Transition from π Radicals to σ Radicals: Substituent-Tuned Cyclization of Hydrazonyl Radicals

Hydrazonyl radicals are known for their π‐electronic structures; however, their σ‐electronic structures have not been reported as yet. Herein, we show that readily accessible β,γ‐ and γ,δ‐unsaturated N‐trichloroacetyl and N‐trifluoroacetyl hydrazones can be conveniently converted into hydrazonyl σ radicals, which subsequently undergo 5‐exo‐trig radical cyclization at the N1 or N2 atom to form pyrazolines and azomethine imines, respectively. Time for a radical career change? Known for their π character, hydrazonyl radicals can be tuned to act as σ radicals with the spin density delocalized on both nitrogen atoms by attaching a trifluoroacetyl or trichloroacetyl group to the N1 atom. The hydrazonyl σ radicals derived from the corresponding N‐acyl‐substituted β,γ‐ and γ,δ‐unsaturated hydrazones underwent CN1 and CN2 bond‐forming 5‐exo‐trig cyclization, respectively (see scheme).