Generation of Alkene Radical Cations by Heterolysis of β-Substituted Radicals: Mechanism, Stereochemistry, and Applications in Synthesis

Abstract

The experimental basis for the formation of alkene radical cations by the heterolysis of alkyl radicals bearing leaving groups at the β position is reviewed, and a general mechanism involving contact alkene radical cation/anion pairs is presented for both fragmentation reactions and rearrangements. The available kinetic data for both fragmentations and migrations are summarized. The β-(acyloxy)alkyl and β-(phosphatoxy)alkyl radical rearrangements, previously viewed as concerted shifts, are reinterpreted in terms of the general mechanism with extremely rapid collapse of the intermediate contact alkene radical cation/anion pair. The reactions of alkene radical cations in the confines of the contact ion pair are reviewed, including radical cyclizations, nucleophilic attack, and tandem nucleophilic attack/radical cyclization processes. Stereochemical memory effects arising from the order within the contact alkene radical cation/anion pair are discussed at the level of both diastereoselectivity and enantioselectivity. Open image in new window