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.

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)(PEt 3 ) 3 ] (1) or via C-H bond activation at [Rh(CH 3 )(PEt 3 ) 3 ] (8). In both cases the formation of [Rh{( E )-CF[double bond, length as m-dash]CHCF 3 }(PEt 3 ) 3 ] (3) was observed. Importantly, the C-F activation product [Rh{( E )-CH[double bond, length as m-dash]CHCF 3 }(PEt 3 ) 3 ] (2) reacts in the presence of Z -1,3,3,3-tetrafluoropropene into 3. The latter converted into [Rh(C[triple bond, length as m-dash]CCF 3 )(PEt 3 ) 3 ] (6) by an unprecedented dehydrofluorination reaction, presumably via a vinylidene complex as intermediate. When the carbonyl complex [Rh(C[triple bond, length as m-dash]CCF 3 )(CO)(PEt 3 ) 3 ] (12) was treated with an excess of NEt 3 ·3HF or HBF 4 at low temperature, the formation of the phosphonioalkenyl compounds [Rh{( Z )-C(PEt 3 )[double bond, length as m-dash]CHCF 3 }(CO)(PEt 3 ) 2 ]X (X = F(HF) x , BF 4 ) (13) was observed. The formation of 13 can be explained by an attack of PEt 3 at the electrophilic α-carbon atom of an intermediate vinylidene complex. The employment of P i Pr 3 derivatives as model compounds allowed for the isolation of the unique fluorido vinylidene complex trans -[Rh(F)([double bond, length as m-dash]C[double bond, length as m-dash]CHCF 3 )(P i Pr 3 ) 2 ] (16), which in the presence of PEt 3 transforms into [Rh(C[triple bond, length as m-dash]CCF 3 )(PEt 3 ) 3 ] (6).