Organic Letters
Letter
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was found to mask this functionality. Installation of additional
methoxy group(s) generally had a negative impact over the
reaction (49−53), especially for meta-substitution (52, 53). In
the case of 2,4,6-trimethoxy substitution (50), the radical cation
is potentially stabilized on the aromatic ring, rendering the
styrene double bond less reactive. In the previous reports, the
aromatic ring of the β-methylstyrenes was electron-rich, e.g.,
substituted with alkoxy group(s), and most examples of this class
were demonstrated by using 1. However, β-methylstyrenes with
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to be productive under TiO2 photocatalysis. Compared with the
positional effect of the methoxy group(s), that of methyl
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electron density in the aromatic ring and steric hindrance around
the styrene double bond should be balanced. Furthermore, a
nonsubstituted β-methylstyrene that was previously reported to
be an unsuccessful substrate under Ru complex photocatalysis
and electrocatalysis gave the cycloadduct 61 in acceptable yield.
In conclusion, we have demonstrated that TiO2 photo-
catalysis in combination with the use of a LiClO4/CH3NO2
solution is a powerful redox option to catalyze radical cation
Diels−Alder reactions. The LiClO4/CH3NO2 solution facili-
tates reactions between carbon-centered radical cations and
carbon nucleophiles, even in the presence of a nucleophilic
oxygen functionality. We believe that TiO2 photocatalysis
should find complementary applications in synthetic organic
chemistry that would be difficult to accomplish by molecular
photocatalysis or electrocatalysis. Further design and develop-
ment of reactions by TiO2 photocatalysis are under investigation
in our laboratory.
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ASSOCIATED CONTENT
* Supporting Information
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S
The Supporting Information is available free of charge on the
Additional figures, general remarks, synthesis and
1
characterization data, including copies of H and 13C
AUTHOR INFORMATION
(8) For selected reviews, see: (a) Nosaka, Y.; Nosaka, A. Y. Generation
and Detection of Reactive Oxygen Species in Photocatalysis. Chem. Rev.
2017, 117, 11302−11336. (b) Dharmaraja, A. T. Role of Reactive
Oxygen Species (ROS) in Therapeutics and Drug Resistance in Cancer
and Bacteria. J. Med. Chem. 2017, 60, 3221−3240. (c) AbdulSalam, S.
F.; Thowfeik, F. S.; Merino, E. J. Excessive Reactive Oxygen Species and
Exotic DNA Lesions as an Exploitable Liability. Biochemistry 2016, 55,
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Corresponding Author
ORCID
Notes
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5341−5352. (d) Apak, R.; Ozyurek, M.; Gucļ u, K.; Çapanoglu, E.
̈ ̈ ̈
Antioxidant Activity/Capacity Measurement. 3. Reactive Oxygen and
Nitrogen Species (ROS/RNS) Scavenging Assays, Oxidative Stress
Biomarkers, and Chromatographic/Chemometric Assays. J. Agric. Food
Chem. 2016, 64, 1046−1070. (e) Forman, H. J.; Maiorino, M.; Ursini,
F. Signaling Functions of Reactive Oxygen Species. Biochemistry 2010,
49, 835−842. (f) Wang, Y. Bulky DNA Lesions Induced by Reactive
Oxygen Species. Chem. Res. Toxicol. 2008, 21, 276−281.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was partially supported by JSPS KAKENHI Grant
Nos. 16H06193, 17K19221 (to Y.O.), and 16H02413 (to H.K.).
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