Vinamidines and Vinamidinium Salts—Examples of Stabilized Push‐Pull Alkenes

The name vinamidines has been proposed for 1,5‐diazapentadienes: these compounds can be regarded as vinylogous amidines and at the same time as push‐pull substituted (and thereby stabilized) alkenes. Vinamidine and vinamidinium structural elements may also form part of a ring system. Characteristic of this class of compounds are their reactivity toward electrophiles (at C β ) and nucleophiles (C α ) and their regenerative character, i.e. their tendency to undergo substitution instead of addition reactions. Reactions of vinamidinium salts with nucleophiles are of special preparative value: they lead e.g. to pyrazoles, oxazoles, pyrimidines, diazepines, quinolines, and quinolizines.