Copper-Catalyzed Defluorinative Thioannulation of Trifluoropropynes for the Synthesis of 1,2-Dithiole-3-thiones

Article abstract of DOI:10.1002/adsc.201800846

A simple and practical strategy for the preparation of 1,2-dithiole-3-thiones via copper-catalyzed defluorinative thioannulation of trifluoropropynes has been developed using elemental sulfur as the sole sulfur source. This reaction displays a wide substrate scope and high functional group tolerance to afford the corresponding S-heterocycles in moderate to good yields and features efficient construction of multiple C?S bonds through C?F bond cleavage of CF₃ groups. (Figure presented.).

Full text of DOI:10.1002/adsc.201800846

Title: Copper-Catalyzed Defluorinative Thioannulation of
Trifluoropropynes for the Synthesis of 1,2-Dithiole-3-thiones
Authors: Feng Wei, Xiao-Qin Shen, Xiao-Hong Zhang, and Xing-Guo
Zhang
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10.1002/adsc.201800846
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Copper-Catalyzed
Defluorinative
Thioannulation
of
Trifluoropropynes for the Synthesis of 1,2-Dithiole-3-thiones
Feng Wei, Xiao-Qin Shen, Xiao-Hong Zhang and Xing-Guo Zhang*
College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, China
Tel./Fax: (+86)-577-8668-9615; e-mail: zxg@wzu.edu.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######. ((Please
delete if not appropriate))
Abstract: A simple and practical strategy for the
preparation of 1,2-dithiole-3-thiones via copper-catalyzed
defluorinative thioannulation of trifluoropropynes has been
developed using elemental sulfur as the sole sulfur source.
This reaction displays a wide substrate scope and high
functional group tolerance to afford the corresponding S-
heterocycles in moderate to good yields and features
efficient construction of multiple C–S bonds through C–F
bond cleavage of CF3 groups.
Keywords: defluorination; thioannulation; C–F bond
activation; sulfur heterocycles.
Scheme 1. Strategies for defluorination of CF3 groups.
As one of the major challenges in modern organic
chemistry,[1] the activation of carbon-fluorine bonds and
Danshensu has some effects on male infertility (Figure
1).[14] The traditional synthetic method involving multi-step
reactions of terminal alkynes or β-ketoesters with
sulfurating reagents suffers from harsh reaction conditions,
low yields and poor functional group tolerance.[15] In 2016,
Singh and co-workers reported an efficient synthesis of
subsequent transformation has profound significance for
organic synthesis, as well as the destruction of atmospheric
pollutants such as CFCs and perfluoroalkanes.
Consequently, a number of methods have been developed
for the conversion of C–F bonds,[2] most of which are
focused on hydrodefluorination[3] and the functionalization
of aryl fluorides.[4] However, the functional group
transformation through activation of three C–F bonds of
trifluoromethylated compounds is less studied.[5] Recently,
1,2-dithiole
derivatives
by
InCl3-catalyzed
heterocyclization of α-enolic dithioesters with elemental
sulfur.[16] As part of our continue interest in the synthetic
utility of fluorine and sulfur-containing molecules,[17] we
wondered if the conversion of fluoride to sulfide could be
achieved through multiple carbon-fluorine bond cleavages
of trifluoromethyl compounds. Herein, we wish to report a
Akiyama
hydrodefluorinative
and
co-workers
cyclization
developed
the
of
trifluorotoluene
derivatives for the synthesis of N-fused indoles and fused
carbocycles via a C–F bond activation and C–H bond
6]
insertion approach (Scheme 1a).[5a,
In past decades,
copper-catalyzed
defluorinative
thioannulation
of
trifluoropropynes with elemental sulfur for the efficient and
practical synthesis of 1,2-dithiole-3-thiones (Scheme 1b).
transition-metal-catalyzed cross-coupling reactions of
organic halides with sulfur-surrogates have been developed
as powerful tools for the synthesis of sulfur-containing
compounds.[7] Nevertheless, the introduction of sulfur to
organofluorine compounds through the cleavage of C–F
bonds is rarely reported [8] due to the thermodynamic and
chemical stability of fluorocarbons,[9] and thus, sulfuration
of the CF3 group remains unexplored.
1,2-Dithiole-3-thiones (D3T) are essential structural
motifs in numerous pharmaceuticals with remarkable
biological and medicinal utility, including chemotherapy,
antioxidant, and radiation protection.[10] For instance,
Oltipraz exhibits anti-HIV activity,[11] NOSH-1 shows
excellent
anti-inflammatory
effects,[12]
Anethole
dithiolethione ADT is used as cholagogue,[13] and S-
Figure 1. Representative drugs containing the D3T moiety.
1
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10.1002/adsc.201800846
Advanced Synthesis & Catalysis
Table 1. Optimization of reaction conditions.[a]
optimal ligand, and an 88% yield was obtained (entry 8).
During the testing of different bases, the reaction yields
were decreased when K2CO3 and K3PO4 were used (entries
9 and 10). The reaction did not proceed without a base,
suggesting that strong base was crucial for the process of
defluorinative thioannulation (entry 11). When the solvent
was changed to DMSO, MeCN or toluene, the reaction
afforded lower yields or trace amounts of product (entries
12-14). Lower yields were also observed when the loading
of CuBr was reduced to 10 mol% or when the reaction
temperature was reduced to 100 °C (entries 15 and 16). A
72% yield was obtained when l.2 equiv of S8 was used
(entry 17), while the desired product was not detected using
Na2S or NaHS as the sulfurating reagent (entry 18).
Entry Catalyst
Ligand
Base
Solvent Yield (%)
1
2
CuOAc
CuCl
CuBr
CuI
-
-
-
-
-
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
49
42
3
59
4
43
5
-
Trace
56
Table 2. Synthesis of 1, 2-dithiole-3-thione derivatives.[a]
6
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
1,10-Phen Cs2CO3
bipyridine Cs2CO3
TMEDA Cs2CO3
7
62
8
88
9
TMEDA
TMEDA
TMEDA
K2CO3
K3PO4
-
62
10
11
12
13
14
15[b]
16[c]
17[d]
18[e]
36
NR
65
TMEDA Cs2CO3 DMSO
TMEDA Cs2CO3 MeCN
TMEDA Cs2CO3 Toluene
23
Trace
52
TMEDA Cs2CO3
TMEDA Cs2CO3
TMEDA Cs2CO3
TMEDA Cs2CO3
DMF
DMF
DMF
DMF
63
72
ND
[a] Reaction conditions: 1a (0.2 mmol), S8 (1.5 equiv), CuBr
(20 mol %), ligand (40 mol %), base (2.0 equiv), solvent
(2 mL) at 120 °C for 12 h, isolated yield.
[b] CuBr (10 mol %).
[c] At 100 °C.
[d] S8 (1.2 equiv).
[e]
Na2S or NaHS was used as the sulfur source; NR = no
reaction; ND = not detected.
Inspired by the high efficiency of C–S bond formation
catalyzed by cuprous salts,[18] we carried out our research
by testing the reaction of (3,3,3-trifluoroprop-1-yn-1-
yl)benzene 1a[19] with sulfur in the presence of 20 mol%
CuOAc and 2 equiv of Cs2CO3 in DMF at 120 °C. We
were pleased to find that the desired product 2a was
isolated in 49% yield (Table 1, entry 1), and the structure
was confirmed by X-ray diffraction analysis. Encouraged
by these results, various copper catalysts were investigated,
and CuBr proved to be the best catalyst for delivering
product 2a in 59% yield (entry 3). However, only trace
amounts of 1,2-dithiole-3-thiones could be detected in the
absence of a catalyst (entry 5). To further enhance the
reaction yield, several N,N-bidentate ligands were
examined, such as 1,10-Phen, bipyridine and TMEDA
[a]
Reaction conditions: 1 (0.2 mmol), S8 (1.5 equiv), CuBr
(20 mol %), TMEDA (40 mol %), Cs2CO3 (2 equiv) in
DMF (2 mL) at 120 °C for 12 h, isolated yields.
[b] 15 mmol scale, 2.55g of 2a was isolated.
Under the optimal conditions, we next explored the
(entries 6-8). The results showed that TMEDA was the substrate scope of this defluorinative thioannulation by
2
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10.1002/adsc.201800846
Advanced Synthesis & Catalysis
testing the reaction with a series of aryl trifluoropropynes.
On the basis of the experimental results and the
As shown in Table 2, the results revealed that various relevant reports,[21] a plausible mechanism was proposed in
functional groups including electron-donating or electron- Scheme 3. It is well-known that S8 can undergo a
withdrawing substituents in different positions of the disproportionation reaction under alkaline conditions to
phenyl group could be well tolerated. For example, methyl produce SO32- and S2-, and S2- could further react with S8 to
or ethyl substituted phenyls afforded the 1,2-dithiole-3- give polysulfides including –S-S-.[22] Next, the substrate
thiones 2b-2f in 64%-82% yield. Even substrates 1b and 1e trifluoropropyne
1
could undergo copper-catalyzed
with a steric ortho-methyl group underwent the reaction defluorinative C–S formation with S2- to form intermediate
smoothly to give products 2b and 2e in 75% and 80% A in the presence of TMEDA and Cs2CO3. The thionyl
yields, respectively. Significantly, meta- or para- fluoride B could be formed through the F- leaving
methylthio and methoxy substituted products 2g-2i were intermediate A.[23] Then, the oxidative addition with CuBr
obtained in 52%-63% yields, especially product 2h, which and ligand exchange with S-S- provides intermediate C,
-
has been applied as a commercial prescription drug. The and the following reductive elimination delivers
para- tert-butyl, phenyl and phenoxy substituted products dithioperoxothioate D. Finally, the intramolecular
2j-2l were isolated in 42%-79% yields. Moreover, F, Cl nucleophilic addition of sulfide to the triple bond and
and Br substituents were compatible for the thioannulation protonation affords the cyclization product 2.
to give products 2m-2p in yields of 64%-83%. Pleasingly,
substrate 1q with an electron-withdrawing trifluoromethyl
group successfully afforded product 2q in 45% yield.
During the examination of other aryl groups, the results
indicated that the reaction could proceed smoothly under
the standard conditions for naphthyl or heteroaryl groups.
For instance, 1-naphthyl and 2-naphthyl provided the
products 2r and 2s with 74% and 72% yields, respectively.
As expected, the thiophen-2-yl and furan-2-yl substituted
2t and 2u were prepared successfully by this defluorinative
Scheme 3. Possible mechanism.
thioannulation, albeit in low yields of 56% and 52%.
In conclusion, we have successfully developed a
copper-catalyzed defluorinative thiocyclization for the
synthesis of 1,2-dithiole-3-thione derivatives using odorless,
nontoxic and readily available S8 as the sole sulfur source.
The reaction provides an efficient pathway for the
construction of the D3T moiety and shows a broad
substrate tolerance to give the corresponding products in
moderate to good yields. This protocol may offer a method
for the formation of C–S bonds from the cleavage of C–F
bonds, and also for the conversion of trifluoromethylated
compounds to sulfur-containing heterocyclic compounds.
In order to probe the mechanism of this defluorinative
thioannulation, the following control experiments were
conducted (Scheme 2). First, we carried out the reaction of
1a with S8 under the standard conditions by adding 2 equiv.
of 2,2,6,6-tetramethylpiperidine oxide (TEMPO), a radical
scavenger. The desired product 2a was still isolated in 84%
yield, which implied that this transformation might not
proceed through a free radical pathway. According to
previous reports,[20] we speculated that thionyl fluoride was
formed as a highly active intermediate before cyclization.
Unfortunately, we did not detect or isolate 3-phenylprop-2-
ynethionyl fluoride owing to its extreme reactivity.
Therefore, the reaction of 3-phenylpropynoyl chloride 3
with S8 was carried out under standard conditions. As
expected, the corresponding product 4 was detected by GC-
MS (eq. 2) these results partly proved our hypothesis.
Moreover, when the reaction of 1a with S8 was conducted
in anhydrous DMF and 0.1 mL D2O under standard
conditions, the deuterated product 2aa was isolated in 83%
yield, suggesting that the proton was derived from water in
the solvent (eq. 3).
Experimental Section
Typical Experimental Procedure for the Synthesis of 2a:
To a flame-dried Schlenk tube with a magnetic stirring bar
was charged with 1a (34.0 mg, 0.2 mmol), S8 (76.8 mg, 0.3
mmol), CuBr (5.7 mg, 20 mol %), TMEDA (9.3 mg, 40
mol %), Cs2CO3 (130.3 mg, 2 equiv) in DMF (2 mL) under
air atmosphere. The reaction mixture was stirred at 120 °C
for 12 hours. After the reaction was completed, the mixture
was poured into ethyl acetate, which was washed with
brine (2 x 15 mL). After the aqueous layer was extracted
with ethyl acetate, the combined organic layers were dried
over anhydrous Na2SO4 and evaporated under vacuum. The
residue was purified by flash column chromatography
(petroleum ether/ethyl acetate) to afford the desired product
o
2a (37.0 mg) as a red solid. Yield 88%; m.p. 122-124 C;
1H NMR (500 MHz, CDCl3) δ 7.57 (d, J = 7.0 Hz, 2H),
7.48-7.47 (m, 1H), 7.42-7.39 (m, 2H), 7.35 (s, 1H); 13C
NMR (125 MHz, CDCl3) δ 215.5, 172.8, 135.9, 132.1,
131.6, 129.5, 126.8; LRMS (EI 70 ev) m/z (%): 210 (M+,
79), 146 (20), 145 (100), 122 (29).
Scheme 2. Control experiments.
3
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10.1002/adsc.201800846
Advanced Synthesis & Catalysis
X-Ray structure: Supplementary crystallographic data
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Acknowledgements
We thank the National Natural Science Foundation of
China (No. 21272177) and Natural Science Foundation of
Zhejiang Province (No. LR15B020002) for financial
support.
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Advanced Synthesis & Catalysis
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5
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10.1002/adsc.201800846
Advanced Synthesis & Catalysis
COMMUNICATION
Copper-Catalyzed Defluorinative Thioannulation
of Trifluoropropynes for the Synthesis of
1,2-Dithiole-3-thiones
Adv. Synth. Catal. Year, Volume, Page – Page
Feng Wei, Xiao-Qin Shen, Xiao-Hong Zhang and
Xing-Guo Zhang*
6
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