Visible-Light-Induced Cyclization/Aromatization of 2-Vinyloxy Arylalkynes: Synthesis of Thio-Substituted Dibenzofuran Derivatives

Article abstract of DOI:10.1021/acs.orglett.0c03876

A visible-light-induced cascade reaction of 2-vinyloxy arylalkynes with thiosulfonates was developed and provided unexpected thio-substituted dibenzofuran derivatives in moderate yields. Mechanistic studies revealed the thiosulfonylation product of 2-vinyloxy arylalkyne was the key intermediate, and the additive disulfide played the role of hydrogen abstraction in the aromatization process to offer the desired product. This reaction presents a new reaction mode for the construction of polycyclic oxygen heterocycles.

Full text of DOI:10.1021/acs.orglett.0c03876

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Letter
Visible-Light-Induced Cyclization/Aromatization of 2‑Vinyloxy
Arylalkynes: Synthesis of Thio-Substituted Dibenzofuran Derivatives
Hui Chen, Yunyun Yan, Niuniu Zhang, Zuyu Mo,* Yanli Xu,* and Yanyan Chen*
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ABSTRACT: A visible-light-induced cascade reaction of 2-vinyl-
oxy arylalkynes with thiosulfonates was developed and provided
unexpected thio-substituted dibenzofuran derivatives in moderate
yields. Mechanistic studies revealed the thiosulfonylation product
of 2-vinyloxy arylalkyne was the key intermediate, and the additive
disulfide played the role of hydrogen abstraction in the
aromatization process to offer the desired product. This reaction
presents a new reaction mode for the construction of polycyclic
oxygen heterocycles.
he dibenzofuran core widely exists in natural products1
Scheme 1. Reported Radical Cascade Reaction of 2-
Vinyloxy Arylalkyne
2
Tand pharmaceutical molecules,
and presents diverse
biological activities; thus, their synthetic methods attract
considerable attention. Traditional methods for the con-
struction of dibenzofurans are via the oxidation/cyclization
of 2-arylphenol.3 The transition-metal-catalyzed intramolecular
cyclization of diphenyl ethers bearing a leaving group in the
ortho position such as nitro4 and carboxyl5 has also been
applied as a promising strategy. However, relying heavily on
transition metals, low functional group tolerance, and harsh
conditions in above-reported methods were not negligible.
Recently, the radical cascade reaction of 1,6-enynes has
emerged as an attractive approach for the synthesis of
heterocycle derivatives.6 Li’s group reported Tempo or nitro
radical induced cyclization of 2-vinyloxy arylalkyne which
provided two different types of benzofurans (Scheme 1a and
1b).7 Jiang8 and Sun’s9 groups achieved the construction of the
sulfonylated benzofuran skeleton by using sodium sulfinate and
DMSO as the source of sulfonyl in a similar radical pathway,
respectively (Scheme 1c and 1d). CF3SO2Na was employed as
the precursor of the trifluoromethyl radical, generating
trifluoromethylated benzofurans (Scheme 1e).10 Xia’s group
developed iron-mediated reductive radical cascade cyclization
to give 3-acylbenzofurans (Scheme 1f).11 Despite the
remarkable advances that have been made for the assembly
of carbonylated benzofuran in the above-mentioned ap-
proaches, the development of an efficient method to introduce
diverse groups into the benzofuran skeleton is still desirable.
Furthermore, the construction of polycyclic benzofurans
through the radical cascade reaction of 1,6-enynes has not
been reported.
Meanwhile, substantial efforts have been devoted to explore
the facile synthesis of thio-substituted heterocycles because of
their potential pharmacological activity.12 Undoubtedly,
cascade reaction is a highly valuable approach for the assembly
of thio-substituted heterocycles in terms of atom economy and
Received: November 23, 2020
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© XXXX American Chemical Society
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operability. The sulfonium ion attacked the CC triple bonds,
followed by the electrophilic cascade cyclization of 2-
alkynylphenol derivatives to give 3-sulfenylbenzofuran.13 The
domino reaction of enoic acid with disulfide14 or thiol15
offered thio-substituted lactones as product. Pan and co-
workers developed electrocatalytic radical [3 + 2] cyclo-
addition of vinyl azides, thiophenols, and pyridines to provide
sulfide imidazo[1,2-a]pyridines.16 Sahoo and co-workers
reported a thio radical induced cyclization of yne-ynamides
to synthesize 4-thioaryl-pyrroles.17 To our knowledge, the
reports of preparing thio-substituted polycyclic heterocycles via
radical cascade reaction were rare. In this context and in
continuation of our interest in photocatalysis,18 we describe a
visible-light-induced cyclization/aromatization of 2-vinyloxy
arylalkynes with thiosulfonates to give thio-substituted
dibenzofuran derivatives. This method presents a new reaction
mode for the construction of polycyclic oxygen heterocycles.
Initially, the reaction of 1a (1-(phenylethynyl)-2-(vinyloxy)-
benzene) and 2a (S-phenyl benzenesulfonothioate) under the
catalysis of 5 mol % Na2-Eosin Y in CH3CN irradiated by 30
W white LEDs at 80 °C under a nitrogen atmosphere was
studied. Delightly, unexpected product 3a was obtained in 55%
yield which was confirmed by X-ray crystal structure analysis.
Then, the reaction conditions were further screened, and the
results are shown in Table 1. Without the addition of a
photocatalyst, only a trace amount of 3a was observed (Table
1, entry 2), indicating that the photocatalyst was necessary.
Other photocatalysts were screened, and Na2-Eosin Y was
proven to be the best catalyst (Table 1, entries 3−9). Solvents
such as DCE, DMF, toluene, EtOAc, and 1,4-dioxane were
unsuitable, resulting in the significant decrease in product yield
(see Supporting Information). The higher temperature 90 °C
was favorable for this transformation, giving 3a in 60% yield
(Table 1, entry 10). However, 3a was only obtained in 56%
yield when the reaction was carried out at 100 °C (Table 1,
entry 11). 1,2-Diphenyldisulfane was used as an additive, and
the yield of 3a was improved to 70% (Table 1, entries 12 and
13). When the reaction was conducted under an air
atmosphere, only a trace amount of 3a was detected, and
almost all of 1a was decomposed (Table 1, entry 14). When
this reaction was carried out in the dark, no 3a was observed
and most of 1a was recovered (Table 1, entry 15). Therefore,
the optimal conditions for the synthesis of 3a were as follows:
5 mol % Na2-Eosin Y and 50 mol % 1,2-diphenyldisulfane in
CH3CN under the irradiation of 30 W white LEDs at 90 °C
under a nitrogen atmosphere with stirring for 12 h.
With the optimized conditions in hand, the substrate scope
of 2-vinyloxyphenylacetylene 1 and sulfonothioate 2 was
investigated, and the results are summarized in Scheme 2.
Scheme 2. Scope of 2-Vinyloxyphenylacetylene and
a
Sulfonothioate
a
Table 1. Condition Optimization
Entry
Photocatalyst
Additive
Temp (°C)
Yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
Na2-Eosin Y
no
Eosin Y
no
no
no
no
no
no
no
no
no
no
no
(PhS)2
(PhS)2
(PhS)2
(PhS)2
80
80
80
80
80
80
80
80
80
90
100
90
90
90
90
55
trace
31
trace
50
43
trace
20
25
60
56
Mes-AcrPh+BF4
Rhodamine B
Bengal Rose
Ru(bpy)3Cl2
Ir(ppy)3
Ir(bpy)(ppy)2PF6
Na2-Eosin Y
Na2-Eosin Y
Na2-Eosin Y
Na2-Eosin Y
Na2-Eosin Y
Na2-Eosin Y
b
65
70
trace
0
c
cd
,
14
15
ce
,
a
Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), and
photocatalyst (5 mol %) in solvent (2.0 mL) under the irradiation
of 30 W white LEDs at 80 °C under a nitrogen atmosphere were
stirred for 12 h; data in parentheses are the yields of isolated products.
a
Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), Na2-Eosin Y (5
b
c
d
(PhS)2 (20 mol %) was used. (PhS)2 (50 mol %) was used. The
mol %) and disulfide (50 mol %) in CH3CN (2.0 mL) under the
irradiation of 30 W white LEDs at 90 °C at nitrogen atmosphere was
stirred for 12 h. 1a was used in mmol scale.
e
reaction was performed under an air atmosphere. The reaction was
carried out in the dark.
b
B
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Aryl alkyne bearing an electron-withdrawing group such as 4-F,
4-Cl, and 4-Br in the R1 position resulted in the slight decrease
in the product yield, affording the desired products 3b−3d in
50−58% yield. The introduction of electron-donating sub-
stituents such as 4-Me, 4-t-Bu, and 4-MeO gave 3e−3g in 60−
71% yield. The 2-Me group was also employed, however,
affording 3h in 44% yield, indicating that steric hindrance had
a significant impact on this reaction. 3,5-Dimethoxyl and 3,5-
dimethyl were tolerant, providing 3i and 3j in 54% and 58%
yields, respectively. The vinyloxybenzene ring was assembled
with 4-F or 4-Me, giving 3k and 3l in 47% and 52% yields,
respectively. In the moiety of sulfonothioate, the benzene ring
bearing an electron-withdrawing group (4-F, 4-Cl and 4-Br)
and electron-donating group (4-Me and 4-MeO) reacted
smoothly, providing the corresponding product 3m−3q in
47−66% yield. The 3-Me, 3,5-dimethyl, and 2-Me group
exerted negligible effect on this transformation, affording 3r−
3t in 59−61% yield. S-(Naphthalen-2-yl) benzenesulfono-
thioate was suitable, generating 3u in 63% yield. 2-
Methylfuran-3-yl was applied and provided 3v in 30% yield.
It is noteworthy that alkyl thio-substituted sulfonothioates
were tolerant, providing 3w−3y in 35−44% yield. Finally, the
reaction of 1a and 2a was performed in 1 mmol scale, and 3a
was obtained 59% yield.
Scheme 4. Controlled Experiments
The vinylthio and vinylamino benzene were considered next
(Scheme 3). Unfortunately, 4a only gave 5a in 30% yield, and
Scheme 3. Other Types of Substrates
most of 4a was decomposed. 4b was applied, but no 5b was
observed, giving difunctional product 5ba in 70% yield. When
the reaction time was prolonged to 12 h, 5ba decomposed.
α,β-Unsaturated ketone 4c was also applied, but no 5c was
observed and most of 4c was recovered.
To probe the possible mechanism, several controlled
experiments were conducted. First, the reaction of 1e and 2a
was carried out under the standard conditions for 45 min. Yet,
only a trace amount of 3e was observed, and intermediate 3ea
was isolated in 81% yield; however, 3ea was seriously inhibited
when the reaction was conducted at lower temperature or
without light irradiation, indicating that light irradiation and
high temperature were necessary for the formation of 3ea
(Scheme 4a and 4b). Then, 3ea was proven as the key
intermediate, and the transformation of 3ea to 3e was
promoted by the sulfur radical through the contrast experi-
ments of additives (Scheme 4c). On the other hand, the one-
pot two-step operation indicated that light irradiation was
more impactful than high temperature in the transformation of
3ea to 3e (Scheme 4d). The reaction of 1e and 2a was
seriously inhibited by radical scavengers. Furthermore, the
radical adducts 4e and 4ea were detected by LC-MS (Scheme
4e). When the reaction of 1e and 2a was conducted under
irradiation of 254 nm light, 3ea and 3e were obtained in 43%
and 14% yield, respectively, meaning a radical pathway induced
by energy transfer was preferred (Scheme 4f). 1,2-Bis(4-
methoxyphenyl)disulfane was used as the additive instead of
1,2-diphenyldisulfane in the reaction of 1a and 2a, which
generated 3a and 3g in 16% and 47% yield (Scheme 4g). Then,
the 1:1 mixture of 2a and 1,2-bis(4-methoxyphenyl)disulfane
was conducted under standard conditions for 45 min, only
C
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giving 2g in 20% yield with recovered 2a and 1,2-bis(4-
methoxyphenyl)disulfane in 75% and 71% yield. The crossing
experiment indicated the thio group in the final compound
mainly came from disulfide.
On the basis of the above controlled experiments and the
reported literature, a possible mechanism was proposed as
described in Scheme 5. First, the photocatalyst Na2-Eosin Y
Scheme 5. Proposed Pathway for the Synthesis of Thio-
Substituted Dibenzofuran
Figure 1. UV−vis absorption (solid line) and normalized fluorescence
spectra (dashed line) of thio-substituted dibenzofuran in DCE (1 ×
10−5 M).
heterocycles. Steady-state absorption and photoluminescence
measurements for the obtained thio-substituted dibenzofurans
were also examined.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03876.
Experimental procedure, characterization data, and
copies of H and 13C NMR spectra (PDF)
1
reached an excited state under the irradiation of white LEDs
which underwent energy transfer to 2a and offered an excited
2a*.19 The excited 2a* underwent homolytic cleavage of the
SO2−S bond, offering a sulfur radical and sulfonyl radical.
Addition of the sulfonyl radical to 1a gave radical intermediate
A; simultaneously, the sulfur radical attacked 2a with the
generation of 1,2-diphenyldisulfane and the sulfonyl radical.
The radical cyclization of A furnished intermediate B which
attacked 1,2-diphenyldisulfane or 2a and gave 3aa. The
intramolecular cyclization of 3aa provided intermediate C
which abstracted a hydrogen atom by the sulfur radical with
the generation of radical D.20 D was oxidized into cation E by
the excited photocatalyst along with the formation of the
photocatalyst radical anion. The photocatalyst radical anion
transformed an electron to the sulfur radical, giving the sulfur
anion and finishing the photocatalyst recycle. Finally, E
underwent deprotonation to provide product 3a.
Accession Codes
CCDC 2032114 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.
AUTHOR INFORMATION
Corresponding Authors
■
Yanyan Chen − College of Pharmacy, Guilin Medical
University, Guilin 541004, People’s Republic of China; Key
Laboratory of Agricultural Resources Chemistry and
Biotechnology, College of Chemistry and Food Science of
Yulin Normal University, Yulin 537000, People’s Republic of
China; orcid.org/0000-0001-5813-9353;
To understand the photophysical properties of the thio-
substituted dibenzofurans, the steady-state absorption and
photoluminescence (PL) measurements for the 3g, 3i, 3q, 3s,
3u, 3w, and 3y in DCE were studied (Figure 1). The
fluorescence spectra of the above-mentioned compounds show
emission maxima in the 382−442 nm range.
In summary, we have developed a visible-light-induced
cascade reaction of thiosulfonates with 2-vinyloxy arylalkynes
which offered thio-substituted dibenzofuran derivatives in
moderate yields. The mechanistic study supported the
thiosulfonylation product of 2-vinyloxy arylalkyne as the key
intermediate, and the following cyclization/aromatization gave
the final product. And the additive disulfide was found to play a
role in hydrogen abstraction in the aromatization process. This
cascade reaction features metal-free conditions, a novel
product, and good regioselectivity and also presents a new
reaction mode for the construction of polycyclic oxygen
Email: chenyy269614@163.com
Yanli Xu − College of Pharmacy, Guilin Medical University,
Guilin 541004, People’s Republic of China; Key Laboratory
of Agricultural Resources Chemistry and Biotechnology,
College of Chemistry and Food Science of Yulin Normal
University, Yulin 537000, People’s Republic of China;
orcid.org/0000-0003-1373-6431; Email: yanli.xu@
163.com
Zuyu Mo − College of Pharmacy, Guilin Medical University,
Guilin 541004, People’s Republic of China;
Email: 18877380547@163.com
Authors
Hui Chen − College of Pharmacy, Guilin Medical University,
Guilin 541004, People’s Republic of China
Yunyun Yan − Key Laboratory of Phytochemistry, College of
Chemistry and Chemical Engineering, Baoji University of
D
https://dx.doi.org/10.1021/acs.orglett.0c03876
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Arylation of Benzoic Acids by Tandem Decarboxylation/C−H
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Niuniu Zhang − College of Pharmacy, Guilin Medical
University, Guilin 541004, People’s Republic of China
Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(21861015), Natural Science Foundation of Guangxi
(2018GXNSFBA138050), Special Fund for Talents of Guangxi
(AD18281028 and AD18281035), Shaanxi Provincial Key
Laboratory Project (15JS010), Guangxi Key Laboratory of
Agricultural Resources Chemistry and Biotechnology
(2019KF07 and 2019KF08), and Cultivation Plan of
Thousands of Young and Middle-aged Backbone Teachers in
Guangxi Colleges and Universities for financial support for
financial support.
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oxidant and metal-free dehydrogenative cascade trifluoromethylation
and oxidation of 1,6-enynes with water. Chem. Sci. 2017, 8, 6633.
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reductive cyclization reaction of 1,6-enynes for the synthesis of 3-
acylbenzofurans and thiophenes. Org. Chem. Front. 2019, 6, 342.
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W. A.; Van Doorslaer, S.; Tehrani, K. A.; Maes, B. U. W.
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https://dx.doi.org/10.1021/acs.orglett.0c03876
Org. Lett. XXXX, XXX, XXX−XXX