A Thermal [2+2] Cycloaddition/Retro-4π-Electrocyclization Reaction of p-Quinone Methides with Alkyne and a Mechanistic Investigation: Ingress of a Vinyl Fuchsone Pharmacophore

Herein, we demonstrate a [2+2] cycloaddition/electrocyclic ring-opening reaction between quinone methide and electronically unbiased terminal alkynes under strictly thermal conditions. The present investigation is a unique example that demonstrates the effect of extended conjugation and cross-conjugation on the energy of the lowest unoccupied molecular orbital. As supported by density functional theory calculations, the reaction proceeds through the biradical pathway, accommodating both electron-deficient and electron-rich alkyne partners. This newly developed strategy offers a novel synthetic route to vinyl fuchsones, which can be further utilized to access incredibly challenging synthons. These vinyl fuchsones not only act as ligands to facilitate sp2–sp2 coupling under mild conditions but also serve as efficient surrogates for synthesizing commercially available triaryl ethanes.