A Three-Component Strategy for Benzoselenophene Synthesis under Metal-Free Conditions Using Selenium Powder

Article abstract of DOI:10.1021/acs.orglett.9b00739

An efficient three-component benzoselenophenes formation has been developed from substituted indoles, acetophenones, and selenium powder under metal-free conditions. 2-Aryl indoles played an important role to promote benzoselenophene formation from acetophenone derivatives and selenium powder. One C-C and two C-Se bonds were selectively formed to provide 40 new benzoselenophenes in good yields.

Full text of DOI:10.1021/acs.orglett.9b00739

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
A Three-Component Strategy for Benzoselenophene Synthesis
under Metal-Free Conditions Using Selenium Powder
Penghui Ni, Jing Tan, Wenqi Zhao, Huawen Huang, Fuhong Xiao,* and Guo-Jun Deng*
Key Laboratory for Green Organic Synthesis and Application of Hunan Province, Key Laboratory of Environmentally Friendly
Chemistry and Application of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, China
S
* Supporting Information
ABSTRACT: An efficient three-component benzoseleno-
phenes formation has been developed from substituted
indoles, acetophenones, and selenium powder under metal-
free conditions. 2-Aryl indoles played an important role to
promote benzoselenophene formation from acetophenone
derivatives and selenium powder. One C−C and two C−Se
bonds were selectively formed to provide 40 new benzosele-
nophenes in good yields.
rganoselenium compounds are of considerable interest,
because of their potential biological activities, such as
Scheme 1. Selected Synthetic Approaches to
Benzoselenophene Derivatives
O
antiviral, antihypertensive, antioxidant, and antitumor proper-
ties.1 In particular, aromatic benzoselenophene derivatives are
attracting much attention as promising electronic materials for
organic light-emitting diodes (OLED), organic conductors,
and field-effect transistors.2 Two general protocols for
constructing C−Se bonds include (1) transition-metal-
catalyzed cross-coupling reaction using active organoselenium
compounds as substrates;3 and (2) elemental selenium as the
selenium source. In recent years, the direct selenation of C−H
bond with various reactive selenium reagents and even
selenium powder has emerged as a very powerful and more
direct approach for C−Se bond construction.4
Substituted benzoselenophenes can be constructed by
annulation of either ring or by functionalization of the
benzoselenophene core.5 In the early days, the benzoseleno-
phene ring construction is mainly dependent on ethylbenzene
via gaseous reaction from phenoselenide and acetylene at high
temperature (400−600 °C).6 Cyclization of alkynyl arenes
bearing an ortho-selenium functional group provides an general
approach (Scheme 1a).7 Besides Larock’s electrophilic
cyclization7a,b and Nakamura’s transition-metal (Au or Pt)-
catalyzed cyclization,8 a few other methodsparticularly the
reaction of o-haloethynylbenzenes and selenating reagents
are also available.9 Although phenylacetylenes can also be used
as the raw materials, multiple steps are required and sensitive
lithium or zinc reagent is usually involved (Scheme 1b).10
Recently, Ranu and co-workers found that substituted
benzoselenophenes can be achieved using styrenyl bromides
and potassium selenocyanate (KSeCN) as the reagents
(Scheme 1c).11 Besides these great progresses, versatile
methods for benzoselenophene formation from readily
available substrates under simple conditions are still highly
desirable. Acetophenones are readily available, inexpensive,
and easy to handle. Therefore, direct insertion of elemental
selenium into acetophenones can provide an ideal approach for
the construction of these heterocycles. However, as far as we
know, there are no examples on direct insertion of selenium
into acetophenones under mild conditions. In our previous
indole-to-carbazole work based on indoles and ketones, we
found that 3-vinylindole was in-situ-generated through
dehydrative condensation, which could be used as the C4
source for heterocycle formation.12 We also realized thieno-
[2,3-b]indole formation through insertion of a S atom into the
C2 position of indole and the vinyl group under metal-free
Received: February 27, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00739
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
conditions (Scheme 1a).13 We envisioned that it might be
possible to insert a Se atom to 3-vinylindole to form
benzoselenophene ring via [4 + 1] cycloaddition when the
C2 position in indole is occupied, which can enable cyclization
reaction occurring at the benzene ring (Scheme 1b). If this
strategy can be realized, it will provide a convenient way for the
construction of the benzoselenophene ring via in situ activation
of acetophenones in the aid of indoles. Furthermore, the target
products contain two important structures: indole and
benzoselenophene, which will provide many new opportunities
for further exploration on their properties. As our continuing
efforts on indole functionalization under simple conditions,14
herein, we describe a three-component strategy for benzose-
lenophene formation from selenium powder and acetophe-
nones under metal-free conditions (Scheme 1d).
Scheme 2. Screening Different 1-Methyl-2-Substituted-1H-
Indoles
a
We studied the three-component cycloaddition of 1-methyl-
2-phenyl-1H-indole (1a) with acetophenone (2a) and
selenium powder (Table 1). Several iodine-containing
a
Table 1. Screening the Reaction Conditions
a
Reaction conditions: 1 (0.3 mmol), 2a (0.2 mmol), Se (0.3 mmol),
IBr (0.24 mmol), NMP (0.5 mL), 140 °C, 8 h, under O2. Yields of
isolated products.
b
entry
oxidant
solvent
yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
HI
I2
NMP
NMP
NMP
NMP
NMP
NMP
NMP
PhCl
11
20
41
59
81
trace
n.d.
n.d.
n.d.
The reaction could not proceed when the indole C-2 position
was substituted by alkyl, silyl, ester, and alkynyl groups. 1-
Methyl-2-phenyl-1H-indole could provide benzoselenophene
3aa in good yield. The 1-methyl-2-(thiophen-2-yl)-1H-indole
gave medium transformation, generating 3at in 57% yield.
Only a trace amount of product was observed via gas
chromatography−mass spectrometry (GC-MS) when the
large sterically hindered aromatic fused ring is substituted at
the indole C-2 position.
ICl3
ICl
IBr
NIS
NH4I
IBr
IBr
IBr
IBr
IBr
IBr
IBr
c
c
c
DMSO
d
c
DMF
DEF
n.d.
e
26
89
66
76
With this three-component cycloaddition procedure in hand,
we investigated the substrate scope of this selenium insertion
process (Scheme 3). The isolated yield of model reaction
product 3aa was 83%. Acetophenones bearing methyl, butyl,
and phenyl on the para position of the benzene ring reacted
smoothly to give the similar yields (3ab−3ae). The structure
of 3ab was confirmed by X-ray crystallography. Note that the
phenylethynyl was well-tolerated under standard conditions,
giving the product 3af in 51% yield. Trimethoxy (2g) and
trifluoromethoxy (2h) were well-tolerated, affording the target
products 3ag−3ah in moderate yields. The results showed that
substrates with a halo group on the phenyl ring are suitable for
this transformation (3ai−3al). Substrates bearing strongly
electron-withdrawing groups (−CN, −NO2, and −SO2CH3)
were also compatible with the reaction conditions, providing
the corresponding products in good yields (3am−3ao),
whereas the ester group is not compatible. As for m-substituted
acetophenones, a mixture of two isomers (near 1:1 ratio) was
obtained and difficult to be separated. The steric effect of the
substituent has great influence on the yield. For example, 1-(2-
methoxyphenyl)ethanone gave a moderate yield of the
benzoselenophene 3ap. The products (3as−3au) were given
in modest yields when bulky aromatic ketones such as 1-
(anthracen-2-yl)ethanone (2s), 1-(phenanthren-3-yl)ethanone
(2t), and 1-(naphthalen-2-yl)ethanone (2u) were used.
Notably, heteroaromatic ketone 2q−2r and 2v also reacted
f
NMP
NMP
NMP
NMP
g
h
c
15
n.d.
a
Reaction conditions: 1a (0.3 mmol), 2a (0.2 mmol), Se (0.3 mmol),
additive (0.24 mmol), solvent (0.5 mL), 140 °C, 8 h, under an air
atmosphere. GC yield based on 2a. n.d. = not detected. DMF =
N,N-dimethylformamide. DEF = N,N-diethylformamide. Under O2.
IBr (50 mol %). 130 °C.
b
c
d
e
f
g
h
reagents, including HI, I2, ICl3, ICl, and IBr were investigated
(Table 1, entries 1−5). IBr was preferable additive for this
reaction, giving the desired product 3aa in 81% yield (Table 1,
entry 5). A sharp decline in yield was observed when NIS or
NH4I was used as the additive (Table 1, entries 6 and 7).
Among the solvents examined, NMP resulted in the best
reactivity (Table 1, entries 8−11). The yield could be slightly
enhanced by performing the reaction under O2 atmosphere
(Table 1, entry 12). Decreasing the amount of IBr or the
reaction temperature both led to lower yields of product
(Table 1, entries 13 and 14). A control experiment was
performed, indicating that the reaction did not proceed in the
absence of IBr (Table 1, entry 15).
We also screened a range of 1-methyl-2-substituted-1H-
indole under the optimized reaction conditions (Scheme 2).
B
DOI: 10.1021/acs.orglett.9b00739
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Organic Letters
Letter
a
a
Scheme 3. Substrate Scope, with Respect to the Ketones
Scheme 4. Substrate Scope, with Respect to the Indoles
a
Conditions: 1 (0.3 mmol), 2b (0.2 mmol), Se (0.3 mmol), IBr (0.24
mmol), NMP (0.5 mL), 140 °C, 8 h, under O2. Yields of isolated
products.
be smoothly introduced into products 4b and 4c in 95% and
72% yields, respectively.15 When 3aa was treated with I2 in
dimethylsulfoxide (DMSO) under an air atmosphere, the
iodization product 4d was obtained in 52% yield. Sub-
sequently, we investigated the same reaction using elemental
sulfur powder and tellurium powder instead of selenium.
Elemental sulfur gave the sulfuration product 4e in only 34%
yield, whereas tellurium powder did not produce the
corresponding product (see Scheme 5).
a
Conditions: 1a (0.3 mmol), 2 (0.2 mmol), Se (0.3 mmol), IBr (0.24
mmol), NMP (0.5 mL), 140 °C, 8 h, under O2. Yields of isolated
b
c
products. 6 mmol scale. 5 mmol scale.
efficiently to afford corresponding 3aq−3ar and 3av in good
yields. Propiophenone and aliphatic ketones were not suitable
substrates for this transformation.
Next, several substituted indoles were evaluated to react with
acetophenone (Scheme 4). 2-Phenyl-substituted indoles
bearing methyl, butyl, trifluoromethyl, and chloro at the
benzene rings were reacted smoothly to give the corresponding
products 3ba−3ia in moderate to good yields. Generally,
indoles with a methyl group at the C-5, C-6, and C-7 positions
underwent the cycloaddition smoothly to afford products 3ka,
3na, and 3oa in 76%, 71%, and 78% yields, respectively.
However, the methyl group (1j) at the C-4 position almost
prevented the transformation, revealing a steric hindrance
effect, and the ester group is also not compatible. Indole
containing a fluoro group gave product 3la in 83% yield,
whereas the bromo substrate was to afford product 3ma in
lower yield (69%). In comparison with N-methylindole, the
unprotected NH-indole (1p) also underwent the reaction
giving product 3pa in 52% yield. N-ethylindoles gave the target
product in 75% yield (3qa). 1-Benzyl-2-phenyl-1H-indole (1r)
and 5-methyl-1,2-diphenyl-1H-indole (1s) gave product yields
of 44% and 35%, respectively. However, N-acetyl-2-phenyl-
indole and N-benzoyl-2-phenylindole are not suitable for this
reaction.
Scheme 5. Application of the Present Work
The benzoselenophene product 3ak was obtained in 61%
yield in a gram-scale reaction. Further synthetic modification of
the bromo group of product 3ak was investigated. Sonogashira
coupling of 3ak with phenylacetylene afforded the adduct 4a in
76% yield. Moreover, Pd-catalyzed Suzuki−Miyaura cross-
coupling and Buchwald−Hartwig amination of 3ak could also
C
DOI: 10.1021/acs.orglett.9b00739
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Letter
Several control experiments were performed to elucidate the
reaction mechanism (Scheme 6). In the absence of selenium
3a via single electron transfer. Then, intermediate A reacts with
elemental selenium to give a selenium free radical B, which
proceeds through intramolecular selenium cyclization to give
intermediate C. The final desired product 3aa is formed
through the oxidation of C.
Scheme 6. Control Experiments under Various Conditions
In summary, we have developed a versatile strategy for the
synthesis of benzoselenophenes. The key 3-vinyl-indole
intermediate was generated in situ through dehydrative
condensation of aryl ketones and indoles under metal-free
conditions. This simple method provides an efficient route for
the preparation of a variety of new bis-heteroaryls containing
both indole and benzoselenophene motifs from readily
available starting materials.
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.9b00739.
1
Experimental procedures, characterization data, and H
NMR and 13C NMR spectra for all new products (PDF)
Accession Codes
CCDC 1849689 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
powder, 1-methyl-2-phenyl-1H-indole (1a) reacts with aceto-
phenone (2a) to form an intermediate 3a in 17% yield after 0.5
h (Scheme 6a). The target benzoselenophene 3aa was
obtained in 72% yield, using the newly formed 3a as a
substrate under standard conditions, whereas no reaction was
occurred in the absence of IBr (Scheme 6b). The reaction was
completely suppressed when 1 equiv of 2,2,4,4-tetramethyl-1-
piperidinyloxy (TEMPO) was added (Scheme 6c), suggesting
that a radical pathway was probably involved in [4 + 1]
cycloaddition with selenium powder.
The control experimental results indicated that 3a is the key
intermediate. Based on the above experimental observations
and related references,4b,c,16 we proposed a plausible
mechanism in Scheme 7. Dehydrative condensation from 2-
phenyl-1H-indole (1a) and acetophenone (2a) generates
intermediate 3a. The process of dual C−H oxidative
selenylation probably proceeded through a free radical. First,
vinyl radical intermediate A was generated from 3-vinylindole
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: fhxiao@xtu.edu.cn (F. Xiao).
*E-mail: gjdeng@xtu.edu.cn (G.-J. Deng).
ORCID
Huawen Huang: 0000-0001-7079-1299
Fuhong Xiao: 0000-0003-0862-0557
Guo-Jun Deng: 0000-0003-2759-0314
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Nos. 21871226, 21572194), the
Collaborative Innovation Center of New Chemical Technol-
ogies for Environmental Benignity and Efficient Resource
Utilization, the China Postdoctoral Science Foundation (No.
2018M632976), and the Hunan Provincial Innovative
Foundation for Postgraduate (No. CX2018B362).
Scheme 7. Possible Reaction Mechanism
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