The simplest azabutadienes, i.e. 1-aza-1,3-butadiene and 2-aza-1,3-butadiene, are generated in their N-protonated forms 1 and 2 via gas-phase dissociative electron ionization of allylamine and piperidine, respectively. Formation of 1 and 2 is suggested by simple dissociation mechanisms, and supported by high-accuracy G2 ab initio calculations, which show the ions to be stable, non-interconverting species. Whereas 1 and 2 are unreactive toward ethylene and cyclohexene, 2 reacts with alkenes activated by electron-donating (OC2H5), electron-withdrawing (CN, COCH3), and vinyl and phenyl substituents most likely by polar [4+ + 2] cycloaddition, as suggested by MS3 experiments and ab initio calculations. The cycloadduct of 2 with ethyl vinyl ether is unstable and dissociates promptly by ethanol loss; hence, net C2H2 addition occurs. This novel vinylation reaction is proposed as a potential structurally diagnostic test for both 2-azabutadienes and vinyl ethers. Isomer 1 is in general much less reactive, and abun...