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

1‐Aza‐2‐azoniaallene salts 3 , prepared in situ from geminal chloroalkylazo compounds 2 with Lewis acids, react with isocyanates 4 to give 4,5‐dihydro‐5‐oxo‐3 H ‐1,2,4‐triazolium salts 6 and 4,5‐dihydro‐5‐oxo‐1 H ‐1,2,4‐triazolium salts 7 , respectively. The intramolecular cyclization of 3u opens a new route to cinnolinium salts 11 . Allenes 3 react with isobutene to give pyrazolium salts 8 . According to AM1 calculations the cycloadditions of 3 to isocyanates proceed in two steps via acylium salts 5 as intermediates. Mechanistically, the rearrangements 6 → 7 resemble Wagner‐Meerwein rearrangements rather than pericyclic [1,5]‐sigmatropic shifts.