Aromatization of pyridinylidenes into pyridines is inhibited by exocyclic delocalization. A theoretical mechanistic assessment

Pyridinylidenes are synthesized but aromatization is not observed. Electronic structure and energetic arguments are presented to rationalize the reluctance of these compounds to undergo an aromatization reaction. Two plausible aromatization mechanisms were evaluated through Density Functional Theory (DFT) calculations. Calculated energy barriers are too high to be reached (50–70 kcal/mol). Ab initio calculations point toward the presence of bonding and anti-bonding electron corridors in which the electron density is strongly delocalized, yielding large barrier heights.