An air-stable copper reagent for nucleophilic trifluoromethylthiolation of aryl halides

Article abstract of DOI:10.1002/anie.201208432

A series of copper(I) trifluoromethyl thiolate complexes have been synthesized from the reaction of CuF₂ with Me₃SiCF ₃ and S₈ (see scheme; Cu red, F green, N blue, S yellow). These air-stable complexes serve as reagents for the efficient conversion of a wide range of aryl halides into the corresponding aryl trifluoromethyl thioethers in excellent yields. Copyright

Full text of DOI:10.1002/anie.201208432

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DOI: 10.1002/anie.201208432
Trifluoromethylthiolation
An Air-Stable Copper Reagent for Nucleophilic
Trifluoromethylthiolation of Aryl Halides**
Zhiqiang Weng,* Weiming He, Chaohuang Chen, Richmond Lee, Davin Tan, Zhiping Lai,
Dedao Kong, Yaofeng Yuan, and Kuo-Wei Huang*
Compounds that contain a trifluoromethylthio group (–SCF3)
are found in many pharmaceutical and agrochemical prod-
ucts, such as Tiflorex and Vaniliprole.[1] Owing to their high
lipophilicity and hydrophobicity, aryl trifluoromethylthio-
ethers (ArSCF3) have attracted increasing attention from
synthetic chemists. There have been considerable efforts in
both academic and industrial laboratories towards achieving
straightforward, efficient, high-yielded, and economical prep-
aration of these molecules. A number of methods are now
available for synthesizing ArSCF3 compounds; examples
include the nucleophilic and radical trifluoromethylation of
aryl sulfides, disulfides, sulfenyl chlorides and thiocyanates[2]
and the reaction of trifluoromethylthiolate with aryl halides.[3]
Togni and co-workers have made major progress in the field
of electrophilic trifluoromethylations of thiolate nucleo-
philes.[4] However, these transformations are encumbered by
significant limitations, for example, the requirement of high
reaction temperatures, the use of expensive and/or toxic
reagents, and the requirement of electron-poor aromatic
groups.
One approach to address these drawbacks would be the
development of metal-mediated or metal-catalyzed trifluor-
omethylthiolation to produce aryl trifluoromethylthioethers.
Although there has been substantial progress in the metal-
mediated or metal-catalyzed trifluoromethylation of aromatic
substituents in recent years,[5–7] fewer studies on trifluorome-
thylthiolation has been reported. Chen et al described a direct
trifluoromethylthiolation of aryl halides by using methyldi-
fluoro(fluorosulfonyl)acetate and elemental sulfur, and medi-
ated by cuprous iodide.[8] Buchwald and co-workers recently
developed Pd-catalyzed reactions of aryl bromides to form
trifluoromethylthioethers with AgSCF3 as the nucleophile.[9]
Notably, Vicic and Zhang published the trifluoromethyl-
thiolation of aryl iodides and aryl bromides with the thermal
sensitive reagent [NMe4][SCF3], catalyzed by a nickel bipyr-
idine complex at room temperature.[10] Most recently, the
Qing and the Vicic groups independently reported Cu-
catalyzed oxidative trifluoromethylthiolation of aryl boronic
acids with Me3SiCF3 and S8[11] or with [NMe4][SCF3].[10b]
However, the use of an efficient and easily available
organometallic reagent for the direct conversion of bromo- or
iodoarenes and heteroarenes into the corresponding
trifluoromethylthioethers would be a simple alternative to
these methods. Such a reagent should be inexpensive to
prepare, and should be stable enough to be stored for an
extended period. Based on the extensive studies into Cu-
[12]
À
mediated C S bond formation reactions, copper trifluoro-
methylthiolate (CuSCF3) is considered to be a promising
reagent for the trifluoromethylthiolation of arenes.[3b,13]
Unfortunately, the practical application of this compound
has not been extensively explored presumably because of its
ill-defined nature. Herein, we report a convenient procedure
for the preparation of a series of air-stable copper reagents
and demonstrate their application for the efficient nucleo-
philic trifluoromethylthiolation of aryl halides.
In our recent investigations into trifluoromethylation[14]
we have demonstrated the cooperative effect of silver in the
copper-catalyzed trifluoromethylation of aryl iodides with
Me3SiCF3. Isolated trifluoromethyl copper complexes con-
taining chelating nitrogen ligands were also found to react
smoothly with aryl halides to produce trifluoromethylated
arenes.[6f] These results, together with our interest in the C S
À
bond formation[15] led us to explore the trifluoromethylth-
iolation of aryl halides by a ligated copper trifluoromethyl-
thiolate species generated in situ from a mixture of simple
copper salts, elemental sulfur, a trifluoromethyl source, and
an electron-donating ligand.
We initially focused on the synthesis of copper(I)
trifluoromethylthiolate complexes. Gratifyingly, the reactions
of CuIIF2 with the Ruppertꢀs reagent (Me3SiCF3) and S8 in the
presence of diimine ligands in CH3CN afforded monomeric,
copper(I) trifluoromethylthiolate complexes ligated by che-
lating nitrogen ligands [(bpy)CuI(SCF3)] (1a), [(dmbpy)CuI-
(SCF3)] (1b), and [(dtbpy)CuI(SCF3)] (1c), and dimeric
complexes [{(phen)CuI(SCF3)}2] (1d) and [{(Me2phen)CuI-
(SCF3)}2] (1e), in 42–49% yields based on Cu (Scheme 1).
CF3H, Me3SiF, and (CF3)2S were observed as major side
products. In the solid state, all of these complexes are stable in
air for several days; in solution, all of the compounds are
unchanged after several hours in air.
[*] Prof. Dr. Z. Weng, W. He, C. Chen, D. Kong, Prof. Dr. Y. Yuan
Department of Chemistry and Fujian Provincial Key Laboratory of
Photocatalysis—State Key Laboratory Breeding Base
Fuzhou University, Fuzhou 350002 (China)
E-mail: zweng@fzu.edu.cn
R. Lee, D. Tan, Prof. Dr. Z. Lai, Prof. Dr. K.-W. Huang
Division of Physical Science and Engineering
King Abdullah University of Science and Technology (KAUST)
Thuwal 23955-6900 (Saudi Arabia)
E-mail: hkw@kaust.edu.sa
[**] We thank Prof. John Hartwig for useful discussions. The financial
support from National Natural Science Foundation of China
(21072030) and Fuzhou University (022318) to Z.W. and KAUST to
K.-W.H. are acknowledged.
The molecular structures of 1a, and 1c–e have been
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208432.
determined by the single crystal X-ray crystallography.[16] The
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Chemie
action (for comparison, the Cu–Cu distance in the copper
metal lattice is equal to 2.56 ꢁ).[18] This Cu–Cu distance is
marginally shorter than the values found in closely related CuI
thiolato
complex
[{Cu(SC6H4CH3-o)(1,10-phenanthro-
line)}2]·CH3CN (2.613(3) ꢁ).[19] The Cu S bond distances of
2.3303(10) ꢁ in 1d are significantly longer than that in the
monomeric complex 1a. The Cu–N distances of 2.0617(19) ꢁ
in 1d are comparable with those found in 1a. The N-Cu-N
angle (81.84(10)8) in 1d is larger than those in 1a (78.69(8)8)
and in (bathophenanthroline)CuCF3 (79.38).[14a]
À
Density functional theory (DFT) calculations based on
the B3LYP functional were performed to analyze the
dimerization behavior of complexes 1c and 1d (see the
Supporting Information for details). The energies of the gas-
phase optimized structures of the copper trimethylthiolate
dimers bearing dtbpy and phen ligands were compared, and it
was observed that the dimerization process for the
[{(phen)Cu(SCF3)}2] complex was exergonic and thermo-
dynamically favorable (DG = À2.8 kcalmolÀ1), whereas that
for the [{(dtbpy)Cu(SCF3)}2] complex was endergonic and not
favored (DG = 2.9 kcalmolÀ1). These data are consistent with
the experimental observations.
Scheme 1. Synthesis of CuSCF3 complexes 1a–1e. bpy=bipyridine,
dmbpy=4,4’-di-methylbipyridine, dtbpy=4,4’-di-tert-butylbipyridine,
phen=1,10-phenanthroline, Me2phen=neocuproine.
ORTEP diagrams of 1a and 1d are shown in Figure 1.
Complexes 1a and 1c possess a three-coordinated trigonal
planar geometry around the copper atom. The metal center is
bound by one neutral bidentate bipyridine ligand and one
À
anionic SCF3 group. The Cu S bond distance of 2.2729(6) ꢁ
After isolation and characterization of copper(I)
trifluoromethylthiolate complexes 1a–e, we evaluated the
reactivity of these complexes for the trifluoromethylthiola-
tion of aryl halides. Initially, 4-iodotoluene was chosen as
model substrate, and various solvents, temperatures, and
reaction times were evaluated to optimize the trifluorome-
thylthiolation conditions (Table 1).
Our results showed that nature of the nitrogen ligands
strongly affected the formation of trifluoromethylthiolation
products. For example, we found that treatment of 1a, ligated
by bipyridine, with 1.2 equivalents of 4-iodotoluene in
CH3CN at 1108C for 15 h afforded 4-(trifluoromethylthio)-
toluene in 84% yield (Table 1, entry 1). Under similar
reaction conditions, complex 1b ligated by 4,4’-dimethylbi-
pyridine gave the desired product in a comparable yield
(Table 1, entry 2). 1c containing 4,4’-di-tert-butylbipyridine
showed excellent activities and afforded 3a in 97% yield
(Table 1, entry 3). However, the reactions of 1d and 1e, which
are ligated by 1,10-phenanthroline and neocuproine, respec-
tively, resulted in much lower yields (Table 1, entries 4 and 5).
These results suggested that bipyridine was the most effective
ligand for the trifluoromethylthiolation, presumably owing to
its preference to maintain the monomeric form, which is
Figure 1. ORTEP diagram of 1a (top) and 1d (bottom). The thermal
ellipsoids are set at 40% probability and hydrogen atoms are omitted
for clarity.
in 1a is comparable with those found in the structures of
acetonitrile-solvated copper(I) trifluoromethylthiolate com-
À
plexes (CF3SCu)10(CH3CN)8 (the average of the Cu S bond
distances in the three-coordinate environment is 2.23 ꢁ) and
the monomeric alkyne-stabilized Cu(SCF3) complex
(2.272(2) ꢁ).[17] The average Cu N bond distance of
À
2.0855(2) ꢁ is also close to the values seen in the previously
reported analogous complex [(bathophenanthroline)CuCF3]
(2.062 ꢁ).[14a] Compounds 1d and 1e form dimeric complexes.
The two Cu atoms are bridged by the S atoms of the
trifluoromethylthiolate anions forming a planar Cu2S2 ring. In
1d, the Cu–Cu separation inside the dimer is 2.5781(9) ꢁ.
This distance suggests the presence of a metal–metal inter-
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Table 1: Optimization of trifluoromethylthiolation of 4-iodotoluene with
1a–e.[a]
Entry
[Cu]
Solvent
t [h]
T [8C]
Yield [%][b]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1a
1a
1a
1a
1a
1a
1a
1a
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
toluene
THF
DMSO
CH2Cl2
DMF
15
15
15
15
15
15
15
15
15
15
15
7
110
110
110
110
110
150
90
150
60
150
150
110
90
84
88
97
53
18
69
21
3
8
4
8
51
55
9
10
11
12
13
NMP
CH3CN
CH3CN
15
[a] Reaction conditions: [Cu] (0.15 mmol), 2a (0.15 mmol), solvent
(1.0 mL), under N2 atmosphere. [b] Yields were determined by the
19F NMR analysis of the crude reaction mixture with (trifluorometh-
oxy)benzene as an internal standard.
active towards the cross coupling reactions, as observed in the
X-ray crystallographic and DFT analyses.
Scheme 2. Scope of trifluoromethylthiolation of aryl and heteroaryl
halides by 1a. Reaction conditions: 1a (0.25 mmol), aryl halides
(0.25 mmol), 15 h, N2, 1108C. Yields of isolated products are shown.
We next examined the effect of different solvents on the
reaction. Reactions performed in toluene or THF proceeded
less efficiently (Table 1, entries 6 and 7) than those in CH3CN.
Reactions in other solvents, such as DMSO, CH2Cl2, DMF, or
NMP, occurred very slowly (Table 1, entries 8–11). Thus,
CH3CN was identified as the best solvent for the reaction. To
further elucidate the factors that affect the reaction rate, the
reaction temperature and time were also evaluated. The yield
of product decreased from 84% to 51% when the reaction
time was shortened from 15 to 7 h (Table 1, entry 12).
Furthermore, a reaction temperature of 1108C was essential
for efficient trifluoromethylthiolation. The reaction con-
ducted at the lower temperature of 908C resulted in
substantially lower product yields (55%; Table 1, entry 13).
Having established this trifluoromethylthiolation method,
we explored the scope of the reactions with various aryl and
heteroaryl halides (Scheme 2). Complex 1a was selected as
the trifluoromethylthiolation reagent and the reactions of
various electronically and structurally diverse aryl and
heteroaromatic halides were evaluated. The reaction pro-
ceeded smoothly with neutral alkyl- and aryl-substituted
aromatic iodides to give the corresponding products 3a–e in
good to excellent yields (76–90%). The reactions para- and
ortho-acetyl substituted iodoarenes also gave good results,
affording the desired products 3 f and 3g in 84% and 82%,
respectively. Reactions of iodoarenes bearing electron-defi-
cient groups, as well as those containing electron-donating
moieties, give products 3h–l and 3n in moderate to good
yields (57–81%). It is noteworthy that the chlorine function-
ality on the aryl iodide was tolerated and produced 3m in
75% yield. Heteroaryl iodides and bromides also reacted to
afford the products 3o–q, again in good yields (71–84%).
As these reactions started with a well-defined CuSCF3
complex, they provided a model for the plausible reaction
pathways to be evaluated by DFT to increase our mechanistic
À
understanding of the C SCF3 bond formation process (see
the Supporting Information for details). Four reaction
mechanisms were considered: oxidative addition/reductive
elimination (OA/RE), single electron transfer (SET), iodine
atom transfer (IAT), and s-bond metathesis (s-bond).[20]
À
Although the mechanism for the Cu-mediated C S bond
cross-coupling reaction is still controversial, the computed
energies for the proposed transition states and intermediates
with solvent effect favored the OA/RE process, supporting
the cycle containing CuI/CuIII species (Scheme 3).[21]
In conclusion, we have prepared a series of copper(I)
trifluoromethylthiolate complexes ligated by bipyridine
ligands. These complexes react with a wide range of aryl
and heteroaryl halides to produce aryl trifluoromethyl
thioethers in modest to excellent yields. The bipyridine
ligated complexes were showed to be the most effective for
the trifluoromethylthiolation. These reactions provide a novel
and convenient approach for trifluoromethylthiolation of aryl
halides starting from a well-defined, readily available copper
reagent prepared from inexpensive materials. Computational
studies suggest that the reaction may proceed through a CuIII
intermediate in an OA/RE pathway.
Experimental Section
Synthesis of [(bpy)CuSCF3] (1a): In a glovebox, CuF2 (714 mg,
7.0 mmol), S8 (224 mg, 7.0 mmol), and 30 mL CH3CN were added to
an oven-dried resealable Schlenk tube possessing a Teflon screw
valve. CF3SiMe3 (2.98 g, 21.0 mmol) was added into this tube and the
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Scheme 3. Four plausible pathways leading to the trifluoromethylthiolated product are shown.
Reported values (DGsol or DG°sol) are the free energies relative to starting materials with
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tube was sealed. The reaction mixture was stirred in a preheated oil
bath at 808C for 10 h. The reaction mixture was then allowed to cool
to room temperature and filtered through Celite. The volatiles were
removed under reduced pressure and the resulting dark-brown solid
was washed with Et2O (3 ꢂ 15 mL). The solid was redissolved in 6 mL
of CH3CN and 2,2’-bipyridine (1.09 g, 7.0 mmol) in 15 mL of Et2O
was carefully added to this solution. The resulting solution was then
kept at À258C for 24 h. The resulting red crystals were washed with
diethyl ether (2 ꢂ 5 mL), and dried to give 984 mg of the desired
product 1a (44% yield). 1H NMR (400 MHz, CD2Cl2): d = 8.80 (s,
2H), 8.30 (d, J = 7.6 Hz, 2H), 7.99 (td, J = 7.9, 1.6 Hz, 2H), 7.60–
7.46 ppm (m, 2H); 19F NMR (376 MHz, CD2Cl2): d = À20.96 ppm (s,
3F); 13C NMR (101 MHz, CD2Cl2): d = 153.3 (s), 149.8 (s), 137.9 (s),
125.2 (s), 121.0 ppm (s), SCF3 was not observed. Elemental analysis
calcd (%) for C11H8CuF3N2S: C, 41.18; H, 2.51; N, 8.73; found: C,
40.98; H, 2.64; N, 8.87.
General Procedure for Trifluoromethylthiolation of Aryl Halides
by complex [(bpy)CuSCF3] (1a): 1a (80 mg, 0.25 mmol), the starting
aryl halides (0.25 mmol), and CH3CN (1.2 mL) were added to
a flame-dried 5 mL test tube with Teflon screw cap equipped with
a magnetic stir bar. The tube was sealed and then placed into
a preheated 1108C oil bath for 15 h. The tube was removed from the
oil bath and allowed to cool. The reaction mixture was then filtered
through SiO2, eluted with diethyl ether, washed with brine, and
concentrated in vacuo. The residue was purified by silica gel column
chromatography with pentane.
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Received: October 19, 2012
Published online: December 13, 2012
À
Keywords: copper · cross-coupling · C S bonds ·
trifluoromethylthiolation
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