Single electron transfer (SET) and iodine-atom transfer radical addition (I-ATRA) induced cyclopropanation reaction: elucidating the role of iodine

Abstract

Mechanistic studies were conducted for the visible-light-mediated cyclopropanation reaction of alkenes with diazoacetate, catalyzed by a phenalenyl-based organic hydrocarbon (PLY) and co-catalysed by iodine, using DFT. In the absence of iodine, the system proceeds towards the hydroalkylation reaction, with the aid of aryl thiol. Both reactions were studied to understand the role of iodine in promoting the formation of cyclopropanes, the key structural units in pharmacophores. Our findings suggest that the single electron transfer (SET) will generate the transient iodine radical species, whereas the iodine atom transfer radical addition (I-ATRA) will construct the iodinated acetate radical moiety for the cyclopropanation reaction. On the other hand, the proton-coupled electron transfer (PCET) mechanism induces the formation of a protonated acetate radical for the hydroalkylation reaction. The negligible back donation from the lone pair orbital of iodine to the antibonding orbital of the newly generated C–C bond makes the iodine a better leaving group, thereby making the cyclisation feasible, validating the role of iodine in the cyclopropanation reaction. Various analysis techniques such as atoms in molecule, activation strain analysis, and energy decomposition analysis reveal that the favorable non-covalent interactions and steric effects promote the regioselective anti-Markovnikov radical addition over the Markovnikov radical addition.