194858-55-4Relevant academic research and scientific papers
Mechanism of the formation of carbyne complexes of rhenium upon protonation of vinylidene precursors
Carvalho, Maria Fernanda N. N.,Almeida, Silvia S. P. R.,Pombeiro, Armando J. L.,Henderson, Richard A.
, p. 5441 - 5448 (1997)
The reactions of trans-[ReX(=C=CHR)(dppe)2] with [NHEt3][BPh4] to form the carbyne complexes trans-[ReX(sCCH2R)(dppe)2]+ (X = Cl; R = Ph, C6H4Me-4, But, 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 β-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 subsquently rearranges to form the carbyne by an intramolecular pathway or by protonation of [Re(H)X-(=C=CHR)(dppe)2]+ at the β-carbon of the vinylidene ligand to give [Re(H)X(S=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 = But 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 β-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.
