On the Reaction of 1‐Aza‐2‐azoniaallene Salts with Acetylenes

1‐Aza‐2‐azoniaallene cations 3 , prepared in situ from geminal chloro(alkylazo) compounds 2 , react with acetylenes 4 to give either 1 H ‐pyrazolium salts 6 or 4 H ‐pyrazolium salts 7 or mixtures of both. 4 H ‐Pyrazolium salts with a hydrogen atom attached to C(4) rearrange to the protonated 1 H ‐pyrazoles 8 , from which the free bases 9 are obtained upon treatment with aqueous NaOH. According to AM1 calculations the cycloaddition of acetylenes to 1‐aza‐2‐azoniaallene cations is a concerted process, which can be classified as a “1,3‐dipolar cycloaddition with reverse electron demand”. The cycloaddition forming the intermediates 5 is followed by an [1,2] alkyl shift to furnish the final products 6–8. The direction of the [1,2] shift has been found to be governed by subtle steric effects.