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

Abstract

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.

Discovery and Mechanistic Study of a Totally Organic C(aryl)–C(alkyl)Oxygen Insertion Reaction

“…The photoreduced product 10 was isolated as the major product (18 mg, 0.04 mmol, 36% yield). Its characterization data were consistent with those reported in the literature …”

“…Our studies began with recently reported p -quinone methide ( p -QM) 1 , which displays a variety of unusual reaction chemistry. We photolyzed p -QM 1 at various wavelengths with the rather vague goal of inducing a dimerization reaction, known to happen in other quinone methides .…”

“…Several conclusions can be drawn from these results: (1) protic solvents are not necessary for successful oxygenation, suggesting that hydrogen atoms (or protons) are not transferred at any point in the process; (2) the lack of a hydrogen-based KIE is also congruent with this fact; (3) nonnitrilic, nonprotic solvents, in contrast, produce no oxygen insertion product; (4) nitrile-based solvents are apparently necessary for success, suggesting a chemical role for them in the mechanism; and (5) in the presence of water, oxygenation is overridden in favor of hydrolysis, a fact that will be discussed below.…”

“…Compound 1 (p-QM) was synthesized following the previously reported method. 5 Photooxygenated Product 2. p-QM 1 (74 mg, 0.17 mmol) was dissolved in 5 mL of CH 3 CN and exposed to 254 nm light in a Rayonet reactor for 16 h. The solution was exposed to air through a needle for complete conversion (>90% yield by 1 H NMR). Solvent was evaporated under reduced pressure, and an analytical sample for characterization was obtained by MPLC as a yellow solid (27 mg, 0.06 mmol): 1 H NMR (CDCl 3 ) δ 7.31−7.38 (m, 4H), 7.22 (m, 1H), 7.05 (dd, 1H, J = 7.5 Hz, 1.7 Hz), 6.79 (m, 1H, J = 7.5 Hz, 1.7 Hz), 6.68 (m, 1H, J = 7.…”

“…Its characterization data were consistent with those reported in the literature. 5 Photohydration Product 11. Compound 1 (95 mg, 0.22 mmol) was dissolved in a mixture of 7 mL of CH 3 CN and 1 mL of H 2 O.…”