Azapyramidinium Ion and iso-Azapyramidinium Ion: Mechanisms for their Synthesis, Interconversion, and Decomposition

The properties and reactivity of azapyramidinium ion 3 were examined using coupled-cluster theory and density functional theory with the goal of identifying synthetic routes and anticipating decay pathways. It is shown that azapyramidinium ion is nearly isoenergetic with an isomer that results from transposition of the apical nitrogen atom with a basal CH group. The latter is termed the “iso-azapyramidinium ion” 5. There is a low energy kinetic barrier (ca. 14 kcal/mol) connecting the two isomers, suggesting they would interconvert rapidly at ambient temperatures. The two isomers show substantial barriers for both isomerization to the lower energy pyrrolenium ion as well as elimination of HCN to form the cyclopropenium ion. Thus 3 and 5 should be isolable (if as an equilibrium mixture) if segregated from reactive partners. Water is shown to react rapidly with the azapyramidinium isomers. Finally, analysis of the potential energy surface for (CH)¬4N+ shows that 5 and/or 3 should be accessible through addition of acetylene to the azirenium ion. Proposed synthetic routes are discussed.