Photoreactions of 3,4,5,6-Tetrafluoro-N-methylphthalimide with Alkenes. Effect of Fluoro Substituents on Photoreaction Pathway

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Abstract
Photoreactions of 3,4,5,6-tetrafluoro-N-methylphthalimide (7) with styrene (8), α-methylstyrene (2), and 1,1-diphenylethylene (11) have been investigated in order to clarify the effect of fluoro substituents on the reaction pathway. The irradiation of 7 with 2, 8, and 11 in benzene under N2 gave oxetanes as exclusive products. The irradiation of 7 with 11 under O2 afforded benzophenone (13) together with the oxetane. The methanol-incorporated addition of 2 and 8 to 7 proceeded in photoreactions of 7 with 2 and 8 in methanol. A consideration of the energy levels of the excited triplet states of 7 and the alkenes, the quenching of the fluorescence of 7 by 8, and the concentration effects of the alkenes indicate that all of the reactions occur from the singlet excited state of 7. The difference between the exclusive oxetane formation of 7 and the preferred insertion of alkenes into the C–N bond of phthalimides is discussed regarding the nature of the singlet excited states of the imides. The methanol-incorporated addition and formation of 13 are rationalized in terms of mechanisms involving electron-transfer. The calculated ΔGet values associated with electron-transfer support the electron-transfer process.