Copper‐Catalyzed Cyclopropenation of Alkynes with Difluoromethyl Carbene^†

Comprehensive Summary The [2+1] cycloaddition of alkynes with fluoroalkyl carbenes represents the most straightforward approach for constructing fluoroalkylated cyclopropenes. However, until now, this strategy has not been applicable to difluoromethyl carbene, as its precursor, difluoromethyl diazomethane, tends to undergo [3+2] cycloaddition with alkynes to form pyrazoles. This study presents the first example of copper‐catalyzed cyclopropenation of alkynes with difluoromethyl carbene, employing difluoroacetaldehyde triftosylhydrazone as the carbene precursor. A wide range of internal and terminal alkynes, featuring diverse functional groups, were efficiently converted into the corresponding difluoromethyl cyclopropenes in good to high yields. Mechanistic investigations, supported by DFT calculations, revealed that the bulky Tp Br3 Cu(NCMe) catalyst plays a pivotal role in facilitating the cyclopropenation of alkynes with difluoromethyl carbenes via a concerted pathway.