2-Alkoxycarbonylpyridinium N-Aminides:  1,3-Dipoles or 1,4-Nucleophile−Electrophile Synthons? Experimental and Theoretical Evidence for the Mechanism of Pyrido[1,2-b]pyridazinium Inner Salt Formation

2-Alkoxycarbonylpyridinium N-aminides behave as 1,3-dipoles toward acetylenic compounds and as 1,4-nucleophile−electrophiles with heterocumulenes in a [4 + 2] cyclocondensation process, yielding in the latter case conjugated mesomeric betaines. These N-aminides also behave as 1,3-dipoles when reacted with olefinic dipolarophiles, producing the corresponding cycloadducts that, depending on their regioisomeric nature, subsequently undergo a ring expansion process to produce pyrido[1,2-b]pyridazinium inner salts. A mechanistic investigation performed using both PM3 frontier molecular orbital (FMO) and potential energy surface (PES) analysis at the RHF/6-31+G* level indicates that both the cycloaddition reaction and the ring expansion occur in a concerted way rather than through a stepwise mechanism via a zwitterionic intermediate.