Formal [4+1] Annulation of Azoalkenes with CF 3 -Imidoyl Sulfoxonium Ylides and Dual Double Bond Isomerization Cascade: Synthesis of Trifluoromethyl-Containing Pyrazole Derivatives.

A straightforward strategy for the metal-free construction of trifluoromethyl-containing pyrazole derivatives has been achieved from readily available α-halo hydrazones and CF 3 -imidoyl sulfoxonium ylides. The cascade transformation proceeds through the formal [4+1] cycloaddition followed by an unexpected dual double bond isomerization. The protocol features mild conditions, easy operation, excellent substrate compatibility, and good regioselectivity. The synthetic utility is demonstrated by scale-up reaction and further elaboration of the obtained pyrazole products.