A free-radical cascade trifluoromethylation/cyclization of N-arylmethacrylamides and enynes with sodium trifluoromethanesulfinate and iodine pentoxide

Article abstract of DOI:10.1021/ol5014747

An I₂O₅-promoted free-radical cascade trifluoromethylation/cyclization of a broad range of N-arylmethacrylamides and enynes with sodium trifluoromethanesulfinate in aqueous medium has been achieved. This strategy allows highly selective access to a variety of CF ₃-containing oxindoles and pyrrolidines. Electron spin resonance (ESR) studies indicate that atom-transfer processes are involved in this system.

Full text of DOI:10.1021/ol5014747

Letter
pubs.acs.org/OrgLett
A Free-Radical Cascade Trifluoromethylation/Cyclization of
N‑Arylmethacrylamides and Enynes with Sodium
Trifluoromethanesulfinate and Iodine Pentoxide
Lizhi Zhang, Zejiang Li, and Zhong-Quan Liu*
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou Gansu 730000, P. R. China
S
* Supporting Information
ABSTRACT: An I2O5-promoted free-radical cascade trifluor-
omethylation/cyclization of a broad range of N-arylmethacry-
lamides and enynes with sodium trifluoromethanesulfinate in
aqueous medium has been achieved. This strategy allows
highly selective access to a variety of CF3-containing oxindoles
and pyrrolidines. Electron spin resonance (ESR) studies
indicate that atom-transfer processes are involved in this system.
pathways. One is mediated by tert-butyl hydroperoxide
wide range of pharmaceuticals and agrochemicals bear a
Atrifluoromethyl (CF ) group mainly because it can (TBHP) with or without copper catalyst.6−8,10,11 The second
3
method is promoted by air or oxygen in the presence of silver
salts and K2S2O8.9 However, these systems required large excess
amounts of potentially explosive peroxide and/or metal catalyst.
Although a very recent example of trifluoromethylation of
arenes with CF3SO2Na and phenyliodine bis(trifluoroacetate)
(PIFA) was reported by Shibata and co-workers, this system
was limited in electron-rich arenes.12 More safe and practical
trifluoromethylation strategies are highly desirable.
dramatically modify their nature, such as their lipophilicity,
electronegativity, metabolic stability, and bioavailability.1 There-
fore, recent advances in exploring more efficient strategies for
trifluoromethylation have been made.2 Recently, significant
developments have been achieved by transition-metal-cata-
lyzed/-promoted3 as well as free-radical-mediated trifluorome-
thylation.4 Among the radical systems, several reagents such as
CF3X (X = Br, I), CF3SO2X, TMSCF3, Umemoto’s reagent [S-
(trifluoromethyl)dibenzothiophenium tetrafluoroborate], and
Togni’s reagent (1-trifluoromethyl-1,2-benziodoxol-3-(1H)-one
and 1-trifluoromethyl-1,3-dihydro- 3,3-dimethyl-1,2-benziodox-
ole), etc., have been used as the trifluoromethyl radical
sources.5 In contrast to these reagents, sodium trifluorometha-
nesulfinate (CF3SO2Na, Langlois reagent), a stable and
inexpensive solid, has gained much attention only in recent
years.6 In 2011, Baran et al. reported a very efficient
trifluoromethylation of heterocycles by using Langlois reagent.7
Later, several radical methods using CF3SO2Na have been
developed by Sanford,8 Maiti,9 Qing,10 and us.11 As depicted in
Scheme 1, in the previous trifluoromethylation reactions with
Langlois reagent, the strategies for generation of the
trifluoromethyl radical can be mainly summarized in two
With particular interest in free-radical-initiated trifluorome-
thylation, we began to hypothesize that a nonmetal inorganic
single-electron oxidant would promote a CF3-derivatived anion
−
(e.g., CF3SO2 ) to generate the trifluoromethyl radical. In our
previous studies, we found that some nonmetal inorganic
iodines such as iodic acid (HIO3, IA) and iodine pentoxide
(I2O5, IP) can be used as reliable and safe single-electron
oxidants.13 Despite their wide applications in chemical industry
because of their low price and particular stability,14 IA and IP
are rarely used in organic synthesis.15 Inspired by these
previous investigations on IA/IP-promoted single-electron-
transfer (SET) reactions, we reasoned that it could trigger a
series of free-radical trifluoromethylations through single-
electron oxidation of CF3SO2Na. Herein, we report an IP-
promoted trifluoromethylation/cyclization of N-arylmethacry-
lamides and enynes with Langlois reagent.
The CF3 motif is becoming an increasingly common unit
among important heterocycles found in agrochemicals and
pharmaceuticals, which supplies a driving force for the
exploration of more powerful and practical trifluoromethylation
protocols.3j,6b,c,7 Recently, several efficient methods have been
developed to construct trifluoromethylated oxindoles and
pyrrolidines by using Togni’s reagent with or without
transition-metal catalysis.3m−o,u In order to realize a safe, low-
Scheme 1. Strategies for Generation of Trifluoromethyl
Radical from CF3SO2Na
Received: May 21, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ol5014747 | Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
Table 1. Modification of the Typical Reaction Conditions
b
entry
1
oxidant (equiv)
solvent (v/v, V)
temp (°C)
yield (%)
CH2Cl2/H2O (2.5/1, 3.5 mL)
CH2Cl2/H2O (2.5/1, 3.5 mL)
CH2Cl2/H2O (2.5/1, 3.5 mL)
CH2Cl2/H2O (2.5/1, 3.5 mL)
CH2Cl2/H2O (2.5/1, 3.5 mL)
CH2Cl2/H2O (2.5/1, 3.5 mL)
H2O (3.5 mL)
50
50
50
50
50
50
50
50
50
25
70
70
70
70
c
2
IP (3)
IP (3)
IA (6)
PIFA (3)
IBX (3)
IP (3)
IP (3)
IP (3)
IP (3)
IP (3)
IP (3)
IP (3)
IP (3)
76
84
51
37
3
4
5
6
7
6
31
52
5
8
DMSO/H2O (2.5/1, 3.5 mL)
CH2Cl2/H2O (1/1, 3.5 mL)
CH2Cl2/H2O (2.5/1, 3.5 mL)
CH2Cl2/H2O (2.5/1, 3.5 mL)
CH2Cl2/H2O (2.5/1, 2 mL)
CH2Cl2/H2O (2.5/1, 5 mL)
CH2Cl2/H2O (2.5/1, 10 mL)
9
10
11
12
13
14
89
12
26
5
a
b
Reaction conditions: N-methyl-N-phenylmethacrylamide (1 equiv, 0.2 mmol), NaSO2CF3 (5 equiv, 1.0 mmol), 22 h, sealed tube. Isolated yields.
c
NaSO2CF3 (3 equiv, 0.6 mmol).
cost, and practical strategy for introduction of CF3 into
heterocycles, we chose N-methyl-N-phenylmethacrylamide and
sodium trifluoromethanesulfinate as the model substrates to
optimize the suitable conditions for this transformation (Table
1 and the Supporting Information). It was found that the
oxidants and solvent critically affect the efficiency of the
reaction. No product was observed without oxidant (Table 1,
entry 1). It was found that several organic hypervalent iodines
such as PhI(OCOCF3)2 and 2-iodoxybenzoic acid (IBX) as
well as HIO3 were much less efficient than IP (Table 1, entries
2−6). We found that a mixed solvent of CH2Cl2/H2O with a
ratio of 2.5/1 was better than others such as H2O, DMSO/
H2O, etc. (Table 1, entries 7−9). When the reaction was
conducted at 25 and 70 °C, the desired product was isolated in
5% and 89% yield, respectively (Table 1, entries 10 and 11).
Finally, the cage effect of this reaction was also investigated
(Table 1, entries 12−14). As a result, higher or lower
concentrations of the alkene (compared to 0.057 mol/L) all
led to much lower yields of the product.
Scheme 2. I2O5-Mediated Trifluoromethylation/Cyclization
of N-arylmethacrylamides
a
As depicted in Scheme 2, trifluoromethylated oxindole and
its derivatives were isolated in moderate to nearly quantitative
yields under the typical reaction conditions (1a−m). The N-
methyl-, N-phenyl-, N-benzyl-, and N-alkyl-substituted N-
phenylmethacrylamides all gave the desired products in good
to high yields (1a−d). Gratifyingly, the amides with halogen
atoms such as F, Cl, Br, and I as well as alkyl substituents on the
para-position of the N-aryl moiety led to moderate to nearly
quantitative yields of the corresponding products (1e−i). It is
noteworthy that the reaction can be scaled-up to gram level
without losing efficiency (1f, 1.0 g, 35 h, 78% yield). The N-
methyl-N-phenylacrylamide gave the CF3 bearing oxindole in
44% yield (1j). In addition, functional groups such as hydroxyl,
ester, and amine at the 2 position of the acrylamide core could
be well tolerated in this system (1k−m).
a
Reaction conditions: N-arylmethacrylamides (1 equiv, 0.2 mmol),
NaSO2CF3 (5 equiv, 1.0 mmol), I2O5 (3 equiv, 0.6 mmol), CH2Cl2/
H2O (2.5/1, 3.5 mL), 70 °C, 22 h, sealed tube. Isolated yields.
b
reaction (Scheme 3). As illustrated in Scheme 3, the
trifluoromethylated five-membered N-heterocycles were ob-
tained with specific regioselectivity, which indicated that the 5-
exo-dig cyclization represented the exclusive pathway (2a−g). It
was found that there was no evident electronic effect since
various enynes containing aromatic cores with both electron-
donating and electron-withdrawing groups resulted in moderate
to high yields of the desired products (2a−f). In the case of
To apply this strategy in other systems, we studied the
trifluoromethylation/cyclization of enynes with IP/NaSO2CF3.
An efficient and highly selective trifluoromethylation−cycliza-
tion−iodination of an array of enynes is also achieved via a
single step under conditions similar to those of the former
B
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Letter
this system are consistent with the reported data.16 In the case
of N-arylmethacrylamide, addition of the CF3 radical to the
double bond would generate radical B, which then adds to the
aromatic core followed by aromatization forming the
trifluoromethylated oxindole. Similarly, the trifluoromethyl
radical adds to the CC double bond of enyne leading to a
β-CF3-alkyl radical intermediates C, which occurs a free-radical
5-exo-dig cyclization to give an alkenyl radical D. The product is
formed via capture of iodine by radical D. We proposed that
the I2 might come from I2O5 through multistep redox
processes. The formation of I2 has also been confirmed by
observation of an obvious color change while starch was added
into the system though the mechanistic details of the redox
processes are not very clear at the presence. Overall, a free-
radical cascade cyclization pathway would be involved in both
of the trifluoromethylation reactions.
Scheme 3. I2O5-Mediated Trifluoromethylation/Cyclization
of Enynes
a
In conclusion, an I2O5-promoted highly selective and scalable
trifluoromethylation/cyclization cascade reaction of N-arylme-
thacrylamides and enynes with NaSO2CF3 in aqueous medium
has been developed. A wide range of trifluoromethylated
oxindoles and pyrrolidines are efficiently prepared by using this
strategy. EPR studies indicate that free radical processes would
be involved in this system. Further studies on the IP-promoted
SET reactions are ongoing in this laboratory.
a
Reaction conditions: enyne (1 equiv, 0.2 mmol), NaSO2CF3 (3 equiv,
ASSOCIATED CONTENT
* Supporting Information
Full experimental details and characterization data for all
products. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
0.6 mmol), I2O5 (3 equiv, 0.6 mmol), CH2Cl2/H2O (2.5/1, 3.5 mL),
S
b
c
110 °C, 20 h, sealed tube. Isolated yields. Ratio of the E/Z isomers
d
determined by 19F NMR spectroscopy. Only one isomer was
observed.
enynes bearing aryls with either meta- or ortho-substituents,
moderate to good yields of the pyrrolidines were obtained,
which suggests the steric effect is not obvious (2d and 2f). It is
noteworthy that thiophene-substituted enyne can also act as an
effective substrate in this system (2g). Interestingly, N-(buta-
2,3-dien-1-yl)-4-methyl-N-(2-methylallyl)benzenesulfonamide
gave the corresponding 2h as the major product, suggesting the
allene group can be tolerated. Finally, 3-phenylprop-2-yn-1-yl
methacrylate also led to the desired product 2i, although the
yield was low. The features of step economy, functional group
tolerance, and high selectivity make this method very attractive.
To gain insight into the details of the mechanism, we carried
out a series of mechanistic studies through spin-trapping
technology and electron-spin resonance (ESR) (Scheme 4).
When 2-methyl-2-nitrosopropane (MNP) was used as a radical
spin trap, the signals of the radical adduct trifluoromethyl tert-
butyl nitroxide radical A were clearly recorded by ESR. the
relatively stable nitroxide radical A would be formed by
addition of CF3 radical to MNP. The g value (2.0061) and the
hyperfine coupling constant (aN = aF = 12.25 G) obtained in
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: liuzhq@lzu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This project is supported by the National Science Foundation
of China (No. 21272096) and the Fundamental Research
Funds for the Central Universities (lzujbky-2014-58). We also
thank Professors Bo Zhou, Xiangfeng Shao, Wei Yu, and Long-
Min Wu (State Key Laboratory of Applied Organic Chemistry,
Lanzhou University, P. R. China) for helpful discussions.
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