Multiplicities of ?-ylide ground states: Computational evidence for a breakdown of aromaticity arguments

Abstract

AM1 and 3–21G computations, on the lowest singlet state of selected π-ylides, support the earlier conclusion that π-ylides are highly polar with highly reactive side-chains. The computations also indicate that the corresponding triplet states are lower in energy and have substantially reduced polarity. Contrary to the expectations derived from Huckel calculations, AM1 and 3–21G calculations suggest that the best π-ylide structures have non-aromatic rings. The heats of isomerization contradict earlier conclusions reached using topological resonance energies. π-Ylides are expected to afford different product structures depending on the electronic state of the π-ylide involved in a given cycloaddition reaction.