Stabilizing Effect of Pre-equilibria: A Trifluoromethyl Complex as a CF₂ Reservoir in Catalytic Olefin Difluorocarbenation

An efficient, catalytic difluorocarbenation of olefins to give 1,1-difluorocyclopropanes is presented. The catalyst, an organobismuth complex, uses TMS-CF3 as a stoichiometric difluorocarbene source. We demonstrate both the viability and robustness of this reaction over a wide range of alkenes and alkynes, including electron-poor alkenes, to generate the corresponding 1,1-difluorocyclopropanes and 1,1-difluorocyclopropenes. Ease of catalyst recovery from the reaction mixture is another attractive feature of this method. In-depth experimental and theoretical studies show that the key difluorocarbene-generating step proceeds through a bismuth nonredox synchronous mechanism generating highly reactive free CF2 in an endergonic pre-equilibrium. The reversible difluorocarbene generationand thus low concentration of active speciesleads to high selectivity while minimizing CF2-recombination side reactions.