Mechanistic investigation of vinylic carbon–fluorine bond activation of perfluorinated cycloalkenes using Cp*2ZrH2 and Cp*2ZrHF

Cp*2ZrH2 (1) (Cp*: pentamethylcyclopentadienyl) reacts with cyclic perfluorinated olefins to give Cp*2ZrHF (2) and hydrodefluorinated products under very mild conditions. Initial C–F bond activation occurs selectively at the vinylic positions of the cycloolefin to exchange fluorine for hydrogen. Several mechanisms are discussed for this H/F exchange: (a) olefin insertion/β-fluoride elimination, (b) olefin insertion/α-fluoride elimination, and (c) hydride/fluoride σ-bond metathesis. Following H/F σ-bond metathesis exchange of both vinylic C–F bonds of perfluorocyclobutene, 1 then reacts with allylic C–F bonds by insertion/β-fluoride elimination. A similar sequence is observed with perfluorocyclopentene. Cp*2ZrHF reacts selectively with vinylic C–F bonds of perfluorocyclobutene to give 3,3,4,4-tetrafluorocyclobutene and Cp*2ZrF2 without further hydrodefluorination occurring. In the presence of excess 1 and H2, perfluorocyclobutene and perfluorocyclopentene are reduced to cyclobutane and cyclopentane in 46% and 16% yield, respectively. DFT calculations exclude the pathway by way of the olefin insertion/α-fluoride elimination and suggest that the pathway by way of hydride/fluoride σ-bond metathesis is preferred.

Cp*2ZrH2 reduces perfluorocycloolefins by a sigma-bond metathesis pathway.Figure optionsDownload as PowerPoint slide