In Situ Generation and [3 + 2] Annulation Reactions of Propiolaldehyde─A Metal-Free, Cascade Route to Pyrazole and Bipyrazole Carboxaldehydes in One Pot

Propiolaldehyde, the most reactive yet less explored electrophilic acetylene, was generated in situ via a base-free Kornblum oxidation from propargyl tosylate and successfully intercepted with hydrazones affording pyrazole-4-carboxaldehyde in one-pot by a [3 + 2] annulation reaction. Further, the pyrazole-4-carboxaldehyde endured a unique cascade reaction with phenylhydrazine and propargyl tosylate, yielding synthetically challenging bipyrazole carboxaldehydes. The method is free of any metal catalyst, base, or additives and offers a convenient protocol to handle the reactive and volatile propiolaldehyde under ambient conditions.