Copper-catalyzed synthesis of 2-acylbenzo[: B] thiophenes from 3-(2-iodophenyl)-1-arylpropan-1-ones and potassium sulfide under aerobic conditions

Article abstract of DOI:10.1039/c8ob02315h

A method was developed for the synthesis of 2-acylbenzo[b]thiophenes via a copper-catalyzed sulfuration of 3-(2-iodophenyl)-1-arylpropan-1-ones with K₂S under aerobic conditions. Mechanistically, this procedure was proved to involve the formati

Full text of DOI:10.1039/c8ob02315h

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Biomolecular Chemistry
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Copper-catalyzed synthesis of 2-acylbenzo[b]
thiophenes from 3-(2-iodophenyl)-1-arylpropan-
1-ones and potassium sulfide under aerobic
conditions†
Cite this: Org. Biomol. Chem., 2018,
16, 8020
Received 19th September 2018,
Accepted 11th October 2018
DOI: 10.1039/c8ob02315h
Zhilei Zheng, Liang Chen, Cheng Qian, Xiaoming Zhu, Yuzhong Yang, Jianbing Liu,*
rsc.li/obc
Yuan Yang * and Yun Liang
*
A method was developed for the synthesis of 2-acylbenzo[b]thio-
2-Acylbenzo[b]thiophenes, as an important derivatives of
phenes via a copper-catalyzed sulfuration of 3-(2-iodophenyl)-1- benzo[b]thiophenes, show their good biological activities, such as
arylpropan-1-ones with K2S under aerobic conditions. antimitotic,9 antitrypanosomal,10 anti-inflammatory activities.11
Mechanistically, this procedure was proved to involve the for- Thus, developing efficient methods for the synthesis of 2-acyl-
mation of a dihydrobenzo[b]thiophene intermediate.
benzo[b]thiophenes is of great value. Generally, the synthesis
of 2-acylbenzo[b]thiophenes by the transition metal-catalyzed
cross-coupling reaction from benzo[b]thiophene-2-yl-boronic
acids was developed.12 However, these methods were limited
by the difficult preparation of starting materials and use of
expensive metal catalysts. Recently, Nguyen developed an
unusual DIPEA-promoted reaction of 2-nitrochalcones with
elemental sulfur for the synthesis of 2-benzoylbenzothio-
phenes. Sekar and coworkers reported an efficient synthetic
method of 2-acylbenzo[b]thiophenes via Cu-catalyzed C(sp2)–H
functionalization of 2-halochalcones.7c Both of them achieved
the double thiolation reaction of 2-substituted chalcones
between C(sp2) and C(sp2) atoms. As part of our ongoing
research towards the copper-catalyzed double thiolation reac-
tion via the cleavage of the C–X bond and C(sp3)–H
Benzo[b]thiophenes are one of the most important classes of
sulfur-containing heterocyclic compounds, which constitute
the key structures of many natural products, pharmaceutical
drugs, and functional materials.1–3 Therefore, a number of
efficient strategies have been devised for the synthesis of
benzo[b]thiophenes via the formation of C–S bonds.4–7 In par-
ticular, inorganic sulfides as a readily available, stable, and
non-toxic sulfur source were used for the synthesis of benzo[b]
thiophenes via double thiolation, which has become a hot
research field for chemists.5–7 Most of these representative
approaches have focused on: (a) the coupling cyclization reac-
tion of o-halo alkynylbenzenes (Scheme 1, eqn (1)),6 and (b)
the coupling cyclization reaction of o-halo alkenylbenzenes
(Scheme 1, eqn (2)).7 These methods have realized double thio-
lation reactions of inorganic sulfides between C(sp2) and C(sp)
atoms and between C(sp2) and C(sp2) atoms, which lead to the
efficient synthesis of benzothiophenes. To the best of our
knowledge, double thiolation reactions of inorganic sulfides
between C(sp2) and C(sp3) atoms for the synthesis of benzo[b]
thiophenes have been rarely reported.8 Accordingly, the syn-
thesis of benzo[b]thiophenes via the construction of double
C–S bonds between C(sp2) and C(sp3) atoms represents a sig-
nificantly bigger challenge (Scheme 1, eqn (3)).
National & Local Joint Engineering Laboratory for New Petro-chemical Materials and
Fine Utilization of Resources, Key Laboratory of Chemical Biology and Traditional
Chinese Medicine Research, Ministry of Education, Key Laboratory of the Assembly
and Application of Organic Functional Molecules of Hunan Province, Hunan Normal
University, Changsha, Hunan 410081, China. E-mail: 1485192973@qq.com,
yangyuan071@126.com, yliang@hunnu.edu.cn
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8ob02315h
Scheme 1 Synthesis of benzo[b]thiophenes.
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and CuI was found to be the best catalyst, and could give 92%
yield of 2a. Continuously, the ligands, including TEMED,
DMEDA, and 1,10-phenanthroline, were examined. However,
these ligands could not promote the coupling cyclization reac-
tion (entries 9–11). Subsequently, solvents, such as DMSO,
NMP, and CH3CN, were also evaluated, and the yields of 2a
decreased slightly (entries 12–14). To improve the efficiency of
the coupling cyclization reaction, different sulfur sources, such
as S8, Na2S, Li2S, and NaS2O3, were screened (entries 15–18).
The results indicated that K2S was the best sulfur source for
this reaction. Finally, it was found that the lower temperature
led to a decrease in the yields (entry 19). The yield of 2a
showed no significant change when the catalyst loading of CuI
reduced to 10 mol% (91% yield, entry 20). Thus, the optimized
reaction conditions are as follows: 1a (0.2 mmol), K2S
(0.6 mmol), and CuI (10 mol%), in DMF (2 mL) under air
atmosphere at 130 °C.
With the optimal reaction conditions for the synthesis of
2-acylbenzo[b]thiophenes in hand, the substrate scope was
investigated (Scheme 3). Initially, under the optimal con-
ditions, an array of substituents on the aryl ring of the 3-(2-
iodophenyl)-1-arylpropan-1-ones were screened. The results
demonstrated that substrates 1b–1h bearing electron-donating
groups (Me, OMe, and NH2) and electron-withdrawing groups
(F and Cl) could be smoothly transformed into the desired pro-
ducts 2b–2h. The steric effect of the methyl and methoxyl
groups at the para- or ortho-position of the phenyl group did
not obviously affect the yield of the product (2b–2e). Notably,
amino substituted 3-(2-iodophenyl)-1-arylpropan-1-one was a
competent reaction partner in this transformation, leading to
2f in 70% yield. Halo-substituted 1g and 1h survived well, and
the corresponding products were obtained in good yields.
Moreover, when the aryl group of 3-(2-iodophenyl)-1-arylpro-
pan-1-ones was displaced with 1-naphthyl, 2-naphthyl, and
4-biphenyl groups, the desired products 2i–2k were obtained
in 94%, 88%, and 87% yields, respectively. To our delight,
3-acylfuran, 3-acylthiophene, 3-acylpyrrole, and 3-acylindole
derived 3-(2-iodophenyl)-1-arylpropan-1-ones could be success-
fully converted into the corresponding heterocycle substituted
products 2l–2o in excellent yields.
Scheme 2 Synthesis of 2-acylbenzo[b]thiophenes.
functionalization,13 this synthetic strategy would be applied to
the synthesis of benzo[b]thiophenes. Herein, we report a
copper-catalyzed synthesis of 2-acylbenzo[b]thiophenes from
the C(sp3)–H functionalization of 3-(2-iodophenyl)-1-arylpro-
pan-1-one (Scheme 2).
To validate our supposition, the reaction of 3-(2-iodophe-
nyl)-1-phenylpropan-1-one 1a and potassium sulfide was
chosen as a model reaction for screening the optimized con-
ditions (Table 1). First of all, we isolated 2a in 43% yield by
simply heating a DMF solution of 1a and potassium sulfide
(130 °C) in the presence of Cu(OAc)2 as the catalyst under a
nitrogen atmosphere (entry 1). When the reaction atmosphere
was replaced with oxygen and air, the desired product 2a was
afforded in 61% and 88% yields, respectively (entries 2 and 3).
Encouraged by these results, a series of copper salts (CuI,
CuBr, CuCl, CuBr2, and CuCl2) were evaluated (entries 4–8),
Table 1 Optimization of reaction conditionsa
Entry
Catalyst
Ligand
[S]
Solvent
Yieldb (%)
1c
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
CuI
CuBr
CuCl
CuBr2
CuCl2
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
—
—
—
—
—
—
—
—
K2S
K2S
K2S
K2S
K2S
K2S
K2S
K2S
K2S
K2S
K2S
K2S
K2S
K2S
S
Na2S
Li2S
Na2S2O3
K2S
K2S
K2S
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
NMP
CH3CN
DMF
DMF
DMF
DMF
DMF
DMF
DMF
43
61
88
92
89
90
89
88
92
86
88
79
78
88
Trace
Trace
21
2d
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18e
19e
20f
21g
In addition, 3-(2-iodophenyl)-1-alkylpropan-1-ones could
react with K2S under the standard conditions, and 2-acetyl-
benzo[b]thiophenes (2p), 2-valerylbenzo[b]thiophenes (2q) and
2-cyclohexanecarbonylbenzo[b]thiophenes (2r) were afforded
in moderate to good yields. It is noteworthy that 3-methyl-2-
benzoyl benzo[b]thiophene could be isolated in 78% yield.
Finally, both the electron-deficient group (F and Cl) and the
electron-rich group (OMe and Me) on the aromatic ring of the
iodobenzene moiety were well-tolerated under the reaction
conditions, and the corresponding products 2t–2w were gener-
ated in good to excellent yields. These results indicated that
the electronic effect of the substituents on the iodobenzene
moiety did not play a significant role in regulating the reac-
tion, and revealed the inherent high reactivity of 2-iodophenyl.
Importantly, 3-(2-bromophenyl)-1-phenylpropan-1-one could
also react with K2S to give 2a in 62% yield.
TEMED
DMEDA
1,10-Phen
—
—
—
—
—
—
—
—
—
—
57
86
91
86
a Reaction conditions: 1a (0.2 mmol), K2S (0.6 mmol), catalyst
(20 mol%), ligand (40 mol%), and DMF (2 mL) under air atmosphere
in a sealed Schlenk tube, at 130 °C for 12 h. b Isolated yields.
c Under N2 atmosphere. d Under O2 atmosphere. e At 100 °C. f CuI
(10 mol%). g CuI (5 mol%).
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Scheme 4 Synthesis of 11H-benzo[4,5]thieno[3,2-b]thiochromen-11-
one.
Scheme 5 Control experiments.
propan-1-one 1a with K2S was decreased to 80 °C, 40% of (2,3-
dihydrobenzo[b]thiophen-2-yl)(phenyl)methanone 3a was iso-
lated (eqn (1)). To our delight, 3a can be translated into the
desired product in 99% yield under air conditions without CuI
and K2S (eqn (2)). These results indicated that the (2,3-dihydro-
benzo[b]thiophen-2-yl)(phenyl)methanone 3a should be the
key intermediate in the reaction of 3-(2-iodophenyl)-1-phenyl-
propan-1-one 1a with potassium sulfide. In addition, when the
radical inhibitor TEMPO was added to the model reaction
(eqn (3)), it was found that the yield of 2a was only slightly
decreased, which proved that this reaction could not undergo
a radical process. Finally, the deuteration experiments indi-
Scheme 3 Synthesis of 2-acylbenzo[b]thiophenes. Reaction con-
ditions: 1a (0.2 mmol), K2S (0.6 mmol), CuI (10 mol%), and DMF (2 mL) in
a sealed Schlenk tube, at 130 °C for 12 h. Isolated yields. a 3-(2-
Bromophenyl)-1-phenylpropan-1-one in place of 3-(2-iodophenyl)-1-
phenylpropan-1-one.
Notably, this approach was applicable to the construction cated that 2-iodochalcone 4 should not be the intermediate
of double sulfur-heterocyclic rings via double C(sp3)–H (eqn (4) and (5)).
functionalization in a step reaction, which provided an
From the above experimental results and previous
efficient route for the assembly of polycyclic sulfur-containing reports,14 a possible mechanism for the formation of 2a was
heterocycles. For example, 1,3-bis(2-iodophenyl)propan-1-one proposed as shown in Scheme 6. First, intermediate A is
1x could efficiently react with K2S in the presence of CuI afforded from the copper-catalyzed traditional coupling reac-
(20 mol%) as the catalyst at 130 °C, and the desired product tion of 3-(2-iodophenyl)-1-phenylpropan-1-one 1a with K2S,
11H-benzo[4,5]thieno[3,2-b]thiochromen-11-one
obtained in 93% yield (Scheme 4).
2x
was which is transformed into enolate B by base-promoting depro-
tonation. Then, enolate B undergoes the copper-catalyzed oxi-
To gain insight into the mechanism of the reaction, two dation reaction and gives intermediate C. Subsequently, 2,3-
control experiments were performed, as shown in Scheme 5. dihydrobenzo[b]thiophene D is formed via the intramolecular
When the reaction temperature of 3-(2-iodophenyl)-1-phenyl- electrophilic addition of intermediate C. Finally, an oxidative
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dehydrogenation of intermediate D gives the desired product
benzo[b]thiophene 2a.
In conclusion, we have developed an efficient approach for
the construction of double C–S bonds via a copper-catalyzed
coupling reaction of the C–X bond and functionalization of
the C(sp3)–H bond using K2S as the sulfur source. In addition,
the reaction has been proved to tolerate a wide variety of func-
tional groups, and the corresponding products are obtained in
good yields. This finding opens a new method which is the
functionalization of the C(sp3)–H bonds applied to the incor-
poration of sulfur into organic frameworks. Further appli-
cations of this method are currently underway.
Conflicts of interest
The authors declare no competing financial interest.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21572051 and 21602057), the Opening
Fund of Key Laboratory of Chemical Biology and Traditional
Chinese Medicine Research (Ministry of Education of China),
Hunan
Normal
University
(KLCBTCMR201707
and
KLCBTCMR201708), and Science and Technology Planning
Project of Hunan Province (2018TP1017).
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