Synthesis of Polysubstituted Thiophenes via Base-Induced [2+2+1] Cycloaddition Reaction of Alkynes and Elemental Sulfur

Article abstract of DOI:10.1002/adsc.201500422

A simple approach to polysubstituted thiophene derivatives is demonstrated. The synthesis is highlighted by the direct [2+2+1] cycloaddition reaction to construct carbon-sulfur and carbon-carbon bonds, without adding oxidizing, reducing and noble metal ag

Full text of DOI:10.1002/adsc.201500422

UPDATES
DOI: 10.1002/adsc.201500422
Synthesis of Polysubstituted Thiophenes via Base-Induced
2+2+1]Cycloaddition Reaction of Alkynes and Elemental Sulfur
[
a,
a
b,
Weibing Liu, * Cui Chen, and Hailing Liu *
a
College of Chemical Engineering, Guangdong University of Petrochemical Technology, 2 Guandu Road, Maoming
525000, Peopleꢀs Republic of China
Fax: (+86)-668-292-3575; phone: (+86)-668-292-3956; e-mail: lwb409@aliyun.com
College Analytical & Testing Centre, Beijing Normal University, No. 19 Xinjiekouwai St., Haidian District, Beijing
b
100875, Peopleꢀs Republic of China
E-mail: liuhailing@bnu.edu.cn
Received: July 11, 2015; Revised: August 27, 2015; Published online: December 4, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500422.
Abstract: A simple approach to polysubstituted
thiophene derivatives is demonstrated. The synthe-
sis is highlighted by the direct [2+2+1]cycloaddi-
tion reaction to construct carbon-sulfur and carbon-
carbon bonds, without adding oxidizing, reducing
and noble metal agents. All tested substrates pro-
vided their corresponding thiophene products in
high isolated yields under the optimized conditions.
ed thiophene derivatives (Scheme 1). Thiophene was
first discovered by Victor Meyer in coal-tar light oil
[3]
in 1882. This heterocyclic moiety is one of the privi-
[4]
leged scaffolds in chemistry and is widely found in
[5]
a variety of biologically active and pharmaceutically
useful compounds, such as plavix, spiriva, raloxifene,
[6]
zileuton, and clopidogrel. Furthermore, thiophene
derivatives also find broad applications in the design
and synthesis of novel organic functional materials
due to their structural rigidity and specific electronic
properties, such as organic semiconductors, organic
solar cells, liquid crystals, field effect transistors, mo-
Keywords: alkynes; carbon-sulfur bond formation;
[
2+2+1]-cycloaddition; elemental sulfur; thio-
phenes
[7]
lecular wires etc. In view of their applications in var-
ious fields, the development of new and efficient
methods for the synthesis of polysubstituted thio-
phenes has received considerable attention and nu-
In recent years, the use of elemental sulfur in organic merous synthetic routes have been reported in recent
[8]
synthesis has received increasing attention for the years. The general synthetic methods for thiophene
construction of CÀS bonds due to its low toxicity, derivatives involve either direct functionalization of
[9]
ready availability, easy handling, lack of odour and the preconstructed thiophene ring or the construc-
yielding no metal waste. A number of unique synthet- tion of the thiophene ring via ring closure of appro-
[10]
ic applications of elemental sulfur have been report- priately substituted precursors. The latter approach
[
1]
ed. The Nguyen group reported the synthesis of 2- is apparently more versatile and thus represents an at-
[
1d]
hetarylbenzothiazoles in 2013
and 2-aroylbenzo- tractive but less well developed methodology. Among
[
1k]
[11] [12] [13]
thiazoles in 2015 via redox reactions. Lei reported others, Paal–Knorr, Fiesselmann, Gewald
a facile base-promoted sulfur-centered radical genera-
and
tion mode and a single-step protocol for the synthesis
[
1j]
of thiophene derivatives in 2014. Singh reported the
synthesis of 2-substituted benzothiazoles via decar-
[
1m]
boxylative redox cyclization in 2015.
Up to now,
direct CÀS bond formation by using elemental sulfur
as the sulfur source has remained relatively undevel-
oped. This is mainly due to catalyst poisoning by ele-
mental sulfur or the susceptibility of sulfur towards
[
2]
oxidative decomposition or oligomerization.
As a case study, we disclose herein an elemental
sulfur-mediated [2+2+1]cycloaddition reaction of al-
kynoates or ynones for the synthesis of polysubstitut- Scheme 1. Synthesis of thiophenes.
4
050
ꢁ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2015, 357, 4050 – 4054

UPDATES
Synthesis of Polysubstituted Thiophenes via Base-Induced [2+2+1]Cycloaddition
[a]
Table 1. Optimization of the reaction conditions.
base, reaction temperature and solvent. The experi-
mental results showed that potassium hydroxide
(
KOH) and sodium methoxide (CH ONa) promoted
3
the conversion to a high extent, but 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU) and other organic or in-
organic bases performed poorly or even resulted in
no desired product 2a. An increase of the potassium
hydroxide loading in the system from 1.0 to 1.5 equiv.
Entry Solvent
Elemental
Sulfur (S)
Time Base
[h]
Yield
[b]
[%]
(equiv.)
(
vs. 1a) did not result in any noticeable increase in
the yield of product, but there was a significant de-
crease of the yield when the KOH loading in system
was decreased to 0.2 equiv. (entries 5, 7, and 8). It was
also noticed that the reaction temperature dramatical-
ly affected the yield of the product. At 1008C, an
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
DMSO
toluene
DCE
S (1.0)
S (1.0)
S (1.0)
S (2.0)
2
3
4
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
KOH
11
16
16
61
82
82
27
82
24
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
S
S(4.0)
8
3
2% yield of the product was obtained in 3 h with
.0 equivalents of elemental sulfur and 1.0 equivalent
[
[
[
c]
d]
e]
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
of KOH (vs. 1a). However, when the temperature was
decreased to 60C, only 24% yield was achieved. On
a further decrease to room temperature, only a trace
of product was obtained (entries 5, 9, and 10). Among
the examined solvents (entries 19–23), toluene is
more suitable than other solvents, leading to 2a in
88% yield (entry 23). Finally, this conversion was also
tested by using NaOH as the base instead of KOH,
the result indicated that NaOH was a good substitute
for KOH, and gave almost the same result, producing
the desired product 2a with 86% GC yield (entry 24).
Therefore, further experimental investigations were
carried out with these optimized conditions [1.0 mmol
substrate scale, elemental sulfur 3.0 mmol (based on S
atom), KOH 1.0 equivalent, toluene 2.0 mL]. The
structure of the product 2a was confirmed by
[f]
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
trace
CH ONa 78
3
NaHCO3 none
Na CO3 none
2
AcONa none
DBU
DABCO trace
none
DMAP trace
trace
PPh3
KOH
KOH
KOH
KOH
KOH
NaOH
65
88
46
31
68
86
[
[
g]
g]
acetonitrile S
DMF
toluene
S
S
[a]
Unless otherwise specified, all the reactions were carried
out on 1a at the 0.25 mmol scale with elemental sulfur
.0 equivalents (based on S atom), base 1.0 equivalent,
solvent 2.0 mL, reaction temperature 1008C.
GC yield.
KOH 0.2 equivalent.
KOH 1.5 equivalent.
Reaction temperature: 608C.
Reaction at room temperature.
Reaction at reflux temperature.
1
13
[15]
H NMR, C NMR and X-ray analysis (Figure 1).
For other new products, they mainly existed in the
form of powders or oily liquids, therefore, their struc-
3
1
13
[
[
[
[
[
[
b]
tures were confirmed by H NMR, C NMR and HR-
MS analyses. Due to restricted rotation in some of the
c]
d]
e]
f]
1
3
sterically congested molecules, their C NMR spectra
showed more than the expected number of signals.
With the optimized conditions in hand, the scope
and versatility of the [2+2+1]cycloaddition reaction
was then thoroughly examined, and the outcome is
given in Table 2. The results indicated that all the
g]
[
14]
Hinsberg reactions are representative of the classi- tested alkynoates and ynones were ideal substrates.
cal methods for thiophene synthesis. Notably, various functional groups, irrespective of
At the outset of our experiments, we chose ethyl 3- their nature whether electron-donating or electron-
phenylpropiolate (1a) and elemental sulfur as a reac- withdrawing, were tolerated under the conditions. Be-
tion partners to optimize the reaction conditions in- sides, the location of the substituents on the aromatic
cluding reaction time, the ratio of reactants, tempera- rings was noticed to have an insignificant effect on
ture, solvent and base (Table 1). From entries 1–3 in the product yield. For example, fluoro-, chloro-,
Table 1, it is seen that the reaction could be complet- methyl-, ethyl-, methoxy- and other substituents on
ed in 3 h and gave diethyl 3,4-diphenylthiophene-2,5- the aromatic rings as well as their locations as ortho-
dicarboxylate 2a in 16% GC yield. By varying the and para-substitution were all found to be compatible
ratio of the reactants, we found that the optimal ratio and all alkynoates or ynones 1 led to their corre-
of 3-phenylpropiolate (1a) to elemental sulfur (based sponding products with excellent yields.
on S atom) is 1:3. We then tried to optimize the yield
We all know that elemental sulfur could undergo
of 2a via changing the type and loading amount of a disproportionation reaction in the presence of base
Adv. Synth. Catal. 2015, 357, 4050 – 4054
ꢁ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
4051

Weibing Liu et al.
UPDATES
Scheme 2. Radical trapping and control experiments.
[16]
to produce a sulfide and a sulfite. However, the sul-
fide ion was not found to be involved in the reaction
based on the following experimental facts. During the
process of the optimization of reaction conditions
(
Table 1), we examined several bases, but only potas-
sium hydroxide (KOH) and sodium methoxide
CH ONa) could promoted the reaction, yielding the
Figure 1. X-ray structure of diethyl 3,4-diphenylthiophene-
,5-dicarboxylate 2a.
(
2
3
product 2a, and other bases, such as sodium bicarbon-
ate (NaHCO ), sodium carbonate (Na CO ) and
3
2
3
Table 2. Reaction of various alkynoates or ynones 1 with sodium acetate (AcONa) failed to do so. If the sulfide
[a]
sulfur to form thiophenes 2a.
ion had participated in the reaction, all investigated
bases should have the capability to mediate the for-
mation of target product 2a.
In order to gain further insight into the nature of
this transformation, two experiments were conducted
(
Scheme 2). Instead of elemental sulfur, sodium sul-
fide (Na S) was used to react with 1a under the stan-
2
Entry 1a–1s
2a– Yield
[b]
dard conditions but no desired product was obtained
in this case, which further confirmed that the sulfide
ion was not involved in the transformation. At the
same time, the radical scavenger TEMPO completely
inhibited the reaction and no product at all was ob-
tained. These results indicated that the mechanism
2s
[%]
1
2
1
2
3
4
5
6
7
R =C H /R =OEt (1a)
2a
2b
2c
2d
2e
2f
2g
84
85
88
76
88
87
84
6
5
1
2
R =CO Et/R =OEt (1b)
2
1
2
R =CO Me/R =OMe (1c)
2
1
2
R =n-pentyl/R =OMe (1d)
1
2
R =R =C H (1e)
6
5
[1k]
1
2
may involve a radical pathway.
R =4-ethylC H /R =C H (1f)
6
4
6
5
1
On the basis of these observations as well as related
reactions, we propose the mechanism shown in
Scheme 3. The process could be initiated by the reac-
R =4-(4-ethylcyclohexyl)C H /
6
4
2
R =C H (1g)
6
5
1
2
8
9
R =4-(4-phenyl)C H /R =C H (1g)
2h
2i
86
86
6
4
6
5
1
R =4-[4-(n-propyl)phenyl]C H /
tion of elemental sulfur with the base to generate the
6
4
2
À
R =C H (1i)
6
5
S C radical anion, which has been confirmed and re-
3
1
2
1
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
9
R =C H /R =4-MeC H (1j)
2j
2k
2l
84
86
89
[1j]
6
5
6
4
ported by Lei and his co-workers. Subsequent addi-
1
2
R =C H /R =4-ClC H (1k)
6
5
6
4
tion of the trisulfur radical to the alkyne 1a would
generate the radical anion intermediate 3. Under
strong basic conditions, the SÀS bond is fragile.
1
2
R =C H /R =2-ClC H (1l)
6
5
6
4
1
2
R =C H /R =4-MeOC H (1m)
2m 83
6
5
6
4
1
2
R =4-MeC H /R =C H (1n)
2n
2o
2p
2q
2r
88
89
85
84
83
85
6
4
6
5
1
2
Therefore compound 3 can easily dissociate to give 4
and 5, which would subsequently undergo intermolec-
ular nucleophilic addition onto another triple bond to
give the labile radical 6. The isomerization of 6 results
in another radical 7. Finally, the radical intermediate
R =4-MeC H /R =4-MeOC H (1o)
6
4
6
4
1
2
R =4-MeC H /R =4-ClC H (1p)
6
4
6
4
1
2
R =4-MeC H /R =4-FC H (1q)
6
4
6
4
1
2
R =4-MeC H /R =2-ClC H (1r)
6
4
6
4
1
2
R =R =4-MeC H (1s)
2s
6
4
[17]
[a]
7 goes through a hydrogen abstraction process by
the trisulfur radical under the reaction conditions to
furnish the desired product.
Unless otherwise specified, all the reactions were carried
out on 1 at the 1.0 mmol scale with elemental sulfur
3.0 equivalents (based on S atom), KOH 1.0 equivalent,
toluene 2.0 mL.
Isolated yield.
In conclusion, we have developed a simple, atom
economic and one-pot method to access polysubstitut-
[b]
4
052
asc.wiley-vch.de
ꢁ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2015, 357, 4050 – 4054

UPDATES
Synthesis of Polysubstituted Thiophenes via Base-Induced [2+2+1]Cycloaddition
Typical Procedure: Diethyl 3,4-Diphenylthiophene-
,5-dicarboxylate (Table 2, entry 1)
2
A
mixture of ethyl 3-phenylpropiolate (1a) (358 mg,
1.0 mmol), elemental sulfur (96 mg, 3.0 mmol), potassium
hydroxide (KOH) (56 mg, 1.0 mmol) and toluene (2.0 mL)
was added successively in a round-bottom flask, and the re-
sulting solution was stirred for 3 h at 1008C. The mixture
was then subjected to purification by preparative thin-layer
chromatography (PE-EtOAc, 20:1) to afford product 2a.
Acknowledgements
We thank the Excellent Young Scientist Fund of Guangdong
Province Education Department (No.2013LYM 0059) for fi-
nancial support.
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ꢁ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2015, 357, 4050 – 4054