Photochemical reaction of anthracene with dioxygen catalyzed by platinum(II) porphyrin

Article abstract of DOI:10.1016/j.tetlet.2019.151081

Visible light irradiation of anthracene in the presence of platinum(II) porphyrin (PtPor) and dioxygen affords anthraquinone as the main product. This reaction involves two steps: the [4+2]cycloaddition of anthracene with singlet oxygen to afford anthracene-9,10-endoperoxide and the degradation of endoperoxide. PtPor catalyzes both reactions.

Full text of DOI:10.1016/j.tetlet.2019.151081

Tetrahedron Letters 60 (2019) 151081
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Tetrahedron Letters
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Photochemical reaction of anthracene with dioxygen catalyzed by
platinum(II) porphyrin
Ken-ichi Yamashita a,b, , Ken-ichi Sugiura a,
⇑
⇑
a
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan
Department of Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Visible light irradiation of anthracene in the presence of platinum(II) porphyrin (PtPor) and dioxygen
affords anthraquinone as the main product. This reaction involves two steps: the [4+2]cycloaddition of
anthracene with singlet oxygen to afford anthracene-9,10-endoperoxide and the degradation of endoper-
oxide. PtPor catalyzes both reactions.
Received 26 June 2019
Revised 22 August 2019
Accepted 23 August 2019
Available online 24 August 2019
Ó 2019 Elsevier Ltd. All rights reserved.
Keywords:
Photocatalysis
Photooxidation
Photosensitization
Porphyrinoids
Photochemical reactions, particularly photocatalytic reactions,
have attracted considerable attention because they utilize solar
energy and/or initiate new reactions. Recently, photoredox cata-
lysts have been extensively studied [1]. Heavy metal complexes,
such as ruthenium(II) or iridium(III), have been widely used as
effective photoredox catalysts owing to their high chemical stabil-
ity, visible-light absorption properties, and long-lived excited
states; thus, these complexes can induce electron transfer
processes.
1
Scheme 1. Reaction of anthracenes with photo-generated singlet oxygen ( O
afford anthracene-9,10-endoperoxides.
2
) to
Another important photocatalytic reaction is the generation of
1
singlet oxygen ( O
2
) resulting from the energy transfer from the
properties derived from their characteristic
p-electron systems.
photoexcited catalyzing (sensitizing) molecule (e.g., tetraphenyl-
Among these complexes, platinum(II) porphyrins (PtPor) have
attracted special attention owing to their unique photophysical
properties, such as their strong red phosphorescence at room tem-
perature. Therefore, many applications, such as organic light-emit-
porphyrin, methylene blue, or rose bengal) to the ground-state tri-
3
) [2]. Because 1
plet oxygen ( O
2
O
2
is highly reactive, it can react
1
with various compounds [3]. The [4+2]cycloaddition of
dienes is a representative reaction, e.g., anthracene derivatives
2
O with
ting diodes (OLEDs) [7], organic solar concentrators [8], O
2
sensors
1
react with
O
2
to afford anthracene-9,10-endoperoxides
[
9], and pressure-sensitive paints [10], employ PtPor. Despite these
(
Scheme 1). Such photocatalysts have also been used in photody-
namic therapy (PDT) [4].
advancements, the photocatalytic properties of PtPor have not yet
been widely explored [11]. Our investigations of the synthetic and
structural chemistry of PtPor [12] motivated us to further study
their photocatalytic activities in photoredox and
reactions. This study reports the photocatalytic properties of a
Porphyrins and their metal complexes have been used as effec-
tive photocatalysts (in both photoredox [5] and 1
O
2
-generating
1
2
O -generating
[
2,4e,6] reactions) because of their intense visible-light absorption
platinum(II) tetraarylporphyrin and its related compounds, 1(M)
(
M = 2H, Pt, Pd, and Ni, Fig. 1), in the reaction of anthracene (2)
with O . The results revealed that the reactions involving 1(Pt) or
(Pd) primarily afforded anthraquinone. This transformation has
rarely been observed in the photocatalytic reactions of 2 [13].
⇑
Corresponding authors at: Department of Chemistry, Graduate School of
Science and Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa,
Hachioji, Tokyo 192-0397, Japan (Ken-ichi Yamashita).
2
1
E-mail addresses: yamashita-k@chem.sci.osaka-u.ac.jp (K.-i. Yamashita),
sugiura@porphyrin.jp (K.-i. Sugiura).
https://doi.org/10.1016/j.tetlet.2019.151081
0
040-4039/Ó 2019 Elsevier Ltd. All rights reserved.

2
K.-i. Yamashita, K.-i. Sugiura / Tetrahedron Letters 60 (2019) 151081
Fig. 1. Chemical structure of tetraarylporphyrin complex 1(M).
The photochemical reactions of 2 with O
M) under various reaction conditions were initially examined.
Typically, a stirred solution of 2 (0.020 mmol) and a catalytic
amount of 1(M) (0.05–0.2 equiv.) in CDCl (1 mL) was irradiated
2
in the presence of 1
(
3
2
using a 120-W halogen lamp at 298 K in an O atmosphere. The
Fig. 2. 1H NMR (500 MHz, CDCl
by 1(Pt) (0.1 equiv.): (a) before irradiation and after irradiation for (b) 15 min and
c) 180 min.
) spectra showing the photoreaction of 2 catalyzed
reaction progress was monitored using 1H NMR spectroscopy.
3
The results are summarized in Table 1. The use of 1(2H) quantita-
(
tively afforded the endoperoxide 3 via the [4+2]cycloaddition of 2
1
with in situ-generated O
2
(Table 1, entry 1), which is in agreement
with previous reports [14]. In contrast, the reaction in the presence
of 1(Pt) for 15 min afforded not only 3 but also a small amount of
anthraquinone (4). In addition, 2 was completely consumed
The amount of 1(Pt) did not affect the yield of 4 but had a profound
effect on the reaction rate because 0.05 and 0.2 equiv. of 1(Pt)
required 240 and 120 min of reaction time, respectively (Table 1,
entries 5 and 6; Fig. 3).
(Table 1, entry 2; Fig. 2b). Further irradiation decreased the yield
of 3 and increased the yields of 4 and several minor byproducts.
Isolation and identification of the byproducts by 1H NMR and/or
GC-MS were failed. Then, 3 was completely consumed after
1
80 min, and 4 was formed in a 25% yield (Table 1, entry 4; Fig. 2c).
Table 1
2
Photochemical reaction of 2 catalyzed by 1(M) in an O atmosphere.
Entry
1(M) (equiv.)
Time (min)
NMR yields (%)a
3
4
1
2
3
4
5
6
7
8
1(2H) (0.10)
1(Pt) (0.10)
1(Pt) (0.10)
1(Pt) (0.10)
1(Pt) (0.05)
1(Pt) (0.20)
1(Pd) (0.10)
1(Ni) (0.10)
60
15
60
180
240
120
300
120
>99
76
26
<1
2
0
5
16
25
27
30
17
0
0
3
0
b
Fig. 3. Time profiles of the product yields (2: black square, 3: red triangle, 4:
blue circle) for the photoreaction of 2 catalyzed by 0.05 (a), 0.1 (b), and 0.2 equiv. (c)
of 1(Pt).
a
Otherwise noted, starting material 2 was completely consumed.
Recovery of 2.
b

K.-i. Yamashita, K.-i. Sugiura / Tetrahedron Letters 60 (2019) 151081
3
Subsequently, we examined the reaction using palladium and
nickel analogs, 1(Pd) and 1(Ni), to evaluate the dependence of
the reaction on the central metal ions of the porphyrins. In the case
of 1(Pd), the starting material 2 was completely consumed within
In a plausible reaction mechanism, the first step is the [4+2]cy-
1
2
cloaddition of O with 2 to afford intermediate 3. During the sec-
ond step, O does not play any role in the formation of 4, and 1(Pt)
2
is not consumed. Thus, the second reaction is the photodispropor-
3
0 min, and formation of 3 and 4 was observed (Fig. 4). However,
tionation of 3 catalyzed by the triplet excited state of 1(Pt), which
the consumption of 3 required a long time (300 min) (Table 1,
entry 7; Fig. 4). In contrast, the anticipated products (3 or 4) were
not obtained in the presence of 1(Ni) (Table 1, entry 8). This lack of
reactivity likely reflects the considerably short-lived excited states
of Ni(II) porphyrins [15].
can be easily quenched by O
2
, thereby inhibiting the second reac-
tion in O atmosphere. In this mechanism, half of 3 is oxidized to 4,
2
and the remaining amount is reduced to several uncharacterized
species. This result is consistent with the low yields of 4 (<50%)
under all reaction conditions.
To probe the reaction mechanism, the photoreaction of 3, which
was purified in advance, was performed under various conditions
The reported redox properties of 3 and the analogous com-
pound of 1(Pt) also support our proposed mechanism involving
photodisproportionation of 3. Considering the redox potential of
(Scheme 2). The reaction of 3 under the same reaction conditions
+
À
+
13
as that of 2 afforded 4 in almost the same yield (24%) for the same
reaction time. No reaction proceeded in the absence of 1(Pt), indi-
cating that 1(Pt) catalyzed the photochemical transformation of 3–
3 (E(3/3 ) = 1.65 V, and E(3 /3) = À1.32 V vs. Fc/Fc in CH
3
CN)
and the photoexcited state of PtTPP (TPP = tetraphenylporphyri-
+
À
nato dianion, E(PtTPP*/[PtTPP] ) = À1.32 V and E([PtTPP] /
+
4. To clarify the role of O
2
, 3 was reacted in the absence of O
2
. Inter-
2 2
PtTPP*) = 0.23 V vs. Fc/Fc in CH Cl ) [5b], single electron transfer
estingly, the reaction reached completion within 15 min, which is
considerably shorter than the time required for reaction comple-
from 3 to 1(Pt)* is thermodynamically unfavorable while that
from 1(Pt)* to 3 is more favorable. This suggests that the first
step of the second reaction is the reduction of 3 by 1(Pt)* to
2
tion in the presence of O . These results strongly indicated that
+Å
O
2
did not act as an oxidant for 3 but acted as an inhibitor.
afford uncharacterized species and 1(Pt) . Then, 4 might be
+Å
Although UV- [16], thermal- [16], silicagel- [17], and base-medi-
ated [14b,18] degradation of 3 has been reported earlier, the
degradation of 3 without 1(Pt) was not observed under our
reaction conditions likely because 3 only absorbs UV light. Thus,
the triplet photoexcited state of 1(Pt) (1(Pt)*) directly interacts
with 3 to form 4.
The formation of 3 from 2 mediated by a photosensitizer has
been extensively studied; however, the transformation of 2 or 3
to 4 by a photoredox catalyst has been rarely reported. Fukuzumi
et al. reported the photooxidation of 2 using the 9-mesityl-10-
methylacridinium ion as the catalyst [13], wherein 3 was detected
formed by the oxidation involving 1(Pt) .
The photocatalytic ability of 1(Pd) to the photodisproportiona-
tion reaction of 3 revealed herein was inferior to that of 1(Pt).
Comparing to 1(Pt), 1(Pd) has 10 times longer excitation life-time
(
s
(PtTPP) = 250
ls and s (PdTPP) = 2500 ls) [19], almost the same
+
reduction potential in the excited state (E(PdTPP*/[PdTPP] ) =
+
À1.30 V vs. Fc/Fc in CH
2 2
Cl ), and lower oxidation potential in
+
the ground state (E(PtTPP/[PtTPP] ) = 0.71 V and E(PdTPP/
+
+
2 2
[PdTPP] ) = 0.64 V vs. Fc/Fc in CH Cl ) [5b]. Therefore, the oxida-
tion potential of 1(M) might be more prominent factor than the
excitation life-time of 1(M) in this reaction.
as an intermediate. However, the formation of 4 required O
2
as the
In conclusion, we demonstrated the photooxidation of 2 to 4
oxidizing agent. Moreover, the yield of 4 was higher (ca. 75%) than
that obtained in this study, suggesting that the two transforma-
tions occur via two distinct mechanisms.
catalyzed by PtPor, which mediated not only the generation of
O but also the photo-induced disproportionation reaction. Con-
2
1
trary to Fukuzumi’s report on the photooxidation of 2 using the
-mesityl-10-methylacridinium ion, the reaction reported herein
does not require O as an oxidizing agent to transform 3 into 4.
9
2
To the best of our knowledge, similar reaction types have not been
reported, and we anticipate that these findings will contribute to
the design of new photocatalysts based on PtPor. Further mecha-
nistic investigations and an exploration of the scope of reactivity
are currently underway.
Acknowledgment
We thank Professor Motoko S. Asano (Gunma University) for
helpful discussion.
Appendix A. Supplementary data
Fig. 4. Time profile of the product yields (2: black square, 3: red triangle, 4: blue
circle) for the photoreaction of 2 catalyzed by 1(Pd) (0.1 equiv.).
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2019.151081.
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