Copper-Catalyzed three-component oxytrifluoromethylation of alkenes with sodium trifluoromethanesulfinate and hydroxamic acid

Article abstract of DOI:10.1002/anie.201307595

Radical paths: The title reaction of olefins with NaSO₂CF ₃ and N-hydroxy-N-phenylacetamide at room temperature is described for the first time (see scheme). This reaction provides a practical and convenient route to a series of trifluoromethylated alcohols bearing a wide range of functional groups. Copyright

Full text of DOI:10.1002/anie.201307595

Angewandte
Chemie
DOI: 10.1002/anie.201307595
Synthetic Methods
Copper-Catalyzed Three-Component Oxytrifluoromethylation of
Alkenes with Sodium Trifluoromethanesulfinate and Hydroxamic
Acid**
Xin-Yi Jiang and Feng-Ling Qing*
[
14]
The trifluoromethyl group, having an enhanced electronega-
tivity, lipophilicity, metabolic stability, and bioavailability, is
widely prevalent in pharmaceuticals and agrochemicals.
phenium tetrafluoroborate]
and Togniꢀs reagents (1-tri-
fluoromethyl-1,2-benziodoxol-3-(1H)-one and 1-trifluoro-
methyl-1,3-dihydro-3,3-dimethyl-1,2-benziodoxole)
[
1]
[15]
have
Therefore, great efforts have been devoted to the develop-
ment of new methodologies for efficient and selective
incorporation of the trifluoromethyl group into organic
compounds. Recently, strategies have been well established
for trifluoromethylation of arenes, including transition-
metal-catalyzed/mediated trifluoromethylation of aryl hal-
attracted attention because these reagents are easy to
handle; they are solids at room temperture. Especially, only
the electrophilic trifluoromethyalting reagents including
Togniꢀs reagent and Umemotoꢀs reagent are employed for
[
2]
[11]
the oxytrifluoromethylation of alkenes (Scheme 1). How-
ever, the high cost and/or lack of ready availability of these
electrophilic trifluoromethylating reagents limits their usage
on a large scale.
[
3]
[4]
[5]
ides, aryl boronic acid derivatives, arynes, aromatic
[
6]
[7]
amines, and even aromatic CÀH bonds. Very recently,
the trifluoromethylation of olefins has received considerable
attention and tremendous progress has been made in this
area. The copper-catalyzed trifluoromethylation of terminal
alkenes or allylsilanes with electrophilic trifluoromethylating
reagents has been developed, thus providing a series of
trifluoromethylated allylic compounds. Our group also
developed a complementary route to trifluoromethylated
allylic compounds through the copper-catalyzed oxidative
trifluoromethylation of terminal alkenes with the Ruppert–
[
8]
[
9]
Prakash reagent (CF SiMe ). The hydrotrifluoromethyla-
3
3
Scheme 1. Copper-catalyzed oxytrifluoromethylation of alkenes.
tions of unactivated alkenes by using nucleophilic, electro-
philic, and radical trifluoromethylating reagents were
reported respectively by our group, as well as those of
Since the pioneering work by Langlois and co-workers on
the oxidation of NaSO CF to the trifluoromethyl radical
[10]
Gouverneur and Nicewicz. Furthermore, the difunctional-
ization-type trifluoromethylation of alkenes including oxy-
2
3
[16]
(CF C) and subsequent addition to electron-rich aromatics,
the trifluoromethylation of arenes, heterocycles, and aryl
3
[
11]
[12]
[7o]
[7g]
trifluoromethylation,
carbotrifluoromethylation,
and
[
13]
[4j,m]
aminotrifluoromethylation
have been achieved with or
boronic acids
using NaSO CF as a stable and inexpensive
2 3
without transition-metal catalysis. These difunctionalization
reactions allow rapid access to a variety of classes of CF3-
containing building blocks including lactones, cyclic ethers,
epoxides, alcohols, ketones, amines, aziridines, and carbo-
cycles. Among these difunctionalization-type trifluoromethyl-
ations of alkenes, the electrophilic and radical transformation
using Umemotoꢀs reagent [S-(trifluoromethyl)dibenzothio-
trifluoromethyl (CF -) source has been recently developed.
3
[10c]
Furthermore, the hydrotrifluoromethylation of alkenes
and decarboxylative trifluoromethylation of a,b-unsaturated
[17]
carboxylic acids with NaSO CF have also been reported.
2
3
In contrast, Alexanian and co-workers have successfully
[
18a,b]
[18c]
developed the dioxygenation,
ketooxygenation
oxyamination,
and
[18d]
of alkenes using hydroxamic acids,
which are readily converted into amidoxyl radicals upon
exposure to mild oxidants or radical initiators. Inspired by
these intriguing studies, we hypothesized that the oxytri-
fluoromethylation of alkenes would be possible using both
NaSO CF and hydroxamic acid, because copper salts could
[
*] X.-Y. Jiang, Prof. Dr. F.-L. Qing
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences
345 Lingling Lu, Shanghai 200032 (China)
2
3
E-mail: flq@mail.sioc.ac.cn
generate both BuOC and BuOOC from tert-butylhydroperoxide
Prof. Dr. F.-L. Qing
College of Chemistry, Chemical Engineering and Biotechnology
Donghua University
[19]
[7g]
(
TBHP),
and the trifluoromethyl radical (CF C)
and
3
[18]
amidoxyl radical [ArN(CO Me)OC] could be formed from
2
NaSO CF and hydroxamic acid in the presence of BuOC and
2
3
2999 North Renmin Lu, Shanghai, 201620 (China)
BuOOC (Scheme 2). However, the regioselectivity of this
process would be a great challenge. When the trifluoromethyl
radical first adds to an alkene, the radical intermediate A is
formed, followed by trapping with an amidoxyl radical to give
the product 3 (Scheme 2, Path 1). In the case of the first
[
**] This work was supported by the National Natural Science
Foundation of China (21072028, 21272036, 21332010) and the
National Basic Research Program of China (2012CB21600).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307595.
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Table 1: Selected optimization of the copper-catalyzed oxytrifluoro-
[
a]
methylation of styrene.
Entry
Cu salts
3a
Yield [%]
[
b]
Scheme 2. Proposed mechanism.
1
2
3
4
5
6
7
8
9
10
CuCl
CuI
CuOAc
CuBr
CuTc
(CuOTf) Ph
[Cu(MeCN) PF ]
[Cu(MeCN) BF ]
CuSO ·5H O
61 (8)
53 (10)
61(9)
addition of an amidoxyl radical to an alkene, the product 3’
would be formed via intermediate B (Scheme 2, Path 2).
Herein, we describe the copper-catalyzed three-component
oxytrifluoromethylation of alkenes by employing sodium
trifluoromethanesulfinate (NaSO CF , Langlois reagent) as
63 (12)
57 (9)
62 (12)
63 (10)
67 (11)
66 (9)
65 (11)
63 (12)
57 (14)
–
2
4
6
4
4
2
3
4
2
[
20]
a trifluoromethyl (CF -) source (Scheme 1). This oxytri-
3
Cu(OTf)2
fluoromethylation proceeds regioselectively to give a single
product and is dictated by the initial trifluoromethyl radical.
To test the hypothesis, we began our study by examining
the oxytrifluoromethylation of styrene (1a) with NaSO CF ,
tBuOOH and N-hydroxy-N-phenylacetamide (2). After
examining the effects of solvent and reaction temperature
11
CuCl2
CuBr2
–
CuSO ·5H O
CuSO ·5H O
12
13
[
[
[
c]
d]
e]
14
15
16
17
72 (–)
77 (–)
84 (–)
4
2
2
3
4
2
CuSO ·5H O
CuSO ·5H O
4
2
[f]
[g]
–
4
2
(
see Table S1 in the Supporting Information), we found that
[
(
a] Reaction conditions: 1a (0.2 mmol), 2 (0.24 mmol), NaSO CF3
0.6 mmol), Cu salts (0.02 mmol). tBuOOH (1.2 mmol), CH Cl /MeOH/
H O (1 mL:1 mL:0.8 mL), 12 h, N , RT. [b] Determined by F NMR
spectroscopy using fluorobenzene as an internal standard. The value
within parentheses is the yield of 4. [c] 1a (0.4 mmol), 2 (0.2 mmol).
[d] 1a (0.6 mmol), 2 (0.2 mmol). [e] 1a (0.6 mmol), 2 (0.2 mmol),
NaSO CF (0.8 mmol). [f] TEMPO (0.2 mmol) was used instead of 2
0.2 mmol). [g] No TEMPO adduct was detected. Tc=thiophene-2-
carboxylate, Tf=trifluoromethanesulfonyl.
2
the oxytrifluoromethylation of 1a proceeded smoothly at
room temperature using CuCl as a catalyst in a mixed solvent
2
19
2
2
2
(
CH Cl /MeOH/H O = 5:5:4), thus giving the desired product
2 2 2
3
a in 61% yield together with the side product N-trifluoro-
1
9
methoxy-N-phenylacetamide 4 (detected by F NMR spec-
troscopy and GC-MS); the regioisomer 3a’ was not observed
2
3
(
(
Table 1, entry 1). We speculated that the faster formation of
the CF radical or the greater stability of the amidoxyl radical
3
may result in the selective fromation of 3a. Switching the
CuCl catalyst to CuI, CuOAc, CuBr, CuTc, or even to a more
ation of alkenes was investigated (Scheme 3). Styrenes
possessing both electron-donating (1e,f, 1j) and electron-
withdrawing (1b–d, 1g–i) groups worked well, thus affording
the corresponding products in good yield. Functionalities
such as chloro, bromo, ether, and ester groups, which serve as
useful reaction handles for additional transformations, were
tolerated under these mild reaction conditions (3c–f, 3h,i).
The substituent at the meta (3i) or ortho (3j) position of the
aromatic ring did not hinder the reaction (72–73% yields).
The b-methylstyrene (1k) was also a suitable substrate with
a moderate level of stereoselection (5:3 d.r.). In addition, the
heterocyclic substrate 2-vinylthiophene yielded the desired
oxytrifluoromethylated compound 3l in 60% yield. Impor-
tantly, the copper-catalyzed oxytrifluoromethylation pre-
sented herein has also been easily extended to unactivated
alkenes. A number of unactivated alkenes such as 4-phenyl-1-
butene, methylenecyclohexane, and cyclohexene proceeded
smoothly to give the corresponding products in moderate
yields (3m–o). Notably, the oxytrifluoromethylation of 4-
pentene-1-ol and 5-hexen-1-ol provided the trifluoromethyl-
Lewis-acidic copper(I) species, such as (CuOTf) Ph, Cu-
2
(
MeCN) PF , and Cu(MeCN) BF , the improvement of the
4 6 4 4
yield of 3a and inhibition of the byproduct 4 (entries 2–8)
were not successful. Subsequently, copper(II) catalysts such as
CuSO ·5H O, Cu(OTf) , CuCl , and CuBr were tested
4
2
2
2
2
(
entries 9–12). Actually, CuSO ·5H O was an effective cata-
4 2
lyst for oxytrifluoromethylation, thus providing 3a in 66%
yield. Notably, 3a was not formed in the absence of a copper
catalyst (entry 13). As CuSO ·5H O is less expensive than
4
2
other copper catalysts, CuSO ·5H O was chosen for the
4
2
preparative experiments. To reduce the formation of 4 and
inhibit polymerization of styrene, the use of 1.0 equivalent of
2
with either 2.0 or 3.0 equivalents of styrene led to improved
yields (entries 14 and 15). To our delight, the yield of 3a was
improved to 84% when the amount of NaSO CF3 was
2
increased to 4.0 equivalents (entry 16). Additionally, the
commercially available aminoxyl radical TEMPO (2,2,6,6-
tetramethyl-1-piperidinyloxy) was examined as a radical
terminator instead of 2 for oxytrifluoromethylation of
styrene, however no TEMPO-trapped product was observed
ated alcohols (3p,q), rather than the CF -containing cyclic
3
(
entry 17).
With the optimized reaction conditions in hand, the
substrate scope of this copper-catalyzed oxytrifluoromethyl-
ethers that were formed from reactions of 4-pentene-1-ol and
[11c]
5-hexen-1-ol with Togniꢀs reagent.
To our delight, a,b-
unsaturated esters, a,b-unsaturated ketone, and a,b-unsatu-
1
4178 www.angewandte.org
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Scheme 4. Synthesis of ß-trifluoromethyl-substitued alcohol.
intermediate was formed in this reaction, 1a was subjected to
our standard reaction conditions in the absence of NaSO CF ,
2
3
and the dioxygenation product 6 was obtained in 68% yield
Scheme 5a). However, was not formed without
CuSO ·5H O (Scheme 5b). These results (Scheme 5 and
(
6
4
2
entry 13 of Table 1) suggested that the catalytic copper salt
played a key role in the formation of both the trifluoromethyl
radical (CF C) and amidoxyl radical [ArN(CO Me)OC].
3
2
Scheme 5. Mechanistic experiments.
In conclusion, we have developed a copper-catalyzed
regioselective oxytrifluoromethylation of alkenes. This reac-
tion employs the stable and inexpensive NaSO CF as the CF
2
3
3
source and CuSO ·5H O as the catalyst. More importantly,
4
2
this system is tolerant of a wide range of functional groups.
The cheapness of reactants and the mild reaction conditions
make this reaction a practical method for oxytrifluoromethyl-
ation of alkenes.
Received: August 28, 2013
Revised: September 23, 2013
Published online: November 19, 2013
Scheme 3. Scope of the copper-catalyzed oxytrifluoromethylation of
alkenes. Reaction conditons: 1a (0.6 mmol), 2 (0.2 mmol), NaSO CF
2
3
(
0.8 mmol), CuSO ·5H O (0.02 mmol), tBuOOH (1.2 mmol), CH Cl /
4 2 2 2
Keywords: alkenes · copper · fluorine · radical chemistry ·
synthetic methods
MeOH/H O (1 mL:1 mL:0.8 mL), 12 h, N , RT. Yields shown are those
of the isolated products. [a] Used CuBr (0.02 mmol) as a catalyst.
2
2
.
2
[b] Obtained as a mixture of diastereoisomers. The diastereomeric
1
9
ratio determined by F NMR spectroscopy.
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