Computational Study of the Intramolecular Pericyclic Reactions of Aldazines and Some Pseudopericyclic Variants
The mechanisms of the complex cascade cyclizations of homoallenyl aldazines and analogues to afford tetracyclic compounds has been computed. Double sequential crisscross 1,3-dipolar cycloaddition through azomethine imine intermediates was characterized both for the homoallenyl aldazine and for its ketene analogue. If conjugation of the two interacting groups (azine and allene/ketene) was introduced through the presence of an olefin system or an oxygen atom in place of the saturated carbon, pseudopericyclic mechanisms were also characterized. Unlike in the parent system, the barrierless cascade electrocyclization of [(Z)-hexa-2,4,5-trien-1-ylidene]hydrazine and of its ketene analogue is interrupted after formation of the 2,2′-methylenebis(dihydropyridine) and bis-2,2′-pyridone, respectively, by pseudoelectrocyclic reactions. The cascade reaction of a hypothetical diazine with an allenylformimidate structure generates a compound containing four fused five-membered heterocyclic rings through a sequence that differs from that of the parent homoallenyl aldazine in the involvement of 6π-electron-5-atom electrocyclization and nucleophile–electrophile reaction steps before the final 1,3-dipolar cycloaddition. The analysis of the aromaticities of the transition structures through the computation of the NICS is consistent with their characterization as pericyclic or pseudopericyclic processes.