Green-light-driven thioxanthylium-based organophotoredox catalysts: Organophotoredox promoted radical cation Diels-Alder reaction

Article abstract of DOI:10.1016/j.tetlet.2018.07.058

Thioxanthylium-based organophotoredox catalysts that operate under irradiation with green light have been developed. These catalysts present high excited-state reduction potentials (E₀′(C^?/C^?) = +1.79–1.94 V vs SCE). They

Full text of DOI:10.1016/j.tetlet.2018.07.058

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Green-light-driven thioxanthylium-based organophotoredox catalysts: Organo-
photoredox promoted radical cation Diels-Alder reaction
Kenta Tanaka, Mami Kishimoto, Mayumi Sukekawa, Yujiro Hoshino, Kiyoshi
Honda
PII:
S0040-4039(18)30949-3
https://doi.org/10.1016/j.tetlet.2018.07.058
TETL 50167
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Reference:
Tetrahedron Letters
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24 July 2018
27 July 2018
Please cite this article as: Tanaka, K., Kishimoto, M., Sukekawa, M., Hoshino, Y., Honda, K., Green-light-driven
thioxanthylium-based organophotoredox catalysts: Organophotoredox promoted radical cation Diels-Alder
reaction, Tetrahedron Letters (2018), doi: https://doi.org/10.1016/j.tetlet.2018.07.058
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Tetrahedron Letters
Green-light-driven thioxanthylium-based organophotoredox catalysts:
Organophotoredox promoted radical cation Diels-Alder reaction
Kenta Tanaka, Mami Kishimoto, Mayumi Sukekawa, Yujiro Hoshino,* Kiyoshi Honda*
Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Thioxanthylium-based organophotoredox catalysts that operate under irradiation with green light
have been developed. These catalysts present high excited-state reduction potentials (E0’
(C*/C−) = +1.79–1.94 V vs SCE). They are able to efficiently activate dienophiles under green
or blue light irradiation afforded the targeted radical cation Diels-Alder cycloadducts in good
yields. The present thioxanthylium-based catalysts provide a new green-light-driven photoredox
catalysis system.
Available online
Keywords:
Organophotoredox catalyst
Green light
2009 Elsevier Ltd. All rights reserved.
Thioxanthylium
Radical cation Diels-Alder reaction
1. Introduction
photoredox catalysts have been reported, despite the desirability
of this approach.7
Chemical transformations mediated by visible light have
attracted much attention due to their potential in sustainable
energy-conversion systems. The current global situation with
regard to the balance between the demand and supply of energy
and its associated environmental implications could be
ameliorated by developing clean systems for the chemical
conversion of solar energy.1 In organic synthesis, photoredox
catalysts can be used to carry out a wide variety of organic
transformations (Figure 1).2 Polypyridyl complexes of ruthenium
and iridium are among the most widely used photoredox
catalysts, as they usually exhibit high activity and versatility.3
However, such iridium and ruthenium catalysts are usually toxic
and expensive. In light of the disadvantages associated with
noble-metal-based catalysts, the recent focus of interest has
shifted toward the development of metal-free photoredox
catalysts, which represent a more cost-effective and sustainable
approach.4 Despite the promising properties of recently reported
catalysts, including metal-based and metal-free photoredox
catalysts, these normally suffer from narrow absorption windows
and require the use of high-energy light sources. On the other
hand, green light is of lower energy (500–600 nm) than blue and
ultraviolet, thus efficiently reducing the risk of retinal damage
and stress.5 Eosin Y is the most widely used organic photoredox
catalyst using green light as the irradiation source, and it can be
employed in a wide variety of organic transformations.6
However, despite its utility, it suffers from a low excited-state
reduction potential, which narrows the scope of substrates, and
from poor solubility in common organic solvents. Moreover, to
the best our knowledge, only a few green-light-activated
Figure 1. Absorption and excited-state reduction potential of
representative photoredox catalysts

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Tetrahedron Letters
Table 1. Synthesis and physical properties of thioxanthylium-based photocatalysts
Photocatalyst
E0’(C*/C−)a (V)
E0’(C/C−)b (V)
Absorption
λmax (nm)/ε × 10-3
(mol-1 dm3 cm-1)
468/31.7
Absorption
λ (nm)/ε × 10-3
(mol-1 dm3 cm-1)
520/5.5
Excitation
Emission
λmax (nm)
λmax (nm)
TXT1
TXT2
TXT3
TXT4
+1.83
+1.94
+1.86
+1.79
-0.31
-0.27
-0.35
-0.35
468
461
463
478
636
659
620
666
461/28.3
520/6.7
463/42.3
520/7.4
478/32.8
520/6.9
a
Excited-state reduction potentials were estimated from ground-state redox potentials and the intersection of the absorption and
emission bands.4a
b Determined by cyclic voltammetry in acetonitrile vs SCE. See the Supporting Information.
Thioxanthylium is a useful compound in the material and biological fields.8 It is known that thioxanthylium has absorption in
green region.9 However, 1-, 3-, 6- and 8-positions of thioxanthylium are amenable to be attacked by nucleophiles, and benzene
group at the 9-position is a key substituent group for stabilizing thioxanthylium core.10 Then, in order to expand the utility of
photoredox catalysts, we report herein thioxanthylium-based organophotoredox catalysts that operate under irradiation with green or
blue light. These organophotoredox catalysts afford radical cation Diels–Alder cycloadducts in high yields.
We initially synthesized a series of thioxanthylium salts to probe the relationship between the structure and physical properties of
these novel catalysts (Table 1). Their synthesis was accomplished through the reaction of a thioether that was treated in
chlorobenzene at 120 ºC for 1 h with benzoyl chloride in the presence of TfOH. These catalysts exhibit high excited-state reduction
potentials (E0’(C*/C−) = +1.79–1.94 V vs SCE). The substrate scope of such catalysts should therefore be larger than that of eosin Y
(E1/2(C*/C−) = +0.83 V vs SCE).6 Importantly, these thioxanthylium salts exhibit an absorption band in the blue/green region of the
visible spectrum (~400–600 nm), which indicates that these catalysts may be activated by visible light. As shown in Table 1, almost
the same physical properties were observed in spite of having different substituent groups on benzene ring at the 9-position. In
addition, when the solvent effects were examined using MeCN, CH3NO2, DMSO and MeOH, no substantial shifts in both UV-vis
and emission spectra were observed (see Supporting Information), which exhibit these catalysts would have -* transition state.
Importantly, it is expected that 9-aromatic substituent on thioxanthylium would a key important group for stabilizing catalysts.10,11
Recently, visible-light-driven radical cation Diels–Alder reactions catalyzed by metal-based photoredox catalysts have been
developed that employ ruthenium complexes under irradiation with visible light or chromium complexes under irradiation with
NUV light.12 From a fundamental perspective, as well as with respect to practical applications, the relatively low cost and the fact
that these photocatalytic systems operate with visible light are highly desirable. Subsequently, the radical cation Diels-Alder
reaction of trans-anethole (1) with 2,3-dimethylbutadiene (2) was screened in the presence of an organophotoredox catalyst (Table
2). First, all blank experiments, i.e., in the absence of a catalyst, light source, and air (degassed CH3NO2 was used under N2),
afforded trace amounts of the product 3a (entries 1 and 2), and 4% yield of the product (entry 3). When the reaction was carried out
with 1 (0.5 mmol), 2 (1.5 mmol), and TXT1 (1.0 mol%) in CH3NO2 (0.0625 M) at room temperature under air and irradiation with
blue light, excellent yield was obtained within 15 minutes (entries 4). This should partly be due to the protective nature of the o-
trifluoromethyl group, which minimize any undesirable reactivity of the thioxanthylium chromophore with exogenous nucleophiles
and transient radicals.11 On the other hand, it was assumed that such p-methoxy substituted salts (TXT2) are easily attacked by
nucleophiles.11c Indeed, good yield of the product was obtained after 15 h (entry 5). The parent phenyl group (TXT3) and halogen
groups on benzene ring (TXT4) were also suitable in the reaction, giving the corresponding product in high yields (entries 6 and 7).
Next, other representative organic photoredox catalysts, such as eosin Y, Acr+-Mes, and TPT were screened. The use of eosin Y,
whose excited-state reduction potential (E1/2(C*/C−) = +0.83 V vs SCE) is lower than that of trans-anethole 1 (Ep/2 = +1.24 V vs
SCE),13 did not result in a reaction (entry 8). Acr+-Mes and TPT, which have high excited-state reduction potentials and can be used
under irradiation with blue light, afforded the product in low yields for 15 min. But when the reaction time was changed to 1.5 h,
starting material 1 was consumed and high yields were observed (entries 9 and 10). Surprisingly, when green light was used as the
light source, all of thioxanthylium catalysts led to the formation of the desired product in excellent yields (entries 11-14). As
expected, eosin Y did not afford any product owing to its low excited-state reduction potential (entry 15). Moreover, Acr+-Mes and
TPT did not efficiently afford the product for 0.25 h or 15 h, given their inefficient absorption of green light (entries 16 and 17).

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Table 2. Optimization of the reaction conditionsa
Entry
1
Time (h)
0.25
Light
Blue
Catalyst
No catalyst
TXT1
Yield (%)
0
0
2
3b,c
0.25
No light
Blue
0.25
TXT1
4
4
0.25
Blue
TXT1
92
5
15
Blue
TXT2
94
6
1.15
Blue
TXT3
89
7
0.75
Blue
TXT4
89
8
0.25
Blue
Eosin Y
Acr+-Mes
TPT
0
9
0.25, (1.5)
0.25, (1.5)
0.25
Blue
5b, (91)
16b, (91)
88
10
11
12
13
14
15
16
17
Blue
Green
Green
Green
Green
Green
Green
Green
TXT1
15
TXT2
92
1.15
TXT3
91
0.5
TXT4
95
0.25, (15)
0.25, (15)
0.25, (15)
Eosin Y
Acr+-Mes
TPT
0, (0)
0, (11)b
0, (0)
a All reactions were carried out with 1 (0.5 mmol), 2 (1.5 mmol), and the catalyst (1.0 mol%) in the specified solvent (0.0625 M)
at room temperature under air and irradiation with visible light. b NMR yield. c Under N2.
With the optimal conditions in hand, a variety of dienes and dienophiles were examined to evaluate the generality of the reaction
(Table 3). When isoprene and myrcene were used as a diene, the desired products were obtained in 94% and 84%, respectively (3b
and 3c). When benzyloxy group was used instead of methoxy group on benzene ring of dienophile, the reaction afforded the
corresponding cycloadduct in high yield (3d). In addition, isosafrole also gave good yield in the reaction (3e). When 1,2-dimethoxy-
4-(1-propenyl)benzene was reacted with isoprene or 2,3-dimethylbutadiene, medium yields were observed (3f and 3g).

4
Tetrahedron Letters
Table 3. Scope and limitation of the reaction
a NMR yield
Scheme 1. Proposed reaction mechanism
A proposed mechanism for this reaction is shown in Scheme 1. Excitation of the photocatalysts (PC*), whose E0’(C*/C−) values
range between +1.79 and +1.94 V (vs SCE), under irradiation with visible light enable the oxidation of trans-anethole 1 (Ep/2
=
+1.24 V vs SCE).13 In addition, the results of Stern-Volmer experiments suggest that the electron transfer from trans-anethole to the
photocatalyst should occur smoothly.14 The reduced photocatalyst (PC¯·) can be regenerated into the original photocatalyst (PC) via
single-electron transfer from O2. The resulting radical cation A undergoes a [4+2] cycloaddition to generate the radical cation
product B. Finally, the transfer of a single electron from the superoxide radical (based on Table 2, entry 3) or trans-anethole 1
afforded the desired product 3.15 According to the reaction quantum yield experiment shown in supporting information, the reaction
would mainly proceed through the radical chain process. The scope and mechanism are currently under investigation and will be
reported in due course.
Finally, when the reaction was performed on the gram scale under sunlight irradiation, the reaction furnished the desired product
after 5 min in 84% yield (1.5 g, Scheme 2). This means that the present thioxanthylium-based catalytic system provides a
sustainable synthetic method using only 1.0 mol% of the organocatalyst and solar energy.
Scheme 2. Reaction carried out under irradiation with sunlight
In conclusion, thioxanthylium-based organophotoredox catalysts that operate under irradiation with green light have been developed.
These organophotoredox catalysts absorb light in the visible region (~400–600 nm) and have the high excited-state reduction
potentials (E0’(C*/C−) = +1.79–1.94 V vs SCE). Catalytic amounts (1.0 mol%) of the thioxanthylium catalysts are able to efficiently

5
promote radical cation Diels-Alder reactions under irradiation with green, which cannot be activated by any of the representative
organic photocatalysts. In addition, the utility of this system was demonstrated on the gram scale under irradiation with solar light.
The present catalysts provide a new green-light-driven photoredox catalytic system.
Acknowledgements
We thank Professor Mahito Atobe of Yokohama National University for the use of the linear sweep voltammetry measurements.
References and notes
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Tetrahedron Letters
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Green-light-driven thioxanthylium-based
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promoted radical cation Diels-Alder reaction
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Kenta Tanaka, Mami Kishimoto, Mayumi Sukekawa, Yujiro Hoshino,* Kiyoshi Honda*