HETEROCYCLES

■ Effect of Nitro-Substitution on the Photochemistry of 3-Piperidino-1,2-benzisothiazole Derivatives: A Mechanistic Investigation

Hiroharu Tanikawa, Kazuhiro Ishii, Shun Kubota, Takashi Sasanuma, Shiki Yagai, Akihide Kitamura, and Takashi Karatsu*

*Department of Applied Chemistry and Biotechnology, Graduate School and Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522,

Abstract

Photochemical isomerization of 3-piperidino-1,2-benzisothiazole (BIT)–benzothiazole (BT) was investigated. In particular, the effect of nitro-substitution on the benzene ring was selectively examined for the parent, 5-nitro-, and 7-nitro-derivatives. 3-Piperidino-BIT and nitro-3-piperidino-BIT isomerized irreversibly to the corresponding 2-piperidino-BT, and this reaction mechanism was investigated using density-functional theory (DFT) and time-dependent (TD)-DFT calculations. These calculations showed that this isomerization goes through an azirine intermediate. In addition, excitation wavelength effect demonstrates that isomerization occurs from the upper excited state of nitro-derivatives. The HOMO-LUMO transition has charge-transfer and photoinactive characters.