Generation of alkene radical cations by heterolysis of β-substituted radicals:: Mechanism, stereochemistry, and applications in synthesis

The experimental basis for the formation of alkene radical cations by the heterolysis of alkyl radicals bearing leaving groups at the 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 beta-(acyloxy)alkyl and beta-(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.