Photoinduced cycloadditions of 1-acetylisatin (1) with alkenes 2â7 give spiroxetanes 8â21 respectively in moderate to high yields, displaying a typical triplet nâÏ* 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 SalemâRowland rules for diradical intersystem crossing explains the diastereoselectivity. For the more electron-rich alkenes, e.g.5â7, 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.
1-乙酰基异靛(1)与烯烃2-7的光诱导环加成反应分别生成螺旋杂环(spiroxetanes)8-21,产率为中等到高,显示出典型的三线态 n-π* 反应活性。反应的区域选择性和立体选择性取决于反应机制。在与高氧化电位的烯烃(2和4)的反应中,如果不涉及三线态激发的单电子转移(
SET)过程,区域选择性可以通过考虑两个加成物的前缘分子轨道相互作用来解释,而萨利姆-罗兰德规则用于双自由基的跨系统交叉则解释了立体选择性。对于更富电子的烯烃,例如5-7,
SET过程与31*和离子自由基对的形成在能量上是可行的,环加成的区域选择性取决于离子自由基中的电荷和自旋密度分布,而立体选择性也由离子自由基对的崩溃决定。