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]ben zoxazine-5,12-dione 7 were synthesized by the reaction of homophthalic anhy dride with the corresponding o-substituted anilines. Reaction mechanisms we re 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 a nd 5 with electron deficient alkenes (acrylonitrile, methyl acrylate, dimet hyl fumarate and dimethyl maleate) gave cyclobutane products (36, 37 and 39 -47 respectively). The regioselectivity in these photocycloadditions was ex amined by FMO interaction considerations. The mechanism of the cycloadditio ns was investigated by fluorescence quenching and triplet quenching experim ents, solvent effect on the reaction and calculation of free energy change for electron transfer (SET) between the excited states of 4a (and 5) and th e alkenes.