Prototropy in the System -ClCH-N=N-; Cycloadditions of 1-Aza-2-azoniaallene Cations Derived from Aldehydes

1,3-Disubstituted 1-aza-2-azoniaallene salts 10 were obtained as reactive intermediates on oxidation of aldehyde hydrazones 7 with tert-butyl hypochlorite and treatment of the resulting (1-chloroalkyl)azo compounds 8 with a Lewis acid (SbCl₅). The allenes 10 underwent cycloadditions to the multiple bonds of nitriles, acetylenes, and olefins affording the heterocyclic salts 12, which gave the free bases 12’ with aqueous NaOH. The critical point of this sequence lies in the prototropic rearrangement of 8 into the hydrazonyl chlorides 9, from which no heterocumulenes 10 could be obtained. However, further chlorination of 7c led to the (1,1-dichloropropyl)azo compound 13, which gave the chloro substituted allenium salt 14 on treatment with SbCl₅. Cycloaddition of 14 to acetonitrile afforded the chloro substituted 1,2,4-triazolium salt 16. Conditions for slow rearrangements 8 → 9 were found for compounds 8 derived from primary or secondary alkanecarbaldehydes. However, for (1-chloroalkyl)azo compounds derived from benzaldehyde or pivaldehyde the prototropy 8 → 9 was too fast to permit preparation of heterocumulenes 10.

1-aza-2-azoniaallene salts - cycloadditions - 1,2,4-triazoles - pyrazoles - 2,3-dihydropyrazoles