Light-induced oxidative dehydrogenation of 12,13-dihydro-5H-indolo[3,2-c]acridine.

Photochemical dehydrogenation of the 1,3-cyclohexadiene-type ring of 12,13-dihydro-5H-indolo[3,2-c]acridine (1) by singlet oxygen was observed in n-hexane, acetonitrile and methanol. UV irradiation of 1 leads exclusively to the formation of 13H-indolo[3,2-c]acridine (2) and hydrogen peroxide, where 1 can itself act as a singlet oxygen sensitiser. Although this type of photooxidation reaction for 1 might be expected also to yield addition products, such as endoperoxides or hydroperoxides, the oxygenated species were not detected. Our findings are rationalised by DFT calculations of relative energies and conversion barriers involving various possible photooxidation products.