Transition-metal-free polycyclic indoline formation via a free radical pathway: a computational mechanistic study

The mechanism of [2 + 2 + 1] cyclization reactions to access polycyclic sulfonyl indolines via Fe(II)-catalyzed or UV-driven has been investigated by density functional theory (DFT) methods. Mechanistic studies aided by the DFT calculations suggest that the reaction might proceed via a free radical pathway with HSO3· serving as the initiator and terminator of the free radical reaction, which has never been reported to our knowledge. The reactions involve the formation of two five-membered rings. The calculated results show that the first five-membered ring formation is the rate-determining step, with an energy barrier of 3.2 kcal/mol. And the second five-membered ring formation is an exergonic process. It is worth noting that HSO3· radicals are involved in the reaction preceding the formation of the second ring, and they facilitate formation of the second ring. Meanwhile, the most active site was predicted by the condensed Fukui function (CFF) and verified by DFT calculations.