Construction of Multi-Substituted gem -Difluoropyran Derivatives: Tension Ring-Expansion of gem -Difluorocyclopropene with Allyl Alcohols

A novel structurally diverse gem-difluorinated 2H- and 4H-pyrans derivatives have been efficiently synthesized by Fe(III)-catalyzed reductive radical cycloexpansion of gem-difluorocyclopropenes with allyl alcohols. Preliminary mechanistic studies suggest that the process via hydrogen atom transfer (HAT) to generate an alkyl radical, which triggers strain release of the gem-difluorocyclopropene ring and subsequent cyclization to afford the desired products. This method has the characteristics of atomic economy, complete retention of CF2 units, mild reaction conditions, broad substrate scope, and high functional group tolerance. The strategy provides new insights into the synthesis of gem-fluorinated pyrans and the ring-expansion chemistry of gem-difluorocyclopropenes.