trifluoromethylthiolation−cyclization reaction.
The corresponding products 2aa−2at were obtained in moderate
to excellent yields under the optimal reaction conditions
(Scheme 2). The structure of 2at was also confirmed by X-ray
a
Scheme 2. Synthesis of Products 2a from 1,6-Enynes 1a
ligand
yield
b
entry
solvent
MeCN
oxidant (equiv)
(mol %)
(%)
1
2
3
4
5
6
7
8
9
10
K2S2O8 (3.0)
K2S2O8 (3.0)
K2S2O8 (3.0)
K2S2O8 (3.0)
42
0
1,2-DCE
DMSO
DMF
36
c
44
MeCN/DMF (1:1) K2S2O8 (3.0)
MeCN/DMF (1:2) K2S2O8 (3.0)
MeCN/DMF (1:1) Na2S2O8 (3.0)
MeCN/DMF (1:1) PhI(OAc)2 (3.0)
MeCN/DMF (1:1) m-CPBA (3.0)
MeCN/DMF (1:1) K2S2O8 (2.5)
MeCN/DMF (1:1) K2S2O8 (3.0)
MeCN/DMF (1:1) K2S2O8 (3.0)
MeCN/DMF (1:1) K2S2O8 (3.0)
MeCN/DMF (1:1) K2S2O8 (3.0)
MeCN/DMF (1:1) K2S2O8 (3.0)
MeCN/DMF (1:1) K2S2O8 (3.0)
MeCN/DMF (1:1) K2S2O8 (3.0)
66
d
54
e
34
trace
f
0
61
54
52
84
65
62
66
87
51
g
11
h
12
13
14
15
16
17
18
19
L1 (20)
L2 (20)
L3 (20)
L4 (20)
L1 (10)
L1 (10)
L1 (10)
L1 (10)
L1 (10)
MeCN
DMF
K2S2O8 (3.0)
K2S2O8 (3.0)
i
59
j
20
MeCN/DMF (1:1) K2S2O8 (3.0)
MeCN/DMF (1:1) K2S2O8 (3.0)
58
78
k
21
a
Unless otherwise noted, all reactions were performed with 1aa (0.2
mmol), AgSCF3 (1.5 equiv), oxidant (3.0 equiv), base (0.5 equiv), and
ligand (10 mol %) in anhydrous solvent (2 mL) under an argon
a
Unless otherwise noted, all reactions were performed with 1a (0.2
b
c
mmol), AgSCF3 (1.5 equiv), K2S2O8 (3.0 equiv), HMPA (0.5 equiv),
and ligand L1 (10 mol %) in anhydrous MeCN/DMF (2 mL, 1:1)
under an argon atmosphere for 12 h. Yields are given for isolated
atmosphere for 12 h. Yields are given for isolated products. 18% of
d
e
1aa was recovered. 6% of 1aa was recovered. 11% of 1aa was
f
g
h
recovered. Decomposed. HMPA (1.0 equiv) was uesd. NaOAc (0.5
equiv) was used instead of HMPA. 21% of 1aa was recovered. This
reaction was performed under an air atmosphere. This reaction was
b
1
i
j
products. The configuration was determined by H NMR spectrum.
c
1
k
The configuration and the ratio were determined by H NMR and
NOE spectra.
performed in the absence of HMPA.
1). A subsequent brief survey of various representative solvents
showed that DMF gave a similar yield of product 2aa with 18% of
substrate 1aa recovered (entries 2−4). Further study on the effect
of solvents revealed that the yield could be increased to 66% in a
mixed solvent of MeCN/DMF (1:1) without substrate recovered
(entry 5). Simply improving the ratio of DMF in the mixed
solvent gave a lower yield with substrate 1aa recovered (entry 6).
We considered that DMF also functioned as a possible ligand to
improve the solubility of AgSCF3 and K2S2O8. And excess ligand
might go against the reaction process as known fact. No better
results were obtained after the replacement of oxidant (entries 7−
9). And the adjustments on K2S2O8 or HMPA failed to give
superior yields (entries 10−12). During subsequent attempts, the
crystal structure analysis (see the Supporting Information (SI)).
The electronic effect of substituent groups shows insensitivity in
this transformation; in general, both electron-rich (1aa−1ac) and
-deficient (1ad−1ai) groups on the para-position of the
substrates could be tolerated. It was noteworthy that substrates
with strong electron-withdrawing groups (NO2 or CN) worked
smoothly and gave the corresponding product in 49% and 70%
yield, respectively (2ag and 2ah). Furthermore, gratifyingly,
substrates bearing electron-deficient groups on the meta-position
achieved a total regioselective reaction and gave the correspond-
ing products in good yields (2al and 2am), whereas a mixture of
the products 2an′ and 2an″ was obtained in the ratio 5:2 when m-
OMe substituted 1,6-enyne was investigated (1an). In addition,
B
DOI: 10.1021/acs.orglett.5b01657
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Letter
these halogenated products may provide other potential
applications for further transformations through orthogonal
cross-couplings. The steric effect for ortho-position substituent
groups was then explored. When substrate 1ao (o-chlorophenly)
was utilized under the optimal conditions, 70% yield of product
2ao was isolated. Compared with 2ao, product 2ap could not be
isolated from a mixture product. Several multisubstituted
substrates were also applied, which all showed excellent tolerance
(2aq−2at). Similarly, a mixture of product 2aq′ with 2aq″ and a
single configuration of product 2ar was isolated. Notably, the
transformation proceeded smoothly for the substrates with a
multiple-ring group (1-naphthyl group, 1au) or a heterocyclic
group (2-thienyl, 1av).
while, the structure of 2cb was confirmed by X-ray crystal
structure analysis (see the SI).
To gain further understanding of the reaction mechanism,
some necessary inhibition experiments were performed (Scheme
4). When 1.5 equiv of TEMPO (2,2,6,6-tetramethylpiperidine-1-
Scheme 4. Verification Experiments for the Mechanism
Encouraged by the above results, the reactions of N-tethered
1,6-enynes were further examined under the optimal conditions.
Generally, as described in Scheme 3, the reactions showed steric
Scheme 3. Synthesis of Products 2b (2c) from 1,6-Enynes 1b
a
(1c)
a
b
97% of 1aa was recovered. 93% of 1aa was recovered.
oxyl) or BHT (2,6-di-tert-butyl-4-methylphenol) were added
into the reaction system, the desired transformation was found to
be completely inhibited with over 90% of 1aa recovered,
respectively. This observation was consistent with the hypothesis
that the reaction proceeds via a radical pathway, which indicated
an SET (single electron transfer) process triggered by a free
radical. Then, a kinetic isotope experiment was carried out with
low kinetic isotope effects detected, which pointed out the C−H
cleavage step was not a rate-limiting step. And the replacement of
a SCF3 source (CuSCF3) led to no product, which indicated that
silver had a vital impact on this transformation.
A plausible mechanism that is consistent with the experimental
results mentioned above and the precedent literature8b,e,f is
proposed in Scheme 5. In fact, AgSCF3 is initially oxidized by
K2S2O8 and generates the SCF3· radical. The intermolecular
regioselective addition of the SCF3· radical onto the triple bond of
substrate 1 affords alkenyl radical intermediate A,13b,c which
undergoes a subsequent 6-exo-trig process to give alkyl radical
a
Unless otherwise noted, all reactions were performed with 1b (0.2
mmol), AgSCF3 (1.5 equiv), K2S2O8 (3.0 equiv), HMPA (0.5 equiv),
and ligand L1 (10 mol %) in anhydrous MeCN/DMF (2 mL, 1:1)
under an argon atmosphere for 12 h. Yields are given for isolated
products.
Scheme 5. Proposed Reaction Mechanism
and electronic effects similar to those of carbon-tethered
substrates. The structure of 2bd was also confirmed by X-ray
crystal structure analysis (see the SI). Substrate 1bg (m-
fluorophenyl) still gave the single configuration. Substrates 1bk
and 1bl bearing a terminal olefinic bond failed to give the
corresponding product due to an unstable radical intermediate B
(see Scheme 5). Several O-tethered 1,6-enyne substrates were
studied next under the optimal conditions (Scheme 3).
Gratifyingly, the corresponding trifluoromethylthio-substituted
2-pyrone derivatives were obtained. 2-Pyrones have been
identified to be significant intermediates and vitally important
flavonoid skeletal structures, which are found in a wide variety of
natural products and pharmaceutically active molecules. Mean-
C
DOI: 10.1021/acs.orglett.5b01657
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ASSOCIATED CONTENT
* Supporting Information
Detailed experimental procedures, spectral data, and CIF for all
new compounds are provided. The Supporting Information is
available free of charge on the ACS Publications website at DOI:
10.1021/acs.orglett.5b01657.
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: liangym@lzu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was received from the National Science
Foundation (NSF21272101, NSF21472073, NSF21472074,
and NSF21302076), the Program for Changjiang Scholars and
Innovative Research Team in University (IRT1138). And also,
we are really grateful to Xin-Hua Hao for providing four
substrates, Xian-Rong Song, and Bo Song for valuable
suggestions to this manuscript.
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