Photochemistry of thiophthalimides

Rapley, Patricia Anne (1986). Photochemistry of thiophthalimides. PhD thesis The Open University.



Thiomide photochemistry is of interest because previous photochemical studies of thiocarbonyl compounds have been restricted largely to thioketones, and because the "alloxygen" imides have proved to have a varied and synthetically useful photochemistry. Successful reactions with thiomides might lead to analogous sulphur products that could be readily modified by further chemical reaction.

Irradiation of the previously unreported thiophthalimide Mannich bases (A) leads to unusual cleavage products (B and C ), unlike the reaction of related phthalimide Mannich bases, which gives photocyclised compounds. Other N-substituted thiophthalimides that might be expected to undergo photocyclisation via hydrogen transfer are unreactive upon irradiation (except to photooxidation).

However, photocycloaddition reactions of N-substituted thiophthalimides with a range of multiply-bonded substrates have proved to be more fruitful. (2+2)-Cycloadducts are obtained with alkenes such as stilbene (adduct structure D ), with alkynes such as diphenylethyne (E), with ketenes such as diphenylketene (F), and with ketenimines. These (2+2)-adducts contain thietane or thiete rings, and in some cases ringcleavage products (G) or ring-opened products (H) are formed. With some alkenes such as methyl acrylate, 2:1 adducts are obtained (I). A substrate (J) designed to produce intramolecular cycloadducts led, surprisingly, to a compound (K) derived by hydrogen abstraction and cyclisation.

A brief study of the photophysical properties of the N-substituted thiophthalimides shows that they are weakly luminescent at 77 K, and that the reactive excited state is most probably the lowest (n,pi*) triplet.

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