Transition from p Radicals to s Radicals: Substituent-Tuned Cyclization of Hydrazonyl Radicals

Hydrazonyl radicals are known for their -electronic structures; however, their sigma-electronic structures have not been reported as yet. Herein, we show that readily accessible ,- and ,-unsaturated N-trichloroacetyl and N-trifluoroacetyl hydrazones can be conveniently converted into hydrazonyl sigma radicals, which subsequently undergo 5-exo-trig radical cyclization at the N-1 or N-2 atom to form pyrazolines and azomethine imines, respectively.