Photodimerization and photooxygenation of 9-vinylcarbazole catalyzed by titanium dioxide and magnesium perchlorate

Article abstract of DOI:10.1016/j.cclet.2009.11.020

Photoreaction of 9-vinylcarbazole in acetonitrile in the presence of titanium dioxide and a catalytic amount of magnesium perchlorate gave 3,6-di(9-carbazolyl)-1,2-dioxane as a photooxygenated product via photodimerization of 9-vinylcarbazole. The photoreaction proceeds via an electron transfer mechanism, where magnesium perchlorate accelerated formation of the photooxygenated product.

Full text of DOI:10.1016/j.cclet.2009.11.020

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 365–368
www.elsevier.com/locate/cclet
Photodimerization and photooxygenation of 9-vinylcarbazole
catalyzed by titanium dioxide and magnesium perchlorate
b
b
b,
Hajime Maeda a, , Mio Yamamoto , Hideyuki Nakagawa , Kazuhiko Mizuno
*
*
a Division of Material Sciences, Graduate School of Natural Science and Technology, Kanazawa University,
Kakuma-machi, Kanazawa, Ishikawa 920-1192, Japan
b Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University,
1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
Received 15 July 2009
Abstract
Photoreaction of 9-vinylcarbazole in acetonitrile in the presence of titanium dioxide and a catalytic amount of magnesium
perchlorate gave 3,6-di(9-carbazolyl)-1,2-dioxane as a photooxygenated product via photodimerization of 9-vinylcarbazole. The
photoreaction proceeds via an electron transfer mechanism, where magnesium perchlorate accelerated formation of the photo-
oxygenated product.
# 2009 Hajime Maeda. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Titanium dioxide; Magnesium perchlorate; 9-Vinylcarbazole; Photooxygenation; Photodimerization
Photoinduced electron transfer reactions by using titanium dioxide (TiO2) have attracted much attention because
TiO2 has high potential as a clean heterogeneous photocatalyst [1–3]. We have recently developed efficient
photooxygenation reactions of alkenes [4], cyclopropanes [5], and carboxylic acids [6], catalyzed by TiO2 to give the
corresponding oxygenated products, respectively. During the course of our study, we investigated the
photooxygenation of 9-vinylcarbazole (1), a representative enamine having relatively low oxidation potential, and
found that photodimerization is a primary step and that a 1,2-dioxane derivative (3) is then formed in the presence of a
catalytic amount of magnesium perchlorate (Mg(ClO4)2).
1. Experimental
TiO2 (ST-01, particle size = 7 nm) was donated by Ishihara Sangyo, Co. Ltd. 9-Vinylcarbazole (1), 9,10-
dicyanoanthracene (DCA), and Mg(ClO4)2 were purchased and used without purification. Acetonitrile was distilled
from P2O5. A 300 W high-pressure mercury lamp (Eikosha, PIH-300) was used as a light source. 1H NMR (300 MHz)
was recorded on Varian Mercury 300 spectrometer. Column chromatography was done by using Merck silica gel 60
* Corresponding author.
E-mail address: maeda-h@t.kanazawa-u.ac.jp (H. Maeda).
1001-8417/$ – see front matter # 2009 Hajime Maeda. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2009.11.020

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H. Maeda et al. / Chinese Chemical Letters 21 (2010) 365–368
Table 1
Photodimerization and photooxygenation of 9-vinylcarbazole (1)a.
Entry
Additive
Atmosphere
Irradiation
time (min)
Yields of productsb (%)
Recovery of 1b (%)
2
3
4
5
1
2
3
4
5
6
7
8
TiO2, Mg(ClO4)2
TiO2, Mg(ClO4)2
DCAc
O2
O2
O2
O2
O2
Ar
O2
O2
20
1
20
93
64
18
76
97
66
88
57
4
20
0
2
trace
8
0
0
20
20
60
60
30
60
21
54
O
0
5
0
DCAc, Mg(ClO4)2
12
14
0
none
trace
trace
0
22
0
TiO2, Mg(ClO4)2
TiO2
0
3
5
0
10
21
trace
TiO2
0
6
a
Compound 1 (57 mg, 0.3 mmol), TiO2 (20 mg), CH3CN (10 mL), DCA (3 mg), Mg(ClO4)2 (5 mg), 300 W high-pressure mercury lamp, stirred
by a magnetic stirrer.
b
Determined by 1H NMR.
c
DCA = 9,10-dicyanoanthracene.
(70–230 mesh). HPLC separation was conducted by Jasco Megapak GEL 201C (GPC, eluent: CHCl3) equipped with
Jasco PU-986 pump and Shodex RI-72 refractometry.
1.1. General procedure for photoreaction of 1
Into a Pyrex vessel (12 mmØ Â 10.5 cm), 1 (57 mg, 0.3 mmol), TiO2 (20 mg), Mg(ClO4)2 (5 mg) acetonitrile
(10 mL), and a stirrer bar were placed and sealed with a rubber septum. After bubbling of oxygen injected through a
needle for 10 min, the suspension was irradiated by a 300 W high-pressure mercury lamp with continuous stirring and
continued bubbling of oxygen for 1–60 min (See Table 1). The temperature of the suspension was kept around room
temperature during irradiation by circulated cooling water. TiO2 was removed by centrifugal separation (3000 rpm,
30 min) and subsequent decantation. The TiO2 was washed with methanol and precipitated by centrifugal separation
1
again. The combined filtrate was evaporated. Yields of products were determined by H NMR spectra of the crude
mixture, comparing with the reported data of 1 [7], 2 [8], 3 [9], 4 [10], and 5 [11]. Data for 9-vinylcarbazole (1) [7]: mp
60–65 8C; 1H NMR (300 MHz, CDCl3): d 5.17 (d, 1H, J = 9.2 Hz), 5.56 (d, 1H, J = 15.7 Hz), 7.30 (t, 2H, J = 7.7 Hz),
7.32 (dd, 1H, J = 15.7, 9.2 Hz), 7.48 (t, 2H, J = 7.7 Hz), 7.67 (d, 2H, J = 7.7 Hz), 8.08 (d, 2H, J = 7.7 Hz). Data for
trans-1,2-di(9-carbazolyl)-cyclobutane (2) [8]: mp 196–198 8C; 1H NMR (300 MHz, CDCl3): d 2.67–2.82 (m, 2H),
3.04–3.19 (m, 2H), 6.26–6.36 (m, 2H), 7.20 (t, 4H, J = 8.0 Hz), 7.39 (t, 4H, J = 8.0 Hz), 7.56 (d, 4H, J = 8.0 Hz), 8.05
(d, 4H, J = 8.0 Hz). Data for 3,6-di(9-carbazolyl)-1,2-dioxane (3) [9]: mp 183–184 8C; 1H NMR (300 MHz, CDCl3): d
2.40 (d-like, 2H, J = 9.0 Hz), 3.36 (t-like, 2H, J = 10.6 Hz), 6.78 (d-like, 2H, J = 8.6 Hz), 7.32 (t, 4H, J = 7.8 Hz), 7.53
(, 4H t, J = 7.8 Hz), 7.78 (d, 4H, J = 7.8 Hz), 8.11 (d, 4H, J = 7.8 Hz).
2. Results and discussion
Photoirradiation to a slurry containing 9-vinylcarbazole (1), TiO2, a catalytic amount of Mg(ClO4)2, and
acetonitrile with continuous bubbling of oxygen and stirring for 20 min gave a mixture containing dimerized product
trans-1,2-di(9-carbazolyl)cyclobutane (2, 20%), 3,6-di(9-carbazolyl)-1,2-dioxane (3, 57%), 9-formylcarbazole (4,
20%) and carbazole (5, 2%) (Scheme 1, entry 1 in Table 1). At the early stage of the photoreaction, dimer 2 was
obtained as a major product (entry 2). When 9,10-dicyanoanthracene (DCA) was used as a homogeneous
photosensitizer, the yield of 3 decreased (entry 3) [12]. When a mixed system of DCAwith Mg(ClO4)2 was employed,
the product yield became comparable with the TiO2/Mg(ClO4)2 system (entry 4) [9]. The photooxygenated product 3
was not produced in the absence of TiO2 (entry 5), under Ar atmosphere (entry 6), and in the absence of Mg(ClO4)2
even in the presence of TiO2 and oxygen (entries 7 and 8).
Photoirradiation to the isolated 2 under the same irradiation conditions as those of entry 1 gave 3 (51%) and 4
(21%). Photoirradiation to the isolated 3 under the same conditions gave some decomposed products containing 4 and
5 mainly without formation of 1 and 2.

H. Maeda et al. / Chinese Chemical Letters 21 (2010) 365–368
367
Scheme 1. Photodimerization and photooxygenation of 9-vinylcarbazole (1).
Scheme 2. Reaction mechanism for the photodimerization and photooxygenation of 9-vinylcarbazole (1) catalyzed by TiO2 and Mg(ClO4)2.
From these results, we propose a reaction mechanism depicted in Scheme 2. It is already reported that the
dimerization of 1 proceeds to give 2 by photoirradiation with or without photocatalyst [9,12–16], and by oxidation
with metal salts [8,17,18]. The dimerization without photocatalyst proceeds via an electron transfer from 1
(Eox = 0.94 V vs. SCE [19]) to O2 (Ered = À0.86 V vs. SCE [20]) even under the conditions without bubbling of O2
because the dissolved oxygen is enough to act as an electron acceptor. Photoexcitation of TiO2 causes the promotion of
an electron into the conduction band (À0.8 V vs. SCE) to generate a hole in the valence band (+2.4 V vs SCE).
Electron transfer from 2 to the valence band takes place as an exoergonic process to produce the radical cations of 2
(2+ꢀ). Mg(ClO4)2 might act to stabilize O2 [20,21] and 2+ꢀ or its open form, hence suppresses the back electron
Àꢀ
transfer to give 2 [22–30]. Another possibility for the effect of Mg(ClO4)2 is that the magnesium salt may accelerate
the desorption of the radical cation from the TiO2 surface due to the enhanced cationic charge on the surface [31].
When the concentration of O2 is sufficiently high, open form of 2+ꢀ attacks O2 and the following ring closure and
electron transfer from neutral 2 affords 3. Further photoirradiation causes decomposition of 3 to give 4 and 5.
In conclusion, photooxygenation of 1 is catalyzed by TiO2 and Mg(ClO4)2 to give a 1,2-dioxane derivative 3 via the
initial formation of dimerized product 2. In the absence of Mg(ClO4)2, photooxygenated product was not obtained
upon irradiation even in the presence of TiO2 and O2. It was demonstrated that TiO2–Mg(ClO4)2–O2 system is a
powerful tool for the effective photooxygenation reaction under mild conditions.
Acknowledgments
This work was financially supported by a Grant-in-Aid for Scientific Research on Priority Areas ‘‘Fundamental
Science and Technology of Photofunctional Interfaces (417)’’ (Nos. 14050085, 15033264, 17029058) and the
Cooperation for Innovative Technology and Advanced Research in Evolutional Area (CITYAREA) program from the
Ministry of Education, Culture, Sports, Science and Technology of Japan. We also thank Ishihara Sangyo Co., Ltd. for
donating the TiO2.

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H. Maeda et al. / Chinese Chemical Letters 21 (2010) 365–368
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