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

Photoinduced cycloadditions of 1-acetylisatin (1) with alkenes 2-7 give spi roxetanes 8-21 respectively in moderate to high yields, displaying a typica l triplet n-pi* reactivity for 1. The regioselectivity and diastereoselecti vity of the reactions depend on the reaction mechanism. In reactions with a lkenes of high oxidation potential (2 and 4) where single-electron transfer (SET) processes with triplet excited 1 are not involved, the regioselectiv ity can be rationalized by consideration of frontier molecular orbital inte ractions of the two addends, and the Salem-Rowland rules for diradical inte rsystem crossing explains the diastereoselectivity. For the more electron-r ich alkenes, e.g. 5-7, SET processes with (3)1* and ion-radical pair format ion are energetically feasible, and the cycloaddition regioselectivity is d ependent on charge and spin-density distribution in the ion-radicals and th e diastereoselectivity is also decided by ion-radical pair collapse.