Synthesis of 2-sulfenylindenones by visible-light-mediated addition of sulfur-centered radicals to 1,3-diarylpropynones

Article abstract of DOI:10.1080/00397911.2019.1580744

A novel, visible-light-mediated method for the synthesis of 2-sulfenylindenones from easily available thiophenols (2.0 equiv.) and 1, 3-diarylpropynones (1.0 equiv.) in the presence of eosin Y (0.02 equiv.) under air atmosphere has been developed. The rea

Full text of DOI:10.1080/00397911.2019.1580744

Synthetic Communications
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ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Synthesis of 2-sulfenylindenones by visible-light-
mediated addition of sulfur-centered radicals to
1,3-diarylpropynones
Chengqun Chen, Qian Xiong & Jie Wei
To cite this article: Chengqun Chen, Qian Xiong & Jie Wei (2019): Synthesis of 2-
sulfenylindenones by visible-light-mediated addition of sulfur-centered radicals to 1,3-
diarylpropynones, Synthetic Communications, DOI: 10.1080/00397911.2019.1580744
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https://doi.org/10.1080/00397911.2019.1580744
Synthesis of 2-sulfenylindenones by visible-light-mediated
addition of sulfur-centered radicals to 1,3-diarylpropynones
Chengqun Chen, Qian Xiong, and Jie Wei
Department of Chemical Engineering, Fuzhou University Zhicheng College, Fuzhou, P.R. China
ABSTRACT
ARTICLE HISTORY
Received 7 October 2018
A novel, visible-light-mediated method for the synthesis of 2-sulfeny-
lindenones from easily available thiophenols (2.0 equiv.) and 1, 3-dia-
rylpropynones (1.0 equiv.) in the presence of eosin Y (0.02 equiv.)
under air atmosphere has been developed. The reaction proceeded
smoothly, for a wide range of derivatives of thiophenols and 3-diary-
lpropynones, to give the expected products in moderate to good
yields. Compared with traditional methods, this method is more con-
venient and avoids using stoichiometric amounts of I2 and K2S2O8.
KEYWORDS
Visible-light-mediate; inden-
one; eosin Y; 1; 3-
diarylpropynone; thiophenol
GRAPHICAL ABSTRACT
Introduction
Indenones are important structural motifs in a great number of pharmaceuticals and
biologically active molecules,[1–4] they also serve as versatile intermediates in organic
and pharmaceutical synthesis.[5,6] Hence, numerous methodologies have been developed
for the synthesis of these structural motifs. Amongst the various approaches to produce
indenones, the most commonly used methodologies are the intramolecular
Friedel–Crafts-type-cyclizations or the addition of Grignard reagents to indandiones.[7,8]
In comparison, Palladium or rhodium-catalyzed annulations of internal alkynes with 2-
halophenyl carbonyl compounds[9–11] or their equivalents[12–17] provided more direct
strategy for the synthesis of substituted indenones. Recently, the cyclization of 1, 3-dia-
rylpropynones triggered by superacids[18–20] has been shown to be an efficient method
for the synthesis of indenones. The cyclization can also be triggered by sulfur-centered
CONTACT Chengqun Chen
chenchq06@qq.com
Department of Chemical Engineering, Fuzhou University
Zhicheng College, Fuzhou 350002, P.R. China.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data for this article is available online at on the publisher's website.
ß 2019 Taylor & Francis Group, LLC

2
C. CHEN ET AL.
Scheme 1. Visible-light-induced synthesis of 2-sulfenylindenone derivatives.
radicals generating from the reaction of manganese(III) acetate with arylthiols, as
reported by the groups of Zou.[21,22] Very recently, similar tandem radical processes
have been developed for the construction of indenones.[23–25] Professor Zhang has
described an efficient approach to the synthesis of 2-sulfenylindenone derivatives
through a tandem annulation of arylpropynols with disulfides,[23] the reaction pathway
involves one-pot tandem Meyer–Schuster rearrangement of arylpropynols and successive
radical cyclization with disulfides. But, the reaction necessitated stoichiometric amounts
of I2 and K2S2O8. While significant progress has been made in the construction of
indenone skeletons, the development of new practical and general protocol for the syn-
thesis of diverse multisubstituted indenones is still strongly desired.
As a clean, inexpensive and sustainable energy source, visible light has shown wide
application prospects in organic synthesis.[26–31] From the viewpoint of energy effi-
ciency, mild reaction conditions, synthetic simplicity, step and cost economy and the
environment, the chemical transformations initiated by visible light (k ¼ 400–700 nm)
sources have attracted considerable attention over a decade.[32] However, the visible-
light-driven catalytic radical cyclization of 1,3-diarylpropynones has not been uncovered.
Herein, we describe a method for the synthesis of 2-sulfenylindenones derivatives from
1, 3-diarylpropynones and thiophenols under visible-light (green LEDs: kmax ¼ 535 nm)
photocatalyzed conditions (Scheme 1). In addition, a tentative mechanism is described
for the reaction.
Results and discussion
Cheap, environmentally friendly and commercially available eosin Y and 1a, 3-diphenyl-
propynone (4a) and thiophenol, respectively, were chosen as photocatalyst and model
substrates to optimize the reaction conditions. The optimization processes included
wavelength of radiation source, solvents, catalyst, and its quantity and atmosphere.
When a hexane solution containing 1, 3-diphenylpropynone (1.0 equiv.), thiophenol
(2.0 equiv.), and a catalytic amount of eosin Y (2.0 mol%) was irradiated by white LEDs
in an open vessel under ambient conditions for 20 h, the desired 3-phenyl-2-(phenyl-
thio)-1H-inden-1-one was obtained in 27% yield (Table 1, entry 1). Encouraged by these
initial results, we optimized the reaction conditions further to increase the reaction
yield. Interestingly, the yield of product enhanced to 38% when the reaction was per-
formed with blue LEDs (kmax ¼ 450 nm, Table 1, entry 2). It was gratifying that the
desired product could be selectively produced in 58% yield when the reaction was per-
formed with green LEDs (kmax ¼ 535 nm, Table 1, entry 3). In contrast, the yield of

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Table 1. The photocatalytic synthesis of 2-sulfenylindenone under various conditions.a
O
visible-light
catalyst
O
SPh
+
PhSH
Ph
solvent, r.t
Ph
Ph
1a
2a
3a
Entry
Catalystb (mol%)
Solvent
Time (h)
Yieldc (%)
1
2
3
4
5
6
7
8
Eosin Y (2)
Eosin Y (2)
Eosin Y (2)
Eosin Y (2)
Eosin Y (2)
Eosin Y (2)
Eosin Y (2)
Eosin Y (2)
Eosin Y (2)
Eosin Y (2)
Rose bengal (2)
Fluorescein (2)
Rhodamine (2)
Eosin B (2)
–
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DMSO
HOAc
CH3OH
CH2Cl2
DMF
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
20
20
20
20
20
27d
38e
58
38f
N.Rg
82
20
20–36
20–36
20–36
20–36
20
20
20
20
20
15
14
9
18
26
32
19
21
8
31
N.R
75
78
77
48h
23i
9
10
11
12
13
14
15
16
17
18
19
20
Eosin Y (4)
Eosin Y (6)
Eosin Y (6)
Eosin Y (2)
Eosin Y (2)
20
20
20
aReaction conditions: 1a (1 mmol), 2a (2 mmol) and 2.0 mol% of photocatalyst in dry solvent under an open-air atmos-
phere at room temperature using high power green LEDs.
bCommercially available high purity catalyst was purchased and utilized as such.
cIsolated yield.
dWhite LEDs used.
eBlue LEDs used.
fXenon lamp used.
gReaction was carried out in the dark.
hThe thiophenol declined to 1 mmol.
iReplacement of air(O2) with 30% H2O2 (1 mmol).
product was lowered to 21% when the reaction was irradiated with a xenon lamp
(Table 1, entries 4). However, no product was detected in a dark environment (Table 1,
entry 5). Afterward, the influence of solvents on the formation of 3-diarylpropynones
was explored. The investigation revealed that DMSO was the most effective medium to
promote the reaction with 82% yield (Table 1, entry 6) while the use of other solvents
resulted in lower yields (Table 1, entries 6–10) even after 36 h of irradiation. In add-
ition, several other photocatalysts including rose bengal, fluorescein, rhodamine, and
eosin B were also investigated (Table 1, entries 11–14). Compared to eosin Y, these cat-
alysts display much less efficiency toward the reaction. However, no product was
observed without photocatalyst (Table 1, entries 15). With an increase in the catalyst
amount from 2.0 mol% to 4.0–6.0 mol%, the reaction was accelerated obviously, but the
yield was not improved (Table 1, entries 16–18). Next, the decline of 2a has led to lower
yields due to the generation of the disulfide byproduct (Table 1, entry 19). Disulfide can
be prepared by the action of hydrogen peroxide.[33] Subsequently, we investigated the
influence of H2O2 on the reaction. If we replaced air (O2) with 30% H2O2, the yield was
declined to 23% (Table 1, entry 20).

4
C. CHEN ET AL.
Table 2. Substrate scope for the transformation of a mixture of thiophenol and a variety of
substituted 1, 3-diarylpropynones into 2-sulfenylindenone derivatives.a
Entry
R1–
R2–
3, yield (%)b
1
2
3
4
5
6
7
8
H
o–CH3O
o–F
o–Cl
o–Br
p–CH3
p–F
p–Cl
p–Br
p–CH3O
H
H
H
H
H
H
H
H
H
H
3a, 82
3b, 80
3c, 56
3d, 62
3e, 66
3f, 75
3g, 56
3h, 67
3i, 71
3j, 84
3k, 71
3l, 79
3m, 65
3n, 60
3o, 70
3p, 71
3q, 56
9
10
11
12
13
14
15
16
17
H
p–CH3
p–CH3O
p–C2H5
p–F
p–Cl
p–Br
o–CH3
H
H
H
H
H
H
aReaction conditions: 1 (1 mmol), 2a (2 mmol) and 2 mol% of eosin Y in DMSO under an open air atmosphere
at room temperature using high power green LEDs (kmax ¼ 535 nm) as a visible-light source for 20–36 h.
bIsolated yield.
Under the optimized reaction conditions, the reactions of a variety of substituted 1,
3-diarylpropynones 1 with thiophenol 2a were carried out (Table 2). Several diarylpro-
pynones bearing substituents at the ortho- and para-position of benzoyl, as well as
alkyne part, were used in the reaction. The results shown in Table 2 indicate that elec-
tron-donating substituents on the benzoyl of 1,3-diphenylpropynones favored the reac-
tions (Table 2, entries 1, 6, and 10), whereas electron-withdrawing groups on the
benzoyl slightly disfavored the reactions (Table 2, entries 3–5 and 7–9). The MeO group
at the ortho- or para-positions of the benzoyl ring gave similar yields (Table 2, entries 2
and 10). Electron-donating or electron-withdrawing groups on the alkyne part also gave
moderate yields (Table 2, entries 11–17).
Next, the scope of thiophenols was examined. As shown in Table 3, thiophenols bear-
ing electron-donating or withdrawing groups were applicable to the coupling with mod-
erate to good yields (Table 3). However, a strong dependence on the position of the
substituents was observed (Table 3, entries 1, 4, 7, and 8). For example, a reaction with
para-methyl substituted thiophenol afforded higher yield than ortho-methyl substituted
thiophenol (Table 3, entries 1 and 7).
To gain insight into the mechanistic profile, a series of control experiments were con-
ducted (Scheme 2). Initially, the present transformation was completely inhibited when
a stoichiometric amount of a radical scavenger TEMPO was added into reaction system
under the standard conditions and a TEMPO-trapped complex (4-MePhS-TEMPO) was

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Table 3. Substrate scope for the transformation of a mixture of various thiophenols and 1,
3-diphenylprop-2-yn-1-one into 2-sulfenylindenone derivatives.a
Entry
R3–
4, yield (%)b
1
2
3
4
5
6
7
8
p–CH3
p–CH3O
p–F
p–Cl
p–Br
p–NO2
o–CH3
o–Cl
4a, 84
4b, 76
4c, 59
4d, 68
4e, 67
4f, 63
4g, 64
4h, 59
aReaction conditions: 1a (1 mmol), 2 (2 mmol) and 2 mol% of eosin Y in DMSO under an open air atmosphere
at room temperature using high power green LEDs (kmax ¼ 535 nm) as a visible-light source for 20–36 h.
bIsolated yield.
Scheme 2. Control experiments.
detected by LC-MS analysis, most of 1a and 2b have been recovered. Thus, suggesting
that this transformation might involve a radical process (Scheme 2a). Furthermore, the
reaction did not proceed under an argon atmosphere, which indicates that air (O2) is
essential for the present transformation (Scheme 2b). Furthermore, when the reaction of
1a and p-Tolyl disulfide was carried out under the standard conditions, no desired
product was obtained (Scheme 2c).
On the basis of the experimental results and the previous reports,[23–25,34,35] we pro-
ꢀ
posed a tentative mechanism, as shown in Scheme 3. Initially, the excited state EY was

6
C. CHEN ET AL.
Scheme 3. Plausible reaction mechanism for the synthesis of 2-sulfenylindenones from the mixture of
thiophenols and 1, 3-diarylpropynones.
produced from Eosin Y under visible-light irradiation. A single-electron transfer (SET)
from the excited state of EY to thiol 2a gave the radical cation A and EYꢁꢂ radical
ꢀ
anion. The oxidation of EYꢁꢂ by oxygen (air) would lead to the formation of the
ground state Eosin Y and O2ꢁꢂ, subsequently, the deprotonation of the radical cation A
ꢁꢂ
by O2 produced the thiyl radical B and hydroperoxide radical species. The thiyl rad-
ical B reacted with the carbon atom of the alkynyl to generate the radical C.
Subsequently, C underwent intramolecular cyclization followed by hydrogen radical loss
to afford the desired product.
Conclusion
In summary, we have developed a novel, visible-light photoredox catalysis for the con-
version of a mixture of thiophenols and 1, 3-diarylpropynones into 2-sulfenylindenones.
This method has some advantages such as easy operation, atom economy, good func-
tional group tolerance, greenness and requires a very small quantity of non-metal cata-
lyst. Further investigation focused on expanding the scope of substrates of this method
is currently in progress.
Experimental
NMR spectra were performed on a Bruker Avance III 400 spectrometer (1H: 400 MHz;
1
13C: 100 MHz). H NMR chemical shifts were determined relative to Me4Si (0.0 ppm) as
an internal standard. 13C NMR chemical shifts were determined relative to CDCl3
(77.0 ppm). HRMS data were determined on a Bruker Daltonics APEXII 47e FT-ICR
spectrometer. Mass spectra were recorded by the EI method on a HP 5998 mass spec-
trometer. Melting points are uncorrected.

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Synthesis of 3-phenyl-2-(phenylthio)-1H-inden-1-one; typical procedure
To an oven-dried round, bottom flask equipped with a magnetic stir bar was charged
with eosin Y (2.0 mol% in respect to amount of 1a), 1a (0.21 g, 1 mmol), 2a (0.22 g,
2 mmol) and DMSO solvent. The mixture was stirred a few minutes to mix well and
then the flask was irradiated under a 5W green LED bulb at a distance of 5 cm. After
stirring at room temperature for 20 h (The progress of the reaction was monitored using
TLC), the solvent was removed under reduced pressure and the residue was then
diluted with water. The aqueous solution was extracted (three times) with ethyl acetate.
The combined organic phase was washed with brine, dried over anhydrous MgSO4, fil-
tered, and concentrated. The residue was purified using column chromatography [SiO2
column chromatography (petroleum ether (bp 60–90 ꢃC)/EtOAc ¼ 50:1)] to give 3-phe-
nyl-2-(phenylthio)-1H-inden-1-one (0.26 g, 82%).
Supplementary data Supporting information associated with this article can be found
via the “Supplementary Content” section of this article’s webpage.
Funding
We are grateful to the Fujian Province Young and Middle-aged Teacher Education Research
Project (JAT160655), Innovation Training Program for College Students (201813470007), and the
Fuzhou University Zhicheng College, for the support of this work.
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