Copper-catalyzed oxidative trifluoromethylthiolation of aryl boronic acids with TMSCF3 and elemental sulfur

Article abstract of DOI:10.1002/anie.201108663

Fluorinated functionality: The copper-catalyzed oxidative trifluoromethylthiolation of aryl boronic acids with TMSCF₃ and elemental sulfur at room temperature is described for the first time. This reaction provides a concise and efficient method for the synthesis of aryl trifluoromethyl thioethers (ArSCF₃) under mild conditions. Phen=Phenanthroline. Copyright

Full text of DOI:10.1002/anie.201108663

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DOI: 10.1002/anie.201108663
Thioether Synthesis
Copper-Catalyzed Oxidative Trifluoromethylthiolation of Aryl Boronic
Acids with TMSCF3 and Elemental Sulfur**
Chao Chen, Yan Xie, Lingling Chu, Ruo-Wen Wang, Xingang Zhang, and Feng-Ling Qing*
Fluorinated functional groups are key structural units found
in various pharmaceuticals and agrochemicals.[1] Approxi-
mately 30% of all agrochemicals and 20% of all pharma-
ceuticals on the market contain fluorine. Among these
(the Ruppert–Prakash reagent, TMSCF3), to access aryl
trifluoromethyl thioethers would be an attractive alternative.
The present study was inspired by our own and Buch-
waldꢀs recent investigations into the copper mediated oxida-
tive trifluoromethylation of arylboronic acid with TMSCF3,[11]
as well as Karlinꢀs observation of the formation of a stable
copper disulfide complex from the reaction of elemental
sulfur (S8) with a CuI complex.[12] We hypothesized that a CuI
disulfide complex generated in situ (II; Scheme 1) would
À
substituents, the trifluoromethylthio group (CF3S ), espe-
cially as an aromatic substituent, plays an important role
because of its strong electron-withdrawing effect and high
lipophilicity. These characteristics are similar to those of
À
À
trifluoromethyl (CF3
) and trifluoromethoxy (CF3O )
groups.[2] Additionally, aryl trifluoromethyl thioethers
(CF3SAr) are also key intermediates in the preparation of
trifluoromethyl sulfoxide and sulfone, which are important
trifluoromethylation reagents.[3] Although impressive prog-
ress has been made in the trifluoromethylation of arenes in
the past several years,[4–7] only a few methods are available for
the synthesis of aryl trifluoromethyl thioethers.[8,9] Generally,
aryl trifluoromethyl thioethers are prepared either by a nucle-
ophilic reaction of trifluoromethylthiolate with aryl halides,[8]
or by a nucleophilic or radical reaction of aryl sulfides and
disulfides with a trifluoromethylation reagent.[9] However,
these methods are variously limited by a combination of high
temperatures, expensive reagents, and low reactivity with
electron-rich aromatic groups. Thus, the development of
general, safe, and efficient methods to access aryl trifluor-
omethyl thioethers is highly desirable. Very recently, Buch-
wald reported a palladium-catalyzed trifluoromethylthiola-
tion of aryl bromides with CF3SAg.[10] This breakthrough for
the preparation of ArSCF3 is highly efficient and compatible
with a variety of functional groups. However, from the point
view of cost-effectiveness and synthetic convenience, using
readily available and inexpensive catalysts and fluorinated
reagents, such as copper and (trifluoromethyl)trimethylsilane
Scheme 1. Copper(I)-catalyzed formation of aryl trifluoromethyl thio-
ether from aryl boronic acid, TMSCF3, and S8.
react with aryl boronic acid to give intermediate III (Path A),
which would subsequently react with TMSCF3, providing the
key intermediate complex LnCu(CF3)(ArS) (V). Finally,
oxidation of complex V to LnCuIII(CF3)(ArS),[13] followed by
reductive elimination would lead to the expected aryl
trifluoromethyl thioether. Alternatively, intermediate IV
could be formed by the reaction of TMSCF3 with complex
II (Path B). The desired product might still be obtained from
oxidation of key intermediate VI, generated from complex
IV.
Herein, we report the first example of the copper-
catalyzed oxidative trifluoromethylthiolation of arylboronic
acids with TMSCF3 and elemental sulfur at room temper-
ature. The notable features of this reaction are its high
efficiency, excellent functional group compatibility (bromide
is also compatible), operational simplicity, inexpensive cata-
lyst, easily accessible starting materials, and mild reaction
conditions.
[*] C. Chen, Y. Xie, L. Chu, Dr. R.-W. Wang, Prof. Dr. X. Zhang,
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)
E-mail: flq@mail.sioc.ac.cn
Prof. Dr. F.-L. Qing
College of Chemistry, Chemical Engineering and Biotechnology,
Donghua University
2999 North Renmin Lu, Shanghai, 201620 (China)
[**] This work was supported by the National Natural Science
Foundation of China (21072028, 20832008) and the National Basic
Research Program of China (2012CB21600). The authors thank Prof.
Qilong Shen of the Shanghai Institute of Organic Chemistry for
helpful discussions.
In accordance with our hypothesis, we began this study by
reacting phenyl boronic acid 1, TMSCF3, and S8 in the
presence of different copper salts, bases, and oxidants to
optimize the reaction conditions. To our delight, when the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108663.
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reaction was conducted with CuI (1.0 equiv) and 1,10-
phenanthroline (phen; 1.1 equiv) in the presence of Ag2CO3
(2.0 equiv) as oxidant, along with K3PO4 (1.0 equiv) and KF
(1.0 equiv) as bases, and 4 ꢁ molecular sieves (4 ꢁ M.S.), in
DMF at room temperature, phenyl trifluoromethyl thioether
2a was formed in 33% yield (Table 1, entry 1). The undesired
formation of 3, 5.0 equivalents of TMSCF3 was used, further
improving the yield to 85% (Table 1, entry 14). Notably,
catalytic amount of CuSCN (10 mol%) and phen
(10 mol%) still furnished 2a in a comparable yield (78%).
Further increasing the loading of phen to 20 mol% provided
the optimum yield of 2a, 95% (entry 16).
a
With the optimum reaction conditions (Table 1, entry 16)
determined, the substrate scope of the reaction was then
investigated (Scheme 2). The mild reaction conditions
Table 1: Optimization of the copper-catalyzed oxidative trifluoro-
methylthiolation of phenyl boronic acid.[a]
Entry
CuX
Base
Oxidant
Yield [%][b]
1[c]
2[c,d]
3[c]
4[c]
5
6
7
8
9
10
11
12
13
14[e]
15[e,f]
16[e,g]
CuI
CuI
–
CuI
CuI
CuI
CuCl
CuBr
CuOAc
(CuOTf)2C6H6
K4[Cu2(CN)6]
CuCN
CuSCN
CuSCN
CuSCN
CuSCN
K3PO4/KF
K3PO4/KF
K3PO4/KF
K3PO4/KF
KF
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
Ag2CO3
Ag2CO3
Ag2CO3
–
33
0
0
0
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
23
43
19
19
21
18
9
61
72
85
78
95
[a] Reaction conditions: 1 (0.2 mmol), S8 (0.6 mmol), TMSCF3
(0.6 mmol), CuX (0.2 mmol). phen (0.22 mmol), base (0.4 mmol),
oxidant (0.4 mmol), 4 ꢀ M.S. (50 mg), DMF (4 mL), 24 h, N2, RT.
[b] Determined by 19F NMR. [c]1.0 equiv of K3PO4 and 1.0 equiv of KF.
[d] Without phen. [e] With 5.0 equiv of TMSCF3 and 3.0 equiv of K3PO4.
[f] With 10 mol% of CuSCN and 10 mol% phen. [g] With 10 mol% of
CuSCN and 20 mol% phen.
products phenyl disulfide (3) and cyclic trimeric phenyl
boronic acid anhydride (4) were observed in the reaction
mixture. Furthermore, reactions without CuI, phen, or
Ag2CO3 failed to afford desired product 2a, thus showing
the pivotal role of these reagents in the reaction (entries 2–4).
Interestingly, when K3PO4 was used as both base and initiator
for TMSCF3, the yield increased to 43% (entry 6). The
formation of 4 indicated that the intermediate III might be
generated slowly. Likewise, the formation of 3 arose from the
homocoupling of copper complex III; an indication that the
reaction of III with TMSCF3 to form V was even slower.
Therefore, if a suitable copper salt was used to accelerate
step 1 (the formation of copper complexes III from inter-
mediate II) and step 2 (formation of V from III), and a proper
oxidant was employed to facilitate the reductive elimination
of V (Scheme 1), the formation of byproducts 3 and 4 would
be inhibited and thus lead to improved yield of desired
product 2a. Accordingly, different copper salts and oxidants
were examined to improve the reaction efficiency. After many
attempts, CuSCN and Ag2CO3 were found to be the best
choices, providing 2a in 72% yield along with a small amount
of phenyl disulfide 3 (Table 1, entry 13). To further inhibit the
Scheme 2. Scope of the Copper(I)-catalyzed oxidative trifluoromethyl-
thiolation of aryl boronic acids. Reaction conditions: 1 (0.2 mmol), S8
(0.6 mmol), TMSCF3 (1.0 mmol), CuSCN (0.02 mmol), Phen
(0.04 mmol), K3PO4 (0.6 mmol), Ag2CO3 (0.4 mmol), 4 ꢀ M.S. (50 mg),
DMF (3 mL), 24 h, N2, RT. Yields shown are of isolated products.
allowed for the trifluoromethylthiolation of aryl boronic
acids containing a range of functional groups, including ester,
unprotected amide, ketone, nitrile, and sulfonyl groups.
Notably, even for the substrates bearing bromide or vinyl
group, which are reactive in the presence of a Pd0 catalyst,
good yields were still obtained, thus providing opportunities
for further transformation.
To investigate the mechanism of the oxidative trifluoro-
methylthiolation, several experiments were performed. First,
the reaction of 3,4,5-trimethoxyphenyl boronic acid (1h) with
S8, CuSCN, and phen in the presence of K3PO4 in [D7]DMF at
1
room temperature was monitored by H NMR spectroscopy
(see the Supporting Information). A new species correspond-
ing to intermediate III was observed. GC/MS analysis of the
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silica gel column chromatography with hexane to provide pure aryl
trifluoromethyl thioether.
reaction mixture showed a peak at m/z = 199, which was
assigned to Ph(OMe)3SÀ, suggesting that the formation of
intermediate III from boronic acid 1, S8, and CuI is a reason-
able proposal. In contrast, neither CF3S-Cu nor CF3-Cu was
observed when a mixture of TMSCF3, CuSCN, phen, S8, and
K3PO4 in DMF was stirred at room temperature (see the
Supporting Information). Furthermore, the reaction of
copper 4-methoxyphenyl thiolate[14] with TMSCF3 in the
presence of Ag2CO3 and K3PO4 at room temperature
proceeded smoothly to give 2q as the only fluorinated
product in 59% yield (determined by 19F NMR; Eq. (1) of
Scheme 3). However, only trace 2q was detected in the
Received: December 8, 2011
Published online: January 27, 2012
Keywords: aryl boronic acids · copper catalysis ·
.
oxidative coupling · sulfur · trifluoromethylthiolation
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iolation of aryl boronic acids with TMSCF3 and S8.[16]
In summary, we have developed the first copper(I)-
catalyzed oxidative trifluoromethylthiolation of arylboronic
acid using TMSCF3 and S8 at room temperature. This reaction
provides an efficient and convenient method for the prepa-
ration of aryl trifluoromethyl thioethers. An organocopper
disulfide complex was proposed as the key intermediate in the
catalytic cycle.
Experimental Section
General procedure for oxidative trifluoromethylthiolation: In a glove
box, 4 ꢁ powdered molecular sieves (50 mg) and K3PO4 (128 mg,
0.6 mmol, 3.0 equiv) were added to a test tube equipped with
a magnetic stir bar. The vessel was sealed with a septum and flame-
dried under vacuum. The tube was cooled to room temperature and
backfilled with argon. Then CuSCN (3 mg, 0.02 mmol, 0.1 equiv),
1,10-phenanthroline (8 mg, 0.04 mmol, 0.2 equiv), S8 (20 mg,
0.6 mmol, 3.0 equiv), aryl boronic acid 1 (0.2 mmol, 1.0 equiv), and
Ag2CO3 (110 mg, 0.4 mmol, 2.0 equiv) were quickly added under a N2
atmosphere. The tube was then evacuated and backfilled with argon
gas. Freshly distilled DMF (5 mL) and TMSCF3 (150 mL, 1.0 mmol,
5.0 equiv) were then added to the reaction tube by syringe, which was
then placed under a balloon of N2 and stirred vigorously for 24 h.
Fluorobenzene (56 mL, 0.6 mmol) was added as an internal standard,
and the yield of the crude reaction was measured by 19F NMR before
workup. The reaction solution was filtered through Celite on silica
and the filter cake was washed with diethyl ether. The filtrate was
then washed with brine and concentrated. The residue was purified by
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[16] The formation of trifluoromethylthiolated arenes by the reaction
of AgSCF3 with aryl boronic acids was also ruled out; see the
Supporting Information.
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