Fluoro-Promoted Thermal Ring Expansion of Cyclopropyl Carbenes to <i>gem</i>-Difluorinated Cyclobutenes

Synthesis of <i>gem</i>-difluorinated cyclobutenes presents certain challenges for considering the compatibility of the fluorine atom introduction with four-membered ring retention. Herein, we develop a transition-metal-free synthetic strategy toward <i>gem</i>-difluorinated cyclobutenes from <i>gem</i>-difluorinated cyclopropyl <i>N</i>-tosylhydrazons via ring expansion reaction. The <i>gem</i>-difluoro substitution alters the properties of the cyclopropane, facilitating the thermal rearrangement of cyclopropyl carbenes into cyclobutenes. This reaction can be easily handled free from inert gas protection, thus offering an efficient route to synthesize <i>gem</i>-difluorinated cyclobutenes.