Pushing Radical Cyclization from Regioselective to Regiospecific: Cyclization of Amidyl Radicals Controlled by Vinylic Halogen Substitution

Efficient and regiospecific 6-exo, 7-endo, 7-exo, and even 8-endo amidyl radical cyclizations can be accomplished by the direct reactions of unsaturated N-H amides if they bear a vinylic halogen (Cl, Br, I) substituent. This remarkable halogen-substitution effect could be rationalized in terms of lone pair−lone pair electron repulsion between the N radical and the vinylic halogen atom, in addition to the well-known steric and radical-stabilizing effect.