A Protonated Quinone Methide Stabilized by a Combination of Partial Aromatization and π-Interaction: Spectroscopic and Crystallographic Analysis.

We have expanded the repertoire of cation-π interactions to include a carbocation-π system resulting from the protonation of a π-stacked para-quinone methide (p-QM). This unusual carbocation is stabilized by a combination of partial aromatization of the QM moiety and through-space interaction with the π-system of the adjacent aromatic ring. Single crystal X-ray analysis of the protonated form reveals a structure consisting of a hydrogen-bound complex involving two molecules of the precursor and one proton.