Addition of a stoichiometric amount of HCl to alkenylaldimines,-formamidines, and-amidines results in the protonation of the sp2-nitrogen atom. The resulting alkenylaldiminium,- formamidinium, and-amidinium salts can be isolated and fully characterized, including single-crystal X-ray diffraction studies. Heating solutions of these salts induces ring closure cleanly and regioselectively via formal “exo” addition of the nitrogen-hydrogen bond to the ...
