A Thermal [2+2] Cycloaddition/Retro-4π-Electrocyclization Reaction of -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 sp-sp coupling under mild conditions but also serve as efficient surrogates for synthesizing commercially available triaryl ethanes.