Transition-Metal-Free Sulfuration/Annulation of Alkenes: Economical Access to Thiophenes Enabled by the Cleavage of Multiple C-H Bonds

Article abstract of DOI:10.1021/acs.orglett.8b03078

A novel, atom economical, and transition-metal-free strategy for the synthesis of thiophenes from substituted buta-1-enes with potassium sulfide has been presented. The reaction achieves double C-S bond formations via cleavage of multiple C-H bonds and provides an efficient approach to access various functionalized thiophenes. Moreover, the strategy can also be used for the synthesis of thiophenes from 1,4-diaryl-1,3-dienes. Mechanistically, DMSO plays a role of oxidant and S₃^?- in situ generated from K₂S is involved.

Full text of DOI:10.1021/acs.orglett.8b03078

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Transition-Metal-Free Sulfuration/Annulation of Alkenes:
Economical Access to Thiophenes Enabled by the Cleavage of
Multiple C−H Bonds
Liang Chen,† Hao Min,† Weilan Zeng, Xiaoming Zhu, Yun Liang,* Guobo Deng,* and Yuan Yang*
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
S
* Supporting Information
ABSTRACT: A novel, atom economical, and transition-metal-free strategy for the
synthesis of thiophenes from substituted buta-1-enes with potassium sulfide has been
presented. The reaction achieves double C−S bond formations via cleavage of
multiple C−H bonds and provides an efficient approach to access various
functionalized thiophenes. Moreover, the strategy can also be used for the synthesis
of thiophenes from 1,4-diaryl-1,3-dienes. Mechanistically, DMSO plays a role of
•−
oxidant and S3 in situ generated from K2S is involved.
Scheme 1. Synthesis of Thiophenes Enabled by Inorganic
Sulfurating Reagents
n recent decades, the sulfuration reaction of functionalized
substrates by use of inorganic sulfurating reagents has
I
emerged as a powerful tool to assemble sulfur-containing
heterocycles.1−4 In particular, the C−H bond sulfuration
reaction has attracted increasing attention in recent years due
to atom economy and the avoidance of prefunctionalization of
the starting materials, and significant progress has already been
made toward the preparation of sulfur-containing hetero-
cycles.3 However, on the one hand, such successful examples
remain scarce. On the other hand, these approaches have
focused on the sulfuration of C(sp2)−H bond and activated
C(sp3)−H bonds adjacent to the nitrogen atom or double
bond. Thus, the direct sulfuration of an inert C(sp3)−H bond
adjacent to the alkyl carbon for the construction of sulfur-
containing heterocycles is not reported and even more
challenging.
The synthesis of thiophenes is interesting to scientific
researchers4,7,8 as they represent an important class of sulfur-
containing heterocycles, which are ubiquitous core structural
motifs found in natural products, pharmaceuticals,5 and
functional materials.6 Recently, several efficient methods for
the synthesis of thiophenes have been developed through the
coupling annulation of 1,4-diiodo-1,3-dienes with potassium
sulfides (Scheme 1-1a)7 or the sulfuration/annulation of 1,3-
diynes and arylacetylenes with inorganic sulfurating reagents
such as elemental sulfur, potassium sulfide, and sodium sulfide
(Scheme 1-1b and -1c)8 by the groups of Xi, Lei, Jiang, Ji, and
so on. However, the method which employs inorganic
sulfurating reagents to prepare thiophenes via C−H bond
functionalization is urgently needed. Very recently, we have
established the C−H bond sulfuration of easily available N-
substituted arylamines for the synthesis of benzothiazoles
under transition-metal-free conditions.9 The method enabled
the cleavage of one C(sp2)−H bond and two C(sp3)−H bonds
adjacent to the nitrogen atom. Accordingly, as a part of our
ongoing interests toward developing efficient methods for
building sulfur-containing heterocycles,10 the results have led
us to investigate the possibility for synthesis of thiophenes via a
similar strategy. Fortunately, functionalized thiophenes could
be synthesized from readily available substituted buta-1-enes
via the cleavage of multiple C−H bonds, including two inert
C(sp3)−H bonds adjacent to alkyl carbon, one C(sp3)−H
bond adjacent to double bond, and one C(sp2)−H bond
(Scheme 1-2). Herein, we wish to detail our results.
We commenced with the sulfuration/annulation reaction of
1,3-diphenyl-1-butene 1a and K2S for exploring the optimal
reaction conditions (Table 1). We were pleased to find that the
Received: September 26, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03078
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Optimization of the Reaction Conditions
Scheme 2. Variation of the Substituted Butenes 1
b
entry
variation from the standard conditions
yield (%)
1
2
3
4
5
6
7
8
9
none
93
23
51
68
54
65
24
93
94
86
Na2S instead of K2S
Li2S instead of K2S
S instead of K2S
DMF instead of DMSO
NMP instead of DMSO
CH3CN instead of DMSO
nitrogen instead of air
in 120 °C, for 60 h
none
c
10
a
Reaction conditions: 1a (0.3 mmol), K2S (0.9 mmol), and DMSO (2
b
c
mL) at 140 °C for 24 h under air. Isolated yield. 1a (1 mmol).
desired product 2a could be obtained in 93% yield through
simple heating to 140 °C in DMSO under transition-metal-free
conditions. Other sulfur sources, namely Na2S, Li2S, and
elemental sulfur, were subsequently screened, but all of them
were inferior to K2S (entries 2−4). The nature of solvents was
found to affect the reaction, and the yields were dramatically
decreased by replacing DMSO with DMF, NMP, or CH3CN
(entries 5−7). Under the nitrogen atmosphere, the reaction
exhibited the identical activity (entry 8). When decreasing the
temperature to 120 °C, the 2,4-diphenylthiophene could be
given in 96% yield, but the reaction time needed to be
extended to 60 h (entry 9). After comprehensive consid-
erations on these results, the optimal conditions are as follows:
1a (0.3 mmol) and K2S (0.9 mmol) in DMSO (2 mL) at 140
°C for 24 h under air. Finally, the reaction scale up to 1 mmol
of 1a could provide 2a in 86% yield (entry 10).
After identifying the optimal reaction conditions, we set out
to investigate the substrate scope of this transition-metal-free
sulfuration/annulation protocol (Scheme 2). We initially
examined the compatibility of various functional groups on
the aromatic ring of 1,3-diaryl butenes 1b−u. Gratifyingly, the
optimal conditions were compatible with a wide range of
substituents, including electron-donating groups (Me, OMe, t-
Bu, and OCF3) and electron-withdrawing groups (F, Cl, Br,
and CF3). For most of the 1,3-diaryl butenes 1b−e and 1h−r,
the sulfuration/annulation protocol proceeded smoothly, and
the corresponding 2,4-diaryl thiophenes (2b−e and 2h−r)
were obtained in good to excellent yields. To this reaction, the
steric effect of the substituents was not obvious. For example,
the para-, meta-, and ortho-methylphenyl substituted thio-
phenes (2b, 2i, and 2j) were afforded in 85%, 92%, and 97%
yields, respectively. Importantly, 1-methyl-4-(3-methylbut-1-
en-1-yl)benzene 1v could successfully be converted into the
desired product 2v, albeit with a diminished yield. Sub-
sequently, the sulfuration/annulation protocol was applicable
to 1,4-diaryl substituted butenes 1w−z, and 2,5-diaryl
substituted thiophenes 2w−z were afforded in moderate
yields. In addition, it was noted that 2,3,5-trisubstituted
thiophenes (2aa and 2at) were synthesized in 51% and 83%
yields, respectively.
a
Reaction conditions: 1 (0.3 mmol), K2S (0.9 mmol), and DMSO (2
b
mL) at 140 °C for 24 h under air. 48 h.
a double C−H bond sulfuration process. Therefore, 1,4-
diphenyl-1,3-dibutene 3w was subjected to the sulfuration/
annulation reaction with K2S under the above standard
reaction conditions. Expectedly, 2,5-diphenylthiophene 2w
was obtained in 82% yield. Further survey revealed that the
yield of 2,5-diphenylthiophene increased to 96% when the
mixture solvent DMSO and NMP in a ratio of 1:1 was used.
Encouraged by this results, the scope of the sulfuration/
annulation reaction with regard to 1,4-diaryl-1,3-dibutenes (3x-
z and 3ac-at) was then examined (Scheme 3). An array of
substrates (3x-z and 3ac-an) with substituents on the aromatic
ring Ar2 or both of the aromatic rings Ar1 and Ar2 could
uneventfully be translated into the conrresponding products
(2x-z and 2ac-an) in moderate to excellent yields. It was
noteworthy that the steric effect of the substituent was obvious
in this reaction. For example, the para-, meta-, and ortho-
methylphenyl substituted thiophenes (2x, 2ac, and 2ad) were
afforded in 87%, 80%, and 50% yields, respectively.
Furthermore, polycyclic naphthyl, heterocyclic pyridyl, and
thienyl substituted 1,3-dibutenes 3ao-at were competent
substrates for the sulfuration/annulation reaction. Interest-
ingly, fully substituted 1,3-diaryl-4,5,6,7-tetrahydrobenzo[c]-
thiophenes 2au and 2av were also synthesized in moderate
yields.
To investigate the possible reaction mechanism, several
control experiments were carried out (Scheme 4). The reaction
of 1,3-diphenyl-1-butene (1a) and K2S detected the
production of dimethyl sulfide by GC analysis under the
standard conditions (eq 1; see the Supporting Information
(SI)). The result reveals that DMSO could play a vital role of
oxidant in the sulfuration/annulation process.9,11 The results of
the isotope experiments indicate that both solvent (DMSO)
and water could act as a hydrogen donor, which participated in
the reaction (eqs 2 and 3). Therefore, we speculated that two
types of H-abstraction, including proton-hydrogen capture8a
Based on the successful sulfuration/annulation strategy of
alkenes, we speculated that the reaction possibly underwent a
diene intermediate, which could deliver the desired product by
B
DOI: 10.1021/acs.orglett.8b03078
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Letter
a
Scheme 3. Variation of the 1,4-Diaryl-1,3-dibutaenes 3
Scheme 5. Possible Reaction Mechanism
1-butene 1a affords 1,3-diphenyl-1,3-dibutene 3a with the aid
•−
of DMSO. Subsequently, radical addition of S3 in situ
generated from K2S to 3a provides the intermediate A, which
undergoes two reaction pathways to form the intermediate C
and C′ (path A: sequential getting an electron and proton-
hydrogen capture; path B: radical-hydrogen capture). Under
the action of DMSO, the second oxidative dehydrogenation of
intermediate C and C′ delivers the intermediate D, which
undergoes intramolecular radical addition to obtain the
intermediate E and release S2•−. Finally, the thiophene 2a is
produced via sequential H-abstraction and further oxidation of
intermediate E. Furthermore, another pathway involving
a
Reaction conditions: 3 (0.3 mmol), K2S (0.9 mmol), and DMSO (2
mL) at 140 °C for 24 h under air.
Scheme 4. Control Experiment
•−
radical addition of S3 to the 4-position of 3a is not ruled
out (for the detailed mechanism, see the SI).
In conclusion, we have described a transition-metal-free
strategy for the synthesis of thiophenes from substituted buta-
1-enes and potassium sulfide. The method provides
straightforward access to diverse functionalized thiophenes in
moderate to excellent yields through cleavage of three types of
C−H bonds such as two inert C(sp3)−H bonds adjacent to
the alkyl carbon, one C(sp3)−H bond adjacent to the double
bond, and one C(sp2)−H bond. Furthermore, the method can
be applicable to 1,4-diaryl-1,3-dibutenes for assembling
functionalized thiophenes by a slight change in the optimal
reaction conditions. Further mechanistic studies and applica-
tions of this method are currently in progress in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03078.
and radical-hydrogen capture,8c occurred in the transforma-
tion.
Based on the previous reports, the S3•− species, which can be
produced by the reaction of elemental sulfur and base or
inorganic sulfides in DMF or DMA, was initially discovered by
Chivers and Shields.12 Recently, Lei and Ji et al. developed
several methods involving S3•−species for the synthesis of
sulfur-containing heterocycles.2d,8a−c Herein, our electron
paramagnetic resonance (EPR) study reveals that this reaction
possibly involved S3•− in situ generated from K2S (see the SI).
Therefore, a hypothetical reaction mechanism involving
S3•−species for the synthesis of thiophenes was proposed
based on the reported literature and our experimental results
(Scheme 5). First, oxidative dehydrogenation of 1,3-diphenyl-
Experimental procedures, full characterization of prod-
ucts, and NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: yliang@hunnu.edu.cn.
*E-mail: gbdeng@hunnu.edu.cn.
*E-mail: yangyuan071@126.com.
ORCID
Yun Liang: 0000-0002-2550-9220
C
DOI: 10.1021/acs.orglett.8b03078
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Organic Letters
Letter
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Guobo Deng: 0000-0002-5470-5706
Yuan Yang: 0000-0002-9061-1758
Author Contributions
†L.C. and H.M. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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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