Alkene metathesis is an indispensable technique in organic synthesis and polymer science. Metathesis with directly fluorinated alkenes is challenging, however, limiting the application of this powerful method in the burgeoning field of fluoro-organic chemistry. We recently reported the first persistent cobalt and nickel difluorocarbene complexes ([M]=CF2; M = Co, Ni), which undergo cycloaddition reactions with tetrafluoroethylene (CF2=CF2) to give perfluorometallacyclobutanes {[M](κ2-CF2CF2CF2-)} – the first step in the Chauvin metathesis mechanism. Until now, however, we had not observed the formal retro-cycloaddition required to advance the metathesis sequence. A new nickel fluoro(trifluoromethyl)carbene ([Ni]=CFCF3) reacts with CF2=CF2 or CF2=CH2 to yield a trifluoromethyl-substituted metallacyclobutane {[Ni][κ2-CF(CF3)CR2CF2-]} and perfluorocarbene metathesis products, [Ni]=CF2 and CR2=CFCF3 (R = F, H). Density functional theoretical calculations (DFT-M06) and experimental evidence establish that the metallacyclobutane is not an intermediate to the observed metathesis products, highlighting a novel variant of the Chauvin mechanism enabled by the disparate four-coordinate transition states. Kinetic simulation provides insights into the product ratio of metathesis vs metallacycle. Finally, reaction of [Ni]=CFCF3 with CH2=CH2 yields exclusively the metathesis alkene complex, [Ni](CH2=CFCF3), resulting presumably from decomposition of the [Ni]=CH2 metathesis intermediate.

Nickel-mediated fluorocarbene metathesis

255th National Meeting (2018)

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