Silver-mediated radical aryltrifluoromethylthiolaton of activated alkenes

Article abstract of DOI:10.1021/ol403739w

The first example of silver-mediated oxidative aryltrifluoromethylthiolation of activated alkenes to produce valuable trifluoromethylthiole-containing oxindoles was developed. Mechanistic investigations indicated that this novel transformation proceeded through a unique F₃CS^? radical addition path, thus providing a practical and easy-handling method to generate a F₃CS ^? radical in the laboratory.

Full text of DOI:10.1021/ol403739w

Letter
pubs.acs.org/OrgLett
Silver-Mediated Radical Aryltrifluoromethylthiolaton of Activated
Alkenes
Feng Yin and Xi-Sheng Wang*
Department of Chemistry, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, China
S
* Supporting Information
ABSTRACT: The first example of silver-mediated oxidative
aryltrifluoromethylthiolation of activated alkenes to produce
valuable trifluoromethylthiole-containing oxindoles was devel-
oped. Mechanistic investigations indicated that this novel
transformation proceeded through a unique F3CS• radical
addition path, thus providing a practical and easy-handling
method to generate a F3CS• radical in the laboratory.
he incorporation of fluorine or fluorine-containing func-
tional groups can effectively improves the physicochemical
CF3SSCF3.13 Inspired by recent advance on silver-mediated or
-catalyzed radical additions,14 we envisioned that the F3CS•
radical could be triggered by the combination of AgSCF3/
oxidant, affording the C(sp3)-SCF3 bond after trapping by
alkenes. Herein, we report a novel silver-mediated aryltrifluor-
omethylthiolation of alkenes, to afford bioologically interesting
oxindoles.15 Mechanistic investigations indicate that this
transformation proceeds through a radical-type pathway.
T
properties of parent molecules. As a result, fluorinated
compounds are widely used in pharmaceuticals, agrochemicals,
and materials because of their unique lipophilicity and
bioactivities.1 Among all such intriguing fluorinated moieties,
the trifluoromethylthio group (CF3S) has attracted increasing
attention because of its special biological properties, such as
enhancement of membrane permeability and absorption rate
and improvement of the stability of parent molecules, due to its
lipophilicity and high electronegativity.2
As an easily prepared and stable trifluoromethylthiolation
reagent, AgSCF3 has been previously widely used as a CF3S−
source.16 Our study commenced by examining the aryltrifluor-
omethylthiolation of N-methyl-N-phenylmethacrylamide (1a)
in the presence of AgSCF3 (1.5 equiv) at 75 °C. A wide range of
oxidants, including PhI(OAc)2, TBHP, NFSI, Oxone,
(NH4)2S2O8, and K2S2O8, were first investigated to trigger the
F3CS• radical. To our delight, the desired product 2a could be
obtained with several oxidants, and K2S2O8 gave the best yield at
47% (Table 1, entries 1−7). A careful survey of solvents and
reagents ratio was then performed, which revealed the
combination of K2S2O8 (3.0 equiv), AgSCF3 (1.5 equiv), and
CH3CN (3.0 mL) was optimal, and the yield was increased to
59% (entry 11). To improve the yield further, a series of
inorganic and organic bases were next investigated. While the
inorganic bases and most organic bases have almost no effect on
this transformation (entries 12−14), HMPA (0.5 equiv)
increased the yield notably to 83% (entry 18). It is worth
pointing out that HMPA functioned not only as a base but also
as a possible ligand to improve the solubility and stability of
AgSCF3. Additionally, we found the yield maintained at 50% and
almost all of the starting material 1a consumed after 12 h with
no addition of HMPA, which indicated that the coordination
between HMPA and AgSCF3 might lower the redox potential of
the high valent silver species, thus preventing the oxidative
decomposition of starting materials and products. Finally, as a
control experiment, no desired product was obtained and an
The classical strategy of choice for the introduction of the
trifluoromethylthio group into organic molecules has been
limited to indirect methods, such as halogen-fluorine exchange
of polyhalogenomethyl thioethers or the trifluoromethylation of
sulfur-containing compounds.3 However, both approaches
suffered from harsh reaction conditions and the requirement
of prefunctionalization. Thus more general and straightforward
methods, in which the C-SCF3 bond was formed directly, are
highly desirable. MSCF3’s, which were previously reported as
SCF3− sources in SN2 reactions,4 have recently been developed
as coupling partners in several efficient transition metals,
including Pd-,5 Ni-,6 and Cu-mediated7 or -catalyzed trifluor-
omethylthiolations. Although CF3SCl has long been used to
react directly with some nucleophiles to transfer the CF3S
moiety,8 its utility was limited as a gaseous and highly toxic
reagent. In recent years, several electrophilic trifluoromethylth-
+
iolation reagents (SCF3 ), including trifluoromethanesulfanyla-
mides,9 trifluoromethylathiolated hypervalent iodine,10 trifluor-
omethanesulfonyl hypervalent iodonium ylide,11 and N-
trifluoromethylthiophthalimide,12 have been developed or
used for efficient formation of C-SCF3, especially the C(sp3)-
SCF3 bond.
Although a diverse array of organic molecules were
trifluoromethylthiolated by the nucleophilic and electrophilic
CF3S reagents, the F3CS• radical-type pathway remains less
explored. The only examples for F3CS• radical additions were
reported half a century ago, and the CF3S resources were limited
to toxic and gaseous (or volatile) CF3SH,8a CF3SCl,8b−d and
Received: December 25, 2013
Published: February 5, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
a
Table 1. Silver-Mediated Aryltrifluoromethylthiolation of Alkenes: Optimization of Reaction Conditions
b
b
entry
oxidant (equiv)
base (equiv)
yield (%)
entry
oxidant (equiv)
base (equiv)
none
yield (%)
c
1
2
Phl(OAc)2 (2.0)
TBHP (2.0)
none
none
none
none
none
none
none
none
none
none
trace
trace
trace
0
11
K2S2O8 (3.0)
K2S2O8 (3.0)
K2S2O8 (3.0)
K2S2O8 (3.0)
K2S2O8 (3.0)
K2S2O8 (3.0)
K2S2O8 (3.0)
K2S2O8 (3.0)
K2S2O8 (3.0)
none
59
49
42
45
63
50
72
83
73
0
c
12
NaHCO3(1.5)
PhCOONa (2.0)
DBU (1.5)
c
3
NFSI (2.0)
13
c
4
Oxone (2.0)
14
c
5
Ce(SO4)2·4H2O (2.0)
K2S2O8 (2.0)
33
15
HMPA (1.5)
HMPA (2.0)
HMPA (1.0)
HMPA (0.5)
HMPA (0.25)
HMPA (0.5)
c
6
47
16
c
7
(NH4)2S2O8 (2.0)
K2S2O8 (1.5)
38
17
c
8
44
18
c
9
K2S2O8 (3.0)
54
19
c
10
K2S2O8 (4.0)
trace
20
a
Reaction conditions: 1a (0.2 mmol, 1.0 equiv), AgSCF3 (1.5 equiv), oxidant (2.0 equiv), and base (1.5 equiv) in CH3CN (2 mL) at 75 °C for 12 h.
Isolated yield. CH3CN (3 mL).
b
c
almost quantitive amount of 1a was recovered when K2S2O8 was
removed from the reaction system (entry 20).
With the optimized conditions in hand, we then investigated
the substrate scope of silver-mediated aryltrifluoromethylth-
iolation of alkenes (Scheme 1). Unsurprisingly, a benzyl group
protecting substrate afforded the oxindole at only slightly
reduced yield (2b, R3), while an electon-withdrawing protecting
group like tosyl inhibited the reacion with only 30% yield (2c,
R3). We then investigated the substitute effect of the aryl ring
(R1). A variety of N-methyl-N-phenylmethacryl amides 1 with
substituents at para, meta, as well as ortho positions in the aniline
ring were aryltrifluoromethylthiolated smoothly to give the
corresponding oxindoles in moderate to good yields. meta-
Substituted N-phenylacrylamides could also be cyclized
successfully in good yields although with inevitable regioisomers
(2n−p, C2:C6 = 1.8:1). To our satisfaction, the less reactive N-
phenylacrylamides substrates with ortho substituents, presum-
ably due to steric effect, afforded the corresponding oxindoles
still with good yields, albeit requiring further increase of the
amount of K2S2O8 and AgSCF3. Both electron-donating groups,
such as OMe and Me, and electron-withdrawing groups, such as
CF3, F, Cl, Br, I, CO2Et, Ac, and NO2, were well-tolerated on the
aryl rings. The presence of halogen atoms (F, Cl, Br, I) in the
oxindoles offers the potential for further synthetic elaboration by
transition-metal-catalyzed cross-coupling reactions. The sub-
stitute effect at the α-position (R2) of the acrylamides was next
explored, and it was found that several α-sustitutents, including
benzyl, methoxyl, and ester, were compatible with this
transformation to furnish the desired products in good yields
(2v−x). Notably, heterocyclic substrates 1y and 1z could also be
cyclized smoothly to afford 2y and 2z with acceptable yields.
To understand the mechanism of this transformation, a series
of experiments were carried out. First, we performed the
reaction in the presence of 1.0 equiv of TEMPO as a radical
scavenger, and only a trace of desired product was obtained with
85% of 1a recovered. This observation was consistent with the
hypothesis that the reaction proceeds via a radical pathway.
Kinetic isotope experiments, including intermolecular and
intramolecular experiments, were next undertaken, and low
kinetic isotope effects (Scheme 2) were observed for both cases,
which indicated the C−H cleavage step was not rate-
determining. While we failed to capture the coupling product
of TEMPO with AgSCF3, the reaction of AgSCF3 with a normal
radical probe vinyl cyclopropane 3,17 in the presence of H-donor
1,4-CHD to facilitate quenching of the cyclic carbon radical
intermediates, afforded the ring-opened products 4 (5.8:1, dr)
and 5 (4.9:1, dr), which were detected by GC−MS and HRMS,
as an inseparable mixture in 58% combined yield (eq 3, Scheme
a
Scheme 1. Scope of Activated Alkenes
a
Reaction conditions: 1 (0.2 mmol, 1.0 equiv), AgSCF3 (1.5 equiv),
K2S2O8 (3.0 equiv), and HMPA (0.5 equiv) in CH3CN (3.0 mL) at 75
b
°C for 12 h. AgSCF3 (2.0 equiv), K2S2O8 (4.0 equiv), HMPA (1.0
c
d
equiv). AgSCF3 (2.0 equiv), K2S2O8 (4.0 equiv). AgSCF3 (2.5
equiv), K2S2O8 (5.0 equiv).
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Organic Letters
Letter
Scheme 2. Kinetic Isotope Effect (KIE) Studies
Scheme 4. Role of Silver Experiments
NMR),4h which ws consumed rapidly in 1 h, and no additional
gain of the product 2a was observed after that. When the
amount of added TBAI was decreased to 0.75 equiv (TBAI/
AgSCF3 = 1:2), an in situ generated F3CS anion species
3). Furthermore, additional direct evidence for the radical
mechanism was obtained by trapping the F3CS• radical with
Scheme 3. Radical Trapping Experiments
(possibly {I[Ag(SCF3)]2}−, resonated at δ = −19.0 ppm in 19
F
NMR) was formed, which released AgSCF3 following the
capture of I− anion by the generated Ag(I) as the reaction went
on, and then 2a increased until the AgSCF3 ran out (see Scheme
4, Supporting Information). Since CF3SSCF313a was found in all
above cases, we further set out to investigate the role of this
species. With the addition of TBAI (1.5 equiv, TBAI/AgSCF3 =
1:1) under the standard conditions, 19F NMR monitoring
showed AgSCF3 was consumed in 1.5 h to give a CF3SSCF3-
containing mixture. It was found the aryltrifluoromethythiolated
product 2a was obtained at 30% yield with the subjection of 1a/
AgNO3 (30 mol %), while no 2a was observed with the
subjection of 1a only to this reaction mixture, which indicated
CF3SSCF3 could regeneratethe F3CS• radical with the help of
silver in the reaction system (eq 9, Scheme 4).
On the basis of these observations and previous reports,14,15
a
plausible mechanism is proposed as in Scheme 5. Initially, the
Scheme 5. Proposed Mechanism
radical clocks 6, 8, and 10, where the desired cyclization
products were isolated in 28%, 15%, and 17%, respectivly (eqs
4−6, Scheme 3).
To gain insight into the role of silver in this radical reaction,
several more experiments were carried out. First, replacement of
AgSCF3 with CuSCF3 in the standard reaction conditions gave
no 2a, which indicated silver was crucial to this transformation
(eq 7, Scheme 4). According to Clark’s4g and Buchwald’s5
reports, n-Bu4NI (TBAI) could activate AgSCF3 in acetonitrile
−
to afford a reactive source of SCF3 via the formation of
[Ag(SCF3)I]−. However, interestingly, the addition of 1.5 equiv
of TBAI (TBAI/AgSCF3 = 1:1) almost quenched the reaction
with only 12% yield, while markedly increased yield was
obtained with slightly reduced TBAI (64% yield, TBAI/AgSCF3
= 0.8:1), which confirmed the key role of the free silver in our
reaction system (eq 8, Scheme 4).
oxidation of AgSCF3 by K2S2O8 affords Ag(II)SCF3 species,
which triggers the F3CS• radical after the following single
electron transfer course. The addition of F3CS• radical to 1a
generates the corresponding alkyl radical intermediate A,
followed by cyclization to give the aryl radical B. Single electron
transfer from B to an additional 1 equiv of Ag(II), followed by β-
H elimination, yields the desired product 2a.
In summary, we have developed the first example of silver-
mediated aryltrifluoromethylthiolation of activated alkenes to
produce valuable trifluoromethylthiole-containing oxindoles,
which had easy handling, was practical and straightforward to
construct the C(sp3)-SCF3 bond, and was of broad functional
group compatibility. Mechanistic investigations indicated that
To obtain more details in the reaction process, 19F NMR was
next used to track these reactions. It was found that the
aryltrifluoromethylation product 2a increased gradually with the
decrease of AgSCF3 (resonated at δ = −22.0 ppm in 19F
NMR),4h while reaction time was prolonged under the standard
conditions. With the addition of 1.5 equiv of TBAI (TBAI/
AgSCF3 = 1:1), the AgSCF3 was promptly converted to
[IAg(SCF3)]− species (resonated at δ = −16.0 ppm in 19F
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Organic Letters
Letter
(8) (a) Harris, J. F.; Stacey, F. W. J. Am. Chem. Soc. 1961, 83, 840.
(b) Harris, J. F. J. Am. Chem. Soc. 1962, 84, 3148. (c) Harris, J. F. J. Org.
Chem. 1966, 31, 931. (d) Andreades, S.; Harris, J. F.; Sheppard, W. A. J.
Org. Chem. 1964, 29, 898. (e) Ippen, J.; Baasner, B.; Marhold, A.;
Kysela, E.; Dutzmann, S.; Reinecke, P. (Bayer A. G., Germany)
DE3836161A1; Chem. Abstr. 1990, 113, 167366. (f) Sheppard, W. A. J.
Org. Chem. 1964, 29, 895. (g) Sivasankaran, K.; Rao, K. S.; Ratnam, K.
R.; Mithyantha, M. S.; Bhat, K. S. (Rallis India Limited, India)
IN178903A1; Chem. Abstr. 1997, 140, 375162. (h) Cherkofsky, S. C.
(E. I. Du Pont de Nemours & Co., United States) US4302461A; Chem.
Abstr. 1981, 96, 104081. (i) Bogdanowicz-Szwed, K.; Kawałek, B.; Lieb,
M. J. Fluorine Chem. 1987, 35, 317. (j) Munavalli, S.; Rohrbaugh, D. K.;
Rossman, D. I.; Waner, W. G.; Durst, H. D. Phosphorus Sulfur Silicon
Relat. Elem. 2002, 177, 1021. (k) Haas, A.; Niemann, U. Chem. Ber.
1977, 110, 67. See also ref 3a.
this novel transformation proceeded through an unreported
F3CS• radical addition path.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedure and characterization of all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: xswang77@ustc.edu.cn.
Notes
(9) (a) Ferry, A.; Billard, T.; Langlois, B. R.; Bacque, E. J. Org. Chem.
2008, 73, 9362. (b) Ferry, A.; Billard, T.; Langlois, B. R.; Bacque,
Angew. Chem., Int. Ed. 2009, 48, 8551. (c) Yang, Y.; Jiang, X. L.; Qing, F.
L. J. Org. Chem. 2012, 77, 7538. (d) Ferry, A.; Billard, T.; Bacque, E.;
Langlois, B. R. J. Fluorine Chem. 2012, 134, 160. (e) Baert, F.; Colomb,
J.; Billard, T. Angew. Chem., Int. Ed. 2012, 51, 10382. (f) Liu, J.; Chu, L.;
Qing, F. L. Org. Lett. 2013, 15, 894. (g) Alazet, S.; Zimmer, L.; Billard,
T. Angew. Chem., Int. Ed. 2013, 52, 10814. See also ref 3b.
(10) (a) Shao, X.; Wang, X.; Yang, T.; Lu, L.; Shen, Q. Angew. Chem.,
Int. Ed. 2013, 52, 3457. (b) Wang, X.; Yang, T.; Cheng, X.; Shen, Q.
Angew. Chem., Int. Ed. 2013, 52, 12860.
́
E.
The authors declare no competing financial interest.
́
ACKNOWLEDGMENTS
■
We gratefully acknowledge the National Science Foundation of
China (21102138, J1030412), the Chinese Academy of
Sciences, the Ministry of Education (SRFDP
20123402110040), University of Science and Technology of
China (USTC) for financial support.
(11) Yang, Y. D.; Azuma, A.; Tokunaga, E.; Yamasaki, M.; Shiro, M.;
Shibata, N. J. Am. Chem. Soc. 2013, 135, 8782.
(12) (a) Bootwicha, T.; Liu, X.; Pluta, R.; Atodiresei, I.; Rueping, M.
Angew. Chem., Int. Ed. 2013, 52, 12856. (b) Munavalli, S.; Rohrbaugh,
D. K.; Rossman, D. I.; Berg, F. J.; Wagner, G. W.; Durst, H. D. Synth.
Commun. 2000, 30, 2847.
REFERENCES
■
(1) (a) Kirsch, P. Modern Fluoroorganic Chemistry; Wiley-VCH:
Weinheim, 2004. (b) O’Hagan, D.; Rzepa, H. S. Chem. Commun. 1997,
645. (c) Muller, K.; Faeh, C.; Diederich, F. Science 2007, 317, 1881.
̈
(d) Hagmann, W. K. J. Med. Chem. 2008, 51, 4359. (e) Purser, S.;
Moore, P. R.; Swallow, S.; Gouverneur, V. Chem. Soc. Rev. 2008, 37,
320.
(13) (a) Haran, G.; Sharp, D. W. A. J. Chem. Soc., Perkin Trans. 1 1972,
34. (b) Dear, R. E. A.; Gilbert, E. E. J. Fluorine Chem. 1974, 4, 107.
(14) For early examples of silver-mediated radical reactions, see:
(a) Minisci, F.; Citterio, A.; Giordano, C. Acc. Chem. Res. 1983, 16, 27.
(b) Kumar, A.; Neta, P. J. Am. Chem. Soc. 1980, 102, 7284.
(15) For selected examples of oxindoles’ synthesis via radical addition,
see: (a) Wei, W. T.; Zhou, M. B.; Fan, J. H.; Liu, W.; Song, R. J.; Liu, Y.;
Hu, M.; Xie, P.; Li, J. H. Angew. Chem., Int. Ed. 2013, 52, 3638. (b) Li, Y.
M.; Sun, M.; Wang, H. L.; Tian, Q. P.; Yang, S. D. Angew. Chem., Int. Ed.
2013, 52, 3972. (c) Xie, J.; Xu, P.; Li, H.; Xue, Q.; Jin, H.; Cheng, Y.;
Zhu, C. Chem. Commun. 2013, 49, 5672. (d) Wei, X. H.; Li, Y. M.;
Zhou, A. X.; Yang, T. T.; Yang, S. D. Org. Lett. 2013, 15, 4158.
(e) Zhou, M. B.; Song, R. J.; Ouyang, X. H.; Liu, Y.; Wei, W. T.; Deng,
G. B.; Li, J. H. Chem. Sci. 2013, 4, 2690. (f) Li, X.; Xu, X.; Hu, P.; Xiao,
X.; Zhou, C. J. Org. Chem. 2013, 78, 7343. (g) Matcha, K.; Narayan, R.;
Antonchick, A. P. Angew. Chem., Int. Ed. 2013, 52, 7985. (h) Wang, H.;
Guo, L. N.; Duan, X. H. Org. Lett. 2013, 15, 5254. (i) Yuan, Y. Z.; Shen,
T.; Wang, K.; Jiao, N. Chem. Asian J. 2013, 8, 10. (j) Zhou, Z. Z.; Hua,
H. L.; Luo, J. Y.; Chen, Z. S.; Zhou, P. X.; Liu, X. Y.; Liang, Y. M.
Tetrahedron 2013, 69, 10030. (k) Fu, W. J.; Xu, F. J.; Fu, Y. Q.; Zhu, M.;
Yu, J. Q.; Xu, C.; Zou, D. P. J. Org. Chem. 2013, 78, 12202. (l) Kong,
W.; Casimiro, M.; Merino, E.; Nevado, C. J. Am. Chem. Soc. 2013, 135,
14480. (m) Kong, W.; Casimiro, M.; Fuentes, N.; Merino, E.; Nevado,
C. Angew. Chem., Int. Ed. 2013, 52, 13086. (n) Li, Y. M.; Wei, X. H.; Li,
X. A.; Yang, S. D. Chem. Commun. 2013, 49, 11701.
(2) (a) Leo, A.; Hansch, C.; Elkins, D. Chem. Rev. 1971, 71, 525.
(b) Yagupol’skii, L. M.; Il’chenko, A. Y.; Kondratenko, N. V. Russ.
Chem. Rev. 1974, 43, 32. (c) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev.
1991, 91, 165. (d) Leroux, F.; Jeschke, P.; Schlosser, M. Chem. Rev.
2005, 105, 827. (e) Manteau, B.; Pazenok, S.; Vors, J. P.; Leroux, F. R. J.
Fluorine Chem. 2010, 131, 140.
(3) For selected reviews of trifluoromethylthiolation, see: (a) Boiko,
V. N. Beilstein J. Org. Chem. 2010, 6, 880. (b) Tlili, A.; Billard, T. Angew.
Chem., Int. Ed. 2013, 52, 6818. (c) Liu, H.; Jiang, X. Chem. Asian J.
2013, 8, 2546.
(4) (a) Yagupolskii, L. M.; Kondratenko, N. V.; Sambur, V. P.
Synthesis 1975, 721. (b) Remy, D. C.; Rittle, K. E.; Hunt, C. A.;
Freedman, M. B. J. Org. Chem. 1976, 41, 1644. (c) Kondratenko, N. V.;
Kolomeytsev, A. A.; Popov, V. I.; Yagupolskii, L. M. Synthesis 1985, 667.
(d) Clark, J. H.; Jones, C. W.; Kybett, A. P.; McClinton, M. A.; Miller, J.
M.; Bishop, D.; Blade, R. J. J. Fluorine Chem. 1990, 48, 249. (e) Clark, J.
H.; Tavener, S. J. J. Fluorine Chem. 1997, 85, 169. (f) Tavener, S. J.;
Adams, D. J.; Clark, J. H. J. Fluorine Chem. 1999, 95, 171. (g) Adams, D.
J.; Clark, J. H. J. Org. Chem. 2000, 65, 1456. (h) Adams, D. J.; Goddard,
A.; Clark, J. H.; Macquarrie, D. J. Chem. Commun. 2000, 987. (i) Tyrra,
W.; Naumann, D.; Hoge, B.; Yagupolskii, Y. L. J. Fluorine Chem. 2003,
119, 101.
(5) Teverovskiy, G.; Surry, D. S.; Buchwald, S. L. Angew. Chem., Int.
Ed. 2011, 50, 7312.
(16) (a) Orda, V. V.; Yagupolskii, L. M.; Bystrov, V. F.; Stepanyati, A.
U. Chem. Abstr. 1965, 63, 17861. (b) Petrova, T. D.; Platnov, V. E.;
Shcheyoleva, L. N.; Maksimov, A. M.; Haas, A.; Schelvis, M.; Lieb, M. J.
Fluorine Chem. 1996, 79, 13. (c) Xiao, Q.; Sheng, J.; Chen, Z. Y.; Wu, J.
Chem. Commun. 2013, 49, 8647. (d) Wang, K. P.; Yun, S. Y.;
Mamidipalli, P.; Lee, D. Chem. Sci. 2013, 4, 3205. See also refs 4h and
5.
(6) Zhang, C. P.; Vicic, D. A. J. Am. Chem. Soc. 2012, 134, 183.
(7) (a) Zhang, C. P.; Vicic, D. A. Chem. Asian J. 2012, 7, 1756.
(b) Weng, Z.; He, W.; Chen, C.; Lee, R.; Tan, D.; Lai, Z.; Kong, D.;
Yuan, Y.; Huang, K. W. Angew. Chem., Int. Ed. 2013, 52, 1548. (c) Kong,
D.; Jiang, Z.; Xin, S.; Bai, Z.; Yuan, Y.; Weng, Z. Tetrahedron 2013, 69,
6046. (d) Tan, J.; Zhang, G.; Ou, Y.; Yuan, Y.; Weng, Z. Chin. J. Chem.
2013, 31, 921. (e) Rueping, M.; Tolstoluzhsky, N.; Nikolaienko, P.
Chem.Eur. J. 2013, 19, 14043. For other recently reported examples
with Cu or metal free, see: (f) Chen, C.; Xie, Y.; Chu, L.; Wang, R. W.;
Zhang, X.; Qing, F. L. Angew. Chem., Int. Ed. 2012, 51, 2492. (g) Chen,
C.; Chu, L.; Qing, F. L. J. Am. Chem. Soc. 2012, 134, 12454. (h) Tran, L.
D.; Popov, I.; Daugulis, O. J. Am. Chem. Soc. 2012, 134, 18237.
(17) For a review on radical probes, see: Renaud, P., Sibi, M. P., Eds.;
Radicals in Organic Synthesis; Wiley-VCH: Weinheim, 2001.
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