Efficient generation of perfluoroalkyl radicals from sodium perfluoroalkanesulfinates and a hypervalent iodine(III) reagent: Mild, metal-free synthesis of perfluoroalkylated organic molecules

Article abstract of DOI:10.1039/c6ob01245k

This article describes an efficient method for the introduction of perfluoroalkyl groups into N-acrylamides, 2-isocyanides, olefins, and other heterocycles using perfluoroalkyl radicals that were generated from the reaction between sodium perfluoroalkanesulfinates and a hypervalent iodine(iii) reagent. This approach represents a simple, scalable perfluoroalkylation method under mild and metal-free conditions.

Full text of DOI:10.1039/c6ob01245k

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DOI: 10.1039/C6OB01245K
Journal Name
COMMUNICATION
Efficient Generation of Perfluoroalkyl Radicals from Sodium
Perfluoroalkanesulfinates and a Hypervalent Iodine(III) Reagent:
Mild, Metal-free Synthesis of Perfluoroalkylated Organic
Molecules
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Ryu Sakamoto, Hirotaka Kashiwagi, Sermadurai Selvakumar, Shin A. Moteki, and Keiji Maruoka*
www.rsc.org/
This article describes an efficient method for the introduction of
perfluoroalkyl groups into N-acrylamides, 2-isocyanides, olefins,
and other heterocycles using perfluoroalkyl radicals that were
generated
from
the
reaction
between
sodium
perfluoroalkanesulfinates and a hypervalent iodine(III) reagent.
This approach represents a simple, scalable perfluoroalkylation
method under mild and metal-free conditions.
Figure 1. Commonly used, commercially available perfluoroalkylating reagents
and their shortcomings
.
Over the last decades, the development of cost-effective,
efficient, and environmentally benign synthetic methods for the
preparation of perfluoroalkylated organic molecules has
attracted much attention in pharmaceutical and agrochemical
research, as well as in materials science. This is predominantly
due to the fact that the introduction of fluoro substituents into
organic molecules has a significant positive impact on their
physical properties, including metabolic stability, solubility, and
1,2
lipophilicity. To date, various methods for the introduction of
trifluoromethyl groups into organic molecules have been
3
developed, and several trifluoromethylating reagents, such as
the Ruppert–Prakash reagent, Togni’s reagent, and Umemoto’s
reagent, are now commercially available.4-6 However, the
Scheme 1. Perfluoroalkylations using perfluoroalkyl analogs of the Langlois
reagent.
efficient introduction of longer perfluoroalkyl groups (C
2) still remains a substantial challenge in current organic
synthesis.
n
F2n+1, n

requires transition metal catalysts, high temperatures, and/or
photo-irradiation, resulting in limitations with respect to the
substrate scope and the practical utility in organic synthesis in
general (Figure 1). Thus, the development of efficient
perfluoroalkylation reactions under mild, metal-free conditions
represents a highly desirable research target.
Recently, the in situ generation of perfluoroalkyl radicals has
been recognized as an attractive method for the direct insertion
of perfluoroalkyl groups. This commonly encountered approach
n
is based on perfluoroalkyl halides (C F2n+1–X; X = I, Cl, Br) as
radical sources, as these are widely available and easily
7
handled. Alternatively, perfluoroalkyl analogs of Togni’s or
Ruppert–Prakash trifluoromethylating reagents, have recently
been used for radical-based perfluoroalkylations. However, the
generation of perfluoroalkyl radicals from these reagents often
3 2
Sodium trifluoromethanesulfinate (CF SO Na, Langlois
8
reagent), a stable and inexpensive solid reagent, has recently
received attention as a convenient trifluoromethylating reagent,
which generates trifluoromethyl radicals under mild conditions
9,10
upon reaction with several oxidants. To our surprise, only a
few perfluoroalkylations using higher perfluoroalkyl analogs of
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo,
Kyoto 606-8502, Japan. E-mail: maruoka@kuchem.kyoto-u.ac.jp;
Fax: +81-75-753-4041; Tel: +81-75-753-4041;
n 2
the Langlois reagent (C F2n+1SO Na, n  2) have been reported
†
Electronic Supplementary Information (ESI) available: Experimental Details. See
11
so far. For example, in 2013, Itho and co-workers reported the
DOI: 10.1039/x0xx00000x
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photolytic radical perfluoroalkylation of arenes using sodium
With optimized conditions in hand, we exVaiemw Ainrtieclde Ontlhinee
perfluoroalkanesulfinates and an organocatalyst. However, this substrate scope of the perfluoroalkylaDtiOoIn: /1c0y.1c0l3iz9a/Cti6oOnB0o1f24N5K-
n 2
system also required photo-irradiation and the substrate scope arylacrylamides (2) with C F2n+1SO Na (1) (Scheme 2). The
was limited to electron-rich arenes (Scheme 1a).11a In this reaction of 2a with various sodium perfluoroalkanesulfinates
context, we would like to report herein the efficient generation proceeded smoothly and furnished the corresponding
of perfluoroalkyl radicals from sodium perfluoroalkan- perfluorinated oxindoles (3a
esulfinates, using a hypervalent iodine(III) reagent as the electron-donating and electron-withdrawing substituents on
oxidant. The present system generates perfluoroalkyl radicals the aryl moiety of did not significantly affect the yield (3f ),
–e) in moderate to high yield. Both
2
–h
under mild, metal-free conditions, and it can be used for the and α-substituents on the alkenes, e.g. phenyl and alcohol
radical-based perfluoroalkylation of N-acrylamides, 2- groups (3i and 3j) were also tolerated well. The reactions of N-
isocyanides, olefins, and heterocycles (Scheme 1b).
benzyl and N-isopropyl acrylamides (2k and 2l) also afforded the
corresponding products (3k and 3l) in good yield. The use of
Sodium perfluoroalkanesulfinates (C Na; n
1
n
F
2n+1SO
2

2) were heterocycle 2m slightly decreased the yield of 3m (47%).
easily prepared as stable solids in large quantities according to the Furthermore, the reaction could also be carried out in the
procedure reported by Hu and DesMarteau.12 With these sulfinates presence of a tetrahydroquinoline moiety (2n), affording
in hand, we initially examined the perfluoroalkylation/cyclization of tricyclic oxytetrahydroquinoline 3n in 83% yield.
N-arylacrylamide
oxindole as a model reaction.13 When the reaction between
SO Na (1a) and N-methyl-N-phenylacrylamide (2a) was carried
out in the presence of oxidants such as K , 2,3-dichloro-5,6-
2 to afford the corresponding perfluorinated
C
4
F
9
2
2 2 8
S O
dicyano-p-benzoquinone (DDQ), or tert-butylhydroperoxide (TBHP),
only trace amounts of the desired product (3a) were obtained
(
Table 1, entries 1–3). Under Baran’s conditions (TBHP in
DCM/H
2
O),10a the reaction furnished 3a in merely 22% yield (entry
4
). Therefore, we subsequently tested hypervalent iodine(III)
oxidants:14 while (diacetoxyiodo)benzene (DIB) did not generate
any 3a (entry 5), [bis(trifluoroacetoxy)iodo]benzene (PIFA) afforded
3a in 72% yield (entry 6). Moreover, we tested several solvents for
this reaction, and acetonitrile was identified as the most efficient.15
By slow addition of the PIFA solution using a syringe pump, the yield
of 3a could be improved to 83% (entry 7). It should also be noted
that the reaction could be scaled up to 3 mmol without any loss in
efficiency, generating 3a (1.03 g) in 87% yield (entry 8).
Table 1. Perfluoroalkylation/cyclization of N-methyl-N-phenylacrylamide 2a with
C F SO
4 9 2
Na 1a.[a]
Scheme 2. Perfluoroalkylation/cyclization of N-Arylacrylamide (2).
entry
oxidant
solvent
MeCN
MeCN
MeCN
Yield 3a (%)[b]
1
2
2 2
K S O
8
<5
<5
DDQ
3
4[c]
TBHP
TBHP
DIB
<5
2
DCM/H O
22
5
MeCN
MeCN
MeCN
MeCN
<5
6
7[d]
8[f]
PIFA
PIFA
PIFA
72
83 (76)[e]
87[e]
[a] Unless otherwise specified, reactions were carried out in the presence of 1a
(
1.2 equiv), 2a (0.1 mmol), oxidant (1.2 equiv.), and the respective solvent (0.05
1
M). [b] The yield was determined by H NMR spectroscopy using 1,1,2,2-
tetrachloroethane as the internal standard. [c] The reaction between 2a and 1a
(
3.0 equiv.) was carried out in the presence of TBHP (5.0 equiv.) in DCM/H
2
O
(2.5/1). [d] A solution of PIFA in acetonitrile was added to the reaction mixture with
a syringe pump over 20 min. [e] Isolated yield. [f] The reaction was conducted on
a 3 mmol scale.
Scheme 3. Radical cyclization of 2-isocyanobiphenyl 4 with sulfinates 1 and PIFA.
2
| J. Name., 2012, 00, 1-3
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The present system was also utilized for the synthesis of 6- derivatives was studied. The reactions involving coumarin
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1
6
DOI: 10.1039/C6OB01245K
perfluoroalkylated phenanthridines. The radical perfluoroalk- derivatives proceeded well, and the corresponding
ylation of 2-isocyanobiphenyl with sulfinates and PIFA perfluorinated coumarins (7i ) were obtained in moderate to
proceeded smoothly to give a variety of 6-perfluoroalkylated good yield. The use of 2-quinolinone or flavone derivatives
4
1
–k
phenanthridines
Scheme 3.
5
in moderate to high yields as shown in afforded 7l and 7m in modest yield. Moreover, the introduction
1
7,18
of perfluoroalkyl groups into uracil and caffeine derivatives
furnished 7n
–p in good yield.
To demonstrate the extended utility of this system, we
subsequently turned our attention to the direct
At present, we consider these reactions to proceed via a
2
perfluoroalkylation of sp –hybridized C–H bonds of olefins and mechanism that is based on the generation of perfluoroalkyl
heterocycles (Scheme 4). Even though the direct radicals. In fact, when 1a was added to a solution containing
2
trifluoromethylation of sp –hybridized C–H bonds of PIFA and the radical scavenger 2,2,6,6-tetramethyl-1-
(hetero)arenes has recently been accomplished by several piperidinyloxy (TEMPO, 8), the perfluoroalkyl radical was
research groups, the direct perfluoroalkylation and successfully trapped (Scheme 5).
2
trifluoromethylation of olefinic sp –hybridized C–H bonds still
remains challenging.1
9,20
Therefore, we initially examined the
trifluoromethylation of 3,3-diphenylacrylate 6a as a model
reaction. The optimization of the reaction conditions revealed
that the use of a mixture of [bis(trifluoroacetoxy)iodo]-
pentafluorobenzene (F
corresponding product in good yield. The use of symmetrical
5
-PIFA) and DDQ afforded the
1
5
1
3
g
3
,3’-diarylacrylates (6a
in moderate to good yield, and in case of an unsymmetrical
,3’-diarylacrylate (6h), both E and Z isomers of 7h were
–
g
) afforded tetrasubstituted olefins 7a
–
4 9
Scheme 5. Trapping C F  radicals with TEMPO. [a] The yield was determined by H NMR
spectroscopy using 1,1,2,2-tetrachloroethane as the internal standard.
obtained in 63% yield (E/Z = 1/1). Additionally, the
perfluoroalkylation of sp –hybridized C–H bonds in several
2
Conclusions
heterocycles such as coumarin, flavone, and 2-quinolinone
In summary, we developed a method for the efficient
generation of perfluoroalkyl radicals using readily available
sodium perfluoroalkanesulfinates and a hypervalent iodine(III)
reagent. This approach allows the introduction of perfluoroalkyl
groups into a wide variety of organic molecules under mild and
metal-free conditions. Further investigations into the detailed
mechanism and synthetic applicability to other, related systems
are currently in progress in our laboratory.
Notes and references
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Scheme 4. Direct perfluoroalkylation of sp –hybridized C–H bonds in olefins and
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