Synthesis of 3,3,6,6-tetraaryl-1,2-dioxanes via TiO2-catalyzed photooxygenation of 1,1-diarylethenes in the presence of Mg(ClO 4)2

Article abstract of DOI:10.1246/cl.2004.462

Photooxygenation of methoxy-substituted 1,1-diarylethenes and 1,1,8,8-tetraaryl-1,7-octadienes catalyzed by titanium dioxide proceeded via photoinduced electron transfer to give 3,3,6,6-tetraaryl-1,2-dioxanes in high yields. The photooxygenation was remarkably accelerated by the addition of Mg(ClO₄)₂.

Full text of DOI:10.1246/cl.2004.462

4
62
Chemistry Letters Vol.33, No.4 (2004)
Synthesis of 3,3,6,6-Tetraaryl-1,2-Dioxanes via TiO -catalyzed Photooxygenation
2
of 1,1-Diarylethenes in the Presence of Mg(ClO4)2
ꢀ
Hajime Maeda, Hisayuki Miyamoto, and Kazuhiko Mizuno
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531
(Received January 20, 2004; CL-040078)
Photooxygenation of methoxy-substituted 1,1-diarylethenes
Table 1. Photooxygenation of 1,1-Diarylethenes Catalyzed by
Titanium Dioxidea
and 1,1,8,8-tetraaryl-1,7-octadienes catalyzed by titanium diox-
ide proceeded via photoinduced electron transfer to give 3,3,6,6-
tetraaryl-1,2-dioxanes in high yields. The photooxygenation was
remarkably accelerated by the addition of Mg(ClO4)2.
b
Substrate Additive Irradiation Products and yields
time/min
Recovery of
substrateb
1
a
a
none
Mg(ClO4)2
none
40
40
40
40
40
960
40
40
40
40
40
40
40
2a (6%), 3a (10%)
2a (97%)
56%
0%
88%c
73%
91%c
0%
1
c
1
b
3b (12%)
3b (8%)
Semiconductor-catalyzed photoreactions of organic mole-
1
1b
4a
Mg(ClO4)2
none
none
c
cules are of current interest. Semiconductors such as TiO2,
5a (9%)
c
c
4a
4a
4a
4a
4a
4a
5a (75%) , 3a (25%)
5a (71%)
CdS, ZnS are utilized as redox type heterogeneous photocata-
lysts for various types of photoinduced electron tranfer reactions
using photoexcitation from their valence bands to conduction
Mg(ClO4)2
KClO4
0%
c
66%c
28%
44%
40%
97%
87%
5a (34%)
NaClO4
LiClO4
LiBF4
5a (25%)
5a (41%)
5a (51%)
—
2
bands. Hitherto various alkenes are exposed to the semiconduc-
tor-catalyzed photoreactions in the presence of oxygen, and it is
already known that the alkenes are converted to ketones, alde-
4
4
b
b
none
Mg(ClO4)2
3
hydes, epoxides, hydroperoxides, and so on. However, the effi-
—
ciency, selectivity, and chemical yields are not necessarily high
in these photoreactions. To explore the synthetic utility for such
reactions, we have investigated the photooxygenation of me-
thoxy-substituted 1,1-diarylethenes and found that 3,3,6,6-tet-
raaryl-1,2-dioxanes were obtained in high yields.
Photoirradiation of an acetonitrile solution containing 1,1-
bis(p-methoxyphenyl)ethene (1a) in the presence of suspended
TiO2 and Mg(ClO4)2 under a constant stream of dioxygen
through a Pyrex filter by a 300 W-high pressure mercury lamp
for 40 min afforded 3,3,6,6-tetrakis(p-methoxyphenyl)-1,2-di-
oxane (2a) in a 97% isolated yield (Scheme 1, Table 1). In the
absence of Mg(ClO4)2, 2a was slowly formed accompanying
methoxy-substituted benzophenone 3a. Photoreaction of p-
chloro derivative 1b gave a small amount of benzophenone de-
rivative 3b without any formation of 2b. Photooxygenation of
a
Conditions: Substrate (0.25 mmol), TiO2 (10 mg), additive (0.125 mmol),
CH3CN (15 mL), oxygen bubbling, rt. b Isolated yields. c Determined by H
1
NMR.
trans-fused 1,2-dioxane derivative 5a in a 71% isolated yield.
In the absence of Mg(ClO4)2, prolonged photoirradiation was re-
quired to obtain 5a. Addition of other inorganic salts such as
KClO4, NaClO4, LiClO4, and LiBF4 also enhanced the rate of
the photooxygenation, but their efficiency was lower than that
of Mg(ClO4)2. Photoreaction of unsubstituted 1,1,8,8-tetraphen-
yl-1,7-octadiene (4b) resulted in almost recovery of 4b.
Irradiation of TiO2 in acetonitrile causes the promotion of an
electron into the conduction band (ꢁ0:8 V vs SCE) to generate a
1
,4
hole in the valence band (þ2:4 V vs SCE). One electron trans-
5
fer from 1a (Eox ¼ 1:32 V vs SCE in CH3CN) and 4a (Eox ¼
þ
6
0
:95 V vs Ag/Ag in CH3CN) to the valence band should take
1,1,8,8-tetrakis(p-methoxyphenyl)-1,7-octadiene (4a) in the
place as an exothermic process to produce the radical cations of
these alkenes, because their oxidation potentials are sufficiently
presence of Mg(ClO4)2 proceeded intramolecularly to give
1
a,b
low.
to O2 (E red ¼ ꢁ0:86 V vs SCE in CH3CN) is thermodynami-
In addition, electron transfer from the conduction band
0
h
ν
(> 280 nm)
7
Ar
Ar
Ar
Ar
ꢂ
1a,3a,b,g
TiO , additive Ar
Ar
Ar
ꢁ
2
cally permissible process to produce O2
.
From these re-
+
O2
+
O
CH CN
Ar
sults, we propose a plausible mechanism for the TiO2-catalyzed
photooxygenation of 1,1-diarylethenes as exemplified in
Scheme 2 using the photoreaction of 4a. The initial step is the
excitation of TiO2 followed by one-electron transfer from 4a
to the valence band of TiO2 to produce monomer radical cation
of 4a. The monomer radical cation of 4a cyclizes intramolecu-
larly to give distonic radical cation 6a via dimer radical cation
of 4a. The attack of triplet dioxygen on the 1,4-radical cation
6
rivative 5a by one-electron reduction. The other possible path-
way for the photooxygenation is the attack of O2 to 6a. Forma-
tion of benzophenone derivative 3a can be explained by the
decomposition of dioxetane 8a, which is produced by the oxy-
genation of monomer radical cation. The enhancement of the ef-
3
O O
1
1
a (Ar = p-MeOC H )
2a
3a-b
6
4
b (Ar = p-ClC H )
6
4
hν (> 280 nm)
Ar
TiO , additive
2
Ar
+
O2
Ar
CH CN
3
Ar
a (Ar = p-MeOC H )
4
4
6
4
a generates 1,6-radical cation 7a which gives 1,2-dioxane de-
b (Ar = C H )
H
H
6
5
Ar
Ar
Ar
Ar
Ar
+
O
ꢂ
ꢁ
Ar
O O
5
a
3a
Scheme 1.
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.4 (2004)
463
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8
a
Scheme 2.
ficiency by the addition of Mg(ClO4)2 can be explained by the
ꢂ
suppression of back electron transfer from TiO2 to 4a due
ꢁ
þ
4
2þ
ꢁ
to the interaction between Mg and ionic species such as TiO2
ꢁ
ꢂ
ꢂ
2r,8–10
ꢁ
þ
and O2 or the interaction between ClO4 and 4a ,
Photooxygenation of 1a and 4a to give 1,2-dioxanes under
photoinduced electron transfer conditions have already been re-
ported in homogeneous solutions using electron accepting pho-
5
6
(
1993).
5
,6,11
tosensitizers such as 9,10-dicyanoanthracene.
But the pres-
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ent system using TiO2 as a heterogeneous photocatalyst has a
notable advantage; the photocatalyst can be removed only by
the filtration of the reaction mixture. For synthetic use, TiO2 is
a clean, low cost, efficient, and facile tool for the synthesis of
,3,6,6-tetraaryl-1,2-dioxanes.
In conclusion, it was demonstrated that 1,2-dioxane deriva-
tives can be synthesized by the photooxygenation of electron-
rich diarylalkenes by use of TiO2 as a heterogeneous photocata-
lyst. Scope and mechanisms are now under investigation.
7
8
3
(
(
1994), Vol. 169, p 301. g) S. Fukuzumi, Bull. Chem. Soc. Jpn., 70, 1
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We would like to thank Prof. M. Anpo, Dr. M. Matsuoka,
Dr. M. Takeuchi, Ms. R. Takeuchi, and Mr. H. Nakagawa at
Osaka Prefecture University for measurement of diffuse reflec-
tance spectra. We also thank Ishihara Sangyo Co., Ltd., for a gift
of TiO2 (ST-21). This work was supported by a Grant-in-Aid for
Scientific Research on Priority Areas (417) from the Ministry of
Education, Culture, Sports, Science and Technology (MEXT) of
Japan.
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(2002). j) S. Fukuzumi, Pure Appl. Chem., 75, 577 (2003).
Diffuse reflectance spectrum of suspension of TiO2 in CH3CN was
not changed by the addition of Mg(ClO4)2.
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9
1
ꢁꢂ
2þ
þ
þ
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2
Published on the web (Advance View) March 20, 2004; DOI 10.1246/cl.2004.462