Visible-light-induced synthesis of benzothiophenes and benzoselenophenes via the annulation of thiophenols or 1,2-diphenyldiselane with alkynes

Article abstract of DOI:10.1016/j.tet.2017.03.053

An effective metal-free photoredox-mediated tandem addition/cyclization reaction of thiophenols or 1,2-diphenyldiselane with alkynes leads to 2,3-disubstituted benzothiophenes and benzoselenophenes. Blue light irradiation of the organic dye, Mes-Acr-Me

Full text of DOI:10.1016/j.tet.2017.03.053

Accepted Manuscript
Visible-light-induced synthesis of benzothiophenes and benzoselenophenes via the
annulation of thiophenols or 1,2-diphenyldiselane with alkynes
Xiao-Feng Xia, Guo-Wei Zhang, Su-Li Zhu
PII:
S0040-4020(17)30303-4
10.1016/j.tet.2017.03.053
TET 28559
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 19 January 2017
Revised Date: 13 March 2017
Accepted Date: 20 March 2017
Please cite this article as: Xia X-F, Zhang G-W, Zhu S-L, Visible-light-induced synthesis of
benzothiophenes and benzoselenophenes via the annulation of thiophenols or 1,2-diphenyldiselane with
alkynes, Tetrahedron (2017), doi: 10.1016/j.tet.2017.03.053.
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Visible-light-induced synthesis of
benzothiophenes and benzoselenophenes via
the annulation of thiophenols or 1,2-
diphenyldiselane with alkynes
Leave this area blank for abstract info.
Xiao-Feng Xiaa , Guo-Wei Zhang a, and Su-Li Zhu a
The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and
Material Engineering, Jiangnan University, Wuxi, Jiangsu, 214122, People’s Republic of China
1

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Tetrahedron
journal homepage: www.elsevier.com
Visible-light-induced synthesis of benzothiophenes and benzoselenophenes via the
annulation of thiophenols or 1,2-diphenyldiselane with alkynes
Xiao-Feng Xiaa , Guo-Wei Zhang a, and Su-Li Zhu a
a
The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi,
Jiangsu, 214122, People’s Republic of China.
ARTICLE INFO
ABSTRACT
An effective metal-free photoredox-mediated tandem addition/cyclization reaction of
thiophenols or 1,2-diphenyldiselane with alkynes leads to 2,3-disubstituted benzothiophenes and
benzoselenophenes. Blue light irradiation of the organic dye, Mes-Acr-Me+, initiates the
photoredox catalysis. A series of functional groups could be tolerated under ambient conditions,
and good to excellent yields were generated.
Article history:
Received
Received in revised form
Accepted
Available online
Keywords:
2009 Elsevier Ltd. All rights reserved
.
Visible-light-induced
Tandem addition/cyclization
Benzothiophenes
Benzoselenophenes
Photoredox catalysis
1. Introduction
Benzothiophene derivatives have attracted considerable attention
from chemists in recent years due to their wide application in
biology,1 pharmacy,2 catalysis,3 and especially in material
science.4 There are several active drugs on the market and
potential active molecules containing the benzothiophene
skeletons. For instance, Zileuton is used as a potent and selective
inhibitor of 5-lipoxygenase, while raloxifene and arzoxifene are
selective estrogen receptor modulators and antitubulin agents.5
FAUC 346 and FAUC 365 can be used as D3 partial agonists,
while benzothiophene biphenyl analogues are presented for
inhibition of protein tyrosine phosphatase 1B (PTP1B) (Figure
1).6 To date, lots of elegant methods have been access to the
synthesis of these substituted benzothiophene cores, including
intramolecular cyclizations of α-arylthioketones,7 o-alkynyl (or
alkenyl or ynol) benzenthiols,3c,8 and alkynyl(aryl)thioethers,9 and
direct arylation of the benzothiophene moiety,10 and other
methods11. Although great progress has been achieved by using
these strategies, prefunctionalized thiophenols have to be
synthesized for the transformations, which limited their pervasive
applications in pharmaceutical synthesis. Therefore, an efficient
and practical method from thiophenols for the construction of
substituted benzothiophenes was urgent in modern organic
synthesis.12
Figure 1. Biologically active molecules containing
benzothiophene moiety.
substituted benzothiophenes. In 2012, König and co-workers
reported
a visible-light-mediated direct C-H arylation of
benzothiophenes with aryl diazonium salts, but unfortunately
mixtures of regioisomers were achieved in low yields (Scheme 1-
a).11a Later, the photocatalytic reaction of o-methylthio-
arenediazonium salts with alkynes for the synthesis of
benzothiophenes was realized by the same group (Scheme 1-
b).11b However, prefunctionalized thiophenols or unstable and
insecure arenediazonium salts resulted in poor selectivity and
difficulties with scale-up. Inspired by our continuous efforts in
radical addition/cyclization reactions14 and based on our previous
work on photoredox catalysis,15 we here report a straightforward
photoredox-mediated tandem addition/cyclization reaction of
thiophenols or 1,2-diphenyldiselane with alkynes for the
Recently, the visible light-induced photoredox catalysis has
attracted renewed interest from synthetic chemists to access
challenging targets and to generate new, structurally-complex
molecules with high levels of practicality and chemoselectivity.13
In the last few years, visible light photocatalysis has also been
used as a powerful tool for mild and selective synthesis of
Corresponding author. Tel.: +86 510 85917763; e-mail: xiaxf@jiangnan.edu.cn
2

synthesis of
2,3-disubstituted benzothiophenes and
O2 (1 atm), CHCl3
2a (3.0 equiv.), PhCOOH (2.0 equiv.),
O2 (1 atm), toluene
2a (2.0 equiv.), PhCOOH (2.0 equiv.),
O2 (1 atm), CHCl3
2a (3.0 equiv.), PhCOOH (1.2 equiv.),
O2 (1 atm), CHCl3
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benzoselenophenes (Scheme 1-c). 16
7
8
9
48
40
59
10
11
2a (3.0 equiv.), O2 (1 atm), CHCl3
2a (3.0 equiv.), PivOH (2.0 equiv.), O2
(1 atm), CHCl3
20
50
12
13
14c
2a (3.0 equiv.), PhCOOH (2.0 equiv.),
air (1 atm), CHCl3
2a (3.0 equiv.), PhCOOH (2.0 equiv.),
Ar (1 atm), CHCl3
2a (3.0 equiv.), PhCOOH (2.0 equiv.),
Ar (1 atm), CHCl3
80
95
80
15
16
Without LED light
Without catalyst
15
16
The best reaction conditions are indicated in bold.
a
Reaction conditions: 1a (0.3 mmol), 2a (3.0 equiv., 0.9 mmol),
Scheme 1. Photocatalytic approaches for the synthesis of
benzothiophenes.
catalyst (5 mol%), oxidant, solvent (3.0 mL), r.t., for 10h.
b
Isolated yields.
c 1,2-di-p-tolyldisulfane was used instead of 1a.
2. Results and Discussion
In previous report, benzenesulfanyl radical can be easily
formed by SET from thiophenol (E1/2ox=+0.95 V versus the
saturated calomel electrode (SCE) in MeCN for thiophenol)17 to
red
the excited state of the photocatalyst Mes-Acr-Me+* (ES
1/2
[Mes-Acr-Me+*/Mes-Acr-Me.]=+2.18 V versus SCE in MeCN).18
So, our initial studies focused on the reaction of the 4-
methylbenzenethiol 1a with dimethyl but-2-ynedioate using Mes-
Acr-Me+ as a photoredox catalyst and benzoic acid as the
additive by irradiating at 450 nm at 1 atm oxygen atmosphere.19
First, several solvents were investigated including DCE, DCM,
CH3CN, CHCl3, DMF, EtOAc, and toluene (Table 1, entries 1-7),
where CHCl3 gave a better result (entry 6). Then, we examined
the amount of additive loading (entries 9 and 10) and different
equivalents of alkyne (entry 8) on this photocatalysis reaction,
and poor results were obtained compared with entry 6. When
PivOH was used as the acid additive, a lower yield was get (entry
11). To our delight, when the reaction was performed in air or
under Ar atmosphere, high yields were obtained (entries 12 and
13). In addition, 1,2-di-p-tolyldisulfane was also suitable in this
transformation (entry 14). To prove the essential role of
photocatalysis for the addition/annulation reaction, experiments
without blue light irradiation or without dye under irradiation
were carried out. As expected, only a 15% and 16% yield was
observed, respectively (entries 15 and 16).
Table 1. Screening of the reaction conditions a
Entry
1
Conditions
2a (3.0 equiv.), PhCOOH (2.0 equiv.),
O2 (1 atm), DCE
Yield (%) b
40 (18)
2
3
4
5
6
2a (3.0 equiv.), PhCOOH (2.0 equiv.),
O2 (1 atm), CH3CN
2a (3.0 equiv.), PhCOOH (2.0 equiv.),
O2 (1 atm), DCM
2a (3.0 equiv.), PhCOOH (2.0 equiv.),
O2 (1 atm), DMF
2a (3.0 equiv.), PhCOOH (2.0 equiv.),
O2 (1 atm), EtOAc
32
44
Scheme 2. Scope of the photoannulation reactions.
With the optimized reaction conditions in hand (Table 1, entry
13), we investigated the reaction scope for thiophenols with
alkynes for the photoannulation reaction (Scheme 2). The
obvious electron-effect on the benzene ring has not been
observed, and both electron-donating and electron-withdrawing
0 (63)
16
substituted
thiophenols
could
produce
the
desired
2a (3.0 equiv.), PhCOOH (2.0 equiv.),
63
benzothiophenes 3a-n in good to excellent yields. Ortho-
3

substituted thiophenols could proceed well under the optimized
Scheme 5. Scale-up experiment and synthetic transformations.
To further demonstrate the utility of the present method, a
scale-up experiment was investigated, and a gram scale of
product 3a can be easily prepared (Scheme 5). In addition,
benzothiophene 8 can be efficiently synthesized via two-steps
from product 3a in good yield. 12a
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reaction conditions (3k-3n). However, a poor regioselectivity
was observed using m-MeO-thiophenol, and the isomers of 3o
and 3o’ can be easily separated. When 3,4-diMeO-thiophenol and
m-Me-thiophenol were submitted to the standard conditions,
mixed isomers were produced (3p and 3q). It was found that o-
ester substituted thiophenol can also yield the target product (3r).
β-Naphthiophenol was also tolerated, and selectively cyclized at
the α-position (3s). Of particular note, p-OH-thiophenol and 1,2-
diphenyldiselane can react with 2a to give the corresponding
products in moderate yields (3t, 3u and 3v). Such molecules are
difficult to synthesize using previous reported methods12,16 and
very useful for further synthetic elaborations. To our
disappointment, heterocyclic thiophenol such as pyridine-2-thiol
failed in this transformation, but a self-coupling product 4a was
produced. In addition, methyl propiolate and methyl 3-
phenylpropiolate were not suitable for this photoannulation
reaction.
3. Conclusions
In summary, we have developed a general and efficient
visible-light photoredox tandem addition/cyclization reaction of
thiophenols or 1,2-diphenyldiselane with alkynes at room
temperature. Interestingly, the reaction can be conducted at argon
or oxygen atmosphere, and the cyclization/oxygen insertion
products can be produced under 1 atm oxygen conditions. In
addition, benzoselenophene derivatives can be easily synthesized
via this transformation.
4. Experimental section
4.1. General information
Column chromatography was carried out on silica gel. Unless
1
noted H NMR spectra were recorded on 400 MHz in CDCl3 or
d-actone, 13C NMR spectra were recorded on 100 MHz in CDCl3
or d-actone. IR spectra were recorded on an FT-IR spectrometer
and only major peaks are reported in cm-1. Melting points were
determined on a microscopic apparatus and were uncorrected. All
new products were further characterized by HRMS (high
resolution mass spectra), high resolution mass spectrometry
(HRMS) spectra was obtained on a micrOTOF-Q instrument
1
equipped with an ESI source; copies of their H NMR and 13C
NMR spectra are provided.
Scheme 3. Photocatalysis oxygen insertion reaction
4.2. Typical procedure for the synthesis of product 3
Interestingly, when the photocatalysis reaction was carried out
at 1 atm oxygen atmosphere, a cyclization/oxygen insertion
phenomenon was observed (Scheme 3). Benzaldehyde and
acetophenone derivatives (5a and 5b) were obtained in moderate
yields.
An oven-dried Schlenk tube (10 mL) was equipped with a
magnetic stir bar, 4-methylbenzenethiol (1a, 0.3 mmol), dimethyl
but-2-ynedioate (2, 0.9 mmol), 5% Mes-Acr-Me+ (0.015 mmol),
PhCOOH (2 equiv., 0.6 mmol). The flask was evacuated and
backfilled with Ar for 3 times. Then CHCl3 (3.0 mL) was added
with syringe. The reaction mixture was then stirred for 10 h at
room temperature. After the reaction, 6 mL water was added to
quench the reaction, and the resulting mixture was extracted
twice with EtOAc. The combined organic extracts were washed
with brine, dried over Na2SO4 and concentrated. Purification of
the crude product by flash column chromatography afforded the
product (petroleum ether/ethyl acetate as eluent (8:1)).
4.2.1 dimethyl 5-methylbenzo[b]thiophene-2,3-dicarboxylate, 3a,
o
1
95%, M. P. = 85-86 C. H NMR (400 MHz, CDCl3): 7.80-7.66
(m, 2H), 7.32 (d, J = 8.3Hz, 1H), 4.02 (s, 3H), 3.93 (s, 3H), 2.47
(s, 3H). 13C NMR (100 MHz, CDCl3): 165.0, 162.2, 137.7, 137.0,
135.7, 132.9, 129.4, 125.1, 124.0, 122.1, 52.8, 52.7, 21.4. IR:
(cm-1): 3021, 2953, 2920, 1913, 1714, 1565, 1536, 1456, 1434,
1358, 1197, 1236, 1168, 1104, 939, 888, 808, 765.
Scheme 4. Control experiments.
To gain more insights into the mechanism, the control
experiments were performed (Scheme 4). The photocatalysis
addition/cyclization reaction was found to be completely
suppressed using 1.0 equiv. of TEMPO (2,2,6,6-tetramethyl-1-
piperidinyloxy) as the typical radical scavenger, and an addition
product of TEMPO with dimethyl but-2-ynedioate was separated
in 55% yield. When 1,1-diphenylethylene was added into the
standard conditions, no product was observed. These results
indicated that a radical addition pathway might be involved in
this transformation.
4.3. Typical procedure for the synthesis of product 5
An oven-dried Schlenk tube (10 mL) was equipped with a
magnetic stir bar, 4-ethylbenzenethiol (0.3 mmol), dimethyl but-
2-ynedioate (2, 0.9 mmol), 5% Mes-Acr-Me+ (0.015 mmol),
PhCOOH (2 equiv., 0.6 mmol). The flask was evacuated and
backfilled with O2 for 3 times. Then CHCl3 (3.0 mL) was added
with syringe. The reaction mixture was then stirred for 24 h at
room temperature. After the reaction, 6 mL water was added to
quench the reaction, and the resulting mixture was extracted
twice with EtOAc. The combined organic extracts were washed
with brine, dried over Na2SO4 and concentrated. Purification of
the crude product by flash column chromatography afforded the
product (petroleum ether/ethyl acetate as eluent (6:1)).
4.3.1 dimethyl 7-formylbenzo[b]thiophene-2,3-dicarboxylate, 5a,
25%, M. P. = 152-153 oC, 1H NMR (400 MHz, CDCl3): 10.24 (s,
1H), 8.30-8.26 (m, 1H), 8.05-8.02 (m, 1H), 7.71-7.65 (m, 1H),
S
S
aq. NaOH (1.0M, 1.5 mL)
S
Ag2CO3 (10 mol%)
AcOH (5 mol%)
DMSO, 120 oC, 6h
85%
COOMe
COOH
THF/MeOH (6/1, 3.5 mL)
r.t., 1h, 75%
COOMe
1.0 mmol, 3a
COOMe
COOMe
8
7
4

4.04 (s, 3H), 3.98 (s, 3H). 13C NMR (100 MHz, CDCl3): 190.7,
7
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164.5, 162.1, 138.2, 137.9, 136.9, 133.7, 131.46, 130.6, 130.5,
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1356, 1293, 1254, 1172, 1110, 1066, 1043, 917, 863, 808, 769,
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We thank the National Science Foundation of China NSF
21402066 and the Natural Science Foundation of Jiangsu
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