Mechanism of the formation of carbyne complexes of rhenium upon protonation of vinylidene precursors

The reactions of trans-[ReX(=C=CHR)(dppe)(2)] with [NHEt3][BPh4] to form the carbyne complexes trans-[ReX(=CCH2R)(dppe)(2)](+) (X = Cl; R = Ph, C6H4Me-4, Bu-t, CO2Me, CO2Et; X = F; R = CO2Et; dppe = Ph2PCH2CH2PPh2) have been studied by stopped-flow spectrophotometry and shown to proceed via three pathways whose relative contribution depends on the nature of R and X. The most direct pathway involves regiospecific protonation at the beta-carbon of the vinylidene. However, under some conditions initial protonation at the metal to form [Re(H)X(=C=CHR)(dppe)(2)](+) is more rapid, and this hydride subsequently rearranges to form the carbyne by an intramolecular pathway or by protonation of [Re(H)X(=C=CHR)(dppe)(2)](+) at the beta-carbon of the vinylidene ligand to give [Re(H)X(=CCH2R)(dppe)(2)](2+), which then undergoes deprotonation to form [ReX(=CCH2R)(dppe)(2)](+). For the R = C6H5 or C6H4Me-4 complexes, kinetic analysis indicates that all three pathways occur, whereas for the bulky R = Bu-t analogue, the pathways that involve direct addition to the vinylidene ligand do not operate. For [ReX(=C=CHCO2R)(dppe)(2)] (R = Et, Me) the strong electron-withdrawing effect of the ester group results in slow proton transfer from [NHEt3](+) to the vinylidene ligand in [Re(H)Cl(=C=CHCO2R)(dppe)(2)](+). The formation of an adduct is evident from the kinetic studies with these complexes and is proposed to be the species in which [NHEt3](+) is hydrogen-bonded to the beta-carbon of the vinylidene ligand. Rate-limiting proton transfer within this adduct completes the reaction and is associated with a large primary isotope effect. The way in which the trans-halide influences this reactivity has also been investigated and fluoride shown to highly promote the rate of protonation.