Calculation of a nine-membered analog of pyrrole and cyclononatetraenylpyridinium ylides by the molecular orbital method

Theπ-electronic system of the cyclononatetraenyl anion is stabilized both by the introduction of a heteroatora, with transition to the neutral nine-membered analog of pyrrole, and by substitution by a pyridinium cation, with transition to the systems of cyclononatetraenyl-pyridinium ylides, with a net neutral charge. However, both the introduction of a heteroatom and substitution by a pyridinium cation stabilize theπ-electronic system of the indenyl anion to an even greater degree. Therefore, we should expect an easy rearrangement of the cyclic nine-membered system to a bicyclic system, especially if we consider that the constriction of the nine-membered ring almost entirely removes the strain in the valence angles.