Competing C–H and C–F bond activation reactions of a fluorinated olefin at Rh: a fluorido vinylidene complex as an intermediate in an unprecedented dehydrofluorination step

Abstract

The hydrofluoroolefin Z-1,3,3,3-tetrafluoropropene has been activated via an initial C–F bond activation and subsequent C–H bond activation using [Rh(H)(PEt₃)₃] (1) or via C–H bond activation at [Rh(CH₃)(PEt₃)₃] (8). In both cases the formation of [Rh{(E)-CFCHCF₃}(PEt₃)₃] (3) was observed. Importantly, the C–F activation product [Rh{(E)-CHCHCF₃}(PEt₃)₃] (2) reacts in the presence of Z-1,3,3,3-tetrafluoropropene into 3. The latter converted into [Rh(CCCF₃)(PEt₃)₃] (6) by an unprecedented dehydrofluorination reaction, presumably via a vinylidene complex as intermediate. When the carbonyl complex [Rh(CCCF₃)(CO)(PEt₃)₃] (12) was treated with an excess of NEt₃·3HF or HBF₄ at low temperature, the formation of the phosphonioalkenyl compounds [Rh{(Z)-C(PEt₃)CHCF₃}(CO)(PEt₃)₂]X (X = F(HF)_x, BF₄) (13) was observed. The formation of 13 can be explained by an attack of PEt₃ at the electrophilic α-carbon atom of an intermediate vinylidene complex. The employment of PⁱPr₃ derivatives as model compounds allowed for the isolation of the unique fluorido vinylidene complex trans-[Rh(F)(CCHCF₃)(PⁱPr₃)₂] (16), which in the presence of PEt₃ transforms into [Rh(CCCF₃)(PEt₃)₃] (6).