Photoinduced [2 + 2] cycloadditions (the Paterno–Büchi reaction) of 1H-1-acetylindole-2,3-dione with alkenes

Photoinduced cycloadditions of 1-acetylisatin (1) with alkenes 2–7 give spiroxetanes 8–21 respectively in moderate to high yields, displaying a typical triplet n–π* reactivity for 1. The regioselectivity and diastereoselectivity of the reactions depend on the reaction mechanism. In reactions with alkenes of high oxidation potential (2 and 4) where single-electron transfer (SET) processes with triplet excited 1 are not involved, the regioselectivity can be rationalized by consideration of frontier molecular orbital interactions of the two addends, and the Salem–Rowland rules for diradical intersystem crossing explains the diastereoselectivity. For the more electron-rich alkenes, e.g.5–7, SET processes with 31* and ion-radical pair formation are energetically feasible, and the cycloaddition regioselectivity is dependent on charge and spin-density distribution in the ion-radicals and the diastereoselectivity is also decided by ion-radical pair collapse.