The simplest azabutadienes in their N-protonated forms. Generation, stability, and cycloaddition-reactivity in the gas phase

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 abundant adducts are only formed in reactions with alkenes activated by electron-withdrawing substituents. In reactions of 1 and 2 with esters (methyl acetate and dimethyl carbonate), hydrogen-bridged ion-neutral complexes are formed as the most abundant and stable products, as suggested by the ab initio calculations. Acetone, fluoroacetone and acetonitrile form abundant adducts with-bath 1 and 2; However, the experimental and theoretical results on these adducts provide nb clear structural information. Reactions of 1 with DMSO occur almost exclusively by proton transfer, whereas 2 forms an abundant complex with DMSO. Limited reactivity is observed for I and 2 with acetyl chloride and thionyl chloride; the minor products observed were those of either dissociative proton transfer or charge exchange.;The distinctive reactivities of 1 and 2 with styrene, ethyl vinyl ether, and dimethyl sulfoxide contrast to their identical low energy CID behavior, and allow their straightforward differentiation in the gas phase.