Syntheses and electronic structures of benzannelated isoquinolinones and their photoinduced cycloaddition reactions with electron deficient alkenes †

Benzoxazolo[3,2-b]isoquinolin-11-ones 4, benzothiazolo[3,2-b]isoquinolin-11-one 5, benzimidazo[1,2-b]isoquinolin-11-ones 6 and isoquino[2,3-a][3,1]benzoxazine-5,12-dione 7 were synthesized by the reaction of homophthalic anhydride with the corresponding o-substituted anilines. Reaction mechanisms were investigated by isolation of the intermediate products under controlled reaction conditions. Electronic structures of 4a and 5 were investigated by ab initio calculations. Photoinduced [2+2] cycloaddition reactions of 4a and 5 with electron deficient alkenes (acrylonitrile, methyl acrylate, dimethyl fumarate and dimethyl maleate) gave cyclobutane products (36, 37 and 39–47 respectively). The regioselectivity in these photocycloadditions was examined by FMO interaction considerations. The mechanism of the cycloadditions was investigated by fluorescence quenching and triplet quenching experiments, solvent effect on the reaction and calculation of free energy change for electron transfer (SET) between the excited states of 4a (and 5) and the alkenes.