Solvent Dependent Divergent Reactivity of Electron-Rich Dienones with and without Visible Light: Access to Cyclopropanated Furans and Butenolides

Visible light excitation of electron-rich dienones in isopropanol solvent under ambient conditions using hexafluoroisopropanol (HFIP) as an additive promotes intramolecular radical cascade cyclization to afford cyclopropanated furans. Molecular dioxygen in the air serves as a redox catalyst in this reaction which is proposed to proceed through a radical cation intermediate generated by single-electron transfer (SET) from a phototransient dienone to oxygen. By changing the solvent from isopropanol to HFIP, exclusive formation of substituted butenolides occurs via ionic cascade cyclization involving E -> Z isomerization/lactonization/thiolate addition. HFIP is assumed to act as a medium, hydrogen bond donor catalyst, and a Lewis acid substitute as well. This reaction may be conducted even in the absence of light and no catalyst or reagent is required.