The 1-aza-2-azoniaallene salts, generated from alpha-chlotoazo compounds by treatment with halophilic Lewis acids, undergo intramolecular C-H amination reactions to form pyrazolines in good to excellent yields. This intra-molecular amination occurs readily at :both benzylic and tertiary aliphatic positions and proceeds at an enantio enriched chiral center with retention Of stereochemistry. Competition experiments show that insertion occurs more readily At an electron-rich benzylic position than it does at an electron-deficient one The C-H amination reaction occurs only with certain tethers connecting the heteroallene cation and the pendant aryl groups. With a longer tether or when the reaction is intermolecular, electrophilic aromatic substitution occurs instead of C-H amination. The mechanism and origins of stereospecificity and chemoselectivity were explored with density functional theory (B3LYP and MO6-2X). The 1-aza-2-azoniaallene cation undergoes C-H amination through a hydride transfer transition state to form the N-H bond, and the subsequent C-N bond formation occurs spontaneously to generate the heterocyclic product. This concerted two-stage mechanism was shown by IRC and quasi-classical molecular dynamics trajectory studies.